TW202509688A - Method and apparatus for measuring a topography of a surface of an object - Google Patents

Abstract

A method for determining a correction for a height profile of a substrate comprises: securing the substrate to a substrate holder; making a first measurement of a first height profile; rotating the substrate about a global normal of the surface; making a second measurement of a second height profile; correcting the first and second measurements for a shape of the substrate holder so as to determine first and second corrected measurements respectively; and combining the first and second corrected measurements so as to determine the correction for the height profile. Making the first and second measurements of a first height profile each comprises: projecting a patterned radiation beam onto a beam spot region; moving the substrate, while in its current orientation, relative to the beam spot region; receiving a portion of the patterned radiation beam reflected from the substrate and determining the first measurement of the first height profile therefrom.

Description

Method and apparatus for measuring the topography of the surface of an object

The present invention relates to a method for determining and correcting the height profile of the surface of a substrate. The present invention also relates to a method for measuring the morphology of the surface of a substrate. The present invention also relates to a corresponding device for measuring the morphology of the surface of a substrate. The present invention is particularly used in the field of lithography. The substrate may be a substrate in a lithography device. This substrate may include a silicon wafer coated with a photoresist. The device may be called a level sensor and may form a part of a lithography device. The present invention also relates to a lithography exposure method, which uses the method or device to measure the morphology of the surface of a substrate.

A lithography apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithography apparatus may be used, for example, in the manufacture of integrated circuits (ICs). A lithography apparatus may, for example, project a pattern (also often referred to as a "design layout" or "design") of a patterned device (e.g., a mask) onto a layer of radiation-sensitive material (resist) disposed on a substrate (e.g., a wafer).

As semiconductor manufacturing processes continue to improve, the size of circuit components has been decreasing over the past few decades, while the number of functional components such as transistors per device has been increasing steadily, following a trend often referred to as "Moore's law." To keep up with Moore's law, the semiconductor industry is pursuing technologies that enable the creation of smaller and smaller features. To project a pattern onto a substrate, lithography equipment can use electromagnetic radiation. The wavelength of this radiation determines the minimum size of the features patterned onto the substrate. Typical wavelengths currently used are 365 nm (i-line), 248 nm, 193 nm, and 13.5 nm. Lithography equipment using extreme ultraviolet (EUV) radiation having a wavelength in the range of 4 nm to 20 nm (eg, 6.7 nm or 13.5 nm) may be used to form smaller features on a substrate compared to lithography equipment using radiation having a wavelength of, for example, 193 nm.

Before exposing the wafer to patterned radiation in a lithography apparatus, a device known as a level sensor may be used to determine the topography of the wafer. This measurement of the topography of the wafer may be performed within the lithography apparatus, for example, once the wafer has been clamped to a wafer stage. This information may be used during subsequent exposure of the wafer in order to keep the exposed portion of the wafer in the plane of best focus.

It may be desirable to provide new methods and/or apparatus for determining the topography of a wafer that may at least partially address one or more problems associated with prior arrangements, whether identified herein or otherwise.

According to a first aspect of the present disclosure, a method for determining a correction for a height profile of a surface of a substrate is provided, the method comprising: fixing the substrate to a substrate holder; performing a first measurement of a first height profile by: projecting a patterned radiation beam onto a beam spot; moving the substrate relative to the beam spot when the substrate is in a first orientation; receiving a portion of the patterned radiation beam reflected from the substrate and determining the first measurement of the first height profile based on the portion; rotating the substrate about a global normal to the surface to a second orientation; and performing a first measurement of the first height profile by: projecting a patterned radiation beam onto a beam spot; moving the substrate relative to the beam spot when the substrate is in a first orientation; receiving a portion of the patterned radiation beam reflected from the substrate and determining the first measurement of the first height profile based on the portion; rotating the substrate about a global normal to the surface to a second orientation; and performing a first measurement of the first height profile by: A second measurement of a second height profile is performed by: projecting a patterned radiation beam onto the beam spot; moving the substrate relative to the beam spot when the substrate is in the second orientation; receiving a portion of the patterned radiation beam reflected from the substrate and determining the second measurement of the second height profile based on the portion; correcting the first measurement and the second measurement for a shape of the substrate holder to determine first and second corrected measurements, respectively; and combining the first and second corrected measurements to determine the correction for the height profile.

The method according to the first aspect is advantageous, as now discussed.

The substrate may be referred to as an object. The substrate may be a substrate within a lithography apparatus. The substrate may comprise a silicon wafer coated with a photoresist. It will be appreciated that the substrate comprises a relatively thin disk of flat material comprising two opposing substantially circular surfaces. It will also be appreciated that the surface of the substrate is not completely flat and therefore there will be some variation in the local normal to the surface of the substrate. However, the global normal to the substrate is the normal to the global or average plane of the surface of the substrate. The lithography apparatus may be used to expose a substrate to radiation that has been patterned by a reticle or mask. Prior to exposing the substrate to the patterned radiation, the topology of the surface of the substrate may be determined. For example, a height map of the surface of the substrate may be determined, for example, using a step sensor. The measured topography of the surface of the substrate can then be used to control the height of the substrate when it is exposed to patterned radiation, for example to keep the substrate in the best focus plane of the image of the patterning device.

A type of level sensor can be used to measure a height profile of a surface of a substrate by projecting a patterned radiation beam onto a beam spot; moving the substrate relative to the beam spot; receiving a portion of the patterned radiation beam reflected from the object, and determining a first measurement of height based on the portion. As the height of the substrate changes, the position of the pattern of reflected radiation can change, for example, relative to a splitting optic configured to split the reflected radiation into a first portion and a second portion.

The inventors of the present invention have recognized that a height map determined in this manner will typically have contributions from both: (a) the shape of the substrate holder (e.g., a clamp) to which the substrate is clamped (since once clamped, the substrate will follow the shape of the clamp); and (b) the process layers formed on the surface. A silicon wafer may include one or more layers that have been previously formed, for example using a lithography process. Generally, there will be a series of features of different materials and/or different densities across the surface of such a wafer. Furthermore, the inventors have recognized that features on the process layers can cause a number of errors in the height measurement and that these layers depend on the orientation of the substrate as it is scanned across the beam spot area. For example, multiple errors in height measurement caused by features on a process layer change sign when the substrate is rotated 180° about its global normal (and may therefore be described herein as anti-symmetric errors). In contrast, the (actual) contribution of the fixture shape to the height measurement is not anti-symmetric. By correcting the first and second measurements of the height for the shape of the fixture in order to determine the first and second corrected measurements of the height of the surface, respectively, the data becomes independent of the shape of the fixture. Subsequently, when the first and second corrected measurements of the height are combined, the anti-symmetric process-related errors may be accurately determined. For example, if the two orientations are opposite (i.e., the substrate has been rotated 180° about its global normal between the two measurements), then the first and second corrected measurements of the height profile may be given by and Given, where The antisymmetric error can be determined as half the difference between the first calibrated measurement and the second calibrated measurement of the height.

Variations in the reflectivity of a wafer across its surface can occur because different materials absorb different rates of incident radiation and features of different densities can cause different amounts of scattering of incident radiation. For example, a 3D-NAND wafer can contain features that can reduce the amount of specular reflection of radiation by up to 50%. There are measured variations in height profiles at transition regions due to variations in substrate reflectivity. .

It should be understood that determining the height profile of an object includes determining the height profile of the object relative to a reference height or position.

Correcting a measurement of a height profile for a shape of the substrate holder so as to determine a corrected measurement may include: securing a reference substrate to the substrate holder; making a reference measurement of the height profile of the reference substrate by: projecting a patterned radiation beam onto a beam spot; moving the substrate relative to the beam spot when the substrate holder is in the same orientation as it was when determining the corrected measurement; receiving a portion of the patterned radiation beam reflected from the substrate and determining the reference measurement of the height profile based on the portion; and determining the corrected measurement of the height profile as a difference between the measurement and the reference measurement.

The reference substrate may be referred to as a reference object. The reference substrate may be a bare wafer. That is, the reference object may be a silicon wafer without a processing layer applied thereto.

Rotating the global normal of the substrate surrounding surface to a second orientation may include rotating the global normal of the substrate surrounding surface by 180°.

For such embodiments, many errors in the height measurement caused by features on the processing layer contribute to the first and second measurements having the same magnitude but opposite signs (i.e., antisymmetric errors). For example, for such embodiments, the antisymmetric error can be determined as half the difference between the first and second corrected measurements of height.

The correction can be determined as half of the difference between the first corrected measurement and the second corrected measurement of the height profile.

Each of the first measurement and the second measurement of the height profile may include measuring the height of the surface at a plurality of locations on the object.

It will be appreciated that combining a first calibrated measurement and a second calibrated measurement of the height profile to determine a calibration for the height profile of a surface of an object may comprise combining measurements from the first calibrated measurement and the second calibrated measurement of the height profile corresponding to the same location on the object.

Each of the first and second measurements of the height profile of the surface may include measuring a height map of the surface for substantially the entire surface of the substrate. That is, the height map (and therefore the correction) may be determined for substantially the entire object (wafer).

Alternatively, each of the first and second measurements of the height profile may include measuring a height map of the surface for one or more of a plurality of regions of the surface of the object to which the same pattern is to be applied.

That is, the height map (and therefore the correction) may be determined for one or more fields or dies of a substrate (wafer). In some embodiments, the height map (and therefore the correction) may be determined for only one (or a relatively small portion) of a plurality of regions of the surface of a substrate to which the same pattern is to be applied.

The method may further include correcting the first and second measurements of the height profile for local variations in slope of a surface of the substrate before correcting the first and second measurements of the height profile for a shape of the substrate holder. The method may further include correcting the first and second reference measurements of the height profile for local variations in slope of a surface of a reference substrate before correcting the first and second measurements of the height profile for a shape of the substrate holder.

In general, variations in the slope of the surface of the substrate will affect the focusing of the patterned radiation beam onto the beam spot. This in turn will produce height measurement errors that depend on the slope or gradient of the surface of the substrate. A key factor causing local variations in the slope of the surface of the substrate may be the shape of the substrate holder (or fixture) to which the substrate is fixed/clamped. Such errors are typically antisymmetric errors (and therefore may form part of the correction determined using the method according to the first embodiment). However, in general, such errors may be subject to significant local variations across the surface of the substrate. For example, the substrate (which may be an etch resist coated silicon wafer) may include a plurality of generally rectangular regions (which may be referred to as fields or grains) to which substantially the same pattern is to be applied (and has been applied when forming previous process layers). There may be significant variations from field to field (or grain to grain) in errors due to local variations in the slope of the surface of the substrate. This is in contrast to some other antisymmetric errors which may depend primarily on features in the process layers on the substrate and which may therefore be substantially the same for all fields or grains.

The method may further include storing the correction in a memory.

The inventors have also recognized that the correction determined by the method according to the first aspect of the present disclosure depends primarily on: (a) the substrate holder (or fixture); and (b) the lithography process (i.e., layer recipe). If, as is typical, multiple wafers are to be processed by a given lithography apparatus using the same lithography process, the correction will be substantially the same for all such wafers. Therefore, in some embodiments, the correction can be determined once and then used for all subsequent wafers processed on the same fixture and using the same lithography process. Furthermore, since the method according to the first aspect comprises correcting the first measurement and the second measurement for the shape of the substrate holder so as to determine the first corrected measurement and the second corrected measurement, respectively, before the first corrected measurement and the second corrected measurement are combined to determine the correction for the height profile, the correction determined by the method is substantially independent of the substrate holder or fixture. Thus, in some embodiments, the correction can be determined once in one lithography apparatus (using a fixture) and then used for all subsequent wafers processed using the same lithography process (even on different fixtures and even in different lithography apparatuses). Furthermore, in some embodiments, the correction can be determined once for a single field or die of a wafer and then used for all fields or die of all subsequent wafers processed using the same lithography process.

According to a second aspect of the present disclosure, a method for measuring a topography of a surface of a substrate is provided, the method comprising: measuring a height profile of the surface by the following operations: projecting a patterned radiation beam onto a beam spot; moving an object relative to the beam spot when the object is in a first orientation; receiving a portion of the patterned radiation beam reflected from the object and determining the height profile based on the portion; and combining the measurement of the height profile with a correction of a height profile for a substrate determined using a method such as any of the aforementioned technical solutions to determine a corrected measurement of the height profile.

The method according to the second aspect is advantageous because it provides calibrated measurements that have been calibrated using the calibration determined by the method according to the first aspect. For example, this may allow accurate correction of antisymmetric errors.

The corrected measurement of the height profile can be determined as the measurement of the height profile minus the correction.

The method may further comprise determining the correction for a height profile using the method of the first aspect of the present disclosure.

For at least some wafers, the method may include first determining a calibration (using a method according to a first aspect) and then using this calibration to calibrate the height measurement (using a method according to a second aspect). For example, the calibration may be determined for at least a first wafer processed using a given lithography process or recipe.

The method may include retrieving the correction from memory.

Combining the measurement of the height profile with a correction for a height profile of a substrate may include combining the measurement with a correction for substantially the same location on the substrate.

For example, in some embodiments, a calibration for the height of a surface of an object may be determined for substantially the entire object (wafer). Similarly, a measurement of the height of a surface may be determined for substantially the entire object (wafer) and may be combined with the calibration to determine a calibrated measurement of the height of the surface.

The substrate may include a plurality of regions to which the same pattern is to be applied, and combining the measurement of the height profile with a correction for the height profile of the substrate may include combining the measurement for each of the plurality of regions with a correction for one of the plurality of regions.

That is, in some embodiments, a calibration may be determined for only one (or a relatively small portion) of a plurality of regions of a surface of an object to which the same pattern is to be applied. The calibration for that single region of the plurality of regions of the object may be used to calibrate height measurements for each of the plurality of regions of the object.

In the method of any of the first and second aspects of the present disclosure, a measurement of a height profile determined based on a portion of a patterned radiation beam reflected from the substrate may include: receiving the portion of the radiation beam reflected from the substrate and splitting the reflected radiation into a first portion and a second portion; determining an intensity of the first portion and the second portion of the radiation; and determining a height of the substrate by combining the intensity of the first portion of the radiation with the intensity of the second portion of the radiation.

Advantageously, by splitting the reflected radiation into a first portion and a second portion and by combining the intensities of the first portion and the second portion to determine the height of the substrate, the height can be determined substantially independently of the intensity of the radiation beam. For example, the height can be determined as a differential measurement.

Splitting the reflected radiation into a first portion and a second portion may include forming an image of the pattern on splitting optics, and using the splitting optics to direct radiation from the first portion and the second portion of the second image so as to be spatially separated.

The position of the second image relative to the splitting optics can determine how much reflected radiation is directed into each of the first and second portions.

The height of the substrate may be proportional to the difference between the first intensity and the second intensity.

For example, the height of an object may be determined to be proportional to the difference between the first intensity and the second intensity divided by the sum of the first intensity and the second intensity.

According to a third aspect of the present disclosure, a lithography exposure method is provided, which includes the following operations: using the method of the second aspect of the present disclosure to measure a morphology of a surface of a substrate; using a patterning device to pattern a radiation beam; and projecting the patterned radiation onto the substrate to form an image of the patterning device on the substrate; wherein a position of the substrate when the patterned radiation is projected onto the substrate is controlled depending on the measured morphology of the surface of the substrate.

Advantageously, the measured topography of the surface of the substrate can be used to control the height of the substrate when the substrate is exposed to patterned radiation, for example to keep the substrate in the best focus plane of the image of the patterned device. It should be understood that the image of the patterned device formed on the substrate can be a diffraction limited image.

The lithographic exposure may be a scanning exposure such that patterning a radiation beam using a patterning device includes moving the patterning device through the radiation beam, and projecting the patterned radiation onto the substrate to form an image of the patterning device on the substrate includes moving the substrate so that the image of the patterning device is substantially stationary relative to the substrate.

That is, in order to image a pattern onto a target area of the substrate, the patterning device is moved in a scanning direction through the illumination area or scanned across the illumination area. It will be appreciated that the substrate is also scanned relative to the illumination area in the plane of the substrate. The movement of the substrate makes the aerial image of the patterning device static relative to the substrate, and it will be appreciated that the direction and/or speed of the substrate may generally be different from the direction and/or speed of the patterning device (e.g., if the image is inverted and/or if the projection system applies a reduced portion).

According to a fourth aspect of the present disclosure, there is provided an apparatus for measuring a morphology of a surface of a substrate, the apparatus comprising: a support member for supporting a substrate, projection optical components for forming a first image of a pattern on a beam spot area by a radiation beam; a moving mechanism for causing relative movement of the support member relative to the beam spot area; detection optical components for receiving a portion of the radiation beam reflected from the substrate; and a controller for determining a height of the substrate based on the radiation beam reflected from the substrate and further for implementing any one of the methods of the first aspect, the second aspect and the third aspect of the present disclosure.

The apparatus may be referred to as a one-step sensor. The apparatus may form part of a lithography apparatus. A support for supporting a substrate may include a substrate holder that may be used to secure the substrate. For example, the support may include a clamp for clamping the substrate to the support. An apparatus according to the fourth aspect is advantageous because it allows calibrated height measurements that have been corrected using a correction determined by the method according to the first aspect. For example, this may allow antisymmetric errors in the height measurement to be accurately corrected.

The moving mechanism can be used to move the support relative to the beam spot. Additionally or alternatively, the moving mechanism can be used to move the beam spot relative to the support. For example, movement of the beam spot can be achieved by moving the projection optics. For such embodiments, the moving mechanism can also be used to move the detection optics.

The detection optics may be used to receive a portion of the radiation beam reflected from the substrate and split the reflected radiation into a first portion and a second portion such that a first portion of the radiation corresponding to a first portion of the first image is spatially separated from a second portion of the radiation corresponding to a second portion of the first image. The apparatus may further include: a first detector configured to determine an intensity of the first portion of the radiation; and a second detector configured to determine an intensity of the second portion of the radiation; and the controller may be used to determine a height profile of the substrate by combining the intensity of the first portion of the radiation with the intensity of the second portion of the radiation.

Advantageously, because the detection optics can be used to split the reflected radiation into a first portion and a second portion, and the controller can be used to determine the height of the substrate by combining the intensities of the first portion and the second portion, the height is determined substantially independent of the intensity of the radiation beam. For example, the height can be determined as a differential measurement.

The projection optical element may include: a projection patterning device; and a first imaging optical element, which is configured to form an image of the projection patterning device on the beam spot area.

The projection patterning device may include a grating. The grating may include a plurality of lines. The lines may have a uniform thickness. The grating may have a 50% duty cycle.

The detection optical element may include: a splitting optical element configured to split the reflected radiation into a first portion and a second portion; and a second imaging optical element configured to receive radiation reflected from an object supported by the support and form a second image of the pattern on the splitting optical element.

The first imaging optical element may be substantially equivalent to the second imaging optical element.

The splitting optics, beam spot, and projection patterning device are all in optically conjugate planes. It will be appreciated that two planes are optically conjugate if all radiation passing through different points in the first plane is imaged onto different points in the second plane.

An image of the projected patterning device is formed on the split optic, the position of the image indicating the height of the object. In particular, the position of the image relative to the split optic indicates the height of the object. As explained above, the projected patterning device may include a grating, the grating including a plurality of lines. The split optic may include a plurality of prisms, and the image of each line may be imaged onto one of a plurality of generally triangular prisms such that a first portion of the line is incident on a first surface of the prism and a second portion of the line is incident on a second surface of the prism. The first portion of the line is directed to a first detector and the second portion of the line is directed to a second detector. As the line moves relative to the prism (due to changes in the height of the object), the amount of radiation directed to each of the detectors changes.

The apparatus may further comprise a radiation source which may be used to generate a radiation beam.

According to a fifth aspect of the present disclosure, a lithography device is provided, which includes the device of the fourth aspect of the present disclosure.

The lithography apparatus may further include: an illumination system, which can be used to illuminate an illumination area; a support structure, which is configured to support a patterning device so that the patterning device can be positioned in the illumination area; a substrate stage, which is configured to support a substrate; and a projection system, which can be used to form an image of a patterning device supported by the support structure on a substrate supported by the substrate stage.

Cross-reference to related applications

This application claims priority to European patent application 23173024.3 filed on May 12, 2023, which is incorporated herein by reference in its entirety.

In this document, the terms "radiation" and "beam" are used to cover all types of electromagnetic radiation, including UV radiation (e.g., having a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultraviolet radiation, e.g., having a wavelength in the range of about 5 to 100 nm).

The terms "reduction mask", "mask" or "patterning device" as used herein may be broadly interpreted as referring to a general patterning device that can be used to impart a patterned cross-section to an incident radiation beam, the patterned cross-section corresponding to the pattern to be produced in a target portion of a substrate. In this context, the term "light valve" may also be used. In addition to classical masks (transmissive or reflective, binary, phase-shifting, hybrid, etc.), other examples of such patterning devices include programmable mirror arrays and programmable LCD arrays.

FIG1 schematically depicts a lithography apparatus LA. The lithography apparatus LA comprises an illumination system (also referred to as an illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation, or EUV radiation); a mask support (e.g., a mask stage) MT constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA according to certain parameters; a substrate support (e.g., a wafer stage) WT constructed to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support according to certain parameters; and a projection system (e.g., a refractive projection lens system) PS is configured to project the pattern imparted to the radiation beam B by the patterning device MA onto a target portion C of the substrate W (eg, comprising one or more dies).

In operation, the illumination system IL receives a radiation beam from a radiation source SO, for example via a beam delivery system BD. The illumination system IL may include various types of optical components for directing, shaping and/or controlling the radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic and/or other types of optical components, or any combination thereof. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross-section at the plane of the patterning device MA.

The term "projection system" PS as used herein should be interpreted broadly as covering various types of projection systems appropriate to the exposure radiation used and/or to other factors such as the use of an immersion liquid or the use of a vacuum, including refractive, reflective, catadioptric, synthetic, magnetic, electromagnetic and/or electro-optical systems, or any combination thereof. Any use of the term "projection lens" herein should be considered synonymous with the more general term "projection system" PS.

The lithography apparatus LA may be of a type in which at least a portion of the substrate may be covered by a liquid with a relatively high refractive index, such as water, in order to fill the space between the projection system PS and the substrate W, which is also known as immersion lithography. More information on immersion technology is given in US6952253, which is incorporated herein by reference.

The lithography apparatus LA may also be of a type having two or more substrate supports WT (also known as a "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel, and/or a step of preparing the substrate W for subsequent exposure may be performed on a substrate W on one of the substrate supports WT while another substrate W on another substrate support WT is being used to expose a pattern on another substrate W.

In addition to the substrate support WT, the lithography apparatus LA may also comprise a measuring stage. The measuring stage is configured to hold sensors and/or cleaning devices. The sensors may be configured to measure properties of the projection system PS or of the radiation beam B. The measuring stage may hold a plurality of sensors. The cleaning device may be configured to clean parts of the lithography apparatus, for example parts of the projection system PS or parts of a system for providing an immersion liquid. The measuring stage may be moved under the projection system PS when the substrate support WT is away from the projection system PS.

In operation, a radiation beam B is incident on a patterning device (e.g. a mask) MA held on a mask support MT and is patterned by a pattern (design layout) present on the patterning device MA. Having traversed the mask MA, the radiation beam B passes through a projection system PS which focuses the beam onto a target portion C of the substrate W. By means of a second positioner PW and a position measurement system IF, the substrate support WT can be accurately moved, for example, in order to position different target portions C at focused and aligned positions in the path of the radiation beam B. Similarly, a first positioner PM and possibly a further position sensor (which is not explicitly depicted in FIG. 1 ) can be used to accurately position the patterning device MA relative to the path of the radiation beam B. The mask alignment marks M1, M2 and substrate alignment marks P1, P2 may be used to align the patterning device MA and the substrate W. Although the substrate alignment marks P1, P2 are shown occupying dedicated target portions, the marks may be located in the space between target portions C. When the substrate alignment marks P1, P2 are located between target portions C, the substrate alignment marks are referred to as scribe line alignment marks.

To illustrate the present invention, a Cartesian coordinate system is used. A Cartesian coordinate system has three axes, namely the x-axis, the y-axis and the z-axis. Each of the three axes is orthogonal to the other two axes. A rotation about the x-axis is called an Rx rotation. A rotation about the y-axis is called an Ry rotation. A rotation about the z-axis is called an Rz rotation. The x-axis and the y-axis define a horizontal plane, while the z-axis is in the vertical direction. The Cartesian coordinate system does not limit the present invention and is only used for illustration. In fact, another coordinate system, such as a cylindrical coordinate system, can be used to illustrate the present invention. The orientation of the Cartesian coordinate system may be different, for example, so that the z-axis has a component along the horizontal plane.

A topography measurement system, a step sensor, or a height sensor that may be integrated into a lithography apparatus is configured to measure the topography of a top surface of a substrate (or wafer). A map of the topography of a substrate (also referred to as a height map) may be generated from such measurements that indicates the height of the substrate as a function of position on the substrate. This height map may then be used to correct the position of the substrate during transfer of the pattern onto the substrate so as to provide an aerial image of the patterned device in focus on the substrate. It should be understood that "height" in this context generally refers to the dimension out of plane to the substrate (also referred to as the Z axis). Typically, a step or height sensor performs measurements at a fixed position (relative to its own optical system), and relative movement between the substrate and the optical system of the step or height sensor results in height measurements at locations across the substrate.

A level or height sensor LS known in the art is schematically shown in FIG2 , which merely illustrates the operating principle. In this example, the level sensor LS comprises an optical system, which includes a projection unit LSP and a detection unit LSD. The projection unit LSP comprises a radiation source LSO providing a radiation beam LSB, which is patterned by a projection grating PGR of the projection unit LSP. The projection grating PGR may alternatively be referred to as a patterning device PGR. The radiation source LSO may be, for example, a narrowband or broadband radiation source, such as a polarized or non-polarized, pulsed or continuous supercontinuum light source, such as a polarized or non-polarized laser beam. The radiation source LSO may comprise a plurality of radiation sources of different colors or wavelength ranges, such as a plurality of LEDs. The radiation source LSO of the level sensor LS is not limited to visible radiation, but may additionally or alternatively cover UV and/or IR radiation and any range of wavelengths suitable for reflection from the surface of the substrate.

The projection grating PGR is a periodic grating comprising a periodic structure which generates a radiation beam BE1 with a periodically varying intensity. The radiation beam BE1 with a periodically varying intensity is directed to a measurement location MLO on the substrate W, the radiation beam having an incident angle ANG between 0 and 90 degrees, typically between 70 and 80 degrees, relative to an axis (Z axis) perpendicular to the incident substrate surface. The measurement location MLO may alternatively be referred to as a beam spot MLO. At the measurement location MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by arrow BE2) and directed to the detection unit LSD.

In order to determine the height level at the measurement location MLO, the level sensor further comprises a detection system, which comprises a detection grating DGR, a detector DET and a processing unit (not shown) for processing an output signal of the detector DET. The detection grating DGR may be equivalent to the projection grating PGR. The detector DET generates a detector output signal, which indicates the received light, for example, indicating the intensity of the received light, such as can be output by a light detector, or represents the spatial distribution of the received intensity, such as can be output by a camera or a sensor array. The detector DET may include any combination of one or more detector types.

By means of triangulation techniques, the height level at the measurement location MLO can be determined. The detected height level is usually related to the signal strength measured by the detector DET, which has a periodicity that depends inter alia on the design of the projection grating PGR and the (tilted) angle of incidence ANG.

The projection unit LSP and/or the detection unit LSD may include further optical elements, such as lenses and/or mirrors, along the path (not shown) of the patterned radiation beam between the projection grating PGR and the detection grating DGR.

In one embodiment, the detection grating DGR may be omitted and the detector DET may be placed at the location where the detection grating DGR is located. This configuration provides a more direct detection of the image of the projection grating PGR.

In order to effectively cover the surface of the substrate W, the level sensor LS may be configured to project an array of measurement beams BE1 onto the surface of the substrate W, thereby generating an array of measurement regions MLO or light spots covering a larger measurement range.

Various height sensors of general type are disclosed in, for example, US7265364 and US7646471, both of which are incorporated by reference. A height sensor using UV radiation rather than visible or infrared radiation is disclosed in US2010233600A1, incorporated by reference. In WO2016102127A1, incorporated by reference, a compact height sensor is described that uses a multi-element detector to detect and identify the position of a grating image without the need for a detection grating.

In general, the detection unit LSD may be configured such that the reflected radiation BE2 is split into a first portion and a second portion, and the height of the substrate W is determined by combining the intensities of the first portion and the second portion. For example, the height may be determined as a differential measurement. Advantageously, with this configuration, the determination of the height of the substrate W may be substantially independent of the intensity of the radiation beam BE1. In practice, the splitting of the radiation into the first portion and the second portion may be achieved in a number of different ways.

For example, in some known configurations, a combination of a polarizer and a shear plate (e.g. in the form of a Wollaston prism) is used to form two laterally displaced images (each with a different polarization state) of a projection grating PGR on a detection grating DGR. An example of such a configuration is schematically shown in FIG. 5 of US2010233600A1. For example, the projection grating PGR may have a pitch P and a duty cycle of 50%, so that a radiation beam BE1 with periodically different intensities comprises a plurality of lines of thickness P/2, with adjacent lines spaced apart by P/2. The polarizer and shear plate are arranged to form two images of the projection grating PGR (each with a different polarization state) on the detection grating DGR, one image being laterally displaced by P/2 relative to the other image. Downstream of the detection grating DGR, the two independent polarization states are each directed to a different detector. The height of the substrate W is determined to be proportional to the difference in the intensity of the two individual polarization states.

In some other known configurations, instead of using two images of the projection grating PGR with different polarization states to split the reflected radiation BE2, a single image of the projection grating PGR is formed on a splitting optic configured to split the single image into a first portion and a second portion. Examples of such configurations are schematically shown in FIG. 6 of US2010233600A1 and FIG. 2 of WO2016102127A1. For example, such configurations typically include a splitting optic configured to split the reflected radiation into a first portion and a second portion. The splitting optic may be a ruled grating having a triangular grating profile that acts as a series of wedges or prisms to redirect the reflected radiation BE2 (according to Snell's law). This type of split optic can be viewed as comprising a plurality of prisms and the image of each line of the projection grating PGR can be imaged onto one of a plurality of generally triangular prisms such that a first portion of the line is incident on a first surface of the prism and a second portion of the line is incident on a second surface of the prism. The first portion of the line is directed to a first detector and the second portion of the line is directed to a second detector. As the line moves relative to the prism (due to changes in the height of the substrate W), the amount of radiation directed to each of the detectors changes. Embodiments of the present disclosure have particular application to position sensors using this type of split optic.

Some embodiments of the present disclosure relate to a method for calibrating height profile determination for a surface of a substrate W, as now discussed with reference to FIG 3. As used herein, a surface of a substrate may refer to a bare (unprocessed) substrate or a surface of a processed substrate having one or more material deposits and/or resist layers.

3 is a schematic representation of a method 100 for calibrating height profile determination of a surface of a substrate, such as substrate W. For example, the method 100 may be implemented using a level sensor LS generally in the form shown in FIG.

The method 100 includes a step 110 of securing a substrate W to a substrate holder. For example, the substrate holder may include a clamp (e.g., an electrostatic clamp) that can be used to clamp the substrate W to a support, such as a wafer stage WT in a lithography apparatus LA.

The method 100 further comprises a step 120 of taking a first measurement of a first height profile when the substrate W is in the first orientation.

The method 100 further comprises a step 130 of rotating the global normal of the surface of the substrate W around to a second orientation. The substrate W may be a substrate W within a lithography apparatus LA. The substrate W may comprise a silicon wafer coated with photoresist. It should be understood that the substrate W comprises a relatively thin disk of flat material comprising two opposing substantially circular surfaces. It should also be understood that the surface of the substrate W is not completely flat and therefore, there will be some variation in the local normal of the surface of the substrate W. However, the global normal of the substrate W is the normal to the global or average plane of the surface of the substrate W.

The method 100 further comprises a step 140 of taking a second measurement of the second height profile when the substrate W is in the second orientation.

The method 100 further comprises a step 150 of correcting the first measurement and the second measurement (as determined at steps 120 and 140) for the shape of the substrate holder to determine a first corrected measurement and a second corrected measurement, respectively.

The method 100 further comprises a step 160 of combining the first calibrated measurement with the second calibrated measurement to determine a correction for the height profile.

Step 120 includes taking a first measurement of the first height profile, and step 140 includes taking a second measurement of the second height profile. Each of these measurements of the height profile includes the following sub-steps: 170a projecting a patterned radiation beam onto a beam spot; 170b moving the substrate W relative to the beam spot when the substrate W is in its current orientation (a first orientation for the first measurement 120 or a second orientation for the second measurement 140); 170c receiving a portion of the patterned radiation beam reflected from the substrate W; and 170d determining the measurement of the height profile based on the portion of the patterned radiation beam reflected from the substrate W.

The step 170a of projecting the patterned radiation beam onto the beam spot may include forming a first image of the pattern on the beam spot with the radiation beam. Forming the first image of the pattern on the beam spot MLO may include: providing the radiation beam LSB; patterning the radiation beam LSB with a patterning device (such as a projection grating PGR); and projecting the patterned radiation BE1 onto the beam spot MLO using projection optics. For example, the projection unit LSP may be used to form a first image of the projection grating PGR (pattern) on the measurement location MLO (beam spot) with the radiation beam LSB.

It should be understood that although step 170b of moving the substrate W relative to the beam spot area (e.g., the measurement position MLO) when the substrate W is in the first orientation is described here as moving the substrate relative to the beam spot area, in alternative embodiments, the beam spot area may be moved relative to the substrate W (e.g., by moving the projection unit LSP and the detection unit LSD while the object W remains stationary).

Moving the substrate W relative to the beam spot MLO may include scanning the substrate W relative to the beam spot MLO. This scanning may be performed at a constant speed or a variable speed. As used herein, scanning of the substrate W means continuous movement of the substrate W. Alternatively, moving the substrate W relative to the beam spot MLO may include stepping the substrate W relative to the beam spot MLO. As used herein, stepping of the substrate W means movement of the substrate W in a plurality of consecutive (separated in time) steps.

The substrate W is secured to a substrate holder (see step 110), such as a wafer stage WT within the lithography apparatus LA. Thus, moving the substrate W relative to the beam spot MLO may include moving the substrate holder WT.

The step 170c of receiving a portion of the patterned radiation beam reflected from the substrate W may include receiving the reflected radiation BE2 with a detection unit LSD of a level sensor LS.

The method 100 for determining and calibrating a height profile of a surface of a substrate (eg, substrate W) shown in FIG. 3 is advantageous as will now be discussed.

The substrate W may be a substrate W within a lithography apparatus LA. This substrate W may comprise a silicon wafer coated with a photoresist. The lithography apparatus LA may be used to expose the substrate W to radiation B which has been patterned by a mask or mask MA. Before exposing the substrate W to the patterned radiation, the topology of the surface of the substrate W may be determined. For example, a height map of the surface of the substrate W may be determined, for example using a level sensor LS. Subsequently, the measured morphology of the surface of the substrate W may be used to control the height of the substrate W when the substrate W is exposed to the patterned radiation, for example to keep the substrate W in the best focus plane for the image of the patterning device MA.

One type of level sensor LS may be used to measure the height profile of the surface of a substrate W by projecting a patterned radiation beam BE1 onto a beam spot MLO, moving the substrate W relative to the beam spot MLO, receiving a portion of the patterned radiation beam BE2 reflected from the substrate W and determining the height of the substrate W based on the portion. As the height of the substrate W changes, the position of the pattern of the reflected radiation BE2 may change, for example, relative to a splitting optic DGR configured to split the reflected radiation into a first portion and a second portion.

The inventors of the present invention have recognized that a height map determined in this way will typically have contributions from both: (a) the shape of the substrate holder (e.g., a clamp or wafer stage WT) to which the substrate W is clamped (since once clamped, the substrate W will follow the shape of the clamp); and (b) the process layers formed on the surface. A silicon wafer W may include one or more layers previously formed, for example using a lithography process. Generally, there will be a range of features of different materials and/or different densities across the surface of such a wafer. Furthermore, the inventors have recognized that features on the process layers can cause a number of errors in the height measurement, and that these layers depend on the orientation of the substrate W as it is scanned across the beam spot area MLO. For example, many errors in the height measurement caused by features on the process layer change sign when the substrate W is rotated 180° about its global normal (and may therefore be described herein as anti-symmetric errors). In contrast, the (actual) contribution of the fixture shape to the height measurement is not anti-symmetric. By correcting the first and second measurements of the height for the shape of the fixture (at step 150) in order to determine, respectively, a first and second corrected measurement of the height of the surface, the data becomes independent of the shape of the fixture. Subsequently, when the first and second corrected measurements of the height are combined, the anti-symmetric process-related errors may be accurately determined. For example, if the two orientations are opposite (i.e., the substrate W has been rotated 180° about its global normal at step 130 between the two measurements at steps 120, 140), then the first corrected measurement and the second corrected measurement of the height profile may be given by and Given, where The antisymmetric error can be determined as half the difference between the first calibrated measurement and the second calibrated measurement of the height.

Variations in the reflectivity of a wafer W across its surface can occur because different materials absorb different rates of incident radiation and features of different densities can cause different amounts of scattering of incident radiation. For example, a 3D-NAND wafer W can contain features that can cause a reduction in the amount of specular reflection of radiation by up to 50%. There are measured variations in height profiles at transition regions due to variations in substrate reflectivity. .

It should be appreciated that determining the height profile of the substrate W includes determining the height profile of the substrate W relative to a reference height or position, as is known in the art.

Although method 100 is shown in Fig. 3 as six separate steps 110, 120, 130, 140, 150, 160, this is only for ease of understanding, and it should be understood that the steps can be performed in any order. In some embodiments, method 100 is performed within an extended measurement time period, and therefore, these steps can be concurrent.

There are three types of step sensor height measurement errors that depend on the orientation of the substrate W when measuring a wafer having a height Z. Each of these three errors depends on the orientation of the wafer W (0° versus 180°) as follows, as described with reference to FIG.

The first error is the phase step error (see Figure 11(a)). This error is due to the actual height step encountered during the measurement. At this height step, there are two features in the raw height measurement, rather than the height simply increasing from one step (on the left side of Figure 11(a)) to another step (on the right side of Figure 11(a)). Due to these features, the height measurement drops to a lower height than on the left side of Figure 11(a) and rises to a higher height than on the left side of Figure 11(b). The magnitudes of these two features can be expressed as rotationally symmetric components and rotationally antisymmetric components It should be noted that if the surface slope error (see FIG. 11( c ) discussed below) is non-zero, then the antisymmetric component is non-zero (i.e. ).

The second error is the reflectivity step error (or reflectivity error) (see FIG. 11( b)). This error is due to the reflectivity step (no change in the height of the wafer W). This reflectivity error is further discussed below with reference to FIG. 6 to FIG. 7B. ) is rotationally antisymmetric, so that for 0°, the error is , and for 180°, the error is .

The third error is the surface slope error (see FIG. 11( c )). This error is due to local variations in the slope of the surface of the wafer W. Generally speaking, variations in the slope of the surface of the substrate W will affect the focusing of the patterned radiation beam onto the beam spot MLO. This in turn will produce height measurement errors that depend on the slope or gradient of the surface of the substrate ( ). This surface slope error is proportional to the local wafer slope and is rotationally antisymmetric, so that for 0°, the error is , and for 180°, the error is .

In addition to these anti-symmetric errors, the contribution of the shape of the substrate holder or fixture to the height measurement also depends on the orientation of the wafer W, but is not anti-symmetric.

The actual height is determined when the orientation of the wafer W is 0° and 180°. Two measurements of the height of the wafer W and It can be written as follows: The antisymmetric term is given by: Promoting antisymmetry The main error is the reflection step error . Just two measurements and For the antisymmetric term is given by:

The method 100 shown in FIG. 3 can be used to determine any combination of antisymmetric errors. For example, in some embodiments, the method 100 shown in FIG. 3 can be used to determine the given by equation (3) is determined to be a correction to the height profile of the surface of the substrate W. For example, a first measurement (at step 120) of a first height profile when the substrate W is in a first orientation may be Similarly, a second measurement of the second height profile (at step 140) when the substrate W is in the second orientation may be The first calibrated measurement and the second calibrated measurement can be respectively obtained by and given.

Alternatively, as explained below, in some embodiments, the method 100 shown in FIG. 3 may be used for a method that does not include surface slope errors. That is, the method 100 of FIG. 3 can be used for determining and correcting the height profile of the surface of the substrate W. The following antisymmetric terms is given by: Just two measurements and For is given by:

For such embodiments, the first and second measurements of the height profile may be considered to be corrected for local variations in the slope of the surface of the substrate W.

In some embodiments, calibrating the measurement of the height profile against the shape of the substrate holder in order to determine the calibrated measurement (eg, at step 150) may use a reference substrate (eg, a bare wafer), as now discussed with reference to FIG. 4 .

As schematically shown in FIG. 4 , a method 200 of correcting a measurement of a height profile for a shape of a substrate holder in order to determine a corrected measurement (e.g., at step 150 ) may include a step 210 of securing a reference substrate to the substrate holder; a step 220 of taking a reference measurement of the height profile of the reference substrate; and a step 230 of determining a corrected measurement of the height profile as a difference between the measurement and the reference measurement.

Step 220 includes taking a reference measurement of a height profile of a reference substrate. Similar to steps 120 and 140 of method 100 shown in FIG3 and described above, this measurement of the height profile includes the following sub-steps: 170a projecting a patterned radiation beam onto a beam spot; 170b moving the reference substrate relative to the beam spot when the reference substrate is in its current orientation; 170c receiving a portion of the patterned radiation beam reflected from the reference substrate; and 170d determining a first measurement of the reference height profile based on the portion of the patterned radiation beam reflected from the reference substrate.

It should be noted that when using the method 200, at step 220, the substrate holder should be in the same orientation as it was when determining the measurement of the height profile for shape correction of the substrate holder. For example, when using the method 200 of FIG. 4 to correct a first measurement for shape of the substrate holder to determine a first corrected measurement, at step 220, the substrate holder should be in the same orientation as it was when determining the first measurement (i.e., the first orientation used in step 120). Similarly, when using the method 200 of FIG. 4 to correct a second measurement for shape of the substrate holder to determine a second corrected measurement, at step 220, the substrate holder should be in the same orientation as it was when determining the second measurement (i.e., the second orientation used in step 140).

It should be noted that step 150 of the method 100 shown in FIG. 3 may be implemented by performing the method 200 shown in FIG. 4 twice, once to correct the first measurement for the shape of the substrate holder to determine the first corrected measurement, and once to correct the second measurement for the shape of the substrate holder to determine the second corrected measurement.

The reference substrate may be referred to as a reference object. The reference substrate may be a bare wafer. That is, the reference substrate may be a silicon wafer without a processing layer applied thereto.

This process using a reference substrate or bare wafer can work as follows. To correct for the shape of the substrate holder (fixture), the bare wafer is measured at the same position as the process wafer during two rotations. Phase step and reflectivity step errors are absent for a (flat) bare wafer. Therefore, the process wafer height measurement can be corrected by subtracting the bare wafer measurement. Therefore, the corrected measurement can be written as follows: as well as: The correction term can be determined as:

In some embodiments, step 170d of determining the measurement of the height profile based on a portion of the patterned radiation beam reflected from the substrate (or reference substrate) may include the following steps, as schematically shown in FIG. 5 .

In detail, step 170d may include a step 310 of receiving a portion of the radiation beam reflected from the substrate W (e.g., reflected radiation BE2) and splitting the reflected radiation into a first portion and a second portion. In detail, the reflected radiation BE2 may be split such that the first portion of the radiation BE2 corresponding to the first portion of the first image (formed at the measurement location MLO) is spatially separated from the second portion of the radiation corresponding to the second portion of the first image.

Step 170d may further comprise a step 320 of determining the intensity _I1 , _I2 of each of the first and second parts of the radiation. For example, this step 320 may be performed by a detector DET which may comprise at least two parts, each part being operable to determine the intensity of one of the first and second parts of the reflected radiation BE2.

Step 170d may further include step 330 of determining the height h of the substrate W by combining the intensity _I1 of the first portion of the radiation BE2 with the intensity _I2 of the second portion of the radiation BE2.

It should be understood that determining the height of object W includes determining the height of the object relative to a reference height or position, as is known in the art. Available with first strength With the second intensity For example, the height h of the object W can be determined to be proportional to the difference between the first intensity With the second intensity The difference between the first intensity With the second intensity That is, the height It can be given by the following formula: in Any measurement of the height of the wafer W mentioned herein (e.g., the measurement discussed above) and ) can be determined according to equation (10).

As the height of the object W changes, the position of at least a portion of the second image of the pattern PGR formed at the detection grating DGR will also change, and this in turn may cause the first intensity and the second intensity For example, as the height of the object W changes, the position of at least a portion of the second image of the pattern PGR may change relative to a splitting optical element (or relative to the detector array) that is configured to split the reflected radiation into a first portion and a second portion.

Advantageously, by splitting the reflected radiation BE2 into a first portion and a second portion and by combining the intensities of the first portion and the second portion to determine the height of the substrate W, the height can be determined substantially independently of the intensity of the radiation beam LSB. For example, the height can be determined as a differential measurement (e.g., according to equation (10)).

The method 170d shown in FIG. 5 is of the type in which the reflected light BE2 is split into two parts corresponding to different parts of a first image (formed on substrate W). The first image is formed in the beam spot or measurement location MLO. In general, there may be a spatial offset between the first and second parts of the first image (although the first and second parts may partially overlap in space). The object W being measured may have some variation in reflectivity across the surface. For example, the object W may have a localized region or feature that has a different reflectivity than surrounding parts of the surface. In the case of this configuration, any spatial offset between the first and second parts of the first image as the feature moves into (or out of) the beam spot MLO (e.g., due to relative movement at step 120) will produce a difference in the first and second intensities due to the variation in reflectivity (rather than the height of the object W). 6-7B , such features may cause errors in the height measurement due to any changes in the reflectivity of the surface of the object W. This is because the first and second portions of the radiation are reflected from different areas of the surface of the object W.

In some embodiments, splitting the reflected radiation BE2 into the first portion and the second portion (at step 310) can include forming a patterned second image on a splitting optic (which can be typically disposed at the location where the detection grating DGR is shown in FIG. 2), and using the splitting optic to direct radiation from the first portion and the second portion of the second image so as to be spatially separated. The position of the second image relative to the splitting optic can determine how much of the reflected radiation is directed to each of the first portion and the second portion.

Alternatively, in some embodiments, splitting the reflected radiation BE2 into a first portion and a second portion (at step 310) may include forming a second image patterned on a detector array comprising a plurality of sensing elements. The detector array may be referred to as a camera, and the individual sensing elements may be referred to as pixels. With this configuration, a first subset of sensing elements may be used to determine the intensity of the first portion of the radiation (at step 320), and a second subset of sensing elements may be used to determine the intensity of the second portion of the radiation (also at step 320).

In some embodiments of the method 100 shown in FIG. 3 , the step 130 of rotating the global normal of the substrate W around the surface to the second orientation may include rotating the global normal of the substrate around the surface by 180°.

For such embodiments, many errors in the height measurement caused by features on the processing layer contribute to the first and second measurements having the same magnitude but opposite signs (i.e., antisymmetric errors). For example, for such embodiments, the antisymmetric error can be determined as half the difference between the first and second corrected measurements of height.

In some embodiments of the method 100 shown in FIG. 3 , the correction (determined at step 160 ) is determined as half the difference between the first corrected measurement and the second corrected measurement of the height profile (e.g., according to any of Equations 4, 6, 9, or 13).

In some embodiments of the method 100 shown in FIG. 3 , taking each of the first and second measurements of the height profile (at steps 120 and 140 ) may include measuring the height of the surface at a plurality of locations on the object.

It will be appreciated that combining the first calibrated measurement of the height profile with the second calibrated measurement (at step 160) in order to determine a correction for the height profile of the surface of the substrate W may include combining measurements from the first calibrated measurement and the second calibrated measurement of the height profile corresponding to the same location on the substrate W.

In some embodiments of the method 100 shown in Figure 3, each of the first measurement and the second measurement of the height profile of the surface (at steps 120 and 140) includes measuring a height map of the surface for substantially the entire surface of the substrate W. That is, the height map (and therefore the correction) may be determined for substantially the entire substrate (wafer W).

In some embodiments of the method 100 shown in FIG. 3 , each of the first and second measurements of the height profile (at steps 120 and 140 ) includes measuring a height map of the surface for one or more of a plurality of regions C of the surface W of the object to which the same pattern is to be applied.

That is, a height map (and therefore a correction) may be determined for one or more fields or dies C of a substrate (wafer W). In some embodiments, a height map (and therefore a correction) may be determined for only one (or a relatively small portion) of a plurality of regions C of a surface of a substrate W to which the same pattern is to be applied.

In some embodiments of the method 100 shown in Figure 3, the method 100 may further include correcting the first and second measurements of the height profile for local variations in the slope of the surface of the substrate W. Additionally, for embodiments of the method 100 shown in Figure 3, wherein step 150 of the method 100 includes the method 200 shown in Figure 4, the first and second reference measurements of the height profile may be corrected for local variations in the slope of the surface of the substrate before correcting the first and second measurements of the height profile for a shape of the substrate holder.

In general, variations in the slope of the surface of the substrate W will affect the focusing of the patterned radiation beam onto the beam spot MLO. This in turn will produce height measurement errors that depend on the slope or gradient of the surface of the substrate W. A key factor causing local variations in the slope of the surface of the substrate W may be the shape of the substrate holder (or fixture) to which the substrate W is fixed/clamped. Such errors are typically anti-symmetric errors (and may therefore form part of the correction determined using the method 100 shown in FIG. 3 ). However, in general, such errors may be subject to significant local variations across the surface of the substrate W. For example, a substrate W (which may be an anti-etchant coated silicon wafer) may include a plurality of generally rectangular regions C (which may be referred to as fields or grains) to which substantially the same pattern is to be applied (and has been applied when forming previous process layers). There may be significant variations from field C to field C (or grain to grain) in errors due to local variations in the slope of the surface of the substrate W. This is in contrast to some other antisymmetric errors which may depend primarily on features in the process layers on the substrate W and which may therefore be substantially the same for all fields C or grains C.

For such embodiments of method 100 in which (a) the first and second measurements of the height profile and (b) the first and second reference measurements of the height profile are corrected for local variations in the slope of the surface of the substrate W, the corrected measurements may be written as follows: as well as: in is the surface slope error of the reference substrate (which may or may not be equal to ), is the height profile of the substrate W that has been corrected for local variations in the slope of the surface of the substrate W, and is the height profile of the reference substrate that has been corrected for local variations in the slope of the reference substrate's surface. It can be determined as:

Optionally, in some embodiments of the method 100 shown in FIG. 3 , the method 100 may further include a step 180 of storing the correction in a memory.

The inventors have also recognized that the correction determined by the method 100 shown in FIG. 3 depends primarily on: (a) the substrate holder (or fixture MT); and (b) the lithography process (i.e., layer recipe). If, as is typical, a plurality of wafers W are to be processed by a given lithography apparatus LA using the same lithography process, then the correction will be substantially the same for all such wafers W. Therefore, in some embodiments, the correction may be determined once and then used for all subsequent wafers W processed on the same fixture WT and using the same lithography process. 3 includes correcting (at step 150) the first and second measurements (as determined at steps 120 and 140) for the shape of the substrate holder so that the first and second corrected measurements, respectively, are determined before they are combined (at step 160) to determine a correction for the height profile, the correction determined at step 160 is substantially independent of the substrate holder or fixture. Thus, in some embodiments, the correction may be determined once in one lithography apparatus LA (using fixture WT) and then used for all subsequent wafers W processed using the same lithography process (even on different fixtures and even in different lithography apparatuses LA). Furthermore, in some embodiments, the calibration may be determined once for a single field C or die of a wafer W and then applied to all fields C or dies of all subsequent wafers W processed using the same lithography process.

FIG6 is a schematic diagram of a portion of a substrate W showing a beam spot or measurement location MLO. A first image of a pattern of a projection grating PGR comprising two lines _L1 , _L2 is also shown. In addition, each of the two lines comprises two parts (the upper half and the lower half of each line _L1 , _L2 in FIG6, respectively). The first image of the projection grating PGR can be considered to comprise a first part 210 (comprising the top part of the two lines _L1 , _L2 ) and a second part 220 (comprising the bottom part of the two lines _L1 , _L2 ).

The first portion 210 and the second portion 220 of the first image correspond to the first portion and the second portion of radiation reflected by the substrate W, which are split, for example, by splitting optics (see 648 in FIG. 10 , as discussed below). That is, the first portion 210 and the second portion 220 indicated in FIG. 6 may be viewed as projections of the splits achieved by the splitting optics back onto the substrate W (indicated here only to illustrate the advantages of the method of the disclosed embodiments). As described above (see equation (10)), the intensities of the reflected radiation from the first portion 210 and the second portion 220 of the first image (which may be referred to as the first intensity _I1 and the second intensity _I2 ) may be used to determine a measure of the height profile (at step 170d) based on the portion of the patterned radiation beam reflected from the substrate. In detail, the first image is divided into the first portion 210 and the second portion 220 shown in FIG. 6 to represent a situation when the height of the substrate W is zero (relative to the reference height) when the first portion 210 and the second portion 220 have substantially equal sizes.

As indicated by arrow 230, during method 100 (at step 120), substrate W is moved in a scanning direction (y-direction in FIG. 6 ) relative to the beam spot region (measurement location MLO). It should be noted that at step 140, substrate W may be moved in the same scanning direction (y-direction in FIG. 6 ) but in a different orientation relative to the beam spot region (measurement location MLO). The x-direction and y-direction shown in FIG. 6 represent the side of a target portion C (e.g., including one or more dies) of substrate W (see FIG. 1 ), and in general, features formed on substrate W tend to be aligned with the x-direction and/or the y-direction. It should be noted that lines L ₁ , L ₂ of projection grating PGR are arranged to make non-zero angles with both the x-direction and the y-direction in FIG. 6 . This will minimize the effect of scattering of the incident radiation beam BE1 from features on the substrate W (other than specular reflections) on the height measurement.

Each of the two lines _L1 , _L2 has a thickness t in the direction in which the substrate W moves relative to the beam spot MLO (i.e., the y-direction). It should be noted that, in general, the thickness t of each of the lines _L1 , _L2 in the first image (formed on the substrate W) will be greater than the thickness of each of the corresponding lines on the projection grating PGR (by a factor of 1/cos(ANG)). The space between the two lines in the y-direction is also t, so that the pitch p of the first image (in the y-direction) is 2t. There is a spatial offset in the y-direction between the first portion 210 and the second portion 220 of the first image equal to t/2 (where t is the thickness t of the lines _L1 , _L2 ).

FIG. 6 also shows a feature 240 on wafer W having a different reflectivity than the rest of substrate W. Specifically, the reflectivity of feature 240 on wafer W may be 80% of the rest of substrate W. In this example, the extent of the lower reflectivity feature 240 in the x-direction is equal to or greater than an extent of beam spot MLO. Thus, feature 240 may be viewed as providing a one-dimensional step in reflectivity (in the y-direction). As the substrate is scanned or stepped in the y-direction, feature 240 will move into and subsequently out of beam spot MLO.

As feature 240 moves into (or out of) beam spot MLO (e.g., due to relative motion at step 120), the spatial offset in the y-direction between first portion 210 and second portion 220 of the first image produces a difference in the first intensity and the second intensity that is due to a reflectivity variation between feature 240 and the rest of substrate W (rather than due to a height of substrate W). This can be seen in FIGS. 7A and 7B .

FIG7A shows a first intensity _I1 and a second intensity _I2 as a function of the position of the substrate in the y-direction, which includes radiation from a first portion 210 and a second portion 220 of a first image, respectively. As can be seen in FIG7A (moving from left to right in FIG7A), as feature 240 moves into beam spot MLO, first intensity _I1 decreases before second intensity _I2 decreases (due to the reduced reflectivity of feature 240). Once feature 240 is completely within beam spot MLO, both first intensity _I1 and second intensity _I2 decrease due to the reduced reflectivity of feature 240. Furthermore, as feature 240 moves out of beam spot MLO, first intensity _I1 increases back to its nominal value before second intensity _I2 .

FIG. 7B shows the height determined by combining the first intensity _I1 and the second intensity _I2 determined at any given time. In detail, the height is proportional to the difference _I2 - _I1 . As mentioned above, this example shown in FIG. 6 represents the situation when the height of the substrate W is zero (relative to the reference height) when the first portion 210 and the second portion 220 have substantially equal sizes. Therefore, when the feature 240 is not in the beam spot region MLO (the left and right sides of FIG. 7A and FIG. 7B), the height is zero. In addition, when the feature 240 is completely within the beam spot region MLO (the central portion of FIG. 7A and FIG. 7B), the height is close to zero. However, due to reflectivity changes caused when feature 240 moves into or out of beam spot 240, combining the two simultaneously determined intensities will produce significant errors in determining the height of substrate W when feature 240 moves into (or out of) beam spot MLO (e.g., due to relative motion at step 120), even though the substrate is at height zero. This is because the first portion 210 and the second portion 220 of the radiation are reflected from different areas of the surface of substrate W.

Some embodiments of the present disclosure relate to a method of measuring the topography of a surface of a substrate W, as will now be discussed with reference to Figure 8. Figure 8 is a schematic representation of a method 400 of measuring the topography of a surface of a substrate W.

The method 400 of Figure 8 includes a step 410 of taking a measurement of a height profile of a surface of a substrate W. Similar to steps 120 and 140 of the method 100 shown in Figure 3 and described above (and step 220 of the method 200 shown in Figure 4), this measurement of the height profile of the substrate W includes the following sub-steps: 170a projecting a patterned radiation beam onto a beam spot; 170b moving the reference substrate relative to the beam spot when the reference substrate is in its current orientation; 170c receiving a portion of the patterned radiation beam reflected from the reference substrate; and 170d determining a first measurement of the reference height profile based on the portion of the patterned radiation beam reflected from the reference substrate.

The method 400 of FIG. 8 further includes the step 410 of combining the measurement of the height profile with a correction for the height profile of the substrate determined using the method 100 of FIG. 3 to determine a corrected measurement of the height profile.

The method 400 shown in FIG8 is advantageous because it provides a calibrated measurement that has been calibrated using the calibration determined by the method 100 shown in FIG3. For example, this can allow accurate correction of antisymmetric errors. In some embodiments of the method 400 shown in FIG8, the calibrated measurement of the height profile is determined as the measurement of the height profile minus the calibration. That is, as follows: Among them, correction For example, or , as described above.

In some embodiments, the method 400 of FIG. 8 may further include using the method 100 shown in FIG. 3 to determine a correction for the height profile.

For at least some wafers W, the method 400 of FIG8 may include first determining a calibration (using the method 100 of FIG3), and then using the calibration to calibrate the height measurement (using the method 400 of FIG8). For example, the calibration may be determined for at least the first wafer W processed using a given lithography process or recipe.

In some embodiments, the method 400 of Figure 8 may include retrieving the correction from memory. For example, in the case where the method 100 of Figure 3 has been previously executed and includes step 180 of storing the correction in memory, the stored correction may be retrieved from memory.

In some embodiments of the method 400 of FIG. 8 , the step 420 of combining the measurement of the height profile with the correction of the height profile of the substrate W may include combining the measurement with the correction at substantially the same location on the substrate W.

For example, in some embodiments, a correction for the height of the surface of the substrate W may be determined for substantially the entire substrate (wafer W). Similarly, a measurement of the height of the surface may be determined for substantially the entire substrate (wafer W) and may be combined with the correction to determine a corrected measurement of the height of the surface of the substrate W.

In some embodiments of the method 400 of Figure 8, the substrate W may include a plurality of regions C to which the same pattern is to be applied. For such embodiments, the step 420 of combining the measurement of the height profile with the correction of the height profile of the substrate may include combining the measurement of each of the plurality of regions C with the correction of one of the plurality of regions C.

That is, in some embodiments, a calibration may be determined for only one (or a relatively small portion) of a plurality of regions C of the surface of a substrate W to which the same pattern is to be applied. Calibration for that single one of the plurality of regions C of the substrate W may be used to calibrate height measurements for each of the plurality of regions C of the substrate.

Some embodiments of the present disclosure relate to lithographic exposure methods. An example of such a lithographic exposure method 500 is schematically shown in FIG9 . The lithographic exposure method 500 comprises measuring the topography of a surface of a substrate W using the method 400 of FIG8 . The lithographic exposure method 500 further comprises: a step 510 of patterning a radiation beam B using a patterning device MA; and a step 520 of projecting the patterned radiation onto the substrate W so as to form an image of the patterning device MA on the substrate W. The position of the substrate W when the patterned radiation is projected onto the substrate W (at step 520) is controlled depending on the measured topography of the surface of the substrate W (measured at step 400 ).

Advantageously, the measured topography of the surface of the substrate may be used to control the height of the substrate W when it is exposed to patterned radiation, for example to keep the substrate W in a best focus plane for the image of the patterning device MA. It will be appreciated that the image of the patterning device MA formed on the substrate W may be a diffraction limited image.

In some embodiments, the lithography exposure method 500 includes a scanning exposure, such that patterning a radiation beam using a patterning device MA includes moving the patterning device MA through the radiation beam B and projecting the patterned radiation onto the substrate W so as to form an image of the patterning device MA on the substrate W, and includes moving the substrate W so that the image of the patterning device MA is substantially stationary relative to the substrate W. That is, in order to image a pattern onto a target area C of the substrate W, the patterning device MA is moved in a scanning direction through an illumination area or is scanned across the illumination area. It should be understood that the substrate W is also scanned relative to the illumination area in the plane of the substrate W. The movement of the substrate W makes the aerial image of the patterning device MA static relative to the substrate W, and it should be understood that the direction and/or speed of the substrate W may generally be different from the direction and/or speed of the patterning device MA (for example, if the image is inverted and/or if the projection system PS applies a reduced portion).

Some embodiments of the present disclosure relate to an apparatus for measuring the topography of a surface of an object W. An embodiment of such an apparatus 600 is shown in FIG. 10 . The apparatus 600 may be referred to as a level sensor. The apparatus 600 may form part of a lithography apparatus LA of the type shown in FIG. 1 and described above. The apparatus 600 is typically in the form of a level sensor LS shown in FIG. 2 .

The device 600 includes: a support member 610; a projection optical component 620; a moving mechanism 630; a detection optical component 640; a first detector 650; a second detector 660 and a controller 670.

The support 610 is adapted to support an object W, such as a substrate W. The support 610 may, for example, include a substrate table WT. The support 610 may include a substrate holder, such as a clamp.

The projection optical element 620 can be used to form a first image of a pattern on the beam spot area 680 with the radiation beam 622. The projection optical element 620 is substantially equivalent to the projection unit LSP shown in Figure 2 and described above, and the beam spot area 680 can be substantially equivalent to the measurement location MLO shown in Figure 2 and described above. The projection optical element 620 may, for example, include: a projection patterning device 624; and a first imaging optical element 626, which is configured to form an image of the projection patterning device 624 on the beam spot area 680. The projection patterning device 624 may include a grating. The grating may include a plurality of lines. The lines may have a uniform thickness. The grating may have a 50% duty cycle. The projection patterning device 624 may be substantially equivalent to the projection grating PGR shown in FIG. 2 and described above.

The movement mechanism 630 may be used to move the support 610 so that an object (eg, a substrate W) supported by the support 610 moves through the beam spot region 680. This movement is schematically indicated by arrow 632.

The detection optics 640 may be used to receive a portion 642 of the radiation beam reflected from the object W and split the reflected radiation 642 into a first portion 644 and a second portion 646 such that the first portion 644 of the radiation corresponding to the first portion of the first image is spatially separated from the second portion 646 of the radiation corresponding to the second portion of the first image. The detection optics 640 is substantially equivalent to the detection unit LSD shown in FIG. 2 and described above.

The first detector 650 is configured to determine the intensity of the first portion 644 of the reflected radiation. The second detector 660 is configured to determine the intensity of the second portion 646 of the reflected radiation.

The detection optics may include a splitting optic 648 configured to split the reflected radiation 642 into a first portion 644 and a second portion 646, and a second imaging optic 649 configured to receive the radiation 642 reflected from the object W supported by the free support 610 and form a second image of the pattern on the splitting optic 648. The first imaging optic 626 may be substantially equivalent to the second imaging optic 649.

The splitting optics 648, the beam spot 680, and the projection patterning device 624 are all in optically conjugate planes. It should be understood that two planes are optically conjugate if all radiation passing through different points in the first plane is imaged onto different points in the second plane.

An image of the projected patterning device 624 is formed on the split optics 648, and the position of the image indicates the height of the object W. In particular, the position of the image relative to the split optics 648 indicates the height of the object W. As explained above, the projected patterning device 624 may include a grating, which includes a plurality of lines. The split optics 648 may include a plurality of prisms, and the image of each line may be imaged onto one of a plurality of generally triangular prisms such that a first portion of the line is incident on a first surface of the prism and a second portion of the line is incident on a second surface of the prism. The first portion of the line is directed to a first detector 650 and the second portion of the line is directed to a second detector 660. As the line moves relative to the prism (due to changes in the height of object W), the amount of radiation directed to each of the detectors 650, 660 changes.

Alternatively, in some embodiments, splitting the reflected radiation 642 into the first portion 644 and the second portion 646 may include forming a second image patterned on a detector array comprising a plurality of sensing elements (e.g., using second imaging optics 649). The detector array may be referred to as a camera, and the individual sensing elements may be referred to as pixels. With this configuration, a first subset of sensing elements may be used to determine the intensity of the first portion of the radiation (at step 320), and a second subset of sensing elements may be used to determine the intensity of the second portion of the radiation (also at step 320). It should be noted that in the case of this configuration (in which the splitting optical element 648, the first detector 650 and the second detector 660 are replaced by a detector array), the detection optical element 640 can be considered to be operable to receive a portion 642 of the radiation beam reflected from the object W and split the reflected radiation 642 into a first portion 644 and a second portion 646, such that by means of forming a second image of the pattern (for example, using a second imaging optical element 649), the first portion 644 of the radiation corresponding to the first portion of the first image is spatially separated from the second portion 646 of the radiation corresponding to the second portion of the first image.

The controller 670 is configured to receive a first signal _s1 from the first detector 450 indicating a first strength and a second signal _s2 from the second detector 460 indicating a second strength.

The controller 670 may be used to determine the height of the object W by combining the intensity of the first portion 644 of the reflected radiation with the intensity of the second portion 646 of the reflected radiation.

The apparatus 600 shown in Figure 10 is advantageous, as will now be discussed. Because the detection optics 640 can be used to split the reflected radiation into a first portion 644 and a second portion 646, and the controller 670 can be used to determine the height of the substrate W by combining the intensities of the first portion 644 and the second portion 646, the height can be determined substantially independent of the intensity of the radiation beam 622. For example, the height can be determined as a differential measurement (e.g., according to equation (10)).

Controller 670 may be used to implement any of the methods of FIGS. 3 , 4 , 5 , and 8 , as discussed above.

In some embodiments, the first detector 650 and the second detector 660 are configured to determine the intensity of the first portion 644 and the second portion 646, respectively, a plurality of times at a sampling frequency f. That is, the first detector 650 and the second detector 660 may be configured so that the intensity of the first portion 644 and the second portion 646 of the radiation may be determined a plurality of times, each determination being separated in time from a previous determination and a subsequent determination. The sampling frequency f may be the reciprocal of the time interval between the start of one determination and the start of a subsequent determination.

In some embodiments, apparatus 600 may further include a radiation source 690 that may be used to generate radiation beam 622.

Some embodiments of the present disclosure relate to a lithography apparatus comprising an apparatus 600 of the type shown in Fig. 10. The lithography apparatus may be of the type shown in Fig. 1. The lithography apparatus LA may further comprise an illumination system IL operable to illuminate an illumination area, a support structure MT configured to support a patterning device MA such that the patterning device MA may be positioned in the illumination area, a substrate table WT configured to support a substrate W, and a projection system PS operable to form an image of the patterning device MA supported by the support structure MT on a substrate W supported by the substrate table WT.

Although some examples of the methods 100, 300 shown in FIG. 3 and FIG. 8 are described herein using two measurements (at steps 120 and 140), alternative embodiments may use additional measurements that are each corrected for the shape of the substrate holder (at step 150) before being combined to determine the correction (at step 160), which additional measurements may be performed using different orientations of the substrate and the substrate holder. For example, some of the examples described above use two measurements at 0° and 180° angles (i.e., the substrate is rotated 180° between measurements). However, in an alternative embodiment, the method 100 of FIG. 3 may use four measurements with the substrate rotated 90° between measurements (i.e., measurements at 0°, 90°, 180°, and 270°). Embodiments using more measurements may produce more accurate calibrations.

Although specific reference may be made herein to the use of lithography equipment in IC manufacturing, it should be understood that the lithography equipment described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guide and detection patterns for magnetic memory, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like.

Although specific reference may be made herein to embodiments of the invention in the context of lithography equipment, embodiments of the invention may be used in other equipment. Embodiments of the invention may form part of a mask inspection equipment, a metrology equipment, or any equipment that measures or processes an object such as a wafer (or other substrate) or a mask (or other patterned device). Such equipment may generally be referred to as a lithography tool. The lithography tool may use vacuum conditions or ambient (non-vacuum) conditions.

Although the above may specifically refer to the use of embodiments of the present invention in the context of optical lithography, it will be appreciated that the present invention is not limited to optical lithography and may be used in other applications (such as imprint lithography) where the context permits.

Where the context permits, embodiments of the present invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the present invention may also be implemented as instructions stored on a machine-readable medium that may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form that is readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read-only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustic, or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.); and others. In addition, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be understood that such descriptions are for convenience only and that such actions are in fact caused by a computing device, processor, controller, or other device executing the firmware, software, routines, instructions, etc., and that doing so may cause actuators or other devices to interact with the physical world.

Although specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced in other ways than those described. The above description is intended to be illustrative and not restrictive. Thus, it will be apparent to one skilled in the art that the invention as described may be modified without departing from the scope of the claims set forth below. Other aspects of the invention are set forth in the following numbered clauses. 1. A method for determining a correction for a height profile of a surface of a substrate, the method comprising: securing the substrate to a substrate holder; performing a first measurement of a first height profile by: projecting a patterned radiation beam onto a beam spot; moving the substrate relative to the beam spot when the substrate is in a first orientation; receiving a portion of the patterned radiation beam reflected from the substrate and determining the first measurement of the first height profile based on the portion; rotating the substrate about a global normal to the surface to a second orientation; A second measurement of a second height profile is performed by: projecting a patterned radiation beam onto the beam spot; moving the substrate relative to the beam spot when the substrate is in the second orientation; receiving a portion of the patterned radiation beam reflected from the substrate and determining the second measurement of the second height profile based on the portion; correcting the first measurement and the second measurement for a shape of the substrate holder to determine a first corrected measurement and a second corrected measurement, respectively; and combining the first corrected measurement and the second corrected measurement to determine the correction for the height profile. 2. The method of clause 1, wherein correcting a measurement of a height profile for a shape of the substrate holder to determine a corrected measurement comprises: securing a reference substrate to the substrate holder; performing a reference measurement of the height profile of the reference substrate by: projecting a patterned radiation beam onto a beam spot; moving the substrate relative to the beam spot when the substrate holder is in the same orientation as it was when determining the corrected measurement; receiving a portion of the patterned radiation beam reflected from the substrate and determining the reference measurement of the height profile based on the portion; and determining the corrected measurement of the height profile as a difference between the measurement and the reference measurement. 3. The method of any preceding clause, wherein the step of rotating the substrate about a global normal to the surface to a second orientation comprises rotating the substrate about a global normal to the surface by 180°. 4. The method of any preceding clause, wherein the correction is determined as half of a difference between the first corrected measurement and the second corrected measurement of the height profile. 5. The method of any preceding clause, wherein each of the first measurement and the second measurement of the height profile comprises measuring the height of the surface at a plurality of locations on an object. 6. The method of any preceding clause, wherein each of the first measurement and the second measurement of the height profile of the surface comprises measuring a height map of the surface for substantially the entire surface of the substrate. 7. A method as in any of clauses 1 to 5, wherein each of the first measurement and the second measurement of the height profile comprises measuring a height map of the surface for one or more of a plurality of regions of the surface of the object to which the same pattern is to be applied. 8. A method as in any of the preceding clauses, further comprising: correcting the first measurement and the second measurement of the height profile for local variations in a slope of the surface of the substrate, and when appended to clause 2, correcting the first reference measurement and the second reference measurement of the height profile for local variations in a slope of the surface of the reference substrate before correcting the first measurement and the second measurement of the height profile for the shape of the substrate holder. 9. A method as in any of the preceding clauses, further comprising storing the correction in a memory. 10.   A method of measuring a topography of a surface of a substrate, the method comprising: measuring a height profile of the surface by: projecting a patterned radiation beam onto a beam spot; moving the object relative to the beam spot when the object is in a first orientation; receiving a portion of the patterned radiation beam reflected from the object and determining the height profile based on the portion; and combining the measurement of the height profile with a correction of a height profile of a substrate determined using a method as in any of the preceding clauses to determine a corrected measurement of the height profile. 11.   The method as in clause 10, wherein the corrected measurement of the height profile is determined as the measurement of the height profile minus the correction. 12.   The method of clause 10 or clause 11, further comprising determining the correction for a height profile using the method of any of clauses 1 to 9. 13.   The method of any of clauses 10 to 12 when directly or indirectly attached to clause 9, wherein the method comprises retrieving the correction from a memory. 14.   The method of any of clauses 10 to 13, wherein combining the measurement of the height profile with a correction for a height profile of a substrate comprises combining the measurement with a correction for substantially the same location on the substrate. 15.   The method of any of clauses 10 to 14, wherein the substrate comprises a plurality of regions to which the same pattern is applied, and wherein combining the measurement of the height profile with a correction for a height profile of a substrate comprises combining the measurement for each of the plurality of regions with a correction for one of the plurality of regions. 16.   The method of any preceding clause, wherein determining a measurement of a height profile based on a portion of a patterned radiation beam reflected from the substrate comprises: receiving the portion of the radiation beam reflected from the substrate and splitting the reflected radiation into a first portion and a second portion; determining an intensity of the first portion and the second portion of the radiation; and determining a height of the substrate by combining the intensity of the first portion of the radiation with the intensity of the second portion of the radiation. 17.   The method of clause 16, wherein splitting the reflected radiation into a first portion and a second portion comprises forming an image of the pattern on a splitting optic, and using the splitting optic to direct radiation from the first portion and the second portion of the second image to be spatially separated. 18.   The method of clause 16 or clause 17, wherein the height of the substrate is proportional to the difference between the first intensity and the second intensity. 19.   A lithographic exposure method comprising: measuring a topography of a surface of a substrate using a method such as any of the preceding clauses when directly or indirectly attached to clause 10; patterning a radiation beam using a patterning device; and projecting the patterned radiation onto the substrate so as to form an image of the patterning device on the substrate; wherein a position of the substrate when the patterned radiation is projected onto the substrate is controlled in dependence on the measured topography of the surface of the substrate. 20.   The lithographic exposure method of clause 19, wherein the lithographic exposure is a scanning exposure, such that patterning a radiation beam using a patterning device comprises moving the patterning device through the radiation beam, and projecting the patterned radiation onto the substrate to form an image of the patterning device on the substrate comprises moving the substrate such that the image of the patterning device is substantially stationary relative to the substrate. 21.   An apparatus for measuring a morphology of a surface of a substrate, the apparatus comprising: a support member for supporting a substrate, projection optics for forming a first image of a pattern on a beam spot by a radiation beam; a moving mechanism for causing relative movement of the support member relative to the beam spot; detection optics for receiving a portion of the radiation beam reflected from the substrate; and a controller for determining a height of the substrate based on the radiation beam reflected from the substrate, and further for implementing the method of any one of clauses 1 to 18. 22.   The apparatus of clause 21, wherein the detection optics are operable to receive a portion of the radiation beam reflected from the substrate and split the reflected radiation into a first portion and a second portion such that a first portion of the radiation corresponding to a first portion of the first image is spatially separated from a second portion of the radiation corresponding to a second portion of the first image, and wherein the apparatus further comprises: a first detector configured to determine an intensity of the first portion of the radiation; and a second detector configured to determine an intensity of the second portion of the radiation; and wherein the controller is operable to determine a height profile of the substrate by combining the intensity of the first portion of the radiation with the intensity of the second portion of the radiation. 23.   The apparatus of clause 21 or clause 22, wherein the projection optics comprises: a projection patterning device; and first imaging optics configured to form an image of the projection patterning device on the beam spot. 24.   The apparatus of any of clauses 21 to 23, wherein the detection optics comprises: splitting optics configured to split the reflected radiation into a first portion and a second portion; and second imaging optics configured to receive radiation reflected from an object supported by the support and to form a second image of the pattern on the splitting optics. 25.   The apparatus of any of clauses 21 to 24, further comprising a radiation source that can be used to generate the radiation beam. 26.   A lithography apparatus comprising an apparatus as claimed in any one of clauses 21 to 25.

100: Method 110: Step 120: Step 130: Step 140: Step 150: Step 160: Step 170a: Sub-step 170b: Sub-step 170c: Sub-step 170d: Sub-step 180: Step 200: Method 210: Step/Part 1 220: Step/Part 2 230: Step/arrow 240: Feature 300: Method 310: Step 320: Step 330: Step 400: Method 410: Step 420: Step 500: Lithography exposure method 510: Step 520: Step 600: Apparatus 610: Support 620: Projection optical element 622: Radiation radiation beam 624: projection patterning device 626: first imaging optical element 630: moving mechanism 632: arrow 640: detection optical element 642: reflected radiation 644: first part 646: second part 648: splitting optical element 649: second imaging optical element 650: first detector 660: second detector 670: controller 680: beam spot 690: radiation source ANG: incident angle B: radiation beam BD: beam delivery system BE1: radiation beam BE2: arrow/reflected radiation C: target part/area/field or grain DET: detector DGR: detection grating f: sampling frequency h: height I ₁ : Intensity I ₂ : Intensity IF: Position measurement system IL: Illumination system L ₁ : Line L ₂ : Line LA: Lithography equipment LS: Level or height sensor LSB: Radiation beam LSD: Detection unit LSO: Radiation source LSP: Projection unit M ₁ : Mask alignment mark M ₂ : Mask alignment mark MA: Patterning device MLO: Measurement position/measurement area MT: Mask support/support structure p: Pitch P: Pitch P ₁ : Substrate alignment mark P ₂ : Substrate alignment mark PGR: Projection grating PM: First positioner PS: Projection system PW: Second positioner s ₁ : First signal s ₂ : Second signal SO: Radiation source t: Thickness W: Substrate/object/wafer WT: Substrate support/wafer stage/fixture Z: Height

Embodiments of the invention will now be described by way of example only with reference to the accompanying schematic drawings, in which: - FIG. 1 depicts a schematic overview of a lithography apparatus; - FIG. 2 is a schematic illustration of a level or height which may form part of the lithography apparatus shown in FIG. 1 ; - FIG. 3 is a schematic representation of a method for determining a correction for a height profile of a surface of a substrate according to an embodiment of the present disclosure; - FIG. 4 is a schematic representation of a method for correcting a measurement of a height profile for the shape of a substrate holder in order to determine a corrected measurement, which method may form part of the method shown in FIG. 3 ; - FIG. 5 is a schematic representation of a method for determining a measurement of a height profile based on a portion of a patterned radiation beam reflected from a substrate (or a reference substrate), which method may form part of the methods shown in FIGS. 3 and 4 ; - FIG. 6 is a schematic diagram of a portion of a substrate showing: a beam spot or measurement location; a first image of a pattern of a projection grating comprising two lines; and a first feature on the substrate having a different reflectivity than the rest of the substrate; - FIG. 7A shows first and second intensities I ₁ , I ₂ as a function of the position of the substrate in the y direction (determined using the method shown in FIG. 5 ), comprising radiation from a first portion and a second portion of the first image shown in FIG. 6 , respectively; - FIG. 7B shows a height determined by combining the first and second intensities I ₁ , I ₂ shown in FIG. 7A ; - FIG. 8 is a schematic representation of a method for measuring the topography of a surface of a substrate according to an embodiment of the present disclosure; - FIG. 9 is a schematic representation of a lithography exposure method according to an embodiment of the present disclosure; - Figure 10 is a schematic representation of an apparatus for measuring the morphology of a surface of an object according to an embodiment of the present disclosure, which apparatus can form part of the lithography apparatus shown in Figure 1 and can implement the methods shown in Figures 3, 4, 5 and 8; and - Figure 11 is a schematic representation of three types of step sensor height measurement errors, which depend on the orientation of the substrate when measuring the surface height on the wafer.

600: Equipment

610: Support parts

620: Projection optics

622: Radiation beam

624: Projection patterning device

626: First imaging optical component

630: Mobile mechanism

632:arrow

640: Detection optics

642: Reflected radiation

644: Part 1

646: Part 2

648: Splitting optics

649: Second imaging optical component

650: First Detector

660: Second detector

670: Controller

680: beam spot area

690: Radiation source

s ₁ : first signal

s ₂ : Second signal

W: Substrate/object/wafer

Claims (15)

A method for determining a correction for a height profile of a surface of a substrate, the method comprising: securing the substrate to a substrate holder; performing a first measurement of a first height profile by: projecting a patterned radiation beam onto a beam spot; moving the substrate relative to the beam spot when the substrate is in a first orientation; receiving a portion of the patterned radiation beam reflected from the substrate and determining the first measurement of the first height profile based on the portion; rotating the substrate about a global normal to the surface to a second orientation; A second measurement of a second height profile is performed by: projecting a patterned radiation beam onto the beam spot; moving the substrate relative to the beam spot when the substrate is in the second orientation; receiving a portion of the patterned radiation beam reflected from the substrate and determining the second measurement of the second height profile based on the portion; correcting the first measurement and the second measurement for a shape of the substrate holder to determine a first corrected measurement and a second corrected measurement, respectively; and combining the first corrected measurement and the second corrected measurement to determine the correction for the height profile. The method of claim 1, wherein correcting a measurement of a height profile for a shape of the substrate holder to determine a corrected measurement comprises: securing a reference substrate to the substrate holder; performing a reference measurement of the height profile of the reference substrate by: projecting a patterned radiation beam onto a beam spot; moving the substrate relative to the beam spot when the substrate holder is in the same orientation as it was when determining the corrected measurement; receiving a portion of the patterned radiation beam reflected from the substrate and determining the reference measurement of the height profile based on the portion; and determining the corrected measurement of the height profile as a difference between the measurement and the reference measurement. A method as in any of claims 1-2, wherein the step of rotating the substrate about a global normal to the surface to a second orientation comprises rotating the substrate about a global normal to the surface by 180°. A method as in any of claims 1-2, wherein the correction is determined as half of a difference between the first corrected measurement and the second corrected measurement of the height profile. A method as in any of claims 1-2, wherein each of the first measurement and the second measurement of the height profile comprises measuring the height of the surface at a plurality of locations on the object. A method as claimed in any one of claims 1 to 2, wherein each of the first measurement and the second measurement of the height profile of the surface comprises measuring a height map of the surface for substantially the entire surface of the substrate. A method as claimed in any one of claims 1 to 2, wherein each of the first measurement and the second measurement of the height profile comprises measuring a height map of the surface for one or more of a plurality of regions of the surface of the object to which the same pattern is to be applied. The method of any of claims 1 to 2, further comprising: correcting the first and second measurements of the height profile for local variations in a slope of the surface of the substrate, and when dependent on claim 2, correcting the first and second reference measurements of the height profile for local variations in a slope of the surface of the reference substrate before correcting the first and second measurements of the height profile for the shape of the substrate holder. A method for measuring a topography of a surface of a substrate, the method comprising: measuring a height profile of the surface by: projecting a patterned radiation beam onto a beam spot; moving an object relative to the beam spot when the object is in a first orientation; receiving a portion of the patterned radiation beam reflected from the object and determining the height profile based on the portion; and combining the measurement of the height profile with a correction of a height profile of a substrate determined using the method of any of claims 1 to 8 to determine a corrected measurement of the height profile. A method as in any of claims 1 or 9, wherein determining a measurement of a height profile based on a portion of a patterned radiation beam reflected from the substrate comprises: receiving the portion of the radiation beam reflected from the substrate and splitting the reflected radiation into a first portion and a second portion; determining an intensity of the first portion and the second portion of the radiation; and determining a height of the substrate by combining the intensity of the first portion of the radiation with the intensity of the second portion of the radiation. A method as in claim 10, wherein splitting the reflected radiation into a first portion and a second portion comprises forming an image of a pattern on a splitting optic, and using the splitting optic to direct radiation from the first portion and the second portion of the second image so as to be spatially separated. A lithographic exposure method comprising: measuring a topography of a surface of a substrate using a method such as any of the preceding claims when directly or indirectly attached to claim 9; patterning a radiation beam using a patterning device; and projecting the patterned radiation onto the substrate so as to form an image of the patterning device on the substrate; wherein a position of the substrate when the patterned radiation is projected onto the substrate is controlled in dependence on the measured topography of the surface of the substrate. A device for measuring a morphology of a surface of a substrate, the device comprising: a support member for supporting a substrate, projection optics for forming a first image of a pattern on a beam spot with a radiation beam; a moving mechanism for causing the support member to move relative to the beam spot; detection optics for receiving a portion of the radiation beam reflected from the substrate; and a controller for determining a height of the substrate based on the radiation beam reflected from the substrate, and further for implementing the method of any one of claims 1 to 11. The apparatus of claim 13, wherein the detection optics is operable to receive a portion of the radiation beam reflected from the substrate and split the reflected radiation into a first portion and a second portion such that a first portion of the radiation corresponding to a first portion of the first image is spatially separated from a second portion of the radiation corresponding to a second portion of the first image, and wherein the apparatus further comprises: a first detector configured to determine an intensity of the first portion of the radiation; and a second detector configured to determine an intensity of the second portion of the radiation; and wherein the controller is operable to determine a height profile of the substrate by combining the intensity of the first portion of the radiation with the intensity of the second portion of the radiation. A lithography apparatus comprising an apparatus as claimed in any one of claims 13 or 14.