CN111801619B

CN111801619B - Lithographic method and apparatus

Info

Publication number: CN111801619B
Application number: CN201880086167.0A
Authority: CN
Inventors: A·B·杰尤恩克; 拜尔拉克·摩艾斯特; E·H·E·C·奥姆梅伦; H·P·T·托尔斯马
Original assignee: ASML Holding NV
Current assignee: ASML Holding NV
Priority date: 2018-01-11
Filing date: 2018-12-12
Publication date: 2023-09-29
Anticipated expiration: 2038-12-12
Also published as: JP2021510425A; WO2019137725A1; CN111801619A; JP7044888B2

Abstract

A lithographic apparatus comprising: a support structure (MT) configured to support a Mask (MA) and an associated pellicle (P), the mask being capable of imparting a radiation beam with a pattern in its cross-section to form a patterned radiation beam (PB); and a Projection System (PS) configured to project the patterned radiation beam onto a target portion of a substrate (W), wherein a wall extends between the support structure and the projection system, the wall comprising an opening (22) allowing transfer of the patterned radiation beam from the mask and pellicle to the projection system, and wherein the wall is provided with a two-dimensional array of pressure sensors (30).

Description

Lithographic method and apparatus

Cross Reference to Related Applications

The present application claims priority from european application 18151235.1 filed on 1 month 11 in 2018 and european application 17202511.6 filed on 11 month 20 in 2017, the entire contents of which are incorporated herein by reference.

Technical Field

The present application relates to a lithographic method and also to a lithographic apparatus.

Background

A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. For example, lithographic apparatus can be used in the manufacture of Integrated Circuits (ICs). In this case, a mask (alternatively referred to as a mask or reticle) may be used to create a circuit pattern corresponding to a single layer of the IC, and this pattern may be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). Typically, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time; in a scanner, each target portion is irradiated by scanning the substrate in a direction parallel or antiparallel to a given direction (the "scanning" direction) while scanning the pattern with a radiation beam.

It is conventional in DUV lithographic apparatus to attach a pellicle to the mask. The pellicle is a transmissive film spaced apart from the pattern of the mask by a few mm (e.g., 5 mm). Contaminant particles received on the pellicle are located in the far field relative to the pattern of the mask and thus have no significant effect on the quality of the image projected onto the substrate by the lithographic apparatus. If a pellicle is not present, contaminant particles will be located on the pattern of the mask and will obscure a portion of the pattern, thereby preventing the pattern from being properly projected onto the substrate. The pellicle is thus effective in preventing contaminant particles from adversely affecting the projection of the pattern by the lithographic apparatus onto the substrate.

While the pellicle provides a useful and valuable function, the pellicle causes undesirable side effects in that: the pellicle itself will have an effect on the image projected onto the substrate by the lithographic apparatus. The pellicle has a finite thickness and has a refractive index greater than that of air. Thus, when the pellicle is not perpendicular to the optical axis of the lithographic apparatus, deviations in the radiation passing through the pellicle will result. This results in distortion of the pattern projected by the lithographic apparatus onto the substrate. It would be desirable to provide, for example, a method that eliminates or alleviates one or more problems of the prior art, whether identified herein or elsewhere.

Disclosure of Invention

According to a first aspect of the invention, there is provided a lithographic apparatus comprising: a support structure configured to support a mask and associated pellicle, the mask being capable of imparting a pattern in a cross-section of a radiation beam to the radiation beam to form a patterned radiation beam; and a projection system configured to project the patterned beam of radiation onto a target portion of a substrate, wherein a wall extends between the support structure and the projection system, the wall comprising an opening that allows the patterned beam of radiation to pass from the mask and pellicle to the projection system, and wherein the wall is provided with a two-dimensional array of pressure sensors.

The signal output from the two-dimensional array of pressure sensors advantageously allows for computation of the shape formed by the pellicle during a scanning exposure (e.g., using shape reconstruction via near-field acoustic holography). This in turn allows correction of image distortion caused by the pellicle. Because the two-dimensional array of pressure sensors does not interfere with the patterned radiation beam, the two-dimensional array can be held in place during production exposure of the substrate.

The two-dimensional array of pressure sensors may extend on either side of the opening in the wall.

The pressure sensor may be located in a recess formed in the wall.

The upper surface of the pressure sensor may be flush with the upper surface of the wall. This may provide a smooth continuous surface when the pressure sensor is level with the upper surface of the wall, such that the pressure sensor does not cause significant turbulence to the gas flowing past the upper surface of the wall.

The pressure sensor may be provided with a spacing of up to 3 cm.

Masks and pellicle may be present in the lithographic apparatus. The spacing between the pressure sensors may generally correspond to a separation distance between the pressure sensors and the pellicle.

The lithographic apparatus may further include a processor configured to receive output signals from the array of pressure sensors and to calculate a shape formed by the pellicle during a scanning motion of the mask and pellicle.

The processor may be configured to reconstruct a shape formed by the pellicle using near-field acoustic holography.

The lithographic apparatus may further include a controller configured to apply an adjustment to a lens element of the projection system during a scanning exposure to compensate for distortion caused by the shape formed by the pellicle.

According to a second aspect of the invention, there is provided a method of measuring pellicle deflection in a lithographic apparatus, the method comprising: loading a mask assembly comprising a mask and a pellicle into a lithographic apparatus according to the first aspect of the invention; performing a scanning motion of the mask assembly and receiving a signal output from the pressure sensor; and calculating a shape formed by the pellicle during the scanning movement using the signal output from the pressure sensor.

Calculating the shape formed by the pellicle during the scanning motion may include reconstructing the shape formed by the pellicle using near-field sonoholography.

The method may further include determining an eigenfrequency of the pellicle and then considering the eigenfrequency and harmonics of the eigenfrequency in calculating a shape formed by the pellicle.

The method may further include filtering the signal output from the pressure sensor to remove frequencies higher than a known maximum movement frequency of the pellicle.

The scanning motion may be a set of scanning motions corresponding to scanning motions to be used during a production exposure performed using the mask and pellicle.

The set of scanning motions may include a scanning motion to be used for exposing a field located at an edge of the substrate and a scanning motion to be used for exposing a field located away from the edge of the substrate.

Before performing production exposure using the mask and pellicle, the shape formed by the pellicle during the scanning motion may be calculated, and the correction to be applied to the lithographic apparatus may be calculated.

The calculated correction may be applied during a production exposure of the substrate using the mask and pellicle.

The output signal from the pressure sensor may continue to be received during the production exposure performed using the mask and pellicle. The output signal may be used to adjust the calculated shape formed by the pellicle.

The correction to be applied to the lithographic apparatus may be adjusted to take into account the adjusted calculated shape formed by the pellicle.

When performing production exposure using the mask and pellicle, the shape formed by the pellicle during the scanning motion may be calculated, and the correction to be applied to the lithographic apparatus may be calculated.

According to a third aspect of the present invention there is provided a computer program comprising computer readable instructions configured to cause a computer to perform the method according to the second aspect of the present invention.

According to a fourth aspect of the present invention, there is provided a computer comprising: a memory storing processor readable instructions; and a processor arranged to read and execute instructions stored in the memory, wherein the processor readable instructions comprise instructions arranged to control the computer to perform the method according to the second aspect of the invention.

Features of one aspect of the invention may be combined with features of a different aspect of the invention.

Drawings

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

FIG. 1 schematically depicts a lithographic apparatus according to an embodiment of the invention;

figure 2 schematically depicts the effect of a non-planar pellicle on a radiation beam;

3A and 3B schematically depict a part of the lithographic apparatus of FIG. 1 in more detail;

figure 4 schematically depicts a deformation of the pellicle during a scanning movement of the pellicle;

FIG. 5 depicts the x, y distortion of an image projected using the lithographic apparatus, which distortion has been caused by pellicle deformation; and is also provided with

Fig. 6 is a flow chart of a method according to an embodiment of the invention.

Detailed Description

Although specific reference may be made in this text to the use of a lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. Those skilled in the art will appreciate that any term "wafer" or "die" as used herein may be considered synonymous with the more general term "substrate" or "target portion", respectively, in the context of such alternative applications. The substrate referred to herein may be processed, before or after exposure, in for example a coating and developing system or track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool, or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. In addition, the substrate may be processed more than once, for example, in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

The terms "radiation" and "beam" used herein encompass all types of electromagnetic radiation, including Ultraviolet (UV) radiation (e.g. having a wavelength of 365nm, 248nm, 193nm, 157nm or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5nm to 20 nm).

The support structure holds a mask (which may also be referred to as a reticle). The support structure holds the mask in a manner that depends on the orientation of the mask, the design of the lithographic apparatus, and other conditions. The support structure may use mechanical clamping, vacuum, or other clamping techniques (e.g., electrostatic clamping under vacuum conditions). The support structure may be a frame or a table, for example, which may be movable as required, and may ensure that the mask is in a desired position, for example, with respect to the projection system.

The term "projection system" used herein should be broadly interpreted as encompassing various types of projection system, including refractive optical systems, reflective optical systems, and catadioptric optical systems, as appropriate, for example, for the exposure radiation being used, or for other factors such as the use of an immersion liquid or the use of a vacuum. Any term "projection lens" used herein may be considered as synonymous with the more general term "projection system".

The illumination system may also encompass various types of optical components, including refractive, reflective, and catadioptric optical components for directing, shaping, or controlling the beam of radiation, and such components may also be referred to below, collectively or singularly, as a "lens".

The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more support structures). In such "multiple stage" machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein: wherein the substrate is immersed in a liquid having a relatively high refractive index (e.g., water) so as to fill a space between a final element of the projection system and the substrate. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems.

FIG. 1 schematically depicts a lithographic apparatus according to a particular embodiment of the invention. The apparatus comprises:

an illumination system (illuminator) IL configured to condition a radiation beam PB (e.g., UV radiation or DUV radiation);

A support structure MT supporting a mask MA, the support structure being connected to a positioning device (not depicted) to accurately position the mask with respect to an article PL;

a substrate table (e.g. a wafer table) WT for holding a substrate (e.g. a resist-coated wafer) W and connected to a positioning device PW for accurately positioning the substrate with respect to item PL; and

a projection system (e.g. a refractive projection system) PL configured to image a pattern imparted to the radiation beam PB by the mask MA onto a target portion C (e.g. comprising a portion of a die, one or more dies) of the substrate W.

As depicted herein, the apparatus is of a transmissive type (e.g., employing a transmissive mask). Alternatively, the device may be of a reflective type (e.g. employing a programmable mirror array of a type as referred to above).

The pellicle P is attached to a frame, which in turn is attached to the mask MA. The pellicle P is a transmissive film spaced apart from the pattern on the mask. The pellicle P prevents contaminant particles from being incident on the pattern of the mask and keeps such contaminant particles away from the mask pattern. The pellicle P may be separated from the mask pattern by a few mm, for example, about 5mm. The mask MA, frame F and pellicle P are all located within the environment defined by the housing 2. A two-dimensional array of pressure sensors 30 is located on a wall 33 of the housing 2. The array of pressure sensors 30 is configured to monitor the pressure of a gas (e.g., air) in the housing during a scanning motion of the mask MA, frame F, and pellicle P.

The illuminator IL receives a radiation beam from a radiation source SO. The source and the lithographic apparatus may be separate entities (for example when the source is an excimer laser). In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD comprising, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjusting device AM for adjusting the angular intensity distribution of the beam. In general, at least an outer radial extent and/or an inner radial extent (commonly referred to as σ -outer and σ -inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. IN addition, the illuminator IL generally includes various other components, such as an integrator IN and a condenser CO. The illuminator provides a conditioned beam PB of radiation having a desired uniformity and intensity distribution in its cross-section.

The radiation beam PB is incident on the mask MA, which is held by the support structure MT. After having traversed the mask MA, the beam PB passes through the pellicle P and then into the projection system PS. The projection system focuses the beam PB onto a target portion C of the substrate W. By means of the positioner PW and position pressure sensor IF (e.g. an interferometric device), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the beam PB. Similarly, the support structure MT may be used to accurately position the mask MA with respect to the path of the beam PB, e.g. during a scanning exposure. Mask MA and substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2.

The lithographic apparatus may be used to perform a scanning exposure. In a scanning exposure, the support structure MT and the substrate table WT are scanned synchronously while a pattern imparted to the beam PB is projected onto a target portion C (i.e. a single dynamic exposure). The speed and direction of the substrate table WT relative to the support structure MT may be determined by the (de-) magnification and image reversal characteristics of the projection system PS.

The lithographic apparatus further comprises a processor PR. The processor is configured to receive signal outputs from the array of pressure sensors and use these signals to calculate the shape formed by the pellicle P during the scanning movement of the mask assembly MS. The processor may calculate an adjustment to be applied to the lithographic apparatus during a scanning exposure in order to reduce distortion caused by the pellicle. The lithographic apparatus further comprises a controller CT. The controller CT is configured to apply adjustments to the lithographic apparatus during a scanning exposure. The adjustment may include an adjustment of a lens element of the projection system PS.

The processor PR and the controller CT may be provided as a single entity. The processor PR and/or the controller CT may comprise a computer. The computer may include a memory storing processor readable instructions. The processor PR may be arranged to read and execute instructions stored in the memory.

It will be appreciated that for some time, the pellicle film P will have an effect on the patterned radiation beam PB propagating through it. However, consideration of the effect of the pellicle has been limited to treating the pellicle as if the pellicle had the form of a planar sheet transverse to the radiation beam PB. It has now been determined that the pellicle P is dynamically deflected during a scanning exposure. This dynamic deformation introduces distortion to the image projected by the lithographic apparatus onto the substrate W. Embodiments of the present invention address this problem and allow distortion to be reduced. This is achieved without any modification of the pellicle P, frame F or mask M.

Fig. 2 schematically illustrates the shift of the radiation beam to be caused by the pellicle P when the pellicle (or a part of a pellicle) is at an angle relative to the optical axis of the lithographic apparatus. To assist in explaining the offset, cartesian coordinates are included in FIG. 2. The cartesian coordinates, which are also used for other figures, conform to the convention for scanning lithographic apparatus. The y-direction is the scanning direction (i.e. the direction of movement during a scanning exposure), the x-direction is in the plane of the mask in the non-scanning direction, and the z-direction is the optical axis of the lithographic apparatus.

The pellicle P has a refractive index n that is greater than the refractive indices n1, n2 of the gases (e.g., air) on either side of the pellicle _p . The pellicle has a thickness d. The offset introduced by the pellicle P conforms to snell's law and is determined in part by the thickness of the pellicle and the refractive index of the pellicle. In addition, since the pellicle is at an angle relative to the XY plane, the XY offset is also determined by the angle of the pellicle relative to the XY plane. Primary ray R of the system _p Is shown as a dash-dot line and ray R is shown relative to primary ray R _p At an angle theta _a . Dotted line R ₁ Showing how the ray R would propagate in the absence of a pellicle. Solid line R ₂ Showing how the line propagates when pellicle P is present. As can be seen, in ray R, compared to the rays that would be seen if the pellicle P were not present ₂ There is a significant offset deltay in the y-direction of (2) _p . As can also be appreciated from fig. 2, the displacement of the ray R depends in part on the angle of the pellicle P relative to the XY plane. Primary ray R _p Will be offset by a smaller amount than ray R. Rays (not shown) perpendicular to the pellicle P will not shift.

FIG. 3A schematically depicts a portion of the lithographic apparatus of FIG. 1 in more detail. As shown in fig. 1, the pellicle P is fixed to a pellicle frame F, which in turn is attached to a mask MA. The mask MA is attached to the support structure MT. The pellicle P, the pellicle frame F and the mask MA may be referred to as a mask assembly MS.

A wall 33 extends between the mask assembly MS and the first lens element 24 of the projection system PS of the lithographic apparatus. The wall 33 is provided with an opening 22 through which opening 22 the patterned radiation beam may travel to the projection system PS. The opening 22 may be referred to as an exposure slit. The wall 33 is depicted as viewed from above in fig. 3B.

The mask assembly and the support structure MT are located in an environment defined by a housing 20. The environment defined by the housing may be referred to as a mask assembly environment 18. The housing 20 has an additional opening 21 at an upper end opposite the mask MA for receiving the radiation beam PB (see FIG. 1).

The wall is provided with a two-dimensional array of pressure sensors 30. The walls 33 face toward the support structure MT and thus the pellicle P of the mask assembly MS when the mask assembly is being held by the support structure MT. The processor PR receives an output from the array of pressure sensors 30. The two-dimensional array of pressure sensors 30 extends on either side of the opening 22 of the wall 33. Although the depicted two-dimensional array includes sixty sensors, the two-dimensional array may have some other number of sensors.

A gas (e.g., air) is present in the mask assembly environment 18. The gas may be provided at a higher pressure than in the projection system PS to inhibit or prevent contaminant particles from traveling from the projection system into the mask assembly environment 18.

The volume 26 is enclosed by the pellicle P, mask MA and frame F. A gas is contained in the volume 26. The volume is connected to the mask assembly environment 18 by a leak path (not depicted) that allows gas (e.g., air) to flow between them. The leak path is limited such that the rate at which gas can travel between the volume 26 and the mask assembly environment 18 is limited. The flow rate is low enough that the amount of gas in the volume 26 during a scanning exposure can be considered to be fixed.

During a scanning exposure, the support structure MT and mask assembly MS quickly move in the y-direction (as indicated by the arrow in FIG. 3A) from one side of the housing 20 to the other. The scanning exposure may be performed, for example, in about 100 milliseconds.

As schematically depicted in fig. 3A, during a left-to-right scanning movement of the mask assembly MS, the gas pressure at the right hand side of the mask assembly MS and support structure MT will increase, as the volume containing the gas is being reduced. At the same time, the pressure on the left hand side of the mask assembly MS and support structure MT is reduced, as the volume containing the gas is increased. As a result, the gas flows around the mask assembly MS and support structure MT until the gas pressure in the mask assembly environment 18 has equilibrated. Such a gas flow may cause dynamic deformation of the pellicle P (i.e. deformation that changes during the scanning movement of the pellicle).

The inertia of the gas (e.g., air) within the volume 26 enclosed by the pellicle P, mask MA, and frame F may also cause dynamic deformation of the pellicle. Referring to fig. 3A, when a scanning motion of the mask assembly MS in the positive y-direction begins, the inertia of the gas within the volume 26 causes the gas to tend to stay in its initial position. Thus, during acceleration of the mask assembly MS, there will be a build-up of gas at the left hand end of the mask assembly. This will cause the pellicle film P to bulge outwardly at the left hand end of the mask assembly MS. The mask assembly MS will slow down when it reaches the right hand end of its scanning movement. The gas is now moving in the positive y-direction within the volume 26 and will tend to remain moving as the mask assembly MS decelerates. Thus, during deceleration of the mask assembly MS, there will be a build-up of gas at the right hand end of the mask assembly. This will cause the pellicle film P to bulge outwardly at the right hand end of the mask assembly MS. As noted above, the scanning exposure may be performed in about 100 milliseconds. During this time, the mask assembly will accelerate from rest, move more than 100mm, and then decelerate to rest. It will be appreciated that the inertia of the gas within the volume 26 of the mask assembly MS will result in considerable deformation of the pellicle.

The dynamic deformation consists of a curvature of the pellicle P and this introduces distortion to the image projected onto the substrate W by the lithographic apparatus LA. As explained above in connection with fig. 2, this introduces an offset into the projected image when the pellicle P is at an angle relative to the mask MA. Because the pellicle is curved and thus has a range of angles relative to the mask, the pellicle does not introduce a simple offset but instead introduces distortion into the projected image. In addition, the distortion introduced by the pellicle changes during the scanning exposure. This is because the patterned beam PB passes along the pellicle P during the scanning movement of the mask assembly MS, and different portions of the pellicle will bend in different ways.

Fig. 4 schematically depicts an example of pellicle deformation that may occur during a scanning motion of the mask assembly. As indicated schematically in fig. 4, the scanning movement of the pellicle P is in this example in the Y-direction.

As mentioned further above, the amount of gas in the volume 26 between the pellicle P and the mask MA is effectively fixed during a scanning exposure. Furthermore, the gas G within the volume will tend to resist compression or expansion. As a result, the total volume enclosed by the pellicle P will remain substantially constant, such that any outward expansion of one portion of the pellicle P will tend to be matched by a corresponding inward movement of the other portion of the pellicle. An example of this form of deformation of the pellicle P is depicted in fig. 4. A portion of the pellicle P toward the lower left end of the figure has an inflated interior, and a portion of the pellicle toward the upper right end of the figure has an inflated exterior by a corresponding amount. Thus, the volume enclosed by the pellicle P remains substantially constant. This form of deformation of the pellicle may be considered to be similar to the movement of the surface of a water bed, i.e. to the movement of a flexible membrane enclosing a volume of substantially incompressible fluid.

The distortion caused by the pellicle in the projected image during a scanning exposure is relatively complex. The distortion may be considered as an aberration that may be expressed as a zernike polynomial, and the distortion includes several orders of the zernike polynomial. However, the distortion is relatively uniform. That is, when a given mask assembly MS and a particular pellicle are used to perform a scanning exposure in a given lithographic apparatus, the distortion caused by the pellicle will typically be the same as the distortion caused during the previous exposure. This would be the case assuming that the speed and direction of the scanning exposure are the same and assuming that the background gas pressure in the mask assembly environment is the same (i.e., the pressure of the gas is unchanged all the time when the mask assembly MS is not moving). To scan expose at the same speed but in opposite directions, the distortion caused by the pellicle will be reversed.

The pressure measurement output from the array of pressure sensors 30 is used to determine the dynamic deformation of the pellicle film P during a scanning exposure. Dynamic deformation may be considered as a shape formed by the pellicle as a function of position during a scanning motion of the mask assembly MS. The dynamic deformation of the pellicle during a scanning exposure may be pre-calculated by the processor by using a plurality of measurements obtained before the scanning exposure of the substrate occurs (as described further below). The calculated dynamic deformation of the pellicle during the scanning exposure is used to determine the adjustment applied to the projection system during the scanning exposure of the substrate. These adjustments reduce the distortion caused by the pellicle.

Referring again to fig. 3, the pressure sensors 30 located in the wall 33 of the housing 20 are arranged in a two-dimensional array. The pressure sensor 30 may be a movable diaphragm attached to the magnet, for example in the form of a microphone. The pressure sensor may be, for example, a MEMs microphone. In an embodiment, the pressure sensor may be an AKU242 digital silicon MEMs microphone available from Akustica inc. Other MEMs microphones (pressure sensors) may be used, such as, for example, the VM101 microphone available from Vespa Technologies, inc.

The pressure sensor 30 may be located in a recess formed in the wall 33. Positioning the pressure sensor 30 in a recess in the wall 33 is advantageous because it prevents the pressure sensor from protruding outwards from the wall and creates significant turbulence to the gas (e.g. air) flowing across the wall. The upper surface of the pressure sensor may be flush with the upper surface of the wall. This may provide a smooth continuous surface when the pressure sensor is level with the upper surface of the wall and also helps to avoid causing turbulence.

Electrical connections from pressure sensor 30 (not depicted) may pass through wall 33 and protrude from the bottom surface of the wall, or may pass into the wall and protrude from the side wall. A wireless connection may alternatively be used. The processor PR receives an output signal from said pressure sensor 30.

In fig. 3, the two-dimensional array of pressure sensors consists of 60 pressure sensors. However, this is merely illustrative and any suitable number of pressure sensors may be used.

The pressure sensors 30 of the pressure sensor array may be located about 2cm away from the pellicle film P. The pressure sensors may be separated from each other by a spacing of about 2cm (e.g., a spacing of up to 3 cm). In general, providing the pressure sensor 30 with a separation (which separation is generally consistent with or less than the distance between the pressure sensor and the pellicle P) may allow for the efficient use of near-field sonoholography to determine pellicle shape.

The pellicle P may for example measure about 110mm x 150mm. The deflection of the pellicle during a scanning exposure may have a relatively low spatial frequency (e.g., 3cm or more). Thus, a spacing of about 2cm (e.g., a spacing of up to 3 cm) of the pressure sensors 30 of the array may provide pressure measurements with a spatial frequency high enough to allow accurate determination of the dynamic pellicle deformation. The surface membrane deflection occurs in millimeter scale. The sensing system 40 may be capable of determining pellicle deflection with micrometer accuracy. This is sufficient to provide an accurate characterization of the millimeter-scale deflection of the pellicle.

The pressure sensor 30 may have a sampling frequency that is higher than the frequency of movement of the pellicle during the scanning motion. The frequency of movement of the pellicle may be, for example, in the range 25Hz to 40 Hz. The pressure sensor 30 may, for example, provide output measurements having frequencies up to about 100Hz (e.g., up to about 200 Hz), and be able to accurately detect frequencies as low as 10 Hz. The pressure sensor 30 may be able to detect frequencies below 10Hz, but the accuracy of such measurements may be reduced.

In general, the spatial spacing of the pressure sensors 30 and the frequency of the output from the pressure sensors may be selected to be high enough to allow the deflection of the pellicle film P to be effectively sampled and determined.

Near field acoustic holography (NAH) may be used by the processor PR to reconstruct the dynamic deformation of the pellicle film P during the scanning motion of the mask assembly MS. In other words, to reconstruct the shape formed by the pellicle film P, the shape is a function of position during the scanning movement of the mask assembly MS. The reconstruction is implemented using calculations performed by the processor PR. The reconstruction of the shape formed by the pellicle during the scanning exposure allows determining the distortion caused by the pellicle. Once the distortions have been determined, a lens model may be used to determine corrections to be applied to the lithographic apparatus (e.g., adjustments of lens elements of the projection system), which reduces those distortions. This advantageously improves the accuracy with which the pattern is projected onto the substrate. Near field acoustic holography is discussed further below.

The array of pressure sensors 30 and the processor PR may be used to determine the dynamic deformation of the pellicle P before starting the exposure of the substrate. This is because the behaviour of the pellicle is uniform and the deformation measured before exposure occurs can be expected to repeat during exposure, assuming that the perimeter of the deformation is determined to remain the same. Thus, the mask assembly MS may be moved with a scanning motion corresponding to a scanning exposure, but the exposure radiation is not incident on the substrate. The dynamic pellicle deformation that occurs is determined by the processor PR based on the output signals from the array of pressure sensors 30. During a subsequent scanning exposure of the substrate, the dynamic deformation of the pellicle P is assumed to be the same and a correction is therefore applied to the lithographic apparatus.

In one example, a lithographic apparatus may be used to expose a substrate with a mask MA and an associated pellicle P that was not previously used in the lithographic apparatus. The desired scan exposure length will be used during exposure of the substrate and the desired scan speed may be used. However, this may vary for exposures at different locations on the substrate. For example, when exposing a field positioned toward the center of the substrate, a full exposure scan length and a maximum scan speed may be used. However, when exposing a field located at the edge of the substrate, a local field may be exposed. Thus, a shorter exposure scan length can be used. The speed of the scanning exposure can also be reduced. Different scan lengths and/or scan speeds may be used to expose different locations around the edge of the substrate.

The array of pressure sensors 30 may be used to obtain measurements for a set of scanning motions prior to exposing the substrate, the measurements including different scanning lengths and speeds to be used during subsequent exposures of the substrate. For each scan length and/or speed of the set of scan motions, measurements obtained using the array of pressure sensors 30 are used to determine the dynamic deformation of the pellicle P that will occur during the scan exposure. A lens model is then used to determine adjustments to be applied to the lens elements during those scanning exposures in order to reduce distortion caused by the pellicle deformation.

In one example, the scanning movement of the mask assembly MS may be performed in the positive Y-direction and the negative Y-direction for each scanning speed and scanning length, so as to allow two sets of data to be obtained. As further noted above, for scanning in the positive Y-direction and the negative Y-direction, the pellicle may be expected to be symmetrical, and thus the measurement of a single scanning motion may be sufficient to characterize the dynamic deflection of the pellicle. However, performing two scan motions provides additional data that may allow for a more accurate determination of the deformation of the pellicle (e.g., by improving the signal-to-noise ratio). For a given scan length, more than two scan motions may be used in order to obtain additional data. This may allow the pellicle deformation to be obtained with greater accuracy (e.g., by further improving the signal-to-noise ratio).

In one example, a simulated exposure of the entire substrate may be performed without directing exposure radiation through the mask MA. In this example, the set of scanning motions includes a simulated exposure of the entire substrate. Data for each scanning movement of the mask assembly MS may be collected from the pressure sensor 30. Performing a simulated exposure of the entire substrate ensures that all scan speeds and scan durations that will occur during production exposure of the substrate have occurred and that data has been generated that can be used to determine the shape that will be formed by the pellicle during those exposures. Once the measurement has been performed, a correction to be applied to the lens element may be calculated. The correction may then be applied while performing the exposure of the substrate. As noted elsewhere in this document, pressure measurements may be performed during exposure of the substrate.

The computational processing power required to determine the pellicle deformation and the correction to be applied to the lens element may be such that real-time computation and application is not possible during exposure of the substrate. For this purpose, in the embodiments described above, they are calculated before exposure of the substrate. However, if a sufficiently high processing power is available, the determination of the pellicle deformation and the associated lens correction may be determined in real time. Thus, the shape formed by the pellicle during the scanning motion can be calculated while the production exposure is performed, and the correction to be applied to the lithographic apparatus can be calculated.

The shape formed by the timetable film at each instant in time during the scanning movement of the mask assembly MS is reconstructed via calculation using near field sonography. The computation used by such reconstruction can be computationally intensive and thus useful for filtering out possible noise. The filtering of the data output from the pressure sensor may be performed using a plurality of frequencies. Near field acoustic holography is described in WO2009/130243A2, US2013/0094678A1 and US 2013/012873 A1, each of which is hereby incorporated by reference.

One parameter that may be used to apply the filtering to the data is the eigenfrequency (resonance frequency) of the time pellicle P. The eigenfrequency of the pellicle will be in the range of 20Hz to 50Hz and will depend on the tension of the pellicle. In practice, the eigenfrequency is likely to be about 25Hz (e.g., plus or minus 5 Hz). The eigenfrequency of the pellicle may be determined by applying vibrations to the mask assembly MS, for example, using an actuator that is used to provide a scanning motion of the mask assembly during exposure of the substrate. While vibration is being applied, the output from the pressure sensor 30 is monitored. The frequency of the vibration initially applied is the eigenfrequency expected hereinafter. The frequency is then increased, for example, in 0.1Hz increments, until a spike is seen in the signal output from the pressure sensor 30. Such spikes indicate the eigenfrequency of the pellicle P. Such eigenfrequencies and harmonics of the eigenfrequencies will be present when the pellicle is deformed during the scanning movement.

Another frequency that will be present during the scanning movement of the mask assembly MS is the frequency of the exposure performed by the lithographic apparatus. Such exposure frequency may be, for example, between 2Hz and 10 Hz. In addition, harmonics of the exposure frequency may also be present in the pellicle deformations. The two scanning movements of the mask assembly MS are sufficient to allow the measurement of pellicle deformations caused by the exposure frequency. In this context, the term "two scan motions" is intended to mean a scan motion in one direction and then a return scan motion in the opposite direction. This may allow for measuring the deformation of the pellicle caused by the exposure frequency by using a single scanning motion.

Typically, because the behavior of the pellicle is highly repeatable, a single scan motion may be sufficient to allow determination of the pellicle deformation during the scan motion. The determined deformation of the pellicle will be repeated for subsequent scanning movements of the same speed and duration.

The dynamic deflection of the pellicle may have a maximum frequency limit significantly below about 200 Hz. Low pass filtering may be applied to the signal output from the pressure sensor 30 so that signals having a frequency greater than about 200Hz are excluded when deflection of the pellicle is being calculated. This provides a further improvement in signal to noise ratio.

Knowledge of the frequencies present in the pellicle deformations may be used to apply filtering to the signal output from the pressure sensor 30, thereby improving the signal-to-noise ratio. For example, frequencies present in the signal received from the pressure sensor 30 that are outside of the expected frequency of movement of the implemented pellicle may be filtered out (e.g., implemented by the processor PR).

When near-field sonography is used to reconstruct the shape formed by the pellicle during a scanning exposure, the phase difference between the signal outputs from the pressure sensors 30 of the pressure sensor array is used. Because the array of pressure sensors 30 is two-dimensional (as opposed to just one row of sensors), the information received from the sensors is sufficient to allow the shape formed by the pellicle to be reconstructed. The processor PR performs a correlation, i.e. correlation, between signals received from the different pressure sensors 30. If a strong signal is observed for a given correlation, this indicates that: the object has caused pressure waves that have been incident at these pressure sensors at different times. The phase difference (time delay) that causes the strongly correlated signal can be used to determine the likely origin of the pressure wave. The position of the mask assembly (and thus the pellicle P) in the Y-direction is known at any given moment in time when a signal is being output from the pressure sensor 30. Thus, since the position of the pellicle P is known (although its deflection is unknown), this information can be used to determine whether the origin of the pressure wave corresponds to the pellicle or to some other device. If the pellicle P is not the origin of a pressure wave, the pressure wave may be ignored. If the pellicle P is not the origin of a pressure wave, the pressure wave is used as part of the reconstruction of the shape formed by the pellicle. This is performed for the pressure sensor 30 across the array of pressure sensors. A number of origin points of the pressure wave are determined. Together, these points of origin indicate the shape formed by the pellicle P.

In more detail, when a pressure wave is generated, the pressure wave propagates according to a propagation function or a propagation operator G.

Wherein z is _s Is a position in one dimension of the source of the pressure wave, andand z _h Is a position in one dimension of the pressure sensor. In an embodiment of the invention, the acoustic wave as received at the pressure sensor 30 is known. A back propagation function or back propagation operator may be used to back propagate the acoustic wave and determine the deflection of the pellicle that caused the pressure wave. The inverse solution may be to deconvolute the measurement plane (the plane in which the pressure sensor 30 is located) with Rayleigh (Rayleigh) propagation kernels.

As indicated further above, once the deformation of the pellicle has been determined for the pellicle in the lithographic apparatus for a particular scan length and scan speed, such known deformation of the pellicle may be expected to occur during subsequent substrate exposures using the same scan in length and scan speed.

As indicated further above, assuming that the other parameters are unchanged, the behaviour of the pellicle P is consistent for a given scan speed and scan length. In practice, there may be some differences between the interior of the housing 20 where the mask assembly MS is located in different lithographic apparatus. Therefore, even when the same scanning length and scanning speed are used, deformation of the pellicle may be different inside different lithographic apparatuses. Thus, the previously determined pellicle deformation may be used for a particular pellicle within a particular lithographic apparatus, but should not be used for a pellicle in a different lithographic apparatus. When a pellicle is present, a previously determined deformation of the pellicle may be used for subsequent scanning exposure. This may occur when the pellicle is attached to the same mask MA, or when the pellicle is attached to a different mask MA (changing the mask does not have a significant effect on the deformation of the pellicle).

The pressure sensor 30 may be installed in all lithographic apparatus in the manufacturing plant. This allows dynamic pellicle deformation to be determined within each lithographic apparatus. This is advantageous because, as indicated above, different dynamic deformations of the same pellicle may occur in different lithographic apparatuses.

The pressure sensor 30 is present during the production exposure of the substrate. Because the pressure sensors 30 are passive (i.e., they have no effect on the pellicle or the mask), they can be used to collect data during the production exposure of the substrate. The processor PR may continue to be used to determine the shape formed by the pellicle by using the data obtained from the pressure sensor 30 during the production exposure of the substrate. This may allow for refinement of the calculated dynamic deformation of the pellicle, for example. In other words, the accuracy of determining the pellicle deformation may be improved over time. Similarly, adjustments applied to the lithographic apparatus to reduce distortion caused by pellicle deformation may be improved over time.

As indicated further above, the deformation of the pellicle is determined at least in part by the tension in the pellicle. This tension will gradually decrease over time. This is because the pellicle will absorb some of the radiation from the patterned beam of radiation and over time this will lead to ageing of the pellicle. This aging causes the pellicle to lose some tension. When the tension is reduced, the deformation of the pellicle will have the same shape during the scanning exposure. However, the shape will be enlarged. In other words, the maximum deflection of the pellicle from a plane passing through the pellicle edge is increased.

Because aging of the pellicle will be incremental and predictable, a simple model may be used to adjust the calculated dynamic deformation of the pellicle to account for aging of the pellicle that has occurred as a result of measuring the pellicle deformation. Alternatively, if the output signal from the pressure sensor 30 is monitored during a production exposure, the dynamic deformation of the pellicle may be calculated periodically. This will include changes in dynamic deformation due to aging.

When reconstructing the shape formed by the pellicle P using the signal output from the pressure sensor 30, then the processor PR may take into account the known constraints of the pellicle. For example, as indicated above, the position of the pellicle is known during the scanning movement, and the processor PR may ignore the pressure signal received from the origin other than the pellicle. In another example, it is known that the edges of the pellicle P do not move in the z-direction, as they are fixed to the frame F.

When reconstructing the shape formed by the pellicle P using the signal output from the pressure sensor 30, then the processor PR may take into account the shape previously observed for the other pellicle. This may be accomplished, for example, using shapes previously observed for other pellicle of the same type. The same type of pellicle may be a pellicle having the same thickness. The same type of pellicle may have the same initial tension as the pellicle P when manufactured. However, due to aging of the pellicle, the tension of the same type of pellicle may decrease over time. Due to the ageing of the pellicle, the shape observed for the pellicle will be scaled in size. The cumulative dose of radiation experienced by the pellicle causes aging of the pellicle. Such a dose may be calculated by a processor. Thus, the ageing and reduced tension of the pellicle caused by the dose can be determined. The processor PR may apply adjustments to the shape previously observed for the same type of pellicle to account for aging of the pellicle. When reconstructing the shape formed by the pellicle P, the resulting shape may be considered by the processor PR. Embodiments of the present invention do not affect the production exposure performed using the lithographic apparatus. As indicated above, this means that measurements of pellicle deflection can be performed during production exposure.

The array of pressure sensors 30 may be post-assembled (i.e., reassembled or reassembled) to an existing lithographic apparatus. This may be achieved, for example, by replacing the existing wall 33 of the mask assembly housing 20 with a new wall within which the array of pressure sensors 30 is disposed.

FIG. 6 is a flow chart setting forth a method according to an embodiment of the invention that may be used to compensate for distortion of a projected image caused by the shape formed by the pellicle P during a scanning exposure performed by a lithographic apparatus. The correction may be applied, for example, during a production exposure of the substrate (e.g., during exposure of a series of wafers and dies that will form an integrated circuit). In summary, the method includes using output from the array of pressure sensors and other information to calculate a shape formed by the pellicle during a scanning motion. The computation may be a reconstruction of the shape formed by the pellicle using near-field acoustic holography. The method further comprises the steps of: using a radiation beam aberration model to determine how the radiation beam PB is deformed by the pellicle; and applying a rolling gaussian slit exposure model to take into account the scanning properties of the exposure and thereby determine the effect of the pellicle on the exposure. The effect of the pellicle on exposure may be referred to as the pellicle's fingerprint. The method further comprises using a lens model to determine an adjustment of the projection system PS to be applied for compensating the pellicle fingerprint. The correction is then applied to the projection system during a production exposure of the substrate.

As indicated further above, the distortion of the pellicle is reversed when the direction of the scanning exposure is reversed. Thus, two sets of adjustments may be stored in memory, one set for each direction of the scanning exposure. When a scanning exposure is performed at the edge of the wafer, the scanning exposure is shorter and/or slower than a scanning exposure performed away from the edge of the wafer. As a result, the pellicle deformation will be different when performing these exposures. Thus, additional sets of adjustments of the projection system PS may be stored in memory for use when exposure occurs at the edge of the wafer.

If the heating of the pellicle P by the radiation beam PB is expected to have a significant effect (e.g. a reduction in tension in the pellicle due to thermal expansion), calibration may be performed when the radiation beam is incident on the pellicle. Alternatively, the effect of heating caused by the radiation beam on the tension of the pellicle P may be calculated and added to the model (using the thermal expansion coefficient of the pellicle). The temperature of the pellicle P may be expected to increase in a known manner as a function of time and may, for example, increase at the same rate as the mask MA temperature. The heating of the mask may be the subject or subject of a separate pre-existing model, and the temperature of the pellicle P may be derived from the model. The pellicle P may be heated directly by absorption of incident radiation or indirectly by thermal conduction from the mask MA to the pellicle P via the frame F.

In more detail, referring to fig. 6, a mask assembly having a new pellicle is loaded into a lithographic apparatus. In this context, "new" may mean that the pellicle was not previously used in this lithographic apparatus (it may have been used in other lithographic apparatus). The eigenfrequency of the pellicle may be determined (as described further above). This is an optional step that may reduce the amount of computation required to reconstruct the shape formed by the pellicle. However, the shape formed by the pellicle may be determined without first determining the eigenfrequency of the pellicle. A scanning motion of a mask assembly to be used during a production exposure is performed. An array of pressure sensors is used to measure pressure during the scanning motion.

Near field acoustic holography is used to reconstruct the shape formed by the pellicle during the scanning motion. The shape formed by the pellicle changes according to the pellicle position during the scanning movement. The eigenfrequency of the pellicle and the exposure scan frequency (along with harmonics of these frequencies) may be used to filter the signal. The filtering may occur prior to or as part of the calculation.

The radiation beam aberration model receives as input the shape of the pellicle during a scanning motion and receives as input an illumination pattern to be used during a production exposure. The radiation beam aberration model may be, for example, a ray deflection model, which may be a model implementing snell's law (described above with respect to fig. 2). Alternatively, the implementation of the radiation beam aberration model may be a more advanced model modeling the zernike aberration of the radiation beam caused by the deformation of the pellicle (this type of model treats the pellicle as a lens element).

The output from the radiation beam aberration model is an input to the rolling gaussian slit exposure model. This model accounts for movement (e.g., as a convolution) of the pellicle and the mask relative to the radiation beam during a scanning exposure and provides as output a pellicle fingerprint caused by a deformation of the pellicle. An example of a pellicle fingerprint is depicted in fig. 5. The pellicle fingerprint indicates how points in the image are displaced due to the effects of the pellicle distortion.

Finally, a lens model is used to determine the correction to be applied to the lens elements of the projection system PS in order to compensate for the pellicle fingerprint. Such lens models are well known in the art and are therefore not described herein. The correction may be, for example, capable of applying a fourth order polynomial correction in the y-direction.

As indicated above, the correction to be applied to the lens element may be determined before the production exposure takes place. The correction may then be applied during production exposures and thus the pellicle fingerprint compensated during those exposures.

The pellicle may be stationary before exposure of a new substrate begins. Vibration of the mask assembly MS will occur when exposure of the substrate begins and will stabilize after about two or three scanning exposures. The effect of these vibrations on the shape formed by the pellicle may be measured using embodiments of the invention. This effect can then be taken into account when applying corrections to the lithographic apparatus during production of a scanning exposure.

The adjustment to be applied by the lithographic apparatus during a scanning exposure may be stored at the lithographic apparatus. Alternatively, the adjustments may be stored remotely and communicated to the lithographic apparatus when the adjustments are required.

The adjustment to compensate for the pellicle fingerprint may be combined with an adjustment to compensate for an aberration source located elsewhere in the lithographic apparatus (e.g. an adjustment to compensate for aberrations caused by heating of lens elements of the projection system during exposure).

Although the described embodiments of the invention relate to a particular form of model, any suitable form of model may be used.

While an adjustment to compensate for the pellicle fingerprint has been explained in terms of lens element adjustment, other adjustments may be used by the lithographic apparatus. For example, the position of the substrate during the scanning exposure may be adjusted by the lithographic apparatus (e.g. a certain movement in the z-direction may be used to compensate for a change in focus).

The Z-direction movement of the pellicle during the scanning exposure may be determined as part of the calculation of the shape formed by the pellicle during the scanning exposure. Such an output can be used to determine the extent of performance degradation of the pellicle caused by z-direction movement. If dust particles are present on the pellicle, a z-direction movement towards the mask will move the dust particles closer to the focal plane of the lithographic apparatus. The calculated pellicle shape may be used to determine the extent to which this occurs. The effect of the z-direction movement of the dust particles can then be determined.

Aspects of the invention can be implemented in a convenient manner, including by appropriate hardware and/or software. For example, a programmable device (which may form part of the controller CT) may be programmed to implement embodiments of the invention. Accordingly, the present invention also provides suitable computer programs for implementing aspects of the present invention. Such a computer program may be carried on a suitable carrier medium, including tangible carrier media (e.g., hard drives, CDROMs, etc.) and intangible carrier media (such as communication signals).

While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The description is not intended to limit the invention.

Claims

1. A lithographic apparatus comprising:

a support structure configured to support a mask and associated pellicle, the mask being capable of imparting a pattern in a cross-section of a radiation beam to the radiation beam to form a patterned radiation beam; and

a projection system configured to project the patterned beam of radiation onto a target portion of a substrate,

wherein a wall extends between the support structure and the projection system, the wall comprising an opening allowing the patterned radiation beam to pass from the mask and pellicle to the projection system, and wherein the wall is provided with a two-dimensional array of pressure sensors.

2. The lithographic apparatus of claim 1, wherein the two-dimensional array of pressure sensors extends on either side of the opening in the wall.

3. The lithographic apparatus of claim 1 or claim 2, wherein the pressure sensor is located in a recess formed in the wall.

4. A lithographic apparatus according to claim 3, wherein an upper surface of the pressure sensor is flush with an upper surface of the wall.

5. The lithographic apparatus of claim 1, further comprising a processor configured to receive output signals from the two-dimensional array of pressure sensors and to calculate a shape formed by the pellicle during a scanning motion of the mask and pellicle.

6. The lithographic apparatus of claim 5, wherein the processor is configured to reconstruct a shape formed by the pellicle using near-field sonoholography.

7. The lithographic apparatus of claim 5 or 6, further comprising a controller configured to apply an adjustment to a lens element of the projection system during a scanning exposure to compensate for distortion caused by the shape formed by the pellicle.

8. A method of measuring pellicle deflection in a lithographic apparatus, the method comprising:

loading a mask assembly comprising a mask and a pellicle into the lithographic apparatus of claim 1;

performing a scanning motion of the mask assembly and receiving a signal output from the pressure sensor; and

the signal output from the pressure sensor is used to calculate the shape formed by the pellicle during the scanning motion.

9. The method of claim 8, wherein calculating a shape formed by the pellicle during the scanning motion comprises reconstructing a shape formed by the pellicle using near-field acoustic holography.

10. The method of claim 8 or 9, wherein the method further comprises determining the eigenfrequency of the pellicle and then taking into account the eigenfrequency and harmonics of the eigenfrequency when calculating the shape formed by the pellicle.

11. The method of claim 8, wherein receiving output signals from the pressure sensor continues during a production exposure performed using the mask and pellicle, and wherein the output signals are used to adjust the calculated shape formed by the pellicle.

12. The method of claim 11, wherein a correction to be applied to the lithographic apparatus is adjusted to account for the adjusted calculated shape formed by the pellicle.

13. A memory storing computer readable instructions configured to cause a computer to perform the method of any one of claims 8 to 12.

14. A computer, comprising:

a memory storing processor readable instructions; and

A processor arranged to read and execute instructions stored in the memory;

wherein the processor readable instructions comprise instructions arranged to control the computer to perform the method according to any of claims 8 to 12.