KR20250005146A - Source selection module and associated instrumentation - Google Patents

Abstract

A source selection module for selecting spectral characteristics of a broadband illumination beam to obtain a modulated illumination beam is disclosed. The source selection module includes a first beam-dispersing element for dispersing the beam along a first direction; a second beam-dispersing element for dispersing the beam along a second direction perpendicular to the first direction; a controllable diffractive element operable to controllably spatially modulate the broadband illumination beam after being dispersed by the first beam-dispersing element and the second beam-dispersing element; and an aperture stop operable to maximize transmission of one of the regularly reflected radiation and the diffracted radiation from the controllable diffractive element and to minimize transmission of the other of the regularly reflected radiation and the diffracted radiation.

Description

Source selection module and associated instrumentation

Cross-reference to related applications

This application claims the benefit of EP application 22169636.2, filed April 25, 2022 and EP application 22174097.0, filed May 18, 2022, which are incorporated herein by reference in their entirety.

The present invention relates to a method and an apparatus usable for manufacturing devices by, for example, lithographic techniques, and to a method for manufacturing devices by using lithographic techniques. The present invention particularly relates to a measurement sensor and a lithographic apparatus having such a measurement sensor, and more particularly to an illumination arrangement for such a measurement sensor.

A lithographic apparatus is a machine that applies a desired pattern onto a substrate, typically onto a target portion of the substrate. A lithographic apparatus may be used, for example, in the manufacture of integrated circuits (ICs). In that example, a patterning device, alternatively referred to as a mask or a reticle, may be used to create a circuit pattern to be formed on an individual layer of the IC. The pattern may be transferred onto a target portion (e.g., comprising a portion of a die, one or more dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically accomplished by imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. Typically, a single substrate will comprise a network of adjacent target portions that are sequentially patterned. These target portions are often referred to as "fields".

In the manufacture of complex devices, typically many lithographic patterning steps are performed, thereby forming functional features in successive layers on a substrate. Therefore, a critical aspect of the performance of a lithographic apparatus is its ability to correctly and accurately place an applied pattern with respect to features laid down in a previous layer (either by the same apparatus or by a different lithographic apparatus). For this purpose, the substrate is provided with a set of one or more alignment marks. Each mark is a structure whose position can be subsequently measured using a position sensor, typically an optical position sensor. The lithographic apparatus comprises one or more alignment sensors, and the positions of the marks on the substrate can be accurately measured by these alignment sensors. Different types of marks and different types of alignment sensors are known from different manufacturers and from different products of the same manufacturer.

In another application, the metrology sensor is used to measure exposed structures on a substrate (within the resist and/or after etching). A fast, non-invasive type of specialized inspection tool is a scatterometer, in which a beam of radiation is directed at a target on the surface of the substrate and the properties of the scattered or reflected beam are measured. Examples of known scatterometers include angle-resolved scatterometers of the type described in US2006/033921A1 and US2010/201963A1. In addition to measuring feature shapes by reconstruction, diffraction-based overlay can be measured using such devices, as described in published patent application US2006/066855A1. Diffraction-based overlay measurement using dark-field imaging of diffraction orders enables overlay measurements on smaller targets. Examples of dark field imaging metrology can be found in international patent applications WO2009/078708 and WO2009/106279, which are incorporated herein by reference in their entireties. Further developments of this technology are described in published patent applications US2011/0027704A, US2011/0043791A, US2011/102753A1, US2012/0044470A, US2012/0123581A, US2013/0258310A, US2013/0271740A and WO2013/178422A1. The targets can be smaller than the illumination spot and can be surrounded by product structures on the wafer. Multiple gratings can be measured within a single image using compound grating targets. The contents of all of these applications are also incorporated herein by reference.

In some metrology applications, such as in some scatterometers or alignment sensors, imperfections in the measurement target can result in wavelength/polarization dependent variations in the measured values from that target. Therefore, compensation and/or mitigation for these variations is sometimes effected by performing the same measurement using a number of different wavelengths and/or polarizations (or more generally, a number of different illumination conditions). For such metrology applications, it would be desirable to improve the switching and selection of the spectral components of the illumination.

The present invention in a first aspect provides a source selection module for selecting spectral characteristics of a broadband illumination beam to obtain a modulated illumination beam, the source selection module comprising: a first beam-dispersing element for dispersing the broadband illumination beam, the first beam-dispersing element being operable to disperse the broadband illumination beam along a first direction; a second beam-dispersing element for dispersing the broadband illumination beam, the second beam-dispersing element being operable to disperse the broadband illumination beam along a second direction perpendicular to the first direction; a controllable diffractive element having controllable elements arranged along the first direction such that a direction of its periodicity includes the first direction, the controllable diffractive element being operable to controllably spatially modulate the broadband illumination beam after being dispersed by the first beam-dispersing element and the second beam-dispersing element; and an aperture stop operable to maximize transmission of one of the regularly reflected radiation and the diffracted radiation from the controllable diffractive element and to minimize transmission of the other of the regularly reflected radiation and the diffracted radiation.

The present invention in a second aspect provides a source selection module for selecting spectral characteristics of a broadband illumination beam to obtain a modulated illumination beam, the source selection module comprising: at least one beam-dispersing element for dispersing the broadband illumination beam, the at least one beam-dispersing element being operable to disperse the broadband illumination beam along a first direction; a controllable diffractive element having controllable elements arranged along a first direction such that a direction of their periodicity includes the first direction, the controllable diffractive element being operable to controllably spatially modulate the broadband illumination beam after being dispersed by the first beam-dispersing element; an aperture stop operable to maximize transmission of one of the regularly reflected radiation and the diffracted radiation from the controllable diffractive element and to minimize transmission of the other of the regularly reflected radiation and the diffracted radiation; and a plurality of lens elements including at least one lens or lens system operable to image the broadband illumination beam after being dispersed by the first beam-dispersing element onto the controllable diffractive element and to collect the modulated illumination beam from the controllable diffractive element; Here, the first beam dispersing element is also arranged to recombine the modulated illumination beam on the return path from the controllable diffractive element.

Also disclosed are a metrology apparatus and a lithography apparatus including a metrology device operable to perform the method of the first aspect or the second aspect.

The above and other aspects of the present invention will be understood in consideration of the examples described below.

An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:
Figure 1 illustrates a lithography apparatus.
Figure 2 illustrates a schematic overview of a lithography cell.
Figure 3 illustrates a schematic representation of holistic lithography, showing the collaboration between three key technologies to optimize semiconductor manufacturing.
FIG. 4 is a schematic diagram of a scatterometry device used as a measuring device that may include a dark-field digital holographic microscope according to an embodiment of the present invention.
Figure 5 includes (a) a schematic diagram of a dark-field scatterometer for use in measuring a target using a first pair of illumination apertures, and (b) a detailed illustration of the diffraction spectrum of the target grating for a given illumination direction.
Figure 6 is a schematic drawing of a grating light valve, showing its basic operation in (a) a plan view, (b) a cross-sectional view of a first configuration, and (c) a cross-sectional view of a second configuration.
Figure 7 is a schematic diagram of the operating principle of a lighting array including a grid light valve in a first configuration.
FIGS. 8A, 8B and 8C are schematic drawings of the operating principle of a lighting array including a grid light valve in a second configuration.
Figure 9 is a pupil representation illustrating the difficulty in optimizing the aperture stop for the array of Figure 7.
FIGS. 10A and 10B are pupil representations for a lighting arrangement according to an embodiment of the present invention.
FIG. 11 is a schematic diagram of the operating principle of a lighting array including a grid light valve according to a first embodiment of the present invention.
FIGS. 12A and 12B are schematic diagrams of the operating principle of a lighting array including a grid light valve according to a second embodiment of the present invention.

Before describing embodiments of the present invention in detail, it is helpful to present an exemplary environment in which embodiments of the present invention may be implemented.

In this specification, the terms “radiation” and “beam” are used to include all types of electromagnetic radiation, including ultraviolet radiation (e.g., having wavelengths of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultraviolet radiation, e.g., having wavelengths in the range of about 5 to 100 nm).

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

Figure 1 schematically illustrates a lithography apparatus (LA). A lithographic apparatus (LA) comprises an illumination system (also referred to as an illuminator) (IL) configured to control a radiation beam (B) (e.g., UV radiation, DUV radiation or EUV radiation), a mask support (e.g., a mask table) (MT) configured 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 specific parameters, a substrate support (e.g., a wafer table) (WT) configured 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 specific parameters, and a projection system (e.g., a refractive projection lens system) (PS) configured to project a pattern imparted to the radiation beam (B) by the patterning device (MA) onto a target portion (C) (e.g., including one or more dies) of the substrate (W).

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, such as refractive, reflective, magnetic, electromagnetic, electrostatic and/or other types of optical components or any combination thereof, to direct, shape and/or control the radiation. The illuminator (IL) may be used to condition the radiation beam (B) to have a desired spatial and angular intensity distribution in the cross-section of the radiation beam in the plane of the patterning device (MA).

The term "projection system" (PS) as used herein should be broadly construed to include various types of projection systems, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and/or electrostatic optical systems or any combination thereof, as suited to the exposure radiation employed and/or other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term "projection lens" within this specification may be considered synonymous with the more general term "projection system" (PS).

The lithographic apparatus (LA) may be of a type in which at least a portion of the substrate may be covered with a liquid having a relatively high refractive index, for example water, to fill the space between the projection system (PS) and the substrate (W) - also referred to as immersion lithography. More information regarding immersion techniques is provided in US Pat. No. 6,952,253, which is incorporated herein in its entirety by reference.

The lithography apparatus (LA) may also be of the type having two or more substrate supports (WT) (also called "dual stage"). In such a "multi-stage" machine, the substrate supports (WT) may be used simultaneously, and/or the preparatory steps for subsequent exposure of the substrate (W) may be performed on a substrate (W) positioned on one of the substrate supports (WT), while another substrate (W) on another substrate support (WT) is used to expose the pattern on the other substrate (W).

In addition to the substrate support (WT), the lithographic apparatus (LA) may include a measurement stage. The measurement stage is arranged to hold sensors and/or cleaning devices. The sensors may be arranged to measure characteristics of the projection system (PS) or characteristics of the radiation beam (B). The measurement stage may hold a plurality of sensors. The cleaning device may be arranged to clean a portion of the lithographic apparatus, for example a portion of the projection system (PS) or a portion of a system providing an immersion liquid. When the substrate support (WT) is away from the projection system (PS), the measurement stage may be moved beneath the projection system (PS).

In operation, a radiation beam (B) is incident on a patterning device (MA), for example a mask, which is held on a mask support (MT), and is patterned by a pattern (design layout) present on the patterning device (MA). The radiation beam (B), having traversed the mask (MA), passes through a projection system (PS), which focuses the beam onto target portions (C) of a substrate (W). With the aid of a second positioner (PW) and a position measuring system (IF), the substrate support (WT) can be moved precisely, for example, in order to position different target portions (C) in focused and aligned positions in the path of the radiation beam (B). Likewise, a first positioner (PM) and possibly further position sensors (not explicitly shown in FIG. 1) can be used to precisely position the patterning device (MA) relative to the path of the radiation beam (B). The patterning device (MA) and the substrate (W) can be aligned using mask alignment marks (M1, M2) and substrate alignment marks (P1, P2). As illustrated, the substrate alignment marks (P1, P2) occupy dedicated target portions, but they can be located in the space between the target portions. The substrate alignment marks (P1, P2) are known as scribe-lane alignment marks when they are located between the target portions (C).

As shown in Fig. 2, a lithography apparatus (LA) may form part of a lithography cell (LC), which is sometimes also referred to as a lithocell or (litho)cluster, and often also includes devices for performing pre-exposure and post-exposure processes on a substrate (W). Typically, these include a spin coater (SC) for depositing a resist layer, a developer (DE) for developing the exposed resist, a cooling plate (CH) and a bake plate (BK) for controlling the temperature of the substrate (W), for example for controlling a solvent in the resist layer. A substrate handler or robot (RO) picks up the substrate (W) from an input/output port (I/O1, I/O2), moves the substrate between different process devices, and delivers the substrate (W) to a loading bay (LB) of the lithography apparatus (LA). The devices within a lithography cell, commonly referred to collectively as tracks, are typically under the control of a track control unit (TCU) which may itself be controlled by a supervisory control system (SCS), which may also control a lithography apparatus (LA), for example via a lithography control unit (LACU).

In order to ensure that a substrate (W) exposed by a lithography apparatus (LA) is exposed accurately and consistently, it is desirable to inspect the substrate to measure characteristics of the patterned structure, such as overlay errors between subsequent layers, line thicknesses, critical dimensions (CDs), etc. For this purpose, an inspection tool (not shown) may be included in the lithocell (LC). If an error is detected, particularly if the inspection is performed before other substrates (W) of the same batch or lot have been exposed or processed, adjustments can be made, for example, for the exposure of subsequent substrates or for other processing steps to be performed on the substrate (W).

An inspection device, which may also be referred to as a metrology device, is used to determine characteristics of a substrate (W), and in particular to determine how characteristics of different substrates (W) vary or how characteristics associated with different layers of the same substrate (W) vary from layer to layer. The inspection device may alternatively be configured to identify defects on the substrate (W) and may also be, for example, part of a lithocell (LC), integrated into a lithographic apparatus (LA), or even a standalone device. The inspection device may measure characteristics of a latent image (an image of a resist layer after exposure) or a semi-latent image (an image of a resist layer after a post-exposure bake step (PEB), or a developed resist image (wherein exposed or unexposed portions of the resist have been removed), or even an etched image (after a pattern transfer step such as etching).

Overall, the patterning process in a lithography apparatus (LA) is one of the most critical steps in the process, requiring high accuracy in dimensioning and placement of structures on a substrate (W). To ensure this high accuracy, three systems can be combined in a so-called "holistic" control environment, as schematically illustrated in FIG. 3. One of these systems is a lithography apparatus (LA) that is (virtually) connected to a metrology tool (MT) (second system) and to a computer system (CL) (third system). The key to this "holistic" environment is to optimize the cooperation between these three systems to enhance the overall process window and to provide a tight control loop to ensure that the patterning performed by the lithography apparatus (LA) remains within the process window. The process window defines the range of process parameters (e.g., dose, focus, overlay) within which a particular manufacturing process produces a specified result (e.g., a functional semiconductor device) - typically, the process parameters of the lithography process or the patterning process are allowed to vary within this range.

The computer system (CL) can use the design layout (part of) to be patterned to predict which resolution enhancement technique to use and can perform computer lithography simulations and calculations to determine which mask layout and lithography apparatus settings will achieve the largest overall process window of the patterning process (illustrated by the double arrows within the first scale (SC1) in FIG. 3). Typically, the resolution enhancement technique is tailored to match the patterning capabilities of the lithography apparatus (LA). The computer system (CL) can also be used to detect where within the process window the lithography apparatus (LA) is currently operating (e.g., using input from the metrology tool (MT)) to predict whether a defect might be present due to, for example, suboptimal processing (illustrated by the arrow pointing to "0" within the second scale (SC2) in FIG. 3).

The metrology tool (MT) can provide input to the computer system (CL) to enable accurate simulations and predictions, and can provide feedback to the lithography apparatus (LA) to identify, for example, possible drifts in the calibration state of the lithography apparatus (LA) (illustrated by the multiple arrows within the third scale (SC3) in Figure 3).

In lithography processes, it is desirable to frequently measure the structures produced, for example for process control and verification. Tools for performing such measurements are typically referred to as metrology tools (MT). Different types of metrology tools (MT) are known for performing such measurements, including scanning electron microscopes or various types of scatterometer metrology tools (MT). A scatterometer is a multipurpose instrument which allows the measurement of parameters of a lithography process, either by having a sensor on the pupil of the objective of the scatterometer or in a plane conjugate to the pupil (in which case the measurement is generally referred to as a pupil-based measurement), or by having a sensor on the image plane or in a plane conjugate to the image plane (in which case the measurement is generally referred to as an image- or field-based measurement). These scatterometers and related measurement techniques are further described in patent applications US2010/0328655, US2011/102753A1, US2012/0044470A, US2011/0249244, US2011/0026032 or EP1,628,164A, which are incorporated herein by reference in their entirety. The aforementioned scatterometers can measure gratings using soft X-rays, extreme ultraviolet (EUV), and light in the visible to near infrared wavelength range.

In a first embodiment, the scatterometer (MT) is an angularly resolved scatterometer. In such a scatterometer, a reconstruction method can be applied to the measured signal to reconstruct or calculate the properties of the grating. Such reconstruction can result, for example, from simulating the interaction of scattered radiation with a mathematical model of the target structure and comparing the simulation results with the results of the measurements. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from the actual target.

In a second embodiment, the scatterometer (MT) is a spectroscopic scatterometer (MT). In such a spectroscopic scatterometer (MT), radiation emitted by a radiation source is directed toward a target, and reflected or scattered radiation from the target is directed to a spectroscopic detector, which measures a spectrum of the reflected radiation (i.e., measures intensity as a function of wavelength). From this data, the structure or profile of the target that generated the detected spectrum can be reconstructed, for example, by rigorous coupled wave analysis and nonlinear regression, or by comparison with a library of simulated spectra.

In a third embodiment, the scatterometer (MT) is an ellipsometric scatterometer. An ellipsometric scatterometer allows for determining parameters of a lithographic process by measuring scattered radiation for each polarization state. Such a metrology device emits polarized radiation (such as linearly, circularly or elliptically), for example by using a suitable polarizing filter within the illumination portion of the metrology device. A source suitable for the metrology device can also provide polarized radiation. Various embodiments of conventional ellipsometric scatterometers are described in U.S. patent applications Nos. 11/451,599, 11/708,678, 12/256,780, 12/486,449, 12/920,968, 12/922,587, 13/000,229, 13/033,135, 13/533,110, and 13/891,410, which are incorporated herein by reference in their entireties.

A measuring device such as a scatterometer is illustrated in Fig. 4. It comprises a broadband (white light) radiation projector (2) that projects radiation (optionally spectrally filtered to a narrow band prior to the substrate (W)) onto a substrate (W). The reflected or scattered radiation passes to a scatterometer detector (4), which measures a spectrum (6) of the reflected radiation (i.e. a measurement of the intensity as a function of wavelength). From this data, the structure or profile (8) that generated the detected spectrum can be reconstructed by a processing unit (PU), for example by rigorous coupled wave analysis and nonlinear regression or by comparison with a library of simulated spectra as shown at the bottom of Fig. 4. Typically, for such a reconstruction, the general shape of the structure is known, some parameters are assumed from knowledge of the process by which the structure was made, leaving only a few parameters of the structure to be determined from the scatterometry data. These scatterometers can be configured as normal-incidence scatterometers or oblique-incidence scatterometers.

The overall measurement quality of a lithographic parameter by measurement of a metrology target is determined at least in part by the measurement recipe used to measure the lithographic parameter. The term "substrate measurement recipe" may include one or more parameters of the measurement itself, one or more parameters of the one or more patterns measured, or both. For example, if the measurement used in the substrate measurement recipe is a diffraction-based optical measurement, one or more of the parameters of the measurement may include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation with respect to the substrate, the orientation of the radiation with respect to the pattern on the substrate, and the like. One of the criteria for selecting the measurement recipe may be, for example, the sensitivity of one of the measurement parameters to processing variations. More examples are described in U.S. Patent Application No. US2016/0161863 and Published U.S. Patent Application No. US 2016/0370717A1, which are incorporated herein by reference in their entirety.

Another type of metrology device is shown in Fig. 5a. The target (T) and the diffracted beam of the measuring radiation used to illuminate the target are shown in more detail in Fig. 5b. The metrology device illustrated is of a type known as a dark-field metrology device. The metrology device illustrated here is purely exemplary in order to provide an explanation of dark-field metrology. The metrology device may be a stand-alone device, or may be incorporated, for example, in a measuring station, in a lithography apparatus (LA), or in a lithography cell (LC). An optical axis having a number of branches throughout the apparatus is represented by a dashed line (O). In this apparatus, light emitted by a source (11) (e.g. a xenon lamp) is directed onto a substrate (W) through a beam splitter (15) by an optical system comprising lenses (12, 14) and an objective lens (16). These lenses are arranged in a double sequence of 4F arrays. Different lens arrangements can be used, provided that the substrate image is still provided on the detector while at the same time allowing access to the intermediate pupil-plane for spatial frequency filtering. The angular range over which the radiation is incident on the substrate can thus be selected by defining the spatial intensity distribution in the plane providing the spatial spectrum of the substrate plane, referred to herein as the (conjugate) pupil plane. In particular, this can be done by inserting an aperture plate (13) of suitable shape between the lenses (12) and (14), in the plane which is the back-projected image of the objective pupil plane. In the illustrated example, the aperture plate (13) has different shapes, denoted 13N and 13S, which allow different illumination modes to be selected. The illumination system in the present example forms an off-axis illumination mode. In the first illumination mode, the aperture plate (13N) provides off-axis illumination from a direction designated "north" for illustrative purposes only. In the second illumination mode, the aperture plate (13S) is used to provide similar illumination, but from the opposite direction, denoted "south". Other illumination modes are possible by using different apertures. The rest of the pupil plane is preferably dark, since any unwanted light outside the desired illumination mode will interfere with the desired measurement signal.

As shown in Fig. 5b, the target (T) is positioned with the substrate (W) perpendicular to the optical axis (O) of the objective lens (16). The substrate (W) may be supported by a support (not shown). A ray (I) of measurement radiation impinging on the target (T) from an angle off the axis (O) produces a zeroth order ray (solid line 0) and two first order rays (dot-dashed line +1 and dot-dashed line -1). It should be remembered that for an overfilled small target, this ray is only one of many parallel rays that cover the area of the substrate including the measurement target (T) and other features. Since the aperture of the plate (13) has a finite width (necessary to allow for a useful amount of light), the incident ray (I) will in fact occupy a wide angular range, and the diffracted rays (0 and +1/-1) will be somewhat spread out. Depending on the point spread function of the small target, each order (+1 and -1) will be further spread over a range of angles rather than a single ideal ray as shown. Note that the grating pitch and illumination angle of the target can be designed or adjusted so that the first order ray entering the objective is closely aligned with the central optical axis. The rays depicted in FIGS. 5A and 3B are shown somewhat off-axis purely to allow the rays to be more easily distinguished in the drawings.

At least the 0 and +1 orders diffracted by the target (T) on the substrate (W) are collected by the objective lens (16) and directed backwards through the beam splitter (15). Returning to Fig. 5a, both the first and second illumination modes are illustrated by designating diametrically opposed apertures, labeled North (N) and South (S). When the incident ray (I) of the measurement radiation comes from the north side of the optical axis, i.e. when the first illumination mode is applied using the aperture plate (13N), the +1 diffracted ray, labeled +1 (N), enters the objective lens (16). In contrast, when the second illumination mode is applied using the aperture plate (13S), the -1 diffracted ray (labeled 1 (S)) is the ray that enters the lens (16).

A second beam splitter (17) splits the diffracted beam into two measurement branches. In the first measurement branch, an optical system (18) uses the 0th and 1st order diffracted beams to form a diffraction spectrum (pupil plane image) of the target onto a first sensor (19) (e.g., a CCD or CMOS sensor). Each diffraction order impinges on a different point on the sensor, so that image processing can compare and contrast the orders. The pupil plane image captured by the sensor (19) can be used to focus a metrology device and/or to normalize the intensity measurement of the 1st order beam. The pupil plane image can also be used for many measurement purposes, such as reconstruction.

In the second measurement branch, the optical system (20, 22) forms an image of the target (T) on a sensor (23) (e.g. a CCD or CMOS sensor). In the second measurement branch, an aperture stop (21) is provided in a plane conjugate to the pupil plane. The aperture stop (21) functions to block the 0th order diffracted beam so that an image of the target formed on the sensor (23) is formed only as the -1st or +1st order beam. The images captured by the sensors (19 and 23) are output to a processor (PU) which processes the images, the function of which will depend on the particular type of measurement being performed. It is noted that the term "image" is used herein in a broad sense. No image of such grating lines will be formed if only one of the -1 and +1 orders is provided.

The specific configuration of the aperture plate (13) and the field stop (21) shown in FIG. 5 are purely exemplary. In another embodiment of the present invention, on-axis illumination of the target is used and an aperture stop having an off-axis aperture is used so that substantially only one first order diffracted light is transmitted to the sensor. In another example, a two quadrant aperture may be used. This may allow simultaneous detection of positive and negative orders, such as described in US2010/201963A1, mentioned above. An embodiment having an optical wedge (a segmented prism or other suitable element) in the detector branch may be used to separate the orders for spatial imaging in a single image, such as described in US2011/102753A1, mentioned above. In another embodiment, instead of or in addition to the first order beam, second, third and higher order beams (not shown in FIG. 5) may be used for measurement. In another embodiment, a segmented prism may be used in place of the aperture stop (21), allowing both the +1 and -1 orders to be captured simultaneously at spatially separated locations on the image sensor (23).

In order to make the measurement radiation adaptable to these different types of measurements, the aperture plate (13) may include a plurality of aperture patterns formed around a disk, which rotates to bring the desired pattern into position. It is noted that the aperture plate (13N or 13S) can only be used to measure gratings oriented in one direction (X or Y, depending on the configuration). For measurements of orthogonal gratings, rotation of the target through 90° and 270° can be implemented.

The metrology application disclosed herein may include a light source that may include any broadband source and a color selection array for selecting one or more colors from the broadband output. As an example, the radiation source may be based on a hollow core or solid core fiber, such as a hollow core photonic crystal fiber (HC-PCF) or a solid core photonic crystal fiber (SC-PCF). For example, in the case of a HC-PCF, the hollow core of the fiber may be filled with a gas that acts as an expanding medium to expand the input radiation. Such a fiber and gas array may be used to create a supercontinuum radiation source. The radiation input to the fiber may be electromagnetic radiation, such as radiation in one or more of the infrared, visible, UV, and extreme UV spectra. The output radiation may be comprised of or may include broadband radiation, which may be referred to herein as white light. This is only one example of a broadband light source technology that may be used in the methods and devices disclosed herein, and other suitable technologies may be used instead.

When using metrology sensors including those described above and/or other types of metrology sensors (e.g., alignment sensors, leveling sensors), it is often desirable to control the illumination spectrum, e.g., switching the illumination between different wavelengths (colors) and/or wavefront profiles.

To perform color selection, a color selection module utilizing grating light valve (GLV) technology, such as that sold by Silicon Light Machines (SLM), for example and incorporated herein by reference, has been proposed. A GLV is an electronically controllable diffraction grating based on micro-electro-mechanical systems (MEMS) technology. FIG. 6 illustrates the principle. FIG. 6 is a schematic diagram of a GLV pixel or component (500) from (a) above and (b), (c) end-on. The GLV component comprises two types of alternating GLV reflective ribbons (510): a static or bias ribbon (510) typically grounded along a common electrode, and a drive or active ribbon (520) driven by an electronic driver channel. A GLV module can include any number of these GLV components (500) arranged in an array. The active and bias ribbons can be essentially identical except for how they are driven. When no voltage is applied to the active ribbons (520), they are coplanar with the bias ribbons, and the configuration is illustrated in FIG. 5b. In this configuration, the GLVs essentially act as mirrors, and incident light is specularly reflected (i.e., forms specular radiation or 0th-order diffraction radiation). When a voltage is applied to the active ribbons (520), they are deflected with respect to the bias ribbons (510), forming a square-well diffraction grating, as illustrated in FIG. 6c. In this state, incident light is diffracted at a fixed diffraction angle. The ratio of reflected light to diffracted light can be continuously varied by controlling the voltage on the active ribbons (520), which controls the magnitude of their deflection. Thus, the amount of light diffracted by the GLVs can be controlled in an analog manner from zero (perfect specular reflection) to all incident light (zero specular reflection). Controlling the amount of reflected radiation relative to the amount of radiation diffracted into nonzero diffraction orders may be referred to as modulating the illumination in the context of the present invention.

The GLV module can be used in 0th order mode so that the diffracted radiation is blocked/dumped and the reflected (0th diffraction order) radiation is provided to the metrology tool. This has the advantage of preserving etendue. Thus, an aperture stop can be provided at the pupil plane aimed at maximizing the transmission of the 0th order and minimizing the blocking of the 1st (and other diffraction orders) (minimizing transmission). However, the diffraction angle is wavelength dependent. Furthermore, the spot size in the pupil plane may also be wavelength dependent, so that each color may have a different spot size in the pupil plane. For example, sources currently used in some metrology applications may contain different etendues for different light colors, and thus have different beam widths for the different colors.

Because of this, it is difficult to configure the aperture stop to maximize the transmission of the 0th order while blocking the 1st order for all wavelengths of interest (e.g., the wavelength band covered by the source selection module). Any particular shape or configuration for the hard aperture stop may be sub-optimal for a particular wavelength range (e.g., causing too much blocking of the 0th order and/or too much leakage of the 1st order in the transmission window). This problem becomes more severe as the wavelength range being utilized increases.

This problem is exacerbated when using beams with high Etendue. High Etendue beams make it difficult to minimize the spot size (per wavelength) at the GLV and have a low numerical aperture (NA) per order. A small spot on the GLV is highly desirable to operate in the flat region of the active ribbon and thus not lose contrast. A low NA per order is beneficial for the separation of the 0th and 1st orders over multiple wavelengths.

An arrangement of known color selection modules based on controllable diffractive elements such as GLVs may include a beam-dispersing element for spreading a broadband illumination beam; a controllable diffractive element or GLV module for spatially modulating the broadband illumination beam after it has been dispersed; an aperture stop in the far field (the pupil plane of the GLV or its conjugate) for removing all but the desired order(s) (e.g., removing all but the 0th order; but this may be reversed so that the 0th order is blocked and the 1st order is transmitted); and a beam-combining element for recombining the spatially modulated broadband illumination beam to obtain an output source beam. The beam-dispersing element may disperse the colors of the white light source across the GLV in a first direction (e.g., when the GLV is included in the image plane or field plane of the system). The combining element and the dispersing element may be different elements or may be a single element.

Fig. 7 illustrates a first color selection module array based on GLV. The top of Fig. 7 is a side view of the array, while the bottom of the drawing shows the same array in plan view form. A broadband source (SO) emits broadband radiation. A lens system, represented by a plurality of lens elements (e.g. lenses L1 and L2), provides access to a pupil plane on which a beam-dispersing element (DE) (e.g. a grating or a prism) is positioned. The beam-dispersing element (DE) disperses the broadband illumination beam, optionally (as shown here) through an intermediate image in a first spectrally dispersed image plane (SDIP) (or field plane) between the lenses L2 and L3, onto the GLV. A lens (L4) focuses the spectrally dispersed radiation onto the GLV module (GLV) in a second spectrally dispersed image plane. (For example, in this embodiment, i.e. when the incident radiation is focused onto the GLV and the spatially modulated reflected radiation is captured at the GLV using the same lens arrangement) the reflected (0th order) radiation from the GLV is captured by the lens L4. An aperture stop (ST ₀ ) is positioned at the pupil plane P2 provided by the lens L4 to block unwanted diffraction orders from the GLV module; for example, blocking the 1st order while allowing the 0th order to pass substantially undistorted. In the arrangement shown, the modulated illumination beam is recombined on the return path using the same dispersing element (DE) that was used to disperse the beam in the outgoing path. The lens (L1) then focuses the output beam onto a measurement device (MET) (for example onto a suitable optical fiber, such as a single mode fiber, for transmitting the radiation to the measurement device (MET).

While this arrangement is shown to illustrate the problems of prior art arrangements that are addressed by the concepts disclosed herein, it should be recognized that the actual arrangement shown is not prior art. An arrangement of source selection modules that uses the same optical components for the outgoing path to the GLV and the return path from the GLV is not known from the prior art. For example, this would involve using a single element (DE) (e.g., a prism) as a dispersing element in the outgoing path and a combining element in the return path, and sharing all lenses, including the imaging lens (L4), for imaging the dispersed radiation onto the GLV and collecting the modulated radiation from the GLV. This is accomplished by having a first off-axis beam path (outgoing path) to the GLV (colored black in the top drawing, note that this aspect is only shown in the top drawing) and a second off-axis beam path (return path) from the GLV (colored gray in the top drawing). Prior art arrangements typically use separate optical branches for the GLV and separate optical branches from the GLV, with dedicated optics for each branch; such arrangements are also within the scope of the present invention.

Also shown in Fig. 7 is a schematic representation of the light distribution at lens L2 (plane (P1)). The top of the representation corresponds to the outgoing path and shows the dispersed source radiation; the bottom of the representation corresponds to the return path and shows the GLV modulated radiation (color selection shown is purely exemplary). Gray scale shading represents different colors/wavelengths. An image representation at the spatially dispersed image plane (SDIP) is also shown, showing the dispersed beam at the image plane.

Also shown is a first pupil plane representation (P2 ₀ ) associated with the pupil plane (P2) of the lens system including lenses (L3 and L4). This defines an aperture (AP 0 ) configured for 0th mode operation, including the position of the (1st order) stop (ST ₀ ); that is, to block the 1st diffraction order (+1, _-1 ) (shown as dashed line in the bottom drawing, which is not visible in the top drawing) and to transmit the 0th order (0) and the source beam (SB). Note the different positions (i.e., diffraction angles) within the pupil plane and the spot sizes/diameters for each wavelength of the 1st diffraction order (+1, -1) (in which case different colors will also have different, different diameters in the source beam SB). It is not necessary that different colors have different spot sizes and the concepts disclosed herein can be used with sources where the spot size is not wavelength dependent.

Fig. 8 illustrates an array of second color selection modules based on GLV. Fig. 8a is a side view of the array, and Fig. 8b is a plan view thereof. In this example of the array, the GLV is configured for first mode operation; that is, to transmit the first diffraction order (for clarity, only two colors (+1 _λ1 , +1 _λ2 , -1 _λ1 , -1 _λ2 ) are shown, both of which are selected by the GLV; of course there may be more and/or a continuous spectrum) and the source beam (SB), and to block the zeroth order (0 _λ1 , 0 _λ2 ). More specifically, in the illustrated example, two wavelengths (λ ₁ , λ ₂ ) are shown being transmitted through the array (i.e., both wavelengths are selected by the GLV), and the resulting diffraction orders (+1 _{λ 1} , -1 _{λ 1} , +1 _{λ 2} , -1 _{λ 2} ) are captured by the lens (L ₃ ).

Many of the components have been described with respect to FIG. 7 and will not be described again. FIG. 8c is a second pupil plane representation (P2 ₁ ) corresponding to the first pupil plane representation (P2 ₀ ) shown in FIG. 7. In this arrangement, the stop (ST ₁ ) is positioned as a 0th order stop, blocking only the 0th order (specular reflected radiation), thereby defining an aperture (AP ₁ ) which transmits the 1st order (and/or other higher orders).

As described, the aperture stop should maximize transmission of the 0th order beam (for all selected wavelengths) and minimize transmission of the 1st order beam for all wavelengths, or vice versa. Maximizing transmission of (e.g., of the 0th order beam or of the diffracted, e.g., 1st order beam for all wavelengths) should be understood to mean increasing transmission as much as possible, given the limitations of the arrangement and trade-offs required to minimize transmission of blocked radiation. Likewise, minimizing transmission of (e.g., of the 1st or 0th order for all wavelengths) should be understood to mean blocking this order as much as possible, given these same limitations and trade-offs. In particular, the fact that the spots have spatially overlapping tails (considering a plot of intensity or amplitude versus pupil position for each spot) necessitates either passing some unwanted light (leading to out-of-band contrast) or blocking the desired 0th order tail, resulting in less signal and therefore less throughput. This problem is exacerbated the larger these spots are (i.e. the larger the NA of the beam) compared to the separation of the orders.

In embodiments, maximizing transmission may include transmitting greater than 90% of the transmitted radiation, greater than 95% of the transmitted radiation, greater than 98% of the transmitted radiation, greater than 99% of the transmitted radiation, greater than 99.9% of the transmitted radiation, or greater than 99.99% of the transmitted radiation. In embodiments, minimizing transmission may include blocking greater than 90% of the blocked radiation, greater than 95% of the blocked radiation, greater than 98% of the blocked radiation, greater than 99% of the blocked radiation, greater than 99.9% of the blocked radiation, or greater than 99.99% of the blocked radiation.

Fig. 9 illustrates the problem in terms of pupil representation, showing an aperture stop (ST ₀ ) (corresponding to, e.g., the 0th mode configuration). It shows a source beam (SB) (represented by shading as before) (only two wavelengths are shown for clarity), and the corresponding 0th order beam (0) and diffraction orders (+1, -1). When the source beam (SB) is diffracted by the GLV, each of the first diffraction orders (+1, -1) is scattered in a first direction (denoted here as X), and when reflected, the source beam is reflected regularly into the 0th order (0) (the ratio of diffraction into the 1st diffraction order (+1, -1) and reflection into the 0th order can be controlled via the GLV as explained). Below the pupil representation is a plot of the intensity versus pupil location in one dimension, through the 0th order beam (0) and the diffraction orders (+1, -1), and spatially aligned with the pupil representation. As can be seen from the intensity plot, the overlapping tails of the 0th and 1st orders mean that it is not possible to optimally block the 1st order in this case; it may be the case that some 0th order radiation is blocked or some 1st order radiation is transmitted. This is especially true for blocking the short wavelength 1st order radiation and transmitting the longer wavelength 0th order radiation.

The same principle applies when the GLV is configured for first-order operation. For example, if an aperture stop (ST ₁ ) is used instead of the aperture stop (ST ₀ ) in Fig. 9, it is not possible to completely block the 0th order and completely transmit the desired 1st order(s).

To solve this problem, it is proposed to provide, in addition to a first beam-dispersing element (e.g. a prism or a diffraction grating) which disperses a broadband illumination beam (e.g. the source beam radiation) in a first direction in the (conjugate) image plane of the controllable (e.g. electrically programmable) diffractive element or GLV, at least one second beam-dispersing element (e.g. a prism or a diffraction grating) which disperses the broadband illumination beam in a second direction in the pupil plane of the controllable diffractive element or GLV. The first and second directions can be orthogonal, the first direction being the direction of the periodicity of the GLV and the second direction being the direction in which each GLV ribbon extends. Thus, the at least one second beam-dispersing element can be positioned in the (conjugate) image plane of the GLV. This allows the aperture stop shape to be optimized for a more optimal blocking of unwanted radiation (e.g. non-0th or first diffraction orders from the GLV).

FIG. 10a is a pupil representation identical to that of FIG. 9, implementing the concepts disclosed herein. Here the source beam (SB) is now dispersed in a second direction (here denoted Y) within this pupil plane due to the second beam spreading element. Although only two wavelengths are shown for clarity, it will be appreciated that the source beam may in fact comprise many more wavelengths, e.g. a continuous wavelength band, and thus the dispersed source beam may in fact comprise a continuous dispersion spectrum (or multiple discrete wavelengths) in the second direction. The position of the source radiation towards the GLV is thus wavelength dependent in this second direction (in the example shown, the shorter WL has a larger angle of incidence to the GLV; this is a design choice and can be the other way around). The resulting zeroth order radiation (0) is similarly dispersed in the second direction. The first order (+1, -1) will also be dispersed in the second direction, and also dispersed in the first direction due to diffraction by the GLV (the dispersion in the first direction will be the same as that in Fig. 9).

This two-dimensional dispersion in the first order allows for a better optimization of the configuration/shape of the aperture stop (ST ₀ '), more specifically so that its effective aperture size (e.g. in the first direction) varies with wavelength. As can be seen in the illustrative example of Fig. 10a, the diffracted radiation of the first (e.g. short) wavelength depends on the effective aperture size b, and the diffracted radiation of the second (e.g. long) wavelength depends on the effective aperture size a. The dashed line represents the compromised fixed aperture size in the example of Fig. 9. Thus, the aperture stop (ST ₀ ') can be configured to include an aperture size in the first direction (e.g. for at least a portion (e.g. approximately half) of the pupil used in the optical return path downstream from the GLV), which varies (increases or decreases) in size along the second direction. These successively increasing aperture widths (or stepwise increasing aperture widths) of smaller apertures can be positioned in the pupil region corresponding to positions in the second direction at which diffraction orders will be diffracted (in the second direction), which also diffract at smaller angles in the first direction and/or for which the spot size is smaller on the pupil. Likewise, the successively increasing apertures or larger ends of the larger apertures of larger aperture widths can be positioned in the pupil region in the second direction corresponding to positions at which diffraction orders will be diffracted (in the second direction), which also diffract at larger angles in the first direction and/or for which the spot size is larger on the pupil.

Fig. 10b shows an equivalent arrangement, but here the stop (ST ₁ ') is configured for first mode operation; that is, to block the 0th order and transmit at least one higher diffraction order (e.g., one 1st order or both 1st orders). Thus, instead of a continuously increasing aperture width, the arrangement comprises a continuously decreasing (0th) stop (ST ₁ ') width.

It will be appreciated that the shape of these successively increasing apertures or successively decreasing (0th-order) stops may differ from that shown, for example the aperture/stop edges need not be straight as shown here; they may be curved from bottom to top (in terms of the drawing) or may increase/decrease in steps, for example. Alternatively or additionally, the aperture stop may include a "soft" edge that transmits/blocks light in a non-binary manner (e.g. the blocking portion of the aperture at the edge may be partially transparent to radiation).

It will be appreciated that the GLV may be used in an inverted arrangement (higher order mode), where the desired radiation is diffracted radiation (e.g., first order radiation) and the unwanted radiation is specularly reflected (0th order) radiation. In such an arrangement, the aperture may be inverted from the aperture seen in the pupil region corresponding to the return path downstream of the GLV (but may not block the source beam in the outward path). For example, in FIG. 10, the stop may block the zeroth order (0) by blocking the central region between the visible stops/blocked portions, while also allowing the first orders (+1, -1) to be transmitted.

FIG. 11 is a schematic drawing of an exemplary color selection module according to a first embodiment. This arrangement is identical to the arrangement illustrated in FIG. 7 except for the addition of a second beam spreading element (DE2) in addition to the first beam spreading element (DE1). In the arrangement illustrated, the second beam spreading element (DE2) is provided in the outward path to spread the source radiation in the second direction at the pupil plane (e.g., P2). In contrast, the first beam spreading element spreads the source radiation in the first direction at the image plane. In the return path, a second beam combining element (CE2) is provided to recombine the illumination in the second direction. It is noted that in this embodiment the first beam spreading element (DE1) is also used as the first beam combining element to recombine the illumination in the first direction; this is optional and a separate first beam combining element may instead be provided to recombine the illumination in the first direction.

The second beam dispersing element (DE2) and the second beam combining element (CE2) can be positioned at the conjugate image plane of the GLV. In addition to these elements (DE2, CE2), a wedge (WG) (e.g. of a low-dispersion material) can be provided at the plane P2 (e.g. a pupil plane defined by the lens system (L3, L4)), which functions to split the illumination of the outward path (e.g. at the approximate positions of the elements (DE2, CE2)) in the first spectrally dispersive image plane (SDIP). The second beam dispersing element (DE2) and the second beam combining element (CE2) can comprise a pair of (e.g. similar but oppositely oriented) dispersing elements or prisms.

FIG. 12a is a schematic drawing of an exemplary color selection module according to a second embodiment. This arrangement is identical to the arrangement illustrated in FIG. 8 except for the addition of a second beam spreading element (DE2) and a second beam combining element (CE2) in addition to the first beam spreading element (DE1) (which is also used as the first beam combining element as before). In the arrangement illustrated, the second beam spreading element (DE2) is provided in the outward path to disperse the source radiation in a second direction at the pupil plane. In contrast, the first beam spreading element disperses the source radiation in a first direction at the image plane. In the return path, the second beam combining element (CE2) is provided to recombine the illumination in the second direction.

FIG. 12b shows the resulting pupil plane representation (P2 ₁ ") of the plane including a zeroth order stop (ST ₁ "). The zeroth order stop (ST ₁ ") has a variable (e.g., continuously decreasing) width defining a first order or diffraction aperture (AP ₁ ") configured to transmit the first diffraction order.

As an alternative to the illustrated positioning of the second beam diverging/combining elements (DE2, CE2) (in both embodiments), they may be positioned directly in front of the GLV (e.g. at the input window of the GLV, or to replace it). The second beam diverging/combining elements (DE2, CE2) (in both embodiments) may optionally comprise a composite prism, such as an Amici prism.

Thus, as can be seen by the respective exemplary pupil representations (P2 ₀ ", P2 ₁ ") of the two embodiments, the source beam (SB) being spectrally dispersed in the second direction and all diffracted/reflected beams enable the use of more optimal aperture shapes (AP ₀ ", AP ₁ ") as described.

For simplicity, the beam paths after the second beam diverging/combining element (DE2, CE2) are not drawn tilted; in reality they may have to be tilted.

A source selection module based on GLV enables control of the transmittance per color or band of the output beam rather than simply switching a color/band on or off.

The advantages of the proposed arrangement include providing the option to use larger etendue sources, enabling higher achievable throughput (signal) for a given out-of-band (OoB) signal or conversely better achievable OoB suppression for a given throughput (signal), and enabling smaller minimum bandwidth (by enabling a smaller spot in the GLV for a given throughput and OoB performance).

Additional embodiments are disclosed in the following numbered list of clauses:

1. A source selection module is disclosed for selecting spectral characteristics of a broadband illumination beam to obtain a modulated illumination beam, the source selection module comprising:

A first beam-dispersing element for dispersing a broadband illumination beam, wherein the first beam-dispersing element is operable to disperse the broadband illumination beam along a first direction;

A second beam spreading element for spreading a broadband illumination beam, wherein the second beam spreading element is operable to spread the broadband illumination beam along a second direction perpendicular to the first direction;

A controllable diffractive element having controllable elements arranged along a first direction such that the direction of their periodicity includes the first direction, wherein the controllable diffractive element is operable to controllably spatially modulate a broadband illumination beam after being dispersed by the first beam-dispersing element and the second beam-dispersing element; and

An aperture stop operable to maximize transmission of one of the regularly reflected radiation and the diffracted radiation from the controllable diffractive element and to minimize transmission of the other of the regularly reflected radiation and the diffracted radiation.

2. In a source selection module as defined in clause 1, the controllable diffractive element comprises a grating light valve module.

3. In a source selection module as defined in clause 1 or 2, the first beam dispersing element is positioned at the pupil plane of the controllable diffractive element or its conjugate to disperse a broadband illumination beam at the image plane of the controllable diffractive element or its conjugate.

4. In a source selection module as defined in any one of clauses 1 to 3, the second beam dispersing element is positioned at the image plane of the controllable diffractive element or its conjugate to disperse the broadband illumination beam at the pupil plane of the controllable diffractive element or its conjugate.

5. A source selection module as defined in any one of clauses 1 to 4 further comprises at least a first beam combining element for recombining the modulated illumination beam in the first direction to obtain an output source beam.

6. In a source selection module as defined in any one of clauses 1 to 4, the first beam diverging element is further arranged to recombine the modulated illumination beam on a return path from the controllable diffractive element.

7. In a source selection module as defined in clause 6, at least one lens element is shared between the outgoing path and the return path of the broadband illumination beam to the controllable diffractive element, the outgoing path comprising a first off-axis path and the return path comprising a second off-axis path through the at least one lens element and the first beam diverging element.

8. In the source selection module as defined in clause 7, at least one lens element comprises at least one lens or lens system operable to image a broadband illumination beam dispersed by the first beam-dispersing element and the second beam-dispersing element onto a controllable diffractive element, and to collect a modulated illumination beam from the controllable diffractive element.

9. The source selection module as defined in any one of clauses 1 to 8 further comprises at least a second beam combining element for recombining the modulated illumination beam in the second direction to obtain an output source beam.

10. In a source selection module as defined in any one of clauses 1 to 9, the aperture stop is positioned at the pupil plane of the controllable diffractive element or its conjugate.

11. In the source selection module specified in clause 10, the aperture stop is operable to maximize transmission of the reflected radiation from the controllable diffractive element and to minimize transmission of the diffracted radiation from the controllable diffractive element.

12. In a source selection module as defined in clause 10, the aperture stop is operable to minimize transmission of the reflected radiation from the controllable diffractive element and to maximize transmission of the diffracted radiation from the controllable diffractive element.

13. In a source selection module as defined in any one of clauses 1 to 12, the aperture stop defines an aperture having an aperture size in the first dimension that varies along the second dimension for at least a portion of the pupil plane corresponding to the return path of the modulated illumination beam.

14. In a source selection module as defined in clause 13, the aperture size and/or the aperture stop size of the first dimension increases or decreases continuously or stepwise along the second dimension.

15. In a source selection module as defined in any one of clauses 1 to 14, each of the first beam spreading element and the second beam spreading element includes a prism.

16. A source selection module for selecting spectral characteristics of a broadband illumination beam to obtain a modulated illumination beam is disclosed, the source selection module:

At least one beam spreading element for spreading a broadband illumination beam, said at least one beam spreading element being operable to spread the broadband illumination beam along a first direction;

A controllable diffractive element having controllable elements arranged along a first direction such that the direction of their periodicity includes the first direction, wherein the controllable diffractive element is operable to controllably spatially modulate a broadband illumination beam after being dispersed by the first beam-dispersing element;

an aperture stop operable to maximize transmission of one of the reflected radiation and the diffracted radiation from said controllable diffractive element and to minimize transmission of the other of the reflected radiation and the diffracted radiation; and

A plurality of lens elements comprising at least one lens or lens system operable to image a broadband illumination beam onto a controllable diffractive element after being dispersed by the first beam dispersing element, and to collect a modulated illumination beam from the controllable diffractive element;

Here, the at least one first beam diverging element is also arranged to recombine the modulated illumination beam on a return path from the controllable diffractive element.

17. In a source selection module as defined in clause 16, each of the plurality of lens elements is shared between an outgoing path and a return path of a broadband illumination beam to the controllable diffractive element, the outgoing path comprising a first off-axis path and the return path comprising a second off-axis path through the plurality of lens elements and at least one beam diverging element.

18. In a source selection module as defined in clause 16 or 17, the controllable diffractive element comprises a grating light valve module.

19. A source selection module as defined in any one of clauses 16 to 18 further comprises a lighting source for providing said input lighting.

21. In a source selection module as defined in clause 19, the illumination source comprises a hollow core fiber for defining an extended medium and an excitation radiation source operable to provide excitation radiation for exciting the extended medium.

21. The measuring device comprises a source selection module according to any one of clauses 1 to 20 for providing measurement illumination.

22. In a measuring device as defined in Article 21, the measuring device comprises a scatterometer.

23. Measuring devices as specified in Article 22:

Support for substrate;

an optical system for directing said measurement light onto a structure on said substrate; and

It includes a detector for detecting measurement radiation scattered by a structure on a substrate.

24. In a measuring device as defined in clause 21, the measuring device comprises an alignment sensor.

25. The lithography apparatus:

A patterning device support for supporting a patterning device;

A substrate support for supporting the substrate; and

A metrology device according to clause 24, operable to perform alignment of the patterning device support and/or the substrate support.

It should be recognized that the term "color" is used throughout this specification synonymously with wavelength or spectral component and may include colors outside the visible band (e.g., infrared or ultraviolet wavelengths).

While specific embodiments of the present invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described.

While specific reference may be made above to the use of embodiments of the present invention in the context of optical lithography, it will be appreciated that the present invention may be used in other applications, for example imprint lithography, and is not limited to optical lithography where the context permits. In imprint lithography, a topography in a patterning device defines a pattern to be created on a substrate. The topography of the patterning device may be pressed into a layer of resist supplied to the substrate, over which the resist is cured by applying electromagnetic radiation, heat, pressure, or a combination thereof. The patterning device is moved out of the resist, leaving the pattern in the resist after the resist has been cured.

The terms “radiation” and “beam” as used herein include all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of about 365, 355, 248, 193, 157, or 126 nm or greater) and extreme ultraviolet (EUV) radiation (e.g., having a wavelength in the range of 1 to 100 nm), as well as particle beams such as ion beams or electron beams.

Where the context permits, the term "lens" may refer to any one or a combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components. Reflective components are likely to be used in devices operating in the UV and/or EUV range.

The breadth and scope of the present invention should not be limited by any of the exemplary embodiments described above, but should be defined only in accordance with the following claims and their equivalents.

Claims (15)

In a source selection module for selecting spectral characteristics of a broadband illumination beam to obtain a modulated illumination beam,
A first beam-dispersing element for dispersing a broadband illumination beam, wherein the first beam-dispersing element is operable to disperse the broadband illumination beam along a first direction;
A second beam spreading element for spreading a broadband illumination beam, wherein the second beam spreading element is operable to spread the broadband illumination beam along a second direction perpendicular to the first direction;
A controllable diffractive element having controllable elements arranged along the first direction such that the direction of the periodicity of the controllable diffractive elements includes the first direction, wherein the controllable diffractive element is operable to controllably spatially modulate a broadband illumination beam after being dispersed by the first beam-dispersing element and the second beam-dispersing element; and
A source selection module including an aperture stop operable to maximize transmission of one of the reflected radiation and the diffracted radiation from the controllable diffractive element and to minimize transmission of the other of the reflected radiation and the diffracted radiation. In the first aspect, the controllable diffractive element is a source selection module including a grating light valve module. A source selection module according to claim 1 or 2, wherein the first beam dispersing element is positioned at the pupil plane of the controllable diffractive element or its conjugate to disperse a broadband illumination beam at the image plane of the controllable diffractive element or its conjugate. A source selection module according to any one of claims 1 to 3, wherein the second beam dispersing element is positioned at the image plane of the controllable diffractive element or its conjugate to disperse a broadband illumination beam at the pupil plane of the controllable diffractive element or its conjugate. A source selection module further comprising at least a first beam combining element for recombining the modulated illumination beam in the first direction to obtain an output source beam, according to any one of claims 1 to 4. A source selection module according to any one of claims 1 to 4, wherein the first beam diverging element is further arranged to recombine the modulated illumination beam on a return path from the controllable diffractive element. In the sixth aspect, a source selection module wherein at least one lens element is shared between the outgoing path and the return path of the broadband illumination beam to the controllable diffractive element, the outgoing path including a first off-axis path and the return path including a second off-axis path through the at least one lens element and the first beam diverging element. In claim 7, the at least one lens element comprises at least one lens or lens system operable to image the broadband illumination beam after being dispersed by the first beam-dispersing element and the second beam-dispersing element onto the controllable diffractive element, and to collect the modulated illumination beam from the controllable diffractive element. A source selection module according to any one of claims 1 to 8, further comprising at least a second beam combining element for recombining the modulated illumination beam in the second direction to obtain an output source beam. A source selection module according to any one of claims 1 to 9, wherein the aperture stop is positioned at a pupil plane of the controllable diffractive element or its conjugate, and is operable to maximize transmission of the reflected radiation from the controllable diffractive element and to minimize transmission of the diffracted radiation from the controllable diffractive element. A source selection module according to any one of claims 1 to 10, wherein the first beam dispersing element and the second beam dispersing element each include a prism. A source selection module according to any one of claims 1 to 11, wherein the aperture stop defines an aperture having an aperture size in the first dimension that varies along the second dimension for at least a portion of the pupil plane corresponding to the return path of the modulated illumination beam. In the 12th paragraph, a source selection module in which the aperture size in the first dimension increases or decreases continuously or stepwise along the second dimension. A source selection module according to any one of claims 1 to 13, wherein the source selection module includes a lighting source for providing the input lighting. A measuring device comprising a source selection module according to any one of claims 1 to 14 for providing measuring illumination.