Source selection module and related metrology device - Patents.com

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 comprising 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 it has been dispersed by the first and second beam dispersing elements, and an aperture stop operable to maximize a transmission of one of the specularly reflected radiation and the diffracted radiation from the controllable diffractive element and minimize a transmission of the other of the specularly reflected radiation and the diffracted radiation.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to European Application No. 22169636.2, filed January 25, 2022, and European Application No. 22174097.0, filed May 18, 2022, both of which are incorporated by reference in their entireties herein. [Background technology]

[0002] The present invention relates to methods and apparatus usable for example in the manufacture of devices by lithographic techniques, and to methods of manufacturing devices using lithographic techniques, more particularly to metrology sensors and lithographic apparatus comprising such metrology sensors, and even more particularly to illumination arrangements for such metrology sensors.

[0003] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A patterning device, also known as a mask or reticle, can be used to generate the circuit pattern formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of a die, or one or more dies) on the substrate (e.g. a silicon wafer). Transfer of the pattern is typically by imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. These target portions are commonly referred to as "fields".

[0004]

[0004] In the manufacture of complex devices, typically numerous lithographic patterning steps are performed, which result in the formation of functional features in successive layers on a substrate. Therefore, an important aspect of the performance of a lithographic apparatus is the ability to accurately and precisely position an applied pattern with respect to features provided in a previous layer (by the same apparatus or 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 includes one or more alignment sensors that can accurately measure the position of the marks on the substrate. Different types of marks and different types of alignment sensors are known from different manufacturers and from different products of the same manufacturer.

[0005]

[0005] In other applications, metrology sensors are used to measure exposed structures on a substrate (either in resist and / or after etching). A scatterometer is a fast and non-invasive form of special inspection tool in which a radiation beam is directed at a target on the surface of the substrate and the properties of the scattered or reflected beam are measured. Known scatterometers include angle resolved scatterometers of the type described in US Patent Application Publication No. 2006033921A1 and US Patent Application Publication No. 2010201963A1. In addition to measuring feature shapes by reconstruction, devices such as those described in US Patent Application Publication No. 2006066855A1 can also be used to measure diffraction-based overlay. Diffraction-based overlay metrology using dark-field imaging of diffraction orders allows overlay measurement of smaller targets. Examples of dark field imaging metrology include those described in WO 2009 / 078708 and WO 2009 / 106279, which are incorporated herein by reference in their entirety. Further developments of the above technology are described in U.S. Patent Application Publication No. 20110027704A, U.S. Patent Application Publication No. 20110043791A, U.S. Patent Application Publication No. 2011102753A1, U.S. Patent Application Publication No. 20120044470A, U.S. Patent Application Publication No. 20120123581A, U.S. Patent Application Publication No. 20130258310A, U.S. Patent Application Publication No. 20130271740A, and WO 2013178422A1. These targets can be smaller than the illumination spot and may be surrounded by product structures on the wafer. Multiple grating targets can also be used to measure multiple gratings in one image.The contents of all of the above applications are also incorporated herein by reference.

[0006]

[0006] In some metrology applications, for example in some scatterometers or alignment sensors, imperfections in the metrology target may cause wavelength / polarization dependent variations in measurements obtained from that target. Correction and / or mitigation of such variations may then be achieved by performing the same measurement using multiple different wavelengths and / or polarizations (or more generally, multiple different illumination conditions). Improved switching and selection of the spectral content of illumination for such metrology applications is desirable. Summary of the Invention

[0007]

[0007] In a first aspect, the present invention 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 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 operable to disperse the broadband illumination beam along a second direction perpendicular to the first direction; a controllable diffractive element, the controllable diffractive element having controllable elements arranged along the first direction such that a direction of a period of the controllable diffractive element includes the first direction, the controllable diffractive element operable to controllably spatially modulate the broadband illumination beam after it is dispersed by the first beam dispersing element and the second beam dispersing element; and an aperture stop operable to maximize a transmittance of one of the specularly reflected radiation and the diffracted radiation from the controllable diffractive element and minimize a transmittance of the other of the specularly reflected radiation and the diffracted radiation.

[0008]

[0008] In a second aspect, the present invention 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 dispersion element for dispersing the broadband illumination beam, the at least one beam dispersion element operable to disperse the broadband illumination beam along a first direction, and a controllable diffractive element having a controllable element arranged along the first direction such that a direction of a period of the controllable diffractive element includes the first direction, and operable to controllably spatially modulate the broadband illumination beam after it has been dispersed by the first beam dispersion element. a controllable diffractive element operable to modulate a transmission of one of the specularly reflected radiation and the diffracted radiation from the controllable diffractive element and to minimize a transmission of the other of the specularly reflected radiation and the diffracted radiation; and a plurality of lens elements including at least a 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, wherein the first beam dispersing element is also arranged to recombine the modulated illumination beam on a return path from the controllable diffractive element.

[0009] Also disclosed is a metrology apparatus and a lithography apparatus comprising a metrology device operable to perform the method of the first or second aspect.

[0010]

[0010] These and other aspects of the present invention will be understood in light of the examples described below. [Brief description of the drawings]

[0011]

[0011] 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] 1 depicts a lithographic apparatus; [Diagram 2] 1 shows a schematic overview of a lithographic cell. [Diagram 3] A schematic diagram of Holistic Lithography is shown, illustrating the collaboration between three key technologies to optimize semiconductor manufacturing. [Figure 4] 1 shows a schematic overview of a scatterometry apparatus used as a metrology device, which may include a dark-field digital holographic microscope according to an embodiment of the present invention. [Diagram 5] FIG. 1A is a schematic diagram of a dark field scatterometer used to measure a target using a first pair of illumination apertures; and FIG. 1B is a detailed diagram of the diffraction spectrum of the target grating for a given illumination direction. [Figure 6] 1A-1C are schematic diagrams of a grating light valve showing its basic operation in (a) a top view, (b) a front view of a first configuration, and (c) a front view of a second configuration. [Figure 7] FIG. 2 is a schematic diagram of the operating principle of an illumination arrangement including a grating light valve in a first configuration; [Figure 8(a)-8(b)-8(c)] FIG. 2 is a schematic diagram of the operating principle of an illumination arrangement including a grating light valve in a second configuration; [Figure 9] FIG. 8 is a pupil diagram illustrating the difficulties in optimizing the aperture stop configuration for the arrangement of FIG. 7. [Figure 10(a)-10(b)] FIG. 2 is a pupil diagram of an illumination arrangement in accordance with an embodiment of the present invention; [Figure 11] FIG. 2 is a schematic diagram of the operating principle of an illumination arrangement including a grating light valve according to a first embodiment of the invention; [Figure 12(a)-12(b)] FIG. 4 is a schematic diagram of the operating principle of an illumination arrangement including a grating light valve according to a second embodiment of the invention; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Before describing embodiments of the present invention in detail, it is beneficial to present an exemplary environment in which embodiments of the present invention may be implemented.

[0013]

[0013] In this document, the terms "radiation" and "beam" are used to encompass 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 a wavelength in the range of about 5 to 100 nm).

[0014]

[0014] The terms "reticle," "mask," or "patterning device" as used herein may be broadly interpreted to refer to any general-purpose patterning device that can be used to impart an incident radiation beam with a patterned cross-section that corresponds to the pattern to be produced in a target portion of a substrate. The term "light valve" may also be used in this context. In addition to the classic mask (transmissive or reflective; binary, phase-shifting, hybrid, etc.), examples of other such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0015]

[0015] Fig. 1 shows a schematic diagram of a lithographic apparatus LA. The lithographic apparatus LA includes an illumination system (also called illuminator) IL configured to condition a radiation beam B (e.g. UV, DUV or EUV radiation), a mask support (e.g. mask table) MT constructed to support a patterning device (e.g. mask) MA and coupled to a first positioner PM configured to accurately position the patterning device MA according to certain parameters, a substrate support (e.g. wafer table) WT constructed to hold a substrate (e.g. resist coated wafer) W and coupled to a second positioner PW configured to accurately position the substrate support according to certain parameters, and a projection system (e.g. 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. comprising one or more dies) of the substrate W.

[0016]

[0016] In operation, the illuminator IL receives a radiation beam from a radiation source SO (e.g. 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, for directing, shaping and / or controlling the radiation. The illuminator IL can be used to condition the radiation beam B so that it has a desired spatial and angular intensity distribution in its cross-section in the plane of the patterning device MA.

[0017]

[0017] The term "projection system" PS as used herein should be broadly interpreted to encompass various types of projection systems including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, appropriate for the exposure radiation being used and / or for other factors such as the use of an immersion liquid or the use of a vacuum. Where the term "projection lens" is used herein, it may be considered as synonymous with the more general term "projection system" PS.

[0018]

[0018] The lithographic apparatus LA may be of a type in which at least a part of the substrate may be covered by a liquid having a relatively high refractive index, such as water, to fill a space between the projection system PS and the substrate W, which is also referred to as immersion lithography. More information about immersion techniques is given in US Patent No. 6,952,253, which is incorporated herein by reference.

[0019] The lithographic apparatus LA may be of a type having two or more substrate supports WT (also referred to as "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel and / or one substrate W on another substrate support WT may be used to expose a pattern onto the other substrate W whilst a process is being performed on a substrate W positioned on the other substrate support WT to prepare the substrate W for a subsequent exposure.

[0020]

[0020] In addition to the substrate support WT, the lithographic apparatus LA may comprise a measurement stage. The measurement stage is arranged to hold a sensor and / or a cleaning device. The sensor may be arranged to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold a number of sensors. The cleaning device may be arranged to clean a part of the lithographic apparatus, for example a part of the projection system PS or a part of a system for providing immersion liquid. The measurement stage may move under the projection system PS when the substrate support WT is spaced apart from the projection system PS.

[0021] In operation, a radiation beam B is incident on a patterning device (e.g. mask) MA, which is held on a mask support MT, and is patterned by a pattern (design layout) present on the patterning device MA. Having passed through the mask MA, the radiation beam B passes through a projection system PS, which focuses the beam onto a target portion C of a substrate W. With the aid of a second positioner PW and a position measurement system IF, the substrate support WT can be accurately moved, for example to position different target portions C at focused and aligned positions in the path of the radiation beam B. Similarly, a first positioner PM, and possibly further position sensors (not explicitly shown in FIG. 1 ), can be used to accurately position the patterning device MA with respect to the path of the radiation beam B. The patterning device MA and substrate W can be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the illustrated substrate alignment marks P1, P2 occupy dedicated target portions, they may be positioned in spaces between the target portions. When the substrate alignment marks P1, P2 are positioned between the target portions C, they are known as scribe-line alignment marks.

[0022]

[0022] As shown in Fig. 2, the lithographic apparatus LA may form part of a lithographic cell LC, sometimes also called a lithocell or (litho)cluster, which may also include one or more apparatus for performing pre-exposure and post-exposure processes on a substrate. Conventionally, 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 (e.g. to adjust the solvent in the resist layer, e.g. to adjust the temperature of the substrate W). A substrate handler or robot RO picks up substrates W from input / output ports I / O1, I / O2, moves them between the different processing devices, and delivers them to a loading bay LB of the lithographic apparatus LA. These devices in the lithocell, sometimes also collectively called a track, are typically under the control of a track control unit TCU, which itself may be controlled by a supervisory control system SCS, which may also control the lithographic apparatus (e.g. via a lithography control unit LACU).

[0023]

[0023] In order for a substrate W to be exposed by the lithographic apparatus LA to be correctly and uniformly exposed, it is desirable to inspect the substrate to measure properties of the patterned structures, such as overlay errors between successive layers, line thickness, critical dimensions (CD), etc. For this purpose, an inspection tool (not shown) may be included in the lithographic cell LC. If an error is detected, adjustments may be made, for example, to the exposure of subsequent substrates or other processing steps to be performed on the substrate W, especially if the inspection is performed before other substrates W of the same batch or lot are subsequently exposed or processed.

[0024]

[0024] The inspection apparatus, sometimes also called metrology apparatus, is used to determine the characteristics of the substrate W, in particular how the characteristics of different substrates W differ, or how the characteristics associated with different layers of the same substrate W differ from layer to layer. The inspection apparatus may alternatively be constructed to identify defects on the substrate, and may for example be part of the lithographic cell LC, integrated in the lithographic apparatus LA, or even be a standalone device. The inspection apparatus may measure characteristics related to the latent image (image in the resist layer after exposure), or the semi-latent image (image in the resist layer after a post-exposure bake step), or the developed resist image (image in which exposed or unexposed parts of the resist have been removed), or even the etched image (image after a pattern transfer step such as etching).

[0025] Typically, the patterning process in the lithography apparatus LA is the most critical step in the process, requiring high accuracy in the dimensioning and placement of structures on the substrate W. To ensure this high accuracy, a so-called "holistic" control environment, as shown diagrammatically in FIG. 3, can combine three systems. One of these systems is the lithography apparatus LA, which is (virtually) connected to a metrology tool MT (second system) and a computer system CL (third system). The key to such holistic lithography is to optimize the coordination between these three systems to enforce the overall process window and provide a tight control loop to ensure that the patterning performed by the lithography apparatus LA stays within the process window. The process window defines the range of process parameters, e.g. dose, focus, overlay, etc., within which a particular manufacturing process will produce a defined result, e.g. a functioning semiconductor device, and within which the process parameters in the lithography process or patterning process can typically be varied.

[0026]

[0026] The computer system CL can use (part of) the design layout to be patterned to predict which resolution enhancement technique should be used and can perform computational lithography simulations and calculations to determine the mask layout and lithography device settings that achieve maximization of the overall process window of the patterning process (shown in FIG. 3 by the double-headed arrow at the first scale SC1). Typically, the resolution enhancement technique is arranged to match the patterning capability of the lithography device LA. The computer system CL can also be used to detect where in the process window the lithography device LA is currently operating (e.g. using input from the metrology tool MT) and predict whether there may be defects due to, for example, suboptimal processing (shown in FIG. 3 by the arrow pointing to "0" at the second scale SC2).

[0027]

[0027] The metrology tool MT can provide input to the computer system CL enabling accurate simulation and prediction, and can also provide feedback to the lithographic apparatus LA identifying possible drifts, for example in the calibration status of the lithographic apparatus LA (as shown by multiple arrows at the third scale SC3 in Figure 3).

[0028]

[0028] In a lithographic process, it is desirable to frequently measure the structures being created, for example for process control and verification. Tools for performing such measurements are typically called metrology tools MT. Different types of metrology tools MT for performing such measurements are known, including scanning electron microscopes or various forms of scatterometer metrology tools MT. A scatterometer is a multipurpose instrument that allows the measurement of parameters of a lithographic process by having a sensor in the pupil or in a conjugate plane to the pupil of the objective lens of the scatterometer (usually called pupil-based measurement) or in the image plane or in a conjugate plane to the image plane (usually called image-based or field-based measurement in this case). Such scatterometers and associated measurement techniques are further described in US Patent Application Publication No. 20100328655, US Patent Application Publication No. 2011102753A1, US Patent Application Publication No. 20120044470A, US Patent Application Publication No. 20110249244, US Patent Application Publication No. 20110026032, or European Patent Application Publication No. 1,628,164A, which are incorporated herein by reference in their entireties. The scatterometers described above are capable of measuring gratings using soft x-rays and light in the visible to near infrared wave range.

[0029]

[0029] In a first embodiment, the scatterometer MT is an angle resolved scatterometer. In such a scatterometer, a reconstruction method for reconstructing or calculating the properties of the grating can be applied to the measurement signal. Such a reconstruction can for example be the result of simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the results of the measurement. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to the diffraction pattern observed from the real target.

[0030]

[0030] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such a spectroscopic scatterometer MT, radiation emitted from a radiation source is directed onto a target, and radiation reflected or scattered from the target is directed to a spectrometer detector, which measures the spectrum of the specularly reflected radiation (i.e., measuring the intensity as a function of wavelength). From this data, the structure or profile of the target giving rise to the detected spectrum can be reconstructed (e.g., by rigorous coupled wave analysis and nonlinear regression, or by comparison with a library of simulated spectra).

[0031]

[0031] In a third embodiment, the scatterometer MT is an ellipsometric scatterometer. The ellipsometric scatterometer allows to determine the parameters of the lithographic process by measuring the scattered radiation for each of the polarization states. Such a metrology apparatus emits polarized light (linear, circular or elliptical, for example) using a suitable polarizing filter in the illumination section of the metrology apparatus. A source suitable for the metrology apparatus can provide polarized radiation as well. Various embodiments of existing ellipsometric scatterometers are described in U.S. Patent Application Publication No. 11 / 451,599, U.S. Patent Application Publication No. 11 / 708,678, U.S. Patent Application Publication No. 12 / 256,780, U.S. Patent Application Publication No. 12 / 486,449, U.S. Patent Application Publication No. 12 / 920,968, U.S. Patent Application Publication No. 12 / 922,587, U.S. Patent Application Publication No. 13 / 000,229, U.S. Patent Application Publication No. 13 / 033,135, U.S. Patent Application Publication No. 13 / 533,110, and U.S. Patent Application Publication No. 13 / 891,410, which are incorporated herein by reference in their entireties.

[0032]

[0032] A metrology apparatus such as a scatterometer is shown in Fig. 4. The metrology apparatus comprises a broadband (white light) radiation projector 2, which projects radiation (optionally after being spectrally filtered to a narrow band prior to the substrate W) onto a substrate W. The reflected or scattered light is passed to a spectrometer detector 4, which measures the spectrum 6 of the specularly reflected radiation (i.e. a measurement of the intensity as a function of wavelength). From this data, the structure or profile 8 giving rise to the detected spectrum can be reconstructed by a processing unit PU (for example by rigorous coupled wave analysis and non-linear regression, or by comparison with a library of simulated spectra as shown in the lower part of Fig. 4). Typically, for reconstruction, the general shape of the structure is known from knowledge of the process that created it, and some parameters are assumed, so that only a few parameters of the structure remain to be determined from the scatterometry data. Such a scatterometer can be configured as a normal incidence scatterometer or an oblique incidence scatterometer.

[0033]

[0033] The overall measurement quality of a lithography parameter by measuring a metrology target is at least partially determined by the measurement recipe used to measure this lithography parameter. The term "substrate measurement recipe" can include one or more parameters of the measurement itself, one or more parameters of one or more patterns to be 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 can include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation to the substrate, the orientation of the radiation to the pattern on the substrate, etc. One of the criteria for selecting the measurement recipe can be, for example, the sensitivity to process variations of one of the measurement parameters. Further examples are described in U.S. Patent Application No. 2016-0161863 and published U.S. Patent Application No. 2016 / 0370717A1, which are incorporated herein by reference in their entirety.

[0034] Another type of metrology apparatus is shown in FIG. 5(a). The target T and the diffracted beams of the measurement radiation used to illuminate the target are shown in more detail in FIG. 5(b). The metrology apparatus shown is of the type known as a dark-field metrology apparatus. The metrology apparatus shown in the figure is merely exemplary, intended to provide an explanation of dark-field metrology. The metrology apparatus may be a free-standing device or may be integrated into the lithography apparatus LA (e.g. a measurement station) or into the lithographic cell LC. An optical axis with several branches throughout the apparatus is represented by a dotted line O. In this apparatus, light emitted by a radiation source 11 (e.g. a xenon lamp) is directed onto a substrate W through a beam splitter 15 by an optical system with lenses 12, 14 and an objective lens 16. These lenses are arranged in a double arrangement in a 4F configuration. Different lens arrangements may be used, provided that an image of the substrate is presented to the detector and at the same time allows access to an intermediate pupil plane for spatial frequency filtering. The angular range at which the radiation is incident on the substrate can therefore be selected by defining a spatial intensity distribution in a plane that presents a spatial spectrum in the substrate plane, referred to herein as the (conjugate) pupil plane. In particular, this can be done, for example, by inserting an aperture plate 13 of suitable form between lenses 12 and 14, in a plane that is a back-projected image of the objective pupil plane. In the illustrated example, the aperture plate 13 has different forms, denoted as 13N and 13S, allowing different illumination modes to be selected. The illumination system of this example forms an off-axis illumination mode. In a first illumination mode, the aperture plate 13N provides off-axis illumination from a direction designated "north" for illustration purposes only. In a second illumination mode, the aperture plate 13S is used to provide a similar illumination from the opposite direction, denoted "south". Other illumination modes are possible by using different apertures. It is desirable for the remainder of the pupil plane to be dark, since unwanted radiation outside the desired illumination mode would interfere with the desired measurement signal.

[0035] As shown in FIG. 5(b), the target T is placed 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). When the measurement illumination ray I strikes the target T at an angle off the axis O, it gives rise to a zero-order ray (solid line 0) and two first-order rays (dash-dotted line +1 and dash-dotted line -1). It should be noted that in the case of an overfilled small target, these rays are only one of many parallel rays covering the area of ​​the substrate containing the metrology target T and other features. Since the aperture in the plate 13 has a finite width (necessary to accept a useful amount of radiation), the incident ray I will actually occupy a range of angles, and the diffracted rays 0 and +1 / -1 will be slightly spread out. According to the point spread function of the small target, each order +1 and -1 will be more spread out 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 rays entering the objective are precisely aligned with the central optical axis. The rays shown in Figures 5(a) and 3(b) are shown slightly off-axis simply to make them easier to distinguish in the figures.

[0036]

[0036] At least the 0th and +1st orders diffracted by the target T on the substrate W are collected by the objective lens 16 and guided back through the prism 15. Returning to Fig. 5(a), both the first and second illumination modes are illustrated by designating diametrically opposite apertures, labeled as North (N) and South (S). If the incident ray I of measurement radiation is from the North side of the optical axis, i.e., if the first illumination mode is applied using the aperture plate 13N, the +1 diffracted ray, labeled as +1(N), enters the objective lens 16. In contrast, if the second illumination mode is applied using the aperture plate 13S, it is the -1 diffracted ray (labeled as 1(S)) that enters the lens 16.

[0037]

[0037] A beam splitter 17 splits the diffracted beam into two measurement branches. In the first measurement branch, an optical system 18 uses the zeroth and first order diffracted beams to form a diffraction spectrum (pupil plane image) of the target on a first sensor 19 (e.g. a CCD or CMOS sensor). Since each diffraction order hits a different spot on the sensor, image processing can compare and contrast the orders. The pupil plane image captured by the sensor 19 can be used to focus the metrology device and / or to normalize the intensity measurement of the first order beam. The pupil plane image can also be used for many measurement purposes, such as reconstruction.

[0038] In the second measurement branch, the optical system 20, 22 forms an image of the target T on the substrate W 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 with the pupil plane. The aperture stop 21 serves to block the zeroth order diffracted beam so that the image of the target formed on the sensor 23 is formed only from the −1 or +1 first order beams. 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 should be noted that the term “image” is used in a broad sense in this specification. If only one of the −1 and +1 orders is present, then the image of the grating lines will not be formed as such.

[0039]

[0039] The particular form of the aperture plate 13 and field stop 21 shown in FIG. 5 is merely an example. In another embodiment of the invention, on-axis illumination of the target is used, and an aperture stop with an off-axis aperture is used to pass substantially only one first order diffracted radiation to the sensor. In another example, a two-quadrant aperture may be used. This allows for simultaneous detection of positive and negative orders, as described in the aforementioned US Patent Application Publication No. 2010201963A1. As described in the aforementioned US Patent Application Publication No. 2011102753A1, an embodiment with an optical wedge (segmented prism or other suitable element) in the detection branch can be used to separate the orders for spatial imaging in a single image. In still other embodiments, second, third and higher order beams (not shown in FIG. 5) can be used for the measurement instead of or in addition to the first order beam. In yet other embodiments, a segmented prism can be used in place of aperture stop 21, and both the +1 and −1 orders can be captured simultaneously at spatially separated locations on image sensor 23.

[0040]

[0040] To make the measurement radiation adaptable to these different types of measurements, the aperture plate 13 can contain several aperture patterns formed around a disk that is rotated to bring the desired pattern into place. It should be 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 setup). For measurements of orthogonal gratings, the target can be rotated 90° to 270°.

[0041]

[0041] The light source for metrology applications that can be used for the concepts disclosed herein may include any broadband light source and any color selection arrangement for selecting one or more colors from the broadband output. As an example, the radiation source can be based on a hollow or solid core, 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 an HC-PCF, the hollow core of the fiber can be filled with a gas that acts as a broadening medium to broaden the input radiation. Such a fiber and gas configuration can be used to create a supercontinuum radiation source. The radiation input to the fiber can be electromagnetic radiation, for example, one or more radiation in the infrared, visible, UV and extreme UV spectrum. The output radiation can consist of or include broadband radiation, sometimes referred to herein as white light. This is just one example of a broadband light source technology that can be used with the methods and apparatus disclosed herein, and other suitable technologies can be employed instead.

[0042]

[0042] When using metrology sensors, including those described above and / or other types (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).

[0043]

[0043] To perform color selection, color selection modules have been proposed that use Grating Light Valve (GLV) technology, such as that sold by Silicon Light Machines (SLM), as described, for example, in US Pat. No. 6,947,613 B, which is incorporated herein by reference. GLVs are electrically programmable diffraction gratings based on Microelectromechanical Systems (MEMS) technology. The principle is illustrated in FIG. 6, which shows a schematic diagram of a GLV pixel or component 500, viewed from above (a) and from end-on (b) and (c). A GLV component alternates between two types of GLV reflective ribbons: static or biased ribbons 510, which are typically grounded together with a common electrode, and driven or active ribbons 520, which are driven by electronic driver channels. A GLV module can include any number of these GLV components 500 arranged in an array. The active and biased ribbons can be essentially identical, except for how they are driven. When no voltage is applied to the active ribbons 520, the active ribbons 520 are flush with the bias ribbons, the configuration shown in FIG. 6(b). In this configuration, the GLV essentially acts as a mirror and the incident light is specularly reflected (i.e., forming specularly reflected or zero-order diffracted radiation). When a voltage is applied to the active ribbons 520, the active ribbons 520 deflect relative to the bias ribbons 510, establishing a well-shaped diffraction grating, as shown in FIG. 6(c). In this state, the incident light is diffracted to a defined diffraction angle. The ratio of reflected light to diffracted light can be continuously varied by controlling the voltage to the active ribbons 520, which controls the magnitude of the deflection of the active ribbons 520. Thus, the amount of light diffracted by the GLV can be controlled in an analog fashion from zero (perfect specular reflection) to all incident light (no specular reflection). This control of the amount of reflected radiation relative to the amount of radiation diffracted into non-zero diffraction orders may be referred to as modulation of illumination in the context of this disclosure.

[0044]

[0044] The GLV module can be used in zero-order mode so that the diffracted radiation is blocked / attenuated and the specularly reflected (zeroth order diffraction) radiation is provided to the metrology tool. This has the advantage of preserving the etendue. Therefore, an aperture stop can be provided at the pupil plane to maximize the blocking (minimizing the transmission) of the first order (and other diffraction orders) and maximize the transmission of the zeroth order. However, the diffraction angle is wavelength dependent. In addition, the spot size at the pupil plane is also wavelength dependent, so each color has a different spot size at the pupil plane. For example, radiation sources currently used in some metrology applications may include different etendues for different colors of light, and therefore the respective beam widths for different colors are different.

[0045]

[0045] For this reason, it is difficult to configure the aperture stop to maximize the zero-order transmission and the first-order blocking for all wavelengths of interest (e.g., the wavelength range covered by the source selection module). Any particular shape or configuration for the hard aperture may be suboptimal for a particular wavelength range (e.g., it may cause excessive blocking of the zero-order and / or excessive leakage of the first order in the transmission window). This problem becomes greater as the wavelength range used increases.

[0046]

[0046] This problem is exacerbated when using beams with high etendue. With high etendue beams, it becomes difficult to minimize the spot size (per wavelength) on the GLV and have a low numerical aperture (NA) per order. A small spot on the GLV is highly desirable in order to operate on the flat area of ​​the activated ribbon and thus not lose contrast. A low NA of the order is beneficial to separate the zeroth and first orders over several wavelengths.

[0047]

[0047] A known color selection module arrangement based on a controllable diffractive element such as a GLV includes a beam dispersion element for dispersing a broadband illumination beam, a controllable diffractive element or GLV module for spatially modulating the dispersed broadband illumination beam, an aperture stop in the far field (at the pupil plane of the GLV or its conjugate) for removing all but the desired order (e.g. removing all but the zeroth order, although this can be reversed so that the zeroth order is blocked and the first 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 can disperse the color of white light across the GLV in a first direction (e.g. where the GLV is included in the image plane or field plane of the system). The combining element and the dispersing element can be different elements or a single element.

[0048]

[0048] Figure 7 shows a first color selection module arrangement based on a GLV. The top of Figure 7 is a side view of the arrangement, and the bottom of the figure is a top-down view of the same arrangement. A broadband light source SO emits broadband radiation. A lens system represented by multiple lens elements (e.g., lenses L1 and L2) provides access to a pupil plane where a beam dispersion element DE (e.g., a grating or prism) is located. The beam dispersion element DE distributes the broadband illumination beam onto the GLV, optionally via an intermediate image of a first spectrally dispersed image plane SDIP (or field plane) between lenses L2 and L3 (as shown). Lens L4 focuses the spectrally dispersed radiation onto the GLV module GLV at the second spectrally dispersed image plane. The reflected (zero order) radiation from the GLV is captured by lens L4 (e.g., in this embodiment, i.e., the incident radiation is focused onto the GLV and the spatially modulated reflected radiation is captured from the GLV using the same lens arrangement). An aperture stop ST0 is positioned at a pupil plane P2 provided by lens L4 to block any unwanted diffraction orders from the GLV (e.g., to block the first order while passing the zeroth order substantially unattenuated). In the illustrated arrangement, the modulated illumination beams are recombined on the return path using the same beam dispersion element DE that was used to disperse the beams on the outbound path. Lens L1 then focuses the output beam into a metrology device MET (e.g., into a suitable optical fiber such as a single mode fiber for carrying the radiation to the metrology device MET).

[0049]

[0049] It should be understood that while this arrangement is shown to illustrate the problems of the prior art arrangements that the concepts disclosed herein address, the actual arrangement shown is not prior art. Source selection module arrangements using the same optical components for the outbound path to the GLV and the return path from the GLV are not known from the prior art. This includes, for example, using a single element DE (e.g., a prism) as the dispersive element in the outbound path and the combining element in the return path, and sharing all lenses, including the imaging lens L4, to image the dispersed radiation onto the GLV and collect the modulated radiation from the GLV. This is achieved by having a first off-axis beam path (outbound path) to the GLV (shown in black in the figure above; note that this aspect is only visible in the figure above) and a second off-axis beam path (return path) from the GLV (shown in grey in the figure above). Prior art arrangements typically use separate optical branches to and from the GLV, with dedicated optics for each branch, and such arrangements are also within the scope of this disclosure.

[0050]

[0050] In Fig. 7, a schematic diagram of the light distribution at lens L2 (plane P1) is also shown. The upper part of the diagram corresponds to the outbound path and shows the dispersed source radiation, while the lower part of the diagram corresponds to the return path and shows the GLV modulated radiation (only an exemplary color selection is shown). The grayscale shading represents different colors / wavelengths. A diagram of the image at the spatially dispersed image plane SDIP is also shown, showing the dispersed beam at the image plane.

[0051]

[0051] A first pupil plane view P20 is also shown for the pupil plane P2 of the lens system including lenses L3 and L4. It includes the location of the (first order) stop ST0, which defines an aperture AP0, configured for zero-order mode operation, i.e., to block the first diffraction orders +1, -1 (shown as dotted lines in the lower view; they are not visible in the upper view) and transmit the zeroth order 0 and the source beam SB. Note the different positions (i.e., diffraction angles) in the pupil plane and the spot sizes / diameters per wavelength of the first diffraction orders +1, -1 (in this case different colors will also have different diameters in the source beam SB). It is not essential that different colors have different spot sizes, and the concepts disclosed herein can also be used with radiation sources whose spot size is independent of wavelength.

[0052]

[0052] Figure 8 shows a second color selection module arrangement based on a GLV. Figure 8(a) is a side view and Figure 8(b) is a top down view of the arrangement. In this example arrangement, the GLV is configured for first mode operation, i.e., first order diffraction (for clarity, two colors + 1 λ1 , +1 λ2 , -1 λ1 , -1 λ2 Only the zero-order 0 is shown, both of which are selected by the GLV. Of course, there may be more colors and / or a continuous spectrum) and transmits the source beam SB, λ1 , 0 λ2 More specifically, in the illustrated example, two wavelengths are shown as being transmitted through configurations λ1 and λ2 (i.e., both are selected by the GLV), resulting in a diffraction order of +1. λ1 , -1 λ1 , +1 λ2 , -1 λ2 is captured by lens L3.

[0053]

[0053] Many of the components have been described with respect to Figure 7 and will not be described again. Figure 8(c) is a view P21 of the second pupil plane corresponding to the view P20 of the first pupil plane shown in Figure 7. In this arrangement, the stop ST1 is a zero order stop arranged to block only the zeroth order (specularly reflected radiation), thereby defining an aperture AP1 that transmits the first order (and / or other higher orders).

[0054]

[0054] As already explained, the aperture stop should maximize the transmission of the zeroth order beam (for all selected wavelengths) and minimize the transmission of the first order beam for all wavelengths, or vice versa. Maximizing the transmission (e.g., of the zeroth order beam or diffraction (e.g., the first order beam for all wavelengths)) should be understood to mean increasing the transmission as much as possible, taking into account the constraints and trade-offs of the geometry required with respect to minimizing the transmission of the radiation to be blocked. Similarly, minimizing the transmission (e.g., of the first or zeroth order for all wavelengths) should be understood to mean blocking these orders as much as possible, taking into account these same constraints and trade-offs. In particular, the fact that the spots have spatially overlapping tails (when considering the plot of intensity or amplitude versus pupil position for each spot) creates the need to pass some unwanted light (leading to poor out-of-band contrast) or block the tail of the wanted zeroth order, which results in less signal and therefore lower throughput. This challenge becomes more severe the larger these spots become compared to the separation of the orders (i.e. the larger the NA of the beam).

[0055] In one embodiment, maximizing the transmission may include transmitting 90% or more of the transmitted radiation, transmitting 95% or more, transmitting 98% or more, transmitting 99% or more, transmitting 99.9% or more, or transmitting 99.99% or more. In one embodiment, minimizing the transmission may include blocking 90% or more of the blocked radiation, blocking 95% or more, blocking 98% or more, blocking 99% or more, blocking 99.9% or more, or blocking 99.99% or more.

[0056]

[0056] Figure 9 illustrates the problem of the illustrated aperture stop ST0 (e.g., corresponding to the zeroth mode configuration) in terms of a pupil diagram. The figure shows a source beam SB with only two wavelengths shown for clarity (represented by shading as before), as well as the corresponding zeroth 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 dispersed in a first direction (here denoted as X), and when reflected, the source beam is specularly reflected back to the zeroth order 0 (as mentioned above, the ratio of diffraction into the first diffraction orders +1, -1 and reflection back to the zeroth order is controllable via the GLV). At the bottom of the pupil diagram is a plot of intensity versus pupil position in one dimension, spatially aligned with the pupil diagram, from the zeroth order beam 0 to the diffraction orders +1, -1. As can be seen from these intensity plots, the overlapping tails of the zeroth and first orders mean that in this case it is not possible to block the first order in an optimal way, and some zeroth order radiation must be blocked or first order radiation must be transmitted. This is especially true when blocking the first order radiation for short wavelengths and when transmitting the zeroth order for longer wavelengths.

[0057]

[0057] If the GLV is configured for first order mode operation, similar principles apply: for example, if aperture stop ST1 is used instead of aperture stop ST0 in Figure 9, it is not possible to completely block the zeroth order and completely transmit the desired first order.

[0058]

[0058] To address this issue, it is proposed to provide at least one second beam dispersion element (e.g., prism or grating) that disperses the broadband illumination beam (e.g., source beam radiation) in a first direction in the (conjugate) image plane of the controllable (e.g., electrically programmable) diffractive element or GLV, in addition to the first beam dispersion element (e.g., prism or grating), which disperses the broadband illumination beam in a second direction in the pupil plane of the controllable diffractive element or GLV. The first direction and the second direction are mutually orthogonal, the first direction being the direction of the period of the GLV and the second direction being the direction in which each GLV ribbon extends. Thus, the at least one second beam dispersion element can be arranged in the (conjugate) image plane of the GLV. This allows the shape of the aperture stop to be optimized for a more optimal blocking of unwanted radiation (e.g., non-zero or first order diffraction from the GLV).

[0059]

[0059] Figure 10(a) is a pupil diagram equivalent to that of Figure 9, implementing the concepts disclosed herein. In this diagram, the source beam SB is now dispersed in this pupil plane in a second direction (here denoted Y) by a second beam dispersion element. Although only two wavelengths are shown for clarity, it will be understood that the source beam may actually contain more wavelengths (e.g., a continuous wavelength band), so that the dispersed source beam may actually contain a continuous dispersion spectrum (or multiple discrete wavelengths) in the second direction. Therefore, the position of the source radiation towards the GLV depends on the wavelength in this second direction (in the illustrated example, a shorter WL has a larger angle of incidence on the GLV; this is a design choice and can also be reversed). The resulting zero-order radiation 0 is similarly dispersed in the second direction. The first orders +1, -1 are also dispersed in this second direction and, due to diffraction by the GLV, also in the first direction (the dispersion in the first direction is the same as in FIG. 9).

[0060]

[0060] This first-order dispersion in two dimensions allows for better optimization of the aperture stop ST0' configuration / shape, more specifically optimization such that the effective aperture size (e.g., in the first direction) is different for each wavelength. As can be seen in the illustrated example of FIG. 10(a), the diffracted radiation of the first (e.g., short) wavelength follows an effective aperture size b, and the diffracted radiation of the second (e.g., long) wavelength follows an effective aperture size a. The dotted lines represent a compromise fixed aperture size in the example of FIG. 9. Thus, the aperture stop ST0' is 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 for the downstream optical return path from the GLV) that varies in size (increases or decreases) along the second direction. The smaller aperture of such a continuously increasing aperture width (or stepwise increasing aperture width) may be located in a pupil region corresponding to a location in the second direction where the diffraction orders are diffracted (in that second direction) and also diffracted to smaller angles in the first direction and / or the spot size in the pupil is smaller. Similarly, a larger aperture or the larger end of the continuously increasing aperture width may be located in a pupil region in the second direction corresponding to a location where the diffraction orders are diffracted (in that second direction) and also diffracted to larger angles in the first direction and / or the spot size in the pupil is larger.

[0061]

[0061] Figure 10(b) shows an equivalent arrangement, but now the stop ST1' is configured for first order mode operation (i.e., to block the zeroth order and transmit at least one higher diffraction order (e.g., one or both of the first orders)). Therefore, instead of a continuously increasing aperture width, this arrangement includes a continuously decreasing (zeroth order) stop ST1' width.

[0062]

[0062] It should be understood that the shape of such a continuously increasing aperture or continuously decreasing (zero order) stop may differ from that shown, e.g. the edges of the aperture / stop need not be straight as shown here, but may bend (from the point of view of the figure) from bottom to top, e.g. increasing / decreasing in steps. Alternatively or additionally, the aperture stop may include "soft" edges that transmit / block in a non-binary manner (e.g. blocking portions at the edges of the aperture may partially transmit radiation).

[0063] It should be understood that the GLV can also be used in the opposite configuration (higher order mode), in which case the desired radiation is diffracted radiation (e.g., first order radiation) and the unwanted radiation is specularly reflected (zero order) radiation. In such a configuration, the aperture may be the inverse of the aperture shown in the pupil region corresponding to the return path downstream of the GLV (but does not block the source beam in the outward path). For example, in FIG. 10, such an aperture may block the central region between the illustrated aperture / blocking portions to block the zeroth order 0 and transmit the first orders +1, -1.

[0064]

[0064] Figure 11 is a schematic diagram of an exemplary color selection module according to a first embodiment. This arrangement is the same as that shown in Figure 7, except that in addition to the first beam dispersion element DE1, a second beam dispersion element DE2 is added. In the illustrated arrangement, the second beam dispersion element DE2 is provided in the outgoing path to disperse the source radiation in a second direction at the pupil plane (e.g. P2). In contrast, the first beam dispersion element disperses the source radiation in a 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 should be noted that in this embodiment, the first beam dispersion element DE1 also doubles as the first beam combining element for recombining the illumination in the first direction. This is optional, and instead, a separate first beam combining element can be provided for recombining the illumination in the first direction.

[0065]

[0065] The second beam dispersion element DE2 and the second beam combining element CE2 can be located at the conjugate image plane of the GLV. In addition to these elements DE2, CE2, a wedge WG (e.g. of low dispersion material) can be provided in the plane P2 (e.g. the pupil plane defined by the lens system L3, L4), which serves to separate the illumination in the outbound path at the first spectrally dispersed image plane SDIP (e.g. the approximate location of the elements DE2, CE2). The second beam dispersion element DE2 and the second beam combining element CE2 can include a pair of dispersive elements or prisms (e.g. similarly and oppositely oriented).

[0066]

[0066] Fig. 12(a) is a schematic diagram of an exemplary color selection module according to a second embodiment. This arrangement is the same as that shown in Fig. 8, except that in addition to the first beam dispersion element DE1 (still serving as the first beam combining element), a second beam dispersion element DE2 and a second beam combining element CE2 are added. In the illustrated arrangement, the second beam dispersion element DE2 is provided in the outbound path to distribute the source radiation in the second direction in the pupil plane. In contrast, the first beam dispersion element distributes the source radiation in the first direction in the image plane. In the return path, the second beam combining element CE2 is provided to recombine the illumination in the second direction.

[0067] FIG. 12(b) shows a view P21″ of the resulting pupil plane in a plane including the zero-order stop ST1″. The zero-order stop ST1″ has a varying (e.g., continuously decreasing) width that defines a first order or diffraction aperture AP1″ configured to transmit first diffraction orders.

[0068] As an alternative to the illustrated location of the second beam dispersion / combining element DE2, CE2 (in either embodiment), they may be located in front of the GLV (e.g., at or instead of the input window of the GLV). The second beam dispersion / combining element DE2, CE2 (in either embodiment) may include a compound prism, such as an Amichi prism.

[0069]

[0069] Therefore, as can be seen in the exemplary pupil diagrams P20'', P21'', respectively, of the two embodiments, the source beam SB and all diffracted / reflected beams that are spectrally dispersed in the second direction make it possible to use more optimal aperture shapes AP0'', AP1'', as described above.

[0070]

[0070] For simplicity, the beam paths after the second beam dispersing / combining elements DE2, CE2 are not drawn tilted, but in practice they may need to be tilted.

[0071]

[0071] A GLV-based source selection module allows control of the transmittance for each color or band of the output beam, rather than simply switching the color / band on and off.

[0072]

[0072] Advantages of the proposed arrangement include providing the option of using a larger etendue light source, allowing a higher achievable throughput (signal) for a given out-of-band (OoB) signal, or conversely, better achievable OoB suppression for a given throughput (signal), and allowing a smaller minimum bandwidth (by allowing a smaller spot on the GLV for a given throughput and OoB performance).

[0073]

[0073] Further embodiments are disclosed in the following list of numbered items: 1. A source selection module for selecting a spectral characteristic of a broadband illumination beam to obtain a modulated illumination beam, comprising: a first beam dispersing element for dispersing the broadband illumination beam, the first beam dispersing element 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 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 a first direction such that a direction of a period of the controllable diffractive element includes the first direction, the controllable diffractive element being operable to controllably spatially modulate the broadband illumination beam after it has been dispersed by the first beam dispersive element and the second beam dispersive element; an aperture stop operable to maximise the transmission of one of the specularly reflected radiation and the diffracted radiation from the controllable diffractive element and to minimise the transmission of the other of the specularly reflected radiation and the diffracted radiation; A source selection module comprising: 2. The source selection module of clause 1, wherein the controllable diffractive element comprises a grating light valve module. 3. A source selection module as described in clause 1 or 2, wherein a first beam dispersion element is positioned at a pupil plane or a conjugate thereof of the controllable diffractive element to disperse the broadband illumination beam at an image plane of the controllable diffractive element or a conjugate thereof. 4. A source selection module described in any one of clauses 1 to 3, wherein a second beam dispersion element is positioned at an image plane or a conjugate thereof of the controllable diffractive element to disperse the broadband illumination beam at a pupil plane of the controllable diffractive element or a conjugate thereof. 5. A source selection module described in any one of clauses 1 to 4, further comprising at least a first beam combining element for recombining the modulated illumination beam in a first direction to obtain an output source beam. 6. A source selection module as described in any one of clauses 1 to 4, wherein the first beam dispersion element is also arranged to recombine the modulated illumination beam on a return path from the controllable diffractive element. 7. A source selection module as described in clause 6, wherein at least one lens element is shared between an outbound path of the broadband illumination beam to the controllable diffractive element and a return path, the outbound path including a first off-axis path and the return path including a second off-axis path through at least one lens element and the first beam dispersion element. 8. The source selection module described in clause 7, wherein at least one lens element includes at least a lens or lens system operable to image the broadband illumination beam after it has been dispersed by the first beam dispersion element and the second beam dispersion element onto the controllable diffractive element and to collect the modulated illumination beam from the controllable diffractive element. 9. A source selection module described in any one of clauses 1 to 8, further comprising at least a second beam combining element for recombining the modulated illumination beam in a second direction to obtain an output source beam. 10. A source selection module according to any one of clauses 1 to 9, wherein the aperture stop is positioned in a pupil plane of the controllable diffractive element or its conjugate. 11. A source selection module as described in clause 10, wherein the aperture stop is operable to maximize transmission of specularly reflected radiation from the controllable diffractive element and minimize transmission of diffracted radiation from the controllable diffractive element. 12. A source selection module as described in clause 10, wherein the aperture stop is operable to minimize transmission of specularly reflected radiation from the controllable diffractive element and to maximize transmission of diffracted radiation from the controllable diffractive element. 13. A source selection module described in any one of clauses 1 to 12, wherein the aperture stop defines an aperture having an aperture size in a first dimension that varies along a second dimension for at least a portion of the pupil surface corresponding to the return path of the modulated illumination beam. 14. A source selection module as described in clause 13, wherein the aperture size and / or aperture stop size in the first dimension increases or decreases continuously or in steps along the second dimension. 15. A source selection module as described in any one of clauses 1 to 14, wherein the first beam dispersion element and the second beam dispersion element each include a prism. 16. A source selection module for selecting a spectral characteristic of a broadband illumination beam to obtain a modulated illumination beam, comprising: at least one beam dispersing element for dispersing the broadband illumination beam, the at least one beam dispersing element 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 a period of the controllable diffractive element includes the first direction, the controllable diffractive element being operable to controllably spatially modulate the broadband illumination beam after it has been dispersed by the first beam dispersion element; an aperture stop operable to maximise the transmission of one of the specularly reflected radiation and the diffracted radiation from the controllable diffractive element and to minimise the transmission of the other of the specularly reflected radiation and the diffracted radiation; a plurality of lens elements including at least a lens or a lens system operable to image the broadband illumination beam after being dispersed by the first beam dispersion element onto the controllable diffractive element and to collect a modulated illumination beam from the controllable diffractive element; The at least one first beam dispersive element is also positioned to recombine the modulated illumination beam on a return path from the controllable diffractive element, a source selection module. 17. A source selection module as described in clause 16, wherein each of the plurality of lens elements is shared between an outbound path of the broadband illumination beam to the controllable diffractive element and a return path, the outbound path including a first off-axis path and the return path including a second off-axis path through the plurality of lens elements and at least one beam dispersion element. 18. A source selection module according to clause 16 or 17, wherein the controllable diffractive element comprises a grating light valve module. 19. A source selection module as described in any one of clauses 1-18, comprising an illumination source for providing input illumination. 20. A source selection module as described in clause 19, wherein the illumination source comprises a hollow fiber for confining the broadening medium and an excitation radiation source operable to provide excitation radiation for exciting the broadening medium. 21. A metrology device comprising a source selection module according to any one of clauses 1 to 20 for providing measurement illumination. 22. A metrology device according to clause 21, comprising a scatterometer. 23. Support for the substrate; an optical system for directing measurement illumination to a structure on the substrate; a detector for detecting measurement radiation scattered by the structure on the substrate; 23. The metrology device of clause 22, comprising: 24. A metrology device as described in clause 21, comprising an alignment sensor. 25. A patterning device support for supporting a patterning device; A substrate support for supporting a substrate; A metrology device according to clause 24, operable to align a patterning device and / or a substrate support; A lithographic apparatus comprising:

[0074]

[0074] The term "color" is used throughout this text as synonymous with wavelength or spectral content, with the understanding that color can include those outside the visible band (e.g., infrared or ultraviolet wavelengths).

[0075]

[0075] Whilst specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described.

[0076]

[0076] Although particular reference has been made above to the use of embodiments of the invention in the context of optical lithography, it should be understood that the invention may also be used in other applications, such as imprint lithography, depending on the context, and is not limited to optical lithography. In imprint lithography, a topography in a patterning device defines a pattern created on a substrate. The topography of the patterning device is imprinted into a layer of resist applied to the substrate, and the resist is hardened by applying electromagnetic radiation, heat, pressure, or a combination thereof. The patterning device is removed from the resist, leaving a pattern therein as the resist hardens.

[0077]

[0077] As used herein, the terms "radiation" and "beam" include all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of (approximately) 365, 355, 248, 193, 157 or 126 nm), extreme ultraviolet (EUV) radiation (e.g., having a wavelength in the range of 1 to 100 nm), and particle beams such as ion beams or electron beams.

[0078] The term "lens", where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components. Reflective components are more likely to be used in devices operating in the UV and / or EUV ranges.

[0079]

[0079] The breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

Claims

1. a source selection module for selecting a spectral characteristic of the broadband illumination beam to obtain a modulated illumination beam, the source selection module comprising: a first beam dispersion element for dispersing the broadband illumination beam, the first beam dispersion element operable to disperse the broadband illumination beam along a first direction; a second beam dispersion element for dispersing the broadband illumination beam, the second beam dispersion element 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 a first direction such that a direction of a period of the controllable diffractive element includes the first direction, the controllable diffractive element being operable to controllably spatially modulate the broadband illumination beam after it has been dispersed by the first beam dispersive element and the second beam dispersive element; an aperture stop operable to maximise the transmission of one of the specularly reflected radiation and the diffracted radiation from the controllable diffractive element and to minimise the transmission of the other of the specularly reflected radiation and the diffracted radiation; A source selection module comprising:

2. The source selection module of claim 1 , wherein the controllable diffractive element comprises a grating light valve module.

3. 3. The source selection module of claim 1, wherein the first beam dispersion element is positioned at a pupil plane of the controllable diffractive element or a conjugate thereof to disperse the broadband illumination beam at an image plane of the controllable diffractive element or a conjugate thereof.

4. 4. The source selection module of claim 1, wherein the second beam dispersion element is positioned at an image plane of the controllable diffractive element or a conjugate thereof to disperse the broadband illumination beam in a pupil plane of the controllable diffractive element or a conjugate thereof.

5. 5. The source selection module of claim 1 , further comprising at least a first beam combining element for recombining the modulated illumination beams in the first direction to obtain an output source beam.

6. 5. A source selection module according to claim 1, wherein the first beam dispersion element is also arranged to recombine the modulated illumination beam on a return path from the controllable diffractive element.

7. at least one lens element is shared between a path of the broadband illumination beam to and from the controllable diffractive element; the outbound path includes a first off-axis path; The source selection module of claim 6 , wherein the return path includes a second off-axis path through the at least one lens element and a first beam dispersion element.

8. 8. The source selection module of claim 7, wherein the at least one lens element comprises at least a lens or lens system operable to image the broadband illumination beam after being dispersed by the first beam dispersion element and the second beam dispersion element onto the controllable diffractive element and to collect the modulated illumination beam from the controllable diffractive element.

9. 9. The source selection module of claim 1, further comprising at least a second beam combining element for recombining the modulated illumination beams in the second direction to obtain an output source beam.

10. 10. 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 the transmittance of the specularly reflected radiation from the controllable diffractive element and minimize the transmittance of the diffracted radiation from the controllable diffractive element.

11. The source selection module of claim 1 , wherein the first beam dispersive element and the second beam dispersive element each comprise a prism.

12. 12. A source selection module according to claim 1, 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 a return path of the modulated illumination beam.

13. The source selection module of claim 12 , wherein the aperture size in the first dimension increases or decreases continuously or in steps along the second dimension.

14. A source selection module according to any preceding claim, comprising an illumination source for providing said input illumination.

15. A metrology device comprising a source selection module according to any one of claims 1 to 14 for providing measurement illumination.

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