Method for measuring parameters of photolithography process

Abstract

The present disclosure relates to measurement of parameters of a lithography process and to a measurement apparatus. In one arrangement, radiation from a radiation source is modified and used to illuminate a target formed on a substrate using a lithography process. Radiation scattered from the target is detected and analyzed to determine the parameter. Modifying the radiation includes modifying a wavelength spectrum of the radiation to have a local minimum between a global maximum and a local maximum, wherein a power spectral density of the radiation at the local minimum is less than 20% of a power spectral density of the radiation at the global maximum, and a power spectral density of the radiation at the local maximum is at least 50% of a power spectral density of the radiation at the global maximum.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to EP application 18212262.2 filed on December 13, 2018, and the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present description relates to the measurement of parameters of a lithography process and to a metrology apparatus. Background Art

[0004] A lithographic apparatus is a machine that applies a desired pattern to a substrate (usually to a target portion of the substrate). For example, a lithographic apparatus can be used to manufacture an integrated circuit (IC). In this example, a pattern forming device (which may alternatively be referred to as a mask or reticle) can be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred to a target portion (e.g., a portion including one or more bare chips) on a substrate (e.g., a silicon wafer). The transfer of the pattern is usually performed via imaging onto a layer of radiation-sensitive material (resist) disposed on the substrate. In general, a single substrate contains a network of adjacent target portions that are continuously patterned. During the lithographic process, it is often desirable to measure the created structure, for example, for process control and verification. Various tools for performing such measurements are known, including scanning electron microscopes that are commonly used to measure critical dimensions (CDs), and special tools for measuring overlay (a measure of the alignment accuracy of two layers in a device). Overlay can be described according to the degree of misalignment between the two layers, for example, a reference to a measured overlay of 1 nm can describe a situation where the two layers are misaligned by 1 nm.

[0005] Various forms of scatterometers have been developed for use in the field of photolithography. These devices direct a radiation beam onto a target and measure one or more characteristics of the scattered radiation to determine a parameter of interest, such as overlay. Interference effects in the target can cause the sensitivity of the measurement of the parameter of interest to vary depending on the wavelength, angle of incidence, and / or azimuth of the radiation. To provide high sensitivity in such scenarios, multiple measurements can be made at different wavelengths, but this can increase system complexity, increase measurement time, and / or reduce measurement throughput. Summary of the invention

[0006] In situations where sensitivity varies significantly with wavelength, it is desirable to improve the measurement of parameters of interest.

[0007] According to one aspect, a method for measuring parameters of a lithography process is provided, comprising: modifying radiation from a radiation source and irradiating a target formed on a substrate using the lithography process with the modified radiation; and detecting radiation scattered from the target and analyzing the detected radiation to determine the parameters, wherein modifying the radiation comprises: modifying a wavelength spectrum of the radiation to have a local minimum between a global maximum and a local maximum, wherein a power spectral density of the radiation at the local minimum is less than 20% of a power spectral density of the radiation at the global maximum, and a power spectral density of the radiation at the local maximum is at least 50% of the power spectral density of the radiation at the global maximum.

[0008] According to one aspect, a metrology apparatus for measuring parameters of a lithography process is provided, comprising an optical system configured to direct radiation from a radiation source onto a target formed on a substrate; and a beam modification device configured to modify radiation received from the radiation source and provide the modified radiation to the optical system, wherein the beam modification device is configured to modify a wavelength spectrum of radiation from the radiation source to have a local minimum between a global maximum and a local maximum, wherein a power spectral density of the radiation at the local minimum is less than 20% of a power spectral density of the radiation at the global maximum, and a power spectral density of the radiation at the local maximum is at least 50% of the power spectral density of the radiation at the global maximum. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Embodiments will now be described, by way of example only, with reference to the accompanying drawings, in which

[0010] Figure 1 A lithography apparatus is depicted which, together with other apparatus, forms a production facility for semiconductor devices;

[0011] Figure 2 Included are (a) a schematic diagram of a dark field scatterometer for measuring a target using a first illumination aperture and (b) details of a diffraction spectrum of a target grating in a given illumination direction;

[0012] Figure 3 Schematically depicts a portion of a target with two layers and the interference effect that causes a periodic variation in the sensitivity curve;

[0013] Figure 4 is a graph showing sensitivity curves and metrology measurements at three different wavelengths;

[0014] Figure 5 An illumination system having a radiation source and a filter is schematically depicted;

[0015] Figure 6 schematically depicts a metrology device for measuring parameters of a lithography process;

[0016] Figure 7 schematically depicts an irradiation system having a radiation source and a beam modification device;

[0017] Figure 8 schematically depicts an illumination system with a radiation source, a filter, and a beam modification device;

[0018] Fig. 9 A tunable multi-band filter including a Fabry–Pérot interferometer is schematically depicted;

[0019] Fig.10 is a graph showing an example modified wavelength spectrum output from a beam modification device;

[0020] Fig.11 yes Fig.10 A graph of a modified wavelength spectrum of the type depicted multiplied by the sensitivity curve.

[0021] Fig.12 schematically depicts a beam-modifying device comprising a spatial light modulator (SLM) configured to modify a radiation wavelength spectrum to vary as a function of the angle of incidence onto a substrate;

[0022] Fig.13 Schematically depicts Fig.12 Example light rays directed onto a target at different angles of incidence generated by a beam modification device. DETAILED DESCRIPTION

[0023] Before describing embodiments of the invention in detail, it is instructive to present an example environment in which embodiments of the invention may be implemented.

[0024] Figure 1 A lithographic apparatus LA is shown at 200 as part of an industrial facility implementing a large-scale lithographic manufacturing process. In this example, the manufacturing process is suitable for manufacturing semiconductor products (integrated circuits) on substrates such as semiconductor wafers. The skilled person will appreciate that a wide variety of products can be manufactured in variations of the process by processing different types of substrates. The production of semiconductor products is purely taken as an example, which is of great commercial significance today.

[0025] In a lithographic apparatus (or simply "lithographic tool" 200), a measurement station MEA is shown at 202 and an exposure station EXP is shown at 204. A control unit LACU is shown at 206. In this example, each substrate visits a measurement station and an exposure station to apply a pattern. For example, in an optical lithographic apparatus, a projection system is used to transfer a product pattern from a patterning device MA to a substrate using conditioned radiation and a projection system. This is done by forming an image of the pattern in a layer of radiation sensitive resist material.

[0026] The term "projection system" as used herein should be broadly interpreted to cover any type of projection system, including refractive optical systems, reflective optical systems, catadioptric optical systems, magnetic optical systems, electromagnetic optical systems and electrostatic optical systems, or any combination thereof, depending on the exposure radiation being used or other factors (such as the use of immersion liquid or the use of vacuum). The pattern forming MA device can be a mask or a reticle that imparts a pattern to a radiation beam transmitted or reflected by the pattern forming device. Well-known operating modes include stepping mode and scanning mode. It is well known that the projection system can cooperate with the support and positioning system for the substrate and the pattern forming device in various ways to apply the desired pattern to many target portions across the substrate. A programmable pattern forming device can be used instead of a reticle with a fixed pattern. The radiation can, for example, include electromagnetic radiation in the deep ultraviolet (DUV) or extreme ultraviolet (EUV) band. The present disclosure may also be applicable to other types of lithography processes, for example, imprint lithography and direct write lithography (e.g., by electron beam).

[0027] The lithographic apparatus control unit LACU controls all movements and measurements of the various actuators and sensors to receive the substrate W and the mask MA and to implement the patterning operation. The LACU also includes signal processing and data processing capabilities to implement the desired calculations related to the operation of the apparatus. In practice, the control unit LACU is implemented as a system with many sub-units, each of which handles real-time data acquisition, processing and control of a subsystem or component within the apparatus.

[0028] Before the pattern is applied to the substrate at the exposure station EXP, the substrate is processed at the measurement station MEA so that various preparatory steps can be performed. The preparatory steps may include: using a leveling sensor to map the surface height of the substrate; and using an alignment sensor to measure the position of alignment marks on the substrate. The alignment marks are nominally arranged in a regular grid pattern. However, due to the lack of precision when the marks are produced and due to the deformation of the substrate during its entire processing, the marks deviate from the ideal grid. Therefore, in addition to measuring the position and orientation of the substrate, if the device is to print product features at the correct position with very high accuracy, the alignment sensor must actually measure the position of many marks across the substrate area in detail. The device can be a so-called dual-stage type, which has two substrate stages, each substrate stage having a positioning system controlled by a control unit LACU. While one substrate on one substrate stage is being exposed at the exposure station EXP, another substrate can be loaded onto the other substrate stage at the measurement station MEA so that various preparatory steps can be performed. Therefore, measuring the alignment marks is very time-consuming, and providing two substrate stages enables the throughput of the device to be greatly improved. If the position sensor IF is unable to measure the position of the substrate table both at the measurement station and at the exposure station, a second position sensor may be provided to enable the position of the substrate table to be tracked at both stations. The lithographic apparatus LA may, for example, be of a so-called dual-stage type having two substrate tables and two stations (an exposure station and a measurement station) between which the substrate table can be exchanged.

[0029] Within a production facility, the device 200 forms part of a "lithography cell" or "lithography cluster", which also includes a coating device 208 for applying photoresist and other coatings to substrates W for patterning by the device 200. A baking device 210 and a developing device 212 are provided at the output side of the device 200 for developing the exposed pattern into a physical resist pattern. Between all these devices, the substrate handling system is responsible for supporting the substrates and transferring them from one device to another. These devices, which are generally referred to as tracks, are under the control of a track control unit, which itself is controlled by a monitoring system SCS, which also controls the lithography device via a lithography device control unit LACU. Therefore, different devices can be operated to maximize throughput and processing efficiency. The monitoring system SCS receives a treatment scheme information R, which provides in detail the definition of the steps performed to create each patterned substrate.

[0030] Once the pattern is applied and developed in the lithography unit, the patterned substrate 220 is transferred to other processing devices, such as those illustrated at 222, 224, and 226. In a typical manufacturing facility, many different processing steps are performed by various devices. By way of example only, device 222 in this embodiment is an etching station, and device 224 performs a post-etching annealing step. Other physical and / or chemical processing steps are applied in other devices 226, etc. The production of a real device may require multiple types of operations, such as depositing materials, modifying surface material features (oxidation, doping, ion implantation, etc.), chemical mechanical polishing (CMP), etc. In practice, device 226 may represent a series of different processing steps performed in one or more devices.

[0031] As is well known, the manufacture of semiconductor devices includes many repetitions of such processing to build up device structures with appropriate materials and patterns layer by layer on the substrate. Thus, the substrates 230 arriving at the lithography cluster may be newly prepared substrates, or they may be substrates that have all been previously processed in the cluster or in another device. Likewise, upon leaving the device 226, the substrates 232 may be returned to the same lithography cluster for subsequent patterning operations, they may be destined for patterning operations in a different cluster, or they may be finished products to be sent for dicing and packaging, depending on the desired processing.

[0032] Each layer of the product structure requires a different set of process steps, and the type of apparatus 226 used at each layer can be completely different. Further, even in the case where the processing steps to be applied by apparatus 226 are nominally the same, in a large facility there may be several supposedly identical machines working in parallel to perform steps 226 on different substrates. Slight differences in the settings or failures between these machines may mean that they affect different substrates in different ways. Even steps that are relatively common to each layer, such as etching (apparatus 222), can be implemented by several etching apparatuses that are nominally the same but working in parallel to maximize throughput. Moreover, in practice, different layers require different etching processes depending on the details of the material to be etched, for example, chemical etching, plasma etching, and special requirements such as, for example, anisotropic etching.

[0033] As just mentioned, the preceding process and / or the subsequent process may be performed in other lithography apparatuses, and may even be performed in different types of lithography apparatuses. For example, some layers that are very demanding on parameters such as resolution and overlay during device fabrication may be performed in more advanced lithography tools compared to other layers that are less demanding. Thus, some layers may be exposed in an immersion lithography tool, while other layers may be exposed in a 'dry' tool. Some layers may be exposed in a tool operating at a DUV wavelength, while other layers are exposed using EUV wavelength radiation.

[0034] In order to correctly and consistently expose the substrate exposed by the lithography apparatus, it is desirable to inspect the exposed substrate to measure characteristics such as overlay error between subsequent layers, line thickness, critical dimension (CD), etc. Therefore, the manufacturing facility where the lithography unit LC is located also includes one or more metrology systems. The metrology system may include an independent metrology device MET 240 and / or an integrated metrology device IM 207. The independent metrology system MET 240 receives some or all of the substrates W that have been processed in the lithography unit for offline measurement. The integrated metrology device IM 207 performs online measurement and is integrated into the track to receive and measure some or all of the substrates W immediately after exposure. The measurement results are provided directly or indirectly to the monitoring system (SCS) 238. If an error is detected, the exposure of the subsequent substrate can be adjusted, especially when the measurement can be completed quickly and quickly enough that other substrates of the same batch are still to be exposed.

[0035] A common example of a metrology device in a modern lithography production facility is a scatterometer, such as an angle-resolved scatterometer or a spectral scatterometer, and it can be used to measure the characteristics of the developed substrate 220 before etching in the device 222. Using the independent metrology device 240 and / or the integrated metrology device 207, it can be determined that important performance parameters such as overlay or critical dimension (CD) do not meet the specified tolerances in the developed resist. Prior to the etching step, there is an opportunity to strip the developed resist and reprocess the substrate 220 by the lithography cluster. It is also known that the metrology results 242 from the device 240 can be used to maintain accurate performance of the patterning operation in the lithography cluster by making small adjustments over time by the monitoring system SCS and / or the control unit LACU 206, thereby minimizing the risk of the product being out of specification and requiring rework. Of course, the metrology device 240 and / or other metrology devices (not shown) can be used to measure the characteristics of the processed substrates 232, 234 and the incoming substrate 230.

[0036] Figure 2 (a) shows a measurement device. The independent measurement device 240 and / or the integrated measurement device 207 may include, for example, such a measurement device or any other suitable measurement device. Figure 2(b) illustrates in more detail the target T and the diffracted rays of the measurement radiation used to illuminate the target. The illustrated measurement device belongs to a type known as a dark field measurement device. The measurement device can be a stand-alone device or incorporated into, for example, a lithography device LA at a measurement station or a lithography unit LC. The optical axis with several branches in the entire device is represented by a dotted line O. In this device, the light emitted by a source 11 (e.g., a xenon lamp) is guided to a substrate W via a beam splitter 15 through an optical system including lenses 12, 14 and an objective lens 16. These lenses are arranged in a double sequence of a 4F arrangement. Different lens arrangements can be used as long as it still provides a substrate image to the detector and allows access to the intermediate pupil plane for spatial frequency filtering at the same time. By defining the spatial intensity distribution in a plane (referred to herein as a (conjugate) pupil plane) that presents the spatial spectrum of the substrate plane, the angular range of radiation incident on the substrate can be selected. For example, this can be performed by inserting an aperture plate 13 of a suitable form between lenses 12 and 14 in a plane that is a reverse projection image of the objective lens pupil plane. In the illustrated example, the aperture plate 13 has different forms, labeled 13N and 13S, allowing different illumination modes to be selected. The illumination system in 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 as 'North' for descriptive purposes only. In a second illumination mode, the aperture plate 13S is used to provide similar illumination, but from the opposite direction, labeled 'South'. Other illumination modes are possible by using different apertures. Ideally, the rest of the pupil plane is dark because any unwanted light outside the desired illumination mode is likely to interfere with the desired measurement signal.

[0037] like Figure 2 As shown in (b), the target T is placed together with the substrate W perpendicular to the optical axis O of the objective lens 16. The substrate W can be supported by a support (not shown). The rays of measurement radiation I that impinge on the target T at an angle offset from the axis O produce a zero-order ray (solid line 0) and two first-order rays (dotted chain line +1 and double-dotted chain line -1). It should be remembered that for the case of an overfilled small target T, these rays are only one of many parallel rays that cover the substrate area (including the metrology target T and other features). Due to the limited width of the hole in the plate 13 (which must allow a useful amount of light to enter), the incident ray I actually occupies a certain angle range, and the diffracted rays 0 and +1 / -1 will be slightly spread out. Each order +1 and -1 will further spread out within a certain angle range, rather than a single ideal ray as shown. It should be noted 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 closely aligned with the central optical axis. Figure 2 (a) and Figure 2(b) The illustrated rays are shown slightly off-axis purely to make them easier to distinguish in the figure.

[0038] At least one of the orders diffracted and reflected by the target T on the substrate W is collected by the objective lens 16 and directed back through the beam splitter 15. Figure 2 (a) illustrates both the first illumination mode and the second illumination mode by specifying diametrically opposed apertures labeled as north (N) and south (S). When the incident ray I of the measurement radiation comes from the north side of the optical axis, that is, when the first illumination mode is applied using aperture plate 13N, the +1 diffracted ray labeled as +1 (N) enters the objective lens 16. In contrast, when the second illumination mode is applied using aperture plate 13S, the -1 diffracted ray labeled as 1 (S) is the ray that enters the lens 16.

[0039] The second beam splitter 17 divides the diffracted beam into two measurement branches. In the first measurement branch, the optical system 18 uses the zero-order diffracted beam and the first-order diffracted beam to form a diffraction spectrum (pupil plane image) of the target on a first sensor 19 (e.g., a CCD sensor or a CMOS sensor). Each diffraction order hits 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 for focusing the metrology device and / or normalizing the intensity measurement of the first-order beam. The pupil plane image can also be used for many measurement purposes, such as reconstruction.

[0040] In the second measurement branch, the optical systems 20, 22 form an image of the target T on a sensor 23 (e.g., a CCD sensor or a CMOS sensor). In the second measurement branch, an aperture stop 21 is arranged in a plane conjugate to the pupil plane. The aperture stop 21 is used to block the zero-order diffraction beam so that the image of the target formed on the sensor 23 is formed only by the -1 order beam or the +1 order beam. The image captured by the sensors 19 and 23 is output to a processor PU that processes the image, the function of which depends on the specific type of measurement being performed. Note that the term 'image' is used in this article in a broad sense. Thus, if only one of the -1 order and the +1 order is present, an image of the grating lines itself is not formed.

[0041] Figure 2 The particular forms of aperture plate 13 and field stop 21 shown are purely exemplary. In another embodiment of the invention, coaxial illumination of the target is used and an aperture stop with an off-axis aperture is used to allow substantially only one first-order diffracted light to pass to the sensor. In still other embodiments, second-order beams, third-order beams and higher-order beams ( Figure 2 not shown).

[0042] In order to adapt the measurement radiation to these different types of measurements, the aperture plate 13 may include several patterns of holes formed around a disk that rotates to place the desired pattern in place. Note that aperture plates 13N or 13S can only be used to measure gratings oriented in one direction (X or Y, depending on the setup). For measuring orthogonal gratings, target rotations of 90° and 270° may be implemented.

[0043] The measurement device may allow the wavelength of the measurement radiation to be selected. The measurement radiation from the source 11 may be filtered so that a specific measurement can be performed. The wavelength may be adjusted once for each full substrate measurement. The optimal wavelength may depend on the layer. Typically, the measurement device includes a small number (e.g., about 7 to 10) of discrete filters to filter the measurement radiation, thereby enabling wavelength selection only from the same number of possible wavelengths. This limits performance. Additionally, the current switching times between the discrete wavelengths are long, so the current switching times can only be adjusted once for each total substrate measurement.

[0044] Therefore, it has been proposed to use one or more linear variable filters (LVFs) instead of discrete filters. An LVF is a filter having a spectral characteristic that varies substantially linearly with position in one (or more) directions along the filter. By moving the filter relative to the radiation beam (by physically moving the filter, the beam, or both) so that the position of the beam through the filter varies, the wavelength of the filtered beam can be varied linearly over a wide wavelength range (e.g., between 400 nm and 900 nm). The output radiation beam can be used, for example, Figure 2 (a) A measuring radiation beam of a metrology device such as the one shown.

[0045] The simplest implementation of LVF is a single adjustable bandpass filter, in which the output wavelength is selected only by the spatial position on the filter. In an embodiment, LVF can include such a single adjustable bandpass filter. However, a more flexible filter can be designed by combining two edge-pass LVFs, a long-wave pass LVF and a short-wave pass LVF, to create an adjustable passband. By moving the two filters together, the center wavelength can be continuously adjusted, and by moving the filters relative to each other, the bandwidth of the combined filter can also be tuned. Therefore, in other embodiments, LVF can include both a short-wave pass filter and a long-wave pass filter in series, thereby providing a bandpass filter that can be continuously tuned across a large center wavelength band (e.g., between 400nm and 900nm) and / or different passbands.

[0046] In diffraction-based overlay metrology, an asymmetry α in the radiation diffracted from the target is used to determine the overlay. For example, the asymmetry a can be obtained based on the difference between components A and B extracted from the diffracted radiation as follows:

[0047]

[0048] The different components A and B may, for example, correspond to the intensities of the +1st and -1st diffraction orders of the scattered radiation, as described above with reference to Figure 2 Typically, the components A and B and the resulting asymmetry a are acquired as an image to allow different regions on the target to be compared with each other. For example, the target may include regions where different pre-applied overlay deviations β are applied. In one embodiment, the asymmetry a is obtained from regions having equal and opposite deviations β, respectively. + and a - These asymmetries can be combined to obtain the quantity b as follows:

[0049]

[0050] Overlay can be derived from b with reduced effects from non-overlay target asymmetries (eg, sidewall angles of the etched structure).

[0051] like Figure 3 As schematically depicted, interference effects occur between radiation (solid arrows) reflected from different layers 51, 52 in the target (e.g., the layers between which the overlay is determined). The interference effect causes the asymmetry a to vary periodically depending on the wavelength of the incident radiation (which can be determined via the wave number denoted by , the wave number is the inverse of the wavelength). In a system such as Figure 3 In the simple case depicted, the measured asymmetry a can take the following wavenumbers And the form of the function of engraving o:

[0052]

[0053] Where K is the proportionality constant, is the phase angle (which does not depend on the overlay o but does depend on other target properties, such as the materials of the layers making up the target), and ΔL is the optical path length difference between the radiation reflected from the upper layer 52 and the radiation reflected from the lower layer 51 (e.g., the example +1 diffracted ray Figure 3 = ABD multiplied by the refractive index of the medium between the layers 51, 52). Thus, the intensity of the overlay signal at the measured asymmetry α is given by the quantity The periodic variation as a function of wave number (or wavelength) can therefore be called the sensitivity S with respect to overlay.

[0054] The sensitivity S is generally a function of the target (e.g., geometry and material) and the measurement conditions (e.g., radiation wavelength, radiation polarization, radiation incidence angle). The variation of the sensitivity S with wavelength may be referred to as a sensitivity curve. The sensitivity curve may be plotted in terms of wavelength or in terms of wave number (the inverse of the wavelength). Figure 4 An example of a sensitivity curve plotted against wavelength is schematically shown, as described below. The oscillation frequency of the sensitivity curve increases with the thickness of the stack. For some applications, such as 3D NAND applications, the distance between adjacent maxima can become less than 30nm, which is comparable to the bandwidth of filters used in some metrology devices.

[0055] In order to achieve high sensitivity S and avoid cancellation of positive and negative sensitivities S within a given asymmetry a measurement, a narrower bandwidth of incident radiation (e.g., less than 30 nm) can be generated and positioned near a peak (local minimum or local maximum) in the sensitivity curve. This can be done using a tuned bandpass filter (such as the LVF described above). Figure 5 An illumination system 120 for achieving this functionality is depicted, which includes a radiation source 11 and a bandpass filter 112. The bandpass filter 112 filters the broadband radiation from the radiation source 11 to provide a narrowband output radiation 130. Using such a filter 112 to create a smaller bandwidth can solve the cancellation problem. However, filtering reduces the amount of light available for measurement, which is proportional to the selected bandwidth. Reducing the amount of light available for measurement increases measurement time and reduces throughput.

[0056] The sensitivity curve may vary between different targets, for example, due to undesired variations in stack thickness across or between different substrates. A solution to this problem is to measure sequentially at multiple wavelengths using multiple corresponding settings of, for example, bandpass filters. The maximum signal or a weighted combination of multiple signals can be used to suppress the effects of stack thickness variations and determine the parameter of interest in a reliable manner. Figure 4 An example of this scheme is depicted for three wavelengths in Figure 4Typical sensitivity curves and positions of three measurements (represented by circles) are schematically shown. Herein, sensitivity S is defined as the relative change in the measurement signal corresponding to a fixed change in the measured parameter of interest. Measuring the asymmetry a using the larger two wavelengths results in a relatively high sensitivity, while a measurement using the lowest wavelength of the three wavelengths will achieve a relatively low sensitivity. Therefore, a combination of three measurements can be used to obtain high sensitivity (because two of the measurements have relatively high sensitivity, even if one of the measurements does not have a relatively high sensitivity). This scheme is robust to changes in the phase and / or frequency of the sensitivity curve, as long as the frequency of the sensitivity curve does not become so high that all three measurement wavelengths may be located near zero sensitivity S at the same time. However, the need to perform sequential measurements at different wavelengths requires additional measurement time and is not conducive to throughput.

[0057] The embodiments described below allow metrology measurements to be performed more efficiently at multiple wavelengths, thereby increasing measurement speed and throughput, particularly where the sensitivity curve changes rapidly as a function of wavelength.

[0058] Figure 6 A measurement device suitable for implementing the embodiments described below is schematically depicted. The measurement device is configured to measure a photolithography process (e.g., as described above with reference to Figure 1 and Figure 2 The measurement device includes an illumination system 120. An optical system 122 directs radiation (solid arrows) from the illumination system 120 to a target T formed on a substrate W. In one embodiment, the target T includes a multi-layer structure and the parameters include overlay errors between different layers of the multi-layer structure. The optical system 122 also directs radiation scattered from the target T to the detector 23. In one embodiment, the optical system 122 uses Figure 2 (a) is implemented by one or more components of the measurement device depicted in (a), including, for example, one or more of lenses 12, 14 and 16, aperture plate 13, beam splitter 15, a first measurement branch (including one or more elements of optical system 18), and a second measurement branch (which includes one or more elements of optical systems 20, 22, and aperture stop 21). In one embodiment, detector 23 is implemented as described above with respect to the second measurement branch and includes a CCD sensor or a CMOS sensor. In one embodiment, optical system 122 is configured to form one or more images of target T on sensor 23.

[0059] The irradiation system 120 comprises a radiation source 11 and a beam modification device 30 (see Figure 7 and Figure 8). The beam modification device 30 modifies the radiation from the radiation source 11 and provides the modified radiation 32 to the optical system 122. The beam modification device 30 modifies the wavelength spectrum of the radiation from the radiation source 11 to have at least one local minimum and two local maxima at the same time (wherein one of the local maxima may also be a global maximum). The radiation wavelength spectrum defines the amount of radiation (e.g., power or intensity) present according to the wavelength. The distribution of power over the wavelength spectrum can be described with reference to the power spectral density, which describes how much power exists in each wavelength interval. At least one local minimum and two local maxima represent significant features in the wavelength spectrum rather than secondary features such as ripples or noise. In one embodiment, the modified wavelength spectrum includes a local minimum between the global maximum and the local maximum, wherein the power spectral density of the radiation at the local minimum is less than 20% (optionally less than 15%, optionally less than 10%, optionally less than 5%) of the power spectral density of the radiation at the global maximum, and the power spectral density of the radiation at the local maximum is at least 50% (optionally at least 60%, optionally at least 70%, optionally at least 80%, optionally at least 90%) of the power spectral density of the radiation at the global maximum. By modifying the wavelength spectrum in this way to have at least two maxima at the same time, the beam modification device 30 makes it possible to irradiate the target T simultaneously using two (or more, depending on the number of maxima provided) radiation bands centered at different wavelengths. As with the above reference Figure 4 In contrast to the described sequential approach of applying radiation of different wavelengths in succession, the beam modification device 30 makes it possible to apply multiple narrowband radiations simultaneously. This approach improves the use of the radiation provided by the radiation source 11 (by discarding a smaller proportion of the radiation) and also allows measurements to be performed in parallel. The above-referenced measurements can be performed, for example, in a single measurement (i.e., simultaneously). Figure 4 A sequence of three measurements at different wavelengths is described. Hence, the beam modification device 30 enables improved measurement speed and throughput.

[0060] In one embodiment, Figure 7 As depicted, the illumination system 120 is configured such that the beam modification device 30 receives broadband radiation (eg, without filtering from the broadband radiation source 11) as the incoming radiation beam 31. In another embodiment, as Figure 8 As depicted, the illumination system 120 is configured such that the beam modification device 30 receives an incoming radiation beam 31 that has been filtered by a filter 112, such as a bandpass filter. Figure 8 In embodiments of the type depicted in FIG. 1 , the bandwidth of filter 112 may be greater than the bandwidth associated with an individual minimum or individual maximum introduced by beam modification device 30 (e.g., as described below). Fig.10 as depicted in ).

[0061] In one embodiment, the beam modification device 30 comprises a tunable multi-band filter that modifies the wavelength spectrum using the interferometric principle (e.g., by arranging interferometric resonances to produce desired maxima and minima). In an example of such an embodiment, Fig. 9 Schematically depicted, the beam modification device 30 comprises a Fabry-Perot interferometer (i.e., an interferometer configured to operate based on the principle of Fabry-Perot interferometer). The Fabry-Perot interferometer filters the input radiation in a manner that produces a comb-like wavelength spectrum, in which local maxima 34 and local minima 36 are regularly spaced. According to the principle of Fabry-Perot interferometer, the Fabry-Perot interferometer comprises two parallel partially reflecting surfaces 38. The transmission wavelength spectrum comprises a peak (local maximum 34) corresponding to the resonance of the space between the partially reflecting surfaces 38. Fig.10 Depicted Fig. 9 1. An example of an output radiation beam 32 of the beam modification device 30. The illumination system 120 in this embodiment is configured as Figure 8 As shown. Through the filter 112 to Fig. 9 The incoming radiation beam 31 of the Fabry-Perot interferometer is bandpass filtered to have a Fig.10 The outgoing radiation beam 32 has a comb-like wavelength spectrum superimposed on the wider wavelength spectrum of the incoming radiation beam 31. Fig.10 In the particular example depicted, the beam modification device 30 thus modifies the wavelength spectrum of the radiation from the radiation source 11 in such a way that it has five local maxima 34 (one of which is a global maximum) and four local minima 36 .

[0062] The spacing (in wavenumber space) between local maxima 34 provided by the Fabry-Perot interferometer It is given by the following expression:

[0063]

[0064] Here, h is the distance between the partially reflecting surfaces 38 .

[0065] In one embodiment, the modified wavelength spectrum can be made to more closely match or follow the sensitivity curve than if the wavelength spectrum were not modified. Matching the wavelength spectrum to the sensitivity curve in this way improves the overall sensitivity, thereby improving accuracy and / or allowing measurements to be performed faster and / or using simpler equipment. In one embodiment, the local maximum 34 and the global maximum 34 located on either side and directly adjacent to the local minimum, and / or at least two other local maxima 34 are matched with the position of the local maximum or local minimum plotted according to the wave number (e.g., so that all relevant maxima in the modified wavelength spectrum are aligned with the local maximum in the sensitivity curve or aligned with the local minimum in the sensitivity curve), and the degree is within 25% of the average interval between local maxima or the average interval between local minima in the sensitivity curve plotted according to the wave number, optionally within 15%, optionally within 10%, optionally within 5%, optionally within 1%. The sensitivity curve can be derived by performing calibration measurements (e.g., by performing measurements at multiple wavelengths and calculating the sensitivity at each wavelength) and / or by mathematical modeling.

[0066] In one embodiment, the wavelength spectrum is modified so that when the wavelength spectrum is plotted as a function of wavenumber, two or more pairs of adjacent local maxima 34 (optionally, including a global maximum) or adjacent local minima 36 are uniformly spaced (e.g., so that the wavelength spectrum is at least locally periodic or pseudo-periodic). In such an embodiment, the spacing between the uniformly spaced maxima (i.e., the period or unit cell of the wavelength spectrum plotted as a function of wavenumber) is tuned to match the average spacing (averaged over the spectral range of the wavelength spectrum) between local maxima in the sensitivity curve plotted as a function of wavenumber or between local minima in the sensitivity curve plotted as a function of wavenumber to within 25%, optionally within 15%, optionally within 10%, optionally within 5%, optionally within 1% of the average spacing between local maxima in the sensitivity curve. In using the method described above with reference to Fig. 9 In the embodiment of the Fabry-Perot interferometer described, this matching can be achieved by tuning the Fabry-Perot interferometer so that h = ΔL / 2. Matching the period in this way will result in a higher sensitivity to overlay at path length differences ΔL, unless the phase is approximately In one embodiment, the Fabry-Perot interferometer is implemented using electrostatic actuation to tune the position of the reflective film that provides the partially reflective surface 38. Configuring the Fabry-Perot interferometer to be adjustable so that h can be varied between 0 microns and about 50 microns (optionally, 0 microns and about 10 microns) covers a wide range of possible applications. In applications where the sensitivity curve is more complex and / or the effects of multiple different parameters (e.g., different overlay parameters) need to be suppressed, more complex modifications to the wavelength spectrum may be required, including modifications where the correlation between the expected positions of maxima and minima in the modified wavelength spectrum and the positions of the maxima or minima in the sensitivity curve is less direct.

[0067] In one embodiment, the phase of the periodic variation corresponding to uniformly spaced local maxima or uniformly spaced local minima in the modified wavelength spectrum plotted against wavenumber is tuned to be in phase with the sensitivity curve plotted against wavenumber (e.g., with the local maxima or local minima, respectively, aligned along a horizontal axis representing wavenumber) to within 25%, optionally within 15%, optionally within 10%, optionally within 5%, optionally within 1% of the average spacing between local maxima in the sensitivity curve plotted against wavenumber (e.g., so that the local maximum in the periodic variation applied by the beam modification device 30 occurs at a wavelength or wavenumber close to or the same as a maximum in the sensitivity curve or a minimum in the sensitivity curve). This is in Fig.11 The short dashed curve depicts the expected sensitivity curve plotted as a function of wavelength in arbitrary units along the vertical axis (the plot as a function of wave number looks similar, but the peak spacing is more regular). The long dashed curve depicts the periodically varying wavelength spectrum produced by the beam modification device 30 in arbitrary units along the vertical axis (which follows Fig.10 ), where the period and phase are tuned so that local maxima in the periodically varying wavelength spectrum correspond to local maxima in the sensitivity curve, and local minima in the periodically varying wavelength spectrum correspond to local minima in the sensitivity curve. The solid line curve shows the product of the periodically varying wavelength spectrum and the sensitivity curve in arbitrary units along the vertical axis, the sensitivity curve being related to the overall sensitivity of the process to the parameter of interest and remaining predominantly positive over the wavelength range considered. In an alternative arrangement, the local maxima in the modified wavelength spectrum may be aligned with the local minima in the sensitivity curve, in which case the solid line curve may be predominantly negative, and similar benefits in terms of increased sensitivity may be obtained.

[0068] Fig.11 The integral below the solid line is equal to approximately 1 / 6 of the bandwidth of filter 112 (similar to Fig.10Since this scheme allows a wider range of settings for the filter 112 without losing signals due to cancellation, the bandwidth of the filter 112 can be set higher (e.g., higher than 6 times the width of a single peak in the sensitivity curve), thereby supporting a reduction in measurement time and an increase in throughput.

[0069] In an embodiment using a Fabry-Perot interferometer, the phase can be adjusted so that the maximum j occurs at the wavenumber:

[0070]

[0071] where j is an integer and f is a number between 0 and 1. This can be achieved, for example, using an appropriate multilayer coating to achieve a partially reflective surface 38 that produces a non-zero f. For example, if a particular overlay parameter is If the sensitivity curve defined is associated, the overall sensitivity can be enhanced by using a partially reflective surface with f = 0.25. In this specific case, a coating can be applied to achieve a partially reflective surface 38 with a phase shift of λ / 8 per reflection over the entire wavelength range. In one embodiment, a chirped dielectric mirror is used for this purpose.

[0072] In an alternative embodiment, the beam modification device 30 uses an acousto-optic tunable filter to modify the radiation wavelength spectrum. Such a filter may also be referred to as a Bragg cell, which uses the acousto-optic effect to diffract radiation passing through it by using acoustic waves. By controlling the frequency of the applied acoustic waves, it is possible to provide a periodic output wavelength spectrum with an adjustable period and an adjustable phase (similar to the spectrum described above with reference to the Fabry-Perot interferometer).

[0073] In one embodiment, the method is repeated to obtain the parameters of the lithography process multiple times using one or both of different targets and different substrates, and the spectrum is modified in different ways to match the different corresponding sensitivity curves of each measurement in at least a subset of the multiple measurements. Thus, as the process changes between different substrates or at different locations on the substrate, the method can maintain a high overall sensitivity by following the changes in the sensitivity curve.

[0074] In another alternative, if Fig.12 and Fig.13As depicted, the beam modification device 30 operates by directing different wavelength components of the radiation to be modified (e.g., using appropriate optical elements, such as prisms or gratings) to different locations and modulating the radiation in different ways at least in a subset of different corresponding locations to perform the modification of the wavelength spectrum. In one embodiment, the different locations correspond to different corresponding regions of a spatial light modulator (which may also be referred to as an SLM) 53. In such an embodiment, the SLM 53 then modulates the radiation in different ways at least in a subset of different corresponding regions to perform the modification of the wavelength spectrum. Thus, in order to produce a beam such as Fig.10 The wavelength spectrum depicted, for example, SLM 53 can be programmed to pass radiation in the region of SLM 53 that receives the wavelength component corresponding to maximum value 34 and is programmed to remove radiation in the region of SLM 53 that receives the wavelength component corresponding to minimum value 36. SLM 53 can define both the period and phase of the wavelength spectrum. SLM 53 can be implemented in various ways. In one embodiment, SLM 53 is configured to modify the polarization of radiation passing through SLM 53 and cooperate with a polarizer to selectively pass or block radiation from different regions of SLM 53. In one embodiment, SLM 53 includes a liquid crystal (LC) mask, a ferroelectric mask, or a tiltable mirror array. In one embodiment, SLM 53 includes a hard mask (e.g., a lightweight metal sheet with an aperture)), which may be mechanically switched faster than a heavier glass substrate filter.

[0075] Fig.12An example beam modifying device 30 of this type is shown. The beam modifying device 30 comprises a first grating 51, a first cylindrical lens 52, an SLM 53, a second cylindrical lens 54 and a second grating 55. The rays in the incoming radiation beam 31 to the beam modifying device 30 can be characterized by their x, y coordinates. The incoming radiation beam 31 is diffracted by the first grating 51. In the embodiment shown, the first grating 51 and the second grating 55 are transmissive, but equivalent configurations can be conceived using reflective gratings without departing from the basic principles. Each wavelength component in the incoming radiation beam 31 is diffracted into a different direction. A representative long wavelength component is labeled 61 and a representative short wavelength component is labeled 62, but in practice there may be a continuous range of wavelength components. The diffracted wavelength components are focused by the first cylindrical lens 52 to the line focal points 43, 44 on the SLM 53. The long wavelength component 61 is focused to the line focal point 43 on the SLM 53. The short wavelength component 62 is focused to the line focal point 44 on the SLM 53. The SLM 53 may alter a property of the transmitted light depending on an x′, y′ coordinate on the SLM 53. In the illustrated configuration of the beam modifying device 30, the x′ coordinate is wavelength dependent (due to the periodicity of the wavelength dependent diffraction spread provided by the first grating 51 being parallel to the x′ direction, with the grating lines extending parallel to the y′ direction), and the y′ coordinate matches the y coordinate in the incident radiation beam 31. In one embodiment, the property of the transmitted radiation altered by the SLM 53 is polarization. Thus, in combination with a polarizer (which may be immediately downstream of the SLM 53 or elsewhere downstream), the SLM 53 may effectively operate as a position dependent attenuator. Thus, the SLM 53 may attenuate radiation depending on x′, y′ or equivalently λ, y′, thereby providing the selective passage or removal of radiation referred to above. The radiation output from the SLM 53 passes through a second cylindrical lens 54 and a second grating 55, which combine the wavelength components into a single collimated outgoing radiation beam 32 having ray coordinates x″ and y″. If the incoming radiation beam 31 has an irradiance distribution I(x, y, λ), where λ is the wavelength, then the irradiance distribution of the outgoing radiation beam 32 is given by the following expression:

[0076] I″(x″,y″,λ)=I(x″,y″,λ)T(y′,λ),

[0077] where T(y′, λ) is a transfer function that can be chosen freely (although usually 0 < T < 1). Similar functionality can be achieved by folding Fig.12 The configuration can be implemented so that there is only one lens and a single grating. Alternatively, the configuration can be adjusted so that the radiation is reflected from the grating two or four times. Fig.12 In the configuration shown, distances 51 - 52 , 52 - 53 , 53 - 54 , and 54 - 55 are all equal to the focal lengths of cylindrical lenses 52 and 54 .

[0078] Fig.12 Embodiments of the type depicted allow for flexible and efficient modification of the wavelength spectrum of the radiation for a given angle of incidence of the radiation onto the target T. Additionally, the geometry allows for modification of the wavelength spectrum to vary as a function of the angle of incidence. This is desirable because for many applications, the sensitivity curve will also vary as a function of the angle of incidence (e.g., changing frequency and / or phase as a function of the angle of incidence). In one embodiment, improved overall measurement sensitivity is achieved by causing the variation of the wavelength spectrum with angle of incidence to more closely follow the expected variation of the sensitivity curve with angle of incidence (e.g., by matching period and / or phase) relative to the case where a constant wavelength spectrum is applied for all angles of incidence.

[0079] Fig.12 and Fig.13 It is shown how a modification of the wavelength spectrum as a function of the angle of incidence can be achieved using an SLM 53. In an embodiment of this type, the modification is applied by the SLM 53 modulating the radiation in different ways in parts of the SLM 53 which contribute to different corresponding angles of incidence of the radiation onto the target T. In the geometry shown, these different parts of the SLM 53 correspond to different positions along the y′ direction. The different positions along the y′ direction correspond to different positions along the y″ direction in the outgoing radiation beam 32. Since the wavelength spectrum can be made dependent on the y″ coordinate, it also depends on the angle of incidence θ on the target. This is in the Fig.13 In the figure, the Fig.13 The interaction between the beam modifying device 30 and selected elements of the optical system 122 (the beam splitter 15 and the objective lens 16) is shown. An example ray of the outgoing radiation beam 32 at the y″ coordinate y 1 ″ and y 2 ″. The optical axis is indicated by a dotted line. The light is redirected by a beam splitter 15 and then focused onto a target T. The scattered radiation passes through the objective lens 16 and the beam splitter 15 to a detector (not shown). The angle of incidence depends on the y″ coordinate: 1 The light ray of θ 1 The incident 2 The light ray of θ 2 Incident.

[0080] Fig.12 and Fig.13The arrangement shown is an example in which an optical element (first grating 51) diffuses different wavelength components across the SLM 53 in parallel only to the first axis (parallel to the x' direction) so that each elongated region (strip) of the SLM 53 orthogonal to the first axis receives a different wavelength component. The optical system 122 is configured so that the radiation received by the SLM 53 at different positions along the second axis (parallel to the y' direction) of the SLM 53 contributes to different corresponding angles of incidence on the target T. Modulating the radiation in different forms in the portions of the SLM 53 that contribute to different corresponding angles of incidence of the radiation on the target T includes modulating the radiation in a non-uniform manner depending on the position along the second axis. In the example shown, the first axis is orthogonal to the second axis, but other arrangements are possible. For example, the first axis may be oriented at an oblique angle relative to the second axis.

[0081] The wavelength spectrum applied at a given angle of incidence can therefore be defined by appropriate actuation of elongated regions (strips) of the SLM 53 that are parallel to x' and pass through the y' coordinate corresponding to the given angle of incidence. Different spectra can be applied to radiation incident on the target at different angles of incidence by actuating corresponding strips passing through different y' coordinates. Matching the spectrum to the expected changes in the sensitivity curve at different angles of incidence improves the overall sensitivity.

[0082] Embodiments of the present disclosure may be applied to situations where it is desired to obtain multiple overlay parameters (e.g., representing offsets between different combinations of layers in the target, respectively). In such situations, different overlay parameters may be expected to have different sensitivity curves, and multiple measurements may be performed using the radiation wavelength spectrum modified independently each time to be optimal for the particular overlay parameter being obtained and / or to reduce sensitivity to other layer pairs and / or offending asymmetric characteristics (e.g., sidewall angle) parameters. For example, where it is desired to obtain N overlay parameters from the target, M (e.g., ) times. In such an embodiment, analysis can be performed based on the asymmetry a without applying a bias in advance, so that the overlay parameter between the reference layer 1 and the other layer j can be obtained according to the following equation:

[0083]

[0084] Where Q is a sparse N×M matrix.

[0085] In which the beam modification device uses a Fabry-Perot interferometer principle (such as Fig. 9 In an embodiment of a tunable multi-band filter (depicted), the partially reflective surface 38 may be configured (eg, via appropriate selection of a coating) to transmit a wavelength spectrum that is The basic Fourier transform components of are maximized, for example, maximizing the following formula:

[0086]

[0087] Although the above may have referenced the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the 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, the topography in the patterning device defines the pattern created on the substrate. The topography of the patterning device may be pressed into a layer of resist supplied to the substrate, after which the resist is cured by applying electromagnetic radiation, heat, pressure, or a combination thereof. After the resist is cured, the patterning device removes the resist, leaving a pattern therein.

[0088] Further embodiments according to the invention are presented in the following numbered clauses:

[0089] 1. A method for measuring parameters of a photolithography process, comprising:

[0090] modifying radiation from a radiation source and irradiating a target formed on a substrate using a photolithography process with the modified radiation; and

[0091] detecting radiation scattered from the target and analyzing the detected radiation to determine a parameter, wherein

[0092] The modification of the radiation includes: modifying the radiation wavelength spectrum to have a local minimum between a global maximum and a local maximum, wherein the power spectral density of the radiation at the local minimum is less than 20% of the power spectral density of the radiation at the global maximum, and the power spectral density of the radiation at the local maximum is at least 50% of the power spectral density of the radiation at the global maximum.

[0093] 2. The method according to clause 1, wherein

[0094] The sensitivity curve defines the variation in sensitivity of the process for determining the parameter as a function of the wavelength of radiation scattered from the target; and

[0095] The modified wavelength spectrum more closely matches the sensitivity curve relative to the case where the wavelength spectrum has not been modified, so that the overall sensitivity of the process of determining parameters by analyzing the detected radiation is improved.

[0096] 3. A method according to claim 2, wherein the positions of the local maximum and the global maximum, or the positions of at least two other local maxima of the modified wavelength spectrum, match the positions of the local minimum or local maximum in the sensitivity curve plotted according to the wave number, and the degree of matching is within 25% of the average interval between local maxima or the average interval between local minima in the sensitivity curve plotted according to the wave number.

[0097] 4. A method according to clause 2 or 3, wherein the wavelength spectrum is modified such that when the wavelength spectrum is plotted as a function of wave number, two or more pairs of adjacent local maxima are evenly spaced.

[0098] 5. A method according to claim 4, wherein the intervals between uniformly spaced local maxima are tuned to match the intervals between local maxima or local minima in a sensitivity curve plotted according to wavenumber, and the degree of matching is within 25% of the average interval between local maxima or the average interval between local minima in the sensitivity curve plotted according to wavenumber.

[0099] 6. A method according to claim 5, wherein the phase of the periodic variation corresponding to uniformly spaced local maxima in the modified wavelength spectrum plotted according to wavenumber is tuned to be in phase with the sensitivity curve plotted according to wavenumber, and the degree of in-phase is within 25% of the average interval between local maxima or the average interval between local minima in the sensitivity curve plotted according to wavenumber.

[0100] 7. A method according to any one of clauses 2 to 6, wherein a parameter of a lithography process is measured multiple times using one or both of different targets and different substrates, and the spectrum is modified in different ways to match different corresponding sensitivity curves for each measurement of at least a subset of the measurements of the parameter of the lithography process.

[0101] 8. A method according to any preceding clause, wherein the wavelength spectrum is modified using an interferometry-based tunable multi-band filter.

[0102] 9. The method of clause 8, wherein the tunable multi-band filter comprises a Fabry-Perot interferometer.

[0103] 10. A method according to any preceding clause, wherein the wavelength spectrum is modified using an acousto-optic tunable filter.

[0104] 11. A method according to any preceding clause, wherein

[0105] Different wavelength components of the radiation to be modified are directed to different respective locations; and

[0106] The radiation is modulated differently in at least a subset of different corresponding positions to perform a modification of the wavelength spectrum.

[0107] 12. A method according to any preceding clause, wherein

[0108] Different wavelength components of the radiation to be modified are directed to different corresponding regions of the spatial light modulator; and

[0109] The spatial light modulator modulates the radiation differently in at least different subsets of corresponding regions to perform a modification of the wavelength spectrum.

[0110] 13. A method according to any preceding clause, wherein modifying the radiation further comprises modifying the wavelength spectrum to vary as a function of the angle of incidence of the radiation onto the target.

[0111] 14. The method according to clause 13, wherein

[0112] The sensitivity curve defines the variation in sensitivity of the process for determining the parameter as a function of the wavelength of radiation scattered from the target; and

[0113] Relative to applying a constant wavelength spectrum for all incident angles, the variation of the wavelength spectrum as a function of incident angle more closely matches the variation of the sensitivity curve as a function of incident angle, so that the overall sensitivity of the process of determining parameters by analyzing the detected radiation is improved.

[0114] 15. A method according to clause 14, wherein the variation of the wavelength spectrum as a function of the angle of incidence is applied using a spatial light modulator.

[0115] 16. A method according to clause 15, wherein the spatial light modulator modulates radiation differently in portions of the spatial light modulator that contribute to different respective angles of incidence.

[0116] 17. A method according to any preceding clause, wherein the target comprises a multi-layer structure and the parameter comprises an overlay error between different layers of the multi-layer structure.

[0117] 18. A measurement device for measuring parameters of a photolithography process, comprising:

[0118] an optical system configured to direct radiation from a radiation source onto a target formed on a substrate; and

[0119] A beam modification device configured to modify radiation received from a radiation source and to provide the modified radiation to an optical system, wherein

[0120] The beam modification device is configured to modify the wavelength spectrum of radiation from the radiation source to have a local minimum between a global maximum and a local maximum, wherein the power spectral density of the radiation at the local minimum is less than 20% of the power spectral density of the radiation at the global maximum, and the power spectral density of the radiation at the local maximum is at least 50% of the power spectral density of the radiation at the global maximum.

[0121] 19. Apparatus according to clause 18, wherein the beam modification device is further configured to modify the wavelength spectrum to vary as a function of the angle of incidence onto the target.

[0122] 20. Apparatus according to any of clauses 18 to 19, wherein the beam modification device comprises one or more of: a tunable multi-band filter, a Fabry-Perot interferometer, an acousto-optic tunable filter.

[0123] 21. Apparatus according to any of clauses 18 to 20, wherein the beam modification device comprises:

[0124] Optical components; and

[0125] A spatial light modulator, wherein

[0126] The optical element is configured to direct different wavelength components of the radiation to be modified onto different corresponding regions of the spatial light modulator; and

[0127] The spatial light modulator is configured to modulate the radiation differently in at least different subsets of the respective regions, thereby modifying the wavelength spectrum.

[0128] 22. The apparatus of clause 21, wherein the spatial light modulator is further configured to modulate radiation differently in portions of the spatial light modulator that contribute to different respective angles of incidence onto the target.

[0129] 23. The device according to clause 22, wherein

[0130] the optical element being configured to diffuse different wavelength components across the spatial light modulator only parallel to the first axis such that each elongated region of the spatial light modulator orthogonal to the first axis receives a different wavelength component; and

[0131] the optical system being configured such that radiation received by the spatial light modulator at different positions along a second axis of the spatial light modulator contributes to different respective angles of incidence onto the target; and

[0132] Modulating the radiation differently in portions of the spatial light modulator that contribute to different respective angles of incidence of the radiation onto the target includes modulating the radiation in a non-uniform manner as a function of position along the second axis.

[0133] 24. The device of clause 23, wherein the first axis is orthogonal to the second axis.

[0134] The terms "radiation" and "beam" as used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of or about 365 nm, 355 nm, 248 nm, 193 nm, 157 nm, or 126 nm) and extreme ultraviolet (EUV) radiation (e.g., having a wavelength in the range of 5 nm to 20 nm), as well as particle beams (e.g., ion beams or electron beams).

[0135] The term "lens," where the context permits, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic, and electrostatic optical components.

[0136] The foregoing description of specific embodiments will reveal the general nature of the invention so fully that others can easily modify and / or adjust these specific embodiments for various applications without undue experimentation by applying the knowledge of those skilled in the art without departing from the general concept of the invention. Therefore, based on the teachings and guidance set forth herein, such adjustments and modifications are intended to fall within the meaning and range of equivalents of the disclosed embodiments. It should be understood that the purpose of the terms or phrases herein is to illustrate by way of example and not by way of limitation, so that the terms or phrases of this specification are to be interpreted by those skilled in the art in accordance with the teachings and guidance.

[0137] 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 method for measuring parameters of a photolithography process, comprising: modifying radiation from a radiation source and irradiating a target formed on a substrate using the photolithography process with the modified radiation; as well as detecting radiation scattered from the target and analyzing the detected radiation to determine the parameter, wherein: The modification of the radiation includes: modifying the wavelength spectrum of the radiation to have a local minimum between a global maximum and a local maximum, wherein the power spectral density of the radiation at the local minimum is less than 20% of the power spectral density of the radiation at the global maximum, and the power spectral density of the radiation at the local maximum is at least 50% of the power spectral density of the radiation at the global maximum.

2. The method according to claim 1, wherein: a sensitivity curve defining a change in sensitivity of a process for determining said parameter as a function of the wavelength of radiation scattered from said target; as well as The modified wavelength spectrum more closely matches the sensitivity curve relative to a situation where the wavelength spectrum is not modified, so that the overall sensitivity of the process of determining the parameter by analyzing the detected radiation is improved.

3. A method according to claim 2, wherein the positions of the local maximum and the global maximum or the positions of at least two other local maxima of the modified wavelength spectrum match the positions of the local minimum or local maximum in the sensitivity curve plotted according to wavenumber, and the degree of matching is within 25% of the average interval between local maxima or the average interval between local minima in the sensitivity curve plotted according to wavenumber.

4. A method according to claim 2 or 3, wherein when the wavelength spectrum is plotted as a function of wave number, the wavelength spectrum is modified so that two or more pairs of adjacent local maxima are evenly spaced.

5. A method according to claim 4, wherein the intervals between the uniformly spaced local maxima are tuned to match the intervals between local maxima or local minima in the sensitivity curve plotted according to wavenumber, and the degree of matching is within 25% of the average interval between local maxima or the average interval between local minima in the sensitivity curve plotted according to wavenumber.

6. A method according to claim 5, wherein the phase of the periodic variation corresponding to the uniformly spaced local maxima in the modified wavelength spectrum plotted according to wavenumber is tuned to be in phase with the sensitivity curve plotted according to wavenumber, and the degree of in-phase is within 25% of the average interval between local maxima or the average interval between local minima in the sensitivity curve plotted according to wavenumber.

7. A method according to any one of claims 2, 3, 5 and 6, wherein the parameter of the lithography process is measured multiple times using one or both of different targets and different substrates, and the wavelength spectrum is modified in different ways to match different corresponding sensitivity curves for each measurement of at least a subset of the measurements of the parameter of the lithography process.

8. The method according to any one of claims 2, 3, 5 and 6, wherein the wavelength spectrum is modified using an interferometry based tunable multi-band filter.

9. The method of claim 8, wherein the tunable multi-band filter comprises a Fabry-Perot interferometer.

10. The method of any one of claims 2, 3, 5, 6 and 9, wherein the wavelength spectrum is modified using an acousto-optic tunable filter.

11. The method according to any one of claims 2, 3, 5, 6 and 9, wherein: Different wavelength components of the radiation to be modified are directed to different corresponding locations; as well as The radiation is modulated differently in at least a subset of the different respective locations to perform the modification of the wavelength spectrum.

12. The method according to any one of claims 2, 3, 5, 6 and 9, wherein: Different wavelength components of the radiation to be modified are directed to different corresponding regions of the spatial light modulator; as well as The spatial light modulator modulates radiation differently in at least subsets of the different corresponding regions to perform the modification of the wavelength spectrum.

13. The method of any one of claims 2, 3, 5, 6, and 9, wherein modifying the radiation further comprises: The wavelength spectrum is modified to vary as a function of the angle of incidence of the radiation onto the target.

14. The method according to claim 13, wherein: a sensitivity curve defining a change in sensitivity of a process for determining said parameter as a function of the wavelength of radiation scattered from said target; as well as The variation in the wavelength spectrum as a function of incident angle more closely matches the variation in the sensitivity curve as a function of incident angle relative to the case where a constant wavelength spectrum is applied for all incident angles, thereby improving the overall sensitivity of the process of determining the parameter by analyzing the detected radiation.

15. The method of claim 14, wherein the variation of the wavelength spectrum as a function of incident angle is applied using a spatial light modulator.

Images