Measuring device and testing equipment

Abstract

A detection device for a measurement device is disclosed, the measurement device being operable to measure a parameter of interest based on scattered radiation that has been scattered from a sample. The detection device comprises a detector, the detector comprising a pixel array. The pixel array comprises: imaging pixels, the imaging pixels being used to detect an image, the parameter of interest being determined based on the image; and direction detection pixels, the direction detection pixels being used to detect an angle of incidence of the scattered radiation on the detector.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to European application 19192986.8 filed on August 22, 2019, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The invention relates to a metrology device or inspection device for determining properties of a structure on a substrate. The invention also relates to a test device for such a metrology device. Background Art

[0004] A lithographic apparatus is a machine configured to apply a desired pattern to a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus can, for example, project a pattern (also commonly referred to as a "design layout" or "design") at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) disposed on a substrate (e.g., a wafer).

[0005] In order to project a pattern onto a substrate, a lithographic apparatus can use electromagnetic radiation. The wavelength of this radiation determines the minimum size of a feature that can be formed on a substrate. Typical wavelengths currently used are 365nm (i-line), 248nm, 193nm, and 13.5nm. Compared to a lithographic apparatus using radiation with a wavelength of, for example, 193nm, a lithographic apparatus using extreme ultraviolet (EUV) radiation with a wavelength in the range of 4nm to 20nm (e.g., 6.7nm or 13.5nm) can be used to form smaller features on a substrate.

[0006] Low k1 lithography can be used to process features with sizes smaller than the classical resolution limit of the lithographic apparatus. In such a process, the resolution formula can be expressed as CD=k1×λ / NA, where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the "critical dimension" (usually the smallest feature size printed, but in this case it is the half pitch), and k1 is an empirical resolution factor. In general, the smaller the k1, the more difficult it is to reproduce on the substrate a pattern of a shape and size similar to that planned by the circuit designer to achieve specific electrical functionality and performance. To overcome these difficulties, complex fine-tuning steps can be applied to the lithographic projection apparatus and / or the design layout. These steps include, for example (but not limited to), optimization of NA, custom illumination schemes, use of phase-shifted patterning devices, various optimizations of the design layout (such as optical proximity correction (OPC, sometimes also referred to as "optical and process correction") of the design layout), or other methods generally defined as "resolution enhancement technology" (RET). Alternatively, a strict control loop for controlling the stability of the lithographic apparatus can be used to improve the reproduction of patterns at low k1.

[0007] During lithography, it is desirable to frequently measure the resulting structure, for example for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes or various forms of metrology equipment, such as scatterometers. A general term used to refer to such tools may be metrology equipment or inspection equipment.

[0008] The measurement device can use the phase obtained in a computational manner to apply aberration correction to an image captured by the measurement device. One described method for phase calculation uses multiple different images, such as multiple images of the same target under different focusing conditions. This typically requires mechanical movement of the sample (e.g., the target) and multiple image captures with a high time cost. In addition, it is assumed that moving the sample through the focus is equivalent to moving the detector through the focus. The validity of this assumption is questionable. In addition, the defocus range for which this assumption is valid may be too short to obtain sufficient information for successful reconstruction. Summary of the invention

[0009] It is desirable to reduce acquisition time and increase throughput when performing phase acquisition in metrology applications.

[0010] Embodiments of the invention are disclosed in the claims and in the detailed description.

[0011] In a first aspect of the present invention, a detection device for a measuring device is provided, the measuring device being operable to measure a parameter of interest based on scattered radiation that has been scattered from a sample; the detection device comprising: a detector, the detector comprising a pixel array, the pixel array comprising: imaging pixels, the imaging pixels being used to detect an image, the parameter of interest being determined based on the image; and direction detection pixels, the direction detection pixels being used to detect an angle of incidence of the scattered radiation on the detector.

[0012] In a second aspect of the present invention, a measuring device is provided, wherein the measuring device comprises the detection device according to the first aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0014] - Figure 1 depicting a schematic overview of a lithographic apparatus;

[0015] - Figure 2 depicts a schematic overview of a lithography cell;

[0016] - Figure 3 depicts a schematic representation of overall lithography showing the collaboration between three key technologies for optimizing semiconductor manufacturing;

[0017] - Figure 4 is a schematic illustration of a scatterometry device;

[0018] - Figure 5 illustrates a detection arrangement with direction detection according to an embodiment of the present invention;

[0019] - Figure 6 FIG. 1 illustrates a direction detection pixel set according to an embodiment of the present invention;

[0020] - Figure 7 is a graph of intensity versus angle of incidence for a set of three adjacent direction detection pixels, the graph illustrating how direction may be inferred from sensed intensity values ​​in a method according to an embodiment of the invention;

[0021] - Figure 8 A detection arrangement with a direction detection function according to another embodiment of the present invention is illustrated; and

[0022] - Fig. 9 is a block diagram illustrating a computer system that can assist in implementing a method according to an embodiment of the present invention. DETAILED DESCRIPTION

[0023] In this document, the terms "radiation" and "beam" are used to cover all types of electromagnetic radiation, including ultraviolet radiation (e.g., having a wavelength of 365nm, 248nm, 193nm, 157nm or 126nm) and EUV (extreme ultraviolet radiation, e.g., having a wavelength in the range of about 5nm to 100nm).

[0024] The terms "reticle", "mask" or "patterning device" as used herein may be broadly interpreted to refer to a general patterning device that can be used to impart a patterned cross-section to an incident beam 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 classical masks (transmissive or reflective, binary, phase-shifting, hybrid, etc.), other examples of such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0025] Figure 1 A lithographic apparatus LA is schematically depicted. The lithographic apparatus LA comprises an illumination system (also referred to as an illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation, or EUV radiation), a mask support (e.g., a mask stage) MT configured to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA according to certain parameters, a substrate support (e.g., a wafer stage) WT configured to hold a substrate (e.g., a wafer coated with resist) W and connected to a second positioner PW configured to accurately position the substrate support according to certain parameters, and a projection system (e.g., a refractive projection lens system) PS 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.

[0026] In operation, the illumination system IL receives a radiation beam from a radiation source SO, for example via a beam delivery system BD. The illumination system IL may include various types of optical components for directing, shaping and / or controlling the radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic and / or other types of optical components, or any combination thereof. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross-section at the plane of the patterning device MA.

[0027] The term "projection system" PS as used herein should be broadly interpreted as covering various types of projection systems suitable for the exposure radiation used and / or for other factors such as the use of immersion liquid or the use of a vacuum, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems or any combination thereof. Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system" PS.

[0028] The lithographic apparatus LA may be of a type in which at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, such as water, so as to fill the space between the projection system PS and the substrate W, which is also known as immersion lithography. More information on immersion techniques is given in US6952253, which is incorporated herein by reference.

[0029] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also known as a "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel, and / or a step of preparing a substrate W for subsequent exposure may be performed on a substrate W located on one of the substrate supports WT while another substrate W on another substrate support WT is being used to expose a pattern on another substrate W.

[0030] In addition to the substrate support WT, the lithographic apparatus LA may also comprise a measurement platform. The measurement platform is arranged to hold sensors and / or cleaning means. The sensors may be arranged to measure properties of the projection system PS or properties of the radiation beam B. The measurement platform may hold a plurality of sensors. The cleaning means may be arranged to clean parts of the lithographic apparatus, for example a part of the projection system PS or a part of the system providing immersion liquid. The measurement platform may be moved under the projection system PS when the substrate support WT is away from the projection system PS.

[0031] In operation, a radiation beam B is incident on a patterning device (e.g. a mask) MA held on a mask support MT and is patterned by a pattern (design layout) present on the patterning device MA. Having traversed the mask MA, the radiation beam B passes through a projection system PS which focuses the beam onto a target portion C of the substrate W. With the aid of a second positioner PW and a position measurement system IF, the substrate support WT can be accurately moved, for example in order to position different target portions C in the path of the radiation beam B in a focused and aligned position. Similarly, the first positioner PM and possibly a further position sensor (possibly another position sensor not present) are provided. Figure 1The patterning device MA may be accurately positioned relative to the path of the radiation beam B using the mask alignment marks M1, M2 and the substrate alignment marks P1, P2. Although the substrate alignment marks P1, P2 occupy dedicated target portions as illustrated, the marks may be located in spaces between target portions. When the substrate alignment marks P1, P2 are located between target portions C, they are referred to as scribe line alignment marks.

[0032] like Figure 2 As shown in FIG. 1 , the lithographic apparatus LA may form part of a lithocell LC (sometimes also referred to as a lithocell or litho cluster), which typically also includes equipment for performing pre-exposure and post-exposure processes on a substrate W. Conventionally, these equipment include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a chill plate CH and a bake plate BK, for example, for regulating the temperature of the substrate W (e.g., for regulating the solvent in the resist layer). A substrate transport device or robot RO picks up the substrate W from the input / output ports I / O1, I / O2, moves the substrate W between the different process devices, and transfers the substrate W to a feed station LB of the lithographic apparatus LA. The devices in the lithocell, which are also generally referred to as tracks or coating and developing systems, are typically under the control of a track control unit TCU, which itself may be controlled by a management control system SCS, which may also control the lithographic apparatus LA, for example, via a litho control unit LACU.

[0033] In order to correctly and consistently expose the substrate W exposed by the lithographic apparatus LA, it is desirable to inspect the substrate to measure properties of the patterned structure, such as overlay errors between subsequent layers, line thickness, critical dimensions (CD), etc. For this purpose, an inspection tool (not shown) may be included in the lithography cell LC. In particular, if an error is detected, the exposure of subsequent substrates or other processing steps to be performed on the substrate W may be adjusted, for example, if an error is detected.

[0034] The inspection apparatus, which may also be referred to as a metrology apparatus, is used to determine properties of a substrate W, and in particular, to determine how properties of different substrates W vary or how properties associated with different layers of the same substrate W vary from layer to layer. The inspection apparatus may alternatively be configured to identify defects on the substrate W, and may for example be part of a lithography cell LC, or may be integrated into a lithography apparatus LA, or may even be a separate device. The inspection apparatus may measure properties of a latent image (the image in the resist layer after exposure), or properties of a semi-latent image (the image in the resist layer after a post-exposure bake step PEB), or properties of a developed resist image (wherein exposed or unexposed parts of the resist have been removed), or even properties of an etched image (after a pattern transfer step such as etching).

[0035] Typically, the patterning process in the lithographic apparatus LA is one of the most critical steps in the process, which requires a high accuracy in the dimensioning and placement of structures on the substrate W. In order to ensure this high accuracy, three systems can be combined in Figure 3 . One of these systems is the lithography apparatus LA, which is (in practice) connected to a metrology tool MET (a second system) and to a computer system CL (a third system). The key to this "holistic" environment is to optimize the cooperation between these three systems to enhance the overall process window and to provide a tight control loop to ensure that the patterning performed by the lithography apparatus LA remains within the process window. The process window defines the range of process parameters (e.g., dose, focus, overlay accuracy) within which a particular manufacturing process produces a defined result (e.g., a functional semiconductor device) - typically the process parameters in the lithography process or patterning process are allowed to vary within the range.

[0036] The computer system CL can use (part of) the design layout to be patterned to predict which resolution enhancement techniques to use and perform computer lithography simulations and calculations to determine which mask layouts and lithography equipment settings achieve the maximum overall process window (in Figure 3 Typically, the resolution enhancement technique is arranged to match the patterning possibilities of the lithographic apparatus LA. The computer system CL may also be used to detect where within the process window the lithographic apparatus LA is currently operating (e.g. using input from a metrology tool MET) to predict whether defects may be present due to, for example, suboptimal processing (e.g., at Figure 3 0 in the figure).

[0037] The metrology tool MET may provide input to the computer system CL to enable accurate simulations and predictions, and may provide feedback to the lithographic apparatus LA to identify, for example, possible drifts in the calibration state of the lithographic apparatus LA (in Figure 3 SC3 is depicted by multiple arrows in the third scale SC3).

[0038] During a lithographic process, it is desirable to make frequent measurements of the structures produced, for example for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes or various forms of metrology equipment, such as scatterometers. Examples of known scatterometers often rely on the supply of dedicated metrology targets, such as underfilled targets (targets in the form of simple gratings or overlaid gratings in different layers, which are large enough that the measurement beam produces a spot smaller than the grating) or overfilled targets (so that the illumination spot partially or completely encompasses the target). In addition, the use of metrology tools (e.g., angle-resolved scatterometers that illuminate underfilled targets such as gratings) allows the use of so-called reconstruction methods, in which the properties of the grating can be calculated by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulated results with the measured results. The parameters of the model are adjusted until the simulated interaction produces a diffraction pattern similar to the diffraction pattern observed from a real target.

[0039] A scatterometer is a multifunctional instrument that allows to measure parameters of a lithographic process by having a sensor in the pupil or a plane conjugated to the pupil of the objective of the scatterometer, which measurement is usually referred to as pupil-based measurement, or by having a sensor in the image plane or a plane conjugated to the image plane, in which case the measurement is usually referred to as image-based or field-based measurement. Such scatterometers and associated measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP1,628,164A, which are incorporated herein by reference in their entirety. The aforementioned scatterometers can measure multiple targets from multiple gratings in one image using light from soft x-rays and visible to near IR wave ranges.

[0040] Figure 4A measurement device such as a scatterometer is depicted in . The measurement device comprises a broadband (white light) radiation projector 2 which projects radiation 5 onto a substrate W. The reflected or scattered radiation 10 is passed to a spectrometer detector 4 which measures the spectrum 6 of the specularly reflected radiation 10 (i.e. the intensity I as a function of wavelength λ). From such data, the structure or profile 8 which produced the detected spectrum can be reconstructed by a processing unit PU, for example by rigorous coupled wave analysis and nonlinear regression or by comparison with a library of simulated spectra. Typically, for the reconstruction, the general form of the structure is known and some parameters are assumed based on knowledge of the process used to manufacture the structure, leaving only a few parameters of the structure to be determined from the scatterometry data. Such a scatterometer can be configured as a normal incidence scatterometer or an oblique incidence scatterometer.

[0041] In a first embodiment, the scatterometer MT is an angle-resolved scatterometer. In such a scatterometer, a reconstruction method can be applied to the measured signal to reconstruct or calculate the properties of the grating. Such a reconstruction can be produced, for example, by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measured results. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to the diffraction pattern observed from a real target.

[0042] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such a spectroscopic scatterometer MT, radiation emitted by a radiation source is directed onto a target, and reflected or scattered radiation from the target is directed to a spectrometer detector which measures the spectrum of the specularly reflected radiation (i.e. measures the intensity as a function of wavelength). From such data, the structure or profile of the target producing the detected spectrum can be reconstructed, for example, by rigorous coupled wave analysis and nonlinear regression or by comparison with a library of simulated spectra.

[0043] In a third embodiment, the scatterometer MT is an ellipsometry scatterometer. An ellipsometry scatterometer allows the parameters of a lithography process to be determined by measuring the scattered radiation for each polarization state. Such a metrology device emits polarized light (such as linear, circular or elliptical) by using, for example, an appropriate polarization filter in the illumination section of the metrology device. A source suitable for the metrology device may also provide polarized radiation. Various embodiments of existing ellipsometry scatterometers are described in US patent applications 11 / 451,599, 11 / 708,678, 12 / 256,780, 12 / 486,449, 12 / 920,968, 12 / 922,587, 13 / 000,229, 13 / 033,135, 13 / 533,110 and 13 / 891,410, which are incorporated herein by reference in their entirety.

[0044] In one embodiment of the scatterometer MT, the scatterometer MT is adapted to measure the overlay of two misaligned gratings or periodic structures by measuring the reflection spectrum and / or an asymmetry in the detection configuration (the asymmetry being related to the extent of the overlay). The two (typically overlaid) grating structures may be applied in two different layers (not necessarily consecutive layers) and the two grating structures may be formed at approximately the same position on the wafer. The scatterometer may have a symmetrical detection configuration as described, for example, in the commonly owned patent application EP1,628,164A so that any asymmetry can be clearly identified. This provides a straightforward way to measure misalignment in the gratings. Further examples of measuring the overlay error between two layers containing a periodic structure as a target via the asymmetry of the periodic structure may be found in PCT Patent Application Publication No. WO 2011 / 012624 or US Patent Application No. US20160161863, which are incorporated herein by reference in their entirety.

[0045] Other parameters of interest may be focus and dose. Focus and dose may be determined simultaneously by scatterometry as described in US patent application US2011-0249244, which is incorporated herein by reference in its entirety (or alternatively by scanning electron microscopy). A single structure with a unique combination of critical dimension and sidewall angle measurements for each point in a focus energy matrix (FEM, also referred to as a focus exposure matrix) may be used. If these unique combinations of critical dimensions and sidewall angles are available, focus and dose may be uniquely determined from these measurements.

[0046] The metrology target may be a composite grating set formed by a lithography process, mainly in a resist, and also after, for example, an etching process. Typically, the pitch and line width of the structures in the gratings depend heavily on the measurement optics (particularly the NA of the optics) to be able to capture the diffraction orders from the metrology target. As indicated earlier, the diffraction signal may be used to determine the phase shift between two layers (also called "overlay") or may be used to reconstruct at least a portion of the original grating as produced by the lithography process. Such a reconstruction may be used to provide guidance on the quality of the lithography process and may be used to control at least a portion of the lithography process. The target may have smaller subsections configured to mimic the size of a functional portion of a design layout in the target. Due to such subsections, the target will behave more similarly to the functional portion of the design layout, so that the overall process parameter measurement more preferably resembles the functional portion of the design layout. The target may be measured in an underfill mode or in an overfill mode. In the underfill mode, the measurement beam produces a spot that is smaller than the overall target. In the overfill mode, the measurement beam produces a spot that is larger than the overall target. In such an overfill mode, it is also possible to measure different targets simultaneously and thus to determine different process parameters simultaneously.

[0047] The overall measurement quality of a lithography parameter using a specific target is at least partially determined by the measurement options used to measure such lithography parameters. The term "substrate measurement options" may include one or more parameters of the measurement itself, one or more parameters of one or more patterns measured, or both. For example, if the measurement used in the substrate measurement options is a diffraction-based optical measurement, one or more of the measured parameters may include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the orientation of the radiation relative to the pattern on the substrate, etc. One of the criteria for selecting a measurement option may, for example, be the sensitivity of one of the measurement parameters to process variations. More examples are described in US patent application US2016-0161863, which is incorporated herein by reference in its entirety, and published US patent application US2016 / 0370717A1.

[0048] A metrology apparatus using a computational imaging / phase acquisition method has been described in US patent publication US2019 / 0107781, which is incorporated herein by reference. Such a metrology apparatus can use relatively simple sensor optics that have ordinary or even relatively common aberration performance. Thus, the sensor optics can be allowed to have aberrations, and therefore the sensor optics produce relatively aberrated images. Of course, unless something is done to compensate for the effects of these optical aberrations, simply allowing larger aberrations in the sensor optics will have an unacceptable impact on image quality. Therefore, computational imaging techniques are used to compensate for the negative impact of the relaxation of aberration performance in the sensor optics.

[0049] In this method, the intensity and phase of the target are obtained from one or more intensity measurements of the target. Phase acquisition can use previous information of the measurement target (for example, to be included in the loss function that forms the starting point to derive / design the phase acquisition algorithm). Alternatively, or in combination with the previous information method, a diversity measurement can be performed. In order to achieve diversity, the imaging system is slightly changed between measurements. An example of a diversity measurement is cross-focus stepping, that is, by obtaining measurement results at different focus positions. Alternative methods for introducing diversity include, for example, using different illumination wavelengths or different wavelength ranges, adjusting the illumination, or changing the angle of incidence of the illumination on the target between measurements. The phase acquisition itself can be based on the aforementioned US2019 / 0107781 or the content described in the patent application EP1 7199764 (also incorporated herein by reference). This describes the determination of the corresponding phase acquisition based on the intensity measurement, so that the interaction of the target with the illumination radiation is described based on the electric field or composite field of the target ("composite" here means that there are both amplitude and phase information). The intensity measurement can have a lower quality than the intensity measurement used in conventional measurement, and can therefore be defocused as described. The described interaction may include a representation of the electric and / or magnetic fields directly above the target. In such an embodiment, the illuminated target electric and / or magnetic field image is modeled as an equivalent source description by means of infinitesimal current and / or magnetic current dipoles on a surface (e.g., two-dimensional) in a plane parallel to the target. Such a plane may, for example, be a plane directly above the target, e.g., a plane in focus according to the Rayleigh criterion, but the location of the model plane is not important: once the amplitude and phase at one plane are known, the amplitude and phase may be propagated computationally to any other plane (in-focus, out-of-focus, or even a pupil plane). Alternatively, the description may include a composite transmission of the target or its two-dimensional equivalent.

[0050] Phase acquisition can include modeling the effect of the interaction between the illuminating radiation and the target on the diffracted radiation to obtain a modeled intensity pattern; and optimizing the phase and amplitude of the electric field / composite field within the model so as to minimize the difference between the modeled intensity pattern and the detected intensity pattern. More specifically, during measurement acquisition, an image (e.g., an image of the target) is captured on a detector (at a detection plane) and its intensity is measured. A phase acquisition algorithm is used to determine the amplitude and phase of the electric field at a plane, for example, parallel to the target (e.g., directly above the target). The phase acquisition algorithm uses a forward model of the sensor (e.g., taking into account aberrations) to computationally image the target to obtain modeled values ​​of the intensity and phase of the field at the detection plane. No target model is required. The difference between the modeled intensity value and the detected intensity value is minimized in terms of phase and amplitude (e.g., iteratively), and the corresponding modeled phase value obtained is regarded as the acquired phase. A specific method for using composite fields in metrology applications is described in PCT application PCT / EP2019 / 052658, which is also incorporated herein by reference.

[0051] Performing measurements at multiple focus levels in succession takes a lot of time. It is therefore slow to obtain progressively defocused images, resulting in slow measurement speeds and low throughput. Furthermore, there are inherent assumptions when using focus diversity measurements that introduce inaccuracies in phase acquisition, such as the assumption that moving the sample through the focus is equivalent to moving the detector through the focus. Therefore, the method described herein eliminates the need for such through-focus multiple measurements.

[0052] The proposed method and apparatus include using at least one set of direction detection pixels to detect the direction of light traveling towards the sensor. Using the information given by these sets of direction detection pixels, a phase distribution can be inferred and used to perform image correction. Each set of direction detection pixels can include at least one direction detection pixel pair, wherein each set of pixels or pair of pixels are at the same nearby position to each other.

[0053] The one or more sets of direction detection pixels may include a subset of pixels on the detector that are used to detect an image. The remaining pixels (imaging pixels) are used to measure the image (eg, intensity (and therefore amplitude)) of the signal.

[0054] It is therefore proposed to infer a complex-valued field at the detector. Such a composite field may be inferred from a combination of signals measured by imaging pixels (which are used to determine the field amplitude) and signals from direction detection pixels (which are used to determine the field phase). It may be necessary to perform interpolation to obtain the entire composite field, for example, interpolating for missing amplitude data (at the detector area occupied by the direction detection pixels) and interpolating for missing phase data (at the detector area occupied by the imaging pixels). Such interpolation may be performed on the field amplitude data and the field phase data before determining the composite field, or on the determined composite field to determine the entire composite field.

[0055] The composite field can be Fourier transformed to reach the pupil plane. The lens aberration phase distribution is conjugated and multiplied with the Fourier transformed composite field. The defocus aberration is then digitally adjusted to maximize image quality and correct for sensor optics aberrations.

[0056] For example, multiple sets of direction detection pixels may be provided at multiple (e.g., equally spaced) locations on the detector array. In an embodiment, these multiple sets of direction detection pixels may include a 1-dimensional or 2-dimensional direction detection array of pixel sets; wherein, for example, the direction detection array generally includes a sub-array of the detector pixel array. Optionally, where the direction detection pixel set includes a 1-dimensional array, a first subset of the set may be oriented to detect the direction of a first axis relative to the detector plane, and a second subset of the set may be oriented to detect the direction of a second (perpendicular) axis relative to the detector plane.

[0057] It should be noted that the terms "direction detection pixels" and "imaging pixels" are used only to distinguish one set of pixels from another set of pixels. It is possible that both of these categories of pixels will be included on a single camera or pixel array, and will therefore operate using the same basic detection principle (e.g., photodiodes or equivalent devices that detect intensity levels). The two categories of pixels are distinguished only by the way the pixels receive signals (e.g., whether via a mask / microlens) and / or the way they process the signals.

[0058] The working principle has similarities to the concept of PDAF (Phase Detection Auto Focus) pixel architecture used in many current commercial consumer cameras. In the PDAF scheme, multiple pixel pairs distributed over the sensor area are used to detect whether the image is in focus. PDAF is achieved by providing "focus pixels", which are pixels on the main camera sensor grid that have been set aside for focusing rather than capturing a small percentage of the image. Focus pixels typically appear in pairs. In each focus pixel pair, the pixels are close to each other on the sensor and are partially shielded so that one pixel only receives light from the top (or left) part of the lens, while the other pixel in the pair only receives light from the opposite part of the lens. Since the pixel pairs are at the same nearby position, the pixel pairs should receive the same amount of light when the image is in focus at that point. If the pixel pairs receive different amounts of light, the difference provides information about the image defocus level. The camera can then use this information to quickly focus the lens. A typical PDAF system has many focus pixel pairs (or equivalent) scattered around the sensor area, because the target object (on which is focused) is not always at the same location on the lens.

[0059] The extension of this principle to 3D imaging applications is disclosed in the following document: "CMOS image sensor for extracting depth information using offset pixel aperture technique", Choi et al.; Proc. SPIE 10376, Novel Optical Systems Design and Optimization XX, 103760Y (August 24, 2017). This document is incorporated herein by reference. In particular, this document (and especially Figure 5 and Figure 6 , and the accompanying text of the document) describes how a partially shaded pixel array can be used to measure the angle of incidence of incident radiation on a pixel array. More specifically, the document describes a pixel set of three shaded pixels:

[0060] • Masking the first pixel using a left-offset pixel aperture LOPA mask; ie, a mask with a light admission aperture offset to the left of the photodiode.

[0061] • The second pixel is masked using a right-offset pixel aperture ROPA mask; ie, a mask having a light admission aperture offset to the left of the photodiode, and • a third pixel, which is masked by a non-offset or centered light admission aperture.

[0062] A graph of light power (or intensity) versus angle of incidence for each of these obscured pixels shows that there are different shifts (corresponding to different angles of incidence) in the position of each peak of a LOPA obscured pixel, a ROPA obscured pixel, and a non-offset obscured pixel. In a particular arrangement described in the literature, the peak point of the LOPA obscured pixel is shifted 12° from the peak point of the non-offset obscured pixel, and the peak point of the ROPA obscured pixel is shifted -12° from the peak point of the non-offset obscured pixel. By comparing the intensity of light detected by these three (e.g., adjacent) pixels, the angle of incidence (light direction) can be classified as 12°, 0, or -12°. Intermediate angles can also be determined from the graph.

[0063] Figure 5 The measurement arrangement according to the embodiment is shown. The arrangement is largely similar to the arrangements described in the aforementioned documents US2019 / 0107781 and PCT / EP2019 / 05265, which utilize computational phase acquisition techniques to correct simpler lens designs with phase differences. Figure 5 In (a), a highly simplified schematic diagram shows a measurement beam MB of radiation (e.g. from a source, not shown) directed onto a target T on a substrate S. Higher order diffracted radiation DR is captured by an objective lens OL and directed to a detector D. No zero order radiation ZR is captured, and therefore the captured image is a dark field image.

[0064] The shield array or shield array OA is located between the objective lens OL and the detector D (for example, directly in front of the detector D). The shield array OA includes a plurality of mask sets or shield sets (it should be noted that the use of the term "mask" here is different from the use of the same term synonymous with the reticle - to avoid confusion, shields will be used instead). Each shield set includes a plurality of partial shields that partially shield the corresponding direction detection pixels. Each partial shield of the shield set has a different shielding direction, position or configuration (for example, different positions or offsets of the shields and / or the holes therein relative to their corresponding pixels).

[0065] The detector D is a camera type detector having a plurality of pixels, for example a two-dimensional pixel matrix. The shield array is configured so that the shield sets are spaced apart and a large portion of the pixel matrix is ​​not shielded. These unshielded pixels are imaging pixels. Each pixel set shielded by the shield set is a direction detection pixel set, each of which includes a plurality of direction detection pixels.

[0066] Figure 5(b) Illustration of an example image (or portion thereof) that may be detected on a detector (e.g., when imaging an overlay type target based on diffraction of "two biases per direction" as is well known in the art). The captured image is represented by shaded regions SR in which higher intensity values ​​are detected relative to the rest of the detector (e.g., due to diffracted radiation DR). These shaded regions correspond to target pads or gratings oriented with a measured illumination profile. A subset of the detector pixel array includes direction detection pixel sets DDP, which are pixels shielded by the shield array. For clarity, the imaging pixels are not depicted individually. It should be noted that this is merely an exemplary arrangement, and that the direction detection pixels may include a detector pixel array of a different (e.g., smaller) scale than that illustrated here, and / or may be arranged or spaced differently.

[0067] Each direction detection pixel set can include, for example, a pixel pair with a corresponding mask set having two different mask offsets; for example, a left direction detection pixel can have a left mask that masks a left portion of the pixel detection area, and a right direction detection pixel can have a right portion of the light detection area masked by the right mask. A microlens array (e.g., a microlens array film) can be used in front of the direction detection pixels to collect more light. More accurate direction detection can be obtained by providing more direction detection pixels per direction detection pixel set.

[0068] In an embodiment, each direction detection pixel set may include a two-dimensional array of direction detection pixels to allow direction detection in two directions of the detector plane. In such a two-dimensional arrangement, the offset may also be two-dimensional: for example, four (2×2) pixel arrays may have a hole offset toward each corner, or nine (3×3) pixel arrays may have the following offsets, respectively: upper left, upper, upper right, right, lower right, lower, lower left, left, and center. The number of pixels of each direction detection pixel set will ultimately be a trade-off between the accuracy or precision of the direction / phase detection and the subsequent loss of the imaging area (imaging pixels). In some practical embodiments, each direction detection pixel set may include a 3×3, 4×4, or 5×5 pixel array. More specifically, assuming a magnification of Mag=100x and a typical pixel size of 5μm, there will be 800×800 pixels for sampling an image of an 8μm target on a wafer. This means that for the direction detection pixel set, the array may include four pixels up to a practical maximum of about 100×100 pixels. The two-dimensional array of direction detection pixels is not necessarily square, especially when there are a large number of pixels per array.

[0069] Figure 6An exemplary arrangement of a 4×4 array of direction detection pixels in a direction detection pixel set is shown, which can detect direction (and therefore phase) in two dimensions. The arrangement includes 16 offset aperture masked pixels P11 to P 44 (The suffix describes the position of the pixel in the array, of which only four pixels P are explicitly marked. 11 , P 41 , P 14 , P 44 ). For example, one of these pixels P 33 As can be seen from the cross-sectional view of , each pixel includes a photodiode PD shielded by an offset pixel hole OPA. The offset of each offset pixel hole OPA is different, so that for each pixel, the hole is in a different position relative to the photodiode. In an embodiment (such as the embodiment shown), the offset and therefore the position of the hole can correspond to the position of the pixel in the array. In other words, the direction of the offset and the magnitude of the offset correspond to the position of the pixel in the array.

[0070] Optionally, each pixel may also include a corresponding microlens ML that helps focus more radiation into each direction-detecting pixel (eg, to compensate for light lost due to absorption and / or reflection by the mask).

[0071] The maximum angle of the light at the detector D in the metrology application considered here is typically 1 degree, and the resolution of the angle detection should be in the range of 0.1 degrees. Therefore, the size and focal length of the pixel should be selected accordingly. By using larger pixels and increasing the focal length of the microlens array, the directional sensitivity can be enhanced linearly.

[0072] It has been proposed that Figure 5 and Figure 6 The arrangement illustrated in , and in particular the set of direction detection pixels may be used to acquire a phase of the diffracted radiation DR at (or very close to) the detector plane.

[0073] Phase acquisition may include a first step of inferring the direction of the incident radiation using a set of direction-detecting pixels. This may include using the methods described in the aforementioned "CMOS Image Sensor Using Offset Pixel Aperture Technique to Extract Depth Information" document; for example, inferring the angle by which sensor in the array detects the radiation and / or by the relative intensity levels measured by neighboring pixels.

[0074] Figure 7 is the intensity or power I relative to the direction or angle α A graph of α (as incident on the detector) is provided to help explain the use of a graph such as Figure 6A method for inferring direction from a set of direction detection pixels illustrated in FIG. The figure shows three curves CP1, CP2, and CP3, each of which corresponds to three adjacent pixels in the set of direction detection pixels (in a single direction). It can be seen that curve CP2 is centered for zero-angle (normal incidence) radiation, and curve CP2 can correspond to the center pixel (for example, in the center row / column) in an odd-numbered (for example, 3×3 or 5×5) direction detection pixel array, where curves CP1 and CP3 correspond to pixels on either side, having peaks at angles α1 and α2. Therefore, angles 0 degrees, α1, and α2 can be easily distinguished by which of the corresponding pixels detects the peak intensity (or an intensity value exceeding a threshold). Intermediate angles can be inferred by the relative intensities sensed by the associated adjacent pixels; for example, angles between 0 and α2 can be inferred by the relative intensity levels on the pixels corresponding to CP1 and CP2 (for example, the ratio of the intensity levels on these pixels). α1 For example, an equal (non-zero) intensity level on each of these two pixels would indicate an angle corresponding to the intersection of curve CP1 and curve CP2. A skilled person will understand how to extend this concept to more than three pixels and two dimensions.

[0075] The key information required to correct aberrations in an image is both the direction of the light rays determined by a set of direction detection pixels and the intensity detected by the imaging pixels. Conventional image sensors can only detect the intensity of light. In such a concept, the main detector chip is provided with a set of direction detection pixels sparsely positioned on the detector area. Therefore, some of the actual area on the detector is used to detect the direction of the rays. The remaining area is used to detect the image, as is conventionally done. The area reserved for detecting the direction will be smaller than the area used for image detection (e.g., 5% to 20% of the total area is used to detect the direction).

[0076] It will be appreciated that some information about the intensity distribution is lost due to the direction detection pixels, and there will be empty gaps in the intensity image. These gaps can be filled by 2D interpolation. Similarly, the direction information from the direction detection pixels is also incomplete, and in practice is measured rather sparsely. It is also possible to interpolate a complete phase map for the entire detector area, since the direction information in such applications varies relatively smoothly and slowly over the image area.

[0077] Thus, once the angles and hence directions of the diffracted radiation are detected at one or more sets of direction detecting pixels, an interpolation of the entire image is performed to find the directions over the entire image. From this, a phase map can be inferred, for the image corrected for the aberrations in the sensor. A similar intensity map can be obtained, and thus a map of the entire composite field can be obtained from a combination of a phase map and an amplitude map (e.g., the amplitude map being the square root of the intensity map). Using a description or model of the sensor optics and its aberrations (e.g., the lens aberration phase distribution), the composite field can then be corrected for these aberrations to obtain an improved image, for example, using the methods already described.

[0078] Figure 8 Another alternative arrangement for performing local direction detection is illustrated. This approach uses a sparse microlens array MLA comprising an array of microlenses ML. The microlens array is sparse, wherein the lenses are arranged at relatively large intervals with a lower duty cycle (e.g., between 5% and 10%). The microlens array is placed at a distance f from the detector plane, wherein the distance f includes the focal length of the microlens array. The set of pixels (e.g., the direction detection pixel set) assigned to each microlens array is dedicated to direction (phase) detection. The remaining pixels are used as conventional imaging pixels.

[0079] The measurement principle of this embodiment is different from the measurement principle described for pixels shielded by offset holes. The radiation falling on the microlens is focused onto an area of ​​the detector (e.g., a set of direction detection pixels), forming a bright spot. The position of the bright spot on the set of direction detection pixels (e.g., in two dimensions) reveals the direction of the radiation beam incident on the microlens. The microlens array is sparse, which means that there are large gaps between each microlens; as mentioned above, the pixels included in these gaps (imaging pixels) measure intensity information in a conventional manner.

[0080] The accuracy of angle detection of such an embodiment can be estimated roughly as follows. Assume that the microlens diameter d is 100 μm, the focal length f of the microlens is 1 mm, the pixel size PS of the sensor is 5 μm, and the radiation wavelength λ is 500 nm. This means that when the angle changes PS / f=5 mrad, the main beam moves 1 pixel. The diffraction-limited spot size is f*λ / d:=5 μm. The spot blurring due to the limited lens diameter is also f*λ / d, and f*λ / d is also 5 μm. Therefore, considering aberrations, each lens can form a spot (e.g., a compact isolated spot) with a diameter of approximately 10 μm to 15 μm. The peak of such a spot can be determined with an accuracy of 1 to 2 pixels, which is sufficient to estimate the local phase (direction) with an accuracy of 10 mrad. Using sub-pixel interpolation of the position of the peak, an accuracy of 1 mrad can be obtained. The accuracy is limited by the signal-to-noise ratio, and there should be good signal strength to accurately detect the peak of the spot. The number of pixels in the direction detection pixel set (or array) can be the same or similar to that in the aforementioned embodiment.

[0081] It will be appreciated that such an embodiment of a microlens comprises an arrangement of a sparse lens array placed at a distance from a conventional image detector. Thus, no special detector chip fabrication is required. In contrast, the first (offset aperture mask) embodiment is an on-chip solution, wherein the on-chip microlenses (if present) have obscured pixels for detecting direction. For this purpose, such an unconventional image sensor requires special fabrication.

[0082] After image acquisition, two images are produced: a low-resolution phase-change image from the set of direction-detecting pixels and a regular image from the imaging pixels. The low-resolution phase-change image is extrapolated to produce a high-resolution phase map. This map can be used as an initial estimate in an iterative algorithm that corrects the aberrations of the image.

[0083] It must be noted that this technique is different from that of plenoptic cameras. In plenoptic cameras, a local pupil image is generated per microlens. The 2D collection of local pupil images is called a 4D light field. In such proposals, microlens-based angle detection is performed sparsely and such a 4D light field is not measured.

[0084] Fig. 91 is a block diagram illustrating a computer system 1600 that can assist in implementing the methods and processes disclosed herein. The computer system 1600 includes a bus 1602 or other communication mechanism for communicating information, and a processor 1604 (or multiple processors 1604 and 1605) coupled to the bus 1602 for processing information. The computer system 1600 also includes a main memory 1606, such as a random access memory (RAM) or other dynamic storage device, coupled to the bus 1602 for storing information and instructions to be executed by the processor 1604. The main memory 1606 can also be used to store temporary variables or other intermediate information during the execution of instructions to be executed by the processor 1604. The computer system 1600 also includes a read-only memory (ROM) 1608 or other static storage device coupled to the bus 1602 for storing static information and instructions for the processor 1604. A storage device 1610, such as a magnetic disk or optical disk, is provided and coupled to the bus 1602 for storing information and instructions.

[0085] The computer system 1600 can be coupled to a display 1612, such as a cathode ray tube (CRT) or a flat panel display or a touch panel display, via a bus 1602 for displaying information to a computer user. An input device 1614 including alphanumeric keys and other keys is coupled to the bus 1602 for communicating information and command selections to the processor 1604. Another type of user input device is a cursor control 1616, such as a mouse, a trackball, or a cursor direction key, for communicating direction information and command selections to the processor 1604 and for controlling cursor movement on the display 1612. Such an input device typically has two degrees of freedom on two axes (a first axis (e.g., x) and a second axis (e.g., y)), which allow the device to specify a position in a plane. A touch panel (screen) display can also be used as an input device.

[0086] As described herein, one or more of the methods may be performed by the computer system 1600 in response to the processor 1604 executing one or more sequences of one or more instructions contained in the main memory 1606. These instructions may be read from another computer-readable medium (such as storage device 1610) into the main memory 1606. The execution of the sequence of instructions contained in the main memory 1606 causes the processor 1604 to perform the process steps described herein. One or more processors in a multi-processing arrangement may also be used to execute the sequence of instructions contained in the main memory 1606. In alternative embodiments, hard-wired circuits may be used instead of or in combination with software instructions. Therefore, the description herein is not limited to any specific combination of hardware circuitry and software.

[0087] As used herein, the term "computer-readable medium" refers to any medium that participates in providing instructions to the processor 1604 for execution. Such a medium can be in many forms, including but not limited to non-volatile media, volatile media, and transmission media. Non-volatile media include, for example, optical disks or magnetic disks, such as storage devices 1610. Volatile media include dynamic memory, such as main memory 1606. Transmission media include coaxial cables, copper wires, and optical fibers, and transmission media include wires containing bus 1602. Transmission media can also take the form of sound waves or light waves, such as, sound waves or light waves generated during radio frequency (RF) and infrared (IR) data communications. Common forms of computer-readable media include, for example, floppy disks, flexible disks, hard disks, magnetic tapes, any other magnetic media, CD-ROMs, DVDs, any other optical media, punch cards, paper tapes, any other physical media with hole patterns, RAMs, PROMs and EPROMs, FLASH-EPROMs, any other memory chips or cassettes, carriers as described below, or any other media that can be read by a computer.

[0088] Various forms of computer-readable media may be involved in carrying one or more sequences of one or more instructions to the processor 1604 for execution. For example, initially the instructions may be carried on a disk of a remote computer. The remote computer may load the instructions into its dynamic memory and send the instructions via a telephone line using a modem. A modem local to the computer system 1600 may receive data on the telephone line and convert the data into an infrared signal using an infrared transmitter. An infrared detector coupled to the bus 1602 may receive the data carried in the infrared signal and place the data on the bus 1602. The bus 1602 carries the data to the main memory 1606, from which the processor 1604 retrieves and executes the instructions. The instructions received by the main memory 1606 may optionally be stored on the storage device 1610 before or after being executed by the processor 1604.

[0089] Computer system 1600 also preferably includes a communication interface 1618 coupled to bus 1602. Communication interface 1618 provides a bidirectional data communication coupling coupled to a network link 1620 connected to a local area network 1622. For example, communication interface 1618 can be an integrated services digital network (ISDN) card or a modem to provide a data communication connection with a telephone line of a corresponding type. As another example, communication interface 1618 can be a local area network (LAN) card to provide a data communication connection with a compatible LAN. Wireless links can also be implemented. In any such embodiment, communication interface 1618 sends and receives electrical signals, electromagnetic signals, or optical signals that carry digital data streams representing various types of information.

[0090] The network link 1620 typically provides data communication with other data devices via one or more networks. For example, the network link 1620 may provide a connection to a host computer 1624 through a local area network 1622 or a connection to data equipment operated by an Internet Service Provider (ISP) 1626. The ISP 1626 in turn provides data communication services via a global packet data communication network (now commonly referred to as the "Internet") 1628. Both the local area network 1622 and the Internet 1628 use electrical, electromagnetic or optical signals that carry digital data streams. The signals through the various networks and the signals on the network link 1620 and through the communication interface 1618, which carry the digital data to and from the computer system 1600, are exemplary forms of carrier waves that transport the information.

[0091] The computer system 1600 can send messages and receive data including program code through the network, network link 1620 and communication interface 1618. In the Internet example, the server 1630 may transmit the requested program code for the application via the Internet 1628, ISP 1626, local area network 1622 and communication interface 1618. For example, one such downloaded application can provide one or more of the technologies described herein. The received code can be executed by the processor 1604 when it is received, and / or stored in the storage device 1610 or other non-volatile storage for later execution. In this way, the computer system 1600 can obtain application code in the form of a carrier wave.

[0092] Further embodiments are disclosed in the following numbered aspects:

[0093] 1. A detection apparatus for a metrology device, the detection apparatus being operable to measure a parameter of interest based on scattered radiation that has been scattered from a sample; the detection apparatus comprising:

[0094] A detector, the detector comprising a pixel array, the pixel array comprising: imaging pixels, the imaging pixels being used to detect an image, the parameter of interest being determined based on the image; and direction detection pixels, the direction detection pixels being used to detect an angle of incidence of the scattered radiation on the detector.

[0095] 2. The detection device according to aspect 1, wherein the direction detection pixel comprises a plurality of direction detection pixel sets spaced apart on the detector, each direction detection pixel set being used to detect the incident angle local to the direction detection pixel set.

[0096] 3. The detection device according to aspect 2, wherein the direction detection pixel sets are placed in the form of a two-dimensional grid.

[0097] 4. The detection device according to aspect 2 or 3, wherein each direction detection pixel set comprises a one-dimensional array of direction detection pixels.

[0098] 5. A detection device according to aspect 4, wherein the direction detection pixel set includes two subsets thereof, one subset is oriented perpendicular to the other subset, and the two subsets are used to detect directions in two dimensions located in the detector plane.

[0099] 6. A detection device according to aspect 2 or 3, wherein each direction detection pixel set comprises a two-dimensional array of direction detection pixels, and the two-dimensional array of direction detection pixels is used to detect directions in two dimensions located in the detector plane.

[0100] 7. A detection device according to any one of aspects 2 to 6, wherein each direction detection pixel comprises a corresponding shield that partially shields the pixel from the scattered radiation.

[0101] 8. A detection device according to aspect 7, wherein the set of direction detection pixels includes direction detection pixels having corresponding shields, each shield having an aperture for allowing some incident radiation to enter, wherein each aperture has a different offset or position relative to its corresponding pixel.

[0102] 9. A detection device according to aspect 8, wherein the offset or position of each hole corresponds to the positioning of the corresponding pixel within its set of direction detection pixels.

[0103] 10. The detection device according to any one of clauses 7 to 9, wherein each of the direction detection pixels comprises a corresponding microlens located in front of a shield.

[0104] 11. A detection device according to any one of aspects 2 to 6, wherein each set of direction detection pixels has a corresponding microlens, and the corresponding microlens is positioned to focus the scattered radiation towards the set of direction detection pixels.

[0105] 12. The detection device according to clause 11, wherein each microlens is positioned at a distance from its corresponding set of direction detection pixels, the distance comprising the focal length of the microlens.

[0106] 13. A detection device according to clause 11 or 12, wherein the microlenses form an array of microlenses sparsely spaced with a duty cycle below 10%.

[0107] 14. A detection device according to any one of aspects 7 to 10, wherein the detection device includes a processor capable of operating to determine the incident angle of the scattered radiation at which one or more direction detection pixels of a direction detection pixel set detect an intensity value exceeding a threshold.

[0108] 15. A detection device according to clause 14, wherein the processor is operable to determine the angle of incidence of the scattered radiation based on relative intensity levels detected at at least two corresponding direction detection pixels in the set of direction detection pixels.

[0109] 16. A detection device according to any one of Aspects 11 to 13, wherein the detection device includes a processor capable of operating to determine the incident angle of the scattered radiation based on the position of the spot focused by the microlens on the set of direction detection pixels.

[0110] 17. A detection device according to any one of aspects 14 to 16, wherein the processor is operable to perform interpolation on an image captured by the detector based on a determined value of the incident angle local to each direction detection pixel in the set of direction detection pixels to interpolate the incident angle or a phase value determined based on the incident angle; and

[0111] The result of the interpolation step is used to determine a phase map describing the phase of the scattered radiation at the detector.

[0112] 18. A detection apparatus according to clause 17, wherein the processor is operable to determine a complex-valued field of the scattered radiation at the detector using the phase map and image data captured by the imaging pixels.

[0113] 19. The detection device of clause 18, wherein the detection device comprises sensor optics for capturing the scattered radiation; and

[0114] The processor is operable to use the complex-valued field to correct aberrations in the sensor optics.

[0115] 20. A measurement device, comprising the detection apparatus according to any preceding aspect.

[0116] 21. A lithography unit, comprising:

[0117] The metrology apparatus of clause 20, the metrology apparatus being configured to perform measurements from which corrections for a lithography process are determined; and

[0118] A lithographic apparatus is operable to perform the lithographic process in accordance with the correction.

[0119] 22. A lithographic apparatus comprising a metrology module, the metrology module comprising a detection apparatus according to any one of aspects 1 to 19.

[0120] Although specific reference may be made herein to the use of lithographic equipment in IC manufacturing, it should be understood that the lithographic equipment described herein may have other applications. Possible other applications include manufacturing integrated optical systems, guidance and detection for magnetic domain memory, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0121] Although specific reference may be made herein to embodiments of the invention in the context of inspection or metrology equipment, embodiments of the invention may be used for other equipment. Embodiments of the invention may form part of a mask inspection equipment, a lithography equipment, or any equipment that measures or processes an object such as a wafer (or other substrate) or a mask (or other pattern forming device). The term "metrology equipment" may also refer to an inspection equipment or an inspection system. For example, an inspection equipment including embodiments of the invention may be used to inspect a substrate for defects or defects in a structure on a substrate. In such embodiments, the characteristic of interest of a structure on a substrate may relate to a defect in the structure, the absence of a particular portion of the structure, or the presence of an undesired structure on the substrate.

[0122] Although specific reference is made to "measurement equipment / tools / systems" or "inspection equipment / tools / systems," these terms may refer to the same or similar types of tools, equipment, or systems. For example, an inspection or measurement equipment including embodiments of the present invention may be used to determine characteristics of a structure on a substrate or on a wafer. For example, a detection equipment or measurement equipment including embodiments of the present invention may be used to detect defects in a substrate or defects in a structure on a substrate or on a wafer. In such embodiments, the characteristic of interest of a structure on a substrate may relate to a defect in the structure, the absence of a particular portion of a structure, or the presence of an unwanted structure on a substrate or on a wafer.

[0123] Although specific reference may be made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention is not limited to optical lithography and may be used in other applications such as imprint lithography where the context permits.

[0124] While the targets or target structures described above (more generally, structures on a substrate) are metrology target structures specifically designed and formed for the purpose of measurement, in other embodiments, the properties of interest may be measured on one or more structures that are functional parts of a device formed on a substrate. Many devices have regular grating-like structures. The terms structure, target grating, and target structure as used herein do not require that the structure has been provided specifically for the measurement being performed. In addition, the pitch P of the metrology target may be close to the resolution limit of the optical system of the scatterometer or may be smaller, but may be much larger than the size of a typical product feature made by a lithography process in the target portion C. In practice, the lines and / or spaces of the overlaid grating within the target structure may be made to include smaller structures that are similar in size to the product features.

[0125] Although specific embodiments of the present invention have been described above, it will be appreciated that the present invention may be practiced in other ways than those described. The above description is intended to be illustrative rather than restrictive. Therefore, it will be appreciated by those skilled in the art that modifications may be made to the described invention without departing from the scope of the claims set forth below.

Claims

1. A detection apparatus for a metrology device operable to measure a parameter of interest based on scattered radiation that has been scattered from a sample; the detection apparatus comprising: A detector comprising a pixel array, the pixel array comprising: imaging pixels and direction detection pixels, the imaging pixels being used to detect an image, the parameter of interest being determined based on the image, the direction detection pixels being used to detect an angle of incidence of the scattered radiation on the detector.

2. The detection device according to claim 1, wherein: The direction detection pixels include a plurality of direction detection pixel sets spaced apart on the detector, each direction detection pixel set being used to detect the incident angle local to the direction detection pixel set.

3. The detection device according to claim 2, wherein: The direction detection pixel sets are placed in the form of a two-dimensional grid.

4. The detection device according to claim 2 or 3, wherein: Each direction detection pixel set includes a one-dimensional array of direction detection pixels.

5. The detection device according to claim 4, wherein: The set of direction detection pixels includes two subsets thereof, one subset being oriented perpendicularly to the other subset, the two subsets being used to detect directions in two dimensions lying in a detector plane.

6. The detection device according to claim 2 or 3, wherein: Each direction detection pixel set includes a two-dimensional array of direction detection pixels for detecting directions in two dimensions in a detector plane.

7. The detection device according to any one of claims 2 to 3 and 5, wherein: Each direction detection pixel comprises a respective shield that partially shields the pixel from the scattered radiation.

8. The detection device according to claim 7, wherein: The set of direction detection pixels includes direction detection pixels having corresponding shields, each shield having an aperture for allowing some incident radiation to enter, wherein each aperture has a different offset or position relative to its corresponding pixel.

9. The detection device according to claim 8, wherein: The offset or position of each aperture corresponds to the positioning of the corresponding pixel within its set of direction detection pixels.

10. The detection device according to claim 7, wherein: Each of the direction detection pixels includes a corresponding microlens located in front of the shield.

11. The detection device according to any one of claims 2 to 3 and 5, wherein: Each set of direction detecting pixels has a corresponding microlens positioned to focus the scattered radiation towards the set of direction detecting pixels.

12. The detection device according to claim 11, wherein: Each microlens is positioned at a distance from its corresponding set of direction detection pixels, the distance comprising the focal length of the microlens.

13. The detection device according to claim 11, wherein: The microlenses form an array of microlenses sparsely spaced with a duty cycle of less than 10%.

14. The detection device according to claim 7, wherein: The detection device comprises a processor operable to determine an angle of incidence of the scattered radiation from which one or more direction detection pixels of a set of direction detection pixels detect an intensity value exceeding a threshold.

15. The detection device according to claim 14, wherein: The processor is operable to determine an angle of incidence of the scattered radiation based on relative intensity levels detected at at least two corresponding direction detection pixels in a set of direction detection pixels.

16. The detection device according to claim 11, wherein: The detection device comprises a processor operable to determine the angle of incidence of the scattered radiation from the position of a spot focused by the microlens onto the set of direction detection pixels.

17. A detection device according to any one of claims 14 to 16, wherein: The processor is operable to perform interpolation on an image captured by the detector based on the determined value of the angle of incidence local to each of the sets of direction detecting pixels to interpolate the angle of incidence or a phase value determined from the angle of incidence; as well as The result of the interpolation step is used to determine a phase map describing the phase of the scattered radiation at the detector.

18. The detection device according to claim 17, wherein: The processor is operable to determine a complex-valued field of the scattered radiation at the detector using the phase map and image data captured by the imaging pixels.

19. The detection device according to claim 18, wherein: The detection device comprises sensor optics for capturing the scattered radiation; and the processor is operable to use the complex-valued field to correct aberrations in the sensor optics.

20. A metrology device comprising a detection apparatus according to any preceding claim.

21. A lithography unit, the lithography unit include ： The metrology apparatus of claim 20, the metrology apparatus being configured to perform measurements based on which corrections for a lithography process are determined; and A lithographic apparatus is operable to perform the lithographic process in accordance with the correction.

22. A lithographic device, comprising a measurement module, wherein the measurement module comprises a detection device according to any one of claims 1 to 19.

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