Method for operating a detection system of a measurement device and associated measurement device

Abstract

A method of reading out a detection arrangement defining a detection area in a plurality of pixels is disclosed. The method includes receiving scattered radiation on the detection arrangement; dividing the detection area into at least two different regions of interest at least based on the measurement parameters of the scattered radiation; and employing a respective different readout scheme for each of the regions of interest in order to read out the detection arrangement.

Description

[0001] Cross - reference to related applications

[0002] This application claims priority to European Patent Application 22200780.9, filed on October 11, 2022, and the entire disclosure of this patent application is incorporated herein by reference. Field of the Invention

[0003] The present invention relates to metrology applications in the manufacture of integrated circuits. Background Art

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

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

[0006] Low k1 1 Lithography can be used to process features smaller than the classical resolution limit of a lithographic apparatus. In such processes, the resolution formula can be expressed as CD = k1 1 ×λ / NA, where λ is the wavelength of the radiation employed, 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 the half-pitch), and k1 1 is an empirical resolution factor. Generally, the smaller k1 1 is, the more difficult it is to reproduce on the substrate a pattern similar in shape and size to that planned by a circuit designer to achieve a particular electrical function and performance. To overcome these difficulties, complex fine-tuning steps can be applied to the lithographic projection apparatus and / or the design layout. These include, for example but not limited to, optimization of NA, customized illumination schemes, use of phase-shifting patterning devices, various optimizations of the design layout such as optical proximity correction (OPC, sometimes also referred to as “optical and process correction”) in the design layout, or other methods generally defined as “resolution enhancement techniques” (RET). Alternatively, a tight control loop for controlling the stability of the lithographic apparatus can be used to improve pattern reproduction at low k1.

[0007] In lithography processes and other manufacturing processes, it is desirable to frequently measure the created structures, for example for process control and verification. Various tools for performing such measurements are known, including scanning electron microscopes commonly used to measure critical dimensions (CD) and dedicated tools for measuring overlay, which is the accuracy of alignment of two layers in a device. More recently, various forms of scatterometers have been developed for the lithography field. The manufacturing process can be, for example, lithography, etching, deposition, chemical mechanical planarization, oxidation, ion implantation, diffusion, or a combination of two or more of them.

[0008] Examples of known scatterometers typically rely on providing dedicated metrology targets. For example, the method may require a target in the form of a simple grating that is large enough such that the measurement beam produces a spot smaller than the grating (i.e., the grating is underfilled). In so-called reconstruction methods, the properties of the grating can be calculated by simulating the interaction of the scattered radiation with a mathematical model of the target structure. The parameters of the model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from the real target.

[0009] In addition to measuring feature shape by reconstruction, such devices can also measure diffraction-based overlay as described in the published patent application US2006066855A1. Diffraction-based overlay metrology using dark field imaging of diffraction orders enables overlay measurements to be made on smaller targets. These targets can be smaller than the illumination spot and can be surrounded by product structures on the wafer. Examples of dark field imaging metrology can be found in many published patent applications such as US2011102753A1 and US20120044470A. Using a composite grating target, multiple gratings can be measured in one image. Known scatterometers tend to use light in the visible or near infrared (IR) band, which requires the pitch of the grating to be coarser than the actual product structures whose properties are actually of interest. Such product features can be defined using much shorter wavelength deep ultraviolet (DUV), extreme ultraviolet (EUV), or X-ray radiation. Unfortunately, such wavelengths are generally not available or cannot be used for metrology.

[0010] On the other hand, modern product structures are so small that they cannot be imaged by optical metrology techniques. Small features include, for example, those formed by multiple patterning processes and / or pitch multiplication. Thus, targets for high-volume metrology typically use features that are much larger than the product with the overlay error or critical dimension as the property of interest. The measurement results are only indirectly related to the dimensions of the real product structure and can be inaccurate because the metrology target does not suffer the same distortion under different treatments such as optical projection in a lithographic apparatus and / or other steps of the manufacturing process. Although a scanning electron microscope (SEM) can directly resolve these modern product structures, SEM is much more time-consuming than optical measurement. In addition, electrons cannot penetrate thick process layers, making them less suitable for metrology applications. Other techniques, such as measuring electrical properties using contact pads, are also known, but they only provide indirect evidence of the real product structure.

[0011] By reducing the wavelength of the measurement radiation used during metrology (e.g., moving towards the "soft X-ray (SXR)" wavelength spectrum), smaller structures can be resolved to increase the sensitivity to structural changes of the structure and / or additionally penetrate the product structure. One such method of generating suitable high-frequency measurement radiation (e.g., hard X-rays, soft X-rays, and / or EUV radiation) can be to use pump radiation (e.g., infrared IR radiation) to excite a generation medium to generate emission radiation, optionally including high harmonic generation of high-frequency radiation.

[0012] In some metrology systems, such as soft X-ray SXR metrology systems, a pixelated detector is used to measure the diffracted light from a sample. The illumination beam can have a relatively low divergence, resulting in diffracted orders that are detected on only a few pixels of the detector.

[0013] It is desirable to optimize the readout of such detectors. Summary of the Invention

[0014] In a first aspect of the present invention, there is provided a method of readout detection arrangement, the detection arrangement defining a detection area according to a plurality of pixels, the method comprising: receiving scattered radiation on the detection arrangement; dividing the detection area into at least two different regions of interest based at least on measurement parameters of the scattered radiation; and applying a corresponding different readout scheme to each of the regions of interest in order to read out the detection arrangement.

[0015] In a second aspect of the present invention, there is provided a detection module, comprising: a detection arrangement defining a detection area according to a plurality of pixels; and a processor operable to: divide the detection area into at least two different regions of interest based at least on measurement parameters of the scattered radiation received on the detection arrangement; and apply a corresponding different readout scheme to each of the regions of interest in order to read out the detection arrangement.

[0016] The above and other aspects of the present invention will be understood by considering the examples described below. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Embodiments will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0018] - Figure 1 Schematic overview depicting a lithographic apparatus;

[0019] - Figure 2 Schematic overview depicting a lithography cell;

[0020] - Figure 3 Schematic representation depicting overall lithography, which represents the collaboration between three key technologies for optimizing semiconductor manufacturing;

[0021] - Figure 4 Schematically illustrate a scatterometry apparatus;

[0022] - Figure 5 Schematic representation depicting a metrology apparatus using EUV and / or SXR radiation; - Figure 6 Simplified schematic of an illumination source, which can be an illumination source for high harmonic generation for a metrology apparatus such as Figure 5 as illustrated;

[0023] - Figure 7 Comprising (a) a schematic of a dark field scatterometer for measuring a target using a first pair of illumination apertures according to an embodiment of the present invention, (b) details of the diffraction spectrum of a target grating for a given illumination direction, (c) a second pair of illumination apertures providing an additional illumination mode when performing diffraction-based overlay measurements using the scatterometer, and (d) a third pair of illumination apertures combining the first pair of apertures and the second pair of apertures;

[0024] - Figure 8 Schematic illustration of a metrology apparatus in a known configuration;

[0025] - Figure 9 Illustration of a 2D diffraction pattern obtainable using a metrology apparatus such as Figure 8 as illustrated;

[0026] - Figure 10 (a) shows Figure 9 the 2D diffraction pattern and Figure 10 (b) illustrates how such a pattern can be divided into two regions of interest according to an embodiment;

[0027] - Figure 11 Illustration of the total acquisition time as a function of a threshold for dividing the incident power into two regions of interest; and

[0028] -Figure 12 Timing diagrams including (a) a conventional rolling shutter readout scheme, (b) an exemplary readout scheme according to a first embodiment, and (c) an exemplary readout scheme according to a second embodiment. Detailed Description

[0029] In this document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic radiation and particle radiation, including ultraviolet radiation (e.g., having a wavelength of 365 nm, 248 nm, 193 nm, 157 nm, or 126 nm), EUV (extreme ultraviolet radiation, e.g., having a wavelength in the range of about 5 nm - 100 nm), X-ray radiation, electron beam radiation, and other particle radiation.

[0030] As used herein, the terms “reticle”, “mask”, or “patterning device” can be broadly interpreted to refer to a general patterning device that can be used to endow an incident radiation beam with a patterned cross-section corresponding to a pattern to be created in a target portion of a substrate. The term “light valve” can also be used in this context. Examples of other such patterning devices include programmable mirror arrays and programmable LCD arrays in addition to classical masks (transmission or reflection, binary, phase-shifting, hybrid, etc.).

[0031] Figure 1 Schematically depicts a lithographic apparatus LA. The lithographic apparatus LA includes: an illumination system (also referred to as an illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation, EUV radiation, or X-ray radiation); a mask support (e.g., a mask table) T 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 table) 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 the pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g., including one or more dies) of the substrate W.

[0032] In operation, the illumination system IL receives a radiation beam from a radiation source SO via, for example, a beam delivery system BD. The illumination system IL can include various types of optical components, such as refractive, reflective, diffractive, magnetic, electromagnetic, electrostatic, and / or other types of optical components, or any combination thereof, for guiding, shaping, and / or controlling the radiation. The illuminator IL can be used to condition the radiation beam B such that it has a desired spatial and angular intensity distribution in a cross-section at the plane of the patterning device MA.

[0033] As used herein, the term "projection system" PS should be interpreted broadly to cover various types of projection systems, including refractive, reflective, diffractive, catadioptric, anamorphic, magnetic, electromagnetic, and / or electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation used and / or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system" PS.

[0034] The lithographic apparatus LA may be of the type in which at least a portion of the substrate is covered by a liquid having a relatively high refractive index (e.g., water) in order to fill the space between the projection system PS and the substrate W - this is also known as immersion lithography. More information on immersion techniques is given in US6952253, which is incorporated herein by reference in its entirety.

[0035] The lithographic apparatus LA may also be of the type having two or more substrate supports WT (also referred to as "dual stage"). In such "multi-stage" machines, the substrate supports WT may be used in parallel, and / or the steps of preparing the substrate W for subsequent exposure on a substrate W located on one of the substrate supports WT may be carried out while another substrate W on another substrate support WT is being used for exposing the pattern on the other substrate W.

[0036] In addition to the substrate support WT, the lithographic apparatus LA may further include a measurement stage. The measurement stage is arranged to hold a sensor and / or a cleaning device. The sensor may be arranged to measure properties of the projection system PS or of the radiation beam B. The measurement stage may hold a plurality of sensors. The cleaning device may be arranged to clean a part of the lithographic apparatus, such as a part of the projection system PS or a part of the system providing the immersion liquid. The measurement stage may move under the projection system PS when the substrate support WT is moved away from the projection system PS.

[0037] In operation, the radiation beam B is incident on a patterning device (e.g., a mask MA) held on a mask support T and is patterned by the pattern (design layout) present on the patterning device MA. After having passed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. By means of a second positioner PW and a position measurement system IF, the substrate support WT can be moved precisely, for example in order to position different target portions C in the focusing and alignment positions in the path of the radiation beam B. Similarly, a first positioner PM and another position sensor (which is Figure 1(not explicitly depicted in the figure) can be used to precisely position the patterning device MA relative to the path of the radiation beam B. Mask alignment marks M1, M2 and substrate alignment marks P1, P2 can be used to align the patterning device MA and the substrate W. Although the substrate alignment marks Pl, P2 as illustrated occupy dedicated target portions, they can be located in the space between the target portions. The substrate alignment marks Pl, P2 are called scribe alignment marks when located between the target portions C.

[0038] As Figure 2 shown, the lithographic apparatus LA can form part of a lithocell LC, sometimes also called a lithographic cell or (lithographic) cluster, which typically also includes apparatus for performing pre-exposure and post-exposure processing on the substrate W. Traditionally, these apparatus include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a cooling plate CH and a baking plate BK, which are used, for example, to regulate the temperature of the substrate W, for example, to regulate the solvent in the resist layer. A substrate handler or robot RO picks up the substrate W from the input / output ports I / O1, I / O2, moves it between the different processing apparatus, and transports the substrate W to the loading bay LB of the lithographic apparatus LA. The apparatus in the lithocell (which are typically also collectively referred to as a track) can be controlled by a track control unit TCU, which itself can be controlled by a supervisory control system SCS, which can also control the lithographic apparatus LA, for example, via a lithography control unit LACU.

[0039] In a lithographic process, it is desirable to frequently measure the structures created, for example, for process control and verification. Tools for performing such measurements can be referred to as metrology tools MT. Different types of metrology tools MT for performing such measurements are known, including scanning electron microscopes or various forms of scatterometer metrology tools MT. A scatterometer is a general-purpose instrument that allows the parameters of a lithographic process to be measured by: placing the sensor in or near the pupil of the scatterometer objective or in a plane conjugate to the pupil, and measuring what is typically referred to as pupil-based measurement, or placing the sensor in or near the image plane or in a plane conjugate to the image plane, in which case the measurement is typically referred to as image or field-based measurement. Such scatterometers and associated measurement techniques are described additionally 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 use light from the hard X-ray (HXR), soft X-ray (SXR), extreme ultraviolet (EUV), visible light to near-IR and IR wavelength ranges to measure gratings. In the case where the radiation is hard X-ray or soft X-ray, the aforementioned scatterometer can optionally be a small angle X-ray scattering metrology tool.

[0040] In order to expose the substrate W by the lithographic apparatus LA correctly and uniformly, it is desirable to inspect the substrate to measure properties of the patterned structure, such as overlay error between subsequent layers, line thickness, critical dimension (CD), structure shape, etc. For this purpose, an inspection tool and / or a metrology tool (not shown) may be included in the lithography cell LC. If an error is detected, then, for example, the exposure of subsequent substrates or other processing steps to be performed on the substrate W can be adjusted, especially if the inspection is completed before the same batch or other substrates W of the same lot still have to be exposed or processed.

[0041] An inspection device, also referred to as a metrology device, is used to determine properties of the substrate W, in particular how the properties of different substrates W vary, or how the properties associated with different layers of the same substrate W vary layer by layer. Alternatively, the inspection device can be configured to identify defects on the substrate W and can, for example, be part of the lithography cell LC, or can be integrated into the lithographic apparatus LA, or can even be a stand-alone device. The inspection device can measure properties on a latent image (an image in the resist layer after exposure), or a semi-latent image (an image in the resist layer after the post-exposure bake step PEB), or a developed resist image (where the exposed or unexposed portions of the resist have been removed), or even an etched image (after a pattern transfer step such as etching).

[0042] In a first embodiment, the scatterometer MT is an angular-resolved scatterometer. In such a scatterometer, a reconstruction method can be applied to the measurement signal to reconstruct or calculate properties of the grating. For example, such a reconstruction can be generated by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measurement results. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from the real target.

[0043] 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 the target, and the reflected, transmitted, or scattered radiation from the target is directed to a spectrometer detector that measures the spectrum of the specularly reflected radiation (i.e., the measured value of the intensity as a function of wavelength). Based on this data, the structure or profile of the target that produced the detected spectrum can be reconstructed, for example, by rigorous coupled-wave analysis and non-linear regression or by comparison with a simulated spectral library.

[0044] In a third embodiment, the scatterometer MT is an ellipsometer. An ellipsometer allows for the determination of parameters of a lithography process by measuring scattered (which can be diffracted, reflected, or transmitted) radiation for each polarization state. Such metrology devices emit polarized light (such as linear, circular, or elliptical) by using, for example, a suitable polarization filter in the illumination section of the metrology device. Sources suitable for metrology devices can also provide polarized radiation. Various embodiments of existing ellipsometers are described in U.S. 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.

[0045] 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 the asymmetry in the detection configuration, the asymmetry being related to the degree of overlay. Two (which can be overlapping) grating structures can be applied in two different layers (not necessarily consecutive layers) and can be formed substantially at the same location on the wafer. The scatterometer can have a symmetric detection configuration, as described, for example, in co-owned Patent Application EP1,628,164A, such that any asymmetry is clearly distinguishable. This provides a direct way to measure grating misalignment. Additional examples of measuring overlay errors between two layers containing periodic structures as targets through the asymmetry of the periodic structures can be found in PCT Patent Application Publication No. WO 2011 / 012624 or U.S. Patent Application US20160161863, which are incorporated herein by reference in their entirety.

[0046] Other parameters of interest can be focus and dose. Focus and dose can be determined simultaneously by scatterometry (or alternatively by scanning electron microscopy), as described in U.S. Patent Application US2011-0249244, which is incorporated herein by reference in its entirety. A single structure can be used, which has a unique combination of critical dimension and sidewall angle measurements for each point in a Focus Energy Matrix (FEM - also known as a Focus Exposure Matrix). If these unique combinations of critical dimension and sidewall angle are available, then the focus and dose values can be uniquely determined from these measurements.

[0047] The measurement target can be a set of composite gratings, which are mainly formed in the resist but also by lithography after other manufacturing processes such as etching. The pitch and line width of the structures in the grating can strongly depend on the measurement optics (especially the NA of the optics) capable of capturing the diffraction orders from the measurement target. As previously indicated, the diffraction signal can be used to determine the displacement between two layers (also called 'overlay'), or can be used to reconstruct at least a part of the original grating generated by the lithography process. Such reconstruction can be used to provide guidance on the quality of the lithography process and can be used to control at least a part of the lithography process. The target can have smaller sub-divisions, which are configured to mimic the dimensions of the functional parts of the design layout in the target. Due to such sub-division, the target will behave more similarly to the functional parts of the design layout, making the overall process parameter measurement better resemble the functional parts of the design layout. The target can be measured in underfill mode or overfill mode. In underfill mode, the spot produced by the measurement beam is smaller than the overall target. In overfill mode, the spot produced by the measurement beam is larger than the overall target. In such overfill mode, different targets can also be measured simultaneously, thereby determining different process parameters simultaneously.

[0048] The overall measurement quality of the lithography parameters using a specific target is at least partly determined by the measurement recipe used to measure this lithography parameter. The term'substrate measurement recipe' can include one or more parameters of the measurement itself, one or more parameters of one or more patterns of the measurement, or both. For example, if the measurement used in the substrate measurement recipe is a diffraction-based optical measurement, then one or more of the parameters of the measurement can include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the orientation of the radiation relative to the pattern on the substrate, etc. For example, one of the criteria for selecting a measurement recipe can be the sensitivity of one of the measurement parameters to process variations. More examples are described in US Patent Application US2016 - 0161863 and Published US Patent Application US2016 / 0370717A1, which are incorporated herein by reference in their entirety.

[0049] The patterning process in the lithography apparatus LA can be one of the most critical steps in the process, which requires high-precision sizing and placement of structures on the substrate W. To ensure such high precision, three systems can be combined in a so-called 'integrated' control environment, as Figure 3Schematically depicted. One of these systems is a lithographic apparatus LA, which is (virtually) connected to a metrology tool MT (second system) and a computer system CL (third system) that runs a computer program, for example. The key to such an "integrated" environment is to optimize the collaboration 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 lithographic apparatus LA remains within the process window. The process window defines the range of process parameters (such as dose, focus, overlay) within which a particular manufacturing process yields a defined result (such as a functional semiconductor device) - the process parameters in the lithographic process or patterning process are allowed to vary within this range.

[0050] The computer system CL can use (a part of) the design layout to be patterned to predict which resolution enhancement techniques are to be used and perform computational lithography simulations and calculations to determine which mask layout and lithographic apparatus settings achieve the maximum overall process window for the patterning process (depicted by the double arrow in the first scale SCI in Figure 3 ). The resolution enhancement techniques can be arranged to match the patterning capabilities of the lithographic apparatus LA. The computer system CL can also be used to detect the current operating position of the lithographic apparatus LA within the process window (for example, using input from the metrology tool MET) to predict whether there may be defects due to, for example, sub-optimal processing (depicted by the arrow pointing to "0" in the second scale SC2 in Figure 3 ).

[0051] The metrology tool MT can provide input to the computer system CL to enable accurate simulation and prediction and can provide feedback to the lithographic apparatus LA to identify possible drifts, for example, in the calibration state of the lithographic apparatus LA (depicted by the multiple arrows in the third scale SC3 in Figure 3 ).

[0052] Many different forms of metrology tool MT can be provided for measuring structures created using a lithographic patterning apparatus. The metrology tool MT can use electromagnetic radiation to interrogate the structure. The properties of the radiation (such as wavelength, bandwidth, power) can affect different measurement characteristics of the tool, where shorter wavelengths generally allow for improved resolution. The radiation wavelength has an impact on the resolution achievable by the metrology tool. Therefore, in order to be able to measure structures with small feature sizes, a metrology tool MT with a short-wavelength radiation source is preferred.

[0053] Another way in which the radiation wavelength can affect the measurement characteristics is the penetration depth, and the transparency / opacity of the material to be inspected at the radiation wavelength. Depending on the opacity and / or penetration depth, the radiation can be used to measure transmission or reflection. The type of measurement can affect whether information about the surface and / or the interior of the structure / substrate is obtained. Therefore, the penetration depth and opacity are another element to consider when selecting the radiation wavelength for a metrology tool.

[0054] To achieve higher resolution in lithographic patterned structure measurements, it is preferable to have a metrology tool MT with a short wavelength. This can include wavelengths shorter than the visible light wavelength, such as in the UV, EUV, and X-ray portions of the electromagnetic spectrum. Hard X-ray methods such as transmission small angle X-ray scattering (TSAXS) use the high resolution and high penetration depth of hard X-rays and can thus operate in transmission. On the other hand, soft X-rays and EUV do not penetrate the target very far but can cause rich optical responses in the material to be probed. This can be due to the optical properties of many semiconductor materials, as well as the fact that the structures are comparable in size to the probing wavelength. As a result, EUV and / or soft X-ray metrology tools MT can operate in reflection, for example, by imaging or by analyzing the diffraction pattern from the lithographic patterned structure.

[0055] For hard X-rays, soft X-rays, and EUV radiation, their applications in high volume manufacturing (HVM) applications can be limited due to the lack of available high brightness radiation sources at the required wavelengths. In the case of hard X-rays, sources commonly used in industrial applications include X-ray tubes. X-ray tubes, including advanced X-ray tubes based on, for example, liquid metal anodes or rotating anodes, can be relatively inexpensive and compact but may lack the brightness required for HVM applications. Currently, there are high brightness X-ray sources such as synchrotron light sources (SLS) and X-ray free electron lasers (XFEL), but their size (>100m) and high cost (hundreds of millions of euros) make them too large and expensive for metrology applications. Similarly, there is a lack of availability of sufficiently bright EUV and soft X-ray radiation sources.

[0056] In a lithography process, it is desirable to frequently measure the created structures, for example for process control and verification. Various tools for performing such measurements are known, including scanning electron microscopes or various forms of metrology devices, such as scatterometers. Examples of known scatterometers typically rely on providing dedicated metrology targets, such as underfilled targets (targets in the form of simple gratings or overlapping gratings in different layers, which are large enough such that the measurement beam produces a spot smaller than the grating) or overfilled targets (whereby the illuminated spot partially or fully encompasses the target). Additionally, using a metrology tool, such as an angular-resolved scatterometer that illuminates an underfilled target such as a grating, allows the use of so-called reconstruction methods, where 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 simulation results with the measurement results. The parameters of the model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from the real target.

[0057] A scatterometer is a general-purpose instrument that allows the measurement of lithography process parameters by placing a sensor in the pupil of the scatterometer objective or in a plane conjugate to the pupil, in which case the measurement is typically referred to as pupil-based measurement, or by placing the sensor in the image plane or in a plane conjugate to the image plane, in which case the measurement is typically referred to as image- or field-based measurement. Such scatterometers and associated measurement techniques are additionally 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 the soft x-ray, extreme ultraviolet, visible light to the near-IR band.

[0058] Figure 4 Depicts an example of a metrology device, such as a scatterometer. It can include a broadband (e.g., white light) radiation projector 2 that projects radiation 5 onto a substrate W. The reflected or scattered radiation 10 is passed to a spectrometer detector 4 that measures the spectrum 6 of the specularly reflected radiation (i.e., the measured value of the intensity I as a function of the wavelength λ). From this data, the structure or profile 8 that produced the detected spectrum can be reconstructed by a processing unit PU, for example, by rigorous coupled-wave analysis and non-linear regression or by comparison with a simulated spectral library as shown at the bottom. Typically, for reconstruction, the general form of the structure is known, and some parameters are assumed from knowledge of the process that fabricated the structure, leaving only a few parameters of the structure to be determined from the scatterometry measurement data. Such scatterometers can be configured as normal-incidence scatterometers or oblique-incidence scatterometers. Figure 4 A transmission version of the example of the metrology device such as

[0059] The transmission version of the example of the metrology device such asFigure 4 The scatterometer shown is another example. The transmitted radiation is passed to a spectrometer detector that measures the spectrum, as Figure 4 discussed. Such scatterometers can be configured as normal incidence scatterometers or grazing incidence scatterometers. Optionally, the transmission version uses hard X-ray radiation with a wavelength < 1 nm, optionally < 0.1 nm, optionally < 0.01 nm.

[0060] As an alternative to optical metrology methods, the use of hard X-rays, soft X-rays or EUV radiation is also considered, for example radiation having at least one of the following wavelength ranges: < 0.01 nm, < 0.1 nm, < 1 nm, between 0.01 nm and 100 nm, between 0.01 nm and 50 nm, between 1 nm and 50 nm, between 1 nm and 20 nm, between 5 nm and 20 nm, and between 10 nm and 20 nm. An example of a metrology tool that operates in one of the wavelength ranges presented above is transmission small angle X-ray scattering (T-SAXS as in US2007224518A, the content of which is incorporated herein by reference in its entirety). Lemaillet et al. discuss the critical dimension (CD) measurements using T-SAXS in "Intercomparison between optical and X-ray scatterometry measurements of FinFET structures", Proc. of SPIE, 2013, 8681. Note that the use of laser-produced plasma (LPP) X-ray sources is described in US Patent Publication No. 2019 / 003988A1 and US Patent Publication No. 2019 / 215940A1, these patent publications being incorporated herein by reference in their entirety. Reflectometry techniques using X-rays (GI-XRS) and extreme ultraviolet (EUV) radiation at grazing incidence can be used to measure the properties of films and layer stacks on a substrate. Within the general field of reflectometry, goniometric and / or spectroscopic techniques can be applied. In goniometry, the variation of the reflected beam with different angles of incidence can be measured. On the other hand, spectroscopic reflectometry measures the wavelength spectrum reflected at a given angle (using broadband radiation). For example, EUV reflectometry has been used to inspect mask blanks before manufacturing a mask blank (patterning device) for EUV lithography.

[0061] The application scope can render the use of wavelengths in, for example, the hard X-ray, soft X-ray or EUV domains insufficient. The published patent applications US20130304424A1 and US2014019097A1 (Bakeman et al. / KLA) describe hybrid metrology techniques in which measurements made using X-rays and optical measurements with wavelengths in the range from 120 nm to 2000 nm are combined to obtain measurements of parameters such as CD. The CD measurement is obtained by coupling an X-ray mathematical model and an optical mathematical model through one or more common points. The content of the cited US patent applications is incorporated herein by reference in its entirety.

[0062] Figure 5 A schematic representation of a metrology device 302 is depicted, in which the aforementioned radiation can be used to measure parameters of structures on a substrate. Figure 5 The presented metrology device 302 can be adapted for the hard X-ray, soft X-ray and / or EUV domains.

[0063] Figure 5 The schematic physical arrangement of the metrology device 302 is illustrated only by way of example and includes a spectroscopic scatterometer that optionally uses hard X-ray, EUV and / or SXR radiation in a grazing incidence manner. Alternative forms of inspection devices can be provided in the form of angularly resolved scatterometers that can use radiation with normal or nearly normal incidence similar to conventional scatterometers operating at longer wavelengths, and which can also use radiation in a direction greater than 1° or 2° with respect to the direction parallel to the substrate. Alternative forms of inspection devices can be provided in the form of transmission scatterometers.

[0064] The inspection device 302 includes a radiation source or so-called illumination source 310, an illumination system 312, a substrate support 316, a detection system 318, 398 and a metrology processing unit (MPU) 320.

[0065] In this example, the illumination source 310 is used to generate EUV, hard X-ray or soft X-ray radiation. The illumination source 310 can be based on Figure 6 the high harmonic generation (HHG) technique as shown, and it can also be other types of illumination sources, such as a liquid metal jet source, an inverse Compton scattering (ICS) source, a plasma channel source, a magnetic undulator source, a free electron laser (FEL) source, a compact storage ring source, a discharge-produced plasma source, a soft X-ray laser source, a rotating anode source, a solid anode source, a particle accelerator source, a microfocus source or a laser-produced plasma source.

[0066] The HHG source can be a gas jet / nozzle source, a capillary / fiber source or a gas cell source.

[0067] For an example of an HHG source, as Figure 6As shown, the main components of the radiation source are a pump radiation source 330 operable to emit pump radiation and a gas delivery system 332. Optionally, the pump radiation source 330 is a laser, and optionally, the pump radiation source 330 is a pulsed high-power infrared or optical laser. The pump radiation source 330 can be, for example, a fiber-based laser with an optical amplifier, generating infrared radiation pulses that can be, for example, less than 1 ns (1 nanosecond) in duration per pulse, with a pulse repetition rate up to several megahertz. The wavelength of the infrared radiation can be in the range of 200 nm to 10 μm, for example, in the region of 1 μm (1 micron). Optionally, the laser pulse is delivered as first pump radiation 340 to the gas delivery system 332, where a portion of the radiation of the gas is converted to a higher frequency than the first radiation, becoming emission radiation 342. A gas supply source 334 supplies a suitable gas to the gas delivery system 332, where the gas is optionally ionized by a power source 336. The gas delivery system 332 can be a cutting tube.

[0068] The gas provided by the gas delivery system 332 defines a gas target, which can be a gas flow or a static volume. The gas can be, for example, air, neon (Ne), helium (He), nitrogen (N 2 ), oxygen (O 2 ), argon (Ar), krypton (Kr), xenon (Xe), carbon dioxide, and combinations thereof. These can be selectable options within the same device. The emission radiation can include multiple wavelengths. If the emission radiation is monochromatic, then measurement calculations (such as reconstruction) can be simplified, but it is easier to generate radiation with several wavelengths. The emission divergence angle of the emission radiation can be wavelength-dependent. For example, when imaging the structures of different materials, different wavelengths will provide different levels of contrast. For example, for the inspection of metal or silicon structures, different wavelengths can be selected for imaging the features of (carbon-based) resists, or for detecting the contamination of such different materials. One or more filtering devices 344 can be provided. For example, filters such as thin films of aluminum (Al) or zirconium (Zr) can be used to prevent the passage of the fundamental IR radiation to the inspection device. A grating (not shown) can be provided to select one or more specific wavelengths from those generated. Optionally, the irradiation source includes a space configured to be evacuated, and the gas delivery system is configured to provide a gas target in that space. Optionally, some or all of the beam paths can be included in a vacuum environment, bearing in mind that SXR and / or EUV radiation is absorbed when propagating in air. The various components of the radiation source 310 and the irradiation optics 312 can be adjustable to achieve different metrology 'formulas' within the same device. For example, different wavelengths and / or polarizations can be selected.

[0069] Depending on the material of the structure being inspected, different wavelengths can provide the desired level of penetration through the lower layers. To resolve the smallest device features and defects among the smallest device features, then shorter wavelengths may be preferred. For example, one or more wavelengths in the range of 0.01 - 20 nm or alternatively in the range of 1 - 10 nm or alternatively in the range of 10 - 20 nm can be selected. When reflected from the materials of interest in semiconductor manufacturing, wavelengths shorter than 5 nm can be affected by very low critical angles. Therefore, selecting wavelengths greater than 5 nm can provide a stronger signal at a higher angle of incidence. On the other hand, if the inspection task is for detecting the presence of a certain material, such as for detecting contamination, then wavelengths up to 50 nm can be useful.

[0070] The filtered beam 342 can enter the inspection chamber 350 from the radiation source 310, where the substrate W including the structure of interest is held in a measurement position by the substrate support 316 for inspection. The structure of interest is labeled T. Optionally, the atmosphere within the inspection chamber 350 can be maintained at near-vacuum by a vacuum pump 352 such that EUV radiation can pass through the atmosphere without excessive attenuation. The illumination system 312 has the function of focusing the radiation into a focused beam 356 and can include, for example, a two-dimensional curved mirror or a series of one-dimensional curved mirrors as described in the above-mentioned published U.S. Patent Application US2017 / 0184981A1, the content of which is incorporated herein by reference in its entirety. When projected onto the structure of interest, focusing is performed to achieve a circular or elliptical spot S with a diameter less than 10 μm. The substrate support 316 includes, for example, an X-Y translation stage and a rotation stage by which any part of the substrate W can be brought to the focus of the beam in a desired orientation. Thus, the radiation spot S is formed on the structure of interest. Alternatively or additionally, the substrate support 316 includes, for example, a tilt stage that can tilt the substrate W at an angle to control the angle of incidence of the focused beam on the structure of interest T.

[0071] Optionally, the illumination system 312 provides a reference radiation beam to a reference detector 314 that can be configured to measure the spectrum and / or intensity of different wavelengths in the filtered beam 342. The reference detector 314 can be configured to generate a signal 315 provided to the processor 320, and the filter can include information about the spectrum of the filtered beam 342 and / or the intensity of different wavelengths in the filtered beam.

[0072] The reflected radiation 360 is captured by the detector 318 and the spectrum is provided to the processor 320 for calculating the properties of the target structure T. Thus, the illumination system 312 and the detection system 318 form an inspection device. The inspection device can include the kind of hard X-ray, soft X-ray, and / or EUV spectroscopic reflectometer described in US2016282282A1, the content of which is incorporated herein by reference in its entirety.

[0073] If the target Ta has a certain periodicity, then the radiation of the focused beam 356 can also be partially diffracted. Compared with the reflected radiation 360, the diffracted radiation 397 follows another path at a clearly defined angle relative to the angle of incidence. In Figure 5 , the diffracted radiation 397 is drawn schematically, and the diffracted radiation 397 can follow many other paths in addition to the drawn path. The inspection device 302 may also include an additional detection system 398 that detects and / or images at least a portion of the diffracted radiation 397. In Figure 5 , a single additional detection system 398 is drawn, but embodiments of the inspection device 302 may also include more than one additional detection system 398, which are arranged at different positions to detect and / or image the diffracted radiation 397 in multiple diffraction directions. In other words, the (higher) diffraction orders of the focused radiation beam impinging on the target Ta are detected and / or imaged by one or more additional detection systems 398. One or more detection systems 398 generate a signal 399 that is provided to the metrology processor 320. The signal 399 may include information about the diffracted light 397 and / or may include an image obtained from the diffracted light 397.

[0074] To assist in aligning and focusing the spot S with the desired product structure, the inspection device 302 may also use auxiliary optics using auxiliary radiation under the control of the metrology processor 320. The metrology processor 320 may also communicate with a position controller 372 that operates a translation stage, a rotation, and / or a tilt stage. The processor 320 receives highly accurate feedback regarding the position and orientation of the substrate via sensors. For example, the sensor 374 may include an interferometer, which can give an accuracy in the picometer range. During operation of the inspection device 302, the spectral data 382 captured by the detection system 318 is conveyed to the metrology processing unit 320.

[0075] As mentioned, the inspection device in the alternative form optionally uses hard X-rays, soft X-rays, and / or EUV radiation, for example, to perform diffraction-based asymmetry measurements in a normal incidence or nearly normal incidence manner. Another alternative form of the inspection device uses hard X-rays, soft X-rays, and / or EUV radiation at an angle greater than 1° or 2° from a direction parallel to the substrate. Both types of inspection devices can be provided in a hybrid metrology system. The performance parameters to be measured can include overlay (OVL), critical dimension (CD), the focus of the lithography apparatus when printing the target structure in the lithography apparatus, coherent diffraction imaging (CDI), and alignment resolution overlay (ARO) metrology. The hard X-rays, soft X-rays, and / or EUV radiation can have, for example, a wavelength in the range from 0.01 nm to 100 nm, optionally from 0.1 nm to 100 nm, optionally from 1 nm to 100 nm, optionally from 1 nm to 50 nm, or optionally from 10 nm to 20 nm. The characteristics of the radiation can be narrowband or broadband. The radiation can have discrete peaks in a specific wavelength band or can have a more continuous characteristic.

[0076] Like the optical scatterometers used in today's production facilities, the inspection device 302 can be used to measure structures in the resist material processed within the lithography cell (post-development inspection or ADI), and / or to measure structures formed in a harder material (post-etch inspection or AEI). For example, the substrate can be inspected using the inspection device 302 after it has been processed by a development device, an etching device, an annealing device, and / or other devices.

[0077] A metrology tool MT, including but not limited to the scatterometer mentioned above, can perform measurements using radiation from a radiation source. The radiation used by the metrology tool MT can be electromagnetic radiation. The radiation can be optical radiation, such as radiation in the infrared, visible, and / or ultraviolet portions of the electromagnetic spectrum. The metrology tool MT can use the radiation to measure or inspect properties and aspects of a substrate, such as a lithographic exposure pattern on a semiconductor substrate. The type and quality of the measurement can depend on several properties of the radiation used by the metrology tool MT. For example, the resolution of an electromagnetic measurement can depend on the wavelength of the radiation. For example, due to the diffraction limit, a smaller wavelength can measure smaller features. To measure features with small dimensions, it can be preferable to use radiation with a short wavelength, such as EUV, hard X-ray, and / or soft X-ray (SXR) radiation, to perform the measurement. To perform metrology at a specific wavelength or wavelength range, the metrology tool MT needs access to a source that provides the radiation of that / those wavelengths. There are different types of sources that provide radiation of different (one or more) wavelengths. Depending on the wavelength provided by the source, different types of radiation generation methods can be used. The source can use high harmonic generation (HHG) or any other type of illumination source mentioned above to obtain radiation of the desired wavelength. One of the challenges faced in the development of these sources is how to effectively couple the emitted radiation out of the generation setup and separate the emitted radiation from the radiation used to drive the process.

[0078] Figure 6 A simplified schematic diagram of an embodiment 600 of an illumination source 310 is shown, which can be an illumination source for high harmonic generation. Refer to Figure 5 One or more of the features of the illumination source in the described metrology tool can also be appropriately present in the illumination source 600. The illumination source 600 includes a chamber 601. The illumination source 600 is configured to receive pump radiation 611 having a propagation direction indicated by an arrow. The pump radiation 611 shown herein is an example of pump radiation 340 from a pump radiation source 330, as Figure 5 shown. The pump radiation 611 can be guided into the chamber 601 through a radiation input end 605, which can be an observation port made of fused quartz or a similar material. The pump radiation 611 can have a Gaussian or hollow (e.g., annular) transverse cross-sectional profile and can be incident (optionally focused) on an air flow 615 within the chamber 601, which has a flow direction indicated by a second arrow. The air flow 615 includes a small volume, called a gas volume or gas target (e.g., a few cubic mm), of a specific gas (e.g., air, neon (Ne), helium (He), nitrogen (N 2 ), oxygen (O 2 ), argon (Ar), krypton (Kr), xenon (Xe), carbon dioxide, and combinations thereof), where the gas pressure is above a certain value. The air flow 615 can be a steady flow. Other media, such as metal plasmas (e.g., aluminum plasma), can also be used.

[0079] The gas delivery system of the irradiation source 600 is configured to provide an air flow 615. The irradiation source 600 is configured to provide pump radiation 611 in the air flow 615 to drive the generation of emitted radiation 613. The region where at least most of the emitted radiation 613 is generated is referred to as the interaction region. The interaction region can vary from a few tens of micrometers (for tightly focused pump radiation) to a few millimeters or centimeters (for moderately focused pump radiation), or even up to several meters (for extremely loosely focused pump radiation). The gas delivery system is configured to provide a gas target for generating emitted radiation in the interaction region of the gas target, and optionally, the irradiation source is configured to receive pump radiation and provide pump radiation in the interaction region. Optionally, the air flow 615 is provided by the gas delivery system into a evacuated or nearly evacuated space. The gas delivery system includes a gas nozzle 609, as Figure 6 shown, which may include an opening 617 in the exit plane of the gas nozzle 609. The air flow 615 is provided from the opening 617. A gas trap is used to confine the air flow 615 within a certain volume by extracting the residual air flow and maintaining a vacuum or nearly vacuum atmosphere inside the chamber 601. Optionally, the gas nozzle 609 can be made of a thick-walled tube and / or a high thermal conductivity material to avoid thermal deformation caused by the high-power pump radiation 611.

[0080] It is also conceivable to use the gas nozzle 609 in scaled-up or scaled-down versions ranging from micrometer-sized nozzles to meter-sized nozzles. This wide range of sizing comes from the fact that the setup is scalable such that the intensity of the pump radiation at the air flow is ultimately within a specific range, which can be beneficial for the emitted radiation. Different sizing is required for different pump radiation energies, which can be pulsed lasers and the pulse energy can vary from a few tens of microjoules to several joules. Optionally, the gas nozzle 609 has a thicker wall to reduce nozzle deformation caused by thermal expansion effects, which can be detected by, for example, a camera. A gas nozzle with a thicker wall can produce a more stable gas volume with reduced variation. Optionally, the irradiation source includes a gas trap that maintains the pressure of the chamber 601 near the gas nozzle.

[0081] Due to the interaction of the pump radiation 611 with the gas atoms of the air flow 615, the air flow 615 converts a portion of the pump radiation 611 into emitted radiation 613, which can be Figure 5 an example of the emitted radiation 342 as shown. The central axis of the emitted radiation 613 can be collinear with the central axis of the incident pump radiation 611. The emitted radiation 613 can have a wavelength in the X-ray or EUV range, where the wavelength is in the range from 0.01 nm to 100 nm, optionally from 0.1 nm to 100 nm, optionally from 1 nm to 100 nm, optionally from 1 nm to 50 nm, or optionally from 10 nm to 20 nm.

[0082] In operation, the emitted radiation 613 beam can pass through the radiation output 607, such as an aperture or window, and can then be manipulated and directed by the illumination system 603 (which can be an example of the illumination system 312 in Figure 5 ) to the substrate to be inspected for metrology measurements. The emitted radiation 613 can be directed (optionally focused) onto a structure on the substrate.

[0083] Since air (and indeed any gas) strongly absorbs SXR or EUV radiation, the volume between the gas stream 615 and the wafer to be inspected can be evacuated or nearly evacuated. Since the central axis of the emitted radiation 613 can be collinear with the central axis of the incident pump radiation 611, it may be necessary to block the pump radiation 611 to prevent it from passing through the radiation output 607 and entering the illumination system 603. This can be done by incorporating the Figure 6 filtering device 344 shown into the radiation output 607, which is placed in the emitted beam path and is opaque or nearly opaque to the pump radiation (e.g., opaque or nearly opaque to infrared or visible light), but at least partially transparent to the emitted radiation beam. The filter can be made of zirconium or a variety of materials combined into a multilayer. When the pump radiation 611 has a hollow, optionally annular, transverse cross-sectional profile, the filter can be a hollow, optionally annular, block. Optionally, the filter is not perpendicular and not parallel to the propagation direction of the emitted radiation beam to have effective pump radiation filtering. Optionally, the filtering device 344 includes a hollow block and a thin film filter, such as an aluminum (Al) or zirconium (Zr) film filter. Optionally, the filtering device 344 can also include a mirror that effectively reflects the emitted radiation but poorly reflects the pump radiation, or a wire mesh that effectively transmits the emitted radiation but poorly transmits the pump radiation.

[0084] Described herein are methods, apparatuses, and components for optionally obtaining emitted radiation at a high harmonic frequency of pump radiation. The radiation generated by this process, optionally HHG, which uses nonlinear effects to generate radiation optionally at a harmonic frequency of the provided pump radiation, can be provided as radiation in a metrology tool MT for inspecting and / or measuring a substrate. If the pump radiation includes short pulses (i.e., a few cycles), the generated radiation does not necessarily lie exactly at the harmonic of the pump radiation frequency. The substrate can be a lithographically patterned substrate. The radiation obtained by this process can also be provided in a lithographic apparatus LA and / or a lithographic cell LC. The pump radiation can be pulsed radiation, which can provide a high peak intensity in a short burst time.

[0085] The pump radiation 611 may include one or more types of radiation having one or more wavelengths that are longer than the wavelengths of the emission radiation. The pump radiation may include infrared radiation. The pump radiation may include radiation having (one or more) wavelengths in the range of 500 nm to 1500 nm. The pump radiation may include radiation having (one or more) wavelengths in the range of 800 nm to 1300 nm. The pump radiation may include radiation having (one or more) wavelengths in the range of 900 nm to 1300 nm. The pump radiation may be pulsed radiation. The pulsed pump radiation may include pulses having a duration in the femtosecond range.

[0086] For some embodiments, the emission radiation, optionally the high harmonic radiation, may include one or more harmonics of the pump radiation wavelength. The emission radiation may include wavelengths in the extreme ultraviolet, soft X-ray, and / or hard X-ray portions of the electromagnetic spectrum. The emission radiation 613 may include wavelengths in one or more of the following ranges: less than 1 nm, less than 0.1 nm, less than 0.01 nm, 0.01 nm to 100 nm, 0.1 nm to 100 nm, 0.1 nm to 50 nm, 1 nm to 50 nm, and 10 nm to 20 nm.

[0087] Radiation, such as the high harmonic radiation described above, may be provided as source radiation in a metrology tool MT. The metrology tool MT may use the source radiation to perform measurements on a substrate that has been exposed by a lithographic apparatus. The measurements may be used to determine one or more parameters of the structures on the substrate. Compared to using longer wavelengths (e.g., visible light radiation, infrared radiation), using shorter wavelengths, such as EUV, SXR, and / or HXR wavelengths included in the wavelength ranges described above, may allow smaller features of the structures to be resolved by the metrology tool. Radiation having a shorter wavelength, such as EUV, SXR, and / or HXR radiation, may also penetrate deeper into materials such as a patterned substrate, which means that measurements can be made on deeper layers on the substrate. These deeper layers may not be reachable by radiation having a longer wavelength.

[0088] In the metrology tool MT, the source radiation may be emitted from a radiation source and directed onto a target structure (or other structure) on the substrate. The source radiation may include EUV, SXR, and / or HXR radiation. The target structure may reflect, transmit, and / or diffract the source radiation incident on the target structure. The metrology tool MT may include one or more sensors for detecting the diffracted radiation. For example, the metrology tool MT may include detectors for detecting the positive (+1) and negative (-1) first diffraction orders. The metrology tool MT may also measure specularly reflected or transmitted radiation (zero-order diffracted radiation). Additional sensors for metrology may be present in the metrology tool MT, for example, to measure additional diffraction orders (e.g., higher diffraction orders).

[0089] In an exemplary lithography metrology application, radiation generated by HHG can be focused onto a target on a substrate using an optical column, which can be referred to as an illuminator that transfers the radiation from the HHG source to the target. Then, the HHG radiation can be reflected from the target, detected, and processed, for example, to measure and / or infer properties of the target.

[0090] Gas target HHG configurations can be broadly divided into three separate categories: gas jets, gas cells, and gas capillaries. Figure 6 An exemplary gas jet configuration is depicted, in which a gas volume is introduced into the drive radiation laser beam. In the gas jet configuration, the interaction of the drive radiation with the solid part is kept to a minimum. The gas volume can include, for example, an air flow perpendicular to the drive radiation beam, where the gas volume is enclosed inside a gas cell. In the gas capillary setup, the dimensions of the capillary structure that holds the gas are small in the lateral direction such that it significantly affects the propagation of the drive radiation laser beam. The capillary structure can be, for example, a hollow-core fiber, where the hollow core is configured to hold the gas.

[0091] The gas jet HHG configuration can provide relatively more freedom to form the spatial profile of the drive radiation beam in the far field, as it is not restricted by the constraints imposed by a gas capillary structure. The gas jet configuration can also have less stringent alignment tolerances. On the other hand, the gas capillary can provide an increased interaction region between the drive radiation and the gaseous medium, which can optimize the HHG process.

[0092] To use the HHG radiation (e.g., in a metrology application), it is separated from the drive radiation downstream of the gas target. For the gas jet and gas capillary configurations, the separation of the HHG and drive radiation can be different. In both cases, the drive radiation suppression scheme can include a metal transmission filter for filtering out any remaining drive radiation from the short-wavelength radiation. However, before such a filter can be used, the intensity of the drive radiation should be significantly reduced from its intensity at the gas target to avoid damaging the filter. The methods available for such intensity reduction vary for the gas jet and capillary configurations. For gas jet HHG, due to the relative freedom in the shape and spatial profile (which can also be referred to as the spatial distribution and / or spatial frequency) of the drive radiation beam focused onto the gas target, this can be designed such that there is low intensity in the far field along the direction of propagation of the short-wavelength radiation. This spatial separation in the far field means that an aperture can be used to block the drive radiation and reduce its intensity

[0093] In contrast, in a gas capillary structure, the spatial profile of the beam as it passes through the gaseous medium can be largely determined by the capillary. The spatial profile of the driving radiation can be determined by the shape and material of the capillary structure. For example, in the case where a hollow core fiber is used as the capillary structure, the shape and material of the fiber structure determine which mode of driving radiation is supported to propagate through the fiber. For most standard fibers, the supported propagation modes result in a spatial profile where the high-intensity driving radiation overlaps with the high-intensity HHG radiation. For example, the intensity of the driving radiation can be centered on a Gaussian or near-Gaussian profile in the far field.

[0094] In Figure 7 (a) shows a further metrology apparatus suitable for embodiments of the present invention. Note that this is only one example of a suitable metrology apparatus. Alternative suitable metrology apparatuses may use, for example, EUV radiation as disclosed in WO2017 / 186483A1. In Figure 7 (b) the target structure T and the diffracted rays of the measurement radiation used to irradiate the target structure are explained in more detail. The metrology apparatus illustrated is of the type known as a dark field metrology apparatus. The metrology apparatus can be a stand-alone device or incorporated into a lithography apparatus LA (e.g., at a measurement station) or a lithography cell LC. The optical axis having several branches throughout the apparatus is indicated by the dashed line O. In this apparatus, light emitted by a source 11 (e.g., a xenon lamp) is directed via a beam splitter 15 through an optical system including lenses 12, 14, and an objective 16 onto a substrate W. These lenses are arranged in a double sequence in a 4F arrangement. Different lens arrangements can be used as long as it still provides an image of the substrate onto the detector and at the same time allows access to the intermediate pupil plane for spatial frequency filtering. Thus, the angular range at which the radiation is incident on the substrate can be selected by defining the spatial intensity distribution in the plane that presents the spatial spectrum of the substrate plane (referred to herein as the (conjugate) pupil plane). In particular, this can be done by inserting an aperture plate 13 of a suitable form between lenses 12 and 14 in the plane that is the back-projected image of the objective pupil plane. In the illustrated example, the aperture plate 13 has different forms, labeled 13N and 13S, to allow selection of different illumination modes. The illumination system in this example forms an off-axis illumination mode. In a first illumination mode, the aperture plate 13N provides an off-axis in a direction designated as 'north' for illustrative purposes only. In a second illumination mode, the aperture plate 13S is used to provide a similar illumination but from the opposite direction, labeled'south'. By using different apertures, other illumination modes can be performed. The remainder of the pupil plane is desirably dark since any unwanted light outside the desired illumination mode will interfere with the desired measurement signal.

[0095] As Figure 7As shown in Fig. (b), the target structure T is placed at a position on the substrate W normal to the optical axis O of the objective lens 16. The substrate W can be supported by a support (not shown). Rays of the measurement radiation I impinging on the target structure T at an angle deviating from the axis O (e.g., including SXR wavelengths) generate a zero-order ray (solid line 0) and two first-order rays (dashed line +1 and double-dashed line -1). It should be remembered that for a small target structure with overfilling, these rays are only one of many parallel rays covering the substrate area including the metrology target structure T and other features. Since the aperture in the plate 13 has a finite width (the width necessary to allow a useful amount of light to enter), the incident ray I will actually occupy an angular range, and the diffracted rays 0 and +1 / -1 will be slightly spread out. According to the point spread function of the small target, each of the orders +1 and -1 will be additionally spread within an angular range, rather than being a single ideal ray as shown in the figure. Note that the grating pitch and illumination angle of the target structure can be designed or adjusted such that the first-order rays entering the objective lens are closely aligned with the central optical axis. Figure 7 The rays illustrated in Figs. 7(a) and 7(b) are shown slightly deviated from the axis only to make them more distinguishable in the figure.

[0096] At least the 0th and +1st orders diffracted by the target structure T on the substrate W are collected by the objective lens 16 and guided back through the beam splitter 15. Return Figure 7 In Fig. 7(a), both the first illumination mode and the second illumination mode are illustrated by specifying diametrically opposite apertures labeled North (N) and South (S). When the incident ray I of the measurement radiation comes from the north side of the optical axis, i.e., when the first illumination mode is applied using the aperture plate 13N, the +1 diffracted ray labeled +1(N) enters the objective lens 16. In contrast, when the second illumination mode is applied using the aperture plate 13S, the -1 diffracted ray (labeled 1(S)) is the ray entering the lens 16.

[0097] The second beam splitter 17 divides the diffracted light beam into two measurement branches. In the first measurement branch, the optical system 18 uses the zero-order and first-order diffracted light beams to form a diffracted spectrum (pupil plane image) of the target structure on the first sensor 19 (e.g., a CCD or CMOS sensor). Each diffracted order hits a different point on the sensor, enabling image processing to 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 light beam. The pupil plane image can also be used for many measurement purposes, such as reconstruction.

[0098] In the second measurement branch, the optical systems 20, 22 form an image of the target structure T on a sensor 23 (e.g., a CCD or CMOS sensor). In the second measurement branch, an aperture stop 21 is provided in a plane conjugate to the pupil plane. The function of the aperture stop 21 is to block the zero-order diffracted beam such that the target image formed on the sensor 23 is formed only by the -1 or +1 first-order beams. The images captured by sensors 19 and 23 are output to a processor PU, which processes the images, the function of which will depend on the particular type of measurement being performed. Note that the term 'image' as used herein is in a broad sense. If only one of the -1 and +1 orders is present, then an image of the grating lines will not be formed accordingly.

[0099] Figure 8 For an example measurement using an SXR metrology device such as Figure 5 shown. In SXR metrology, a beam ILL of SXR radiation (e.g., in the wavelength range of 10 - 20 nm) can be used to irradiate a structure or target T (in the structure plane or target plane). The diffraction pattern DIFF is captured by a (at least one) detector DET, which can be an image sensor, such as a CCD or CMOS image sensor. The target T coordinate system is labeled x, y, z (where x and y describe the structure plane / target plane), and the detector coordinate system is labeled x', y', z'. The irradiation angle of incidence is labeled θ, and the irradiation azimuth angle is labeled The irradiation polarization vector PV has a polarization angle γ. The detector DET can actually be a detection arrangement including two or more detectors that are not necessarily coplanar. Thus, the method involves any detection arrangement that can include a single detector or more than one detector. Any reference to a detector herein can be extended to and understood to mean a detection arrangement. Such a detection arrangement can be part of a detection module of the metrology device, the detection module further including a processor.

[0100] Figure 9 An example 2D diffraction image as can be captured on a detector DET or detection arrangement (which can include one or more individual detectors) including a detection region defined by a pixel array (i.e., the detection arrangement defines the detection region in terms of a plurality of pixels). In this particular example, the x and y axes are labeled in terms of pixels (e.g., a 1024×1024 pixel detector). The details of the detector are of course only examples. As can be seen, many diffraction orders include curves, the curvature of which depends on the orientation and position of the detector DET and the angle of incidence of the incident beam on the inspection target.

[0101] The larger the angular space (detection NA) captured by the detector, the better the measurement accuracy, and the shorter the product pitch supported by the metrology setup. Due to the nature of high-brightness sources (high intensity in a small diverging beam), the detector is irradiated very unevenly. The diffraction orders are high-intensity spots on the detector, but most of the detector is not irradiated. This can be seen immediately from Figure 9 . However, the spread in the positions of all diffraction orders is large, and thus a large detection area is required to capture them all (large detection NA). As a result, a few pixels on the detector that are irradiated quickly saturate in an otherwise blank image. To capture enough light for an accurate single measurement and to prevent those limited number of irradiated pixels from saturating, several frames must be taken. In this case, a frame can be a separate image capture. In one embodiment, the signal to be detected is processed frame by frame at non-overlapping intervals. In one embodiment, the signal to be detected is processed frame by frame at overlapping intervals.

[0102] Reading out these frames from the camera takes time. For example, performing a 14-bit readout using a current detector can take over 150 ms. Current detector technology optionally enables a rolling shutter operation. In such a rolling shutter operation, photons incident on the detector are recorded / detected in parallel with the readout of the immediately preceding frame. This is particularly useful when the exposure time Te of a single frame is similar to the readout time Tf of that frame, because this means that the total time required for a single frame will not exceed the maximum of the exposure time and the readout time (i.e., max(Te,Tf)), rather than the sum of these durations (i.e., Te + Tf).

[0103] The exposure time required to saturate a single frame depends on the source power and source divergence, the transmittance and magnification of the optical system, the detector configuration, and the diffraction efficiency of the target. The latter is the fraction indicating how much of the light incident on the target is in a certain diffraction order. The detector configuration describes the size of the pixels, the electron well depth per pixel, the distance between the detector and the target, and their mutual orientation, all of which affect how many photons are required to saturate a pixel.

[0104] Until recently, the source power was relatively low, resulting in an exposure time significantly longer than the readout time of the detector. However, the source power has increased, which means that the exposure time is expected to decrease, making the readout time of the detector the limiting factor rather than the exposure time. The result of this can be to set an absolute minimum limit on the total acquisition time, for example about 5 - 20 seconds, that can be obtained using SXR metrology, thus making the acquisition time insensitive to any additional source development.

[0105] Currently, the entire frame (all detected pixels) is read out because, especially in the case of a 2D object grating, scattered radiation or diffraction orders are scattered across the entire detector (e.g., scattered by structures such as the object on the substrate). However, the detector readout time depends on how many rows (or individual pixels, if the detector allows) are read out. Rows that include bright pixels (e.g., illumination from diffraction orders) are typically significantly fewer than rows that include relatively dark pixels. Rows that include darker pixels (or at least some of them) may still need to be read out because they contain information, but not necessarily at the frame rate determined by the brightest pixels / rows.

[0106] Therefore, it is proposed to apply different acquisition modes to different regions or zones of the detector (e.g., different regions of interest ROI). In this case, although one or more zones may be continuous, each different region of interest does not necessarily have to be continuous (i.e., the "region of interest" may include multiple separate zones / regions on one or more detectors). In one embodiment, the different regions of interest may be non - overlapping, e.g., such that each pixel is assigned to only one of the regions of interest. The frame for each ROI may include a separate image capture for the corresponding ROI.

[0107] The division of the detection area into ROIs each having a corresponding acquisition mode can be determined based on the intensity distribution on the detector. The detector can rapidly switch between acquisition modes during a single total acquisition (a single target measurement including an exposure period and a readout period) such that each ROI is read out individually. Alternatively, the detector can support parallel exposure and acquisition of various ROIs.

[0108] By way of a specific example, it can be assumed that a certain target requires Nf frames to be read out in a single complete acquisition. Each pixel (which can be described as the i - th row and j - th column) will have a corresponding power Pij incident on it. It can be understood that although the description will be in terms of power for the incident signal amplitude on the detector, throughout the description, any other measurement parameter related to the intensity or amplitude of the incident radiation can be used, and power can be (consistently) replaced by any such other measurement parameter. There is a pixel (i,j)=(m,n) that has the highest incident power Pmax. To avoid saturation, the exposure time Te of this pixel (m,n) (and thus the full frame using traditional methods) is inversely proportional to the highest incident power Pmax; i.e., Te = Dsat / Pmax, where the proportionality constant Dsat describes the saturation dose of the pixel. Dsat is a property of the detector and is largely proportional to the surface area of an individual pixel and its full - well capacity. Then the total acquisition time TAT of the traditional readout scheme (taking into account the rolling - shutter method already described) c is:

[0109] TAT c= Nf * max(Te, Tf); where Tf is the readout time per frame.

[0110] In an example without using a rolling shutter, the total acquisition time will be the product of the number of frames to be read out and the sum of the exposure time and the readout time per frame.

[0111] The proposed method may include defining at least one threshold measurement parameter value to be used as a threshold for comparison with the measured pixel measurement parameter, such that it can be classified into different ROIs based on the comparison according to the measurement parameter incident on the detector. The at least one threshold measurement parameter value may be defined according to a corresponding threshold fraction Td of the maximum detected measurement parameter value. For example, the threshold fraction may include a value between 0 and 1, which, combined with the maximum detected measurement parameter value, determines the threshold measurement parameter value or the cut-off measurement parameter value that can be compared with all measured pixel measurement parameter values. The threshold measurement parameter value may be calculated, for example, as the product of the threshold fraction and the maximum detected measurement parameter value.

[0112] Using the specific example described, the threshold power Ptd can be determined from the maximum detected power Pmax and the threshold fraction Td; for example, as the product of the maximum detected power: Ptd = Td * Pmax. All pixels with a corresponding power Pij greater than the threshold power Ptd (i.e., Pij > Ptd) can be attributed to the first ROI, while the remaining pixels (i.e., those with a corresponding power Pij less than or equal to the threshold power Ptd (i.e., Pij ≤ Ptd)) can be attributed to the second ROI. However, as will be described below, such per-pixel assignment is not always possible.

[0113] It can be understood that more than two ROIs can be defined by setting two or more threshold measurement parameter values (e.g., as corresponding different fractions of the maximum detected value), where each ROI has a corresponding different acquisition mode. Such embodiments are not described in detail but will be obvious to those skilled in the art. For example, a first ROI can be defined for pixel powers in a lower range, a second ROI for pixel powers in an intermediate range, and a third ROI for pixel powers in a higher range. Then each of these can be read out using corresponding different acquisition modes as described below, or a particular ROI can be selected not to be read out at all.

[0114] Many detectors have separate readout control only for subsets where the detector pixels > 1, such as row (or column) readout. In such cases, any decision on whether to read out a detector section can be made only at the subset or row / column level and not at the pixel level. Thus, an ROI can be defined only by rows, columns, or subsets and not by pixels, where the subset represents the smallest unit of the detection area that can be read out individually. However, the concepts disclosed herein also apply to detector arrangements that support per-pixel level control (i.e., where each pixel can be read out individually), and thus, the ROI can be defined by pixels for additional improvement.

[0115] In an example of row level control in the above context, a first ROI can include all rows where there are corresponding power values Pij greater than a threshold power Ptd, and a second ROI can include all rows where there are no corresponding power values Pij greater than the threshold power Ptd. In such examples, the first ROI can be assigned a first ROI exposure time Te1, e.g., an exposure time that can be expected to saturate the first ROI. This can be the exposure time Te of the pixels m, n that are subject to the maximum incident power (i.e., the same exposure time as the entire detector in the conventional scheme). However, a longer exposure time can be assigned to the second ROI as these rows (or pixels) take longer to saturate. This longer exposure time Te2 can be based on the threshold and the exposure time Te, e.g., it can be calculated as Te2 = Te / Td. In cases where more than two ROIs are defined, each ROI can have an associated different exposure time, e.g., as defined by the respective threshold of each ROI. Thus, if a pixel or a subset of pixels has an associated measurement parameter value greater than the threshold measurement parameter value, then the pixel or subset of pixels can be assigned to the region of interest associated with the corresponding readout scheme having the shortest readout period in the readout scheme.

[0116] Thus, for a given threshold fraction Td (e.g., in the example of two ROIs with per-row readout control), the number of rows in the first ROI will be N1, and the number of rows in the second ROI will be N2. Each row (or pixel) can have an associated readout time Trow, e.g., which can depend on the analog-to-digital converter (ADC) resolution; additionally, there can be a constant readout time contribution for each ROI. Thus, the respective readout times Tf1, Tf2 of the first and second ROIs can be calculated separately. The number of frames Nf1 to be read out for the first ROI can be the same as in the conventional readout scheme, i.e., Nf frames as described above. However, the number of frames Nf2 to be read out for the second ROI can be less than Nf; e.g., by a factor depending on the threshold fraction. For example, the number of frames Nf2 can be determined by the formula: Nf2 = ceil(Td * Nf), where ceil is the ceiling function indicating rounding up to the nearest integer, as only an integer number of frames can be read out.

[0117] Figure 10 Illustrates how the Figure 9 detected image can be divided into two ROIs. Figure 10 (a) is the detected image and Figure 10 (b) shows the image divided into white rows and black rows. The white rows together define the first ROI (i.e., those rows including at least one pixel with incident power higher than the threshold determined as described above), and the black rows define the second ROI (i.e., those rows not including pixels with incident power higher than the threshold).

[0118] In an embodiment employing a rolling shutter, such that a frame can be exposed in parallel with the readout of the previous frame, the total acquisition time can now be the maximum of the respective ROI total acquisition times of the first ROI and the second ROI, where the exposure and readout time of a single frame of each respective ROI is the maximum of the exposure time and readout time of that ROI (or the sum in an embodiment without a rolling shutter). Thus, the total acquisition time TAT of the proposed method according to the embodiment can be given by the following equation:

[0119] TAT = max(Nf * max(Te1, Tf1), ceil(Td * Nf) * max(Te2 / Td, Tf2))

[0120] The parameters Nf, Te1 depend on the original detector and the target configuration. The optimal choice of the other parameters depends on the choice of the threshold fraction Td, which can be optimized for each target and is thus a tuning parameter. Setting the threshold fraction Td to be at or near 1 or 0 is essentially a traditional readout scheme. An optimal intermediate value can be found such that the total acquisition time TAT is significantly lower than the total acquisition time TAT using traditional methods c .

[0121] It can be understood that this equation for TAT is only an example and can be more complex in some cases, for example. This can occur when there is a constant contribution to the readout time that is independent of the number of rows / pixels of each ROI. Additionally, to make the (semi)-parallel readout work smoothly in the rolling shutter mode, the readout frequencies can be matched such that, for example, one readout time is an integer multiple of the other readout time. Such constraints will also modify the TAT equation.

[0122] Figure 11 A plot of the total acquisition time TAT versus the threshold fraction Td for an example intensity distribution on the detector, illustrating how the threshold fraction Td can be optimized according to an embodiment. Such methods can include determining the function TAT(Td) (solid black line) and determining the optimized threshold fraction Td OPT as corresponding to the minimum TAT of this function TAT(Td) min . It can be seen that this minimum total acquisition time TATmin Much shorter than the total acquisition time TAT of traditional methods c The function TAT(Td) can be determined as the first ROI TAT above the minimum TAT min (e.g., above its intersection point) ROI1 (Td) and the second ROI TAT ROI2 (Td) as a combination of the corresponding parts of the total acquisition time that are functions of the threshold fraction. The parts of these functions below the minimum are shown as dashed lines to illustrate this. Thus, the method can include determining the threshold fraction as corresponding to the intersection point of a first region of interest function that describes the total acquisition time of a first region of interest as a function of the threshold fraction and a second region of interest function that describes the total acquisition time of a second region of interest as a function of the threshold fraction.

[0123] For example, depending on the hardware characteristics, the exposure time can be limited to the minimum total acquisition time TAT at the exposure limit etl This is illustrated by the gray dashed line, resulting in the total acquisition time function TAT etl (td) including only the function TAT etl above this value ROI1 (Td), TAT ROI2 (Td) such that the function does not go below TAT etl . In such embodiments, the total acquisition time function TAT etl (td) with the smallest any threshold fraction Td will be optimal, and there will be no difference in the total exposure time between any of the values within this threshold fraction range. In any case, as the source power increases, the value of TAT etl will decrease, and the influence of the exposure time will be reduced.

[0124] In one embodiment, this threshold optimization can be performed for each target and / or stack, as it depends on the target / stack. Subsequently, the ROI definition is set in the detector software. However, each target only needs to be done once (e.g., during calibration), and thus does not impose any significant overhead.

[0125] As already described, the above method can be extended to more than two ROIs, where the ROIs are defined not by rows but by pixels (2D-ROI). Additionally, in an optional embodiment, it can be decided not to read out the ROI within the defined ROI, for example because the pixel power or other measurement parameters do not exceed a low threshold, and thus the pixel does not contain significant information. In the context of the present disclosure, not reading out the ROI is an adopted readout scheme (i.e., not reading out itself is a readout scheme).

[0126] Figure 12Illustrated are (a) a conventional rolling shutter readout scheme and (b), (c) two example readout schemes according to the methods disclosed herein. Each figure is a timing diagram with time extending from left to right.

[0127] In Figure 12 (a), a conventional rolling shutter readout scheme is illustrated, where a first frame is exposed during an initial period TeF1, and subsequently, exposure of a second frame TeF2 and readout of the first frame TfF1 are performed in parallel. Once both are completed (i.e., as determined by the longer of the exposure time and the readout time), this can be repeated in the next frame (i.e., exposure of the third frame TeF3 and readout of the second frame TfF2), and so on.

[0128] Figure 12 (b) illustrates a readout scheme implementation for an embodiment, where two ROIs are defined as described above. This arrangement can be applicable to hardware / firmware arrangements (as the case may be) that do not support fully parallel readout. In this arrangement, there are separate readout sub - schemes for the first ROI (ROI 1) and the second ROI (ROI 2). Each of these readout sub - schemes generally follows the same or a similar rolling shutter scheme as Figure 12 (a) described, although the second ROI has a longer exposure time Te ROI 2 (and optionally a longer readout time Tf ROI2) than the first ROI (Te ROI 1, Tf ROI 1). However, in the case of an implementation that does not support full parallelism, the fast readout / frame rate of ROI 1 is temporarily stopped to read ROI 2. This may be because, for example, the same ADC is used to read both ROIs. The additional total acquisition time over the fully parallelized scheme (i.e., in addition to the time TAT according to the above equation) is ceil(Td*Nf)*Tf2.

[0129] Figure 12 (c) illustrates a readout scheme implementation similar to the readout scheme implementation illustrated in Figure 12 (b), but uses a fully parallel implementation. This can be achieved by providing an ADC for each ROI. It is in this implementation that the total acquisition time TAT is the time described in the above equation.

[0130] For providing rolling shutter readout with multiple ROIs, the detector firmware can be provided with mechanisms for controlling which readout mode to execute at a given time and for quickly switching between these modes. For example, this can be provided in the synchronous digital TTL (transistor-transistor logic) lines and the trigger for the start of detector readout. A more efficient implementation can include providing the detector firmware with a suggested readout scheme such that the firmware runs the desired scheme based on a defined input vector or line numbers representing its respective ROIs. This will also be beneficial for starting the acquisition sequence as frame 0 will include all the radiation prior to the start of readout and can thus be read out to zero the detector. Thus, for multiple ROIs and readout modes, this results in at least one round of all ROIs being read out before any detected radiation is due to measurement (i.e., it is only background radiation). Running the detector via a scheme and ignoring unwanted frames (e.g., during movement between two targets) can be more efficient than starting and stopping the acquisition scheme.

[0131] Additional embodiments are disclosed in the following numbered clauses:

[0132] 1. A method of reading out a detection arrangement, the detection arrangement defining a detection area in terms of a plurality of pixels, the method comprising:

[0133] Receiving scattered radiation on the detection arrangement;

[0134] Dividing the detection area into at least two different regions of interest based at least on measurement parameters of the scattered radiation; and

[0135] Applying a respective different readout scheme to each of the regions of interest so as to read out the detection arrangement.

[0136] 2. The method according to clause 1, wherein the readout of at least one of the regions of interest is performed over a plurality of frames of the respective region of interest.

[0137] 3. The method according to clause 2, wherein the readout schemes differ at least in terms of exposure time per frame / and or readout time.

[0138] 4. The method according to clause 2 or 3, wherein for each region of interest, the exposure of the current frame is performed in parallel with the readout of the immediately preceding frame of at least some of the plurality of frames.

[0139] 5. The method according to any of the preceding clauses, wherein each region of interest is defined in terms of one or more individual pixels, where each pixel can be read out individually.

[0140] 6. The method according to any one of clauses 1 to 4, wherein each region of interest is defined according to one or more subsets of pixels, and the subset represents the smallest unit of the detection region that can be read out individually.

[0141] 7. The method according to clause 6, wherein each subset includes an individual row or column of pixels.

[0142] 8. The method according to any one of the preceding clauses, wherein the partitioning step includes:

[0143] Determining at least one threshold measurement parameter value; and

[0144] Comparing the measurement parameter of each pixel with the at least one threshold measurement parameter value.

[0145] 9. The method according to clause 8, including assigning a pixel or a subset of pixels to one of the regions of interest based on the comparison step.

[0146] 10. The method according to clause 9, wherein the assignment step includes assigning the pixel or the subset of pixels to the region of interest associated with the corresponding readout scheme having the shortest readout period in the readout scheme if the pixel or the subset of pixels has an associated measurement parameter value greater than the threshold measurement parameter value.

[0147] 11. The method according to any one of clauses 8 to 10, wherein the step of determining at least one threshold measurement parameter value includes:

[0148] Determining the maximum measurement parameter value incident on the detection arrangement;

[0149] Determining at least one threshold fraction; and

[0150] Determining the at least one threshold measurement parameter value as a combination of the maximum measurement parameter value and the corresponding threshold fraction of the at least one threshold fraction.

[0151] 12. The method according to clause 11, including determining the at least one threshold fraction as the minimum total acquisition time corresponding to a function describing the total acquisition time as a function of the threshold fraction.

[0152] 13. The method according to clause 12, wherein the minimum total acquisition time is limited by the exposure time of the acquisition frame.

[0153] 14. The method according to clause 11, 12 or 13, comprising determining the threshold score as the intersection of a first region-of-interest function that describes the total acquisition time of a first region of interest corresponding to at least two different regions of interest described as a function of the threshold score and a second region-of-interest function that describes the total acquisition time of a second region of interest corresponding to at least two different regions of interest described as a function of the threshold score.

[0154] 15. The method according to any one of the preceding clauses, wherein there is no overlap between the respective regions of interest of the at least two regions of interest.

[0155] 16. The method according to any one of the preceding clauses, wherein one of the readout schemes in the readout scheme comprises not reading the corresponding region of interest of the readout scheme.

[0156] 17. The method according to any one of the preceding clauses, wherein the number of regions of interest is two.

[0157] 18. The method according to any one of clauses 1 to 16, wherein the number of regions of interest is more than two.

[0158] 19. The method according to any one of the preceding clauses, wherein the regions of interest are read out in parallel.

[0159] 20. The method according to any one of clauses 1 to 18, wherein the regions of interest are read out individually.

[0160] 21. The method according to any one of the preceding clauses, wherein the total acquisition time of the acquisition is the maximum value of the total acquisition time of the respective regions of interest of each of the regions of interest.

[0161] 22. The method according to any one of the preceding clauses, wherein the scattered radiation comprises photons and / or electrons.

[0162] 23. The method according to any one of the preceding clauses, wherein the scattered radiation has been scattered by a structure on the substrate.

[0163] 24. The method according to any one of the preceding clauses, wherein the measurement parameter comprises the intensity, amplitude or power of the scattered radiation.

[0164] 25. The method according to any one of the preceding clauses, wherein the readout frequencies for each respective different readout scheme are matched such that the readout frequencies all include integer multiples of each other.

[0165] 26. A computer program comprising computer-readable instructions operable to perform the method according to any one of clauses 1 to 25.

[0166] 27. A processor and an associated storage medium, the storage medium including a computer program according to clause 26, such that the processor is operable to execute the method according to any one of clauses 1 to 25.

[0167] 28. A measuring device, including the processor and the associated storage medium according to clause 21, so as to be operable to execute the method according to any one of clauses 1 to 27.

[0168] 29. A detection module, including:

[0169] A detection arrangement that defines a detection area in terms of a plurality of pixels; and

[0170] A processor operable to:

[0171] Divide the detection area into at least two different regions of interest based at least on measurement parameters of scattered radiation received on the detection arrangement; and

[0172] Adopt a corresponding different readout scheme for each of the regions of interest to read out the detection arrangement.

[0173] 30. The detection module according to clause 29, wherein the processor is operable to read out at least one of the regions of interest in the region of interest on a plurality of frames corresponding thereto.

[0174] 31. The detection module according to clause 30, wherein the readout schemes are different at least according to the exposure time per frame / and or the readout time.

[0175] 32. The detection module according to clause 30 or 31, wherein for each region of interest, the processor is operable to expose the current frame in parallel with the readout of the immediately preceding frame of at least some of the plurality of frames.

[0176] 33. The detection module according to any one of clauses 29 to 32, wherein the processor is operable to define each region of interest in terms of one or more individual pixels, and each pixel can be read out individually.

[0177] 34. The detection module according to any one of clauses 1 to 32, wherein the processor is operable to define each region of interest in terms of one or more subsets of pixels, and the subset represents the smallest unit of the detection area that can be read out individually.

[0178] 35. The detection module according to clause 34, wherein each subset includes a separate row or column of pixels.

[0179] 36. The detection module according to any one of clauses 29 to 35, wherein the processor is operable to perform the partitioning by:

[0180] Determining at least one threshold measurement parameter value; and

[0181] Comparing the measurement parameter of each pixel with the at least one threshold measurement parameter value.

[0182] 37. The detection module according to clause 36, wherein the processor is operable to assign a pixel or a subset of pixels to one of the regions of interest in the region of interest based on the comparison step.

[0183] 38. The detection module according to clause 37, wherein if the pixel or the subset of pixels has an associated measurement parameter value greater than the threshold measurement parameter value, then the processor is operable to assign the pixel or the subset of pixels to the region of interest associated with the corresponding readout scheme having the shortest readout period in the readout scheme.

[0184] 39. The detection module according to any one of clauses 36 to 38, wherein the processor is operable to determine at least one threshold measurement parameter value by:

[0185] Determining the maximum measurement parameter value incident on the detection arrangement;

[0186] Determining at least one threshold fraction; and

[0187] Determining the at least one threshold measurement parameter value as a combination of the maximum measurement parameter value and the corresponding threshold fraction of the at least one threshold fraction.

[0188] 40. The detection module according to clause 39, including wherein the processor is operable to determine the at least one threshold fraction as a function of the minimum total acquisition time corresponding to a function describing the total acquisition time of the measurement as a function of the threshold fraction.

[0189] 41. The detection module according to clause 40, wherein the minimum total acquisition time is limited by the exposure time of the acquisition frame.

[0190] 42. The detection module according to clause 39, 40 or 41, wherein the processor is operable to determine the threshold fraction as the intersection of a first region-of-interest function of the total acquisition time of the first region of interest describing at least two different regions of interest as a function of the threshold fraction and a second region-of-interest function of the total acquisition time of the second region of interest describing at least two different regions of interest as a function of the threshold fraction.

[0191] 43. The detection module according to any one of clauses 29 to 42, wherein there is no overlap between the corresponding regions of interest of the at least two regions of interest.

[0192] 44. The detection module according to any one of clauses 29 to 43, wherein one of the readout schemes in the readout scheme includes not reading the corresponding region of interest of the readout scheme.

[0193] 45. The detection module according to any one of clauses 29 to 44, wherein the number of regions of interest is two.

[0194] 46. The detection module according to any one of clauses 1 to 44, wherein the number of regions of interest is more than two.

[0195] 47. The detection module according to any one of clauses 29 to 46, capable of operating such that the regions of interest are read out in parallel.

[0196] 48. The detection module according to clause 47, wherein the detection arrangement includes a corresponding analog-to-digital converter for each region of interest in the regions of interest.

[0197] 49. The detection module according to any one of clauses 1 to 46, wherein the regions of interest are read out individually.

[0198] 50. The detection module according to clause 49, wherein the detection arrangement includes a shared analog-to-digital converter for each region of interest in the regions of interest.

[0199] 51. The detection module according to any one of clauses 29 to 50, wherein the processor is capable of operating to determine the total acquisition time of the acquisition as the maximum value of the corresponding total acquisition times of the regions of interest for each region of interest in the regions of interest.

[0200] 52. The detection module according to any one of clauses 29 to 51, wherein the scattered radiation includes photons and / or electrons.

[0201] 53. The detection module according to any one of clauses 29 to 52, wherein the scattered radiation has been scattered by a structure on the substrate.

[0202] 54. The detection module according to any one of clauses 29 to 53, wherein the measurement parameter includes the intensity, amplitude or power of the scattered radiation.

[0203] 55. The detection module according to any one of clauses 29 to 54, wherein the processor is capable of operating to match the readout frequencies of each corresponding different readout scheme such that the readout frequencies all include integer multiples of each other.

[0204] 56. A metrology device includes a detection module according to any one of clauses 29 to 55.

[0205] Although the above methods have been disclosed in the context of SXR metrology, the methods are not limited to metrology using any specific wavelength or wavelength range, and can thus equally be used for visible light and / or metrology methods at any other wavelength. The metrology device can be an electron microscope device and instead of using electromagnetic radiation, and thus any mention of scattered radiation or measurement radiation can be understood to include photons and / or electrons.

[0206] Although specific reference may be made herein to the detection arrangement of the metrology device, it should be understood that the detection arrangement described herein can be part of a lithographic apparatus or an inspection apparatus. An example of an inspection apparatus having the above-mentioned embodiments is a charged particle (e.g., electron) beam microscope, such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM).

[0207] The methods described herein can be embodied in the form of a computer program including computer-readable instructions.

[0208] Although specific reference may be made herein to the use of a lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein can have other applications. Other possible applications include the manufacture of integrated optical systems, the guiding and detecting of patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0209] Although specific reference may be made herein to embodiments in the context of a lithographic apparatus, the embodiments can be used in other devices. The embodiments can form part of a mask inspection apparatus, a metrology apparatus, or any apparatus for measuring or processing an object such as a wafer (or other substrate) or a mask (or other patterning device). These devices are generally referred to as lithographic tools. Such lithographic tools can use vacuum conditions or ambient (non-vacuum) conditions.

[0210] Although specific reference may be made herein to embodiments in the context of an inspection or metrology device, the embodiments can be used in other devices. The embodiments can form part of a mask inspection apparatus, a lithographic apparatus, or any apparatus for measuring or processing an object such as a wafer (or other substrate) or a mask (or other patterning device). The term "metrology device" (or "inspection device") can also refer to an inspection apparatus or inspection system (or metrology device or metrology system). For example, an inspection apparatus including the embodiments can be used to detect defects on a substrate or defects in a structure on a substrate. In such embodiments, the characteristic of interest of the structure on the substrate can relate to defects in the structure, the absence of a particular part of the structure, or the presence of an unwanted structure on the substrate.

[0211] Although the above may specifically refer to the use of embodiments in the context of optical lithography, it should be understood that, where the context permits, the present invention is not limited to optical lithography and may be used in other applications, such as imprint lithography.

[0212] Although the above-described targets or target structures (more generally, structures on a substrate) are metrology target structures designed and formed specifically for measurement purposes, in other embodiments, an attribute of interest may be measured on one or more structures of the functional part of a device that may be formed on a substrate. Many devices have regular grating-like structures. As used herein, the terms structure, target, target grating, and target structure do not require that the structure be provided specifically for the measurement being performed. Additionally, the pitch of a metrology target may approach the resolution limit of the optical system of a scatterometer, or may be smaller, but may be much larger than the size of a typical non-target structure (optionally, a product structure fabricated in target portion C by a lithography process). In practice, the lines and / or spaces of an overlay grating within a target structure may include smaller structures having dimensions similar to non-target structures.

[0213] Although specific embodiments have been described above, it should be understood that the present invention may be practiced in a manner different from that described. The above description is intended to be illustrative and not limiting. Thus, it will be apparent to those skilled in the art that the present invention as described may be modified without departing from the scope of the claims set forth below.

[0214] Although specific reference is made to a “metrology device / instrument / system” or an “inspection device / instrument / system”, these terms may refer to the same or similar types of tools, devices, or systems. For example, an inspection or metrology device including an embodiment of the present invention may be used to determine the characteristics of a structure on a substrate or a wafer. For example, an inspection device or a metrology device including an embodiment of the present invention may be used to detect a defect of a substrate or a defect of a structure on a substrate or a wafer. In such embodiments, the attribute 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 unwanted structure on the substrate or the wafer.

[0215] Although specific reference is made to HXR, SXR, and EUV electromagnetic radiation, it should be understood that, where the context permits, the present invention may be practiced using all electromagnetic radiation, including radio waves, microwaves, infrared rays, (visible) light, ultraviolet light, X-rays, and gamma rays.

[0216] Although specific embodiments have been described above, it should be understood that one or more features of the features in one embodiment may also be present in different embodiments and that features in two or more different embodiments may also be combined.

Claims

1. A method of a readout detection arrangement, the detection arrangement defining a detection area according to a plurality of pixels, the method comprising: receiving scattered radiation on the detection arrangement; dividing the detection area into at least two different regions of interest based at least on measurement parameters of the scattered radiation; and applying a corresponding different readout scheme to each of the regions of interest to read out the detection arrangement.

2. The method according to claim 1, wherein reading out is performed on a plurality of frames in the corresponding region of interest for at least one of the regions of interest.

3. The method according to claim 2, wherein the readout schemes are different at least according to the exposure time per frame / and or readout time.

4. The method according to claim 2 or 3, wherein for each region of interest, the exposure of the current frame is performed in parallel with the readout of at least some of the immediately preceding frames among the plurality of frames.

5. The method according to any one of the preceding claims, wherein each region of interest is defined according to one or more individual pixels, and wherein each pixel can be read out individually.

6. The method according to any one of claims 1 to 4, wherein each region of interest is defined according to one or more subsets of pixels, wherein the subset represents the smallest unit of the detection area that can be read out individually, and wherein optionally each subset includes an individual row or column of pixels.

7. The method according to any one of the preceding claims, wherein the dividing step comprises: determining at least one threshold measurement parameter value; and comparing the measurement parameter of each pixel with the at least one threshold measurement parameter value.

8. The method according to claim 7, comprising assigning a pixel or a subset of pixels to one of the regions of interest based on the comparing step, and wherein optionally the assigning step comprises: if the pixel or the pixels in the subset of pixels have an associated measurement parameter value greater than the threshold measurement parameter value, then assigning the pixel or the subset of pixels to the region of interest associated with the corresponding readout scheme having the shortest readout period among the readout schemes.

9. The method according to any one of the preceding claims, wherein there is no overlap between the corresponding regions of interest of at least two of the regions of interest.

10. The method according to any one of the preceding claims, wherein one of the readout schemes includes not reading out the corresponding region of interest of the readout scheme.

11. The method according to any one of the preceding claims, wherein the regions of interest are read out in parallel.

12. The method according to any one of claims 1 to 10, wherein the regions of interest are read out individually.

13. A computer program comprising computer-readable instructions operable to execute the method according to any one of claims 1 to 12.

14. A processor and an associated storage medium, the storage medium comprising the computer program according to claim 13, such that the processor is operable to execute the method according to any one of claims 1 to 12.

15. A detection module, comprising: A detection arrangement that defines a detection area in terms of a plurality of pixels; and a processor operable to: divide the detection area into at least two different regions of interest based at least on measurement parameters of scattered radiation received on the detection arrangement; and apply a respective different readout scheme to each of the regions of interest in order to read out the detection arrangement.

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