Measurement methods

Abstract

A method and apparatus for determining a parameter of a structure fabricated in or on a substrate that is compensated for drift error. The method comprises: illuminating at least a portion of the structure with electromagnetic radiation at a plurality of times, the at least portion of the structure being in a first orientation; sensing a plurality of average reflectances of the at least portion of the structure at the plurality of times; and determining an estimate of the parameter at one or more other times based on the plurality of average reflectances.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority from European application 20162286.7, filed on March 11, 2020, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to a method and apparatus for measuring parameters of structures fabricated in or on a substrate. The particular arrangement may involve, but need not be limited to, measurement of overlay or critical dimensions. Background Art

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

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

[0006] Low k1 lithography can be used to process features with dimensions smaller than the classical resolution limit of the lithographic apparatus. In this process, the resolution formula can be expressed as CD = k1 × λ / NA, where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the "critical dimension" (usually the smallest feature size printed, but in this case it is half the pitch), and k1 is the empirical resolution factor. In general, the smaller k1 is, the more difficult it becomes to reproduce a pattern on the substrate that resembles the shape and dimensions planned by the circuit designer in order to achieve specific electrical functionality and performance. To overcome these difficulties, complex fine-tuning steps can be applied to the lithographic projection apparatus and / or the design layout. These steps include, for example, but are not limited to: optimization of the 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 strict control loop for controlling the stability of the lithographic apparatus can be used to improve the reproduction of patterns at low k1.

[0007] During photolithography, measurements of the resulting structures frequently need to be taken, for example, for process control and verification. Various tools are known for making such measurements, including scanning electron microscopes, which are often used to measure critical dimensions (CDs), and specialized tools for measuring overlay (the accuracy of alignment of two layers in a device). Recently, various forms of scatterometers have been developed for use in the field of photolithography.

[0008] Known examples of scatterometers often rely on the availability of specialized measurement targets. For example, one approach might require a target in the form of a simple grating, large enough so 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 a real target.

[0009] In addition to measuring feature shape by reconstruction, this device can also be used to measure diffraction-based overlay, as described in published patent application US2006066855A1. Diffraction-based overlay measurement using darkfield imaging of diffraction orders enables overlay measurement of smaller targets. These targets can be smaller than the illumination spot and can be surrounded by product structures on the wafer. Examples of darkfield imaging metrology can be found in numerous published patent applications, such as US2011102753A1 and US20120044470A. Composite grating targets can be used to measure multiple gratings in a single image. Known scatterometers tend to use light in the visible or near-IR wavelength range, which requires the grating pitch to be much coarser than the actual product structure, whose characteristics are of actual interest. Deep ultraviolet (DUV), extreme ultraviolet (EUV), or X-ray radiation with much shorter wavelengths can be used to define these product features. Unfortunately, these wavelengths are generally unavailable or unsuitable for measurement.

[0010] On the other hand, the dimensions of modern product structures are so small that they cannot be imaged by optical measurement techniques. Small features include, for example, features formed by multiple patterning processes and / or pitch multiplication. Therefore, targets for high-volume measurement often use features that are much larger than the product where overlay error or critical dimensions are the characteristics of interest. The measurement results are only indirectly related to the dimensions of the real product structure and may not be accurate because the measurement target is not subject to the same distortion under optical projection in the lithography equipment and / or different processing in other steps of the manufacturing process. Although scanning electron microscopy (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, which makes electrons less suitable for measurement applications. Other techniques such as using contact pads to measure electrical properties are also known, but these other techniques only provide indirect evidence of the real product structure.

[0011] By reducing the wavelength of the radiation used during measurement (i.e., moving towards the "soft X-ray" wavelength spectrum), it is possible to resolve smaller structures to increase sensitivity to structural changes in the structure and / or to further penetrate the product structure. One such method of generating suitable high-frequency radiation (e.g., hard X-rays and / or soft X-rays and / or EUV radiation) may use pump radiation (e.g., infrared radiation) to excite the production medium, thereby generating emitted radiation, optionally including the generation of higher-order harmonics of the high-frequency radiation.

[0012] In a specific known arrangement, overlay measurements can be performed by illuminating an overlapping target or other structure with electromagnetic radiation and measuring the radiation diffracted or reflected from the overlapping target. The target can include two gratings on top of each other. The asymmetry in the diffracted radiation is defined as the difference between the intensity of a negative diffraction order and the corresponding positive diffraction order, for example the difference between the -1 diffraction order and the +1 diffraction order. This asymmetry depends on the lateral shift (overlay shift) between the top and bottom gratings of the overlapping target. Therefore, the asymmetry of the overlapping gratings allows the overlay to be assessed.

[0013] As used herein, the term "intensity" encompasses the incident power (in Watts) of radiation (which may be SXR radiation) per unit area. In the disclosed exemplary arrangements, the area may be the detector or sensor area. The term "signal" encompasses the charge collected by the detector (or sensor) pixels during exposure. The signal may be expressed in Coulombs or in analog digital units (ADU). The signal is proportional to the irradiance and the exposure time (the proportionality constant is wavelength dependent). The term "reflectance" encompasses the ratio of the diffracted spectral flux to the spectral flux incident on the target. The reflectance may depend on target characteristics, target orientation, wavelength and / or diffraction order. The reflectance of the target may vary (drift) over time. The reflectance may be measured as an average value over the entire exposure time.

[0014] This assessment typically requires calibrating the relationship between asymmetry and overlay (in other words, extracting the sensitivity of overlay to asymmetry). This can be done using measurements of multiple overlay targets with known overlay shifts (overlay biases). One exemplary calibration method uses measurements of two overlay targets with different overlay shifts to extract overlay (and sensitivity).

[0015] In the absence of system (or tool) asymmetries (e.g., sensor asymmetries), a single measurement of the diffracted radiation from the target is sufficient for overlap extraction. System asymmetries (e.g., different gain of the detector for -1 order compared to 1 order) add non-overlap asymmetry to the asymmetry determined based on the diffracted radiation. In order to remove this tool-induced asymmetry, a second measurement is taken of the same target after it has been rotated 180 degrees in the plane. The first measurement is referred to as the nominal target orientation measurement and the second measurement is referred to as the rotated target orientation measurement. The rotated measurement results in diffracted radiation from a target that is also rotated. However, the tool-induced asymmetry will not be rotated. Therefore, the combination of the nominal measurement and the rotated measurement allows to distinguish overlap asymmetry from system asymmetry.

[0016] Typically, (components of) a metrology tool can drift over time, for example due to thermal effects. The drift can affect the intensity of the diffracted light and possibly the asymmetry. This is known as tool drift. Summary of the Invention

[0017] The methods and apparatus disclosed herein are directed to solving one or more problems in the art.

[0018] In particular, it is advantageous to extract the overlay (using the methods outlined above) from an overlay target having a grating width similar to the critical dimension of the stack. Promising methods for doing this are to use hard X-ray (HXR) radiation, soft X-ray (SXR) radiation, or to use EUV radiation (e.g., having a wavelength in the range of 10 nm to 20 nm). However, it is expected that, for example, the reaction of SXR or extreme ultraviolet (EUV) radiation with hydrocarbons will result in the deposition of material, such as carbon, on the target during exposure of the target.

[0019] It is also advantageous to measure the overlay on after-development inspection (ADI) targets, where the top grating is only present in the developed resist. However, exposure of such ADI targets to, for example, SXR or EUV radiation results in exposure of the resist, which is expected to result in changes to the resist, in particular shrinkage of the resist.

[0020] Material deposition (optionally, carbon deposition), resist shrinkage, and / or other sources of error may be referred to as target drift errors. Current methods for determining parameters such as overlay are insensitive to these target drift errors. Target drift (e.g., due to structural changes in the irradiated target) affects the intensities of the -1 and +1 (or higher-order) diffraction orders and may therefore affect asymmetry. If asymmetry is affected, this will in turn affect (e.g., degrade) the accuracy of the determined overlay. Thus, the determined overlay may differ from the actual overlay due to target drift. Therefore, there is a need for methods for compensating for drift errors when determining overlay, or generally determining parameters of a structure.

[0021] According to one aspect of the present invention, there is provided a method for determining a parameter of a structure fabricated in or on a substrate that is compensated for drift error, the method comprising: irradiating at least a portion of the structure with electromagnetic radiation at a plurality of times, the at least portion of the structure being in a first orientation; sensing a plurality of average reflectances of the at least portion of the structure at the plurality of times, wherein the average reflectances are indicative of the parameter at the plurality of times; irradiating the at least portion of the structure with electromagnetic radiation at one or more other times, the at least portion of the structure being in a second orientation; and sensing one or more other average reflectances of the at least portion of the structure at the one or more other times, wherein the other average reflectances are indicative of the parameter at the one or more other times; and determining an estimate of the parameter at the one or more other times based on the plurality of average reflectances and the one or more other average reflectances.

[0022] Optionally, the method further includes: estimating the drift error based on the multiple average reflectances.

[0023] Optionally, in the second orientation, the at least a portion of the structure is rotated about a z-axis perpendicular to the plane of the substrate, the rotation being relative to a source of the electromagnetic radiation, and optionally wherein the rotation is one of 180 degrees and 90 degrees.

[0024] Optionally, the one or more other times are multiple other times.

[0025] Optionally, the estimate of the parameter is further determined based on a total radiation dose associated with the irradiation of the at least a portion of the structure in the first orientation and a total radiation dose associated with the irradiation of the at least a portion of the structure in the second orientation.

[0026] Optionally, the total radiation dose associated with the irradiating the at least a portion of the structure in the first orientation is substantially equal to the total radiation dose associated with the irradiating the at least a portion of the structure in the second orientation.

[0027] Optionally, the plurality of average reflectances and / or the further average reflectance comprises an integral of the radiation intensity over at least a portion of the illumination time.

[0028] Optionally, the estimate of the parameter is further determined based on a linear relationship between the drift error of the at least part of the structure and the illumination time.

[0029] Optionally, the method further comprises determining a rate of change of the drift error for use in determining an estimate of the parameter of one or more structures fabricated in or on one or more other substrates.

[0030] Optionally, the estimate of the parameter is determined further based on a relationship between the drift error of the at least part of the structure and the illumination time, the relationship comprising one or more polynomial functions.

[0031] Optionally, determining said estimate of said parameter comprises determining a weighted average of said plurality of average reflectances.

[0032] Optionally, determining said estimate of said parameter comprises determining a weighted average of said plurality of other average reflectances.

[0033] Optionally, the weights applied when determining the weighted average are determined based on a time between irradiations of the at least part of the structure.

[0034] Optionally, the irradiation at the multiple times includes: irradiation of at least a portion of the first structure at a first time; irradiation of at least a portion of the second structure at a second time; and irradiation of at least a portion of the first structure at a third time, wherein the multiple average reflectances indicate the parameter at the first time, the parameter at the second time, and the parameter at the third time.

[0035] Optionally, determining the estimate of the parameter comprises determining an estimate of target drift and / or an estimate of system drift based on the plurality of average reflectances.

[0036] Optionally, the estimate of the system drift is determined based on an intensity of diffracted radiation corresponding to illumination of the first structure at the first time and an intensity of diffracted radiation corresponding to illumination of the second structure at the second time.

[0037] Optionally, the estimate of the target drift is determined based on an intensity of diffracted radiation corresponding to illumination of the first structure at the first time and an intensity of diffracted radiation corresponding to illumination of the first structure at the third time.

[0038] Optionally, the electromagnetic radiation used to illuminate the at least a portion of the structure in the first orientation is one of p-polarized electromagnetic radiation and s-polarized electromagnetic radiation.

[0039] Optionally, the electromagnetic radiation used to illuminate the at least a portion of the structure in the second orientation is the other of p-polarized electromagnetic radiation and s-polarized electromagnetic radiation.

[0040] Optionally, the electromagnetic radiation used to illuminate at least a portion of the structure in the first orientation includes electromagnetic radiation in a first spectrum, and wherein the electromagnetic radiation used to illuminate at least a portion of the structure in the second orientation includes electromagnetic radiation in a second spectrum.

[0041] Optionally, the structure comprises a measurement target.

[0042] Optionally, the parameters include overlap.

[0043] Optionally, the electromagnetic radiation comprises electromagnetic radiation having a wavelength in the range of 0.1 nm to 100 nm.

[0044] According to an aspect of the present invention, there is provided a computer program product comprising instructions which, when executed on at least one processor, cause the at least one processor to control an apparatus to implement a method according to any of the contents described herein.

[0045] According to one aspect of the present invention, there is provided an apparatus for determining a parameter of a structure fabricated in or on a substrate that is compensated for drift error, the apparatus comprising a computer processor configured to control the apparatus to implement the following method: irradiating at least a portion of the structure with electromagnetic radiation at a plurality of times, the at least a portion of the structure being in a first orientation; sensing a plurality of average reflectances of the at least a portion of the structure at the plurality of times, wherein the average reflectances are indicative of the parameter at the plurality of times; irradiating the at least a portion of the structure with electromagnetic radiation at one or more other times, the at least a portion of the structure being in a second orientation; and sensing one or more other average reflectances of the at least a portion of the structure at the one or more other times, wherein the other average reflectances are indicative of the parameter at the one or more other times; and determining an estimate of the parameter at the one or more other times based on the plurality of average reflectances and the one or more other average reflectances.

[0046] According to an aspect of the present invention, there is provided a metrology tool comprising the apparatus according to claim 25 .

[0047] According to an aspect of the present invention, there is provided a lithography system comprising the measurement tool according to claim 26.

[0048] According to an aspect of the present invention, there is provided a lithography cell comprising the lithography system according to claim 27.

[0049] According to one aspect of the present invention, there is provided a method for determining a parameter of a structure fabricated in or on a substrate that is compensated for drift error, the method comprising: irradiating at least a portion of the structure with electromagnetic radiation at a plurality of times, the at least a portion of the structure being in a first orientation; sensing a plurality of average reflectances of the at least a portion of the structure at the plurality of times, wherein the average reflectances are indicative of the parameter at the plurality of times; and determining an estimate of the parameter at one or more other times based on the plurality of average reflectances.

[0050] According to one aspect of the present invention, there is provided an apparatus for determining a parameter of a structure fabricated in or on a substrate that is compensated for drift error, the apparatus comprising a computer processor configured to control the apparatus to implement the following method: irradiating at least a portion of the structure with electromagnetic radiation at a plurality of times, the at least a portion of the structure being in a first orientation; sensing a plurality of average reflectances of the at least a portion of the structure at the plurality of times, wherein the average reflectances are indicative of the parameter at the plurality of times; and determining an estimate of the parameter at one or more other times based on the plurality of average reflectances. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Embodiments will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0052] - Figure 1 A schematic overview diagram depicting a lithographic apparatus;

[0053] - Figure 2 A schematic overview depicting a lithography cell;

[0054] - Figure 3 A schematic diagram depicting overall lithography showing the collaboration between three key technologies used to optimize semiconductor manufacturing;

[0055] - Figure 4 schematically illustrates a scatterometry device;

[0056] - Figure 5 Describe a schematic diagram of the measurement equipment;

[0057] - Figure 6 A simplified schematic diagram showing an illumination source;

[0058] - Figure 7 a graph showing reflectance at multiple wavelengths versus illumination time of a structure or target;

[0059] - Figures 8 to 10showing graphs of reflectance versus time for different illumination and measurement schemes with the structure in a first orientation and a second orientation; and

[0060] - Figure 11 Flowchart including steps in a method of generating emitted radiation. DETAILED DESCRIPTION

[0061] In this document, the terms "radiation" and "beam" are used to cover 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 to 100 nm), X-ray radiation, electron beam radiation and other particle radiation.

[0062] As used herein, the terms "reticle," "mask," or "patterning device" should be broadly interpreted to refer to a general patterning device that can be used to impart a patterned cross-section to an incident radiation beam that corresponds to the pattern to be produced in a target portion of the substrate. In this context, the term "light valve" may also be used. In addition to classical masks (transmissive or reflective; binary, phase-shift, hybrid, etc.), examples of other such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0063] Figure 1 A lithographic apparatus LA is schematically depicted. The lithographic apparatus LA comprises an illumination system (also referred to as an illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation, 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 stage) WT configured to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support according to certain parameters, and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0064] In operation, the illumination system IL receives a radiation beam from a radiation source SO, for example, via a beam delivery system BD. The illumination system IL may include various types of optical components for directing, shaping, and / or controlling the radiation, such as refractive, reflective, diffractive, magnetic, electromagnetic, electrostatic, and / or other types of optical components, or any combination thereof. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross-section at the plane of the patterning device MA.

[0065] The term "projection system" PS as used herein should be broadly interpreted as covering various types of projection systems, including refractive, reflective, diffractive, catadioptric, synthetic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, as appropriate to the experiential radiation used and / or other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term "projection lens" herein should be considered synonymous with the more general term "projection system" PS.

[0066] The lithographic apparatus LA may be of a type in which at least a portion of the substrate may be covered by a liquid having a relatively high refractive index (e.g., water) so as 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 US Pat. No. 6,952,253, which is incorporated herein by reference in its entirety.

[0067] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also designated as a "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel, and / or a step of preparing a substrate W for subsequent exposure may be performed on a substrate W on one of the substrate supports WT while another substrate W on another substrate support WT is being used to expose a pattern on the other substrate W.

[0068] In addition to the substrate support WT, the lithographic apparatus LA may further comprise a measurement platform. The measurement platform is configured to hold sensors and / or cleaning devices. The sensors may be configured to measure characteristics of the projection system PS or characteristics of the radiation beam B. The measurement platform may hold multiple sensors. The cleaning devices may be configured to clean a portion of the lithographic apparatus, such as a portion of the projection system PS or a portion of a system for providing immersion liquid. The measurement platform may be movable beneath the projection system PS when the substrate support WT is away from the projection system PS.

[0069] In operation, a radiation beam B is incident on a pattern forming device (e.g. a mask) MA held on a mask support T and is patterned by a pattern (design layout) present on the pattern forming device MA. Having passed through the mask MA, the radiation beam B passes through a projection system PS which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and the position measurement system IF, the substrate support WT can be accurately moved, for example so as to position different target portions C in the path of the radiation beam B at focused and aligned positions. Similarly, the first positioner PM and possibly a further position sensor (the further position sensor not being in position) are arranged to move the radiation beam B in a manner which is consistent with the present invention. Figure 1 , (explicitly depicted in FIG) can be used to accurately position the patterning device MA relative to the path of the radiation beam B. The mask alignment marks M1, M2 and the substrate alignment marks P1, P2 can be used to align the patterning device MA and the substrate W. Although the substrate alignment marks P1, P2 occupy dedicated target portions as illustrated, the marks can be located in spaces between target portions. When the substrate alignment marks P1, P2 are located between target portions C, they are referred to as scribe lane alignment marks.

[0070] like Figure 2 As shown in , the lithography apparatus LA may form part of a lithography cell LC, which is sometimes also referred to as a lithography unit or (lithography) cluster, which often also includes equipment for performing pre-exposure and post-exposure processes on a substrate W. Typically, these equipment include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a chill plate CH and a bake plate BK, for example, for regulating the temperature of the substrate W (e.g., for regulating the solvent in the resist layer). A substrate handler or robot RO picks up the substrate W from input / output ports I / O1, I / O2, moves the substrate W between the different process equipment, and transfers the substrate W to a feed table LB of the lithography apparatus LA. The devices in the lithography cell, which are often also collectively referred to as tracks, may be under the control of a track control unit TCU, which itself may be controlled by a supervisory control system SCS, which may also control the lithography apparatus LA, for example, via a lithography control unit LACU.

[0071] During a lithographic process, it is desirable to perform frequent measurements of the structures being produced, for example, for process control and verification. The tool used to perform such measurements may be referred to as a metrology tool 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. Scatterometers are multifunctional instruments that allow parameters of the lithographic process to be measured either by having a sensor in the pupil or in a plane conjugate to the pupil of the scatterometer objective (measurements often referred to as pupil-based measurements), or by having a sensor in the image plane or in a plane conjugate to the image plane, in which case measurements often referred to as image- or field-based measurements. Such scatterometers and associated measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032, or EP 1,628,164 A, which are incorporated herein by reference in their entirety. The aforementioned scatterometers can measure gratings using light from soft x-rays, extreme ultraviolet, hard x-rays, and visible to near IR wavelength ranges.

[0072] In order to correctly and consistently expose the substrate W exposed by the lithographic apparatus LA, it is desirable to inspect the substrate to measure characteristics of the patterned structure, such as overlay errors between subsequent layers, line thickness, critical dimensions (CD), etc. For this purpose, inspection tools and / or metrology tools (not shown) may be included in the lithographic cell LC. In particular, when inspecting before other substrates W from the same batch or lot are yet to be exposed or processed, if errors are detected, adjustments may be made to the exposure of subsequent substrates or other processing steps to be performed on the substrates W.

[0073] The inspection apparatus, which may also be referred to as a metrology apparatus, is used to determine properties of the substrate W, and in particular to determine how properties vary between different substrates W or how properties associated with different layers of the same substrate W vary between the different layers. The inspection apparatus may alternatively be configured to identify defects on the substrate W and may, for example, be part of the lithography cell LC, or may be integrated into the lithography apparatus LA, or may even be a separate device. The inspection apparatus may measure properties on a latent image (the image in the resist layer after exposure), or on a semi-latent image (the image in the resist layer after a post-exposure bake step PEB), or on a developed resist image (wherein the exposed or unexposed parts of the resist have been removed), or even on an etched image (after a pattern transfer step such as etching).

[0074] In a first embodiment, the scatterometer MT is an angle-resolving scatterometer. In this scatterometer, reconstruction methods can be applied to the measured signals to reconstruct or calculate the properties of the grating. This reconstruction can be obtained, for example, by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measured results. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to the diffraction pattern observed from a real target.

[0075] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In this spectroscopic scatterometer MT, radiation emitted by a radiation source is directed onto a target, and reflected or scattered radiation from the target is directed onto a spectrometer detector, which measures the spectrum of the specularly reflected radiation (i.e., a measure of the intensity as a function of wavelength). From 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 nonlinear regression, or by comparison with a library of simulated spectra.

[0076] In a third embodiment, the scatterometer MT is an ellipsometry scatterometer. An ellipsometry scatterometer allows parameters of a lithographic process to be determined by measuring scattered radiation for each polarization state. This metrology device emits polarized light (such as linear, circular, or elliptically) by using, for example, appropriate polarization filters in the illumination section of the metrology device. A source suitable for the metrology device can also provide polarized radiation. Various embodiments of existing ellipsometry scatterometers are described in U.S. patent applications Ser. Nos. 11 / 451,599, 11 / 708,678, 12 / 256,780, 12 / 486,449, 12 / 920,968, 12 / 922,587, 13 / 000,229, 13 / 033,135, 13 / 533,110, and 13 / 891,410, which are incorporated herein by reference in their entireties.

[0077] 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 spectrum of the diffracted radiation and / or an asymmetry in the detection configuration (the asymmetry being related to the extent of the overlap). It should be noted that the diffracted radiation may include zero-order diffraction as well as higher-order diffraction. The two (possibly overlapping) grating structures may be applied in two different layers (not necessarily consecutive layers), and the two grating structures may be formed to be in substantially the same position on the wafer. The scatterometer may have a symmetrical detection configuration, such as described in commonly owned patent application EP1,628,164A, so that any asymmetry is clearly distinguishable. This provides a straightforward way to measure misalignment in the gratings. Further examples of measuring overlay error between two layers comprising a periodic structure by targeting an asymmetry of the periodic structure may be found in PCT Patent Application Publication No. WO 2011 / 012624 or U.S. Patent Application US 20160161863, both of which are incorporated herein by reference in their entirety.

[0078] Other parameters of interest may be focus and dose. Focus and dose may be determined simultaneously by scatterometry as described in U.S. Patent Application US2011-0249244, incorporated herein by reference in its entirety (or alternatively by scanning electron microscopy). A single structure may be used that has a unique combination of critical dimension and sidewall angle measurements for each point in the focus energy matrix (FEM - also known as the focus exposure matrix). If these unique combinations of critical dimension and sidewall angle can be obtained, focus and dose values ​​may be uniquely determined based on these measurements.

[0079] The metrology target can be a composite grating formed primarily in resist by a photolithography process and also formed after, for example, an etching process. The pitch and linewidth of the structures in the grating can depend heavily on the measurement optics (particularly the NA of the optics) to be able to capture the diffraction orders from the metrology target. As previously indicated, the diffracted radiation can be used to determine the shift between two layers (also known as "overlay") or to reconstruct at least a portion of the original grating as produced by the photolithography process. This reconstruction can be used to provide guidance on the quality of the photolithography process and can be used to control at least a portion of the photolithography process. The target can have smaller sub-segments that are configured to mimic the dimensions of a functional portion of the design layout in the target. Due to this sub-segmentation, the target will behave more similarly to the functional portion of the design layout, so that the overall process parameter measurement better resembles the functional portion of the design layout. The target can be measured in an underfill mode or in an overfill mode. In underfill mode, the measurement beam produces a spot that is smaller than the overall target. In overfill mode, the measurement beam produces a spot that is larger than the overall target. In this overfill mode, it is also possible to measure different targets simultaneously and thus determine different processing parameters simultaneously.

[0080] The overall measurement quality of a lithography parameter performed using a particular target is determined at least in part by the measurement profile used to measure the lithography parameter. The term "substrate measurement profile" can include one or more parameters of the measurement itself, one or more parameters of the measured pattern(s), or both. For example, if the measurement used in the substrate measurement profile is an optical measurement based on diffraction, one or more of the measured parameters 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, and the like. One of the criteria for selecting a measurement profile can be, for example, the sensitivity of one of the measurement parameters to process variations. More examples are described in U.S. patent application US2016-0161863, which is incorporated herein by reference in its entirety, and published U.S. patent application US 2016 / 0370717A1.

[0081] The patterning process in the lithographic apparatus LA can be one of the most critical steps in the process, requiring high accuracy in the dimensioning and placement of structures on the substrate W. In order to ensure this high accuracy, the three systems can be combined in a so-called "holistic" control environment, such as Figure 3Schematically depicted. One of these systems is the lithography apparatus LA, which is (in practice) connected to a metrology tool MT (a second system) and to a computer system CL (a third system). The key to this "holistic" environment is to optimize the cooperation between these three systems to enhance the overall process window and provide a tight control loop to ensure that the patterning performed by the lithography apparatus LA remains within the process window. The process window defines the range of process parameters (e.g., dose, focus, overlay) within which a particular manufacturing process produces a defined result (e.g., a functional semiconductor device)—perhaps within which process parameter variations in the lithography process or patterning process are tolerated.

[0082] The computer system CL can use (a portion of) the design layout to be patterned to predict which resolution enhancement technology to use and perform computational lithography simulations and calculations to determine which mask layout and lithographic equipment settings achieve the maximum overall process window (in Figure 3 The resolution enhancement technique may be configured to match the patterning possibilities of the lithographic apparatus LA. The computer system CL may also be used (e.g. using input from a metrology tool MT) to detect where within the process window the lithographic apparatus LA is currently operating in order to predict whether defects may be present (e.g. due to suboptimal processing) Figure 3 ) is depicted by an arrow pointing to “0” in the second scale SC2.

[0083] The metrology tool MT may provide input to the computer system CL to enable accurate simulations and predictions, and may provide feedback to the lithographic apparatus LA to identify, for example, possible drift in the calibration state of the lithographic apparatus LA (e.g., Figure 3 ) depicted by multiple arrows in a third scale SC3.

[0084] In photolithographic processes, it is desirable to perform frequent measurements of the structures produced, for example for process control and verification. Various tools are known for performing such measurements, including scanning electron microscopes or various forms of measurement devices, such as scatterometers. Examples of known scatterometers often rely on the provision of dedicated metrology targets, such as underfilled targets (targets in the form of simple gratings or overlapping gratings in different layers, which are large enough that the measurement beam produces a spot smaller than the grating) or overfilled targets (whereby the illumination spot partially or completely encompasses the target). In addition, the use of metrology tools (e.g., angle-resolved scatterometers that illuminate underfilled targets such as gratings) allows the use of so-called reconstruction methods, in which the properties of the grating can be calculated by simulating the interaction of scattered radiation with a mathematical model of the target structure and comparing the simulated results with the measured results. The parameters of the model are adjusted until the simulated interaction produces a diffraction pattern similar to the diffraction pattern observed from a real target.

[0085] Scatterometers are multifunctional instruments that allow for measurement of parameters of a lithographic process by having a sensor in the pupil or a plane conjugated to the pupil of the scatterometer's objective (measurements often referred to as pupil-based measurements), or by having a sensor in the image plane or a plane conjugated to the image plane, in which case measurements are often referred to as image- or field-based measurements. Such scatterometers and associated measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032, or EP1,628,164A, which are incorporated herein by reference in their entirety. The aforementioned scatterometers can measure multiple targets from multiple gratings in a single image using light from hard X-rays, soft X-rays, extreme ultraviolet light, and visible to near-IR wavebands.

[0086] Figure 4 A measurement device, such as a scatterometer, is depicted in FIG. The measurement device comprises a broadband (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, which measures the spectrum 6 of the specularly reflected radiation (i.e., a measure of the intensity as a function of wavelength). From this data, for example, by rigorous coupled wave analysis and nonlinear regression, or by comparing Figure 4The structure or profile of the detected spectrum can be reconstructed by the processing unit PU by comparing it with the simulated spectral library shown at the bottom of FIG. In general, for the reconstruction, the general form of the structure is known, and some parameters are assumed based on knowledge of the process used to make the structure, leaving only a few parameters of the structure to be determined based on the scatterometry data. The scatterometer can be configured as a normal incidence scatterometer or an oblique incidence scatterometer. As an alternative to the method described above, the asymmetries of multiple targets with different known overlap shifts (overlap biases) can be obtained, and the overlap can be determined based on these asymmetries without the need for reconstruction.

[0087] There are transmissive versions of examples of metrology equipment, such as Figure 4 The transmitted radiation is passed to the spectrometer detector, which measures the Figure 4 The spectrum in question. This scatterometer can be configured as a normal incidence scatterometer or an oblique incidence scatterometer. Optionally, a transmission version of hard X-ray radiation is used with a wavelength <1 nm, optionally <0.1 nm, optionally <0.01 nm.

[0088] As an alternative to optical metrology methods, the use of hard X-rays, soft X-rays, or EUV radiation has also been 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 functions in one of the wavelength ranges presented above is transmission small-angle X-ray scattering (e.g., T-SAXS in US 2007224518A, the entire contents of which are incorporated herein by reference). Lemaillet et al. discuss profile (CD) measurements using T-SAXS in "Intercomparison between optical and X-ray scatterometry measurements of FinFET structures" (Proc. of SPIE, 2013, 8681). It should be noted that the use of laser produced plasma (LPP) x-ray sources is described in U.S. Patent Publication No. 2019 / 003988A1 and U.S. Patent Publication No. 2019 / 215940A1, which are incorporated herein by reference in their entirety. Reflectometry techniques using X-rays at grazing incidence (GI-XRS) and extreme ultraviolet (EUV) radiation can be used to measure the properties of films and layer stacks on substrates. Within the general field of reflectometry, goniometric and / or spectroscopic techniques can be applied. In goniometric techniques, the variation of the reflected beam at different angles of incidence can be measured. On the other hand, spectroscopic reflectometry (using broadband radiation) measures the spectrum of wavelengths reflected at a given angle. For example, EUV reflectometry has been used for inspection of mask substrates prior to fabrication of reticles (patterning devices) for use in EUV lithography.

[0089] The range of applications may make the use of wavelengths in the soft X-ray or EUV domain inadequate. Therefore, published patent applications US 20130304424A1 and US 2014019097A1 (Bakeman et al. / KLA) describe hybrid measurement techniques in which measurements using X-rays and optical measurements using wavelengths between 120 nm and 2000 nm are combined to obtain measurements of parameters such as CD. CD measurements are obtained by coupling X-ray and optical mathematical models via one or more common components. The entire contents of the cited US patent applications are incorporated herein by reference.

[0090] Figure 5A schematic diagram of a metrology apparatus 302 is depicted, wherein radiation in the wavelength range of 0.1 nm to 100 nm can be used to measure parameters of structures on a substrate. Figure 5 The metrology device 302 presented in FIG is suitable for use in the soft X-ray or EUV domain. In the case of using hard X-rays, the metrology device 302 needs to be adapted to a transmission geometry, such as small-angle X-ray scattering (SAXS).

[0091] Figure 5 A schematic physical arrangement of a metrology apparatus 302 comprising a spectroscopic scatterometer using, for example, EUV and / or SXR radiation at grazing incidence is illustrated purely by way of example. An alternative form of inspection apparatus may be provided in the form of an angle-resolved scatterometer using radiation at normal or near normal incidence similar to conventional scatterometers operating at longer wavelengths.

[0092] The inspection apparatus 302 includes a radiation source or so-called illumination source 310 , an illumination system 312 , a substrate support 316 , detection systems 318 , 398 and a metrology processing unit (MPU) 320 .

[0093] In this example, the radiation source 310 is used to generate EUV or soft x-ray radiation, which can be based on high-order harmonic generation (HHG) technology. 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, thereby generating pulses of infrared radiation that can last, for example, less than 1 ns (1 nanosecond) per pulse, with a pulse repetition rate of up to several megahertz as needed. The wavelength of the infrared radiation can be, for example, approximately 1 μm (1 micrometer). Optionally, the laser pulses are delivered to the gas delivery system 332 as first pump radiation 340, where a portion of the radiation is converted in the gas to a higher frequency than the first radiation and becomes emitted radiation 342. A gas supply 334 supplies a suitable gas to a gas delivery system 332 where the suitable gas is optionally ionized by a power source 336. The gas delivery system 332 may be a cutting tube as will be discussed below.

[0094] The emitted radiation can contain multiple wavelengths. If the emitted radiation is monochromatic, measurement calculations (e.g., reconstruction) can be simplified, but it is easier to generate radiation with multiple wavelengths. The emission divergence angle of the emitted radiation can be wavelength-dependent. The gas provided by the gas delivery system 332 defines a gas target, which can be a gas flow or a static volume. For example, the gas can be an inert gas such as neon (Ne), helium (He), or argon (Ar). N2, O2, Ar, Kr, and Xe gases are all considered. These gases can be selectable options within the same device. Different wavelengths will provide different levels of contrast, for example, when imaging structures made of different materials. For example, to inspect metal structures or silicon structures, different wavelengths can be selected for imaging features in (carbon-based) resists or for detecting contamination of these different materials. One or more filtering devices 344 can be provided. For example, filters such as aluminum (Al) or zirconium (Zr) thin films can be used to cut off fundamental IR radiation to prevent it from being further transmitted into the inspection device. A grating (not shown) can be provided to select one or more specific harmonic wavelengths from the generated wavelengths. Some or all of the beam path may be contained within a vacuum environment, keeping in mind that SXR / EUV radiation is absorbed when traveling through air. Various components of the radiation source 310 and illumination optics 312 may be adjustable to implement different measurement "configurations" within the same device. For example, different wavelengths and / or polarizations may be selectable.

[0095] Depending on the material of the structure under inspection, different wavelengths may provide a desired level of penetration into the underlying layers. In order to distinguish between the smallest device features and defects in the smallest device features, short wavelengths may be preferred. For example, one or more wavelengths in the range of 1 nm to 20 nm, or alternatively in the range of 1 nm to 10 nm, or alternatively in the range of 10 nm to 20 nm may be selected. Wavelengths shorter than 5 nm may suffer from very low critical angles when reflected from materials of interest in semiconductor manufacturing. Therefore, selecting a wavelength greater than 5 nm will provide a stronger signal at higher angles of incidence. On the other hand, if the inspection task is to detect the presence of a certain material, for example to detect contamination, wavelengths up to 50 nanometers may be useful.

[0096] From the radiation source 310, a filtered beam 342 enters an inspection chamber 350, in which a substrate W including a structure of interest is held by a substrate support 316 for inspection at a measurement position. The structure of interest is labeled T. The atmosphere within the inspection chamber 350 is maintained at a near vacuum by a vacuum pump 352, so that EUV radiation can pass through the atmosphere without undue attenuation. The illumination system 312 has the function of focusing the radiation into a focused beam 356 and may include, for example, a two-dimensional curved mirror or a series of one-dimensional curved mirrors, as described in the previously mentioned published U.S. patent application US2017 / 0184981A1 (the entire contents of US2017 / 0184981A1 are incorporated herein by reference). Focusing is performed to achieve a circular or elliptical spot S with a diameter of less than 10 μm when projected onto the structure of interest. The substrate support 316 includes, for example, an XY translation stage and a rotation stage, by which any portion of the substrate W can be brought to the focus of the beam at a desired orientation. Thus, a radiation spot S is formed on the structure of interest. Alternatively or additionally, the substrate support 316 includes, for example, a tilting stage that can tilt the substrate W at a certain angle to control the angle of incidence of the focused beam on the structure of interest T.

[0097] Optionally, illumination system 312 provides a reference radiation beam to a reference detector 314, which can be configured to measure the spectrum and / or intensity of different wavelengths in filtered beam 342. Reference detector 314 can be configured to generate a signal 315, which is provided to processor 310 and the filter can include information about the spectrum of filtered beam 342 and / or the intensity of different wavelengths in the filtered beam.

[0098] The reflected (or diffracted 0th order) radiation 360 is captured by the detector 318 and the spectrum is provided to the processor 320 for use in calculating the properties of the target structure T. Thus, the illumination system 312 and the detection system 318 form an inspection apparatus. This inspection apparatus may include a soft X-ray and / or EUV spectroscopic reflectometer of the type described in US2016282282A1, the entire contents of which are incorporated herein by reference.

[0099] If the target has a certain periodicity, the radiation of the focused beam 356 may also be partially diffracted. The diffracted radiation 397 then follows another path at a well-defined angle relative to the angle of incidence and then relative to the reflected radiation 360. Figure 5In FIG, the diffracted radiation 397 is drawn in a schematic manner, and the diffracted radiation 397 may follow many other paths besides the path drawn. The inspection device 302 may also include another detection system 398 that detects at least a portion of the diffracted radiation 397 and / or images at least a portion of the diffracted radiation 397. Figure 5 , although a single further detection system 398 is depicted, embodiments of the inspection apparatus 302 may also include more than one further detection system 398, which are arranged at different locations 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 are detected and / or imaged by one or more further detection systems 398. The one or more detection systems 398 generate a signal 399, which 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.

[0100] To assist in aligning and focusing the light spot S with the desired product structure, the inspection apparatus 302 may also be provided with auxiliary optics that utilize auxiliary radiation under the control of the metrology processor 320. The metrology processor 320 may also communicate with a position controller 372 that operates the translation, rotation, and / or tilt stages. The processor 320 receives highly accurate feedback regarding the position and orientation of the substrate via sensors. The sensors 374 may include, for example, interferometers that can provide accuracy on the order of several picometers. During operation of the inspection apparatus 302, spectral data 382 captured by the detection system 318 is transmitted to the metrology processing unit 320.

[0101] As mentioned, alternative forms of inspection equipment use soft X-ray and / or EUV radiation at normal incidence or near normal incidence, for example to perform diffraction-based asymmetry measurements. Both types of inspection equipment can be provided in a hybrid metrology system. Performance parameters to be measured may include: overlay (OVL), critical dimension (CD), focus of the lithography apparatus as it prints a target structure, coherent diffraction imaging (CDI), overlay at resolution (ARO) measurement, and edge placement error (EPE). The soft X-ray and / or EUV radiation may, for example, have a wavelength of less than 100 nm, for example using radiation in the range of 5 nm to 30 nm, optionally in the range of 10 nm to 20 nm. The radiation may be narrowband or broadband in nature. The radiation may have discrete peaks in a particular wavelength band or may have a more continuous nature.

[0102] Similar to optical scatterometers used in today's production facilities, inspection device 302 can be used to measure structures in resist materials processed within a photolithography cell (post-development inspection or ADI) and / or to measure structures after they have been formed in harder materials (post-etch inspection or AEI). For example, inspection device 302 can be used to inspect a substrate after it has been processed by a developer, etcher, annealer, and / or other equipment.

[0103] Metrology tool MT, including but not limited to the scatterometers mentioned above, can use radiation from a radiation source to perform measurements. The radiation used by 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. Metrology tool MT can use 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 characteristics of the radiation used by metrology tool MT. For example, the resolution of electromagnetic measurements can depend on the wavelength of the radiation, with smaller wavelengths enabling measurement of smaller features, for example due to diffraction limitations. To measure features with smaller dimensions, it may be preferable to perform measurements using radiation with a short wavelength, such as EUV and / or soft X-ray (SXR) and / or hard X-ray (HXR) radiation. To perform measurements at a specific wavelength or wavelength range, metrology tool MT requires access to a source that provides radiation at that wavelength(s). Different types of sources exist for providing radiation of different wavelengths. Depending on the wavelength provided by the source, different radiation generation methods can be used. For extreme ultraviolet (EUV) radiation (e.g., 1 nm to 100 nm), and / or soft X-ray (SXR) radiation (e.g., 0.1 nm to 10 nm), sources can use high-order harmonic generation (HHG) to obtain radiation at the desired wavelength. One of the challenges faced in the development of these sources is how to efficiently couple the emitted radiation from the generation device and separate the emitted radiation from the radiation used to drive the process.

[0104] Figure 6 A simplified schematic diagram of an embodiment 600 of an illumination source 310 is shown, which may be an illumination source for high-order harmonic generation. Figure 5 One or more of the features of the illumination source in the described metrology tool may also be present in the illumination source 600, where appropriate. The illumination source 600 comprises 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 here is an example of pump radiation 340 from the pump radiation source 330, as shown in FIG. Figure 5. Pump radiation 611 can be introduced into the chamber 601 via a radiation input end 605, which can be an inspection region that can be made of molten silica or a comparable 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 a gas flow 615 within the chamber 601, which has a flow direction indicated by a second arrow. The gas flow 615 includes a small volume (e.g., several cubic millimeters) of a specific gas (e.g., an inert gas, optionally helium, argon or neon, nitrogen, oxygen or carbon dioxide) in which the gas pressure is above a certain value. The gas flow 615 can be a steady flow. Other media can also be used, such as metal plasma (e.g., aluminum plasma).

[0105] The gas delivery system of the irradiation source 600 is configured to provide a gas flow 615. The irradiation source 600 is configured to provide pump radiation 611 in the gas flow 615 to drive the generation of emission radiation 613. The region where at least a large portion of the emission radiation 613 is generated is called the interaction region. The interaction region can vary from tens of micrometers (for tightly focused pump radiation) to several millimeters or centimeters (for moderately focused pump radiation) or even up to several meters (for extremely loosely focused pump radiation). Optionally, the gas flow 615 is provided by the gas delivery system into an evacuated or nearly evacuated space. The gas delivery system includes a gas nozzle 609, such as Figure 6 As shown in FIG, the gas nozzle includes an opening 617 in the outlet plane of the gas nozzle 609. A gas flow 615 is provided from the opening 617. In almost all prior art, the gas nozzle has a cutting tube geometry, which is a uniform cylindrical internal geometry, and the shape of the opening in the outlet plane is circular. An elongated opening has also been used as described in patent application CN101515105B.

[0106] The dimensions of the gas nozzle 609 can also conceivably be scaled up or down in versions ranging from micrometer-sized nozzles to meter-sized nozzles. This wide range of sizing results from the fact that the device should be scaled so that the intensity of the pump radiation at the gas flow is ultimately within a specific range that is beneficial for the emitted radiation, which requires different sizing for different pump radiation energies, which can be pulsed lasers, and the pulse energies can vary from tens of microjoules to several joules.

[0107] Due to the interaction of the pump radiation 611 with the gas atoms of the gas flow 615, the gas flow 615 will convert a portion of the pump radiation 611 into emission radiation 613, which can be Figure 5, an example of emitted radiation 342 is shown in . The central axis of the emitted radiation 613 may be collinear with the central axis of the incident pump radiation 611. The emitted radiation 613 may have a wavelength in the X-ray or EUV range, wherein the wavelength is in the range of 0.01 nm to 100 nm, optionally 0.1 nm to 100 nm, optionally 1 nm to 100 nm, optionally 1 nm to 50 nm, or optionally 10 nm to 20 nm.

[0108] In operation, the beam of emitted radiation 613 may pass through the radiation output port 607 and may then be manipulated and directed to the wafer under inspection for metrology measurement by the illumination system 603, which may be Figure 5 6. Emitted radiation 613 may be directed (and optionally focused) to a target on the wafer.

[0109] Because air (and indeed any gas) absorbs SXR or EUV radiation to a great extent, the volume between the gas flow 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, the pump radiation 611 may need to be blocked to prevent it from passing through the radiation output 607 and entering the illumination system 603. This can be achieved by placing Figure 5 The filter arrangement 344 shown in FIG is incorporated into the radiation output end 607, which is placed in the emission beam path and is opaque or nearly opaque to the drive radiation (e.g., opaque or nearly opaque to infrared or visible light) but at least partially transparent to the emission radiation beam. The filter can be made of zirconium. When the pump radiation 611 has a hollow (optionally annular) transverse cross-sectional profile, the filter can be a hollow (optionally annular) block.

[0110] It should be noted that Figure 6 The illumination source 600 shown in FIG. 6 is merely an example, and in practice any other illumination source with other arrangements may be used as Figure 5 For example, a liquid metal jet X-ray source, an inverse Compton scattering source, or a source having a capillary tube instead of a Figure 6 In the case where the emitted radiation 342 has a wavelength in the hard X-ray region, Figure 5 The measurement equipment in can be adapted to transmission geometry.

[0111] Methods, apparatus, and assemblies are described herein for obtaining emitted radiation, optionally at higher harmonic frequencies of pump radiation. The radiation generated by the process (optionally using nonlinear effects to generate HHG of radiation at harmonic frequencies of the provided pump radiation) can be provided as radiation in a metrology tool MT for inspection and / or measurement of a substrate. The substrate can be a lithographically patterned substrate. The radiation obtained by the 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 high peak intensity in a short pulse duration.

[0112] The pump radiation 611 may include radiation having one or more wavelengths higher than one or more wavelengths of the emitted radiation. The pump radiation may include infrared radiation. The pump radiation may include radiation having a wavelength(s) in the range of 800 nm to 1500 nm. The pump radiation may include radiation having a wavelength(s) in the range of 900 nm to 1300 nm. The pump radiation may include radiation having a wavelength(s) in the range of 100 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.

[0113] In some embodiments, the emitted radiation (optionally higher order harmonic radiation) may include one or more harmonics of the pump radiation wavelength(s). The emitted radiation may include wavelengths in the extreme ultraviolet (EUV), soft X-ray (SXR), and / or hard X-ray portion of the electromagnetic spectrum. The emitted radiation 613 may include wavelengths in the range of 0.01 nm to 100 nm. The emitted radiation 613 may include wavelengths in the range of 0.1 nm to 100 nm. The emitted radiation 613 may include wavelengths in the range of 0.1 nm to 50 nm. The emitted radiation 613 may include wavelengths in the range of 1 nm to 50 nm. The emitted radiation 613 may include wavelengths in the range of 10 nm to 20 nm.

[0114] Radiation such as the higher order harmonic radiation described above can be provided as source radiation in the measurement tool MT. The measurement tool MT can use the source radiation to perform measurements on a substrate exposed by the lithographic apparatus. The measurements can be used to determine one or more parameters of a structure on the substrate. Compared to using longer wavelengths (e.g., visible radiation, infrared radiation), the use of radiation at shorter wavelengths (e.g., at wavelengths of EUV and / or SXR and / or HXR included in the wavelength ranges described above) can allow smaller features of the structure to be resolved by the measurement tool. Radiation with shorter wavelengths (such as EUV and / or SXR and / or HXR radiation) can also penetrate deeper into materials such as patterned substrates, which means that measurement of deeper layers on the substrate is possible. These deeper layers may not be accessible by radiation with longer wavelengths.

[0115] In the measurement tool MT, source radiation can be emitted from a radiation source and directed onto a target structure (or other structure) on a substrate. The source radiation may include EUV and / or SXR and / or HXR radiation. The target structure may reflect and / or diffract the source radiation incident on the target structure. The measurement tool MT may include one or more sensors for detecting diffracted radiation. For example, the measurement tool MT may include detectors for detecting positive (+1) and negative (-1) first diffraction orders or positive (+3) and negative (-3) third diffraction orders. The measurement tool MT may also measure specularly reflected radiation (0th order diffraction radiation). Other sensors for measurement may be present in the measurement tool MT, for example to measure other diffraction orders (e.g., higher diffraction orders).

[0116] In general, the methods and apparatus disclosed herein allow for compensation for target drift and / or system drift when measuring parameters of structures fabricated in or on a substrate. As stated above, the term "target drift" encompasses errors in the measurement of parameters due to material deposition and / or resist changes (particularly resist shrinkage). The term "system drift" encompasses errors caused by elements of the system, such as thermal errors.

[0117] With respect to ADI overlay targets measured using, for example, SXR, it has recently been discovered that after irradiating a target (or other structure, such as a product structure) with electromagnetic radiation, the intensity of the first-order diffracted radiation from the target is time-dependent, and in particular irradiation time-dependent. That is, the intensity of the diffracted radiation varies depending on the amount of time the target spends under irradiation, e.g. Figure 7 As shown in Figure 7A graph of the diffracted radiation intensity as the Y-axis versus the irradiation time as the X-axis is presented for radiation of three separate wavelengths. This is believed to result from the contribution of the target drift mentioned above, for example from material deposition, such as carbon deposition, and / or resist shrinkage. This time-dependent intensity introduces errors into the correction determined based on measurements of the target in a different orientation (such as a 180-degree rotation), because the diffracted radiation intensity during the second (e.g., rotated target orientation) measurement is no longer the same as the diffracted radiation intensity during the first (nominal) measurement. This can lead to incorrect asymmetry values, i.e., asymmetry values ​​that do not represent the overlap of the target.

[0118] An example of this is shown in Figure 8 In the Figure 8 Graphs of reflectance (Re) versus time (Ti) are shown for a target in a first nominal orientation (N) and a second rotated orientation (R). Figure 8 and Figure 9 In the example, the proportionality constant in the signal is considered to be unity for the purpose of illustration. The intensity of the reflectance from the target oriented for N and R varies over time as indicated by Figure 8 , as indicated by the curves labeled N and R in FIG. A target is illuminated with electromagnetic radiation in a first orientation, and the reflectance from the target is sensed and integrated over time, resulting in an area A1 under the curve. The target is then illuminated with electromagnetic radiation in a second orientation, and the reflectance from the target is sensed and integrated over time, resulting in an area A2 under the curve. The N signal is offset from the R signal, and therefore, integrating A1 and A2 will lead to incorrect conclusions about the target asymmetry and, subsequently, about the parameters to be measured (e.g., overlay and CD).

[0119] In an exemplary arrangement, at least a portion of a structure fabricated in or on a substrate is irradiated with electromagnetic radiation in a first orientation (e.g., a nominal orientation) and in a second orientation (e.g., a rotated orientation). Reflectance from the structure is measured in the nominal orientation and the rotated orientation, wherein multiple illuminations and reflectance measurements are performed for at least one of the target orientations. The reflectance measurements in the first and / or second orientations can then be combined to obtain the corrected asymmetry. In some exemplary methods and apparatus, the reflectance measurements are combined such that the measurements at the first and / or second orientations at which the multiple measurements have been performed provide an estimate of the reflectance intensity at its target orientation during a time interval over the total measurement time.

[0120] The error due to target drift is reduced in the determined asymmetry, as explained above with respect to the prior art (nominal-rotation).

[0121] exist Figure 9In one example shown in , a measurement scheme comprising nominal-rotated-nominal (NRN) measurements is performed. That is, the first orientation is the nominal orientation, and at multiple times (optionally as Figure 9 At least a portion of the structure is illuminated at two times as shown in FIG. 1 . The signal is measured at the multiple times and the reflectance can be estimated based on the measurements. The measurement of the parameter is determined based on the reflectance at the multiple times. In other arrangements, the signal can be used to determine the asymmetry and the parameter can be determined based on the asymmetry. The intensity of the reflectance can be defined as the area under the curve divided by the exposure time (A5, A6 and A7). The average of the reflectance at the two nominal intensity measurements is taken as an estimate of the reflectance at the time of measurement at the rotational orientation. That is, by Figure 9 The drift shown in the curves for the N and R lines in FIG is considered to be primarily linear. The effect of target drift on asymmetry is reduced by taking the average of two nominal reflectance measurements.

[0122] Figure 9 The example shown in provides full correction in the case of a primarily linear illumination time dependence of reflectance. Other illumination and measurement schemes, such as illuminating and measuring reflectance in nominal-nominal-rotated-nominal-nominal or other situations mentioned below, allow correction beyond a primarily linear relationship (e.g., a primarily second-order polynomial in time). It should also be noted that the illumination and measurement scheme can illuminate and measure reflectance in any order at the first orientation and the second orientation. The order can be determined to maximize throughput.

[0123] Each of the plurality of reflectance measurements at the first orientation may be shorter in duration than the reflectance measurements at the second orientation. For example, the total duration of the measurements at the first orientation may be the same as the total duration of the measurements at the second orientation. Figure 9 In the example shown in FIG. 1 , a first nominal reflectance measurement (associated with A5) may take a first time period, a rotated reflectance measurement (associated with A6) may take a second time period that may be substantially twice the first time period, and a second nominal reflectance measurement (associated with A7) may take a third time period that may be substantially the same as the first time period. In this way, the ratio of the total measurement time per orientation may be the same as in known methods. For signal-to-noise ratio, it may be advantageous to have the total reflectance measurement time spent on the first and second orientations be approximately equal.

[0124] There may be multiple measurements at a first (e.g., nominal) orientation and a second (e.g., rotated) orientation of a target (or other structure). For example, an illumination and measurement scheme may include illumination and reflectance measurements at a nominal-rotated-nominal-rotated-nominal (nom-rot-nom-rot-nom) orientation. When utilizing a scheme with multiple illumination and reflectance measurements at a first orientation and a second orientation, information about the time dependence of reflectance at both the nominal orientation and the rotated orientation may be obtained, from which estimates of the nominal and rotated orientation strengths may be obtained.

[0125] In the above example (nominal-rotated-nominal-rotated-nominal (nom-rot-nom-rot-nom)), it is possible, for example, to obtain estimates of the reflectance at the nominal and rotated orientations during the first half of the second nominal measurement. Therefore, the time interval for estimating the reflectance is not necessarily equal to the integration time of the initial measurement (here, five subsequent measurements). Instead, estimates can be made for the reflectance at each time period during the first rotated measurement, the second nominal measurement, and the second rotated measurement.

[0126] If the reflectance is not expected to vary in a linear fashion with cumulative illumination time, an exemplary arrangement may include an illumination and measurement scheme that functions correctly for signals that vary according to a model that includes primarily polynomial terms, such as primarily second-order polynomials. Figure 10 An example is shown in Figure 10 shows a drift error that is not linear with illumination time but includes major polynomial terms. Figure 10 In an example of , the illumination and measurement scheme includes an illumination and measurement sequence with the target in a first (e.g., nominal) orientation and a second (e.g., rotated) orientation, the illumination and measurement sequence being NRNRNR (i.e., a total of six exposures (illumination and measurement) with equal exposure times, with no interval measurements between the total six exposures). In such an arrangement, a weighted average of the measured reflectances for "N" and "R" can be used to model drift errors using illumination time. For example, a weighted average of the measured reflectances for "N" with weights [0.813, 0.625, 1.563], respectively, and a weighted average of the measured reflectances for "R" with weights [1.563, 0.625, 0.8125], respectively. The weights can be adjusted if there is an interval between measurements, for example to allow for substrate rotation. For example, if the gap time is 25% of the illumination time, the weights for the "N" exposure can be [0.75, 0.75, 1.5], and for the "R" exposure, the weights can be of reverse order.

[0127] In some examples disclosed herein, the weighting factors for the "N" and "R" exposures are selected so as to obtain an estimate of the average "N" reflectance over the entire measurement sequence and the average "R" reflectance over the entire measurement sequence. To do this, assuming that the temporal dependence is described by a principal second-order (quadratic) polynomial, these examples may include at least three "N" exposures and at least three "R" exposures. In other examples, the measurement sequence has only one "R" exposure and multiple "N" exposures (or otherwise). The duration of the "R" exposure (or "N" if there are more "R" exposures than "N" exposures) can be longer than that of the individual "N" exposures, so that the contribution from shot noise is mitigated. The weighting factors for the "N" exposures can be selected so as to obtain an estimate of the "N" reflectance during the "R" exposure. For example, assume that the measurement sequence is NRNN with relative exposure times 1-2-1-1. If the exposures are back-to-back without a gap, the weights for the "N" reflectance can be considered to be [0.333, 1.167, -0.500]. If the reflectance varies over time as a primarily second-order polynomial, the sum of the products of the reflectance and the weights will equal the average "N" reflectance during the "R" exposure. For example, if the N reflectance is 0.5, 0.48, and 0.42, the estimated N reflectance during the time window of the "R" exposure is 0.333*0.5+1.167*0.48-0.500*0.42=0.517. This method can be generalized to other sequences, such as NNRN, NNRNN, and with different exposure times and different intervals between exposures. This method can also be applied to sequences such as NRNRN with two "R" exposures; the weighting factors are chosen so that we can take the average "R" reflectance from the two "R" exposures and estimate the average "N" reflectance during the "R" exposure.

[0128] In addition to correcting drift errors in the manner described herein, exemplary methods and apparatus can also determine a drift error rate based on multiple measured reflectances at the same orientation. In an exemplary arrangement, the drift error rate can be determined during setup of a metrology tool's configuration and then applied to correct measurements performed during high-volume tool operation. This approach has the advantage that only a single measurement per target orientation is required during high-volume tool operation, which increases tool throughput.

[0129] There may be multiple nominal-rotational orientations, for example, measurements at 0 and 180 degrees of in-plane rotation, and measurements at 90 and 270 degrees of in-plane rotation. A model of the time-dependent reflectance change (e.g., a sinusoidal change in intensity over time) may be applied to the multiple measurements to provide an estimate. Alternatively, numerical integration techniques (e.g., a (weighted) average) may be used to provide an estimate.

[0130] In some exemplary methods and apparatus, the reflectance from multiple structures (such as overlapping targets) can be measured at a first (e.g., nominal) orientation. The reflectance from the same overlapping targets (some of them) can be measured at a second rotated orientation. Then, the reflectance from the same overlapping targets (some of them) is measured again at at least one of the nominal or rotated orientations. Thus, the reflectance from at least some of the multiple targets is measured at least three times, including measurements performed at both the nominal orientation and the rotated orientation. The measured reflectances are then combined to obtain a corrected asymmetry. The measured reflectances can be combined so that the measurements at the target orientations where the multiple measurements are performed provide an estimate of the intensity during a time interval of the total measurement time.

[0131] In such an arrangement, multiple overlapping targets are illuminated with radiation and the resulting reflectance is measured. This can be done sequentially for each of the multiple targets in a first orientation (e.g., measuring one target per measurement). The substrate (and therefore the target) can then be rotated relative to the illumination source. It should be noted here that rotating the target to a different orientation relative to the illumination source can include physical rotation of the substrate, physical rotation of the illumination source, and / or a change in the properties (e.g., polarization) of the radiation emitted by the illumination source.

[0132] In addition to target drift, tool drift can also be extracted when illuminating and measuring multiple targets in one or more of a first orientation and a second orientation because there may be a longer time interval between measurements of the same target in different orientations. In some exemplary arrangements, the illumination and measurement schemes can allow for the differentiation of target drift from tool drift. Compared to rotating the substrate (wafer) after each target measurement, sequentially measuring multiple overlapping targets in the same orientation reduces the total number of substrate rotations, thereby allowing for greater throughput and reducing measurement errors (e.g., alignment errors).

[0133] Figure 11 A flow chart illustrating an exemplary method for determining parameters of a structure fabricated in or on a substrate. The determined parameters are used to compensate for or are compensated for drift errors. The method includes irradiating 1100 at least a portion of the structure with electromagnetic radiation a plurality of times while the structure is in a first orientation.

[0134] Irradiation of at least a portion of the structure can be performed using an exemplary radiation source such as those disclosed herein. Additionally, radiation having one or more wavelengths or within a range of wavelengths can be used for irradiation. For example, the radiation can include radiation having a wavelength within a range of 0.01 nm to 100 nm. Other more specific ranges, such as those disclosed herein, can be used.

[0135] The structure is in a first orientation. This may be related to an orientation relative to an illumination source. The orientation may be a physical orientation, or parameters of the radiation, such as polarization, may be used to generate the orientation.

[0136] The multiple times at which the structure is illuminated can be sequential. Alternatively, additional illumination can occur between one or more pairs of illuminations at a first orientation. This will be apparent from the illumination and measurement schemes disclosed herein, including schemes that include multiple alternating orientations.

[0137] The method also includes sensing 1102 a plurality of reflectances from at least a portion of the structure. The reflectances are the result of illuminating the structure with the radiation discussed above. The reflectances are sensed (measured) at a plurality of times, i.e., during illumination of the structure. The reflectances indicate a parameter at the plurality of times, which may be, for example, overlap or CD. It should be noted that in the exemplary arrangement, the reflectance may comprise an integral of the intensity over a time period. The time period may be a portion or all of the time period during which the structure is illuminated.

[0138] Based on the plurality of reflectances, an estimate of a parameter is determined 1104 at another time different from the plurality of times. The other time may be between two times in the plurality of times, such as in an illumination and measurement scheme that includes the nominal orientation, the rotational orientation, or the nominal orientation of the structure. Alternatively, the other time may be a time before or after the plurality of times. In some exemplary arrangements, an estimate of a drift error may be determined at the other time based on the plurality of sensed reflectances. This determination may be made in addition to or as an alternative to determining the parameter at the other time. The determined drift error may be a target drift error or a combination of target drift and system drift.

[0139] In some exemplary arrangements, determining 1104 an estimate of the parameter at another time may include illuminating at least a portion of the structure at another time when the structure is in the second orientation, and sensing a resulting reflectance. There may be multiple illuminations of the at least a portion of the structure, and the resulting reflectance may be sensed multiple times at multiple other times. The estimate of the parameter at one or more other times may be based on the sensed reflectance for the structure in the second orientation at one or more other times.

[0140] As discussed above, the difference between the first and second orientations of a structure (e.g., a target) can be achieved by using a separate illumination source, rotating the illumination source, or using radiation with different parameters (e.g., p-polarized radiation and s-polarized radiation), each of which can achieve relative rotation of the structure relative to the illumination source. However, in a typical arrangement, the substrate can be rotated on the substrate support WT. The rotation can be about a z-axis that is orthogonal to the plane of the substrate. Although the rotation can be 180 degrees, in some arrangements the rotation can be 90 degrees or any other angle suitable for measurement of the parameter.

[0141] In an exemplary arrangement, determining parameters 1104 is based on a total radiation dose to at least a portion of the structure in a first orientation and a second orientation. The total radiation dose may be proportional to the irradiation time of the structure in the first and / or second orientations. The total radiation dose for a particular orientation may include the sum of the irradiation times of the structure in that orientation at multiple times or at multiple other times. Examples of arrangements are discussed above in which the total radiation doses in the first and second orientations are substantially equal. This can be used in conjunction with the assumption that drift error varies primarily linearly with irradiation time. Other examples include using a weighted sum of reflectances, as discussed above. This can be used in conjunction with the assumption that drift error varies with irradiation time based on a relationship including one or more polynomial terms. The weights applied in determining one or more weighted averages may be based on the time between irradiations of at least a portion of the structure. For example, if the substrate is rotated between irradiations, the weights may be determined accordingly, as discussed in the examples above.

[0142] In an exemplary arrangement where the relationship between drift error and illumination time can be assumed to be primarily linear, a drift error rate can be determined based on the primarily linear relationship. This can then be used to illuminate and measure subsequent structures, which may be on the same substrate and / or on other substrates. Using the drift error rate allows for parameter estimation without having to repeat multiple illuminations and measurements performed at multiple times in the first orientation.

[0143] Illuminating 1100 at a plurality of times may include irradiating a plurality of structures (eg, targets) on a substrate, wherein the substrate, and therefore the structures, are in a first orientation.

[0144] Examples of illumination and measurement schemes may be N1a, N1b, R1a, R1b, N2a, N2b, where N represents a first (nominal) orientation and R represents a second (rotated) orientation, the numbers 1 and 2 represent a first illumination and a second illumination and subsequent measurement of a structure, and a and b represent two different structures. This may allow for determining an estimate of target drift and / or an estimate of system drift, respectively, based on reflectance.

[0145] In the above scheme, the average of the sensed reflectances of N1a and N2a can, for example, provide an estimate of the reflectance at the nominal orientation at R1a. Since the time interval N1a-R1a is substantially identical to the time interval N2a-R1a (assuming the same illumination and measurement times for each), this average provides (partial) compensation for both target drift and tool drift. In this simple case, it may not be necessary to distinguish between tool drift and target drift.

[0146] If the time intervals mentioned above are different, a weighted average can be used. For example, the time interval between N1a and R1a is t 1a And the time interval between R1a-N2a is t 2a In the case of 2a / (t 1a +t 2a ) and t 1a / (t 1a +t 2a ) are applied to measurements N1a and N2a, respectively.

[0147] In some arrangements, only the first structure "a" may be measured multiple times, for example three times, and the drift error of structure "b" may be calculated based on the multiple measurements of structure "a". The illumination and measurement scheme in this example may be N1a, N1b, R1a, N2a. Since the time interval R1a-N1a is now longer than N2a-R1a (assuming the illumination and measurement times are the same for each), the estimate of the reflectance at the nominal orientation when measuring the reflectance under the rotated orientation can distinguish target drift from system (or tool) drift. This can be done as follows: from the difference in intensity between N1b and N1a, the tool drift rate is estimated (assuming a constant radiation dose, the target drift is the same for targets "a" and "b"; for simplicity, noise is assumed to be negligible). The system drift rate is then subtracted from N2a-N1a multiplied by the time interval between these measurements to obtain the target drift rate. The tool drift rate and target drift rate can then be used to calculate the reflectance at the nominal orientation when rotating the directional illumination and measurement, as mentioned above. Note that this assumes that the correction for structure "a" extracted using the system drift rate estimate and the target drift rate estimate can be applied to structure "b".

[0148] The system drift rate and target drift rate can be determined as described above during the setup of the tool (calibration) configuration. During operation of the tool for high-volume manufacturing, a single illumination and measurement of a structure is then sufficient, which increases throughput. It may be beneficial to periodically re-evaluate the system drift rate and target drift rate as described above during high-volume tool operation. Even if a correction needs to be performed for each structure (thus requiring two measurements (N and R) per structure), the target rate and drift rate extracted according to the above scheme can be used to allow this correction without requiring a third measurement of each target.

[0149] The other features discussed above also apply to this scheme, such as the use of multiple structure orientations and multiple repetitions of illumination and measurement to compensate for higher-order effects in drift.

[0150] The methods and apparatus disclosed herein can be applicable to overlay and / or CD, but are also generally applicable to estimating a single measurement parameter in the presence of target drift, which can be derived from a structure or target based on multiple illuminations and measurements of the target under different conditions. Thus, multiple illuminations and measurements under different conditions are combined to obtain the measurement parameter. While the measurement parameter can still use reflectance and the measurements can still be at different orientations (e.g., 0 degree in-plane rotation and 90 degree in-plane rotation), in some arrangements, the measurements can be combined to obtain a combined intensity detector image. This can be done because the detector is more elongated in one direction, so that for a 2D diffraction target, more diffraction orders are captured in one direction (e.g., x-direction) than in another direction (e.g., y-direction). Rotating 90 degrees allows more orders to be captured in the other direction. In the case of the 0 and 90 degree measurements, although the signals are not subtracted to account for asymmetry, they are fed as a pair to the analysis algorithm.

[0151] In some arrangements, the measurement may be made after illumination with s-polarized radiation for the first measurement and p-polarized radiation for the second measurement.The two measurements are fed as a pair into the analysis algorithm.

[0152] In other arrangements, measurement parameters may be determined based on reflectance, and measurements may be made after illumination with radiation including wavelengths in a first spectrum for a first measurement and illumination with radiation including wavelengths in a second spectrum for a second measurement.

[0153] The methods and apparatus disclosed herein differ from the CD-SEM context in a number of ways. The methods and apparatus may differ with respect to the purpose and / or problem under consideration in the following respects: The methods and apparatus disclosed herein may be intended to enable correction of overlap extraction in the presence of sample drift. This correction typically uses two measurements of the same target, which cannot be made simultaneously and may therefore include target drift. The purpose of a CD-SEM is to obtain an overlap of the unexposed state. The methods and apparatus disclosed herein may also differ from a CD-SEM in how the method is applied. In a CD-SEM, extrapolation of the shrinkage model may be performed in combination with a model of resist shrinkage that varies as a function of time and / or dose. The methods and apparatus disclosed herein do not involve extrapolation, but rather may use techniques based on numerical integration to computationally obtain measurements at both a nominal sample orientation and a rotated sample orientation simultaneously.

[0154] Another application of the embodiments mentioned above may be the following situation: there may be a situation where the positive and negative orders of radiation (e.g. SXR diffraction patterns) cannot be detected at once, because the positive and negative orders of radiation do not fit on one or more detectors, and therefore at least two acquisitions are required, wherein the one or more detectors are translated between these acquisitions. In order to take into account the target drift during these multiple, optionally two, acquisitions, the embodiments mentioned above may be applied. An example is that after the second acquisition, the one or more detectors return to the detector position of the first acquisition and acquire a third image. Averaging the first and third images at the first position may (partially) correct the target drift. It should be noted that in this embodiment the detector changes orientation, whereas in the embodiments mentioned above the sample changes orientation. An alternative to changing the detector position may be to change the target orientation. Another alternative may be to slightly change the pointing of the beam incident on the sample.

[0155] The illumination source may be provided in, for example, a metrology apparatus MT, optionally a metrology apparatus, an inspection apparatus, a lithographic apparatus LA and / or a lithocell LC in a lithographic apparatus.

[0156] The properties of the emitted radiation used to perform the measurement can affect the quality of the resulting measurement. For example, the shape and size of the lateral beam profile (cross-section) of the radiation beam, the intensity of the radiation, the power spectral density of the radiation, etc. can affect the measurement performed by the radiation. Therefore, it is beneficial to have a source that provides radiation with properties that result in high-quality measurements.

[0157] Further embodiments are disclosed in subsequently numbered aspects (Aspect Group A):

[0158] 1. A method for determining parameters of a structure produced in or on a substrate, which are compensated for drift errors, the method comprising:

[0159] irradiating at least a portion of the structure with electromagnetic radiation at a plurality of times, the at least a portion of the structure being in a first orientation;

[0160] sensing a plurality of average reflectances of the at least a portion of the structure at the plurality of times, wherein the average reflectances are indicative of the parameter at the plurality of times;

[0161] irradiating the at least a portion of the structure with electromagnetic radiation at one or more other times, the at least a portion of the structure being in a second orientation; and

[0162] sensing one or more other average reflectances of the at least a portion of the structure at the one or more other times, wherein the other average reflectances are indicative of the parameter at the one or more other times; and

[0163] An estimate of the parameter at the one or more other times is determined based on the plurality of average reflectances and the one or more other average reflectances.

[0164] 2. The method according to clause 1, further comprising: estimating the drift error based on the plurality of average reflectances.

[0165] 3. A method according to any of the preceding aspects, wherein, in the second orientation, at least a portion of the structure is rotated about a z-axis perpendicular to the plane of the substrate, the rotation being relative to the source of the electromagnetic radiation, and optionally wherein the rotation is one of 180 degrees and 90 degrees.

[0166] 4. The method according to any preceding aspect, wherein the one or more other times is a plurality of other times.

[0167] 5. A method according to any of the preceding aspects, wherein the estimate of the parameter is further determined based on a total radiation dose associated with the irradiation of at least a portion of the structure in the first orientation, and a total radiation dose associated with the irradiation of at least a portion of the structure in the second orientation.

[0168] 6. A method according to aspect 5, wherein the total radiation dose associated with the irradiation of at least a portion of the structure in the first orientation is substantially equal to the total radiation dose associated with the irradiation of at least a portion of the structure in the second orientation.

[0169] 7. The method according to any of the preceding aspects, wherein the plurality of average reflectances and / or the further average reflectances comprise an integral of the radiation intensity over at least a portion of the illumination time.

[0170] 8. The method according to any of the preceding aspects, wherein the estimation of the parameter is further determined based on a predominantly linear relationship between the drift error of the at least part of the structure and the illumination time.

[0171] 9. The method of clause 8, further comprising determining a rate of change of the drift error for use in determining an estimate of the parameter of one or more structures fabricated in or on one or more other substrates.

[0172] 10. A method according to any one of aspects 1 to 7, wherein the estimate of the parameter is further determined based on a relationship between the drift error of the at least part of the structure and the illumination time, the relationship comprising one or more principal polynomial functions.

[0173] 11. The method of any one of aspects 1 to 7 and 10, wherein determining the estimate of the parameter comprises determining a weighted average of the plurality of average reflectances.

[0174] 12. The method of clause 11, wherein determining the estimate of the parameter comprises determining a weighted average of the plurality of other average reflectances.

[0175] 13. A method according to clause 11 or 12, wherein the weights applied when determining the weighted average are determined based on the time between irradiations of the at least part of the structure.

[0176] 14. The method according to any one of the preceding aspects, wherein irradiating at the plurality of times comprises:

[0177] irradiating at least a portion of the first structure at a first time;

[0178] irradiating at least a portion of the second structure at a second time; and

[0179] Illuminating at least a portion of the first structure at a third time, wherein the plurality of average reflectances are indicative of the parameter at the first time, at the second time, and at the third time.

[0180] 15. The method according to any of the preceding aspects, wherein determining the estimate of the parameter comprises determining an estimate of target drift and / or an estimate of system drift based on the plurality of average reflectances.

[0181] 16. A method according to clause 15, wherein the estimate of the system drift is determined based on the intensity of diffracted radiation corresponding to illumination of the first structure at the first time and the intensity of diffracted radiation corresponding to illumination of the second structure at the second time.

[0182] 17. A method according to aspects 15 or 16, wherein the estimate of the target drift is determined based on the intensity of diffracted radiation corresponding to the illumination of the first structure at the first time and the intensity of diffracted radiation corresponding to the illumination of the first structure at the third time.

[0183] 18. The method of any preceding aspect, wherein the electromagnetic radiation used to illuminate the at least a portion of the structure in the first orientation is one of p-polarized electromagnetic radiation and s-polarized electromagnetic radiation.

[0184] 19. The method of clause 18, wherein the electromagnetic radiation used to illuminate the at least a portion of the structure in the second orientation is the other of p-polarized electromagnetic radiation and s-polarized electromagnetic radiation.

[0185] 20. The method according to any preceding aspect, wherein the electromagnetic radiation used to illuminate the at least a portion of the structure in the first orientation comprises electromagnetic radiation in a first spectrum,

[0186] And wherein the electromagnetic radiation used to illuminate the at least a portion of the structure in the second orientation comprises electromagnetic radiation in a second spectrum.

[0187] 21. A method according to any preceding aspect, wherein the structure comprises a metrology target.

[0188] 22. The method of any preceding aspect, wherein the parameter comprises one of overlay and critical dimension.

[0189] 23. The method of any preceding aspect, wherein the electromagnetic radiation comprises electromagnetic radiation having a wavelength in the range of 0.01 nm to 100 nm.

[0190] 24. The method according to any of the preceding aspects, wherein the drift error is caused by material deposition or target shrinkage.

[0191] 25. A computer program product comprising instructions, which when executed on at least one processor cause the at least one processor to control a device to implement the method according to any preceding aspect.

[0192] 26. An apparatus for determining parameters of a structure fabricated in or on a substrate, compensated for drift errors, the apparatus comprising a computer processor configured to control the apparatus to implement the following method:

[0193] irradiating at least a portion of the structure with electromagnetic radiation at a plurality of times, the at least a portion of the structure being in a first orientation;

[0194] sensing a plurality of average reflectances of the at least a portion of the structure at the plurality of times, wherein the average reflectances are indicative of the parameter at the plurality of times;

[0195] irradiating the at least a portion of the structure with electromagnetic radiation at one or more other times, the at least a portion of the structure being in a second orientation; and

[0196] sensing one or more other average reflectances of the at least a portion of the structure at the one or more other times, wherein the other average reflectances are indicative of the parameter at the one or more other times; and

[0197] An estimate of the parameter at the one or more other times is determined based on the plurality of average reflectances and the one or more other average reflectances.

[0198] 27. A metrology tool comprising the apparatus according to clause 26.

[0199] 28. A lithographic system comprising the apparatus of clause 26.

[0200] 29. A lithocell comprising the apparatus of clause 26.

[0201] Further embodiments are disclosed in subsequently numbered aspects (Aspect Group B):

[0202] 1. A method for determining parameters of a structure produced in or on a substrate, which are compensated for drift errors, the method comprising:

[0203] irradiating at least a portion of the structure with electromagnetic radiation at a plurality of times, the at least a portion of the structure being in a first orientation;

[0204] sensing a plurality of average reflectances of the at least a portion of the structure at the plurality of times, wherein the average reflectances are indicative of the parameter at the plurality of times; and

[0205] An estimate of the parameter at one or more other times is determined based on the plurality of average reflectances.

[0206] 2. The method according to aspect 1, further comprising:

[0207] irradiating the at least a portion of the structure with electromagnetic radiation at the one or more other times, the at least a portion of the structure being in a second orientation; and

[0208] One or more other average reflectances of the at least a portion of the structure are sensed at the one or more other times, wherein the other average reflectances are indicative of the parameter at the one or more other times.

[0209] 3. The method of clause 2, further comprising determining the estimate of the parameter based on the one or more other average reflectances.

[0210] 4. The method according to any of the preceding aspects further comprises: estimating the drift error based on the multiple average reflectances.

[0211] 5. A method according to any one of aspects 2 to 4, wherein, in the second orientation, at least a portion of the structure is rotated about a z-axis perpendicular to the plane of the substrate, the rotation being relative to the source of the electromagnetic radiation, and optionally wherein the rotation is one of 180 degrees and 90 degrees.

[0212] 6. A method according to any one of aspects 2 to 5, wherein the estimate of the parameter is further determined based on a total radiation dose associated with the irradiation of at least a portion of the structure in the first orientation, and a total radiation dose associated with the irradiation of at least a portion of the structure in the second orientation.

[0213] 7. A method according to any one of aspects 2 to 6, wherein the plurality of average reflectances and / or the further average reflectances comprise an integral of the radiation intensity over at least a portion of the illumination time.

[0214] 8. A method according to any preceding aspect, wherein determining the estimate of the parameter comprises determining a weighted average of the plurality of average reflectances.

[0215] 9. A method according to clause 8 when directly or indirectly dependent on clause 2, wherein determining the estimate of the parameter comprises determining a weighted average of the plurality of other average reflectances.

[0216] 10. A method according to clause 8 or 9, wherein the weight applied when determining the weighted average is determined based on the time between irradiations of the at least part of the structure.

[0217] 11. The method according to any of the preceding aspects, wherein determining the estimate of the parameter comprises determining an estimate of target drift and / or an estimate of system drift based on the plurality of average reflectances.

[0218] 12. The method according to any preceding aspect, wherein irradiating at the plurality of times comprises:

[0219] irradiating at least a portion of the first structure at a first time;

[0220] irradiating at least a portion of the second structure at a second time; and

[0221] Illuminating at least a portion of the first structure at a third time, wherein the plurality of average reflectances are indicative of the parameter at the first time, at the second time, and at the third time.

[0222] 13. A method according to clause 12, wherein the estimate of the system drift is determined based on the intensity of diffracted radiation corresponding to illumination of the first structure at the first time and the intensity of diffracted radiation corresponding to illumination of the second structure at the second time.

[0223] 14. A method according to aspects 12 or 13, wherein the estimate of the target drift is determined based on the intensity of diffracted radiation corresponding to the illumination of the first structure at the first time and the intensity of diffracted radiation corresponding to the illumination of the first structure at the third time.

[0224] 15. A method according to any of the preceding claims, wherein the electromagnetic radiation used to illuminate at least a portion of the structure in the first orientation includes electromagnetic radiation in a first spectrum, and wherein the electromagnetic radiation used to illuminate at least a portion of the structure in the second orientation includes electromagnetic radiation in a second spectrum.

[0225] 16. A computer program product comprising instructions, which when executed on at least one processor cause the at least one processor to control a device to implement the method according to any preceding aspect.

[0226] 17. An apparatus for determining parameters of a structure fabricated in or on a substrate, which are compensated for drift errors, the apparatus comprising a computer processor configured to control the apparatus to implement the following method:

[0227] irradiating at least a portion of the structure with electromagnetic radiation at a plurality of times, the at least a portion of the structure being in a first orientation;

[0228] sensing a plurality of average reflectances of the at least a portion of the structure at the plurality of times, wherein the average reflectances are indicative of the parameter at the plurality of times;

[0229] An estimate of the parameter at one or more other times is determined based on the plurality of average reflectances.

[0230] 18. A metrology tool comprising the apparatus according to clause 17.

[0231] 19. A lithographic system comprising the apparatus according to clause 17.

[0232] 20. A lithocell comprising the apparatus of clause 17.

[0233] According to certain arrangements disclosed herein, a method of determining a drift error in a measurement of a parameter of a structure fabricated in or on a substrate is provided, the method comprising: illuminating at least a portion of the structure with electromagnetic radiation at a plurality of times, the at least a portion of the structure being in a first orientation; sensing a plurality of reflectances from the at least a portion of the structure at the plurality of times, wherein the reflectances are indicative of the parameter at the plurality of times; and determining an estimate of the drift error in the measurement of the parameter at one or more other times based on the plurality of reflectances. A corresponding apparatus is also disclosed.

[0234] Although specific reference may be made herein to the use of lithographic equipment in IC manufacturing, it should be understood that the lithographic equipment described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like.

[0235] Although specific reference may be made herein to embodiments in the context of lithographic apparatus, the embodiments may be used in other apparatuses. The embodiments may form part of mask inspection equipment, metrology equipment, or any apparatus that measures or processes an object such as a wafer (or other substrate) or a mask (or other patterning device). These apparatuses may generally be referred to as lithographic tools. The lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0236] Although specific reference may be made herein to embodiments in the context of inspection or metrology equipment, embodiments may be used in other equipment. Embodiments may form part of mask inspection equipment, lithographic equipment, or any equipment that measures or processes objects such as wafers (or other substrates) or masks (or other pattern forming devices). The term "metrology equipment" (or "inspection equipment") may also refer to an inspection equipment or a detection system (or a metrology equipment or a metrology system). For example, an inspection equipment including an embodiment may be used to detect defects in a substrate or defects in a structure on a substrate. In this embodiment, the characteristic of interest of a structure on a substrate may be a defect in the structure, the absence of a particular portion of the structure, or the presence of an undesirable structure on the substrate.

[0237] Although specific reference may be made above to the use of embodiments in the context of optical lithography, it will be appreciated that the invention is not limited to optical lithography but may be used in other applications (eg imprint lithography) where the context permits.

[0238] While the targets or target structures (more generally, structures on a substrate) described above are metrology target structures that are specifically designed and formed for the purpose of measurement, in other embodiments, the properties of interest may be measured on one or more structures that are functional parts of a device formed on the substrate. Many devices have regular grating-like structures. The terms "structure," "target grating," and "target structure" as used herein do not require that the structure has been provided specifically for the measurement being performed. Additionally, while the pitch of the metrology target may be close to the resolution limit of the scatterometer's optical system or possibly smaller, it may be much larger than the size of a typical non-target structure (optionally a product structure) in the target portion C that is made by a photolithographic process. In practice, the lines and / or spaces of the overlapping gratings within the target structure may be made to include smaller structures that are similar in size to the non-target structures.

[0239] Although specific embodiments have been described above, it will be appreciated that the present invention may be practiced in other ways than those described. The above description is intended to be illustrative, not restrictive. Therefore, it will be apparent to those skilled in the art that modifications may be made to the described invention without departing from the scope of the claims set forth below.

[0240] Although specific reference is made to "measurement equipment / tools / systems" or "inspection equipment / tools / systems," these terms may refer to the same or similar types of tools, equipment, or systems. For example, an inspection or measurement equipment including embodiments of the present invention may be used to determine characteristics of structures on a substrate or on a wafer. For example, an inspection or measurement equipment including embodiments of the present invention may be used to detect defects in a substrate or defects in a structure on a substrate or on a wafer. In this embodiment, the characteristic of interest of a structure on a substrate may relate to a defect in the structure, the absence of a particular portion of the structure, or the presence of an undesirable structure on the substrate or on the wafer.

[0241] Although specific reference is made to SXR and / or EUV electromagnetic radiation, it will be understood that the present invention can be practiced with all electromagnetic radiation, including radio waves, microwaves, infrared, (visible) light, ultraviolet, X-rays and gamma rays, where the context permits. As an alternative to optical metrology methods, the use of X-rays, optionally hard X-rays, for example radiation in the wavelength range between 0.01 nm and 10 nm, or alternatively between 0.01 nm and 0.2 nm, or alternatively between 0.1 nm and 0.2 nm, has also been considered for metrology measurements.

Claims

1. A method for determining parameters of a structure produced in or on a substrate, which are compensated for drift errors, the method comprising: irradiating at least a portion of the structure with electromagnetic radiation at a plurality of times, the at least a portion of the structure being in a first orientation; sensing a plurality of average reflectances of the at least a portion of the structure at the plurality of times, wherein the average reflectances are indicative of the parameter at the plurality of times and the average reflectances encompass a ratio of a diffracted spectral flux to a spectral flux incident on a target; as well as An estimate of the parameter at one or more other times is determined based on the plurality of average reflectances.

2. The method according to claim 1, further comprising: irradiating the at least a portion of the structure with electromagnetic radiation at the one or more other times, the at least a portion of the structure being in a second orientation; as well as One or more other average reflectances of the at least a portion of the structure are sensed at the one or more other times, wherein the other average reflectances are indicative of the parameter at the one or more other times.

3. The method according to claim 2, further comprising: The estimate of the parameter is determined based on the one or more other average reflectances.

4. The method according to any one of the preceding claims, further comprising: The drift error is estimated based on the plurality of average reflectances.

5. The method according to any one of claims 2 to 3, wherein In the second orientation, the at least a portion of the structure is rotated about a z-axis perpendicular to the plane of the substrate, the rotation being relative to a source of the electromagnetic radiation, and optionally wherein the rotation is one of 180 degrees and 90 degrees.

6. The method according to any one of claims 2 to 3, wherein: The estimate of the parameter is determined further based on a total radiation dose associated with the irradiating the at least a portion of the structure in the first orientation and a total radiation dose associated with the irradiating the at least a portion of the structure in the second orientation.

7. The method according to any one of claims 2 to 3, wherein: The plurality of average reflectances and / or the further average reflectance comprise an integral of the radiation intensity over at least a portion of the illumination time.

8. The method according to any one of claims 2 to 3, wherein: Determining the estimate of the parameter includes determining a weighted average of the plurality of average reflectances.

9. The method according to claim 8, wherein Determining the estimate of the parameter comprises determining a weighted average of the plurality of other average reflectances.

10. The method according to claim 8, wherein A weight applied when determining the weighted average is determined based on a time between irradiations of the at least a portion of the structure.

11. The method according to any one of claims 1 to 3, 9 and 10, wherein Determining the estimate of the parameter includes determining an estimate of target drift and / or an estimate of system drift based on the plurality of average reflectances.

12. The method according to claim 11, wherein The irradiation at the plurality of times includes: irradiating at least a portion of the first structure at a first time; irradiating at least a portion of the second structure at a second time; and Illuminating at least a portion of the first structure at a third time, wherein the plurality of average reflectances are indicative of the parameter at the first time, the parameter at the second time, and the parameter at the third time.

13. The method according to claim 12, wherein: An estimate of the system drift is determined based on an intensity of diffracted radiation corresponding to illumination of the first structure at the first time and an intensity of diffracted radiation corresponding to illumination of the second structure at the second time.

14. The method according to claim 12, wherein: An estimate of the target drift is determined based on an intensity of diffracted radiation corresponding to illumination of the first structure at the first time and an intensity of diffracted radiation corresponding to illumination of the first structure at the third time.

15. The method according to any one of claims 2 to 3, 9 and 10, wherein The electromagnetic radiation used to illuminate the at least a portion of the structure in the first orientation comprises electromagnetic radiation in a first spectrum, and wherein the electromagnetic radiation used to illuminate the at least a portion of the structure in the second orientation comprises electromagnetic radiation in a second spectrum.

16. A computer program product comprising instructions which, when executed on at least one processor, cause the at least one processor to control a device to implement the method according to any one of the preceding claims.

17. An apparatus for determining parameters of a structure fabricated in or on a substrate, which are compensated for drift errors, the apparatus comprising a computer processor configured to control the apparatus to implement the following method: irradiating at least a portion of the structure with electromagnetic radiation at a plurality of times, the at least a portion of the structure being in a first orientation; sensing a plurality of average reflectances of the at least a portion of the structure at the plurality of times, wherein: The average reflectance is indicative of the parameter at the plurality of times, and the average reflectance encompasses a ratio of a diffracted spectral flux to a spectral flux incident on a target; An estimate of the parameter at one or more other times is determined based on the plurality of average reflectances.

18. A metrology tool comprising the apparatus according to claim 17.

19. A lithography system comprising the apparatus according to claim 17.

20. A lithocell comprising an apparatus according to claim 17.

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