Patent Application
20250189904
This application claims priority of EP application 22161160.1 which was filed on 2022 Mar 9 and which is incorporated herein in its entirety by reference.
The present invention relates to metrology applications in the manufacture of integrated circuits.
A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern (also often referred to as “design layout” or “design”) at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer).
To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm and 13.5 nm. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.
Low-k1 lithography may be used to process features with dimensions smaller than the classical resolution limit of a lithographic apparatus. In such process, the resolution formula may be expressed as CD=k1×λ/NA, where λ is the wavelength of radiation employed, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the “critical dimension” (generally the smallest feature size printed, but in this case half-pitch) and k1 is an empirical resolution factor. In general, the smaller k1 the more difficult it becomes to reproduce the pattern on the substrate that resembles the shape and dimensions planned by a circuit designer in order to achieve particular electrical functionality and performance. To overcome these difficulties, sophisticated fine-tuning steps may be applied to the lithographic projection apparatus and/or design layout. These include, for example, but not limited to, optimization of NA, customized illumination schemes, use of phase shifting patterning devices, various optimization 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, tight control loops for controlling a stability of the lithographic apparatus may be used to improve reproduction of the pattern at low k1.
In lithographic processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes, which are often used to measure critical dimension (CD), and specialized tools to measure overlay, the accuracy of alignment of two layers in a device. Recently, various forms of scatterometers have been developed for use in the lithographic field.
Examples of known scatterometers often rely on provision of dedicated metrology targets. For example, a method may require a target in the form of a simple grating that is large enough that a measurement beam generates a spot that is smaller than the grating (i.e., the grating is underfilled). In so-called reconstruction methods, properties of the grating can be calculated by simulating interaction of scattered radiation with a mathematical model of the target structure. Parameters of the model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from the real target.
In addition to measurement of feature shapes by reconstruction, diffraction-based overlay can be measured using such apparatus, as described in published patent application US2006066855A1. Diffraction-based overlay metrology using dark-field imaging of the diffraction orders enables overlay measurements on smaller targets. These targets can be smaller than the illumination spot and may be surrounded by product structures on a wafer. Examples of dark field imaging metrology can be found in numerous published patent applications, such as for example US2011102753A1 and US20120044470A. Multiple gratings can be measured in one image, using a composite grating target. The known scatterometers tend to use light in the visible or near-IR wave range, which requires the pitch of the grating to be much coarser than the actual product structures whose properties are actually of interest. Such product features may be defined using deep ultraviolet (DUV), extreme ultraviolet (EUV) or X-ray radiation having far shorter wavelengths. Unfortunately, such wavelengths are not normally available or usable for metrology.
On the other hand, the dimensions of modern product structures are so small that they cannot be imaged by optical metrology techniques. Small features include for example those formed by multiple patterning processes, and/or pitch-multiplication. Hence, targets used for high-volume metrology often use features that are much larger than the products whose overlay errors or critical dimensions are the property of interest. The measurement results are only indirectly related to the dimensions of the real product structures, and may be inaccurate because the metrology target does not suffer the same distortions under optical projection in the lithographic apparatus, and/or different processing in other steps of the manufacturing process. While scanning electron microscopy (SEM) is able to resolve these modern product structures directly, SEM is much more time consuming than optical measurements. Moreover, electrons are not able to penetrate through thick process layers, which makes them less suitable for metrology applications. Other techniques, such as measuring electrical properties using contact pads is also known, but it provides only indirect evidence of the true product structure.
By decreasing the wavelength of the radiation used during metrology (i.e. moving towards the “soft X-ray (SXR)” wavelength spectrum), it is possible to resolve smaller structures, to increase sensitivity to structural variations of the structures and/or penetrate further into the product structures. One such method of generating suitably high frequency radiation (e.g., soft X-ray and/or EUV radiation) may be using a pump radiation (e.g., infra-red radiation) to excite a generating medium, thereby generating an emitted radiation, optionally a high harmonic generation comprising high frequency radiation.
It is desirable to eliminate or mitigate the signal impact imposed by the source when performing SXR metrology or similar metrology techniques.
In a first aspect of the invention there is provided a method of correcting a measured spectrum for the effects of a source spectrum resulting from an illumination source; the method comprising: obtaining a measured spectrum in terms of a measurement parameter, the measured spectrum being obtained from captured diffracted radiation from a periodic structure following illumination of said periodic structure using source radiation from said illumination source, the periodic structure being the spectrometer grating and an object being measured; determining an estimate of the source spectrum from the measured spectrum; and correcting the measured spectrum using the estimate of the source spectrum.
Also disclosed is a computer program, metrology apparatus and a lithographic apparatus being operable to perform the method of the first aspect.
The above and other aspects of the invention will be understood from a consideration of the examples described below.
Embodiments will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:
In the present document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic radiation and particle radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm), EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range of about 5-100 nm), X-ray radiation, electron beam radiation and other particle radiation.
The term “reticle”, “mask” or “patterning device” as employed in this text may be broadly interpreted as referring to a generic patterning device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Besides the classic mask (transmissive or reflective, binary, phase-shifting, hybrid, etc.), examples of other such patterning devices include a programmable mirror array and a programmable LCD array.
In operation, the illumination system IL receives a radiation beam from a radiation source SO, e.g. via a beam delivery system BD. The illumination system IL may include various types of optical components, such as refractive, reflective, diffractive, magnetic, electromagnetic, electrostatic, and/or other types of optical components, or any combination thereof, for directing, shaping, and/or controlling radiation. 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 a plane of the patterning device MA.
The term “projection system” PS used herein should be broadly interpreted as encompassing various types of projection system, including refractive, reflective, diffractive, catadioptric, anamorphic, magnetic, electromagnetic and/or electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, and/or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system” PS.
The lithographic apparatus LA may be of a type wherein 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 a space between the projection system PS and the substrate W—which is also referred to as immersion lithography. More information on immersion techniques is given in U.S. Pat. No. 6,952,253, which is incorporated herein by reference in its entirety.
The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also named “dual stage”). In such “multiple stage” machine, the substrate supports WT may be used in parallel, and/or steps in preparation of a subsequent exposure of the substrate W may be carried out on the substrate W located on one of the substrate support WT while another substrate W on the other substrate support WT is being used for exposing a pattern on the other substrate W.
In addition to the substrate support WT, the lithographic apparatus LA may comprise a measurement stage. The measurement stage is arranged to hold a sensor and/or a cleaning device. The sensor may be arranged to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold multiple sensors. The cleaning device may be arranged to clean part of the lithographic apparatus, for example a part of the projection system PS or a part of a system that provides the immersion liquid. The measurement stage may move beneath the projection system PS when the substrate support WT is away from the projection system PS.
In operation, the radiation beam B is incident on the patterning device, e.g. mask, MA which is held on the mask support T, and is patterned by the pattern (design layout) present on patterning device MA. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and a position measurement system IF, the substrate support WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B at a focused and aligned position. Similarly, the first positioner PM and possibly another position sensor (which is not explicitly depicted in
As shown in
In lithographic processes, it is desirable to make frequently measurements of the structures created, e.g., for process control and verification. Tools to make such measurement are typically called metrology tools MT. Different types of metrology tools MT for making such measurements are known, including scanning electron microscopes or various forms of scatterometer metrology tools MT. Scatterometers are versatile instruments which allow measurements of the parameters of a lithographic process by having a sensor in the pupil or a conjugate plane with the pupil of the objective of the scatterometer, measurements usually referred as pupil based measurements, or by having the sensor in an image plane or a plane conjugate with the image plane, in which case the measurements are usually referred as image or field based measurements. Such scatterometers and the associated measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP1,628,164A, incorporated herein by reference in their entirety. Aforementioned scatterometers may measure gratings using light from soft x-ray, extreme ultraviolet (EUV) and visible to near-IR wavelength range.
In order for the substrates W exposed by the lithographic apparatus LA to be exposed correctly and consistently, it is desirable to inspect substrates to measure properties of patterned structures, such as overlay errors between subsequent layers, line thicknesses, critical dimensions (CD), etc. For this purpose, inspection tools and/or metrology tools (not shown) may be included in the lithocell LC. If errors are detected, adjustments, for example, may be made to exposures of subsequent substrates or to other processing steps that are to be performed on the substrates W, especially if the inspection is done before other substrates W of the same batch or lot are still to be exposed or processed.
An inspection apparatus, which may also be referred to as a metrology apparatus, is used to determine properties of the substrates W, and in particular, how properties of different substrates W vary or how properties associated with different layers of the same substrate W vary from layer to layer. The inspection apparatus may alternatively be constructed to identify defects on the substrate W and may, for example, be part of the lithocell LC, or may be integrated into the lithographic apparatus LA, or may even be a stand-alone device. The inspection apparatus may measure the properties on a latent image (image in a resist layer after the exposure), or on a semi-latent image (image in a resist layer after a post-exposure bake step PEB), or on a developed resist image (in which 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).
In a first embodiment, the scatterometer MT is an angular resolved scatterometer. In such a scatterometer reconstruction methods may be applied to the measured signal to reconstruct or calculate properties of the grating. Such reconstruction may, for example, result from simulating interaction of scattered radiation with a mathematical model of the target structure and comparing the simulation results with those of a measurement. Parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from the real target.
In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such spectroscopic scatterometer MT, the radiation emitted by a radiation source is directed onto the target and the reflected or scattered radiation from the target is directed to a spectrometer detector, which measures a spectrum (i.e. a measurement of intensity as a function of wavelength) of the specular reflected radiation. From this data, the structure or profile of the target giving rise to the detected spectrum may be reconstructed, e.g. by Rigorous Coupled Wave Analysis and non-linear regression or by comparison with a library of simulated spectra.
In a third embodiment, the scatterometer MT is an ellipsometric scatterometer. The ellipsometric scatterometer allows for determining parameters of a lithographic process by measuring scattered radiation for each polarization states. Such metrology apparatus emits polarized light (such as linear, circular, or elliptic) by using, for example, appropriate polarization filters in the illumination section of the metrology apparatus. A source suitable for the metrology apparatus may provide polarized radiation as well. Various embodiments of existing ellipsometric scatterometers are described in U.S. patent application 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 incorporated herein by reference in their entirety.
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 asymmetry in the reflected spectrum and/or the detection configuration, the asymmetry being related to the extent of the overlay. The two (typically overlapping) grating structures may be applied in two different layers (not necessarily consecutive layers), and may be formed substantially at the same position on the wafer. The scatterometer may have a symmetrical detection configuration as described e.g. in co-owned patent application EP1,628,164A, such that any asymmetry is clearly distinguishable. This provides a straightforward way to measure misalignment in gratings. Further examples for measuring overlay error between the two layers containing periodic structures as target is measured through asymmetry of the periodic structures may be found in PCT patent application publication no. WO 2011/012624 or US patent application US 20160161863, incorporated herein by reference in its entirety.
Other parameters of interest may be focus and dose. Focus and dose may be determined simultaneously by scatterometry (or alternatively by scanning electron microscopy) as described in US patent application US2011-0249244, incorporated herein by reference in its entirety. A single structure may be used which has a unique combination of critical dimension and sidewall angle measurements for each point in a focus energy matrix (FEM—also referred to as Focus Exposure Matrix). If these unique combinations of critical dimension and sidewall angle are available, the focus and dose values may be uniquely determined from these measurements.
A metrology target may be an ensemble of composite gratings, formed by a lithographic process, mostly in resist, but also after etch process for example. Typically the pitch and line-width of the structures in the gratings strongly depend on the measurement optics (in particular the NA of the optics) to be able to capture diffraction orders coming from the metrology targets. As indicated earlier, the diffracted signal may be used to determine shifts between two layers (also referred to ‘overlay’) or may be used to reconstruct at least part of the original grating as produced by the lithographic process. This reconstruction may be used to provide guidance of the quality of the lithographic process and may be used to control at least part of the lithographic process. Targets may have smaller sub-segmentation which are configured to mimic dimensions of the functional part of the design layout in a target. Due to this sub-segmentation, the targets may behave more similar to the functional part of the design layout such that the overall process parameter measurements resemble the functional part of the design layout better. The targets may be measured in an underfilled mode or in an overfilled mode. In the underfilled mode, the measurement beam generates a spot that is smaller than the overall target. In the overfilled mode, the measurement beam generates a spot that is larger than the overall target. In such overfilled mode, it may also be possible to measure different targets simultaneously, thus determining different processing parameters at the same time.
Overall measurement quality of a lithographic parameter using a specific target is at least partially determined by the measurement recipe used to measure this lithographic parameter. The term “substrate measurement recipe” may include one or more parameters of the measurement itself, one or more parameters of the one or more patterns measured, or both. For example, if the measurement used in a substrate measurement recipe is a diffraction-based optical measurement, one or more of the parameters of the measurement may include the wavelength of the radiation, the polarization of the radiation, the incident angle of radiation relative to the substrate, the orientation of radiation relative to a pattern on the substrate, etc. One of the criteria to select a measurement recipe may, for example, be to ensure that a measurement is sensitive to processing variations. More examples are described in US patent application US2016-0161863 and published US patent application US 2016/0370717A1 incorporated herein by reference in its entirety.
Typically the patterning process in a lithographic apparatus LA is one of the most critical steps in the processing which requires high accuracy of dimensioning and placement of structures on the substrate W. To ensure this high accuracy, three systems may be combined in a so called “holistic” control environment as schematically depicted in
The computer system CL may use (part of) the design layout to be patterned to predict which resolution enhancement techniques to use and to perform computational lithography simulations and calculations to determine which mask layout and lithographic apparatus settings achieve the largest overall process window of the patterning process (depicted in
The metrology tool MET may provide input to the computer system CL to enable accurate simulations and predictions, and may provide feedback to the lithographic apparatus LA to identify possible drifts, e.g. in a calibration status of the lithographic apparatus LA (depicted in
In lithographic processes, it is desirable to make frequently measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes or various forms of metrology apparatuses, such as scatterometers. Examples of known scatterometers often rely on provision of dedicated metrology targets, such as underfilled targets (a target, in the form of a simple grating or overlapping gratings in different layers, that is large enough that a measurement beam generates a spot that is smaller than the grating) or overfilled targets (whereby the illumination spot partially or completely contains the target). Further, the use of metrology tools, for example an angular resolved scatterometter illuminating an underfilled target, such as a grating, allows the use of so-called reconstruction methods where the properties of the grating can be calculated by simulating interaction of scattered radiation with a mathematical model of the target structure and comparing the simulation results with those of a measurement. Parameters of the model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from the real target.
Scatterometers are versatile instruments which allow measurements of the parameters of a lithographic process by having a sensor in the pupil or a conjugate plane with the pupil of the objective of the scatterometer, measurements usually referred as pupil based measurements, or by having the sensor in the image plane or a plane conjugate with the image plane, in which case the measurements are usually referred as image or field based measurements. Such scatterometers and the associated measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP1,628,164A, incorporated herein by reference in their entirety. Aforementioned scatterometers can measure in one image multiple targets from multiple gratings using light from soft x-ray, extreme ultraviolet and visible to near-IR wave range.
A metrology apparatus, such as a scatterometer, is depicted in
As an alternative to optical metrology methods, it has also been considered to use soft X-rays or EUV radiation (may be referred as SXR in the following text), for example radiation in a wavelength range between 0.1 nm and 100 nm, or optionally between 1 nm and 50 nm or optionally between 10 nm and 20 nm. One example of metrology tool functioning in one of the above presented wavelength ranges is transmissive small angle X-ray scattering (T-SAXS as in US 2007224518A which content is incorporated herein by reference in its entirety). Profile (CD) measurements using T-SAXS are discussed by Lemaillet et al in “Intercomparison between optical and X-ray scatterometry measurements of FinFET structures”, Proc. of SPIE, 2013, 8681. Reflectometry techniques using X-rays (GI-XRS) and extreme ultraviolet (EUV) radiation at grazing incidence are known for measuring properties of films and stacks of layers on a substrate. Within the general field of reflectometry, goniometric and/or spectroscopic techniques can be applied. In goniometry, the variation of a reflected beam with different incidence angles is measured. Spectroscopic reflectometry, on the other hand, measures the spectrum of wavelengths reflected at a given angle (using broadband radiation). For example, EUV reflectometry has been used for inspection of mask blanks, prior to manufacture of reticles (patterning devices) for use in EUV lithography.
It is possible that the range of application makes the use of wavelengths in the soft X-rays or EUV domain not sufficient. Therefore published patent applications US 20130304424A1 and US2014019097A1 (Bakeman et al/KLA) describe hybrid metrology techniques in which measurements made using x-rays and optical measurements with wavelengths in the range 120 nm and 2000 nm are combined together to obtain a measurement of a parameter such as CD. A CD measurement is obtained by coupling and x-ray mathematical model and an optical mathematical model through one or more common. The content of the cited US patent application are incorporated herein by reference in their entirety.
Inspection apparatus 302 comprises a radiation source 310, illumination system 312, substrate support 316, detection systems 318, 398 and metrology processing unit (MPU) 320.
Source 310 in this example comprises a generator of EUV or soft x-ray radiation based on high harmonic generation (HHG) techniques. Main components of the radiation source are a drive laser 330 and an HHG gas cell 332. A gas supply 334 supplies suitable gas to the gas cell, where it is optionally ionized by an electric source 336. The drive laser 300 may be, for example, a fiber-based laser with an optical amplifier, producing pulses of infrared radiation that may last for example less than 1 ns (1 nanosecond) per pulse, with a pulse repetition rate up to several megahertz, as required. The wavelength of the infrared radiation may be for example in the region of 1 μm (1 micron). The laser pulses are delivered as a first radiation beam 340 to the HHG gas cell 332, where in the gas a portion of the radiation is converted to higher frequencies than the first radiation into a beam 342 including coherent second radiation of the desired wavelength or wavelengths.
The second radiation may contain multiple wavelengths. If the radiation were monochromatic, then measurement calculations (for example reconstruction) may be simplified, but it is easier with HHG to produce radiation with several wavelengths. The volume of gas within the gas cell 332 defines an HHG space, although the space need not be completely enclosed and a flow of gas may be used instead of a static volume. The gas may be for example a noble gas such as neon (Ne) or argon (Ar). N2, O2, He, Ar, Kr, Xe gases can all be considered. These may be selectable options within the same apparatus. Different wavelengths may, for example, provide different levels of contrast when imaging structure of different materials. For inspection of metal structures or silicon structures, for example, different wavelengths may be selected to those used for imaging features of (carbon-based) resist, or for detecting contamination of such different materials. One or more filtering devices 344 may be provided. For example a filter such as a thin membrane of Aluminum (Al) or Zirconium (Zr) may serve to cut the fundamental IR radiation from passing further into the inspection apparatus. A grating (not shown) may be provided to select one or more specific harmonic wavelengths from among those generated in the gas cell. Some or all of the beam path may be contained within a vacuum environment, bearing in mind that SXR radiation is absorbed when traveling in air. The various components of radiation source 310 and illumination optics 312 can be adjustable to implement different metrology ‘recipes’ within the same apparatus. For example different wavelengths and/or polarization can be made selectable.
Depending on the materials of the structure under inspection, different wavelengths may offer a desired level of penetration into lower layers. For resolving the smallest device features and defects among the smallest device features, then a short wavelength is likely to be preferred. For example, one or more wavelengths in the range 1-20 nm or optionally in the range 1-10 nm or optionally in the range 10-20 nm may be chosen. Wavelengths shorter than 5 nm suffer from very low critical angle when reflecting off materials typically of interest in semiconductor manufacture. Therefore to choose a wavelength greater than 5 nm will provide stronger signals at higher angles of incidence. On the other hand, if the inspection task is for detecting the presence of a certain material, for example to detect contamination, then wavelengths up to 50 nm could be useful.
From the radiation source 310, the filtered beam 342 enters an inspection chamber 350 where the substrate W including a structure of interest is held for inspection at a measurement position by substrate support 316. The structure of interest is labeled T. The atmosphere within inspection chamber 350 is maintained near vacuum by vacuum pump 352, so that EUV radiation can pass with-out undue attenuation through the atmosphere. The Illumination system 312 has the function of focusing the radiation into a focused beam 356, and may comprise for example a two-dimensionally curved mirror, or a series of one-dimensionally curved mirrors, as described in published US patent application US2017/0184981A1 (which content is incorporated herein by reference in its entirety), mentioned above. The focusing is performed to achieve a round or elliptical spot S under 10 μm in diameter, when projected onto the structure of interest. Substrate support 316 comprises for example an X-Y translation stage and a rotation stage, by which any part of the substrate W can be brought to the focal point of beam to in a desired orientation. Thus the radiation spot S is formed on the structure of interest. Alternatively, or additionally, substrate support 316 comprises for example a tilting stage that may tilt the substrate W at a certain angle to control the angle of incidence of the focused beam on the structure of interest T.
Optionally, the illumination system 312 provides a reference beam of radiation to a reference detector 314 which may be configured to measure a spectrum and/or intensities of different wavelengths in the filtered beam 342. The reference detector 314 may be configured to generate a signal 315 that is provided to processor 310 and the filter may comprise information about the spectrum of the filtered beam 342 and/or the intensities of the different wavelengths in the filtered beam.
Reflected radiation 360 is captured by detector 318 and a spectrum is provided to processor 320 for use in calculating a property of the target structure T. The illumination system 312 and detection system 318 thus form an inspection apparatus. This inspection apparatus may comprise a soft X-ray and/or EUV spectroscopic reflectometer of the kind described in US2016282282A1 which content is incorporated herein by reference in its entirety.
If the target T has a certain periodicity, the radiation of the focused beam 356 may be partially diffracted as well. The diffracted radiation 397 follows another path at well-defined angles with respect to the angle of incidence then the reflected radiation 360. In
To aid the alignment and focusing of the spot S with desired product structures, inspection apparatus 302 may also provide auxiliary optics using auxiliary radiation under control of metrology processor 320. Metrology processor 320 can also communicate with a position controller 372 which operates the translation stage, rotation and/or tilting stages. Processor 320 receives highly accurate feedback on the position and orientation of the substrate, via sensors. Sensors 374 may include interferometers, for example, which can give accuracy in the region of picometers. In the operation of the inspection apparatus 302, spectrum data 382 captured by detection system 318 is delivered to metrology processing unit 320.
As mentioned an alternative form of inspection apparatus uses soft X-ray and/or EUV radiation at normal incidence or near-normal incidence, for example to perform diffraction-based measurements of asymmetry. Both types of inspection apparatus could be provided in a hybrid metrology system. Performance parameters to be measured can include overlay (OVL), critical dimension (CD), focus of the lithography apparatus while the lithography apparatus printed the target structure, coherent diffraction imaging (CDI) and at-resolution overlay (ARO) metrology. The soft X-ray and/or EUV radiation may for example have wavelengths less than 100 nm, for example using radiation in the range 5-30 nm, of optionally in the range from 10 nm to 20 nm. The radiation may be narrowband or broadband in character. The radiation may have discrete peaks in a specific wavelength band or may have a more continuous character.
Like the optical scatterometer used in today's production facilities, the inspection apparatus 302 can be used to measure structures within the resist material treated within the litho cell (After Develop Inspection or ADI), and/or to measure structures after they have been formed in harder material (After Etch Inspection or AEI). For example, substrates may be inspected using the inspection apparatus 302 after they have been processed by a developing apparatus, etching apparatus, annealing apparatus and/or other apparatus.
Metrology tools MT, including but not limited to the scatterometers mentioned above, may use radiation from a radiation source to perform a measurement. The radiation used by a metrology tool MT may be electromagnetic radiation. The radiation may be optical radiation, for example radiation in the infrared, visible, and/or ultraviolet parts of the electromagnetic spectrum. Metrology tools MT may use radiation to measure or inspect properties and aspects of a substrate, for example a lithographically exposed pattern on a semiconductor substrate. The type and quality of the measurement may depend on several properties of the radiation used by the metrology tool MT. For example, the resolution of an electromagnetic measurement may depend on the wavelength of the radiation, with smaller wavelengths able to measure smaller features, e.g. due to the diffraction limit. In order to measure features with small dimensions, it may be preferable to use radiation with a short wavelength, for example EUV and/or Soft X-Ray (SXR) radiation, to perform measurements. In order to perform metrology at a particular wavelength or wavelength range, the metrology tool MT requires access to a source providing radiation at that/those wavelength(s). Different types of sources exist for providing different wavelengths of radiation. Depending on the wavelength(s) provided by a source, different types of radiation generation methods may 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), a source may use High Harmonic Generation (HHG) to obtain radiation at the desired wavelength(s). One of the challenges faced in the development of these sources is how to couple the emitted radiation out of the generating setup efficiently and separate the emitted radiation from the radiation used to drive the process.
The gas delivery system of the illumination source 600 is configured to provide the gas flow 615. The illumination source 600 is configured to provide the pump radiation 611 in the gas flow 615 to drive the generation of emitted radiation 613. The region where at least a majority of the emitted radiation 613 is generated is called an interaction region. The interaction region may vary from several tens of micrometers (for tightly focused pump radiation) to several mm or cm (for moderately focused pump radiation) or even up to a few 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 comprises a gas nozzle 609, as shown in
The dimensions of the gas nozzle 609 may conceivably also be used in scaled-up or scaled-down versions ranging from micrometer-sized nozzles to meter-sized nozzles. This wide range of dimensioning comes from the fact that the setup should be scaled such that the intensity of the pump radiation at the gas flow ends up in the particular range which may be beneficial for the emitted radiation, which requires different dimensioning for different pump radiation energies, which may be a pulse laser and pulse energies can vary from tens of microjoules to joules.
Due to interaction of the pump radiation 611 with the gas atoms of the gas flow 615, the gas flow 615 will convert part of the pump radiation 611 into the emitted radiation 613, which may be an example of the emitted radiation 342 shown in
In operation the emitted radiation 613 beam may pass through a radiation outlet 607, e.g. an aperture or window, and may be subsequently manipulated and directed by an illumination system 603, which may be an example of the illumination system 312 in
Because air (and in fact any gas) heavily absorbs SXR or EUV radiation, the volume between the gas flow 615 and the wafer to be inspected may be evacuated or nearly evacuated. Since the central axes of the emitted radiation 613 may be collinear with the central axes of the incident pump radiation 611, the pump radiation 611 may need to be blocked to prevent it passing through the radiation outlet 607 and entering the illumination system 603. This may be done by incorporating a filtering device 344 shown in
Described herein are methods, apparatuses, and assemblies to obtain emitted radiation optionally
at a high harmonic frequency of pump radiation. The radiation generated through the process, optionally the HHG which uses non-linear effects to generate radiation at a harmonic frequency of provided pump radiation, may be provided as radiation in metrology tools MT for inspection and/or measurement of substrates. The substrates may be lithographically patterned substrates. The radiation obtained through the process may also be provided in a lithographic apparatus LA, and/or a lithographic cell LC. The pump radiation may be pulsed radiation, which may provide high peak intensities for short bursts of time.
The pump radiation 611 may comprise radiation with one or more wavelengths higher than the one or more wavelengths of the emitted radiation. Throughout the description and claims, “radiation comprising one or more wavelengths” may describe, for example, radiation having a spectrum comprising energy or power in one or more wavelength ranges; e.g., the illumination radiation may have substantial energy (e.g., above a nominal threshold) in a specific wavelength range). The pump radiation may comprise infrared radiation. The pump radiation may comprise radiation with wavelength(s) (e.g., substantial energy) in the range of 700 nm to 1500 nm. The pump radiation may comprise radiation with wavelength(s) in the range of 900 nm to 1300 nm. The pump radiation may comprise radiation with wavelength(s) in the range of 100 nm to 1300 nm. The pump radiation may be pulsed radiation. Pulsed pump radiation may comprise pulses with a duration in the femtosecond range.
For some embodiments, the emitted radiation, optionally the high harmonic radiation, may comprise one or more harmonics of the pump radiation wavelength(s). The emitted radiation may comprise wavelengths in the extreme ultraviolet (EUV), soft X-Ray (SXR), and/or hard X-Ray part of the electromagnetic spectrum. The emitted radiation 613 may comprise wavelengths in the range of 0.01 nm to 100 nm. The emitted radiation 613 may comprise wavelengths in the range of 0.1 nm to 100 nm. The emitted radiation 613 may comprise wavelengths in the range of 0.1 nm to 50 nm. The emitted radiation 613 may comprise wavelengths in the range of 1 nm to 50 nm. The emitted radiation 613 may comprise wavelengths in the range of 10 nm to 20 nm.
A further metrology apparatus suitable for use in embodiments of the invention is shown in
As shown in
At least the 0 and +1 orders diffracted by the target structure T on substrate W are collected by objective lens 16 and directed back through beam splitter 15. Returning to
A second beam splitter 17 divides the diffracted beams into two measurement branches. In a first measurement branch, optical system 18 forms a diffraction spectrum (pupil plane image) of the target structure on first sensor 19 (e.g. a CCD or CMOS sensor) using the zeroth and first order diffractive beams. Each diffraction order hits a different point on the sensor, so that image processing can compare and contrast orders. The pupil plane image captured by sensor 19 can be used for focusing the metrology apparatus and/or normalizing intensity measurements of the first order beam. The pupil plane image can also be used for many measurement purposes such as reconstruction.
In the second measurement branch, optical system 20, 22 forms an image of the target structure T on sensor 23 (e.g. a CCD or CMOS sensor). In the second measurement branch, an aperture stop 21 is provided in a plane that is conjugate to the pupil-plane. Aperture stop 21 functions to block the zeroth order diffracted beam so that the image of the target formed on sensor 23 is formed only from the −1 or +1 first order beam. The images captured by sensors 19 and 23 are output to processor PU which processes the image, the function of which may depend on the particular type of measurements being performed. Note that the term ‘image’ is used here in a broad sense. An image of the grating lines as such may not be formed, if only one of the −1 and +1 orders is present.
The particular forms of aperture plate 13 and field stop 21 shown in
In order to make the measurement radiation adaptable to these different types of measurement, the aperture plate 13 may comprise a number of aperture patterns formed around a disc, which rotates to bring a desired pattern into place. Note that aperture plate 13N or 13S can only be used to measure gratings oriented in one direction (X or Y depending on the set-up). For measurement of an orthogonal grating, rotation of the target through 90° and 270° might be implemented. Different aperture plates are shown in
Another type of metrology tool used in IC manufacture is a topography measurement system, level sensor or height sensor. Such a tool may be integrated in the lithographic apparatus, for measuring a topography of a top surface of a substrate (or wafer). A map of the topography of the substrate, also referred to as height map, may be generated from these measurements indicating a height of the substrate as a function of the position on the substrate. This height map may subsequently be used to correct the position of the substrate during transfer of the pattern on the substrate, in order to provide an aerial image of the patterning device in a properly focus position on the substrate. It will be understood that “height” in this context refers to a dimension broadly out of the plane to the substrate (also referred to as Z-axis). Typically, the level or height sensor performs measurements at a fixed location (relative to its own optical system) and a relative movement between the substrate and the optical system of the level or height sensor results in height measurements at locations across the substrate.
An example of a level or height sensor LS as known in the art is schematically shown in
The projection grating PGR is a periodic grating comprising a periodic structure resulting in a beam of radiation BE1 having a periodically varying intensity. The beam of radiation BE1 with the periodically varying intensity is directed towards a measurement location MLO on a substrate W having an angle of incidence ANG with respect to an axis perpendicular (Z-axis) to the incident substrate surface between 0 degrees and 90 degrees, typically between 70 degrees and 80 degrees. At the measurement location MLO, the patterned beam of radiation BE1 is reflected by the substrate W (indicated by arrows BE2) and directed towards the detection unit LSD.
In order to determine the height level at the measurement location MLO, the level sensor further comprises a detection system comprising a detection grating DGR, a detector DET and a processing unit (not shown) for processing an output signal of the detector DET. The detection grating DGR may be identical to the projection grating PGR. The detector DET produces a detector output signal indicative of the light received, for example indicative of the intensity of the light received, such as a photodetector, or representative of a spatial distribution of the intensity received, such as a camera. The detector DET may comprise any combination of one or more detector types.
By means of triangulation techniques, the height level at the measurement location MLO can be determined. The detected height level is typically related to the signal strength as measured by the detector DET, the signal strength having a periodicity that depends, amongst others, on the design of the projection grating PGR and the (oblique) angle of incidence ANG.
The projection unit LSP and/or the detection unit LSD may include further optical elements, such as lenses and/or mirrors, along the path of the patterned beam of radiation between the projection grating PGR and the detection grating DGR (not shown).
In an embodiment, the detection grating DGR may be omitted, and the detector DET may be placed at the position where the detection grating DGR is located. Such a configuration provides a more direct detection of the image of the projection grating PGR.
In order to cover the surface of the substrate W effectively, a level sensor LS may be configured to project an array of measurement beams BE1 onto the surface of the substrate W, thereby generating an array of measurement areas MLO or spots covering a larger measurement range.
Various height sensors of a general type are disclosed for example in U.S. Pat. Nos. 7,265,364 and 7,646,471, both incorporated by reference. A height sensor using UV radiation instead of visible or infrared radiation is disclosed in US2010233600A1, incorporated by reference. In WO2016102127A1, incorporated by reference, a compact height sensor is described which uses a multi-element detector to detect and recognize the position of a grating image, without needing a detection grating.
Another type of metrology tool used in IC manufacture is an alignment sensor. A critical aspect of performance of the lithographic apparatus is therefore the ability to place the applied pattern correctly and accurately in relation to features laid down in previous layers (by the same apparatus or a different lithographic apparatus). For this purpose, the substrate is provided with one or more sets of marks or targets. Each mark is a structure whose position can be measured at a later time using a position sensor, typically an optical position sensor. The position sensor may be referred to as “alignment sensor” and marks may be referred to as “alignment marks”.
A lithographic apparatus may include one or more (e.g. a plurality of) alignment sensors by which positions of alignment marks provided on a substrate can be measured accurately. Alignment (or position) sensors may use optical phenomena such as diffraction and interference to obtain position information from alignment marks formed on the substrate. An example of an alignment sensor used in current lithographic apparatus is based on a self-referencing interferometer as described in U.S. Pat. No. 6,961,116. Various enhancements and modifications of the position sensor have been developed, for example as disclosed in US2015261097A1. The contents of all of these publications are incorporated herein by reference.
Radiation diffracted by the alignment mark AM is collimated (in this example via the objective lens OL) into an information-carrying beam IB. The term “diffracted” is intended to include zero-order diffraction from the mark (which may be referred to as reflection). A self-referencing interferometer SRI, e.g. of the type disclosed in U.S. Pat. No. 6,961,116 mentioned above, interferes the beam IB with itself after which the beam is received by a photodetector PD. Additional optics (not shown) may be included to provide separate beams in case more than one wavelength is created by the radiation source RSO. The photodetector may be a single element, or it may comprise a number of pixels, if desired. The photodetector may comprise a sensor array.
The diverting optics, which in this example comprises the spot mirror SM, may also serve to block zero order radiation reflected from the mark, so that the information-carrying beam IB comprises only higher order diffracted radiation from the mark AM (this is not essential to the measurement, but improves signal to noise ratios).
Intensity signals SI are supplied to a processing unit PU. By a combination of optical processing in the block SRI and computational processing in the unit PU, values for X-and Y-position on the substrate relative to a reference frame are output.
A single measurement of the type illustrated only fixes the position of the mark within a certain range corresponding to one pitch of the mark. Coarser measurement techniques are used in conjunction with this to identify which period of a sine wave is the one containing the marked position. The same process at coarser and/or finer levels may be repeated at different wavelengths for increased accuracy and/or for robust detection of the mark irrespective of the materials from which the mark is made, and materials on and/or below which the mark is provided. The wavelengths may be multiplexed and de-multiplexed optically so as to be processed simultaneously, and/or they may be multiplexed by time division or frequency division.
In this example, the alignment sensor and spot SP remain stationary, while it is the substrate W that moves. The alignment sensor can thus be mounted rigidly and accurately to a reference frame, while effectively scanning the mark AM in a direction opposite to the direction of movement of substrate W. The substrate W is controlled in this movement by its mounting on a substrate support and a substrate positioning system controlling the movement of the substrate support. A substrate support position sensor (e.g. an interferometer) measures the position of the substrate support (not shown). In an embodiment, one or more (alignment) marks are provided on the substrate support. A measurement of the position of the marks provided on the substrate support allows the position of the substrate support as determined by the position sensor to be calibrated (e.g. relative to a frame to which the alignment system is connected). A measurement of the position of the alignment marks provided on the substrate allows the position of the substrate relative to the substrate support to be determined.
In SXR metrology, a periodic target (or more generally a periodic structure e.g., a periodic pattern on a wafer) is illuminated with a soft X-ray beam. The diffraction pattern is captured by one or more image sensors; for example, in some arrangements there may be three separate sensors, respectively for the positive diffraction orders, the zeroth (specular) order and the negative diffraction orders. Each wavelength component in the SXR beam may create diffraction spots in a different location on a respective sensor.
The measured diffraction pattern or signal may be represented as a diffraction-efficiency spectrum Rmλ(mx, my, λ), which is a function of wavelength λ and integer diffraction orders mx, my (e.g., assuming a 2D target). Parameters of interest such as overlay and layer thicknesses can be inferred from the diffraction-efficiency spectrum Rmλ of target, e.g., by using any of the methods described in PCT application WO2021/121906A1 (which is incorporated herein by reference) or otherwise. The methods described in WO2021/121906A1 comprise performing Fourier transforms to transform the measured spectra, resulting in an autocorrelation function as a function of x, y, z in the target, where z is perpendicular to the target surface.
When estimating target parameters of interest from a measured diffraction pattern, it is generally necessary or at least desirable to eliminate or mitigate the signal impact due to other factors. One such factor is the impact of the source spectrum. The raw measurement signal Iλ(mx, my, λ) (where I is intensity or spectral power of the measured signal) is not a direct measure of the target diffraction efficiency spectrum Rmλ(mx, my, λ), but a product of this spectrum and a source power spectrum or source spectral power Φλ(λ) (in W/nm units):
The source spectral power as a function of wavenumber {tilde over (v)} may comprise a comb-like shape, where wavenumber {tilde over (v)} is the reciprocal of wavelength (in a vacuum). This shape is because the SXR generation process, in a typical process, generates mostly odd-numbered harmonics of the drive-laser wavenumber {tilde over (v)}d. However, the methods disclosed herein are applicable to any comb spectrum (e.g., where the comb spacing is approximately known and sufficiently wide such that its individual teeth can be captured in the measured signal.
A Fourier transform of a measurement with such a spectrum may lead to strong source-related contributions. Dividing out the source spectrum is difficult because:
Failure to correct for or divide out the source spectrum may lead to aliasing effects in the Fourier transform. This may result in the autocorrelation measurement having depth (z) features at less than e.g., 140 nm aliased to greater than 140 nm and vice versa. (This is a typical value for a drive-laser wavelength 1030 nm and 30-degree angle of incidence upon the target.) The ability of being sensitive to features at depth at an order of magnitude of 100 nm is one of the perceived advantages of SXR metrology.
In the methods proposed herein, rather than attempting to estimate the unknown measurement errors in the two spectra Φ and R, it is proposed that the source spectrum is estimated from the diffraction pattern using the target, which is a periodic pattern, as a spectrometer.
To implement such a method, it is proposed to represent the source spectrum as a distance spectrum in distance space where distance in this context is the Fourier conjugate of the wavenumber. As such, this distance spectrum is Fourier transformed from a wavenumber spectrum in wavenumber space (i.e., which is already in the frequency domain). This is because the source spectrum is approximately periodic in wavenumber space. As such, the following Fourier pair is considered: ƒ({tilde over (v)})°{circumflex over (ƒ)}(Λ), where {tilde over (v)} is wavenumber, Λ is distance and ƒ is any function of wavenumber (e.g., measured intensities, a related measured parameter from the diffraction pattern and/or components thereof). Also note that ƒ({tilde over (v)}) is real valued and therefore the Fourier sign convention is not critical.
The source spectrum may be approximated as the product of an infinite comb spectrum ({tilde over (v)})=
({tilde over (v)}λd/2)*h({tilde over (v)}) and a smooth envelope spectrum ε({tilde over (v)}), where
is a Dirac comb function, λd is the drive laser wavelength and h({tilde over (v)}) a point-spread function. The infinite comb may be approximately periodic (hence the ≈ sign), as if the point-spread function h({tilde over (v)}) varies across the spectrum. In the distance domain, the spectrum is a comb
(Λ)=
(2Λ/λd) (ignoring a constant pre-factor), convolved with the Fourier transform {circumflex over (ε)}(Λ) of the envelope and multiplied by h(Λ). The convolution blurs the peaks in the distance spectrum, whereas the multiplication causes peaks at higher distances to be weaker. Because comb function
is not exactly a convolution, the peaks of
are slightly broadened (compared to Dirac-delta peaks), but much less so than the broadening due to the convolution with {circumflex over (ε)}. The comb may have an offset, e.g.
(2Λ/λd+a) with 0≤a<1. In the following a=0, but the method is equally applicable to a nonzero a.
Beside each of steps 1000-1040 is an equivalent spectrum plot (in the wavenumber or distance domain as appropriate) for an example measured diffraction spectrum, where the target diffraction efficiency has components a, b, and c.
At step 1000, a measured intensity (or other related measurement parameter) spectrum Iλ(m, λ) may be determined from the captured diffraction pattern and converted to wavenumber representation:
The measurement parameter here is intensity, although any suitable measurement parameter (e.g., a parameter from a measurement signal) may be used, such as any spectral quantity where a spectral quantity may be inter alia: intensity, spectral intensity, spectral radiant flux, spectral irradiance, or pixel value at a given wavelength.
An example resulting wavenumber spectrum is shown beside this step as a plot of spectral power against wavenumber. Shown is the resultant intensity spectrum (dotted line); and in addition the diffraction efficiency signal (solid line), and the product of the diffraction efficiency signal and envelope signal (dashed line).
At step 1010, This intensity spectrum is then Fourier transformed from wavenumber {tilde over (v)} space to distance Λ space to obtain intensity distance spectrum IΛ(m, Λ). For each diffraction order m, this distance spectrum may show strong peaks at integer multiples of half the drive-laser wavelength, i.e., λd/2, for a typical source generation process which generates mostly odd-numbered harmonics of the drive-laser wavenumber. However, it is possible to generate a mix of even and odd harmonics, in which case the method may be trivially modified for such a source generation process. The general method may work for any comb spectrum where the comb spacing is approximately known and wide enough so that its individual teeth can be captured in the measured spectrum I(λ).
Optionally, a window function (e.g., for example a Hamming window) may be applied to the Fourier transform of this step.
The equivalent resulting distance spectrum is shown for this step (i.e., spectral power against distance Λ). This distance spectrum shows not only peaks around Λ=a, b, c, but also at aliased distances a′, a″, b′, b″, c′, c″. The comb-like spectrum in the {tilde over (v)} domain leads to a Nyquist distance Nyq=λd/4 in the Λ domain.
At step 1020, at least a comb distance spectrum is constructed (m, Λ) to comprise all the strong peaks of the intensity distance spectrum IΛ(m, Λ). For example, the spectrum may be constructed to comprise all of the peaks at integer multiples n of half the drive-laser wavelength λd or drive radiation wavelength: nλd/2. Alternatively or in addition, depending on the generation process, the spectrum may be constructed to comprise peaks at integer multiples n of the drive-laser wavelength (e.g., where the source generation process generates mostly even-numbered harmonics of the drive-laser wavenumber), or peaks at both of integer multiples n of the drive-laser wavelength and integer multiples n of half of the drive-laser wavelength (e.g., where the source generation process generates both odd and even-numbered harmonics of the drive-laser wavenumber). This comb spectrum may comprise an estimate of a Dirac comb combined with the relevant point spread function.
An envelope distance spectrum {circumflex over (ε)}(m, Λ) may also be constructed to comprise only the peak of the intensity distance spectrum IΛ(m, Λ) at integer multiple 0 (e.g., n=0 or nλd/2=0). This is shown in the accompanying distance spectrum plot.
More specifically, the comb distance spectrum (m, Λ) may be constructed as follows. A bandwidth ΔΛ is selected, such that the range nλd/2−ΔΛ≤Λ<nλd/2+ΔΛ (assuming peaks at nλd/2) captures a peak around integer multiple n. The function
may be set to be equal to IΛ for Λ in one of these ranges, and zero otherwise. For the example in
At step 1030, the comb distance spectrum (m, Λ), and when constructed also the envelope distance spectrum {circumflex over (ε)}(m, Λ), is/are inverse-Fourier-transformed from the distance domain back to the wavenumber domain to obtain an estimate
″ (m, {tilde over (v)}) of comb wavenumber spectrum
′(m, {tilde over (v)}) and (where applicable) as estimate ε″(m, {tilde over (v)}) of envelope wavenumber spectrum ε(m, {tilde over (v)}). These functions are both shown on the accompanying spectrum plot (solid line is the comb wavenumber spectrum and the dashed line is the envelope wavenumber spectrum).
At step 1040, the source-corrected diffraction efficiency spectrum may be estimated. In an embodiment, the comb wavenumber spectrum ″(m, {tilde over (v)}) may be used alone as an estimate for the source spectrum. As such, a source-corrected diffraction efficiency spectrum may be determined from the measured spectrum Iv(m, {tilde over (v)}) as:
However, comb spectrum ″(m, {tilde over (v)}) typically has values close to zero near its edge and therefore a better behaved estimate may be obtained by estimating the source spectrum as
As such, the source-corrected diffraction efficiency spectrum may be determined from the measured spectrum Iv(m, {tilde over (v)}) as:
The accompanying spectrum plot shows the source-corrected diffraction efficiency spectrum, which can be seen to be very similar to the intensity spectrum (dotted line) of the first plot.
At step 1050, the source-corrected diffraction efficiency spectrum may be used in further analysis to determine a parameter of interest; e.g., according to the methods in the aforementioned WO2021/121906A1.
It can be appreciated that the correction step 1040, and in particular the division by the comb spectrum (m, {tilde over (v)}), fails if there are near-zero values in
(m, {tilde over (v)}). Where this is the case, the correction step may further comprise interpolating around these near-zero values (e.g., such that the spectrum does not fall below a certain value) or clipping the ratio ε/
(or reciprocal 1/
) to exclude nonsensical results. Another approach, which may be used additionally or as an alternative to clipping and/or interpolating, may comprise configuring one or more parameters of the source to reduce or minimize the number of near-zero values in
(m, {tilde over (v)}).
In an embodiment, there may be an initial step of transforming the detected diffraction pattern from a position-dependent intensity Ixy(x, y) to obtain said representation Imλ(mx, my, λ), where mx, my are integer diffraction-order numbers and λ is the wavelength. For example, the detected intensity data may be transformed to pupil space as Iκ(κx, κy) (this concept is described in the aforementioned WO 2021121906 A1 and will not be described further here). Then, for my=0:
where d is the numerical aperture of the illumination beam, p is the pitch of the target, and κx(0)=0 is a nominal value, e.g., κy(0)=−0.5. Equation (4) relies on an implicit assumption that κx(0)=0, which is not essential in the general case but assumed in the discussion below. Where this is not so, an additional +κx(0) term should be added to the argument of Iκ.
In a further optional embodiment, the projection in Equation (4) may be modified to ensure that parts of the diffraction pattern with equal λ/cos ϑ values (i.e., which have equal diffraction efficiencies) are mapped to the same wavelength value. This may comprise applying the following projection:
The ± sign is + or − depending on the sign of m and κy(0). For example, if the zeroth order is located at (κx(0), κy(0))=(0, 0.5), which corresponds to ϑ=30° and the target's x-axis is parallel to the pupil axis, then in Equation (5) there is a + sign for mx>0 and a − sign for mx<0.
Another concept will now be described which may make use of the aforementioned method of determining a source-corrected diffraction efficiency spectrum. As has been described, the source spectrum may comprise a comb-shape. Because of this, there is no target response information in the detected signal at wavenumbers or wavelengths with zero intensity in the source spectrum. Physics-based analysis of the diffraction efficiency, such as the autocorrelation method disclosed in WO 2021121906 A1 (incorporated herein by reference) are impacted by this.
It may be possible to change the optical characteristics of the system optics to address this, e.g., so as to provide an elongated spot shape which effectively compensates for the missing wavelength components, or called missing spectral components, in the spectrum such that at least some of the missing wavelength components are obtained after the compensation step. In one embodiment, the missing wavelength components may be missing parts of the spectrum or parts with lower intensity compared with neighbouring wavelengths. In one embodiment, the missing wavelength components may be local minimum wavelengths. For example, the illumination system may be modified such that the NA of the illumination is larger in the plane of incidence (κy direction in pupil space (k-space or Fourier space)) than perpendicular to the plane of incidence (κx direction in pupil space). However, it is not desirable to change the optical design in this manner. As such, a method will be described which addresses this issue without the need for such a change in optical design.
The aforementioned disclosed method for obtaining source-corrected diffraction efficiency spectrum addresses the situation where the illumination spectral intensity drops to zero between the comb teeth via interpolation, which introduces errors. Alternative methods for estimating the target response, such as model-based reconstruction, requires a thorough simulation model that captures sufficient physics, requires (unavailable) detailed knowledge of the measurement tool and requires a parametrized target model that can (with the proper parameter values) accurately describe the measured sample/target. None of these requirements is trivial. Data-driven methods require a sufficient amount of training data, preferably labeled, which may be difficult to obtain.
This embodiment describes a method for estimating the target response Rmv(m, {tilde over (v)}) at wavenumbers corresponding to troughs in the source spectrum (i.e., corresponding to low values/minima of the source spectrum).
Such a method may comprise method of determining a reconstructed spectrum comprising: obtaining a measured spectrum in terms of a measurement parameter, the measured spectrum being obtained from captured diffracted radiation from a periodic structure following illumination of said periodic structure using source radiation from said illumination source, said source radiation comprising a source spectrum having a plurality of maxima and minima; determining derivatives of the measured spectrum; and determining the reconstructed spectrum from the measured spectrum and the derivatives.
The target response Rmv(m, {tilde over (v)}) in wavenumber {tilde over (v)} domain may be sampled at fixed intervals, e.g., δ{tilde over (v)}˜2 μm−1. In terms of signal processing, this means that there is a Nyquist ‘frequency’ (which actually has dimension of length in this case), with aliasing of ‘frequency’ components above the Nyquist ‘frequency’ to values below. This maps directly to vertical distances in the target (i.e., normal to the target surface); e.g., in the specific example of a sampling interval of 2 μm−1 (and assuming ϑ=30°, the Nyquist ‘frequency’ is 140 nm, meaning that a target stack thicker than 140 nm may suffer from ‘frequency’ aliasing. For example, if the target has a first layer having a thickness 100 nm and a second layer with a thickness 180 nm, the signal contributions from small variations in these layers (thickness, overlay, CD) may appear very similar and may therefore be difficult to separate from each other.
In signal processing, if both the signal and its derivative are sampled at equidistant intervals, it is possible to reconstruct a signal with a bandwidth up to 2× the Nyquist ‘frequency’ rather than up to 1× the Nyquist ‘frequency’. This is illustrated in
As described in aforementioned WO 2021121906 A1, the target response can be written as a function of wavenumber {tilde over (v)} and angle of incidence ϑ as:
for ϑ, ϑ′ between 20 and 70 deg, where ϑ, ϑ′ are two different values of the angle of incidence.
As such, the diffraction efficiency Rm{tilde over (v)} (mx, my, {tilde over (v)}, ϑ) at angle of incidence ϑ can be described in good approximation as:
where ϑ0 is a nominal or reference angle of incidence, for example ϑ0=30°. In WO 2021121906 A1, this relation was used to fit the angle of incidence ϑ in case of an alignment error; for example if the actual value is ϑ=30.01 deg, it enabled correction for the 0.01 deg error without knowing this error a priori.
In this embodiment, this principle may be used to determine an estimate of the derivative of Rmv(m, {tilde over (v)}) at the center of each peak of the source spectrum and use that derivative to interpolate the missing information in the troughs between the peaks (corresponding to near zero or zero diffraction efficiency).
If wavenumber {tilde over (v)} is fixed (e.g., on one narrow peak of the source spectrum) and a small variation in the angle of incidence ϑ allowed (for example, the variation in angle of incidence within the beam divergence), using the principle embodied in Equation (6), it is possible to vary the effective wavenumber over a small range via this small variation in the angle of incidence ϑ. This may comprise performing the transformation of Equation (7) and Fourier transforming this transformation. Such an approach makes it possible to obtain the target response Rmv for {tilde over (v)}j−ϵ<{tilde over (v)}<{tilde over (v)}j+ϵ for a small value ϵ<<{tilde over (v)}0. Effectively, this enables measurement of both the value of R and the derivative dR/d{tilde over (v)} around each {tilde over (v)}j value.
In an optional step 1310, the method may comprise projecting the detected intensity distribution into pupil space, for example according to Equation (4) or the modified projection according to Equation (5). In the latter case, this may modify the projection to ensure that parts of the diffraction pattern with equal λ/cos ϑ (equal {tilde over (v)} cos ϑ values, noting the transformation of Equation (7)) are mapped to the same wavelength value.
At step 1320, the diffraction pattern Imv(m, {tilde over (v)}) may be converted to diffraction efficiency per order and per wavenumber, i.e., a corrected spectrum Rmv(m, {tilde over (v)}), using an estimate of the source spectrum (e.g., dividing intensity by the estimated source spectrum) as has been disclosed above. This converted diffraction efficiency data may have gaps in between the teeth of the comb (corresponding to zero or near-zero local minima).
At step 1330, at each peak wavenumber, the derivative R′mv(m, {tilde over (v)})=dRmv/d{tilde over (v)} may be determined. For example, a cluster of samples (e.g., two samples R(m, {tilde over (v)}j+ϵ), R(m, {tilde over (v)}j−ϵ)) may be determined, and the derivative evaluated as:
for a small ϵ. R(m, {tilde over (v)}j+ϵ) and R(m, {tilde over (v)}j−ϵ) may be determined via the transformation of Equation (7) on the diffraction pattern Rmv(m, {tilde over (v)}), and fitting the diffraction orders m (shorthand for mx, my). An alternative method is described below. In an embodiment, the small value ϵ may be half of the Full-width at half-maximum (FWHM) of the peaks; e.g., less than 0.5 μm−1, less than 0.4 μm−1, less than 0.35 μm−1, less than 0.3 μm−1, between 0.5 μm−1 and 0.1 μm−1 or between 0.5 μm−1 and 0.2 μm−1, for example.
To determine the derivative robustly, a good estimate of the spectrum and beam profile of the illumination beam may be obtained, for example, using ptychography and/or using the method for estimating the source spectrum described above. With respect to a beam profile estimate, it may be appreciated that a single wavelength component may cover a nonzero area in pupil space because of the nonzero numerical aperture. The relation κx=mλ/p in Equations (4)/(5) is only correct for the chief ray of the illumination beam. In the transformation from κ space to λ space this may be accounted for by a kind of deconvolution. Technically, it is a deconvolution if the beam profile is wavelength independent; since this is not the case in practice, it would be a deconvolution-like operation.
At step 1340, a reconstructed signal or reconstructed spectrum Rrecon(m, {tilde over (v)}) may be determined as a continuous or at least well oversampled signal. This may be done, for example, using signal processing techniques; examples of such techniques will be described below.
At step 1350, the reconstructed signal may be used for obtaining estimates of one or more parameters of interest such as overlay, CD, and layer thickness; e.g., using any of the methods described in the aforementioned WO 2021121906 A1.
A number of example implementations of some of the steps of the method depicted in
where the m parameter is implied. The weighting α is optional (or trivially may be set to 1). However, because it is likely that the derivatives R′ are less accurate than the values R, the weighting a may comprise a value of 0<α<1, with α close to zero if the derivatives are considered to be inaccurate. Optionally, α may be variable and different for each point, depending on the estimated accuracy of the derivative; e.g., to comprise a high (e.g., 1) weighting a; for high-accuracy derivative points and low (or zero) weighting αj for low-accuracy derivative points.
More generally, given N samples (not derivatives) spread out over a range Δ{tilde over (v)}, a function can be reconstructed up to a Nyquist ‘frequency’
regardless or whether the samples are uniformly spaced or not. This is known as non-uniform sampling in advanced signal processing. Suppose that three (or more) samples (e.g., one at the top of the peak and one on each side of the peak) are taken around every source spectrum peak, such that N=3M where M is the number of peaks. The samples may be enumerated as Rij with i=0, . . . , M−1 and j=−1, 0, 1. These may be taken at wavenumbers {tilde over (v)}ij={tilde over (v)}00+iδ{tilde over (v)}+jϵ where δ{tilde over (v)}=2{tilde over (v)}d is the large sampling interval and ϵ is a small sampling interval (ϵ<<δ{tilde over (v)}). Three kernels Kj can be constructed such that the reconstructed signal is:
These kernels are described in the literature and would be straightforward for the skilled person.
As with the derivatives in Equations (8) and (9), Equation (10) may be sensitive to noise. to address this, each cluster of three samples may be transformed into a new cluster Qik (k=0, 1, 2):
which are approximations of the function value, the first derivative, and the second derivatives (The denominators 3, 2ϵ,ϵ2 are for mathematical convenience only and are not essential; their purpose is to make Qi0, Qi1, Qi2 correspond approximately to the 0th, 1st, 2nd derivatives (regardless of the exact value of ϵ)). New kernels K′k may be constructed straightforwardly as linear combinations of the kernels Kj as follows. The transformation in Equation (11) can be written in matrix-vector notation as Qik=ΣjTkjRij, where T is a 3×3 matrix. The matrix can be inverted and the new kernels are then:
The reconstruction formula then becomes:
where weighting coefficients αik are introduced in a manner analogous to Equation (9). For a noise-free signal, all αik may be set to αik=1, which would make Equation (13) equivalent to Equation (10). In the presence of noise (or measurement errors in general), αik may be reduced: for example, all αi1 may be set αi1=0, or the values varied individually for the different ij combinations.
If error estimates are known for all Rij, the weights αik can be chosen such that the expected RMS error (or some other error metric as desired) on the reconstructed signal Rrecon is minimized (this also applies to embodiment described by Equation (9)). Weight optimization for minimizing the RMS error is straightforward and negligible in computation time.
The above relates to a non-uniform but regular sampling in clusters of three samples. It can be generalized to clusters of a different size (for example 2 or 4) and also to irregular sampling. This is relevant since, typically, the SXR spectrum has zero intensity between only some pairs of comb teeth but not between others.
An alternative method to obtain the derivative R′mv in step 1330 will now be described. Assume that gi0({tilde over (v)}) is a function describing the shape of a single peak in the illumination spectrum, centered at some wavenumber {tilde over (v)}i0, and that g is known accurately. If a spectrum with this peak shape illuminates a target that locally has a diffraction-efficiency spectrum:
then the diffracted spectrum R({tilde over (v)})gi0({tilde over (v)}) may have a shifted peak. This is illustrated in
Let hi be the peak shift, i.e. hi={tilde over (v)}shifted−{tilde over (v)}i0. Then the derivative in diffraction efficiency is given by
This formula works for smooth peak shapes. As circumstances demand, it could be adapted for non-smooth peak shapes and be based on the centroid shift of the peak rather than the position shift.
In the above disclosure, it was mentioned that it was possible, if not desirable from a practical viewpoint, to provide an elongated spot shape to compensate for the missing wavelength components in the spectrum; e.g., to modify the illumination system such that the NA of the illumination is larger in the plane of incidence than perpendicular to the plane of incidence. Another approach to fill the missing parts in the spectrum may comprise creating new wavelength components in the high harmonic spectrum. This requires a source redesign to generate a continuous HHG spectrum or to at least double the number of harmonics in the spectrum. While it is possible to double the number of harmonics in the spectrum, it requires a significant overhaul of the source design and adds considerable complexity. Moreover, the beam divergence in the κx direction blurs the spectral resolution; it may be necessary to shrink the beam divergence in the κx direction in order to achieve the benefits from the doubled number of harmonics.
In an embodiment, the images DP and DP′ may be converted separately into first and second diffraction-efficiency spectra Rmv(mx, my, {tilde over (v)}, δ) and R′mv(mx, my, {tilde over (v)}, δ−α), respectively. Using equation (7), the second diffraction-efficiency spectra R′ may be converted to an estimate R″(mx, my, {tilde over (v)}, δ). The two spectra Rmv and R″mv may now complement each other and can be combined into a single spectrum Rcombi(mx, my, {tilde over (v)}). One way to do this is:
where w({tilde over (v)}) is a weight function that is close to 1 if R is well defined and close to 0 if R″ is well defined.
Alternatively, the second image DP′ may be a combination or aggregate image obtained from the two image captures which is then converted into a diffraction-efficiency spectrum.
The difference between the first tilt angle and second tilt angle, and therefore between a first angle of incidence for the first acquisition and a second angle of incidence for the second acquisition, may be between 1 mrad and 50 mrad (or between 10 mrad and 50 mrad) to compensate for the gaps in the illumination spectrum. This is a typical value; the exact value may depend on the pitch of the target. More specifically, with a drive-laser wavenumber {tilde over (v)}d˜1 μm−1, the spacing between the teeth of the comb is 2{tilde over (v)}d. By varying the angle of incidence ϑ from the nominal value ϑ0 the sample response mid-way between the teeth (at angle ϑ0) may be estimated by varying the angle as:
where {tilde over (v)} is approximately the center of the spectrum. For example δθ=23 mrad with {tilde over (v)}=75 μm−1 (as in
As such, in one measurement scheme, the target may be exposed in a first acquisition with a first angle of incidence ϑ=ϑ0, and at a second acquisition with a second angle of incidence θ=ϑ0+23 mrad. In another measurement scheme, the target may be exposed in a first acquisition with a first angle of incidence at ϑ=ϑ0−11.5 mrad and at a second acquisition with a second angle of incidence ϑ=ϑ0+11.5 mrad.
Additional embodiments are disclosed in the subsequent numbered list of clauses:
Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus 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, etc.
Although specific reference may be made in this text to embodiments in the context of a lithographic apparatus, embodiments may be used in other apparatus. Embodiments may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatuses may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.
Although specific reference may be made in this text to embodiments in the context of an inspection or metrology apparatus, embodiments may be used in other apparatus. Embodiments may form part of a mask inspection apparatus, a lithographic apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). The term “metrology apparatus” (or “inspection apparatus”) may also refer to an inspection apparatus or an inspection system (or a metrology apparatus or a metrology system). E.g. the inspection apparatus that comprises an embodiment may be used to detect defects of a substrate or defects of structures on a substrate. In such an embodiment, a characteristic of interest of the structure on the substrate may relate to defects in the structure, the absence of a specific part of the structure, or the presence of an unwanted structure on the substrate.
Although specific reference may have been made above to the use of embodiments in the context of optical lithography, it will be appreciated that the invention, where the context allows, is not limited to optical lithography and may be used in other applications, for example imprint lithography.
While the targets or target structures (more generally structures on a substrate) described above are metrology target structures specifically designed and formed for the purposes of measurement, in other embodiments, properties of interest may be measured on one or more structures which are functional parts of devices 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. Further, pitch of the metrology targets may be close to the resolution limit of the optical system of the scatterometer or may be smaller, but may be much larger than the dimension of typical non-target structures optionally product structures made by lithographic process in the target portions C. In practice the lines and/or spaces of the overlay gratings within the target structures may be made to include smaller structures similar in dimension to the non-target structures.
While specific embodiments have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.
Although specific reference is made to “metrology apparatus/tool/system” or “inspection apparatus/tool/system”, these terms may refer to the same or similar types of tools, apparatuses or systems. E.g. the inspection or metrology apparatus that comprises an embodiment of the invention may be used to determine characteristics of structures on a substrate or on a wafer. E.g. the inspection apparatus or metrology apparatus that comprises an embodiment of the invention may be used to detect defects of a substrate or defects of structures on a substrate or on a wafer. In such an embodiment, a characteristic of interest of the structure on the substrate may relate to defects in the structure, the absence of a specific part of the structure, or the presence of an unwanted structure on the substrate or on the wafer.
Although specific reference is made to SXR and EUV electromagnetic radiations, it will be appreciated that the invention, where the context allows, may be practiced with all electromagnetic radiations, includes radio waves, microwaves, infrared, (visible) light, ultraviolet, X-rays, and gamma rays. As an alternative to optical metrology methods, it has also been considered to use X-rays, optionally hard X-rays, for example radiation in a wavelength range between 0.01 nm and 10 nm, or optionally between 0.01 nm and 0.2 nm, or optionally between 0.1 nm and 0.2 nm, for metrology measurements. As such any of the aforementioned references to SXR radiation should be explicitly understood to optionally encompass radiation with wavelengths (e.g., substantial energy) between 0.01 nm and 0.2 nm as an alternative to only SXR (and/or EUV) wavelengths according to other definitions herein.
Additional objects, advantages and features of the present invention are set forth in this specification, and in part will become apparent to those skilled in the art on examination of the following, or may be learned by practice of the invention. The inventions disclosed in this application are not limited to any particular set of, or combination of, objects, advantages and features. It is contemplated that various combinations of the stated objects, advantages and features make up the inventions disclosed in this application.
| Number | Date | Country | Kind |
|---|---|---|---|
| 22161160.1 | Mar 2022 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2023/054352 | 2/22/2023 | WO |
