Method for manufacturing a reflector and associated reflector

Abstract

A method of manufacturing a reflector is disclosed. The method includes at least polishing the uppermost surface of the uppermost substantially flat substrate among a plurality of substantially flat substrates, deforming each substantially flat substrate into a desired shape, and joining the deformed substrates together to form the reflector. In an embodiment, a mold is used to perform the deformation and joining together.

Description

[0001] Cross - Reference to Related Applications

[0002] This application claims priority to EP application 19159257.5, filed on February 26, 2019, which is incorporated herein by reference in its entirety. Technical Field

[0003] The present invention relates to, but is not exclusively used for, optical systems and associated methods for metrology devices. Background Art

[0004] A lithographic apparatus is a machine configured to apply a desired pattern onto a substrate. For example, a lithographic apparatus can be used to manufacture integrated circuits (ICs). For example, a lithographic apparatus can project a pattern (also often referred to as a "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).

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

[0006] Low-k 1 Lithography can be used to process features having dimensions smaller than the classical resolution limit of a lithographic apparatus. In such a process, the resolution formula can be expressed as CD = k 1 x λ / NA, where λ is the wavelength of the radiation employed, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the "critical dimension" (usually the smallest feature size printed, but in this case the half pitch) and k 1 is an empirical resolution factor. Typically, k 1The smaller it is, the more difficult it is to replicate on a substrate a pattern similar in shape and size to that planned by the circuit designer in order to achieve specific electrical functions and performance. To overcome these difficulties, precise fine-tuning steps can be applied to the lithographic projection apparatus and / or the design layout. For example, these include 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 pattern reproduction at low k 1 during pattern reproduction.

[0007] A metrology device, or an inspection device, can be used to determine the characteristics of the pattern fabricated on a substrate by the lithographic apparatus. Nowadays, various forms of optical metrology techniques are known, and as the critical dimensions in fabricated patterns continue to shrink, these optical metrology techniques may lack resolution. One option is to use radiation having a relatively low wavelength (e.g., in the soft X-ray or extreme ultraviolet (EUV) spectral range) in such a metrology device. The relatively low wavelength can be in the range from 0.1 nm to 100 nm, or in the range from 1 nm to 50 nm, or in the range from 10 nm to 20 nm. Such radiation of this wavelength can be generated by using the high harmonic generation (HHG) principle: short pulses of infrared (IR) radiation are focused in an HHG medium (e.g., a specific gas), and the HHG medium converts a portion of the received IR radiation into soft X-ray or EUV radiation. The radiation generated by HHG can include multiple peaks at different wavelengths in a relatively broad spectrum.

[0008] In a metrology device, a radiation beam is guided by an illumination subsystem towards the area of interest on the substrate. For example, a target is provided at the area of interest. Preferably, the radiation beam is focused on the area of interest or the target. Such an illumination subsystem can include reflective optical components, i.e., reflectors or mirrors, which guide the radiation beam, for example, at grazing incidence. Such a mirror can be capable of guiding broadband radiation in the soft X-ray and / or EUV spectral range onto a sensor and, in some cases, can be capable of focusing such broadband radiation.

[0009] There is a desire to improve the existing methods for manufacturing such mirrors. SUMMARY OF THE INVENTION

[0010] According to one aspect or embodiment, a method for manufacturing a reflector is provided, the method comprising: polishing at least the uppermost surface of the uppermost substantially flat substrate among a plurality of substantially flat substrates; deforming each substantially flat substrate into a desired shape, and bonding the deformed substrates together to form the reflector.

[0011] A reflector manufactured according to this method is also disclosed, as well as a lithographic apparatus including such a reflector illumination subsystem, a metrology device, and a lithographic device.

[0012] At least one feature of any aspect or embodiment described herein may replace any corresponding feature of any aspect or embodiment described herein. At least one feature of any aspect or embodiment described herein may be combined with any other aspect or embodiment described herein. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0014] Figure 1 A schematic diagram of a lithographic apparatus is depicted;

[0015] Figure 2 A schematic diagram of a lithography cell is depicted;

[0016] Figure 3 A schematic diagram of overall lithography is depicted, showing the cooperation between three key technologies to optimize semiconductor manufacturing;

[0017] Figure 4 A schematic diagram of a metrology device using radiation in the soft X-ray or EUV spectral range is depicted; and

[0018] Figure 5 A flowchart of a proposed method for manufacturing a reflector according to an embodiment of the present invention is depicted. DETAILED DESCRIPTION

[0019] In this document, the terms "radiation" and "beam" are used to encompass all types of electromagnetic radiation, including, for example, ultraviolet radiation and EUV (extreme ultraviolet radiation, for example having a wavelength in the range of approximately 5 nm - 100 nm) with wavelengths of 365 nm, 248 nm, 193 nm, 157 nm, or 126 nm.

[0020] The terms "reticle", "mask", or "patterning device" as used herein may be broadly interpreted to refer to a general patterning device that can be used to impart an incident radiation beam having a patterned cross-section corresponding to a pattern to be created in a target portion of a substrate. The term "light valve" may also be used in the context. In addition to classical masks (transmission or reflection, binary, phase-shift, hybrid, etc.), examples of other such patterning devices include programmable reflector arrays and programmable LED arrays.

[0021] Figure 1Schematically depicts a lithographic apparatus LA. The lithographic apparatus LA includes: an illumination system (also referred to as an illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation, or EUV radiation); a mask support (e.g., a mask table) T constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA according to certain parameters; a substrate support (e.g., a wafer table) WT constructed 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., including one or more dies) of the substrate W.

[0022] In operation, the illumination system IL (e.g., via a beam delivery system BD) receives the radiation beam from a radiation source SO. The illumination system IL may include various types of optical components for guiding, shaping, and / or controlling the radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and / or other types of optical components or any combination thereof. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross-section at the plane of the patterning device MA.

[0023] The term "projection system" PS as used herein should be interpreted broadly to encompass various types of projection systems, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic, and / or electrostatic optical systems, or any combination thereof, suitable for exposing the radiation being used, and / or other factors (such as the use of immersion or the use of a vacuum). Any use herein of the term "projection lens" may be considered synonymous with the more general term "projection system" PS.

[0024] 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) in order to fill the space between the projection system PS and the substrate W - this is also referred to as immersion lithography. More information on immersion techniques is given in US6952253, which is incorporated herein by reference.

[0025] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also referred to as "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel, and / or steps for preparing the substrate W for a subsequent exposure may be performed on a substrate W located on one of the substrate supports WT while another substrate W on another substrate support WT is used for exposing a pattern on the other substrate W.

[0026] In addition to the substrate support WT, the lithographic apparatus LA may include a measurement stage. The measurement stage is arranged to hold sensors and / or cleaning devices. The sensors may be arranged to measure properties of the projection system PS or of the radiation beam B. The measurement stage may hold a plurality of sensors. The cleaning device may be arranged to clean a part of the lithographic apparatus, for example, a part of the projection system PS or a part of the immersion fluid providing system. When the substrate support WT is moved away from the projection system PS, the measurement stage may move beneath the projection system PS.

[0027] In operation, the radiation beam B is incident on a patterning device (e.g., a mask MA held on a mask support T), and is patterned by a pattern (design layout) present on the patterning device MA. After passing through 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 help of the second positioner PW and the position measurement system IF, the substrate support WT can be accurately moved, for example, 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 another position sensor ( Figure 1 not explicitly depicted in the figure) may be used to accurately position the patterning device MA relative to the path of the radiation beam B. Mask alignment marks M 1 、M 2 and substrate alignment marks P 1 、P 2 may be used to align the patterning device MA and the substrate W. Although the substrate alignment marks P 1 、P 2 occupy dedicated target portions as shown, they may be located in the spaces between the target portions. When the substrate alignment marks P 1 、P 2 are located between the target portions C, they are referred to as scribe alignment marks.

[0028] As Figure 2As shown, the lithography apparatus LA can form part of a lithography cell LC, sometimes also referred to as a lithography cell or (lithography) cluster, which typically also includes means for performing pre-exposure and post-exposure processes on a substrate W. Generally, these means 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 adjusting the temperature of the substrate W, for example, for adjusting the solvent in the resist layer. A substrate handler or robot RO picks up the substrate W from the input / output ports I / O1, I / O2, moves the substrate between different processing means and transfers the substrate W to the feed table LB of the lithography apparatus LA. The devices in the lithography cell (usually also collectively referred to as the track) are generally under the control of a track control unit TCU, which itself can be controlled by a management control system SCS, which can also control the lithography apparatus LA (e.g., via a lithography control unit LACU).

[0029] In order to expose the substrate W exposed by the lithography apparatus LA correctly and consistently, it is desirable to inspect the substrate to measure properties of the patterned structures, such as overlay errors between subsequent layers, line widths, critical dimensions (CD), etc. For this purpose, an inspection tool (not shown) can be included in the lithography cell LC. If an error is detected, especially if the inspection is completed before other substrates W of the same batch are still to be exposed or processed, for example, then the exposure of subsequent substrates or other processing steps to be performed on the substrate W can be adjusted.

[0030] An inspection device (which can also be referred to as a metrology device or metrology tool MT) is used to determine the properties of the substrate W, and in particular how the properties of different substrates W vary or how the properties associated with different layers of the same substrate W vary with the layer. The inspection device can alternatively be configured to identify defects on the substrate W and can, for example, be part of the lithography cell LC, or can be integrated into the lithography apparatus LA, or can even be a stand-alone device. The inspection device can measure properties on a latent image (the image in the resist layer after exposure), or a semi-latent image (the image in the resist layer after a post-exposure bake step PEB), or a developed resist image (where the exposed or unexposed portions of the resist have been removed), or even on an etched image (after a pattern transfer step, such as etching).

[0031] Generally, the patterning process in the lithography apparatus LA is one of the most critical steps in the process, which requires a high degree of accuracy in the dimensions and placement of the structures on the substrate W. As Figure 3Schematically depicted in [the figure], to ensure this high precision, three systems can be combined in a so-called "integrated" control environment. One of these systems is a lithographic apparatus LA, which is (in fact) connected to a metrology tool MT (the second system) and a computer system CL (the third system). The key to such an "integrated" environment is to optimize the cooperation between these three systems to enhance the overall process window and provide a strict control loop to ensure that the patterning performed by the lithographic apparatus LA remains within the process window. The process window defines a range of process parameters (e.g., dose, focus, overlay), within which a specific manufacturing process yields a defined result (e.g., a functional semiconductor device) - typically allowing process parameter changes in the lithography process or patterning process within this process window. The metrology tool MT can provide information that can be used for various purposes. The information provided by the metrology tool MT depends on the stage of the metrology measurements performed during the manufacturing process. A feedback loop can be created between the metrology tool MT and other tools used in the manufacturing process, for example, as part of a lithography, etching, or chemical mechanical polishing (CMP) step. The information provided by aspects or embodiments of the present invention can be used by the metrology tool MT as part of a feedback loop, or by any other tool used in the manufacturing process.

[0032] The computer system CL can use (a part of) the design layout to be patterned to predict which resolution enhancement techniques are to be used and perform computational lithography simulations and calculations to determine which mask layouts and lithographic apparatus settings achieve the maximum overall process window of the patterning process (depicted by the double arrow in the first scale SC1 in [the figure]). Typically, the resolution enhancement techniques are arranged to match the patterning capabilities of the lithographic apparatus LA. The computer system CL can also be used to detect where the lithographic apparatus LA is currently operating within the process window (e.g., using input from the metrology tool MT) to predict whether there might be defects due to, for example, suboptimal processing (depicted by the arrow pointing to "0" in the third scale SC3 in [the figure]). Figure 3 The metrology tool MT can provide input to the computer system CL to enable accurate simulations and predictions, and can provide feedback to the lithographic apparatus LA to identify possible drifts, for example, in the calibration state of the lithographic apparatus LA (depicted by the multiple arrows in the third scale SC3 in [the figure]). Figure 3

[0033] Figure 3

[0034] In a lithography process, it is desirable to frequently measure the created structures, for example for process control and verification. Tools for performing such measurements are commonly referred to as metrology tools MT. Different types of metrology tools MT for performing such measurements are known, including scanning electron microscopes or various forms of scatterometer metrology tools MT. A scatterometer is a versatile instrument that allows the measurement of lithography process parameters by sensors in the pupil or in a plane conjugate to the pupil of the scatterometer objective (measurements are typically referred to as pupil-based measurements), or by sensors in the image plane or in a plane conjugate to the image plane (in this case, the measurements are typically 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, the entire contents of which are incorporated herein by reference. The above scatterometers can use light from the soft x-ray and visible to near-infrared wavelength ranges to measure gratings.

[0035] In a first embodiment, the scatterometer MT is an angle-resolved scatterometer. In such a scatterometer, a reconstruction method can be applied to the measured signal to reconstruct or calculate the properties of the grating. For example, such a reconstruction can be produced by simulating the interaction of the scattered radiation using a mathematical model of the target structure and comparing the simulation results with those measurements. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to the diffraction pattern observed from the real target.

[0036] In a second embodiment, the scatterometer MT is a spectral scatterometer MT. In such a spectral scatterometer MT, the radiation emitted by a radiation source is directed onto a target, and the radiation reflected or scattered from the target is directed to a spectrometer detector that measures the spectrum of the specularly reflected radiation (i.e., a measurement of intensity as a function of wavelength). From this data, for example, by rigorous coupled-wave analysis and non-linear regression, or by comparison with a simulated spectral library, the structure or profile of the target that produced the detected spectrum can be reconstructed.

[0037] In a third embodiment, the scatterometer MT is an ellipsometric scatterometer. An ellipsometric scatterometer allows for the determination of parameters of a lithography process by measuring scattered radiation for each polarization state. Such metrology apparatus emits (such as linear, circular or elliptical) polarized light by using, for example, a suitable polarization filter in the illumination section of the metrology apparatus. Sources suitable for the metrology apparatus can also provide polarized radiation. Various embodiments of existing ellipsometric scatterometers are described in U.S. Patent Applications 11 / 451,599, 11 / 708,678, 12 / 256,780, 12 / 486,499, 12 / 920,968, 12 / 922,587, 13 / 000,299, 13 / 033,135, 13 / 533,110 and 13 / 891,410, the entire contents of which are incorporated herein by reference.

[0038] In one embodiment of the scatterometer MT, the scatterometer MT is adapted to measure the overlay of two misaligned gratings or periodic structures by measuring the reflection spectrum and / or detecting an asymmetry in the detection configuration, the asymmetry being related to the degree of overlay. Two (usually overlapping) grating structures can be applied in two different layers (not necessarily consecutive layers) and can be formed substantially at the same location on the wafer. The scatterometer can have a symmetric detection configuration as described, for example, in co-owned patent application EP1,628,164A such that any asymmetry can be clearly distinguished. This provides a direct method for measuring misalignment in the gratings. Further examples for measuring the overlay error between two layers containing a periodic structure as a target by the asymmetry of the periodic structure can be found in PCT Patent Application Publication No. WO2011 / 012624 or U.S. Patent Application US20160161863, the entire contents of which are incorporated herein by reference.

[0039] Other parameters of interest can be the focal length and the dose. As described in U.S. Patent Application US2011-0249244, the focal length and the dose can be determined simultaneously by scatterometry (or alternatively by scanning electron microscopy), the entire contents of which are incorporated herein by reference. A single structure can be used that has a unique combination of critical dimension and sidewall angle measurements for each point in a focal energy matrix (FEM - also known as a focal exposure matrix). If these unique combinations of critical dimensions and sidewall angles are available, the focal length and dose values can be uniquely determined from these measurements.

[0040] The metrology target can be primarily in the resist, but can also be a collection of composite gratings formed, for example, by a lithography process after an etching process. These gratings diffract the radiation captured by the metrology optics. The metrology optics can be designed such that the wavelength and the NA of the optics used by the scatterometer can capture the diffraction orders from the metrology target, so that parameters such as the pitch and linewidth of the grating can be determined. As previously described, the diffraction signal can be used to determine the offset between two layers (also referred to as "overlay accuracy") or can be used to reconstruct at least a portion of the original grating produced by the lithography process. This reconstruction can be used to provide guidance on the quality of the lithography process and can be used to control at least a portion of the lithography process. The target may have a smaller sub-segment that is configured to mimic the dimensions of a functional portion of the design layout in the target. Due to this sub-segment, the target will behave more like the functional portion of the design layout, making the overall process parameter measurement more similar to the functional portion of the design layout. The target can be measured in an underfill mode or an overfill mode. In the underfill mode, the measurement beam generates a spot that is smaller than the overall target. In the overfill mode, the measurement beam generates a spot that is larger than the overall target. In this overfill mode, different targets can also be measured simultaneously, so that different process parameters can be determined simultaneously.

[0041] The overall measurement quality of lithography parameters using a particular target is at least partially determined by the measurement recipe used to measure the lithography parameter. The term "substrate metrology scheme" can include one or more parameters of the measurement itself, one or more parameters of the one or more patterns being measured, or both. For example, if the measurement used in the substrate metrology scheme is a diffraction-based optical measurement, one or more of the parameters measured can include: the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the orientation of the radiation relative to the pattern on the substrate, etc. For example, one of the criteria for selecting a measurement recipe can be the sensitivity of one of the measurement parameters to process variations. More examples are described in U.S. Patent Application US2016-0161863 and the unpublished U.S. Patent Application 15 / 181,126, the entire contents of which are incorporated herein by reference.

[0042] As an alternative to optical metrology methods, the use of soft X-rays or EUV radiation is also considered, such as radiation in the 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. An example of a metrology tool that operates in one of the above wavelength ranges is transmission small angle X-ray scattering (such as T-SAXS in US2007224518A, the entire content of which is incorporated herein by reference). Lemaillet et al. discussed the critical dimension (CD) measurements using T-SAXS in the National Natural Science Foundation of China project 2013, 8681 "Intercomparison between optical and X-ray scatterometry measurements of FinFET structures". It is well known that reflection metrology techniques using grazing-incidence X-rays (GI-XRS) and extreme ultraviolet (EUV) radiation can be used to measure the properties of thin films and stacks on a substrate. Within the general field of reflection metrology, goniometric techniques and / or spectroscopic techniques can be applied. In goniometry, the variation of the reflected beam with different incident angles is measured. On the other hand, spectroscopic reflectometry (using broadband radiation) measures the spectrum of the wavelengths reflected at a given angle. For example, EUV reflectometry has been used to inspect mask blanks before manufacturing a mask blank (pattern forming device) for EUV lithography.

[0043] The scope of applications may render the use of wavelengths in the soft X-ray or EUV domain insufficient. Thus, the published patent applications US20130304424A1 and US2014019097A1 (Bakeman et al. / KLA) describe hybrid metrology techniques, in which measurements made using X-rays and optical measurements with wavelengths in the range of 120 nm and 2000 nm are joined together to obtain measurements of parameters such as CD. The CD measurement is obtained by coupling an X-ray mathematical model and an optical mathematical model through one or more common points.

[0044] Figure 4 A metrology device 200 is shown that can use soft X-rays or EUV radiation to determine the characteristics of a substrate, e.g., the characteristics of a target T on a wafer W. The metrology device 200 includes: an infrared (IR) laser 202, a HHG mechanism 204, an optional IR blocking element 206, an illumination subsystem 732, which can include a reference detector 714, a higher-order detector 750, and a spectrometer 700. The illumination subsystem 732 includes an optical system OS, an example of which is described in further detail herein.

[0045] The IR laser 202 seeds the high harmonic generation (HHG) mechanism 204. The IR laser 202 generates short driving pulses of IR radiation that are focused within the HHG mechanism 204 in the HHG medium. The HHG medium can be a gas. The HHG medium converts a portion of the IR radiation into soft X-ray and / or EUV radiation according to the high harmonic generation principle. A compact source of SXR radiation includes an HHG source, where infrared pump radiation from a laser is converted into shorter wavelength radiation through interaction with a gaseous medium. For example, an HHG source can be obtained from KMLabs in Boulder, Colorado, USA (http: / / www.kmlabs.com / ).

[0046] The generated soft X-ray and / or EUV radiation enters the illumination subsystem 732. Before entering the illumination subsystem, an optional IR blocking element 206 can block most of the IR driving beam of the IR driving beam. The illumination subsystem 732 can include a reference measurement branch that includes a reference detector 714 that generates a reference measurement signal SR. The reference detector 714 can be part of a spectrometer that measures the intensities of different wavelengths in the generated soft X-ray and / or EUV radiation.

[0047] The metrology device 200 can include a subsystem to receive and hold a substrate at a specific location, such as a wafer W. In one embodiment, the subsystem is a wafer stage. The wafer W can include a target T, and one or more characteristics of the target T can be determined. The illumination subsystem 732 is arranged to direct illumination radiation 704 onto the target T on the wafer W during use, and the illumination subsystem 732 can be arranged to focus the illumination radiation 704 onto the target T.

[0048] The target T or any other structure on the wafer W can scatter or diffract the illumination radiation 704. The reflected radiation 708 (i.e., specularly reflected radiation) is received by the spectrometer 700. The spectrometer can include a grating 712 that reflects the radiation 708 into a reflected spectrum 710 of different wavelengths. The reflected spectrum 710 is captured by a detector 713 that generates a reflected measurement signal ST. Higher order diffracted radiation from the target T impinges on a higher order detector 750 that generates a higher order measurement signal SF.

[0049] Some or all of the devices of the device 200 can be evacuated, and the evacuated area can include the wafer W.

[0050] The measurement device 200 may include a processor (not shown) and / or a controller that receives a reference measurement signal SR, a high-order measurement signal SF, and / or a reflection measurement signal ST. The processor and / or controller may be arranged to process these signals to determine a measured value of an attribute of interest of the target T. Optionally, the processor and / or controller may also control the generation of soft X-ray and / or EUV radiation by controlling the IR laser 202 and / or the HHG mechanism 204. The processor and / or controller may also control the subsystem that receives and holds the wafer W.

[0051] Optical systems and additional optical systems suitable for focusing a radiation beam on an area of interest including the target T are discussed below. The optical systems discussed may be used in the illumination subsystem 732 of the measurement device 200. Note that it may also be used in an inspection device including an illumination radiation source and a substrate stage for receiving and holding a substrate.

[0052] The potential requirements for the optical system OS or additional optical system OS may be summarized into five: the optical system OS may focus radiation in the soft X-ray or EUV spectral range, the optical system OS may focus radiation with broadband characteristics (or, including multiple wavelength peaks in a relatively wide spectrum, or with narrowband characteristics but allowing selection of a central wavelength, for example, by using a tunable monochromator), the optical system OS may have a relatively large reduction with a diffraction-limited focal length, the optical system OS may be adapted within a relatively small volume such that the measurement device has a relatively small footprint, and the optical system OS may use a reference grating in a reference measurement branch. As further described herein, the reduction of the optical system may be determined by the ratio between the apparent source size and the corresponding spot at the area of interest, where the optical system images the apparent source onto the area of interest to form a spot.

[0053] In one embodiment, one or more reflectors are used to manipulate the radiation beam B generated by the source. The radiation may be incident on the reflector at a grazing incidence angle or a near-normal incidence angle. It will be understood that the term "reflector" may include or refer to a reflector. In some embodiments, at least one reflector may include a reflective element that may serve as a reflector. In some embodiments, at least one reflector may include a diffractive element that may be used for at least one of the following: reflecting radiation, diffracting radiation, and scattering radiation. Thus, the diffractive element may also serve as a reflector and diffract radiation. When the term "reflector" or "mirror" is used herein, this may be understood to refer to the general term "reflector".

[0054] Radiation in the EUV and SXR ranges is difficult to direct or focus because refractive optical elements can only be applied to some very specific applications for that wavelength (e.g., Fresnel zone plates). For most applications, the direction and focusing of EUV and soft X-rays must therefore be performed using reflectors. For example, such reflectors can include grazing incidence (e.g., at about 15 degrees or less relative to the surface) reflectors, or narrow-band multilayer-coated near-normal incidence reflectors (commonly referred to as Bragg mirrors). Because the wavelengths of EUV and soft X-ray radiation are very short, the figure error, slope error, and surface roughness requirements for reflectors of this radiation are very strict in order to focus the scintillating light with low wavefront aberration and surface scattering due to surface defects. The surface quality of such reflectors is typically specified on different length scales, either by power spectral density (PSD), or in a more compact way by the root mean square (RMS) integral values referring to three different mechanisms: very low spatial frequency or figure error (usually the spatial frequency is close to the size of the beam footprint or clear aperture and / or <1mm -1 ), medium spatial frequency or slope error (usually 0.1mm -1 to 1mm -1 ), and high spatial frequency or roughness (usually >1mm -1 ). The exact definition of these mechanisms is typically application-driven.

[0055] Therefore, to focus radiation, a high-precision optical reflector is required with very low surface roughness (e.g., less than 0.1 nm RMS), low slope error (e.g., below 1 μrad RMS), and low form error (e.g., peak-to-valley value below 1 nm). Additionally, the reflector substrate should include a material with low thermal expansion, as EUV radiation is easily absorbed and converted into heat. Existing methods for manufacturing such reflectors include advanced polishing techniques on the substrate, followed optionally by the application of one or more coatings. For example, the coating can include iridium, ruthenium, gold, or a multilayer stack (such as molybdenum-silicon), or any other suitable coating. The substrate surface is made very smooth by polishing; and to achieve the necessary smoothness, advanced polishing techniques or super-polishing techniques can be used. Such super-polishing techniques can include magnetorheological finishing (MRF), ion beam figuring (IBF), or elastic emission machining (EEM). In MRF, a slurry of magnetic particles suspended in a carrier fluid is used for polishing, which allows the use of a magnetic field to control some properties of the fluid. IBF is based on the impact of ions onto the substrate surface, which transfers some of their energy to the atoms on the surface, causing the atoms to be ejected from the solid; this process is also known as sputtering. EEM includes a non-contact machining process, where fine powder particles are brought to the mirror surface by a water stream. The interaction between surface atoms (preferably atomic clusters or peaks protruding from the surface) results in the removal of surface atoms, thus smoothing the surface on an atomic scale.

[0056] Although such manufacturing techniques using MRF, IBF, or EEM have been shown to be effective in manufacturing reflectors for radiation in the EUV and SXR ranges, they are difficult, expensive, and time-consuming. This is especially true when the surface to be polished is uneven. The polishing process that results in low roughness also has a very low material removal rate. This makes it very time-consuming to remove larger volumes (e.g., improving the surface quality in mid- and low-spatial frequencies). Additionally, improving the mirror surface in mid- and low-spatial frequencies is a deterministic process. This means that in an iterative process, the surface is measured by a surface metrology tool (such as a white light interferometer), and the resulting data is fed into the polishing machine to selectively remove material in the desired regions. Surface metrology is increasingly difficult to perform on substrates with large curvatures, thus severely limiting the maximum curvature of the mirrors that can be polished to the surface quality required at EUV or SXR wavelengths. Therefore, an improved and simpler method for manufacturing such reflectors will now be described.

[0057] The proposed method involves decoupling the polishing / smoothing step from the shaping of the reflector to obtain the necessary profile. It is proposed to polish the flat surface until it is very smooth, which can be done using known techniques, and then shape the reflector to create the curvature in the reflector surface. Thus, creating the curvature and creating a very smooth surface become independent steps. This method takes advantage of the relative simplicity of smoothing a flat surface compared to smoothing a curved surface with a specific defined profile.

[0058] It is proposed that the method includes bonding together a collection of flat substrates (such as, silicon substrates or wafers). For example, the substrates can be substantially similar to those used in IC manufacturing that are subjected to exposure, etching, and dicing. In one embodiment, when it is deemed necessary to do so (e.g., depending on the required degree of curvature), an individual substrate can be initially thinned to support the required curvature.

[0059] Figure 5 The conceptual map illustrates the proposed manufacturing process. At step 500, a plurality of silicon substrates are obtained. At the optional step 510, the silicon substrates can be made thinner. For example, this step can be performed by wafer backgrinding or etching.

[0060] At step 520, the top surface of at least one substrate (i.e., the main uppermost reflective surface when the substrates are bonded) is polished to the desired extent. This step can include polishing the top surface of the substrate that will be at the top of the stack to a maximum, while the other surfaces only need to be polished to the extent required for the selected bonding process (and may not need to be polished at all if the bonding process is based on an adhesive). It is highly advantageous to polish the surface while it is still flat because it is much more difficult to polish a curved surface. In addition to roughness, the thickness uniformity on the substrate should also be controlled during the polishing step. Each substrate should have a very uniform thickness because thickness variations will cause shape variations in the reflector surface.

[0061] Any polishing technique among the aforementioned polishing techniques or super-polishing techniques can be used for at least the main uppermost surface. Other bonding surfaces can use less extensive polishing techniques, e.g., using the wafer polishing tools and apparatus already used for wafer preparation in IC manufacturing. For example, chemical mechanical polishing (CMP) can be used for the bonding surfaces.

[0062] At steps 530 and 540, the substrates are shaped, stacked, and bonded together. In certain embodiments, this can include shaping and bonding each substrate individually to the stack using cold bonding techniques, direct bonding techniques, or fusion bonding techniques (i.e., without adhesives). Direct bonding describes a wafer bonding process without any additional intermediate layers. The bonding process is based on the formation of chemical bonds between two surfaces that meet various requirements when materials such as silicon are bonded together. Surface requirements for bonding include that the wafer surfaces be sufficiently clean, flat, and smooth. Thus, in the case of using direct bonding, the foregoing polishing steps can include polishing of all substrates to facilitate such bonding. Direct bonding can include steps of pre-bonding at room temperature and annealing at elevated temperature.

[0063] Thus, the method can include placing a first substrate in a mold and applying sufficient force such that the first substrate is shaped by the mold 530. Then the next substrate is placed on top of the shaped first substrate such that the next substrate is also shaped by the mold and bonded to the first substrate. Then this operation is repeated for all substrates. The mold illustrated herein is a positive mold (i.e., the shape of the mold is replicated to the final reflector shape), rather than a negative mold that would result in an inversion of the curvature. Either type of mold can be used.

[0064] Finally, the bonded substrates are released from the mold (step 550). The final result resembles a curved silicon mirror, but without the active support required to maintain its shape and with an infinite number of support points. Then the mold can be reused to fabricate another reflector. In one embodiment, the final reflector can have some and preferably all of the following qualities: very low surface roughness (e.g., less than 0.1 nm RMS), low slope error (e.g., below 1 μrad RMS), and low profile error (e.g., below 1 nm peak-to-valley).

[0065] It should be understood that when the bonded substrates are released from the mold, the bonded substrates may deform slightly. Thus, the shape of the mold should include a pre-correction for such deformation. For example, this can include (e.g., using finite element modeling packages or similar tools) modeling the deformation after release and calculating the mold shape for the best correction for the desired reflector shape.

[0066] It should be understood that the methods described herein are purely exemplary and different bonding, polishing, and / or molding techniques can be used. Alternatively or additionally, the order of any process steps can be changed where logical or possible. The manufacturing process is not necessarily a linear process, and steps can be performed in parallel and / or in other orders if there is no direct dependency between the steps.

[0067] The proposed method, especially the use of the flat silicon wafer in the manufacture of reflectors, can bring additional benefits. For example, the proposal opens up the option of patterning gratings on the top layer while the wafer is still flat, for example to create spectrometer gratings. In another example, the patterned grating can include a variable line spacing (VLS) grating; for example, for shaping into a flat field spectrometer. Such as Figure 1 the lithography apparatus or scanner illustrated in, or alternatively using holographic techniques instead of lithography, or alternatively using an optical or electron beam-based direct write lithography process to perform the patterning of such gratings.

[0068] Actuators can be utilized to functionalize one or more of the wafer layers in the wafer stack to deform the mirror. Since each layer in the stack should maintain a well-defined and uniform thickness, in one embodiment, such actuators can be embedded in one or more of the substrates within the substrate. Such actuators can also be used to correct residual manufacturing errors (e.g., slope errors or profile errors), such as those resulting from thickness variations in the component substrates. Alternatively or additionally, actuators can be used to impose additional (e.g., graphical) features on the surface shape that are not present in the mold. For example, the actuator can add 2D freeform corrections on top of a shape (e.g., a cylindrical shape), which is imposed by the bending process of the mold. For complex shapes, it is convenient to consider the shape of the optical surface as the sum of various shape or graphical components. In principle, it should be possible to achieve control and correction down to the sub-millimeter lateral length scale.

[0069] For example, the actuator can include a thermal actuator or a piezoelectric actuator. Among them, thermal actuators are easier to integrate into standard wafer processing technologies because they are simple resistive elements. However, applying such thermal actuators requires careful management of both mechanical stress and thermal gradients within the substrate. On the other hand, piezoelectric actuators can enable finer-grained control because there is no thermal gradient to maintain, but require stronger electric fields and more exotic materials.

[0070] An image sensor can also be embedded inside the reflector. For example, such a sensor can be used to detect the beam coverage area. Additionally, an additional coating can be provided for each pixel to generate spectral information. A simple example is to cover some of the pixels in the pixel with a thin layer of zirconium and cover other pixels with an aluminum layer, which have very different transmission characteristics between 5 nm and 50 nm. More materials can be used, such as boron, silicon, molybdenum, ruthenium. A second type of example includes applying a periodic multilayer stack (e.g., a MoSi multilayer), which can be tuned to suppress a selected narrowband spectrum. Even more complex stacks can be envisioned, such as non-periodic multilayers, multilayers terminated with several cover layers, etc. These are only a few examples, and many other examples can be envisioned. Such a sensor can be used for advanced dose control (including pupil resolution and spectral information) (e.g., in a scanner or metrology tool).

[0071] Further embodiments are disclosed in the following numbered clauses:

[0072] 1. A method of manufacturing a reflector, comprising:

[0073] At least polishing the uppermost surface of the uppermost substantially flat substrate among a plurality of substantially flat substrates;

[0074] Deforming each substantially flat substrate into a desired shape, and

[0075] Bonding the deformed substrates together to form the reflector.

[0076] 2. The method according to clause 1, wherein a mold is used to perform the deformation and bonding steps together.

[0077] 3. The method according to clause 2, wherein the deformation and bonding steps include repeating for each substrate:

[0078] Individually deforming the substrate in the mold; and

[0079] Bonding the substrate to the previous substrate to form a stack defining the reflector.

[0080] 4. The method according to clause 3, including forming a pre-corrected mold having additional deformation for releasing the bonded substrate from the mold.

[0081] 5. The method according to any of the preceding clauses, wherein the bonding of the substrates includes a fusion bonding method without an adhesive.

[0082] 6. The method according to clause 5, including polishing all the bonded surfaces of the substantially flat substrates to facilitate the fusion bonding method.

[0083] 7. The method according to clause 6, wherein the uppermost surface of the uppermost substrate is polished to a higher standard than the other bonding surfaces.

[0084] 8. The method according to clause 7, wherein the other bonding surfaces are polished using a chemical polishing method.

[0085] 9. The method according to any of the preceding clauses, wherein the method enables the reflector to have at least some of the following qualities: a surface roughness of less than 0.1 nm RMS, a slope error of less than 1 μrad RMS, and a form error of less than 1 nm peak-to-valley.

[0086] 10. The method according to any of the preceding clauses, wherein the substrate comprises a silicon substrate.

[0087] 11. The method according to any of the preceding clauses, wherein polishing of at least the uppermost surface of the uppermost substrate is performed using a magnetorheological finishing, ion beam figuring, or elastic emission machining process.

[0088] 12. The method according to any of the preceding clauses, including a step of thinning the substrate before the bonding step.

[0089] 13. The method according to any of the preceding clauses, including a step of patterning a grating on a top layer before the uppermost surface of the uppermost substantially flat substrate before the deformation step.

[0090] 14. The method according to clause 13, wherein the grating comprises a spectrometer grating.

[0091] 15. The method according to clause 13, wherein the grating comprises a variable line spacing grating; for a flat field spectrometer.

[0092] 16. The method according to clause 13, 14 or 15, wherein the step of patterning the grating is performed using a lithographic apparatus.

[0093] 17. The method according to clause 13, 14 or 15, wherein the step of patterning the grating is performed using holographic techniques.

[0094] 18. The method according to any of the preceding clauses, including embedding at least one actuator in one or more of the substantially flat substrates, the actuator being operable to deform the reflector.

[0095] 19. The method according to clause 18, wherein the actuator is operable to correct residual manufacturing errors.

[0096] 20. The method according to clause 18 or 19, wherein the actuator is operable to apply additional figure components (such as, freeform correction).

[0097] 21. The method according to clause 18, 19 or 20, wherein the actuator comprises a thermal actuator embedded within the bonded substrate.

[0098] 22. The method according to clause 18, 19 or 20, wherein the actuator comprises a piezoelectric actuator.

[0099] 23. The method according to any of the preceding clauses, comprising embedding an image sensor within the reflector.

[0100] 24. The method according to clause 23, wherein the image detector is operable to detect a coverage area of a light beam incident thereon.

[0101] 25. The method according to clause 23 or 24, wherein the image detector comprises a plurality of coatings per pixel, operable to generate spectral information from a light beam incident thereon.

[0102] 26. A reflector manufactured according to the method of any of the preceding clauses.

[0103] 27. An illumination subsystem comprising at least one reflector according to clause 26.

[0104] 28. A metrology device comprising an illumination subsystem according to clause 27.

[0105] 29. A lithographic apparatus comprising an illumination subsystem according to clause 27.

[0106] Although specific reference may be made herein to the use of a lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include: the manufacture of integrated optical systems, guiding and detecting patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like.

[0107] Although specific reference may be made herein to embodiments of the present invention in the context of a lithographic apparatus, embodiments of the present invention may be used in other apparatuses. Embodiments of the present invention may form part of a mask inspection apparatus, a metrology device or any apparatus for measuring or processing an object, such as a wafer (or other substrate) or a mask (or other patterning device). These apparatuses are generally referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0108] Although specific reference has been made above to the use of embodiments of the present invention in the context of optical lithography, it should be understood that, where the context allows, the present invention is not limited to optical lithography and may be used in other applications (e.g., imprint lithography).

[0109] Although specifically referred to herein as a "measurement device", the term can also refer to an inspection device or an inspection system. For example, an inspection device including an embodiment of the present invention can be used to detect defects in a substrate or defects in a structure on the substrate. In such an embodiment, the property of interest of the structure on the substrate can relate to a defect in the structure, the absence of a particular portion of the structure, or the presence of an unwanted structure on the substrate.

[0110] In the context of the above document, the term HHG or HHG source is introduced. HHG refers to high harmonic generation or sometimes to higher harmonic generation. HHG is a non-linear process in which a target (e.g., a gas, plasma or solid sample) is irradiated with a strong laser pulse. Subsequently, the target can emit radiation at a frequency that is a multiple of the radiation frequency of the laser pulse. Such a frequency, i.e., the multiple, is called the harmonic of the radiation of the laser pulse. The generated HHG radiation can be defined as harmonics higher than the fifth harmonic, and these harmonics are called high harmonics. The physical process underlying the HHG process is different from the physical process associated with the generation of radiation of lower harmonics (usually the second to fifth harmonics). The generation of radiation of lower harmonics is related to perturbation theory. The trajectory of the (bound) electrons in the atoms in the target is basically determined by the Coulomb potential of the host ions. In HHG, the trajectory of the electrons contributing to the HHG process is basically determined by the electric field of the incident laser. In the so-called "three-step model" describing HHG, the electrons cross the Coulomb barrier, at which time the Coulomb barrier is basically suppressed by the laser field (step 1), follow a trajectory determined by the laser field (step 2), and recombine with a certain probability when releasing their kinetic and ionization energies in the form of radiation (step 3). Another way to describe the difference between HHG and the generation of radiation of lower harmonics is to define all radiation with a photon energy higher than the ionization energy of the target atoms as "high harmonic" radiation (e.g., the radiation generated by HHG), and all radiation with a photon energy lower than the ionization energy as non-HHG generated radiation. If neon is used as the gas target, all radiation with a wavelength less than 62 nm (with a photon energy higher than 20.18 eV) is generated by the HHG process. For argon as the gas target, all radiation with a photon energy higher than approximately 15.8 eV is generated by the HHG process.

[0111] Throughout this disclosure and where appropriate, the term "reflected radiation" can be considered to refer to "specularly reflected radiation". Throughout this disclosure and where appropriate, the term "diffracted radiation" can be considered to refer to first-order radiation or higher diffracted-order radiation.

[0112] Some embodiments described herein relate to an optical system OS including a first reflector m1, a second reflector m2, and a third reflector m3. In such embodiments, the second reflector m2 and the third reflector m3 may respectively refer to examples of additional reflectors and further additional reflectors. Some embodiments described herein refer to an optical system OS including a first reflector m1, a second reflector m2, a third reflector m3, and a fourth reflector m4. In such embodiments, the third reflector m3 and the fourth reflector m4 may respectively refer to examples of additional reflectors and further additional reflectors.

[0113] Although specific embodiments of the invention have been described above, it should be understood that the invention may be practiced in other ways different from those described. The above description is intended to be illustrative rather than limiting. Thus, it will be apparent to those skilled in the art that the invention may be modified as described without departing from the scope of the claims set forth below.

Claims

1. A method of manufacturing a reflector for a metrology device, the method comprises: at least polishing the uppermost surface of the uppermost flat substrate among a plurality of flat substrates; deforming each flat substrate into a desired shape, and bonding the deformed substrates together to form the reflector; wherein a mold is used to perform the deformation step and the bonding step together; wherein the uppermost surface of the uppermost substrate is polished to a higher standard than other bonding surfaces.

2. The method according to claim 1, wherein the deformation step and the bonding step comprise repeating for each substrate: individually deforming the substrate in the mold; and bonding the substrate to a previous substrate to form a stack defining the reflector.

3. The method according to claim 2, further comprising forming the mold, the mold having a pre-correction for additional deformation of the bonded substrates due to release of the bonded substrates from the mold.

4. The method according to any one of the preceding claims, wherein the bonding of the substrates comprises a fusion bonding method performed without an adhesive.

5. The method according to claim 4, comprising polishing all bonding surfaces of the flat substrates to facilitate the fusion bonding method.

6. The method according to claim 5, wherein the other bonding surfaces are polished using a chemical mechanical polishing method.

7. The method according to any one of claims 1-3 and 5-6, wherein the method enables the reflector to have at least some of the following qualities: a surface roughness of less than 0.1 nm RMS, a slope error of less than 1 μrad RMS, and a profile error of less than 1 nm peak-to-valley.

8. The method according to any one of claims 1-3 and 5-6, wherein at least the uppermost surface of the uppermost substrate is polished using a magnetorheological finishing, ion beam figuring, or elastic emission machining process.

9. The method according to any one of claims 2-3 and 5-6, comprising a step of thinning the substrates before the bonding step.

10. The method according to any one of claims 2-3 and 5-6, comprising a step of patterning a grating on the uppermost surface of the uppermost flat substrate in the top layer before the deformation step.

11. The method according to claim 10, wherein the grating comprises a variable pitch grating; the grating is for a flat field spectrometer.

12. The method according to any one of claims 1-3, 5-6, and 11, comprising embedding at least one actuator in one or more of the flat substrates, the actuator being operable to deform the reflector.

13. The method according to claim 12, wherein the actuator comprises a thermal actuator embedded within the bonded substrates.

14. The method according to any one of claims 1-3, 5-6, and 11, comprising embedding an image sensor within the reflector.

15. The method according to claim 14, wherein the image sensor is operable to detect a coverage area of a light beam incident thereon.

16. A reflector manufactured by the method according to any one of the preceding claims, the reflector being for a measuring device.

17. A measuring device comprising an illumination subsystem, the illumination subsystem including at least one reflector according to claim 16.

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