Metrology system with optical element(s) having surface structures

Abstract

A metrology system includes an illumination system, at least one optical element, a detector, and a controller. The illumination system directs a beam of radiation at a target disposed on a substrate. The at least one optical element has a plurality of surface structures comprising at least one of nanopillars, nano-hemispheres, or submicron-scale cones. The plurality of surface structures are arranged with a random or periodic distribution to produce a modified refractive index profile to receive and condition at least a portion of the beam of radiation. The detector receives radiation scattered by the target and generates a measurement signal based on the received radiation. The controller is coupled to the illumination system and the detector. The controller controls a characteristic of the beam of radiation and determines a characteristic of the target based on the measurement signal.

Description

METROLOGY SYSTEM WITH OPTICAL ELEMENT(S) HAVING SURFACE STRUCTURESCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63 / 636,324 which was filed on April 19, 2024 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present disclosure relates to metrology systems, for example, optical elements for controlling radiation propagation in lithographic apparatuses and systems.BACKGROUND

[0003] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which can be a mask or a reticle, can be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiationsensitive material (photoresist or simply “resist”) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatuses include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”- direction) while synchronously scanning the target portions parallel or anti-parallel to this scanning direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.

[0004] During lithographic operation, different processing steps can entail different layers to be sequentially formed on the substrate. Accordingly, it can be necessary to position the substrate relative to prior patterns formed thereon with a high degree of accuracy. Generally, alignment marks are placed on the substrate to be aligned and are located with reference to a second object. A lithographic apparatus can use an alignment apparatus for detecting positions of the alignment marks and for aligning the substrate using the alignment marks to ensure accurate exposure from a mask. Misalignment between the alignment marks at two different layers is measured as overlay error.

[0005] In order to monitor the lithographic process, parameters of the patterned substrate are measured. Parameters can include, for example, the overlay error between successive layers formed in or on the patterned substrate and critical linewidth of developed photosensitive resist. This measurement can be performed on a product substrate and / or on a dedicated metrology target. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including theuse of scanning electron microscopes and various specialized tools. A fast and non-invasive form of a specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. By contrast, angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.

[0006] Such optical scatterometers can be used to measure parameters, such as critical dimensions of developed photosensitive resist or overlay error (OV) between two layers formed in or on the patterned substrate. Properties of the substrate can be determined by comparing the properties of an illumination beam before and after the beam has been reflected or scattered by the substrate.

[0007] To ensure effective performance for the inspection tools, a lithographic apparatus can use optical elements with modified surfaces to control the propagation of the beam of radiation. Such optical elements can be modified with thin film coatings or reactive-ion etching (RIE) to condition a beam of radiation. However, these existing surface modification methods have limitations.

[0008] Thin film coatings (or physical vapor deposition) can produce an optical quality to reduce image distortions associated with reflections off a surface of the substrate. However, many layer coatings are used to address a wavelength range from about 400 nmto about 1600 nm. Using multiple layer coatings can add to the complexity of the inspection apparatus, impact performance of the sensor by adding stress to optics, and produce aberrations. As the number of layers increases, there are diminishing returns in reducing image distortions. The reflection that can be achieved over a broad wavelength range can be limited from about 0.5% to about 1%. Current thin film coatings neither effectively address infrared (IR) wavelengths nor achieve a low reflectivity of 1 % over this extended range.

[0009] Reactive ion etching with inductively coupled plasma (ICP-RIE) involves placing a sample inside a vacuum chamber. Reactive gases are introduced into the chamber, and plasma is generated by applying an electric field with electrodes. The plasma can comprise a mixture of ions, free electrons, and neutral particles. The electric field accelerates the ions in the plasma toward the sample, resulting in both chemical reactions and physical sputtering at the surface which operate to removes the material at the target sites. These sites form microstructures that can reduce reflectivity over an extended wavelength range. However, the effectiveness of RIE is limited to material types receptive to chemical etching and is limited to surfaces with minimal curvature. Also, RIE does not enable selective surface treatment on a local scale because the process is governed by the freely moving gases in the chamber. Additionally, RIE can only etch the surface of the sample, so the process does not change the bulk properties of the material.

[0010] As a result, the limitations with these surface modifications methods can make it challenging to control radiation propagation within a lithographic apparatus. Accordingly, the current surface modification methods pose numerous problems for controlling radiation, and thus impeding alignment processes and throughput.SUMMARY

[0011] Accordingly, it is desirable to better control radiation within a lithographic apparatus to improve fabrication speed and throughput. For example, optical inspection processes can be performed faster based on aspects described herein.

[0012] In some aspects, a metrology system can comprise an illumination system, at least one optical element, a detector, and a controller. The illumination system can be configured to direct a beam of radiation at a target disposed on a substrate. The at least one optical element can have a plurality of surface structures comprising at least one of nanopillars, nano-hemispheres, or submicron-scale cones. The plurality of surface structures can be arranged with a random or periodic distribution to produce a modified refractive index profile configured to receive and condition at least a portion of the beam of radiation. The detector can be configured to receive radiation scattered by the target and to generate a measurement signal based on the received radiation. The controller can be coupled to the illumination system and the detector. The controller can be configured to control a characteristic of the beam of radiation and to determine a characteristic of the target based on the measurement signal.

[0013] In some aspects, a lithographic apparatus can comprise a metrology system. The metrology system can comprise an illumination system, at least one optical element, a detector, and a controller. The illumination system can be configured to direct a beam of radiation at a target disposed on a substrate. The at least one optical element can have a plurality of surface structures comprising at least one of nanopillars, nano-hemispheres, or submicron-scale cones. The plurality of surface structures can be arranged with a random or periodic distribution to produce a modified refractive index profile configured to receive and condition at least a portion of the beam of radiation. The detector can be configured to receive radiation scattered by the target and to generate a measurement signal based on the received radiation. The controller can be coupled to the illumination system and the detector. The controller can be configured to control a characteristic of the beam of radiation and to determine a characteristic of the target based on the measurement signal.

[0014] In some aspects, a method can comprise directing, with an illumination system, a beam of radiation at a target disposed on a substrate. The method can further comprise receiving and conditioning at least a portion of the beam of radiation at a plurality of surface structures on at least one optical element within a metrology system. The plurality of surface structures can comprise at least one of nanopillars, nano-hemispheres, or submicron-scale cones arranged with a random or periodic distribution. The method can further comprise receiving a scattered radiation from the target at a detector. The method can further comprise generating a measurement signal with the detector based onthe scattered radiation from the target. The method can further comprise determining a characteristic of the target based on the measurement signal.

[0015] Further features of various aspects of the present disclosure are described in detail below with reference to the accompanying drawings. It is noted that the present disclosure is not limited to the specific aspects described herein. Such aspects are presented herein for illustrative purposes only. Additional aspects will be apparent to those skilled in the relevant art(s) based on the teachings contained herein.BRIEF DESCRIPTION OF THE DRAWINGS / FIGURES

[0016] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present disclosure and, together with the description, further serve to explain the principles of the present disclosure and to enable those skilled in the relevant art(s) to make and use aspects described herein.

[0017] FIG. 1 A shows a reflective lithographic apparatus, according to some aspects.

[0018] FIG. IB shows a transmissive lithographic apparatus, according to some aspects.

[0019] FIG. 2 shows more details of a reflective lithographic apparatus, according to some aspects.

[0020] FIG. 3 shows a lithographic cell, according to some aspects.

[0021] FIGS. 4 A and 4B show inspection apparatuses, according to some aspects.

[0022] FIG. 5 shows a treatment system that forms surface structures on an optical element, according to some aspects.

[0023] FIG. 6 shows a metrology system with an optical element having surface structures, according to some aspects.

[0024] FIG. 7 shows an optical element having surface structures, according to some aspects.

[0025] FIGS. 8A-8C show various aspects of surface structures, according to some aspects.

[0026] FIG. 9 shows a metrology system with an optical element having an aperture surrounded by surface structures, according to some aspects.

[0027] FIG. 10 shows a metrology system with optical elements having surface structures as interior walls and mounting devices, according to some aspects.

[0028] FIG. 11 shows a metrology system with optical elements having surface structures that tune a wettability characteristic of the optical elements, according to some aspects.

[0029] FIG. 12 shows a method for using an optical element(s) having surface structures within a metrology system, according to some aspects.

[0030] The features of the present disclosure will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and / or structurally similar elements. Additionally, generally, the left-most digit(s) of a reference number identifies the drawing in which the reference number first appears. Unlessotherwise indicated, the drawings provided throughout the disclosure should not be interpreted as to- scale drawings.DETAILED DESCRIPTION

[0031] The aspects described herein, and references in the specification to “one aspect,” “an aspect,” “an exemplary aspect,” “an example aspect,” etc., indicate that the aspects described can include a particular feature, structure, or characteristic, but every aspect may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same aspect. Further, when a particular feature, structure, or characteristic is described in connection with an aspect, it is understood that it is within the knowledge of those skilled in the art to effect such feature, structure, or characteristic in connection with other aspects whether or not explicitly described.

[0032] Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “on,” “upper” and the like, can be used herein for ease of description to describe one element or feature’s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus can be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein can likewise be interpreted accordingly.

[0033] The terms “about,” “approximately,” or the like can be used herein to indicate the value of a given quantity that can vary based on a particular technology. Based on the particular technology, the terms “about,” “approximately,” or the like can indicate a value of a given quantity that varies within, for example, 10-30% of the value (e.g., ±10%, ±20%, or ±30% of the value).

[0034] Aspects of the present disclosure can be implemented in hardware, firmware, software, or any combination thereof. Aspects of the disclosure can also be implemented as instructions stored on a computer-readable medium, which can be read and executed by one or more processors. A machine- readable medium can include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium can include read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), and others. Furthermore, firmware, software, routines, and / or instructions can be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. The term “machine-readable medium” can be interchangeable with similar terms, for example, “computer program product,” “computer-readable medium,” “non-transitory computer- readable medium,” or the like. The term “non-transitory” can be used herein to characterize one or more forms of computer readable media except for a transitory, propagating signal.

[0035] Before describing such aspects in more detail, however, it is instructive to present an example environment in which aspects of the present disclosure can be implemented.

[0036] Example Lithographic Systems

[0037] FIGS. 1A and IB show a lithographic apparatus 100 and a lithographic apparatus 100’, respectively, in which aspects of the present disclosure can be implemented. Lithographic apparatus 100 and lithographic apparatus 100’ each include the following: an illumination system (illuminator) IL configured to condition a radiation beam B (for example, deep ultra violet or extreme ultra violet radiation); a support structure (for example, a mask table) MT configured to support a patterning device (for example, a mask, a reticle, or a dynamic patterning device) MA and connected to a first positioner PM configured to accurately position the patterning device MA; and, a substrate table (for example, a wafer table) WT configured to hold a substrate (for example, a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate W. Lithographic apparatus 100 and 100’ also have a projection system PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion (for example, comprising one or more dies) C of the substrate W. In lithographic apparatus 100, the patterning device MA and the projection system PS are reflective. In lithographic apparatus 100’, the patterning device MA and the projection system PS are transmissive.

[0038] The illumination system IL can include various types of optical components, such as refractive, reflective, catadioptric, magnetic, electromagnetic, electrostatic, or other types of optical components, or any combination thereof, for directing, shaping, or controlling the radiation beam B.

[0039] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device MA with respect to a reference frame, the design of at least one of the lithographic apparatus 100 and 100’, and other conditions, such as whether or not the patterning device MA is held in a vacuum environment. The support structure MT can use mechanical, vacuum, electrostatic, or other clamping techniques to hold the patterning device MA. The support structure MT can be a frame or a table, for example, which can be fixed or movable. By using sensors, the support structure MT can ensure that the patterning device MA is at a desired position, for example, with respect to the projection system PS.

[0040] The term “patterning device” MA should be broadly interpreted as referring to any device that can be used to impart a radiation beam B with a pattern in its cross-section, such as to create a pattern in the target portion C of the substrate W. The pattern imparted to the radiation beam B can correspond to a particular functional layer in a device being created in the target portion C to form an integrated circuit.

[0041] The patterning device MA can be transmissive (as in lithographic apparatus 100’ of FIG. IB) or reflective (as in lithographic apparatus 100 of FIG. 1 A). Examples of patterning devices MA include reticles, masks, programmable mirror arrays, or programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase shift, or attenuated phase shift, aswell as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in the radiation beam B, which is reflected by a matrix of small mirrors.

[0042] The term “projection system” PS can encompass any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors, such as the use of an immersion liquid on the substrate W or the use of a vacuum. A vacuum environment can be used for EUV or electron beam radiation since other gases can absorb too much radiation or electrons. A vacuum environment can therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

[0043] Lithographic apparatus 100 and / or lithographic apparatus 100’ can be of a type having two (dual stage) or more substrate tables WT (and / or two or more mask tables). In such “multiple stage” machines, the additional substrate tables WT can be used in parallel, or preparatory steps can be carried out on one or more tables while one or more other substrate tables WT are being used for exposure. In some situations, the additional table may not be a substrate table WT.

[0044] The lithographic apparatus can also be of a type wherein at least a portion of the substrate can be covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system and the substrate. An immersion liquid can also be applied to other spaces in the lithographic apparatus, for example, between the mask and the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems. The term “immersion” as used herein does not mean that a structure, such as a substrate, must be submerged in liquid. For example, a liquid can be located between the projection system and the substrate during exposure.

[0045] Referring to FIGS. 1A and IB, the illuminator IL receives a radiation beam from a radiation source SO. The source SO and the lithographic apparatus 100, 100’ can be separate physical entities, for example, when the source SO is an excimer laser. In such cases, the source SO is not considered to form part of the lithographic apparatus 100 or 100’, and the radiation beam B passes from the source SO to the illuminator IL with the aid of a beam delivery system BD (in FIG. IB) including, for example, suitable directing mirrors and / or a beam expander. In other cases, the source SO can be an integral part of the lithographic apparatus 100, 100’, for example, when the source SO is a mercury lamp. A radiation system can comprise the source SO, the illuminator IL, and / or the beam delivery system BD.

[0046] The illuminator IL can include an adjuster AD (in FIG. IB) for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and / or inner radial extent (commonly referred to as “a-outcr” and “o-inner,” respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL can comprise various other components (inFIG. IB), such as an integrator IN and a condenser CO. The illuminator IL can be used to condition the radiation beam B to have a desired uniformity and intensity distribution in its cross section.

[0047] Referring to FIG. 1A, the radiation beam B is incident on the patterning device (for example, mask) MA, which is held on the support structure (for example, mask table) MT, and is patterned by the patterning device MA. In lithographic apparatus 100, the radiation beam B is reflected from the patterning device (for example, mask) MA. After being reflected from the patterning device (for example, mask) MA, the radiation beam B passes through the projection system PS, which focuses the radiation beam B onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF2 (for example, an interferometric device, linear encoder, or capacitive sensor), the substrate table WT can be moved accurately (for example, so as to position different target portions C in the path of the radiation beam B). Similarly, the first positioner PM and another position sensor IF1 can be used to accurately position the patterning device (for example, mask) MA with respect to the path of the radiation beam B. Patterning device (for example, mask) MA and substrate W can be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2.

[0048] Referring to FIG. IB, the radiation beam B is incident on the patterning device (for example, mask MA), which is held on the support structure (for example, mask table MT), and is patterned by the patterning device. 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. The projection system has a pupil conjugate PPU to an illumination system pupil IPU. Portions of radiation emanate from the intensity distribution at the illumination system pupil IPU and traverse a mask pattern without being affected by diffraction at the mask pattern and create an image of the intensity distribution at the illumination system pupil IPU.

[0049] The projection system PS projects an image of the mask pattern MP, where the image is formed by diffracted beams produced from the mark pattern MP by radiation from the intensity distribution, onto a photoresist layer coated on the substrate W. For example, the mask pattern MP can include an array of lines and spaces. A diffraction of radiation at the array and different from zeroth order diffraction generates diverted diffracted beams with a change of direction in a direction perpendicular to the lines. Undiffracted beams (i.e., so-called zeroth order diffracted beams) traverse the pattern without any change in propagation direction. The zeroth order diffracted beams traverse an upper lens or upper lens group of the projection system PS, upstream of the pupil conjugate PPU of the projection system PS, to reach the pupil conjugate PPU. The portion of the intensity distribution in the plane of the pupil conjugate PPU and associated with the zeroth order diffracted beams is an image of the intensity distribution in the illumination system pupil IPU of the illumination system IL. The aperture device PD, for example, is disposed at or substantially at a plane that includes the pupil conjugate PPU of the projection system PS.

[0050] The projection system PS is arranged to capture (e.g., using a lens or lens group L) the zeroth order diffracted beams, first order diffracted beams, and / or higher order diffracted beams (not shown).In some aspects, dipole illumination for imaging line patterns extending in a direction perpendicular to a line can be used to utilize the resolution enhancement effect of dipole illumination. For example, first- order diffracted beams interfere with corresponding zeroth-order diffracted beams at the level of the wafer W to create an image of the line pattern MP at highest possible resolution and process window (i.e., usable depth of focus in combination with tolerable exposure dose deviations). In some aspects, astigmatism aberration can be reduced by providing radiation poles (not shown) in opposite quadrants of the illumination system pupil IPU. Further, in some aspects, astigmatism aberration can be reduced by blocking the zeroth order beams in the pupil conjugate PPU of the projection system associated with radiation poles in opposite quadrants. This is described in more detail in US 7,511,799 B2, issued Mar. 31 , 2009, which is incorporated by reference herein in its entirety.

[0051] With the aid of the second positioner PW and position sensor IFD (for example, an interferometric device, linear encoder, or capacitive sensor), the substrate table WT can be moved accurately (for example, so as to position different target portions C in the path of the radiation beam B). Similarly, the first positioner PM and another position sensor (not shown in FIG. IB) can be used to accurately position the mask MA with respect to the path of the radiation beam B (for example, after mechanical retrieval from a mask library or during a scan).

[0052] In general, movement of the mask table MT can be realized with the aid of a long-stroke module (coarse positioning) and a short- stroke module (fine positioning), which form part of the first positioner PM. Similarly, movement of the substrate table WT can be realized using a long-stroke module and a short- stroke module, which form part of the second positioner PW. In the case of a stepper (as opposed to a scanner), the mask table MT can be connected to a short-stroke actuator or can be fixed. Mask MA and substrate W can be aligned using mask alignment marks Ml, M2, and substrate alignment marks Pl, P2. Although the substrate alignment marks (as illustrated) occupy dedicated target portions, they can be located in spaces between target portions (known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the mask MA, the mask alignment marks can be located between the dies.

[0053] Mask table MT and patterning device MA can be in a vacuum chamber V, where an in-vacuum robot IVR can be used to move patterning devices such as a mask in and out of vacuum chamber. Alternatively, when mask table MT and patterning device MA are outside of the vacuum chamber, an out-of-vacuum robot can be used for various transportation operations, similar to the in- vacuum robot IVR. Both the in-vacuum and out-of-vacuum robots can be calibrated for a smooth transfer of any payload (e.g., mask) to a fixed kinematic mount of a transfer station.

[0054] The lithographic apparatus 100 and 100’ can be used in at least one of the following modes:1. In step mode, the support structure (for example, mask table) MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam B is projected onto a target portion C at one time (i.e., a single static exposure). The substrate table WT is then shifted in the X and / or Y direction so that a different target portion C can be exposed.2. In scan mode, the support structure (for example, mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam B is projected onto a target portion C (i.e., a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure (for example, mask table) MT can be determined by the (de-)magnification and image reversal characteristics of the projection system PS.3. In another mode, the support structure (for example, mask table) MT is kept substantially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam B is projected onto a target portion C. A pulsed radiation source SO can be employed and the programmable patterning device is updated as needed after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes a programmable patterning device, such as a programmable mirror array.

[0055] Combinations and / or variations on the described modes of use or entirely different modes of use can also be employed.

[0056] In some aspects, lithographic apparatus 100 includes an extreme ultraviolet (EUV) source, which is configured to generate a beam of EUV radiation for EUV lithography. In general, the EUV source is configured in a radiation system, and a corresponding illumination system is configured to condition the EUV radiation beam of the EUV source.

[0057] In some aspects, lithographic apparatus 100’ includes a deep ultraviolet (DUV) source, which is configured to generate a beam of DUV radiation for DUV lithography. In general, the DUV source is configured in a radiation system, and a corresponding illumination system is configured to condition the DUV radiation beam of the DUV source.

[0058] FIG. 2 shows the lithographic apparatus 100 in more detail, including the source collector apparatus SO, the illumination system IL, and the projection system PS. The source collector apparatus SO is constructed and arranged such that a vacuum environment can be maintained in an enclosing structure 220 of the source collector apparatus SO. An EUV radiation emitting plasma 210 can be formed by a discharge produced plasma source. EUV radiation can be produced by a gas or vapor, for example Xe gas, Li vapor, or Sn vapor in which EUV radiation emitting plasma 210 is created to emit radiation in the EUV range of the electromagnetic spectrum. The EUV radiation emitting plasma 210 is created by, for example, an electrical discharge causing at least a partially ionized plasma. Partial pressures of, for example, 10 Pa of Xe, Li, Sn vapor, or any other suitable gas or vapor can be used for efficient generation of the radiation. In some aspects, a plasma of excited tin (Sn) (e.g., excited via a laser) is provided to produce EUV radiation.

[0059] The radiation emitted by the EUV radiation emitting plasma 210 is passed from a source chamber 211 into a collector chamber 212 via an optional gas barrier or contaminant trap 230 (in some cases also referred to as contaminant barrier or foil trap), which is positioned in or behind an opening in source chamber 211. The contaminant trap 230 can include a channel structure. Contamination trap230 can also include a gas barrier or a combination of a gas barrier and a channel structure. The contaminant trap or contaminant barrier 230 further indicated herein at least includes a channel structure.

[0060] The collector chamber 212 can include a radiation collector CO, which can be a so-called grazing incidence collector. Radiation collector CO has an upstream radiation collector side 251 and a downstream radiation collector side 252. Radiation that traverses collector CO can be reflected off a grating spectral filter 240 to be focused in a virtual source point INTF. The virtual source point INTF is commonly referred to as the intermediate focus, and the source collector apparatus is arranged such that the intermediate focus INTF is located at or near an opening 219 in the enclosing structure 220. The virtual source point INTF is an image of the EUV radiation emitting plasma 210. Grating spectral filter 240 is used in particular for suppressing infra-red (IR) radiation.

[0061] Subsequently the radiation traverses the illumination system IL, which can include a faceted field mirror device 222 and a faceted pupil mirror device 224 arranged to provide a desired angular distribution of the radiation beam 221, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA. Upon reflection of the beam of radiation 221 at the patterning device MA, held by the support structure MT, a patterned beam 226 is formed and the patterned beam 226 is imaged by the projection system PS via reflective elements 228, 229 onto a substrate W held by the wafer stage or substrate table WT.

[0062] More elements than shown can generally be present in illumination optics unit IL and projection system PS. The grating spectral filter 240 can optionally be present, depending upon the type of lithographic apparatus. Further, there can be more mirrors present than those shown in the FIG. 2, for example there can be one to six additional reflective elements present in the projection system PS than shown in FIG. 2.

[0063] Collector optic CO, as illustrated in FIG. 2, is depicted as a nested collector with grazing incidence reflectors 253, 254, and 255, just as an example of a collector (or collector mirror). The grazing incidence reflectors 253, 254, and 255 are disposed axially symmetric around an optical axis O and a collector optic CO of this type is preferably used in combination with a discharge produced plasma source, often called a DPP source.

[0064] Example Lithographic Cell

[0065] FIG. 3 shows a lithographic cell 300, also sometimes referred to a lithocell or cluster, according to some aspects. Lithographic apparatus 100 or 100’ can form part of lithographic cell 300. Lithographic cell 300 can also include one or more apparatuses to perform pre- and post-exposure processes on a substrate. Conventionally these include spin coaters SC to deposit resist layers, developers DE to develop exposed resist, chill plates CH, and bake plates BK. A substrate handler, or robot, RO picks up substrates from input / output ports I / Ol, I / O2, moves them between the different process apparatuses and delivers them to the loading bay LB of the lithographic apparatus 100 or 100’ . These devices, which are often collectively referred to as the track, are under the control of a track control unit TCU, whichis itself controlled by a supervisory control system SCS, which also controls the lithographic apparatus via lithography control unit LACU. Thus, the different apparatuses can be operated to maximize throughput and processing efficiency.

[0066] Example Inspection Apparatus

[0067] In order to control the lithographic process to place device features accurately on the substrate, alignment marks are generally provided on the substrate, and the lithographic apparatus includes one or more inspection apparatuses for accurate positioning of marks on a substrate. These alignment apparatuses are effectively position measuring apparatuses. Different types of marks and different types of alignment apparatuses and / or systems are known from different times and different manufacturers. A type of system widely used in current lithographic apparatus is based on a self-referencing interferometer as described in U.S. Patent No. 6,961,116 (den Boef et al.). Generally marks are measured separately to obtain X- and Y-positions. A combined X- and Y-measurement can be performed using the techniques described in U.S. Publication No. 2009 / 195768 A (Bijnen et al.), however. The full contents of both of these disclosures are incorporated herein by reference.

[0068] FIG. 4A shows a cross-sectional view of an inspection apparatus 400 that can be implemented as a part of lithographic apparatus 100 or 100’, according to some aspects. In some aspects, inspection apparatus 400 can be configured to align a substrate (e.g., substrate W) with respect to a patterning device (e.g., patterning device MA). Inspection apparatus 400 can be further configured to detect positions of alignment marks on the substrate and to align the substrate with respect to the patterning device or other components of lithographic apparatus 100 or 100’ using the detected positions of the alignment marks. Such alignment of the substrate can ensure accurate exposure of one or more patterns on the substrate.

[0069] The terms “inspection apparatus,” “metrology system,” or the like can be used herein to refer to, e.g., a device used for measuring a property of a structure (e.g., overlay sensor, critical dimension sensor, or the like), a device or system used in a lithographic apparatus to inspect an alignment of a wafer (e.g., alignment sensor), or the like.

[0070] In some aspects, inspection apparatus 400 can include an illumination system 412, a beam splitter 414, an interferometer 426, a detector 428, a beam analyzer 430, and a processor 432. Illumination system 412 can be configured to provide an electromagnetic narrow band radiation beam 413 having one or more passbands. In an example, the one or more passbands can be within a spectrum of wavelengths between about 500 nm to about 900 nm. In another example, the one or more passbands can be discrete narrow passbands within a spectrum of wavelengths between about 500 nm to about 900 nm. Illumination system 412 can be further configured to provide one or more passbands having substantially constant center wavelength (CWL) values over a long period of time (e.g., over a lifetime of illumination system 412). Such configuration of illumination system 412 can help to prevent the shift of the actual CWL values from the desired CWL values, as discussed above, in current alignmentsystems. And, as a result, the use of constant CWL values can improve long-term stability and accuracy of alignment systems (e.g., inspection apparatus 400) compared to the current alignment apparatuses.

[0071] In some aspects, beam splitter 414 can be configured to receive radiation beam 413 and split radiation beam 413 into at least two radiation sub-beams. For example, radiation beam 413 can be split into radiation sub-beams 415 and 417, as shown in FIG. 4A. Beam splitter 414 can be further configured to direct radiation sub-beam 415 onto a substrate 420 placed on a stage 422. In one example, the stage 422 is movable along direction 424. Radiation sub-beam 415 can be configured to illuminate an alignment mark or a target 418 located on substrate 420. Alignment mark or target 418 can be coated with a radiation sensitive film. In some aspects, alignment mark or target 418 can have one hundred and eighty degrees (i.e., 180°) symmetry. That is, when alignment mark or target 418 is rotated 180° about an axis of symmetry perpendicular to a plane of alignment mark or target 418, rotated alignment mark or target 418 can be substantially identical to an unrotated alignment mark or target 418. The target 418 on substrate 420 can be (a) a resist layer grating comprising bars that are formed of solid resist lines, or (b) a product layer grating, or (c) a composite grating stack in an overlay target structure comprising a resist grating overlaid or interleaved on a product layer grating. The bars can alternatively be etched into the substrate. This pattern is sensitive to chromatic aberrations in the lithographic projection apparatus, particularly the projection system PL, and illumination symmetry and the presence of such aberrations will manifest themselves in a variation in the printed grating. One in-line method used in device manufacturing for measurements of line width, pitch, and critical dimension makes use of a technique known as “scatterometry”. Methods of scatterometry are described in Raymond et al., “Multiparameter Grating Metrology Using Optical Scatterometry”, J. Vac. Sci. Tech. B, Vol. 15, no. 2, pp. 361-368 (1997) and Niu et al., “Specular Spectroscopic Scatterometry in DUV Lithography”, SPIE, Vol. 3677 (1999), which are both incorporated by reference herein in their entireties. In scatterometry, light is reflected by periodic structures in the target, and the resulting reflection spectrum at a given angle is detected. The structure giving rise to the reflection spectrum is reconstructed, e.g. using Rigorous Coupled- Wave Analysis (RCWA) or by comparison to a library of patterns derived by simulation. Accordingly, the scatterometry data of the printed gratings is used to reconstruct the gratings. The parameters of the grating, such as line widths and shapes, can be input to the reconstruction process, performed by processing unit PU, from knowledge of the printing step and / or other scatterometry processes.

[0072] In some aspects, beam splitter 414 can be further configured to receive diffraction radiation beam 419 and split diffraction radiation beam 419 into at least two radiation sub-beams, according to an aspect. Diffraction radiation beam 419 can be split into diffraction radiation sub-beams 429 and 439, as shown in FIG. 4A.

[0073] It should be noted that even though beam splitter 414 is shown to direct radiation sub-beam 415 towards alignment mark or target 418 and to direct diffracted radiation sub-beam 429 towards interferometer 426, the disclosure is not so limiting. Other optical arrangements can be used to obtainthe similar result of illuminating alignment mark or target 418 on substrate 420 and detecting an image of alignment mark or target 418.

[0074] As illustrated in FIG. 4A, interferometer 426 can be configured to receive radiation sub-beam 417 and diffracted radiation sub-beam 429 through beam splitter 414. In an example aspect, diffracted radiation sub-beam 429 can be at least a portion of radiation sub-beam 415 that can be reflected from alignment mark or target 418. In an example of this aspect, interferometer 426 comprises any appropriate set of optical-elements, for example, a combination of prisms that can be configured to form two images of alignment mark or target 418 based on the received diffracted radiation sub-beam 429. It should be appreciated that a good quality image need not be formed. It can be enough to have the features of alignment mark 418 resolved. Interferometer 426 can be further configured to rotate one of the two images with respect to the other of the two images 180° and recombine the rotated and unrotated images interferometrically.

[0075] In some aspects, detector 428 can be configured to receive the recombined image via interferometer signal 427 and detect interference as a result of the recombined image when alignment axis 421 of inspection apparatus 400 passes through a center of symmetry (not shown) of alignment mark or target 418. Such interference can be due to alignment mark or target 418 being 180° symmetrical, and the recombined image interfering constructively or destructively, according to an example aspect. Based on the detected interference, detector 428 can be further configured to determine a position of the center of symmetry of alignment mark or target 418 and consequently, detect a position of substrate 420. According to an example, alignment axis 421 can be aligned with an optical beam perpendicular to substrate 420 and passing through a center of image rotation interferometer 426. Detector 428 can be further configured to estimate the positions of alignment mark or target 418 by implementing sensor characteristics and interacting with wafer mark process variations.

[0076] In a further aspect, detector 428 determines the position of the center of symmetry of alignment mark or target 418 by performing one or more of the following measurements:1. measuring position variations for various wavelengths (position shift between colors);2. measuring position variations for various orders (position shift between diffraction orders);3. measuring position variations for various polarizations (position shift between polarizations); and4. measuring intensity difference between opposite orders of a diffraction order pair (e.g., to characterize and correct for asymmetry).

[0077] This data can be obtained using any type of alignment sensor, for example, a SMASH (SMart Alignment Sensor Hybrid) sensor, as described in U.S. Patent No. 6,961,116 that employs a selfreferencing interferometer with a single detector and four different wavelengths, and extracts the alignment signal in software, or Athena (Advanced Technology using High order ENhancement of Alignment), as described in U.S. Patent No. 6,297,876, which directs each of seven diffraction orders to a dedicated detector, which are both incorporated by reference herein in their entireties.

[0078] In some aspects, beam analyzer 430 can be configured to receive and determine an optical state of diffracted radiation sub-beam 439. The optical state can be a measure of beam wavelength, polarization, or beam profile. Beam analyzer 430 can be further configured to determine a position of stage 422 and correlate the position of stage 422 with the position of the center of symmetry of alignment mark or target 418. As such, the position of alignment mark or target 418 and, consequently, the position of substrate 420 can be accurately known with reference to stage 422. Alternatively, beam analyzer 430 can be configured to determine a position of inspection apparatus 400 or any other reference element such that the center of symmetry of alignment mark or target 418 can be known with reference to inspection apparatus 400 or any other reference element. Beam analyzer 430 can be a point or an imaging polarimeter with some form of wavelength-band selectivity. In some aspects, beam analyzer 430 can be directly integrated into inspection apparatus 400, or connected via fiber optics of several types: polarization preserving single mode, multimode, or imaging, according to other aspects.

[0079] In some aspects, beam analyzer 430 can be further configured to determine the overlay data between two patterns on substrate 420. One of these patterns can be a reference pattern on a reference layer. The other pattern can be an exposed pattern on an exposed layer. The reference layer can be an etched layer already present on substrate 420. The reference layer can be generated by a reference pattern exposed on the substrate by lithographic apparatus 100 and / or 100’. The exposed layer can be a resist layer exposed adj cent to the reference layer. The exposed layer can be generated by an exposure pattern exposed on substrate 420 by lithographic apparatus 100 or 100’. The exposed pattern on substrate 420 can correspond to a movement of substrate 420 by stage 422. In some aspects, the measured overlay data can also indicate an offset between the reference pattern and the exposure pattern. The measured overlay data can be used as calibration data to calibrate the exposure pattern exposed by lithographic apparatus 100 or 100’, such that after the calibration, the offset between the exposed layer and the reference layer can be minimized.

[0080] In some aspects, beam analyzer 430 can be further configured to determine a model of the product stack profile of substrate 420, and can be configured to measure overlay, critical dimension, and focus of target 418 in a single measurement. The product stack profile contains information on the stacked product such as alignment mark, target 418, or substrate 420, and can include mark process variation-induced optical signature metrology that is a function of illumination variation. The product stack profile can also include product grating profile, mark stack profile, and mark asymmetry information. An example of beam analyzer 430 is Yieldstar™, manufactured by ASML, Veldhoven, The Netherlands, as described in U.S. Patent No. 8,706,442, which is incorporated by reference herein in its entirety. Beam analyzer 430 can be further configured to process information related to a particular property of an exposed pattern in that layer. For example, beam analyzer 430 can process an overlay parameter (an indication of the positioning accuracy of the layer with respect to a previous layer on the substrate or the positioning accuracy of the first layer with respective to marks on the substrate), a focus parameter, and / or a critical dimension parameter (e.g., line width and its variations) of the depictedimage in the layer. Other parameters are image parameters relating to the quality of the depicted image of the exposed pattern.

[0081] In some aspects, an array of detectors (not shown) can be connected to beam analyzer 430, and allows the possibility of accurate stack profile detection as discussed below. For example, detector 428 can be an array of detectors. For the detector array, a number of options are possible: a bundle of multimode fibers, discrete pin detectors per channel, or CCD or CMOS (linear) arrays. The use of a bundle of multimode fibers enables any dissipating elements to be remotely located for stability reasons. Discrete PIN detectors offer a large dynamic range but each need separate pre-amps. The number of elements is therefore limited. CCD linear arrays offer many elements that can be read-out at high speed and are especially of interest if phase-stepping detection is used.

[0082] In some aspects, a second beam analyzer 430’ can be configured to receive and determine an optical state of diffracted radiation sub-beam 429, as shown in FIG. 4B. The optical state can be a measure of beam wavelength, polarization, or beam profile. Second beam analyzer 430’ can be identical to beam analyzer 430. Alternatively, second beam analyzer 430’ can be configured to perform one or more of the functions of beam analyzer 430, such as determining a position of stage 422 and correlating the position of stage 422 with the position of the center of symmetry of alignment mark or target 418. As such, the position of alignment mark or target 418 and, consequently, the position of substrate 420, can be accurately known with reference to stage 422. Second beam analyzer 430’ can also be configured to determine a position of inspection apparatus 400, or any other reference element, such that the center of symmetry of alignment mark or target 418 can be known with reference to inspection apparatus 400, or any other reference element.

[0083] In some aspects, second beam analyzer 430’ can be directly integrated into inspection apparatus 400, or it can be connected via fiber optics of several types: polarization preserving single mode, multimode, or imaging, according to other aspects. Alternatively, second beam analyzer 430’ and beam analyzer 430 can be combined to form a single analyzer (not shown) configured to receive and determine the optical states of both diffracted radiation sub-beams 429 and 439.

[0084] In some aspects, processor 432 receives information from detector 428 and beam analyzer 430. Processor 432 can create a basic correction algorithm based on the information received from detector 428 and beam analyzer 430, including but not limited to the optical state of the illumination beam, the alignment signals, associated position estimates, and the optical state in the pupil, image, and additional planes. The pupil plane is the plane in which the radial position of radiation defines the angle of incidence and the angular position defines the azimuth angle of the radiation. Processor 432 can utilize the basic correction algorithm to characterize the inspection apparatus 400 with reference to wafer marks and / or alignment marks 418.

[0085] Example Treatment System

[0086] FIG. 5 shows a treatment system 500 that forms surface structures on an optical element, according to some aspects. In some aspects, treatment system 500 can comprise a laser source 501, aconditioning element 504, and an optical element 506. Treatment system 500 can use laser source 501 to perform surface modifications of optical element 506. For example, laser source 501 can form surface structures 508 on optical element 506 that change the bulk properties of the material of optical element 506 by altering effective refractive indices at selected portions of optical element 506.

[0087] In some aspects, laser source 501 can be an ultrafast pulsed radiation source that emits a laser beam 502. For example, laser source 501 can be a femtosecond laser source. Laser source 501 can have an ultrashort pulse width and an ultrahigh peak intensity, which allow laser source 501 to ablate a wide variety of materials. Laser source 501 can have adjustable parameters of at least: pulse energy, laser fluence, pulse duration, wavelength, repetition rate, light polarization, scan speed, or spot size. Additionally, the ambient environment (e.g., vacuum, gas, liquid media) can affect performance of laser source 501.

[0088] In some aspects, conditioning element 504 can condition laser 502 into a focused laser beam 505 for processing of optical element 506. Conditioning element 504 can be at least one of a mirror, a beamsplitter, or a lens.

[0089] Conditioning element 504 can focus pulses of focused laser beam 505 onto a surface of optical element 506 so that part of the laser energy is directly absorbed by electrons through a nonlinear effect, such as multiphoton absorption and avalanche ionization. As a result of the energy transfer, a high- temperature / pressure plasma forms above the surface of optical element 506. When the plasma bursts out of the laser-ablated spot, the material at the laser-focused spot is removed as ejected particles. This treatment process of optical element 506 can form surface structures 508. Laser source 501 can have a pulse duration shorter than the electron-phonon interaction time (e.g., about 1 ps) of most materials, so any heat transfer can be confined to the ablation spot. Therefore, laser source 501 can precisely remove material from optical element 506, thus enabling smooth etching of surface structures 508.

[0090] In some aspects, laser source 501 can treat a single spot on a stationary optical element 506. In some aspects, laser source 501 can process a pattern on optical element 506 by moving relative to optical element 506. For example, optical element 506 can be moved in a plane by a translational or rotational stage while laser source 501 remains fixed. In another example, laser source 501 can scan across optical element 506 while optical element 506 remains fixed on a stage. A larger irradiance and / or slower scan speed of laser source 501 can produce deeper surface structures 508, and a lower laser irradiance and / or faster scan speed of laser source 501 can produce shallower surface structures 508. Additionally, the number of repeat scans on a treated region can affect the size and shape of surface structures 508.

[0091] In some aspects, optical element 506 can be configured to receive focused laser beam 505 for ablation. Optical element 506 can be disposed on a stage. In some aspects, optical element 506 can be stationary or be moved relative to laser source 501. In some aspects, optical element 506 can comprise at least one of the following materials: a metal (e.g., stainless steel, aluminum, copper), a semiconductor (e.g., silicon), a dielectric (e.g., ceramic, plastic), glass, or a polymer.

[0092] In some aspects, surface structures 508 can be formed within a material of optical element 506. Surface structures 508 can comprise a metamaterial, which is a class of functional material designed using micro and / or nanoscale patterns or structures. The size and density of surface structures 508 produced at a given position can depend on the laser irradiance of laser source 501 and the number of pulses of focused laser beam 505 at that position. In some aspects, surface structures 508 can be arranged in a random pattern or a periodic pattern. A periodic pattern can be determined by the relative scanning motion between laser source 501 and optical element 506. The structure patterns can influence an illumination to interact with surface structures 508 in a manner that is different from a conventional non-patterned material (an example of an interaction with a conventional non-patterned material is refraction at a glass interface).

[0093] In some aspects, surface structures 508 can be engineered to adjust phase, amplitude, and / or polarization of illumination that is received at the surface of optical element 506. The surface structures 508 can control a direction of the received radiation based on the adjusting of the phase, amplitude, and / or polarization of the incident beam. In some aspects, surface structures 508 can change the bulk properties of optical element 506. As a result, the presence of surface structures 508 on optical element 506 can cause optical element 506 to absorb incident radiation, diffract radiation in scattered directions, or transmit radiation.

[0094] In some aspects, the use of surface structures 508 can circumvent the use of bulky lenses to condition illumination. In conventional optical sensors, bulk optics like lenses tend to increase the size of the sensor, which is counter to the goal of miniaturization. Because surface structures 508 can be capable of altering phase, amplitude, and / or polarization of illumination in a predictable manner, surface structures 508 can condition illumination (e.g., collimate, focus, phase correction, correction of optical aberration, or the like) without relying on bulk optics.

[0095] In some aspects, surface structures 508 can transform a surface of optical element 506 to become a selective or broadband optical absorber. For example, unprocessed metals can highly reflective for electromagnetic waves, especially at far-infrared and terahertz frequencies. The presence of surface structures 508 can transform a reflective metal into a highly absorptive metal over an ultrabroad electromagnetic spectrum, ranging from ultraviolet to terahertz.

[0096] In some aspects, the presence of surface structures 508 on optical element 506 can change the color of optical element 506 and an absorptive optical quality associated with that color to control optical properties of optical element 506. For example, surface structures 508 can change the color of optical element 506 to black, which has a greater absorptive quality compared to other colors. Surface structures 508 can trap photons so that the material appears black.

[0097] In some aspects, the presence of surface structures 508 on optical element 506 can improve radiative and convective cooling power of the material of optical element 506. In some aspects, optical element 506 can be used as a heatsink, which is a passive heat exchanger that can transfer heat to a fluid (e.g., air) through convection, radiation, or conduction. The presence of surface structures 508 cantransform optical element 506 into an ultra-broadband perfect light absorber with near-unity emissivity for a wide range of temperatures. Accordingly, the presence of surface structures 508 can increase a spectral emissivity of a material (e.g., aluminum) of optical element 506 used as a heatsink, thereby increasing radiative cooling power. The presence of surface structures 508 can increase a surface area of a material (e.g., aluminum) of optical element 506 due to the area of side walls for each one of surface structures 506. This increase in surface area of optical element 506 can thereby increase convective and radiative cooling power as well. These features can be useful for manufacturing heatsinks with more cooling power than untreated heatsinks.

[0098] In some aspects, the presence of surface structures 508 on optical element 506 can tune a wettability characteristic of optical element 506. For example, surface structures 508 can be arranged in a pattern that is superhydrophilic or superhydrophobic such that water can be wicked away as a coolant for optical element 506.

[0099] Treatment system 500 can improve over existing surface modification methods in numerous ways. First, the small beam size of laser source 501 can enable treatment of local areas on optical element 506. As a result, treatment system 500 can properly treat a curved surface of optical element 506 because laser source 501 can adjust the angle of incidence of focused laser beam 505 when etching on the local scale. In contrast, the existing method of RIE is a random process with minimal to no control on a local scale. Second, treatment system 500 can penetrate the material of optical element 506 with a subtractive process deeper than existing surface modification methods, thereby modifying the bulk properties of optical element 506. In contrast, thin film coatings can only affect the surface of a treated material through an additive process. Third, treatment system 500 can enable fine control over creation of surface structures 508 of different shapes and sizes by changing parameters for laser source 501. For example, a power density, a repetition rate of pulses, and a scan pattern can be tuned to produce a designed pattern (e.g., a random pattern or a periodic pattern). Last, treatment system 500 can process nearly any material, such as materials that are suitable for a clean vacuum environment.

[0100] Example Metrology System and Method

[0101] In some aspects, the term “throughput” can be used to describe the rate at which a wafer clears a particular fabrication step and moves to the next step. Throughput can be a performance marker of marketability of a lithographic system. It is desirable for lithographic systems to output as many products as possible in as little time as possible. Lithographic fabrication can comprise several complex processes. Each process encompasses choices in technology that balance desired qualities (e.g., subnanometer accuracy, high yield) and drawbacks (e.g., slower fabrication, cost). Such processes can involve inspection of printed marks on a substrate. A metrology system can be used in connection with a lithographic process, for example, to ascertain a conformity of a printed pattern on a substrate or to align a substrate in order to properly receive a new pattern. It should be appreciated that an inspection process can greatly enhance mass production using lithographic processes. It should also be appreciated that the inspection process can have an associated time cost, thereby reducing throughput.

[0102] In existing metrology systems, one or more optical elements can guide radiation toward a detector. The one or more optical elements can be supported by support structures such as mounting devices, for example. The support structures can comprise a metal such as stainless steel, for example. Scattered radiation may reflect off surfaces of the support structures and travel in undesired directions within the metrology system.

[0103] Existing metrology systems can control the propagating radiation by using additional support structures to block the undesired propagation of scattered radiation. However, these additional support structures can add complexity to the metrology system and increase the manufacturing cost. Existing metrology systems can control the propagating radiation with anti-reflective coating applied to optical surfaces. However, these anti-reflective coatings can have a limited effectiveness such that ghost imaging still appears and can present a contamination risk in a clean vacuum environment. Existing metrology systems can be encapsulated in a box that controls outgassing and contamination. However, encapsulating the metrology system in this manner can be costly, bulky, complicated, and introduce more potential failure points for monitoring.

[0104] Some aspects herein include devices and functions to enhance speeds of inspection processes by improving control of scattered radiation within a metrology system. The disclosed aspects can use surfaces processed to have desirable optical properties that control propagation of radiation. The disclosed aspects can have one or more optical elements having a wettability characteristic to maintain a clean vacuum environment free from contamination. Therefore, the aspects herein offer numerous improvements over existing metrology systems.

[0105] FIG. 6 shows a metrology system 610 with an optical element 606 having surface structures 608, according to some aspects. In some aspects, metrology system 610 can be implemented as a part of lithographic apparatus 100 or 100’ (as shown in and described with regard to FIGS. 1 A and IB). In some aspects, metrology system 610 can be implemented as a part of inspection apparatus 400 (as shown in and described with regard to FIGS. 4A and 4B). The discussion of the components of lithographic apparatus 100 or 100’ and inspection apparatus 400 can apply to the discussion of the components and functions of metrology system 610.

[0106] In some aspects, metrology system 610 can comprise an illumination system 612, at least one optical element 606, a detector 614, and a controller 616. In some aspects, metrology system 610 can comprise structures and functions similar to inspection apparatus 400 that were described in reference to FIGS. 4A and 4B. For example, metrology system 610 can be configured to align a substrate 622 (e.g., substrate W) with respect to a patterning device (e.g., patterning device MA shown in FIGS. 1A and IB). Metrology system 610 can be further configured to perform an alignment process based on inspections of positions of alignment marks 620 on the substrate 622. Metrology system 610 can be further configured to align the substrate 622 with respect to the patterning device or other components of lithographic apparatus 100 or 100’ (as shown in and described with regard to FIG. 1A and IB) using the detected positions of alignment marks 620. Such alignment of the substrate 622 can ensure accurateexposure of one or more patterns on the substrate 622. In another example, metrology system 610 can be configured to perform overlay error analysis based on inspections of alignment marks 620.

[0107] In some aspects, illumination system 612 can be configured to direct a radiation beam 618 at an alignment mark 620 disposed on a substrate 622. In some aspects, illumination system 612 can comprise structures and functions similar to illumination system 412 that were described in reference to FIGS. 4 A and 4B.

[0108] In some aspects, radiation beam 618 can be configured to illuminate alignment mark 620 located on substrate 622. In some aspects, radiation beam 618 can have one or more passbands. In some aspects, the one or more passbands can be within a visible spectrum of wavelengths between about 380 nm and about 750 nm. For example, the wavelength can be in a range of about 450 nm to about 550 nm, for example, about 500 nm. In some aspects, the one or more passbands can be within an ultraviolet spectrum of wavelengths between about 100 nm and about 380 nm. For example, the wavelength can be in a range of about 350 nm to about 380 nm, for example, about 365 nm. In some aspects, the one or more passbands can be within an infrared spectrum of wavelengths between about 750 nm and about 2 microns. For example, the wavelength can be in a range of about 750 nm to about 900 nm, for example, about 850 nm.

[0109] In some aspects, substrate 622 can be an aspect of substrate W described in reference to FIGS. 1A and IB or substrate 420 described in reference to FIGS. 4 A and 4B. Though the example aspect shown in FIG. 6 illustrates only one alignment mark 620, it should be appreciated that substrate 622 can include a pattern that comprises a larger plurality of alignment marks 620 (e.g., several tens or hundreds of gratings used in alignment and / or overlay inspection).

[0110] In some aspects, alignment marks 620 can be an aspect of substrate alignment marks Pl, P2 described in reference to FIGS. 1A and IB or alignment mark or a target 418 described in reference to FIGS. 4A and 4B. Alignment marks 620 can be coated with a radiation sensitive film. Substrate alignment marks 620 can be (a) a resist layer grating comprising bars that are formed of solid resist lines, or (b) a product layer grating, or (c) a composite grating stack in an overlay target structure comprising a resist grating overlaid or interleaved on a product layer grating.

[0111] In some aspects, optical element 606 can be an aspect of optical element 506 described in reference to FIG. 5. In some aspects, optical element 606 can be configured to receive and condition at least a portion of radiation beam 618. In some aspects, optical element 606 can include can include any optic (e.g., mirror, lens, prism, etc.). In some aspects, optical element 606 can be a support structure within metrology system 610. Optical element 606 can comprise at least one of the following materials: a metal (e.g., stainless steel, aluminum, copper), a semiconductor (e.g., silicon), a dielectric (e.g., ceramic, plastic), glass, or a polymer. For example, optical element 606 can comprise at least one of glass, plastic, ceramic, or stainless steel.

[0112] In some aspects, optical element 606 can include a plurality of surface structures 608. In some aspects, surface structures 608 can be an aspect of surface structures 508 described in reference to FIG.5. Surface structures 608 can be arranged on a receiving surface of optical element 606. Surface structures 608 can comprise various etching features, including grooves, voids, ridges, protrusions, or aggregated particles, for example. Surface structures 608 can comprise various shapes, including at least one of pillars, hemispheres, or cones, for example. Surface structures 608 can be formed at various scales or sizes, such as micro-scale or nano-scale, for example. Accordingly, in some aspects, surface structures 608 can comprise at least one of nanopillars, nano-hemispheres, or submicron-scale cones, for example.

[0113] In some aspects, surface structures 608, as a structural feature of optical element 606, can comprise at least one of the following materials: a metal (e.g., stainless steel, aluminum, copper), a semiconductor (e.g., silicon), a dielectric (e.g., ceramic, plastic), glass, or a polymer. For example, surface structures 608 can comprise at least one of glass, plastic, ceramic, or stainless steel.

[0114] In some aspects, the plurality of surface structures 608 can be arranged with a random or periodic distribution to produce a modified refractive index profile configured to receive and condition at least a portion of radiation beam 618. The modified refractive index profile can modify the bulk properties of the material of optical element 606. In some aspects, the modified refractive index profile can produce at least one of anti-reflective, absorptive, or transmissive optical properties, for example, in optical element 606. In the example aspect shown in FIG. 6, surface structures 608 can produce a modified refractive index profile creating a transmissive optical property in optical element 606. In this example aspect, radiation beam 618 can be guided by optical element 606 to transmit through optical element 606 toward alignment mark 620. It should be appreciated that one portion of optical element 606 can have one optical property and other portions of optical element 606 can have other optical properties. For example, a first portion of optical element 606 can have an absorptive optical property while a second portion of optical element 606 can have a transmissive optical property.

[0115] In some aspects, surface structures 608 can assist with reducing stray radiation propagation inside metrology system 610. For example, the modified refractive index profile(s) provided by surface structures 608 can be beneficial for controlling stray radiation in a vacuum environment that is highly susceptible to contamination. In existing metrology systems, for example, stainless steel can be used as a material for structural supports because it can be free from contaminants, but stainless steel can have a high reflectivity. Few anti-reflective coatings can meet cleanliness standards for a vacuum environment, so stainless steel in existing systems cannot simply receive an anti-reflective coating that would provide a sufficiently low reflectivity. In contrast, surface structures 608 can produce a desirable anti-reflective optical quality in stainless steel without introducing a contamination risk. Therefore, surface structures 608 can provide a stainless steel that is both clean and anti-reflective for use in a metrology system operating in a vacuum environment. It should be appreciated that stainless steel is one example, and that surface structures 608 can enable use of any desired material with desired optical properties inside metrology system 610.

[0116] In some aspects, a plurality of sensors 615a, 615b can be configured to inspect an alignment of substrate 622 (e.g., alignment sensor). For example, sensors 615a, 615b can be configured to perform inspections of alignment marks 620 on substrate 622. Each of the sensors 615a, 615b can receive scattered radiation 624a, 624b from a plurality of alignment marks 620 (e.g., as described in reference to FIGS. 4A and 4B).

[0117] In some aspects, detector 614 can be coupled to sensors 615a, 615b and can be configured to receive scattered radiation 624a, 624b from alignment mark 620. Detector 614 can be configured to measure scattered radiation 624a, 624b and to generate a measurement signal based on the received scattered radiation 624a, 624b.

[0118] In some aspects, controller 616 can be coupled to illumination system 612 and detector 614. In some aspects, controller 616 can be configured to determine a characteristic of alignment mark 620 based on the measurement signal. For example, controller 616 can determine a position and / or asymmetry of alignment mark 620 based on an asymmetry (e.g., difference between first-order diffraction intensities I- 1 , 1+ 1 , for example) of scattered radiation 624a, 624b from alignment mark 620. In some aspects, controller 616 can be configured to control a characteristic of radiation beam 618. Controller 616 can control the characteristic of radiation beam 618 based on the determined characteristic of alignment mark 620 based on the measurement signal. For example, controller 616 can adjust energy, duration, incidence angle, wavelength, repetition rate, light polarization, scan speed, or spot size of radiation beam 618.

[0119] FIG. 7 shows an optical element 706 having surface structures 708, according to some aspects. In some aspects, optical element 706 can be an aspect of optical element 506 described in reference to FIG. 5 and optical element 606 described in reference to FIG. 6. In some aspects, surface structures 708 can be an aspect of surface structures 508 described in reference to FIG. 5 and surface structures 608 described in reference to FIG. 6. The discussion of optical elements 506, 508 and surface structures 508, 608 can apply to the discussion of the structure and functions of optical element 706 and surface structures 708.

[0120] In some aspects, optical element 706 can include a receiving surface 728 configured to receive at least a beam portion 726. In some aspects, beam portion 726 can be a portion of radiation beam 618 discussed in reference to FIG. 6. Beam portion 726 can be a portion of radiation beam 618 that reflected off a surface of a structural support within metrology system 610 discussed in reference to FIG. 6.

[0121] In some aspects, surface structures 708 can be arranged with a random or periodic distribution on receiving surface 728. The example aspect shown in FIG. 7 shows a detailed view of surface structures 708 that may not be visible to the naked eye, for example. In the example aspect of FIG. 7, surface structures 708 can be pillars arranged with a periodic distribution.

[0122] In some aspects, surface structures 708 can condition beam portion 726 by imparting a phase shift to beam portion 726 with a modified refractive index profile having at least one of anti-reflective, absorptive, or transmissive optical properties.

[0123] In some aspects, surface structures 708 can reduce reflectance with several absorption mechanisms. When surface structures 708 are smaller than a wavelength of beam portion 726, surface structures 708 can absorb beam portion 726 due to an antireflection effect caused by the surface texturing having a graded refractive index at the material transition interface. When surface structures 708 are smaller than a wavelength of beam portion 726, surface structures 708 can enhance absorptance due to a plasmonic effect. Additionally, the various sizes and shapes of surface structures 708 can broaden a surface-plasmon absorption spectra to improve broadband absorptance. When surface structures 708 are greater than a wavelength of beam portion 726, surface structures 708 can trap photons in cavities and interfere with reflection angles.

[0124] In some aspects, a depth of surface structures 708 can be directly correlated with absorptance of a spectrum of wavelengths. For example, surface structures 708 that extend deeper into optical element 706 can absorb longer wavelengths. In another example, surface structures 708 that are shallower in optical element 706 can absorb shorter wavelengths. Accordingly, optical element 706 can include surface structures 708 having a high aspect ratio to control a larger wavelength bandwidth.

[0125] FIGS. 8A-8C show various aspects of surface structures, according to some aspects. FIG. 8A shows pillar surface structures 830, FIG. 8B shows hemisphere surface structures 832, and FIG. 8C shows cone surface structures 834, according to some aspects. The example aspects shown in FIGS. 8A-8C show a detailed view of pillar surface structures 830, hemisphere surface structures 832, and cone surface structures 834 that may not be visible to the naked eye, for example.

[0126] In some aspects, pillar surface structures 830, hemisphere surface structures 832, and cone surface structures 834 can be alternative aspects of surface structures 508 described in reference to FIG. 5, surface structures 608 described in reference to FIG. 6, and surface structures 708 described in reference to FIG. 7. The discussion of surface structures 508, 608, 708 can apply to the discussion of the structure and functions of pillar surface structures 830, hemisphere surface structures 832, and cone surface structures 834.

[0127] In some aspects, pillar surface structures 830, hemisphere surface structures 832, and cone surface structures 834 can have various shapes, dimensions, and aspect ratios. The shape, base size, and height of pillar surface structures 830, hemisphere surface structures 832, and cone surface structures 834 can be controlled through laser fluence of laser source 501 and the number of laser pulses of focused laser beam 505. It should be appreciated that the surface structures can be formed in any other shapes beyond the aspects disclosed herein.

[0128] In some aspects, pillar surface structures 830, hemisphere surface structures 832, and cone surface structures 834 can be formed at various scales or sizes, such as micro-scale or nano-scale, for example. Accordingly, in some aspects, pillar surface structures 830 can comprise nanopillars, hemisphere surface structures 832, and cone surface structures 834 can comprise submicron- scale cones.

[0129] In some aspects, pillar surface structures 830, hemisphere surface structures 832, and cone surface structures 834 can gradually change the effective refractive index at the material transition interface to reduce optical reflection. As a result, the bulk refractive index of the material changes layer by layer depending on the shape of the surface structures. Accordingly, in some aspects, pillar surface structures 830 can have an associated pillar refractive index profile 831, hemisphere surface structures 832 can have an associated hemisphere refractive index profile 833, and cone surface structures 834 can have an associated cone refractive index profile 835. Each one of pillar refractive index profile 831, hemisphere refractive index profile 833, and cone refractive index profile 835 can have different optical properties, such as anti-reflection, absorption, or transmission, for example, as compared to the other refractive index profiles. Therefore, pillar refractive index profile 831, hemisphere refractive index profile 833, and cone refractive index profile 835 can provide an optimized refractive index profile that is suited to a particular application for the surface structures.

[0130] FIG. 9 shows a metrology system 910 with an optical element 906 having an aperture 936 surrounded by surface structures 908, according to some aspects. The discussion of the components discussed in reference to FIGS. 5-8 can apply to the discussion of the like-numbered components and functions of metrology system 910.

[0131] In some aspects, optical element 906 can comprise an aperture 936 and a surrounding surface 938 around aperture 936. In some aspects, a plurality of surface structures 908 can be disposed on surrounding surface 938. In the example aspect shown in FIG. 9, surface structures 908 can be pillars, such as pillar surface structures 830, for example. It should be appreciated that surface structures 908 can be other shapes, such as hemisphere surface structures 832 or cone surface structures 834, for example.

[0132] In some aspects, optical element 906 can be a stop configured to control propagation of radiation in metrology system 910. Optical element 906 can prevent undesirable collection of light that did not reflect from an alignment mark, but rather reflected from structural components of metrology system 910. Optical element 906 can comprise at least one of the following materials: a metal (e.g., stainless steel, aluminum, copper), a semiconductor (e.g., silicon), a dielectric (e.g., ceramic, plastic), glass, or a polymer. In one example aspect, optical element 906 can be stainless steel, which is suitable for a clean vacuum environment.

[0133] In some aspects, aperture 936 can be configured to receive radiation beam 918 from illumination system 912. Accordingly, aperture 936 can permit radiation beam 918 to transmit through aperture 936 of optical element 906. In some aspects, surrounding surface 938 around apertures 936 can be configured to receive beam portions 926a, 926b. In this configuration, a desired amount of radiation beam 918 can pass through aperture 936 toward an alignment mark while beam portions 926a, 926b are blocked by surrounding surface 938. In some aspects, surface structures 908 on surrounding surface 938 can modify an anti-reflective, absorptive, or transmissive optical property of surrounding surface 938. In the example aspect where optical element 906 is stainless steel, surface structures 908can produce an absorptive optical property in surrounding surface 938 to absorb beam portions 926a, 926b in a vacuum environment. It should be appreciated that the presence of surface structures 908 on surrounding surface 938 can improve the performance of aperture 936 in limiting radiation propagation.

[0134] FIG. 10 shows a metrology system 1010 with optical elements having surface structures as interior walls 1040a, 1040b and mounting devices 1042a, 1042b, according to some aspects. The discussion of the components discussed in reference to FIGS. 5-9 can apply to the discussion of the like-numbered components and functions of metrology system 1010.

[0135] In some aspects, optical element 1006 can be configured to receive radiation beam 1018 from illumination system 1012. Accordingly, optical element 1006 can have transmissive surface structures 1041 to permit radiation beam 918 to transmit through optical element 1006 toward alignment mark 1020 on substrate 1022. In some aspects, transmissive surface structures 1041 can be an alternative aspect of surface structures 608 discussed in reference to FIG. 6, such that transmissive surface structures 1041 produce a transmissive optical property in optical element 1006.

[0136] In some aspects, metrology system 1010 can have at least one optical element that comprises at least one mounting device configured to support another optical element metrology system 1010. In the example aspect shown in FIG. 10, the at least one optical element comprising at least one mounting device is represented as mounting devices 1042a, 1042b. In some aspects, mounting devices 1042a, 1042b can be configured to support optical element 1006.

[0137] In some aspects, mounting devices 1042a, 1042b can comprise at least one of the following materials: a metal (e.g., stainless steel, aluminum, copper), a semiconductor (e.g., silicon), a dielectric (e.g., ceramic, plastic), glass, or a polymer. In one example aspect, mounting devices 1042a, 1042b can be stainless steel, which is suitable for a clean vacuum environment.

[0138] In some aspects, mounting devices 1042a, 1042b can have absorptive surface structures 1043 disposed on a receiving surface. In some aspects, absorptive surface structures 1043 can be an alternative aspect of surface structures 908 discussed in reference to FIG. 9, such that absorptive surface structures 1043 produce an absorptive optical property in the optical elements termed mounting devices 1042a, 1042b. Mounting devices 1042a, 1042b can be configured to receive beam portions 1026b, 1026c. In this example configuration, mounting devices 1042a, 1042b can absorb beam portions 1026b, 1026c to control undesirable radiation propagation within metrology system 1010.

[0139] In some aspects, metrology system 1010 can have at least one optical element that comprises at least one interior wall of the metrology system 1010. In the example aspect shown in FIG. 10, the at least one optical element is represented as interior walls 1040a, 1040b. In some aspects, interior walls 1040a, 1040b can be configured to support components of metrology system 1010. For example, interior walls 1040a, 1040b can support mounting devices 1042a, 1042b.

[0140] In some aspects, interior walls 1040a, 1040b can comprise at least one of the following materials: a metal (e.g., stainless steel, aluminum, copper), a semiconductor (e.g., silicon), a dielectric(e.g., ceramic, plastic), glass, or a polymer. In one example aspect, interior walls 1040a, 1040b can be stainless steel, which is suitable for a clean vacuum environment.

[0141] In some aspects, interior walls 1040a, 1040b can have absorptive surface structures 1043 disposed on a receiving surface. In some aspects, absorptive surface structures 1043 can be an alternative aspect of surface structures 908 discussed in reference to FIG. 9, such that absorptive surface structures 1043 produce an absorptive optical property in the optical elements termed interior walls 1040a, 1040b. Interior walls 1040a, 1040b can be configured to receive beam portions 1026a, 1026d. In this example configuration, interior walls 1040a, 1040b can absorb beam portions 1026a, 1026d to control undesirable radiation propagation within metrology system 1010.

[0142] FIG. 11 shows a metrology system 1110 with optical elements 1106a, 1106b having surface structures 1144 that tune a wettability characteristic of the optical elements 1106a, 1106b, according to some aspects. The discussion of the components discussed in reference to FIGS. 5-10 can apply to the discussion of the like-numbered components and functions of metrology system 1110.

[0143] In some aspects, illumination system 1112 can be configured to project radiation beam 1118 toward alignment mark 1020 on substrate 1022. Meanwhile, in some aspects, optical elements 1106a, 1106b can control transport of foreign media 1146a, 1146b through metrology system 1110. In some aspects, optical elements 1106a, 1106b can have wettable surface structures 1144 disposed on a receiving surface for foreign media 1146a, 1146b.

[0144] In some aspects, foreign media 1146a, 1146b can comprise at least one of contaminating media or lubricating media. For example, foreign media 1146a, 1146b can comprise at least one of water, oil, or a polymer.

[0145] In some aspects, wettable surface structures 1144 can be an alternative aspect of surface structures 508 discussed in reference to FIG. 5, such that wettable surface structures 1144 are arranged with a random or periodic distribution to tune a wettability characteristic of optical elements 1106a, 1106b to control foreign media 1146a, 1146b. The wettability characteristic can include at least one of superhydrophobic, superhydrophilic, superoleophobic, superoleophilic, superpolymphobic, or superpolymphilic characteristics. In some aspects, wettable surface structures 1144 can be arranged with a distribution such that foreign media 1146a, 1146b can slide along the peaks of wettable surface structures 1144. In some aspects, wettable surface structures 1144 can be arranged with a distribution such that foreign media 1146a, 1146b can run through wicking capillary action channels in between wettable surface structures 1144. Therefore, wettable surface structures 1144 can improve the wetting properties of the material of optical elements 1106a, 1106b and can allow for efficient cooling, lubrication, or decontamination of metrology system 1110.

[0146] FIG. 12 shows a method 1200 for using an optical element(s) having surface structures within a metrology system, according to some aspects.

[0147] In some aspects, in step 1202 an illumination system can direct a beam of radiation at a target disposed on a substrate.

[0148] In some aspects, in step 1204 at least a portion of the beam of radiation can be received and conditioned at a plurality of surface structures on at least one optical element within a metrology system. At least a portion of the beam of radiation can be received on a receiving surface of the at least one optical element. At least a portion of the beam of radiation can be conditioned by a modified refractive index profile having at least one of anti-reflective, absorptive, or transmissive optical properties. The plurality of surface structures can comprise at least one of nanopillars, nano-hemispheres, or submicron- scale cones arranged with a random or periodic distribution. In some aspects, the plurality of surface structures can be arranged with a random or periodic distribution to tune a wettability characteristic of the at least one optical element to control at least one of contaminating media or lubricating media.

[0149] In some aspects, in step 1206 a scattered radiation can be received from the target at a detector.

[0150] In some aspects, in step 1208 a measurement signal can be generated with the detector based on the scattered radiation from the target.

[0151] In some aspects, in step 1210 a characteristic of the target can be determined by a controller based on the measurement signal.

[0152] The method steps of FIG. 12 can be performed in any conceivable order and it is not required that all steps be performed. Moreover, the method steps of FIG. 12 described above merely reflect an example of steps and are not limiting. That is, further method steps and functions are envisaged based aspects described in reference to FIGS. 1-11.

[0153] The embodiments may further be described using the following clauses:1. A metrology system comprising: an illumination system configured to direct a beam of radiation at a target disposed on a substrate; at least one optical element having a plurality of surface structures comprising at least one of nanopillars, nano-hemispheres, or submicron- sc ale cones, the plurality of surface structures arranged with a random or periodic distribution to produce a modified refractive index profile configured to receive and condition at least a portion of the beam of radiation; a detector configured to receive radiation scattered by the target and to generate a measurement signal based on the received radiation; and a controller coupled to the illumination system and the detector, the controller configured to control a characteristic of the beam of radiation and to determine a characteristic of the target based on the measurement signal.2. The metrology system of clause 1 , wherein the plurality of surface structures are arranged with a random or periodic distribution on a receiving surface of the at least one optical element configured to receive the at least a portion of the beam of radiation.3. The metrology system of clause 1, wherein the at least one optical element comprises an aperture and a surface surrounding the aperture and having the plurality of surface structures disposed on the surface.4. The metrology system of clause 1, wherein the at least one optical element comprises at least one interior wall of the metrology system.5. The metrology system of clause 1, wherein the at least one optical element comprises at least one mounting device configured to support another optical element in the metrology system.6. The metrology system of clause 1 , wherein the modified refractive index profile is configured to produce at least one of anti-reflective, absorptive, or transmissive optical properties of the at least one optical element.7. The metrology system of clause 1, wherein the plurality of surface structures are arranged with a random or periodic distribution to tune a wettability characteristic of the at least one optical element to control at least one of contaminating media or lubricating media.8. The metrology system of clause 1, wherein the at least one optical element comprises at least one of glass, plastic, ceramic, or stainless steel.9. A lithographic apparatus comprising: a metrology system comprising: an illumination system configured to direct a beam of radiation at a target disposed on a substrate; at least one optical element having a plurality of surface structures comprising at least one of nanopillars, nano-hemispheres, or submicron- sc ale cones, the plurality of surface structures arranged with a random or periodic distribution to produce a modified refractive index profile configured to receive and condition at least a portion of the beam of radiation; a detector configured to receive radiation scattered by the target and to generate a measurement signal based on the received radiation; and a controller coupled to the illumination system and the detector, the controller configured to control a characteristic of the beam of radiation and to determine a characteristic of the target based on the measurement signal.10. The lithographic apparatus of clause 9, wherein the plurality of surface structures are arranged with a random or periodic distribution on a receiving surface of the at least one optical element configured to receive the at least a portion of the beam of radiation.11. The lithographic apparatus of clause 9, wherein the at least one optical element comprises an aperture and a surface surrounding the aperture and having the plurality of surface structures disposed on the surface.12. The lithographic apparatus of clause 9, wherein the at least one optical element comprises at least one interior wall of the metrology system.13. The lithographic apparatus of clause 9, wherein the at least one optical element comprises at least one mounting device configured to support another optical element in the metrology system.14. The lithographic apparatus of clause 9, wherein the modified refractive index profile is configured to produce at least one of anti-reflective, absorptive, or transmissive optical properties of the at least one optical element.15. The lithographic apparatus of clause 9, wherein the plurality of surface structures are arranged with a random or periodic distribution to tune a wettability characteristic of the at least one optical element to control at least one of contaminating media or lubricating media.16. The lithographic apparatus of clause 9, wherein the at least one optical element comprises at least one of glass, plastic, ceramic, or stainless steel.17. A method comprising : directing, with an illumination system, a beam of radiation at a target disposed on a substrate; receiving and conditioning at least a portion of the beam of radiation at a plurality of surface structures on at least one optical element within a metrology system, the plurality of surface structures comprising at least one of nanopillars, nano-hemispheres, or submicron- scale cones arranged with a random or periodic distribution; receiving a scattered radiation from the target at a detector; generating a measurement signal with the detector based on the scattered radiation from the target; and determining a characteristic of the target based on the measurement signal.18. The method of clause 17, wherein the receiving the at least a portion of the beam of radiation comprises receiving the at least a portion of the beam of radiation on a receiving surface of the at least one optical element.19. The method of clause 17, wherein the conditioning the at least a portion of the beam of radiation comprises imparting a phase shift to the at least a portion of the beam of radiation by a modified refractive index profile having at least one of anti-reflective, absorptive, or transmissive optical properties.20. The method of clause 17, further comprising arranging the plurality of surface structures with a random or periodic distribution to tune a wettability characteristic of the at least one optical element to control at least one of contaminating media or lubricating media.

[0154] The terms “radiation,” “beam,” “light,” “illumination,” or the like can be used herein to refer to one or more types of electromagnetic radiation, for example, ultraviolet (UV) radiation (for example, having a wavelength X of 365, 248, 193, 157 or 126 nm), extreme ultraviolet (EUV or soft X-ray) radiation (for example, having a wavelength in the range of 5-100 nm such as, for example, 13.5 nm), or hard X-ray working at less than 5 nm, as well as particle beams, such as ion beams or electron beams. Generally, radiation having wavelengths between about 400 to about 700 nm is considered visible radiation; radiation having wavelengths between about 780-3000 nm (or larger) is considered IR radiation. UV refers to radiation with wavelengths of approximately 100-400 nm. Within lithography, the term “UV” also applies to the wavelengths that can be produced by a mercury discharge lamp: G- line 436 nm; H-line 405 nm; and / or, I-line 365 nm. Vacuum UV, or VUV (i.e., UV absorbed by gas), refers to radiation having a wavelength of approximately 100-200 nm. Deep UV (DUV) generally refers to radiation having wavelengths ranging from 126 nm to 428 nm, and in some aspects, an excimer laser can generate DUV radiation used within a lithographic apparatus. It should be appreciated that radiationhaving a wavelength in the range of, for example, 5-20 nm relates to radiation with a certain wavelength band, of which at least part is in the range of 5-20 nm.

[0155] Although some aspects of the present disclosure are described in the context of lithographic apparatuses in the manufacture of ICs, it should be understood that lithographic apparatuses described herein can be used in other applications, for example, in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, LCDs, thin-film magnetic heads, etc. Those skilled in the art will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein can be considered as specific examples of the more general terms “substrate” or “target portion”, respectively. A substrate can be processed before or after exposure in, for example, a track unit (a tool that typically applies a layer of resist to a substrate and develops the exposed resist) and / or a metrology unit. Where applicable, aspects disclosed herein can be applied to such and other substrate processing tools. Furthermore, a substrate can be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein can also refer to a substrate that already contains multiple processed layers.

[0156] Furthermore, although some aspects of the present disclosure are described in the context of optical lithography, it should be understood that aspects of the present disclosure are not limited to optical lithography. For example, in imprint lithography, a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device can be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist leaving a pattern in it after the resist is cured.

[0157] It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by those skilled in relevant art(s) in light of the teachings herein.

[0158] The present disclosure has been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. The foregoing description of specific aspects will so fully reveal the general nature of the present disclosure that others can, by applying knowledge within the skill of the art, readily modify and / or adapt for various applications such specific aspects, without undue experimentation and without departing from the general concept of the present disclosure. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed aspects, based on the teaching and guidance presented herein.

[0159] It is to be understood that the Detailed Description section, and not the Summary and Abstract sections, is intended to be used to interpret the claims. The Summary and Abstract sections can set forth one or more, but not necessarily all, aspects of the present disclosure as contemplated by the inventor(s),and thus, are not intended to limit the present disclosure and the appended claims in any way. The breadth and scope of the protected subject matter should not be limited by any of the above-described aspects, but should be defined in accordance with the following claims and their equivalents.

Claims

CLAIMS1. A metrology system comprising: an illumination system configured to direct a beam of radiation at a target disposed on a substrate; at least one optical element having a plurality of surface structures comprising at least one of nanopillars, nano-hemispheres, or submicron-scale cones, the plurality of surface structures arranged with a random or periodic distribution to produce a modified refractive index profile configured to receive and condition at least a portion of the beam of radiation; a detector configured to receive radiation scattered by the target and to generate a measurement signal based on the received radiation; and a controller coupled to the illumination system and the detector, the controller configured to control a characteristic of the beam of radiation and to determine a characteristic of the target based on the measurement signal.

2. The metrology system of claim 1, wherein the plurality of surface structures are arranged with a random or periodic distribution on a receiving surface of the at least one optical element configured to receive the at least a portion of the beam of radiation.

3. The metrology system of claim 1, wherein the at least one optical element comprises an aperture and a surface surrounding the aperture and having the plurality of surface structures disposed on the surface.

4. The metrology system of claim 1, wherein the at least one optical element comprises at least one interior wall of the metrology system.

5. The metrology system of claim 1, wherein the at least one optical element comprises at least one mounting device configured to support another optical element in the metrology system.

6. The metrology system of claim 1, wherein the modified refractive index profile is configured to produce at least one of anti-reflective, absorptive, or transmissive optical properties of the at least one optical element.

7. The metrology system of claim 1, wherein the plurality of surface structures are arranged with a random or periodic distribution to tune a wettability characteristic of the at least one optical element to control at least one of contaminating media or lubricating media.

8. The metrology system of claim 1, wherein the at least one optical element comprises at least one of glass, plastic, ceramic, or stainless steel.

9. A lithographic apparatus comprising: a metrology system comprising: an illumination system configured to direct a beam of radiation at a target disposed on a substrate; at least one optical element having a plurality of surface structures comprising at least one of nanopillars, nano-hemispheres, or submicron-scale cones, the plurality of surface structures arranged with a random or periodic distribution to produce a modified refractive index profile configured to receive and condition at least a portion of the beam of radiation; a detector configured to receive radiation scattered by the target and to generate a measurement signal based on the received radiation; and a controller coupled to the illumination system and the detector, the controller configured to control a characteristic of the beam of radiation and to determine a characteristic of the target based on the measurement signal.

10. The lithographic apparatus of claim 9, wherein the plurality of surface structures are arranged with a random or periodic distribution on a receiving surface of the at least one optical element configured to receive the at least a portion of the beam of radiation.

11. The lithographic apparatus of claim 9, wherein the at least one optical element comprises an aperture and a surface surrounding the aperture and having the plurality of surface structures disposed on the surface.

12. The lithographic apparatus of claim 9, wherein the at least one optical element comprises at least one interior wall of the metrology system.

13. The lithographic apparatus of claim 9, wherein the at least one optical element comprises at least one mounting device configured to support another optical element in the metrology system.

14. The lithographic apparatus of claim 9, wherein the modified refractive index profile is configured to produce at least one of anti-reflective, absorptive, or transmissive optical properties of the at least one optical element.

15. The lithographic apparatus of claim 9, wherein the plurality of surface structures are arranged with a random or periodic distribution to tune a wettability characteristic of the at least one optical element to control at least one of contaminating media or lubricating media.

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