Optical etendue matching method and system for extreme ultraviolet metrology

Abstract

An optical characterization system is disclosed. The optical characterization system may include a synchrotron light source, an optical characterization subsystem, and a sensor configured to receive a projected image from a set of imaging optics. The optical characterization subsystem may include at least a set of illumination optics, a set of imaging optics, and a diffractive optical element, a temporal modulator, or an optical waveguide configured to align the etendue of a light beam output from the synchrotron light source to the set of illumination optics. A method for aligning the etendue of a light beam is also disclosed.

Description

[Technical field]

[0001] The present disclosure relates generally to optical characterization of wafers and masks, and more particularly to a semiconductor wafer inspection system, actinic EUV mask inspection system or actinic mask review system with a synchrotron radiation source. [Background technology]

[0002] (CROSS REFERENCE TO RELATED APPLICATIONS) This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application No. 62 / 854,785, entitled “OPTICAL ETENDUE MATCHING METHODS FOR EUV METROLOGY,” filed May 30, 2019, inventors Zefram D. Marks, Larissa Juschkin, and Daniel C. Wack, which is incorporated by reference in its entirety.

[0003] Plasma-based light sources are commonly implemented to generate extreme ultraviolet (EUV) and vacuum ultraviolet (VUV) light for wafer and mask inspection systems. For example, discharge-produced plasma (DPP) and laser-produced plasma (LPP) are traditional components in EUV illumination source technology.

[0004] DPP and LPP based sources have several drawbacks, such as the need for time-multiplexing of multiple sources to achieve higher levels of in-band EUV brightness, particularly in technologies implementing DPP sources, which limits inspection throughput while simultaneously increasing cost and complexity.

[0005] Furthermore, the spectral output in DPP and LPP sources cannot be directly controlled in design or operation, and the pulse repetition frequency in LPP and DPP systems is limited by several physical constraints, resulting in significant pulse-to-pulse energy variations. Furthermore, surfaces and materials in close proximity to the plasma of an LPP or DPP source are typically exposed to high-energy neutral and ion fluxes, which sputter and spall the material. Therefore, debris and contamination mitigation is commonly required to protect associated downstream optics and masks.

[0006] US Patent No. 5,399,633, the entirety of which is incorporated herein by reference, discloses a synchrotron apparatus, an alternative illumination source that overcomes the above-mentioned deficiencies. The synchrotron apparatus includes an electron beam generator, an electron storage ring, and a wiggler. The wiggler generates a periodic alternating magnetic field using multiple magnetic poles. To generate radiation, the alternating magnetic field accelerates the electrons in a direction perpendicular to their direction of travel.

[0007] Due to the low emittance of the electron beam into the wiggler, the etendue of EUV light emitted from a typical synchrotron can be orders of magnitude lower than the etendue of the illumination optics of a mask or wafer inspection system, and therefore the angular spread of the beam must be large to fill the illumination pupil of the mask or wafer inspection system while still maintaining a sufficiently large field of view required for high throughput.

[0008] US Patent No. 5,399,633 describes a method for increasing the EUV etendue output of a synchrotron wiggler. Using adaptive electron optics, the emittance of the electron beam can be manipulated to increase the etendue of the emitted EUV. The electron beam (or its trajectory) can be manipulated to increase the electron beam size (or the beam size averaged within the detector response time) when entering the wiggler and to decrease the electron beam size when entering the synchrotron track again. This manipulation can be done, for example, by increasing the spatial spread of the electron beam passing through the wiggler or by steering the electron beam at high frequency to achieve the desired time-averaged beam spatial characteristics. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] U.S. Pat. No. 8,941,336 Summary of the Invention [Problem to be solved by the invention]

[0010] Adaptive electron optics, while effective, are costly and complex to implement. Adaptive electron optics suffer from aberrations as the beam size increases, even at much smaller aperture sizes than photo-optics. To match the etendue of the illumination optics of the wafer / mask inspection system, the emittance of the electron beam must be increased by orders of magnitude. This requires a commensurate increase in the size of the electron beam. As the beam size increases, it becomes very difficult to fabricate accelerator and wiggler channels with the appropriate electric and magnetic fields. The additional electron optics require precision electrostatic or magnetic assembly, increasing system complexity, cost, and introducing reliability risks.

[0011] Thus, alternatives that address the shortcomings of previous solutions are desirable. [Means for solving the problem]

[0012] An optical characterization system is disclosed in accordance with one or more exemplary embodiments of the present disclosure. In one exemplary embodiment, the optical characterization system comprises a synchrotron light source. In another exemplary embodiment, the optical characterization system comprises an optical characterization subsystem including at least one set of illumination optics, a set of imaging optics, and a diffractive optical element configured to align an etendue of a light beam output by the synchrotron light source to the set of illumination optics. In another exemplary embodiment, the optical characterization system comprises a sensor configured to receive a projected image from the set of imaging optics.

[0013] An optical characterization system is disclosed in accordance with one or more exemplary embodiments of the present disclosure. In one exemplary embodiment, the optical characterization system comprises a synchrotron light source. In another exemplary embodiment, the optical characterization system comprises an optical characterization subsystem including at least one set of illumination optics, a set of imaging optics, and a temporal modulator configured to align an etendue of a light beam output by the synchrotron light source to the set of illumination optics. In another exemplary embodiment, the optical characterization system comprises a sensor configured to receive a projected image from the set of imaging optics.

[0014] In accordance with one or more exemplary embodiments of the present disclosure, a method for matching the etendue of a light beam in an optical characterization system is disclosed. In one exemplary embodiment, the method includes generating the light beam using a synchrotron light source. In another exemplary embodiment, the method includes directing the light beam to a diffractive optical element or a time modulator. In another exemplary embodiment, the method includes using the diffractive optical element or the time modulator to increase the etendue of the light beam to match a set of illumination optics.

[0015] An optical characterization system is disclosed in accordance with one or more exemplary embodiments of the present disclosure. In one exemplary embodiment, the optical characterization system comprises a synchrotron light source. In another exemplary embodiment, the optical characterization system comprises an optical characterization subsystem including at least one set of illumination optics, a set of imaging optics, and a hollow optical waveguide configured to align an etendue of a light beam output by the synchrotron light source to the set of illumination optics. In another exemplary embodiment, the optical characterization system comprises a sensor configured to receive a projected image from the set of imaging optics.

[0016] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not necessarily restrictive of the invention as claimed. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the general description, serve to explain the principles of the invention.

[0017] The many advantages of the present disclosure may be better understood by those skilled in the art by reference to the following accompanying drawings. [Brief description of the drawings]

[0018] [Figure 1A] FIG. 1 is a simplified schematic diagram of a synchrotron radiation source having a wiggler insertion source, in accordance with one or more embodiments of the present disclosure. [Figure 1B] FIG. 1 is a simplified schematic diagram of a racetrack storage ring in combination with a racetrack microtron device in accordance with one or more embodiments of the present disclosure. [Diagram 2] FIG. 1 is a schematic diagram of a multi-pole wiggler insertion source in accordance with one or more embodiments of the present disclosure. [Diagram 3] FIG. 1 is a block diagram of two optical characterization subsystems with a wiggler synchrotron radiation source in accordance with one or more embodiments of the present disclosure. [Figure 4]FIG. 1 is a block diagram of four optical characterization subsystems with a wiggler synchrotron radiation source in accordance with one or more embodiments of the present disclosure. [Diagram 5] FIG. 2 is a scanning electron micrograph (SEM) image of a block copolymer surface, according to one or more embodiments of the present disclosure. [Figure 6] FIG. 2 is a scanning electron micrograph (SEM) image of a random nanoparticle surface, according to one or more embodiments of the present disclosure. [Figure 7] FIG. 2 is a diagram of a diffractive optical element configured for etendue matching in accordance with one or more embodiments of the present disclosure. [Figure 8] 1 illustrates a diffractive optical element including a diffractive material patterned on a transmissive film in accordance with one or more embodiments of the present disclosure. [Figure 9] FIG. 1 illustrates a beam diffuser having a Gaussian scattering distribution illuminated with a zero divergence beam in accordance with one or more embodiments of the present disclosure. [Figure 10] FIG. 10 illustrates the calculated beam divergence after the beam diffuser of FIG. 9 is illuminated, in accordance with one or more embodiments of the present disclosure. [Figure 11] FIG. 1 illustrates a grazing incidence mirror configured for speckle destruction in accordance with one or more embodiments of the present disclosure. [Figure 12] FIG. 1 illustrates a reflective surface configured for speckle disruption in accordance with one or more embodiments of the present disclosure. [Figure 13A] FIG. 1 illustrates an actuatable steering mirror configured for etendue matching in accordance with one or more embodiments of the present disclosure. [Figure 13B] 13B illustrates the path of a light beam reflected by the mirror shown in FIG. 13A in accordance with one or more embodiments of the present disclosure. [Figure 14] FIG. 1 is a schematic diagram of a beam homogenizer in accordance with one or more embodiments of the present disclosure. [Figure 15] 1 is a flowchart illustrating a method for matching the etendue of a light beam in an optical characterization system in accordance with one or more embodiments of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not necessarily restrictive of the invention as claimed. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the general description, serve to explain the principles of the invention. Reference will now be made in detail to the disclosed subject matter which is illustrated in the accompanying drawings.

[0020] Synchrotron sources are very bright and deliver enough radiation power to sustain some inspection or metrology systems. However, the combination of low etendue and corresponding high brightness can result in an instantaneous sample irradiance that is too high. This high irradiance can be undesirable as it can damage the mask or wafer being inspected or characterized. In a typical inspection system, the large etendue (field size × angular spread) means that for the same throughput, pixel exposure times are longer and the instantaneous peak sample irradiance is reduced (e.g., well below the damage threshold).

[0021] To overcome the deficiencies of adaptive electron optics, the optics of the EUV light output by the synchrotron may be directly manipulated to increase the etendue of the light and match it to the etendue of the illumination optics of the wafer or mask inspection system. This direct manipulation of the light beam may be performed by diffractive or scattering optics and / or time modulators.

[0022] Optical etendue matching on EUV optics allows the use of synchrotron light sources in inspection systems. Etendue matching elements can also serve the function of homogenizing the illumination or generating arbitrary pupil patterns. The use of purely adaptive electron optics requires complex magnetic designs and increased magnetic fields. Using optical techniques to modify the angular spread of the EUV light in one or more dimensions can reduce this complexity. The ease of beam delivery of low etendue "laser-like" radiation is advantageous because beam distribution between inspection or metrology systems can be realized using smaller size and cheaper optical elements.

[0023] There are several challenges in designing optical elements for etendue matching in the EUV spectral range, including (a) the high absorption of EUV light in many materials, (2) the lack of refractive lenses for EUV light, and (3) the short wavelength of EUV light, which requires high surface and structural quality and high manufacturing costs. As a result, most of the solutions applied in the visible spectral range are not applicable or directly transferable to the EUV spectral range. Thus, the present disclosure provides the ability to manipulate the etendue of a light beam in the EUV spectral range.

[0024] 1A-2, a wiggler-based synchrotron radiation source 100 suitable for serving as an illumination source for a semiconductor wafer optical characterization system or a mask optical characterization system is described in accordance with the present disclosure.

[0025] Synchrotron radiation results from highly relativistic electron acceleration, such as that of electrons in a magnetic field. Due to size and power limitations within the spectral bandpass of EUV lithography (e.g., about 1-2% at λ=13.5 nm), it has generally proven impractical to use synchrotron sources in EUV-based semiconductor manufacturing processes. However, the unique requirements of broadband semiconductor wafer inspection and narrowband actinic EUV mask inspection (as well as mask metrology [e.g., actinic mask review or EUV reflectometry]) provide a good match between an optimized synchrotron radiation source and the inspection / metrology system.

[0026] The present disclosure provides a compact synchrotron radiation source with one or more wiggler-type insertion sources suitable for delivering light (e.g., narrowband or broadband light) to an optical input (e.g., illumination optics) of a wafer inspection system, an actinic EUV mask inspection (AMI) system, an actinic mask review (AMR) system, a mask reflectometer system, and the like. Additionally, the present disclosure provides a solution for improving the optical coupling (e.g., etendue matching) between a synchrotron radiation source and an optical subsystem of an optical characterization system. This improved optical coupling can be achieved by directly manipulating the light output by the synchrotron to increase the etendue of the light (e.g., using diffractive optical elements and / or temporal modulators) and / or by manipulating an electron beam to indirectly increase the etendue of the light.

[0027] In this regard, embodiments of the present disclosure provide an "étendue match" between a synchrotron radiation source and the optical input of an inspection / metrology system to maximize utilization of the light source and reduce technical requirements of the synchrotron itself. In this manner, the etendue of the light emitted by a wiggler-based synchrotron radiation source can be matched, within acceptable tolerance levels, to the optical input of at least one of the disclosed wafer optical characterization systems or mask optical characterization systems.

[0028] The present disclosure provides a broadband, selectable and scalable emission distribution whereby narrowband illumination can be selected for actinic EUV mask inspection (or EUV metrology) and broadband illumination can be utilized for broadband wafer inspection. The wavelength bands can be selected via various filtering elements (e.g., spectral filters, diffraction gratings, etc.) positioned between the photon beam output of a wiggler synchrotron light source and an optical input, such as the input of the illumination optics of a given optical characterization system.

[0029] Additionally, the present disclosure is directed to the implementation of multiple optical characterization systems optically coupled to a single compact wiggler-based synchrotron light source. For purposes of this disclosure, an optical characterization system should be interpreted as any optical characterization or measurement system suitable for inspecting, reviewing, characterizing or measuring one or more features of a semiconductor wafer or a lithography mask. For example, the optical characterization system may be, but is not limited to, an actinic EUV mask inspection system, a wafer inspection system, or an EUV metrology system such as an actinic mask review system or an EUV mask reflectometer.

[0030] FIG. 1A shows a simplified schematic diagram of a synchrotron radiation source 100 equipped with a wiggler, according to one or more embodiments of the present disclosure. The synchrotron radiation source 100 may include an electron beam generator 104 suitable for generating one or more beams 105 of electrons. The electron beam generator 104 may include an electron gun 105 configured to generate electrons, and an electron accelerator 107. The electron gun 105 may be operatively coupled to the electron accelerator 107 such that electrons generated by the electron gun are injected into the accelerator 107 for acceleration. The electron accelerator 107 may include, but is not limited to, a LINAC device (i.e., a linear electron accelerator). The electron accelerator 107 may also include a miniature microtron device. As shown in FIG. 1B, the miniature microtron device 114 may accelerate electrons utilizing a racetrack-shaped modified linear accelerator 116 enabled via a pair of D-shaped magnets 118 (e.g., D-shaped electromagnets). In this regard, the electron gun 105 may emit electrons into the microtron device 114. The electrons are then accelerated via the linear accelerator 116 as they traverse a "racetrack" path in the microtron device 114. In some embodiments, the plane of the microtron device may be horizontal or vertical. In some embodiments, the microtron device plane may be positioned to be coplanar with the storage ring 101.

[0031] Referring again to FIG. 1A, according to some embodiments of the present disclosure, the radiation source 100 of the present disclosure may include a compact electron storage ring 101. In some embodiments, the electron storage ring 101 of the radiation source 100 may include, but is not limited to, a plurality of magnets configured to bend the trajectory of electrons emitted from the electron beam generator 104 into a closed path around the storage ring 101. For example, as shown in FIG. 1A, when electrons 105 are injected into the storage ring, the electrons 105 may circulate around a closed path 103 around the storage ring 101. In some embodiments, the curved magnet 106 may include, but is not limited to, one or more permanent magnets. In some embodiments, the curved magnet 106 may include, but is not limited to, one or more electromagnets. In some embodiments, the curved magnet 106 may include, but is not limited to, one or more superconducting magnets.

[0032] In some embodiments, the plane of the electron storage ring 101 may be oriented horizontally or vertically. In further embodiments, the microtron device plane 114 may be oriented coplanar with the storage ring 101.

[0033] In some embodiments, synchrotron radiation source 100 includes a radio frequency (RF) system 110 configured to provide energy to electrons as they circulate around electron storage ring 101. RF system 110 may include any RF system known in the art.

[0034] In some embodiments, radiation source 100 includes one or more wiggler insertion sources 108. In some embodiments, each wiggler insertion source 108 of radiation source 100 is positioned along a linear portion of electron storage ring 101, as depicted in FIG. 1A.

[0035] 2 shows a schematic diagram of a wiggler insertion light source 108 according to one or more embodiments of the present disclosure. In some embodiments, one or more of the wiggler insertion light sources 108 of the radiation source 100 include a plurality of magnetic poles adapted to generate a periodic alternating magnetic field for generating a light beam 112 emitted along an electron traveling direction 113. The one or more of the wiggler insertion light sources 108 generate the light beam 112 along the traveling direction by periodically accelerating electrons passing through a gap 115 of a given wiggler insertion light source 108 (e.g., a gap between an upper portion and a lower portion of the wiggler insertion light source 108) along a direction perpendicular to the traveling direction.

[0036] In some embodiments, the one or more wiggler insertion light source 108 includes a set of main magnetic poles 202 and a set of soft magnetic poles 206. In further embodiments, each of the set of soft magnetic magnets 206 is interposed between each adjacent pair of the set of main magnetic poles 202. In some embodiments, the main magnetic poles 202 are formed from any permanent magnetic material known in the art. In some embodiments, the soft magnetic poles 206 may be formed from any soft magnetic material known in the art. For example, the soft magnetic magnets 206 may include a cobalt-iron based magnetic alloy, such as, but not limited to, PERMENDUR. In further embodiments, the one or more wiggler insertion light source 108 includes a set of end magnetic poles 204. In some embodiments, the set of end magnetic poles 204 may be formed from a permanent magnetic material.

[0037] It is noted herein that the compact features of the synchrotron radiation source 100 of the present disclosure are enabled in part through a compact wiggler design and / or a compact electron storage ring design. In some embodiments, for a permanent magnet (PM) based wiggler insertion source, the size of the PM wiggler insertion source may be fabricated to have a length of about 1-3 m. It is further recognized that the size required for a superconducting magnet (SM) wiggler insertion source suitable for implementing the present disclosure may be shorter than a PM based wiggler insertion source.

[0038] In some embodiments, the compact design of the synchrotron light source 100 of the present disclosure is made possible by various design characteristics of the electron storage ring 101. In some embodiments, the storage ring 101 of the synchrotron light source 100 may be configured to generate an electron beam having a "low" energy substantially between 100-600 MeV and a small amount of current of about 100-900 mA. It is noted herein that the choice of low energy of the electron storage ring design allows for a compact electron ring 101 and accelerator 107. In some embodiments, a bending radius of about 0.5 m may allow for a compact design of the storage ring 101.

[0039] In some embodiments, the synchrotron light source 100 may include a set of adaptive electron-optical elements 109a-b configured to manipulate one or more spatial properties of the electron beam as it passes through one or more wiggler insertion sources 108. It is noted herein that the etendue of the light 112 output from one or more wiggler devices is a function of the spatial spread of the electron beam (as it passes through one or more wiggler insertion sources 108) and the angular spread of the photon beam caused by the action of one or more wiggler insertion sources 108 on the electron beam. Thus, by manipulating the spatial spread of the electron beam, the etendue of the light beam 112 may be controlled. In some embodiments, etendue-matching optical elements (e.g., diffractive optical elements and / or time modulators) may control the etendue of the light beam 112 after it exits the synchrotron light source 100, in addition to or without the adaptive electron-optical elements 109a-b.

[0040] In some embodiments, a set of adaptive electron-optical elements 109a-b may be configured to match the etendue of the light beam 112 emitted by one or more wiggler insertion sources 108 to the light input (e.g., illumination optics) of an associated optical characterization system by adjusting the spatial extent of the electron beam of the electron storage ring 101 within the one or more wiggler insertion sources 108.

[0041] The adaptive electron-optical elements 109a-b may be arranged at both ends of the one or more wiggler insertion light sources 108. In a further embodiment, the adaptive electron-optical elements 109a-109b may include a pair of entrance optics 109a and exit optics 109b arranged respectively before and after the one or more wiggler insertion light sources 108. It is mentioned again here that the expansion of the spatial spread of the electron beam as the electrons pass through the one or more wiggler insertion light sources 108 may cause an increase in the etendue of the light beam 112 emitted by the one or more wiggler insertion light sources 108. In this regard, the spatial spread of the electron beam may be expanded by the entrance optics 109a so that the etendue of the light beam 112 emitted by the one or more wiggler insertion light sources 108 matches the input of one or more illumination optics of the optical characterization subsystem.

[0042] The one or more exit electron optical elements 109b can then collapse the spatial extent of the electron beam upon exiting the one or more wiggler devices 108. In some embodiments, the one or more exit electron optical elements 109b may collapse the spatial extent of the exit electron beam so that it has a stable orbit within the electron storage ring. In some embodiments, the one or more exit electron optical elements 109b may collapse the spatial extent of the exit electron beam so that it has a spatial extent substantially similar to the electron beam before entering the one or more wiggler insertion light source 108. It is noted herein that it is not a requirement of the present disclosure that the exit spatial extent of the electron beam substantially matches the spatial extent of the entrance spatial extent.

[0043] 3 illustrates a block diagram of two optical characterization subsystems 310a-b with a wiggler synchrotron radiation source 100 in accordance with one or more embodiments of the present disclosure. Each optical characterization subsystem 310a-b may include a respective etendue matching optics 312a-b, illumination optics 314a-b, mask or wafer 316a-b, imaging optics 318-b, and sensor 320a-b, respectively.

[0044] After the light beam 112 exits the wiggler synchrotron 100, the beam delivery optics 301 may distribute the light beam 112 to the optical characterization subsystems 310a-b. The beam delivery optics 301 may include any beam delivery optics known in the art. For example, the beam delivery optics 301 may include, but are not limited to, beam steering optics (e.g., mirrors, beam splitters, lenses, etc.) configured to direct the light beam 112 from the wiggler 108 of the synchrotron light source 100 to the etendue matching optics 312a-b. For example, the beam steering optics of the beam delivery optics 301 may include a set of relay mirrors suitable for directing light from the synchrotron light source 100 to the etendue matching optics 312a-b.

[0045] As another example, the beam delivery optics 301 may include spectral filtering optics, such as, but not limited to, one or more gratings configured to transmit or direct a selected wavelength band of the light beam 112 from the wiggler 108 to the etendue matching optics 312a-b. Due to the narrow band requirements of an actinic EUV mask inspection system (or actinic mask review system), the spectral filtering optics or gratings of the beam delivery optics 301 may be used to selectively filter the broadband illumination from the wiggler 108 of the light source 100 such that only a selected wavelength band (e.g., 13.5 nm EUV) is provided to the illumination optics 314a-b of the mask inspection system 310.

[0046] In another embodiment, the spectral filtering optics or diffraction gratings of the beam delivery optics 301 may be used to select the centroid wavelength to transmit to the illumination optics 314a-b. The use of centroid wavelengths in the inspection (or review) process may lead to improved accuracy of defect printability estimation. In one aspect, careful selection of the inspection or review wavelength for image acquisition, coupled with computational models for "wafer-side or mask-side" review, may lead to simpler and more accurate estimation of defect printing. In another aspect, tailoring the illumination used in the inspection (or review) process to a broader range, yet within the spectral bandwidth of the multi-layered surfaces of the components of the optical subsystem 310a-b coupled with photon and electron detectors or spectrometers, provides additional information (e.g., compositional information) to the defect classification process.

[0047] Within each of the optical characterization subsystems 310a-b, the low etendue light beam 112 is sent through a respective etendue matching optic 312a-b and then fills the pupil of a respective illumination optic 314a-b. The illumination optic 314a-b may expose a respective mask or wafer 316a-b under inspection or characterization, and the reflected photon beam 112 may be collected by a respective imaging optic 318a-b and imaged to a respective sensor 320a-b.

[0048] The etendue matching optical elements 312a-b may, in cooperation with or in the absence of the adaptive electron-optical elements 109a-b, match the etendue of the light beam 112 to the etendue of the respective illumination optical elements 314a-b (e.g., by increasing the etendue of the light beam 112). The etendue matching optical elements 312a-b may be diffractive optical elements and / or temporal modulators, as will be further described with respect to Figures 5-13.

[0049] The illumination optics 314a-b may direct illumination from the delivery optics 301 to a respective mask or wafer 316a-b disposed on the stage. In one embodiment, the mask or wafer 316a-b may include, but is not limited to, a reflective mask. For example, the mask or wafer 316a-b may include a reflective mask having a patterned absorbing layer disposed on a resonantly reflective substrate (e.g., a Mo-Si multilayer).

[0050] It is noted herein that the illumination optics 314a-b may include any optical components suitable for use in a set of EUV illumination optics. For example, the illumination optics 314a-b may include, but are not limited to, an EUV mirror (e.g., a multi-layer mirror for reflecting EUV light (e.g., 13.5 nm light)) suitable for directing illumination from the beam delivery optics 301 to the mask or wafer 316a-b. In some embodiments, light reflected and / or scattered from one or more regions of the mask or wafer 316a-b is collected by the respective imaging optics 318a-b. The imaging optics 318a-b may then project images of one or more illuminated portions of the respective mask or wafer 316a-b to the respective sensors 320a-b. In some embodiments, the imaging optics 318a-b may include any optical components suitable for use in a set of EUV projection optics. For example, the projection optics may include, but are not limited to, a series of EUV mirrors suitable for projecting an image of one or more portions of the respective mask or wafer 316a-b onto the imaging area of ​​the respective sensor 320a-b.

[0051] In some embodiments, the sensors 320a-b may include any EUV-sensitive sensor known in the art. For example, the sensors 320a-b may include, but are not limited to, CCD sensors. As another example, the sensors 320a-b may include, but are not limited to, TDI-enabled CCD sensors (e.g., TDI-enabled silicon CCD sensors). It is noted herein that the presence of defects in or on the mask or wafer 316a-b may cause changes in the reflection or scattering of photons by the mask or wafer 316a-b, thereby altering one or more portions of the images acquired by the sensors 320a and 320b. It is further noted that the sensitivity of the actinic EUV mask inspection process is linked to the number of photons that interact with a given defect through the statistical variation of the recorded photons per image pixel (i.e., the shot noise effect). Therefore, it is desirable to utilize a high brightness illumination source for the inspection system.

[0052] In some embodiments, one or more computing systems (not shown) may be communicatively coupled to the sensors 320a-b. For example, the one or more computing systems may include one or more computer processors, memory for storing data, and program instructions for directing the one or more processors to perform various data processing steps described throughout this disclosure. In this regard, the one or more processors of the computing system may receive one or more signals from the sensors 320a-b indicative of the inspection data received by the sensors 320a-b. The computing system may then execute one or more processing and analysis routines to process and analyze the detected data for mask (or wafer) inspection (e.g., pattern inspection or defect detection).

[0053] The etendue of the light beam 112 emitted by the multipole wiggler 108 of the synchrotron light source 100 may be controlled in any manner known in the art. In some embodiments, a set of etendue matching optics 312a-b of the subsystems 310a-b may match the etendue of the light beam emitted by the wiggler 108 of the wiggler synchrotron light source 100 with the light input required for the inspection or review process. It is recognized herein that the etendue matching optics 312a-b may help maximize light source utilization while reducing technical demands on the synchrotron light source 100.

[0054] In some embodiments, the etendue of the light beam 112 from the wiggler 108 of the synchrotron light source 100 may be controlled by manipulating the spatial characteristics of the electron beam of the synchrotron 100. In this regard, the phase spatial distribution of the electron beam in the storage ring 101 may be manipulated to control the etendue of the EUV light beam 112 emitted by the synchrotron 100. Manipulation of the phase spatial distribution provides a flexible and loss-free method for filling the illumination pupil of the illumination optics 314a-b of the subsystems 310a-b. In this regard, the etendue of the light beam 112 incident on the illumination optics 314a-b may be set to a selected level through selection of the focusing magnet design of the storage ring 101. For example, the etendue of the light beam 112 incident on the illumination optics 314a-b may be set to a level required for a given inspection system architecture through selection of the focusing magnet design in the storage ring 101. It is noted herein that this capability arises from the fact that the angular distribution of the light beam 112 emitted by the wiggler 108 (e.g., on the order of 1 mrad) is generated by the periodic excursion of the electron beam passing through the wiggler 108. It is further noted that the periodic excursion of the wiggler beam is much larger than the opening angle of the electron beam (e.g., on the order of 1 μrad).

[0055] As previously described, the wiggler synchrotron light source 100 may include a set of adaptive electron-optical elements 109a-b configured to match the etendue of the light beam 112 emitted by one or more wiggler insertion light sources 108 to the respective optical inputs (e.g., the set of illumination optics 314a-b) of the subsystems 310a-b. Again, the adaptive electron-optical elements 109a-b may match the etendue of the light beam 112 to the set of illumination optics 314a-b by adjusting one or more spatial characteristics of the electron beam of the electron storage ring 101. For example, the set of adaptive electron beam optics 109a-b may increase the etendue of the light beam 112 by expanding the spatial spread of the electron beam as it passes through one or more wiggler insertion light sources 108.

[0056] In one embodiment, the entrance optics 109a and exit optics 109b of the adaptive electron optics may include, but are not limited to, a pair of skewed quadrupole magnets positioned respectively before and after one or more wiggler insertion sources 108. In this regard, the skewed quadrupole magnets are suitable for generating an electron beam having a size sufficient to generate a light beam 112 that meets the requirements of subsystems 310a-b.

[0057] In some embodiments, the adaptive electron optics 109a-b may include, but is not limited to, a set of electromagnetic beam modulators positioned before and after one or more wiggler insertion sources 108. In this regard, the set of electromagnetic beam modulators is configured to steer the electron beam at high frequency such that the averaged (within the detector response time) beam spatial characteristics are sufficient to generate a light beam 112 that meets the requirements of the subsystems 310a-b.

[0058] In some embodiments, in the balanced or "natural" state of electron beam emittance, the design of the magnet "grid" of the synchrotron light source 100 can provide the required etendue of the light emitted by the wiggler insertion source 108. The use of the magnet grid of a synchrotron light source to control the etendue is described in David Attwood, Soft X-Rays and Extreme Ultraviolet Radiation, 1st ed, Cambridge University Press, 1999, pp. 172-188, which is incorporated herein by reference in its entirety.

[0059] In some embodiments, the etendue of the light beam from the wiggler 108 of the synchrotron light source 100 may be controlled via the spectral dispersion of the light emitted by the wiggler insertion source 108. The bandwidth required by the actinic EUV mask inspection system (e.g., 1-2%) may be selected from the broadband output of the wiggler 108 via first order diffraction from a diffraction grating. In some embodiments, for a given bandwidth, the angular dispersion of the diffraction grating (not shown) is matched to the entrance (pupil) aperture for the illumination optics 314a-b.

[0060] In some embodiments, the power required for each of the subsystems 310a-b is given by the product of the required brightness of each of the subsystems 310a-b and the etendue of each of the systems 310a-b. The etendue of each of the systems 310a-b is then composed of the product of the acceptance solid angle and the irradiated area of ​​each of the subsystems 310a-b.

[0061] In the context of actinic EUV mask inspection, a given mask is typically raster scanned with only a small area of ​​the given mask being illuminated at a given time. The etendue of actinic EUV mask inspection systems is typically small, leading to a simultaneous realization of high brightness with a small amount of average power for the inspection system. For example, to support a high throughput of 1 hour / mask while using only a few TDI-CCD-based sensors, a typical actinic EUV mask inspection system illumination source requires approximately 100 W / (mm 2 -Sr) brightness. In contrast, Xe-based discharge-produced plasma (DPP) sources have a brightness of about 5-10 W / (mm 2 -Sr), and Xe-based laser-produced plasma (LPP) sources have a usable brightness of about 25 W / (mm 2 -Sr) available brightness. However, the small etendue (e.g., 1-4×10) required by a typical actinic EUV mask inspection system is -3 mm 2 ), the average power requirement of a typical actinic EUV mask inspection system is approximately 0.2-2W.

[0062] It is noted herein that the etendue of a typical actinic EUV mask inspection system is generally orders of magnitude higher than the etendue of a typical synchrotron radiation light beam. It is further noted that the etendue of a synchrotron radiation source is generated by the convolution of the emittance of the electron beam (widely adjustable) and the synchrotron radiation (which has a small angular distribution). Thus, the design of the compact synchrotron source 100 of the present disclosure may be optimized (or at least improved to a sufficient level) for actinic EUV mask inspection (and broadband wafer inspection) through the design of the electron optics of the electron storage ring 101 and the wiggler insertion source 108.

[0063] It is further noted herein that the multi-pole wiggler insertion source 108 of the present disclosure allows for the generation of EUV light in a full period. Thus, the multi-pole wiggler insertion source 108 provides a direct path to increase broadband power in a given optical characterization system (e.g., actinic EUV mask inspection system or wafer inspection system). The in-band EUV power (in watts) for actinic EUV mask inspection (e.g., 2.5% bandwidth at 13.5 nm) from a wiggler insertion source is given as follows:

number

[0064] In the formula, L is the total length of the wiggler (m), and L=NL w where N is the total number of periods and L w is the size of the wiggler period (m). Also, B is the peak magnetic field in the wiggler (T), I is the beam current (A), G1 is the energy dependence coefficient, E is the photon energy (e.g., for 13.5 nm light, the energy is 92 eV), and E c represents the critical energy, and E c =(665eV)·E e 2 Given by B, E e is the electron beam energy in GeV. Wiggler insertion sources are described in Soft X-Rays and Extreme Ultraviolet Radiation, 1st ed, Cambridge University Press, 1999, pp. 172-188, by David Attwood, which is incorporated herein by reference in its entirety.

[0065] Furthermore, in settings where the energy of the storage ring 101 is selected for peak power for actinic EUV mask inspection purposes, significant broadband power at longer wavelengths (e.g., 100-200 nm) suitable for wafer inspection is also produced.

[0066] In one embodiment, a compact low energy wiggler synchrotron light source 100 as described throughout this disclosure may have the following parameters: E e =270MeV, B=2T (permanent magnet level), L=1m, N=6, L w = 16 cm, and I = 200 mA. Based on the above equations, such a set of parameters can yield 0.4 W of EUV in-band power, sufficient for current actinic EUV mask inspection needs. Applicants note that the listed synchrotron source parameters are provided for illustrative purposes only and should not be construed as limitations of the present disclosure.

[0067] In one embodiment, a compact low energy wiggler synchrotron source with the above characteristics may have a beam divergence angle such as 1.9(v)×101(h) mrad. For actinic EUV mask inspection, etendue requirements may require a beam size of approximately 4400(v)×100(h) μm. Applicants note that such beams are typically large by typical synchrotron standards. Such a configuration allows for increased current collection by effectively eliminating the etendue limit of the electron gun, which translates into faster fill times for the storage ring.

[0068] It is described herein that the total radiant power of the wiggler 108 when integrated over all frequencies may be on the order of 10-100 W. It is further recognized herein that the power input to the illumination optics 314a-b of the subsystems 310a-b may be adjusted utilizing various filtering methods. For example, an absorber integrated into the beam delivery optics 301 may be used to adjust the power input to the illumination optics 314a-b. As another example, a diffraction grating may be used to adjust the power input to the illumination optics 314a-b.

[0069] In some embodiments, an undulator (not shown) may be placed in the path of the electron storage ring 101 of the synchrotron light source 100. It is noted herein that in narrow band systems where out-of-band emissions are a concern (e.g., actinic EUV mask inspection systems or actinic mask review systems), an undulator may be utilized within the storage ring 101 of the synchrotron light source 100. Utilizing an undulator within the storage ring 101 may provide improved radiation characteristics. For example, the use of an undulator may provide cleaner, more tunable radiation centered around a desired wavelength emitted by the light source 100.

[0070] In some embodiments, the light beam 112 emitted by the wiggler insertion light source 108 is linearly polarized. It is noted herein that the Brewster angle of EUV light (e.g., λ=13.5 nm) is close to 45°. Therefore, for an unpolarized beam incident on a turning mirror at 45°, the p-polarized light in the incident beam is absorbed. As a result, the angle of incidence of light on a multilayer coated mirror is limited to small values ​​(i.e., a "normal incidence" mirror). In one embodiment, these losses can be avoided for large angles of incidence by orienting the wiggler field to limit the beam turning to a single plane and provide an S-polarized output.

[0071] 4 illustrates a block diagram of a system 400 including multiple optical characterization subsystems 410a-d. Each of the optical characterization subsystems 410a-d may include etendue matching optics 412a-d, illumination optics 414a-d, masks or wafers 416a-d, imaging optics 417a-d, and sensors 420a-d. The components of the optical characterization subsystems 410a-d may be substantially identical in form and function to the similarly named components of the optical characterization subsystems 310a-b described with respect to FIG.

[0072] The optical characterization subsystems 410a-d may be optically coupled to a single wiggler synchrotron radiation source 100 with an electron storage ring 101 disposed below the multiple optical characterization subsystems 410a-d. The electron storage ring 101 of the synchrotron radiation source 100 may be disposed below the fab floor on a sub-fab floor. The beam delivery optics 401a-b may deliver light via pass-throughs 408a-d to respective etendue matching optics 412a-d of each optical characterization subsystem 410a-d. The beam delivery optics 401a-b may be substantially identical in form and function to the beam delivery optics 301 described with respect to FIG.

[0073] It is noted herein that any number of inspection / metrology systems may be implemented within the context of system 410. System 410 may implement any of the various optical characterization subsystems described throughout this disclosure. Additionally, system 410 may simultaneously implement different types of optical characterization subsystems. Each of optical characterization subsystems 410a-d may be a semiconductor wafer inspection system, an actinic EUV mask inspection system, or an actinic mask review system. In one example, subsystems 410a-b are narrowband (e.g., narrowband achieved using spectral filtering) actinic EUV mask inspection systems, subsystem 410c is an EUV mask review system, and subsystem 410d is a broadband wafer inspection system.

[0074] As discussed above, the adaptive electron-optical elements 109a-b in the synchrotron 100 may indirectly manipulate the etendue of the light beam 112 (e.g., by manipulating the electron beam in the synchrotron 100). In some embodiments, the etendue matching optics 312a-b (described with respect to FIG. 3) and the etendue matching optics 412a-d (described with respect to FIG. 4) may directly manipulate the etendue of the light beam 112 in conjunction with the adaptive electron-optical elements 109a-b or in the absence of the adaptive electron-optical elements 109a-b.

[0075] In some embodiments, the etendue matching optical elements 312a-b and 412a-d may include diffractive optical elements (DOEs). Diffractive optical elements (DOEs) are surface structures patterned on a transmissive or reflective optical element. The pattern spatially modulates the phase and / or amplitude of the light waves. The DOEs may be patterned on the substrate of a reflective EUV mirror (normal or oblique incidence), on the multilayer (ML) of a reflective mirror, or on a transmissive thin film. The diffractive pattern may be binary (e.g., the height of the structure depends on the etch depth), grayscale (more than one step), or blazed (triangular shaped), or may exhibit another more complex height profile.

[0076] Possible functional DOEs for etendue matching may include beam diffusers or diffractive optical elements. An example of a transmission mode EUV beam diffuser includes a thin film filter with spin-coated nanoparticles, and an example of a reflection mode EUV beam diffuser includes a mirror with known (suitable) roughness characteristics. Examples of diffractive optical elements include phase zone plates and specially designed scattering holograms with transmission or reflection geometries.

[0077] The diffractive structures may be made in various patterns of binary or variable thickness. The diffractive structures may be absorbing or phase-shifting (and less absorbing). DOEs may be used in transmission mode or in reflection (grazing or normal incidence) mode. Experimentally, the diffraction efficiency of phase gratings in transmission mode can be 40% or more.

[0078] Structured multilayer mirrors may be used as highly efficient diffractive elements in reflective geometry. When a DOE in grazing incidence geometry is used, the natural asymmetry in the x and y directions of the synchrotron output beam (e.g., light beam 112) may be compensated for by the asymmetry of the in-plane and out-of-plane (conical) diffraction / scattering patterns to provide a symmetric pupil fill.

[0079] If the phase-modulating DOE is constructed of a low-absorbing material, the diffraction efficiency in different diffraction orders can be optimized by adjusting the height of the structure to achieve the desired pupil fill. For example, by choosing the height of the structure to provide a phase shift close to π, the zeroth order beam intensity can be significantly reduced and most of the energy can be redirected into the higher diffraction orders.

[0080] In some embodiments, the optical surface of the DOE may be a one-dimensional regular pattern (spatially varying in one direction, e.g., lines or spaces) or a two-dimensional regular pattern (spatially varying in two spatial directions, e.g., dots or other regularly spaced features).

[0081] In some embodiments, the optical surface of the DOE may be a random pattern with a programmed size distribution. The size distribution of features on the surface may be characterized using a power spectral density function. The random pattern may include deposited nanoparticles (monodisperse or polydisperse) of known size distribution. The size distribution, material composition (which determines the phase shift), and density of the nanoparticles may be selected and optimized to achieve the desired pupil fill. The random pattern may be generated by controlling the surface finish, i.e., surface roughening, for example, by chemical or plasma etching or laser ablation. The random (or regular) pattern may be generated by using self-assembly techniques on unpatterned (or pre-patterned) surfaces, for example, ordering of block copolymers or quantum dot growth.

[0082] Figure 5 shows a scanning electron micrograph (SEM) image 500 of a block copolymer surface that can be used as a diffuse diffractive surface in a DOE (SOKim et al., Nature 424, 411

[2003] ). Figure 6 shows an SEM image 600 of a random nanoparticle pattern (quantum dots grown on an unpatterned substrate) that can be used as a diffuse diffractive surface in a DOE (C. Dais, Templated Self-Assembly of SiGe quantum dots, PhD thesis, RWTH Aachen University

[2009] ).

[0083] In some embodiments, the surface of the DOE may be a programmed pattern, called a holographic pattern. A holographic pattern is generated from a desired projected pattern by calculating the inverse of the diffraction pattern. The programmed pattern may be a continuous pattern or a binary or grayscale approximation of the pattern (digital holography). The programmed pattern may be created using lithography.

[0084] In some embodiments, the DOE may be an optical diffuser of the EUV light. The diffuser is designed to uniformly fill the illumination pupil (e.g., illumination optics 314a-b or 414a-d) and match the etendue of the inspection system. The DOE may create any projection pattern. For example, the DOE may project a discrete number of uniform tiled illumination fields with a programmed separation. These tiled illumination fields may illuminate a set of tiled sensors (e.g., sensors 320a-b and 420a-d) in the camera of the inspection system (each field corresponds to a separate detector). The DOE may create any fill pattern to simulate different pupil apertures (e.g., annular, bipolar, etc.) of the illumination system (e.g., subsystems 310a-b and 410a-d).

[0085] In addition to increasing the etendue of the illumination system, a DOE may be used to split an incident beam (e.g., beam 112) into a discrete number of beams in a regular pattern. A DOE (e.g., a conical diffraction grating) may be used to evenly split the incident beam 112 into one or more beams. This splitting may illuminate a tiled sensor or may split the light beam 112 from the synchrotron 100 to distribute optical power to multiple inspection subsystems (e.g., subsystems 310a-b, 410a-d).

[0086] 7 shows a DOE 702 configured to match (e.g., increase) the etendue of a low etendue, high aspect ratio asymmetric light beam 112 incident on the DOE 702. The output light beam 704 is designed to fill the illumination pupil of a set of illumination optics. The projected output light may be a homogenously filled area (pattern 706), a set of discrete points (pattern 708), or an arbitrary pattern (pattern 710).

[0087] 8 shows a transmissive DOE 800 configured to match (e.g., increase) the etendue of a low etendue, high aspect ratio asymmetric light beam 112 incident on the DOE 800. The output light beam 610 is designed to fill the illumination pupil of a set of illumination optics. The transmissive DOE 800 may include a back thinned substrate 802, a transmissive thin film material 804 (which may be composed of multiple layers of different materials to optimize mechanical stability and transmission in the desired spectral bands), an absorbing material 806 (which may define an aperture), and a diffractive structure composed of an absorbing or phase shifting material 808 (which may also be layered). The diffractive structure 808 may be patterned on the transmissive thin film material 804. The substrate 802 may include silicon, the thin film material 804 may be made of silicon nitride, the transmissive material may be made of molybdenum, and the absorptive or phase shifting material 808 may be made of nickel (if absorptive) or molybdenum (if phase shifting). It is noted that the substrate 802 and thin film material 804, as well as the absorptive or phase shifting material 808, are not limited to the materials described herein. Rather, the scope of the present disclosure may be extended to any substrate, thin film, absorptive and phase shifting materials known in the art.

[0088] Figure 9 shows a (reflective) beam diffuser 905 with a Gaussian scattering distribution illuminated with a zero divergence light beam 112 in an oblique incidence geometry, and an output beam 910. Figure 10 shows the calculated beam divergence (in degrees) of the output beam 910. The beam diffuser 905 may include a ruthenium coating with a thin capping layer, which results in a grazing incidence reflectivity of about 80% at a 15 degree angle of incidence on the surface. The incident power is 1 W, and the total reflected or scattered power hitting the detector is 0.79 W.

[0089] A low etendue light source (e.g., synchrotron 100) means that the output light has high spatial coherency. Illumination of a surface containing a diffraction pattern with high coherency light (e.g., light beam 112) can lead to speckle patterns in the projected image due to the unique nanostructure of the optical elements. To generate a uniform projected light wave field with a diffuser, the speckle pattern needs to be destroyed or mitigated. The speckle pattern can be mitigated by varying the illuminated pattern in time such that the source beam is incident on unique surfaces creating new unique speckle patterns or views. The time-averaged projected pattern is the sum of all these unique views, creating a uniformly illuminated light wave field.

[0090] In some embodiments, the speckle pattern can be mitigated by vibrating the DOE over an amplitude. This vibration can be 1, 2, 3, 4, 5 or 6 degrees of freedom. The vibration can be implemented using, for example, galvanometric, piezo, or motor actuators, depending on the amplitude and frequency required to generate a sufficient number of views.

[0091] In some embodiments, the speckle pattern may be mitigated by using a rotating surface, where the diffraction pattern is on a large surface and only a portion of the total surface is illuminated at each moment in time. By rotating the DOE at a certain speed, a sufficient number of unique views are generated. These views may be generated by patterning a DOE on the surface of a disk or the edge of a wheel. Light may be transmitted through or reflected (normal or oblique incidence) from these rotating or oscillating elements.

[0092] 11 shows a grazing incidence mirror 1102 configured to destroy the speckle pattern of a light beam 112. The grazing incidence mirror 1102 is a rotating drum with a reflective surface. The speckle pattern of the output light beam 1110 is mitigated after the input light beam 112 is reflected from the grazing incidence mirror 1102.

[0093] In some embodiments, the speckle pattern may be mitigated by reflecting light off the surface of a liquid mirror. The liquid mirror may be composed of a material in a liquid state that has sufficient EUV reflectivity. The liquid may be a eutectic metal alloy or a suspension of nanoparticles in a liquid medium. The liquid mirror may be rotated to create the desired optical surface, or it may be vibrated to create a standing wave pattern to generate a sufficient number of unique views to destroy the speckle pattern.

[0094] 12 illustrates a rotating mirror 1202 configured to destroy the speckle pattern of the light beam 112. The rotating mirror 1202 may be composed of a solid or liquid reflective material. The speckle pattern of the output light beam 1210 is mitigated after the input light beam 112 is reflected from the rotating mirror 1202.

[0095] In some embodiments, the speckle pattern may be mitigated by transmitting the light through a region having a flow of gas. The gas may be at a sufficient flow rate and pressure to cause sufficient spatial refraction of the EUV light. Pulsed or turbulent flow may be used to generate a unique view to mitigate the speckle pattern.

[0096] In some embodiments, the speckle pattern may be mitigated by acousto-optic modulation, where mechanical excitation of the material generates waves in the material that can spatially and temporally change the density of the material and, consequently, its optical properties.

[0097] In some embodiments, MEMS-based digital micromirror devices (DMDs) may be used as speckle-busting surfaces, dynamic DOEs, and / or beam steering elements. Mirrors may be used in grazing incidence reflecting, or normal incidence reflecting geometry if coated with multilayer mirror materials. Dynamic random patterns can be generated on the DMD, and diffusive DOEs can be created with varying views to bust speckle. Digital holography may be used to generate arbitrary diffraction patterns from the DMD, which may include splitting and steering the incident beam. DMDs coated with multilayer mirrors may be used as programmable diffraction gratings and spatial light modulators with adjustable phase difference. In grazing incidence geometry with pixel tilt modes, the DMD may adjust the blaze angle of the diffraction grating, thus improving diffraction efficiency.

[0098] In some embodiments, a multi-layer mirror may be deposited on the (back-thinned) membrane to create a beam splitter with a distribution of transmitted and reflected power that may be used to distribute light or as a pick-off for power or beam monitoring.

[0099] In some embodiments, the beam splitter may be formed from a well-defined thin wedge, with a portion of the light reflected from one side of the wedge (e.g., in a grazing incidence geometry) and a portion transmitted or reflected from the other side of the wedge. Such a wedge may be formed by anisotropically etching a smooth silicon wafer of sufficient thickness (e.g., hundreds of microns) through lithographically predefined windows / openings. To improve the (grazing incidence) EUV reflectivity on at least one surface of the wedge, the wafer may be coated with a layer of EUV reflective material (e.g., molybdenum).

[0100] As described above, etendue matching optical elements 312a-b (described with respect to FIG. 3) and etendue matching optical elements 412a-d (described with respect to FIG. 4) may directly manipulate the etendue of light beam 112 in conjunction with adaptive electro-optical elements 109a-b, or without adaptive electro-optical elements 109a-b.

[0101] In some embodiments, the etendue matching optics 312a-b and 412a-d may include a temporal modulator that uses temporal methods to manipulate (e.g., increase) the etendue of the light beam 112, thereby matching the etendue of the light beam 112 to the illumination pupil of the illumination optics 314a-b and 414a-d. Temporal methods may include using any method of steering the beam, including using fast steering mirrors (piezo or electromagnetic actuators), diffractive pointing, or a rotating set of mirrors.

[0102] The temporal method involves rastering a low etendue beam (e.g., light beam 112) over the illumination field or pupil aperture such that the time-averaged illumination simulates a beam with a larger etendue. This rastering may be achieved by using mirrors (normal or oblique incidence) on actuators to steer the source beam over programmed areas (e.g., "pupil painting"). Mirror actuation may be achieved using actuators such as hexapods or voice coils, or DMDs (see above). The steering is fast enough that the source beam can fill the pupil or be steered over a discrete number of tiled sensors within the integration time of the inspection system. Steering mirrors may also be used to split the source beam to distribute the light output to multiple inspection systems.

[0103] Fill of the illumination pupil or field in the time domain (e.g., matching the etendue of the illumination optics 314a-b and 414a-d) can also be achieved by rotating a set of mirrors on a single mount so that each reflection points to a different focal position. The rotating mirrors can also be continuous surfaces with variable focal positions across the surface of the rotating element. Additionally, the rotating mirrors can split the source beam to distribute optical power to multiple inspection subsystems (e.g., subsystems 310a-b and 410a-d).

[0104] Figure 13A illustrates a temporal modulator including an actuatable steering mirror 1302 configured to match (e.g., increase) the etendue of a light beam 112 to the etendue of a set of illumination optics by filling an illumination pupil (or field) 1320. An input light beam 112 is incident on the steering mirror 1302, and an output light beam 1310 is rastered across the illumination pupil (or field) 1320. Figure 13B illustrates a path that the output light beam 1310 may take when rastered across the illumination pupil (or field) 1320, beginning at a first point 1312 and ending at a second point 1314.

[0105] FIG. 14 is a schematic diagram of a hollow optical waveguide 1404 configured to homogenize the output light beam 112 and match the etendue of the light beam 112 to a set of illumination optics (e.g., illumination optics 314a-b and 414a-d). The inner surface of the waveguide 1404 is reflective to EUV light. The waveguide 1404 may be constructed of grazing incidence mirrors and may have a cross section of different shapes (e.g., square cross section). Using a set of waveguide matching optics 1402, the light beam 112 is focused at the first end (input side) of the waveguide 1404 such that the opening angle after focusing fills the angle space matching the required numerical aperture (NA). The light 112 is reflected multiple times inside the waveguide 1404. The second end (output side) of the waveguide 1404 is imaged (focused) onto a mask or wafer 1408 (e.g., mask or wafer 316a-b or 416a-b) using a set of illumination optics 1406 (e.g., illumination optics 314a-b or 414a-d). The spatial irradiance at the mask or wafer 1408 is represented by a homogenized illumination field 1410 of a desired (e.g., square) shape. In angular space, the irradiance is a repeating pattern 1412 of the source beam 112 resulting in a structured pupil fill.

[0106] Exemplary Beam Delivery System with Etendue Matching Optics In one embodiment, a multi-layer mirror with two grazing incidence mirrors and a diffractive optical element can be used to split and direct a light beam from a sub-fab (e.g., a synchrotron) to an optical characterization subsystem (e.g., a mask or wafer inspection tool). The light beam is coupled into the illumination optics, the spectral filtering optics, and the etendue matching optics of the subsystem. The light beam may be incident on the grazing incidence mirrors at a 20 degree angle of incidence, and one of the grazing incidence mirrors may be combined with a diffraction grating in an out-of-plane configuration for beam splitting.

[0107] The transmission efficiency of such a beam delivery system may be about 10-20%. Considering all losses (including the inspection system optics), actinic inspection of EUV masks requires a source power of about 140 mW and a power of about 60 W / mm 2 A brightness of 1000 nm / sr is required. If four inspection tools are used, a total power of about 600 mW may be required. Defect review tools require similar brightness and lower source power (1 mW). These specifications are achievable with compact synchrotron-based radiation sources. Further optimization of the beam delivery and illumination optics can result in an improvement of about 2-3 times.

[0108] (Exemplary Method for Matching the Etendue of a Light Beam in an Optical Characterization System) FIG. 15 is a flow chart 1500 illustrating a method for matching the etendue of a light beam in an optical characterization system.

[0109] At step 1502, a light beam is generated using a wiggler synchrotron light source. The light beam may be, for example, the light beam 112 generated by the wiggler synchrotron light source 100. One or more wiggler insertion sources may generate the light beam by periodically accelerating electrons passing through a gap between an upper surface and a lower surface of a given wiggler insertion source along a direction perpendicular to the electron's direction of travel.

[0110] In step 1504, the light beam is directed to a diffractive optical element or a time modulator or a homogenizing optical waveguide in an optical characterization system (e.g., one of the optical characterization subsystems 410a-d). After the light beam exits the wiggler synchrotron, beam delivery optics (e.g., beam delivery optics 401a-b) may distribute the light beam to the optical characterization subsystems. The beam delivery optics may include beam steering optics (e.g., mirrors, beam splitters, lenses, etc.) configured to direct the light beam from the synchrotron light source to the diffractive optical element or the time modulator or the homogenizing optical waveguide (e.g., any of the etendue matching optics 412a-d). The beam steering optics may include a set of relay mirrors suitable for directing the light from the synchrotron light source 100 to the diffractive optical element or the time modulator or the homogenizing optical waveguide. The beam delivery optics may include spectral filtering optics, such as one or more gratings configured to transmit or direct a selected wavelength band of the light beam. The spectral filtering optics or gratings may be used to select the centroid wavelength. Use of the centroid wavelength in the inspection (or review) process may lead to improved accuracy of defect printability estimation.

[0111] In step 1506, a diffractive optical element or a temporal modulator or a homogenizing optical waveguide is used to match the etendue of the light beam to a set of illumination optics of the optical characterization system.

[0112] Diffractive optical elements (DOEs) are surface structures patterned on transmissive or reflective optical elements. The pattern spatially modulates the phase and / or amplitude of light waves. DOEs may be patterned on the substrate of a reflective EUV mirror (normal or oblique incidence), on the multilayer (ML) of a reflective mirror, or on a transmissive thin film. The diffractive pattern may be binary (e.g., structure height determined by etch depth), grayscale (more than one step), or blazed (triangular shaped), or may exhibit another more complex height profile.

[0113] The temporal modulator uses temporal methods to align the etendue of the light beam to the illumination pupil of the illumination optics. Temporal methods may include using any method for steering the beam, including using fast steering mirrors (piezo or electromagnetic actuators), diffractive pointing, or a set of rotating mirrors. Temporal methods include rastering a low etendue light beam over the illumination field or pupil aperture such that the time-averaged illumination simulates a beam with a larger etendue. This rastering may be achieved by using mirrors (normal or oblique incidence) on the actuator to steer the source beam over a programmed area (e.g., "pupil painting"). The steering is fast enough that the source beam can fill the pupil or be steered over a discrete number of tiled sensors within the integration time of the inspection system. Diffractive optical elements and temporal modulators are further described with respect to Figures 5-14.

[0114] Those skilled in the art will recognize that it is common in the art to describe devices and / or processes in the manner described herein and then use engineering practices to integrate such described devices and / or processes into a data processing system. That is, at least a portion of the devices and / or processes described herein may be integrated into a data processing system through a reasonable amount of experimentation. Those skilled in the art will recognize that a typical data processing system generally includes one or more of a system unit housing, a video display device, memory such as volatile and non-volatile memory, a processor such as a microprocessor and a digital signal processor, a computing entity such as an operating system, a driver, a graphical user interface, and an application program, one or more interaction devices such as a touchpad or screen, and / or a control system including feedback loops and control motors (e.g., feedback for sensing position and / or velocity, control motors for moving and / or adjusting components and / or quantities). A typical data processing system may be implemented utilizing any suitable commercially available components such as those found in data computing / communication and / or network computing / communication systems.

[0115] The subject matter described herein may show different components being included in or connected with different other components. It should be understood that the architectures depicted are merely exemplary, and that in fact many other architectures that achieve the same functionality may be implemented. Conceptually, the arrangement of components to achieve the same functionality is effectively "associated" such that the desired functionality is achieved. Thus, for purposes of this specification, any two components that are combined to achieve a particular functionality may be considered to be "associated" with one another such that the desired functionality is achieved, regardless of the architecture or intermediate components. Similarly, any two components that are associated in this manner may also be considered to be "connected" or "coupled" with one another to achieve the desired functionality, and any two components that can be associated in this manner may also be considered to be "coupleable" with one another to achieve the desired functionality. Specific examples of coupleable include, but are not limited to, components that are physically matable and / or physically interacting, and / or components that can wirelessly interact and / or wirelessly interacting, and / or components that can logically interact and / or logically interacting.

[0116] The present disclosure and many of its attendant advantages will be understood from the foregoing description, and it will be apparent that various changes in form, construction and arrangement of the elements may be made without departing from the disclosed subject matter or sacrificing all of its important advantages. The described forms are merely illustrative, and it is the intent of the following claims to encompass all such modifications. It will be further understood that the invention is defined by the appended claims.

Claims

1. a synchrotron light source; at least, a set of illumination optics; a set of imaging optics; and a diffractive optical element configured to match the etendue of a light beam output from the synchrotron light source to the set of illumination optics; an optical characterization subsystem including: a sensor configured to receive a projected image from the set of imaging optics; the diffractive optical element includes an optical surface that is a random pattern having a programmed size distribution, the random pattern including nanoparticles or block copolymers; Optical characterization system.

2. 10. The optical characterization system of claim 1, wherein the optical characterization system comprises at least one of an actinic EUV mask inspection system, a wafer inspection or metrology system, an actinic mask review system, or an EUV mask reflectometer system.

3. The optical characterization system of claim 1 , wherein the optical characterization subsystem includes a rotating surface configured to destroy a speckle pattern in the light beam.

4. The optical characterization system of claim 3 , wherein the surface of rotation comprises a grazing incidence mirror having a reflective surface.

5. The optical characterization system of claim 3 , wherein the surface of rotation comprises a eutectic metal alloy or a suspension of nanoparticles in a liquid medium.

6. The optical characterization system of claim 1 , wherein the synchrotron light source comprises a set of adaptive electron-optical elements.

7. The optical characterization system of claim 6 , wherein the set of adaptive electron-optical elements is configured to match an etendue of the light beam to the set of illumination optics.

8. The optical characterization system of claim 6 , wherein the synchrotron light source comprises a wiggler synchrotron light source.

9. 2. The optical characterization system of claim 1, wherein the optical characterization subsystem includes at least one of a galvanometric actuator, a piezoelectric actuator, or a motor actuator configured to vibrate the diffractive optical element to disrupt a speckle pattern in the light beam.

10. a synchrotron light source; at least, a set of illumination optics; a set of imaging optics; and a diffractive optical element configured to match the etendue of a light beam output from the synchrotron light source to the set of illumination optics; an optical characterization subsystem including: a sensor configured to receive a projected image from the set of imaging optics; and An optical characterization system including a digital micromirror device (DMD) coated with a multilayer mirror configured to destroy the speckle pattern of the light beam, and acting as a programmable diffraction grating or spatial light modulator with adjustable phase difference.

11. a synchrotron light source; at least, a set of illumination optics; a set of imaging optics; and a temporal modulator configured to align the etendue of a light beam output from the synchrotron light source to the set of illumination optics; an optical characterization subsystem including: a sensor configured to receive a projected image from the set of imaging optics; the temporal modulator comprising an actuatable steering mirror; the actuatable steering mirror is configured to steer the light beam onto an illumination pupil of the set of illumination optical elements. Optical characterization system.

12. A synchrotron light source; at least, a set of illumination optics; a set of imaging optics; and a temporal modulator configured to align the etendue of a light beam output from the synchrotron light source to the set of illumination optics; an optical characterization subsystem including: a sensor configured to receive a projected image from the set of imaging optics; the temporal modulator comprising an actuatable steering mirror; The actuatable steering mirror is configured to steer the light beam onto an illumination field of the set of illumination optics.

13. A synchrotron light source; at least, a set of illumination optics; a set of imaging optics; and a temporal modulator configured to align the etendue of a light beam output from the synchrotron light source to the set of illumination optics; an optical characterization subsystem including: a sensor configured to receive a projected image from the set of imaging optics; the temporal modulator comprising an actuatable steering mirror; An optical characterization system, wherein a hexapod actuator or a voice coil actuator is configured to actuate the actuatable steering mirror.

14. A synchrotron light source; at least, a set of illumination optics; a set of imaging optics; and a temporal modulator configured to align the etendue of a light beam output from the synchrotron light source to the set of illumination optics; an optical characterization subsystem including: a sensor configured to receive a projected image from the set of imaging optics; the temporal modulator comprising an actuatable steering mirror; The synchrotron light source comprises a set of adaptive electron-optical elements.

15. The optical characterization system of claim 14 , wherein the set of adaptive electron-optical elements is configured to match an etendue of the light beam to the set of illumination optics.

16. a synchrotron light source; at least, a set of illumination optics; a set of imaging optics; and a temporal modulator configured to align the etendue of a light beam output from the synchrotron light source to the set of illumination optics; an optical characterization subsystem including: a sensor configured to receive a projected image from the set of imaging optics; and An optical characterization system including a digital micromirror device (DMD) coated with a multilayer mirror configured to destroy the speckle pattern of the light beam, and acting as a programmable diffraction grating or spatial light modulator with adjustable phase difference.

17. 1. A method for matching etendue of a light beam in an optical characterization system, comprising: generating the light beam using a synchrotron light source; directing the light beam towards a diffractive optical element or a time modulator; using the diffractive optical element or the temporal modulator to increase the etendue of the light beam to match a set of illumination optics; Including, The method of claim 1, wherein the diffractive optical element comprises an optical surface that is a random pattern with a programmed size distribution, the random pattern comprising nanoparticles or block copolymers, and the temporal modulator comprises an actuatable steering mirror.

18. 20. The method of claim 17, wherein the optical characterization system comprises at least one of an actinic EUV mask inspection system, a wafer inspection or metrology system, an actinic mask review system, or an EUV mask reflectometer system.

19. a synchrotron light source; at least, a set of illumination optics; a set of imaging optics; and a diffractive optical element configured to match the etendue of a light beam output from the synchrotron light source to the set of illumination optics; an optical characterization subsystem including: a sensor configured to receive a projected image from the set of imaging optics; the diffractive optical element includes an optical surface; the optical surface includes a programmed holographic pattern; Optical characterization system.

20. 20. The optical characterization system of claim 19, wherein the programmed hologram pattern is generated from a projected pattern by calculating the inverse of a diffraction pattern.

21. 20. The optical characterization system of claim 19, wherein the programmed hologram pattern is at least one of a continuous pattern, a binary pattern, or a grayscale approximation.

22. 1. A method for matching etendue of a light beam in an optical characterization system, comprising: generating the light beam using a synchrotron light source; directing the light beam towards a diffractive optical element or a time modulator; using the diffractive optical element or the temporal modulator to increase the etendue of the light beam to match a set of illumination optics; Including, the diffractive optical element includes an optical surface; the optical surface includes a programmed holographic pattern; method.

23. 23. The method of claim 22, wherein the programmed hologram pattern is generated from a projected pattern by calculating the inverse of a diffraction pattern.

24. 23. The method of claim 22, wherein the programmed hologram pattern is at least one of a continuous pattern, a binary pattern, or a grayscale approximation.

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