Metrology apparatus for mirror

Abstract

A metrology apparatus includes: a light source generating radiation in the deep ultraviolet wavelength range; an optical module including one or more reference mirrors, each reference mirror at a respective reference mount and a test mount configured to receive a test mirror arranged relative to the reference mirror when installed to thereby form an optical cavity, the optical module placed in a path of the generated radiation; and a measurement module. The measurement module is configured to: receive radiation transmitted through the optical cavity; and estimate a reflectivity of a test mirror received at the test mount based on the received radiation.

Description

METROLOGY APPARATUS FOR MIRRORCROSS-REFERENCE TO REALTED APPLICATIONS

[0001] This application claims priority to US Application No. 63 / 653,088, filed May 29, 2024, titled METROLOGY APPARATUS FOR MIRROR, which is incorporated herein by reference in its entirety.TECHNICAL FIELD

[0002] The disclosed subject matter relates to a metrology apparatus for characterizing a property such as a reflectivity of a mirror for use in a deep ultraviolet light source.BACKGROUND

[0003] Excimer optical sources can be used to supply a light beam to a lithography exposure apparatus. The light beam produced from the excimer optical source can have an ultraviolet (UV) wavelength, such as a deep ultraviolet (DUV) wavelength. The DUV wavelength range is between 10 nanometers (nm) and 400 nm. An excimer optical source can be built using a single gas discharge chamber or using a plurality of gas discharge chambers.

[0004] The body of the gas discharge chamber can be any shape and configured to house, within its cavity a gas mixture that includes a gain medium. Optical amplification occurs in the gain medium when enough energy is provided by an energy source (which can include electrodes within the cavity of the body). The gas mixture can be any suitable gas mixture configured to produce a light beam (or a laser beam) around the required wavelengths and bandwidth. For example, the gas mixture can include argon fluoride (ArF), which emits light at a wavelength of about 193 nm, or krypton fluoride (KrF), which emits light at a wavelength of about 248 nm. Moreover, an optical feedback mechanism can be arranged or configured relative to the body to provide an optical resonator. The optical source also includes high reflectivity mirrors that are reflective to wavelengths in the DUV range.SUMMARY

[0005] In some general aspects, a metrology apparatus includes: a light source generating radiation in the deep ultraviolet wavelength range; an optical module including one or more reference mirrors, each reference mirror at a respective reference mount and a test mount configured to receive a test mirror arranged relative to the reference mirror when installed to thereby form an optical cavity, the optical module placed in a path of the generated radiation; and a measurement module. The measurement module is configured to: receive radiation transmitted through the optical cavity; and estimate a reflectivity of a test mirror received at the test mount based on the received radiation.

[0006] Implementations can include one or more of the following features. For example, the optical module can be defined inside an optical enclosure that is sealed, the interior of which ismaintained as a controlled environment. The interior can be configured as a gas purged or vacuum environment. The interior can be configured as a vacuum environment that has a negative pressure level that is at a pressure that is lower than 0.1 kilopascals (kPa). The interior can be configured as a nitrogen purged environment or a helium purged environment.

[0007] The generated radiation can have a wavelength below 196 nanometers (nm). The generated radiation can have a wavelength of about 193 nm.

[0008] Each reference mirror can be a concave, convex, or flat mirror, and the test mirror can be a concave, convex, or a flat mirror. Each reference mirror and the test mirror can both be concave mirrors having a focal length that is greater than 0.5 meters (m), greater than 1.0 m, greater than 1.3 m, or about 1.4 m.

[0009] The light source can be configured to generate the radiation as pulsed radiation.

[0010] The metrology apparatus can further include a beam delivery module that is configured to: receive the generated radiation from the light source, align the generated radiation with the optical cavity, and direct the generated radiation to the optical module. The beam delivery module can be configured to adjust a beam pointing of the generated radiation relative to the optical cavity. The beam delivery module can include a polarization module configured to adjust a polarization of the generated radiation. The module can be configured to output the generated radiation with a linear polarization.

[0011] The metrology apparatus can further a beam delivery module that is configured to: receive the generated radiation from the light source. The beam delivery module can include a mode control configured to effectively filter out spatial modes of the generated radiation from the light source. The mode control can include a beam divergence module configured to modify a beam divergence of the generated radiation to match an optical cavity mode of the optical cavity. The mode control can include an aperture.

[0012] The metrology apparatus can also include an actuation module mechanically coupled to each reference mount and the test mount. The actuation module can be configured to mechanically adjust one or more of each reference mount and the test mount to thereby adjust one or more of each reference mirror and the test mirror when the test mirror is received at the test mount and adjust a cavity length. The actuation module being configured to mechanically adjust a reference mount can thereby adjust one or more of a translational location and a rotational state of the reference mirror and the actuation module being configured to mechanically adjust the test mount can thereby adjust one or more of a translational location and a rotational state of the test mirror.

[0013] The generated radiation can be a pulsed laser beam. The pulses of the pulsed laser beam can have a pulse width of that is less than or equal to a round trip time through the optical cavity. The measurement module can include: a detector configured to receive the radiation transmitted through the optical cavity and to produce an electrical signal based on the received radiation; and a processing apparatus configured to estimate the reflectivity of the test mirror based on the produced electricalsignal. The detector can include a photomultiplier tube, and an oscilloscope configured to digitize and store an electrical signal produced by the photomultiplier tube. The oscilloscope can be configured to graphically display the electrical signal produced by the photomultiplier tube. The detector can include an optical filter configured to block interference radiation at wavelengths not in the DUV range from reaching the photomultiplier tube. The processing apparatus can be configured to estimate the reflectivity of the test mirror by fitting the produced electrical signal to a decay function. The decay function can be an exponential function. The processing apparatus can be configured to, prior to fitting the produced electrical signal to the single exponential function: determine an amplitude of each peak in the produced electrical signal due to a pulse of the transmitted radiation and a peak time associated with each peak; and calculate a time constant for the decay of the produced electrical signal based on the determined amplitudes, a cavity length defined between the reference mirror and the test mirror, and the peak times. The processing apparatus can be configured to estimate the reflectivity of the test mirror based at least on the calculated time constant. The processing apparatus can be configured to estimate the reflectivity of the test mirror also based on at least the reflectivity of each reference mirror, an absorption coefficient within the optical cavity, and the cavity length.

[0014] The optical cavity can be a stable optical cavity configured to contain the radiation in the stable optical cavity.

[0015] The measurement module can be configured to estimate the reflectivity of the test mirror with a sensitivity that is better than 0.1%.

[0016] The metrology apparatus can also include a mode control within the optical cavity of the optical module, the mode control configured to effectively filter out high order spatial modes in the generated radiation.

[0017] The optical module can include a single reference mirror and the optical cavity can be a linear optical cavity. The optical module can include two reference mirrors and the optical cavity can be a folded optical cavity.

[0018] In other general aspects, a method is performed for analyzing a reflectivity of a test mirror. The method includes: generating a pulsed radiation having a wavelength in the deep ultraviolet range; delivering the generated pulsed radiation into an optical cavity formed between a reference mirror at a reference location and the test mirror at a test location; receiving radiation transmitted through the optical cavity; and estimating a reflectivity of the test mirror based on the received radiation.

[0019] Implementations can include one or more of the following features. For example, the method can further include: receiving the test mirror that has been removed from a deep ultraviolet optical source; installing the test mirror at the second location to form the optical cavity; and categorizing a quality of the test mirror based on the estimated reflectivity of the test mirror.

[0020] The method can also include: receiving the test mirror from a supplier who manufactured the test mirror; installing the test mirror at the second location to form the optical cavity; and categorizing a quality of the test mirror based on the estimated reflectivity of the test mirror.

[0021] The pulsed radiation can have a wavelength below 196 nanometers (nm).

[0022] The generated pulsed radiation can be delivered into the optical cavity by receiving the generated pulsed radiation, aligning the generated pulsed radiation with the optical cavity, and directing the generated pulsed radiation to the optical cavity. The generated pulsed radiation can be delivered into the optical cavity by receiving the generated pulsed radiation and modifying a beam divergence of the generated radiation to match an optical cavity mode of the optical cavity.

[0023] The method can also include, prior to delivering the generated pulsed radiation into the optical cavity: adjusting one or more of a translational location and a rotational state of the reference mirror; adjusting one or more of a translational location and a rotational state of the test mirror; and adjusting a cavity length that is a distance between the reference mirror and the test mirror.

[0024] The radiation transmitted through the optical cavity can be received by: producing an electrical signal based on the received radiation; and estimating the reflectivity of the test mirror based on the produced electrical signal. The radiation transmitted through the optical cavity can be received by blocking interference radiation at wavelengths not in the DUV range. The reflectivity of the test mirror can be estimated by fitting the produced electrical signal to a decay function. The method can further include, prior to fitting the produced electrical signal to the single exponential function: determining an amplitude of each peak in the produced electrical signal due to a pulse of the transmitted radiation and a peak time associated with each peak; and calculating a time constant for decay of the produced electrical signal based on the determined amplitudes, a cavity length defined between the reference mirror and the test mirror, and the peak times. The reflectivity of the test mirror can be estimated based at least on the calculated time constant. The reflectivity of the test mirror can be estimated based on at least the reflectivity of the reference mirror, an absorption coefficient within the optical cavity, and the cavity length. The reflectivity of the test mirror can be estimated by estimating the reflectivity with a sensitivity that is better than 0.1%. The method can further include effectively filtering out high order spatial modes in the generated radiation prior to delivering the generated pulsed radiation into an optical cavity.

[0025] In other general aspects, a metrology apparatus includes: a light source generating pulsed radiation having a wavelength in the deep ultraviolet range; an optical module including a reference mirror at a reference mount and a test mount configured to receive a test mirror facing the reference mirror to thereby form an optical cavity within a controlled and gas purged environment; a beam delivery module that receives the generated radiation from the light source, aligns the generated radiation with the optical cavity, and directs the generated radiation to the optical module; and a measurement module. The measurement module includes: a detector configured to receive the radiation transmitted through the optical cavity and to produce an electrical signal based on thereceived radiation; and a processing apparatus configured to estimate the reflectivity of a test mirror received at the second mount based on the produced electrical signal.

[0026] The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims.DRAWING DESCRIPTION

[0027] 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 a person skilled in the relevant art(s) to make and use implementations described herein.

[0028] Fig. 1 is a schematic block diagram of a metrology apparatus configured to estimate a reflectivity of a test mirror, the metrology apparatus including a light source, an optical module, and a measurement module;

[0029] Fig. 2A is a schematic block diagram of an optical module of a metrology apparatus, the optical module including a flat reference mirror and a flat test mirror;

[0030] Fig. 2B is a schematic block diagram of an optical module of a metrology apparatus, the optical module including a concave reference mirror and a concave test mirror;

[0031] Fig. 2C is a schematic block diagram of an optical module of a metrology apparatus, the optical module including a concave reference mirror and a flat test mirror;

[0032] Fig. 2D is a schematic block diagram of an optical module of a metrology apparatus, the optical module including a flat reference mirror and a concave test mirror;

[0033] Fig. 2E is a schematic block diagram of an optical module of a metrology apparatus, the optical module including a convex reference mirror and a concave test mirror;

[0034] Fig. 2F is a schematic block diagram of an optical module of a metrology apparatus, the optical module including a concave reference mirror and a convex test mirror;

[0035] Fig. 3 is a schematic block diagram of an implementation of a metrology apparatus configured to estimate a reflectivity of a test mirror, the metrology apparatus including a light source, a beam delivery module, an optical module, an actuation module, and a measurement module;

[0036] Fig. 4A is a schematic block diagram of an optical module and an actuation module configured to send a signal to a test mount to translate a test mirror;

[0037] Fig. 4B is a schematic block diagram of an optical module and an actuation module configured to send a signal a reference mount to translate a reference mirror;

[0038] Fig. 4C is a schematic block diagram of an optical module and an actuation module configured to send a signal to a test mount to cause it to tip and / or tilt a test mirror;

[0039] Fig. 4D is a schematic block diagram of an optical module and an actuation module configured to send a signal to a reference mount to cause it to tip and / or tilt a reference mirror;

[0040] Fig. 5 is a bock diagram of an implementation of a measurement module that can be used in the metrology apparatus of Figs. 1 or 3;

[0041] Fig. 6 is a graph of an amplitude of an electrical signal produced by a detector in a measurement module such as the measurement module of Fig. 5 plotted versus time;

[0042] Fig. 7 is a flow chart of a procedure that can be performed by a metrology apparatus of Figs. 1 or 3 for analyzing a reflectivity of a test mirror;

[0043] Fig. 8 is a schematic block diagram of an optical source in which a test mirror can be used; and

[0044] Fig. 9 is a schematic block diagram of an implementation of an optical module that can be used in the metrology apparatus of Figs. 1 or 3.

[0045] 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. Unless otherwise indicated, the drawings provided throughout the disclosure should not be interpreted as to-scale drawings.DESCRIPTION

[0046] Referring to Fig. 1, a metrology apparatus 100 is designed to estimate a reflectivity of a test mirror 105. The test mirror 105 can be a high reflectivity mirror that reflects light having a wavelength in the deep ultraviolet (DUV) range, for example, between 10 nanometers (run) and 400 nm. The test mirror 105 has a high reflectivity to radiation in the DUV range, which means the test mirror 105 is configured to reflect greater than 95% of DUV radiation impinging upon the reflective surface of the test mirror 105. Such high reflectivity DUV mirrors can be used in an optical source 170 that generates optical radiation 171 having a wavelength in the DUV range. The operation and performance of the optical source 170 greatly depends on the reflectivity of the DUV mirrors within the optical source 170. In order to troubleshoot issues and ensure quality (that is, acceptable reflectivity) of the DUV mirrors, the metrology apparatus 100 is configured to characterize the reflectivity of the test mirror 105 with a sensitivity that is better than 1%, better than 0.5%, better than 0.3%, or about 0.2% at DUV wavelengths. Moreover, the sensitivity improves as the reflectivity of the DUV mirror approaches 100%. The metrology apparatus 100 is designed such that the signal to noise ratio is large, which reduces measurement errors. The metrology apparatus 100 is configured to characterize the reflectivity of the test mirror 105 that is either flat, convex, or concave (for example, having a focal length that is greater than 1 meters (m)). The metrology apparatus 100 is configured to characterize the reflectivity of the test mirror 105 within a short period of time, such as less than 60 minutes, less than 30 minutes, or less than 10 minutes. Once the reflectivity of the test mirror 105 ischaracterized to better than 1%, better than 0.5%, better than 0.3%, or about 0.2% by the metrology apparatus 100 and is found to be in an acceptable range for use in the optical source 170, the test mirror 105 can be installed in the optical source 170.

[0047] The metrology apparatus 100 includes a light source 110, an optical module 120, and a measurement module 140. The light source 110 generates radiation 111 that has a wavelength in the DUV wavelength range. The optical module 120 includes at least one reference mount 121 and a test mount 123. The optical module 120 includes a reference mirror 122 positioned at or fixed to the reference mount 121. The test mount 123 is configured to receive the test mirror 105. When the test mirror 105 is mounted to or installed at the test mount 123, a reflective surface 105r of the test mirror 105 faces a reflective surface 122r of the reference mirror 122. An optical cavity is formed between the test mirror 105 and the reference mirror 122. The optical cavity can be a stable optical cavity in which the radiation 112 directed to the optical cavity formed between the test mirror 105 and the reference mirror 122 is contained (or partially trapped) in the optical cavity. Some of the radiation 112 leaks out through one of the mirrors 105 or 122 as radiation 125. In particular, the position and orientation of each of the test mirror 105 and the reference mirror 122 can be adjusted using the respective test mount 123 and reference mount 121. By making these adjustments (using the test mount 123 and / or the reference mount 121), the cavity length (which is the distance between the reflective surfaces 105r, 122r) can be adjusted to form the optical cavity. In some implementations, the optical cavity is a marginally unstable cavity in which a dwell time (of the radiation 112 within the optical cavity is large enough such that there are enough round trips of the radiation 112 and the output signal is measurable by the measurement module 140.

[0048] The optical module 120 is placed in a path of generated radiation 112. The generated radiation 112 can be the radiation 111 directly produced from the light source 110 or it can be radiation that has been formed from the radiation 111.

[0049] The measurement module 140 is configured to: receive radiation 125 transmitted through the optical cavity (formed in the optical module 120) and characterize or estimate a reflectivity of a test mirror 105 that is received at the test mount 123 based on the received radiation 125. The measurement module 140 is configured to characterize the reflectivity of the test mirror 105 with a sensitivity that is better than 1% at DUV wavelengths. Once the reflectivity of the test mirror 105 is characterized, a determination can be made either manually by an installer or operator of the optical source 170 or automatically by a processing device as to whether the test mirror 105 has an acceptable reflectivity for use in the optical source 170. If it is determined that the characterized reflectivity of the test mirror 105 (by the measurement module 140 of the metrology apparatus 100) is acceptable or within a suitable range, then the test mirror 105 can therefore be installed within the optical source 170 at the appropriate location, such as shown in the example of Fig. 1 .

[0050] The metrology apparatus 100 uses the technique of cavity ring -down spectroscopy (CRDS) to determine the reflectance of the highly reflective test mirror 105 by measuring the totalloss of the optical cavity. The loss through the optical cavity includes loss by way of absorption, transmission, reflectance, and scattering of the radiation 112. The reflectivity of the test mirror 105 can then be determined by calculating the loss at the test mirror 105 (knowing the values of the other types of losses) and then subtracting the measured calculated loss at the test mirror 105 from 100%. The resonant cavity (the optical cavity) is formed by the two highly reflective mirrors, the reference mirror 122 and the test mirror 105 (when the test mirror 105 is fixed to the test mount 123). The reference mirror 122 and the test mirror 105 can have reflectance values of at least 98% or even as large as or about 99.99%. The pulse of the radiation 122 is sent into the optical cavity of the optical module 120, and this pulse oscillates between the mirrors 122 and 105, losing a small amount of intensity at each reflection due to transmission, absorption, and scattering. An intensity detector within the measurement module 140 is placed after one of the mirrors (in the example of Fig. 1, it is placed after the test mirror 105) to record the decreasing intensity of the reflected light as a function of time. This allows the total loss of both mirrors 122, 105 to be determined by the decay time, or “ring down,” of the reflected light inside of the optical cavity.

[0051] As discussed above, the radiation 111 has a wavelength in the DUV wavelength range. In some implementations, the wavelength of the radiation 111 is below 196 nm. In some implementations, the wavelength of the radiation 111 is about 193 nm. The light source 110 includes a set of optical elements configured to produce the radiation 111 in the DUV wavelength range. The light source 110 can be a pulsed light source such that the radiation 111 is in the form of pulses of light. The pulses of the radiation 111 can have a pulse width (or pulse duration) that is less than or equal to 100 nanoseconds (ns). In some examples, the pulse duration is less than a round trip time through the optical cavity. For such scenarios, the signal contrast at the measurement module 140 is high, which reduces the amount of signal processing needed to analyze the data. If the cavity round trip time is about 10 ns, then the pulse duration can be less than 10 ns. In other examples, the pulse duration is about the same as the cavity round trip time. For such scenarios, the measurement module 140 may need to perform signal processing to separate peaks and their time delays due to possible lower signal contrast. And, the coherence of radiation 111 can also be considered in the data processing algorithm of the measurement module 140.

[0052] For example, the light source 110 can be a two-stage excimer light source that uses ArF as the gain medium in both stages and produces pulsed radiation 111 at a wavelength of about 193 nm. An example of such a light source 110 is the XLA-165 by Cymer, Inc. of San Diego, CA, USA. As another example, the light source 110 can be a single frequency all -solid-state laser system operating a wavelength of about 193 nm. An example of such a light source 110 is the IXION 193 SUM by Xiton Photonics GmbH of Kaiserslautern, Germany.

[0053] The reference mirror 122 can be made of a material that is able to reflect radiation that is in the DUV wavelength range. For example, the reference mirror 122 can be made of a crystalline substrate or any substrate such as calcium fluoride (CaF2), magnesium fluoride (MgF2), or fusedsilica. The reflective surface 122r can be formed as a polished surface on the substrate. In some implementations, the reflective surface 122r is formed as a separate polished coating on the substrate.

[0054] As discussed above, when the reference mirror 122 and the test mirror 105 are positioned or fixed to their respective reference mount 121 and test mount 123, and are adjusted to form the optical cavity, the generated radiation 112 fed into the optical module 120 is transmitted through one of the mirrors, is bounced back and forth within the optical cavity between the reference mirror 122 and the test mirror 105, and leaks through the other mirror as the transmitted radiation 125. In the example of Fig. 1, the generated radiation 112 is initially transmitted through the reference mirror 122 and is leaked through the test mirror 105 as the transmitted radiation 125. In other implementations, such as shown in Fig. 3, the generated radiation 112 is initially transmitted through a test mirror 305 and is leaked through a reference mirror 322.

[0055] With reference to Fig. 2A, in some implementations, an optical module 220A includes a reference mirror 222A that is a flat mirror having an infinite focal length and is configured to receive a test mirror 205A that is a flat mirror having an infinite focal length. With reference to Fig. 2B, in other implementations, an optical module 220B includes a reference mirror 222B that is concave and is configured to receive a test mirror 205B that is concave. For example, the focal length of the reference mirror 222B or the test mirror 205B can be greater than 0.5 meters (m), greater than 1 m, or greater than 1.3 m. In some implementations, the focal lengths can be 0.675 m, 0.805 m, 0.83 m, 1.392 m, or 1.4 m. With reference to Fig. 2C, an optical module 220C includes a reference mirror 222C that is concave and is configured to receive a test mirror 205 C that is flat. With reference to Fig. 2D, an optical module 220D includes a reference mirror 222D that is flat and is configured to receive a test mirror 205D that is concave. With reference to Fig. 2E, an optical module 220E includes a reference mirror 222E that is convex and is configured to receive a test mirror 205E that is concave. And, with reference to Fig. 2F, an optical module 220F includes a reference mirror 222F that is concave and is configured to receive a test mirror 205F that is convex. Other configurations are possible.

[0056] Referring to Fig. 3, in some implementations, a metrology apparatus 300 is designed to estimate a reflectivity of a test mirror 305, which can correspond to the test mirror 105 of Fig. 1. Thus, the test mirror 305 can be a high reflectivity mirror that reflects light having a wavelength in the deep ultraviolet (DUV) range, for example, between 10 nanometers (nm) and 400 run. The metrology apparatus 300 includes a light source 310, an optical module 320, and a measurement module 340. The light source 310 generates radiation 311 that has a wavelength in the DUV wavelength range. The optical module 320 includes a reference mount 321 and a test mount 323. The optical module 320 includes a reference mirror 322 positioned at or fixed to the reference mount 321. The test mount 323 is configured to receive the test mirror 305. When the test mirror 305 is mounted to or installed at the test mount 323, a reflective surface 305r of the test mirror 305 faces a reflective surface 322r of thereference mirror 322. As with the optical cavity of Fig. 1, an optical cavity is formed between the test mirror 305 and the reference mirror 322.

[0057] The metrology apparatus 300 includes a beam delivery module 330 configured to receive the generated radiation 311 from the light source 310. The beam delivery module 330 is configured to direct generated radiation 312 to the optical module 320. The beam delivery module 330 can include one or more of a beam pointing module 331, a mode control module 332, and a polarization module 333.

[0058] The beam pointing module 331 includes one or more optical elements that are configured to adjust a beam pointing of the generated radiation 311 to a desired beam pointing in the generated radiation 312. The beam pointing of the generated radiation 311 indicates the direction of the generated radiation 311. By adjusting the beam pointing of the generated radiation 311, the beam pointing module 331 is configured to control where the generated radiation 312 interfaces with the optical module 320 and thus controls the alignment between the generated radiation 312 and the optical module 320 to ensure the generated radiation 312 efficiently couples with the optical cavity. The beam pointing of the generated radiation 311 can fluctuate due to, for example, mechanical vibrations, air currents, drifts, and thermal effects including effects within the light source 310. The beam pointing module 331 therefore makes adjustments to the generated radiation 311 using optical elements such as refractive or reflective optical elements.

[0059] For example, at least two mirrors can be used in the beam pointing module 331. Indeed, the two mirrors can adjust both the pointing and position in both axes, if they are properly mounted. The beam pointing module 331 can further include an alignment detection system that is configured to measure alignment / beam pointing and adjust the mirrors to thereby establish the optimum or improved alignment. For example, such a detection system can include cameras, photodiode arrays, or other types of detectors for establishing the pointing.

[0060] The mode control module 332 can include one or more optical elements that are configured to filter out undesirable spatial modes in the generated radiation 311 to produce the generated radiation 312. The mode control module 332 can include one or more optical elements that are configured to ensure that a lowest order spatial mode in the generated radiation 311 is excited efficiently.

[0061] In some implementations, the mode control module 332 includes a beam divergence adjuster that adjusts a divergence of the generated radiation 311 to a desired divergence in the generated radiation 312. The beam divergence of the generated radiation 311 is a measure for how fast the radiation 311 expands far from its focus or beam waist. Because the focus is within the light source 310, the beam divergence of the radiation 311 indicates how fast the radiation 311 expands as it travels away from the light source 310 and toward the optical module 320. The beam divergence adjuster can include one or more refractive optical elements such as lenses. The generated radiation 312 can have a modified beam divergence from the generated radiation 311. In particular, the beamdivergence adjuster can modify the beam divergence of the generated radiation 311 to form generated radiation 312 that matches an optical cavity mode of the optical cavity of the optical module 320. The adjustment to the beam divergence of the generated radiation 312 enables control of the mode coupling into the optical cavity to assure a more rapid loss in higher order modes does not contribute to uncertainty in the measurement performed by the measurement module 340. Specifically, this can ensure that the lowest order spatial mode in the generated radiation 311 is excited efficiently, and avoid exciting higher order spatial modes. In this way, marginally-stable higher order spatial modes within the generated radiation 311 can be effectively filtered out of the generated radiation 312 that is directed to the optical module 320.

[0062] In other implementations, the mode control module 332 includes an aperture configured to spatially filter out the higher-order spatial modes in the generated radiation 311 to thereby form the generated radiation 312. In some implementations, the aperture of the mode control module 332 can be within the beam delivery module 330. In other implementations, an aperture 334 can be configured inside the optical cavity (between the test mirror 305 and the reference mirror 322 of the optical module 320). In some implementations, the aperture can be configured to control beam size to perfectly match the cavity mode.

[0063] The polarization module 333 is configured to adjust an optical polarization of the generated radiation 311 to thereby form the generated radiation 312 having a desired polarization. For example, the polarization module 333 can be configured to form generated radiation 312 having a linear polarization.

[0064] In some implementations, the optical module 320 is defined inside an optical enclosure 325 that is sealed. In this way, an interior or cavity 326, defined by the optical enclosure 325 and in which the optical module 320 is configured, can be maintained as a controlled environment. For example, the interior 326 can be gas purged or a vacuum environment. As a vacuum environment, the interior 326 can be kept at a negative pressure level that is at a pressure that is lower than 0.1 kilopascals (kPa). By controlling the environment of the interior 326, Rayleigh scattering from nitrogen can be reduced. Moreover, gas purging the interior 326 with a pure non-absorbing gas such as nitrogen, helium, or argon can flush out impurities in the optical module 320 and the optical cavity, thus reducing errors in the measurements of the reflectivity by the measurement module 340. Errors in measurements can occur whenever impurities on surfaces in the optical module 320 boil off and absorb the generated radiation 312. For pressures below 0. 1 Pa, gas purging may not be needed.

[0065] The metrology apparatus 300 also includes an actuation module 350 mechanically coupled to the reference mount 321 and the test mount 323. The actuation module 350 is configured to provide one or more signals to one or more of the reference mount 321 and the test mount 323 to thereby make mechanical adjustments to the respective reference mirror 322 and test mirror 305. In this way, a relative position and angle between the reference mirror 322 and the test mirror 305 can be adjusted. For example, with reference to Fig. 4A, the actuation module 350 sends a signal 351A to thetest mount 323 to cause it to translate the test mirror 305 along a cavity direction Zc. As another example, as shown in Fig. 4B, the actuation module 350 sends a signal 35 IB to the reference mount 321 to translate the reference mirror 322 along the cavity direction Zc. As a further example, and with reference to Fig. 4C, the actuation module 350 sends a signal 351C to the test mount 323 to cause it to tip and / or tilt the test mirror 305 relative to the cavity direction Zc. Specifically, the test mirror 305 can be rotated about one or more axes that are perpendicular to the cavity direction Zc. Lastly, with reference to Fig. 4D, the actuation module 350 sends a signal 35 ID to the reference mount 321 to cause it to tip and / or tilt the reference mirror 322 relative to the cavity direction Zc. Specifically, the reference mirror 322 can be rotated about one or more axes that are perpendicular to the cavity direction Zc. By making these adjustments, the actuation module 350 is able to adjust the cavity length Lc, which is the distance between the reference mirror 322 and the test mirror 305.

[0066] Referring again to Fig. 3, the measurement module 340 includes a detector 327 configured to receive radiation 325 transmitted through the optical cavity of the optical module 320 and a processing apparatus 345. The detector 327 is configured to produce an electrical signal 328 based on the received radiation 325 and this electrical signal 328 is provided to the processing apparatus 345. The processing apparatus 345 is configured to estimate a reflectivity of the test mirror 305 based on the electrical signal 328.

[0067] Referring to Fig. 5, an implementation of a measurement module 540 is shown. The measurement module 540 is configured to receive radiation 525 (which can correspond to the radiation 125 of Fig. 1 or the radiation 325 of Fig. 3) and to estimate a reflectivity of a test mirror (such as the test mirror 105 of Fig. 1 or the test mirror 305 of Fig. 3) based on the received radiation 525. The measurement module 540 includes a detector 527 and a processing apparatus 545. The detector 527 is configured to receive the radiation 525, which is transmitted through the optical cavity of the optical module (such as the optical module 120 of Fig. 1 or the optical module 320 of Fig. 3). The detector 527 is configured to produce an electrical signal 528 based on the received radiation 525 and this electrical signal 528 is provided to the processing apparatus 545. The processing apparatus 545 is configured to estimate a reflectivity of the test mirror based on the electrical signal 528.

[0068] The detector 527 includes a light detector (such as a photomultiplier tube) 541 and an oscilloscope 542. The photomultiplier tube 541 can be suitable for detection of the radiation 525 in applications that require low noise and high sensitivity. The photomultiplier tube 541 includes a photocathode material on which the radiation 525 impinges, and electrons are ejected from the photocathode material as a result. These electrons are multiplied in a manner that amplifies the electrical signal, which eventually reaches an anode. The electrical signal (in the form of a current) 543 produced at the anode is detected by the oscilloscope 542. The oscilloscope 542 is configured to digitize and store the electrical signal 543. The oscilloscope 542 can also graphically display the electrical signal 543. The detector 527 has a response time that is short relative to the round -trip time of the optical cavity and the pulse duration.

[0069] The detector 527 can also include an optical filter 544 configured to block interference radiation within the radiation 525 that is at wavelengths not in the DUV range from reaching the light detector 541.

[0070] With reference to Fig. 6, an example of a graph 660 of the amplitude 661 of the electrical signal 528 is plotted versus time 662. The graph 660 is discussed with reference to the metrology apparatus 100 of Fig. 1, but it can also be discussed with reference to the metrology apparatus 300 of Fig. 3. In general, the electrical signal 528 is a series of peaks starting with peak Pl that decay over time. Each peak corresponds to a pulse of the radiation 525 that is transmitted from the optical cavity of the optical module 120 to the measurement module 540. The series of peaks in the electrical signal 528 are produced from a single pulse of the radiation 111 produced from the light source 110. Because energy is lost in the optical cavity of the optical module 120 with each roundtrip between the reference mirror 122 and the test mirror 105, the amplitude 661 of each peak in the electrical signal 528 is reduced with the passage of time 662.

[0071] The processing apparatus 545 includes a set of software modules 546, 547, 548, 549 that can each perform one or more tasks on the electrical signal 528. Some of the modules perform preprocessing tasks while others perform tasks directly related to calculating the reflectivity of the test mirror. The processing apparatus 545 can include other features not shown in Fig. 5. The processing apparatus 545 can include one or more of digital electronic circuitry, computer hardware, firmware, and software. The processing apparatus 545 can include or can have access to memory, which can be read-only memory and / or random-access memory. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including, by way of example, semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; and CD-ROM disks. The processing apparatus 545 can also include one or more input devices (such as, for example, one or more of a keyboard, touch screen, microphone, mouse, hand-held input device) and one or more output devices (such as, for example, a speaker or a monitor). The processing apparatus 545 can include one or more programmable processors, and one or more computer program products tangibly embodied in a machine-readable storage device for execution by a programmable processor. The one or more programmable processors can each execute a program of instructions to perform desired functions by operating on input data and generating appropriate output such as using the software modules 546, 547, 548, 549. Each of the modules 546, 547, 548 can be a set of computer program products executed by one or more processors. Each of these processing modules can communicate with each other.

[0072] For example, the software module 546 is configured to receive the raw data in the electrical signal 528 and filter out the information that does not cross a threshold value. A threshold value T628 is displayed as a dashed line. For example, if an average value of the amplitude of the electrical signal 528 does not cross the threshold value T628, then the processing apparatus 545 doesnot perform tasks on the electrical signal 528 because there is not enough information to use in the electrical signal 528.

[0073] The module 547 is configured to determine an amplitude or maximum value Ip of each peak in the electrical signal 528 and a peak time tp associated with each peak by analyzing the graph 660. The peak time tp corresponds to the time at which the peak reaches its maximum value Ip. The output of the module 547 is therefore a series of pairs of values [Ip, tp].

[0074] The module 548 is configured to calculate a time constant r for the decay of the electrical signal 528. The time constant r can be referred to as the decay rate. The time constant r is calculated based on the determined amplitudes Ip and the determined peak times tp. Additionally, the module 548 can calculate the time constant r based also on the cavity length Lc (see Figs. 4A-4D) between the reference mirror 122 and the test mirror 105. The electrical signal 528 is shaped like a decay function, and the module 548 can calculate the time constant r by fitting the electrical signal 528 to a decay function. The type of decay function that can be fitted can be based on the type of medium within the optical cavity as well as the properties of the test mirror. In some implementations, the decay function is an exponential function given by:Equation 1 where I is the amplitude of the electrical signal 528, Io is the amplitude of the first peak (as shown in Fig. 6), r is the time constant, c is the speed of light in a vacuum, Lc is the optical cavity length, and t is the time. The module 548 determines or calculates the time constant r by fitting the electrical signal 528 data (the pairs of values [Ip, tp]) to the decay function given by the exponential function in Equation 1.

[0075] The module 549 is configured to estimate or calculate a reflectivity of the test mirror 105 based on the time constant r calculated by the module 548 in accordance with a relationship between the reflectivity Rios and the time constant r as follows:Equation 2 where Rios is the reflectivity of the test mirror 105, R122 is the known reflectivity of the reference mirror 122, a is an absorption coefficient of the optical cavity, and Lc is the optical cavity length. The reflectivity Rios of the test mirror 105 can therefore be calculated from the known values of R122, a, and Lc. The reflectivity Rios of the test mirror 105 can be calculated (by the module 549 within the measurement module 540) using Equation 2 with a sensitivity that is better than 0.1%.

[0076] Referring to Fig. 7, a procedure 780 is performed by the metrology apparatus 100 (or the metrology apparatus 300) for analyzing a reflectivity of a test mirror. Reference is made to themetrology apparatus 300 of Fig. 3 when discussing the procedure 780 but the metrology apparatus 100 of Fig. 3 can perform the procedure 780. Initially, pulsed radiation is generated (781). For example, the light source 310 generates (781) the pulsed radiation 311. The pulsed radiation 311 can have a wavelength in the DUV wavelength range. For example, the pulsed radiation 311 can have a wavelength below 196 nm or about 193 nm.

[0077] The generated pulsed radiation is delivered into an optical cavity formed between a reference mirror at a reference location and the test mirror at a test location (782). For example, the beam delivery module 330 delivers (782) the generated pulsed radiation 312 into the optical cavity formed by the optical module 320 and the optical cavity is formed between the reference mirror 322 attached or fixed to the reference mount 321 and the test mirror 305 attached or fixed to the test mount 323. In some implementations, the beam delivery module 330 aligns the pulsed radiation 312 with the optical cavity. In some implementations, the beam delivery module 330 additionally filters out unwanted spatial modes in the pulsed radiation 312 prior to delivering the pulsed radiation 312 into the optical cavity, as discussed above.

[0078] Radiation transmitted through the optical cavity is received (783). For example, radiation 325 that is transmitted through the optical cavity of the optical module 320 is received (783) at the measurement module 340. In some implementations, the measurement module 340 (specifically the detector 327) produces an electrical signal 328 based on the received radiation (at 783) and this electrical signal 328 is provided to a processing apparatus 345.

[0079] Next, the reflectivity of the test mirror is estimated or calculated based on the received radiation (784). For example, the measurement module 340 can calculate the reflectivity R305 of the test mirror 305 in accordance with Equation 2. In some implementations, the processing apparatus 345 estimates or calculates the reflectivity R305 based on the electrical signal 328 provided by the detector 327.

[0080] Prior to the start of the procedure 780, the test mirror can be received and placed at the test location. For example, the test mirror 305 can be received from a supplier who manufactured the test mirror. As another example, the test mirror 305 can be removed from a DUV optical source (such as the optical source 170). The test mirror 305 can then be installed at the second location by fixing the test mirror to the test mount 323. Moreover, the test mount 323 and / or the reference mount 321 can be configured to adjust one or more of a translational location and a rotational state of either or both of the test mirror 305 and the reference mirror 322 in order to adjust the cavity length Lc and ensure that an optical cavity is created.

[0081] After the procedure 780, if the measurement module 340 determines that the reflectivity of the test mirror 305 is acceptable, then the test mirror 305 can be mounted within the optical source 170 (Fig. 1) for operation in the optical source 170. The quality of the test mirror 305 can be categorized as good or unacceptable based on the estimated reflectivity determined at 784.

[0082] Referring to Fig. 8, an implementation of an optical source 870 that can receive a test mirror 805 (such as the test mirror 105 or 305) that has been determined to have an acceptable reflectivity with the metrology apparatus 100 or 300 is shown. The optical source 870 is a multi-stage optical source that produces pulsed optical radiation 871 as a light beam under control of a control system 872. As shown in the example of Fig. 8, the optical source 870 is a two-stage system that includes a master oscillator (MO) 873 that provides a seed light beam 874 to a power amplifier (PA) 875. The master oscillator 873 typically includes a gain medium in which amplification occurs and an optical feedback mechanism such as an optical resonator. The power amplifier 875 typically includes a gain medium in which amplification occurs when seeded with the seed laser beam 874 from the master oscillator 873. If the power amplifier 875 is designed as a regenerative ring resonator then it is described as a power ring amplifier (PRA), and in this case, enough optical feedback can be provided from the ring design. The master oscillator 873 enables tuning of spectral parameters such as the center wavelength and the bandwidth at relatively low output pulse energies (when compared with the output of the power amplifier 875). The power amplifier 875 receives the seed light beam 874 from the master oscillator 873 and amplifies the seed light beam 874 to attain the necessary powers in the optical radiation 871 for output to use in an output apparatus 890 (for example, for photolithography).

[0083] The master oscillator 873 includes a gas discharge chamber having two elongated electrodes, a gas that serves as the gain medium, and a fan for circulating the gas between the electrodes. A laser resonator is formed between a second actuatable apparatus 876 (which acts as a spectral feature selection system) on one side of the discharge chamber and an output coupler 877 on a second side of the discharge chamber. The second actuatable apparatus 876 receives a light beam 878 from the master oscillator 873, and finely tunes the spectral output of the light beam 871 produced by the optical source 870 by finely tuning the spectral features of the light beam 878 based on the input from the control system 872. Spectral features (such as the wavelength and the bandwidth) of the seed light beam 874 (and therefore the output light beam 871) are determined by the configuration of the master oscillator 873, and these spectral features can be adjusted by adjusting the light beam 878 that is produced within the master oscillator 873.

[0084] The power amplifier 875 includes a power amplifier discharge chamber, and, if it is a regenerative ring amplifier, the power amplifier also includes a beam return (such as a reflector) 879 that returns (via reflection, for example) the light beam back into the discharge chamber to form a circulating and looped path (in which the input into the ring amplifier intersects the output out of the ring amplifier). The power amplifier discharge chamber includes a pair of elongated electrodes, a gas that serves as the gain medium, and a fan for circulating the gas between the electrodes. The seed light beam 874 is amplified by repeatedly being passed through the power amplifier 875. A beam modification optical system 891 provides a way (for example, a partially-reflecting mirror) to incouple the seed light beam 874 to the power amplifier 875 and to out-couple a portion of the amplified radiation from the power amplifier 875 to form the output light beam 871.

[0085] The optical source 870 can also include a line center analysis module (LAM) 892 that receives an output from the output coupler 877. The LAM 892 can be used to measure the wavelength (for example, the center wavelength) of the seed light beam 874 or the output light beam 871.

[0086] The optical source 870 can also include a bandwidth analysis module (BAM) 893 that is configured to analyze the output light beam 871. The BAM 893 can be used to measure the bandwidth of the output light beam 871. In some implementations, the BAM 893 can include an etalon assembly that produces interference fringes at a photodiode array and the bandwidth can be determined from these fringes.

[0087] The gas used in the discharge chambers can be any suitable gas for producing a light beam (for example, a laser beam) around the required wavelengths and bandwidth. For example, the gas can be argon fluoride (ArF), which emits light at a wavelength of about 193 run, or krypton fluoride (KrF), which emits light at a wavelength of about 248 nm.

[0088] The optical source 870 also includes an optical pulse stretcher 894, where copies of the output light beam 871 are delayed and recombined to thereby reduce speckle in the output light beam 871 that is directed to the output apparatus 890. The optical pulse stretcher 894 further reduces a rate of optical damage to optical components downstream of the optical pulse stretcher 894. Examples of an optical pulse stretcher 894 are described in WO 2021 / 076658, published on April 22, 2021 by applicant Cymer, LLC, the disclosure of which is incorporated herein by reference in its entirety. In some implementations, the optical pulse stretcher 894 includes a single pulse stretcher. In other implementations, the optical pulse stretcher 894 includes several stages of pulse stretchers. For example, a pulse stretcher can include a plurality (at least two) of concave mirrors arranged relative to each other to form a confocal resonator. In some implementations, as discussed in detail in WO 2021 / 076658, the optical pulse stretcher 894 includes a first optical pulse stretcher device and a second optical pulse stretcher device arranged as a series of stacked optical pulse stretcher devices. In these implementations, the first optical pulse stretcher device receives the input pulsed light beam (from the power amplifier 875) and delays and recombines copies of the input pulsed light beam to generate a first stretched pulsed light beam. This first stretched pulsed light beam is then input to the second optical pulse stretcher device, which delays and recombines copies of the first stretched pulsed light beam to generate a second stretched pulsed light beam. This second stretched pulsed light beam is input to the first optical pulse stretcher device, where it is then redirected out as the output light beam 871. Each optical pulse stretcher device can include one or more optical pulse stretchers, with each optical pulse stretcher including one or more confocal resonators. A confocal resonator includes reflecting surfaces (for example, on mirrors) that generally face each other and are arranged relative to each other so that a pulsed light beam (such as the second pulsed light beam or the first stretched pulsed light beam) is reflected back and forth in a region between the reflecting surfaces. Thus, the optical pulse stretcher 894 can include many highly-reflective mirrors, and the metrology apparatus100 or 300 can be used to characterize the reflectivity of one or more of these highly -reflective mirrors.

[0089] As another example, the optical source 870 can also include a beam turning module 895 configured to redirect the seed light beam 874 along a path toward the beam modification optical system 891. The beam turning module 895 includes one or more highly -reflective mirrors. Thus, the metrology apparatus 100 or 300 can be used to characterize the reflectivity of these highly-reflective mirrors.

[0090] The optical source 870 can include other components not shown in Fig. 8. The test mirror 805 can be placed in any of the components of the optical source 870. In some implementations, the test mirror 805 is placed in the power amplifier 875 or the beam modification optical system 891.

[0091] In other implementations, the light detector 541 is a semiconductor device such as a silicon photomultiplier, an avalanche photodiode, or a vacuum photodiode.

[0092] In some implementations, as shown in Fig. 9, a metrology apparatus 900 (similar in design to the metrology apparatus 100) can be configured with two reference mirrors 922a, 922b arranged to create an optical module 920 in which an optical cavity is formed at a non-linear angle 0 (that is greater than 0° and less than 180°). The reference mirrors 922a, 922b are both concave. A pivot point is created and a test mirror 905 is positioned at the pivot point. The test mirror 905 is a flat mirror. A measurement module 940 receives radiation 925 transmitted through the reference mirror 922b. The measurement module 940 can determine the reflectivity of the test mirror 905 by 1. setting up the reference mirrors 922a, 922b in the linear optical cavity (without the test mirror 905) and calculating the decay rate; 2. setting up the reference mirrors 922a, 922b with the test mirror 905 in the angled optical module 920 and calculating a decay rate in the angled optical cavity; and 3. comparing the decay rates to determine the reflectivity of the test mirror 905.

[0093] In some implementations, the metrology apparatus 100 can be configured to measure an absorption of the test mirror 105 or a transmissive optic using a non-normal angle of incidence.

[0094] The implementations can be further described using the following clauses:1. A metrology apparatus comprising: a light source generating radiation in the deep ultraviolet wavelength range; an optical module including one or more reference mirrors, each reference mirror at a respective reference mount and a test mount configured to receive a test mirror arranged relative to the reference mirror when installed to thereby form an optical cavity, the optical module placed in a path of the generated radiation; and a measurement module configured to: receive radiation transmitted through the optical cavity; and estimate a reflectivity of a test mirror received at the test mount based on the received radiation.2. The metrology apparatus of clause 1, wherein the optical module is defined inside an optical enclosure that is sealed, the interior of which is maintained as a controlled environment.3. The metrology apparatus of clause 2, wherein the interior is configured as a gas purged or vacuum environment.4. The metrology apparatus of clause 2, wherein the interior is configured as a vacuum environment that has a negative pressure level that is at a pressure that is lower than 0. 1 kilopascals (kPa).5. The metrology apparatus of clause 3, wherein the interior is configured as a nitrogen purged environment or a helium purged environment.6. The metrology apparatus of clause 1, wherein the generated radiation has a wavelength below 196 nanometers (nm).7. The metrology apparatus of clause 6, wherein the generated radiation has a wavelength of about 193 nm.8. The metrology apparatus of clause 1, wherein each reference mirror is a concave, convex, or flat mirror, and the test mirror is a concave, convex, or a flat mirror.9. The metrology apparatus of clause 1, wherein each reference mirror and the test mirror are both concave mirrors having a focal length that is greater than 0.5 meters (m), greater than 1.0 m, greater than 1.3 m, or about 1.4 m.10. The metrology apparatus of clause 1, wherein the light source is configured to generate the radiation as pulsed radiation.11. The metrology apparatus of clause 1, further comprising a beam delivery module that is configured to: receive the generated radiation from the light source, align the generated radiation with the optical cavity, and direct the generated radiation to the optical module.12. The metrology apparatus of clause 11, wherein the beam delivery module is configured to adjust a beam pointing of the generated radiation relative to the optical cavity.13. The metrology apparatus of clause 11, wherein the beam delivery module comprises a polarization module configured to adjust a polarization of the generated radiation.14. The metrology apparatus of clause 13, wherein the polarization module is configured to output the generated radiation with a linear polarization.15. The metrology apparatus of clause 1, further comprising a beam delivery module that is configured to: receive the generated radiation from the light source, the beam delivery module comprising a mode control configured to effectively filter out spatial modes of the generated radiation from the light source.16. The metrology apparatus of clause 15, wherein the mode control comprises a beam divergence module configured to modify a beam divergence of the generated radiation to match an optical cavity mode of the optical cavity.17. The metrology apparatus of clause 15, wherein the mode control comprises an aperture.18. The metrology apparatus of clause 1, further comprising an actuation module mechanically coupled to each reference mount and the test mount, the actuation module configured to mechanically adjust one or more of each reference mount and the test mount to thereby adjust one or more of eachreference mirror and the test mirror when the test mirror is received at the test mount and adjust a cavity length.19. The metrology apparatus of clause 18, wherein the actuation module being configured to mechanically adjust a reference mount thereby adjusts one or more of a translational location and a rotational state of the reference mirror and the actuation module being configured to mechanically adjust the test mount thereby adjusts one or more of a translational location and a rotational state of the test mirror.20. The metrology apparatus of clause 1, wherein the generated radiation is a pulsed laser beam.21. The metrology apparatus of clause 20, wherein the pulses of the pulsed laser beam have a pulse width of that is less than or equal to a round trip time through the optical cavity.22. The metrology apparatus of clause 20, wherein the measurement module comprises: a detector configured to receive the radiation transmitted through the optical cavity and to produce an electrical signal based on the received radiation; and a processing apparatus configured to estimate the reflectivity of the test mirror based on the produced electrical signal.23. The metrology apparatus of clause 22, wherein the detector comprises a photomultiplier tube, and an oscilloscope configured to digitize and store an electrical signal produced by the photomultiplier tube.24. The metrology apparatus of clause 23, wherein the oscilloscope is configured to graphically display the electrical signal produced by the photomultiplier tube.25. The metrology apparatus of clause 23, wherein the detector further comprises an optical filter configured to block interference radiation at wavelengths not in the DUV range from reaching the photomultiplier tube.26. The metrology apparatus of clause 22, wherein the processing apparatus is configured to estimate the reflectivity of the test mirror by fitting the produced electrical signal to a decay function.27. The metrology apparatus of clause 26, wherein the decay function is an exponential function.28. The metrology apparatus of clause 26, wherein the processing apparatus is configured to, prior to fitting the produced electrical signal to the single exponential function: determine an amplitude of each peak in the produced electrical signal due to a pulse of the transmitted radiation and a peak time associated with each peak; and calculate a time constant for the decay of the produced electrical signal based on the determined amplitudes, a cavity length defined between the reference mirror and the test mirror, and the peak times.29. The metrology apparatus of clause 28, wherein the processing apparatus is configured to estimate the reflectivity of the test mirror based at least on the calculated time constant.30. The metrology apparatus of clause 29, wherein the processing apparatus is configured to estimate the reflectivity of the test mirror also based on at least the reflectivity of each reference mirror, an absorption coefficient within the optical cavity, and the cavity length.31. The metrology apparatus of clause 1, wherein the optical cavity is a stable optical cavity configured to contain the radiation in the stable optical cavity.32. The metrology apparatus of clause 1, wherein the measurement module is configured to estimate the reflectivity of the test mirror with a sensitivity that is better than 0. 1%.33. The metrology apparatus of clause 1, further comprising a mode control within the optical cavity of the optical module, the mode control configured to effectively filter out high order spatial modes in the generated radiation.34. The metrology apparatus of clause 1, wherein the optical module includes a single reference mirror and the optical cavity is a linear optical cavity.35. The metrology apparatus of clause 1, wherein the optical module includes two reference mirrors and the optical cavity is a folded optical cavity.36. A method for analyzing a reflectivity of a test mirror, the method comprising: generating a pulsed radiation having a wavelength in the deep ultraviolet range; delivering the generated pulsed radiation into an optical cavity formed between a reference mirror at a reference location and the test mirror at a test location; receiving radiation transmitted through the optical cavity; and estimating a reflectivity of the test mirror based on the received radiation.37. The method of clause 36, further comprising: receiving the test mirror that has been removed from a deep ultraviolet optical source; installing the test mirror at the second location to form the optical cavity; and categorizing a quality of the test mirror based on the estimated reflectivity of the test mirror.38. The method of clause 36, further comprising: receiving the test mirror from a supplier who manufactured the test mirror; installing the test mirror at the second location to form the optical cavity; and categorizing a quality of the test mirror based on the estimated reflectivity of the test mirror.39. The method of clause 36, wherein generating the pulsed radiation comprises generating pulsed radiation having a wavelength below 196 nanometers (nm).40. The method of clause 36, wherein delivering the generated pulsed radiation into the optical cavity comprises receiving the generated pulsed radiation, aligning the generated pulsed radiation with the optical cavity, and directing the generated pulsed radiation to the optical cavity.41. The method of clause 36, wherein delivering the generated pulsed radiation into the optical cavity comprises receiving the generated pulsed radiation and modifying a beam divergence of the generated radiation to match an optical cavity mode of the optical cavity.42. The method of clause 36, further comprising, prior to delivering the generated pulsed radiation into the optical cavity: adjusting one or more of a translational location and a rotational state of the reference mirror; adjusting one or more of a translational location and a rotational state of the test mirror; and adjusting a cavity length that is a distance between the reference mirror and the test mirror.43. The method of clause 36, wherein receiving radiation transmitted through the optical cavity comprises: producing an electrical signal based on the received radiation; and estimating the reflectivity of the test mirror based on the produced electrical signal.44. The method of clause 43, wherein receiving the radiation transmitted through the optical cavity comprises blocking interference radiation at wavelengths not in the DUV range.45. The method of clause 43, wherein estimating the reflectivity of the test mirror comprises fitting the produced electrical signal to a decay function.46. The method of clause 45, further comprising, prior to fitting the produced electrical signal to the single exponential function: determining an amplitude of each peak in the produced electrical signal due to a pulse of the transmitted radiation and a peak time associated with each peak; and calculating a time constant for decay of the produced electrical signal based on the determined amplitudes, a cavity length defined between the reference mirror and the test mirror, and the peak times.47. The method of clause 46, wherein estimating the reflectivity of the test mirror is based at least on the calculated time constant.48. The method of clause 47, wherein estimating the reflectivity of the test mirror is also based on at least the reflectivity of the reference mirror, an absorption coefficient within the optical cavity, and the cavity length.49. The method of clause 46, wherein estimating the reflectivity of the test mirror comprises estimating the reflectivity with a sensitivity that is better than 0.1%.50. The method of clause 46, further comprising effectively filtering out high order spatial modes in the generated radiation prior to delivering the generated pulsed radiation into an optical cavity.51. A metrology apparatus comprising: a light source generating pulsed radiation having a wavelength in the deep ultraviolet range; an optical module including a reference mirror at a reference mount and a test mount configured to receive a test mirror facing the reference mirror to thereby form an optical cavity within a controlled and gas purged environment; a beam delivery module that receives the generated radiation from the light source, aligns the generated radiation with the optical cavity, and directs the generated radiation to the optical module; anda measurement module comprising: a detector configured to receive the radiation transmitted through the optical cavity and to produce an electrical signal based on the received radiation; and a processing apparatus configured to estimate the reflectivity of a test mirror received at the second mount based on the produced electrical signal.

[0095] Other implementations are within the scope of the following claims.

Claims

CLAIMS1. A metrology apparatus comprising: a light source generating radiation in the deep ultraviolet wavelength range; an optical module including one or more reference mirrors, each reference mirror at a respective reference mount and a test mount configured to receive a test mirror arranged relative to the reference mirror when installed to thereby form an optical cavity, the optical module placed in a path of the generated radiation; and a measurement module configured to: receive radiation transmitted through the optical cavity; and estimate a reflectivity of a test mirror received at the test mount based on the received radiation.

2. The metrology apparatus of claim 1, wherein the optical module is defined inside an optical enclosure that is sealed, the interior of which is maintained as a controlled environment.

3. The metrology apparatus of claim 2, wherein the interior is configured as a gas purged or vacuum environment.

4. The metrology apparatus of claim 2, wherein the interior is configured as a vacuum environment that has a negative pressure level that is at a pressure that is lower than 0. 1 kilopascals (kPa).

5. The metrology apparatus of claim 3, wherein the interior is configured as a nitrogen purged environment or a helium purged environment.

6. The metrology apparatus of claim 1, wherein the generated radiation has a wavelength below 196 nanometers (nm).

7. The metrology apparatus of claim 6, wherein the generated radiation has a wavelength of about 193 nm.

8. The metrology apparatus of claim 1, wherein each reference mirror is a concave, convex, or flat mirror, and the test mirror is a concave, convex, or a flat mirror.

9. The metrology apparatus of claim 1, wherein each reference mirror and the test mirror are both concave mirrors having a focal length that is greater than 0.5 meters (m), greater than 1.0 m, greater than 1.3 m, or about 1.4 m.

10. The metrology apparatus of claim 1, wherein the light source is configured to generate the radiation as pulsed radiation.

11. The metrology apparatus of claim 1, further comprising a beam delivery module that is configured to: receive the generated radiation from the light source, align the generated radiation with the optical cavity, and direct the generated radiation to the optical module.

12. The metrology apparatus of claim 11, wherein the beam delivery module is configured to adjust a beam pointing of the generated radiation relative to the optical cavity.

13. The metrology apparatus of claim 11, wherein the beam delivery module comprises a polarization module configured to adjust a polarization of the generated radiation.

14. The metrology apparatus of claim 13, wherein the polarization module is configured to output the generated radiation with a linear polarization.

15. The metrology apparatus of claim 1, further comprising a beam delivery module that is configured to: receive the generated radiation from the light source, the beam delivery module comprising a mode control configured to effectively filter out spatial modes of the generated radiation from the light source.

16. The metrology apparatus of claim 15, wherein the mode control comprises a beam divergence module configured to modify a beam divergence of the generated radiation to match an optical cavity mode of the optical cavity.

17. The metrology apparatus of claim 15, wherein the mode control comprises an aperture.

18. The metrology apparatus of claim 1, further comprising an actuation module mechanically coupled to each reference mount and the test mount, the actuation module configured to mechanically adjust one or more of each reference mount and the test mount to thereby adjust one or more of each reference mirror and the test mirror when the test mirror is received at the test mount and adjust a cavity length.

19. The metrology apparatus of claim 18, wherein the actuation module being configured to mechanically adjust a reference mount thereby adjusts one or more of a translational location and a rotational state of the reference mirror and the actuation module being configured to mechanically adjust the test mount thereby adjusts one or more of a translational location and a rotational state of the test mirror.

20. The metrology apparatus of claim 1, wherein the generated radiation is a pulsed laser beam.

21. The metrology apparatus of claim 20, wherein the pulses of the pulsed laser beam have a pulse width of that is less than or equal to a round trip time through the optical cavity.

22. The metrology apparatus of claim 20, wherein the measurement module comprises: a detector configured to receive the radiation transmitted through the optical cavity and to produce an electrical signal based on the received radiation; and a processing apparatus configured to estimate the reflectivity of the test mirror based on the produced electrical signal.

23. The metrology apparatus of claim 22, wherein the detector comprises a photomultiplier tube, and an oscilloscope configured to digitize and store an electrical signal produced by the photomultiplier tube.

24. The metrology apparatus of claim 23, wherein the oscilloscope is configured to graphically display the electrical signal produced by the photomultiplier tube.

25. The metrology apparatus of claim 23, wherein the detector further comprises an optical filter configured to block interference radiation at wavelengths not in the DUV range from reaching the photomultiplier tube.

26. The metrology apparatus of claim 22, wherein the processing apparatus is configured to estimate the reflectivity of the test mirror by fitting the produced electrical signal to a decay function.

27. The metrology apparatus of claim 26, wherein the decay function is an exponential function.

28. The metrology apparatus of claim 26, wherein the processing apparatus is configured to, prior to fitting the produced electrical signal to the single exponential function:determine an amplitude of each peak in the produced electrical signal due to a pulse of the transmitted radiation and a peak time associated with each peak; and calculate a time constant for the decay of the produced electrical signal based on the determined amplitudes, a cavity length defined between the reference mirror and the test mirror, and the peak times.

29. The metrology apparatus of claim 28, wherein the processing apparatus is configured to estimate the reflectivity of the test mirror based at least on the calculated time constant.

30. The metrology apparatus of claim 29, wherein the processing apparatus is configured to estimate the reflectivity of the test mirror also based on at least the reflectivity of each reference mirror, an absorption coefficient within the optical cavity, and the cavity length.

31. The metrology apparatus of claim 1, wherein the optical cavity is a stable optical cavity configured to contain the radiation in the stable optical cavity.

32. The metrology apparatus of claim 1, wherein the measurement module is configured to estimate the reflectivity of the test mirror with a sensitivity that is better than 0.1%.

33. The metrology apparatus of claim 1, further comprising a mode control within the optical cavity of the optical module, the mode control configured to effectively filter out high order spatial modes in the generated radiation.

34. The metrology apparatus of claim 1, wherein the optical module includes a single reference mirror and the optical cavity is a linear optical cavity.

35. The metrology apparatus of claim 1, wherein the optical module includes two reference mirrors and the optical cavity is a folded optical cavity.

36. A method for analyzing a reflectivity of a test mirror, the method comprising: generating a pulsed radiation having a wavelength in the deep ultraviolet range; delivering the generated pulsed radiation into an optical cavity formed between a reference mirror at a reference location and the test mirror at a test location; receiving radiation transmitted through the optical cavity; and estimating a reflectivity of the test mirror based on the received radiation.

37. The method of claim 36, further comprising:receiving the test mirror that has been removed from a deep ultraviolet optical source; installing the test mirror at the second location to form the optical cavity; and categorizing a quality of the test mirror based on the estimated reflectivity of the test mirror.

38. The method of claim 36, further comprising: receiving the test mirror from a supplier who manufactured the test mirror; installing the test mirror at the second location to form the optical cavity; and categorizing a quality of the test mirror based on the estimated reflectivity of the test mirror.

39. The method of claim 36, wherein generating the pulsed radiation comprises generating pulsed radiation having a wavelength below 196 nanometers (nm).

40. The method of claim 36, wherein delivering the generated pulsed radiation into the optical cavity comprises receiving the generated pulsed radiation, aligning the generated pulsed radiation with the optical cavity, and directing the generated pulsed radiation to the optical cavity.

41. The method of claim 36, wherein delivering the generated pulsed radiation into the optical cavity comprises receiving the generated pulsed radiation and modifying a beam divergence of the generated radiation to match an optical cavity mode of the optical cavity.

42. The method of claim 36, further comprising, prior to delivering the generated pulsed radiation into the optical cavity: adjusting one or more of a translational location and a rotational state of the reference mirror; adjusting one or more of a translational location and a rotational state of the test mirror; and adjusting a cavity length that is a distance between the reference mirror and the test mirror.

43. The method of claim 36, wherein receiving radiation transmitted through the optical cavity comprises: producing an electrical signal based on the received radiation; and estimating the reflectivity of the test mirror based on the produced electrical signal.

44. The method of claim 43, wherein receiving the radiation transmitted through the optical cavity comprises blocking interference radiation at wavelengths not in the DUV range.

45. The method of claim 43, wherein estimating the reflectivity of the test mirror comprises fitting the produced electrical signal to a decay function.

46. The method of claim 45, further comprising, prior to fitting the produced electrical signal to the single exponential function: determining an amplitude of each peak in the produced electrical signal due to a pulse of the transmitted radiation and a peak time associated with each peak; and calculating a time constant for decay of the produced electrical signal based on the determined amplitudes, a cavity length defined between the reference mirror and the test mirror, and the peak times.

47. The method of claim 46, wherein estimating the reflectivity of the test mirror is based at least on the calculated time constant.

48. The method of claim 47, wherein estimating the reflectivity of the test mirror is also based on at least the reflectivity of the reference mirror, an absorption coefficient within the optical cavity, and the cavity length.

49. The method of claim 46, wherein estimating the reflectivity of the test mirror comprises estimating the reflectivity with a sensitivity that is better than 0.1%.

50. The method of claim 46, further comprising effectively filtering out high order spatial modes in the generated radiation prior to delivering the generated pulsed radiation into an optical cavity.

51. A metrology apparatus comprising: a light source generating pulsed radiation having a wavelength in the deep ultraviolet range; an optical module including a reference mirror at a reference mount and a test mount configured to receive a test mirror facing the reference mirror to thereby form an optical cavity within a controlled and gas purged environment; a beam delivery module that receives the generated radiation from the light source, aligns the generated radiation with the optical cavity, and directs the generated radiation to the optical module; and a measurement module comprising: a detector configured to receive the radiation transmitted through the optical cavity and to produce an electrical signal based on the received radiation; and a processing apparatus configured to estimate the reflectivity of a test mirror received at the second mount based on the produced electrical signal.

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