Calibration of bandwidth metrology apparatus

Abstract

An apparatus includes: a cell defining an interior including a calibration material, the cell, in operation, placed in a calibration path; an optical arrangement including one or more optical elements configured to produce a pair of calibration light beams directed along the calibration path along opposite directions through the cell; a detector configured to sense light produced from an interaction between the pair of calibration light beams and the calibration material; and a control system in communication with the detector and with a bandwidth metrology apparatus. The control system is configured to calibrate the bandwidth metrology apparatus based at least on an output from the detector.

Description

CALIBRATION OF BANDWIDTH METROLOGY APPARATUSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US Application No. 63 / 699,250, filed on September 26, 2024, titled CALIBRATION OF BANDWIDTH METROLOGY APPARATUS and US Application No. 63 / 744,905, filed on January 14, 2025, titled CALIBRATION OF BANDWIDTH METROLOGY APPARATUS, which are incorporated herein by reference in their entireties.TECHNICAL FIELD

[0002] The disclosed subject matter relates to an apparatus and method for calibration of a bandwidth metrology apparatus.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 100 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 a combination of materials that, when excited, form an excimer such as, for example, 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.SUMMARY

[0005] In some general aspects, an apparatus includes: a cell defining an interior including a calibration material, the cell, in operation, placed in a calibration path; an optical arrangement including one or more optical elements configured to produce a pair of calibration light beams directed along the calibration path along opposite directions through the cell; a detector configured to sense light produced from an interaction between the pair of calibration light beams and the calibration material; and a control system in communication with the detector and with a bandwidth metrology apparatus. The control system is configured to calibrate the bandwidth metrology apparatus based at least on an output from the detector.

[0006] Implementations can include one or more of the following features. For example, the calibration material can include molecular hydrogen. The optical arrangement can include a beam splitter in a primary path of a light beam, the beam splitter configured to separate out a portion of the light beam, and a subset of optical elements configured to form the pair of calibration light beams from the separated-out portion of the light beam to define a calibration path through first and second ends of the cell. The subset of optical elements can include a mirror arranged with its reflective surface facing the second end of the cell along the calibration path, the mirror configured to form a second calibration light beam from a reflection of a first calibration light beam directed through the first and second ends of the cell. The subset of optical elements can include a pair of mirrors arranged to split the separated-out portion of the light beam into first calibration light beam and second calibration light beam directed along opposite directions toward the cell through the respective first and second ends along the calibration path. The subset of optical elements can include a pair of lenses positioned at the first and second ends of the cell. The optical arrangement can include a beam homogenizer configured to spatially homogenize the pair of calibration light beams, and the subset of optical elements can include a pair of lenses positioned at first and second ends of the cell. The detector can include a photodiode or a photomultiplier tube. The detector can be configured to sense fluorescence released from a known excited state of the calibration material that is produced from a two-photon interaction between the pair of calibration light beams and the calibration material. The detector can include a filter configured to block light of wavelengths outside a range that is centered on the wavelength of the fluorescence and that includes the spectral width of the fluorescence. A width of the two-photon interaction can be narrower than a bandwidth of the calibration light beams. The two-photon interaction can be configured to not be subject to Doppler broadening. The detector can be arranged adjacent to the cell. The detector can include two detectors, each configured with a respective filter to sense, independently, the light produced from the decay to eigenstates of an intermediate level.

[0007] The control system can calibrate the bandwidth metrology apparatus by: comparing a bandwidth measurement in the sensed light from the detector with a bandwidth measurement output from the bandwidth metrology apparatus; and determining an adjustment to one or more calibration parameters of the bandwidth metrology apparatus based on the comparison. The control system can compare the bandwidth measurement in the sensed light from the detector with the bandwidth measurement output from the bandwidth metrology apparatus by comparing a width of the intensity distribution of fluorescence produced from the interaction with a bandwidth measurement output from the spectral feature metrology apparatus.

[0008] The cell can include a region transparent to the wavelength of the calibration light beams and a region transparent to the light produced from the interaction between the calibration light beams and the calibration material. The cell can be hermetically sealed after being filled with the calibration material. The cell can include a pump configured to pass calibration material into the interior. Thecontrol system can calibrate the bandwidth metrology apparatus by: comparing at least one width measurement in the sensed light from the detector with at least one bandwidth measurement output from the bandwidth metrology apparatus; and determining an adjustment to one or more calibration parameters of the bandwidth metrology apparatus based on the comparison. The apparatus can further include a spectral feature control apparatus in communication with the control system and configured to scan a wavelength of the calibration light beams across a two-photon energy transition in the calibration material. The detector can sense the light by sensing a fluorescence released from the excited state of the calibration material produced from the two-photon energy transition induced by the interaction between the calibration light beams and the calibration material. The optical arrangement can include a pair of polarization optics, each polarization optic in the path of one of the calibration light beam, and configured to adjust the polarization of the calibration light beams. The pair of polarization optics can be configured to convert the calibration light beams into oppositely- handed circular polarization.

[0009] In other general aspects, a method includes: producing a pair of calibration light beams and directing the calibration light beams along a calibration path along opposite directions through an interior of a cell, the interior comprising a calibration material; sensing light produced from an interaction between the pair of calibration light beams and the calibration material; and calibrating a bandwidth metrology apparatus based at least on the sensed light.

[0010] Implementations can include one or more of the following features. For example, the calibration material can include molecular hydrogen. The pair of calibration light beams can be produced by separating out a portion of a primary light beam, the primary light beam configured for use in photolithography; and forming the pair of calibration light beams from the separated-out portion of the light beam. The calibration light beams can be directed by forming overlapping beam waists of the calibration light beams within the interior of the cell. Forming the overlapping beam waists includes focusing the calibration light beams such that their beam waists overlap in a focal plane within the interior of the cell. The calibration light beams can be directed by: directing a first of the calibration light beams through a first end of the cell; and directing a second of the calibration light beams through the first end of the cell, through the interior, and through a second end of the cell, and reflecting the second of the calibration light beams off a mirror, and then directing the reflection of the second of the calibration light beams through the second end of the cell through the interior. The pair of calibration light beams can be produced by spatially homogenizing the pair of calibration light beams.

[0011] The light produced from the interaction between the pair of calibration light beams and the calibration material can be sensed by sensing fluorescence released from a known excited state of the calibration material that is produced from a two-photon interaction between the pair of calibration light beams and the calibration material. The light produced from the interaction between the pair of calibration light beams and the calibration material can be sensed by spatially blocking stray lightand / or by filtering out light of other wavelengths, such as wavelengths outside a range that is centered on the wavelength of the fluorescence and that includes the spectral width of the fluorescence. For example, the calibration light beams can be spatially blocked or filtered out. A width of the two- photon interaction can be less than a bandwidth of the calibration light beams.

[0012] The bandwidth metrology apparatus can be calibrated by: comparing a spectral width measurement in the sensed light (for example, a bandwidth of the fluorescence) with a bandwidth measurement output from the bandwidth metrology apparatus; and determining an adjustment to one or more calibration parameters of the bandwidth metrology apparatus based on the comparison. The spectral width measurement in the sensed light can be compared with the bandwidth measurement output from the bandwidth metrology apparatus by comparing a width of the intensity distribution of fluorescence produced from the interaction with the bandwidth measurement output from the bandwidth metrology apparatus.

[0013] In other general aspects, an apparatus includes: an optical arrangement including one or more optical elements configured to produce a pair of light beams directed along opposite directions through a gas material held within a cell, the pair of light beams being produced from an output light beam of a light source apparatus; a scanning device configured to scan a wavelength of the pair of light beams across a two-photon energy transition in the gas material; a detector configured to sense fluorescence emitted from the gas material due to the two-photon energy transition as a function of the wavelength of the pair of light beams; and a control system in communication with the detector and configured to adjust the light source apparatus based on the sensed fluorescence.

[0014] Implementations can include one or more of the following features. For example, control system being configured to adjust the light source apparatus based on the sensed fluorescence can include adjusting a bandwidth metrology apparatus of the light source apparatus based on the sensed fluorescence. The control system can be configured to adjust the bandwidth metrology apparatus by calibrating the bandwidth metrology apparatus. The control system can be configured to estimate a spectral feature of the pair of light beams based on the sensed fluorescence. The control system can be configured to adjust the light source apparatus based on the sensed fluorescence by adjusting a spectral feature actuation apparatus to thereby adjust a spectral feature of the output light beam. The estimated spectral feature of the pair of light beams can be a bandwidth of the pair of light beams and the spectral feature of the output light beam that is adjusted can be a bandwidth of the output light beam.

[0015] 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

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

[0017] Fig. 1 is a schematic block diagram of an apparatus including a cell, an optical arrangement, a detector, and a control system configured relative to a bandwidth metrology apparatus associated with a light source apparatus that produces a light beam;

[0018] Fig. 2 is a graph of an example of an optical spectrum of the light beam of Fig. 1 ;

[0019] Fig. 3A is a schematic illustration of a portion of an energy level diagram of a material within the cell of Fig. 1;

[0020] Fig. 3B is a schematic illustration of a portion of an energy level diagram of atomic krypton within the cell of Fig. 1;

[0021] Fig. 4 is a schematic illustration of an implementation of the cell of Fig. 1;

[0022] Fig. 5 is a schematic illustration of an implementation of the cell of Fig. 1;

[0023] Fig. 6A is a schematic illustration of an implementation of the cell of Fig. 4;

[0024] Fig. 6B is a schematic illustration of an implementation of the cell of Fig. 4;

[0025] Fig. 7 is a schematic illustration of an implementation of the cell and the optical arrangement of Fig. 1;

[0026] Fig. 8 is a schematic illustration of an implementation of the cell and the optical arrangement of Fig. 1;

[0027] Fig. 9 is a schematic illustration of an implementation of the cell and the detector of Fig. 1;

[0028] Fig. 10 is a schematic block diagram of another implementation of the apparatus of Fig.1;

[0029] Fig. 11 is a schematic block diagram of an implementation of a light source apparatus and an implementation of a spectral feature control apparatus associated with an optical source within the light source apparatus;

[0030] Fig. 12 is a schematic block diagram of an implementation of a bandwidth metrology apparatus of Fig. 1;

[0031] Fig. 13 is a flow chart of an implementation of a procedure performed by the apparatus of Figs. 1-12;

[0032] Fig. 14 is a flow chart of an implementation of a procedure that can be performed during the procedure of Fig. 13; and

[0033] Fig. 15 is a schematic illustration of an implementation of the cell and the optical arrangement of Fig. 1.

[0034] 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

[0035] Referring to Fig. 1, an apparatus 100 includes a cell 105, an optical arrangement 120, a detector 140, and a control system 150. The cell 105 defines an interior 106 that includes a material 107 (which, in some implementations, can be a calibration material). The cell 105, in operation, is placed in a path 108. The optical arrangement 120 includes one or more optical elements configured to produce a pair of light beams 12 li, 12 lii directed along the path 108 along opposite and overlapping directions through the material 107 in the interior 106 of the cell 105. The detector 140 is arranged relative to the cell 105 and is configured to sense light 141 produced from an interaction between the pair of light beams 12 li, 12 lii and the material 107. The control system 150 is in communication with the detector 140 and with a bandwidth metrology apparatus 160. In some implementations, the control system 150 is configured to calibrate the bandwidth metrology apparatus160 based at least on an output 142 from the detector 140, as discussed next.

[0036] The bandwidth metrology apparatus 160 measures or senses a bandwidth of a light beam161 produced during operation of an optical source 170 of a light source apparatus 171. The bandwidth metrology apparatus 160 can estimate a value of a metric from a measured optical spectrum of the light beam 161. An operator or an automated system can use this measured or sensed bandwidth of the light beam 161 to adjust properties of the optical source 170 to thereby adjust the optical spectrum of the light beam 161. During operation, the accuracy of the bandwidth metrology apparatus 160 can deteriorate because of various unwanted effects. In one unwanted effect, the bandwidth metrology apparatus 160 may have been calibrated in an inaccurate manner using an external spectrometer. For example, an external spectrometer can have a calibration accuracy of about 50 femtometers (fin), which limits the accuracy of the calibrations performed. In another effect, optical components (such as an etalon or a lens) within the bandwidth metrology apparatus 160 can degrade overtime due to interaction between these optical components and the light beam 161. For example, the finesse of an etalon within the bandwidth metrology apparatus 160 can degrade upon repeated interaction between the etalon and the light beam 161. Moreover, the resolving power of the etalon is proportional to the etalon's finesse and inversely proportional to the free spectral range of the etalon. Thus, the resolving power of the etalon within the bandwidth metrology apparatus 160 generally deteriorates if the finesse of the etalon is degraded by exposure to the light beam 161. Theseeffects deteriorate the accuracy of the measurements made by the bandwidth metrology apparatus 160, and thus make it difficult to accurately determine the spectral feature (such as the bandwidth) of the light beam 161, and accordingly, it becomes difficult to control the spectral features of the light beam 161 for use at a downstream output apparatus (such as a photolithography exposure apparatus).

[0037] In order to improve the accuracy of measurements of a spectral feature or features such as the measurement of bandwidth by the bandwidth metrology apparatus 160, the apparatus 100 can provide a calibration that provides an absolute reference for the bandwidth. This absolute reference can serve as a benchmark for the bandwidth metrology apparatus 160, which can subsequently use the absolute reference as a basis for measuring bandwidth of the light beam 161. As a result, the apparatus 100 can provide an absolute reference for the bandwidth of the light beam 161, which can be referred to as an absolute bandwidth reference (ABR). The apparatus 100 uses a spectral profde of an energy transition between quantum states that are intrinsic properties of the material 107. The energy transition occurs due to an interaction between the pair of light beams 12 li, 12 lii, which can be considered calibration light beams, and the material 107, which can be considered a calibration material. The spectral profde of this energy transition serves as the basis for the absolute bandwidth reference, as discussed below. The absolute bandwidth reference is related to fundamental properties of the calibration material 107, and is not an artifact or a secondary, but rather a primary absolute bandwidth reference. Moreover, the control system 150 can compare the absolute bandwidth reference (ABR) with the spectral profde or the bandwidth of the light beam 161 that is sensed by the bandwidth metrology apparatus 160 to determine whether the bandwidth metrology apparatus 160 needs to be calibrated or recalibrated (if previously calibrated). Calibration of the bandwidth metrology apparatus 160 can involve, for example, adjusting the scale of the measurement component or components of the bandwidth metrology apparatus 160 so as to improve the accuracy of the measurements of the spectral feature (such as the bandwidth) provided by the bandwidth metrology apparatus 160.

[0038] In particular, in a calibration operation, the apparatus 100 operates using the pair of calibration light beams 12 li, 12 lii to excite a two-photon absorption within the calibration material 107. The calibration light beams 121i, 12 lii are produced or generated from the light beam 161. Thus, the calibration light beams 12 li, 12 lii can, for example, correspond to a picked off portion of the light beam 161. The calibration material 107 can be selected so that it does not significantly degrade with use. Thus, the two-photon absorption by the calibration material 107 is stable; it does not significantly change over time. The calibration material 107 can be selected such that a finite range of wavelengths that produces the fluorescence (the light 141) is significantly narrower than the bandwidth of the light beam 161 (and the bandwidth of the calibration light beams 12 li, 12 lii, which are generated from the light beam 161).

[0039] In implementations in which the calibration material 107 is a gas, the two-photon nature of the absorption and the counter-propagating configuration of the calibration light beams 12 li, 12 liican mitigate or largely eliminate Doppler broadening of the transition. This mitigation occurs because a moving molecule (or atom) in the gas (of the calibration material 107) is slightly red-shifted with respect to one of the counter-propagating beams 12 li or 12 lii, and blue-shifted with respect to the other of the counter-propagating beams 12 lii or 12 li. Such a molecule in motion absorbs photons from each of the counter-propagating beams only if the sum of their energies matches the energy of the transition, which is the same condition as for a molecule that is not in motion. This mitigation of Doppler broadening can contribute to making the absorption’s accessible linewidth narrower than the bandwidths of the calibration light beams 12 li, 12 lii. In particular, the absolute bandwidth reference provided by the interaction between the calibration light beams 12 li, 12 lii and the calibration material 107 produces a spectral feature in which Doppler broadening is mitigated or eliminated, thus ensuring that the spectral width of the detected fluorescence (at the detector 140) is small relative to the bandwidth of the light beam 161.

[0040] The net spectral shape of the light 141 that is sensed by the detector 140 is a convolution of the spectrum of the light beams 12 li, 12 lii (generated from the light beam 161) with the intrinsic linewidth of the two-photon absorption in the calibration material 107. The linewidth of the two- photon absorption in the calibration material 107 can be broadened beyond the “intrinsic” linewidth. For example, the linewidth of the two-photon absorption can be broadened in dependence on the power of the light beams 12 li, 12 lii, the quality of the overlap between the light beams 12 li, 12 lii, the pressure in the interior 106 applied to the material 107, and the magnetic field environment. Nevertheless, the broadening mechanisms are still likely negligible compared with the spectrum of the light beams 12 li, 12 lii. In applications where the spectral width of the fluorescence is much narrower than the bandwidth of the light beams 12 li, 12 lii, the net spectral shape of the light 141 is dominated by the bandwidth of the light beams 12 li, 12 lii (and the light beam 161). The apparatus 100 is compact in that it only requires the use of the cell 105 and the detector 140 (which can be adjacent to the cell 105 or remote from the cell 105, as discussed below), is optomechanically robust, and is environmentally robust when using a calibration material 107 that is environmentally robust.

[0041] In operation, the bandwidth metrology apparatus 160 outputs the optical spectrum 262 (Fig. 2) of the light beam 161 (or from a portion of the light beam 161 separated out from the light beam 161). To be clear, the optical spectrum 262 also includes a spectral width of the bandwidth metrology apparatus 160. From the optical spectrum 262, a controller (which can be within the bandwidth metrology apparatus 160, external to the bandwidth metrology apparatus 160, or within the control system 150) estimates the actual, instantaneous bandwidth of the light beam 161. The optical spectrum 262 contains information about how the optical energy or power (spectral intensity) 263 of the light beam 161 is distributed over different wavelengths (or frequencies) 264. In Fig. 2, the optical spectrum 262 of the light beam 161 is depicted in the form of a diagram in which the spectral intensity 263 is plotted as a function of the wavelength 264. Spectral properties or features of the light beam 161 include any aspect or representation of this optical spectrum 262. For example, bandwidth is aspectral feature. The bandwidth of the light beam 161 is a measure of the width W of this optical spectrum 262, and this width W can be given in terms of wavelength or frequency of the laser light. Any suitable mathematical construction (for example, metric) related to the details of the optical spectrum 262 can be used to estimate a value that characterizes the bandwidth of the light beam 161. For example, the full width of the optical spectrum 262 at a fraction (X) of the maximum peak intensity of the spectral shape (referred to as FWXM) can be used to characterize the bandwidth of the light beam 161. In an example, the fraction X is 50% and the corresponding bandwidth metric is referred to as the full-width at half-maximum (FWHM) bandwidth. As another example, the width of the optical spectrum 262 that contains a fraction (Y) of the integrated spectral intensity (referred to as EY) can be used to characterize the bandwidth of the light beam 161. In an example, the fraction Y is 95%, and the corresponding bandwidth metric is referred to as the E95 bandwidth.

[0042] In one particular implementation, the bandwidth metrology apparatus 160 calculates the bandwidth using an E95 metric that relies on at least two aspects of the optical spectrum 262 (because the bandwidth is based on a width W). For example, the bandwidth metrology apparatus 160 can calculate the bandwidth using an E95 metric that relies on a full width of the optical spectrum 262 at a first fraction XI of the maximum peak intensity 263 and a full width of the optical spectrum 262 at a second fraction X2 of the maximum peak intensity 263. The E95 metric can further rely on a set of two or more linear parameters (for example, three linear parameters A, B, C). These linear parameters A, B, C can characterize the free spectral range of the etalon within the bandwidth metrology apparatus 160 or a dispersion of the etalon, which can be considered the scale of the optical spectrum 262 (a measure of a relative distance between the ticks of the wavelength scale 264). In this particular implementation, the accuracy of the measurement of the bandwidth W of the light beam 161 depends, at least in part, on whether the parameters A, B, and C are accurate.

[0043] The calibration apparatus 100 (Fig. 1) provides the spectral profile of the known energy transition (which can be referred to as a reference spectral profile) to provide the absolute bandwidth reference ABRto the control system 150, and the control system 150 uses this reference spectral profile (that provides an absolute bandwidth reference or ABR) to determine how to adjust the parameters A, B, C. In general, because the calibration apparatus 100 is used to provide an absolute bandwidth reference, and the bandwidth is determined based on two or more aspects of the optical spectrum (such as the two values of the width W discussed above), the comparison between the reference spectral profile (which provides the absolute bandwidth reference) and the sensed spectral profile (of the optical spectrum 262) involves comparing at least two data points from the reference spectral profile with at least two data points from the sensed spectral profile. Additionally, because the E95 metric relies on the adjustable parameters A, B, C, the control system 150 can rely on three sets of measurements at three distinct bandwidths of the light beam 161, and the calibration light beams 12 li, 12 lii in order to solve a set of three linear equations with three unknowns (the adjustable parameters A, B, C).

[0044] This process will be discussed in more detail below after a discussion of the calibration apparatus 100.

[0045] Referring to Fig. 3A, in an implementation, the relevant energy levels of the calibration material 107 are shown, noting that there are other energy levels present but not represented in Fig. 3 A. As mentioned above, in the calibration operation, the apparatus 100 operates using the pair of calibration light beams 321Ai, 321 Aii to excite the two-photon absorption (depicted by two upward arrows) within the calibration material 107. Specifically, the material 107 in a lower (or ground) state 3O7_1A absorbs two photons of the same wavelength (from each of the light beams 321Ai, 321 Aii) and is thereby excited to a higher (excited) state 307_2A. The material 107 releases photons (energy) when in the excited state 307_2A and decays to an intermediate level 307_3A. The released energy from the decay to the intermediate level 307_3A is emitted as fluorescence 341 A, which is detected or sensed by the detector 140. While not shown, the intermediate level 307_3A for various calibration materials 107 can include sub-states (eigenstates). In such situations, the wavelength of the fluorescence 341 A for this two-photon transition depends on which eigenstate of the intermediate level 307_3A is populated. This is discussed in more detail below with reference to Fig. 3B.

[0046] For example, in some implementations, the calibration material 107 includes molecular hydrogen (H2). Molecular hydrogen is a chemically basic neutral molecule with a distinct two-photon transition at 193 nanometers (nm). Thus, molecular hydrogen is suitable for use as the calibration material 107 with a light beam 161 that is produced at 193 nm. The range of wavelengths of the fluorescence 341 A for this two-photon transition in molecular hydrogen is about 746 to 840 nm. Thus, the detector 140 can be configured for sensing wavelengths in this range. The wavelength (746 to 840 nm) of the detection is distinct from the wavelength of the two-photon transition (193 nm) and this further reduces noise within the detector 140, which can be selected to be insensitive to the 193 nm wavelength.

[0047] As another example, with reference to Fig. 3B, in other implementations, the calibration material 107 includes atomic krypton (Kr). The relevant energy levels of atomic krypton used as the calibration material 107 are shown in Fig. 3B, noting that other energy levels can be present though not represented in Fig. 3B. Atomic krypton has a distinct two-photon transition at 193 nanometers (nm) and a decay path to an intermediate state that can provide usable fluorescence. Thus, atomic krypton is suitable for use as the calibration material 107 with a light beam 161 that is produced at 193 nm. The apparatus 100 operates using the pair of calibration light beams 321Bi, 32 IBii to excite the two-photon absorption (depicted by two upward arrows) within the calibration material 107. Specifically, the material 107 in a lower (or ground) state 3O7_1B absorbs two photons of the same wavelength (from each of the light beams 321Bi, 32 IBii) and is thereby excited to a higher (excited) state 307_2B. The material 107 releases photons (energy) when in the excited state 307_2B and decays to an intermediate level 307_3B. This released energy is emitted as fluorescence, which is detected or sensed by the detector 140. The intermediate level 307_3B includes sub-states(eigenstates) 307_3Ba, 307_3Bb. The wavelength of fluorescence 341Ba, 341Bb for this two-photon transition in atomic krypton depends on which eigenstate 307_3Ba, 307_3Bb of the intermediate level 307_3B is populated. Thus, for example, the wavelength of the fluorescence 341Ba can be in the range of 425-430 nm and the wavelength of the fluorescence 341Bb can be in the range of 440-450 nm. A particular eigenstate can be selected to reduce noise in the fluorescence detected by the detector 140 due to Doppler broadening from a two-photon absorption of a single beam. Specifically, for example, if the light beams 321Bi, 32 IBii are linearly polarized, the eigenstate 307_3Ba is populated. In this case, there will be Doppler-broadened noise in the fluorescence 341Ba detected by the detector 140. If, on the other hand, the light beams 321Bi, 32 IBii are opposite-handed circular polarization, the absorption and decay will populate the eigenstate 307_3Bb of the intermediate level 307_3B. In this case, there is no Doppler broadening noise in the fluorescence 341Bb detected by the detector 140. An example of an optical arrangement 1520 for selecting the eigenstate 307_3Bb is shown in Fig. 15, and is discussed below.

[0048] Referring to Fig. 4, an implementation 405 of the cell 105 is shown. The cell 405 includes a set of walls 401 extending in three dimensions, the walls 401 defining an interior 406 in which the material 407 is contained. The walls 401 are made of a material that is not reactive with the material 407 contained within the interior 406. The cell 405 includes regions 402A, 402B in respective side walls (or ends) 401 A, 40 IB that are optically transparent to the wavelength of the light beams 12 li, 12 lii. The regions 402A, 402B can be optical plates or windows that are made of a material that is not reactive with the material 407 contained within the interior 406. The windows 402A, 402B can be oriented at Brewster’s angle such that parasitic reflection losses are reduced. The cell 405 includes a region 403 in a side wall 404 that is optically transparent to the wavelength of the light 141 produced from the interaction between the light beams 12 li, 12 lii and the material 407. The region 403 can be an optical window that is made of a material that is not reactive with the material 407 contained within the interior 406 and is transmissive to the wavelength of the fluorescence 141. The cell 405 is hermetically sealed after it is filled with the material 407. The interior 406 is maintained by this seal to hold or retain the material 407. In the implementation discussed above in which the material 407 contained within the interior 406 is molecular hydrogen, the walls 401 can be made of, for example, aluminum, stainless steel, or nickel; the regions 402A, 402B can be made of, for example, fused silica, calcium fluoride (CaF2), barium fluoride (BaF2), magnesium fluoride (MgF2), or quartz; and the region 403 can be made of for example, fused silica, calcium fluoride (CaF2), barium fluoride (BaF2), magnesium fluoride (Mgp2), quartz, or a high quality optical glass such as BK7 glass. In some implementations, an interior surface of the walls of the cell 405 or a portion of the interior surface of the walls of the cell 405 is reflective so as to increase the fluorescence 141 directed toward the detector 140.

[0049] Referring to Fig. 5, another implementation 505 of the cell 105 is shown. The cell 505 includes a body 501 that defined an interior 506 that retains a material 507. For example, the cell 505can be a gas wavelength reference cell defined by the body 501 that contains the material 507. The walls of the body 501 are made of a material that is not reactive with the material 507 contained within the interior 506. The cell 505 can include side walls (or ends) 501 A, 50 IB that are optically transparent to the wavelength of the light beams 12 li, 12 lii, such walls 501A, 501B being maintained in the body 501. In some implementations, the side walls 501 A, 50 IB can be oriented at Brewster’s angle such that parasitic reflection losses are reduced. The cell 505 is hermetically sealed after being filled with the material 507. The interior 506 is maintained by this seal to hold or retain the material 507. In the implementation discussed above in which the material 507 contained within the interior 506 is molecular hydrogen, the body 501 can be made of fused quartz and the side walls 501A, 501B can be made of UV-grade fused silica (such as Coming 7890). The materials of the side walls 501A, 50 IB and the body 501 are compatible so that they can be bonded together to form a hermetically- sealed cell 505. For example, the materials of the side walls 501A, 501B and the body 501 can have a similar coefficient of thermal expansion or CTE. And, if the walls 501 A, 50 IB are not glued to the body, 501, then they can made of similar materials to enable optical bonding. In optical bonding, the two joining surfaces are polished and cleaned and then placed together, at which point they join without the use of an adhesive.

[0050] Referring to Fig. 6A, an implementation 605A of the cell 405 of Fig. 4 is shown. The cell 605A is includes a source 609As of the material 407 and a pump 609Ap configured to circulate the material 407 between the source 609As and the interior 406 to thereby ensure a fresh source of material 407 within the interior 406. A measurement and control apparatus 610A can be utilized to maintain the material 406 at a stable pressure within the cell 605A during operation.

[0051] Referring to Fig. 6B, an implementation 605B of the cell 505 of Fig. 5 is shown. The cell 605B is includes a source 609Bs of the material 507 and a pump 609Bp configured to circulate the material 507 between the source 609Bs and the interior 506 to thereby ensure a fresh source of material 507 within the interior 506. A measurement and control apparatus 610B can be utilized to maintain the material 506 at a stable pressure within the cell 605B during operation.

[0052] Referring to Fig. 7, an implementation 720 of the optical arrangement 120 is shown relative to the cell 405. The optical arrangement 720 includes a beam splitter 722 arranged in a primary path of a light beam 762. The light beam 762 can correspond to the light beam 161, or it can be formed from the light beam 161. For example, the light beam 762 can be a portion or a redirected component of the light beam 161. The beam splitter 722 is configured to separate out a portion 723 of the light beam 762. The optical arrangement 720 further includes a subset of optical elements configured to form light beams 72 li, 72 lii from the separated-out portion 723 of the light beam 762. These optical elements can include a set of lenses 724i, 724ii positioned relative to respective regions 402A, 402B at respective side walls 401 A, 40 IB of the cell 405. The optical elements include a pair of mirrors 725i, 725ii positioned relative to respective lenses 724i, 724ii. In some implementations, the lenses 724i, 724ii are not needed, and the use of the lenses 724i, 724ii can depend on parametersor properties (such as the intensity) of the light beam 723, properties of the calibration material 407, and the type of calibration material 407 as well as whether a diffuser is positioned in the path of the portion 723.

[0053] The mirror 725i redirects the portion 723 to form the light beam 72 li directed to the lens 724i along a path 708. The path 708 is defined by the mirror 725i and the mirror 725ii. The path 708 extends through the lens 724i, through the region 402A, through the interior 406 of the cell 405, through the region 402B, and through the lens 724ii. The lens 724i acts to focus the light beam 72 li to a beam waist at a location 726 inside the interior 406. The light from the beam waist of the light beam 72 li continues through the interior 406 along the path 708 and passes through the region 402B, is collimated by the lens 724ii, and impinges upon the mirror 725ii, where it is retroreflected and forms the light beam 72 lii. The light beam 72 lii is directed back through the lens 724ii, which focuses the light beam 72 lii to the beam waist at the location 726. The geometry of the optics depicted in Fig. 7 thus forms a double-pass arrangement; it directs light beam portion 723 to make a first pass through the cell 405 in one direction, and to make a second pass through the cell 405 in an opposite direction. The mirrors 725i, 725ii and the lenses 724i, 724ii can be arranged to ensure that the light beams 72 li, 72 lii counterpropagate (or nearly counterpropagate to the greatest extent feasible) along the path 708 and also that the waists of the light beams 72 li, 72 lii overlap at the location 726. The light beams 72 li, 72 lii can be considered to counterpropagate if they are traveling along opposite directions along the path 708. This arrangement supports atwo-photon excitation by the light beams 721i, 721ii in the material 407 at the location 726. In other implementations, the mirror 725ii can be replaced with a comer cub to reduce the chances of mis-alignment.

[0054] In some implementations, the optical arrangement 720 also includes a beam homogenizer 729 positioned to spatially homogenize the light beam portion 723, which forms the light beams 72 li, 72 lii. The beam homogenizer 729 improves beam homogenization of the light beams 72 li, 72 lii such that the full spectral content can be present in all of portions of the overlapping light beams 72 li, 72 lii at the location 726. The beam homogenizer 729 can be any optical device configured to modify the light beam portion 723 (and therefore the light beams 72 li, 72 lii) such as to obtain a nearly uniform optical intensity over some area, and negligible intensity outside that area of the beam profile. The beam homogenizer 729 can include a microlens array or a diffractive optic (which can be transmissive or reflective).

[0055] The optical arrangement 720 can also include a diffuser system in the path of one or more of the light beams 72 li, 72 lii. The diffuser system includes one or more optical elements that are configured to evenly diffuse the light beam 72 li and / or 72 lii. The diffuser system causes the light beam 72 li and / or 72 lii to spread evenly across a transverse plane, thus reducing high intensity bright spots. The diffuser system can alter the angular divergence of the light beam 72 li and / or 72 lii, and when used, the lenses 724i, 724ii can be configured to offset the effects of the diffuser system. The diffuser system can also or alternatively act as a beam homogenizer.

[0056] Referring to Fig. 8, another implementation 820 of the optical arrangement 120 is shown relative to the cell 405. The optical arrangement 820 includes a beam splitter 822’ arranged in a path of a separated out light beam 823. The light beam 823 can be formed from the light beam 161. For example, the light beam 823 can be a portion or a redirected component of the light beam 161. The beam splitter 822’ is configured to separate the light beam 823 into a reflected portion 827A and a transmitted portion 827B. The optical arrangement 820 further includes optical elements configured to form light beams 82 li, 82 lii from the respective portions 827A, 827B of the light beam 823. These optical elements include a set of lenses 824i, 824ii positioned relative to respective regions 402A, 402B at respective side walls 401A, 401B of the cell 405; a set of mirrors 825i, 825ii positioned relative to respective lenses 824i, 824ii; and a mirror 825C. A path 808 is defined by the mirror 825i and the mirror 825ii, the path 808 passing through the lenses 824i, 824ii, the interior 406 of the cell 405, and the regions 402A, 402B. The mirror 825 C is positioned to redirect the light beam 827B toward the path 808.

[0057] Similar to the optical arrangement 120, the lens 824i focuses the light beam 82 li to a beam waist at a location 826 inside the interior 406. The light from the beam waist of the light beam 82 li continues through the interior 406 along the path 808 and passes through the region 402B, is collimated by the lens 824ii, and impinges upon the mirror 825ii. The lens 824ii focuses the light beam 82 lii to the beam waist at the location 826. The mirrors 825i, 825ii and the lenses 824i, 824ii are arranged to ensure that the light beams 82 li, 82 lii counterpropagate (or nearly counterpropagate to the greatest extent feasible) along the path 808 and also that the waists of the light beams 82 li, 82 lii overlap at the location 826. The light beams 82 li, 82 lii can be considered to counterpropagate if they are traveling along opposite directions along the path 808. These constraints promote a two- photon excitation from the beam waists of the light beams 82 li, 82 lii interacting with the material 407 in the location 826.

[0058] The optical arrangement 820 can also include a diffuser system in the path of one or more of the light beams 82 li, 82 lii. The diffuser system includes one or more optical elements that are configured to evenly diffuse the light beam 82 li and / or 82 lii. The diffuser system causes the light beam 82 li and / or 82 lii to spread evenly across a transverse plane, thus reducing high intensity bright spots. The diffuser system can alter the angular divergence of the light beam 82 li and / or 82 lii, and when used, the lenses 824i, 824ii can be configured to offset the effects of the diffuser system. The diffuser system can act as a beam homogenizer configured to spatially homogenize the light beam. A beam homogenizer can be beneficial if different portions of the beam profile have different center wavelengths (which can arise due to the way bandwidth is narrowed (such as by the spectral feature control apparatus 1072 of Fig. 10).

[0059] Referring again to Fig. 1, the detector 140 can include a photodiode or photomultiplier tube. The detector 140 is configured to sense fluorescence (the light 141) that is released from a known excited state of the calibration material 107, the fluorescence being produced from a two-photon interaction between the pair of light beams 12 li, 12 lii and the calibration material 107. Accordingly, the detector 140 needs to be able to sense or detect the light 141 at the wavelength of the fluorescence for the particular calibration material 107. In the example above in which the calibration material 107 is molecular hydrogen, the wavelength of the fluorescence 141 is in a range of about 746 to 840 nm and the detector 140 is configured for sensing wavelengths in this range.

[0060] Referring to Fig. 9, an implementation 940 of the detector 140 is shown relative to the cell 405 of Fig. 4. Light beams 92 li, 92 lii are directed along opposite directions (counterpropagating directions) along path 908 through respective lenses 924i, 924ii so that their waists overlap at a location 926 within the interior 406 of the cell 405. The calibration material 407 absorbs the photons of the same wavelength in the overlapping light beams 92 li, 92 lii in the location 926 and then emits photons (energy) in the form of fluorescence 941. The fluorescence 941 is detected or sensed by the detector 940. The fluorescence 941 can be emitted in all directions. The detector 940 includes a lens 943 placed to capture as much of the fluorescence 941 as feasible, and redirect this captured fluorescence 941 to a sensor 945 (which can be a photodiode or a photomultiplier tube). In some implementations, a reflector can be positioned inside the cell to redirect fluorescence 941 emitted in a direction away from the detector 940 in order to further increase the amount of captured fluorescence 941 at the detector 940. As mentioned above, the fluorescence 941 is typically at a wavelength that is distinct from the wavelength of the light beams 92 li, 92 lii. Nevertheless, it can be helpful to reduce unwanted light, that is, any light that is not the fluorescence 941, from reaching the sensor 945. To this end, the detector 940 can include an optical filter 944 configured to block light having wavelengths outside a range that is centered on the wavelength of the fluorescence 941 and that includes the spectral width of the fluorescence 941.

[0061] There are many different ways to detect the fluorescence 941. For example, the fluorescence 941 can be collected and directed into an optical fiber, which relays the light to a detector located more remotely from the cell 405.

[0062] With reference again to Fig. 1, as discussed above, the bandwidth metrology apparatus 160 measures or senses the bandwidth of the light beam 161 produced during operation of the optical source 170 of the light source apparatus 171. The bandwidth metrology apparatus 160 can estimate a value of a metric from the measured optical spectrum of the light beam 161. Then, an operator or an automated system can use this measured or sensed bandwidth of the light beam 161 to adjust properties of the optical source 170 to thereby adjust the optical spectrum of the light beam 161. An implementation 1071 of the light source apparatus 171 is shown in Fig. 10, in which the automated system for adjustment includes a spectral feature control apparatus 1072 communicating with and controlling the optical source 170. The spectral feature control apparatus 1072 is configured to adjust spectral properties (such as the wavelength and the bandwidth) of the light beam 161. These adjustments can be in response to instructions from a controller 1069 (that can be in the bandwidthmetrology apparatus 160, external to the bandwidth metrology apparatus 160, or within the control system 150).

[0063] For example, the spectral feature control apparatus 1072 can receive instructions from the bandwidth metrology apparatus 160 (or the controller of the bandwidth metrology apparatus 160) to adjust a bandwidth of the light beam 161 because the measured bandwidth is not within a reasonable range of the bandwidth value desired by a downstream output apparatus 1075.

[0064] As another example, the spectral feature control apparatus 1072 communicates with the control system 150 and receives instructions from the control system 150. Specifically, the control system 150 instructs the spectral feature control apparatus 1072 to scan a wavelength of the light beam 161, or the light beams 12 li, 12 lii across the two-photon energy transition (for example, 3O7_1A to 307_2A, Fig. 3A) in the material 107. As noted above, the two-photon energy transition can have a spectral width that is narrower than the bandwidth of the light beams 121 i, 121 ii . The narrowness of the transition makes the material 107 sensitive to a narrow range of wavelengths. Thus, the resulting fluorescence (that is, the light 141) results from only a narrow slice of the spectrum of the light beams 12 li, 12 lii. As the spectral feature control apparatus 1072 scans the wavelength of the light beams 12 li, 12 lii, changing their center wavelength over time, the amplitude of the detected fluorescence also changes overtime. The amplitude of the detected fluorescence is proportional to (or otherwise indicative of) the amplitude of the slice of the spectrum of the light beams 12 li, 12 lii that matches the transition spectrum. By recording this changing amplitude over time, the control system 150 can measure a spectral shape of the light beams 12 li, 12 lii. More precisely, as noted above, the control system 150 can measure a convolution of the spectral shape of the beams 12 li, 12 lii and the spectral shape of the two-photon energy transition.

[0065] Referring to Fig. 11, an implementation 1171 of the light source apparatus 171 is shown. The light source apparatus 1171 includes an optical source 1170 that is a pulsed laser source that produces a pulsed laser beam as the light beam 1161. The optical source 1170 is a two-chamber gas discharge system as a nonlimiting example. The principles explained herein are equally applicable to a single chamber system or a laser system having more than two chambers. The gas discharge laser system can include, for example, a solid state or a gas discharge master oscillator (MO) system 1110; an amplification stage, for example, a power oscillator or a power ring amplifier (PRA) system 1115, relay optics 1162, and an output subsystem 1164.

[0066] The MO system 1110 includes, for example, a MO gas discharge chamber 1111 that includes a power source such as a pair of electrodes 1112A and 1112B. The MO system 1110 also includes an optical output coupler 1113, which can include a partially reflective mirror. An oscillator cavity is defined, in part, by a reflective grating 1173 in a spectral feature control apparatus 1172. The relay optics 1162 redirects the output of the MO system 1110 toward the PRA system 1115. The relay optics 1162 can include, for example, a multi prism beam expander (not shown) and an optical delay path (not shown).

[0067] The PRA system 1115 includes, for example, a PRA gas discharge chamber 1116. An oscillator can be formed by seed beam injection and output coupling optics 1117 and a beam reverser 1118. The optics 1117 can incorporate a partially reflective input / output coupler (not shown) and a maximally reflective mirror for the nominal operating wavelength (for example, at around 193 nm for an ArF system) and one or more prisms. The PRA gas discharge chamber 1116 includes a power source such as a pair of electrodes 1119A and 1119B.

[0068] The light beam from the PRA gas discharge chamber 1116 passes through a bandwidth metrology apparatus 1160. The bandwidth metrology apparatus 1160 is arranged to receive the light beam from the PRA gas discharge chamber and divert a portion of the radiation beam for metrology purposes, for example, to measure the output linewidth, bandwidth, and pulse energy.

[0069] The remainder of the light beam then passes to output subsystem 1164. The output subsystem 1164 can include, for example, an optical pulse stretcher and an autoshutter, in this case a combined autoshutter metrology module, which can also be the location of a pulse energy meter. In some implementations, the bandwidth metrology apparatus 1160 may be placed downstream of the optical pulse stretcher within the output subsystem 1164.

[0070] The PRA gas discharge chamber 1116 and the MO gas discharge chamber 1111 are configured as chambers in which electrical discharges between the respective sets of electrodes cause lasing gas discharges in a lasing gas to create an inverted population of high energy molecules or excimers, including, for example, ArF, KrF, F2, XeF, and / or XeCl, to produce relatively broad band radiation that may be line narrowed to a relatively very narrow bandwidth and center wavelength selected in the spectral feature control apparatus 1172.

[0071] The spectral feature control apparatus 1172 includes the grating 1173 and four prisms 1174 1 , 1174_2, 1174 3, 1174_4. While four prisms are shown in the apparatus 1172, it is possible for there to be fewer than four or more than four prisms. The grating 1173 and the four prisms 1174 1 , 1174_2, 1174 3, 1174_4 are configured to interact with a pre-cursor light beam 1175 produced by the MO gas discharge chamber 1111 in the optical source 1170. The light beam 1175 travels along a path from the chamber 1111, through the prism 1174 1 , the prism 1174_2, the prism 1174 3, the prism 1174_4, and then is reflected from the grating 1173, and back through the prisms in reverse order before exiting the apparatus 1172 and entering the chamber 1111.

[0072] The prisms 1174 1 , 1174_2, 1174 3, 1174_4 can be right-angled prisms through which the pulsed pre-cursor light beam 1175 is transmitted so that the pulsed pre-cursor light beam 1175 changes its optical magnification as it passes through each right-angled prism. One or more of the prisms 1174 1 , 1174_2, 1174 3, 1174_4 can be rotated to thereby change this optical magnification. The wavelength of the pre-cursor light beam 1175 can be changed by adjusting an angle of incidence at which the light beam 1175 impinges upon the diffractive surface of the grating 1173. Specifically, this can be done by rotating one or more of the prisms 1174 1 , 1174_2, 1174 3, 1174_4 and the grating 1173 to thereby adjust the angle of incidence of the light beam 1175. Moreover, the bandwidthof the light beam 1175 can be adjusted by adjusting the optical magnification of the light beam 1175. Thus, the bandwidth of the light beam 1175 can be adjusted by rotating one or more of the prisms 1174 1 , 1174_2, 1174 3, 1174_4, which causes the optical magnification of the light beam 1175 to change. Because the rotation of a particular prism causes a change in both the local beam refraction angle and the local optical magnification at that prism, the control of wavelength and bandwidth are coupled in this design. Additionally, the bandwidth of the light beam 1175 can be relatively sensitive to the rotation of the prism 1174 1 and relatively insensitive to rotation of the prism 1174_4. This is because any change in the local optical magnification of the light beam 1175 due to the rotation of the prism 1174 1 is multiplied by the product of the change in the optical magnification in the other prisms because those prisms are between the rotated prism 1174 1 and the grating 1173, and the light beam 1175 must travel through these other prisms after passing through the prism 1174 1. On the other hand, the wavelength of the light beam 1175 is relatively sensitive to the rotation of the prism 1174_4 and relatively insensitive to the rotation of the prism 1174 1.

[0073] In some implementations, the prism 1174 1 can be rotated by a stepper motor that can be a direct-drive stepper motor. A direct drive stepper motor is a conventional electromagnetic motor that uses a built-in step motor functionality for position control. In other implementations in which a higher resolution in motion may be needed, the stepper motor can use a piezoelectric motor technology. The stepper motor can be a rotary stage that is controlled with a motor controller using a variable-frequency drive control method to provide rapid rotation of the prism 1174 1 to control the bandwidth. The rotation of each of the prisms and the grating can be performed under control of an actuator in communication with the control system 150 and / or the controller 1069 associated with the bandwidth metrology apparatus 160.

[0074] An implementation 1260 of the bandwidth metrology apparatus 1160 is shown in Fig. 12. The bandwidth metrology apparatus 1260 includes, for example, a spectrometer (such as an etalon spectrometer) 1251 and a sensor 1255. The spectrometer interacts 1251 with the light beam 1161 and outputs spatial components that correspond to the spectral components of the light beam 1161. The sensor 1255 estimates the spectral feature (bandwidth) based on the outputted spatial components. The spectrometer 1251 includes an optical frequency separation apparatus (an etalon 1252) as well as a pair of lenses 1253A, 1253B. The etalon 1252 can include a pair of partially reflective optical flats that can be spaced a short distance apart, with reflective surfaces facing each other. As the light beam 1161 passes through the etalon 1252, it is multiply reflected, and produces a plurality of transmitted rays, which are collected and brought to the sensor 1255. A plurality of spatial components impinges upon the sensor 1255, with the spatial components corresponding to the spectral components of the light beam 1161. These spatial components show up as an interference pattern at the sensor 1255, such pattern taking the appearance of a set of concentric rings 1256 in two dimensions or as a fringe pattern 1257 in one dimension. The bandwidth metrology apparatus 1260 can include other components such as a beam homogenizer (not shown) positioned before the etalon 1252 to ensure thatall parts of the light beam 1161 impinge on all parts of the etalon 1252 and to evenly distribute the intensity of the light beam 1161 at the sensor 1255.

[0075] Referring to Fig. 13, a procedure 1380 is performed to calibrate a bandwidth metrology apparatus. The bandwidth metrology apparatus can correspond to the bandwidth metrology apparatus 160 (Figs. 1 or 10), 1160 (Fig. 11), or 1260 (Fig. 12). The procedure 1380 can be performed by the apparatus 100 (Figs. 1 or 10). The procedure 1380 includes producing a pair of calibration light beams (1381) and directing the calibration light beams along a calibration path along opposite directions through an interior of a cell (1382). The interior of the cell includes a calibration material such as molecular hydrogen. The procedure 1380 includes sensing light produced from an interaction between the pair of calibration light beams and the calibration material (1383). The procedure 1380 includes calibrating a bandwidth metrology apparatus based at least on the sensed light (1384).

[0076] As discussed above, the calibration light beams 12 li, 12 lii can be produced (1381) by the optical arrangement 120 (Fig. 1). In other implementations, the calibration light beams 72 li, 72 lii can be produced (1381) using the optical arrangement 720. In still other implementations, the calibration light beams 82 li, 82 lii can be produced (1381) using the optical arrangement 820. In particular, the calibration light beams 12 li, 12 lii can be produced by separating out a portion of the primary light beam 161, which is configured for use in photolithography (such as by the output apparatus 1075 of Fig. 10), and then forming the calibration light beams 12 li, 12 lii from this separated-out portion of the light beam 161.

[0077] In some implementations, the calibration light beams are directed along the calibration path 108 along opposite directions through the interior 106 of the cell 105 (1382) such that their beam waists are overlapping within the interior 106 of the cell 105. For example, as shown in Fig. 7, the calibration light beam 72 li is directed through the first end 401 A of the cell 405, then it traverses the region 402A, the interior 406, and the region 402B of the cell 405. The calibration light beam 72 lii is formed from the retroreflection of the calibration light beam 72 li off the mirror 725ii, and this calibration light beam 72 lii is directed through the second end 40 IB of the cell 405 through the interior 406.

[0078] In some implementations, the beam homogenizer 729 spatially homogenizes the light beams 72 li, 721ii (Fig. 7).

[0079] With reference to the implementations of Figs. 3 and 9, the light that is sensed (1383) at the sensor 945 can be fluorescence 141, 941 released from a known excited state 307_2A of the calibration material 407 that is produced from the two-photon interaction between the pair of calibration light beams 12 li, 12 lii or 92 li, 92 lii and the calibration material 405. A bandwidth of the two-photon interaction and the fluorescence 141, 941 is less than a bandwidth of the calibration light beams 92 li, 92 lii. Additionally, the detector 940 can include a filter 944 that is configured to filter out light of wavelengths outside a range that is centered on the wavelength of the fluorescence 941 and that includes the spectral width of the fluorescence 941.

[0080] As mentioned, the procedure 1380 includes calibrating a bandwidth metrology apparatus based at least on the sensed light (1384). The bandwidth metrology apparatus 160 can be calibrated by the control system 150 by comparing a spectral width measurement in the light 141 sensed at the detector 140 (for example, a bandwidth of the fluorescence) with a bandwidth measurement output from the bandwidth metrology apparatus 160. The control system 150 can then determine an adjustment to one or more calibration parameters of the bandwidth metrology apparatus 160 based on the comparison.

[0081] For example, as discussed above, in one particular implementation, the bandwidth metrology apparatus 160 calculates the bandwidth BW using an E95 metric that relies on at least two aspects of the optical spectrum 262 (because the bandwidth is based on a width W). For example, the bandwidth metrology apparatus 160 can calculate the bandwidth using an E95 metric that relies on a full width (FW) of the optical spectrum 262 at a first fraction XI of the maximum peak intensity 263 and a full width (FW) of the optical spectrum 262 at a second fraction X2 of the maximum peak intensity 263. The E95 metric can further rely on a set of two or more linear parameters (for example, three linear parameters A, B, C). These linear parameters A, B, C can characterize the free spectral range and finesse of an etalon within the bandwidth metrology apparatus 160 or a dispersion of the etalon, which can be considered the scale of the optical spectrum 262 (a measure of a relative distance between the pixels of the wavelength scale 264). In this particular implementation, the accuracy of the measurement of the bandwidth BW of the light beam 161 depends, at least in part, on whether the parameters A, B, and C are accurate. For example, the bandwidth metrology apparatus 160 can calculate the bandwidth (BW) using the following E95 metric:BW = AX(FWX1) + BX(FWX2) + C [1]

[0082] The calibration apparatus 100 (Fig. 1) provides the spectral profile of the known energy transition (which can be referred to as a reference spectral profile) to provide the absolute bandwidth reference ABRto the control system 150, and the control system 150 uses this reference spectral profile (that provides an absolute bandwidth reference or ABR) to determine how to adjust the parameters A, B, C. In general, because the calibration apparatus 100 is used to provide an absolute bandwidth reference, and the bandwidth is determined based on two or more aspects of the optical spectrum (such as the two values of the width W discussed above), the comparison between the reference spectral profile (which provides the absolute bandwidth reference) and the sensed spectral profile (of the optical spectrum 262) involves comparing at least one data point from the reference spectral profile with at least one data point from the sensed spectral profile. In some implementations, the comparison involves comparing two or more data points from the reference spectral profile with two or more data points from the sensed spectral profile. Additionally, because the E95 metric relies on the adjustable parameters A, B, C, the control system 150 can rely on three sets of measurements atthree distinct bandwidths of the light beam 161, and the calibration light beams 121i, 121ii in order to solve a set of three linear equations with three unknowns (the adjustable parameters A, B, C). In order to obtain measurements at three distinct bandwidths of the light beam 161, the control system 150 (or the controller 1069) can instruct the spectral feature control apparatus 1172 to adjust the bandwidth of the light beam 1175 by rotating the prism 1174 1 to provide for three distinct optical magnifications impinging on the grating 1173 (Fig. 11). There are three equations (corresponding to Equation 1) for each measurement at each distinct bandwidth as follows:BW1 = Ax(FWXl l) + Bx(FWX2_l) + C [2_1]BW2 = Ax(FWXl_2) + Bx(FWX2_2) + C [2_2]BW3 = Ax(FWXl_3) + Bx(FWX2_3) + C [2_3], where the values of BW1, BW2, BW3 are the actual values of the bandwidth measured by the control system 150 from the output of the detector 150, and the subscripts refer to each of the distinct measurements (by adjusting the prism 1174 1). With three equations 2_1, 2_2, 2_3, and three “variables” (the calibration parameters A, B, C), the values for A, B, C can be solved.

[0083] While three calibration parameters are described, it is possible for there to be fewer than three or more than three. The control system 150 can adjust the number of measurements to obtain enough information to solve the linear equations for the calibration parameters.

[0084] Referring again to the procedure 1380 of Fig. 13, additional steps can be performed. In particular, prior to producing the calibration light beams (1381), the calibration material 107 and the two-photon transition for the calibration material 107 can be selected to ensure that the two-photon transition of the calibration material 107 has a linewidth that is substantially narrower than the expected bandwidth of the calibration light beams, as discussed above.

[0085] While the calibration light beams are produced ( 1381), the center wavelength of the calibration light beams is scanned. In particular, as discussed above, the control system 150 instructs the spectral feature control apparatus 1072 to scan the wavelength of the light beam 161 (and the light beams 12 li, 12 lii) across the two-photon energy transition (such as 3O7_1A to 307 _2A, Fig. 3A) in the calibration material 107. As discussed above, the light that is sensed (1383) at the sensor 945 can be fluorescence 141, 941 released from the known excited state 307_2A of the calibration material 407 that is produced from the two-photon interaction between the pair of calibration light beams 12 li, 12 lii or 92 li, 92 lii and the calibration material 405. Specifically, during the sensing (1383), the detector 140 can record the intensity of the sensed light 141 as a function of the center wavelength (which is being scanned) of the calibration beams 121i, 12 lii.

[0086] In some implementations, and with reference to Fig. 14, the bandwidth metrology apparatus 160 can be calibrated (1384) in accordance with a procedure 1484. A bandwidth of the calibration beams is measured or calculated based on the recorded intensities of the sensed light (1485). For example, the control system 150 can calculate the bandwidth BW of the calibration beams 12 li, 12 lii from the output of the detector 150 using any suitable metric discussed above. Moreover, the control system 150 can calculate the bandwidth at three distinct values BW1, BW2, BW3 from the output of the detector 150.

[0087] The bandwidth of the light beam output from the light source apparatus is measured (1486). For example, the bandwidth metrology apparatus 160 can calculate the bandwidth BW of the light beam 161 using the E95 metric, which is provided in Equation 1. As discussed above, the accuracy of the measurement of the bandwidth BW of the light beam 161 depends, at least in part, on whether the parameters A, B, and C are accurate.

[0088] The measured bandwidth BW (reported from the bandwidth metrology apparatus) is compared with the calculated or measured bandwidth from the detected fluorescence (1487). For example, the control system 150 can compare the measured bandwidth BW (reported from the bandwidth metrology apparatus 160) against the calculated BW (reported from the detector 140) to determine how to adjust the parameters A, B, C. Based on the comparison (1487), aspects of the bandwidth metrology apparatus are adjusted (1488). For example, the control system 150 can instruct adjustments or changes to the parameters A, B, C of the bandwidth metrology apparatus 160 so that the bandwidth metrology apparatus 160 reports a measured bandwidth BW within a suitable range of that calculated from the fluorescence 141.

[0089] Referring again to Fig. 10, in some implementations, the apparatus 100 can be configured to make an adjustment to the light source 170 based on the sensed fluorescence 141. In particular, to this end, the apparatus 100 includes the spectral feature control apparatus 1072, which acts as a scanning device that is configured to scan the wavelength of the pair of light beams 12 li, 12 lii across the known two-photon energy transition in the gas material 107. For example, in the spectral feature control apparatus 1172 of Fig. 11, the control system 150 or the controller 1069 can instruct an actuator within the apparatus 1172 to rotate the prism 1174_4 across a range of angles to thereby scan the wavelength across the known two-photon energy transition in the gas material 107. The detector 140 is arranged relative to the cell 105 and is configured to sense the fluorescence 141 emitted from the gas material 107 due to the two-photon energy transition as a function of the wavelength of the pair of light beams 121i, 121ii.

[0090] As discussed above, with reference to Fig. 1, in some implementations, the control system 150 adjusts the calibration parameters associated with the bandwidth metrology apparatus 160 to thereby calibrate the bandwidth metrology apparatus 160.

[0091] In other implementations, the control system 150 is configured to estimate a spectral feature of a light beam. For example, a spectral feature of the light beam 161 can be analyzed bysplitting or otherwise separating off a portion or portions of the light beam 161 into the light beams 12 li, 12 lii and then sensing the fluorescence 141. For example, the control system 150 can calculate a bandwidth of the light beams 12 li, 12 lii (and thus a bandwidth of the light beam 161) by analyzing the spectrum of the fluorescence 141 detected by the detector 140. This calculation of the bandwidth can use any of the metrics described above. The control system 150 can determine whether the measured bandwidth is within an acceptable range of a bandwidth that is desired by the downstream output apparatus 1075. If the bandwidth measured by the control system 150 is outside of this range, then the control system can communicate with the spectral feature actuation apparatus 1072 to thereby adjust the bandwidth of the output light beam 161 (Fig. 10). In various implementations of the light source apparatus 171, a metrology apparatus such as bandwidth metrology apparatus 160 can employ components such as the cell 105, the optical arrangement 120, and the detector 140 instead of an etalon for measuring the bandwidth of light produced by the light source apparatus 171.

[0092] Referring again to Fig. 7, in other implementations, the beam splitter 722 is a fiilly- reflective optical element (such as a mirror). In such implementations, the reflector is removable so that it can be removed once calibration is complete to let the beam 762 pass.

[0093] Referring to Fig. 15, another implementation 1520 of the optical arrangement 120 is shown relative to the cell 405. The optical arrangement 1520 is designed similarly to the optical arrangement 720 of Fig. 7, except for the following differences. A pair of polarization optics 1528i, 1528ii are placed in the path of the respective light beams 72 li, 72 lii. In some implementations, a retroreflector 1525ii can be translated along the path 708 to thereby adjust the location of the pulses in the light beams 72 li, 72 lii in the interior 406. The retroreflector 1525ii can be, for example, a mirror or a comer cube. If the light beam 721 i is linearly polarized, then the polarization optic 1528i can be a X / 4 waveplate that converts the light beam 72 li into circular polarization having a first handedness.

[0094] Moreover, if the light beam 72 lii is linearly polarized, then the polarization optic 1528ii can be a X / 4 waveplate that converts the light beam 72 lii into circular polarization having a second handedness. The polarization optic 1528i may be disposed along the path of the light beam portion 723 upstream of the cell 405, so that the light beam portion 723 interacts with the polarization optic 1528i prior to a first pass through the cell 405. The polarization optic 1528ii may be disposed along the path of the light beam portion 723 downstream of the cell 405, so that the light beam portion 723 interacts with the polarization optic 1528ii after a first pass through the cell 405 and / or prior to a second pass through the cell 405.

[0095] In some implementations, a detector system 1540 can be configured with two detectors 1540a, 1540b, each configured with respective filters 1544a, 1544b to detect, independently, the light 341Ba and 341Bb (Fig. 3B). Because fluorescence is detected at two different wavelengths using two separate detectors 1540a, 1540b, an independent channel can be used to improve the signal -to-noise ratio, or to reveal additional spectroscopy information.

[0096] Referring again to Figs. 7 and 15, in other implementations, the detector 940 of Fig. 9 can be positioned behind optic 725ii (Fig. 7) or optic 1525ii (Fig. 15). In such implementations, the optic 725ii or 1525ii can be configured to filter out light of the calibration light beams 72 li, 72 lii but transmit the fluorescence 941 produced in the location 926.

[0097] The implementations can be further described using the following clauses:1. An apparatus comprising: a cell defining an interior comprising a calibration material, the cell, in operation, placed in a calibration path; an optical arrangement comprising one or more optical elements configured to produce a pair of calibration light beams directed along the calibration path along opposite directions through the cell; a detector configured to sense light produced from an interaction between the pair of calibration light beams and the calibration material; and a control system in communication with the detector and with a bandwidth metrology apparatus, the control system configured to calibrate the bandwidth metrology apparatus based at least on an output from the detector.2. The apparatus of clause 1, wherein the calibration material comprises molecular hydrogen or atomic krypton.3. The apparatus of clause 1, wherein the optical arrangement comprises a beam splitter in a primary path of a light beam, the beam splitter configured to separate out a portion of the light beam, and a subset of optical elements configured to form the pair of calibration light beams from the separated- out portion of the light beam to define a calibration path through first and second ends of the cell.4. The apparatus of clause 3, wherein the subset of optical elements comprises a mirror arranged with its reflective surface facing the second end of the cell along the calibration path, the mirror configured to form a second calibration light beam from a reflection of a first calibration light beam directed through the first and second ends of the cell.5. The apparatus of clause 3, wherein the subset of optical elements comprises a pair of mirrors arranged to split the separated-out portion of the light beam into first calibration light beam and second calibration light beam directed along opposite directions toward the cell through the respective first and second ends along the calibration path.6. The apparatus of clause 3, wherein the subset of optical elements comprise a pair of lenses positioned at the first and second ends of the cell.7. The apparatus of clause 1, wherein the optical arrangement comprises a beam homogenizer configured to spatially homogenize the pair of calibration light beams, and the subset of optical elements comprise a pair of lenses positioned at first and second ends of the cell.8. The apparatus of clause 1, wherein the detector comprises a photodiode or a photomultiplier tube.9. The apparatus of clause 1, wherein the detector is configured to sense fluorescence released from a known excited state of the calibration material that is produced from a two-photon interaction between the pair of calibration light beams and the calibration material.10. The apparatus of clause 9, wherein the detector comprises a filter configured to block light of wavelengths outside a range that is centered on the wavelength of the fluorescence and that includes the spectral width of the fluorescence.11. The apparatus of clause 9, wherein a spectral width of the two-photon interaction is narrower than a bandwidth of the calibration light beams.12. The apparatus of clause 9, wherein the two-photon interaction is not subject to Doppler broadening.13. The apparatus of clause 1, wherein the detector is arranged adjacent to the cell.14. The apparatus of clause 1, wherein the detector comprises two detectors, each configured with a respective filter to sense, independently, the light produced from the decay to eigenstates of an intermediate level.15. The apparatus of clause 1, wherein the control system being configured to calibrate the bandwidth metrology apparatus comprises the control system being configured to: compare a bandwidth measurement in the sensed light from the detector with a bandwidth measurement output from the bandwidth metrology apparatus; and determine an adjustment to one or more calibration parameters of the bandwidth metrology apparatus based on the comparison.16. The apparatus of clause 15, wherein the control system being configured to compare the bandwidth measurement in the sensed light from the detector with the bandwidth measurement output from the bandwidth metrology apparatus comprises the control system being configured to compare a spectral width of the intensity distribution of fluorescence produced from the interaction with a bandwidth measurement output from the spectral feature metrology apparatus.17. The apparatus of clause 1, wherein the cell includes a region transparent to the wavelength of the calibration light beams and a region transparent to the light produced from the interaction between the calibration light beams and the calibration material.18. The apparatus of clause 1, wherein the cell is hermetically sealed.19. The apparatus of clause 1, wherein the cell includes a pump configured to pass calibration material into the interior.20. The apparatus of clause 1, wherein the control system being configured to calibrate the bandwidth metrology apparatus comprises the control system being configured to: compare at least one spectral width measurement in the sensed light from the detector with at least one bandwidth measurement output from the bandwidth metrology apparatus; and determine an adjustment to one or more calibration parameters of the bandwidth metrology apparatus based on the comparison.21. The apparatus of clause 1, further comprising a spectral feature control apparatus in communication with the control system and configured to scan a wavelength of the calibration light beams across a two-photon energy transition in the calibration material; wherein the detector being configured to sense the light comprises the detector being configured to sense a fluorescence released from the excited state of the calibration material produced from the two- photon energy transition.22. The apparatus of clause 1, wherein the optical arrangement comprises a pair of polarization optics, each polarization optic in the path of one of the calibration light beam, and configured to adjust the polarization of the calibration light beams.23. The apparatus of clause 22, wherein the pair of polarization optics is configured to convert the calibration light beams into oppositely-handed circular polarization.24. A method comprising: producing a pair of calibration light beams and directing the calibration light beams along a calibration path along opposite directions through an interior of a cell, the interior comprising a calibration material; sensing light produced from an interaction between the pair of calibration light beams and the calibration material; and calibrating a bandwidth metrology apparatus based at least on the sensed light.25. The method of clause 24, wherein the calibration material comprises molecular hydrogen or atomic krypton.26. The method of clause 24, wherein producing the pair of calibration light beams comprises separating out a portion of a primary light beam, the primary light beam configured for use in photolithography; and forming the pair of calibration light beams from the separated-out portion of the light beam.27. The method of clause 26, wherein directing the calibration light beams comprises forming overlapping beam waists of the calibration light beams within the interior of the cell.28. The method of clause 27, wherein directing the calibration light beams comprises: directing a first of the calibration light beams through a first end of the cell; and directing a second of the calibration light beams through the first end of the cell, through the interior, and through a second end of the cell, and reflecting the second of the calibration light beams off a mirror, and then directing the reflection of the second of the calibration light beams through the second end of the cell through the interior.29. The method of clause 24, wherein producing the pair of calibration light beams comprises spatially homogenizing the pair of calibration light beams.30. The method of clause 24, wherein sensing light produced from the interaction between the pair of calibration light beams and the calibration material comprises sensing fluorescence released from aknown excited state of the calibration material that is produced from a two-photon interaction between the pair of calibration light beams and the calibration material.31. The method of clause 30, wherein sensing light produced from the interaction between the pair of calibration light beams and the calibration material comprises fdtering out light of wavelengths outside a range that is centered on the wavelength of the fluorescence and that includes the spectral width of the fluorescence.32. The method of clause 30, wherein a spectral width of the two-photon interaction is less than a bandwidth of the calibration light beams.33. The method of clause 24, wherein calibrating the bandwidth metrology apparatus comprises: comparing a spectral width measurement in the sensed light with a bandwidth measurement output from the bandwidth metrology apparatus; and determining an adjustment to one or more calibration parameters of the bandwidth metrology apparatus based on the comparison.34. The method of clause 33, wherein comparing the spectral width measurement in the sensed light with the bandwidth measurement output from the bandwidth metrology apparatus comprises comparing a width of the intensity distribution of fluorescence produced from the interaction with the bandwidth measurement output from the bandwidth metrology apparatus.35. An apparatus comprising: an optical arrangement comprising one or more optical elements configured to produce a pair of light beams directed along opposite directions through a gas material held within a cell, the pair of light beams being produced from an output light beam of a light source apparatus; a scanning device configured to scan a wavelength of the pair of light beams across a two-photon energy transition in the gas material; a detector configured to sense fluorescence emitted from the gas material due to the two-photon energy transition as a function of the wavelength of the pair of light beams; and a control system in communication with the detector and configured to adjust the light source apparatus based on the sensed fluorescence.36. The apparatus of clause 35, wherein the control system is configured to adjust the light source apparatus based on the sensed fluorescence comprises adjusting a bandwidth metrology apparatus of the light source apparatus based on the sensed fluorescence.37. The apparatus of clause 36, wherein the control system is configured to adjust the bandwidth metrology apparatus by calibrating the bandwidth metrology apparatus.38. The apparatus of clause 35, wherein the control system is configured to estimate a spectral feature of the pair of light beams based on the sensed fluorescence.39. The apparatus of clause 38, wherein the control system being configured to adjust the light source apparatus based on the sensed fluorescence comprises the control system being configured to adjust a spectral feature actuation apparatus to thereby adjust a spectral feature of the output light beam.40. The apparatus of clause 39, wherein the estimated spectral feature of the pair of light beams is a bandwidth of the pair of light beams and the spectral feature of the output light beam that is adjusted is a bandwidth of the output light beam.41. An apparatus comprising: an optical arrangement configured to receive a pair of light beams from a light source and to counterpropagate the pair of light beams through a gas held within a cell; a control module configured to instruct a variation of a wavelength of the light beams around a two- photon energy transition in the gas; and a detector configured to sense fluorescence emitted from the gas due to the two-photon energy transition, wherein the control module is configured to adjust, based on the sensed fluorescence, (a) a metrology apparatus, and / or (b) the light source.42. The apparatus of clause 41, wherein the control module is configured to estimate a bandwidth of the pair of light beams based on the sensed fluorescence.43. The apparatus of clause 41, wherein the gas is hydrogen vapor. 44. The apparatus of clause 41, wherein the gas is krypton vapor.

[0098] Other implementations are within the scope of the following claims.

Claims

CLAIMS1. An apparatus comprising: a cell defining an interior comprising a calibration material, the cell, in operation, placed in a calibration path; an optical arrangement comprising one or more optical elements configured to produce a pair of calibration light beams directed along the calibration path along opposite directions through the cell; a detector configured to sense light produced from an interaction between the pair of calibration light beams and the calibration material; and a control system in communication with the detector and with a bandwidth metrology apparatus, the control system configured to calibrate the bandwidth metrology apparatus based at least on an output from the detector.

2. The apparatus of claim 1, wherein the calibration material comprises molecular hydrogen or atomic krypton.

3. The apparatus of claim 1, wherein the optical arrangement comprises a beam splitter in a primary path of a light beam, the beam splitter configured to separate out a portion of the light beam, and a subset of optical elements configured to form the pair of calibration light beams from the separated-out portion of the light beam to define a calibration path through first and second ends of the cell.

4. The apparatus of claim 3, wherein the subset of optical elements comprises a mirror arranged with its reflective surface facing the second end of the cell along the calibration path, the mirror configured to form a second calibration light beam from a reflection of a first calibration light beam directed through the first and second ends of the cell.

5. The apparatus of claim 3, wherein the subset of optical elements comprises a pair of mirrors arrange to split the separated-out portion of the light beam into first calibration light beam and second calibration light beam directed along opposite directions toward the cell through the respective first and second ends along the calibration path.

6. The apparatus of claim 1, wherein the detector is configured to sense fluorescence released from a known excited state of the calibration material that is produced from a two-photon interaction between the pair of calibration light beams and the calibration material, and , wherein a spectral width of the two-photon interaction is narrower than a bandwidth of the calibration light beams .

7. The apparatus of claim 6. wherein the detector comprises a filter configured to block light of wavelengths outside a range centered on the wavelength of the fluorescence.

8. The apparatus of claim 6, wherein the two-photon interaction is not subject to Doppler broadening.

9. The apparatus of claim 1, wherein the detector comprises two detectors, each configured with a respective filter to sense, independently, the light produced from the decay to eigenstates of an intermediate level.

10. The apparatus of claim 1, wherein the control system being configured to calibrate the bandwidth metrology apparatus comprises the control system being configured to: compare a bandwidth measurement in the sensed light from the detector with a bandwidth measurement output from the bandwidth metrology apparatus; and determine an adjustment to one or more calibration parameters of the bandwidth metrology apparatus based on the comparison.

11. The apparatus of claim 10, wherein the control system being configured to compare the bandwidth measurement in the sensed light from the detector with the bandwidth measurement output from the bandwidth metrology apparatus comprises the control system being configured to compare a spectral width of the intensity distribution of fluorescence produced from the interaction with a bandwidth measurement output from the spectral feature metrology apparatus.

12. The apparatus of claim 1, wherein the cell includes a region transparent to the wavelength of the calibration light beams and a region transparent to the light produced from the interaction between the calibration light beams and the calibration material.

13. The apparatus of claim 1, wherein the control system being configured to calibrate the bandwidth metrology apparatus comprises the control system being configured to: compare at least one spectral width measurement in the sensed light from the detector with at least one bandwidth measurement output from the bandwidth metrology apparatus; and determine an adjustment to one or more calibration parameters of the bandwidth metrology apparatus based on the comparison.

14. The apparatus of claim 1, further comprising a spectral feature control apparatus in communication with the control system and configured to scan a wavelength of the calibration light beams across a two-photon energy transition in the calibration material;wherein the detector being configured to sense the light comprises the detector being configured to sense a fluorescence released from the excited state of the calibration material produced from the two-photon energy transition.

15. The apparatus of claim 1, wherein the optical arrangement comprises a pair of polarization optics, each polarization optic in the path of one of the calibration light beam, and configured to adjust the polarization of the calibration light beams, and wherein the pair of polarization optics is configured to convert the calibration light beams into oppositely-handed circular polarization.

16. A method comprising: producing a pair of calibration light beams and directing the calibration light beams along a calibration path along opposite directions through an interior of a cell, the interior comprising a calibration material; sensing light produced from an interaction between the pair of calibration light beams and the calibration material; and calibrating a bandwidth metrology apparatus based at least on the sensed light.

17. The method of claim 16, wherein the calibration material comprises molecular hydrogen or atomic krypton.

18. The method of claim 16, wherein producing the pair of calibration light beams comprises separating out a portion of a primary light beam, the primary light beam configured for use in photolithography; and forming the pair of calibration light beams from the separated-out portion of the light beam; and directing the calibration light beams comprises forming overlapping beam waists of the calibration light beams within the interior of the cell.

19. The method of claim 18, wherein directing the calibration light beams comprises: directing a first of the calibration light beams through a first end of the cell; and directing a second of the calibration light beams through the first end of the cell, through the interior, and through a second end of the cell, and reflecting the second of the calibration light beams off a mirror, and then directing the reflection of the second of the calibration light beams through the second end of the cell through the interior.

20. The method of claim 16, wherein sensing light produced from the interaction between the pair of calibration light beams and the calibration material comprises sensing fluorescence releasedfrom a known excited state of the calibration material that is produced from a two-photon interaction between the pair of calibration light beams and the calibration material.

21. The method of claim 20, wherein a spectral width of the two-photon interaction is less than a bandwidth of the calibration light beams.

22. An apparatus comprising: an optical arrangement comprising one or more optical elements configured to produce a pair of light beams directed along opposite directions through a gas material held within a cell, the pair of light beams being produced from an output light beam of a light source apparatus; a scanning device configured to scan a wavelength of the pair of light beams across a two- photon energy transition in the gas material; a detector configured to sense fluorescence emitted from the gas material due to the two- photon energy transition as a function of the wavelength of the pair of light beams; and a control system in communication with the detector and configured to adjust the light source apparatus based on the sensed fluorescence.

23. The apparatus of claim 22, wherein the control system is configured to adjust the light source apparatus based on the sensed fluorescence comprises adjusting a bandwidth metrology apparatus of the light source apparatus based on the sensed fluorescence, and wherein the control system is configured to adjust the bandwidth metrology apparatus by calibrating the bandwidth metrology apparatus, and wherein the control system is configured to estimate a spectral feature of the pair of light beams based on the sensed fluorescence.

24. The apparatus of claim 23, wherein the control system being configured to adjust the light source apparatus based on the sensed fluorescence comprises the control system being configured to adjust a spectral feature actuation apparatus to thereby adjust a spectral feature of the output light beam, and wherein the estimated spectral feature of the pair of light beams is a bandwidth of the pair of light beams and the spectral feature of the output light beam that is adjusted is a bandwidth of the output light beam.

25. An apparatus comprising: an optical arrangement configured to receive a pair of light beams from a light source, and counter-propagate the pair of light beams through a gas held within a cell; a control module configured to instruct a variation of a wavelength of the light beams around a two-photon energy transition in the gas; anda detector configured to sense fluorescence emitted from the gas due to the two-photon energy transition; wherein the control module is configured to adjust, based on the sensed fluorescence, one or more of. a metrology apparatus, or the light source.

26. The apparatus of claim 25, wherein the control module is configured to estimate a bandwidth of the pair of light beams based on the sensed fluorescence.

27. The apparatus of claim 25, wherein the gas is hydrogen vapor.

28. The apparatus of claim 25 wherein the gas is krypton vapor.

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