Method and apparatus for measuring a gas species in a system

Abstract

Disclosed is a method and apparatus for determining one or more characteristics of a gas species in a system (e.g., an EUV lithographic system). The method and apparatus use optical diagnostics, in particular cavity ring-down spectroscopy (CRDS), to accurately quantify in-situ the gas species in the system. Also disclosed are various configurations of an optical system many of which allow simultaneous measurements of the gas species in different locations of the system.

Description

METHOD AND APPARATUS FOR MEASURING A GAS SPECIES IN A SYSTEMCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 24192555.1 which was filed on 02 August 2024 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to a method and apparatus for measuring one or more characteristics of a gas species in a system, in particular in a EUV lithographic system.BACKGROUND

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.

[0004] To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] Such a lithographic apparatus works in conjunction with a projection optics system that has a narrow imaging slot. Rather than expose the entire wafer at one time, individual fields are scanned onto the wafer one at a time . Moving the wafer and reticle simultaneously such that the imaging slot is moved across the field during the scan. The wafer stage is then asynchronously stepped between field exposures to allow multiple copies of the reticle pattern to be exposed over the wafer surface. In this manner, the quality of the image projected onto the wafer is maximized.

[0006] Conventional lithographic systems and methods form images on a semiconductor wafer. The system typically has a lithographic chamber that is designed to contain an apparatus that performs the process of image formation on the semiconductor wafer. The chamber can be designed to have different grades of vacuum depending on the wavelength of light being used. A reticle is positioned inside the chamber. A beam of light is passed from an illumination source (located outside the system) through an optical system, onto an image outline on the reticle, and a second optical system before interacting with a semiconductor wafer. The reticle may be transmissive or reflective depending on the wavelength of light used; EUV systems for example use a reflective reticle within a vacuum chamber to prevent absorption of the EUV radiation.

[0007] The reticle can be placed on a platform or stage (hereinafter, both are referred to as “stage”). The stage can be positioned according to parameters of the lithographic system. Similarly, the semiconductor wafer can be placed on a stage. The stage supporting either the reticle or the semiconductor wafer can be moved one or more directions and / or one or more degrees of freedom depending on how the image is to be formed on the semiconductor wafer.

[0008] At present, residual gas analysis (RGA) is the state-of-the-art technique for measuring partial pressures of different gas species (e.g., H2O) in a lithographic apparatus (e.g., an EUV scanner). RGA analyzes the composition of gases present within (a vacuum environment of) a lithographic apparatus. With the help of RGA, it is possible to obtain valuable information on vacuum quality and process conditions which is crucial for understanding contamination sources and evaluating vacuum performance.

[0009] Despite RGA’s high sensitivity in identifying and / or quantifying elemental species, it is however used ex-situ via differential pumping in a (e.g., EUV) lithographic apparatus. Residual gas analyzers typically operate at pressures up to IxlO-3Pa. However, some internals of the lithographic apparatus are maintained at about 1-10 Pa. Therefore, when operated within a lithographic apparatus, a residual gas analyzer needs to be differentially pumped, rendering measurements ex situ. Differential pumping reduces the sensitivity of RGA, makes it a localized technique, and necessitates models to extract partial pressures of gas species (e.g., ITO) in the lithographic apparatus. The latter can be particularly challenging after venting because the measured partial pressure is then dominated by the outgassing of the residual gas analyzer itself. Additionally, identifying plasma-related species is challenging due to recombination at the walls and mass differentiation. Quantification is further complicated of any transient species present due to their short lifetimes.

[0010] Objects and aspects of the present claimed invention seek to alleviate at least the above- mentioned problems with the prior art.SUMMARY

[0011] In the first aspect of the invention there is provided a method and apparatus for determining one or more characteristics of a gas species in a system, comprising: providing a first laser beam with a first center frequency stabilized to a reference frequency; providing at least one second laser pulse with a second center frequency stabilized with reference to the first center frequency of the first laser beam; placing at least one optical resonator into an interior space of the system such that the at least one optical resonator is filled with the gas species; wherein the at least one optical resonator is configured to receive and resonate the second laser pulse and is further configured such that a portion of the second laser pulse exits the at least one optical resonator after each circulation around the at least one optical resonator; coupling the at least one second laser pulse into the at least one optical resonator, wherein the at least one second laser pulse interacts with the gas species while circulating around the at least one optical resonator; measuring an intensity of the portion of the at least one second laser pulseexiting the at least one optical resonator over a predefined period to obtain a set of intensity data; and determining the one or more characteristics of the gas species based on the obtained intensity data.

[0012] In a second aspect of the invention there is provided an optical system for determining one or more characteristics of a gas species, comprising: a first laser source for providing a first laser beam with a first center frequency stabilized to a reference frequency; a second laser source for providing second laser pulses with a second center frequency stabilized with reference to the first center frequency of the first laser beam; at least one optical resonator placed in an interior space of a system and filled with the gas species; wherein the at least one optical resonator is configured to receive and resonate the second laser pulses and is further configured such that a portion of the second laser pulses exits the at least one optical resonator after each circulation around the at least one optical resonator; and a detector for detecting the second laser pulses; wherein the optical system is configured to: couple at least one second laser pulse into the at least one optical resonator; measure, by the detector, an intensity of the portion of the at least one second laser pulse exiting the at least one optical resonator over a predefined period to obtain a set of intensity data; and determine the one or more characteristics of the gas species based on the obtained intensity data.

[0013] Other aspects of the invention comprise a lithographic system (e.g., an EUV lithographic system), a metrology apparatus, or an inspection apparatus comprising the optical system of the second aspect.BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:Figure 1 depicts a lithographic system comprising a lithographic apparatus and a radiation source;Figure 2 shows a flowchart of a method of determining one or more characteristics of a gas species in a system in accordance with an embodiment;Figure 3A schematically depicts an optical system for determining one or more characteristics of a gas species in accordance with an embodiment;Figure 3B schematically depicts an example implementation of the second laser source of the optical system shown in Figure 3A;Figure 4A schematically depicts a first configuration of the proposed optical system;Figure 4B schematically depicts a second configuration of the proposed optical system; Figure 4C schematically depicts a third configuration of the proposed optical system; Figure 4D schematically depicts a fourth configuration of the proposed optical system;Figure 4E schematically depicts a fifth configuration of the proposed optical system; and Figure 4F schematically depicts a sixth configuration of the proposed optical system;DETAILED DESCRIPTION

[0015] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask or reticle), a projection system PS and a substrate table WT configured to support a substrate W.

[0016] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.

[0017] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13,14 which are configured to project the patterned EUV radiation beam B’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13,14 in Figure 1, the projection system PS may include a different number of mirrors (e.g. six or eight mirrors).

[0018] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.

[0019] A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and / or in the projection system PS.

[0020] The radiation source SO may be a laser produced plasma (LPP) source, a discharge produced plasma (DPP) source, a free electron laser (FEL) or any other radiation source that is capable of generating EUV radiation.

[0021] As described above, it is desirable to have a method and apparatus that allow in-situ measurement of one or more characteristics of a gas species in a lithographic system while simultaneously providing a comparable sensitivity to differentially pumped residual gas analyzers.

[0022] Accordingly, a first aspect of the present disclosure provides a method of determining one or more characteristics of a gas species in a system (e.g., an EUV lithographic system). The gas species may be a volatile gas species. The proposed method uses optical diagnostics, in particular cavity ringdown spectroscopy (CRDS), to accurately quantify in-situ gas species (e.g., Hf . H2O) in the system. More details about the CRDS can be found in the following paragraphs.

[0023] The (e.g., volatile) gas species to be measured in the system (e.g., an EUV lithographic system) may be one of the gas species listed in Table 1 below. Detection of species in this list requires access to the electromagnetic spectrum including vacuum ultraviolet (VUV), ultraviolet (UV), visible (Vis), near-, mid- and far-infrared (NIR, MIR, FIR), and terahertz (THz) regions.Table 1

[0024] With reference to Figure 2, in an embodiment, the method 200 may comprise steps 210 to 260. The method 200 may be implemented with an optical system 300 shown in Figures 3A and 3B.

[0025] Method step 210 may comprise providing a first laser beam with a first center frequency stabilized to a reference frequency which may be a resonance frequency of the gas species as probed in-situ. The resonance frequency of the gas species GS may be one of the transitions of the gas species GS to be measured. By way of example, when quantifying trihydrogen cations (H< ) in the system, thefirst center frequency of the first laser beam LB 1 may be stabilized or locked to a transition of H , at 2051.505 cm'1. This may mean that the first center frequency is stabilized within a frequency range of between 61442.6 GHz and 61562.5 GHz.

[0026] In a different embodiment, the reference frequency may be a resonance frequency of a different species as probed in-situ. In another different embodiment, the reference frequency may be a frequency of e.g., the global positioning system (GPS) or an atomic clock.

[0027] With reference to Figure 3, the optical system 300 may comprise a first laser source LS 1 which may be configured to output the first laser beam LB 1. The first laser beam LB 1 may be a narrowlinewidth continuous wave laser beam. The spectral linewidth of the first laser beam LB1 may be for example less than 10 MHz, less than 5 MHz, or less than 1 MHz. The first center frequency of the first laser beam LB1 will be widely different for different gas species.

[0028] The stabilization of the first center frequency of the first laser source LSI to the reference frequency (e.g., the resonance frequency of the gas species) may be achieved through one of the following methods. The first method is based on lamb dip spectroscopy or saturation spectroscopy of the analyte which targets the probed transition, other transitions in the spectral range, other transitions in the spectral range at elevated or lowered temperatures, or other transitions in the spectral range by a different isotope of the analyte. The second method is based on lamb dip spectroscopy or saturation spectroscopy of a different species which targets any transitions in the spectral range at any temperature . The third method is based on stability transfer from established commercial sources (e.g., atomic clocks, crystal oscillator references, global positioning system references) to the spectral range e.g., via a frequency comb. Note that all the above-mentioned frequency stabilization methods are well known to a person skilled in the art and thus are not described in detail here for the sake of simplicity.

[0029] Method step 220 may comprise providing at least one second laser pulse with a second center frequency stabilized with reference to the first center frequency of the first laser beam.

[0030] As shown in Figure 3 A, the optical system 300 may comprise a second laser source LS2 which may be configured to output the at least one second laser pulse SP. The second center frequency of the second laser pulse SP may be stabilized to the first center frequency of the first laser beam LB 1 via for example optical heterodyne detection. Figure 3B schematically depicts an example implementation of the second laser source LS2.

[0031] The second laser source LS2 may comprise an optical oscillator OS configured to output a second laser beam LB2. The second laser beam LB2 may be a narrow-linewidth CW laser beam. The spectral linewidth of the first laser beam LB1 may be for example less than 10 MHz, less than 5 MHz, or less than 1 MHz. The second laser source LS2 may comprise a beam splitter BS configured to split the second laser beam LB2 into a first (e.g., reflected) portion LB2-1 which is used for optical heterodyne detection and a second (e.g., transmitted) portion LB2-2 which is used for generating the second laser pulse SP. Similar to the first laser beam LB 1, the center frequency of the second laser beamLB2 (or the second laser pulse SP) may be will be widely different for different gas species (e.g., between 61442.6 GHz and 61562.5 GHz for quantifying H, ).

[0032] Referring back to Figure 3 A, the first laser beam LB 1 and the second laser beam LB2 may be spatially combined by an optical beam combiner OBC and the spatially combined two laser beams LB 1 and LB2 may be incident on an optical heterodyne detector OHD (e.g., a photodiode detector). The signal generated by the optical heterodyne detector OHD may be used to stabilize the second center frequency of the second laser beam LB2 and thus the second laser pulse SP at an adjustable (or predefined) frequency offset to the first center frequency of the first laser beam LB 1 (which itself is locked to the resonance frequency of the gas specifies to be measured).

[0033] The second laser source LS2 may comprise an optical modulator (e.g., an electro-optic modulator (EOM) or an acousto-optic modulator (AOM)) operable to modulate an intensity of the second portion LB2-1 of the second laser beam LB2 to generate the at least one second laser pulse SP. In an example application, the optical modulator OM may periodically modulate the intensity of the second portion LB2-1 of the second laser beam LB2 to generate a train of second laser pulses. Each second laser pulse may comprise a boxcar-shaped temporal profile and may have a temporal width of for example at least 1 ps, at least 10 ps, at least 20 ps, at least 30 ps, at least 40 ps, or at least 50 ps. The train of second laser pulses may have a duty cycle of for example at least 10%, at least 20%, at least 30%, or at least 40%.

[0034] A single laser source locked to the target transition cannot take into account a) a broadband change of cavity properties (e.g., reflectivity, scatter, misalignment); and b) the impact of broadening on the target profile, i.e., the increase in temperature at constant concentration may erroneously be interpreted as a decrease in concentration as the species temperature is not tracked. Advantageously, two referenced lasers enable to compare effectively in-vacuo performance of the sample cavity Off- Resonance with the On-Resonance performance. Therefore, the analysis is rendered self-calibrating and self-consistent, with no requirement to hold the process for calibration measurements.

[0035] Method step 230 may comprise placing at least one optical resonator (or optical cavity) into an interior space of the system such that the at least one optical resonator is filled with the gas species. The at least one optical resonator may be configured to receive and resonate the at least one second laser pulse and may be further configured such that a portion of the at least one second laser pulse exits the at least one optical resonator after each circulation around the at least one optical resonator.

[0036] With reference to Figure 3A, the optical system 300 may comprise a single optical resonator OR which may comprise a first cavity mirror CM1 and a second cavity mirror CM2. The first cavity mirror CM1 and the second cavity mirror CM2 may be configured to form a stable linear optical resonator for the second laser pulse SP, i.e. the radiation pattern is reproduced on every round-trip of the second laser pulse SP through the optical resonator OR. The radius of curvatures of and the distance between the two cavity mirrors CM1 and CM2 may be flexibly chosen depending on application needs. In other configurations, the optical resonator OR may comprise three or more cavity mirrors which maybe configured to form for example a ring resonator or a bow-tie resonator. The optical resonator OR may be placed within an interior space of for example the radiation source, the illumination system, or the projection system of an EUV lithographic system (e.g., the EUV lithographic system shown in Figure 1). In such a case, the cavity mirrors CM1, CM2 of the optical resonator OR may comprise a protective coating configured to prevent plasma degradation. The protective coating may be composed of for example AI2O3. When placed into such an interior space, the optical resonator OR may be filled with the gas molecules present inside the space.

[0037] Method step 240 may comprise coupling the at least one second laser pulse into the at least one optical resonator. The at least one second laser pulse may interact with the gas species while circulating around the at least one optical resonator.

[0038] With reference to Figures 3A and 3B, in an embodiment, the coupling step may comprise enabling the modulation of (the transmitted portion of) the second laser beam LB2 to generate a second laser pulse SP and coupling the second laser pulse SP into the optical resonator OR. The modulation of the second laser beam LB2 may be enabled by sending a control signal to the optical modulator OM. The second laser pulse SP may be coupled into the optical resonator OR via an optical lens OL1 (or lens assembly) and the output portion SP-1 of the second laser pulse SP exiting the optical resonator OR may be focused onto a detector DT via a second optical lens OL2 (or lens assembly). The first optical lens OL1 may be configured to improve or optimize the mode matching between the second laser pulse SP and the optical resonator OR and the second optical lens OL2 may be configured to focus the output portion SP-1 of the second laser pulse SP into a desired focused spot.

[0039] After being coupled into the optical resonator OR, the second laser pulse SP may travel back and forth between the first cavity mirror CM 1 and the second cavity mirror CM2 of the optical resonator OR. During each passage across the optical resonator OR, the second laser pulse SP may interact with the gas species GS present within the space of the optical resonator OR which may result in absorption of the second laser pulse SP (when the second center frequency of the second laser pulse SP matches the resonance frequency of the gas species GS) and thus a loss of power or intensity in the output portion SP-1 of the second laser pulse SP that is incident on the detector DT (see method step 250 for more details).

[0040] In the configuration shown in Figure 3A, the second laser pulse SP propagates in free space before entering and after exiting the optical resonator OR. In a different configuration such as the configuration 400 shown in Figure 4A, the second laser pulse SP may propagate within a first optical fiber OF1 before being coupled into the optical resonator OR via the first optical lens OL1 and the output portion SP- 1 of the second laser pulse SP may propagate within a second optical fiber OF2 before being focused onto the detector DT. In other configurations, there may comprise no first optical lens OL1 and second optical lens OL2.

[0041] In some configurations, the optical system may comprise two or more optical resonators, each of which may be placed at a different location in the lithographic system. Comparing with the single-resonator configuration, multi-resonator configurations may allow different locations of a lithographic system to be sampled or measured simultaneously and therefore obtained results are directly comparable to each other. Furthermore, multi-resonator configurations, in particular those where a single second laser source is used to provide the second laser pulses for all the resonators, may also provide a cost-effective solution for simultaneously measuring the gas species at various locations of a lithographic system.

[0042] Figures 4B-4F schematically depict five example configurations 410-450 of the proposed optical system, each configuration comprising two optical resonators OR, OR’. In these example configurations, a single second laser source LS2 is used to provide second laser pulses SP to both optical resonators OR, OR’ either sequentially or parallelly.

[0043] With reference to Figure 4B, the optical system 410 may comprise an optical path switching arrangement (which in this particular example is a fiber optical switch FSW) which may be optically connected to the second laser source LS2 and operable to switch or direct the second laser pulse SP into a first optical path comprising a first optical lens OL1, a first optical resonator OR1 and a second optical lens OL2 or a second optical path comprising a third optical lens OL3, a second optical resonator OR2 and a fourth optical lens OL4. The second optical resonator OR2 may comprise a third cavity mirror CM3 and a fourth cavity mirror CM4 which may be identical to the first cavity CM1 and the second cavity mirror CM2 of the first optical resonator OR1. The output portion SP-1 of the second laser pulse SP exiting the optical resonator OR1 or OR2 may be coupled into a fiber combiner FCM configured to guide the laser pulse SP-1 to the detector DT.

[0044] In an alternative configuration, the optical path switching may be achieved for example by manually placing the output end of an optical fiber from which the second laser pulse SP emits to one of the two optical paths.

[0045] With reference to Figures 4C-4F, the optical system 420 or 430 or 440 or 450 may comprise an optical beam splitting arrangement BSA which may be optically connected to the second laser source LS2 and operable to split the second laser pulse SP into a first portion SP1 and a second portion SP2. The first portion SP1 of the second laser pulse SP and the second portion SP2 of the second laser pulse SP may be directed to the first optical path comprising a first optical lens OL1, a first optical resonator OR1 and a second optical lens OL2 and the second optical path comprising a third optical lens OL3, a second optical resonator OR2 and a fourth optical lens OL4, respectively. The output portion SP1-1 of the first portion SP1 of the second laser pulse SP may be focused by the second optical lens OL2 into a second optical fiber OF2 which guides the pulse to a first detector DTI. The output portion SP2-1 of the second portion SP2 of the second laser pulse SP may be focused by the fourth optical lens OL4 into a fourth optical fiber OF4 which guides the pulse to a first detector DT2.

[0046] Specifically, in the optical system 420, the optical beam splitting arrangement BSA may comprise a fiber optic splitter FSP comprising an input fiber for receiving the second laser pulse SP from the second laser source LS2, a first output fiber for outputting the first portion of the second laserpulse SP and a second output fiber for outputting the second portion SP2 of the second laser pulse SP. The splitting ratio (i.e. the intensity or power of the first portion SP1 over the intensity or power of the second portion SP2) of the fiber optic splitter FSP may be customized depending on applications. The splitting ratio may be for example 30 / 70, 40 / 60, 50 / 50, or 60 / 40.

[0047] In the optical system 430, the optical beam splitting arrangement BSA may comprise a free space beam splitter BS (e.g., a dichroic mirror) and an optical mirror OM. The beam splitter BS may be configured to transmit a first portion SP 1 of the second laser pulse SP and reflect a second portion SP2 of the second laser pulse SP. The first portion SP1 of the second laser pulse SP may be coupled into a first optical fiber OF1 which may guide the pulse towards the first optical resonator OR1. The optical mirror OM may be configured to reflect the second portion SP2 of the second laser pulse SP towards a third optical fiber OF2. The second portion SP2 of the second laser pulse SP may be coupled into the third optical fiber OF2 which may guide the pulse towards the second optical resonator OR2.

[0048] In the optical system 440, the optical beam splitting arrangement BSA may comprise an electro-optic modulator EOM, a polarization beam splitter PBS (e.g., a Brewster plate, or a polarization beam cube) and an optical mirror OM. The EOM may be configured to impose a phase modulation onto the second laser pulse SP so as to controllably vary the polarization state (e.g., the orientation of a linear polarization) of the second laser pulse SP. The polarization beam splitter PBS may be configured to transmit a first portion SP 1 in a first polarization state (e.g., the p-polarization state) and reflect a second portion SP2 of the second laser pulse SP in a second polarization state (e.g., s-polarization state). The first portion SP1 of the second laser pulse SP may be coupled into a first optical fiber OF1 which may guide the pulse towards the first optical resonator ORE The optical mirror OM may be configured to reflect the second portion SP2 of the second laser pulse SP towards a third optical fiber OF3. The second portion SP2 of the second laser pulse SP may be coupled into the third optical fiber OF3 which may guide the pulse towards the second optical resonator OR2.

[0049] In the optical system 450, the optical beam splitting arrangement BSA may comprise a first acousto-optic modulator A0M1 and a second acousto-optic modulator A0M2, each of which may act as a beam splitter. Similar to the foregoing configurations, the second laser pulse SP may be split into a first portion SP1 and a second portion SP2. The first portion SP1 of the second laser pulse SP may be coupled into a first optical fiber OF1 which may guide the pulse towards the first optical resonator OR1. The second portion SP2 of the second laser pulse SP towards a third optical fiber OF3. The second portion SP2 of the second laser pulse SP may be coupled into the third optical fiber OF3 which may guide the pulse towards the second optical resonator OR2.

[0050] Method step 250 may comprise measuring an intensity of the portion of the at least one second laser pulse exiting the at least one optical resonator over a predefined period to obtain a set of intensity data.

[0051] Referring back to Figure 3 A, after the second laser pulse SP is coupled into the optical resonator OR, a detector DT may be configured to observes / measures the intensity of the output portion SP-1 ofthe second laser pulse SP exiting the optical resonator OR. The intensity of the trapped second laser pulse SP will decrease by a fixed percentage during each round trip within the optical resonator OR due to absorption and scattering by the gas species within optical resonator OR and cavity losses due to transmission and / or scattering at the cavity mirrors CM1, CM2. This may result in an exponential decay of the intensity detected by the detector DT (see the example intensity I over time t plot shown in Figure 3A). If the gas species GS present in the optical resonator OR is resonant with the radiation (i.e. the second laser pulse SP), this presents an additional loss mechanism that increases the decay rate. The exponential decay in the intensity of the output portion SP-1 of the second laser pulse SP can be quantified by its decay rate, also known as decay constant, which measures how long it takes for the light to decay to 1 / e of its initial intensity. This so-called “cavity ringdown time” or “ringdown time” can be used to calculate the concentration of the absorbing gas species GS in the optical resonator OR.

[0052] In an embodiment, the predefined period may be equal to or longer than the time for the intensity of the portion of the second laser pulse measured by the detector falls to 1 / e of an initial intensity.

[0053] Method step 260 may comprise determining the one or more characteristics of the gas species based on the obtained intensity data.

[0054] In an embodiment, the determining step may comprise determining a decay constant of the set of intensity data by fitting an exponential decay function of time to the set of intensity data and determining a concentration of the gas species based on the determined decay constant and a reference decay constant.

[0055] The exponential function of time may be expressed as:I(t) = / oexp (-^), [1] where IQdenotes the initial intensity of the light (before decaying) and T denotes the decay constant. For an empty optical resonator (i.e. containing no absorbing gas species), the decay constant T may be expressed as:where n denotes the index of refraction within the optical resonator, c denotes the speed of light in vacuum, I denotes the length of the optical resonator (or cavity length), R denotes the mirror reflectivity, and X denotes other optical losses. When the optical resonator is filled with absorbing gas species, then the decay constant may be expressed as:> c 1-R+X+afL Jwhere a is the absorption coefficient for a specific analyte concentration at the center frequency of the light. The ratio between the determined decay constant and the reference decay constant is directly related to the absolute concentration of the gas species in the lithographic system (assuming X is negligible):where e denotes molar absorptivity and C denotes analyte concentration. The dynamic range of the optical resonator OR can be changed by selecting a weaker optical transition for the gas species within the available spectral range. Further, the pulsed probing of the optical resonator can be synchronized with a pulsed signal e.g., the EUV light source. Via more involved processes, the dynamics in concentration of the target species between EUV pulses can then be inferred from the observed ringdown. Further information about how to determine the concentration of a gas using CRDS can be found in Giel Berden, Rudy Peeters, and Gerard Meijer “Cavity ring-down spectroscopy: Experimental schemes and applications ”, International Reviews in Physical Chemistry, 2000, Vol. 19, No. 4. 565- 607, which is incorporated herein by reference.

[0056] In an embodiment, the one or more characteristics of the gas species may comprise a translational temperature of the gas species. In such a case, the method 200 may further comprise sequentially tuning the second center frequency of the second laser pulse across a predefined frequency range comprising the resonance frequency of the gas species; and performing the measuring step (i.e. method step 250) at each tuned frequency so as to obtain a plurality sets of intensity data, each set of intensity data corresponding to a respective one of the tuned frequencies. The second center frequency of the second laser pulse SP may be varied by adjusting its frequency offset to the first center frequency of the first laser beam LB 1.

[0057] In this embodiment, the determining step may comprise determining a decay constant for each set of intensity data by fitting the data set with an exponential function of time; determining an absorbance value for each set of intensity data based on the corresponding determined decay constant and a reference decay constant; repeating the above two determining steps to obtain a plurality of absorbance values, each absorbance value corresponding to a respective one of the plurality of frequencies; and determining the translational temperature of the gas species by fitting a Gaussian lineshape function to the plurality of absorbance values.

[0058] The translational temperature of a target species may be derived from the line profile. At low pressure and ambient or elevated temperatures, the line profile typically simplifies towith an the half-width-at-half-maximum (HWHM) of the observed Gaussian profde, vy the wavenumber of the spectral transition in vacuum, c the speed of light in vacuum, NA the Avogadro constant, k the Boltzmann constant, T the translational temperature, and M the molar mass of the species associated with the transition. Thus, the temperature can be determined by solving the equation for T. Further information about how to determine the translational temperature of a gas species can be found at R. A. McClatchey, W. S. Benedict, S. A. Clough et al., "AFCRL Atmospheric Absorption Line Parameters Compilation", AFCRL-TR-73-0096, Environmental Research Papers, No. 434, Optical Physics Laboratory, Air Force Cambridge Research Laboratories (1973), which is incorporated herein by reference.

[0059] Being capable of determining the translational temperature of the gas species may prevent erroneously interpreting an increase in temperature at constant concentration as a decrease in concentration. Thus, the plurality sets of intensity data may be used for calibrating a measured concentration of the gas species.

[0060] Note that the optical systems 300, 400, 410-450 are only examples that are suitable for implementing the proposed method. Other different implementations may also be applicable.

[0061] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc.

[0062] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatuses may be generally referred to as lithographic apparatuses or lithographic tools. Such a lithographic apparatus may use vacuum conditions or ambient (non-vacuum) conditions.

[0063] Aspects of the invention are described in the following numbered clauses.1. A method of determining one or more characteristics of a gas species in a system, comprising: providing a first laser beam with a first center frequency stabilized to a reference frequency; providing at least one second laser pulse with a second center frequency stabilized with reference to the first center frequency of the first laser beam; placing at least one optical resonator into an interior space of the system such that the at least one optical resonator is filled with the gas species; wherein the at least one optical resonator isconfigured to receive and resonate the second laser pulse and is further configured such that a portion of the second laser pulse exits the at least one optical resonator after each circulation around the at least one optical resonator; coupling the at least one second laser pulse into the at least one optical resonator, wherein the at least one second laser pulse interacts with the gas species while circulating around the at least one optical resonator; measuring an intensity of the portion of the at least one second laser pulse exiting the at least one optical resonator over a predefined period to obtain a set of intensity data; and determining the one or more characteristics of the gas species based on the obtained intensity data.2. A method according to clause 1, wherein the one or more characteristics of the gas species comprise a concentration of the gas species.3. A method according to clause 1 or 2, wherein the reference frequency is a resonance frequency of the gas species.4. A method according to clause 3, wherein the second center frequency of the at least one second laser pulse is configured to substantially coincide with the reference frequency.5. A method according to clause 4, wherein the determining step comprises: determining a decay constant of the set of intensity data by fitting an exponential decay function of time to the set of intensity data; and determining a concentration of the gas species based on the determined decay constant and a reference decay constant.6. A method according to any preceding clause, wherein the one or more characteristics of the gas species comprise a translational temperature of the gas species.7. A method according to clause 6, further comprising: tuning the second center frequency of the at least one second laser pulse across a predefined frequency range comprising the reference frequency; and performing the measuring step at each tuned frequency so as to obtain a plurality sets of intensity data.8. A method according to clause 7, wherein the determining step comprises: determining a decay constant for each set of intensity data by fitting the data set with an exponential function of time; determining an absorbance value for each set of intensity data based on the corresponding determined decay constant and a reference decay constant; repeating the above two determining steps to obtain a plurality of absorbance values, each absorbance value corresponding to a respective one of the tuned frequencies; and determining the translational temperature of the gas species by fitting a Gaussian line-shape function to the plurality of absorbance values.9. A method according to clause 8, further comprising calibrating a measured concentration of the gas species using the plurality sets of intensity data.10. A method according to any preceding clause, wherein the step of providing the at least one second laser pulse comprises modulating an intensity of a second laser beam. 11. A method according to clause 10, wherein the coupling step comprises: enabling the modulation of the second laser beam to generate a second laser pulse; and coupling the at least one second laser pulses into the at least one optical resonator.12. A method according to any preceding clause, wherein the at least one second laser pulse comprises a boxcar-shaped temporal profile. 13. A method according to any preceding clause, wherein the at least one second laser pulse comprises a temporal width of at least 1 ps.14. A method according to any of clauses 9 to 13, wherein the first laser beam and / or the at least one second laser pulse comprise a spectral linewidth of less than 10 MHz.15. A method according to any preceding clause, wherein the first laser beam and / or the at least one second laser pulse comprise a spectral linewidth of less than 1 MHz.16. A method according to any preceding clause, wherein the gas species comprises one of the gas species listed below:17. A method according to any preceding clause, wherein the at least one optical resonator comprises two or more optical resonators, each at a different location in the system.18. A method according to clause 17, wherein the coupling step comprises coupling one second laser pulse into each of the two or more optical resonators in a sequential manner.19. A method according to clause 18, wherein the sequential coupling of one second laser pulse into each of the two or more optical resonators is achieved using an optical path switching arrangement.20. A method according to clause 19, wherein the optical path switching arrangement comprises a fiber-optic switch operable to direct one second laser pulse into an optical fiber optically coupled to a corresponding optical resonator.21. A method according to clause 17, wherein the coupling step comprises coupling one second laser pulse into each of the two or more optical resonators in a parallel manner.22. A method according to clause 21, wherein the parallel coupling of one second laser pulse into each of the two or more optical resonators is achieved using an optical beam splitting arrangement.23. A method according to clause 22, wherein the optical beam splitting arrangement comprises one or more beam splitters.24. A method according to any preceding clause, wherein the system is an EUV lithographic system.25. A method according to clause 24, wherein the at least one optical resonator is placed within to one or more of: an illumination system, a projection system, and a radiation source of the EUV lithographic system.26. A method according to any preceding clause, wherein the at least one second laser pulse is coupled into the at least one optical resonator using one or more optical fibers.27. A method according to any preceding clause, wherein the second center frequency of the at least one second laser pulse is stabilized with reference to the first center frequency of the first laser beam using optical heterodyne detection.28. A method according to any preceding clause, wherein the predefined period is equal to or longer than the time for the intensity of the portion of the second laser pulse measured by the detector falls to 1 / e of an initial intensity.29. An optical system for determining one or more characteristics of a gas species, comprising: a first laser source for providing a first laser beam with a first center frequency stabilized to a reference frequency; a second laser source for providing second laser pulses with a second center frequency stabilized with reference to the first center frequency of the first laser beam; at least one optical resonator configured to be placed in an interior space of a system and filled with the gas species; wherein the at least one optical resonator is configured to receive and resonate the second laser pulses and is further configured such that a portion of the second laserpulses exits the at least one optical resonator after each circulation around the at least one optical resonator; and a detector for detecting the second laser pulses; wherein the optical system is configured to: couple at least one second laser pulse into the at least one optical resonator; measure, by the detector, an intensity of the portion of the at least one second laser pulse exiting the at least one optical resonator over a predefined period to obtain a set of intensity data; and determine the one or more characteristics of the gas species based on the obtained intensity data.30. An optical system according to clause 29, wherein the second laser source comprises a laser oscillator configured to output a second laser beam and an optical modulator operable to modulate an intensity of the second laser beam to obtain the second pulses.31. An optical system according to clause 30, wherein both the first laser beam and the second laser beam are CW laser beams.32. An optical system according to any of clauses 29 to 31, wherein the second laser pulses have a boxcar shaped temporal profile.33. An optical system according to any of clauses 29 to 32, wherein the second laser pulses comprise a temporal width of at least 1 ps.34. An optical system according to any of clauses 29 to 33, wherein the reference frequency is a resonance frequency of the gas species.35. An optical system according to any of clauses 29 to 34, wherein the second laser pulses comprise a spectral linewidth of less than 10 MHz.36. An optical system according to any of clauses 29 to 35, wherein the first laser beam and / or the second laser pulses comprise a spectral linewidth of less than 1 MHz.37. An optical system according to any of clauses 29 to 36, wherein the gas species comprises one of the gas species listed below:38. An optical system according to any of clauses 29 to 37, wherein the at least one optical resonator comprises two or more optical resonators, each placed at a different location in the system.39. An optical system according to clause 38, wherein the coupling step comprises coupling one second laser pulse into each of the two or more optical resonators in a sequential manner.40. An optical system according to clause 39, wherein the sequential coupling of one second laser pulse into each of the two or more optical resonators is achieved using an optical path switching arrangement.41. An optical system according to clause 40, wherein the optical path switching arrangement comprises a fiber-optic switch operable to direct one second laser pulse into an optical fiber optically coupled to a corresponding optical resonator.42. An optical system according to clause 41, wherein the coupling step comprises coupling one second laser pulse into each of the two or more optical resonators in a parallel manner.43. An optical system according to clause 42, wherein the parallel coupling of one second laser pulse into each of the two or more optical resonators is achieved using an optical beam splitting arrangement.44. An optical system according to clause 43, wherein the optical beam splitting arrangement comprises one or more beam splitters.45. An optical system according to any of clauses 29 to 44, wherein the system is an EUV lithographic system.46. An optical system according to any of clauses 29 to 45, further comprises one or more optical fibers configured to transport the second laser pulse into and / or out of the at least one optical resonator.47. An optical system according to any of clauses 29 to 46, further comprises a heterodyne detection arrangement configured to stabilize the second center frequency of the second laser pulses with reference to the first center frequency of the first laser beam.48. An optical system according to any of clauses 29 to 47, wherein the predefined period is equal to or longer than the time for the intensity of the portion of the second laser pulse measured by the detector falls to 1 / e of an initial intensity.49. An optical system according to any of clauses 29 to 48, wherein the at least optical resonator comprises two or more optical mirrors, each optical mirror being coated with a protective coating configured to prevent plasma degradation.50. An EUV lithographic system comprising an optical system according to any of clauses 29 to 49.51. An EUV lithographic system according to clause 50, wherein each of the at least one optical resonator is located within a different one of: a radiation source of the EUV lithographic system, an illumination system, a projection system, and a radiation source of the EUV lithographic system.52. A metrology or inspection system comprising an optical system according to any of clauses 29 to 49.

[0064] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. A method of determining one or more characteristics of a gas species in a system, comprising: providing a first laser beam with a first center frequency stabilized to a reference frequency; providing at least one second laser pulse with a second center frequency stabilized with reference to the first center frequency of the first laser beam; placing at least one optical resonator into an interior space of the system such that the at least one optical resonator is filled with the gas species; wherein the at least one optical resonator is configured to receive and resonate the second laser pulse and is further configured such that a portion of the second laser pulse exits the at least one optical resonator after each circulation around the at least one optical resonator; coupling the at least one second laser pulse into the at least one optical resonator, wherein the at least one second laser pulse interacts with the gas species while circulating around the at least one optical resonator; measuring an intensity of the portion of the at least one second laser pulse exiting the at least one optical resonator over a predefined period to obtain a set of intensity data; and determining the one or more characteristics of the gas species based on the obtained intensity data.

2. A method as claimed in claim 1, wherein the one or more characteristics of the gas species comprise a concentration of the gas species.

3. A method as claimed in claim 1 or 2, wherein the reference frequency is a resonance frequency of the gas species.

4. A method as claimed in claim 3, wherein the second center frequency of the at least one second laser pulse is configured to substantially coincide with the reference frequency.

5. A method as claimed in claim 4, wherein the determining step comprises: determining a decay constant of the set of intensity data by fitting an exponential decay function of time to the set of intensity data; and determining a concentration of the gas species based on the determined decay constant and a reference decay constant.

6. A method as claimed in any preceding claim, wherein the one or more characteristics of the gas species comprise a translational temperature of the gas species.

7. A method as claimed in claim 6. further comprising: tuning the second center frequency of the at least one second laser pulse across a predefined frequency range comprising the reference frequency; and performing the measuring step at each tuned frequency so as to obtain a plurality sets of intensity data.

8. A method as claimed in claim 7, wherein the determining step comprises: determining a decay constant for each set of intensity data by fitting the data set with an exponential function of time; determining an absorbance value for each set of intensity data based on the corresponding determined decay constant and a reference decay constant; repeating the above two determining steps to obtain a plurality of absorbance values, each absorbance value corresponding to a respective one of the tuned frequencies; and determining the translational temperature of the gas species by fitting a Gaussian line-shape function to the plurality of absorbance values.

9. A method as claimed in any preceding claim, wherein the step of providing the at least one second laser pulse comprises modulating an intensity of a second laser beam.

10. A method as claimed in claim 9, wherein the coupling step comprises: enabling the modulation of the second laser beam to generate a second laser pulse; and coupling the at least one second laser pulses into the at least one optical resonator.

11. A method as claimed in any preceding claim, wherein the at least one second laser pulse comprises a temporal width of at least 1 ps.

12. A method as claimed in any of claims 9 to 11, wherein the first laser beam and / or the at least one second laser pulse comprise a spectral linewidth of less than 10 MHz.

13. A method as claimed in any preceding claim, wherein the gas species comprises one of the gas species listed below:

14. A method as claimed in any preceding claim, wherein the system is an EUV lithographic system.

15. An optical system for determining one or more characteristics of a gas species, comprising: a first laser source for providing a first laser beam with a first center frequency stabilized to a reference frequency; a second laser source for providing second laser pulses with a second center frequency stabilized with reference to the first center frequency of the first laser beam; at least one optical resonator configured to be placed in an interior space of a system and filled with the gas species; wherein the at least one optical resonator is configured to receive and resonate the second laser pulses and is further configured such that a portion of the second laser pulses exits the at least one optical resonator after each circulation around the at least one optical resonator; and a detector for detecting the second laser pulses; wherein the optical system is configured to: couple at least one second laser pulse into the at least one optical resonator; measure, by the detector, an intensity of the portion of the at least one second laser pulse exiting the at least one optical resonator over a predefined period to obtain a set of intensity data; and determine the one or more characteristics of the gas species based on the obtained intensity data.

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