Optics damage metrology in excimer light sources

Abstract

A system includes: an excimer laser configured to generate light having a first wavelength, the excimer laser including at least one optical element interacting with the generated light; a sensor device configured to detect light emanating from the optical element, the detected light having one or more wavelengths that are distinct from the first wavelength; and an assessment module in communication with the sensor device. The assessment module is configured to: analyze the detected light; and generate an estimate of damage to the optical element based on the analysis.

Description

OPTICS DAMAGE METROLOGY IN EXCIMER LIGHT SOURCESCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to US Application No. 63 / 738,008, filed December 23, 2024, titled OPTICS DAMAGE METROLOGY IN EXCIMER LIGHT SOURCES, which is incorporated herein by reference in its entirety.TECHNICAL FIELD

[0002] The disclosed subject matter relates to maintenance of light sources such as those used for integrated circuit photolithographic manufacturing processes.BACKGROUND

[0003] Laser radiation for semiconductor photolithography is typically supplied by a system referred to as a light source. These light sources produce radiation as a series of pulses at specified repetition rates, for example, in the range of about 500 Hz to about 6 kHz. The light sources conventionally have expected useful lifetimes measured in terms of the number of pulses they are projected to be able to produce before requiring repair or replacement, typically expressed as billions of pulses. The light beam produced from the excimer light 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 light source can be built using a single gas discharge chamber or using a plurality of gas discharge chambers.

[0004] For example, one system for generating radiation at frequencies useful for semiconductor photolithography (DUV wavelengths) involves use of a master oscillator power amplifier (MOPA) dual gas discharge chamber configuration. This configuration has two chambers, a master oscillator chamber (MO chamber) and a power amplifier chamber (PA chamber). These chambers and many other system components can be regarded as being modules, and the light source overall can be regarded as an ensemble of modules. In general, each module has a lifetime that is shorter than the lifetime of the overall system. Thus, over the course of the lifetime of the system, the health of individual modules is evaluated to determine if they should be repaired or replaced.SUMMARY

[0005] In some general aspects, a system includes: an excimer laser configured to generate light having a first wavelength, the excimer laser including at least one optical element interacting with the generated light; a sensor device configured to detect light emanating from the optical element, the detected light having one or more wavelengths that are distinct from the first wavelength; and an assessment module in communication with the sensor device. The assessment module is configuredto: analyze the detected light; and generate an estimate of damage to the optical element based on the analysis.

[0006] Implementations can include one or more of the following features. For example, the optical element can be a reflective optical element, a refractive optical element, or a diffractive optical element. The optical element can be a mirror, a prism, a grating, a window, a lens, or a beam splitter. The optical element can include a substrate and a coating applied to at least a portion of the substrate. The optical element can include an uncoated substrate. The excimer laser can be configured to generate light at a first wavelength that is between 192 nanometers (run) and 194 nm or between 247 nm and 249 nm. The wavelength of the detected light can be a value or in a range from 100 nm to 1000 nm. The sensor device can include one or more of: an optical fiber; a spectrometer; a photodiode detector; a camera; an optical filter; and a shutter. The sensor device can include a fiber-coupled spectrometer or a photodiode detector with an optical filter. The sensor device can include an imaging device, and one or more imaging lenses, apertures, and shutters between the imaging device and the optical element. The imaging device can include a camera, a charge coupled device (CCD), or a complementary metal-oxide-semiconductor (CMOS) device. The sensor device can include an optical filter configured to block light at the first wavelength. The assessment module being configured to analyze the detected light can include analyzing an output signal produced by the sensor device. The output signal can include one or more of: an image of at least a portion of a surface of the optical element; a total signal count relating to the image of at least the portion of the optical element surface; and an optical spectrum, a region of the optical spectrum, or an integrated signal of the optical spectrum. The output signal can include an analysis signal in which a secondary signal is subtracted. The secondary signal can include a background signal, a signal at a specific moment in time, or an historical signal. The output signal can be a current signal or a voltage signal. The output signal can be an analog signal or a digital signal.

[0007] The assessment module being configured to generate the estimate of damage to the optical element based on the analysis can include one or more of: assessing a health status of the optical element; assessing a likelihood of failure of the optical element prior to a scheduled maintenance event; and assessing a likelihood of failure of the optical element prior to a subsequent scheduled maintenance event that follows the next scheduled maintenance event. The assessment module being configured to generate the estimate of damage to the optical element based on the analysis can include monitoring, over a period of time, a history of an output signal from the sensor device, the output signal relating to the detected light. The period of time can be 30 seconds or less, 1 minute or less, 30 minutes or less, 1 hour or less, 1 day or less, 1 week or less, 1 month or less, or 1 year or less. At least a portion of the sensor device can be within an enclosure of the excimer laser or at least a portion of the sensor device can be external to the enclosure with optical access to an interior by way of a through optical device.

[0008] The system can further include a temperature sensor device configured to detect a thermal property associated with the optical element, the assessment module being in communication with the temperature sensor device. The assessment module can be configured to: also analyze the detected thermal property; and generate the estimate of damage to the optical element based on the analysis of both the detected light and the detected thermal property. The thermal property associated with the optical element can be a thermal property of the optical element and / or of a mount to which the optical element is coupled. The temperature sensor device can include a thermocouple, or a thermistor or an infrared camera (IR thermal camera), or an infrared photodetector (IR thermometer), for example. The excimer laser can include a first stage and a second stage in series with the first stage. The optical element can be within a spectral feature selection apparatus of the first stage. The optical element can be within an optical transfer apparatus between the first stage and the second stage. The optical element can be within an optical pulse stretcher that follows the second stage.

[0009] In other general aspects, a system includes: an excimer laser configured to generate light, the excimer laser including at least one optical element interacting with the generated light; a sensor device configured to detect fluorescence emanating from the optical element; and an assessment module in communication with the sensor device. The assessment module is configured to: analyze the detected fluorescence; and generate an estimate of damage to the optical element based on the analysis.

[0010] Implementations can include one or more of the following features. For example, the optical element can be a mirror, a prism, a grating, a window, or a beam splitter. The optical element can include a substrate and a coating applied to at least a portion of the substrate. The excimer laser can be configured to generate light at a wavelength that is between 192 nanometers (nm) and 194 nm or between 247 nm and 249 nm. The sensor device can include one or more of: an optical fiber; a spectrometer; a photodiode detector; a camera; an optical filter; and a shutter. The sensor device can include one or more of a data acquisition module, an analysis module, and a display module. The assessment module being configured to analyze the detected fluorescence can include analyzing an output signal produced by the sensor device. At least a portion of the sensor device can be within an enclosure of the excimer laser or at least a portion of the sensor device can be external to the enclosure with optical access to an interior by way of a through optical device. The excimer laser can include a first stage and a second stage in series with the first stage. The optical element can be within a spectral feature selection apparatus of the first stage, an optical transfer apparatus between the first stage and the second stage, or an optical pulse stretcher that follows the second stage.

[0011] In other general aspects, a method includes: detecting light emanating from an optical element that interacts with a light beam generated by an excimer laser, the light beam at a first wavelength and the detected light having one or more wavelengths that are different from the first wavelength; producing an output signal based on the detected light; analyzing the output signal and calculating a metric associated with the detected light; storing the calculated metric; and determining,based on a history of the metric and the current detected light, an estimate of damage to the optical element.

[0012] Implementations can include one or more of the following features. For example, the estimate of damage can be determined by one or more of: analyzing features of an image in the output signal, the imaging being of a region of interest of an optical surface of the optical element; determining whether an intensity of a total signal count of the image extends beyond a threshold value; analyzing features of a spectrum in the output signal; and subtracting a secondary signal from an analysis signal of the output signal. The estimate of damage to the optical element can be determined by one or more of: assessing a health status of the optical element; assessing a likelihood of failure of the optical element prior to a scheduled maintenance event; and assessing a likelihood of failure of the optical element prior to a subsequent scheduled maintenance event that follows the next scheduled maintenance event.

[0013] The method can also include receiving a signal from a temperature sensor device configured to detect a thermal property associated with the optical element and recording a history of a metric based on the detected thermal property. The estimate of damage to the optical element can be determined determining based on a history of the metric relating to the detected thermal property and the current detected thermal property. The method can further include scheduling a service event of the optical element based on the damage estimate. The service event can include replacing the optical element. The service event can include replacing a module that includes the optical element. The service event can be scheduled by advancing or retarding a previously determined schedule of the service event. The service event can be scheduled by setting an alert for an operator to schedule the service event.

[0014] In other general aspects, a system includes: a lithography apparatus including an excimer laser configured to generate light and a substrate printing apparatus configured to receive the generated light, the lithography apparatus including at least one optical element interacting with the generated light; a sensor device configured to detect fluorescence emanating from the optical element; and an assessment module in communication with the sensor device. The assessment module is configured to: analyze the detected fluorescence; and generate an estimate of damage to the optical element based on the analysis.

[0015] In other general aspects, a system includes: at least one optical element positioned to be illuminated with laser light having a first wavelength; a sensor device configured to detect light emanating from the optical element, the detected light having one or more wavelengths that are distinct from the first wavelength; and an assessment module in communication with the sensor device. The assessment module is configured to: analyze the detected light; and generate an estimate of damage to the optical element based on the analysis.

[0016] Implementations can include one or more of the following features. For example, the optical element can be mounted in a laser configured to produce the laser light. The optical elementcan be mounted in a lithography apparatus configured to receive the laser light from a laser source. The optical element can be mounted in a diagnostic tool for semiconductor wafer inspection.

[0017] 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

[0018] 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.

[0019] Fig. 1 is a diagrammatic representation of a photolithography system including an illumination system that supplies a light beam to a photolithography exposure apparatus, the illumination system including one or more metrology apparatuses, each metrology apparatus associated with an optical element of the photolithography system;

[0020] Fig. 2 is a diagrammatic representation of an implementation of an illumination system of a photolithography system;

[0021] Fig. 3 is a diagrammatic representation of an implementation of a metrology apparatus associated with an optical element of a photolithography system, the metrology apparatus including a sensor device and an assessment module in communication with the sensor device;

[0022] Fig. 4 is a diagrammatic representation of an implementation of an illumination system of a photolithography system, including possible locations of one or more sensor devices;

[0023] Figs. 5A-5F are diagrammatic representations of implementations of a metrology apparatus;

[0024] Fig. 6A is a perspective schematic view of an implementation of an optical element of the photolithography system, in which the optical element is undamaged;

[0025] Fig. 6B is a perspective schematic view of the implementation of the optical element of Fig. 6A, in which the optical element is damaged;

[0026] Fig. 7A is a perspective schematic view of an implementation of an optical element of the photolithography system, in which the optical element is undamaged;

[0027] Fig. 7B is a perspective schematic view of the implementation of the optical element of Fig. 7A, in which the optical element is damaged;

[0028] Fig. 8 is a graph of an example of electrical signals that can be output from a sensor device, which is a spectrometer, relative to a wavelength of fluorescence emitted from an optical element, a first electrical signal taken when the optical element is undamaged and a second electrical signal taken when the optical element is damaged;

[0029] Fig. 9 is a graph of an example of electrical signals that can be output from a sensor device, which is a spectrometer, relative to a wavelength of fluorescence emitted from an optical element, a first electrical signal taken when the optical element is undamaged and a second electrical signal taken when the optical element is damaged;

[0030] Fig. 10 is a diagrammatic representation of an implementation of a metrology apparatus associated with an optical element of a photolithography system, the metrology apparatus including a sensor device and an assessment module in communication with the sensor device and a temperature sensor device associated with the optical element;

[0031] Fig. 11 is a flow chart of an implementation of a procedure performed to determine damage to an optical element based on a detection of fluorescence emanating from the optical element; and

[0032] Fig. 12 is a flow chart of an implementation of a procedure performed to determine damage to an optical element based on a detection of fluorescence emanating from the optical element as well as a detection of a thermal property associated with the optical element.

[0033] 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

[0034] Referring to Fig. 1, electronic devices are constructed of circuits that are formed on a substrate 105 while processing in a photolithography system 100. The photolithography system 100 includes an illumination system 110 (which can include an excimer laser system) that produces and supplies light (such as a light beam) 115 to a photolithography exposure apparatus or scanner 160. The substrate 105 is typically made of a semiconductor material (such as, for example, silicon) and is often referred to as a wafer. Many circuits can be formed together on the same piece of silicon and are called integrated circuits or ICs. With advancements in technology, the size of these circuits has decreased dramatically so that many more of them can fit on the substrate. For example, an IC chip in a smart phone can be as small as a fingernail and yet may include over 2 billion transistors, the size of each transistor being less than 1 / 1, 000th the width of a human hair.

[0035] Making these ICs with extremely small structures or components is a complex, timeconsuming, and expensive process, often involving hundreds of individual steps. Errors in even one step have the potential to result in defects in the finished IC, rendering it useless. Thus, one goal of themanufacturing process is to avoid such defects to maximize the number of functional ICs made in the process, that is, to improve the overall yield of the process.

[0036] Yield is a metric that characterizes failure rate in device fabrication. Yield relates to cost and efficiency. Yield can be defined as a ratio of all the wafers that are produced by a fab to the number of wafers that were introduced to the fab. Or, yield can be the number of working chips that survive the device fabrication process performed on a wafer to the number of potential chips that can be fabricated from that wafer in the ideal case of zero failure. As some wafers or chips fail during fabrication, the overall yield is less than 100%. For example, to obtain a 75% yield for a 50-step process (where a step can be indicative of the number of layers formed on a wafer), each individual step should have a yield greater than 99.4%. In contrast, if individual steps have a yield of 95%, the compounding errors at each step result in an overall process yield as low as 7-8%. Every wafer or chip lost during fabrication is a sunk cost and lost time for the fab.

[0037] Speed, or throughput, has been a traditionally important metric alongside yield.Throughput is a measurable quantity that characterizes the manufacture speed of a semiconductor fabrication facility (fab). For example, throughput can be quantified as the number of IC units produced per unit time. Throughput can become even more important if there are global chip shortages. As there are multiple steps in the fabrication of a chip device (for example, multiple steps for multiple layers), each step can have a characteristic throughput. For example, a throughput value can be assigned to how quickly a photolithographic system (such as system 100) can conduct an illumination optimization process. Innovations in the design or functions of source optimizers can increase throughput or resolve problems in another aspect while mitigating adverse impact to throughput.

[0038] Monitoring health of modules within the illumination system 110 and the scanner 160 and maintaining specifications at the scanner 160 are important for maintaining the yield and throughput and for reducing costs associated with device fabrication. Conventional monitoring techniques can be insufficient for accurate in-situ, direct damage analysis of individual optical elements or modules within the illumination system 110 or 210 or for pinpointing a problematic module or optical element. For example, monitoring can be performed indirectly by evaluating various performance characteristics in the illumination system 110 and inferring the health of one or more modules accordingly within the illumination system 110. For example, the optics of a module such as OPuS 227 can be evaluated according to laser output properties. But such methods, while useful in laser issue diagnosis, may not yield direct damage assessment of each individual optic or allow for module lifetime prediction. Additionally, some testing of the illumination system 110 must be done during service events, using dedicated laser firing patterns or other controlled testing conditions, rather than by in-situ measurement of an output beam (such as the light beam 110). Unfortunately, extensions of service events or other system downtime take extra time during time-constrained service events and can negatively impact yield and throughput.

[0039] Alternatively, in the past, some optics can be visually inspected to determine any damage on the optical surfaces. But this approach is merely qualitative and may not be reliable for determining precise damage levels or predicting the remaining lifetime of the optics. Further, while some of these inspections can be performed through a viewport, others can require service engineers to swap suspect modules during a service event. In addition to increasing downtime in the illumination system 110, such inspections risk exposure to air, humidity, and contamination, and these risks can lead to optics damage.

[0040] As technology advances and the complexity of modules within the illumination system 110 increases, it is desirable to perform direct damage inspection and lifetime prediction of individual optics and modules, with high accuracy and minimal downtime, while maintaining the optics and modules in an unperturbed purge environment.

[0041] Implementations of the present disclosure provide systems and methods for performing direct damage inspection and lifetime prediction of individual optics and modules within the photolithography system 100, including the illumination system 110 and the scanner 160 and other systems. Individual optical elements such as, for example, lenses, mirrors, prisms, gratings, fdters, etc. can each be coupled to a dedicated sensor device configured to continuously monitor fluorescence emanating from the optical element during periods of irradiation and non-irradiation of that optical element. A history of variations in the fluorescence of each optical element can be recorded and used to provide a real-time assessment of optical damage and lifetime prediction for that optical element. To this end, the system 100 includes one or more metrology apparatuses 130, each metrology apparatus 130 associated with an optical element 131 and configured to monitor the fluorescence associated with that optical element 131. While the apparatus 130 is shown as being configured relative to the optical element 131 within the illumination system 110, it is alternatively or additionally possible for there to be metrology apparatuses 130 associated with optical elements within the scanner 160.

[0042] Referring again to Fig. 1, the photolithography system 100 includes the illumination system 110. As described more fully below, the illumination system 110 produces a pulsed light beam 115 and directs the light beam 115 to the photolithography exposure apparatus or scanner 160, which then patterns microelectronic features on the substrate or wafer 105. The wafer 105 is placed on a wafer table 162 constructed to hold or fix the wafer 105. The wafer table 162 is connected to a positioner (not shown) that is configured to position the wafer 105 accurately in accordance with certain parameters.

[0043] The photolithography system 100 can produce a light beam 115 having a wavelength in the deep ultraviolet (DUV) range. For example, the wavelength of the light beam 115 can be between 192 nanometers (nm) and 194 nm or can be between 247 nm and 249 nm. The size of the microelectronic features patterned on the wafer 105 depends on the wavelength of the light beam 115, with a lower wavelength resulting in a smaller minimum feature size. When the wavelength of thelight beam 115 is 248 nm or 193 nm, the minimum size of the microelectronic features can be, for example, 50 nm or less. The bandwidth of the light beam 115 can be the actual, instantaneous bandwidth of its optical spectrum (or emission spectrum), which contains information on how the optical energy of the light beam 115 is distributed over different wavelengths.

[0044] The scanner 160 includes an optical arrangement 164 having, for example, one or more condenser lenses, a mask, and an objective arrangement. The mask is movable along one or more directions, such as along an optical axis of the light beam 115 or in a plane that is perpendicular to the optical axis. The objective arrangement includes a projection lens and enables an image transfer to occur from the mask to the photoresist on the wafer 105. The illumination system 110 adjusts the range of angles for the light beam 115 impinging on the mask. The illumination system 110 also homogenizes (makes uniform) the intensity distribution of the light beam 115 across the mask.

[0045] The scanner 160 can include, among other features, a lithography controller 166, temperature control devices, and power supplies for the various electrical components. The lithography controller 166 controls how layers are printed on the wafer 105. The lithography controller 166 includes memory that stores information such as process recipes. A process program or recipe determines the length of the exposure on the wafer 105, the mask used, and other factors that affect the exposure. During lithography, a plurality of pulses of the light beam 115 illuminates the same area of the wafer 105 to together constitute an illumination dose.

[0046] The photolithography system 100 also includes a control system 102. In general, the control system 102 includes one or more of digital electronic circuitry, computer hardware, firmware, and software. The control system 102 also includes memory, which can be read-only memory or random-access memory. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including, by way of example, semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto -optical disks; and CD-ROM disks.

[0047] The control system 102 can also include one or more input devices (such as a keyboard, touch screen, microphone, mouse, hand-held input device, etc.) and one or more output devices (such as a speaker or a monitor). The control system 102 also can include components to enable wireless communication including Bluetooth, NFC, and Wi-Fi. In particular, the control system 102 can include components that permit the control system to exchange data, instructions, etc. with other data management systems that may be local, remote, or accessible in a distributed network (“cloud”). The control system 102 also includes one or more programmable processors, and one or more computer program products tangibly embodied in a machine -readable storage device for execution by one or more programmable processors. The one or more programmable processors can each execute a program of instructions to perform desired functions by operating on input data and generating appropriate outputs. Generally, the processors receive instructions and data from the memory. Any of the foregoing may be supplemented by, or incorporated in, specially designed ASICs (application-specific integrated circuits). The control system 102 can be centralized or be partially or wholly distributed throughout the photolithography system 100.

[0048] Fig. 2 illustrates an exemplary illumination system 210 that can correspond to, for example, illumination system 110 of Fig. 1. The illumination system 210 includes a pulsed laser source that produces a pulsed laser beam as a light beam 115. While Fig. 2 depicts one particular assemblage of components or modules and optical path strictly for purposes of facilitating the description of the broad principles of the disclosed implementations in general, it is appreciated that these principles can be advantageously applied to lasers having other modules and configurations.

[0049] The illumination system 210 can include, for example, a first stage subsystem 211, a second stage subsystem 221 in series with the first stage subsystem 211, a relay optics subsystem 216, and an output subsystem 225. The first stage subsystem 211 can include, for example, a solid state or gas discharge seed laser. The second stage subsystem 221 can include a power amplification (PA) stage, which can be, for example, a single-pass amplifier stage, a double-pass amplifier stage, a power ring amplifier (PRA) stage, or a power oscillator (PO) stage.

[0050] The first stage subsystem 211 can include, for example, a master oscillator (MO) chamber module 212, in which electrical discharges between electrodes (not shown) cause lasing gas discharges in a lasing gas to create an inverted population of high energy molecules or excimers (or “exciplexes”) such as F2, ArF, KrF, XeCl). The discharges can produce relatively broad band radiation that may be line narrowed to a relatively very narrow bandwidth around a center wavelength selected in a spectral feature selection (or line narrowing) module (LNM) 213. The first stage subsystem 211 can also include a master oscillator output coupler (MO OC) 214, which can include a partially reflective mirror. Along with a reflective grating (not shown) in the LNM 213, the MO output coupler 214 can define an oscillator cavity in which discharge light oscillates to form a seed light beam 218. The first stage subsystem 211 can also include a metrology module such as a linecenter analysis module (LAM) 215. The LAM 215 can include, for example, an etalon spectrometer for fine wavelength measurement and a coarser resolution grating spectrometer. A wavefront engineering module (WEB) 217 can serve to redirect the seed light beam 218 toward the second stage subsystem 221. The WEB 217 can include, for example, beam expansion with, for example, a multi prism beam expander (not shown) and coherence busting, for example, in the form of an optical delay path (not shown).

[0051] The second stage subsystem 221 can include, for example, a PRA chamber module 222, which can be an oscillator, for example, formed by seed light beam injection and output coupling optics (not shown) that can be incorporated into a PRA WEB 223 and can be redirected back through a gain medium in the PRA chamber module 222 by a beam reverser 224. The PRA WEB 223 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.

[0052] Another metrology module such as a bandwidth analysis module (BAM) 226 at the output of the second stage subsystem 221 can receive the output light beam of pulses from the second stage subsystem 221 and pick off a portion of the light beam for metrology purposes, for example, to measure the output bandwidth and pulse energy. The output light beam of pulses then passes through an optical pulse stretcher (OPuS) 227 and another metrology module, which can be an output combined autoshutter metrology module (CASMM) 228. The CASMM 228 can also be the location of a pulse energy meter.

[0053] One purpose of the OPuS 227 can be, for example, to convert a single output light pulse into a pulse train. Secondary pulses created from the original single output light pulse can be delayed with respect to each other. By distributing the original pulse energy into a train of secondary pulses, the effective pulse length of the light beam can be expanded and at the same time the peak pulse intensity reduced. The OPuS 227 can thus receive the light beam from the PRA WEB 223 by way of the BAM 226 and direct the output of the OPuS 227 to the CASMM 228.

[0054] The overall availability of the light source (for example, the illumination system 210) is the direct result of the respective availabilities of individual modules making up the light source. In other words, the light source cannot be available unless all of the critical modules making up the light source are available. This necessitates the use of some form of a maintenance strategy. One approach to maintenance of the light source is referred to as umbrella maintenance, in which a group of multiple modules, some of which may not have failed, are all replaced at the same time in order to optimize light source availability and thereby fab productivity. Using an umbrella maintenance strategy, each module is assumed to have a minimum lifetime, or a lifetime that is an integer multiple of another module lifetime. For example, the nominal minimum lifetime of module A is six months and the nominal minimum lifetime of module B is eighteen months. In such a scenario, module B would be replaced with every third replacement of module A.

[0055] An umbrella maintenance strategy is disrupted if an actual module lifetime is less than a rated or expected minimum lifetime, which can also cause a cascading impact by breaking the synchronous maintenance schedule for other modules. A module can also have a potential or actual lifetime that exceeds its rated minimum lifetime, and in these cases umbrella maintenance involves the deinstallation of a module that is still capable of providing additional satisfactory operation. System maintenance events require that a light source be taken out of production. Thus, umbrella maintenance can cause an unnecessary interruption in productivity when the fab operations otherwise could have continued.

[0056] Therefore, it can be desirable to monitor individual optics or optical modules to determine or predict when a service event is actually required. For example, a predictive maintenance (PdM) strategy can include monitoring the condition of in-service equipment to predict when equipment will fail. The future behavior / condition of machine components can be approximated, and this makes it possible to optimize maintenance tasks (for example, prognostic health monitoring). Accordingly,machine downtime and maintenance costs can be reduced significantly while undertaking maintenance as infrequently as possible.

[0057] For example, at the beginning of an irradiation period, an optical element is impinged by the laser radiation. The optical element includes a substrate and optionally a coating applied to the substrate. One or more of the coating and the substrate can emit fluorescence when the laser radiation impinges upon them. Specifically, as this laser radiation or light reflects off a coating or transmits through a substrate, a finite absorption of the laser radiation, either through linear absorption by dopants and impurities or by nonlinear absorption, can lead to fluorescence emission at wavelengths that range from the ultraviolet to the near infrared. As damage occurs to the coating and / or substrate of the optical element, the coating and substrate can emit stronger or weaker fluorescence or start to emit fluorescence at a new wavelength. By monitoring the progression of the fluorescence spectrum induced by the light beam 115 impinging on the optical element, it is possible to measure damage to the optical element.

[0058] These changes to the fluorescence spectra serve as early indicators of the need for a service event, such as maintenance or replacement of an optical element or module within the illumination system 110, 210. By monitoring and recording the fluorescence history of each optical element, changes in the health indicator can be used to assess optical damage and make accurate lifetime predictions. This monitoring may be performed continuously without any need for dedicated downtimes, and it may be more accurate than performing visual inspection or measuring laser performance parameters.

[0059] Referring to Fig. 3, a metrology system 330 is schematically illustrated relative to an optical element 331. The metrology system 330 includes a sensor device 332 configured to detect fluorescence 333 emanating from the optical element 331 when the optical element 331 interacts with a light beam 334 generated by or within a light source (such as the illumination system 110 or 210). In some implementations, the light beam 334 corresponds to a pre-cursor light beam within the light source 110, 210 that forms the light beam 115 for use by the scanner 160 (Fig. 1). In other implementations, the light beam 334 corresponds to the light beam 115 within the scanner 160. In yet other implementations, the light beam 334 corresponds to a light beam in a diagnostic tool for semiconductor wafer inspection. Additionally, the fluorescence 333 is not merely a reflection of the light beam 334 from the optical element 331. Rather, it is the emission of light from molecules or substances within the optical element 331 that have been excited by the light beam 334 into a different excited state. The metrology system 330 also includes an assessment module 335 in communication with the sensor device 332. The assessment module 335 can be integrated within the control system 102 or can be a separate module from the control system 102. The assessment module 335 is configured to receive an output signal 341 from the sensor device 332, the output signal providing information relating to the detected fluorescence 333, to analyze the detected fluorescence 333, and to generate an estimate of damage to the optical element 331 based on this analysis.

[0060] Referring to Fig. 4, a metrology system 430 can be associated with one or more optical elements 331 (not shown in Fig. 4) within one or more of the modules within the illumination system 210. As discussed above, each metrology system 430 includes a dedicated sensor device 432 associated with an optical element 331. In this implementation, each dedicated sensor device 432 communicates with a shared assessment module 435. Each sensor device 432 is represented by a diamond shape in Fig. 4 and thus each diamond shape is associated with an optical element 331.

[0061] While each module of the illumination system 210 is schematically illustrated as having a particular number and locations of sensor devices 432, it should be understood that this is merely illustrative. In practice, a module can include, for example, no sensor devices 432, one sensor device 432 for a particular optical element being monitored, or more than one sensor device 432, with each sensor device 432 associated with a respective optical element being monitored. In some implementations, a single optical element can be connected to multiple sensor devices 432 to provide a plurality of signals for, for example, performing a weighted average measurement or for redundancy. In some implementations, two or more optical elements within a given module can be associated with at least one sensor device 432 for monitoring. In some implementations, not all optical elements within a given module are monitored. For example, if it is determined that certain optical elements within a module consistently fail before others, and failure of these elements would necessitate replacement of the entire module, then it may be sufficient to monitor only the more failure-prone optical elements. Additionally, other locations at which the sensor device 432 can be arranged within the illumination system 210 are possible.

[0062] The first stage subsystem 211 and the wavefront engineering module 217 can include a plurality of sensor devices 432 for monitoring fluorescence of optical elements and providing continuous fluorescence measurements of the optical elements. For example, each sensor device can be configured to transmit a first signal corresponding to a fluorescence emanating from an optical element. Each of the sensor devices 432 can be coupled to an individual mirror, lens, beamsplitter, or other optical element within a module. For example, any of line narrowing module 213, the MO chamber module 212, the MO output coupler 214, the line-center analysis module 215, or the MO wavefront engineering module 217 can have at least one optical element that is monitored by a sensor device 432.

[0063] Like the first stage subsystem 211 and the wavefront engineering module 217 discussed above, the second stage subsystem 221 and the output subsystem 225 can include a plurality of sensor devices 432 for monitoring fluorescence of optical elements and providing continuous fluorescence measurements of these optical elements. For example, any of beam reverser 224, PRA chamber module 222, PRA WEB 223, bandwidth analysis module 226, optical pulse stretcher 227, or output combined autoshutter metrology module 228, can include at least one sensor device 432. The types and arrangements or sensor devices within second stage subsystem 221 and the output subsystem 225can be similar to the types and arrangements described with respect to the first stage subsystem 211 and the wavefront engineering module 217 above.

[0064] Each of the sensor devices 432 is configured to deliver a signal to an assessment module 435. As mentioned above, the assessment module 435 can include, be apart of, or be similar to, for example, the control system 102 of Fig. 1. The assessment module 435 can be configured to, for example, receive and process signals while the illumination system 210 is operating; analyze the detected fluorescence emanating from the various optical elements; record a history of fluorescence variations of each of the optical elements; calculate or estimate, based on the fluorescence emanating from the optical elements and the history of variations of the fluorescence of the optical elements, damage associated with the optical elements; and schedule service events based on the calculation. The service events can include, for example, replacing an individual optical element or module, or performing a maintenance operation on an optical element or module. In some implementations, scheduling the service event includes, for example, initiating the service event, or advancing or retarding a previously determined schedule of the service event. In some implementations, scheduling the service event includes setting an alert for an operator to schedule the service event.

[0065] Referring again to Fig. 3, as mentioned above, the optical element 331 can be a reflective optical element, a refractive optical element, or a diffractive optical element. For example, the optical element 331 can be a mirror, a prism, a grating, a window, or a beam splitter. In some implementations, as shown in inset 3A, the optical element includes a substrate 336 and a coating 337 applied to at least a portion of the substrate 336. In other implementations, as shown in inset 3B, the optical element includes an uncoated substrate 338.

[0066] The illumination system 210, and each of the modules (such as modules 212, 213, 214, 215, 217, 222, 223, 224, 226, 227, 228) and subsystems (such as 211, 221, or 225) within the illumination system 210, can be contained within an enclosure that is sealed.

[0067] As shown in Fig. 5 A, in some implementations of an apparatus 330A, a sensor device 332A is contained entirely within an enclosure 539 in which a module or a subsystem is contained. An output signal 541 A from the sensor device 332A can be transmitted by way of a wired connection through an electrical vacuum feedthrough 540 to the assessment module 335. The feedthrough 540 can be sealed within a wall of the enclosure 539. In other implementations, the output signal 541 A can be transmitted by way of a wireless connection to the assessment module 335. The sensor device 332A can include a photodiode detector, a camera, or a camera array. In some implementations, each camera is a complementary metal -oxide semiconductor (CMOS) having an array of rectangular or square pixels. In other implementations, the camera is a charged coupled device (CCD) or an infrared camera. The photodiode detector and the camera are configured to detect light having the wavelength of the fluorescence 333. In these implementations, the output signal 541 A can include an image of at least a portion of a surface of the optical element 331.

[0068] As shown in Fig. 5B, in other implementations of an apparatus 330B, a sensor device 332B is contained entirely outside the enclosure 539. The fluorescence 333 passes through the enclosure 539 by way of a through optical device 542 such as, for example, a window that is transparent to light having the wavelength of the fluorescence 333. The window 542 can be sealed within a wall of the enclosure 539. And output signal 54 IB from the sensor device 332B is transmitted by way of a wired connection or a wireless connection to the assessment module 335. In these implementations, the output signal 54 IB can include an image of at least a portion of a surface of the optical element 331.

[0069] As shown in Fig. 5C, in still other implementations of an apparatus 330C, a portion 332C1 of a sensor device 332C is contained entirely within the enclosure 539 while a portion 332C2 of the sensor device 332C is contained entirely external to the enclosure 539. For example, the portion 332C1 can constitute an optical sensor element having a surface upon which the fluorescence 333 impinges, and this sensor element can convert the optical energy into an electrical signal 543C. The portion 332C2 can include processing electronics positioned on a substrate, the electronics configured to receive and process the electrical signal 543C and output an output signal 541C for use by the assessment module 335. The sensor device 332C can be formed by a camera, camera array, or photodiode detector, with the optical sensor element physically separated from the processing electronics. In these implementations, the electrical signal 543C can include an image of at least a portion of a surface of the optical element 331 and the output signal 541C can include a total signal count relating to the image.

[0070] In other implementations of an apparatus 330D, as shown in Fig. 5D, a portion 332D1 of a sensor device 332D is within the enclosure 539 and extends through a vacuum sealed feedthrough in a wall of the enclosure 539. The portion 332D1 can be an optical fiber having a first end that captures the fluorescence 333 and a second end that outputs the fluorescence 333 to a portion 332D2 that is external to the enclosure 539. The optical fiber 332D1 is fed through the wall of the enclosure 539 by way of a vacuum sealed feedthrough 545. The detector 332D2 senses the light from the fluorescence 333 and converts the optical energy of the fluorescence 333 into an electrical signal 543D that can be provided to a portion 332D3. The portion 332D3 can include processing electronics positioned on a substrate, the electronics configured to receive and process the electrical signal 543D and output an output signal 541D for use by the assessment module 335.

[0071] The detector 332D2 can include a photodiode detector, a camera, or a camera array. In some implementations, each camera is a complementary metal -oxide semiconductor (CMOS) having an array of rectangular or square pixels. In other implementations, the camera is a charged coupled device (CCD) or an infrared camera. The photodiode detector and the camera are configured to detect light having the wavelength of the fluorescence 333. In these implementations, the electrical signal 543D (and the output signal 54 ID) can include an image of at least a portion of a surface of the optical element 331.

[0072] In other implementations, the detector 332D2 includes a spectrometer configured to measure properties of fluorescence 333 over a specific portion of the electromagnetic spectrum. For example, the spectrometer 332D2 can measure an intensity of the fluorescence relative to the wavelength of the fluorescence. In these implementations, the electrical signal 543D can include an optical spectrum, a region of an optical spectrum, or an integrated signal of the optical spectrum.

[0073] In other implementations of the apparatus 330E, as shown in Fig. 5E, the sensor device 332A of Fig. 5A is arranged downstream (relative to the fluorescence 333) of an optical filter 529E and an optional optical shutter 519E. The optical filter 529E is configured to block light having a wavelength of the light beam 334. The optical filter 529E can include, for example, a long pass filter, a bandpass filter, or a notch filter.

[0074] In other implementations of the apparatus 330F, as shown in Fig. 5F, the sensor device 332A of Fig. 5A is arranged downstream (relative to the fluorescence 333) of one or more imaging lenses 509F and an optional optical aperture 519F. The imaging lenses 509F can be arranged to change the vergence (the angle formed by rays of light that are not perfectly parallel to one another) of the fluorescence 333 in order to cover the sensor area of the sensor device 332A. The aperture 519F can be configured to further adjust the vergence (for example, a convergence or a divergence) of the fluorescence 333.

[0075] In any of the above implementation of the apparatus 330A, 330B, 330C, 330D, 330E, 330F, the assessment module 335 can receive an output signal in which a secondary signal is subtracted. For example, the secondary signal can be a background signal (within the enclosure 539), a signal at a specific moment in time (perhaps at the start of the operation of the optical element 331), or an historical signal (that quantifies the history of the fluorescence emanating from the optical element 331). The output signal received at the assessment module 335 can be a current signal or a voltage signal. The output signal received at the assessment module 335 can be an analog signal or a digital signal.

[0076] Referring to Fig. 6A, an example of an output signal 541A, 541B, 541C (Figs. 5A, 5B, 5C, 5E, 5F) from a camera, a camera array, or a photodiode detector (such as can be used in the device 332A, 332B, or 332C) is shown. The output signal 541A, 541B, 541C constitutes an image 650A of a surface 603 of an optical element 631, which can be a mirror within one of the modules or subsystems of the illumination system 110 or 210. The mirror in this example includes a coating applied to a substrate (such as shown in inset 3A of Fig. 3) and thus the surface 603 is a surface of the coating. In this example, the mirror is undamaged; for example, the mirror has not been previously exposed to the generated light beam 334. The size of the image 650A depends on the transverse size of the light beam 334 that impinges upon the sensor element of the sensor device. The fluorescence 333 emanating from the surface 603 is depicted in the image 650A as 633A. The fluorescence 633A shows up as a homogeneous and weak pattern at the camera sensor. As is evident from the size of the captured fluorescence 633 A relative to the overall size of the surface 603, the transverse extent of thefluorescence 333 in this particular example is much smaller than the area of the surface 603 because the transverse extent of the light beam 334 is much smaller than the area of the surface 603. In other implementations, the transverse extent of the light beam 334 can be larger or as large as the area of the surface 603.

[0077] Referring to Fig. 6B, an example of an output signal 541A, 541B, 541C (Figs. 5A, 5B, 5C, 5E, 5F) from a camera, a camera array, or a photodiode detector (such as can be used in the device 332A, 332B, or 332C) is shown. The output signal 541A, 541B, 541C constitutes an image 650B of the surface 603 of the optical element 631, which, as noted above, can be a mirror within one of the modules or subsystems of the illumination system 110 or 210. The mirror in this example includes a coating applied to a substrate (such as shown in inset 3A of Fig. 3) and thus the surface 603 is a surface of the coating. In this example, the mirror has been damaged; for example, the mirror has been exposed for a significant amount of time to the generated light beam 334. The size of the image 650B depends on the transverse size of the light beam 334. The fluorescence 333 emanating from the surface 603 is depicted in the image 650B as 633B. The fluorescence 633B shows up with a stronger pattern than the fluorescence 633A. The fluorescence includes inhomogeneous and irregular patterns at the camera sensor. These strong patterns that are evident from the image 650B indicate that there is damage on the coating of the mirror 631 when compared with the coating of the mirror 631 of Fig.6A.

[0078] Referring to Fig. 7A, another example of an output signal 541A, 541B, 541C (Figs. 5A, 5B, 5C, 5E, 5F) from a camera, a camera array, or a photodiode detector (such as can be used in the device 332A, 332B, or 332C) is shown. The output signal 541A, 541B, 541C constitutes an image 750A of a surface 703 of an optical element 731, which can be a mirror within one of the modules or subsystems of the illumination system 110 or 210. The mirror in this example includes a coating applied to a substrate (such as shown in inset 3A of Fig. 3) and thus the surface 703 is a surface of the coating. In this example, the mirror is undamaged; for example, the mirror has not been previously exposed to the generated light beam 334. The size of the image 750A depends on the transverse size of the light beam 334 that impinges upon the sensor element of the sensor device. The fluorescence 333 emanating from the surface 703 is depicted in the image 750A as 733A. The fluorescence 733A shows up as a homogeneous and weak pattern at the camera sensor. As is evident from the size of the captured fluorescence 733A relative to the overall size of the surface 703, the transverse extent of the fluorescence 333 in this particular example is much smaller than the area of the surface 703 because the transverse extent of the light beam 334 is much smaller than the area of the surface 703. In other implementations, the transverse extent of the light beam 334 can be larger or as large as the area of the surface 703.

[0079] Referring to Fig. 7B, an example of an output signal 541A, 541B, 541C (Figs. 5A, 5B, 5C, 5E, 5F) from a camera, a camera array, or a photodiode detector (such as can be used in the device 332A, 332B, or 332C) is shown. The output signal 541A, 541B, 541C is an image 750B of thesurface 703 of the optical element 731, which, as noted above, can be a mirror within one of the modules or subsystems of the illumination system 110 or 210. The mirror in this example includes a coating applied to a substrate (such as shown in inset 3A of Fig. 3) and thus the surface 703 is a surface of the coating. In this example, the mirror has been damaged; for example, the mirror has been exposed for a significant amount of time to the generated light beam 334. The size of the image 750B depends on the transverse size of the light beam 334. The fluorescence 333 emanating from the surface 703 is depicted in the image 750B as 733B. The fluorescence 733B shows up with a stronger pattern than the fluorescence 733A. The fluorescence 733B includes a stronger pattern that includes discrete circular patterns at the camera sensor. This strong circular pattern that is evident from the image 750B indicates that there is damage that forms on the coating of this mirror 731 when compared with the mirror 731 at Fig. 7A.

[0080] Referring to Fig. 8, a graph 850 shows an example of the electrical signal 543D (Fig. 5D) that can be output from the spectrometer 332D2 relative to an optical element 331 such as a mirror, which includes a coating applied to a substrate (such as shown in inset 3A of Fig. 3). The graph 850 shows the electrical signal 543D from the spectrometer 332D2 relative to a wavelength of the fluorescence 333. A first electrical signal 843i is an example of the fluorescence 333 detected by the spectrometer 332D2 when the mirror is undamaged (for example, the mirror has not been previously exposed to the generated light 334). A second electrical signal 843ii is an example of the fluorescence 333 detected by the spectrometer 332D2 after the mirror has been damaged (for example, the mirror has been exposed for a significant amount of time to the generated light 334). It is evident that the electrical signal 843ii exhibits a higher fluorescence intensity at wavelength 78. which indicates that there likely is damage on the coating of the mirror. On the other hand, the electrical signal 843 i exhibits a weaker and relatively flatter fluorescence intensity, which indicates that there is likely no or little damage on the coating of the mirror.

[0081] Referring to Fig. 9, a graph 950 shows another example of the electrical signal 543D (Fig.5D) that can be output from the spectrometer 332D2 relative to another optical element 331 such as a mirror, which includes a coating applied to a substrate (such as shown in inset 3 A of Fig. 3). The graph 950 shows the electrical signal 543D from the spectrometer 332D2 relative to a wavelength of the fluorescence 333. A first electrical signal 943i is an example of the fluorescence 333 detected by the spectrometer 332D2 when the mirror is undamaged (for example, the mirror has not been previously exposed to the generated light 334). A second electrical signal 943ii is an example of the fluorescence 333 detected by the spectrometer 332D2 after the mirror has been damaged (for example, the mirror has been exposed for a significant amount of time to the generated light 334). It is again evident that the electrical signal 943ii exhibits a higher fluorescence intensity at a wavelength 79. which indicates that there is likely damage on the coating of the mirror. On the other hand, the electrical signal 943i exhibits a weaker and relatively more homogeneous fluorescence pattern.

[0082] In general, the sensor device 332 can include one or more optical fibers, a spectrometer, a photodiode detector, a camera, a camera array, an optical filter, a shutter, one or more lenses, and / or an optical aperture.

[0083] Referring to Fig. 10, in other implementations, a metrology system 1030 includes the sensor device 332 configured to detect fluorescence 333 emanating from the optical element 331, as discussed above, and also includes a temperature sensor device 1004 in communication with the assessment module 1035. The temperature sensor device 1004 can include one or more temperature sensors 1006, 1007. In some implementations, the one or more temperature sensors 1006, 1007 include thermocouples encased in a cleanroom -compatible material such as stainless-steel tubing. The thermocouples can be inserted into the enclosure (such as enclosure 539) by, for example, vacuum seal fittings. One of the temperature sensor 1007 can be in contact with a surface of the optical element 331 or with a mount 1008 (to which the optical element 331 is fixed), while the other temperature sensor 1006 can be in contact with an environment within the enclosure 539.

[0084] As the optical element 331 is irradiated with the light beam 334 over time, damage can be caused to, for example, its coating or its bulk material (substrate). This damage can cause more of the incoming radiation of the light beam 334 to be absorbed, and less of it to be reflected or transmitted. This can lead to an increase in temperature as well as damage of the optical element 331.Additionally, an environmental temperature measured at the temperature sensor 1006 can be subtracted from temperature measured at the temperature sensor 1007 to eliminate the influence of temperature variations in the environment of the enclosure 539.

[0085] The assessment module 1035 receives the output from the temperature sensor device 1004, such output indicating a thermal property associated with the optical element 331. The assessment module 1035 can analyze this thermal property and generate the estimate of damage to the optical element 331 based on an analysis of both the output from the sensor device 332 and the output from the temperature sensor device 1004.

[0086] Referring to Fig. 11, a procedure 1170 is performed to determine a damage to an optical element, such as the optical element 131, 331, 631, or 731. The procedure 1170 can be performed by the metrology apparatus 130 associated with the optical element. Initially, while a light source is operating to generate a light beam at a first wavelength, light emanating from an optical element that interacts with the light beam is detected (1171). For example, with reference to Fig. 3, the light beam 334 interacts with the optical element 331 and the sensor device 332 detects the fluorescence 333.

[0087] Next, an output signal is produced based on the detected light at 1171 (1172). For example, the sensor device 332 produces the output signal 341 based on the detected fluorescence 333.

[0088] The output signal is analyzed, and a metric associated with the detected light is calculated (1173). For example, the assessment module 335 receives the output signal 341 and performs an analysis of the output signal 341. The assessment module 335 can calculate a metric that representsone or more aspects of the fluorescence 333 based on the data in the output signal 341. As an example, if the sensor device includes a spectrometer, as described in Fig. 5D, then the metric can correspond to a total intensity at a specific wavelength of the optical spectrum or the metric can correspond to a total intensity averaged across all wavelengths of the optical spectrum. As another example, if the sensor device includes a camera, as described in Fig. 5A, then the metric can correspond to a sum of the brightness levels of each pixel in the camera sensor. The metric that is calculated is stored (1173). For example, the metric can be stored within memory in the assessment module 335.

[0089] An estimate of damage to the optical element is determined (1174) based on the history of the metric and also currently -detected light at 1171. For example, if the optical element 331 is a reflective optical element, then the assessment module 335 can determine a reduction in the reflectivity as a percentage of the reflectivity of a new optical element 331. As another example, if the optical element 331 is a prism, then the assessment module 335 can determine a reduction in transmission through the optical element 331. The damage estimate at 1174 can also include one or more of: assessing a health status of the optical element 331; assessing a likelihood of failure of the optical element 331 prior to a scheduled maintenance event; and assessing a likelihood of failure of the optical element 331 prior to a subsequent scheduled maintenance event that follows the next scheduled maintenance event. The assessment module 335 can monitor, over a period of time, a history of an output signal 341 from the sensor device 332. The period of time can be 30 seconds or less, one minute or less, 30 minutes or less, one hour or less, one day or less, one week or less, one month or less, or one year or less.

[0090] Next, the apparatus 130 determines whether the damage estimate exceeds a threshold (1175), and if it does, then a service event of the optical element is scheduled (1176). For example, the service event can include replacing the optical element 331 or replacing a module or sub-system containing the optical element 331. The scheduling of the service event at 1176 can include advancing or retarding a previously determined schedule of the service event. The scheduling of the service events at 1176 can include setting an alert for an operator to schedule the service event.

[0091] Referring to Fig. 12, a procedure 1280 is performed to determine a damage to an optical element, such as the optical element 131, 331, 631, or 731. The procedure 1280 can be performed by the metrology apparatus 1030 associated with the optical element 331 (Fig. 10). Steps that are similar to those in the procedure 1170 are not repeated here.

[0092] In parallel with steps 1171, 1172, 1173 (discussed above), the procedure includes steps 1281, 1282, 1283, that can be performed in parallel with steps 1171, 1172, 1173. Initially, while a light source is operating to generate a light beam at a first wavelength, a thermal property associated with the optical element 331 is detected (1281). For example, with reference to Fig. 10, the light beam 334 interacts with the optical element 331 and the temperature sensor device 1004 by way of 1006, 1007 detects a thermal property. Next, an output signal is produced based on the detected thermalproperty at 1281 (1282). For example, the temperature sensor device 1004 produces an output signal based on the detected thermal property that can be provided to the assessment module 1035. The output signal from the temperature sensor device 1004 is analyzed, a metric associated with the detected thermal property is calculated and stored (1283). For example, the assessment module 1035 receives the output signal from the temperature sensor device 1004 and performs an analysis of the output signal and stores the metric.

[0093] An estimate of damage to the optical element is determined (1274) based on the history of the metrics and also currently-detected properties (such as detected light or detected thermal properties). Next, the apparatus 1030 determines whether the damage estimate at 1274 exceeds a threshold (1275), and if it does, then a service event of the optical element is scheduled (1276), as discussed above. In some implementations, a service event is scheduled at 1276 if both metrics (calculated at 1173 and 1283) fail. In this case, service events are initiated if both damage estimates exceed a threshold at 1275. In other implementations, a service event is scheduled at 1276 if either of the metrics (calculated at 1173 and 1283) fails. In this case, service events are initiated if one of the damage estimates at exceeds a threshold (1275). In other implementations, a service event is scheduled at 1276 if a hybrid metric, based on a linear or other numerical combination of the two metrics (calculated at 1173 and 1283) fails. In this case, service events are initiated if a combination of the damage estimates exceeds a threshold at 1275.

[0094] Implementations of the components of the system 100 can be implemented in hardware, firmware, software, or any combination thereof. Implementations of the components also be implemented as instructions stored on a machine -readable medium, such as a computer-readable medium, which can be read and executed by one or more processors. A machine -readable medium can include any mechanism for storing or transmitting information in a form readable by a machine (for example, a computing device). For example, a machine -readable medium can include read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; electrical, optical, acoustical, or other forms of propagated signals (for example, carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions can be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc.

[0095] The implementations can be further described using the following clauses:1. A system comprising:an excimer laser configured to generate light having a first wavelength, the excimer laser comprising at least one optical element interacting with the generated light;a sensor device configured to detect light emanating from the optical element, the detected light having one or more wavelengths that are distinct from the first wavelength; andan assessment module in communication with the sensor device, the assessment module configured to:analyze the detected light; andgenerate an estimate of damage to the optical element based on the analysis.2. The system of clause 1, wherein the optical element is a reflective optical element, a refractive optical element, or a diffractive optical element.3. The system of clause 1, wherein the optical element is a mirror, a prism, a grating, a window, a lens, or a beam splitter.4. The system of clause 1, wherein the optical element comprises a substrate and a coating applied to at least a portion of the substrate.5. The system of clause 1, wherein the optical element comprises an uncoated substrate.6. The system of clause 1, wherein the excimer laser is configured to generate light at a first wavelength that is between 192 nanometers (nm) and 194 nm or between 247 nm and 249 run.7. The system of clause 6, wherein the wavelength of the detected light is a value or in a range from 100 nm to 1000 nm.8. The system of clause 1, wherein the sensor device comprises one or more of:an optical fiber;a spectrometer;a photodiode detector;a camera;an optical filter; anda shutter.9. The system of clause 1, wherein the sensor device comprises a fiber-coupled spectrometer or a photodiode detector with an optical filter.10. The system of clause 1, wherein the sensor device comprises an imaging device, and one or more imaging lenses, apertures, and shutters between the imaging device and the optical element.11. The system of clause 10, wherein the imaging device comprise a camera, a charge coupled device (CCD), or a complementary metal-oxide-semiconductor (CMOS) device.12. The system of clause 1, wherein the sensor device comprises an optical filter configured to block light at the first wavelength.13. The system of clause 1, wherein the assessment module being configured to analyze the detected light comprises analyzing an output signal produced by the sensor device.14. The system of clause 13, wherein the output signal comprises one or more of:an image of at least a portion of a surface of the optical element;a total signal count relating to the image of at least the portion of the optical element surface; and an optical spectrum, a region of the optical spectrum, or an integrated signal of the optical spectrum.15. The system of clause 13, wherein the output signal comprises an analysis signal in which a secondary signal is subtracted.16. The system of clause 15, wherein the secondary signal is a background signal, a signal at a specific moment in time, or an historical signal.17. The system of clause 13, wherein:the output signal is a current signal or a voltage signal; andthe output signal is an analog signal or a digital signal.18. The system of clause 1, wherein the assessment module being configured to generate the estimate of damage to the optical element based on the analysis comprises one or more of: assessing a health status of the optical element; assessing a likelihood of failure of the optical element prior to a scheduled maintenance event; and assessing a likelihood of failure of the optical element prior to a subsequent scheduled maintenance event that follows the next scheduled maintenance event.19. The system of clause 1, wherein the assessment module being configured to generate the estimate of damage to the optical element based on the analysis comprises monitoring, over a period of time, a history of an output signal from the sensor device, the output signal relating to the detected light. 20. The system of clause 19, wherein the period of time is 30 seconds or less, 1 minute or less, 30 minutes or less, 1 hour or less, 1 day or less, 1 week or less, 1 month or less, or 1 year or less.21. The system of clause 1, wherein at least a portion of the sensor device is within an enclosure of the excimer laser or at least a portion of the sensor device is external to the enclosure with optical access to an interior by way of a through optical device.22. The system of clause 1, further comprising a temperature sensor device configured to detect a thermal property associated with the optical element, wherein the assessment module is in communication with the temperature sensor device, the assessment module configured to:also analyze the detected thermal property; andgenerate the estimate of damage to the optical element based on the analysis of both the detected light and the detected thermal property.23. The system of clause 22, wherein the thermal property associated with the optical element is a thermal property of the optical element and / or of a mount to which the optical element is coupled. 24. The system of clause 23, wherein the temperature sensor device comprises a thermocouple. 25. The system of clause 1, wherein the excimer laser comprises a first stage and a second stage in series with the first stage.26. The system of clause 25, wherein the optical element is within a spectral feature selection apparatus of the first stage.27. The system of clause 25, wherein the optical element is within an optical transfer apparatus between the first stage and the second stage.28. The system of clause 25, wherein the optical element is within an optical pulse stretcher that follows the second stage.29. A system comprising:an excimer laser configured to generate light, the excimer laser comprising at least one optical element interacting with the generated light;a sensor device configured to detect fluorescence emanating from the optical element; and an assessment module in communication with the sensor device, the assessment module configured to:analyze the detected fluorescence; andgenerate an estimate of damage to the optical element based on the analysis.30. The system of clause 29, wherein the optical element is a mirror, a prism, a grating, a window, or a beam splitter.31. The system of clause 29, wherein the optical element comprises a substrate and a coating applied to at least a portion of the substrate.32. The system of clause 29, wherein the excimer laser is configured to generate light at a wavelength that is between 192 nanometers (nm) and 194 nm or between 247 nm and 249 nm.33. The system of clause 29, wherein the sensor device comprises one or more of:an optical fiber;a spectrometer;a photodiode detector;a camera;an optical filter; anda shutter.34. The system of clause 29, wherein the sensor device comprises one or more of a data acquisition module, an analysis module, and a display module.35. The system of clause 29, wherein the assessment module being configured to analyze the detected fluorescence comprises analyzing an output signal produced by the sensor device.36. The system of clause 29, wherein at least a portion of the sensor device is within an enclosure of the excimer laser or at least a portion of the sensor device is external to the enclosure with optical access to an interior by way of a through optical device.37. The system of clause 29, wherein:the excimer laser comprises a first stage and a second stage in series with the first stage, and the optical element is within a spectral feature selection apparatus of the first stage, an optical transfer apparatus between the first stage and the second stage, or an optical pulse stretcher that follows the second stage.38. A method comprising:detecting light emanating from an optical element that interacts with a light beam generated by an excimer laser, the light beam at a first wavelength and the detected light having one or more wavelengths that are different from the first wavelength;producing an output signal based on the detected light;analyzing the output signal and calculating a metric associated with the detected light;storing the calculated metric; anddetermining, based on a history of the metric and the current detected light, an estimate of damage to the optical element.39. The method of clause 38, wherein determining the estimate of damage comprises one or more of: analyzing features of an image in the output signal, the imaging being of a region of interest of an optical surface of the optical element;determining whether an intensity of a total signal count of the image extends beyond a threshold value;analyzing features of a spectrum in the output signal; andsubtracting a secondary signal from an analysis signal of the output signal.40. The method of clause 38, wherein determining the estimate of damage to the optical element comprises one or more of: assessing a health status of the optical element; assessing a likelihood of failure of the optical element prior to a scheduled maintenance event; and assessing a likelihood of failure of the optical element prior to a subsequent scheduled maintenance event that follows the next scheduled maintenance event.41. The method of clause 38, further comprising receiving a signal from a temperature sensor device configured to detect a thermal property associated with the optical element and recording a history of a metric based on the detected thermal property; wherein determining the estimate of damage to the optical element further comprises determining based on a history of the metric relating to the detected thermal property and the current detected thermal property.42. The method of clause 38, further comprising scheduling a service event of the optical element based on the damage estimate.43. The method of clause 42, wherein the service event comprises replacing the optical element. 44. The method of clause 42, wherein the service event comprises replacing a module comprising the optical element.45. The method of clause 42, wherein scheduling the service event comprises advancing or retarding a previously determined schedule of the service event.46. The method of clause 42, wherein scheduling the service event comprises setting an alert for an operator to schedule the service event.47. A system comprising:a lithography apparatus comprising an excimer laser configured to generate light and a substrate printing apparatus configured to receive the generated light, the lithography apparatus comprising at least one optical element interacting with the generated light;a sensor device configured to detect fluorescence emanating from the optical element; andan assessment module in communication with the sensor device, the assessment module configured to:analyze the detected fluorescence; andgenerate an estimate of damage to the optical element based on the analysis.48. A system comprising:at least one optical element positioned to be illuminated with laser light having a first wavelength; a sensor device configured to detect light emanating from the optical element, the detected light having one or more wavelengths that are distinct from the first wavelength; andan assessment module in communication with the sensor device, the assessment module configured to:analyze the detected light; andgenerate an estimate of damage to the optical element based on the analysis.49. The system of clause 48, wherein the optical element is mounted in a laser configured to produce the laser light.50. The system of clause 48, wherein the optical element is mounted in a lithography apparatus configured to receive the laser light from a laser source.51. The system of clause 48, wherein the optical element is mounted in a diagnostic tool for semiconductor wafer inspection.

[0096] Other implementations are within the scope of the following claims.

Claims

CLAIMS1. A system comprising:an excimer laser configured to generate light having a first wavelength, the excimer laser comprising at least one optical element interacting with the generated light;a sensor device configured to detect light emanating from the optical element, the detected light having one or more wavelengths that are distinct from the first wavelength; andan assessment module in communication with the sensor device, the assessment module configured to:analyze the detected light; andgenerate an estimate of damage to the optical element based on the analysis.

2. The system of claim 1, wherein the optical element comprises a substrate and a coating applied to at least a portion of the substrate.

3. The system of claim 1, wherein the excimer laser is configured to generate light at a first wavelength that is between 192 nanometers (nm) and 194 nm or between 247 nm and 249 nm and wherein the wavelength of the detected light is a value or in a range from 100 nm to 1000 nm.

4. The system of claim 1, wherein the sensor device comprises one or more of:an optical fiber;a spectrometer;a photodiode detector;a camera;an optical filter; anda shutter.

5. The system of claim 1, wherein the sensor device comprises an optical filter configured to block light at the first wavelength.

6. The system of claim 1, wherein the assessment module being configured to analyze the detected light comprises analyzing an output signal produced by the sensor device.

7. The system of claim 6, wherein the output signal comprises one or more of:an image of at least a portion of a surface of the optical element;a total signal count relating to the image of at least the portion of the optical element surface; andan optical spectrum, a region of the optical spectrum, or an integrated signal of the optical spectrum.

8. The system of claim 1, wherein the assessment module being configured to generate the estimate of damage to the optical element based on the analysis comprises one or more of: assessing a health status of the optical element; assessing a likelihood of failure of the optical element prior to a scheduled maintenance event; and assessing a likelihood of failure of the optical element prior to a subsequent scheduled maintenance event that follows the next scheduled maintenance event.

9. The system of claim 1, wherein the assessment module being configured to generate the estimate of damage to the optical element based on the analysis comprises monitoring, over a period of time, a history of an output signal from the sensor device, the output signal relating to the detected light.

10. The system of claim 1, wherein at least a portion of the sensor device is within an enclosure of the excimer laser or at least a portion of the sensor device is external to the enclosure with optical access to an interior by way of a through optical device.

11. The system of claim 1, further comprising a temperature sensor device configured to detect a thermal property associated with the optical element, wherein the assessment module is in communication with the temperature sensor device, the assessment module configured to:also analyze the detected thermal property; andgenerate the estimate of damage to the optical element based on the analysis of both the detected light and the detected thermal property.

12. A system comprising:an excimer laser configured to generate light, the excimer laser comprising at least one optical element interacting with the generated light;a sensor device configured to detect fluorescence emanating from the optical element; and an assessment module in communication with the sensor device, the assessment module configured to:analyze the detected fluorescence; andgenerate an estimate of damage to the optical element based on the analysis.

13. The system of claim 12, wherein the optical element is a mirror, a prism, a grating, a window, or a beam splitter.

14. The system of claim 12 wherein the optical element comprises a substrate and a coating applied to at least a portion of the substrate.

15. The system of claim 12, wherein the excimer laser is configured to generate light at a wavelength that is between 192 nanometers (nm) and 194 nm or between 247 nm and 249 nm.

16. The system of claim 12, wherein the sensor device comprises one or more of a data acquisition module, an analysis module, and a display module.

17. The system of claim 12, wherein the assessment module being configured to analyze the detected fluorescence comprises analyzing an output signal produced by the sensor device.

18. The system of claim 12, wherein at least a portion of the sensor device is within an enclosure of the excimer laser or at least a portion of the sensor device is external to the enclosure with optical access to an interior by way of a through optical device.

19. The system of claim 12, wherein:the excimer laser comprises a first stage and a second stage in series with the first stage, and the optical element is within a spectral feature selection apparatus of the first stage, an optical transfer apparatus between the first stage and the second stage, or an optical pulse stretcher that follows the second stage.

20. A method comprising:detecting light emanating from an optical element that interacts with a light beam generated by an excimer laser, the light beam at a first wavelength and the detected light having one or more wavelengths that are different from the first wavelength;producing an output signal based on the detected light;analyzing the output signal and calculating a metric associated with the detected light; storing the calculated metric; anddetermining, based on a history of the metric and the current detected light, an estimate of damage to the optical element.

21. A system comprising:a lithography apparatus comprising an excimer laser configured to generate light and a substrate printing apparatus configured to receive the generated light, the lithography apparatus comprising at least one optical element interacting with the generated light;a sensor device configured to detect fluorescence emanating from the optical element; andan assessment module in communication with the sensor device, the assessment module configured to:analyze the detected fluorescence; andgenerate an estimate of damage to the optical element based on the analysis.

22. A system comprising:at least one optical element positioned to be illuminated with laser light having a first wavelength;a sensor device configured to detect light emanating from the optical element, the detected light having one or more wavelengths that are distinct from the first wavelength; andan assessment module in communication with the sensor device, the assessment module configured to:analyze the detected light; andgenerate an estimate of damage to the optical element based on the analysis.

Images