In-SITU monitoring of laser modules and temperature-based lifetime prediction

Abstract

A system for in-situ monitoring of optical element health in a laser apparatus comprises direct measurement of optical element temperature and recording a history of variations of the optical element temperature during successive periods of irradiation and non-irradiation. The direct measurement may be achieved by a thermocouple inserted into the laser system and pace di contact with the optical element. The history of temperature variations may be used to calculate as trajectory of the change in optical absorption coefficient, which may be used to predict the remaining lifetime of the optical element and schedule service event.

Description

IN-SITU MONITORING OF LASER MODULES AND TEMPERATURE-BASED LIFETIME PREDICTIONCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to US Application No. 63 / 738,160, filed December 23, 2024, titled IN-SITU MONITORING OF LASER MODULES AND TEMPERATURE-BASED LIFETIME PREDICTION, which is incorporated herein by reference in its entirety.TECHNICAL FIELD

[0002] The subject matter disclosed herein 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. They 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.

[0004] One system for generating laser radiation at frequencies useful for semiconductor photolithography (deep-ultraviolet (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 may be regarded as being modules, and the light source overall may be regarded as an ensemble of modules. Each module will in general have 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 must be evaluated to determine if they should be repaired or replaced. Conventional monitoring techniques, however, may be insufficient for pinpointing a problematic module or optical element.SUMMARY

[0005] The following presents a simplified summary of one or more embodiments in order to provide a basic understanding of the present invention. This summary is not an extensive overview of all contemplated embodiments and is not intended to identify key or critical elements of all embodiments nor delineate the scope of any or all embodiments. Its sole purpose is to present some concepts of one or more embodiments in a simplified form as a prelude to the more detailed description that is presented later.

[0006] Some embodiments of the present disclosure provide method. The method may comprise: operating a laser; while operating the laser, receiving a first signal from a first temperature sensor configured to measure a temperature of one of an optical element of the laser or a mount coupled to the optical element; determining a temperature of the optical element or the mount based on the first signal; recording a history of a metric based on the temperature of one of the optical element or the mount; and determining, based on the temperature of the optical element and the history of the metric, a health indicator of the optical element.

[0007] Further features and advantages of the disclosed technology, as well as the structure and operation of various embodiments, are described in detail below with reference to the accompanying drawings. It is noted that the disclosed embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein and their understanding of the underlying technology.BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Fig. 1 is a diagrammatic representation of an example photolithography system, consistent with embodiments of the present disclosure.

[0009] Figs. 2A-C are diagrammatic representations of an example illumination system, consistent with embodiments of the present disclosure.

[0010] Fig. 3 is a diagrammatic representation of an example test setup of an optics module, consistent with embodiments of the present disclosure.

[0011] Figs. 4A-B illustrates an example graph of the change in temperature of an optical element with operation time, consistent with embodiments of the present disclosure.

[0012] Fig. 5A illustrates example graphs, for a plurality of optical elements, of the change in temperature as a function of time and laser duty cycle, consistent with embodiments of the present disclosure.

[0013] Fig. 5B illustrates an example graph of a linear fit of data from the graphs of Fig. 5A, consistent with embodiments of the present disclosure.

[0014] Fig. 6 is a diagrammatic representation of an example method of monitoring a health indicator of a laser optical element, consistent with embodiments of the present disclosure.DETAILED DESCRIPTION

[0015] Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings. The following description refers to the accompanying drawings in which the same numbers in different drawings represent the same or similar elements unless otherwise represented. The implementations set forth in the following description of exemplary embodiments do not represent all implementations consistent with the disclosure. Instead, they are merely examples of apparatuses and methods consistent with aspects related to the subject matterrecited in the appended claims. For example, although some embodiments are described in the context of DUV-based lithographic apparatuses, the present disclosure is not so limited. Unless infeasible, embodiments described herein can be implemented in any type of lithographic apparatus.

[0016] As used herein, unless specifically stated otherwise, the term “or” encompasses all possible combinations, except where infeasible. For example, if it is stated that a component can comprise A or B, then, unless specifically stated otherwise or infeasible, the component can comprise A, or B, or A and B. As a second example, if it is stated that a component can comprise A, B, or C, then, unless specifically stated otherwise or infeasible, the component can comprise A, or B, or C, or A and B, or A and C, or B and C, or A and B and C.

[0017] Electronic devices are constructed of circuits formed on a substrate. The substrate is typically of a semiconductor material (e.g., silicon) and is often referred to as a wafer by persons of skill in the art. Many circuits may 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.

[0018] 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 the manufacturing 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.

[0019] Speed, or throughput, has been a traditionally important metric alongside yield. Throughput is a measurable quantity that characterizes the manufacture speed of a fab (e.g., number of IC units produced per unit time). Throughput has become even more important in view of recent global chip shortages. As there are multiple steps in the fabrication of a chip device (e.g., multiple steps for multiple layers), each step can have a characteristic throughput. For example, a throughput value can be assigned to how quickly a lithographic system 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.

[0020] Yield is a metric that characterizes failure rate in device fabrication, which 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%, thecompounding 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.

[0021] As stated above, conventional monitoring techniques may be insufficient for pinpointing a problematic module or optical element. For example, monitoring may be performed indirectly by evaluating various laser performance characteristics and inferring the health of one or more modules accordingly. But such methods may not yield direct damage assessment of each individual optic or allow for accurate lifetime prediction. Additionally, some laser testing must be done during service events, using dedicated laser firing patterns or other test conditions, rather than by in-situ measurement of an output beam. But extensions of service events or other system downtime negatively impact yield and throughput.

[0022] Alternatively, some optics may 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 may be performed through a viewport, others may require service engineers to open the modules during a service event. In addition to increasing downtime in the laser system, such inspections risk exposure to air, humidity, and contamination, which itself may lead to optics damage.

[0023] As laser technology advances and the complexity of modules increases, it may be 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.

[0024] Embodiments of the present disclosure provide systems and methods for performing direct damage inspection and lifetime prediction of individual optics and modules. Individual optical elements such as, e.g., lenses, mirrors, prisms, gratings, fdters, etc. may each be coupled to a dedicated temperature sensor configured to continuously monitor a temperature of the optical element during periods of irradiation and non-irradiation. A history of temperature variations of each optical element may thus be recorded and used to provide a real-time assessment of optical damage and lifetime prediction.

[0025] Embodiments of the present invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the present invention may also be implemented as instructions stored on a machine -readable medium, such as a computer-readable medium, which may be read and executed by one or more processors. A machine -readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may 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 (e.g., carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that suchdescriptions 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.

[0026] Referring to Fig. 1, a photolithography system 100 includes an illumination system 105. As described more fully below, the illumination system 105 produces a pulsed light beam 110 and directs it to a photolithography exposure apparatus or scanner 115 that patterns microelectronic features on a wafer 120. The wafer 120 is placed on a wafer table 125 constructed to hold wafer 120 and connected to a positioner configured to position the wafer 120 accurately in accordance with certain parameters.

[0027] The photolithography system 100 may use a light beam 110 having a wavelength in the deep ultraviolet (DUV) range, for example, with wavelengths of 248 nanometers (nm) or 193 nm. The size of the microelectronic features patterned on the wafer 120 depends on the wavelength of the light beam 110, with a lower wavelength resulting in a smaller minimum feature size. When the wavelength of the light beam 110 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 110 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 110 is distributed over different wavelengths.

[0028] The scanner 115 includes an optical arrangement 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 110 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 120. The illumination system 105 adjusts the range of angles for the light beam 110 impinging on the mask. The illumination system 105 also homogenizes (makes uniform) the intensity distribution of the light beam 110 across the mask.

[0029] The scanner 115 can include, among other features, a lithography controller 130, temperature control devices, and power supplies for the various electrical components. The lithography controller 130 controls how layers are printed on the wafer 120. The lithography controller 130 includes a memory that stores information such as process recipes. A process program or recipe determines the length of the exposure on the wafer 120, the mask used, and other factors that affect the exposure. During lithography, a plurality of pulses of the light beam 110 illuminates the same area of the wafer 120 to together constitute an illumination dose.

[0030] The photolithography system 100 also preferably includes a control system 135. In general, the control system 135 includes one or more of digital electronic circuitry, computer hardware, firmware, and software. The control system 135 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.

[0031] The control system 135 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 135 also can include components to enable wireless communication including Bluetooth, NFC, and Wi-Fi. In particular, the control system 135 may 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 135 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 (applicationspecific integrated circuits). The control system 135 can be centralized or be partially or wholly distributed throughout the photolithography system 100.

[0032] Figs. 2A-C illustrate an exemplary illumination system 205, consistent with embodiments of the present disclosure. Illumination system 205 may correspond to, e.g., illumination system 105 of Fig. 1. Referring to Fig. 2A, illumination system 205 may comprise a pulsed laser source that produces a pulsed laser beam as a light beam 110. Fig. 2A 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 embodiments in general, and it is appreciated that the principles of the disclosed embodiments may be advantageously applied to lasers having other modules and configurations.

[0033] The illumination system 205 may include, e.g., a solid state or gas discharge seed laser subsystem 140, a power amplification ("PA") subsystem 145 (such as, e.g., a single-pass amplifier stage, a double-pass amplifier stage, a power ring amplifier ("PRA") stage, or a power oscillator (“PO”) stage), relay optics subsystem 150, and output subsystem 160. The seed laser subsystem 140 may include, e.g., a master oscillator ("MO") chamber module 165, in which electrical discharges between electrodes (not shown) may 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 may produce relatively broad band radiation that may be line narrowed to a relatively very narrow bandwidth around a center wavelength selected in a line narrowing module ("LNM") 170, as is known in the art.

[0034] The seed laser subsystem 140 may also include a master oscillator output coupler ("MO OC") 175, which may comprise a partially reflective mirror. Along with a reflective grating (not shown) in the LNM 170, the MO output coupler 175 may define an oscillator cavity in which discharge light oscillates to form the seed laser output pulse. The system may also include a line-center analysis module ("LAM") 180. The LAM 180 may include, for example, an etalon spectrometer for finewavelength measurement and a coarser resolution grating spectrometer. A MO wavefront engineering box ("WEB") 185 may serve to redirect the output of the MO seed laser subsystem 140 toward the PA subsystem 145, and may include, e.g., beam expansion with, e.g., a multi prism beam expander (not shown) and coherence busting, e.g., in the form of an optical delay path (not shown).

[0035] The PA subsystem 145 may include, e.g., a PRA lasing chamber module 200, which may also be an oscillator, e.g., formed by seed beam injection and output coupling optics (not shown) that may be incorporated into a PRA WEB 210 and may be redirected back through the gain medium in the PRA lasing chamber module 200 by a beam reverser 220. The PRA WEB 210 may incorporate a partially reflective input / output coupler (not shown) and a maximally reflective mirror for the nominal operating wavelength (e.g., at around 193 nm for an ArF system) and one or more prisms.

[0036] A bandwidth analysis module ("BAM") 230 at the output of the PA subsystem 145 may receive the output laser light beam of pulses from the amplification subsystem and pick off a portion of the light beam for metrology purposes, e.g., to measure the output bandwidth and pulse energy. The laser output light beam of pulses then passes through an optical pulse stretcher ("OPuS") 240 and an output combined autoshutter metrology module ("CASMM") 250, which may also be the location of a pulse energy meter. One purpose of the OPuS 240 may be, e.g., to convert a single output laser pulse into a pulse train. Secondary pulses created from the original single output pulse may be delayed with respect to each other. By distributing the original laser pulse energy into a train of secondary pulses, the effective pulse length of the laser can be expanded and at the same time the peak pulse intensity reduced. The OPuS 240 can thus receive the laser beam from the PRA WEB 210 via the BAM 230 and direct the output of the OPuS 240 to the CASMM 250.

[0037] The overall availability of the light source (e.g., illumination system 205) 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 lifetime of module A is six months and the nominal lifetime of module B is eighteen months. In such a scenario, module B would be replaced with every third replacement of module A.

[0038] 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 may 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 which is still capable of providing additional satisfactory operation. Systemmaintenance events require that a light source be taken out of production. Thus, umbrella maintenance may cause an unnecessary interruption in productivity when the fab operations otherwise could have continued.

[0039] Therefore, it may 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 may comprise monitoring the condition of in-service equipment to predict when equipment will fail. The future behavior / condition of machine components may be approximated, which makes it possible to optimize maintenance tasks (e.g., prognostic health monitoring). Accordingly, machine downtime and maintenance costs can be reduced significantly while undertaking maintenance as infrequently as possible.

[0040] However, some monitoring techniques may be insufficient for accurate, in-situ, direct damage analysis of individual optical elements or modules. Instead, the health of individual modules may be, e.g., monitored indirectly by evaluating various laser performance characteristics and inferring the health of one or more modules accordingly. For example, the mirrors and beamsplitters of a module such as OPuS 240 may be evaluated according to, e.g., a laser output pulse duration or the output polarization of CASMM 250. Post data processing based on pulse waveforms acquired during health check can allow for measurements of average reflectivity in the various mirrors and beamsplitters. 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. Further, in-laser testing for optics and module health diagnosis may requires running test scripts, such as dedicated firing patterns and controlled testing conditions, to diagnose. The trouble shooting could take extra time during the time constrained service events

[0041] Alternatively, some optics may be directly inspected, such as by, e.g., visual inspection through a viewport to determine any damage on the optical surfaces. But this qualitative approach is not reliable for determining precise damage levels or predicting the remaining lifetime of the optics. Further, some direct inspections may require service engineers to open the modules during a service event to inspect the optics damage. In addition to increasing downtime in the laser system, such inspections risk exposure to air, humidity, and contamination, which itself may lead to optics damage.

[0042] Embodiments of the present disclosure provide systems and methods for performing direct damage inspection and lifetime prediction of individual optics and modules by directly measuring a history of temperature variations of each optical element. The history of temperature variations may be used to provide a real-time assessment of optical damage and lifetime prediction.

[0043] For example, at the beginning of an irradiation period, an optical element may be heated by laser irradiation until it reaches an equilibrium temperature, and it may begin to cool again at the end of the irradiation period. The result is a temperature variation profile having a series of peaks and valleys between periods of irradiation and non-irradiation. Certain parameters of this temperature variation profile may be closely correlated with a health indicator of the optical element. For example,the health indicator may relate to the optical absorption coefficient of the optical element, and may comprise an indication of the need to schedule a service event for the optical element, such as maintenance or replacement. For example, the rate of change in peak temperature variations over multiple cycles of irradiation and non-irradiation periods, or the rate of temperature change within a single irradiation period, may indicate a change in the optical absorption coefficient. These changes may serve as early indicators of the need for a service event, such as maintenance or replacement of an optical element or module. By monitoring and recording the temperature history of each optical element, changes in the health indicator may 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.

[0044] Fig. 2B schematically illustrates seed laser subsystem 140 and relay optics subsystem 150 of illumination system 205, consistent with embodiments of the present disclosure. Seed laser subsystem 140 and relay optics subsystem 150 may comprise a plurality of first temperature sensors 236 and a plurality of second temperature sensors 237 for providing continuous temperature measurements of the optical elements and their environments, respectively. For example, each first temperature sensor 236 may be configured to transmit a first signal corresponding to a temperature of an optical element, and each second temperature sensor 237 may be configured to transmit a second signal corresponding to a temperature of an environment of an optical element.

[0045] Each of the first temperature sensors 236 may be coupled to an individual mirror, lens, beamsplitter, or other optical element within a module, and may therefore be alternatively referred to as optics temperature sensors 236. For example, any of line narrowing module 170, master oscillator chamber module 165, master oscillator output coupler 175, line-center analysis module 180, or MO wavefront engineering box 185 may have at least one optical element that is monitored by a temperature sensor 236. The first temperature sensors 236 may comprise, e.g., a thermocouple placed in contact with the optical element, such as at a surface or within a bulk material of the optical element. Alternatively, the thermocouple may be in contact with a body that is thermally coupled to the optical element, such as a mechanical mount of the optical element. The thermocouple may be, e.g., enclosed in stainless steel tubing or another material that meets cleanroom requirements of the laser system, and may be inserted into the purged environment of an optical module by, e.g., vacuum seal fittings. Alternatively or additionally, other types of optics temperature sensors 236 may be used such as, e.g., a thermistors, or infrared cameras (IR thermal cameras), or infrared photodetectors (IR thermometers). For example, an infrared camera may be configured to monitor the temperature of one or more optical elements from inside a module or, e.g., through an IR-transmissive viewport.

[0046] While each module of the seed laser subsystem 140 and relay optics subsystem 150 is schematically illustrated as having a single optics temperature sensor 236, it should be understood that this is merely illustrative. In practice, the modules may comprise, e.g., at least one optics temperaturesensor 236 for each optical element being monitored. In some embodiments, a single optical element may be connected to multiple optics temperature sensors 236 to provide a plurality of first signals for, e.g., performing a weighted average temperature measurement or for redundancy. In some embodiments, all optical elements within a given module may be connected to at least one first temperature sensor 236 for temperature monitoring. In some embodiments, not all optical elements within a given module may be 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.

[0047] The plurality of second temperature sensors 237 may be configured to measure a temperature of an environment within a module of seed laser subsystem 140 or relay optics subsystem 150, or within a main chamber of the subsystems themselves. As further discussed below, the environmental temperature may be used as a reference for determining the amount of temperature change in an optical element that is attributable to, e.g., optical absorption rather than contact with the local environment. Therefore, second temperature sensors 237 may alternatively be referred to as reference temperature sensors 237. In some embodiments, the plurality of second temperature sensors 237 may have a design that is similar to the plurality of first temperature sensors 236. For example, in some embodiments the reference temperature sensors 237 may comprise one or more thermocouples inserted into a module environment.

[0048] Fig. 2C further schematically illustrates PA subsystem 145 and output subsystem 160 of illumination system 205, consistent with embodiments of the present disclosure. Like seed laser subsystem 140 and relay optics subsystem 150 discussed above, PA subsystem 145 and output subsystem 160 may comprise a plurality of first temperature sensors 236 and second temperature sensors 237 for monitoring the temperatures of optical elements and their environments. For example, any of beam reverser 220, PRA lasing chamber module 200, PRA WEB 210, bandwidth analysis module 230, optical pulse stretcher 240, or output combined autoshutter metrology module 250, may comprise at least one optics temperature sensor 236 or one reference temperature sensor 237.Alternatively or additionally, reference temperature sensors 237 may be located within main chambers of the PA subsystem 145 or output subsystem 160. The types and arrangements or temperature sensors within PA subsystem 145 and output subsystem 160 may be similar to the types and arrangements described with respect to seed laser subsystem 140 and relay optics subsystem 150 in Fig. 2B above. While FIGs. 2B and 2C depict the use of temperature sensors for optics mounted in illumination system 205, such sensors may also be used in other environments. For example, temperature sensors may be used for damage monitoring and / or prediction for optical elements within scanner 115, or within a diagnostic tool for semiconductor wafer inspection, or within other devices.

[0049] Each of the first and second temperature sensors 236 and 237 of Figs. 2A and 2B may be configured to deliver first and second signals, respectively, to a controller 235. Controller 235 maycomprise, be part of, or be similar to, e.g., control system 135 of Fig. 1. Controller 235 may be configured to, e.g., receive and process first or second signals while the laser in operating; determine temperature values of the various optical elements or their environments; record a history of temperature variations of the optical elements or their environments; calculate, based on the temperatures of the optical elements or their environments, and the history of variations of the temperatures of the optical elements or their environments, one or more health indicators of the optical elements; and schedule service events based on the health indicators. The service events may comprise, e.g., replacing an individual optical element or module, or performing a maintenance operation on an optical element or module. In some embodiments, scheduling the service event may comprise, e.g., initiating the service event, or advancing or retarding a previously determined schedule of the service event. In some embodiments, scheduling the service event may comprise setting an alert for an operator to schedule the service event.

[0050] Fig. 3 schematically illustrates an example test setup of an optics module, consistent with embodiments of the present disclosure. The test setup may be used, e.g., in a calibration operation. For example, the calibration operation may be used to determine the relationships between, e.g., laser irradiation characteristics of an optical element, such as duration, wavelength, intensity, operation mode timing, pulse energy, pulse repetition rate, nominal power, average power, duty cycle, etc.; temperature variation profile of the optical element; health indicators of the optical element such as damage assessments based on an optical absorption coefficient; or damage to the optical element, such as, e.g., film coating degradation, or compaction or rarefaction of bulk materials. Furthermore, a discussion of the setup of Fig. 3, as well as the graphs of Figs. 4A-6, may serve to illustrate the principles upon which monitoring systems and methods of embodiments of the present disclosure may be based. The test setup may comprise an actual module of a laser apparatus, or may be configured to simulate a module of a laser apparatus. For example, the test setup may comprise: first temperature sensor 336; second temperature sensor 337; vacuum seal fittings 338; test optical element 390 on an optical element mount 395; module chamber 391; entrance optics 392; exit optics 393; and power meter 394.

[0051] The test optical element 390 may comprise an optical element of a laser apparatus, such as an element configured for use in one the modules discussed above. The condition of the test element may be known. For example, in some embodiments, the test setup may be used to test optical elements having a range of different optical damage levels, for example, ranging from new optical elements to those needing replacement. In the illustration of Fig. 3, test optical element 390 may comprise a mirror or other reflective element, such as an OPuS mirror for use in optical pulse stretcher module 240. However, embodiments of the present disclosure are not limited to this. For example, in some embodiments, test optical element 390 may comprise a lens or other transmissive optical element. In such case, the arrangement of components, such as exit optics 393 and power meter 394, may be configured accordingly.

[0052] First temperature sensor 336 and second temperature sensor 337 may be constructed and arranged in a same or similar manner as first and second temperature sensors 236 and 237 of Figs. 2B and 2C. For example, in some embodiments, first and second temperature sensors 336 and 337 may comprise, e.g., thermocouples encased in a cleanroom-compatible material such as stainless steel tubing. The thermocouples may be inserted into module chamber 391 by, e.g., vacuum seal fittings 338. First temperature sensor 336 may be in contact with a surface of test optical element 390 or with mount 395, while second temperature sensor 337 may be in contact with an environment of module chamber 391.

[0053] A test laser may irradiate test optical element 390 with a test beam 310. The test laser may comprise, e.g., a light source that is similar to the laser in which the test optical element is designed to be used. For example, for the case of illumination system 205 of Figs. 2A-C, test beam 310 may comprise a beam of DUV radiation such as light beam 110 of Fig. 1. In some embodiments, test beam 310 may be designed to simulate the character of light that would pass through the particular module in which the test optical element 390 is designed to be located. For example, when calibrating an optical element designed for use in MO wavefront engineering box 185 of Figs. 2A-C, test beam 310 may be designed to simulate an output of seed laser subsystem 140.

[0054] Entrance optics 392 may be designed to direct or focus the test beam 310 onto test optical element 390. For example, entrance optics 392 may be designed to simulate an illumination distribution that the test optical element 390 would receive when installed in an actual module of a laser apparatus. Exit optics 393 may be designed to receive the illumination passed from test optical element 390 and direct it onto power meter 394. Power meter 394 may be configured to monitor output power or intensity of light reflected or transmitted from test optical element 390.

[0055] As test optical element 390 is irradiated overtime, damage may be caused to, e.g., coatings or bulk material. This damage may cause more of the incoming radiation to be absorbed, and less of it to be reflected or transmitted. Thus, a decrease of incident radiation at power meter 394 may correspond to an increase in temperature as well as damage of test optical element 390. Additionally, as further discussed below, an environmental temperature measured at second temperature sensor 337 may be subtracted from an optical element temperature measured at first temperature sensor 336 to eliminate the influence of temperature variations in the environment of module chamber 391. By irradiating test optical element 390 under controlled conditions and simultaneously measuring the various temperatures as well as the reflected or transmitted power, it may be possible to determine the change in optical absorption coefficient, or another health indicator, as a function of temperature, exposure duration, or laser parameters. In some embodiments, this may be coupled with periodic inspection of test optical element 390 to confirm damage levels. In some embodiments, a plurality of test optical elements 390 of varying damage levels may be examined.

[0056] The calibration system of Fig. 3 may be used to determine a progression of temperature profile variations over a plurality of irradiation and non-irradiation periods, and to correlate thatprogression with health indicators of the optical element. Thus it may be possible to determine damage levels and make lifetime predictions on the optical elements under similar conditions, such as within modules of an illumination system 105 of Fig. 1 or 205 of Figs. 2A-C.

[0057] Figs. 4A-B schematically illustrate example graphs of optical element temperature changes as a function of operation time, consistent with embodiments of the present disclosure. For example, the optical element may comprise a lens, mirror, or other element of a module of a laser system. While the example graphs show a history of temperature changes in terms of minutes of operation time, embodiments of the present disclosure are not limited to this. For example, in some embodiments operation time may be monitored on the order of, e.g., the past 1 second, 1 minute, 1 hour, 1 day, 1 month, 1 year, etc. of the optical element. The illustrated temperature profdes may correspond to, e.g., an OPuS mirror for use in optical pulse stretcher module 240 of the illumination systems 105 or 205. As indicated by the peaks and valleys in the temperature profdes, the graphs correspond to a plurality of alternating irradiation and non-irradiation periods of the illumination system. Fig. 4A shows three temperature profdes corresponding to an optical element temperature TOE, a reference temperature TR, and a temperature difference AT = TOE - TR. Reference temperature TR may correspond to an environmental temperature of the module as measured by one or more reference temperature sensors (such as second temperature sensors 237 of Figs. 2B-C). Optical element temperature TOE may correspond to a temperature of the optical element as measured by one or more optics temperature sensors (such as first temperature sensors 236 of Figs. 2B-C). As seen in Fig. 4A, the temperature inside a module may drift gradually upward throughout the illustrated time interval, even when the laser is not operating. This may affect a baseline temperature of the optical element independently of the temperature changes that are induced by optical absorption. For this reason, in some embodiments it may be desirable to eliminate this environmental influence using temperature difference AT to determine a temperature of the optical element.

[0058] Fig. 4B illustrates this value AT as a temperature variation profile of an optical element as it is successively heated and cooled, for both a healthy optical element (profile ATH) and a damaged optical element (profile ATD). For example, the temperature variation profile ATH may represent an optical element near the beginning of its lifetime, and the temperature variation profile ATD may represent the same optical element near the end of its lifetime. In the illustrated embodiment, by way of example only, the healthy optical element may have a relatively higher reflectivity of, e.g., approximately 98%, resulting in a relatively low amount of light absorption. Meanwhile, the damaged optical element may have a relatively lower reflectivity of, e.g., approximately 88%, resulting in a relatively higher amount of light absorption. A number of intermediate temperature profiles could be taken throughout the lifetime of the optical element as the history of temperature variations is recorded use of the laser. These intermediate temperature profiles may appear, e.g., as contours that fit between the two illustrated extremes, although they may not necessarily be distributed at regularintervals. For example, it has been found that certain features of the temperature variation profile become increasingly pronounced as damage occurs to the optical element.

[0059] First, a total magnitude of the temperature variation (V) between two successive nonirradiation and irradiation periods may become more pronounced as an optical element becomes damaged. As seen in Fig. 4B, the variation VH of an optical element when it is healthy is measurably smaller than a variation VD when the optical element is damaged. Thus, a basic implementation of an analysis may predict the remaining lifetime of an optical element based on a comparison of measured temperature variation V to a predetermined threshold temperature VD. However, every optical element may differ in its specific material properties such as, e.g., material absorption, defect or impurity levels, thermal conductivity of coatings and bulk material, etc. Therefore each optical element may exhibit different optical absorption coefficients and temperature changes even under identical irradiation conditions. Due to such differences, it may not be possible to precisely monitor for damage based on a static predetermined threshold. Instead, it may be more suitable to use a history of the temperature variations V to monitor the progression of optical damage and make lifetime predictions based on the trajectory of temperature variations over a plurality of irradiation cycles.

[0060] Second, the rate of change of temperature variation within individual irradiation periods may become more pronounced as damage occurs. For example, at the beginning of an irradiation period of the laser, an initial temporal slope (rate of change with respect to time) SD of the temperature variation profile ATD of a damaged optical element may be steeper than an initial temporal slope SH of the temperature variation profile ATH of a healthy optical element. Therefore the history of temperature variations in the optical element may be monitored for changes in the slopes. For example, lifetime prediction may be made based on, e.g., a comparison between an initial rate of change SH and a current rate of change SD, or based on a trajectory of rates of change over a plurality of irradiation cycles. Alternatively or additionally, a downward slope during cooling in a non-irradiation period may be used in an analogous manner.

[0061] Figs. 5A-B schematically illustrate example graphs showing relationships between laser operation time, laser duty cycle, optical element temperature, and optical absorption coefficient, consistent with embodiments of the present disclosure. In particular, Fig. 5A illustrates example temperature profiles for a plurality of test optical elements, and Fig. 5B then illustrates an example linear fit of the data from one test optical element of Fig. 5A to illustrate a relationship between optical element temperature and optical absorption coefficient.

[0062] Fig. 5A presents graphs showing the evolution of temperature differences over time for fourteen optical elements in a test setup. The fourteen optical elements are in five separate subcompartments of an OPuS module (such as OPuS 240 of Figs. 2A-C): a first sub-compartment having two test optical elements and four other sub-compartments each with three test optical elements. Each optical element in this example is configured with a temperature sensor to measure the temperature of the optic (such as first temperature sensors 236 or 336 of Figs. 2B-3). The local environments in thisexample are each configured with a temperature sensor to measure the reference temperature (such as second temperature sensors 237 or 337 of Figs. 2B-3) for the optics in those environments. Each optical element is irradiated at progressively higher duty cycles over a period of hours. In some embodiments, the evolution of temperature differences over time could be monitored with respect to another laser parameter such as, e.g., pulse energy, repetition rate, nominal power, or average power. As illustrated by the stepped profiles, each increase in duty cycle corresponds to a rapid increase in the difference AT between the optic temperatures and the corresponding reference temperatures before reaching an equilibrium value. These equilibrium temperature differences may be plotted as a function of duty cycle (or another laser parameter), as shown for one example optical element in Fig.5B. The plot of AT to duty cycle may be fitted to a linear function according to eqn. 1 :AT = KDC + b (eqn. 1)where the slope K of this linear function may relate to a number of parameters including an optical absorption coefficient a, an optical element thermal conductivity and thermal contact, as well as environmental parameters such as purge gas pressure, flow rate, and temperature etc. With the assumption that all these conditions aside from optical absorption coefficient a remain substantially constant, the slope K may be taken as proportional to a. Thus the optical absorption coefficient a can be determined from the slope of the linear fit by:(eqn. 2)where ao and Ko represent the optical absorption coefficient and line slope at the beginning of an optical element’s lifetime, and atand Ktrepresent the optical absorption and slope at evaluated at a subsequent time t. For an optical absorption at the end of lifeit has been found that OE / OO » 1. Thus an optical element temperature may be correlated to an optical absorption coefficient, and a temperature history and temperature trajectory may be used for accurate damage assessment and lifetime prediction of optical elements. This may be done without requiring any downtime for inspection or dedicated testing sequences. Further, it may offer direct measurements of each individual element without any need for complex post-processing to isolate the effects on laser performance of one element from another.

[0063] Fig. 6 illustrates a flowchart of an example method 600 of monitoring a health indicator of an optical element in a laser system, consistent with embodiments of the present disclosure. For example, the laser system may comprise a DUV excimer laser illumination system, such as illumination system 105 or 205 of Figs. 1-2C. The optical element may comprise, e.g., an optical element within a module of illumination system 105 or 205. Alternatively, or in addition, the optical element may comprise anoptical element in a device outside of a laser system or other illumination system, such as an optical element within scanner 115, or within a diagnostic tool for semiconductor wafer inspection, or within another device. Method 600 may be carried out by one or more processors and memory of a controller configured to control the charged particle beam apparatus to perform the method. For example, the controller may comprise control system 135 of Fig. 1 or controller 235 of Figs. 2B-C.

[0064] At step 601, the controller may control the laser system to run through a series of irradiation and non-irradiation periods. By way of example only, the irradiation and non-irradiation periods may be on the order of, e.g., approximately 1 minute, 3 minute, 10 minute, 30 minute, or 1 hour intervals. During irradiation periods, radiation emitted within the laser system (such as a DUV output laser, or a precursor output from an upstream module such as, e.g., a seed laser, etc.) may irradiate the monitored optical element, causing it to heat up until it reaches a maximum or equilibrium temperature. During non-irradiation periods, the optical element may cool down again to a base temperature substantially lower than the most recent equilibrium temperature.

[0065] At step 602, the controller may determine a temperature of the optical element during operation of the laser. For example, the laser operation may comprise an irradiation period in which the laser is generating radiation, or it may comprise a non-irradiation period in which the laser is not generating radiation. Determining the temperature may comprise receiving a signal from a temperature sensor, such as a thermocouple. The temperature sensor may comprise, e.g., an optics temperature sensor (such as first temperature sensors 236 of Figs. 2B-C) or a reference temperature sensor (such as second temperature sensors 237 of Figs. 2B-C). The determined temperature may be based directly on a signal from the optics temperature sensor, or it may be based upon the optics temperature sensor and the reference temperature sensor. The determined temperature may be recorded with a history of temperature variations of the optical element. For example, the history may comprise a temperature variation profile spanning, e.g., a series of alternating irradiation and non-irradiation periods.

[0066] At step 603, the controller may compare the determined temperature of the optical element to the history of variations of the optical element. For example, the comparison may comprise determining a rate of change of temperature variations. For example, the rate of change may comprise a trajectory of temperature variations of the optical element over a plurality of irradiation or non-irradiation periods. Alternatively or additionally, the rate of change may comprise a slope of temperature variation over time within a single irradiation or non-irradiation period, or it may comprise comparing multiple temporal slopes over a plurality of irradiation or non-irradiation periods.

[0067] At step 604, based on the comparison, the controller may determine a health indicator of the optical element. For example, the health indicator may comprise an assessment or prediction of optical damage based on, e.g., the change in optical absorption coefficient of the optical element. The optical absorption coefficient may be related to the optical element temperature through, e.g., prior modelling, experiment or calibration as discussed with respect to Figs. 5A-B above.

[0068] At step 605, based on the determined health indicator, the controller may schedule a service event. The service event may comprise, e.g., replacing the optical element or the entire module in which the optical element is located. Alternatively or additionally, the service event may comprise performing a maintenance operation on the optical element or module. In some embodiments, scheduling the service event may comprise, e.g., initiating the service event, or advancing or retarding a previously determined schedule of the service event. In some embodiments, scheduling the service event may comprise setting an alert for an operator to schedule the service event.

[0069] A non-transitory computer-readable medium may be provided that stores instructions for one or more processors of a controller (e.g., control system 135 in Fig. 1 or controller 235 in Figs. 2B-C) for monitoring optical element health using embodiments of the present disclosure. For example, the instructions stored in the non-transitory computer-readable medium may be executed by the circuitry of the controller for performing method 600 in part or in entirety. Common forms of non-transitory media include, for example, a floppy disk, a flexible disk, hard disk, solid-state drive, magnetic tape, or any other magnetic data storage medium, a Compact Disc Read-Only Memory (CD-ROM), any other optical data storage medium, any physical medium with patterns of holes, a Random Access Memory (RAM), a Programmable Read-Only Memory (PROM), and Erasable Programmable Read-Only Memory (EPROM), a FLASH-EPROM or any other flash memory, Non-Volatile Random Access Memory (NVRAM), a cache, a register, any other memory chip or cartridge, and networked versions of the same. The one or more processors can include any combination of any number of a central processing unit (“CPU”), a graphics processing unit (“GPU”), a neural processing unit (“NPU”), a microcontroller unit (“MCU”), an optical processor, a programmable logic controller, a microcontroller, a microprocessor, a digital signal processor, an intellectual property (IP) core, a Programmable Logic Array (PLA), a Programmable Array Logic (PAL), a Generic Array Logic (GAL), a Complex Programmable Logic Device (CPLD), a Field-Programmable Gate Array (FPGA), a System On Chip (SoC), an Application-Specific Integrated Circuit (ASIC), or the like. In some embodiments, the one or more processors can also be a set of processors grouped as a single logical component.

[0070] The embodiments may further be described using the following clauses:1. A method, comprising:operating a laser;while operating the laser, receiving a first signal from a first temperature sensor configured to measure a temperature of one of an optical element of the laser or a mount coupled to the optical element; determining a temperature of the optical element or the mount based on the first signal; recording a history of a metric based on the temperature of one of the optical element or the mount; anddetermining, based on the temperature of the optical element and the history of the metric, a health indicator of the optical element.2. The method of clause 1, wherein the metric is indicative of the temperature of one of the optical element or the mount.3. The method of clause 1, wherein the metric is indicative of an instantaneous difference between the temperature of one of the optical element or the mount and a temperature of an environment of the optical element.4. The method of clause 1, wherein determining the health indicator is further based on one of a duty cycle, pulse energy, repetition rate, nominal power, or average power of the laser.5. The method of clause 1, wherein determining the health indicator comprises calculating an optical absorption coefficient of the optical element.6. The method of clause 5, wherein calculating the optical absorption coefficient of the optical element is based on the determined temperature of the optical element and one of a duty cycle, pulse energy, repetition rate, nominal power, or average power of the laser.7. The method of clause 1, wherein determining the health indicator is based on a rate of change of the metric.8. The method of clause 7, wherein the rate of change of the metric comprises a rate of change, over a plurality of irradiation periods and non-irradiation periods of the laser, of the metric between successive irradiation periods and non-irradiation periods.9. The method of clause 7, wherein the rate of change of the metric comprises a temporal slope of the metric during an irradiation period of the laser.10. The method of clause 7, wherein the rate of change of the temperature of the optical element comprises a temporal slope of the metric during a non-irradiation period of the laser.11. The method of clause 1, wherein determining the health indicator is based on comparing the temperature of one of the optical element or the mount to a prior temperature of the one of the optical element or the mount during a prior irradiation period or non-irradiation period.12. The method of clause 1, wherein the first temperature sensor comprises a thermocouple mechanically coupled to one of the optical element or the mount.13. The method of clause 1, further comprising:while operating the laser, receiving a second signal from a second temperature sensor configured to measure a temperature of an environment of the optical element;wherein determining the temperature of the one of the optical element or the mount is further based on the second signal.14. The method of clause 13, wherein the second temperature sensor comprises a thermocouple located in the environment of the optical element.15. The method of clause 1, further comprising:scheduling a service event of the optical element based on the health indicator.16. The method of clause 15, wherein the service event comprises replacing the optical element.17. The method of clause 15, wherein the service event comprises replacing a module comprising the optical element.18. The method of clause 15, wherein scheduling the service event comprises advancing or retarding a previously determined schedule of the service event.19. The method of clause 15, wherein scheduling the service event comprises setting an alert for an operator to schedule the service event.20. A laser apparatus, comprising:a laser comprising an optical element coupled to a mount;a first temperature sensor configured to measure a temperature of one of the optical element or the mount; anda controller comprising one or more processors and configured to cause the laser apparatus to perform operations comprising:while operating the laser, receiving a first signal from the first temperature sensor;determining a temperature of one of the optical element or the mount based on the first signal; recording a history of a metric based on the temperature of one of the optical element or the mount; anddetermining, based on the temperature of the optical element and the history of the metric, a health indicator of the optical element.21. The laser apparatus of clause 20, wherein the metric is indicative of the temperature of one of the optical element or the mount.22. The laser apparatus of clause 20, wherein the metric is indicative of an instantaneous difference between the temperature of one of the optical element or the mount and a temperature of an environment of the optical element.23. The laser apparatus of clause 20, wherein determining the health indicator is further based on one of a duty cycle, pulse energy, repetition rate, nominal power, or average power of the laser. 24. The laser apparatus of clause 20, wherein determining the health indicator comprises calculating an optical absorption coefficient of the optical element.25. The laser apparatus of clause 24, wherein calculating the optical absorption coefficient of the optical element is based on the determined temperature of the optical element and one of a duty cycle, pulse energy, repetition rate, nominal power, or average power of the laser.26. The laser apparatus of clause 20, wherein determining the health indicator is based on a rate of change of the metric.27. The laser apparatus of clause 26, wherein the rate of change of the metric comprises a rate of change, over a plurality of irradiation periods and non-irradiation periods of the laser, of the metric between successive irradiation periods and non-irradiation periods.28. The laser apparatus of clause 26, wherein the rate of change of the metric comprises a temporal slope of the metric during an irradiation period of the laser.29. The laser apparatus of clause 26, wherein the rate of change of the temperature of the optical element comprises a temporal slope of the metric during a non-irradiation period of the laser.30. The laser apparatus of clause 29, wherein determining the health indicator is based on comparing the temperature of one of the optical element or the mount to a prior temperature of the one of the optical element or the mount during a prior irradiation period or non-irradiation period.31. The laser apparatus of clause 20, wherein the first temperature sensor comprises a thermocouple mechanically coupled to one of the optical element or the mount.32. The laser apparatus of clause 20, the operations further comprising:while operating the laser, receiving a second signal from a second temperature sensor configured to measure a temperature of an environment of the optical element;wherein determining the temperature of the one of the optical element or the mount is further based on the second signal.33. The laser apparatus of clause 32, wherein the second temperature sensor comprises a thermocouple located in the environment of the optical element.34. The laser apparatus of clause 20, the operations further comprising:scheduling a service event of the optical element based on the health indicator.35. The laser apparatus of clause 34, wherein the service event comprises replacing the optical element.36. The laser apparatus of clause 34, wherein the service event comprises replacing a module comprising the optical element.37. The laser apparatus of clause 34, wherein scheduling the service event comprises advancing or retarding a previously determined schedule of the service event.38. The laser apparatus of clause 34, wherein scheduling the service event comprises setting an alert for an operator to schedule the service event.39. A non-transitory computer-readable medium that stores a set of instructions that is executable by at least one processor of an apparatus to cause the apparatus to perform operations comprising: while operating a laser, receiving a first signal from a first temperature sensor configured to measure a temperature of one of an optical element of the laser or a mount coupled to the optical element; determining a temperature of the optical element or the mount based on the first signal; recording a history of a metric based on the temperature of one of the optical element or the mount; anddetermining, based on the temperature of the optical element and the history of variations of the temperature of the optical element, a health indicator of the optical element.40. The non-transitory computer-readable medium of clause 39, wherein the metric is indicative of the temperature of one of the optical element or the mount.41. The non-transitory computer-readable medium of clause 39, wherein the metric is indicative of an instantaneous difference between the temperature of one of the optical element or the mount and a temperature of an environment of the optical element.42. The non-transitory computer-readable medium of clause 39, wherein determining the health indicator is further based on one of a duty cycle, pulse energy, repetition rate, nominal power, or average power of the laser.43. The non-transitory computer-readable medium of clause 39, wherein determining the health indicator comprises calculating an optical absorption coefficient of the optical element.44. The non-transitory computer-readable medium of clause 43, wherein calculating the optical absorption coefficient of the optical element is based on the determined temperature of the optical element and one of a duty cycle, pulse energy, repetition rate, nominal power, or average power of the laser.45. The non-transitory computer-readable medium of clause 39, wherein determining the health indicator is based on a rate of change of the metric.46. The non-transitory computer-readable medium of clause 45, wherein the rate of change of the metric comprises a rate of change, over a plurality of irradiation periods and non-irradiation periods of the laser, of the metric between successive irradiation periods and non-irradiation periods.47. The non-transitory computer-readable medium of clause 45, wherein the rate of change of the metric comprises a temporal slope of the metric during an irradiation period of the laser.48. The non-transitory computer-readable medium of clause 45, wherein the rate of change of the temperature of the optical element comprises a temporal slope of the metric during a non-irradiation period of the laser.49. The non-transitory computer-readable medium of clause 39, wherein determining the health indicator is based on comparing the temperature of one of the optical element or the mount to a prior temperature of the one of the optical element or the mount during a prior irradiation period or non-irradiation period.50. The non-transitory computer-readable medium of clause 39, wherein the first temperature sensor comprises a thermocouple mechanically coupled to one of the optical element or the mount.51. The non-transitory computer-readable medium of clause 39, the operations further comprising:while operating the laser, receiving a second signal from a second temperature sensor configured to measure a temperature of an environment of the optical element;wherein determining the temperature of the one of the optical element or the mount is further based on the second signal.52. The non-transitory computer-readable medium of clause 51, wherein the second temperature sensor comprises a thermocouple located in the environment of the optical element.53. The non-transitory computer-readable medium of clause 39, the operations further comprising:scheduling a service event of the optical element based on the health indicator.54. The non-transitory computer-readable medium of clause 53, wherein the service event comprises replacing the optical element.55. The non-transitory computer-readable medium of clause 53, wherein the service event comprises replacing a module comprising the optical element.56. The non-transitory computer-readable medium of clause 53, wherein scheduling the service event comprises advancing or retarding a previously determined schedule of the service event.57. The non-transitory computer-readable medium of clause 53, wherein scheduling the service event comprises setting an alert for an operator to schedule the service event.58. An apparatus, comprising:an optical element coupled to a mount;a first temperature sensor configured to measure a temperature of one of the optical element or the mount; anda controller comprising one or more processors and configured to cause the apparatus to perform operations comprising:while operating a laser configured to illuminate the optical element, receiving a first signal from the first temperature sensor;determining a temperature of one of the optical element or the mount based on the first signal; recording a history of a metric based on the temperature of one of the optical element or the mount; anddetermining, based on the temperature of the optical element and the history of the metric, a health indicator of the optical element.59. The apparatus of clause 58, wherein the metric is indicative of the temperature of one of the optical element or the mount.60. The apparatus of clause 58, wherein the metric is indicative of an instantaneous difference between the temperature of one of the optical element or the mount and a temperature of an environment of the optical element.61. The apparatus of clause 58, wherein determining the health indicator comprises calculating an optical absorption coefficient of the optical element.62. The apparatus of clause 58, wherein determining the health indicator is based on a rate of change of the metric.63. The apparatus of clause 58, wherein the rate of change of the metric comprises a temporal slope of the metric during an irradiation period of the laser.64. The apparatus of clause 58, wherein determining the health indicator is based on comparing the temperature of one of the optical element or the mount to a prior temperature of the one of the optical element or the mount during a prior irradiation period or non-irradiation period.65. The apparatus of clause 58, wherein the first temperature sensor comprises a thermocouple mechanically coupled to one of the optical element or the mount.66. The apparatus of clause 58, the operations further comprising:scheduling a service event of the optical element based on the health indicator.

[0071] Some embodiments of the present disclosure have been described with respect to DUV excimer laser systems, such as for use in photolithography illumination systems. However, the present disclosure is not limited to such systems. It should be understood that the above disclosed embodiments may be applicable to other laser systems, such as other non-DUV or non-lithography illumination systems, and that other classes of lasers are contemplated within the scope of the present disclosure.

[0072] Block diagrams in the figures may illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer hardware or software products according to various exemplary embodiments of the present disclosure. In this regard, each block in a schematic diagram may represent certain arithmetical or logical operation processing that may be implemented using hardware such as an electronic circuit. Blocks may also represent a module, segment, or portion of code that comprises one or more executable instructions for implementing the specified logical functions. It should be understood that in some alternative implementations, functions indicated in a block may occur out of the order noted in the figures. For example, two blocks shown in succession may be executed or implemented substantially concurrently, or two blocks may sometimes be executed in reverse order, depending upon the functionality involved. Some blocks may also be omitted. It should also be understood that each block of the block diagrams, and combination of the blocks, may be implemented by special purpose hardware -based systems that perform the specified functions or acts, or by combinations of special purpose hardware and computer instructions.

[0073] It will be appreciated that the embodiments of the present disclosure are not limited to the exact construction that has been described above and illustrated in the accompanying drawings, and that various modifications and changes can be made without departing from the scope thereof. For example, a DUV laser system may be but one example of a laser system consistent with embodiments of the present disclosure.

Claims

CLAIMS1. A method, comprising:operating a laser;while operating the laser, receiving a first signal from a first temperature sensor configured to measure a temperature of one of an optical element of the laser or a mount coupled to the optical element; determining a temperature of the optical element or the mount based on the first signal; recording a history of a metric based on the temperature of one of the optical element or the mount; anddetermining, based on the temperature of the optical element and the history of the metric, a health indicator of the optical element.

2. The method of claim 1, wherein the metric is indicative of the temperature of one of the optical element or the mount.

3. The method of claim 1, wherein the metric is indicative of an instantaneous difference between the temperature of one of the optical element or the mount and a temperature of an environment of the optical element.

4. The method of claim 1, wherein determining the health indicator is further based on one of a duty cycle, pulse energy, repetition rate, nominal power, or average power of the laser.

5. The method of claim 1, wherein determining the health indicator is based on a rate of change of the metric, wherein the rate of change of the metric comprises a rate of change, over a plurality of irradiation periods and non-irradiation periods of the laser, of the metric between successive irradiation periods and non-irradiation periods.

6. A laser apparatus, comprising:a laser comprising an optical element coupled to a mount;a first temperature sensor configured to measure a temperature of one of the optical element or the mount; anda controller comprising one or more processors and configured to cause the laser apparatus to perform operations comprising:while operating the laser, receiving a first signal from the first temperature sensor;determining a temperature of one of the optical element or the mount based on the first signal;recording a history of a metric based on the temperature of one of the optical element or the mount; anddetermining, based on the temperature of the optical element and the history of the metric, a health indicator of the optical element.

7. The laser apparatus of claim 6, wherein the metric is indicative of the temperature of one of the optical element or the mount.

8. The laser apparatus of claim 6, wherein the metric is indicative of an instantaneous difference between the temperature of one of the optical element or the mount and a temperature of an environment of the optical element.

9. The laser apparatus of claim 6, wherein determining the health indicator is further based on one of a duty cycle, pulse energy, repetition rate, nominal power, or average power of the laser.

10. The laser apparatus of claim 6, wherein determining the health indicator comprises calculating an optical absorption coefficient of the optical element.

11. The laser apparatus of claim 10, wherein calculating the optical absorption coefficient of the optical element is based on the determined temperature of the optical element and one of a duty cycle, pulse energy, repetition rate, nominal power, or average power of the laser.

12. The laser apparatus of claim 6, wherein determining the health indicator is based on a rate of change of the metric.

13. The laser apparatus of claim 12, wherein the rate of change of the metric comprises a temporal slope of the metric during an irradiation period of the laser.

14. The laser apparatus of claim 12, wherein the rate of change of the temperature of the optical element comprises a temporal slope of the metric during a non-irradiation period of the laser.

15. The laser apparatus of claim 14, wherein determining the health indicator is based on comparing the temperature of one of the optical element or the mount to a prior temperature of the one of the optical element or the mount during a prior irradiation period or non-irradiation period.

16. The laser apparatus of claim 6, wherein the first temperature sensor comprises a thermocouple mechanically coupled to one of the optical element or the mount.

17. The laser apparatus of claim 6. the operations further comprising:while operating the laser, receiving a second signal from a second temperature sensor configured to measure a temperature of an environment of the optical element;wherein determining the temperature of the one of the optical element or the mount is further based on the second signal.

18. The laser apparatus of claim 17, wherein the second temperature sensor comprises a thermocouple located in the environment of the optical element.

19. The laser apparatus of claim 6, the operations further comprising:scheduling a service event of the optical element based on the health indicator.

20. The laser apparatus of claim 19, wherein the service event comprises replacing the optical element.

21. The laser apparatus of claim 19, wherein the service event comprises replacing a module comprising the optical element.

22. The laser apparatus of claim 19, wherein scheduling the service event comprises advancing or retarding a previously determined schedule of the service event.

23. The laser apparatus of claim 19, wherein scheduling the service event comprises setting an alert for an operator to schedule the service event.

24. A non-transitory computer-readable medium that stores a set of instructions that is executable by at least one processor of an apparatus to cause the apparatus to perform operations comprising: while operating a laser, receiving a first signal from a first temperature sensor configured to measure a temperature of one of an optical element of the laser or a mount coupled to the optical element; determining a temperature of the optical element or the mount based on the first signal; recording a history of a metric based on the temperature of one of the optical element or the mount; anddetermining, based on the temperature of the optical element and the history of variations of the temperature of the optical element, a health indicator of the optical element.

25. The non-transitory computer-readable medium of claim 24, wherein the metric is indicative of the temperature of one of the optical element or the mount.

26. An apparatus, comprising:an optical element coupled to a mount;a first temperature sensor configured to measure a temperature of one of the optical element or the mount; anda controller comprising one or more processors and configured to cause the apparatus to perform operations comprising:while operating a laser configured to illuminate the optical element, receiving a first signal from the first temperature sensor;determining a temperature of one of the optical element or the mount based on the first signal; recording a history of a metric based on the temperature of one of the optical element or the mount; anddetermining, based on the temperature of the optical element and the history of the metric, a health indicator of the optical element.

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