Repetition rate agile laser apparatus and method

Abstract

In an apparatus for generating laser radiation from discharges in a lasing gas in a discharge chamber in which the discharges produce acoustic waves which would disrupt operation of the apparatus at certain repetition rates, a system and method for shifting the acoustic power spectrum of the discharge chamber by altering the temperature of the lasing gas in the discharge chamber.

Description

REPETITION RATE AGILE LASER APPARATUS AND METHODCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to US Application No. 63 / 662,108, filed June 20, 2024, titled REPETITION RATE AGILE LASER APPARATUS AND METHOD, which is incorporated herein by reference in its entirety.FIELD

[0002] The disclosed subject matter relates to laser sources configured to generate pulses of laser radiation at a selectable commanded repetition rate in which the discharges in a discharge region that produce laser radiation also produce acoustic disturbances which may be undesirably reflected back to the discharge region.BACKGROUND

[0003] Photolithography is a process by which semiconductor circuitry is patterned on a substrate such as a silicon wafer. A photolithography radiation source provides the deep ultraviolet (DUV) radiation (wavelengths in a range of about 100 nanometers (nm) to about 400 nm) used to expose a photoresist on the wafer. Often, the radiation source is a laser source and the radiation is a pulsed laser beam. The radiation beam is passed through a beam delivery unit, then a reticle or a mask, and then projected onto the wafer coated with photoresist. In this way, a chip design is patterned onto a photoresist that is then developed and cleaned.

[0004] In many systems that produce a laser beam (such as a laser generator) or employ a laser beam (such as a photolithography system), there is an optical train that includes one or more optical components (such as mirrors, gratings, prisms, optical switches, filters, etc.). Optical components in the optical train may, wholly or partially, reflect, process, filter, modify, focus, expand, etc. the laser beam to obtain one or more desired laser beam outputs.

[0005] In such systems the laser beam is produced by causing discharges in an interelectrode discharge region of one or more laser discharge chambers. One challenge in the design and use of these systems is that the discharge which produces the laser radiation also generates strong acoustic waves inside the discharge region. These acoustic waves create gas density modulations that propagate within the laser discharge chamber. Surfaces in the laser discharge chamber may reflect these acoustic waves back into the discharge region and adversely affect the laser’s performance. In particular these reflected waves may result in a round trip time-of-flight (TOF) resonance depending on the inter-pulse delay or discharge repetition rate at which the laser system is operated.

[0006] It would be advantageous to mitigate the adverse effects of these acoustic disturbances. It is in this context that the need for the subject matter of the present disclosure arises.SUMMARY

[0007] The following presents a succinct summary of one or more embodiments in order to promote a basic understanding of the presently disclosed subject matter. This summary is not an extensive overview of all contemplated embodiments and is not intended to identify as key or critical any elements of any embodiments nor delineate the scope of any or all embodiments. Its sole purpose is to present some concepts relating to one or more embodiments in a streamlined form as a prelude to the more detailed description that is presented later.

[0008] According to an aspect of an embodiment there is disclosed a radiation source comprising a discharge chamber containing a lasing gas, discharges in the lasing gas in a discharge gap in the discharge chamber producing pulses of radiation at a commanded repetition rate, a lasing gas temperature regulating system for governing a temperature of the lasing gas in the discharge chamber, a spectral property measuring device arranged to measure a spectral property of one or more of the pulses of radiation, and a controller arranged to control the lasing gas temperature regulating system to govern the temperature of the lasing gas in the discharge chamber based at least partially on the spectral property as measured by the spectral property measuring device. The spectral property of the radiation may be a bandwidth of the pulses of radiation. The controller may be adapted to cause the lasing gas temperature regulating system to alter a temperature of the lasing gas in the discharge chamber when the controller determines based on the spectral property of the radiation that a commanded change in repetition rate would result in a new repetition rate differing from a resonance repetition rate by less than a predetermined amount.

[0009] The bandwidth measuring device may measure bandwidth as an average of a bandwidth of n pulses of respective m bursts, n and m being positive integers. The average may be a moving average. The bandwidth measuring device may measure bandwidth sigma (BWS). The bandwidth measuring device may measure BWS based on an average bandwidth of the n pulses over the m bursts, n and m being positive integers. The bandwidth measuring device may measure BWS using measured bandwidths BWnaveraged over m bursts according to the formulaswhere Pnm is the measured bandwidth for the nth pulse of the mth burst,AndBWS = J[BWavg- BWn]2 / n.

[0010] The spectral property measuring device may measure a burst transient (BT). The spectral property measuring device may measure the BT based on a polynomial fit.

[0011] The bandwidth measuring device may measure BT using a measured bandwidth BWnaveraged over m bursts according to the formula

[0012] According to another aspect of an embodiment there is disclosed a method of operating a radiation source, the radiation source being configured to generate of m bursts of pulses, each burst having n pulses at a commanded repetition rate, m and n being positive integers, the method comprising measuring a bandwidth of the pulses of radiation and controlling a temperature of the lasing gas in the discharge chamber based at least partially on the measured bandwidth. Controlling the temperature may comprise causing altering a temperature of the lasing gas in the discharge chamber when it is determined that a commanded change in repetition rate would result in a new repetition rate smaller than a resonance repetition rate by less than a predetermined amount. Measuring the bandwidth may comprise determining a moving average of a bandwidth of n first pulses of respective m bursts, n and m being positive integers.

[0013] The method may further comprise measuring a bandwidth sigma (BWS) based on an average bandwidth of the n pulses over the m bursts, n and m being positive integers. Measuring BWS may be based on measured bandwidths BWnaveraged over m bursts according to the formulaswhere Pnm is the measured bandwidth for the nth pulse of the mth burst,AndBWS = / n.

[0014] The method may further comprise measuring a burst transient (BT), wherein BT is measured based at least in part on a measured bandwidth BWnaveraged over m bursts according to the formula

[0015] According to another aspect of an embodiment there is disclosed a method of generating deep ultraviolet (DUV) radiation using a source including a master oscillator (MO) discharge chamber and a power amplifier (AMP) discharge chamber, the method comprising using a bandwidth measuring device to measure at least one of bandwidth (BW), bandwidth sigma (BWS), and burst transient (BT) of the DUV radiation, using a photodiode array or spectrometer to measure beam pointing and beam divergence of the DUV radiation, adjusting an MO temperature of a lasing gas in the MO discharge chamber to optimize at least one of BW, BWS, and BT, and adjusting an AMP temperature of a lasing gas in the AMP discharge chamber to optimize at least one of beam pointing and beam divergence.

[0016] The beam pointing and the beam divergence may be averaged over a predetermined number k of pulses, k being a positive integer, not including a first pulse of burst.

[0017] According to another aspect of an embodiment there is disclosed a method of mitigating acoustic effects in a chamber of a laser radiation source, the method comprising measuring a bandwidth value of laser radiation from the laser radiation source, comparing the bandwidth value with a predetermined threshold value, and altering a temperature setpoint of the chamber if the bandwidth value is smaller than the predetermined threshold value.

[0018] Altering the temperature setpoint of the chamber may include lowering the temperature setpoint in a range from about 5 degrees Celsius to about 15 degrees Celsius. Measuring a bandwidth value of laser radiation from the laser radiation source may comprise averaging a plurality of a bandwidth values of pulses across a sequence of bursts of the laser to obtain the bandwidth value.

[0019] Further embodiments, features, and advantages of the subject matter of the present disclosure, as well as the structure and operation of the various embodiments, are described in detail below with reference to the accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the presently disclosed subject matter and, together with the description, further serve to explain the principles of the presently disclosed subject matter and to enable a person skilled in the relevant art(s) to make and use the presently disclosed subject matter. The drawings are not to scale unless otherwise indicated.

[0021] FIG. 1 is a schematic diagram of an overall broad conception of a photolithography system.

[0022] FIG. 2 is a schematic diagram of an overall broad conception of an illumination system such as might be used in the photolithography system of FIG. 1 .

[0023] FIG. 3 is a diagram of a pulse parameter measuring system such as might be used in the illumination system of FIG. 2.

[0024] FIG. 4 is a cross section of a discharge chamber such as might be used in the illumination system of FIG. 2.

[0025] FIG. 5 is a diagram illustrating the principles of shifting the acoustic power spectrum of a discharge chamber in accordance with an aspect of an embodiment.

[0026] FIG. 6 is a functional block diagram of a feedback control circuit for the control of a setpoint temperature of a discharge chamber for thermal tuning of repetition rate resonance acoustic effects in accordance with an aspect of an embodiment.

[0027] FIG. 7 is a diagram illustrating an example of a pattern for the emission of pulses of laser radiation in a laser radiation source.

[0028] FIG. 8 is a flow chart illustrating a method of feedback control using chamber temperature setpoint tuning in accordance with an aspect of an embodiment.

[0029] FIG. 9 is another flow chart illustrating a method of feedback control using AMP chamber temperature setpoint tuning in accordance with an aspect of an embodiment.

[0030] FIG. 10 is a qualitative graph showing the effects of acoustic wave dispersion on tuning ability in accordance with an aspect of an embodiment.

[0031] Further features and advantages of the presently disclosed subject matter, as well as the structure and operation of various embodiments of the presently disclosed subject matter, are described in detail below with reference to the accompanying drawings. It is noted that the scope of the presently disclosed subject matter is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art based on the teachings presented herein.DETAILED DESCRIPTION

[0032] Various embodiments are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to promote a thorough understanding of one or more embodiments. It may be evident in some or all instances, however, that any embodiment described below can be practiced without adopting the specific design details associated with it below. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate description of one or more embodiments. This summary is not an extensive overview of all contemplated embodiments and is not intended to single out as key or critical any elements of any embodiments nor delineate the scope of any or all embodiments.

[0033] The lithographic system, metrology / inspection system and radiation source described herein can be used in a method for manufacturing a semiconductor device. The semiconductor device manufacturing method includes receiving a substrate or a wafer with a photoresist layer. The method further includes directing a radiation beam from the radiation source to transfer a pattern from a mask onto the photoresist layer. This could be achieved by a patterning device which is configured to form a patterned radiation beam, imparting the patterned radiation beam onto the photoresist layer. The method further comprises the step of partially removing the photoresist layer to form the pattern over the substrate.

[0034] The substrate may be made of silicon or other semiconductor materials. Alternatively or additionally, the substrate may include other semiconductor materials such as germanium (Ge) or carbon (C). In some embodiments, the substrate is made of a compound semiconductor such as III-V compound semiconductors, II-V compound semiconductors, and / or any suitable integration of Group IV materials. In some embodiments, the substrate may be a silicon-on-insulator (SOI) or a germanium- on-insulator (GOI) substrate.

[0035] The semiconductor device made from the substrate may have various device elements. Examples of semiconductor device elements that are formed over the substrate include transistors (e.g., planar or non-planar metal oxide semiconductor field effect transistors (MOSFET), bipolar junction transistors (BJT), high-voltage transistors, high-frequency transistors, etc.), diodes, CMOS image sensors, passive devices, and / or other applicable elements. Various processes may be performed to form the semiconductor device elements, such as deposition, etching, implantation, epitaxial growth, polishing, thermal treatment, and / or other suitable processes.

[0036] Systems such as those described herein may render benefits in a wide range of applications and implementations. For the sake of having a specific nonlimiting example to facilitate description, one such application is in semiconductor photolithography. FIG. 1 shows a photolithography system 100 that includes an illumination system 105. As described more fully below, the illumination system 105 includes a radiation source that produces a pulsed radiation beam 110 and directs it to a photolithography exposure apparatus such as an exposure tool 115 (e.g., a scanner or a stepper) that patterns microelectronic features on a wafer 120. The wafer 120 is placed on a wafer table 125 constructed to hold the wafer 120 and connected to a positioner 130 configured to accurately position the wafer 120 in accordance with certain parameters.

[0037] The pulsed radiation beam 110 may have a wavelength in the DUV range. The exposure tool 115 includes an optical arrangement 135 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 pulsed radiation 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 photoresist on the wafer 120. The illumination system 105 adjusts the range of angles for thepulsed radiation beam 110 impinging on the mask. The illumination system 105 also homogenizes (makes uniform) the intensity distribution of the pulsed radiation beam 110 across the mask.

[0038] The exposure tool 115 can include, among other features, a lithography controller 140 that controls how layers are printed on the wafer 120. The lithography controller 140 may include a memory that stores information such as process recipes that determine the parameters of the beam including a length of the exposure on the wafer 120 based on, for example, the mask used, as well as other factors that affect exposure. During lithography, a burst of pulses of the pulsed radiation beam 110 illuminates the same area of the wafer 120 to constitute an illumination dose.

[0039] The photolithography system 100 also includes a control system 145. In general, the control system 145 includes one or more of digital electronic circuitry, computer hardware, firmware, and software. The control system 145 can be centralized or be partially or wholly distributed throughout the photolithography system 100.

[0040] FIG. 2 shows a pulsed laser source that produces a pulsed laser beam as the radiation beam 110 as an example of an illumination system 105. FIG. 2 shows a two-chamber laser system as a nonlimiting example but it will be understood that the principles explained herein are equally applicable to a single chamber laser system or a laser system having more than two chambers. The gas discharge laser system may include a seed laser system 200, an amplification system 205( e.g., a power amplifier (“PA”), a power oscillator (“PO”) or a power ring amplifier (“PRA”)), relay optics 210, and laser system output subsystem 215. The seed laser system 200 may include, e.g., a solid state or gas discharge master oscillator (“MO”) chamber 220 which includes a pair of electrodes 222 and 224.

[0041] The seed laser system 200 may also include a master oscillator output coupler (“MO OC”) 230, which may comprise a partially reflective mirror, forming an MO discharge chamber 220 with an oscillator cavity, defined in part by a reflective grating (not shown) in a line narrowing module (“LNM”) 235, that oscillates to form the seed laser output pulse. The MO seed laser system 200 may also include a line-center analysis module (“LAM”) 240. A MO wavefront engineering box (“WEB”) 245 may serve to redirect the output of the MO seed laser system 200 toward the amplification system 205, and may include, e.g., a multi prism beam expander (not shown) and an optical delay path (not shown). The beam path through the LNM 235, the MO discharge chamber 220, the MO OC 230, and the LAM 240 defines an optical axis 237 for each of these components.

[0042] The amplification system 205 may include, e.g., an amplification (“AMP”) discharge chamber 250, which also may be an oscillator, e.g., formed by seed beam injection and output coupling optics (not shown) that may be incorporated into a AMP WEB 255. The beam may be redirected back through the gain medium in the AMP discharge chamber 250 by a beam reverser (“BR”) 260. The AMP WEB 255 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. The AMP discharge chamber 250 may also include a pair of electrodes 252 and 254.

[0043] A pulse parameter measuring systemin the form of a bandwidth analysis module (“BAM”) 265 may receive the output laser radiation beam of pulses from AMP discharge chamber 250 and pick off a portion of the radiation beam for metrology purposes, e.g., to measure the output bandwidth and pulse energy. The laser output radiation beam of pulses then passes through the AMP WEB 255 to an optical pulse stretcher (“OPuS”) 270 and an autoshutter, in this case a combined autoshutter metrology module (“CASMM”) 275, which may also be the location of a pulse energy meter. One purpose of the OPuS 270 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 270 may accordingly be arranged to receive the laser beam from the AMP WEB 255 and direct its output to the CASMM 275.

[0044] The beam path through the BR 260, the AMP discharge chamber 250, and the BAM 265 defines an optical axis 267 for each of these components.

[0045] The AMP discharge chamber 250 and the MO discharge chamber 220 are configured as chambers in which electrical discharges between the electrodes cause lasing gas discharges in a lasing gas to create an inverted population of high energy molecules or excimers, including, e.g., ArF, KrF, F2, XeF, and / or XeCl, to produce relatively broad band radiation that may be line narrowed to a center wavelength and relatively very narrow bandwidth selected in the LNM 235.

[0046] As shown in FIG. 3, the BAM 265 may be placed to receive and pick off a portion of the radiation beam to create a metrology beam 310. The metrology beam 310 propagates to the BAM 265 which measures the spectral parameters of interest and generates a signal indicative of the measured parameters which is supplied, for example, to the control system 145 which uses the signal to control the spectral property of the beam 110. These and other details of this arrangement are described in U.S. Patent No. 9,983,060 (the ’060 patent), issued May 29, 2018, and titled “Calibration of a Spectral Analysis Module.”

[0047] All patent applications, patents, and printed publications cited herein are incorporated herein by reference in their entireties, except for any definitions, subject matter disclaimers, or disavowals, and except to the extent that the incorporated material is inconsistent with the express disclosure herein, in which case the language in this disclosure controls.

[0048] Spectral parameters of the radiation beam include any aspect or feature of its optical spectrum. For example, bandwidth and center wavelength are spectral parameters. The bandwidth of the radiation beam is a measure of the width of its optical spectrum, and this width can be given in terms of wavelength or frequency of the laser radiation. Any suitable mathematical construction (for example, metric) related to the details of its optical spectrum can be used to estimate a value that characterizes the bandwidth of the radiation beam.

[0049] The BAM 265 includes an aperture 320 arranged to receive the metrology beam 310 and placed at a focal plane of an input lens 330 within the BAM 265. By locating the aperture 320 of theBAM 265 at the focal plane of the input lens 330, each point from the focal plane acts as a point source and accordingly, the input lens 330 acts to collimate the metrology beam 310 before the metrology beam 310 enters an optical frequency separation apparatus, which in the example arrangement shown in FIG. 3, is an etalon 340. An output lens 350 is positioned to receive the output of the etalon 340 so that the focal plane of the output lens 350 overlaps an active area of a sensor 360.

[0050] In some implementations, the etalon 340 includes a pair of partially reflective glass or optical flats 340A and 340B, which can be spaced a short distance (for example, millimeters to centimeters) apart, with their respective reflective surfaces facing each other. In other implementations, the etalon 340 includes a single plate with two parallel reflecting surfaces. The flats 340A, 340B can be made in a wedge shape to prevent their rear surfaces from producing interference fringes; the rear surfaces often also have an anti -reflective coating. The metrology beam 310 is multiply reflected as it passes through the paired flats 340A and 340B to produce transmitted rays. The output lens 350 collects the transmitted rays and directs them to the active region of a sensor 360.

[0051] The etalon 340 interacts with the metrology beam 310 and outputs a number of spatial components 370 that correspond to the spectral components of the metrology beam 310. The spectral components of the metrology beam 310 are in the optical spectrum 380 of the metrology beam 310. They therefore correspond to how the values of the optical energy or power (the spectral intensity 385) of the metrology beam 310 are distributed over the different wavelengths 387. The spatial components 370 correspond to these intensities mapped onto a two-dimensional space. Thus, the etalon 340 converts the spectral information (such as the wavelength) of the metrology beam 310 into spatial information that can be sensed or detected by the sensor 360. In other words, the conversion maps the spectral information (such as the wavelength) to different positions in space so that the spectral information can be sensed by the sensor 460.

[0052] While the entire interference pattern is shown, not all of the interference pattern is necessarily needed to permit the control system 145 to perform the calculations or estimates just described; it is alternatively possible to generate only fringes within a region that is slightly larger than an active area of the sensor 360.

[0053] The sensor 360 receives and senses the output spatial components 370. The sensor 360 can be defined by a plane that indicates generally the active area of its sensing region. The plane of the sensing region can be perpendicular to the direction of propagation of the spatial components 370.

[0054] The sensor 360 can be a detector that receives and senses the output spatial components 370. For example, one type of suitable detector that can be used to measure along one dimension is a linear photodiode array. The linear photodiode array consists of multiple elements of the same size, formed in a linear arrangement at an equal spacing in one package. The photodiode array is sensitive to the wavelength of the metrology beam 310, and if the metrology beam 310 has a wavelength in the DUV range, then the photodiode array is sensitive to light having a wavelength in the DUV range. As another example, the sensor 360 can be a two dimensional sensor such as a two-dimensional chargedcoupled device (CCD) or a two-dimensional complementary metal oxide semiconductor (CMOS) sensor. The sensor 360 should be able to read out data at a sufficiently fast rate, for example, at about 6 kHz.

[0055] The control system 145 is connected to the output of the sensor 360. The control system 145 measures a property of the spatial components 370, and analyzes these measured properties to calculate an estimate of the spectral feature of the light beam 110. The control system 145 can perform the measurement, analysis, and calculation for each pulse of the light beam 110 or for a set of pulses (burst) of the light beam 110.

[0056] The parameter or property P that is measured can be a scalar quantity (which is fully described by a magnitude or numerical value) alone or a vector quantity (which is fully described by both a magnitude and a direction). An example of a scalar property P is a metric such as the bandwidth W of the optical spectrum 380 as shown in FIG. 3. In this example, it is possible that the overall shape of the optical spectrum 380 is not known but the metric is known and so the metric is used to estimate the shape of the optical spectrum 380. An example of a vector property P is the entire waveform that describes the optical spectrum 380. In this example, one can calculate any metric from the entire spectrum and, by having the entire spectrum, one can make a more accurate calculation. The sensed spatial components can be measured for a range of one or more pulses of the pulsed metrology beam 310.

[0057] The control system 185 can measure the bandwidth W as the property P of the optical spectrum 380. The bandwidth W of the optical spectrum 380 can provide an estimate of the bandwidth (the spectral feature) of the metrology beam 310. In some implementations, the bandwidth W of the optical spectrum 380 is determined using a metric such as the FWHM (full-width half-maximum of the spectrum 380). In other implementations, the bandwidth W of the optical spectrum 480 is determined using a metric such as EY (the width of the spectrum that contains a fraction Y of the integrated spectral intensity) such as E95 which is the width of the spectrum that contains 95% of the integrated spectral intensity or beam energy. Other metrics are suitable for measuring the property of the optical spectrum 380.

[0058] Thus the laser metrology system uses measurements of etalon fringe widths generated by an integrated etalon to estimate spectral bandwidth. The etalon fringe width measurements are correlated to the laser bandwidth as part of an initial calibration procedure using, e.g., an echelle grating spectrometer that has itself been characterized for resolution (slit function) with an extremely narrowband solid state laser.

[0059] Turning now to FIG. 4 there is shown a laser discharge chamber 400 which may serve, for example, as the AMP discharge chamber 250 or the MO discharge chamber 220. The chamber 400 may be composed, e.g., of an upper chamber body 405 and a lower chamber body 410, which may, when connected to each other by suitable means, e.g., by bolting, serve to define a chamber interior 415. Theupper chamber body 405 and lower chamber body 410 also define a chamber interior vertical wall 420 with the lower chamber body 410 defining a chamber interior horizontal bottom wall 425.

[0060] In this specification including the claims the terms “up,” “down,” “upper,” “lower,” “top,” “bottom,” “vertical,” “horizontal,” and like terms are intended to connote relative orientation only and not any absolute orientation such as orientation with respect to gravity unless otherwise indicated or clear from context.

[0061] Contained within the chamber interior 415 is, e.g., a gas discharge system comprising two elongated (along the X axis out of the plane of the figure) opposing electrodes, a cathode 430 and an anode 435, defining between them an elongated gas discharge gap or region 340, wherein, in response to a sufficient voltage being imposed across the cathode 430 and anode 435, a discharge occurs in the gas discharge region 440 resulting in the production of radiation at or near a characteristic center wavelength, that is optically directed along the optical axis of the output laser radiation pulse generally aligned to the longitudinal optical axis of the laser discharge chamber 400 along the X axis (out of the plane of the figure) as indicated by the inset.

[0062] Also within the chamber interior 415 may be, e.g., an anode support bar 445. The anode 435 may be electrically connected to the upper chamber body 405 through a plurality of current returns 406, with the upper chamber body 405, along with lower chamber body 410, being kept at a common voltage, e.g., at ground voltage.

[0063] The cathode 430 may, e.g., be connected to an electrical discharge high voltage feed through an assembly 450, e.g., by a high voltage feed through 455, which passes through a main insulator 460. The main insulator 460 may keep the cathode 430 electrically isolated from the upper chamber body 405. Also within the chamber interior 415 may be, e.g., a preionizer 465, which may include, for example, a preionizer tube, adjacent the cathode 430. The preionizer 465 may be configured as an elongate hollow, tube made of dielectric material that is aligned parallel to the discharge electrodes and positioned near the discharge region. A conductive preionization electrode (typically made of copper or brass), is positioned in the bore of the tube and used to create a potential difference between the preionization electrode and one of the main discharge electrodes. This potential difference extends across the dielectric tube radially and results in a substantially uniform emission of photons that are emitted from the outer surface of the tube.

[0064] Also within the chamber interior 415 may be a gas circulation system comprising a gas circulation fan 470, which may be, e.g., a generally cylindrical crossflow fan 470. The fan 470 serves to move gas within the chamber interior 415, generally in a circular fashion as seen in the cross-sectional view of FIG. 4, in order to remove gas that contains ionized particles and debris and is depleted of F2 from the discharge region 440 between successive discharges, and thus to replenish the discharge region 440 with fresh gas before the next gas discharge.

[0065] The gas circulation system may also include a temperature control system which in the example shown in FIG. 4 is implemented as a plurality of heat exchangers 475 in the generally circulargas flow path to remove heat added to the gas, e.g., by the discharges and the operation of the fan 470. The amount of heat removed by the heat exchangers 475 is regulated by fluid volume controller 477 controlling a volume and rate of flow of fluid in the heat exchangers 475. The fluid is in thermal communication with the gas in the chamber 400. The fluid can carry off more or less heat depending on the flow volume and rate in the heat exchangers 475.

[0066] The gas circulation system may also have a plurality of curved baffles 480 which may serve to shape the generally circular gas flow path out of the discharge region 440 toward the heat exchangers 475 and ultimately the intake of the fan 470, and from the output of the fan 470 to the discharge region 440, respectively. The upper chamber body 405 may also have an attached metal fluoride trap 490 in fluid communication with the chamber interior 415.

[0067] As mentioned, a byproduct of the generation of laser pulses is the production of acoustic waves (also referred to as compression waves or pressure waves) in the lasing gas in the chamber. The amount of time it takes for these waves to propagate to a surface in the chamber and be reflected back to the discharge region may be referred to as the “echo” period of these waves. If the echo period coincides too closely with the repetition time period of the laser then the wave echoes will return to the discharge region just as a new discharge is forming. This phenomenon is sometimes referred to as a time-of-flight or TOF resonance, or, equivalently, acoustic resonance or bandwidth (BW) resonance. This can cause density fluctuations during discharge formation which can in turn lead to beam steering / wavefront distortion, thereby undesirably increasing bandwidth. Depending on the strength of compression waves there might also be a negative impact on pointing stability, center wavelength, EMO, ESIGMA, and BWSIGMA (or BWS). There may also be a negative impact on metrics that include in part feedback from the exposure tool such as the convolved bandwidth (CBW) which is a convolution of exposure tool’s influence with E95 and / or FWHM. The nature and extent of the impacts will depend on whether the acoustic TOF resonance is occurring in the MO chamber or / and the AMP chamber.

[0068] In some embodiments,, EMO is the E95 spectral purity range of the pulses, i.e., the spectral range that contains 95% of the total energy of the laser pulse from the MO chamber, ESIGMA is the standard deviation of the energy variation, and BWS is the standard deviation of the pulse spectrum line-center variation. In one embodiment, EMO is the spectral range of the pulses where the intensity is at least half its maximum value, i.e., FWHM.

[0069] “Repetition rate agility” refers to enabling operation at commanded repetition rates that would otherwise be forbidden as being too close to a repetition rate causing an acoustic resonance. Commanded repetition rates can change wafer-to-wafer or even during processing of a single wafer (die-to-die) when, for example, it is desired to use a different repetition rate at the edges of the wafer. A user will in general want the flexibility to operate at any one of a wide range of commanded repetition rates selected on the basis of manufacturing demands.

[0070] One currently employed method of providing some repetition rate agility involves tuning the relative timing of respective pulse triggers for the MO chamber and the AMP chamber, this relativetiming being referred to as dtMOPA. The tuning of dtMOPA, however, affects the pulse energy which in turn limits the range of tuning available using this technique. The magnification in the LNM can be adjusted to move the dtMOPA setpoint to its nominal value to somewhat mitigate this effect but such adjustment requires physically moving the prisms in the LNM which is a relatively slow process.

[0071] Additional measures for mitigating the effects of TOF resonances involve modifying the morphology, i.e., orientation or shapes, of surfaces inside the chamber such as tilting the preionizer tube or providing the surface of the cathode support with acoustic baffling structures. Such measures, however, may have unintended consequences. Also, such measures may become increasingly ineffective at higher repetition rates due to the complex three dimensional structure of the acoustic flow field within the chamber. In addition, fabricating baffled parts may add costs and the modified surfaces may cause a loss of flow efficiency.

[0072] According to an aspect of an embodiment, repetition rate agility is facilitated by measuring a parameter and then feeding the measured value back in a feedback loop to effect chamber temperature setpoint control. The temperature setpoint of the chamber is in general controlled by the heat exchangers. The degree to which the heat exchangers can remove heat from the discharge chamber is controlled by regulating the flow of a cooling medium such as water through the heat exchangers. In some embodiment the chamber is held at about 65°C. By changing the water flow in response to changes in heat load it is possible to keep the chamber at a constant temperature.

[0073] According to an aspect of an embodiment the effects of TOF resonances are alleviated by controlling the temperature setpoint of the gas in the chamber based on feedback from a BAM such as a high resolution (HR) BAM as an additional control mechanism or “knob.” The feedback may be derived from pulse-to-pulse BW measurements. If the measured BW exceeds a specified value and / or indicates that a new commanded repetition rate is too close to (for example, less than .1 kHz from) a resonance repetition rate at which a TOF resonance will occur then the feedback loop will change the temperature setpoint of the chamber to a new temperature setpoint that shifts the TOF resonance rate away from the new commanded repetition rate. This gives the user the freedom to use anew commanded repetition rate that would otherwise be unavailable as a permitted choice.

[0074] Changing the temperature setpoint in the chamber has the effect of shifting the acoustic power spectrum in the chamber. More specifically, the pressure waves in the chamber propagate at the speed of sound which varies as the square root of the temperature of the gas through which the pressure waves are traveling. Thus, changing the temperature in turn changes the echo period of the acoustic wave and so the values of the resonance repetition rates. Thus, the new commanded repetition rate will no longer be too close to a repetition rate at which a TOF resonance occurs. In some implementations a temperature change of about 10°C can be expected to shift the acoustic spectrum by about 100 Hz. For example, if the temperature were reduced from 65 °C to 55 °C then the acoustic peak would shift from 8 kHz to 7.9 kHz. This is sufficient for detuning off the resonance, i.e., shifting the resonancerepetition rate away from the commanded repetition rate. In some embodiments the temperature change may be in a range of about 5 °C to 15 °C.

[0075] An example of acoustic power spectrum shifting is shown in FIG. 5, which qualitatively plots the strength of the acoustic resonance effect, for example, the effect on E95 and / or FWHM, against proximity to a resonance repetition rate. For example, suppose an operator wishes to run a source at a 5.6 kHz repetition rate. Before the acoustic spectrum is shifted, 5.6 kHz would correspond to a resonance repetition rate as indicated by the solid line peak at 5.6 kHz. By changing the temperature in the chamber, however, the resonance repetition rate that was at 5.6 kHz is shifted as indicated by the dotted peak to closer to 5.7 kHz. The operator would then be free to operate at 5.6 kHz without risking unwanted acoustic resonance effects.

[0076] FIG. 6 is a functional block diagram of a system for temperature control of acoustic resonance effects in accordance with an aspect of an embodiment. The arrangement of FIG. 6 includes a discharge chamber 605 which could be, for example, the MO chamber 220 or the AMP chamber 250 of FIG. 2. The discharge chamber 605 emits a main pulse 600. A part of the main pulse 600 is split off from the main pulse 600 as metrology radiation 610. In general, the metrology radiation 610 will be a small percentage of the energy from the main pulse 600. The meteorology radiation 610 passes to the bandwidth analysis module BAM 265. While the arrangement of FIG. 6 shows the metrology radiation 610 being split off, it will be apparent to one of ordinary skill in the art that arrangements in which the main pulse 600 passes through the bandwidth metrology system BAM 265 are also possible.

[0077] The bandwidth analysis module BAM 265 generates a signal indicative of a beam parameter such as FWHM and / or E95. The signal is supplied to a controller 620. The controller 620 controls a chamber temperature regulation unit 375 which may be implemented as heat exchangers as described above. The signal from the controller 620 controls the amount of fluid passing through the heat exchangers thereby controlling the rate at which the heat exchangers can remove from the chamber 605. This establishes the temperature setpoint of the gas inside the chamber. The temperature control mechanism for BW can applied in conjunction with the dtMOPA control mechanism and LNM control mechanisms so that all three mechanisms including temperature (either TMO or TAMP or both), dTMOPA, and LNM can be used for BW control. These control mechanisms may be employed seriatim or two or more of the control mechanisms may be employed concurrently.

[0078] The arrangement of FIG.6 may also include a temperature sensor 630 supplying a signal indicative of the temperature of the gas in the discharge chamber 605 to the controller 620. The temperature sensor 630 may be any suitable device for measuring temperature of gas in the discharge chamber 605 including but not limited to one or more thermocouples, resistive temperature measuring devices, infrared sensors, bimetallic devices, thermometers, change -of-state sensors, and semiconductor sensors such as silicon diodes. The controller 620 may be a separate dedicated controller or its functions may be carried out by an overall system controller.

[0079] One aspect of some implementations in accordance with this disclosure is that the temperature of the discharge chamber will in general exhibit a finite settling time, that is, it will take some non-negligible amount of time for the temperature in the chamber to reach a new temperature setpoint. In general gas heating is more rapid than gas cooling because the energy for heating is supplied by the plasma formed in the discharge region. On the other hand, cooling is limited by the rate at which the heat exchangers can carry off heat and is slower. In some implementations, for example, it may take up to thirty seconds for the temperature in the chamber to decrease to a lower intended temperature setpoint. In some implementations this settling time will be taken into account in implementing a control system in accordance with this disclosure.

[0080] Temperature tuning can be accelerated, for example, at least in the direction of decreasing temperature, by increasing the rate of heat removal from the chamber as achieved by the heat exchangers. In a given implementation the time the temperature shift can be accelerated without performance trade-offs because such acceleration will not influence plasma characteristics insofar as the reduced electric field (electric field / number density) is unchanged.

[0081] The use of a temperature feedback loop can potentially be integrated with the other control methods (e.g., dtMOPA tuning) for repetition rate agility. Temperature tuning may in principle be less rapid than dtMOPA tuning but temperature tuning can be used for greater excursions from BW resonances than may be possible using dtMOPA tuning. This is because of the effect of dtMOPA tuning on energy which limits the amount of tuning available without degrading performance. Also, temperature tuning does not influence the energy output of the MO chamber as does changing the LNM magnification to compensate for energy changes caused by dtMOPA tuning.

[0082] The parameter at issue affects the choice of which chamber is to be thermally tuned. BW is primarily affected by acoustic resonances in the MO chamber. Acoustic resonances in the MO chamber can also affect the burst transient (BT) which is the change in the line-center wavelength over the course of a burst of pulses. In other words, BT is a pulse-to-pulse systematic shift in the pulse characteristics over the course of a burst and so the change of bandwidth with pulse number.

[0083] Acoustic resonances in the AMP chamber can influence at least one of beam pointing (BP) and beam divergence (BD). BP is the horizontal or vertical deviation of the laser beam from the optical axis of the system. In some embodiments, BP can also refer to the position of the laser beam with respect to the optical axis. BD is a measure of the amount by which the beam spreads out in the horizontal or vertical direction. BD and BP can be measured for every shot by, for example, a PDA or a spectrometer in the CASSM.

[0084] This disclosure uses an example BD as a measure of the spreading out of the laser beam as it propagates, and BP as a measure of the deviation of the beam from its desired path as it propagates, as examples. One of ordinary skill in the art will readily appreciate that spreading and / or divergence may be measured in the vertical direction, the horizontal direction, or in other or additional directions.

[0085] Thus, the temperature setpoint of the gas in the MO chamber, TMO, and the temperature setpoint of the gas in the AMP chamber, T MP, can serve as independent controls for mitigating the adverse acoustical effects described above. In some embodiments a high-resolution BAM (HR-BAM) is used for measuring BW, BWS, and the BT. Then, the temperature setpoint of the gas in the MO chamber, TMO, is tuned to optimize BW, BWS, and BT. In addition or alternatively, a sensor such as a photodiode array or spectrometer which may be located in a CASMM is used for measuring BP and BD and the temperature setpoint of the gas in the AMP chamber, TAMP, is tuned to optimize BP and BD.

[0086] The pulse metric may be based on a pulse-by-pulse measurement or by some statistical combination of pulse measurements. As stated above, in one irradiation procedure pulses are generated as a series of bursts with each burst containing multiple pulses. This is depicted in FIG. 7 which shows a series of bursts Bi through Bmwith each burst containing Pnpulses, with Pnm being used to designate the nth pulse of the mth burst. As indicated by the dashed boxes, the pulses can be grouped for analysis purposes either as multiple pulses in the same burst (horizontally in the figure) or as the same pulse number in each of several bursts (vertically in the figure). In some embodiments, the number for burst measurement ranges from one to six because more than six measurements increases uncertainty in measurement asymptotes.

[0087] As a nonlimiting example, a laser repetition rate may be from 5 kHz to 8 kHz, with 1000 pulses per burst, with an interburst interval of from about 0.1 to about 0.3 seconds. Assuming five burst measurements (m = 5) are desired for an adequate signal-to-noise ratio (SNR). The measurement would thus require from about 1.03 to about 2.2 seconds. BWnmay be calculated as an average [Em(Pnm / m)] with Pnm being equal to the measured BW for the nth pulse of the mth burst, thus averaging Pn’s BW over the m bursts. BWS may be calculated as [BWavg-BWn]2 / n where BWavg= S(BWn) / n.

[0088] Phase averaging is averaging the same pulse number across different bursts. An example of a phase averaged parameter is BT which arise because pulses over the course of a burst see different acoustic fields. BT may be obtained from a polynomial fit of [BWnto Bwi] as a function of n. A moving average can be implemented which will give next BW and BT every from about 0.3 to about 0.5 seconds. As an example, for measuring BT, five bursts may be measured which may take from about 1.03 to about 2.2 seconds. The derivative of change in the line center wavelength with time, that is, the slope of line center wavelength versus pulse number, is determined. When the slope increases more rapidly than a predetermined value then it is inferred that the source is operating at a repetition rate that is approaching a resonance repetition rate, that is, a repetition rate at which an acoustic resonance occurs. In other words, the slope of the change of bandwidth increases when the measure bandwidth exceeds the specification or is approaching a resonance. It is the slope of the change of bandwidth that determines the imminence of a bandwidth resonance.

[0089] There are also a variety of metrics for assessing BP and BD. For example, a photodiode array (PDA) is capable of acquiring data in a single laser shot. BP and BD can be computed as [E2-k(Pnm / (k- 1 )] with Pnmbeing a measurement of one or both of BP and BD for the nth shot of the mth burst in a single shot measurement, thus averaging Pn’s over k-1 pulses (including pulse 2 and k). In some embodiments, the value for the first pulse of a burst is not used because the first pulse of a burst is not subject to acoustic effects caused by an earlier pulse.

[0090] Another metric for BD is the standard deviation over 20 pulses with BD being determined to be out of specification (outside the predetermined acceptance criteria or specification) when the standard deviation of BD exceeds a particular value. In other words, some number of pulses is averaged and when the average or the standard deviation of the BD or BP is out-of-spec for those pulses then an error condition is determined.

[0091] For example, BP or BD or both may be measured as [S(Pnm / (k- 1 )] .averaging Pn’s over k-1 pulses (including pulse 2 and pulse k), with Pnbeing a single pulse measurement. Alternatively, if measured BP is greater than a certain threshold for a second pulse and then is also greater than that threshold for a third pulse and so on for a number k (e.g., from 15 to 35) consecutive pulses then temperature tuning is performed for tire AMP chamber. The same metric may be applied for BD.

[0092] Another phenomenon or metric is BW chirp. Chirp is determined by varying the repetition rate. The energy of an nth, e.g., fourth, pulse in a burst at a first repetition rate, for example, 4000 Hertz, is measured. Then the energy of the nth pulse (fourth) of a burst having a second repetition rate different from the first repetition rate, e.g., 5000 Hertz, is measured. Then energy of an nth pulse in a burst at a third repetition rate, for example, 6000 Hertz, is measured.

[0093] The effectiveness of a thermal setpoint feedback control system according to an aspect of an embodiment can be further improved by performing periodic repetition rate scans, i.e., scanning the repetition rate at which the source operates through a series of repetition rates, measuring the value of a resulting pulse metric such as E95 and / or FWHM at each scanned repetition rate, and associating the measurements with the repetition rates to derive the measured pulse metric as a function of repetition rate.

[0094] FIG. 8 is a flow chart describing an example of the use of feedback to control bandwidth, bandwidth sigma, and burst transients using MO chamber temperature setpoint tuning. In a step S10, the repetition rate (reprate) is set by the exposure tool. In a step S20 at least one of the bandwidth, bandwidth sigma, and burst transient are measured. In a step S30 it is determined whether the bandwidth is within specification or the bandwidth sigma is within specification or the burst transient is within specification. During execution of these steps and of the following steps the laser continues to operate as indicated by the surrounding box labeled S40 unless a laser error condition is determined as described below

[0095] If any of these parameters, e.g., the bandwidth, bandwidth sigma, and burst transient, are not within specification then in a step S50 dtMOPA is adjusted, the LNM magnification is adjusted, and the setpoint temperature of the MO chamber is adjusted. Then, in a step S60 a measurement counter which keeps track of the number of times tuning has been attempted is incremented. Then, in a stepS70, is again determined whether the bandwidth is within specification or the bandwidth sigma is within specification or the burst transient is within specification. If the determination is that all of these parameters are within specification, then the process reverts to step S20. If, however, any of these parameters are not within specification then the process proceeds to step S80 in which it is determined whether the number of times tuning has been attempted as counted by the measurement counter exceeds an attempt limit. If the attempt limit has not been exceeded then then the process reverts to step S50 for another tuning attempt. If, however, it is determined in step S80 that the count exceeds the attempt limit then in a step S90 a laser error is flagged indicating that the laser is in an error state that cannot be rectified by the maximum number of tuning attempts. .

[0096] FIG. 9 is a flow chart describing an example of the use of feedback to control BP and BD using AMP chamber temperature setpoint tuning. In a step S 10, the repetition rate is set by the exposure tool. In a step SI 00 BP and BD are measured. In a step SI 10 it is determined whether BP is within specification and / or BD is within specification. Again, during execution of these steps and of the following steps the laser continues to operate as indicated by the surrounding box labeled S40 unless a laser error condition is determined as described below

[0097] If either of these parameters is not within specification then in a step S 120 the setpoint temperature of the AMP chamber is adjusted. Then, in a step S 130 a measurement counter which keeps track of the number of times tuning has been attempted is incremented. Then, in a step S 140, it is again determined whether BP and BD are within specification. If the determination is that both of these parameters are within specification, then the process reverts to step S 100.. If, however, any of these parameters are not within specification then the process proceeds to step S80 in which it is determined whether the number of times tuning has been attempted as counted by the measurement counter exceeds an attempt limit. If the attempt limit has not been exceeded then then the process reverts to step S 120 for another tuning attempt. If, however, it is determined in step S80 that the number of attempted tunings exceeds the attempt limit then in a step S90 a laser error is flagged indicating that the laser is in an error state that cannot be rectified by a set number of tuning attempts.

[0098] As mentioned, some of the measures mentioned above for mitigating the effects of acoustic resonances involve engineering the acoustic reflecting surfaces in the chamber to redirect acoustic energy. These measures tend to broaden the acoustic energy spectrum. A performance metric for this broadening is the contrast ratio calculated as the measure of the peak value divided by the minimum value. The detuning effect will be the most pronounced in chambers in which the acoustic spectrum exhibits sharp peaks. The detuning then leads to better differentiation of off-resonance intensity versus on-resonance intensity (the “contrast ratio”). FIG. 10 is a graph qualitatively illustrating some principles regarding sharpening the resonance for better off-resonance tuning. The x-axis is arbitrary repetition rates for illustration purposes. The broken curve is qualitatively representative of pulse width and shape in a discharge chamber in which morphological acoustic mitigation measures such as surface reorientation and reshaping have been adopted in order to diffuse acoustical energy returning to thedischarge region. The solid curve is qualitatively representative of pulse width and shape in a discharge chamber in which no such acoustic mitigation measures have been adopted. As can be seen, thermally tuning off the broken curve obtains better contrast on laser performance. Thus, in some embodiments, it is technologically beneficial and less costly to use thermal tuning without the use of morphological acoustic mitigation measures that smear the acoustic waves. In other embodiments thermal tuning may be used in conjunction with morphological acoustic mitigation measures.

[0099] Bandwidth measuring devices such as HRB AM may be implemented in any one of a number of different ways. For example, the bandwidth measuring device may use an etalon. A high resolution bandwidth analysis module may use an etalon with a fast light sensor such as a camera, that is, a light sensitive device which operates at a high rate, e.g., the megahertz range, and can be read out very quickly. Another possible implementation of a bandwidth measuring devices such as a BAM includes a spectrometer with a high-speed CMOS camera.

[0100] The methods described herein are also capable of compensating for BW resonances induced by chamber misalignment.

[0101] The foregoing description is in terms of control mechanisms to minimize or avoid the effects of TOF resonances by controlling the illumination source based on feedback from the illumination system. The characteristics of the radiation that is ultimately delivered to a wafer, however, is also affected by operational parameters of the exposure apparatus, for example, the exposure tool 115 (FIG. 1). For example, as mentioned above, one control parameter may be convolved bandwidth (CBW). This is the bandwidth of the radiation provided by the illumination system convolved with effects on that radiation arising from characteristics of the exposure tool. Control parameters based both on the characteristics of the illumination system and of the exposure tool such as CBW can be used in conjunction with temperature control to minimize or avoid the effects of TOF residences in one or both of the MO Chamber and AMP chamber.

[0102] Some of the above description is in terms of functional block diagrams with some functions allocated to some blocks and other functions allocated to other blocks. It will be understood that the division between blocks and the allocations are arbitrary and that different divisions and allocations are possible so long as the overall functions are carried out as described above.

[0103] The above description includes examples of multiple embodiments. It is, of course, not possible to describe every conceivable combination of components or methodologies for each of these embodiments, but one of ordinary skill in the art may recognize that many further combinations and permutations of elements of the various embodiments are possible based on the disclosure. Accordingly, the described embodiments are intended to be representative of and encompass all such alterations, modifications, and variations that fall within the spirit and scope of the appended claims.

[0104] Furthermore, to the extent that the term “includes” is used in either the detailed description or the claims, such term is intended to be inclusive in a manner similar to the term “comprising” as “comprising” is construed when employed as a transitional word in a claim. Also, although elements ofthe described aspects and / or embodiments may be described or claimed in the singular, the plural is contemplated unless limitation to the singular is explicitly stated. Additionally, all or a portion of any aspect and / or embodiment may be utilized with all or a portion of any other aspect and / or embodiment, unless stated otherwise.

[0105] Aspects and implementations of the present disclosure can be further described using the following clauses:1. A radiation source comprising: a discharge chamber containing a lasing gas, discharges in the lasing gas in a discharge gap in the discharge chamber producing pulses of radiation at a commanded repetition rate; a lasing gas temperature regulating system for governing a temperature of the lasing gas in the discharge chamber; a spectral property measuring device arranged to measure a spectral property of one or more of the pulses of radiation; and a controller arranged to control the lasing gas temperature regulating system to govern the temperature of the lasing gas in the discharge chamber based at least partially on the spectral property as measured by the spectral property measuring device.2. The radiation source as in clause 1 wherein the spectral property of the radiation is a bandwidth of the pulses of radiation.3. The radiation source as in clause 2 wherein the controller is adapted to cause the lasing gas temperature regulating system to alter a temperature of the lasing gas in the discharge chamber when the controller determines based on the spectral property of the radiation that a commanded change in repetition rate would result in a new repetition rate differing from a resonance repetition rate by less than a predetermined amount.4. The radiation source of clause 3 wherein the bandwidth measuring device measures bandwidth as an average of a bandwidth of n pulses of respective m bursts, n and m being positive integers.5. The radiation source of clause 4 wherein the average is a moving average.6. The radiation source of clause 1 wherein the bandwidth measuring device measures bandwidth sigma (BWS).7. The radiation source of clause 6 wherein the bandwidth measuring device measures BWS based on an average bandwidth of the n pulses over the m bursts, n and m being positive integers.8. The radiation source of clause 7 wherein the bandwidth measuring device measures BWS using measured bandwidths BWnaveraged over m bursts according to the formulas red bandwidth for the nth pulse of the mth burst,andBWS = [BWavg— BWn]2 / n.9. The radiation source of clause 1 wherein the spectral property measuring device measures a burst transient (BT).10. The radiation source of clause 9 wherein the spectral property measuring device measures the BT based on a polynomial fit.11. The radiation source of clause 9 wherein the bandwidth measuring device measures BT using a measured bandwidth BWnaveraged over m bursts according to the formula12. A method of operating a radiation source, the radiation source being configured to generate of m bursts of pulses, each burst having n pulses at a commanded repetition rate, m and n being positive integers, the method comprising: measuring a bandwidth of the pulses of radiation; and controlling a temperature of the lasing gas in the discharge chamber based at least partially on the measured bandwidth.13. The method as in clause 12 wherein controlling the temperature comprises causing altering a temperature of the lasing gas in the discharge chamber when it is determined that a commanded change in repetition rate would result in a new repetition rate smaller than a resonance repetition rate by less than a predetermined amount.14. The method of clause 12 wherein measuring the bandwidth comprises determining a moving average of a bandwidth of n first pulses of respective m bursts, n and m being positive integers.15. The method of clause 12 further comprising measuring a bandwidth sigma (BWS) based on an average bandwidth of the n pulses over the m bursts, n and m being positive integers.16. The method of clause 15 wherein measuring BWS is based on measured bandwidths BWnaveraged over m bursts according to the formulas red bandwidth for the nth pulse of the mth burst,andBWS = [BWavg— BWn]2 / n.17. The method of clause 14 further comprising measuring a burst transient (BT), wherein BT is measured based at least in part on a measured bandwidth BWnaveraged over m bursts according to the formula18. A method of generating deep ultraviolet (DUV) radiation using a source including a master oscillator (MO) discharge chamber and a power amplifier (AMP) discharge chamber, the method comprising: using a bandwidth measuring device to measure at least one of bandwidth (BW), bandwidth sigma (BWS), and burst transient (BT) of the DUV radiation; using a photodiode array or spectrometer to measure beam pointing and beam divergence of the DUV radiation; adjusting an MO temperature of a lasing gas in the MO discharge chamber to optimize at least one of BW, BWS, and BT; and adjusting an AMP temperature of a lasing gas in the AMP discharge chamber to optimize at least one of beam pointing and beam divergence.19. The method of clause 18 wherein the beam pointing and the beam divergence are averaged over a predetermined number k of pulses, k being a positive integer, not including a first pulse of burst.20. A method of mitigating acoustic effects in a chamber of a laser radiation source, the method comprising: measuring a bandwidth value of laser radiation from the laser radiation source; comparing the bandwidth value with a predetermined threshold value; and altering a temperature setpoint of the chamber if the bandwidth value is smaller than the predetermined threshold value.21. The method of clause 20 wherein altering the temperature setpoint of the chamber includes lowering the temperature setpoint in a range from about 5 degrees Celsius to about 15 degrees Celsius.22. The method of clause 20, wherein measuring a bandwidth value of laser radiation from the laser radiation source comprises averaging a plurality of a bandwidth values of pulses across a sequence of bursts of the laser to obtain the bandwidth value.

[0106] The above-described aspects and implementations and other implementations are within the scope of the following claims.

Claims

CLAIMS1. A radiation source comprising: a discharge chamber containing a lasing gas, discharges in the lasing gas in a discharge gap in the discharge chamber producing pulses of radiation at a commanded repetition rate; a lasing gas temperature regulating system for governing a temperature of the lasing gas in the discharge chamber; a spectral property measuring device arranged to measure a spectral property of one or more of the pulses of radiation; and a controller arranged to control the lasing gas temperature regulating system to govern the temperature of the lasing gas in the discharge chamber based at least partially on the spectral property as measured by the spectral property measuring device.

2. The radiation source as in claim 1 wherein the spectral property of the radiation is a bandwidth of the pulses of radiation.

3. The radiation source as in claim 2 wherein the controller is adapted to cause the lasing gas temperature regulating system to alter a temperature of the lasing gas in the discharge chamber when the controller determines based on the spectral property of the radiation that a commanded change in repetition rate would result in a new repetition rate differing from a resonance repetition rate by less than a predetermined amount.

4. The radiation source of claim 3 wherein the bandwidth measuring device measures bandwidth as an average of a bandwidth of n pulses of respective m bursts, n and m being positive integers.

5. The radiation source of claim 4 wherein the average is a moving average.

6. The radiation source of claim 1 wherein the bandwidth measuring device measures bandwidth sigma (BWS).

7. The radiation source of claim 6 wherein the bandwidth measuring device measures BWS based on an average bandwidth of the n pulses over the m bursts, n and m being positive integers.

8. The radiation source of claim 7 wherein the bandwidth measuring device measures BWS using measured bandwidths BWnaveraged over m bursts according to the formulaswhere Pnm is the measured bandwidth for the nth pulse of the mth burst,andBWS = J[BWavg- BWn]2 / n.

9. The radiation source of claim 1 wherein the spectral property measuring device measures a burst transient (BT).

10. The radiation source of claim 9 wherein the spectral property measuring device measures the BT based on a polynomial fit.

11. The radiation source of claim 9 wherein the bandwidth measuring device measures BT using a measured bandwidth BWnaveraged over m bursts according to the formula12. A method of operating a radiation source, the radiation source being configured to generate of m bursts of pulses, each burst having n pulses at a commanded repetition rate, m and n being positive integers, the method comprising: measuring a bandwidth of the pulses of radiation; and controlling a temperature of the lasing gas in the discharge chamber based at least partially on the measured bandwidth.

13. The method as in claim 12 wherein controlling the temperature comprises causing altering a temperature of the lasing gas in the discharge chamber when it is determined that a commanded change in repetition rate would result in a new repetition rate smaller than a resonance repetition rate by less than a predetermined amount.

14. The method of claim 12 wherein measuring the bandwidth comprises determining a moving average of a bandwidth of n first pulses of respective m bursts, n and m being positive integers.

15. The method of claim 12 further comprising measuring a bandwidth sigma (BW S) based on an average bandwidth of the n pulses over the m bursts, n and m being positive integers.

16. The method of claim 15 wherein measuring BWS is based on measured bandwidths BWn averaged over m bursts according to the formulaswhere Pnm is the measured bandwidth for the nth pulse of the mth burst,andBWS = J[BWavg- BWn]2In.

17. The method of claim 14 further comprising measuring a burst transient (BT), wherein BT is measured based at least in part on a measured bandwidth BWnaveraged over m bursts according to the formula18. A method of mitigating acoustic effects in a chamber of a laser radiation source, the method comprising: measuring a bandwidth value of laser radiation from the laser radiation source; comparing the bandwidth value with a predetermined threshold value; and altering a temperature setpoint of the chamber if the bandwidth value is smaller than the predetermined threshold value.

19. The method of claim 18 wherein altering the temperature setpoint of the chamber includes lowering the temperature setpoint in a range from about 5 degrees Celsius to about 15 degrees Celsius.

20. The method of claim 18, wherein measuring a bandwidth value of laser radiation from the laser radiation source comprises averaging a plurality of a bandwidth values of pulses across a sequence of bursts of the laser to obtain the bandwidth value.

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