Pulsed laser and method of operating

Abstract

A method of operating a laser includes, after the laser generates a first pulse, setting an attenuation of an attenuator in the laser such that a gain of the laser exceeds a loss of the laser to allow the laser to generate a first continuous beam; after generating the first continuous beam, increasing the attenuation such that the loss of the laser exceeds the gain of the laser; and after increasing the attenuation, decreasing the attenuation such that the laser generates a second pulse. A system for generating pulses of laser radiation comprises: an optical modulator controlled by a signal applied to the optical modulator, the modulator connected to a laser; and a control system configured to provide a signal to the modulator according to the method.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. Application No. 63 / 346,645, filed on May 27, 2022, entitled “PULSED LASERS AND METHODS OF OPERATION,” which is incorporated herein by reference in its entirety. Technical Field

[0003] The present invention relates generally to pulsed lasers and methods of operating pulsed lasers, and more particularly to Q-switched seed lasers in methods of operating extreme ultraviolet light sources. Background Art

[0004] The semiconductor industry continues to develop lithography techniques that enable printing of smaller integrated circuit dimensions. Using shorter wavelengths of light, such as extreme ultraviolet (EUV) light (sometimes referred to as soft x-rays and generally defined as electromagnetic radiation having a wavelength between 10 nanometers and 120 nanometers (nm)), enables smaller features compared to longer wavelengths.

[0005] Currently, EUV lithography typically uses EUV light with wavelengths in the range of 10 to 14 nanometers (nm) to produce features as small as 10 nm or even 7 nm on or in substrates such as silicon wafers. To be commercially useful, it is desirable that systems that produce these extremely small features be highly reliable and provide cost-effective throughput with reasonable process latitude.

[0006] Methods for producing EUV light include, but are not necessarily limited to, converting a material into a plasma state with one or more emission lines in the EUV range, the plasma state having one or more elements, such as xenon, lithium, tin, indium, antimony, tellurium, aluminum, etc. In one such method, often referred to as laser produced plasma (LPP), the desired plasma may be produced by irradiating a target material, such as a droplet, stream, or cluster of material capable of producing the desired line emission, with a laser beam at an irradiation site. The line emitting material may be an element and may be delivered to the irradiation site in pure form or in alloy form, such as an alloy that is liquid at a desired temperature, or may be mixed or dispersed with another material, such as a liquid.

[0007] In some LPP systems, respective targets of the target stream are illuminated by respective laser pulses to generate plasma from each target. Alternatively, each target may be illuminated sequentially by more than one light pulse. In some cases, each target may be exposed to a so-called "pre-pulse" to heat, expand, vaporize, vaporize, and / or ionize the target material and / or generate a weak plasma, and then a strong plasma may be generated by a so-called "main pulse" and convert most or all of the material affected by the pre-pulse into plasma and thereby generate EUV light. It will be appreciated that the functions of the pre-pulses and main pulses may overlap to some extent.

[0008] Since the EUV output power in an LPP system is typically scaled with the laser power irradiating the target material, laser pulses with higher power are required. On the other hand, synchronizing the pulses with the target to be irradiated so that they meet at the desired irradiation location at the desired time requires precise pulse timing. Assuming that both higher power and precision control are required in the pulses used to irradiate the target, it may be useful to use a device including a relatively low-power "seed laser" and one or more amplifiers for amplifying the pulses from the seed laser. The use of one or more amplifiers allows a well-controlled but relatively low-power seed laser to be used for precise timing control while still providing relatively high-power pulses for the LPP process through amplification.

[0009] However, even where a seed laser and amplifier are used, it may still be desirable to produce relatively higher power seed laser pulses so that the need for amplification is mitigated, or so that higher power pulses can be produced from the same amplifier(s) or amplifier system. Summary of the invention

[0010] In some general aspects, a method of operating a laser includes (1) after the laser generates a first pulse, setting the attenuation of an attenuator in the laser to a first attenuation value so that the gain of the laser exceeds the loss of the laser to allow the laser to generate a first continuous beam; (2) after the first continuous beam is generated, increasing the attenuation of the attenuator to a second attenuation value so that the loss of the laser exceeds the gain of the laser; and (3) after increasing the attenuation to the second value, reducing the attenuation of the attenuator to a third attenuation value so that the laser generates a second pulse. Embodiments of the method may also include setting the attenuation to the first attenuation value after the laser generates the second pulse. In embodiments, the second value may cause the laser to turn off.

[0011] Embodiments of the method may include one or more of the following. After setting the attenuation to the second attenuation value and before setting the attenuation to the third attenuation value, the attenuation is reduced to an intermediate attenuation value that is higher than the third attenuation value and low enough to allow the laser to produce a second beam. The attenuator may be or may be included in the laser or an optical modulator connected to the laser. The attenuator may be or may be included in the laser or an acousto-optic modulator (AOM) connected to the laser. Setting the attenuation of the attenuator may include setting the RF power level supplied to the AOM. Increasing the attenuation of the attenuator may include increasing the RF power supplied to the AOM. Reducing the attenuation of the attenuator may include reducing the RF power supplied to the AOM. The attenuator may be or may be included in the laser or an electro-optic modulator (EOM) connected to the laser. The laser may be a CO2 laser. The laser may be a seed laser in an extreme ultraviolet (EUV) light source and / or a main pulse seed laser in an EUV light source.

[0012] Setting the attenuation to a second attenuation value may include setting the attenuation to the second attenuation value and leaving the attenuation at the second attenuation value for a time in a range of 100 nanoseconds to 1000 nanoseconds (ns). Setting the attenuation to an intermediate attenuation value may include setting the attenuation to the intermediate attenuation value and leaving the attenuation at the intermediate attenuation value for a time in a range of 0 ns to 300 ns. Setting the attenuation to a third attenuation value may include setting the attenuation to the third value and leaving the attenuation at the third value for a time period in a range of 400 ns to 700 ns.

[0013] Embodiments of the method may also include one or more of the following: Monitoring a duration from the first pulse to the generation of the first continuous beam, and adjusting the cavity length of the laser to minimize the duration Monitoring a duration between the first pulse and the generation of the first continuous beam, and adjusting the first attenuation value based on the duration.

[0014] The third attenuation value may be a maximum attenuation value. The first attenuation value may be equal to the third attenuation value. (1) increasing the attenuation of the attenuator to the second attenuation value so that the loss of the laser exceeds the gain of the laser, and (2) reducing the attenuation of the attenuator to the third attenuation value after increasing the attenuation to the second value so that the laser generates a second pulse is or may include: performing Q-value switching on the laser.

[0015] In another general aspect, a method of operating a laser including an optical modulator controlled by a signal applied to the optical modulator may include: (1) setting the amplitude of the signal to a first value so that the laser operates in a mode in which laser gain exceeds resonator loss; (2) setting the amplitude of the signal to a second value so that the laser is turned off; and (3) setting the amplitude of the signal to a third value so that the laser generates pulses.

[0016] Embodiments of the method may include one or more of the following: The laser may include an output coupler having a piezoelectric transducer, and the method may include using the output of the laser to control a voltage applied to the piezoelectric transducer during the step of setting the amplitude of the signal to the first value.

[0017] The optical modulator may be or include an acousto-optic modulator (AOM). The signal may be an RF power level applied to the AOM. The optical modulator may be or include an electro-optic modulator (EOM). The steps of (1) setting the amplitude of the signal to a second value so that the laser is turned off, and (2) setting the amplitude of the signal to a third value so that the laser generates a pulse may be or include: Q-switching the laser.

[0018] In another general aspect, a system for generating pulses of laser radiation includes: (1) a laser including an optical modulator controlled by a signal applied to the optical modulator; and (2) a control system configured and adapted to sequentially: set the amplitude of the signal to a first value so that the laser operates in a mode in which the laser gain exceeds the resonator loss, then set the amplitude of the signal to a second value so that the laser is turned off, and then set the amplitude of the signal to a third value so that the laser generates pulses.

[0019] Embodiments of the system may include one or more of the following: The laser may include an output coupler having a piezoelectric transducer, and the control system may be additionally configured and adapted to use the output of the laser to control a voltage applied to the piezoelectric transducer when the signal is at a first value.

[0020] The optical modulator may be or may include an acousto-optic modulator (AOM). The signal applied to the AOM may be an RF power level. The optical modulator may be or may include an electro-optic modulator (EOM). The control system may be or may be configured and adapted to perform Q-value switching.

[0021] In another general aspect, a laser system includes: (a) a laser having a laser cavity; (b) an optical modulator configured to control a Q factor of the laser cavity; (c) a power sensor positioned outside the laser cavity and configured to detect a power level of radiation emitted from the laser and to generate power level data and / or signals related to the power level of the radiation emitted from the laser; and (d) a control system connected to receive the power level data or signals and to control the optical modulator, wherein the control system is configured to: (1) set the Q factor of the cavity of the laser to a first value that is sufficiently high to allow lasing to occur, (2) at a time after lasing is detected by the power sensor, set the Q factor of the cavity to a second value that is less than the second value and sufficiently low to prevent lasing from occurring, and (3) after setting the Q factor of the cavity to the second value, set the Q factor of the cavity to a third value to cause the laser to emit pulses. The control system is or can be configured to perform Q value switching.

[0022] The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 is a schematic diagram of aspects of an extreme ultraviolet (EUV) light source.

[0024] Figure 2 It is a schematic diagram of EUV light source and lithography exposure equipment.

[0025] Figure 3 is a schematic diagram of aspects of a seed laser module that can be used in an EUV light source.

[0026] Figure 4 is a schematic diagram of a laser used as a seed laser in an EUV light source.

[0027] Figure 5A is a graph of an embodiment of a periodically varying Q-factor for generating periodically Q-switched pulses from a laser such as a seed laser in an EUV light source.

[0028] Figure 5B can be applied to the AOM to produce Figure 5A A graph of the periodic variation of the Q factor versus the RF power level.

[0029] Figure 5C Such as can be Figure 5A Periodic variation of the Q factor and / or Figure 5B A graph of periodically varying RF power producing periodically varying laser output power.

[0030] Fig. 6A is a graph of an embodiment of a periodically varying Q-factor for producing periodic Q-switched pulses with increased power output from a laser such as a seed laser in an EUV light source.

[0031] Figure 6B can be applied to the AOM to produce Fig. 6A A graph of the periodic variation of the Q factor versus the RF power level.

[0032] Figure 6C Such as can be Fig. 6A Periodic variation of the Q factor and / or Figure 6B A graph of periodically varying RF power producing periodically varying laser output power.

[0033] Fig.6D is a flow chart describing the steps of a process for varying the Q factor according to aspects of the embodiments.

[0034] Fig. 7A is a graph of another embodiment of a periodically varying Q-factor for producing periodic Q-switched pulses with increased peak power output from a laser such as a seed laser in an EUV light source.

[0035] Figure 7B Such as can be Fig. 7A A graph of the periodic variation of the Q factor resulting in the periodic variation of the laser output power. DETAILED DESCRIPTION

[0036] The methods, apparatus and systems of the present disclosure relate to operating a Q-switched laser. Q-switching, sometimes referred to as giant pulse formation, is a known technique for both controlling a laser to operate in a pulsed mode and for increasing the peak power of the laser. Q-switching, such as by switching to form "pulses" using a beam blanker or beam splitter, allows the laser to produce pulses with much greater peak power than the peak power of a pulse formed by a continuous beam.

[0037] Q-switching is generally accomplished by placing some type of variable attenuator within the optical cavity of the laser (a "Q-switch"). The Q-switch acts as a type of shutter, and may be, for example, an acousto-optic modulator (AOM) or an electro-optic modulator (EOM), either of which may be adjusted by application of a control signal to pass different amounts of light incident on the Q-switch. In basic Q-switching, the Q-switch is initially closed, i.e., set to pass little or no light, which prevents the laser from lasing and allows the energy stored in the laser medium in the form of an inversion population to increase to a level higher than that achievable during continuous lasing. The Q-switch is then quickly opened, allowing substantially all of the energy stored in the laser medium to be released very quickly in a relatively short pulse.

[0038] For example, by using Q switching, a laser might produce pulses that are each ½ microsecond (μs) long at a rate in the range of 50,000 to 100,000 pulses / second (50 kHz to 100 kHz), thus allowing power to build up for about 10 μs to 20 μs between pulses. In this way, a laser that can produce, for example, 50 Watts of power in a continuous laser mode can produce pulses with, for example, 500 Watts to 1 kW of peak power.

[0039] Q-switching as described above may suffer from some timing variability. When the Q-switch is turned on, allowing a Q-switched pulse to be emitted, there is a statistical inaccuracy about the time at which the (multiple) first photons (first light) will begin to be emitted along the optical path within the cavity of the laser. Therefore, the precise timing of the Q-switched pulse itself is slightly variable and not as predictable as would be expected. For example, after turning on the Q-switch for 100 to 200 nanoseconds (ns) and sometimes for up to 400 ns, there may be little or no energy emitted by the laser. This timing variation of the start of the Q-switched pulse, sometimes referred to as "time jitter", is not a shutter problem, because the operating timing of the Q-switch shutter effect of an AOM or EOM, for example, is not significantly variable. However, the timing of the start of lasing is variable.

[0040] One modification of Q-switching is to use "pre-lasing," i.e., to allow the laser to lase continuously at a lower level prior to Q-switching. Typically, to allow pre-lasing, the Q-switch (attenuator) is not fully "closed" during the time between pulses (the "inter-pulse interval"), but is instead set to provide partial attenuation of the laser energy. While the onset of pre-lasing after the Q-switched pulse is also subject to timing jitter, if pre-lasing is already occurring when the Q-switch is opened wider (i.e., when the attenuation of the attenuator is reduced to zero or a lower value), the larger Q-switched pulse will occur essentially instantly without any significant timing jitter. Thus, when using pre-lasing, the timing of the Q-switched pulse is significantly more predictable than with normal Q-switching.

[0041] In Q-switching using pre-lasing, there is a certain amount of partial decay after the pulse that determines the average level of stored power required in the laser before pre-lasing next kicks in. Therefore, the less decay caused by the Q-switch, the sooner pre-lasing starts on average after the previous Q-switched pulse.

[0042] Between pulses, the Q switch or attenuator is ideally set to a level that does not use very much power during pre-lasing (ie, a relatively high attenuation level) so that the small signal gain can be accumulated as much as possible for the Q-switched pulses.

[0043] Although relatively low attenuation is used for pre-lasing between pulses, the improved timing produced by Q-switching with pre-lasing comes at the expense of reduced power in the Q-switched pulses. For example, if a laser produces 1 kW pulses with typical Q-switching, it may produce only about 500 watts or even less power when using pre-lasing.

[0044] If pre-lasing occurs too early, the gain of the laser (and the peak power of the pulses produced) will be lower because the lower attenuation level that produces the earlier pre-lasing will result in a lower total peak power accumulation during the pre-lasing. Therefore, higher pulse-to-pulse attenuation levels that tend to delay the start of pre-lasing are desirable for higher peak pulse powers. However, if pre-lasing occurs too late, pre-lasing may not occur accidentally before the Q switch is fully turned on, and untimely pulses will occur, or in extreme cases, weaker pulses or even no pulses at all may occur during the opening of the Q switch. To avoid these problems, the time from the Q-switched pulse to the start of the subsequent pre-lasing can be monitored, and the attenuation of the Q switch between pulses is gradually increased if the pre-lasing occurs on average faster than the target time, and the attenuation of the Q switch between pulses is gradually reduced if the pre-lasing occurs on average later than the target time.

[0045] A problem separate from and not solved by Q-switching or pre-lasing may be referred to as "mode instability". In a given laser medium, the wavelength bandwidth over which light amplification can occur may be described as the "amplification bandwidth" or "gain bandwidth" or "gain curve", and is a characteristic of the laser medium. A laser cavity with a given laser medium has multiple possible "cavity modes" or frequencies whose wavelengths evenly divide the optical length of the cavity ("resonance frequency") and are within the amplification bandwidth of the laser medium. Therefore, the specific cavity modes available depend on the optical length of the laser cavity and the amplification properties of the laser medium.

[0046] If the cavity mode falls at or near the peak of the gain curve of the laser medium, the mode will dominate the laser emission to the exclusion of other modes, and the laser will operate in a "single longitudinal mode", with generally high efficiency and stable and consistent wavelength and power output. However, if two cavity modes are equidistant from the peak of the gain curve or both are sufficiently close to the peak of the gain curve, multimode (multi-wavelength) operation may occur. Then, instabilities (or "mode beats") between the two (or more) modes may occur, resulting in varying wavelength and power output. Even if stable multimode operation is achieved, the resulting power output is still significantly reduced by dividing the available gain into two (or more) modes.

[0047] If the relationship between the optical path length of the laser's cavity and the gain curve of the laser medium changes over time, such as due to changes in cavity length, due to, for example, thermal effects, then a single mode or single mode that was previously in production may lose power, and the laser may even change from single mode to multi-mode operation and / or become unstable, causing the available power of the laser to be significantly reduced. For example, even a change in cavity length of a few microns may have a large impact on the laser output power.

[0048] Thus, the laser optical cavity may, for example, use a movable optical component, such as a mirror, in the optical cavity such that moving the optical component changes the optical length of the optical cavity. The position of the optical component and the resulting cavity length is continuously jittered (slightly varied) at a relatively high rate (relative to the rate of the attenuation adjustment described above) during operation of the laser, while the duration from each pulse to the subsequent start of prelasing is measured. The position of the optical component (or a more accurate average or reference position of the optical component, such as the center position of the jitter) can then be gradually shifted in a direction that produces the shortest average duration from the pulse to the start of the subsequent prelasing. Because the shortest time until prelasing occurs is when the cavity mode of the laser is centered on the maximum (or peak) of the gain curve of the laser medium (producing stronger single-mode lasing) (all other things being equal), this method allows the laser to maintain its cavity length or have its cavity length continuously adjusted toward a length that centers the cavity mode on the peak of the gain, thereby maintaining single-mode operation of the laser at a resulting higher efficiency and power.

[0049] Figure 1 is a simplified schematic diagram of some components of an embodiment of the LPP EUV light source 110. Figure 1 As shown in FIG. 1 , EUV light source 110 includes a laser source 112 for generating a laser pulse beam and transmitting the beam from laser source 112 along one or more beam paths 113 and into chamber 114 to irradiate a corresponding one of targets 123 at irradiation location 116. Figure 1 Aspects of an example of a laser device of system 112 are shown in FIG.

[0050] Also like Figure 1 As shown in , EUV light source 110 may also include a target material delivery system 122 that delivers a target 123 into interior 115 of chamber 114 to reach irradiation site 116 , where target 123 will interact with one or more laser pulses to ultimately create plasma 124 and generate EUV light 125 .

[0051] The material of target 123 is or includes an EUV emitting material, such as, but not necessarily limited to, a material including tin, lithium, xenon, or a combination thereof. The target material may be in the form of a droplet, or alternatively may be a solid particle or a solid particle contained within a droplet. For example, elemental tin may be present as a target material as pure tin, as a tin compound (such as SnBr4, SnBr2, SnH4), as a tin alloy (e.g., a tin-gallium alloy, a tin-indium alloy, or a tin-indium-gallium alloy), or a combination thereof.

[0052] The EUV light source 110 may also include a collector 118, such as a near normal incidence collector mirror, having a reflective surface 120 in the form of an oblong ellipsoid (i.e., an ellipse rotated about its major axis), such that the optical element 118 has a first focus in or near the illumination location 116 and a second focus at a so-called intermediate focus 121, where EUV light 125 can be output from the EUV light source 110 and input to an apparatus utilizing EUV light, such as a lithography exposure apparatus ( Figure 2 ). The collector 118 is formed with an aperture 119 to allow laser pulses generated by the laser source 112 to pass through the aperture 119 along the beam path 113 and reach the irradiation site 116. To reflect EUV light, the collector surface 120 can have a graded multilayer coating having alternating layers of molybdenum and silicon, and in some cases one or more high temperature diffusion barrier layers, smoothing layers, cap layers, and / or etch stop layers. Other surface shapes can also be used for the surface 120, such as a parabola rotated about its long axis. In an embodiment, the surface 110 can be configured to deliver a beam having a circular cross-section at the intermediate focus region 121. In other embodiments, the surface 120 can utilize coatings and layers different from or in addition to the coatings and layers described above.

[0053] like Figure 1 As shown in , the EUV light source 110 may include a focusing unit 111 including one or more optical elements for focusing the laser beam to a focused light spot or a beam waist at an irradiation location 116. The EUV light source 110 may also include a beam conditioning unit 117 having one or more optical elements between the laser source 112 and the focusing unit 111 for expanding, steering and / or shaping the laser beam and / or shaping the laser pulse.

[0054] Figure 22 is a schematic diagram showing an embodiment of an EUV light source 210, such as the EUV light source 110 or another EUV source, and a lithography exposure apparatus 271. The lithography exposure apparatus 271 receives EUV light 225 generated by the EUV light source 210, and reflects the EUV light in one or more illumination mirrors 272 so as to illuminate a reflective pattern or mask 273. The EUV light reflected from the pattern or mask 273 is further reflected and reduced by one or more reduction mirrors 274, and irradiates onto a substrate or wafer 275 (or onto one or more photosensitive layers on the substrate or wafer 275, not shown) to allow a patterned structure to be formed in or on the substrate or wafer 275.

[0055] As mentioned above and again with reference to Figure 1 In some cases, EUV light source 110 uses one or more seed lasers to generate laser pulses, which can then be amplified to become laser pulses that illuminate target 123 at irradiation site 116 to form plasma 124 , which generates EUV light 125 . Figure 3 is a simplified schematic diagram of some portions of an embodiment of a seed laser module 330 that can be used as a laser source 112 in EUV, such as Figure 1 A portion of the EUV light source 110 .

[0056] like Figure 3 As shown in FIG. 1 , an exemplary embodiment of the seed laser module 330 includes two seed lasers, namely a pre-pulse seed laser 332 and a main pulse seed laser 334. In this embodiment including two seed lasers, the target 123 ( Figure 1 ) may be first irradiated by one or more pulses starting with a pre-pulse seed laser 332 and then irradiated by one or more pulses starting with a main pulse seed laser 334.

[0057] The seed lasers 332 and 334 include relatively fragile optical components inside them that are not shown in the figure, such as output couplers, polarizers, mirrors, gratings, AOMs or EOMs, etc. Therefore, it is desirable to prevent any light that may propagate back toward the seed lasers 332 and 334 (such as light reflected from the target 123 at the irradiation site 116) or light from any other source from reaching and damaging these components or otherwise interfering with the stable operation of the seed lasers 332 and 334.

[0058] exist Figure 3In an embodiment of the present invention, a respective beam 333, 335 in the form of a pulse from each seed laser 332, 334, respectively, is first passed through a respective EOM 336, 336'. The EOM 336, 336' is used together with the seed lasers 332, 334 as a pulse shaping unit to trim the pulses generated by the seed lasers into pulses having a shorter duration and a faster rise time and fall time. The shorter pulse duration and relatively faster fall time can increase the EUV light source output and efficiency because of the shorter interaction time between the pulse and the target 123, and because the trimmed unwanted portion of the pulse does not enter the downstream amplifier (not shown) and unnecessarily consume the amplifier gain of the downstream amplifier. Although two separate pulse shaping units (EOM 336, 336') are shown, alternatively, a common pulse shaping unit can be used to trim the pre-pulse seed pulse and the main pulse seed pulse.

[0059] The beam from the seed laser is then passed through the corresponding AOM 337, 337' and 338, 338'. The AOM 337, 337' and 338, 338' effectively act as a one-way gate by shunting the reverse propagating light reflected from the target 123 or elsewhere, thereby preventing the light from reaching the seed lasers 332, 334. In the embodiment shown here, the beam 333, 335 in the form of a pulse from each seed laser is passed through two AOMs respectively. Each successive AOM causes a frequency and wavelength shift in the beam passing through, and the second AOM 338, 338' on each beam path is oriented so that the shift is opposite to the first AOM 337, 337', and thus the shift of the first AOM 337, 337' is reversed. Other embodiments can use only a single AOM on each path, or even one AOM for two paths when needed.

[0060] After passing through the AOMs 337, 337' and 338, 338', the two pulses are "combined" by a beam combiner 339. Since in one embodiment the pre-pulse seed laser and the main pulse seed laser may have slightly different wavelengths, the beam combiner 339 may be a dichroic beam splitter. Since the pulses from each seed laser 332, 334 are generated at slightly different times, the two temporally separated pulses (one from each seed laser 332, 334) are placed on a common beam path 331 for further processing and use.

[0061] After being placed on the common beam path 331, the pulses from the seed laser may be passed through various components such as a preamplifier, a beam expander, a polarizer, and various redirecting and / or focusing components (not shown). Thereafter, before being passed to a laser such as Figure 1Prior to the focusing unit 111 and the target 123, the pulses may pass through an amplification system, which typically includes a plurality of amplifier stages (not shown) and such as Figure 1 The beam adjustment unit 117 is a beam adjustment unit.

[0062] Figure 4 is a simplified block diagram of a laser 434, which is Figure 3 An example of an implementation of the main pulse seed laser 332 is shown in FIG. Figure 4 In the laser 434 of FIG. 4 , a housing 426 contains a laser medium (or "gain medium") 427 maintained at sub-atmospheric pressure. Energy in the form of an oscillating electric field generated by an RF source 470 between electrodes 428, 428' can be provided to the laser medium 427. A resonant optical cavity or "resonator" is provided along an optical axis 447 by mirrors 429, 429' and a reflective grating 445 along with a movable output coupler or extraction mirror 441. A window 446 allows the beam to exit the housing 426 while retaining the laser medium 427. Lasing typically occurs when the energy added by the laser medium to a beam traveling through the resonator or optical cavity via a round trip ("laser gain") matches or exceeds the energy lost by the beam in the same trip ("resonator loss") .

[0063] A variable attenuator or Q-switch 440, which may take the form of an optical modulator such as an AOM or EOM, is controlled by a signal 440a from a control module or control system 444. In the case where an AOM is used for the Q-switch 440, the signal 440a may be an RF power level. Typically, with low or zero RF power levels applied to the AOM, low attenuation (or low resonator loss and high Q factor) is produced. With higher RF power levels applied to the AOM, higher attenuation (or higher resonator loss and lower Q factor) is produced. In the case where an EOM is used for the Q-switch 440, the signal 440a may be a voltage level. Whether the attenuation increases or decreases with the applied voltage level depends on the design or type of the EOM. The Q-switch 440 is controlled to provide attenuation (lower Q factor or higher resonator loss) to allow power to accumulate in the seed laser 434 as described above, and is then switched to provide lower attenuation or zero attenuation (higher Q factor or lower resonator loss) to Q-switched 434 the laser, thereby allowing the laser 434 to generate pulses.

[0064] The sensor 442 measures one or more parameters of the output beam 443, such as, for example, the output beam power, and provides relevant data or signals 442a to the control module 444. The control module or control system 444 uses the data or signals 442a to determine appropriate adjustments to the Q switch 440, such as the level of attenuation applied between pulses, and to determine certain appropriate adjustments to the length of the laser cavity 447. The control module or control system 444 sends a command or signal 448a to the actuator 448 to move the movable extraction mirror 441 according to the determined adjustments. The actuator 448 can be or can include a piezoelectric transducer (PZT) as a driving element. The command or signal can be a voltage level for the PZT. The actuator 448 is capable of moving the movable extraction mirror 441 over an adjustment range that includes at least 3 cavity modes.

[0065] In addition to controlling the length of the laser cavity, it is desirable to synchronously control the timing of pre-lasing. As described above, there are two factors that affect the time when pre-lasing begins. First, as described above, the lower the Q switch attenuation between pulses (the higher the Q factor), the faster the lasing threshold will be achieved and the faster pre-lasing will occur. Second, when the cavity mode is at the peak of the gain bandwidth, the effective gain will accumulate more quickly than when there are only offset modes away from the gain peak. Therefore, when a partially open Q switch is used to provide attenuation, as described above, the lasing threshold will be achieved, and pre-lasing will therefore start earlier than the time when the cavity mode is at the gain peak compared to when there are only offset modes. Having the cavity mode at the gain peak also results in maximum output power from the laser.

[0066] The cavity mode of the laser 434 can be maintained at or near a gain peak by dithering (i.e., slightly varying back and forth) the position of the mirror 441 by the control module 444 while monitoring the duration from the Q-switched pulse to the corresponding subsequent start of prelasing using, for example, the sensor 442. Then, the position of the mirror 441 along the optical axis 447 (or a more accurate average or reference position of the mirror, such as the center position of the dither) can be gradually shifted toward a direction that produces the shortest average duration from the Q-switched pulse to the corresponding subsequent start of prelasing. Because the shortest duration until prelasing occurs when the cavity mode of the laser is centered on the maximum (or peak) of the gain curve (all other things being equal), this method or process allows the laser to maintain its cavity length at a position, or continuously adjust its cavity length toward a position that centers the cavity mode at the peak of the gain, thereby maintaining single-mode operation of the laser with a resulting higher efficiency and power and a shorter time to the start of prelasing.

[0067] The control module 444 can also adjust the time toward the start of pre-lasing on a longer time scale than the mode centering process by gradually increasing the pulse-to-pulse attenuation of the Q switch 440 if the average time to start or pre-lasing is shorter than the target time, and gradually decreasing the pulse-to-pulse attenuation if the average time to start is longer than the target time. By adjusting the attenuation of the Q switch 440 at a relatively gradual rate by the control module 444, the mode centering process described above can continuously center the cavity mode at or near the gain peak during the attenuation adjustment.

[0068] Figure 5A is a graph of an embodiment of a periodically varying Q factor in arbitrary units as a function of time in nanoseconds for a Q factor from a sample such as that described above with reference to Figure 4 Laser 434 is depicted as a laser that produces Q-switched pulses.

[0069] Figure 5B is a corresponding graph of a periodically varying RF power that may be applied to the AOM, such as by control module 444, to achieve Figure 4 The Q switch 440 (when the AOM is used for the Q switch 440) to generate Figure 5A As can be seen from the various figures, the RF power to the AOM is inversely proportional to the Q factor of the laser and therefore directly proportional to the attenuation. Therefore, Figure 5B It can also be read and understood as a graph of the attenuation (arbitrary scale, as indicated by the vertical axis on the right) produced by the Q switch 440 over time.

[0070] exist Figure 5A The top of the graph is indicated and labeled Figure 5A and Figure 5B The periodic Q factor or periodic RF power or attenuation of the two stages. Figure 5A The Q factor and Figure 5B The corresponding attenuation shown in and / or the RF power to the AOM is periodic, so Figure 5A The Q factor and Figure 5B The corresponding attenuation and / or RF power to the AOM shown in FIG. 1 is repeated from pulse to pulse of the laser. Thus, as shown by Figure 5A Phase 1, indicated by the arrow at the top of the enclosed graph, begins at the end of the previous phase 2 and extends to the beginning of the subsequent phase 2. In this example, there is then a 10,000 ns (or 10 µs) gap between pulses in the various graphs as visible from the x-axis time scale, so the pulse rate shown is 100 kHz.

[0071] exist Figure 5AThe higher Q factor starts after 550a, Figure 5B The lower RF power or lower attenuation value 550b (Q-switching window) is centered at time zero, and a first Q factor 549a and a corresponding first attenuation value 549b are set in stage 1. The value of the Q factor 549a or attenuation value 549b in stage 1 is set high enough (i.e., the level at which the gain of the laser medium exceeds the cavity loss of the laser) to allow the start of lasing to occur and low enough to still provide attenuation to allow the power accumulated in the laser to exceed the power level of non-attenuated continuous lasing. This stage 1 or "inter-pulse" attenuation level 549b can be adjusted as discussed above to keep the average time of the start of pre-lasing at or near the desired target time. Next, in stage 2, a second Q factor 550a and / or a second RF power level or attenuation value 550b is set, such as at the maximum Q factor or lowest attenuation for a particular laser 434, to Q-switch the laser 434 and allow pulse generation.

[0072] Figure 5C is produced in Figure 5A and Figure 5B Graph of the output beam power (on an arbitrary scale) for a two-stage or two-level periodic decay of . Shortly after pulse 551 with a peak at time 0, the laser output drops to almost zero, although the decay in stage 1 is Figure 5A and Figure 5B is set at a value low enough to allow lasing. This is because the larger Q-switching pulse at time 0 consumes almost all of the stored energy in the laser (in the form of a population inversion in the laser medium). Only after duration D1 is sufficient energy accumulated again to cause pre-lasing (or lower level continuous lasing) with an initial peak 552 to begin and be followed by a relatively low power constant beam 553. As mentioned, in the case of higher laser efficiency resulting from the alignment of the cavity mode with the gain peak, duration D1 is relatively short, and thus duration D1 can be used to maintain single-mode operation of the laser, allowing the cavity length to be varied as needed to minimize D1 as described above. Note, however, that in a pulsed laser that repeats at a fixed frequency, duration D1 is equivalent to the period of repetition minus duration D2, so duration D2 from pre-lasing peak 552 to the peak of Q-switching pulse 551 can also be used and maximized rather than minimized to maintain single-mode operation.

[0073] As mentioned above, it is desirable to increase the power output of Q-switched lasers, particularly the power output of seed lasers in EUV light sources. Greater power from the seed laser of the EUV light source results in more amplified laser power delivered to the target in the EUV light source, and thus produces higher power of EUV emitting plasma, so that higher power EUV light can be received by the associated EUV lithography exposure equipment and by wafers being processed in the associated EUV lithography exposure equipment, so that more wafers can be exposed in less time, thereby generating substantial time savings in high-value processes.

[0074] Although it is possible to increase the power of a given seed laser (such as a laser diode) by (1) increasing the frequency of the pulses, or (2) increasing the power of each pulse, or (3) a combination of increasing the frequency of the pulses and increasing the power of each pulse, Figure 4 Lasers 434) have the potential to produce higher powers, but may be difficult to use Figure 5A and Figure 5B The two-stage periodic decay variation of in particular realizes these operations in a combined manner.

[0075] As the pulse repetition rate increases from 50 kHz to 100 kHz, such as in FIG. 5A to FIG. 5C The repetition frequency represented in , there is less time between pulses for the start of pre-lasing, and there is less time for gain recovery. In order to solve the above-mentioned problem of late pre-lasing or no pre-lasing, FIG. 5A to FIG. 5C The attenuation in stage 1 of can be gradually adjusted downward (thus adjusting the Q factor in stage 1 upward) to allow for earlier averaging to begin (lower average for duration D1 and longer average for duration D2). With lower attenuation (higher Q factor) in stage 1, a smaller amount of energy is stored and available for use by the Q-switched pulse 551. For example, the peak pulse power at 100 kHz may thus be reduced by a factor of 2 or more than the peak pulse power at 50 kHz.

[0076] It can be done by Fig. 6A and Figure 6B Periodic Q factor values ​​or attenuation values ​​of the type shown in Figure 6C The periodic laser power output power in operates the laser 434 at a higher repetition rate to produce higher peak pulse powers.

[0077] Fig. 6A is a graph of an embodiment of a periodically varying Q factor in arbitrary units as a function of time in nanoseconds for a Q factor from a sample such as that described above with reference to Figure 4 Laser 434 is depicted as a laser that produces Q-switched pulses.

[0078] Figure 6B is a corresponding graph of a periodically varying RF power that may be applied to the AOM, such as by control module 444 in the form of signal 440a, to reach Figure 4 The Q switch 440 (when the AOM is used for the Q switch 440) to generate Fig. 6A As can be seen from the various figures and as mentioned above, the RF power to the AOM becomes inversely proportional to the Q factor of the laser, and therefore directly corresponds to the attenuation. Therefore, Figure 6B It can also be read and understood as a graph (on an arbitrary scale, as indicated by the right vertical axis) of the attenuation produced by the Q switch 440 over time.

[0079] Fig. 6A and Figure 6B The periodic Q factor or periodic RF power or attenuation in three stages Fig. 6A The top of the graph is indicated and labeled 1 to 3 plus an optional intermediate stage labeled I. Figure 6C Is composed of Fig. 6A and Figure 6B Graph of the (arbitrarily scaled) output beam power produced by three stages of intermediate stage I and periodic Q factor or periodic attenuation values, where stages 1 to 3 and I are also labeled Figure 6C at the top of the .

[0080] because Fig. 6A The Q factor and Figure 6B The corresponding attenuation and / or RF power to the AOM shown in is periodic, so Fig. 6A The Q factor and Figure 6B The corresponding attenuation and / or RF power to the AOM shown in FIG. 4 repeats from pulse to pulse from the laser 434. Thus, as shown by Fig. 6A Phase 1, indicated by the arrow at the top of the enclosed graph, begins at the end of the previous phase 3 and extends to the beginning of the subsequent phase 2. Then, as described above, there are intervals of 10,000 ns (or 10 µs) between pulses as visible from the x-axis time scale in the various graphs, so the pulse rate shown is 100 kHz.

[0081] exist Fig. 6A A higher Q factor starts after 659a, or Figure 6B The lower RF power or lower attenuation value 659b (Q value switching window) is centered at time zero, and the first Q factor 654a and / or the corresponding first attenuation value 654b are set in stage 1. The value of the Q factor 654a in stage 1 ( Fig. 6A ) or attenuation value 654b ( Figure 6B) is set high enough to allow lasing 656 ( Figure 6C ) begins to occur (i.e., the level at which the gain of the laser medium exceeds the cavity losses of the laser), and can be set low enough to still provide attenuation to allow the power accumulated in the laser to exceed the power level in the unattenuated CW lasing, but this is optional, as will be explained below. The attenuation level 654b of this stage 1 ( Fig. 6A ) (or Q factor 654a ( Figure 6B )) may optionally be adjusted as discussed above to maintain the pre-laser 656 ( Figure 6C ) starts (at the initial peak 657) ( Figure 6C ) is at or near the desired target time.

[0082] Next, in stage 2, the second Q factor 658a ( Fig. 6A ) or the second attenuation value 658b ( Figure 6B ) is set to the level at which cavity losses exceed medium gain, which may even be the minimum Q factor or maximum attenuation for a given laser 434 (including until the laser is "turned off"), thereby allowing the power accumulated in the inversion population of the laser medium 427 to reach a higher level than if the laser continued to lase at an even lower level. The laser output thus goes to a substantially zero level 660 ( Figure 6C Phase 2 may last for a time that is selected to be sufficiently long, for example in the range of 100 ns to 900 ns or more, so that the stored power in the inversion population reaches a maximum or saturation value during Phase 2.

[0083] After stage 2, in stage 3, a third Q factor 659a ( Fig. 6A ) or a third attenuation value 659b such as the lowest attenuation value of attenuator 440 ( Figure 6B ) is set, thereby allowing the Q-value switching pulse 655 ( Figure 6C ) is generated. Because of the higher power level (higher small signal gain) stored in the inversion population of the laser medium 427 during phase 2, compared to that shown in FIG. 5A to FIG. 5C Compared to the method in , even with the same repetition rate, pulse 655 has both higher peak power and shorter duration.

[0084] Optionally, between the second attenuation value or Q factor and the third attenuation value or Q factor, the attenuation may be set to a sufficiently low intermediate value 661b during the intermediate phase I ( Figure 6B ) (or set to a sufficiently high intermediate Q factor of 661a ( Fig. 6A)), so that pre-lasing can start again just before the Q-value switch in stage 3 (i.e., to the level where the gain of medium 427 along cavity 447 exceeds the loss of cavity 447) to allow a brief second pre-lasing 662 ( Figure 6C ) to remove the Q-switching pulse 655 ( Figure 6C ) to avoid any potential timing jitter.

[0085] Although the Q factor or attenuation value in stage 1 is as described above for Fig. 6A and Figure 6B As described, the pulse 655 ( Figure 6C ), the laser output drops to zero or nearly zero. This is because the larger Q-switched pulse 655 consumes nearly all of the stored energy in the laser 434 (in the form of a population inversion in the laser medium 427). Only after duration D1 is enough energy accumulated again to cause the start of pre-lasing (or lower level continuous lasing) to occur at the initial peak 657 and to be followed by a relatively low power constant beam 656. As mentioned above, in the case of higher laser efficiency resulting from alignment of the cavity mode with the gain peak, duration D1 is shorter, and thus the duration D1 from the Q-switched pulse 655 to the initial pre-lasing peak 657 can be used to maintain single-mode operation of the laser, allowing the cavity length to be varied as needed to minimize D1, as described above. As discussed with respect to Figure 5C As mentioned, in a pulsed laser that repeats at a fixed frequency, duration D1 is equivalent to the repetition period minus duration D2, so duration D2 from the pre-laser peak 657 to the peak of the Q-switched pulse 655 can also be used and maximized rather than minimized to maintain single-mode operation.

[0086] and Figure 5A and Figure 5B In contrast to the periodic Q factor or attenuation value, Fig. 6A and Figure 6BIn the embodiment of the present invention, the first (stage 1) Q factor 654a or attenuation value 654b is set to produce more gain than the losses in the laser so that pre-lasing can occur without being maintained or extended until the Q-value switching now in stage 3. In contrast, before the Q-value switching, the second Q factor 658a or second attenuation value 658b is set in stage 2 to a level where the losses exceed the gain or even effectively shuts down the laser, thereby allowing the power (or gain) accumulated in the inversion population of the laser medium 427 to increase to a higher level, even up to a maximum or saturation value. Therefore, the power level available in the Q-value switching pulse 655 during stage 3 is effectively decoupled from the Q factor 654a or attenuation value 654b of stage 1.

[0087] Fig.6D A method of operating the laser 434 is described. Although the method is described in terms of setting the Q factor of the laser, it will be appreciated that the method is equally applicable to setting the attenuation of the laser. In step S10, the Q factor of the laser 434 is set to a first Q factor (e.g., Fig. 6A 654a (in stage 1)), so that the gain of laser 434 exceeds the loss of laser 434 to allow laser 434 to produce a first continuous "pre-lasing" beam 656. After the first continuous beam 656 is produced, i.e., after the beginning of the first continuous beam 656 at some time in its initial peak 657, in step S20, the Q factor is reduced to a second Q factor value (e.g., value 658a (in stage 2)). This stops the lasing of the first continuous beam 656 (as in Figure 6C ) and the concomitant consumption of power accumulated in the reverse population of the laser medium 427 of the laser 434. Next, in step S30, the Q factor is increased to a value (e.g., value 659a in stage 3) such that the laser 434 produces pulses 655.

[0088] After the laser generates a pulse 655, the process may begin to repeat by again setting the attenuation to the Q factor 654a (in a second or repetition phase 1).

[0089] In an embodiment of the method, the second attenuation value 658b or the second Q factor 658a can turn off the laser. The method can also include: after setting the attenuation to the second attenuation value 658b (in stage 2) and before setting the attenuation to the third attenuation value 659b (in stage 3), reducing the attenuation to an intermediate attenuation value 661b (in intermediate stage I), wherein the intermediate attenuation value 661b is higher than the third attenuation value 659b and is low enough to allow the laser 434 to produce the second beam 662.

[0090] In an embodiment of the method, the attenuator 440 may include an optical modulator within or connected to the laser 434. The optical modulator may be an AOM or an EOM. The laser 434 may be a CO2 laser. The laser may be a seed laser 332, 334 in an extreme ultraviolet (EUV) light source 110, such as a main pulse seed laser 334 in the EUV light source 110.

[0091] In other embodiments, setting the attenuation to the second attenuation value 658b may include setting the attenuation to the second attenuation value 658b and leaving the attenuation at the second attenuation value 658b for a time in the range of 200 s to 1000 s or 100 ns to 1000 ns. Setting the attenuation to the intermediate attenuation value 661b may include setting the attenuation to the intermediate attenuation value 661b and leaving the attenuation at the intermediate attenuation value 661b for a time in the range of 0 ns to 300 ns. Setting the attenuation to the third attenuation value 659b may include setting the attenuation to the third value 659b and leaving the attenuation at the third value 659b for a time in the range of 400 ns to 700 ns.

[0092] In other additional embodiments, the method may include monitoring a duration D1 from the first pulse 655 to the generation of the first continuous beam 656, such as represented by an initial peak 657 of the first continuous beam 656, and adjusting the cavity length of the laser to minimize the duration D1. The method may include monitoring the duration D1, and adjusting the first attenuation value 654b based on the duration D1. The third attenuation value may be a maximum attenuation value.

[0093] In another aspect of the present disclosure, reference Figure 4 , Figure 6B and Figure 6C , the laser 434 includes an optical modulator 440, which is controlled by a signal 440a applied to the optical modulator 440. The laser 434 can be operated by the following methods: setting the amplitude of the signal 440a to a first value 654b so that the laser operates in a mode in which the laser gain exceeds the resonator loss; setting the amplitude of the signal 440a to a second value 658b so that the laser 434 is turned off; and setting the amplitude of the signal 440a to a third value 659b so that the laser 434 generates a pulse 655.

[0094] In an embodiment, the laser may include an output coupler 441 having a PZT 448, and the method may include a step performed during the step of setting the amplitude of the signal 440a to a first value 654b using the output of the laser to control the voltage 448a applied to the piezoelectric transducer 448. The optical modulator 440 may include or may be in the form of an AOM or an EOM.

[0095] In other aspects, a system for generating pulses of laser radiation includes: a laser 434, including an optical modulator 440, which is controlled by a signal 440a applied to the optical modulator 440; and a control system 444, which is configured and adapted to sequentially set the amplitude of the signal 440a to a first value 654b, so that the laser 434 operates in a mode in which the laser gain exceeds the resonator loss, then set the amplitude of the signal 440a to a second value 658b, so that the laser 434 is turned off, and then set the amplitude of the signal 440a to a third value 659b, so that the laser 434 generates pulses.

[0096] In an embodiment of the system, the laser 434 may include an output coupler 441 having a PZT 448, and wherein the control system 444 is further configured and adapted to use the output of the laser 434 to control a voltage 448a applied to the PZT 448 when the signal 440a is at the first value 654b.

[0097] In additional aspects and with reference to Figure 4 and Fig. 6A , a laser system, comprising: a laser 434 having a laser cavity 427; an optical modulator 440, the optical modulator 440 being configured to control a Q factor of the laser cavity 447; a power sensor 442, the power sensor 442 being positioned outside the laser cavity 427 and being configured to detect a power level of radiation emitted from the laser 434 and to generate power level data and / or a signal 442a related to the power level of the radiation emitted from the laser 434; and a control system 444, the control system 444 being connected to receive a power level signal. The control system 444 is configured to: (1) set the Q factor of the cavity 427 of the laser 434 to a first value 654a that is high enough to allow lasing to occur, (2) at a time after lasing occurs, set the Q factor of the cavity 427 to a second value 658a that is low enough to prevent lasing from occurring, and (3) after the Q factor of the cavity 427 is set to the second value 658a, set the Q factor of the cavity 427 to a third value 659a so that the laser 434 emits a pulse.

[0098] Fig. 7A and Figure 7B Reference Fig. 6A , Figure 6B and Figure 6C Graphs of additional embodiments and variations of the described methods. Fig. 7A The variation(s) of the Q factor over time are shown. Figure 7B The resulting laser output power is shown over time.

[0099] refer to Fig. 7A and Figure 7B As shown, each of stages 1 to 3 and I can have a different Q factor (and corresponding attenuation). It is not required that the Q factors of any stage match each other. In addition, the Q factor of stage 2 (for energy storage) does not need to be zero (nor does it need to be 100% of the corresponding attenuation). Because the Q-value switching pulse 767 ( Figure 7B ) The power available from the first Q factor in stage 1 is 761 ( Fig. 7A ) decoupling (as above combined Figure 6B and Figure 6C ), and thus also from the timing of the start of the pre-laser, such as by setting an earlier target time for the start (initial peak 762) of the pre-laser continuous beam 763, or even setting the first Q factor 761 equal to the third Q factor 764 in stage 3 (a higher Q factor with lower or zero attenuation during Q-value switching), as indicated by the first Q factor 764. Fig. 7A As shown by the alternating dotted values ​​in , the first Q factor 761 in stage 1 can be set to be relatively high.

[0100] Increasing the first Q factor 761 can have the effect of producing an earlier and less time jittered start 762 of the pre-laser beam 763, thereby giving more time in the inter-pulse interval, such as during the second Q factor 765 of stage 2, for energy storage when necessary or desired. More time in the inter-pulse interval can also be used in the intermediate stage I when necessary to ensure that a "second pre-laser" or continuous beam 768 can be reliably produced during the intermediate Q factor 766 and before the Q-switched pulse 767 to minimize the time jitter of the Q-switched pulse 767. The intermediate Q factor 766 can also be set lower than the typical lasing threshold and still allow the second pre-laser or continuous beam 768 to be produced, because the laser 434 is at this point in time approaching the lasing threshold from an energy saturated state rather than from an energy depleted state, thereby providing a higher initial gain and allowing lasing to start more easily (and with a lower degree of time jitter). In such an embodiment, a shift in the terms may even be appropriate, with the first pre-fire laser 763 being relabeled as "post-fire laser" (immediately following the Q-switched pulse to allow cavity length and cavity mode optimization), and with the second pre-fire laser 768 being simply relabeled as "pre-fire laser" (immediately preceding the Q-switched pulse to absorb any timing jitter).

[0101] The embodiments may be further described using the following aspects:

[0102] 1. A method of operating a laser, the method comprising:

[0103] After the laser generates a first pulse, setting the attenuation of an attenuator in the laser to a first attenuation value so that the gain of the laser exceeds the loss of the laser to allow the laser to generate a first continuous beam;

[0104] After generating the first continuous beam, increasing the attenuation of the attenuator to a second attenuation value so that the loss of the laser exceeds the gain of the laser; and

[0105] After increasing the attenuation to the second value, the attenuation of the attenuator is reduced to a third attenuation value to cause the laser to generate a second pulse.

[0106] 2. The method according to aspect 1, further comprising: after the laser generates the second pulse, setting the attenuation to the first attenuation value.

[0107] 3. The method of clause 1, wherein the second value turns the laser off.

[0108] 4. The method as described in aspect 1 also includes: after setting the attenuation to the second attenuation value and before setting the attenuation to the third attenuation value, reducing the attenuation to an intermediate attenuation value, the intermediate attenuation value being higher than the third attenuation value and low enough to allow the laser to produce a second beam.

[0109] 5. The method of clause 1, wherein the attenuator comprises an optical modulator within or connected to the laser.

[0110] 6. The method of clause 1, wherein the attenuator comprises an acousto-optic modulator (AOM) within or connected to the laser.

[0111] 7. A method as described in aspect 1, wherein the attenuator includes an acousto-optic modulator (AOM) within or connected to the laser, and wherein setting the attenuation of the attenuator includes: setting the RF power level supplied to the AOM, increasing the attenuation of the attenuator includes: increasing the RF power supplied to the AOM, and reducing the attenuation of the attenuator includes: reducing the RF power supplied to the AOM.

[0112] 8. The method of clause 1, wherein the attenuator comprises an electro-optical modulator (EOM) within or connected to the laser.

[0113] 9. The method of clause 1, wherein the laser is a CO2 laser.

[0114] 10. The method of clause 1, wherein the laser is a seed laser in an extreme ultraviolet (EUV) light source.

[0115] 11. The method of clause 1, wherein the laser is a main pulse seed laser in an EUV light source.

[0116] 12. The method of clause 1, wherein setting the attenuation to the second attenuation value comprises setting the attenuation to the second attenuation value and maintaining the attenuation at the second attenuation value for a time in a range of 100 nanoseconds to 1000 nanoseconds (ns).

[0117] 13. The method of clause 1, wherein setting the attenuation to the intermediate attenuation value comprises setting the attenuation to the intermediate attenuation value and leaving the attenuation at the intermediate attenuation value for a time in the range of 0 ns to 300 ns.

[0118] 14. The method of clause 1, wherein setting the attenuation to the third attenuation value comprises setting the attenuation to the third value and maintaining the attenuation at the third value for a time period in the range of 400 ns to 700 ns.

[0119] 15. The method of clause 1, further comprising: monitoring a duration from the first pulse to the generation of the first continuous beam, and adjusting a cavity length of the laser to minimize the duration.

[0120] 16. The method of clause 1, further comprising: monitoring a duration between the first pulse and the generating of the first continuous beam, and adjusting the first attenuation value based on the duration.

[0121] 17. The method of clause 1, wherein the third attenuation value is a maximum attenuation value.

[0122] 18. The method of clause 1, wherein the first attenuation value is equal to a third attenuation value.

[0123] 19. A method as described in aspect 1, wherein (1) the step of increasing the attenuation of the attenuator to a second attenuation value so that the loss of the laser exceeds the gain of the laser, and (2) the step of reducing the attenuation of the attenuator to a third attenuation value after increasing the attenuation to the second value so that the laser generates a second pulse includes: Q-value switching of the laser.

[0124] 20. A method of operating a laser, the laser comprising an optical modulator, the optical modulator being controlled by a signal applied to the optical modulator, the method comprising:

[0125] setting the amplitude of the signal to a first value to cause the laser to operate in a mode in which laser gain exceeds resonator loss;

[0126] setting the amplitude of the signal to a second value so that the laser is turned off; and

[0127] The amplitude of the signal is set to a third value to cause the laser to pulse.

[0128] 21. The method of clause 20, wherein the laser comprises an output coupler having a piezoelectric transducer, and the method further comprises: using the output of the laser to control a voltage applied to the piezoelectric transducer during the step of setting the amplitude of the signal to a first value.

[0129] 22. The method of clause 20, wherein the optical modulator comprises an acousto-optic modulator (AOM).

[0130] 23. The method of clause 20, wherein the optical modulator comprises an acousto-optic modulator (AOM) and the signal comprises an RF power level.

[0131] 24. The method of clause 20, wherein the optical modulator comprises an electro-optic modulator (EOM).

[0132] 25. The method of clause 20, wherein (1) setting the amplitude of the signal to a second value so that the laser is turned off, and (2) setting the amplitude of the signal to a third value so that the laser generates a pulse comprises: performing Q-value switching on the laser.

[0133] 26. A system for generating pulses of laser radiation, the system comprising:

[0134] a laser comprising an optical modulator controlled by a signal applied to the optical modulator; and

[0135] A control system configured and adapted to sequentially: set the amplitude of the signal to a first value so that the laser operates in a mode in which the laser gain exceeds the resonator loss, then set the amplitude of the signal to a second value so that the laser is turned off, and then set the amplitude of the signal to a third value so that the laser generates a pulse.

[0136] 27. The system of aspect 26, wherein the laser comprises an output coupler having a piezoelectric transducer, and wherein the control system is further configured and adapted to control a voltage applied to the piezoelectric transducer using an output of the laser when the signal is at the first value.

[0137] 28. The system of clause 26, wherein the optical modulator comprises an acousto-optic modulator (AOM).

[0138] 29. The system of clause 26, wherein the optical modulator comprises an acousto-optic modulator (AOM) and the signal applied to the optical modulator comprises an RF power level.

[0139] 30. The system of clause 26, wherein the optical modulator comprises an electro-optical modulator (EOM).

[0140] 31. The system of aspect 26, wherein the control system is configured and adapted to perform Q-value switching.

[0141] 32. A laser system comprising:

[0142] a laser having a laser cavity;

[0143] an optical modulator configured to control a Q factor of the laser cavity;

[0144] a power sensor positioned outside the laser cavity and configured to detect a power level of radiation emitted from the laser and to generate power level data and / or a signal related to the power level of radiation emitted from the laser; and

[0145] a control system connected to receive the power level data or signal and to control the optical modulator, the control system being configured to: (1) set the Q factor of the cavity of the laser to a first value that is sufficiently high to allow lasing to occur, (2) at a time after lasing is detected by the power sensor, set the Q factor of the cavity to a second value that is less than the second value and sufficiently low to prevent lasing from occurring, and (3) after setting the Q factor of the cavity to the second value, set the Q factor of the cavity to a third value to cause the laser to emit a pulse.

[0146] 33. The laser system of clause 32, wherein the control system is configured to perform Q-value switching.

[0147] The embodiments described above and other embodiments are within the scope of the following claims.

Claims

1. A method of operating a laser, the method comprising: After the laser generates a first pulse, setting the attenuation of an attenuator in the laser to a first attenuation value so that the gain of the laser exceeds the loss of the laser to allow the laser to generate a first continuous beam; After producing the first continuous beam, increasing the attenuation of the attenuator to a second attenuation value so that the loss of the laser exceeds the gain of the laser; as well as After increasing the attenuation to the second value, the attenuation of the attenuator is reduced to a third attenuation value to cause the laser to generate a second pulse.

2. The method of claim 1, further comprising: After the laser generates the second pulse, the attenuation is set to the first attenuation value.

3. The method of claim 1, wherein: The second value turns the laser off.

4. The method of claim 1, further comprising: After setting the attenuation to the second attenuation value and before setting the attenuation to the third attenuation value, the attenuation is reduced to an intermediate attenuation value that is higher than the third attenuation value and low enough to allow the laser to produce a second beam.

5. The method of claim 1, wherein: The attenuator comprises an optical modulator within the laser or connected to the laser.

6. The method of claim 1, wherein: The attenuator includes an acousto-optic modulator (AOM) within or connected to the laser.

7. The method of claim 1, wherein: The attenuator comprises an acousto-optic modulator (AOM) within or connected to the laser, and wherein setting the attenuation of the attenuator comprises setting an RF power level supplied to the AOM, increasing the attenuation of the attenuator comprises increasing the RF power supplied to the AOM, and decreasing the attenuation of the attenuator comprises decreasing the RF power supplied to the AOM.

8. The method of claim 1, wherein: The attenuator comprises an electro-optical modulator (EOM) within or connected to the laser.

9. The method of claim 1, wherein: The laser is a CO2 laser.

10. The method of claim 1, wherein: The laser is a seed laser in an extreme ultraviolet (EUV) light source.

11. The method of claim 1, wherein: The laser is a main pulse seed laser in an EUV light source.

12. The method of claim 1, wherein: Setting the attenuation to the second attenuation value includes setting the attenuation to the second attenuation value and maintaining the attenuation at the second attenuation value for a time in a range of 100 nanoseconds to 1000 nanoseconds (ns).

13. The method of claim 1, wherein: Setting the attenuation to the intermediate attenuation value includes setting the attenuation to the intermediate attenuation value and leaving the attenuation at the intermediate attenuation value for a time in the range of 0 ns to 300 ns.

14. The method of claim 1, wherein: Setting the attenuation to the third attenuation value includes setting the attenuation to the third value and leaving the attenuation at the third value for a time period in the range of 400 ns to 700 ns.

15. The method of claim 1, further comprising: A duration from the first pulse to the generation of the first continuous beam is monitored, and a cavity length of the laser is adjusted to minimize the duration.

16. The method of claim 1, further comprising: A duration between the first pulse and the generating of the first continuous beam is monitored, and the first attenuation value is adjusted based on the duration.

17. The method of claim 1, wherein: The third attenuation value is a maximum attenuation value.

18. The method of claim 1, wherein: The first attenuation value is equal to the third attenuation value.

19. The method of claim 1, wherein: The steps of (1) increasing the attenuation of the attenuator to a second attenuation value so that the loss of the laser exceeds the gain of the laser, and (2) reducing the attenuation of the attenuator to a third attenuation value after increasing the attenuation to the second value so that the laser generates a second pulse include: performing Q value switching on the laser.

20. A method of operating a laser, the laser comprising an optical modulator, the optical modulator being controlled by a signal applied to the optical modulator, the method comprising: setting the amplitude of the signal to a first value to cause the laser to operate in a mode in which laser gain exceeds resonator loss; setting the amplitude of the signal to a second value so that the laser is turned off; as well as The amplitude of the signal is set to a third value to cause the laser to pulse.

21. The method of claim 20, wherein: The laser includes an output coupler having a piezoelectric transducer, and the method further includes using the output of the laser to control a voltage applied to the piezoelectric transducer during the step of setting the amplitude of the signal to a first value.

22. The method of claim 20, wherein: The optical modulator includes an acousto-optic modulator (AOM).

23. The method of claim 20, wherein: The optical modulator comprises an acousto-optic modulator (AOM), and the signal comprises an RF power level.

24. The method of claim 20, wherein: The optical modulator includes an electro-optic modulator (EOM).

25. The method of claim 20, wherein: The steps of (1) setting the amplitude of the signal to a second value so that the laser is turned off, and (2) setting the amplitude of the signal to a third value so that the laser generates a pulse include: performing Q value switching on the laser.

26. A system for generating pulses of laser radiation, the system comprising: a laser comprising an optical modulator controlled by a signal applied to the optical modulator; as well as A control system configured and adapted to sequentially: set the amplitude of the signal to a first value so that the laser operates in a mode in which the laser gain exceeds the resonator loss, then set the amplitude of the signal to a second value so that the laser is turned off, and then set the amplitude of the signal to a third value so that the laser generates a pulse.

27. The system of claim 26, wherein: The laser comprises an output coupler having a piezoelectric transducer, and wherein the control system is further configured and adapted to control a voltage applied to the piezoelectric transducer using an output of the laser when the signal is at the first value.

28. The system of claim 26, wherein: The optical modulator includes an acousto-optic modulator (AOM).

29. The system of claim 26, wherein: The optical modulator comprises an acousto-optic modulator (AOM), and the signal applied to the optical modulator comprises an RF power level.

30. The system of claim 26, wherein: The optical modulator includes an electro-optic modulator (EOM).

31. The system of claim 26, wherein: The control system is configured and adapted to perform Q-value switching.

32. A laser system comprising: a laser having a laser cavity; an optical modulator configured to control a Q factor of the laser cavity; a power sensor positioned outside the laser cavity and configured to detect a power level of radiation emitted from the laser and to generate power level data and / or a signal related to the power level of radiation emitted from the laser; as well as a control system connected to receive the power level data or signal and to control the optical modulator, the control system being configured to: (1) set the Q factor of the cavity of the laser to a first value that is sufficiently high to allow lasing to occur, (2) at a time after lasing is detected by the power sensor, set the Q factor of the cavity to a second value that is less than the second value and sufficiently low to prevent lasing from occurring, and (3) after setting the Q factor of the cavity to the second value, set the Q factor of the cavity to a third value to cause the laser to emit a pulse.

33. The laser system of claim 32, wherein: The control system is configured to perform Q-value switching.

Images