Pulse Laser and Operating Method

Abstract

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

Description

Technical Field

[0001] Cross - reference to Related Applications

[0001] This application claims the priority of U.S. Patent Application No. 63 / 346,645, filed on May 27, 2022, with the title "Pulse Laser and Method of Operation", the entire disclosure of which is incorporated herein by reference.

[0002]

[0002] The present invention generally relates to a pulse laser and a method of operating a pulse laser, and more particularly, to a Q - switched seed laser in an extreme ultraviolet light source and a method of operating the same.

Background Art

[0003]

[0003] In the semiconductor industry, the development of lithography techniques for printing smaller integrated circuit dimensions continues. Using light with a shorter wavelength, such as extreme ultraviolet ( "EUV") light (also referred to as soft x - rays and generally defined as electromagnetic radiation having a wavelength in the range of 10 - 120 nanometers (nm)), smaller features than those achievable with longer wavelengths can be realized.

[0004]

[0004] Currently, EUV lithography generally employs EUV light with a wavelength in the range of 10 - 14 nanometers (nm) to generate small features such as 10 nm or 7 nm on or within a substrate such as a silicon wafer. For commercial utility, systems that generate these very small features are desirably highly reliable, provide cost - effective throughput, and have a reasonable process tolerance.

[0005]

[0005] Methods for generating EUV light are not necessarily limited, but include converting a material having one or more elements, such as xenon, lithium, tin, indium, antimony, tellurium, aluminum, etc., having one or more emission lines in the EUV range, into a plasma state. In one such method, often called laser-produced plasma (“LPP”), the required plasma can be generated by irradiating a target material, such as droplets, a stream, or clusters of a material capable of generating the desired line emission, with a laser beam at the irradiation site. The line emission material can be an element and can be delivered to the irradiation site in the form of a pure element or an alloy (e.g., in the form of an alloy that is liquid at the desired temperature), or can be something that can be mixed and dispersed with another material, such as a liquid.

[0006]

[0006] In some LPP systems, in the target stream, each target is irradiated by a respective laser pulse, and a plasma is formed from each target. Alternatively, each target may be sequentially illuminated by two or more light pulses. In some cases, each target is exposed to a so-called “pre-pulse” to heat, expand, gasify, evaporate, and / or ionize the target material and / or generate a weak plasma, and then exposed to a so-called “main pulse” to generate a strong plasma and convert most or all of the material affected by the pre-pulse into a plasma, thereby generating EUV light. Of course, the functions of the pre-pulse and the main pulse can overlap to some extent.

[0007]

[0007] In the LPP system, since the EUV output power generally balances the laser power irradiating the target material, a laser pulse with high power is required. On the other hand, in order to synchronize with the target to be irradiated with the pulse so that the pulse and the target encounter at the desired irradiation site at the desired timing, precise pulse timing is required. In the pulse used to irradiate the target, if there are needs for both high power and precise control, it may be useful to adopt a configuration including a relatively low-power "seed laser" and one or more amplifiers for amplifying the pulse from this seed laser. By using one or more amplifiers, it becomes possible to perform precise timing control using a seed laser that is appropriately controlled and has relatively low power, while amplification enables the provision of a pulse with relatively high power to the LPP process.

[0008]

[0008] However, even when using a seed laser and an amplifier, it is more desirable to generate a relatively high-power seed laser so that the need for amplification is reduced, or so that a pulse with higher power can be generated from the same amplifier or amplifier system.

SUMMARY OF THE INVENTION

[0009]

[0009] 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 and enables 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. The implementation of this method may further include setting the attenuation to the first attenuation value after the laser generates the second pulse. In implementation, the second value may shutter the laser.

[0010]

[0010] The implementation of the above method may include one or more of the following. After setting the attenuation to a second attenuation value and before setting the attenuation to a third attenuation value, reducing the attenuation to an intermediate attenuation value that is higher than the third attenuation value and low enough to enable the laser to generate a second beam. The attenuator may be inside the laser, or an optical modulator connected to the laser, or may include the optical modulator. The attenuator may be an acousto-optic modulator (AOM) inside the laser or connected to the laser, or may include the AOM. 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 an electro-optic modulator (EOM) inside the laser or connected to the laser, or may include the EOM. 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.

[0011]

[0011] Setting the attenuation to the second attenuation value may include setting the attenuation to the second attenuation value over a time range of 100 to 1000 nanoseconds (ns). Setting the attenuation to the intermediate attenuation value may include setting the attenuation to the intermediate attenuation value over a time range of 0 to 300 ns. Setting the attenuation to the third attenuation value may include setting the attenuation to the third value over a duration in the range of 400 to 700 ns.

[0012]

[0012] The implementation of the above method may include one or more of the following. Monitoring the 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 the duration between the first pulse and the generation of the first continuous beam and adjusting the first attenuation value based on the duration.

[0013]

[0013] The third attenuation value can be the 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 such that the loss of the laser exceeds the gain of the laser, and (2) after increasing the attenuation to the second value, decreasing the attenuation of the attenuator to the third attenuation value such that the laser generates a second pulse may be or may include Q-switching the laser.

[0014]

[0014] In a further general aspect, a method of operating a laser including an optical modulator controlled by an applied signal includes (1) setting the magnitude of the signal to a first value such that the laser operates in a mode where the laser gain exceeds the resonator loss, and (2) setting the magnitude of the signal to a second value such that the laser is closed, and setting the magnitude of the signal to a third value such that the laser generates a pulse.

[0015]

[0015] Implementing the above 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 controlling the voltage applied to the piezoelectric transducer using the output of the laser during the step of setting the magnitude of the signal to the first value.

[0016]

[0016] The optical modulator may be or may include an acousto-optic modulator (AOM). The signal may be the RF power level applied to the AOM. The optical modulator may be or may include an electro-optic modulator (EOM). (1) Setting the magnitude of the signal to a second value such that the laser is closed, and (2) setting the magnitude of the signal to a third value such that the laser generates a pulse may be or may include Q-switching the laser.

[0017]

[0017] In another general aspect, a system for generating pulses of laser radiation includes: (1) a laser including an optical modulator controlled by an applied signal; and (2) a control system configured and adapted to sequentially set the magnitude of the signal to a first value such that the laser operates in a mode where the laser gain exceeds the resonator loss, then set the magnitude of the signal to a second value such that the laser is closed, and then set the magnitude of the signal to a third value such that the laser generates a pulse.

[0018]

[0018] The implementation of the above 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 further configured and adapted to use the output of the laser when the signal is at the first value to control the voltage applied to the piezoelectric transducer.

[0019]

[0019] The optical modulator may be an acousto-optic modulator (AOM) and may include the AOM. The signal applied to the AOM may be an RF power level. The optical modulator may be an electro-optic modulator (EOM) and may include the EOM. The control system may be configured and adapted to perform Q-switching or may be configured and adapted to perform Q-switching.

[0020]

[0020] In another general aspect, a laser system includes: (a) a laser having a laser cavity; (b) an optical modulator configured to control the Q value of the laser cavity; (c) a power sensor positioned outside the laser cavity and configured to detect the power level of the radiation emitted from the laser and generate power level data and / or a signal regarding the power level of the radiation emitted from the laser; and (d) a control system connected to receive the power level data or signal and configured to control the optical modulator, the control system being configured to: (1) set the Q value of the laser cavity to a first value high enough to cause lasing; (2) set the Q value of the cavity to a second value lower than the first value and low enough to stop lasing at a timing after lasing is detected by the power sensor; and (3) after setting the Q value of the cavity to the second value, set the Q value of the cavity to a third value such that the laser emits a pulse. The control system is configured to perform, or may be configured to perform, Q-switching.

[0021]

[0021] One or more embodiments will be described in detail in the accompanying drawings and the following description. Other features will be apparent from the following description, the drawings, and the claims.

Brief Description of the Drawings

[0022]

Figure 1

[0022] A diagram showing an aspect of an extreme ultraviolet (EUV) light source.

Figure 2

[0023] A diagram showing an EUV light source together with a lithographic exposure apparatus.

Figure 3

[0024] A diagram showing an aspect of a seed laser module useful in an EUV light source.

Figure 4

[0025] A diagram showing a laser useful as a seed laser in an EUV light source.

Figure 5A

[0026] A graph showing the implementation of a periodically varying Q value for generating periodic Q - switched pulses from a laser such as a seed laser in an EUV light source.

Figure 5B

[0027] A graph showing a periodically varying RF power level that can be applied to an AOM to generate a periodically varying Q value as shown in FIG. 5A.

Figure 5C

[0028] A graph showing a periodically varying laser output power that can be generated by the periodically varying Q value of FIG. 5A and / or the periodically varying RF power of FIG. 5B.

Figure 6A

[0029] A graph showing the implementation of a periodically varying Q value for generating periodic Q - switched pulses with an increased power output from a laser such as a seed laser in an EUV light source.

Figure 6B

[0030] A graph showing a periodically varying RF power level that can be applied to an AOM to generate a periodically varying Q value as shown in FIG. 6A.

Figure 6C

[0031] A graph showing a periodically varying laser output power that can be generated by the periodically varying Q value of FIG. 6A and / or the periodically varying RF power of FIG. 6B.

Figure 6D

[0032] A flowchart explaining the steps of a process for varying a Q value according to an aspect of an embodiment.

Figure 7A

[0033] A graph showing a further implementation of a periodically varying Q value for generating periodic Q - switched pulses with an increased peak power output from a laser such as a seed laser in an EUV light source.

Figure 7B

[0034] A graph showing a periodically varying laser output power that can be generated by the periodically varying Q value of FIG. 7A.

DETAILED DESCRIPTION OF THE INVENTION

[0023]

[0035] The methods, apparatuses, and systems of the present disclosure involve the operation of a Q - switched laser. Q - switching, also known as giant - pulse formation, is a well - known technique for both controlling a laser to operate in a pulsed mode and increasing the peak power of the laser. In Q - switching, by using a beam blocker or a beam diverter to perform switching to form pulses, etc., pulses with much higher peak power than those formed from a continuous beam can be generated compared to the peak power of the pulses formed from a continuous beam.

[0024]

[0036] Q - switching is generally achieved by placing a certain type of variable attenuator within the optical cavity of the laser (the "Q - switch"). The Q - switch functions as a type of shutter and can be, for example, an acousto - optic modulator (AOM) or an electro - optic modulator (EOM) that can be adjusted by applying a control signal to pass different amounts of incident light. In basic Q - switching, the Q - switch is initially closed, i.e., set to block almost or all light, preventing laser lasing and thereby allowing the energy stored in the form of an inverted population in the laser medium to increase beyond the levels reachable during continuous lasing. Then, when the Q - switch is quickly opened, almost all of the energy stored in the laser medium can be very quickly released within a relatively short pulse.

[0025]

[0037] For example, when using Q - switching, a laser can generate pulses with a length of 1 / 2 microseconds (μs) each at a rate within the range of 50,000 - 100,000 times per second (50 - 100 kHz), so that power can be accumulated during the approximately 10 - 20 μs between pulses. In this way, a laser that can generate a power of 50 watts in continuous lasing, for example, can generate pulses with a peak power of 500 - 1 kW.

[0026]

[0038] Q - switching as described above may face the problem of timing variability. When the Q - switch is opened and the Q - switch pulse becomes available for emission, there is a statistical uncertainty as to when the first photon (first light) will begin to be emitted along the optical path within the laser cavity. Therefore, the exact timing of the Q - switch pulse itself is slightly variable and not as predictable as desired. For example, when the Q - switch is opened, there may be little or no energy emitted by the laser between 100 - 200 nanoseconds (ns), or sometimes even up to 400 ns. Such variability in the start timing of the Q - switch pulse is sometimes known as "temporal jitter", but it is not a shutter problem. This is because the timing of the operation of the Q - switch shutter effect of the AOM or EOM is not significantly variable, for example. Nevertheless, the start timing of the lasing is variable.

[0027]

[0039] One improvement to Q - switching is the use of "pre - lasing", that is, continuously lasing the laser at a low level before Q - switching. Generally, to enable pre - lasing, the Q - switch (attenuator) is set to provide partial attenuation of the laser energy without completely "closing" it during the time between pulses (pulse - to - pulse interval). The occurrence of pre - lasing after the Q - switch pulse also faces the problem of temporal jitter, but if pre - lasing has already occurred when the Q - switch is widely open (i.e., when the attenuation of the attenuator has decreased to zero or a low value), a large Q - switch pulse will occur substantially promptly without significant temporal jitter. Therefore, using pre - lasing makes the timing of the Q - switch pulse much more predictable than in the case of normal Q - switching.

[0028]

[0040] In Q-switching using pre-lasing, the amount of partial attenuation present after the pulse determines the average level of the stored power required in the laser before the next pre-lasing starts. Therefore, the smaller the attenuation by the Q-switch, generally, the earlier the pre-lasing starts after the previous Q-switch pulse.

[0029]

[0041] Between pulses, the Q-switch or attenuator is set to a level that does not use too much power during pre-lasing (i.e., a relatively high attenuation level), and it is ideal to be able to accumulate as much as possible with a small signal gain for use in the Q-switch pulse.

[0030]

[0042] Despite using relatively low attenuation for pre-lasing between pulses, the timing improvement brought about by Q-switching using pre-lasing is accompanied by a reduction in power in the Q-switch pulse. For example, in normal Q-switching, if a laser can generate a 1 kW pulse, when pre-lasing is used, it may only be able to generate about 500 watts or less.

[0031]

[0043] If the onset of pre-lasing occurs too early, the laser gain (and the peak power of the generated pulses) will be lower, because the overall peak power accumulated during pre-lasing is lower due to the lower attenuation level that generates earlier pre-lasing. Therefore, in order to obtain a higher peak pulse power, a higher inter-pulse attenuation level that tends to delay the onset of pre-lasing is desirable. However, if the onset of pre-lasing occurs too late, in some cases, pre-lasing may not occur before the Q-switch is fully opened, resulting in pulses with a timing shift, or, in extreme cases, only weak pulses may be generated while the Q-switch is open, or no pulses may be generated at all. To avoid these problems, the time from the Q-switch pulse to the onset of subsequent pre-lasing is monitored, and if pre-lasing occurs earlier than the target time on average, the attenuation of the Q-switch between pulses is gradually increased, and if pre-lasing occurs later than the target time on average, the attenuation of the Q-switch between pulses can be gradually decreased.

[0032]

[0044] Separate from and not solved by Q-switching or pre-lasing, there may be known a problem of "mode instability". In a given laser medium, the wavelength band in which optical amplification can occur can be expressed as the "amplification band" or "gain band" or "gain profile", which is specific to the laser medium. A laser cavity with a given laser medium has a number of possible "cavity modes", that is, frequencies whose wavelengths are evenly divided by the optical path length of the cavity (the "resonance frequency") and are within the amplification band of the laser medium. Therefore, the specific cavity modes available depend on the optical path length of the laser cavity and the amplification characteristics of the laser medium.

[0033]

[0045] When a certain cavity mode hits the peak of the gain profile of the laser medium or is near this peak, that mode will dominate the laser emission by excluding other modes, and the laser will operate in a "single longitudinal mode", that is, generally with high efficiency and in a state with a stable and consistent wavelength and power output. However, if two cavity modes are equidistant from the peak of the gain profile or both are sufficiently close to this peak, multimode (multi-wavelength) operation can occur. Subsequently, instability (or "mode beating") between the two (or more) modes can occur, which may cause fluctuating wavelengths and power outputs. Even if stable multimode operation is achieved, the available gain is split into two (or more) modes, and as a result, the resulting power output can be significantly reduced.

[0034]

[0046] When the relationship between the optical path length of the laser cavity and the gain profile of the laser medium changes over time, for example, due to a change in the cavity length caused by thermal effects, the single mode that was operating during the previous operation loses power, and the laser may change from single-mode to multimode operation and / or become unstable, thereby significantly reducing the available power of the laser. Even a change in the cavity length of a few microns can, for example, have a significant impact on the laser output power.

[0035]

[0047] Therefore, the laser optical cavity may employ a movable optical component such as a mirror in the optical cavity, and the optical path length in the optical cavity may be changed by moving the optical component. The position of the optical component and the resulting cavity length can be continuously dithered (slightly varied) at a relatively high speed (compared to the speed in the above-described attenuation adjustment) during the operation of the laser, while the duration from each pulse to the generation of the next pre-pulsing is measured. The position of the optical component (or more precisely, the average position or reference position of the optical component, such as the center position of the dithering) can then be gradually shifted in the direction that results in the shortest average duration from the pulse to the generation of the next pre-pulsing. The shortest time to pre-pulsing occurs when (assuming other conditions are the same) the laser cavity mode is centered on the maximum (or peak) of the gain profile of the laser medium (generating strong single-mode lasing). Thus, by this method, it is possible to maintain the laser cavity length at a length where the cavity mode is centered on the gain peak, or to continuously adjust the laser cavity length towards the length where the cavity mode is centered on the gain peak to maintain the single-mode operation of the laser, resulting in high efficiency and power.

[0036]

[0048] FIG. 1 is a simplified schematic diagram showing some components of an embodiment of an LPP EUV light source 110. As shown in FIG. 1, the EUV light source 110 includes a laser source 112 that generates a beam of laser pulses and conveys the beam from the laser source 112 into the chamber 114 along one or more beam paths 113 to illuminate each of the targets 123 at the irradiation site 116. Examples of aspects of the laser configuration that may be suitable for use in the system 112 shown in FIG. 1 will be described in more detail below.

[0037]

[0049] As further shown in FIG. 1, the EUV light source 110 can also include a target material transport system 122 that transports the target 123 to the irradiation site 116 inside the chamber 114. At the irradiation site 116, the target 123 interacts with one or more laser pulses, ultimately generating a plasma 124 and generating EUV light 125.

[0038]

[0050] The material of the target 123 is, but not necessarily limited to, an EUV-emitting material such as a material containing tin, lithium, xenon, or a combination thereof, or includes this EUV-emitting material. The target material may be in the form of droplets, or may be solid particles, or solid particles contained within the droplets. For example, elemental tin can exist as a target material in the form of pure tin, or a tin compound such as SnBr4, SnBr2, SnH4, or a tin alloy such as a tin-gallium alloy, a tin-indium alloy, or a tin-gallium-indium alloy, or a combination thereof.

[0039]

[0051] The EUV light source 110 can also include a collector 118 such as a near-normal incidence condenser mirror having a reflecting surface 120 in the form of a prolate spheroid (an ellipse rotated about its major axis). The optical element 118 has a first focus within or near the irradiation site 116 and a second focus at a so-called intermediate focus 121. At the intermediate focus 121, EUV light 125 is output from the EUV light source 110 and can be input to a device that uses EUV light, such as a lithography exposure apparatus (shown in FIG. 2). The collector 118 is formed with an aperture 119 that allows a laser light pulse generated by the laser source 112 to pass 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 optionally one or more high-temperature diffusion barrier layers, smoothing layers, cap layers, and / or etch stop layers. Other surface shapes, such as a parabola rotated about its major axis, may be used for the surface 120. In practice, the surface 110 can be configured to convey a beam having a ring-shaped cross-section at the intermediate focus 121. In other embodiments, the surface 120 can utilize coatings or layers different from or in addition to the layers described above.

[0040]

[0052] As shown in FIG. 1, the EUV light source 110 can include a focusing unit 111 that includes one or more optical elements for focusing the laser beam at a focus or beam waist at the irradiation site 116. The EUV light source 110 can also include a beam adjustment 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 for shaping the laser pulse.

[0041]

[0053] FIG. 2 is a diagram showing an implementation of an EUV light source 210 such as the EUV light source 110 or another EUV light 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 it with one or more illumination mirrors 272 so as to illuminate a reflection pattern or a reticle 273. The EUV light reflected from the pattern or reticle 273 is further reflected and reduced by one or more reduction mirrors 274 and irradiated onto a substrate or wafer 275 (or on one or more photosensitive layers (not shown) on the substrate or wafer 275), so that a patterned structure can be formed on or in the substrate or wafer 275.

[0042]

[0054] As described above, referring back to FIG. 1, in some cases, the EUV light source 110 uses one or more seed lasers to generate laser pulses, which are then amplified to become laser pulses that irradiate the target 123 at the irradiation site 116, forming a plasma 124 that generates EUV light 125. FIG. 3 is a simplified schematic diagram showing a part of an implementation of a seed laser module 330 that can be used as part of an EUV laser source 112 such as the EUV light source 110 in FIG. 1.

[0043]

[0055] As shown in FIG. 3, a seed laser module 330 according to an example implementation includes two seed lasers, namely a prepulse seed laser 332 and a main pulse seed laser 334. In such an implementation, by including two seed lasers, the target 123 (FIG. 1) can first be irradiated by one or more pulses emitted from the prepulse laser 332 and then irradiated by one or more pulses emitted from the main pulse seed laser 334.

[0044]

[0056] Seed lasers 332 and 334 contain relatively fragile optical components (not shown) such as output couplers, polarizers, mirrors, gratings, AOMs or EOMs inside them. Therefore, it is desirable to prevent any light that can propagate back towards 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 these components and causing damage or otherwise interfering with the stable operation of the seed lasers 332, 334.

[0045]

[0057] In the implementation of FIG. 3, each of the pulsed beams 333 and 335 from each seed laser 332, 334 first passes through the respective EOMs 336, 336'. The EOMs 336, 336' are used together with the seed lasers 332, 334 as pulse shaping units for trimming the pulses generated by the seed lasers into pulses having a shorter duration and faster rise and fall times. Since the interaction time between the pulse and the target 123 is short when the duration is shorter and the fall time is relatively fast, and also because the trimmed unnecessary portions of the pulse do not enter the downstream amplifier (not shown) and do not unnecessarily deplete the amplifier gain of the downstream amplifier, the output and efficiency of the EUV light source can be increased. Although two separate pulse shaping units (EOMs 336, 336') are shown, instead, a common pulse shaping unit may be used to trim both the prepulse and the main pulse seed pulses.

[0046]

[0058] The beam from the seed laser then passes through respective AOMs 337, 337' and 338, 338'. AOMs 337, 337' and 338, 338' effectively act as one-way gates by guiding the backward-propagating light due to reflection from target 123 or other locations and preventing this light from reaching seed lasers 332, 334. In the implementation illustrated here, the pulsed beams 333, 335 from each seed laser pass through two AOMs respectively. Each successive AOM causes a shift in frequency and wavelength in the passing beam, and the second AOMs 338, 338' on each beam path are oriented such that this shift is in the opposite direction to that of the first AOMs 337, 337', and thus reverses the shift caused by the first AOMs 337, 337'. In other implementations, only a single AOM may be employed in each path, or only one AOM may be employed for both paths if necessary.

[0047]

[0059] After passing through AOMs 337, 337' and 338, 338', the two pulses are "combined" by beam combiner 339. In one implementation, since the pre-pulse seed laser and the main-pulse seed laser may have slightly different wavelengths, beam combiner 339 may be a dichroic beam splitter. Since the pulses from each seed laser 332, 334 are generated at slightly different timings, the two temporally separated pulses generated from each seed laser 332, 334 are placed on a common beam path 331 for further processing and use.

[0048]

[0060] After being placed on the common beam path 331, the pulses from the seed laser may pass through various components such as, for example, a pre-amplifier, a beam expander, a polarizer, and various re-guiding and / or focusing components (not shown). Subsequently, the pulses typically pass through an amplification system including a plurality of amplifier stages (not shown) and a beam adjustment unit such as beam adjustment unit 117 shown in FIG. 1 before being conveyed to a focusing unit such as focusing unit 111 of FIG. 1 and further to target 123.

[0049]

[0061] Figure 4 is a simplified block diagram of a laser 434, which is an example of the implementation of the main pulse seed laser 332 in FIG. 3. In the laser 434 of FIG. 4, the enclosure 426 contains a laser medium (or "gain medium") maintained at a pressure below atmospheric pressure. Energy can be supplied to the laser medium 427 in the form of an oscillating electric field generated between the electrodes 428, 428' by the RF source 470. The resonant optical cavity, or "resonator", is provided along the optical axis 447 by the mirrors 429, 429' and the reflection grating 445, together with the movable output coupler or extraction mirror 441. The window 446 holds the laser medium 427 while allowing the beam to exit the enclosure 426. Lasing usually occurs when the energy ("laser gain") added to the optical beam reciprocating through the resonator or optical cavity by the laser medium equals or exceeds the energy ("resonator loss") lost by the beam in the same reciprocation.

[0050]

[0062] A variable attenuator or Q-switch 440, which can 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. When the Q-switch 440 is an AOM, the signal 440a can be an RF power level. Usually, when the RF power level applied to the AOP is low or zero, low attenuation (or low resonator loss and high Q value) occurs. When a high RF power level is applied to the AOM, high attenuation (or high resonator loss and low Q value) occurs. When the Q-switch 440 is an EOM, the signal 440a can 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 (low Q value or high resonator loss) so that power can accumulate in the seed laser 434 as described above, and then switched to provide low attenuation or zero attenuation (high Q value or low resonator loss) to enable Q-switching of the laser 434 so that this laser 434 can generate pulses.

[0051]

[0063] Sensor 442 measures one or more parameters of output beam 443, such as output beam power, and provides associated data or signal 442a to control module 444. Control module or control system 444 uses this data or signal 442a to determine appropriate adjustments for Q-switch 440 (such as the level of attenuation applied between pulses), and to determine a predetermined appropriate adjustment for the length of laser cavity 447. Control module or control system 444 transmits command or signal 448a to actuator 448 according to the determined adjustment to move movable extraction mirror 441. Actuator 448 may be a piezoelectric transducer (PZT) as a driving element, or may include this piezoelectric transducer. The command or signal may be a voltage level for the PZT. Actuator 448 can move movable extraction mirror 441 over an adjustment range including at least three cavity modes.

[0052]

[0064] In addition to controlling the length of the laser cavity, it is desirable to simultaneously control the timing of pre-lasing. As described above, there are two factors that affect the timing at which pre-lasing starts. First, as described above, the lower the Q-switch attenuation between pulses (the higher the Q value), the earlier the lasing threshold is reached and pre-lasing occurs. Second, when the cavity mode is located at the peak of the gain band, the effective gain accumulates more quickly compared to the case where there is only an offset mode away from the gain peak. Therefore, when using a partially open Q-switch to provide attenuation as described above, when the cavity mode is located at the gain peak, compared to the case where only the offset mode exists, the lasing threshold is reached earlier, so pre-lasing starts earlier. By positioning the cavity mode at the gain peak, the largest output power from the laser can be obtained.

[0053]

[0065] The cavity mode of laser 434 is maintained at or near the gain peak by a control module 444 that dithers the position of mirror 441 (i.e., changes it slightly back and forth), while monitoring the duration from each Q-switch pulse to the occurrence of each subsequent pre-lasing, for example using sensor 442. The position of mirror 441 along the optical axis 447 (or more precisely, the average or reference position of the mirror, such as the center position of the dithering) can then be gradually shifted in the direction that creates the shortest average duration from the Q-switch pulse to the occurrence of each subsequent pre-lasing. Since the shortest duration to pre-lasing occurs when the cavity mode is centered on the maximum (or peak) of the gain profile (assuming other conditions are the same), according to this method or process, the laser can hold its cavity length at the position where the cavity mode is centered on the gain peak, or continuously adjust the cavity length towards the position where the cavity mode is centered on the gain peak, maintaining the single-mode operation of the laser, resulting in high efficiency and power, and a shorter time to the occurrence of pre-lasing.

[0054]

[0066] If the average time to the occurrence of pre-lasing is shorter than the target time, control module 444 can adjust the inter-pulse attenuation of Q-switch 440 by gradually increasing it, and if the average time to occurrence is longer than the target time, by gradually decreasing the inter-pulse attenuation, to adjust the time to the occurrence of pre-lasing over a longer time scale than the mode centering process. By the control module 444 adjusting the attenuation of Q-switch 440 at a relatively gradual rate, during the attenuation adjustment, the cavity mode can be continuously centered at or near the gain peak in the mode centering process described above.

[0055]

[0067] FIG. 5A is a graph showing an implementation of a periodically varying Q value (in arbitrary units) as a function of time (nanoseconds) for generating a Q-switch pulse from a laser such as laser 434 described above with reference to FIG. 4.

[0056]

[0068] FIG. 5B is a corresponding graph showing a periodically varying RF power that can be applied to an AOM (when an AOM is used as the Q switch 440) such that it is applied to the Q switch 440 by the control module 444 in FIG. 4 to generate the Q value shown in the graph of FIG. 5A. As can be seen from these figures, the RF power for the AOM is inversely proportional to the Q value of the laser and thus corresponds positively to the attenuation. Therefore, FIG. 5B can also be read and understood as a graph of the attenuation (on an arbitrary scale as shown on the right vertical axis) generated over time by the Q switch 440.

[0057]

[0069] Two phases of the periodic Q value, or periodic RF power or attenuation, shown in FIGS. 5A and 5B are described and labeled at the top of the graph in FIG. 5A. Since the Q value shown in FIG. 5A and the corresponding attenuation and / or RF power for the AOM shown in FIG. 5B are periodic, they are repeated from pulse to pulse of the laser. Therefore, phase 1, indicated by the arrow at the top of FIG. 5A, starts at the end of the preceding phase 2 and wraps around the graph so as to extend to the start of the subsequent phase 2. In this example, there is an interval of 10,000 ns (or 10 μs) between pulses as can be seen from the time scale of the x-axis in the drawing, so the illustrated pulse rate is 100 kHz.

[0058]

[0070] After the high Q value 550a of FIG. 5A or after the low RF power value or low attenuation value 550b (Q-switching window) centered at time 0 in FIG. 5B, a first Q value 549a and a corresponding first attenuation value 549b are set to phase 1. The Q value 549a and the attenuation value 549b in phase 1 are high enough to cause the occurrence of lasing (i.e., at a level where the gain of the laser medium exceeds the cavity loss of the laser), and are set low enough to provide attenuation that allows power accumulation in the laser to exceed the power level in continuous lasing without attenuation. This phase 1, or "inter-pulse" attenuation level 549b, can be adjusted as described above to maintain the average time of the occurrence of pre-lasing at or near a desired target time. Next, in phase 2, a second Q value 550a and / or a second RF power level or attenuation value 550b are set to be the maximum Q value or minimum attenuation value for a specific laser 434 to perform Q-switching of the laser 434 and generate pulses.

[0059]

[0071] FIG. 5C is a graph showing the output beam power (on an arbitrary scale) resulting from the two-phase or two-step periodic attenuation of FIGS. 5A and 5B. In FIGS. 5A and 5B, although the attenuation in phase 1 is set to a value low enough to cause lasing, immediately after pulse 551 having a peak at time 0, the laser output drops to near 0. This is because the large Q-switch pulse at time 0 consumes almost all of the energy stored in the laser (in the form of an inverted distribution of the laser medium). Only after duration D1 does the energy with the first peak 552 accumulate again, which is sufficient to cause pre-lasing (or low-level continuous lasing), followed by beam 553 with a relatively low power constant. As described above, duration D1 is relatively short and has a higher laser efficiency generated by matching the cavity mode to the gain peak. Therefore, duration D1 can be used to maintain a single operating mode of the laser and, as described above, vary the cavity length as needed to minimize D1. However, in the repetition of a pulsed laser at a fixed frequency, since duration D1 corresponds to the time obtained by subtracting duration D2 from the repetition period, duration D2 from the pre-lasing peak 552 to the peak of the Q-switch pulse 551 is used to maintain a single-mode operation and can be maximized rather than minimized.

[0060]

[0072] As described above, it is desirable to increase the power output of a Q-switch laser, particularly a seed laser in an EUV light source. The greater the power from the seed laser of the EUV light source, the more laser power amplified by the target in the EUV light source will be carried, resulting in a higher-power EUV-emitting plasma. Thus, the associated EUV lithography exposure apparatus and the wafers processed in the apparatus can receive higher-power EUV light, enabling more wafers to be exposed in a shorter time, creating a significant time reduction in a high-value process.

[0061]

[0073] From a given seed laser such as the laser 434 of FIG. 4, higher power can potentially be generated by (1) increasing the frequency of the pulses, or (2) increasing the power of each pulse, or (3) a combination of both, but these (especially the combination) are difficult to achieve using the two-phase periodic attenuation variations shown in FIGS. 5A and 5B.

[0062]

[0074] As the pulse repetition rate increases, such as the repetition frequency from 50 kHz to 100 kHz shown in FIGS. 5A - 5C, the time between pulses for the occurrence of pre-lasing becomes shorter, and the time for gain recovery becomes shorter. To address the problems where the above-mentioned pre-lasing is delayed or does not occur, the attenuation in phase 1 of FIGS. 5A - 5C can be gradually adjusted downward (and thus the Q value in phase 1 can be adjusted upward), and the average occurrence can be made earlier (for the shorter average of duration D1 and the longer average of duration D2). The lower the attenuation in phase 1 (the higher the Q value), the less the amount of energy accumulated and made available by the Q-switch pulse 551. Thus, for example, the peak pulse power at 100 kHz can be half as small as the peak pulse power at 50 kHz.

[0063]

[0075] By operating the laser 434 with a periodic Q value or attenuation value of the type shown in FIGS. 6A and 6B, and a periodic laser power output power as shown in FIG. 6C, higher peak pulse power can be generated at a higher repetition rate.

[0064]

[0076] FIG. 6A is a graph showing an implementation of a periodically varying Q value (in arbitrary units) as a function of the time (nanoseconds) for generating a Q-switch pulse from a laser such as the laser 434 described above with reference to FIG. 4.

[0065]

[0077] Figure 6B is a corresponding graph showing the periodically varying RF power that can be applied to an AOM (when an AOM is used as the Q switch 440) in the form of a signal 440a applied to the Q switch 440 by the control module 444 in FIG. 4 to generate the Q value shown in the graph of FIG. 6A. As can be seen from these figures and as described above, since the RF power for the AOM is inversely proportional to the Q value of the laser, it positively corresponds to the attenuation. Therefore, FIG. 6B can also be read and understood as a graph of the attenuation (on an arbitrary scale as shown on the right vertical axis) generated over time by the Q switch 440.

[0066]

[0078] The three phases of the periodic Q value, or periodic RF power or attenuation, shown in FIGS. 6A and 6B are labeled 1 to 3 at the top of the graph in FIG. 6A, and furthermore, any intermediate phase is labeled I. FIG. 6C is a graph showing the output beam power (on an arbitrary scale) resulting from the periodic Q value or periodic attenuation of the three-phase + intermediate phase I of FIGS. 6A and 6B, and is also labeled with phases 1 to 3 and I at the top of FIG. 6C.

[0067]

[0079] Since the Q value shown in FIG. 6A and the corresponding attenuation and / or RF power for the AOM shown in FIG. 6B are periodic, they are repeated from pulse to pulse of the laser 434. Therefore, phase 1, indicated by an arrow at the top of FIG. 6A, starts at the end of the preceding phase 3 and the graph is folded back so as to extend to the start of the subsequent phase 2. Similar to the above, as can be seen from the time scale of the x-axis in the drawing, since there is an interval of 10,000 ns (or 10 μs) between pulses, the illustrated pulse rate is 100 kHz.

[0068]

[0080] After the high Q value 659a of FIG. 6A or after the low RF power value or low attenuation value 659b (Q-switching window) centered at time 0 in FIG. 6B, the first Q value 654a and / or the corresponding first attenuation value 654b are set within phase 1. The Q value 654a (FIG. 6A) or attenuation value 654b (FIG. 6B) in phase 1 is set high enough to cause the occurrence of lasing 656 (FIG. 6C) (i.e., at a level where the gain of the laser medium exceeds the cavity losses of the laser), and further, it can also be set low enough to provide attenuation that allows power accumulation within the laser to exceed the power level in non-attenuated continuous lasing, although this is optional as will be described later. This attenuation level 654b (FIG. 6A) (or Q value 654a (FIG. 6B)) in phase 1 can be optionally adjusted as described above to maintain the average time of the occurrence (FIG. 6C) (at the first peak 657) of pre-lasing 656 (FIG. 6C) at or near a desired target time.

[0069]

[0081] Next, in phase 2, the second Q value 658a (FIG. 6A) or the second attenuation value 658b (FIG. 6B) is set at a level where the cavity losses exceed the medium gain, and this level may be the minimum Q value or the maximum attenuation (including up to the "shuttering" of the laser) level for a given laser 434. With such a setting, power accumulation in the inversion distribution of the laser medium 427 becomes possible at a higher level than if the laser continued to lase at a low level. Thus, the laser output substantially reaches the 0 level 660 (FIG. 6C). Phase 2 can continue for a time selected such that the power accumulated in the inversion distribution reaches a maximum or saturation value during phase 2 (e.g., in the range of 100 - 900 ns).

[0070]

[0082] Following phase 2, in phase 3, a third Q value 659a (FIG. 6A) such as the maximum Q value of the laser 434 or a third attenuation value 659b (FIG. 6B) such as the minimum attenuation value of the attenuator 440 is set, and a Q-switch pulse 655 (FIG. 6C) is generated. Since the power level accumulated in the inversion distribution of the laser medium 427 during phase 2 is higher (the small-signal gain is higher), pulse 655 has both a higher peak power and a shorter duration despite the same repetition rate compared to the methods shown in FIGS. 5A-5C.

[0071]

[0083] Optionally, between the second attenuation value or Q value and the third attenuation value or Q value, in an intermediate phase I, an intermediate value 661b (FIG. 6B) can be set low enough (or an intermediate Q value 661a (FIG. 6A) can be set high enough) so that pre-lasing resumes immediately before the Q-switching of phase 3 (i.e., the gain of the medium 427 along the cavity 447 rises to a level exceeding the loss of the cavity 447), thereby causing a short second pre-lasing 662 (FIG. 6C) and making it possible to remove any potential temporal jitter of the Q-switch pulse 655 (FIG. 6C).

[0072]

[0084] Even though the Q value or attenuation value in phase 1 is set to a value sufficient to enable lasing as described above with reference to FIGS. 6A and 6B, immediately after pulse 655 (FIG. 6C) having a peak at approximately time 0, the laser output drops to 0 or near 0. This is because the large Q-switch pulse 655 consumes almost all of the energy stored in the laser 434 (in the form of the inverted population of the laser medium 427). Only after the duration D1 has elapsed does enough energy accumulate again to cause pre-lasing (or low-level continuous lasing) at the first peak 657, followed by a beam 656 having a relatively low power constant. As described above, the duration D1 is relatively short and has a higher laser efficiency generated by aligning the cavity mode with the gain peak. Therefore, the duration D1 from the Q-switch pulse 655 to the first pre-lasing peak 657 is used to maintain a single operating mode of the laser, and the cavity length can be varied as needed to minimize D1 as described above. As described above with reference to FIG. 5C, in the repetition of a pulsed laser at a fixed frequency, since the duration D1 corresponds to the time obtained by subtracting the duration D2 from the repetition period, the duration D2 from the pre-lasing peak 657 to the peak of the Q-switch pulse 655 is used to maintain a single-mode operation and can be maximized rather than minimized.

[0073]

[0085] Unlike the periodic Q - value or attenuation value of FIGS. 5A and 5B, in FIGS. 6A and 6B, the Q - value 654a or attenuation value 654b of the first (phase 1), which is set to generate a gain greater than the loss in the laser so that a pre - lasing peak can occur, does not persist or extend until the Q - switching located in phase 3. Instead, prior to Q - switching, in phase 2, a second Q - value 658a or a second attenuation value 658b is set to a level where the loss exceeds the gain, or to a level that effectively closes the laser, thereby allowing the accumulation of power (or gain) in the inversion distribution of the laser medium 427 to increase to a higher level (up to a maximum or saturation value). Thus, the power level available in the Q - switch pulse 655 during phase 3 is effectively decoupled from the Q - value 654a or attenuation value 654b of phase 1.

[0074]

[0086] FIG. 6D shows a method of operating the laser 434. This method is described in terms of setting the Q - value of the laser, but it will be understood that it is equally applicable to setting the attenuation of the laser. In step S10, the Q - value of the laser 434 is set to a first Q - value (e.g., 654a in FIG. 6A (at phase 1)) such that after the laser 434 generates a first pulse 655, the gain of the laser 434 exceeds the loss of the laser 434 and the laser 434 can generate a first continuous “pre - lasing” beam 656. After the first continuous beam 656 is generated, i.e., some time after the occurrence of the first continuous beam 656 at the first peak 657, in step S20, the Q - value is decreased to a second Q - value (e.g., value 658a (at phase 2)). This stops the depletion of the power accumulation in the inversion distribution of the laser medium 427 of the first continuous beam 656 (as can be seen from the low or zero power output level 660 in the graph of FIG. 6C) and the associated first continuous beam 656. Next, in step S30, the Q - value is increased to a value (e.g., value 659a at phase 3) such that the laser 434 generates a pulse 655.

[0075]

[0087] After the laser generates pulse 655, the process can start the iteration by setting the attenuation again to Q value 654a (at the second or after-iteration phase 1).

[0076]

[0088] In the implementation of this method, the second attenuation value 658b or the second Q value 658a can close the laser. This method may also include reducing the attenuation to the intermediate attenuation value 661b (at the intermediate phase I) after setting the attenuation to the second attenuation value 658b (at phase 2) and before setting the attenuation to the third attenuation value 659b (at phase 3), where the intermediate attenuation value 661b is higher than the third attenuation value 659b and low enough for the laser 434 to generate the second beam 662.

[0077]

[0089] In the implementation of this method, the attenuator 440 may also include an optical modulator within the laser 434 or connected to the laser 434. The optical modulator can be an AOM or an EOM. The laser 434 can be a CO2 laser. The laser can be the seed lasers 332, 334 within the extreme ultraviolet (EUV) light source 110, for example, the main pulse seed laser 334 within the EUV light source 110.

[0078]

[0090] In an additional implementation, setting the attenuation to the second attenuation value 658b can include setting the attenuation to the second attenuation value 658b over a time range of 200 - 1000 seconds or 100 - 1000 ns. Setting the attenuation to the intermediate attenuation value 661b can include setting the attenuation to the intermediate attenuation value 661b over a time range of 0 - 300 ns. Setting the attenuation to the third attenuation value 659b can include setting the attenuation to the third attenuation value 659b over a time range of 400 - 700 ns.

[0079]

[0091] In other additional implementations, this method may include monitoring a duration D1 from the first pulse 655 until the generation of the first continuous beam 656 (represented by the first peak 657 of the first continuous beam 656), and adjusting the cavity length to minimize this duration D1. This method may include monitoring the duration D1 and adjusting a first attenuation value 654b based on this duration D1. The third attenuation value may be the maximum attenuation value.

[0080]

[0092] In another aspect of the present disclosure, referring to FIGS. 4, 6B, and 6C, a laser 434 including an optical modulator 440 controlled by a signal 440a applied to the optical modulator 440 may be operated by a method including setting the magnitude of the signal 440a to a first value 654b such that the laser operates in a mode where the laser gain exceeds the resonator loss; setting the magnitude of the signal 440a to a second value 658b such that the laser 434 is closed; and setting the magnitude of the signal 440a to a third value 659b such that the laser 434 generates a pulse 655.

[0081]

[0093] In a plurality of implementations, the laser may include an output coupler 441 having a PZT 448, and the method may include a step executed during the step of setting the magnitude of the signal 440a to the first value 654b, the step including controlling a voltage 448a applied to the piezoelectric transducer 448 using the output of the laser. The optical modulator 440 may include or be in the form of an AOM or an EOM.

[0082]

[0094] In another aspect, a system for generating a pulse of laser radiation includes a laser 434 that includes an optical modulator 440 controlled by an applied signal 440a and a control system 444, the control system 444 configured and adapted to set the magnitude of the signal 440a to a first value 654b such that the laser 434 operates in a mode where the laser gain exceeds the resonator losses, then set the magnitude of the signal 440a to a second value 658b such that the laser 434 is turned off, and then set the magnitude of the signal 440a to a third value 659b such that the laser 434 generates a pulse.

[0083]

[0095] In an implementation of this system, the laser 434 may include an output coupler 441 having a PZT 448, and the control system 444 is further configured and adapted to control the voltage 448a applied to the PZT 448 using the output of the laser 434 when the signal 440a is at the first value 654b.

[0084]

[0096] In a further aspect, referring to FIGS. 4 and 6A, a laser system includes a laser cavity 427, an optical modulator 440 configured to control the Q value of the laser cavity 447, a power sensor 442 positioned external to the laser cavity 427 and configured to detect the power level of the radiation emitted from the laser 434 and generate power level data and / or a signal 442a regarding the power level of the radiation emitted from the laser 434, and a control system 444 connected to receive the power level data and / or the signal 442a and control the optical modulator 440, the control system 444 configured to: (1) set the Q value of the cavity 427 of the laser 434 to a first value 654a that is high enough to cause lasing; (2) at a timing after lasing has occurred, set the Q value of the cavity 427 to a second value 658a that is low enough to stop the occurrence of lasing; and (3) after setting the Q value of the cavity 427 to the second value 658a, set the Q value of the cavity 427 to a third value 659a such that the laser 434 emits a pulse.

[0085]

[0097] FIGS. 7A and 7B are graphs showing further implementations and variations of the method described above with reference to FIGS. 6A, 6B, and 6C. FIG. 7A shows the variation of the Q value over time. FIG. 7B shows the resulting laser output power over time.

[0086]

[0098] Referring to FIGS. 7A and 7B, as shown, each of phases 1 to 3 and I may have different Q values (and corresponding attenuation). The Q value of any phase does not have to match the Q value of another phase. Also, the Q value of phase 2 (regarding energy accumulation) does not have to be 0 (nor does the corresponding attenuation have to be 100%). Since the power available in the Q - switch pulse 767 (FIG. 7B) is decoupled from the first Q value 761 (FIG. 7A) in phase 1 and thus from the timing of the occurrence of pre - lasing, the first Q value 761 in phase 1 can be set relatively high, for example, by setting a timing earlier with respect to the occurrence of the pre - lasing continuous beam 763 (the first peak 762), or by setting the first Q value 761 to the third Q value 764 in the third phase (a high Q value with low attenuation or 0 attenuation during Q - switching), as indicated by the alternate dotted - line value in FIG. 7A with respect to the first Q value 764.

[0087]

[0099] Increasing the first Q value 761 causes the occurrence 762 of pre-lasing 763 with less time dithering at an earlier timing, and can bring about the effect of taking a longer time with the inter-pulse interval for energy accumulation etc. between the second Q values 765 in phase 2 as necessary or desired. Taking a longer time with the inter-pulse interval can also be used for the intermediate phase I as necessary, so that the "second pre-lasing" or continuous beam 768 surely occurs between the intermediate Q values 766 and before the Q-switch pulse 767, minimizing the time dithering of the Q-switch pulse 767. The intermediate Q value 766 is set lower than the typical lasing threshold, and it is also possible to enable the second pre-lasing or continuous beam 768 to still occur. This is because at this point, the laser 434 approaches the lasing threshold from an energy saturation state rather than an energy depletion state, provides a higher initial gain, and enables lasing to occur more easily (and with less time dithering). In such an implementation, it may be more appropriate to shift the period by reclassifying the first pre-lasing 763 as "post-lasing" (subsequent immediately after the Q-switch pulse, enabling optimization of the cavity length and cavity mode), and simply reclassifying the second pre-lasing 768 as "pre-lasing" (placed immediately before the Q-switch pulse to absorb any time dithering).

[0088]

[0100] This embodiment will be further described using the clauses shown below. 1. A method of operating a laser, 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, enabling the laser to generate a first continuous beam; 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; After increasing the attenuation to a second value, lowering the attenuation of the attenuator to a third attenuation value such that the laser generates a second pulse; A method comprising. 2. The method according to clause 1, further comprising setting the attenuation to a first attenuation value after the laser generates a second pulse. 3. The method according to clause 1, wherein the second value closes the laser. 4. The method according to clause 1, further comprising lowering the attenuation to an intermediate attenuation value that is higher than the third attenuation value and low enough to enable the laser to generate a second beam after setting the attenuation to the second attenuation value and before setting the attenuation to the third attenuation value. 5. The method according to clause 1, wherein the attenuator includes an optical modulator inside the laser or connected to the laser. 6. The method according to clause 1, wherein the attenuator includes an acousto-optic modulator (AOM) inside the laser or connected to the laser. 7. The method according to clause 1, wherein the attenuator includes an acousto-optic modulator (AOM) inside the laser or connected to the laser, 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 lowering the attenuation of the attenuator includes lowering the RF power supplied to the AOM. 8. The method according to clause 1, wherein the attenuator includes an electro-optic modulator (EOM) inside the laser or connected to the laser. 9. The method according to clause 1, wherein the laser is a CO2 laser. 10. The method according to clause 1, wherein the laser is a seed laser in an extreme ultraviolet (EUV) light source. 11. The method according to clause 1, wherein the laser is a main pulse seed laser in an EUV light source. 12. The method according to clause 1, wherein setting the attenuation to the second attenuation value includes setting the attenuation to the second attenuation value over a time in the range of 100 to 1000 nanoseconds (ns). 13. Setting the attenuation to the intermediate attenuation value includes setting the attenuation to the intermediate attenuation value over a time in the range of 0 to 300 ns, the method according to clause 1. 14. Setting the attenuation to the third attenuation value includes setting the attenuation to the third value over a duration in the range of 400 to 700 ns, the method according to clause 1. 15. Further including monitoring the 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, the method according to clause 1. 16. Further including monitoring the duration between the first pulse and the generation of the first continuous beam and adjusting the first attenuation value based on the duration, the method according to clause 1. 17. The third attenuation value is the maximum attenuation value, the method according to clause 1. 18. The first attenuation value is equal to the third attenuation value, the method according to clause 1. 19. (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) after increasing the attenuation to the second value, reducing the attenuation of the attenuator to the third attenuation value so that the laser generates a second pulse, which includes Q-switching the laser, the method according to clause 1. 20. A method of operating a laser including an optical modulator controlled by an applied signal, Setting the magnitude of the signal to a first value so that the laser operates in a mode where the laser gain exceeds the resonator loss, Setting the magnitude of the signal to a second value so that the laser is closed, Setting the magnitude of the signal to a third value so that the laser generates a pulse, The method including. 21. The laser includes an output coupler having a piezoelectric transducer, and the method further includes controlling the voltage applied to the piezoelectric transducer using the output of the laser during the step of setting the magnitude of the signal to the first value, the method according to clause 20. 22. The optical modulator includes an acousto-optic modulator (AOM), the method according to clause 20. 23. The method according to clause 20, wherein the optical modulator includes an acousto-optic modulator (AOM), and the signal includes an RF power level. 24. The method according to clause 20, wherein the optical modulator includes an electro-optic modulator (EOM). 25. The method according to clause 20, which includes Q-switching the laser by (1) setting the magnitude of the signal to a second value so that the laser is closed, and (2) setting the magnitude of the signal to a third value so that the laser generates a pulse. 26. A system for generating a pulse of laser emission, a laser including an optical modulator controlled by an applied signal, a control system configured and adapted to sequentially set the magnitude of the signal to a first value so that the laser operates in a mode where the laser gain exceeds the resonator loss, then set the magnitude of the signal to a second value so that the laser is closed, and then set the magnitude of the signal to a third value so that the laser generates a pulse. A system including the above. 27. The system according to clause 26, wherein the laser includes an output coupler having a piezoelectric transducer, and the control system is further configured and adapted to control the voltage applied to the piezoelectric transducer using the output of the laser when the signal is at the first value. 28. The system according to clause 26, wherein the optical modulator includes an acousto-optic modulator (AOM). 29. The system according to clause 26, wherein the optical modulator includes an acousto-optic modulator (AOM), and the signal applied to the optical modulator includes an RF power level. 30. The system according to clause 26, wherein the optical modulator includes an electro-optic modulator (EOM). 31. The system according to clause 26, wherein the control system is configured and adapted to perform Q-switching. 32. A laser system, a laser having a laser cavity, an optical modulator configured to control the Q value of the laser cavity. A power sensor positioned outside a laser cavity, configured to detect a power level of radiation emitted from a laser and generate power level data and / or a signal regarding the power level of the radiation emitted from the laser. A control system connected to receive the power level data or signal and control an optical modulator, the control system configured to: (1) set a Q value of the laser cavity to a first value high enough to cause lasing; (2) at a timing after lasing is detected by the power sensor, set the Q value of the cavity to a second value less than the first value and low enough to stop lasing from occurring; and (3) after setting the Q value of the cavity to the second value, set the Q value of the cavity to a third value such that the laser emits a pulse. A laser system comprising the above. 33. The laser system according to clause 32, wherein the control system is configured to perform Q-switching.

[0089]

[0101] The above and other embodiments are included in the claims described below.

Claims

1. A method of operating a laser, comprising: after the laser generates a first pulse, setting the attenuation of an attenuator in the laser to a first attenuation value such that the gain of the laser exceeds the loss of the laser, enabling the laser to generate a first continuous beam; after the first continuous beam is generated, increasing the attenuation of the attenuator to a second attenuation value such that the loss of the laser exceeds the gain of the laser; after increasing the attenuation to the second value, reducing the attenuation of the attenuator to a third attenuation value such that the laser generates a second pulse; A method comprising the above steps.

2. The method according to claim 1, further comprising setting the attenuation to the first attenuation value after the laser generates the second pulse.

3. The method according to claim 1, wherein the second value closes the laser.

4. The method according to claim 1, further comprising reducing the attenuation to an intermediate attenuation value that is higher than the third attenuation value and low enough to enable the laser to generate a second beam, after setting the attenuation to the second attenuation value and before setting the attenuation to the third attenuation value.

5. The method according to claim 1, wherein the attenuator includes an optical modulator inside the laser or connected to the laser.

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

7. The method according to claim 1, wherein the attenuator includes an acousto-optic modulator (AOM) inside the laser or connected to the laser, 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.

8. The method according to claim 1, wherein the attenuator includes an electro-optic modulator (EOM) inside the laser or connected to the laser.

9. The laser is a CO ２ laser, the method according to claim 1.

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

11. The method according to claim 1, wherein the laser is a main pulse seed laser in an EUV light source.

12. The method according to claim 1, wherein setting the attenuation to the second attenuation value includes setting the attenuation to the second attenuation value over a time range of 100 to 1000 nanoseconds (ns).

13. The method according to claim 1, wherein setting the attenuation to the intermediate attenuation value includes setting the attenuation to the intermediate attenuation value over a time range of 0 to 300 ns.

14. The method according to claim 1, wherein setting the attenuation to the third attenuation value includes setting the attenuation to the third value over a duration in the range of 400 to 700 ns.

15. The method according to claim 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.

16. The method according to claim 1, further comprising 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.

17. The method according to claim 1, wherein the third attenuation value is a maximum attenuation value.

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

19. (1) increasing the attenuation of the attenuator to a second attenuation value such that the loss of the laser exceeds the gain of the laser; and (2) after increasing the attenuation to the second value, decreasing the attenuation of the attenuator to a third attenuation value such that the laser generates a second pulse, which includes Q-switching the laser. The method according to claim 1.

20. A method of operating a laser including an optical modulator controlled by an applied signal, setting a magnitude of the signal to a first value such that the laser operates in a mode where the laser gain exceeds the resonator loss; setting the magnitude of the signal to a second value such that the laser is closed; setting the magnitude of the signal to a third value such that the laser generates a pulse; The method includes.

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

22. The method according to claim 20, wherein the optical modulator includes an acousto-optic modulator (AOM).

23. The method according to claim 20, wherein the optical modulator includes an acousto-optic modulator (AOM), and the signal includes an RF power level.

24. The method according to claim 20, wherein the optical modulator includes an electro-optic modulator (EOM).

25. (1) setting a magnitude of the signal to a second value so that the laser is closed, and (2) setting a magnitude of the signal to a third value so that the laser generates a pulse, including Q-switching the laser, the method according to claim 20.

26. A system for generating pulses of laser radiation, a laser including an optical modulator controlled by an applied signal, a control system configured and adapted to sequentially set a magnitude of the signal to a first value such that the laser operates in a mode where laser gain exceeds resonator loss, then set the magnitude of the signal to a second value such that the laser is closed, and then set the magnitude of the signal to a third value such that the laser generates a pulse, a system including.

27. The laser includes an output coupler having a piezoelectric transducer, and 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 the first value, the system according to claim 26.

28. The system according to claim 26, wherein the optical modulator includes an acousto-optic modulator (AOM).

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

30. The system according to claim 26, wherein the optical modulator includes an electro-optic modulator (EOM).

31. The system according to claim 26, wherein the control system is configured and adapted to perform Q-switching.

32. A laser system, A laser having a laser cavity, an optical modulator configured to control the Q value of the laser cavity, a power sensor positioned outside the laser cavity, the power sensor configured to detect a power level of radiation emitted from the laser and generate power level data and / or a signal regarding the power level of the radiation emitted from the laser, a control system connected to receive the power level data or signal and control the optical modulator, the control system configured to: (1) set the Q value of the cavity of the laser to a first value high enough to cause lasing; (2) at a timing after lasing is detected by the power sensor, set the Q value of the cavity to a second value less than the second value and low enough to stop the lasing from occurring; and (3) after setting the Q value of the cavity to the second value, set the Q value of the cavity to a third value such that the laser emits a pulse, A laser system comprising:

33. The laser system according to claim 32, wherein the control system is configured to perform Q-switching.

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