Methods and apparatuses for independent rarefication pulse for co2 laser driven EUV source

Abstract

A pulsed laser system includes a laser that delivers a light pulse to an optical shutter configured to transmit, of the pulse: (a) a first transmitted portion having a rise time of less than 20 ns and a first portion peak power, then (b) a second transmitted portion having an initial power less than the first portion peak power, and then (c) a third transmitted portion having an energy of at least 100 nJ and a duration of less than or equal to 200 ns. A low voltage driver (to control transmission of the first transmitted portion) can be connected to an EOM in the optical shutter in parallel with a high voltage driver (to control transmission of the third transmitted portion), with electrical isolators positioned the outputs of the respective voltage drivers to protect the low voltage driver.

Description

METHODS AND APPARATUSES FOR INDEPENDENT RAREFICATION PULSE FOR CO2 LASER DRIVEN EUV SOURCECROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US Application No. 63 / 673,979, filed on July 22, 2024, and US Application No. 63 / 707,707, filed on October 15, 2024, which are incorporated herein by reference in their entirety.TECHNICAL FIELD

[0002] The disclosed subject matter relates to methods and processes and apparatuses for generating CO2-laser-sourced light pulses useful for interacting with a target in an extreme ultraviolet (EUV) light source.BACKGROUND

[0003] Extreme ultraviolet (EUV) light is used for various processes in which high-resolution imaging is needed. Such processes include inspection and formation of structures in the fabrication of electronic devices, such as fabrication semiconductor processors and memories, and electronic displays and inspection of imaging masks.

[0004] Methods to produce EUV light include, but are not necessarily limited to, converting a source material that has a chemical element with an emission line in the EUV range into a plasma state. Elements can include, but are not necessarily limited to, xenon, lithium, and tin. The EUV range includes wavelengths from about 120 nm down to about 10 nm, including the 13.5 nm emission from tin plasmas.

[0005] In one such method, often termed laser-produced plasma ("LPP"), the desired plasma can be produced by irradiating a source material, for example, in the form of a droplet, stream, or wire, with pulses of light produced using a laser. In another method, often termed discharge produced plasma ("DPP"), the plasma can be generated by positioning source material having an appropriate emission line between a pair of electrodes and causing an electrical discharge to occur between the electrodes.

[0006] EUV optics used in EUV imaging and inspection are relatively inefficient, with as much as 30% or more of total energy lost at each reflecting surface. Thus a general and ongoing need in LPP technology is and has been improving the efficiency and maximum power of EUV light production in an EUV light source.SUMMARY

[0007] In some general aspects, a pulsed laser system includes: a laser configured to deliver pulses of light; an optical shutter configured to receive a pulse of light from the laser and to transmit, of the pulse: (a) a first transmitted portion, the first transmitted portion having a first portion start time, afirst portion duration, a first portion peak power, a first portion energy, and a first portion rise time, the first portion rise time being less than 20 nanoseconds (ns), (b) a second transmitted portion subsequent to the first transmitted portion, the second transmitted portion having a second portion start time, a second portion duration, a second portion energy, a second portion peak power, and a second portion initial power, the second portion initial power being less than the first portion peak power, and (c) a third transmitted portion subsequent to the second transmitted portion, the third transmitted portion having a third portion start time, a third portion duration, and a third portion energy, the third portion energy being at least 100 nJ and the third portion duration being less than or equal to 200 ns.

[0008] Implementations can include one or more of the following.

[0009] The second portion peak power can be in the range of 1 / 1000 to 1 / 10 of the first portion peak power. The second portion peak power is greater than the second portion initial power and is within the range of from 50% to 400% of the first portion peak power. The second portion peak power can occur within a last 30% of the second portion duration. The power of the second transmitted portion can increase over the second portion duration. The third portion start time can be more than 30 ns and less than 1000 ns after the first portion start time. The first portion can have a first portion energy, with the first portion energy being between 100 femtojoules (fJ) and 3000 fj. The first portion peak power can be in the range of 1 / 1,000,000 to 1 / 3 of the third portion peak power.

[0010] The optical shutter can be configured to receive one or more signals and can be configured to independently control the second portion start time and the second portion duration in response to the one or more signals. The optical shutter can be configured to receive one or more signals and to independently control the first portion start time, relative to the third portion start time, and the first portion duration in response to the one or more signals. The optical shutter can be configured to receive the one or more signals and to independently control the first portion energy. The optical shutter can be configured to receive one or more signals and to control one or more of the first portion duration, the first portion peak power, the first portion energy, a temporal shape of the first transmitted portion, the second portion start time, the second portion duration, the second portion energy, the second portion peak power, the second portion initial power, a temporal shape of the second transmitted portion, the third portion start time, and the third portion duration.

[0011] The laser can be a CO2 laser and the optical shutter can include an electro-optical modulator (EOM).

[0012] The system can further include a low-voltage driver connected to supply a low voltage to the EOM; a high-voltage driver connected to supply a high voltage to the EOM; and one or more isolators connected to the low -voltage driver and the high-voltage driver and configured to isolate the low voltage driver from the high voltage supplied by the high voltage driver. The low -voltage driver can be configured to drive the EOM during the first transmitted portion. The high-voltage driver can beconfigured to drive the EOM during the third transmitted portion. The low-voltage driver can be configured to drive the EOM during the second transmitted portion.

[0013] One or more of the low-voltage driver and the high-voltage driver can be configured to apply, before the arrival of the pulse of light at the EOM, one or more voltage signals to the EOM configured to reduce or control a vibration of the EOM. The EOM is configured to receive, before the arrival of the pulse of light at the EOM, one or more voltage signals configured to reduce or control a vibration of the EOM.

[0014] The system can include a multistage amplifier configured to receive the first and third transmitted portions from the optical shutter, and the multistage amplifier can be configured such that, in operation, the first transmitted portion can be amplified in the multistage amplifier and delivered to a target in an extreme ultraviolet (EUV) light source, the target including a material capable of emitting EUV light, the target having been previously shaped by a pre -pulse, the amplified first transmitted portion interacting with the previously shaped target to increase a rarefication of the previously shaped target, and such that, in operation, the third transmitted portion can be amplified in the multistage amplifier and delivered to the target, subsequent to the amplified first-transmitted portion, to convert the target or a portion thereof to an EUV -light-producing plasma.

[0015] The system can include a multistage amplifier configured to receive the first, second, and third transmitted portions from the optical shutter, and configured such that, in operation, the first transmitted portion is amplified in the multistage amplifier and delivered to a target in an extreme ultraviolet (EUV) light source, the target including a material capable of emitting EUV light when in a plasma state, the target having been previously shaped by a pre -pulse, the amplified first transmitted portion interacting with the previously shaped target to increase a rarefication of the previously shaped target, and such that, in operation, the second transmitted portion is amplified by the multistage amplifier and delivered to the target, the amplified second transmitted portion interacting with the target subsequent to the amplified first transmitted portion to increase the vaporization of the target, and such that, in operation, the third transmitted portion is amplified by the multistage amplifier and delivered to the target, subsequent to the amplified second transmitted portion, to convert the target or a portion thereof to a an EUV -light-producing plasma.

[0016] In additional general aspects, a pulsed laser system includes: a laser configured to deliver pulses of light; an optical shutter configured to receive a pulse of light from the laser and to transmit, of that pulse: (a) a first transmitted portion, the first transmitted portion having a first portion start time, a first portion duration, a first portion peak power, a first portion energy, and a first portion rise time; (b) a second transmitted portion subsequent to the first transmitted portion, the second transmitted portion having a second portion start time, a second portion duration, a second portion peak power, and a second portion initial power, the second portion initial power being less than the first portion peak power, and (c) a third transmitted portion subsequent to the second transmitted portion, the third transmitted portion having a third portion start time, a third portion duration, a thirdportion energy, and a third portion peak power, the third portion energy being at least 100 nJ and the third portion duration being less than or equal to 200 ns, wherein the first portion peak power is in the range of 1 / 1,000,000 to 1 / 3 of the third portion peak power.

[0017] Implementations can include one or more of the following.

[0018] The second portion peak power can be in the range of 1 / 1000 to less than 1 / 10 of the first portion peak power. The second portion peak power can be greater than the second portion initial power and can be within the range of from 50% to 400% of the first portion peak power. The second portion peak power can occur within a last 30% of the duration of the second portion. The power of the second transmitted portion can increase over the second portion duration. The third portion start time can be more than 30 ns and less than 1000 ns after the first portion start time. The first portion can have a first portion energy between 100 fj and 3000 fj.

[0019] The optical shutter can be configured such that the first portion rise time is less than 20 ns. The optical shutter can be configured to receive one or more signals and is configured to independently control the second portion start time and the second portion duration in response to the one or more signals. The optical shutter can be configured to receive one or more signals and to independently control the first portion start time, relative to the third portion start time, and the first portion duration in response to the one or more signals. The optical shutter can be configured to receive the one or more signals and to independently control the first portion energy. The optical shutter can be configured to receive one or more signals and to control one or more of the first portion duration, the first portion peak power, the first portion energy, a temporal shape of the first transmitted portion, the second portion start time, the second portion duration, the second portion energy, the second portion peak power, the second portion initial power, a temporal shape of the second transmitted portion, the third portion start time, and the third portion duration.

[0020] The laser can be a CO2 laser and the optical shutter can include an electro-optical modulator (EOM).

[0021] The system can include a low -voltage driver connected to supply a low voltage to the EOM; a high-voltage driver connected to supply a high voltage to the EOM; and one or more isolators connected to the low-voltage driver and to the high-voltage driver and configured to isolate the low voltage driver from the high voltage supplied by the high voltage driver. The low -voltage driver can be configured to control the first transmitted portion and the high-voltage driver can be configured to control the third transmitted portion. The low-voltage driver can be configured to control the second transmitted portion.

[0022] One of the low-voltage driver and the high-voltage driver can be configured to apply, before the arrival of the pulse of light at the EOM, a voltage signal to the EOM configured to stop or reduce a vibration of the EOM. The EOM can be configured to receive, before the arrival of the pulse of light at the EOM, a voltage signal configured to stop or reduce a vibration of the EOM.

[0023] The system can include a multistage amplifier configured to receive the first and third transmitted portions from the optical shutter, and configured such that, in operation, the first transmitted portion is amplified in the multistage amplifier and delivered to a target in an extreme ultraviolet (EUV) light source, the target including a material capable of emitting EUV light, the target having been previously shaped by a pre-pulse, the amplified first transmitted portion interacting with the previously shaped target to increase a rarefication of the previously shaped target, and such that, in operation, the third transmitted portion is amplified in the multistage amplifier and delivered to the target, subsequent to the amplified first-transmitted portion, to convert the target or a portion thereof to an EUV -light-producing plasma.

[0024] The system can include a multistage amplifier configured to receive the first, second, and third transmitted portions from the optical shutter, and configured such that, in operation, the first transmitted portion is amplified in the multistage amplifier and delivered to a target in an extreme ultraviolet (EUV) light source, the target including a material capable of emitting EUV light when in a plasma state, the target having been previously shaped by a pre-pulse, the amplified first transmitted portion interacting with the previously shaped target to increase a rarefication of the previously shaped target, and such that, in operation, the second transmitted portion is amplified by the multistage amplifier and delivered to the target, the amplified second transmitted portion interacting with the target subsequent to the amplified first transmitted portion to increase the vaporization (and produce some ionization) of the target, and such that, in operation, the third transmitted portion is amplified by the multistage amplifier and delivered to the target, the amplified third transmitted portion interacting with the target, subsequent to the amplified second transmitted portion, to convert the target or a portion thereof to a an EUV -light-producing plasma.

[0025] In additional general aspects, a pulsed laser system includes a laser configured to deliver pulses of light; an optical shutter including an electro-optic modulator (EOM) configured to receive a pulse of light from the laser; a low-voltage driver connected to supply a low voltage to the EOM; a high-voltage driver connected to supply a high voltage to the EOM; and one or more isolators connected to the low -voltage driver and the high-voltage driver and configured to isolate the low voltage driver from the high voltage supplied by the high voltage driver.

[0026] Implementations can include one or more of the following.

[0027] The low-voltage driver can be a variable -voltage driver. The EOM, the low-voltage driver, and the high voltage driver can be configured to transmit, of the pulse of light from the laser: (a) with the EOM driven by the low-voltage driver, a first transmitted portion, the first transmitted portion having a first portion peak power and a first portion rise time of less than 20 ns, (b) a second transmitted portion subsequent to the first transmitted portion, the second transmitted portion having a second portion start time, a second portion duration, a second portion peak power, and a second portion initial power less than the first portion peak power, and (c) with the EOM driven by the high- voltage driver, a third transmitted portion subsequent to the second transmitted portion, the thirdtransmited portion having a third portion energy of at least 100 nJ and a third portion duration of less than 200 ns.

[0028] The EOM, the low-voltage driver, and the high voltage driver can be configured to transmit, of the pulse of light from the laser: (a) with the EOM driven by the low -voltage driver, a first transmited portion, the first transmited portion having a first portion start time, a first portion duration, and a first portion peak power; (b) a second transmited portion subsequent to the first transmited portion, the second transmited portion having a second portion start time, a second portion duration, a second portion peak power, and a second portion initial power less than the first portion peak power, and (c) with the EOM driven by the high-voltage driver, a third transmited portion subsequent to the second transmited portion, the third transmited portion having a third portion peak power, a third portion energy of at least 100 nJ, and a third portion duration of less than 200 ns, wherein the first portion peak power is in the range of 1 / 1,000,000 to 1 / 3 of the third portion peak power. The EOM and the low-voltage driver can be further configured to transmit, with the EOM driven by the low-voltage driver, the second transmited portion. The second portion peak power can be greater than the second portion initial power and within the range of from 50% to 400% of the first portion peak power. The second portion peak power can occur within a last 30% of the duration of the second portion. The optical shuter can be configured to receive one or more signals and to independently control a start time and a duration of the second portion in response to the one or more signals. The optical shuter can be configured to receive one or more signals and to independently control the first portion peak power and / or the first portion total energy and / or a temporal shape of the first transmited portion, in response to the one or more signals. The optical shuter can be configured to receive one or more signals and to control a temporal shape of the second transmited portion in response to the one or more signals.

[0029] The laser can be s a CO2 laser.

[0030] The EOM can be configured to receive, before the arrival of the pulse of light at the EOM, a voltage signal configured to stop or reduce a vibration of the EOM. The low-voltage driver can be configured to provide to the EOM a voltage signal configured to stop or reduce the vibration of the EOM.

[0031] According to additional general aspects, in a method for improving the performance potential of a pulsed laser system that provides light to a target of an extreme ultraviolet (EUV) light source and of the associated EUV light source, where the pulsed laser system includes a laser configured to provide a light pulse to an optical shuter including a high-voltage driver connected via a high-voltage output or outputs to an electro-optic modulator (EOM), the method includes connecting a low-voltage driver to the laser system via a low voltage output or outputs connected to the EOM in parallel with the high-voltage output or outputs of the high-voltage driver and positioning one or more voltage isolators electrically between the output or outputs of the low -voltage driver and the output or outputs of the high-voltage driver to isolate the low -voltage driver from the high voltage driver so as to allowthe low-voltage driver and the high-voltage driver to individually drive the EOM to produce from the light pulse (1) with the EOM driven by the low-voltage driver, a first transmitted portion useful to produce a rarefication pulse for the EUV source and (2) with the EOM driven by the high-voltage driver, a subsequent transmitted portion useful to produce a main pulse for the EUV source.

[0032] Implementations can include one or more of the following. The low-voltage driver can be a variable voltage driver. The one or more voltage isolators can include a Zener diode. The one or more voltage isolators can include an insulated-gate bipolar transistor (IGBT). The one or more voltage isolators can include a MOSFET.

[0033] According to additional general aspects, a method of operating a pulsed laser in use with an EUV source includes using an optical shutter to pass a first portion of a laser pulse, the first portion having a rise time of 20 ns or less; passing the first portion through an amplifier to generate a rarefication pulse for an EUV source; using the optical shutter to pass a subsequent portion of the laser pulse, the subsequent portion having an energy of at least 100 nJ and a duration less than or equal to 200 ns; and passing the subsequent portion through the amplifier to generate a main pulse for the EUV source.

[0034] Implementations can include one or more of the following.

[0035] Using the optical shutter to pass the first portion can include using the optical shutter in partially open state. Using the optical shutter to pass the subsequent portion can include using the optical shutter in a fully open state.

[0036] The method can include using the optical shutter to block an intermediate portion of the laser pulse after the first portion and prior to the subsequent portion. The method can include using the optical shutter to pass an intermediate portion of the laser pulse after the first portion and prior to the subsequent portion, with the intermediate portion having an initial power less than a peak power of the first portion. The method can include passing the intermediate portion through the amplifier to generate, at least in part, a vaporization pulse for the EUV source.

[0037] The optical shutter can include an electro-optic modulator (EOM). The method can include, preceding the laser pulse at the optical shutter, applying one or more voltage signals to the EOM to reduce or control a vibration of the EOM. The first portion can have a rise time of 20 ns or less.

[0038] According to additional general aspects, a method of operating a pulsed laser in use with an EUV source includes using an optical shutter to pass a first portion of a laser pulse; passing the first portion through an amplifier to generate a rarefication pulse for an EUV source; using the optical shutter to pass a subsequent portion of the laser pulse, the subsequent portion having an energy of at least 100 nJ, and a duration less than or equal to 200 ns and a peak power of at least 10 times the peak power of the first portion; and passing the subsequent portion through the amplifier to generate a main pulse for the EUV source.

[0039] Implementations can include one or more of the following.

[0040] Using the optical shutter to pass the first portion can include using the optical shutter in partially open state. Using the optical shutter to pass the subsequent portion can include using the optical shutter in a fully open state. The method can include using the optical shutter to block an intermediate portion of the laser pulse after the first portion and prior to the subsequent portion. The method can include using the optical shutter to pass an intermediate portion of the laser pulse after the first portion and prior to the subsequent portion. The intermediate portion can have an initial power less than a peak power of the first portion. The method can include passing the intermediate portion through the amplifier to generate, at least in part, a vaporization pulse for the EUV source.

[0041] The optical shutter can include an electro-optic modulator (EOM). The method can include, preceding the laser pulse at the optical shutter, applying one or more voltage signals to the EOM to reduce or control a vibration of the EOM. The subsequent portion can have a peak power of at least 1000 times a peak power of the first portion.

[0042] According to additional general aspects, a pulse-timing control system includes an electrooptic modulator (EOM) crystal suitable for controlling a polarization of a light beam; a first EOM driver coupled to the crystal and configured to provide a first voltage pulse greater than 4 kV; a second EOM driver coupled to the crystal and configured to provide a second voltage pulse greater than 50 V; and an isolator coupled to the first and second EOM drivers and configured to prevent transmission of the first voltage pulse into the second EOM driver.

[0043] Implementations can include one or more of the following.

[0044] The control system can further include a controller configured to receive a timing input and to control a timing of the first voltage pulse relative to the second voltage pulse. The light beam can have a wavelength in the range of 9 to 11 pm. The isolator can include one or more insulated-gate bipolar transistors (IGBTs). The isolator can include one or more metal-oxide-semiconductor field-effect transistors (MOSFETs). The isolator can include one or more Zener diodes.

[0045] According to additional general aspects, a pulsed laser includes laser configured to provide a laser pulse and an optical shutter configured to receive the laser pulse and to pass a first portion of the laser pulse and a subsequent portion of the laser pulse, and configured such that, in operation, the first portion has a rise time of 20 ns or less and the subsequent portion has an energy of at least 100 nJ and a duration less than or equal to 200 ns.

[0046] According to additional general aspects, a pulsed laser includes a laser configured to provide a laser pulse and an optical shutter configured to receive the laser pulse and to pass a first portion of the laser pulse and a subsequent portion of the laser pulse and configured such that, in operation, the subsequent portion has an energy of at least 100 nJ and a duration less than or equal to 200 ns and a peak power of at least 10 times the power of the first portion.

[0047] Further implementations include one or more of the following.

[0048] The optical shutter is configured to transmit the third transmitted portion during an optical shutter time window centered before a peak power of the pulse of light.

[0049] The pulsed laser system can further include an acousto-optical modulator (AOM) downstream of the optical shutter with the AOM configured to be operated for an AOM time window during which an optical pulse can pass the AOM with a minimum attenuation, with the AOM time window beginning at an AOM start time, with the first portion start time and the AOM start time set relative to each other such that energy transmitted by the optical shutter at and / or before a leading edge of the first transmitted portion is attenuated by the AOM.

[0050] The AOM and the optical shutter can be timed relative to each other to cause the AOM start time to begin when a leading edge of the first transmitted portion arrives at the AOM or up to 100 nanoseconds (ns) before.

[0051] The optical shutter can be configured to independently control the second portion energy. The optical shutter can include a first linear polarizer upstream of the EOM and a second linear polarizer downstream of the EOM, wherein the second linear polarizer is configured to be optically orthogonal to the first linear polarizer so as to minimize leakage of the optical shutter. The optical shutter can include a first linear polarizer upstream of the EOM and a second linear polarizer downstream of the EOM wherein the second linear polarizer is configured vary from ideal optical orthogonally to the first linear polarizer to provide a leakage of the optical shutter. A power of the leakage can be a percentage of the incident power range of 0.002% to 0.03%, or 0.005% to 0.02%, or 0.008% to 0.015%.

[0052] The pulsed laser can include a discharge circuit connected to the EOM configured to discharge, at and / or after the end of the first transmitted portion, a voltage present at the EOM. The discharge circuit can include a switch configured to discharge the voltage at and / or after the end of the first transmitted portion and before the third transmitted portion. The discharge circuit can include a variable resistor across which the voltage is discharged. The variable resistor can be configured to receive a control signal and to adjust its resistance in accordance with the control signal. The variable resistor can be configured to control or assist in controlling the energy of the second transmitted portion by controlling the rate of discharge of the voltage during the second transmitted portion.

[0053] The pulsed laser system can include multiple acousto-optical modulators (AOMs) downstream of the optical shutter, each configured to be operated at a respective limited AOM time window in which an optical pulse can pass the respective AOM with a minimum attenuation, and the optical shutter can be configured to transmit the first transmitted portion at time offset toward the early side of the respective AOM time windows of the respective AOMs to attenuate energy arriving at the respective AOM at and / or preceding a temporally leading edge of the first transmitted portion.

[0054] The above-mentioned method can include providing a discharge circuit connected between the EOM and an electrical ground, the discharge circuit including a switch and a variable resistor in series between the EOM and the electrical ground and operating the variable resistor to control a discharge of a voltage at the EOM after the first transmitted portion and before the subsequenttransmited portion, thereby producing a controllable intermediate transmited portion between the first and subsequent transmited portions.

[0055] In the method, the optical shuter can include an electro-optic modulator (EOM), with the method further including: using a discharge circuit between the EOM and an electrical ground to discharge a voltage present at the EOM at the end of passing the first portion; using a variable resistor in the discharge circuit control the discharge of the voltage and to pass an intermediate portion of the laser pulse after the first portion and prior to the subsequent portion; and passing the intermediate portion through the amplifier to generate, at least in part, a vaporization pulse for the EUV source.

[0056] In the method, the pulsed laser can include multiple acousto -optical modulators (AOMs) positioned optically downstream of the optical shuter, and the method can include: operating each respective AOM at a respective limited time window during which an optical pulse can pass the respective AOM with a minimum atenuation, and passing the first transmited portion at the optical shuter at a time offset toward the early side(s) of the respective time windows of the respective AOMs so as to atenuate energy arriving at the respective AOM at and / or preceding a temporally leading edge of the first transmited portion.

[0057] The method can include passing the subsequent transmited portion during a time window centered at a time preceding a peak of the laser pulse. The method can include transitioning the optical shuter or allowing the optical shuter to transition from a partially open state to a closed or less partially open state while transmiting an intermediate portion of the laser pulse. In the method, the optical shuter can include an EOM and transitioning the optical shuter or allowing the optical shuter to transition from a partially open state to a closed or less partially open state can include discharging a voltage present at the EOM across a resistance to an electrical ground. In the method, the resistance can include a variable resistor. The method can include adjusting a resistance of the variable resistor to improve the performance of the EUV source.

[0058] The pulse-timing control system can inlcude a variable-rate discharge circuit coupled to the EOM, the discharge circuit including a switch configured to provide essentially zero discharge when open and a variable discharge resistor configured to control a rate of discharge when the switch is closed. The controller can be configured to control a timing and a rate of discharge of the variable-rate discharge circuit.

[0059] In still more general aspects, a method of operating a pulsed laser source includes: producing a pulse from a pulsed laser; receiving the pulse at an optical shuter including an electro-optic modulator (EOM); passing a portion of the pulse through the optical shuter in a partially open state of the optical shuter; passing the portion of the pulse through two or more acousto-optic modulators (AOMs); respectively actuating the two or more AOMs sufficiently late relative to a respective arrival of the portion of the pulse at the respective AOMs to atenuate energy arriving at the respective AOMs at and / or preceding a temporally leading edge of the first transmited portion.

[0060] Implementations can include one or more of the following.

[0061] The optical shutter can include pre-and post-EOM polarizers angularly positioned relative to each other so as to produce an extinction less than a maximum possible extinction of the pulse by the optical shutter in a closed state of the optical shutter. The produced extinction can be in the range of 0.002% to 0.03%, or 0.005% to 0.02%, or 0.008% to 0.015%.

[0062] The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims.DRAWING DESCRIPTION

[0063] FIG. 1 is a schematic cross-sectional diagram of an EUV light source as part of a processing apparatus that uses EUV light.

[0064] FIG. 2 is a schematic diagram illustrating the production of certain pulses in an EUV light source such as the EUV light source of FIG. 1.

[0065] FIG. 3 diagram of an optical shutter including an electro-optic modulator (EOM) having both a low-voltage and a high-voltage driver connected in parallel.

[0066] FIG. 4 is a diagram of an N-channel MOSFET useful as an isolator as an alternative to the isolators shown in FIG. 3.

[0067] FIG. 5 is a graph of some examples of voltage signals applied to an EOM of an optical shutter such as the optical shutter of FIG. 3.

[0068] FIG. 5 A is an enlarged view of a portion of FIG. 5.

[0069] FIG. 6 is a graph of examples of pulse(s) that can be applied to a fuel target within an EUV source.

[0070] FIG. 7A is a graph showing a relative timing of an operation of an AOM and an optical shutter.

[0071] FIG 7B is a graph showing another relative timing of an operation of an AOM and an optical shutter.

[0072] FIG. 7C is a graph showing yet another relative timing of an operation of an AOM and an optical shutter.

[0073] FIG. 8 is a graph illustrating a shift of the signals applied to an optical shutter to an earlier time relative to a temporally Gaussian (or approximately temporally Gaussian) pulse.

[0074] FIG. 9 is a graph of examples of pulse(s) that can be applied to a fuel target within an EUV source including a pre-pulse, together with an illustration of the effects of the example pulses on a target.DETAILED DESCRIPTION

[0075] FIG. 1 schematically depicts an extreme ultraviolet (EUV) light source 110 as part of a processing apparatus 100 that uses EUV light. The processing apparatus can be a lithographicexposure apparatus, for example. The EUV light source 110 is of a type that may be referred to as a laser produced plasma (LPP) source. The EUV light source 110 may receive light beams from a laser 112. The laser 112 can be a gas-medium, solid-state-medium, or other laser. The laser 112 can be a CO2 laser. The laser 112 can be arranged to deliver and deposit light energy into fuel or fuel targets 116, such as tin (Sn) or Xenon (Xe) droplets, for example, provided from a fuel emitter 114. The laser 112 can deposit energy into the fuel targets 116 via a one or more light pulses 118 that can be referred to as one or more pre-pulses 118. The laser 112 can deposit energy into the fuel 116 via second and third light pulses 120 and 122. For example, the laser 112 can deposit energy into the fuel via a second light pulse 120 that can be referred to as a rarefication pulse 120 (sometimes termed a rarefaction pulse). The laser 112 can additionally deposit energy into the fuel via a third laser pulse 122 that can be referred to as a main pulse 122. The laserl 12 may be arranged in one or more different configurations; the arrangement shown in FIG. 1 is not intended to be limiting, for example, more than one laser can be used.

[0076] Although tin is referred to in the following description, any suitable fuel material may be used. The fuel may for example be in liquid form, such as in the form of tin or xenon droplets, and may for example be a metal or alloy. In general, the fuel may be a material that, when in an ionized state, emits EUV radiation. The fuel emitter 114 may comprise a nozzle (not shown) configured to direct tin, such as tin droplets, along a trajectory towards a plasma formation region 124.

[0077] The pre-pulse or pre-pulses 118, the rarefication pulse 120, and the main pulse 122 are incident upon a fuel target 116 such as a fuel droplet at the plasma formation region 124. The deposition of laser energy into a fuel target creates a plasma 126 at the plasma formation region 124. Radiation, including EUV light, is emitted from the plasma 126 during de-excitation and recombination of ions of the plasma. A collector 128 reflects EUV light from the plasma 126 along an optical axis A of the collector, forming an EUV beam 130. The EUV beam can pass through an intermediate focus 132 of the EUV light source 110 to be received by an illumination system 134 of the processing apparatus 100.

[0078] The pre-pulse or pre-pulses 118 generated by the laser 112 can be used to improve the geometry of a fuel droplet before subsequent pulses arrive at the fuel droplet. For example, the prepulse or pre-pulses 118 can flatten the fuel droplet. The pre-pulse(s) 118 can also slightly disperse the fuel droplet, e.g., so that some atoms of the fuel target or droplet are in a vapor or plasma phase, which can enable the atoms of the fuel target or droplet to more easily absorb laser energy from subsequent pulses. The pre-pulse(s) can have low energy relative to the subsequent pulses to help achieve a desired flattening of a fuel target or droplet (hereinafter “droplet”).

[0079] The rarefication pulse 120 may be used to further improve the geometry of the droplet. For example, the rarefication pulse 120 may further disperse the droplet, such as in the general direction of the optical axis A of the collector 128, decreasing the density of the droplet and allowing more of the fuel material to be reached by a subsequent pulse. The rarefication pulse 122 can also generatesome ions from the droplet, which can increase an "inverse Bremsstrahlung" effect that contributes to generation of EUV light.

[0080] The main pulse 120 can be used to fully or mostly ionize the droplet. The ionized fuel generates radiation including the desired EUV light.

[0081] The present disclosure involves processes, methods, and apparatuses for generating a rarefication pulse 122 and main pulse 120 from a single approximately Gaussian pulse, but with rarefication pulse properties and timing substantially independent of the properties and timing of the main pulse. For example, independent control of one or more rarefication pulse properties can be achieved, such as timing relative to the main pulse, duration, average power, peak power, temporal (waveform) shape, and the like. Such independent control can be used to increase the efficiency and / or effectiveness of the rarefication pulse and the resulting efficiency useful output power of an LPP EUV light source.

[0082] FIG. 2 is a schematic diagram illustrating the production of certain pulses for contacting fuel targets in an EUV light source such as the EUV light source of FIG. 1. Optical components are shown along an optical path OP. Graphs G1-G4 illustrate certain features of pulses traveling along the optical path, and certain features of the operation of an optical shutter 242, at the general locations indicated by the outlined double-headed arrows. The laser 112 can take the form of a laser system 212 shown in FIG. 2.

[0083] As shown in FIG. 2, the laser system 212 includes a pulsed laser system 250 including a laser 240 configured to deliver pulses of light. The laser 240 can be a Q-switched CO2 laser producing pulses whose temporal profile is Gaussian or approximately Gaussian. An example of these Gaussian- profile (or approximately Gaussian-profile) pulses 241 is shown in graph Gl, which shows an example of the power of a single pulse 241 emerging from laser 240 over time. The optical shutter 242 receives pulses of light from the laser 240 and blocks or heavily attenuates undesired portions of the pulses 241. Graph G3 shows an example of the power emerging from optical shutter 242 over time. This graph and its inset show a first transmitted portion 252 of the original pulse to be used to produce, after amplification, a rarefication pulse to be delivered to a fuel target in an EUV source. An immediately earlier portion and an immediately later portion of the original pulse 241 have been blocked or highly attenuated. The optical shutter 242 does this by transmitting, of the given Gaussian pulse 241, the first transmitted portion 252 with a first portion start time 252st, a first portion duration 252d, a first portion peak power 252pp, and a first portion rise time 252rt of less than 20 nanoseconds (ns). The first portion rise time 252rt, defined as the time for the first transmitted portion to transition from 10% to 90% of an initial amplitude 252ia, can also be less than 10 ns, or less than 5 ns. These features are shown enlarged for easier representation in the inset of graph G3. By using the optical shutter 242 to individually select a portion of the original pulse 241 to produce, after amplification, a rarefication pulse, various properties of the resulting rarefication pulse can be independentlycontrolled relative to the main pulse, and a well-defined rarefication pulse with a short rise time can be reliably produced.

[0084] The optical shutter 242 also transmits, at least nominally (as some small degree of leakage is generally always present even when the optical shutter 242 is nominally fully closed) a second transmitted portion 254 subsequent to the first transmitted portion 252. The second transmitted portion has, in implementations, a second portion start time, a second portion duration, a second portion energy, a second portion peak power, and a second portion initial power, the second portion initial power being less than the first portion peak power 252pp. (In the implementation of FIG. 2, the second transmitted portion 254 is nominally zero, and is so small as to be not representable at the scales shown (e.g., in graph G3), which scales are approximate. Accordingly, no detailed features of the second transmitted portion 254 are shown in FIG. 2.)

[0085] The optical shutter also transmits, for producing the main pulse to be delivered to the target, a third transmitted portion 256 subsequent to the second transmitted portion 254. The third transmitted portion has a third portion start time 256st, a third portion duration 256d, and a third portion energy, with the third portion energy being at least 100 nanojoules (nJ), 200 nJ, or 300 nJ, or even at least 400 nJ, or in ranges among these values. The third portion has a third portion duration 256d less than or equal to 200 ns, e.g., less than or equal to 100 ns, in the range of 60 ns to 100 ns, or 70 ns to 90 ns, or such as 80 ns. The third portion start time can be within the range of more than 20 ns and less than 1000 ns after the first portion start time, e.g., within the range of 20 to 300 ns after the first portion start time.

[0086] Graph G2 shows the temporal relationships among the original pulse 241, the first transmitted portion 252, and the third transmitted portion 256. As shown in the figure in graph G2, the third transmitted portion 256, as the seed for the main pulse, is selected from a region at or near the peak power of the Gaussian pulse 241. In contrast, the first transmitted portion 252 is selected from the Gaussian pulse 241 at a much lower-power portion of the pulse. Further, a signal 256v to the optical shutter 242 for the third transmitted portion 256 is a high value signal represented by the height hv of the signal 256v - corresponding to the optical shutter 242 being fully open or otherwise highly transmissive. In contrast, a signal 252v to the optical shutter for the first transmitted portion 252 is a low value signal represented by the low height Iv of the signal 252v - corresponding to the optical shutter being only partially (or slightly) open or otherwise less transmissive. Selecting the first transmitted portion 252 from the lower power region(s) of the Gaussian pulse 241, together with only partially or slightly opening the optical shutter 242 to perform the selection, ensures that the first transmitted portion 252 (which will serve as the seed for the rarefication pulse) has a much lower peak power than the third transmitted portion 256 (which will serve as the seed for the main pulse). Thus the third transmitted portion 256 can have a third portion peak power 256pp as many as 100, 1000, or even 10,000 times or more greater than the first transmitted portion 252. (This large difference in scale is not represented in the relative scale in FIG. 2.)

[0087] Such a large peak power difference between the transmitted portion used for the main pulse (third transmitted portion 256 of the Gaussian pulse 241) and the transmitted portion used for the rarefication pulse (first transmitted portion 252 of the Gaussian pulse 241) is needed in part because of a different effective level of amplification experienced by the two first and third transmitted portions. After each respective transmitted portion is generated, it is passed through an amplifier 247. The amplifier 247 can be a multistage amplifier. The amplifier 247 can additionally or alternatively be a multi-pass amplifier. The amplifier 247 and the third transmitted portion for the main pulse are generally designed and sized relative to each other to allow the third transmitted portion to utilize as much of the total available energy in the amplifier 247 as possible. As a result the amplifier 247 can tend to operate in saturation or near-saturation when amplifying the third transmitted portion 256., On the other hand, the amplifier 247 is far from saturation when amplifying the first transmitted portion 252 for the rarefication pulse. As a result, the amplifier 254 amplifies the first transmitted portion 252 by a significantly greater factor than the third transmitted portion 254. This is reflected graphically in the graph G4, which shows an example of the light power emerging from amplifier 247 over time. Graph G4 shows a rarefication pulse 252amp and a main pulse 254amp. The rarefication pulse 252amp is the result of passing the first transmitted portion 252 through the amplifier 247. The main pulse 254amp is the result of passing the third transmitted portion 254 through the amplifier 247. Graph G4 shows these two amplified pulses 252amp and 254amp in approximate relative power scale. From the amplifier 247, the amplified pulses can be passed through a beam delivery system 248 to an EUV light source 210 and a fuel target therein (not shown). The EUV light source 210 can be a part of a processing apparatus 200.

[0088] Prior to entering the amplifier 247, the respective first, second, and third transmitted portions 252, 254, and 256 can pass through a preamplifier 249 (or two or more preamplifiers), as well two or more acousto-optic modulators (AOMs) 251 (four shown in this example) before the preamplifier 249 and one or more acousto-optic modulators 253 (one shown in this example) after the preamplifier 249.

[0089] A controller CTR can be in communication (one or two-way) with the various parts of the laser system 212 and can coordinate and control the operation and relative timing of the optical shutter 242, as well as the relative timing of the optical shutter and the AOMs 251 and the AOM or AOMs 253. The controller CTR can be in hardware, in software, in a virtual computing environment, in a single real or virtual location, or distributed over two or more real and / or virtual locations, or combinations of these. Communication with the controller CTR can be any of various methods, including but not limited to wired, optical, wireless, and so forth.

[0090] FIG. 3 is a diagram of an implementation of an optical shutter 342 which can be useful as the optical shutter 242 of FIG. 2, for example. The optical shutter 342 receives pulses Pu from a pulsed laser (not shown) along an optical path OP. Generally, the incoming pulses already have a linear polarization (horizontal linear polarization in the implementation shown). The incoming pulses are (nonetheless) passed through a horizontal linear polarizer 344 such as a thin-film polarizer (TFP)to reduce any polarization noise and to have the polarizer 344 positioned and arranged to be able to reduce or prevent back-propagating light. Incoming pulses are then passed through an electro-optic modulator (EOM) 345. The EOM 345 may include an electro-optic material such as lithium niobate crystal controllably supplied with voltage by one or more voltage drivers 360, 362. When no voltage is applied to the EOM 345, the light passing through is not polarization-rotated and is then rejected (reflected away from the optical path OP) by a downstream vertical linear polarizer 346. When the EOM 345 is fully energized (when the EOM 345 has its designed 90-degree rotation voltage applied), the incoming pulses are polarization-rotated by 90 degrees as they pass through, allowing the pulses to pass the vertical linear polarizer 346 and continue along the optical path OP. Note that “horizontal” and “vertical” are used here to indicate relative optical orientations of polarizers only, not actual orientations relative to gravity or some other reference. In additional implementations, first and second alternate (or alternately-aligned) polarizers 344a, and 346a are used. The first alternate linear polarizer 344a upstream of the EOM and the second linear polarizer 346a downstream of the EOM can be configured such that the second alternate linear polarizer varies slightly from ideal optical orthogonality relative to the first alternate linear polarizer. This can provide a desired leakage of light through the optical shutter which can be used to help provide relatively more power in the first transmitted portion. The leakage can be in the range of 0.002% to 0.03%, or 0.005% to 0.02%, or 0.008% to 0.015% of incident power, for example.

[0091] In the implementation shown, a low-voltage driver 362 and a high-voltage driver 360 are connected in parallel to the EOM 345. Each driver supplies a positive and a negative voltage in this implementation. In some other implementations, only a positive or only a negative voltage can be supplied, with the opposite surface of the EOM 345 connected to ground. One or more isolators, two in this example, a first isolator 364a and a second isolator 364b, are positioned between the low- voltage driver 362 and the high-voltage driver 360 to shield the low voltage driver 362 from the high voltage output of the high voltage driver 360. This is beneficial in that a low voltage driver such as low voltage driver 362 is typically not designed to receive a high voltage on its outputs. Also, the isolation ensures that the operation of the high voltage driver 360 is not affected by any significant current or voltage leakage to the low voltage driver 362.

[0092] Given that the isolators 364a, 364b prevent or limit current flow back toward the low voltage driver 362, a voltage supplied to the EOM 345 by the low voltage driver 362 tends to persist at the EOM 345 or to dissipate only relatively slowly. A discharge circuit 370 is thus useful to cut off or bring down the effective voltage of the signal 252v (FIG. 2) at a desired time and at a desired rate. Discharge is generally needed for each side of the EOM not operated at ground voltage, that is, both sides in the implementation shown in FIG. 3. The discharge circuit 370 thus includes a first discharge subcircuit 370dl and a second discharge subcircuit 370d2. The respective first and second discharge subcircuits 370dl, 370d2 electrically connect a respective side of the EOM 345 to electrical ground through respective first and second switches 372a, 372b. The first and second switches 372a, 372b areprogrammed, or signaled, or otherwise operated or controlled to be open (non-conducting), such that the level of the signal 252v is not reduced by connection to ground, except starting at or soon after the nominal or desired end of the signal 252v for the first transmitted portion 252 (FIG. 2). The first and second switches 372a, 372b are programmed, or signaled, or otherwise operated or controlled to be closed (conducting) to allow the voltage on the EOM to drain away, at a time before the beginning of the third transmitted portion 256 (FIG. 2) Insulated-gate bipolar transistors (IGBTs) can be used for the first and second switches 372a, 372b.

[0093] In at least some implementations, the respective first and second discharge subcircuits 370dl, 370d2 include respective first and second fixed-value resistors 374a, 374b. The first and second fixed- value resistors 374a, 374b can have a value in the range of 20 to 100 Ohms, 30 to 80 Ohms, or 50 to 60 Ohms, for example, and can help provide a stable and repeatable minimum fall time and fall rate of the voltage signal 252v. In at least some implementations, the respective first and second discharge subcircuits 370dl, 370d2 include respective first and second variable resistors 376a, 376b, which can have a variable resistance range such as 0 to 2000 Ohms, or 0 to 1000 Ohms, for example. Providing a discharge of the low-voltage-driver-produced voltage at the EOM across a variable resistance allows the fall or decay of the voltage signal 252v of FIG. 2 to be controlled and / or tuned as desired to produce a desired level of control signal to produce the second transmitted portion, such as signal 254vb discussed below with respect to FIG. 5, resulting in a “vaporization pulse” such as the vaporization pulse 254amp-b discussed below with respect to FIG. 6.

[0094] In principle a single high-voltage-capable variable -voltage driver could be used as generally as described herein, but the overall performance envelope is challenging. As high as 5000-6000 volts or more can typically needed for producing a full 90-degree rotation in a wide-aperture EOM such as the EOM 345, and, in contrast, as low as 100 volts or even less, with both fast rise time and good resolution, can be needed to transmit the first transmitted portion for the rarefication pulse through the EOM 345. Thus, the use of two voltage drivers is significantly more economical and practical. Further, a low-voltage driver with isolators, optionally in the form of a low-voltage variable-voltage driver, can be added relatively easily to systems that are already qualified, and even to systems that are already in service, offering an economical upgrade and performance-improvement path.

[0095] In the embodiment shown in FIG. 3, the isolators 364a, 364b are in the form of Zener diodes, as indicated. High voltage high speed Zener diodes are used. Alternatively, an insulated-gate bipolar transistor (IGBT) can be used. For example, an N-channel MOSFET 464, as shown in diagrammatic / symbolic form in FIG. 4, can be used as an isolator in an optical shutter like the optical shutter 342 of FIG. 3. The positive terminal of the low voltage driver (LV+) of the MOSFET 464 can be connected to the source S, while the corresponding (positive) terminal of the high voltage driver (HV+) can be connected to the drain D. In various implementations of the optical shutter 342, a gate G of the MOSFET may be left floating / unconnected.

[0096] The low-voltage driver 362 can drive the EOM 345 (FIG. 3) during a first transmitted portion such as the first transmitted portion 252 of FIG. 2, and the high-voltage driver 360 can drive the EOM 345 (FIG. 3) during a third transmitted portion such as the third transmitted portion 256 of FIG. 2. This allows a well-controlled partial or slight opening of the optical shutter (by a small polarization rotation of the incoming pulse) during the low-voltage operation of the EOM 345 and a fully or more fully open operation of the optical shutter (by a full 90-degree polarization rotation) during the high- voltage operation of the EOM 345. In some implementations, the low-voltage driver 362 can be a variable-voltage driver allowing for real-time adjustment of the properties of rarefication pulse(s) and other pulses to be discussed below.

[0097] Additional aspects and variations are illustrated in FIG. 5, which is a graph of some examples of signals, in the form of voltage signals or voltages in this implementation, that can be applied to an EOM of an optical shutter such as the EOM 345 of the optical shutter 342 of FIG. 3, in order to produce both an independently controlled rarefication pulse and a main pulse from the same pulsed laser. FIG. 5 A shows an enlarged view of features on the right side of FIG. 5.

[0098] With reference to FIGS. 5 and 5 A, two successive sets of signals are shown graphically as relative voltage values (V axis) overtime (T axis). A first successive approximately Gaussian light pulse 241a from the pulsed laser and a second successive such pulse 241b are represented in dashed outline as a function of relative power over time T. A signal 252va to an optical shutter, such as a voltage signal to an EOM such as EOM 345, selects a temporal slice of the pulse 241a to be transmitted through the optical shutter. This selection forms an optical pulse such as the first transmitted portion 252 (from FIG. 2) of the first pulse 241a. The temporal slice selected by signal 252va can serve as a pre -amplification optical pulse for producing a rarefication pulse. Similarly, a signal 256vato the optical shutter, such as another voltage signal to the EOM 345, selects another temporal slice of the pulse 241a to be transmitted through the optical shutter. This selection forms an optical pulse such as the third transmitted portion 256 (from FIG. 2) of the pulse 241a. The temporal slice selected by signal 256va can serve as a pre-amplification optical pulse for producing a main pulse.

[0099] Optionally, during the time between the successive approximately Gaussian pulses 241a and 241b, one or more additional voltage signals 266vb can be applied to the EOM 345. The one or more additional voltage signals 266b can be sized and shaped to reduce or substantially eliminate any vibration of the EOM that may be present, such as vibration caused by the immediately previous relatively high-voltage signal 256va. Note that because they are applied to the EOM 345 outside the envelope of the optical pulse 241b, the one or more additional signals 266vb do not produce “transmitted portions” of the pulse 241b. Similar additional signals 266va can be used in advance of the pulse 241a. For example, signals 266vb and 266va can be in the range of 10 to 100V.

[0100] As a further option, as shown on the right side of FIG. 5 and in the enlarged view of FIG. 5 A, a signal 254bv of more than nominally zero volts can be used between the signals 252vb and 256vb,such that a second transmitted portion 254 (from FIG. 2) of the signal 241b passes the optical shutter 342 with more than nominally zero power. This second transmitted portion, when more than nominally zero power, can be used to generate, after amplification, a “vaporization” pulse that assists in vaporizing remaining droplets present in the target after the rarefication pulse and / or in pre-ionizing some the target material which can increase an "inverse Bremsstrahlung" effect that contributes to generation of EUV light. As shown in FIG. 5A, the signal 254vb (as well as the resulting second transmitted portion (not shown)) can begin at an initial value lower than the value of the previous signal 252vb. The signal 254vb can also remain steady, increase, or decrease over time (as in the example shown) leading up to the signal 256vb. The signal 254bv selects a temporal slice of the pulse 241b to be transmitted through the optical shutter at a lower transmission rate relative to a transmission rate produced by the signal 252bv. This results in a second transmitted portion 254 having a lower initial amplitude or power 254ia than the peak power of the first transmitted portion. The temporal slice selected by the signal 241b is positioned within a relatively steep upward ramp of the pulse 241b. This can bias the distribution of the energy in the selected slice toward the time of the signal 256vb used for generating the third transmitted portion for the main pulse. The resulting biasing of the energy of a vaporization pulse to be close in time to the start of an associated main pulse helps to prevent the vaporization pulse from significantly altering the geometry of the target, while still allowing some desired amount of vaporization and / or ionization to take place before the main pulse begins.

[0101] FIG. 6 is a graph of two traces showing pulse power after amplification (after the multistage amplifier 247 of FIG. 2) for two variations. The lower trace shows a first variation that uses a rarefication pulse 252amp-a such as one produced by the signals 252va, 254va, 256va shown at the left of FIG. 5 to produce a rarefication pulse 252amp-a followed by a low level or nominal vaporization pulse 254amp-a leading to a main pulse 256amp-a (with full height not shown on the represented scale). The vaporization signal 254vb is not intentionally generated in this variation; it is only nominally present as a result of leakage such as leakage produced by acoustic vibrations that remain in the EOM from the shuttering control of a previous optical pulse. Such vibrations can generate leakage through the optical shutter 242, the result of which after amplification is shown as a vaporization pulse 254amp-a of the lower trace. The upper trace shows a second variation in which a rarefication pulse 242amp-b is produced, for example, by the signals 252vb, 254vb, 256vb shown at the right of FIG. 5 and in FIG. 5A, to produce a rarefication pulse 252amp-b, then a vaporization pulse 254amp-b, followed by a main pulse 256amp-b (with full height not shown on the represented scale). The deliberate signal 254vb, that may be produced at least in part and adjusted by a discharge circuit such as the discharge circuit 370 shown in FIG. 3 discussed above, results in a controllable vaporization pulse 245amp-b that is more than nominally present in this variation. The vaporization pulse 245amp-b can assist in reducing the presence of un-vaporized remnant droplets of target material and / or in providing some pre-ionized target material at the time of the arrival of the mainpulse 245amp-b. In implementations such as shown on the left side of FIG. 5 and in the lower trace of FIG. 6, a second portion peak power can be in the range of 1 / 10 to 1 / 2 , or 1 / 50 to 1 / 2, or 1 / 50 to 1 / 4 or even 1 / 50 to 1 / 10 of the first portion peak power.

[0102] In implementations using signals such as shown on the right side of FIG. 5, in FIG. 5A, and resulting in fully amplified pulses as shown in the upper trace of FIG. 6, the second portion peak power can be greater than the second portion initial power and can be within the range of from 50% to 400% (e.g., 90% to 300%, 120% to 200%) of the first portion peak power. The second portion peak power can occur within a last 30% of the second portion duration. The power of the second transmitted portion can increase over the second portion duration.

[0103] Benefits can be obtained as described above from energy present in a “vaporization pulse” in a time just after a rarefication pulse as in the upper trace of FIG. 6. But energy transmitted to the plasma formation region in a time just before the rarefication pulse is ideally kept to a relatively low level (energy to the left of pulses 252amp-a and 252amp-b), as represented in FIG. 6. Otherwise rarefication can begin too early and the resulting rarefied target may be nonuniform and / or of variable shape and / or or too large in the z-direction (optical-axis direction) for efficient generation and utilization of EUV light. As discussed above, reducing or cancelling acoustic vibrations of the EOM can help reduce power transmitted leading up to a rarefication pulse.

[0104] Another way to help ensure low power of light transmission or low leakage prior to a rarefication pulse is to carefully align and / or preserve the alignment of the polarizers 344 and 346 of the optical switch 342 of FIG. 3 above for maximum extinction. This has the effect of minimizing leakage (maximizing extinction) through the EOM-based optical shutter in its closed state. In some embodiments, however, it can be desirable use alternate polarizers 344a and 344b (or to tune or adjust the polarizers 344 and 346 into an alternative position(s) of slight “misalignment” as illustrated with respect to alternate polarizers 344a and 344b) to provide a relatively more leakage (or lower than maximum extinction) through the EOM-based optical switch 342. The intentionally provided leakage, such as in the range of .002% to 0.03%, or 0.005% to 0.02%, or 0.008% to 0.015% of incident power as mentioned above, produces a “pedestal” (a rise in the light energy passing the closed optical shutter just prior to the main pulse). A higher or more energetic pedestal can help contribute energy to the formation of a vaporization pulse having a desired energy level. But a higher or more energetic pedestal can also tend to produce an undesired amount of optical-shutter-transmitted energy in the time prior to the rarefication pulse.

[0105] An additional way to help ensure relatively low transmitted energy prior to the rarefication pulse is to adjust the relative timing of (1) the first transmitted portion 252 or rarefication pulse seed (FIG. 2), (i.e., the timing of the signal 252v (FIG. 2) that selects the first transmitted portion 252 from the original pulse 241), and (2) the timing of the opening of AOM windows of one or more of the two or more AOMs 251 and the one or more AOMs 253 of FIG. 2. (Typically the timing each of the firsttransmited portion, the second transmited portion, and the third transmited portion would be adjusted together by essentially the same amount.)

[0106] By adjusting the timing of the signal 252v and / or the timing of the opening of one or more downstream AOM windows such that the one or more AOMs are biased toward late opening relative to the signal 252v, the one or more AOMs atenuate energy arriving at the respective AOM at and / or before a temporally leading edge of the first transmited portion 252, and thus decrease any energy transmited before the first transmited portion 252 or “rarefication pulse seed. Two, three, four, five or more AOMs can be used to atenuate in this manner, with each contributing to the total atenuation. Atenuating the temporally leading edge of the first transmited portion by positioning the windows of one or more AOMs relatively later in time compared to the signal 252v for the first transmited portion can also allow use of slight intentional misaligning of polarizers 344 and 346 (FIG. 3) for increased power in a vaporization pulse such as vaporization pulse 254amp-b of FIG. 6, as well as in the rarefication pulse 252amp-b, while still preserving a desired low level of transmited power prior to the rarefication pulse. Advancing the AOM timing of two or more AOMs, and / or retarding the first (and second and third) transmited portion timing to produce a total time shift the range of 20 to 150 ns, or 40 to 100 ns, or 60 to 80 ns can allow a reduction of the energy present in the optical path immediately prior to the first transmited pulse by a percentage in the range of 10 to 50%, or 10 to 30%, or 10 to 20%.

[0107] FIGS. 7A, 7B, and 7C are graphs illustrating aspects of the operation of an acousto-optic modulator (AOM) and an optical shuter with relative timing adjusted as discussed above. Time is on the x-axis, increasing from left to right, and transmited optical power is represented (in an arbitrary logarithmic scale) on the y axis. Note that the peak of a main pulse 256amp-c cannot be shown at the scale used, and so is shown in truncated form, with the actual peak being above the upper limits of the figures. FIGS. 7A-7C show a trace of post-multistage amplifier laser energy or laser-sourced energy including a rarefication pulse 252amp-c, a vaporization pulse 254amp-c, and a main pulse 256amp-c. The rarefication pulse 252amp-c and the vaporization pulse 254amp-c of FIGS. 7A-7C include (and accordingly are increased in relative power by) a “pedestal” or “natural pedestal” NP (shaded region) which is produced by an amount of light or energy leakage through the closed optical shuter prior to the main pulse 256amp-c, which leakage is then amplified such as by the multi-stage amplifier. The amount of leakage can be adjusted or tuned by adjusting the relative positions of pre- and post- EOM polarizers , as described above in relation to FIG. 3.

[0108] FIG. 7A shows operation of an AOM with essentially normal timing. In the operation of an AOM, an acoustic (or acousto-optic) wave (or wave group) is generated and travels across a beam aperture at the AOM. The AOM or the generated wave effectively has an AOM time window TW (FIGS. 7A and 7B) during which all or essentially all of a beam or pulse incident at the beam aperture on the AOM is passed onward along an optical path (such as optical path OP of FIG. 2), that is, a time window during which a beam or pulse incident at the beam aperture is passed with a minimumatenuation. A start Stw of the time window TW is marked on the graph by the long -dashed line. The time window TW is preceded by an up-transition period UT and a down-transition period DT of the AOM during which a portion of beam or pulse incident on the AOM can be passed onward along the optical path (FIG. 7A).

[0109] FIG. 7B illustrates the effects of advancing the timing of an optical shuter, such as an EOM, (arrow Adv), or retarding the timing of an AOM (arrow Ret), or using a combination of such advancing and retarding sufficiently to bring the start Stw of the of the AOM time window TW to a position at or close before the main body of the rarefication pulse 252amp-c. This results in the leading edge of the pre-rarefication-pulse, and / or pre -rarefication pulse energy from leakage at the optical shuter, being reduced or atenuated, producing a reduced energy level Rel (indicated by the small-dashed line relative to the original trace) prior to the main body of the rarefication pulsed 252amp-c.

[0110] FIG. 7C illustrates that when the timing of the optical shuter has been advanced, or the timing of the AOM has been retarded, or a combination thereof, then, if desired, a reduced-length time window RTW can be used for the AOM. As in FIG. 7B, the start Stw of the (reduced) time window RTW is positioned at or close before the main body of the rarefication pulse 252amp-c, resulting in the leading edge of the pre-rarefication-pulse, and / or pre-rarefication pulse energy from leakage at the optical shuter, having a reduced energy level Rel. Because the AOM time window is a reduced time window RTW, which is open essentially no longer than necessary to allow the main pulse 256amp-c to pass the AOM, there is less time for potential undesired transmissions to pass the AOM relative to the timing shown in FIG. 7B.[oni] FIGS. 7B and 7C illustrate the effects of applying a desired “shifted” relative timing between an optical shuter and an AOM. Multiple AOMs, such as two, three, or four or more AOMs of the two or more AOMs 251, and one or more of the one or more AOMs 253 can all be used in the same fashion to increase the total atenuation of the leading edge of the pre-rarefication pulse and / or the pre-rarefication pulse energy.

[0112] Note that the pulses and energies represented in FIGS. 7A-7B are all shown post-multi-stage- amplification, while the actual operation of the optical shuter and the AOMs takes place pre-multi- stage-amplification, such as by the multi-stage amplifier 247 of FIG. 2.

[0113] FIG. 8 is a graph representing the timing of an implementation in which the above-described time shift between the AOMs and the optical shuter 242, or at least a significant portion thereof, is produced by shifting the first, second, and third transmited portions 252, 254, and 256 of FIG. 2 earlier in time relative to both the timing of the AOMs (not shown) and relative to the timing of the original Gaussian-profile (or approximately Gaussian-profile) pulse 241b. (A comparison of FIG. 8 with the right half of FIG. 5 shows the time shift in FIG. 8 of a rarefication signal 252vbs, a main pulse signal 256vbs, and an associated vaporization signal 254vbs.) The main pulse signal or “optical shuter time window” (or the center thereof) would generally typically be timed to coincide with thepeak amplitude of the Gaussian pulse 24 lb. Moving the main pulse signal or optical shutter time window to an earlier time in the form of a shifted main pulse signal 256vbs can result in a small decrease in power selected out from the Gaussian pulse 241b to form the seed pulse of the main pulse. But the shift can also move the timing of the shifted rarefication signal 252vbs and the resulting rarefication pulse toward the leading tail of the Gaussian pulse 241b, where less energy is present preceding the rarefication pulse, reducing pre-rarefication pulse energy. Specifically, a center of the optical shutter time window can be positioned before the peak of the Gaussian pulse 241b.

[0114] FIG. 9 is a graph showing an example of a relationship between a rarefication pulse 252amp, a main pulse 256amp, and an associated pre-pulse PP. Above the graph are illustrations of a target, beginning in the form of a droplet DT, showing an example of the effects on the target of the pulses in the graph. As seen in FIG. 9, the pre-pulse PP arrives well before the rarefication pulse 252amp, at a time in the range of 2000 to 4000 ns before the start of the main pulse 256amp. The pre-pulse PP irradiates a target (or “pre-target”) in the form of a droplet DT. The pre-pulse causes the droplet DT to change shape into a disk that progresses through disk shapes DK1, DK2, DK3, with increasing widths DK1W. DK2W, DK3W over time. Once the target has reached a desired shape DK3 and width DK3W, the rarefication pulse 252amp irradiates the target, causing a decrease in the target’s density and producing a rarefied target RT in part by distributing the material of the target, over time, in a direction approximately perpendicular to the disk width DK3W. The rarefied target RT is fully formed, with a rarefied target width RTW and a rarefied target depth RTD, by the time (time 0 in the figure) of the main pulse 256amp.

[0115] The pre-pulse can be provided by a different laser from the laser, such as a CO2 laser, that provides the rarefication and main pulses. Alternatively, the pre-pulse can also be provided by the CO2 laser. If provided by the CO2 laser, the pre-pulse PP can have a duration in the range of 60 ns to 100 ns, or 70 ns to 90 ns, such as approximately 60, 70, 80, 90, or 100 ns, with an energy in the range of 50 mJ to 120 mJ, such as approximately 50, 60, 70, 80, 90, 100, 110, or 120 mJ, or in ranges among these values. The rarefication pulse 252amp can have a duration in the range of 10 ns to 50 ns or 20 ns to 40 ns, such as approximately 10, 15, 20, 25, 30, 35, 40, 45, 50 ns, or in ranges among these values, with an energy of 0.2 mJ to 3 mJ such as approximately 0.2, 0.5, 1, 1.5, 2, 2.5. or 3 mJ, or in ranges among these values, or such as approximately 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, or 2.0 mJ, or in ranges among these values. The main pulse 256amp can have a duration shorter than 200 ns, such as in the range of 40 ns to 80 ns or 50 ns to 60 ns, such as approximately 40, 50, 60, 70, 80, 90, 100, 120, 140, 160, 180, 190 ns, or in ranges among these values, and an energy in the range of 400 mJ to 800 mJ or 500 mJ to 600 mJ, such as approximately 400, 450, 500, 550, 600, 650, 700, 750, or 800 mJ, or in ranges among these values. The width DK3W of the disk-shape of the target at the time of the arrival of the rarefication pulse (at time 0) can be 400 pm to 600 pm such as 500 pm. The width RTW of the rarefied target RT at time 0 can be 400 pm to 600 pm such as 500 pm. The depth RTD of the rarefied target RT at time 0 can be in the range of 500 pm to 2000 pm.

[0116] Aspects and implementations of the present disclosure can be further described using the following numbered clauses:1. A pulsed laser system including: a laser configured to deliver pulses of light; an optical shutter configured to receive a pulse of light from the laser and to transmit, of the pulse: (a) a first transmitted portion, the first transmitted portion having a first portion start time, a first portion duration, a first portion peak power, a first portion energy, and a first portion rise time, the first portion rise time being less than 20 ns, (b) a second transmitted portion subsequent to the first transmitted portion, the second transmitted portion having a second portion start time, a second portion duration, a second portion energy, a second portion peak power, and a second portion initial power, the second portion initial power being less than the first portion peak power, and (c) a third transmitted portion subsequent to the second transmitted portion, the third transmitted portion having a third portion start time, a third portion duration, and a third portion energy, the third portion energy being at least 100 nJ and the third portion duration being less than or equal to 200 ns.2. The pulsed laser system of clause 1, wherein the second portion peak power is in the range of 1 / 1000 to 1 / 10 of the first portion peak power.3. The pulsed laser system of clause 1, wherein the second portion peak power is greater than the second portion initial power and is within the range of from 50% to 400% of the first portion peak power.4. The pulsed laser system of clause 3, wherein the second portion peak power occurs within a last 30% of the second portion duration.5. The pulsed laser system of clause 3, wherein the power of the second transmitted portion increases over the second portion duration.6. The pulsed laser system of clause 1, wherein the third portion start time is more than 30 ns and less than 1000 ns after the first portion start time.7. The pulsed laser system of clause 1, wherein the first portion has a first portion energy, the first portion energy being between 100 femtojoules (fJ) and 3000 fj.8. The pulsed laser system of clause 1, wherein the first portion peak power is in the range of 1 / 1,000,000 to 1 / 3 of the third portion peak power.9. The pulsed laser system of clause 1, wherein the optical shutter is configured to receive one or more signals and is configured to independently control the second portion start time and the second portion duration in response to the one or more signals.10. The pulsed laser system of clause 1, wherein the wherein the optical shutter is configured to receive one or more signals and to independently control the first portion start time, relative to the third portion start time, and the first portion duration in response to the one or more signals.11. The pulsed laser system of clause 10, wherein the optical shutter is further configured to receive the one or more signals and to independently control the first portion energy.12. The pulsed laser system of clause 1, wherein the optical shutter is configured to receive one or more signals and to control one or more of the first portion duration, the first portion peak power, the first portion energy, a temporal shape of the first transmitted portion, the second portion start time, the second portion duration, the second portion energy, the second portion peak power, the second portion initial power, a temporal shape of the second transmitted portion, the third portion start time, and the third portion duration.13. The pulsed laser system of clause 1, wherein the laser is a CO2 laser and the optical shutter includes an electro-optical modulator (EOM).14. The pulsed laser system of clause 13, further including: a low -voltage driver connected to supply a low voltage to the EOM; a high-voltage driver connected to supply a high voltage to the EOM; and one or more isolators connected to the low-voltage driver and to the high-voltage driver and configured to isolate the low voltage driver from the high voltage supplied by the high voltage driver.15. The pulsed laser system of clause 14, wherein the low -voltage driver is configured to drive the EOM during the first transmitted portion and the high-voltage driver is configured to drive the EOM during the third transmitted portion.16. The pulsed laser system of clause 15, wherein the low -voltage driver is further configured to drive the EOM during the second transmitted portion.17. The pulsed laser system of clause 14, wherein one or more of the low-voltage driver and the high- voltage driver is configured to apply, before the arrival of the pulse of light at the EOM, one or more voltage signals to the EOM configured to reduce or control a vibration of the EOM.18. The pulsed laser system of clause 13, wherein the EOM is configured to receive, before the arrival of the pulse of light at the EOM, one or more voltage signals configured to reduce or control a vibration of the EOM.19. The pulsed laser system of clause 1, further including a multistage amplifier configured to receive the first and third transmitted portions from the optical shutter, and wherein, in operation, the first transmitted portion is amplified in the multistage amplifier and delivered to a target in an extreme ultraviolet (EUV) light source, the target including a material capable of emitting EUV light, the target having been previously shaped by a pre-pulse, the amplified first transmitted portion interacting with the previously shaped target to increase a rarefication of the previously shaped target, and wherein, in operation, the third transmitted portion is amplified in the multistage amplifier and delivered to the target, subsequent to the amplified first-transmitted portion, to convert the target or a portion thereof to an EUV-light-producing plasma.20. The pulsed laser system of clause 1, further including a multistage amplifier configured to receive the first, second, and third transmitted portions from the optical shutter, and wherein, in operation, the first transmitted portion is amplified in the multistage amplifier and delivered to a target in an extreme ultraviolet (EUV) light source, the target including a material capable of emitting EUV light when in a plasma state, the target having been previously shaped by a pre -pulse, the amplified first transmittedportion interacting with the previously shaped target to increase a rarefication of the previously shaped target, and wherein, in operation, the second transmitted portion is amplified by the multistage amplifier and delivered to the target, the amplified second transmitted portion interacting with the target subsequent to the amplified first transmitted portion to increase the vaporization of the target, and wherein, in operation, the third transmitted portion is amplified by the multistage amplifier and delivered to the target, subsequent to the amplified second transmitted portion, to convert the target or a portion thereof to a an EUV-light-producing plasma.21. A pulsed laser system including: a laser configured to deliver pulses of light; an optical shutter configured to receive a pulse of light from the laser and to transmit, of that pulse: (a) a first transmitted portion, the first transmitted portion having a first portion start time, a first portion duration, a first portion peak power, a first portion energy, and a first portion rise time; (b) a second transmitted portion subsequent to the first transmitted portion, the second transmitted portion having a second portion start time, a second portion duration, a second portion peak power, and a second portion initial power, the second portion initial power being less than the first portion peak power, and (c) a third transmitted portion subsequent to the second transmitted portion, the third transmitted portion having a third portion start time, a third portion duration, a third portion energy, and a third portion peak power, the third portion energy being at least 100 nJ and the third portion duration being less than or equal to 200 ns, wherein the first portion peak power is in the range of 1 / 1,000,000 to 1 / 3 of the third portion peak power.22. The pulsed laser system of clause 21, wherein the second portion peak power is in the range of 1 / 1000 to less than 1 / 10 of the first portion peak power.23. The pulsed laser system of clause 21, wherein the second portion peak power is greater than the second portion initial power and is within the range of from 50% to 400% of the first portion peak power.24. The pulsed laser system of clause 23, wherein the second portion peak power occurs within a last 30% of the duration of the second portion.25. The pulsed laser system of clause 23, wherein the power of the second transmitted portion increases over the second portion duration.26. The pulsed laser system of clause 21, wherein the third portion start time is more than 30 ns and less than 1000 ns after the first portion start time.27. The pulsed laser system of clause 21, wherein the first portion has a first portion energy, the first portion energy being between 100 fj and 3000 fj.28. The pulsed laser system of clause 21, wherein the optical shutter is configured such that the first portion rise time is less than 20 ns.29. The pulsed laser system of clause 21, wherein the optical shutter is configured to receive one or more signals and is configured to independently control the second portion start time and the second portion duration in response to the one or more signals.30. The pulsed laser system of clause 21, wherein the wherein the optical shutter is configured to receive one or more signals and to independently control the first portion start time, relative to the third portion start time, and the first portion duration in response to the one or more signals.31. The pulsed laser system of clause 30, wherein the optical shutter is further configured to receive the one or more signals and to independently control the first portion energy.32. The pulsed laser system of clause 21, wherein the optical shutter is configured to receive one or more signals and to control one or more of the first portion duration, the first portion peak power, the first portion energy, a temporal shape of the first transmitted portion, the second portion start time, the second portion duration, the second portion energy, the second portion peak power, the second portion initial power, a temporal shape of the second transmitted portion, the third portion start time, and the third portion duration.33. The pulsed laser system of clause 21, wherein the laser is a CO2 laser and the optical shutter includes an electro-optical modulator (EOM).34. The pulsed laser system of clause 33, further including: a low-voltage driver connected to supply a low voltage to the EOM; a high-voltage driver connected to supply a high voltage to the EOM; and one or more isolators connected to the low-voltage driver and to the high-voltage driver configured to isolate the low voltage driver from the high voltage supplied by the high voltage driver.35. The pulsed laser system of clause 34, wherein the low -voltage driver is configured to control the first transmitted portion and the high-voltage driver is configured to control the third transmitted portion.36. The pulsed laser system of clause 35, wherein the low -voltage driver is further configured to control the second transmitted portion.37. The pulsed laser system of clause 34, wherein one of the low-voltage driver and the high-voltage driver is configured to apply, before the arrival of the pulse of light at the EOM, a voltage signal to the EOM configured to stop or reduce a vibration of the EOM.38. The pulsed laser system of clause 33, wherein the EOM is configured to receive, before the arrival of the pulse of light at the EOM, a voltage signal configured to stop or reduce a vibration of the EOM.39. The pulsed laser system of clause 21, further including a multistage amplifier configured to receive the first and third transmitted portions from the optical shutter, and wherein, in operation, the first transmitted portion is amplified in the multistage amplifier and delivered to a target in an extreme ultraviolet (EUV) light source, the target including a material capable of emitting EUV light, the target having been previously shaped by a pre-pulse, the amplified first transmitted portion interacting with the previously shaped target to increase a rarefication of the previously shaped target, and wherein, in operation, the third transmitted portion is amplified in the multistage amplifier and delivered to the target, subsequent to the amplified first-transmitted portion, to convert the target or a portion thereof to an EUV-light-producing plasma.40. The pulsed laser system of clause 21, further including a multistage amplifier configured to receive the first, second, and third transmitted portions from the optical shutter, and wherein, in operation, the first transmitted portion is amplified in the multistage amplifier and delivered to a target in an extreme ultraviolet (EUV) light source, the target including a material capable of emitting EUV light when in a plasma state, the target having been previously shaped by a pre -pulse, the amplified first transmitted portion interacting with the previously shaped target to increase a rarefication of the previously shaped target, and wherein, in operation, the second transmitted portion is amplified by the multistage amplifier and delivered to the target, the amplified second transmitted portion interacting with the target subsequent to the amplified first transmitted portion to increase the vaporization (and produce some ionization) of the target, and wherein, in operation, the third transmitted portion is amplified by the multistage amplifier and delivered to the target, the amplified third transmitted portion interacting with the target, subsequent to the amplified second transmitted portion, to convert the target or a portion thereof to a an EUV -light-producing plasma.41. A pulsed laser system including: a laser configured to deliver pulses of light; an optical shutter comprising an electro-optic modulator (EOM) configured to receive a pulse of light from the laser; a low-voltage driver connected to supply a low voltage to the EOM; a high-voltage driver connected to supply a high voltage to the EOM; and one or more isolators connected to the low -voltage driver and the high-voltage driver and configured to isolate the low voltage driver from the high voltage supplied by the high voltage driver.42. The pulse laser system of clause 41, wherein the low-voltage driver is a variable -voltage driver.43. The pulsed laser system of clause 41, wherein the EOM, the low -voltage driver, and the high voltage driver are configured to transmit, of the pulse of light from the laser: (a) with the EOM driven by the low-voltage driver, a first transmitted portion, the first transmitted portion having a first portion peak power and a first portion rise time of less than 20 ns, (b) a second transmitted portion subsequent to the first transmitted portion, the second transmitted portion having a second portion start time, a second portion duration, a second portion peak power, and a second portion initial power less than the first portion peak power, and (c) with the EOM driven by the high-voltage driver, a third transmitted portion subsequent to the second transmitted portion, the third transmitted portion having a third portion energy of at least 100 nJ and a third portion duration of less than 200 ns.44. The pulsed laser system of clause 41, wherein the EOM, the low -voltage driver, and the high voltage driver are configured to transmit, of the pulse of light from the laser: (a) with the EOM driven by the low-voltage driver, a first transmitted portion, the first transmitted portion having a first portion start time, a first portion duration, and a first portion peak power; (b) a second transmitted portion subsequent to the first transmitted portion, the second transmitted portion having a second portion start time, a second portion duration, a second portion peak power, and a second portion initial power less than the first portion peak power, and (c) with the EOM driven by the high-voltage driver, a third transmitted portion subsequent to the second transmitted portion, the third transmitted portion havinga third portion peak power, a third portion energy of at least 100 nJ, and a third portion duration of less than 200 ns, wherein the first portion peak power is in the range of 1 / 1,000,000 to 1 / 3 of the third portion peak power.45. The pulsed laser system of clause 44, wherein the EOM and the low -voltage driver are further configured to transmit, with the EOM driven by the low-voltage driver, the second transmitted portion, wherein the second portion peak power is greater than the second portion initial power and within the range of from 50% to 400% of the first portion peak power.46. The pulsed laser system of clause 45, wherein the second portion peak power occurs within a last 30% of the duration of the second portion.47. The pulsed laser system of clause 41, wherein the optical shutter is configured to receive one or more signals and to independently control a start time and a duration of the second portion in response to the one or more signals.48. The pulsed laser system of clause 41, wherein the optical shutter is configured to receive one or more signals and to independently control the first portion peak power and / or the first portion total energy and / or a temporal shape of the first transmitted portion, in response to the one or more signals.49. The pulsed laser system of clause 41, wherein the optical shutter is configured to receive one or more signals and to control a temporal shape of the second transmitted portion in response to the one or more signals.50. The pulsed laser system of clause 41, wherein the laser is a CO2 laser.51. The pulsed laser system of clause 41 , wherein the EOM is configured to receive, before the arrival of the pulse of light at the EOM, a voltage signal configured to stop or reduce a vibration of the EOM.52. The pulsed laser system of clause 51, wherein the low-voltage driver is configured to provide to the EOM the voltage signal configured to stop or reduce the vibration of the EOM.53. A method for improving the performance potential of a pulsed laser system that provides light to a target of an extreme ultraviolet (EUV) light source and of the associated EUV light source, the pulsed laser system including a laser configured to provide a light pulse to an optical shutter including a high-voltage driver connected via a high-voltage output or outputs to an electro-optic modulator (EOM), the method including: connecting a low-voltage driver to the laser system via a low voltage output or outputs connected to the EOM in parallel with the high-voltage output or outputs of the high-voltage driver; and positioning one or more voltage isolators electrically between the output or outputs of the low-voltage driver and the output or outputs of the high-voltage driver to isolate the low-voltage driver from the high voltage driver so as to allow the low -voltage driver and the high- voltage driver to individually drive the EOM to produce from the light pulse (1) with the EOM driven by the low-voltage driver, a first transmitted portion useful to produce a rarefication pulse for the EUV source and (2) with the EOM driven by the high-voltage driver, a subsequent transmitted portion useful to produce a main pulse for the EUV source.54. The method of clause 53, wherein the low-voltage driver is a variable voltage driver.55. The method of clause 53, wherein the one or more voltage isolators comprise Zener diodes.56. The method of clause 53, wherein the one or more voltage isolators comprise an insulated-gate bipolar transistor (IGBT).57. The method of clause 53, wherein the one or more voltage isolators comprise MOSFETs.58. A method of operating a pulsed laser in use with an EUV source, the method including: using an optical shutter to pass a first portion of a laser pulse, the first portion having a rise time of 20 ns or less; passing the first portion through an amplifier to generate a rarefication pulse for an EUV source; using the optical shutter to pass a subsequent portion of the laser pulse, the subsequent portion having an energy of at least 100 nJ and a duration less than or equal to 200 ns; and passing the subsequent portion through the amplifier to generate a main pulse for the EUV source.59. The method of clause 58, wherein using the optical shutter to pass the first portion includes using the optical shutter in partially open state and wherein using the optical shutter to pass the subsequent portion includes using the optical shutter in a fully open state.60. The method of clause 58, further including using the optical shutter to block an intermediate portion of the laser pulse after the first portion and prior to the subsequent portion.61. The method of clause 58, further including: using the optical shutter to pass an intermediate portion of the laser pulse after the first portion and prior to the subsequent portion, the intermediate portion having an initial power less than a peak power of the first portion; and passing the intermediate portion through the amplifier to generate, at least in part, a vaporization pulse for the EUV source.62. The method of clause 61, wherein the optical shutter includes an electro-optic modulator (EOM) and wherein the method further includes, preceding the laser pulse at the optical shutter, applying one or more voltage signals to the EOM to reduce or control a vibration of the EOM.63. The method of clause 58, wherein the optical shutter includes an electro-optic modulator (EOM) and wherein the method further includes, preceding the laser pulse at the optical shutter, applying one or more voltage signals to the EOM to reduce or control a vibration of the EOM.64. The method of clause 58, wherein the first portion has a rise time of 20 ns or less.65. A method of operating a pulsed laser in use with an EUV source, the method including: using an optical shutter to pass a first portion of a laser pulse; passing the first portion through an amplifier to generate a rarefication pulse for an EUV source; using the optical shutter to pass a subsequent portion of the laser pulse, the subsequent portion having an energy of at least 100 nJ, and a duration less than or equal to 200 ns and a peak power of at least 10 times the peak power of the first portion; and passing the subsequent portion through the amplifier to generate a main pulse for the EUV source.66. The method of clause 65, wherein using the optical shutter to pass the first portion includes using the optical shutter in partially open state and wherein using the optical shutter to pass the subsequent portion includes using the optical shutter in a fully open state.67. The method of clause 65, further including using the optical shutter to block an intermediate portion of the laser pulse after the first portion and prior to the subsequent portion.68. The method of clause 65, further including: using the optical shutter to pass an intermediate portion of the laser pulse after the first portion and prior to the subsequent portion, the intermediate portion having an initial power less than a peak power of the first portion; and passing the intermediate portion through the amplifier to generate, at least in part, a vaporization pulse for the EUV source.69. The method of clause 65, wherein the optical shutter includes an electro-optic modulator (EOM) and wherein the method further includes, preceding the laser pulse at the optical shutter, applying one or more voltage signals to the EOM to reduce or control a vibration of the EOM.70. The method of clause 65, wherein the optical shutter includes an electro-optic modulator (EOM) and wherein the method further includes, preceding the laser pulse at the optical shutter, applying one or more voltage signals to the EOM to reduce or control a vibration of the EOM.71. The method of clause 65, wherein the subsequent portion has a peak power of at least 1000 times a peak power of the first portion.72. A pulse-timing control system including: an electro-optic modulator (EOM) crystal suitable for controlling a polarization of a light beam; a first EOM driver coupled to the crystal and configured to provide a first voltage pulse greater than 4 kV; a second EOM driver coupled to the crystal and configured to provide a second voltage pulse greater than 50 V; and an isolator coupled to the first and second EOM drivers and configured to prevent transmission of the first voltage pulse into the second EOM driver.73. The pulse-timing control system of clause 72, further including a controller configured to receive a timing input and to control a timing of the first voltage pulse relative to the second voltage pulse.74. The pulse-timing control system of clause 72, wherein the light beam has a wavelength in the range of 9 to 11 pm.75. The pulse-timing control system of clause 72, wherein the isolator includes one or more insulated- gate bipolar transistors (IGBT).76. The pulse-timing control system of clause 72, wherein the isolator includes one or more metal - oxide-semiconductor field-effect transistors (MOSFET).77. The pulse-timing control system of clause 72, wherein the isolator includes one or more Zener diodes.78. A pulsed laser including: a laser configured to provide a laser pulse; and an optical shutter configured to receive the laser pulse and to pass a first portion of the laser pulse and a subsequent portion of the laser pulse; wherein the first portion has a rise time of 20 ns or less and the subsequent portion has an energy of at least 100 nJ and a duration less than or equal to 200 ns.79. A pulsed laser including: a laser configured to provide a laser pulse; and an optical shutter configured to receive the laser pulse and to pass a first portion of the laser pulse and a subsequentportion of the laser pulse; wherein the subsequent portion has an energy of at least 100 nJ and a duration less than or equal to 200 ns and a peak power of at least 10 times the power of the first portion.80. The pulsed laser system of clause 1, wherein the optical shutter is configured to transmit the third transmitted portion during an optical shutter time window centered before a peak power of the pulse of light.81. The pulsed laser system of clause 80, further including an acousto-optical modulator (AOM) downstream of the optical shutter, the AOM configured to be operated for an AOM time window during which an optical pulse can pass the AOM with a minimum attenuation and with the AOM time window beginning at an AOM start time and with the first portion start time and the AOM start time set relative to each other such that energy transmitted by the optical shutter at and / or before a leading edge of the first transmitted portion is attenuated by the AOM.82. The pulsed laser system of clause 80, further including an acousto-optical modulator (AOM) positioned to receive the first transmitted portion from the optical shutter, the AOM configured to be operated for an AOM time window during which an optical pulse can pass the AOM with a minimum attenuation, the AOM time window beginning at an AOM start time, the AOM and the optical shutter timed relative to each other to cause the AOM start time to begin when a leading edge of the first transmitted portion arrives at the AOM or up to 100 nanoseconds (ns) before.83. The pulsed laser system of clause 1, further including an acousto-optical modulator (AOM) downstream of the optical shutter, the AOM configured to be operated for an AOM time window during which an optical pulse can pass the AOM with a minimum attenuation and with the AOM time window beginning at an AOM start time and with the first portion start time and the AOM start time set relative to each other such that energy transmitted by the optical shutter at and / or before a leading edge of the first transmitted portion is attenuated by the AOM.84. The pulsed laser system of clause 1, further including an acousto-optical modulator (AOM) positioned to receive the first transmitted portion from the optical shutter, the AOM configured to be operated for an AOM time window during which an optical pulse can pass the AOM with a minimum attenuation, the AOM time window beginning at an AOM start time, the AOM and the optical shutter timed relative to each other to cause the AOM start time to begin when leading edge of the first transmitted portion arrives at the AOM or up to 100 nanoseconds (ns) before.85. The pulsed laser system of clause 11, wherein the optical shutter is further configured to independently control the second portion energy.86. The pulsed laser system of clause 13, wherein the optical shutter further includes a first linear polarizer upstream of the EOM and a second linear polarizer downstream of the EOM and wherein the second linear polarizer is configured to be optically orthogonal to the first linear polarizer so as to minimize leakage of the optical shutter.87. The pulsed laser system of clause 13, wherein the optical shutter further includes a first linear polarizer upstream of the EOM and a second linear polarizer downstream of the EOM and wherein the second linear polarizer is configured vary from ideal optical orthogonally to the first linear polarizer to provide a leakage of the optical shutter.88. The pulsed laser system of clause 87 wherein a power of the leakage is a percentage of the incident power, with the percentage being in the range of 0.005% to 0.02%.89. The pulsed laser system of clause 15, wherein the system further includes a discharge circuit connected to the EOM configured to discharge, at and / or after the end of the first transmitted portion, a voltage present at the EOM.90. The pulsed laser system of clause 89 wherein the discharge circuit includes a switch configured to discharge the voltage at and / or after the end of the first transmitted portion and before the third transmitted portion.91. The pulsed laser system of clause 89 wherein the discharge circuit includes a variable resistor across which the voltage is discharged.92. The pulsed laser system of clause 91 wherein the variable resistor is configured to receive a control signal and to adjust its resistance in accordance with the control signal.93. The pulsed laser system of clause 91 wherein the variable resistor is configured to control or assist in controlling the energy of the second transmitted portion by controlling the rate of discharge of the voltage during the second transmitted portion.94. The pulsed laser system of clause 21, wherein the optical shutter is configured to transmit the third transmitted portion within a time window centered before a peak power of the pulse of light.95. The pulsed laser system of clause 94 further including multiple acousto -optical modulators (AOMs) downstream of the optical shutter, each configured to be operated at a respective limited AOM time window in which an optical pulse can pass the respective AOM with a minimum attenuation, and wherein the optical shutter is configured to transmit the first transmitted portion at time offset toward the early side of the respective AOM time windows of the respective AOMs to attenuate energy arriving at the respective AOM at and / or preceding a temporally leading edge of the first transmitted portion.96. The pulsed laser system of clause 31, wherein the optical shutter is further configured to independently control the second portion energy.97. The pulsed laser system of clause 35, wherein the system further includes a discharge circuit connected to the EOM configured to discharge, at and / or after the end of the first transmitted portion, a voltage present at the EOM.98. The pulsed laser system of clause 97 wherein the discharge circuit includes a switch configured to discharge the voltage at and / or after the end of the first transmitted portion and before the third transmitted portion.99. The pulsed laser system of clause 97 wherein the discharge circuit includes a variable resistor across which the voltage is discharged.100. The pulsed laser system of clause 99 wherein the variable resistor is configured to receive a control signal and to adjust its resistance in accordance with the control signal.101. The pulsed laser system of clause 99 wherein the variable resistor is configured to control the energy of the second transmitted portion by controlling the rate of discharge of the voltage during the second transmitted portion.102. The pulsed laser system of clause 41, wherein the system further includes a discharge circuit connected to the EOM.103. The pulsed laser system of clause 102 wherein the discharge circuit includes a switch between the EOM and an electrical ground.104. The pulsed laser system of clause 103 wherein the discharge circuit includes a variable resistor between the EOM and the switch.105. The pulsed laser system of clause 104 wherein the variable resistor is configured to receive a control signal and to adjust its resistance in accord with the control signal.106 . The pulsed laser system of clause 43, wherein the EOM is configured to transmit the third transmitted portion from a time range centered before a peak power of the pulse of light.107. The pulsed laser system of clause 106, further including multiple acousto -optical modulators (AOMs) downstream of the EOM, each configured to be operated at a respective limited time window in which an optical pulse can pass the respective AOM with a minimum attenuation, and wherein the EOM is configured to transmit the first transmitted portion at time offset toward the early side of the respective AOM time windows of the respective AOMs to attenuate energy arriving at the respective AOM at and / or preceding a temporally leading edge of the first transmitted portion.108. The pulsed laser system of clause 43, further including multiple acousto -optical modulators (AOMs) downstream of the EOM, each configured to be operated at a respective limited time window in which an optical pulse can pass the respective AOM with a minimum attenuation, and wherein the EOM is configured to transmit the first transmitted portion at time offset toward the early side of the respective AOM time windows of the respective AOMs to attenuate energy arriving at the respective AOM at and / or preceding a temporally leading edge of the first transmitted portion.109. The pulsed laser system of clause 44, wherein the system further includes a discharge circuit connected to the EOM, and wherein the discharge circuit includes a variable resistor electrically positioned between the EOM and an electrical ground, and wherein the variable resistor is configured to control the energy and / or power of the second transmitted portion by controlling a rate of discharge of a voltage at the EOM during the second transmitted portion.110. The method of clause 53, further including providing a discharge circuit connected between the EOM and an electrical ground, the discharge circuit including a switch and a variable resistor in series between the EOM and the electrical ground and operating the variable resistor to control a dischargeof a voltage at the EOM after the first transmitted portion and before the subsequent transmitted portion, thereby producing a controllable intermediate transmitted portion between the first and subsequent transmitted portions.111. The method of clause 58, wherein the optical shutter includes an electro-optic modulator (EOM), the method further including: using a discharge circuit between the EOM and an electrical ground to discharge a voltage present at the EOM at the end of passing the first portion; and using a variable resistor in the discharge circuit control the discharge of the voltage and to pass an intermediate portion of the laser pulse after the first portion and prior to the subsequent portion; and passing the intermediate portion through the amplifier to generate at least in part a vaporization pulse for the EUV source.112. The method of clause 58, wherein the pulsed laser further includes multiple acousto -optical modulators (AOMs) positioned optically downstream of the optical shutter, and wherein the method further includes: operating each respective AOM at a respective limited time window during which an optical pulse can pass the respective AOM with a minimum attenuation, and passing the first transmitted portion at a time offset toward the early side of the respective time windows of the respective AOMs so as to attenuate energy arriving at the respective AOM at and / or preceding a temporally leading edge of the first transmitted portion.113. The method of clause 58, further including: passing the subsequent transmitted portion during a time window centered at a time preceding a peak of the laser pulse.114. The method of clause 65, further including transitioning the optical shutter or allowing the optical shutter to transition from a partially open state to a closed or less partially open state while transmitting an intermediate portion of the laser pulse.115. The method of clause 114, wherein the optical shutter includes an EOM and transitioning the optical shutter or allowing the optical shutter to transition from a partially open state to a closed or less partially open state includes discharging a voltage present at the EOM across a resistance to an electrical ground.116. The method of clause 115 wherein the resistance includes a variable resistor.117. The method of clause 116 further including adjusting a resistance of the variable resistor to improve the performance of the EUV source.118. The pulse-timing control system of clause 73 further including a variable-rate discharge circuit coupled to the EOM, the discharge circuit including a switch configured to provide zero discharge when open and a variable discharge resistor configured to control a rate of discharge when the switch is closed.119. The pulse-timing control system of clause 118 wherein the controller is configured to control a timing and a rate of discharge of the variable-rate discharge circuit.120. A method of operating a pulsed laser source, the method including: producing a pulse from a pulsed laser; receiving the pulse at an optical shutter including an electro -optic modulator (EOM);passing a portion of the pulse through the optical shutter in a partially open state of the optical shutter; passing the portion of the pulse through two or more acousto-optic modulators (AOMs); respectively actuating the two or more AOMs sufficiently late relative to a respective arrival of the portion of the pulse at the respective AOMs to attenuate energy arriving at the respective AOMs at and / or preceding a temporally leading edge of the first transmitted portion.121. The method of clause 120, wherein the optical shutter further includes pre-and post-EOM polarizers angularly positioned relative to each other so as to produce an extinction less than a maximum possible extinction of the pulse by the optical shutter in a closed state of the optical shutter.122. The method of clause 121 wherein the produced extinction is in the range of 0.005% to 0.02%.123. A method of manufacturing a semiconductor device comprising the acts of: generating radiation; directing the radiation to a mask and a photoresist layer, to transfer a pattern from a mask onto the photoresist layer; and removing a portion of the photoresist layer to form the pattern over the substrate, wherein the generating the radiation comprises: using an optical shutter to pass a first portion of a laser pulse; passing the first portion through an amplifier to generate a rarefication pulse for an EUV source; using the optical shutter to pass a subsequent portion of the laser pulse, the subsequent portion having an energy of at least 100 nJ and a duration less than or equal to 200 ns; and passing the subsequent portion through the amplifier to generate a main pulse for the EUV source; wherein the first portion has a rise time of 20 ns or less and / or the first portion has a peak power in the range of 1 / 1,000,000 to 1 / 3 of a peak power of the subsequent portion.

[0117] The above-described aspects and implementations and other implementations are within the scope of the following claims.

Claims

CLAIMS1. A pulsed laser system comprising: a laser configured to deliver pulses of light; an optical shutter configured to receive a pulse of light from the laser and to transmit, of the pulse:(a) a first transmitted portion, the first transmitted portion having a first portion start time, a first portion duration, a first portion peak power, a first portion energy, and a first portion rise time, the first portion rise time being less than 20 ns,(b) a second transmitted portion subsequent to the first transmitted portion, the second transmitted portion having a second portion start time, a second portion duration, a second portion energy, a second portion peak power, and a second portion initial power, the second portion initial power being less than the first portion peak power,(c) a third transmitted portion subsequent to the second transmitted portion, the third transmitted portion having a third portion start time, a third portion duration, and a third portion energy, the third portion energy being at least 100 nJ and the third portion duration being less than or equal to 200 ns.

2. The pulsed laser system of claim 1, wherein the second portion peak power is in the range of 1 / 1000 to 1 / 10 of the first portion peak power.

3. The pulsed laser system of claim 1, wherein the second portion peak power is greater than the second portion initial power and is within the range of from 50% to 400% of the first portion peak power.

4. The pulsed laser system of claim 3, wherein the second portion peak power occurs within a last 30% of the second portion duration.

5. The pulsed laser system of claim 3, wherein the power of the second transmitted portion increases over the second portion duration.

6. The pulsed laser system of claim 1, wherein the first portion has a first portion energy, the first portion energy being between 100 femtojoules (fJ) and- 3000 fj.

7. The pulsed laser system of claim 1, wherein the first portion peak power is in the range of 1 / 1,000,000 to 1 / 3 of the third portion peak power.

8. The pulsed laser system of claim 1, wherein the optical shutter is configured to receive one or more signals and is configured to independently control the second portion start time and the second portion duration in response to the one or more signals.

9. The pulsed laser system of claim 1, wherein the wherein the optical shutter is configured to receive one or more signals and to independently control the first portion start time, relative to the third portion start time, and the first portion duration in response to the one or more signals.

10. The pulsed laser system of claim 9, wherein the optical shutter is further configured to receive the one or more signals and to independently control the first portion energy.

11. The pulsed laser system of claim 1, wherein the laser is a CO2 laser and the optical shutter comprises an electro-optical modulator (EOM).

12. The pulsed laser system of claim 11, further comprising: a low-voltage driver connected to supply a low voltage to the EOM; a high-voltage driver connected to supply a high voltage to the EOM; and one or more isolators connected to the low-voltage driver and to the high-voltage driver and configured to isolate the low voltage driver from the high voltage supplied by the high voltage driver, wherein the low-voltage driver is configured to drive the EOM during the first transmitted portion and the high-voltage driver is configured to drive the EOM during the third transmitted portion..

13. The pulsed laser system of claim 1, further comprising a multistage amplifier configured to receive the first, second, and third transmitted portions from the optical shutter, and wherein, in operation, the first transmitted portion is amplified in the multistage amplifier and delivered to a target in an extreme ultraviolet (EUV) light source, the target comprising a material capable of emitting EUV light when in a plasma state, the target having been previously shaped by a pre-pulse, the amplified first transmitted portion interacting with the previously shaped target to increase a rarefication of the previously shaped target, and wherein, in operation, the second transmitted portion is amplified by the multistage amplifier and delivered to the target, the amplified second transmitted portion interacting with the target subsequent to the amplified first transmitted portion to increase the vaporization of the target, and wherein, in operation, the third transmitted portion is amplified by the multistage amplifier and delivered to the target, subsequent to the amplified second transmitted portion, to convert the target or a portion thereof to a an EUV -light-producing plasma.

14. A pulsed laser system comprising: a laser configured to deliver pulses of light; an optical shutter configured to receive a pulse of light from the laser and to transmit, of the pulse:(a) a first transmitted portion, the first transmitted portion having a first portion start time, a first portion duration, a first portion peak power, a first portion energy, and a first portion rise time;(b) a second transmitted portion subsequent to the first transmitted portion, the second transmitted portion having a second portion start time, a second portion duration, a second portion peak power, and a second portion initial power, the second portion initial power being less than the first portion peak power,(c) a third transmitted portion subsequent to the second transmitted portion, the third transmitted portion having a third portion start time, a third portion duration, a third portion energy, and a third portion peak power, the third portion energy being at least 100 nJ and the third portion duration being less than or equal to 200 ns, wherein the first portion peak power is in the range of 1 / 1,000,000 to 1 / 3 of the third portion peak power.

15. The pulsed laser system of claim 14, wherein the second portion peak power is greater than the second portion initial power and is within the range of from 50% to 400% of the first portion peak power, and wherein the power of the second transmitted portion increases over the second portion duration.

16. The pulsed laser system of claim 14, wherein the first portion has a first portion energy, the first portion energy being between 100 fj and 3000 fj, and wherein the optical shutter is configured such that the first portion rise time is less than 20 ns.

17. The pulsed laser system of claim 14, wherein the optical shutter is configured to receive one or more signals and is configured to independently control the second portion start time and the second portion duration in response to the one or more signals.

18. The pulsed laser system of claim 14, wherein the optical shutter is configured to receive one or more signals and to control one or more of the first portion duration, the first portion peak power, the first portion energy, a temporal shape of the first transmitted portion, the second portion start time, the second portion duration, the second portion energy, the second portion peak power, the second portion initial power, a temporal shape of the second transmitted portion, the third portion start time, and the third portion duration.

19. A method for improving the performance potential of a pulsed laser system that provides light to a target of an extreme ultraviolet (EUV) light source and of the associated EUV light source, the pulsed laser system including a laser configured to provide a light pulse to an optical shutter including a high-voltage driver connected via a high-voltage output or outputs to an electro-optic modulator (EOM), the method comprising: connecting a low -voltage driver to the laser system via a low voltage output or outputs connected to the EOM in parallel with the high-voltage output or outputs of the high-voltage driver; and positioning one or more voltage isolators electrically between the output or outputs of the low-voltage driver and the output or outputs of the high-voltage driver to isolate the low-voltage driver from the high voltage driver so as to allow the low-voltage driver and the high-voltage driver to individually drive the EOM to produce from the light pulse(1) with the EOM driven by the low-voltage driver, a first transmitted portion useful to produce a rarefication pulse for the EUV source and(2) with the EOM driven by the high-voltage driver, a subsequent transmitted portion useful to produce a main pulse for the EUV source.

20. The method of claim 19, further comprising using the optical shutter to block an intermediate portion of the laser pulse after the first portion and prior to the subsequent portion.

21. A method of manufacturing a semiconductor device comprising the acts of: generating radiation; directing the radiation to a mask and a photoresist layer, to transfer a pattern from a mask onto the photoresist layer; and removing a portion of the photoresist layer to form the pattern over the substrate, wherein the generating the radiation comprises: using an optical shutter to pass a first portion of a laser pulse; passing the first portion through an amplifier to generate a rarefication pulse for an EUV source; using the optical shutter to pass a subsequent portion of the laser pulse, the subsequent portion having an energy of at least 100 nJ and a duration less than or equal to 200 ns; and passing the subsequent portion through the amplifier to generate a main pulse for the EUV source; wherein the first portion has a rise time of 20 ns or less and / or the first portion has a peak power in the range of 1 / 1,000,000 to 1 / 3 of a peak power of the subsequent portion.

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