Optical Isolation Module

Abstract

To provide an optical isolation module.SOLUTION: A photolithographic tool optical source includes: a radiation source constituted so as to emit a first light beam having a first wavelength and a second optical beam having a second wavelength different from the first wavelength; an amplifier constituted so as to amplify the first optical beam and the second optical beam in order to form a first amplification light beam and a second amplification light beam; and an optical isolator between the radiation source and the amplifier, including a plurality of dichroic optical elements and an optical modulator between two dichroic optical elements.SELECTED DRAWING: Figure 7

Description

Technical Field

[0001] Cross - Reference to Related Applications This application claims the benefit of U.S. Provisional Application No. 62 / 236,056, filed Oct. 1, 2015, and U.S. Application No. 14 / 970,402, filed Dec. 15, 2015, both entitled "OPTICAL ISOLATION MODULE", which are hereby incorporated by reference in their entirety.

[0002] This disclosure relates to an optical isolation module. The optical isolation module can be used in an extreme ultraviolet (EUV) light source.

Background Art

[0003] Electromagnetic radiation having an extreme ultraviolet (EUV) light, for example, a wavelength of about 50 nm or less (sometimes also referred to as soft X - rays) and having a wavelength of about 13 nm, can be used in a photolithography process to create extremely small features within a substrate, such as a silicon wafer.

[0004] Methods for generating EUV light include, but are not necessarily limited to, converting a material having an element, such as xenon, lithium, or tin, that has emission lines within the EUV range in a plasma state. In one such method, an essential plasma, often referred to as a laser - produced plasma (LPP), can be generated by irradiating a target material in the form of, for example, a droplet, plate, tape, stream, or cluster of the material, with an amplified light beam that can be referred to as a driver laser. In this process, the plasma is typically generated within a sealed container, such as a vacuum chamber, and monitored using various types of metrology equipment.

Summary of the Invention

[0005] In one general aspect, an optical source for a photolithography tool is a radiation source configured to emit a first light beam and a second light beam, the first light beam having a first wavelength, the second light beam having a second wavelength, the first wavelength and the second wavelength being different, a radiation source, and amplifiers respectively configured to amplify the first light beam and the second light beam to generate a first amplified light beam and a second amplified light beam, and an optical isolator between the radiation source and the amplifiers, the optical isolator including a plurality of dichroic optical elements and an optical modulator between two of the dichroic optical elements.

[0006] The implementation can include one or more of the following features. The optical modulator can include an acousto-optic modulator. Each of the dichroic optical elements can be configured to reflect light having the first wavelength and transmit light having the second wavelength, the acousto-optic modulator can be positioned on the beam path between two of the dichroic optical elements, the acousto-optic modulator can be positioned to receive light reflected from the two dichroic optical elements, the acousto-optic modulator can be configured to transmit the received light when the received light propagates in a first direction with respect to the acousto-optic modulator and deflect the received light away from the beam path when the received light propagates in a second direction with respect to the acousto-optic modulator, the second direction being different from the first direction. The first light beam and the second light beam can be pulsed light beams. The energy of the first amplified light beam can be less than the energy of the second amplified light beam. The first amplified light beam can have sufficient energy to deform the target material in the target material droplet into a modified target, the modified target including target material having a geometric distribution different from the distribution of the target material in the target material droplet, the target material including a material that emits extreme ultraviolet (EUV) light when in a plasma state, and the second amplified light beam can have sufficient energy to convert at least a portion of the target material in the modified target into a plasma that emits EUV light.

[0007] The acousto-optic modulator can be positioned on the beam path between two dichroic optical elements and can be positioned to receive light reflected from the two dichroic optical elements. The acousto-optic modulator can be configured to receive a trigger signal and, in response to receiving the trigger signal, can be configured to deflect the light received from the beam path and, alternatively, to transmit the received light onto the beam path.

[0008] The optical source can also include a second optical modulator between the radiation source and the amplifier. The second optical modulator is between two dichroic optical elements and is on a beam path different from that of the optical modulator.

[0009] The radiation source can include a laser source. The radiation source can include a plurality of radiation sources, with the first optical beam being generated by one radiation source and the second optical beam being generated by another radiation source. The radiation source can include one or more pre-amplifiers.

[0010] In another general aspect, an apparatus for an extreme ultraviolet (EUV) light source includes a plurality of dichroic optical elements, each of the dichroic optical elements being configured to reflect light having a wavelength within a first wavelength band and to transmit light having a wavelength within a second wavelength band; an acousto-optic modulator positioned on a beam path between two of the dichroic optical elements, the acousto-optic modulator being positioned to receive light reflected from the two dichroic optical elements and being configurable to transmit the received light when the received light propagates in a first direction on the beam path and to deflect the received light away from the beam path when the received light propagates in a second direction on the beam path, the second direction being different from the first direction; and wherein the first wavelength band includes the wavelength of a prepulse beam and the second wavelength band includes the wavelength of a main beam.

[0011] The implementation can include one or more of the following features. The optical modulator can be an acousto-optic modulator. The apparatus can include a control system configured to provide a trigger signal to the acousto-optic modulator, and the acousto-optic modulator can be configured to deflect light away from the beam path and, alternatively, transmit light onto the beam path in response to receipt of the trigger signal.

[0012] The apparatus can also include a second optical modulator, the second optical modulator being positioned between two dichroic optical elements and receiving light transmitted by the two dichroic optical elements. The optical modulator and the second optical modulator can be between the same two dichroic optical elements, and the second optical modulator can be on a second beam path different from the beam path.

[0013] In another general aspect, the method includes reflecting a first light beam with a first dichroic optical element, the reflected first light beam passing through an optical modulator and an amplifier to generate an amplified first light beam; transmitting a second light beam through the first dichroic optical element, the second dichroic optical element, and the amplifier to generate an amplified second beam; receiving, with the second dichroic optical element, a reflection of the amplified first light beam, an interaction between the reflection of the amplified first light beam and the second dichroic optical element guiding the reflected amplified first light beam to the optical modulator; and deflecting, with the optical modulator, the reflection of the amplified first light beam, thereby guiding the reflection of the amplified first light beam away from the source of the first light beam.

[0014] The implementation can include one or more of the following features. The trigger signal can be provided to the optical modulator after the first optical beam passes through the optical modulator and before the amplified first optical beam is reflected by the optical modulator. The trigger signal can cause the optical modulator to be in a state where the incident light is deflected.

[0015] The amplified first optical beam can propagate towards an initial target area. The reflection of the first amplified optical beam can be generated via the interaction between the first amplified optical beam and the target material droplets in the initial target area. The second amplified optical beam can propagate towards the target area, and the interaction between the target material and the second amplified optical beam can be capable of generating the reflection of the second amplified optical beam. The method further includes transmitting the reflection of the second amplified optical beam through a second dichroic optical element and deflecting the reflection of the second amplified optical beam with a second optical modulator, thereby inducing the reflection of the second amplified optical beam away from the radiation source of the second optical beam. The radiation source of the first optical beam and the radiation source of the second optical beam can be the same radiation source. The radiation source of the first optical beam can be a first optical subsystem within the radiation source, and the radiation source of the second optical beam can be a second optical subsystem within the radiation source.

[0016] Any implementation of the foregoing techniques can include a method, a process, a kit or an assembled system for retrofitting an existing EUV light source, or an apparatus. The details of one or more implementations are shown in the accompanying drawings and the following description. Other features will become apparent from the description and drawings, as well as from the claims.

Brief Description of the Drawings

[0017]

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Embodiments for Carrying Out the Invention

[0018] Referring to FIG. 1, a block diagram of an exemplary optical system 100 is shown. The optical system 100 is part of an extreme ultraviolet (EUV) light source. The optical system 100 includes an optical source 102 that generates an optical beam 110. The optical beam 110 is emitted from the optical source 102 and propagates along path 112 in the z - direction towards the target region 115.

[0019] The target region 115 receives a target 120 that includes a material that emits EUV light when converted to a plasma. The target 120 reflects at the wavelength or wavelengths of the optical beam 110. Because the target 120 reflects, when the optical beam 110 interacts with the target 120, all or part of the beam 110 can be reflected along path 112 in a direction different from the z - direction. The reflected portion of the beam 110 is labeled as reflection 113. The reflection 113 travels on path 112 in the direction opposite to the z - direction and can return into the optical source 102. The reflection of a forward beam (a beam propagating from the optical source 102 towards the target region 115), such as reflection 113, is called "backward reflection".

[0020] The optical source 102 includes a light generation module 104, an optical isolator 106, and an optical amplifier 108. The light generation module 104 is a light emission source (such as one or more lasers, lamps, or any combination of such elements). The optical amplifier 108 has a gain medium (not shown) on the beam path 112. When the gain medium is excited, the gain medium provides photons to the light beam 110 and amplifies the light beam 110 to generate an amplified light beam 110. The optical amplifier 108 can include a plurality of optical amplifiers arranged with their respective gain media on the path 112. The optical amplifier 108 can be all or part of a drive laser system, such as the drive laser system 880 of FIG. 8B.

[0021] The light generation module 104 emits the light beam 110 on the beam path 112 toward the optical isolator 106. The optical isolator 106 passes the light beam 110 in the z direction to the optical amplifier 108 and toward the target region 115. However, the optical isolator 106 blocks the back reflection 113. Thus, as will be discussed in more detail below, the optical isolator 106 prevents the back reflection from entering the light generation module 104. By preventing the back reflection from entering the light generation module 104, it is possible to deliver additional optical power to the target 120, which can lead to an increase in the amount of EUV light generated.

[0022] Referring to FIG. 2, a block diagram of an EUV light source 200 including an exemplary optical source 202 is shown. The optical source 202 can be used in place of the optical source 102 within the optical system 100 (FIG. 1). The optical source 202 includes a light generation module 204 including two optical subsystems 204a, 204b, the optical amplifier 108, and the optical isolator 106. The optical isolator 106 is on the path 112 and between the optical amplifier 108 and the light generation module 204.

[0023] Optical subsystems 204a and 204b each generate a first optical beam 210a and a second optical beam 210b. In the example of FIG. 2, the first optical beam 210a is represented by a solid line and the second optical beam 210b is represented by a dashed line. The optical subsystems 204a and 204b can be, for example, two lasers. In the example of FIG. 2, the optical subsystems 204a and 204b are two carbon dioxide (CO2) lasers. However, in other implementations, the optical subsystems 204a and 204b are different types of lasers. For example, optical subsystem 204a can be a solid-state laser and optical subsystem 204b can be a CO2 laser.

[0024] The first optical beam 210a and the second optical beam 210b have different wavelengths. For example, in an implementation where the optical subsystems 204a and 204b include two CO2 lasers, the wavelength of the first optical beam 210a can be about 10.26 micrometers (μm), and the wavelength of the second optical beam 210b can be between 10.18 μm and 10.26 μm. The wavelength of the second optical beam 210b can be about 10.59 μm. In these implementations, the optical beams 210a and 210b are generated from different lines of CO2, and even if both beams are generated from the same type of radiation source, the resulting optical beams 210a and 210b will have different wavelengths. The optical beams 210a and 210b can also have different energies.

[0025] The optical generation module 204 also includes a beam combiner 209 that directs the first optical beam 210a and the second optical beam 210b onto the beam path 112. The beam combiner 209 can be any optical element or collection of optical elements capable of directing the first optical beam 210a and the second optical beam 210b onto the beam path 112. For example, the beam combiner 209 can be a collection of mirrors, some of which are positioned to direct the first beam 210a onto the beam path 112 and others of which are positioned to direct the second optical beam 210b onto the beam path 112. The optical generation module 204 can also include a preamplifier 207 that amplifies the first optical beam 210a and the second optical beam 210b within the optical generation module 204.

[0026] The first optical beam 210a and the second optical beam 210b can propagate on the path 112 at different times, but the first optical beam 210a and the second optical beam 210b follow the path 112 and both beams 210a, 210b traverse substantially the same spatial region to the optical isolator 106 and through the optical amplifier 108. As will be discussed with respect to FIGS. 3 and 6, the first optical beam 210a and the second optical beam 210b are separated within the optical isolator 106 and then propagate on the path 112 to the optical amplifier 108.

[0027] The first optical beam 210a and the second optical beam 210b are angularly distributed by the beam delivery system 225, with the first beam 210a being directed towards the initial target region 215a and the second beam 210b being directed towards a modified target region 215b that is displaced in the y - direction relative to the initial target region 215a. In some implementations, the beam delivery system 225 also focuses the first optical beam 210a and the second optical beam 210b at locations within or near the initial and modified target regions 215a, 215b, respectively.

[0028] In the example shown in FIG. 2, the initial target area 215a receives the initial target 220a and the first beam 210a. The first beam 210a has sufficient energy to modify the geometric distribution of the target material within the initial target 220a into a modified target received within the modified target area 215b (or to initiate a spatial reconfiguration of the target material). The second beam 210b is also received within the modified target area 215b. The second beam 210b has sufficient energy to convert at least a portion of the target material within the modified target 220b into a plasma that emits EUV light. In this example, the first beam 210a can be referred to as a "pre-pulse" and the second beam 210b can be referred to as a "main pulse".

[0029] The first beam 210a is reflectable from the initial target 220a, producing a retro-reflection 213a that can propagate in a direction other than the z-direction and into the optical amplifier 108 along the path 112. The first beam 210a is used to modify the spatial characteristics of the initial target 220a and is not intended to convert the initial target 220a into a plasma that emits EUV light, so the first beam 210a has a lower energy than the second beam 210b. However, the reflection of the first optical beam 210a can have more energy than the reflection of the second optical beam 201b.

[0030] The first beam 210a (and reflection 213a) propagates through the optical amplifier 108 before the second beam 210b. Thus, the gain medium of the optical amplifier 108 can still be excited when the reflection 213a passes through the gain medium of the optical amplifier 108. As a result, the reflection 213a can be amplified by the amplifier 108. Further, the initial target 220a can be substantially spherical in shape, dense, and highly reflective, while the modified target 220b can be disk-like in shape (or other non-spherical shape), non-dense, and low-reflective. Due to the non-spherical shape, the modified target 220b can be positioned to reduce the amount of light retroreflected into the path 112 due to the interaction between the second beam 210b and the modified target 220b. For example, the modified target 220b can be tilted in the x-z and / or y-z plane with respect to the propagation direction of the light beam 210b, or the modified target 220b can be moved away from the focus of the second beam 210b.

[0031] In some implementations, the modified target 220b is not tilted in the x-z and / or y-z planes. Instead, the modified target 220b is oriented such that the side surface of the modified target 220b having the maximum spatial extent is in a plane perpendicular to the propagation direction of the second beam 210b. By orienting the modified target 220b in this way (which can be referred to as "flat" target orientation), the absorption of the second beam 210b can be enhanced. In some implementations, such an orientation can increase the absorption of the second beam 210b by about 10% compared to an instance where the modified target 220b is tilted 20 degrees (°) with respect to the plane perpendicular to the propagation direction of the second beam 210b. Orienting the modified target 220b in a flat orientation can increase the amount of light that is reflected and propagates backward into the optical source 202. However, since the optical source 202 includes the optical isolator 106, and the optical isolator 106 acts to reduce the impact of reflections that may occur from the modified target 220b in a flat orientation, the modified target 220b can have a flat orientation.

[0032] Finally, since the second beam 210b has relatively high energy, the forward propagation of the second beam 210b through the amplifier 108 saturates the gain medium, leaving little energy available for the amplifier 108 to provide for the backward reflection of the second beam 210b. Thus, even if the first beam 210a has less energy than the second beam 210b, the backward reflection 213a resulting from the first beam 210a can be quite large and may be larger than the backward reflection resulting from the second beam 210b.

[0033] As will be discussed below, the optical isolator 106 prevents back reflections originating from the first beam 210a from entering the light generation module 204. The optical isolator 106 can also prevent back reflections originating from the second beam 210b from entering the light generation module 204, and an example of such an implementation is shown in FIG. 6. Since the optical isolator 106 prevents potentially damaging back reflections from reaching the light generation module 204, it is possible to generate high-energy light beams from the light generation module 204, and as a result, more energy and more EUV light will be delivered to the modified target 220b. In some implementations, the average amount of EUV light generated can be increased by about 20% by using the optical isolator 106.

[0034] Referring to FIG. 3, a block diagram of an exemplary optical isolator 306 is shown. The optical isolator 306 can be used as the optical isolator 106 within the optical source 102 (FIG. 1), the optical source 202 (FIG. 2), or any other optical source. Consider the optical isolator 306 with respect to the optical source 202.

[0035] The optical isolator 306 includes dichroic optical elements 331, a reflective element 332, an optical modulator 335, and a dichroic element 336. The optical isolator 306 can also include optical arrangements 333, 334. The dichroic optical elements 331 and 336 are on the beam path 112. The dichroic elements 331 and 336 can be any optical component capable of separating or filtering light according to its wavelength. For example, the dichroic elements 331 and 336 can be dichroic mirrors, dichroic filters, dichroic beam splitters, or combinations of such elements. The dichroic elements 331 and 336 can be identical to each other or can have different configurations. In the example of FIG. 3, the dichroic elements 331 and 336 reflect the wavelength (or wavelengths) of the first beam 210a and transmit the wavelength (or wavelengths) of the second beam 210b.

[0036] The first beam 210a is reflected from the dichroic element 331 to the beam path 314 that is between the dichroic elements 331 and 336 and has a spatial range and shape defined by the reflective element 332. The beam path 314 is different from the beam path 112. Thus, in the optical isolator 306, the first beam 210a does not remain on the beam path 112, and the first optical beam 210a and the second optical beam 210b are spatially separated from each other. After the first beam 210a propagates on the beam path 314 through the optical arrangements 333, 334, and the optical modulator 335, it reaches the dichroic element 336, which reflects the beam 210a back onto the beam path 112. The second beam 210b passes through the dichroic element 331 and the dichroic element 336 and remains on the beam path 112 while propagating through the optical isolator 306.

[0037] The optical modulator 335 is on the beam path 314 between the dichroic elements 331 and 336. The optical modulator 335 is an optical element capable of deflecting incident light away from the path 314. The optical modulator 335 is adjustable between an open state and a closed state such that the optical modulator 335 transmits the first beam 210a and blocks the reflection 213a (the reflection of the first beam 210a from the initial target 220a).

[0038] The optical modulator 335 can be, for example, an acousto-optic modulator (AOM). The acousto-optic modulator includes a medium (such as quartz or glass) connected to a transducer (such as a piezoelectric transducer). The movement of the transducer forms a sound wave in the medium, creating a spatially varying refractive index in the medium. When the medium contains a sound wave, the light incident on the medium is deflected. When there is no sound wave in the medium, the acousto-optic modulator transmits the incident light without deflection. It is possible to use other optical modulators as the modulator 335. For example, the optical modulator 335 can be a Faraday rotator or an electro-optic modulator (EOM). The modulator 335 can be a combination of such devices and can include multiple devices of the same type.

[0039] In an implementation where the optical modulator 335 is an acousto-optic modulator, the transducer moves when it is predicted that the reflection 213a will enter the path 314. At other times, the transducer does not move or vibrate. Thus, the beam 210a (forward "pulse") passes through the optical modulator 335, remains on the path 314, and finally rejoins the path 112. However, the reflection 213a is deflected away from the path 314 (shown as deflection 217a in FIG. 3). As a result, the reflection 213a does not reach the optical generation module 204 (FIG. 2).

[0040] Since the optical modulator 335 can be configured to transmit incident light only at certain times, the optical isolator 306 provides a time gate based on isolation as opposed to polarization-based techniques. In addition, the optical isolator 306 can be used in combination with polarization-based isolation techniques. For example, the polarization of the backward reflection can be made different from the polarization of the forward beams 210a, 210b, and a polarization isolator 303 including a polarization element (such as a thin-film polarizer) can be placed between the optical isolator 306 and the optical amplifier 108 (FIGS. 1 and 2) to provide an additional block of the backward reflection. The polarization element of the polarization isolator 303 can be configured to mainly block the reflection of the second optical beam 210b, and the optical isolator 306 can be adjusted to block the reflection of the first optical beam 210a. By using different techniques to block the reflections of the first optical beam 210a and the second optical beam 210b, the total amount of reflection reaching the optical generation module 204 from any radiation source can be reduced.

[0041] In some implementations, the optical isolator 306 includes a first optical arrangement 333 and a second optical arrangement 334. The first beam 210a passes through the first optical arrangement 333 before reaching the optical modulator 335. The first optical arrangement 333 can be any optical element or set of optical elements that reduces the beam diameter of the first optical beam 210a. After passing through the optical modulator 335, the first beam 210a passes through the second optical arrangement 334. The second optical arrangement 334 can be any optical element or set of optical elements that enlarges the beam diameter of the second optical beam 210b. The speed at which the optical modulator 335 can transition between being open (where incident light is transmitted by the optical modulator 335) and closed (where incident light is deflected or blocked by the optical modulator 335) increases as the beam diameter decreases. Thus, by reducing the diameter of the first beam 210a, the first optical arrangement 333 enables the optical modulator 335 to switch between being open and closed, and vice versa, more quickly than in an implementation without the first optical arrangement 333. In some implementations, it is possible to reduce the beam diameter of the beam 210a to about 3 millimeters (mm).

[0042] The second optical arrangement 334 enlarges the diameter of the first optical beam 210a prior to directing the first optical beam 210a onto the path 112. Additionally, the second optical arrangement 334 reduces the beam diameter of the reflection 213a before the reflection 213a reaches the optical modulator 335. By reducing the beam diameter of the reflection 213a, the speed at which the optical modulator 335 has to transition between the open and closed states to block the reflection 213a is reduced.

[0043] Referring to FIGS. 4A and 4B, block diagrams of exemplary optical arrangements 433 and 434 are shown respectively. The optical arrangements 433, 434 can be used as optical arrangements 333, 334 respectively in the optical isolator 306 (FIG. 3). The optical arrangements 433, 434 are Galilean telescopes having one convex lens and one concave lens. In the optical arrangement 433, the concave lens 442 is between the convex lens 441 and the optical modulator 335. In the optical arrangement 434, the concave lens 443 is between the optical modulator 335 and the convex lens 444. Both configurations 433, 434 reduce the diameter of the beam propagating towards the optical modulator 335. When the optical arrangements 433, 434 are used together within the configuration shown in FIG. 3, the beam diameter of the beam 210a is reduced prior to being incident on the optical modulator 335, and the beam diameter of the beam 210a is enlarged by the optical arrangement 434 after passing through the optical modulator 335. The beam diameter of the reflection 213a is reduced by the optical arrangement 434 prior to reaching the optical modulator 335. Since the optical modulator 335 deflects the reflection 213a from the beam path 314, the reflection 213a does not pass through the optical arrangement 433.

[0044] The optical arrangements 433 and 434 can be Galilean telescopes, or the arrangements 433 and 434 can include lenses having different characteristics (such as different focal lengths).

[0045] Referring to FIG. 5A, an exemplary plot showing the state of the optical modulator 335 as a function of time is shown. FIG. 5B shows the relative arrangement of the pulses of the beam 510a and the reflection 513a on the same time axis shown in FIG. 5A. The pulse 510a is a pulse of a pulsed light beam that is used as a "pre-pulse" to shape the initial target 220a and propagates through the system 200 (FIG. 2) when the system 200 is configured to use the optical isolator 306 (FIG. 3) as the optical isolator 106, and the reflection 513a is the reflection of the pulse 513a from the initial target 220a.

[0046] The optical modulator 335 is closed from time t1 to time t2 (either deflecting light from the path 314 or otherwise preventing incident light from remaining on the path 314). At time t2, the optical modulator 335 begins to transition to the open state. The optical modulator 335 is open between time t2 and t3, and during this time range, the optical modulator 335 transmits the incident light. The optical modulator 335 transitions to the closed state at time t3 and opens again at time t4. As discussed above, the transition times (the time between time t2 and t3 and the time between t3 and t4) can be reduced by reducing the beam diameter of the light gated by the optical modulator 335.

[0047] Referring also to FIG. 5B, times t2 and t3 are selected such that the pulse 510a is incident on the optical modulator 335 when the modulator 335 is open. Thus, the pulse 510a passes through the optical modulator 335 to reach the initial target 220a. Times t3 and t4 are selected such that the optical modulator 335 begins to close after the pulse 510a has passed through and is closed when the reflection 513a is incident on the optical modulator 335. In this way, the optical modulator 335 provides time-gate-based isolation of the prepulse reflection 513a.

[0048] In some implementations, the beam diameters of the prepulse 510a and the reflection 513a can be 3 mm. In an implementation where the optical modulator 335 is an acousto-optic modulator, the time required for the optical modulator to transition from open to closed and vice versa is determined by the beam diameter of the incident light and the speed of sound in the material of the optical modulator. The material can be, for example, germanium (Ge) having a sound wave speed of 5500 meters per second (m / s). In this example, the transition time (the time for the optical modulator to transition from closed to open) is 375 nanoseconds (ns). The delay between the prepulse 510a and the reflection 513a can be, for example, 400 ns. Thus, the prepulse 510a is transmitted by the optical modulator 335 and the reflection 513a is deflected off the path 314.

[0049] In some implementations, the optical modulator 335 is closed except for the time period during which the pulse 510a is predicted. By remaining closed at other times, the optical modulator 335 prevents the reflection 513a from entering the light generation module 204. Additionally, by remaining closed, the modulator 335 also prevents or reduces the effect of the secondary reflection of the pulse 510a. Elements on the path 112, such as filters, pinholes, lenses, and tubes, are flash sources and reflect the incident light. These elements can reflect the pulse 510b and cause secondary reflections that propagate on the path 112 and the path 314, and these secondary reflections are additional to the reflection 513a. By keeping the modulator 335 closed except when the pulse 510a is incident on it, the secondary reflections are also prevented from entering the light generation module 204. Further, the secondary reflections are removed from the path 314 and thus prevented from propagating back onto the path 112. In this way, the secondary reflections cannot reach the initial target area 215a, the modified target area 215b, or the area between the areas 215a and 215b. If the secondary reflections can reach these areas, the reflections may harm the target by splitting before the target reaches the modified target area 215b. The secondary reflections can be called forward pulses excited by the reverse pulse (FER). The optical isolator 306 can help with self-lasing relaxation and limit the maximum value regarding the optical power delivered to the target area 215b.

[0050] Referring to FIG. 6, a block diagram of another exemplary optical isolator 606 is shown. Optical isolator 606 can be used in place of optical isolator 106 within system 100 (FIG. 1) or system 200 (FIG. 2). Additionally, optical isolator 606 can be used within any other optical system where back reflection is desirable. Optical isolator 606 is considered with respect to a configuration in which optical isolator 606 is used as optical isolator 106 within system 200 (FIG. 2). Optical isolator 606 can be used in conjunction with polarization isolator 303 discussed above with respect to FIG. 3. In an implementation that includes polarization isolator 303, polarization isolator 303 is between optical isolator 606 and optical amplifier 108 (FIGS. 1 and 2) to provide an additional block of back reflection.

[0051] Optical isolator 606 is similar to optical isolator 306 (FIG. 3), except that optical isolator 606 includes a second optical modulator 637. The second optical modulator 637 is on path 112 and is positioned between dichroic optical element 331 and dichroic optical element 336. Similar to optical modulator 335, the second optical modulator 637 transmits incident light when in the open state and deflects or blocks incident light when in the closed state. A second light beam 210b is emitted from the light generation module 204 and propagates on path 112 to dichroic optical element 331.

[0052] As discussed above, the dichroic optical element 331 transmits the wavelength of the second light beam 210b. Thus, the second light beam 210b passes through the dichroic optical element 331 and is incident on the second optical modulator 637. The second optical modulator 637 is controlled to be in an open state when the second light beam 210b is incident on the modulator 637. The second light beam 210b passes through the modulator 637 and the dichroic optical element 336, is maintained on the path 112, and reaches the modified target area 215b (Figure 2). A part of the second light beam 210b is reflected from the modified target 220b (in addition to converting at least a part of the target material into a plasma that emits EUV light), and can propagate as a reflection 213b in a direction other than the z - direction along the path 112.

[0053] The reflection 213b is transmitted by the dichroic optical element 336 and is maintained on the path 112. The optical modulator 637 is closed when the reflection 213b is incident on the modulator 637, and the reflection 213b is deflected from the path 112 as a deflected light 217b. Thus, the second modulator 637 prevents the reflection 213b from reaching the light generation module 204 or reduces the amount of the reflection 213b reaching the light generation module 204, reduces or eliminates self - lasing from the light generation module 404, and enables the second light beam 210b to have a greater energy. In some implementations, the optical modulator 637 deflects 30 - 40% of the reflection 213b. The amount of self - lasing can be further reduced by reducing the time during which the optical modulator 637 is open. For example, by reducing the open time from 20 microseconds (μs) to 2 μs, the self - lasing can be reduced by 90%.

[0054] The second modulator 637 is closed except for the time period during which the beam 210b is predicted. By remaining closed at other times as well, the second modulator 637 prevents the reflection 213b from entering the light generation module 204. Additionally, by remaining closed, the second modulator 637 also prevents or reduces the effect of secondary reflections from the second beam 210b. Elements on the path 112, such as filters, pinholes, lenses, and tubes, are flash sources and reflect the incident light. These elements can reflect the second beam 210b and cause additional secondary reflections in the reflection 213b (resulting from the interaction between the second beam 210b and the modified target 220b). By keeping the modulator 637 closed except when the second optical beam 210b is incident on it, secondary reflections are also prevented from entering the light generation module 204, and the secondary reflections are removed from the path 112.

[0055] The second optical modulator 637 can be the same as the modulator 335, or the second optical modulator 637 and the modulator 335 can be different types of modulators.

[0056] Referring to FIG. 7, a block diagram of the system 700 is shown. The system 700 includes a light generation module 704, a control system 740, and an optical modulator 735. The light generation module 704 can be the light generation module 104 (FIG. 1), the light generation module 204 (FIG. 2), or any other system that generates an optical beam having a different wavelength. The optical modulator 735 can be the optical modulator 335 (FIG. 3) and / or the optical modulator 637 (FIG. 6).

[0057] The control system 740 provides a trigger signal 747 to the optical modulator 735. The trigger signal 747 is sufficient to cause the optical modulator 735 to change state or initiate a state change. For example, in an implementation where the optical modulator 735 is an acousto-optic modulator, the trigger signal 747 can cause the transducer to vibrate to form a sound wave in the modulator, thereby transitioning the modulator to a closed state. The control system 740 can receive data from the optical generation module 704 via signal 741 and can provide data to the optical generation module 704 via signal 742. Further, the control system 740 can also receive data from the optical module 735 via signal 742.

[0058] The control system 740 includes an electronic storage 743, an electronic processor 744, and an input / output (I / O) interface 745. The electronic processor 744 includes one or more processors suitable for the execution of a computer program, such as a general-purpose or special-purpose microprocessor, and any one or more processors of any type of digital computer. Generally, the processor receives instructions and data from read-only memory or random access memory, or both. The electronic processor 744 can be any type of electronic processor.

[0059] The electronic storage 743 can be a volatile memory such as RAM, or a non-volatile memory. In some implementations, the electronic storage 743 can also include both non-volatile and volatile portions or components. The electronic storage 743 can be data and information used in the operation of the optical modulator 735. For example, the electronic storage 743 can store timing information that specifies when the first optical beam 210a and the second optical beam 210b are expected to propagate through the system 200 (FIG. 2). The electronic storage 743 can also store instructions, perhaps as a computer program that, when executed, causes the processor 744 to communicate with other components within the control system 740, the optical generation module 704, and / or the optical modulator 735. For example, the instructions can cause the electronic processor 744 to provide the trigger signal 747 to the optical modulator 735 at a point in time specified by the timing information stored on the electronic storage 743.

[0060] The I / O interface 745 is any type of electronic interface that enables the control system 740 to receive and / or provide data and signals using an operator, an automated process operating on the optical generation module 704, the optical modulator 735, and / or another electronic device. For example, the I / O interface 745 can include one or more of a visual display, a keyboard, or a communication interface.

[0061] Referring to FIG. 8A, an LPP EUV light source 800 is shown. The optical systems 100 and 200 can be part of an EUV light source such as the radiation source 800. The LPP EUV light source 800 is formed by irradiating a target mixture 814 at a target location 805 with an amplified light beam 810 that travels along a beam path towards the target mixture 814. The target location 805, also referred to as the emission site, is inside the vacuum chamber 830 at 807. When the amplified light beam 810 hits the target mixture 814, the target material within the target mixture 814 is converted into a plasma state having elements with spectral lines in the EUV range. The created plasma has certain characteristics that depend on the composition of the target material within the target mixture 814. These characteristics can include the wavelength of the EUV light generated by the plasma, as well as the type and amount of debris released from the plasma.

[0062] The light source 800 also includes a target material delivery system 825 that delivers, controls, and directs the target mixture 814 in the form of droplets, liquid streams, solid particles or clusters, solid particles contained within droplets, or solid particles contained within liquid streams. The target mixture 814 includes a target material such as, for example, water, tin, lithium, xenon, or any material that has spectral lines in the EUV range when converted to a plasma state. For example, the element tin can be used as pure tin (Sn), as a compound such as SnBr4, SnBr2, SnH4, as a tin alloy such as a tin-gallium alloy, a tin-indium alloy, a tin-indium-gallium alloy, or any combination of these alloys. The target mixture 814 can also include impurities such as non-target particles. Thus, in the situation where no impurities are present, the target mixture 814 is composed of only the target material. The target mixture 814 is delivered by the target material delivery system 825 to the interior 807 of the chamber 630 and the target location 605.

[0063] The light source 800 includes a drive laser system 815 that generates an amplified optical beam 810 due to a gain medium or an inversion distribution within the medium of the laser system 815. The light source 800 includes a beam delivery system between the laser system 815 and the target location 805, and the beam delivery system includes a beam transmission system 820 and a focus assembly 822. The beam transmission system 820 receives the amplified optical beam 810 from the laser system 815, steers and corrects the amplified optical beam 810 as necessary, and outputs the amplified optical beam 810 to the focus assembly 822. The focus assembly 822 receives the amplified optical beam 810 and focuses the beam 810 onto the target location 805.

[0064] In some implementations, the laser system 815 can include one or more optical amplifiers, lasers, and / or lamps for providing one or more main pulses and, in some cases, one or more prepulses. Each optical amplifier includes a gain medium, an excitation source, and internal optics that are capable of optically amplifying with high gain at a desired wavelength. The optical amplifier may or may not have a laser mirror or other feedback device that forms a laser cavity. Thus, the laser system 815 generates the amplified optical beam 810 due to an inversion distribution within the gain medium of the laser amplifier, even if no laser cavity is present. Further, the laser system 815 is capable of generating the amplified optical beam 810, which is a coherent laser beam, if a laser cavity is present to provide sufficient feedback to the laser system 815. The term "amplified optical beam" encompasses one or more of light from the laser system 815 that is merely amplified but not necessarily a coherent laser oscillation, and light from the laser system 815 that is also an amplified coherent laser oscillation.

[0065] The optical amplifier within the laser system 815 can include a filling gas containing CO2 as a gain medium and can amplify light having a wavelength between about 9100 and about 11000 nm, and in particular at about 10600 nm, with a gain greater than or equal to 800. Amplifiers and lasers suitable for use within the laser system 815 can generate radiation, for example, using DC or RF excitation, at about 9300 nm or about 10600 nm and operate at relatively high power, for example 10 kW or more, and at a high pulse repetition rate, for example 40 kHz or more, and can include pulsed laser devices, such as pulsed gas discharge CO2 laser devices. The optical amplifier within the laser system 815 can also include a cooling system, such as water, that can be used when the laser system 815 operates at higher power.

[0066] FIG. 8B shows a block diagram of an exemplary drive laser system 880. The drive laser system 880 can be used as part of the drive laser system 815 within the radiation source 800. The drive laser system 880 includes three power amplifiers 881, 882, and 883. Any or all of the power amplifiers 881, 882, and 883 can include internal optical elements (not shown).

[0067] Light 884 exits the power amplifier 881 through the output window 885 and is reflected by the curved mirror 886. After reflection, the light 884 passes through the spatial filter 887, is reflected by the curved mirror 888, and enters the power amplifier 882 through the input window 889. The light 884 is amplified within the power amplifier 882 and is redirected out of the power amplifier 882 as light 891 through the output window 890. The light 891 is directed towards the amplifier 883 using the folding mirror 892 and enters the amplifier 883 through the input window 893. The amplifier 883 amplifies the light 891 and directs the light 891 out of the amplifier 883 as the output beam 895 through the output window 894. The folding mirror 896 directs the output beam 895 upward (out of the page) and towards the beam transmission system 820 (FIG. 8A).

[0068] Referring again to FIG. 8B, the spatial filter 887 defines an aperture 897, which can be, for example, circular with a diameter between about 2.2 mm and 3 mm. The curved mirrors 886 and 888 can be, for example, off-axis parabolic mirrors with focal lengths of about 1.7 m and 2.3 m, respectively. The spatial filter 887 can be positioned such that the aperture 897 coincides with the focus of the drive laser system 880.

[0069] Referring again to FIG. 8A, the light source 800 includes a condenser mirror 835 having an aperture 840 to allow the amplified light beam 810 to pass through and reach the target location 805. The condenser mirror 835 can be, for example, an elliptical mirror having a primary focus at the target location 805 and a secondary focus (also called an intermediate focus) at the intermediate location 845, where EUV light is output from the light source 800 and can be input, for example, into an integrated circuit lithography tool (not shown). The light source 800 can also include an open-ended hollow conical shroud 850 (e.g., a gas cone) that tapers from the condenser mirror 835 towards the target location 805 to reduce the amount of plasma generation debris entering the focus assembly 822 and / or the beam transmission system 820 while allowing the amplified light beam 810 to reach the target location 805. For this purpose, a gas flow can be provided within the shroud that is directed towards the target location 805.

[0070] The light source 800 can also include a master controller 855 connected to a droplet position detection feedback system 856, a laser control system 857, and a beam control system 858. The light source 800 can provide an output indicating the position of the droplets with respect to, for example, a target location 805, and can include one or more target or droplet imagers 860 capable of calculating droplet positions and trajectories that can calculate, for example, a droplet position error for each droplet or on average. This output is provided to the droplet position detection feedback system 856. Thus, the droplet position detection feedback system 856 provides the droplet position error as an input to the master controller 855. Thus, the master controller 855 can provide position, direction, and timing correction signals to, for example, the laser control system 857 that can be used to control a laser timing circuit, and / or to the beam control system 858 to control the position and shape of the amplified light beam of the beam transmission system 820 to change the location and / or focus power of the beam focus within the chamber 830.

[0071] The target material delivery system 825 includes a target material delivery control system 826 operable to respond to a signal from the master controller 855 to modify, for example, the release point of droplets released by the target material supply device 827 to correct for errors within the droplets reaching a desired target location 805.

[0072] In addition, the light source 800 can include light source detectors 865 and 870 that measure one or more EUV light parameters including, but not limited to, pulse energy, energy distribution as a function of wavelength, energy within a specific band of wavelengths, energy outside a specific band of wavelengths, and angular distribution of EUV intensity and / or average power. The light source detector 865 generates a feedback signal for use by the master controller 855. The feedback signal can indicate errors in parameters such as the timing and focus of the laser pulses, for example, to properly block the droplets at the right place and time for effective and efficient EUV light generation.

[0073] The light source 800 can also include a guide laser 875 that can be used to align various sections of the light source 800 or to assist in steering the amplified light beam 810 to the target location 805. In connection with the guide laser 875, the light source 800 includes a metrology system 824 disposed within the focus assembly 822 to sample a portion of the light from the guide laser 875 and the amplified light beam 810. In other implementations, the metrology system 824 is disposed within the beam transmission system 820. The metrology system 824 can include optical elements that sample or redirect a subset of the light, and such optical elements are composed of any material capable of withstanding the power of the guide laser beam and the amplified light beam 810. The master controller 855 analyzes the light sampled from the guide laser 875 and uses this information to adjust components within the focus assembly 822 via the beam control system 858, such that a beam analysis system is formed from the metrology system 824 and the master controller 855.

[0074] Thus, in summary, the light source 800 generates an amplified light beam 810 that is directed along a beam path to irradiate a target mixture 814 at a target location 805 and convert the target material within the mixture 814 into a plasma that emits light within the EUV range. The amplified light beam 810 operates at a specific wavelength (also referred to as the drive laser wavelength) that is determined based on the design and characteristics of the laser system 815. Additionally, the amplified light beam 810 can be a laser beam when the target material provides sufficient feedback into the laser system 815 or when the drive laser system 815 includes an optical feedback suitable for forming a laser cavity to provide coherent laser light.

[0075] Referring to FIG. 9, a plot 900 of exemplary test data for an optical isolator, such as the optical isolator 306 (FIG. 3), is shown. The plot 900 shows the measured power of the backward prepulse beam as a function of time using the optical isolator in the on and off states. The backward prepulse beam can be a beam, such as the reflection 213a (FIG. 2), resulting from the interaction between the first beam 210a (FIG. 2) and the initial target 220a (FIG. 2) as discussed above. In the on state, the optical isolator blocks or reduces the effect of the reflection 213a by deflecting all or a portion of the reflection 213a from the beam path 314 such that the reflection 213a reaching the light generation module 204 is reduced or eliminated. In the on state, the optical isolator is operable as discussed, for example, with respect to FIGS. 5A and 5B. In the off state, the optical isolator is inactive and the system operates as if the optical isolator were not present.

[0076] In the example of FIG. 9, the optical isolator is in the off state between times 905 and 910 and is in the on state otherwise. When the optical isolator is in the on state, the power of the reflection 213a reaching the light generation module 204 is very low and close to zero watts (W). For example, the power of the reflection 213a reaching the light generation module 204 can be about 0.1 W or less. As discussed above, it is desirable to reduce the power of the reflection 213a reaching the light generation module 204. In contrast, when the optical isolator is in the off state, the power of the reflection 213a reaching the light generation module 204 is greater than 0 and can be between about 4.2 W and 18.2 W. Further, when the optical isolator is in the off state, the power of the reflection 213a reaching the light generation module 204 varies considerably, which can lead to instability in the system. Therefore, in addition to reducing the amount of power in the reflection 213a, the optical isolator also reduces the variation in the power of the reflection, resulting in a more stable system.

[0077] Referring to FIGS. 10A and 10B, additional exemplary test data is shown. FIG. 10A shows the energy of EUV light generated as a function of the number of pulses when no optical isolator (such as optical isolator 306) is present in the system, and FIG. 10B shows the energy of EUV light generated as a function of the number of pulses when an optical isolator is present in the system. When no optical isolator is present, the average energy of the EUV light is 3.4 millijoules (mJ). When an optical isolator is present, the average EUV energy increases to 4.1 mJ.

[0078] Referring also to FIGS. 11A and 11B, when an optical isolator is present in the system, the EUV light generated is also more stable. FIG. 11A shows the distribution of specific values of the energy of EUV light generated when no optical isolator is present, and FIG. 11B shows the distribution of specific values of the energy of EUV light generated when an optical isolator is present. The distribution of the energy values in FIG. 11B (when an optical isolator is used) shows that more higher energy values occur and that all the energy values are contained within a smaller range compared to a system without an optical isolator. Thus, using an optical isolator (such as optical isolator 306) will result in generating EUV light of higher energy and also in generating more stable (less variable) EUV light.

[0079] Referring to FIGS. 12A - 12C and FIGS. 13A - 13C, additional exemplary test data is shown. FIGS. 12A - 12C show the target 1200 three times in a system without an optical isolator such as optical isolator 306, and FIGS. 13A - 13C show the target 1300 three times in a system including an optical isolator such as optical isolator 306. The targets 1200 and 1300 include target materials that emit EUV light when in a plasma state. The targets 1200 and 1300 are shown at the point in time when they coincide with the locations that receive the prepulse (such as the initial target region 215a in FIG. 2) and the main pulse (such as the modified target region 215b in FIG. 2).

[0080] As discussed above with respect to FIGS. 5A and 5B, an optical isolator can reduce or eliminate secondary reflections from objects such as pinholes, lenses, tubes, and optical elements. Secondary reflections, if present, can reach the target as the target moves from the initial target region 215a to the modified target region 215b. FIGS. 12A-12C show examples of secondary reflections that interact with the target 1200 over time. As shown in FIGS. 12B and 12C compared to FIG. 12A, the target 1200 spatially diffuses and splits over time. FIGS. 13A-13C show examples of systems that use an optical isolator (such as optical isolator 306) to reduce or eliminate secondary reflections. Compared to the target 1200 (FIGS. 12A-12C), the target 1300 (FIGS. 13A-13C) has a clearer spatial profile, which leads to an increase in the absorption of the incident light beam and more target material being available for interaction with the first beam 210b (and thus more EUV light being generated). Additionally, since the target 1300 is used with an optical source that includes an optical isolator, the target 1300 can have a flat orientation with respect to the propagation direction of the incident light beam while still reducing or eliminating the effects of back reflections and secondary reflections on the optical source.

[0081] Other implementations are within the scope of the claims.

[0082] In an implementation where the optical subsystems 204a and 204b (FIG. 2) are different types of optical subsystems, the optical subsystem 204a can be a rare earth doped solid laser (such as Nd:YAG or erbium doped fiber (Er:glass)), and the wavelength of the first optical beam 210a can be 1.06 μm. The optical subsystem 204b can be a CO2 laser, and the wavelength of the optical beam 210b can be, for example, 10.26 μm. In these implementations, the first optical beam 210a and the second optical beam 210b can be amplified in separate optical amplifiers and can follow separate paths through the system 200. Also, it is possible to use two separate optical isolators, one for the first optical beam 210a and its corresponding reflections, and the other for the optical beam 210b and its corresponding reflections.

[0083] The preamplifier 207 (FIG. 2) can have multiple stages. In other words, the preamplifier 207 can include a plurality of amplifiers arranged in series and on the path 112.

[0084] The optical beams 110, 210a, and 210b can be pulsed optical beams. The power of the pulses of the first optical beam 210a (or pulse 510a) can be, for example, 20 to 40 watts (W). The power of the pulses of the second optical beam 210b can be, for example, 300 to 500 W.

[0085] The first light beam 210a can be any type of radiation capable of acting on the initial target 220a to form the modified target 220b. For example, the first light beam 210 can be a pulsed light beam generated by a laser. The first light beam 210 can have a wavelength of about 1 to 10.6 μm. The pulse duration of the first light beam 210a can be, for example, 20 to 70 nanoseconds (ns), less than 1 ns, 300 picoseconds (ps), between 100 and 300 ps, between 10 and 50 ps, or between 10 and 100 ps. The pulse energy of the first light beam 210a can be, for example, 15 to 60 millijoules (mJ). When the pulse of the first light beam 210a has a duration of 1 ns or less, the pulse energy can be 2 mJ. The time between the pulse of the first light beam 210a and the pulse of the second light beam 210b can be, for example, 1 to 3 microseconds (μs).

[0086] The initial target 220a and the target 115 can have any features of the target mixture 814. For example, the initial target 220a and the target 115 can include tin.

[0087] The optical systems 100 and 200 can include a polarization isolator 303. In these implementations of the optical system 100, the polarization isolator 303 is between the optical isolator 106 and the optical amplifier 108.

Claims

1. An apparatus for an extreme ultraviolet (EUV) light source, comprising a plurality of dichroic optical elements, each of said dichroic optical elements being configured to reflect light having a wavelength within a first wavelength band and transmit light having a wavelength within a second wavelength band, said plurality of dichroic optical elements including at least a first dichroic optical element and a second dichroic optical element, said first dichroic optical element being positioned to receive at least a first pulse of a first pulsed light beam and a first pulse of a second pulsed light beam, said first dichroic optical element being configured to direct said first pulsed light beam onto a first beam path and said second pulsed light beam onto a second beam path, a plurality of dichroic optical elements, a plurality of other optical elements configured to be positioned on said first beam path between said first dichroic optical element and said second dichroic optical element, a first optical modulator configured to be positioned on said first beam path between said first dichroic optical element and said second dichroic optical element, comprising, said optical modulator comprising an acousto-optic modulator, said first optical modulator being adjustable between an open state and a closed state, said first optical modulator transmitting incident light onto said first beam path when in said open state and removing incident light from said first beam path when in said closed state, during use in operation, said first dichroic optical element and said second dichroic optical element form part of an optical isolator and are between a pre-amplifier of a light generation module and an optical amplifier, said second dichroic optical element receiving the reflection of said first pulsed light beam from a target comprising a target material and being positioned to direct said reflection onto said first beam path such that said optical modulator substantially prevents the reflection of said first pulsed light beam from reaching said pre-amplifier, the wavelength of said first pulsed light beam being within said first wavelength band, the wavelength of said second pulsed light beam being within said second wavelength band, an apparatus.

2. The optical modulator is configured to transmit light propagating in a first direction on the first beam path and deflect light propagating in a second direction on the first beam path so as to remove incident light propagating in the second direction from the first beam path, wherein the second direction is different from the first direction, the apparatus according to claim 1.

3. The apparatus further comprises a control system configured to provide a trigger signal to the acousto-optic modulator, wherein the acousto-optic modulator deflects light from the first beam path in response to reception of the trigger signal and removes the light, or otherwise transmits the light on the first beam path, the apparatus according to claim 1.

4. The acousto-optic modulator deflects light away from the first beam path in response to reception of the trigger signal, the apparatus according to claim 3.

5. The apparatus according to claim 1, further comprising a second optical modulator.

6. The second optical modulator is on the second beam path between the first dichroic optical element and the second dichroic optical element, the apparatus according to claim 5.

7. The plurality of other optical elements comprises a plurality of reflective optical elements, and the first dichroic optical element, the second dichroic optical element, and one or more of the reflective optical elements define the first beam path, the apparatus according to claim 1.

8. The apparatus further comprises a control system configured to provide a trigger signal to the acousto-optic modulator, wherein the acousto-optic modulator is configured to transition from one of the open state and the closed state to the other of the open state and the closed state in response to reception of the trigger signal, the apparatus according to claim 1.

9. The first dichroic optical element is configured to transmit light having a wavelength within the second wavelength band onto the second beam path, the apparatus according to claim 1.

10. The second beam path is between the first dichroic optical element and the second dichroic optical element, and there are no optical elements between the first dichroic optical element and the second dichroic optical element, the apparatus according to claim 9.

11. The apparatus according to claim 10, further comprising a second optical modulator on the second beam path.

12. The apparatus according to claim 1, wherein the optical isolator includes an optical isolator based on polarization.

13. The reflection of the first pulsed light beam has a first polarization state, the reflection of the second pulsed light beam has a second polarization state, and the optical isolator based on polarization is configured to substantially block light having the second polarization state. The apparatus according to claim 12.

14. The apparatus according to claim 1, further comprising at least one other optical modulator.

15. The apparatus according to claim 14, wherein the at least one other optical modulator includes an acousto-optic modulator.

16. The plurality of other optical elements are a first optical arrangement on the first beam path, between the first dichroic optical element and the first optical modulator, and configured to reduce the beam diameter of the first light beam; a first optical arrangement; a second optical arrangement on the first beam path, between the second dichroic optical element and the acousto-optic modulator, and configured to expand the beam diameter of the first light beam; a second optical arrangement; The apparatus according to claim 1, comprising.

17. The apparatus according to claim 1, wherein the first pulsed light beam includes a prepulse light beam, and the second pulsed light beam includes a main pulse light beam.

18. The target is substantially a flat target, the first pulsed light beam has a first energy, the second pulsed light beam has a second energy greater than the first energy, and the reflection of the first pulsed light beam from the target has an energy greater than the reflection of the second pulsed light beam. The apparatus according to claim 1.

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