Systems and methods for gain elements in isolated laser systems

Abstract

A method and apparatus for protecting a seed laser in a laser-generated plasma (LPP) extreme ultraviolet (EUV) optical system are disclosed. An isolation stage positioned in the optical path deflects light reflected from further components in the LPP / EUV optical system to prevent it from reaching the seed laser. The isolation stage includes two isolation elements separated by a delay line. The AOMs, when open, guide light into the optical path, and when closed, guide light away from the optical path. The delay introduced by the delay line is determined such that the opening and closing of the AOMs can be timed to guide forward-moving pulses into the optical path and deflect reflected light at other times. The isolation stage can be positioned between gain elements to prevent amplified reflected light from reaching the seed laser and other potentially harmful effects.

Description

[0001] This application is a divisional application of the invention patent application filed on November 6, 2015, with international application number PCT / US2015 / 059573, which entered the Chinese national phase on May 25, 2017, with Chinese national application number 201580064107.5, and entitled "System and method for gain elements in isolated laser systems". Technical Field

[0002] This application generally relates to laser-generated plasma (LPP) extreme ultraviolet (EUV) light sources, and more specifically to methods and systems for preventing feedback through gain elements within such light sources. Background Technology

[0003] The semiconductor industry continues to develop photolithography technologies capable of printing increasingly smaller integrated circuits. Extreme ultraviolet (“EUV”) light (sometimes also called soft X-rays) is generally defined as electromagnetic radiation with wavelengths between 6 nanometers (nm) and 50 nm. EUV lithography is currently generally considered to include EUV light with wavelengths in the range of 5 nm to 7 nm and is used to create extremely small features, such as sub-10 nm features, in substrates such as silicon wafers. For commercial use, these systems are expected to be highly reliable and offer cost-effective throughput and reasonable process latitude.

[0004] Methods for generating EUV light include, but are not limited to, converting a material into a plasma state having one or more elements (e.g., xenon, lithium, tin, indium, antimony, tellurium, aluminum, etc.) using one or more emission lines in the EUV range. In one such method, the desired plasma, often referred to as laser-generated plasma (“LPP”), can be generated by irradiating a target material (such as droplets, streams, or clusters of material having the desired line-emitting element) at the irradiation site using a laser beam. The line-emitting element can be in pure form or alloy form, such as an alloy that is liquid at the desired temperature, or it can be mixed or dispersed with another material such as a liquid.

[0005] In some prior art LPP systems, droplets in a stream of microdroplets are irradiated with separate laser pulses to form plasma from each droplet. Alternatively, some prior art systems are disclosed in which droplets are sequentially irradiated with more than one light pulse. In some cases, each droplet may be exposed to a so-called “prepulse” to heat, expand, vaporize, evaporate, and / or ionize the target material and / or generate a weak plasma, followed by exposure to a so-called “main pulse” to generate a strong plasma and convert most or all of the material affected by the prepulse into plasma, thereby producing EUV light emission. It should be understood that more than one prepulse and more than one main pulse can be used, and the functions of the prepulse and the main pulse can overlap to some extent.

[0006] Since the EUV output power in an LPP system is generally scaled proportionally to the power of the driving laser used to irradiate the target material, in some cases it may be desirable to employ an arrangement that includes a relatively low-power oscillator or "seed laser" and one or more amplifiers for amplifying the pulses from the seed laser. The use of large amplifiers allows for the use of a seed laser while still providing relatively high-power pulses for use in the LPP process.

[0007] However, irradiation of droplets by a laser pulse can cause reflections and thus backpropagate light through the gain elements toward the seed laser. This can cause undesirable modulation of the forward laser pulse and gain loss in the preamplifier. Furthermore, the seed laser may include sensitive optics, and since the pulse from the seed laser has been amplified, the backpropagating light may be strong enough to damage the relatively fragile seed laser.

[0008] For example, in some cases, the amplifier can have 100,000 (i.e., 10). 5 The signal gain is on the order of magnitude of 10^3. In such cases, typical protective devices of the prior art, such as polarization-discriminating optical isolators that can, for example, block approximately 93% to 99% of backpropagating light, may be insufficient to protect the seed laser from damage.

[0009] Therefore, there is a need for improved systems and methods for isolating gain elements in such EUV light sources and protecting seed lasers. Summary of the Invention

[0010] As described in this article, AOMs are used to provide isolation between a series of preamplifiers by adding a time delay between pairs of AOMs.

[0011] According to some embodiments, a system includes: a laser seed module for generating laser light in an optical path; a first gain element positioned along the optical path; a second gain element positioned along the optical path after the first gain element; and an isolation stage positioned along the optical path between the first and second gain elements, the isolation stage being configured to redirect light reflected back from the second gain element along the optical path, the isolation stage including: a first acousto-optic modulator (AOM) configured to transition between a first state in which light is guided along the optical path and a second state in which light is not guided along the optical path during a first time period; a second AOM configured to transition between the first state in which light is guided along the optical path and the second state in which light is not guided along the optical path during a time period, the transition of the second AOM occurring after a time delay; and a delay device positioned between the first and second AOMs and configured to cause a transmission delay of the laser beam between the first and second AOMs based on a time period selected from the time periods of transition between the two first states and the two second states and a predetermined time period in which both the first and second AOMs remain in the first state.

[0012] According to some embodiments, a method includes: generating a laser beam in an optical path; passing a laser pulse generated from the laser beam through a first gain element positioned along the optical path; passing the laser pulse through an isolation stage positioned between the first and second gain elements along the optical path, the isolation stage being configured to redirect light reflected back from the second gain element along the optical path, the isolation stage including: a first acousto-optic modulator (AOM) configured to transition between a first state in which the light is guided along the optical path and a second state in which the light is not guided along the optical path during a time period; a second AOM configured to transition between the first and second states in which the light is guided along the optical path and a second state in which the light is not guided along the optical path during a time period, the transition occurring after a time delay; and a delay device positioned between the first and second AOMs and configured to cause a transmission delay of the laser beam between the first and second AOMs based on a time period selected from the time periods of transition between the two first states and the two second states and the time period in which both the first and second AOMs remain in the first state; and passing the laser pulse through a second gain element positioned after the first gain element along the optical path. Attached Figure Description

[0013] Figure 1 This is a diagram of some components of an embodiment of the LPP EUV system.

[0014] Figure 2 This is an illustration of some components of an embodiment of a seed laser module that can be used in an LPP EUV system.

[0015] Figure 3This is a simplified block diagram of one embodiment of a pulse generation system using a seed laser module.

[0016] Figures 4A to 4E This is a simplified block diagram of one embodiment of an acousto-optic modulator.

[0017] Figures 5A to 5B This is a simplified block diagram of one embodiment of the isolation level.

[0018] Figure 6 It is a simplified timing diagram depicting how light is redirected by an isolation stage in one embodiment.

[0019] Figure 7 This is a flowchart of an embodiment of a method for redirecting reflected light.

[0020] Specific implementation

[0021] In LPP EUV generation systems, a seed laser typically generates a seed pulse, which is shaped, amplified, and otherwise modified by various elements before irradiating the target material. The seed laser can be fragile, and light can be reflected from the target material and back to the seed laser. Along the reverse path, the reflected light can be added, amplified, and modified by the same elements that modify the seed pulse. An acousto-optic modulator (AOM) is therefore often used as a switch to redirect or transmit light traveling in both directions.

[0022] One challenge in using an AOM is that a Bragg AOM requires a time interval (e.g., one microsecond) to transition from an on state (deflecting light along the optical path) to an off state (redirecting light from the optical path). This time can be significantly longer than the length of the seed pulse, potentially damaging other components that could allow reflected light to pass through the AOM during the length of the seed pulse.

[0023] To protect the seed laser and other components in the LPP EUV system, an isolation stage is positioned between certain components. The isolation stage includes a delay line positioned between two AOMs (Optical Oscillators). The AOMs are timed such that each AOM allows the forward propagation pulse generated by the seed laser to travel along the optical path and, at other times, deflects reflected light from the optical path. When the first AOM deflects the pulse into the optical path, the second deflects the reflected light, and vice versa. The delay line is used to delay the light that has already passed through one of the AOMs while the other AOM transitions to the desired state.

[0024] Figure 1 This is a simplified schematic diagram of some components of one embodiment of the LPP EUV light source 10. (See diagram below.) Figure 1As shown, the EUV source 10 includes a laser source 12 for generating a beam of laser pulses and directing the beam from the laser source 12 along one or more optical paths into the chamber 14 to irradiate a corresponding target, such as a microdroplet, at an irradiation area 16. The following describes in more detail what is suitable for use as... Figure 1 An example of the laser arrangement of the laser source 12 in the EUV light source 10 shown.

[0025] Or as Figure 1 As shown, the EUV source 10 may also include a target material delivery system 26, which, for example, delivers microdroplets of target material into the interior of chamber 14 to irradiation region 16, where the microdroplets will interact with one or more laser pulses to ultimately generate plasma and produce EUV emission. Various target material delivery systems already exist in the prior art, and their relative advantages will be apparent to those skilled in the art.

[0026] As described above, the target material is an EUV emitting element, which may include, but is not limited to, materials including tin, lithium, xenon, or combinations thereof. The target material may be in the form of droplets, or alternatively, may be solid particles contained within droplets. For example, elemental tin may be presented as pure tin, as tin compounds such as SnBr4, SnBr2, SnH4, etc., or as a tin alloy (e.g., tin-gallium alloy, tin-indium alloy, or tin-indium-gallium alloy, or combinations thereof). Depending on the material used, the target material may be presented to the irradiated region 16 at various temperatures including room temperature or near room temperature (e.g., tin alloys or SnBr4), at temperatures above room temperature (e.g., pure tin), or at temperatures below room temperature (e.g., SnH4). In some cases, these compounds may be relatively volatile, such as SnBr4. Similar alloys and compounds of EUV emitting elements other than tin, and the relative advantages of such materials and those described above, will be apparent to those skilled in the art.

[0027] Return to Figure 1 The EUV light source 10 may also include optical elements 18, such as near-normal incident collector mirrors having a reflective surface in the form of a long ellipsoid (i.e., an ellipse rotating about its principal axis), such that the optical element 18 has a first focal point in or near the irradiated region 16 and a second focal point at a so-called intermediate region 20, wherein EUV light can be output from the EUV light source 10 and input to devices utilizing EUV light, such as integrated circuit lithography tools (not shown). Figure 1 As shown, the optical element 18 is formed with an aperture to allow the laser pulse generated by the laser source 12 to pass through and reach the irradiated area 16.

[0028] Optical element 18 should have a suitable surface for collecting EUV light and guiding it to intermediate region 20 for subsequent transmission to a device utilizing EUV light. For example, optical element 18 may have a gradient multilayer coating with alternating layers of molybdenum and silicon, and in some cases one or more high-temperature diffusion barrier layers, smoothing layers, capping layers, and / or etch stop layers.

[0029] Those skilled in the art will appreciate that optical elements other than the elongated ellipsoidal mirror can be used as optical element 18. For example, optical element 18 may alternatively be a parabola rotating about its principal axis, or it may be configured to deliver a beam of light with an annular cross-section to an intermediate position. In other embodiments, optical element 18 may utilize coatings and layers different from or other than those described herein. Those skilled in the art will be able to select appropriate shapes and compositions for optical element 18 in specific situations.

[0030] like Figure 1 As shown, the EUV light source 10 may include a focusing unit 22, which includes one or more optical elements for focusing the laser beam onto a focal spot at the irradiated area. The EUV light source 10 may also include a beam conditioning unit 24 with one or more optical elements between the laser source 12 and the focusing unit 22, for expanding, manipulating, and / or shaping the laser beam and / or shaping the laser pulse. Various focusing units and beam conditioning units are known in the art and can be appropriately selected by those skilled in the art.

[0031] As mentioned above, in some cases, the LPP EUV system uses one or more seed lasers to generate laser pulses, which can then be amplified to become a laser beam that irradiates the target material at the irradiation site 16 to form a plasma that generates EUV emission. Figure 2 This is a simplified schematic diagram of one embodiment of a seed laser module 30 that can be used as part of a laser source in an LPP EUV system.

[0032] like Figure 2 As illustrated, the seed laser module 30 includes two seed lasers: a pre-pulse seed laser 32 and a master-pulse seed laser 34. Those skilled in the art will appreciate that, in using such an embodiment comprising two seed lasers, the target material can be irradiated first with one or more pulses from the pre-pulse seed laser 32, followed by one or more pulses from the master-pulse seed laser 34.

[0033] The seed laser module 30 is shown with a “folded” arrangement rather than arranging the components in a straight line. In practice, such an arrangement is typical to limit the size of the module. To achieve this, the beam generated by the laser pulses from the pre-pulse seed laser 32 and the main pulse seed laser 32 is guided into the desired optical path by a plurality of optical components 36. Depending on the specific configuration desired, the optical components 36 may be elements such as lenses, filters, prisms, mirrors, or any other element that can be used to guide the beam in the desired direction. In some cases, the optical components 36 may also perform other functions, such as changing the polarization of the transmitted beam.

[0034] exist Figure 2 In this embodiment, the beams from the various sub-lasers are first passed through an electro-optic modulator 38 (EOM). The EOM 38, together with the seed laser, serves as a pulse shaping unit to trim the pulses generated by the seed laser into pulses with shorter durations and faster fall times. The shorter pulse duration and relatively fast fall time can increase EUV output and source efficiency because of the short interaction time between the pulse and the target, and because unwanted portions of the pulse do not consume amplifier gain. Although two separate pulse shaping units (EOM 38) are shown, alternatively, a common pulse shaping unit can be used to trim both the pre-pulse and the main pulse seed.

[0035] The beam from the seed laser is then passed through an isolation stage comprising acousto-optic modulators (AOMs) 40 and 42 and a beam delay device 41. As will be explained below, AOMs 40 and 42 act as “switches” or “optical valves” that deflect any reflections of the laser beam from the target material to prevent them from reaching the seed laser; as mentioned above, the seed laser typically contains sensitive optics, and AOMs 40 and 42 thus prevent any reflections from causing damage to the seed laser components. The delay device 41 is as is known in the art; as can be seen more clearly in delay device 48, delay device 41 has a beam-folding optical arrangement including optical components such as mirrors, prisms, etc., such that the light passing through the unit travels an optical delay distance d. 延迟 Use approximately 3×10 8 The estimated beam velocity is given in meters per second, with each meter of beam delay adding an additional approximately 3.33 ns of travel time to the light along the optical path. More details are provided below (especially with...). Figure 3 Additional details regarding the delay device 41 and the isolation stage will be discussed in relation to the first isolation stage 33. In the embodiment shown here, the beams from the various sub-lasers pass through two AOMs. Furthermore, as will be discussed elsewhere herein, the isolation stage may be located elsewhere in the seed laser module 30.

[0036] After passing through AOM 42, the two beams are "combined" by beam combiner 44. Since the pulses from the various sub-lasers are generated at different times, this effectively means that the two time-separated beams are placed on a common optical path 46 for further processing and use.

[0037] After being placed in a common optical path, the beam from one of the seed lasers (again, only one at a time) passes through another beam delay device 48 with a beam-folding optical arrangement. Next, the beam is guided through at least one preamplifier 50 and then through a beam expander 52. Thereafter, the beam passes through a thin-film polarizer 54 and is then guided upwards by an optical element 56, which again guides the beam to the next stage in the LPP EUV system and may also perform other functions. From the optical element 56, the beam is typically passed to one or more optical amplifiers and other components, as will be explained below.

[0038] Various wavelength-tunable seed lasers suitable for use as both pre-pulse and main-pulse seed lasers are known in the art. For example, in one embodiment, the seed laser may be a CO2 laser with a hermetically sealed filling gas comprising CO2 at sub-atmospheric pressure (e.g., 0.05 to 0.2 atmospheres) and pumped by radio frequency discharge. In some embodiments, a grating may be used to help define the optical cavity of the seed laser, and the grating may be rotated to tune the seed laser to a selected rotation line.

[0039] Figure 3 This is a simplified block diagram of one embodiment of the seed pulse generation system 60. Similar to the seed laser module 30, the seed pulse generation system 60 generates, shapes, and amplifies seed pulses. However, the seed pulse generation system 60 includes two preamplifiers 74 and 84 instead of... Figure 2 A preamplifier 50 is provided in the seed laser module 30. The addition of a second preamplifier and the additional gain provided by the second preamplifier can cause a higher probability that the power amplifier, located outside the seed pulse generation system 60, will emit its own laser, thereby inducing modulation of the forward laser pulse and causing gain loss in the preamplifiers 74 and 84 in the seed laser generation system 60. The spontaneous laser generated in the power amplifier has been observed to be a pulse with a wide duration of several microseconds. In order to attenuate these effects of adding a second preamplifier, Figure 3 The seed pulse generation system 60 includes a seed pulse generator positioned in Figure 2 An additional isolation stage is added between the components of the seed laser module 30 to prevent reflected light from reaching the seed laser and the second preamplifier. The isolation stage of the seed pulse generation system 60 can be added to... Figure 2The seed laser module 30, or implemented therein, as will be obvious to those skilled in the art.

[0040] exist Figure 3 In the middle, although the seed laser 62 is depicted as a single unit, it is like... Figure 2 The pre-pulse seed laser 32 and the main pulse seed laser 34 are described in relation to generate a beam. Again, as those skilled in the art will understand, the seed pulse generation system 60 may include more than one seed laser 62. EOM 64 is as described above. Figure 2 The EOM38 describes the shaping pulse as described above.

[0041] A first isolation stage 66 is positioned between an EOM 64 and a first preamplifier 74. The first isolation stage 66 includes a first AOM 68, a delay device 70, and a second AOM 72; the delay device 70 again has a beam-folding optical arrangement. (Image...) Figure 2 The first isolation stage 66 operates in the same manner as AOM40 and 42 and delay line 41 to redirect any reflections of the laser pulse from the target material so as not to reach the seed laser 62. As further detailed herein, isolation stage 66 provides improved isolation from the amplified pulse that has passed through the first preamplifier 74.

[0042] To amplify the seed pulse generated by the seed laser 62, the seed pulse is passed through two or more preamplifiers, instead of... Figure 2 The diagram shows only one preamplifier. Using more than one preamplifier allows for graded amplification of the seed pulse, which offers several advantages. The use of separate amplifiers with smaller individual gains prevents spontaneous lasing of the optics. Another benefit derived from the use of an isolation stage with multiple preamplifiers is that the reflected light can be amplified and redirected in the middle before the gain is so high that even 1% of the reflected light remains strong enough to damage the seed laser 62, after 99% of the reflected light has been redirected.

[0043] The first preamplifier 74 is followed by a second isolation stage 76, which includes a first AOM 78, a delay device 80, and a second AOM 82. The second isolation stage 76 is capable of redirecting reflected light originating from parts of the LPP EUV system other than the first isolation stage. Since the second preamplifier 84 follows the second isolation stage 76 for the pulse traveling to the irradiated area, all reflected light reaching the second isolation stage 76 will also be amplified by the second preamplifier 84.

[0044] Although not depicted, a further isolation stage may follow the second preamplifier 84 before the beam is directed to further components of the LPP EUV generating system. Such a further isolation stage may deflect reflected light arriving from further components in the LPP EUV system before the reflected light is amplified by the second preamplifier 84.

[0045] Figures 4A to 4E Such as Figure 2 Seed pulse generation system 30 and Figure 3 A simplified block diagram of one embodiment of the AOM 90, such as those depicted in 60. The AOM 90 may be a Bragg AOM, as is familiar to those skilled in the art, and is depicted at five points in time during its operation. As described above relative to... Figure 2 As described in AOMs 40 and 42, AOM 90 acts as a “switch” or “light valve” to deflect or redirect light depending on its current state. AOM 90 uses an acousto-optic effect, in which acoustic (sound) waves within the material cause changes in the material’s optical properties, causing the frequency of light passing through AOM 90 to diffract and deflect.

[0046] As is known in the art, an AOM 90 is typically activated by a piezoelectric transducer (PZT) attached to one end of the AOM. Power (typically radio frequency (RF) power) is applied to the PZT as an oscillating electrical signal, which causes the PZT to vibrate and create an acoustic wave 92 in the AOM. When no power is applied, there is therefore no acoustic wave 92, and light is transmitted directly through the AOM; when power is applied, an acoustic wave is present and the AOM operates in a “deflection mode” in which the incident beam is deflected onto the beam path and offset in frequency. In deflection mode, the amplitude of the RF power applied to the PZT is sufficient to deflect the light onto the beam path. As will be apparent to those skilled in the art, the amplitude only needs to guide the light to a degree sufficient for practical deflection. Power is typically applied to the PZT in the direction of the processor or controller due to the desired switching speed.

[0047] like Figures 4A to 4E As depicted, acoustic wave 92 travels across AOM 90. Acoustic wave 92 has a known length based on the time period T during which power is applied to PZT and its velocity V. AOM 90 is positioned in the optical path to intercept the pulse at beam aperture 94. Beam aperture 94 is depicted in the figure as a circle with a diameter “d”, but is not necessarily a physical feature of AOM 90. The amount by which acoustic wave 92 overlaps with beam aperture 94 to allow pulse delivery for the time T (called the minimum acoustic packet size) can be calculated from the beam diameter and pulse duration using the following equation:

[0048] T = D / V + dT

[0049] Where D is the beam diameter, V is the velocity of the sound wave as it propagates through AOM 90 (a constant for AOM), and dT is the optical pulse duration (also a constant for AOM). When the beam diameter is 4 mm, the acoustic packet velocity is 5500 m / s, and the optical pulse duration is 200 nanoseconds, the minimum acoustic packet size produced is 927 nanoseconds.

[0050] Once such Figure 4A As shown in the diagram, sound wave 90 propagates across AOM 90 in one direction. When sound wave 90 overlaps with the beam aperture 94 of AOM 90 (as shown in the diagram), Figure 4C (As shown), the beam is deflected into the optical path to continue to other components. When the acoustic wave 92 does not overlap with the beam aperture 94, light from any direction in the seed generation system 60 is transmitted so as not to follow the optical path. Thus, when there is no acoustic wave at the beam aperture 94, the reflected light is unlikely to reach the seed laser 32, as... Figure 4A and Figure 4E As shown.

[0051] When the sound wave is 92 Figure 4B and Figure 4D When the portion of the beam overlaps with the beam aperture 94, a portion of the light striking the portion with acoustic wave 92 is deflected onto the optical path, while the remainder passes through AOM 90. Therefore, a portion of the reflected light traveling from the chamber toward the seed pulse generator can pass through the portion where acoustic wave 92 overlaps with the beam aperture 94 and is guided onto the optical path. The remainder of the reflected light is prevented from following the optical path in the absence of acoustic wave. In some cases, the deflected portion of the beam exhibits a phenomenon known as "beam imaging," in which the deflected portion retains the shape of a portion of the beam when deflected. Beam imaging is observed as a deflection of the beam from the center of beam aperture 94 and can have a non-circular, oval, or semi-circular shape.

[0052] Figure 5A and Figure 5B This is a simplified block diagram of one embodiment of isolation levels such as isolation levels 66 and 76. Figure 5A In the diagram, the isolation level is shown as consisting of AOM 106 and 112 and delay device 110. Figure 5A and Figure 5B Together, the relative states of the AOM when the seed pulse and reflected light pass through the isolation stage are depicted. As described above, when the acoustic wave 92 overlaps with the beam aperture 94, the light is deflected onto the optical path depicted as optical path 104. When the acoustic wave 92 does not overlap with the beam aperture 94, the light is guided away from optical path 104. As is known in the art, light passes through the AOM when the acoustic wave 92 is absent; however, for simplicity, Figure 5 depicts optical path 104 as a straight line.

[0053] like Figure 5A As seen in the diagram, during operation, the pulse 102 generated by the seed laser 62 reaches the first AOM 106 when the acoustic wave 92, which propagates along direction 108 across AOM 106, reaches the beam aperture 94. The pulse 102 is then transmitted along the optical path 104 to the delay device 110. As the pulse 102 passes through AOM 106, the second AOM 112, which is positioned immediately following the delay device 110, is in a state that prevents reflected light originating from outside the isolation stage from entering the delay device 110 and proceeding back to the seed laser 62.

[0054] As pulse 102 travels through delay device 110, acoustic wave 92 continues to propagate in the first aperture 106 and the second aperture 112. In the second aperture 112, acoustic wave 92 is generated after it is generated in the first aperture 106, such that it is delayed by a predetermined amount of time. The delay between the generation of the acoustic wave and the amount of delay introduced into the optical path by delay device 110 are coordinated such that when pulse 102 arrives at the second aperture 112, acoustic wave 92 is at beam aperture 94 and deflected so that it can continue further along optical path 104.

[0055] While the second AOM 112 deflects the pulse 102 onto the optical path 104, the first AOM 106 is in the opposite state, preventing light from following the optical path 104. Therefore, as Figure 5B As seen in the diagram, if any reflected light 114 passes through the second AOM 112 while the second AOM 112 guides part or all of the forward pulse onto the optical path 104, the reflected light 114 continues to pass through the delay device 110 while the acoustic wave 92 in the first AOM 106 propagates out of the beam aperture 94. After the acoustic wave 92 exits from the beam aperture 94 on the first AOM 106, the reflected light 114 is prevented from continuing to travel back to the seed laser on the optical path 104.

[0056] Figure 6 Timing diagram 600 depicts how reflected light is redirected via isolation levels (e.g., isolation levels 66 and 76). Timing diagram 600 depicts one embodiment of a timing pattern that can be used. Based on the description provided below, those skilled in the art will be able to generate and implement alternative timing patterns to prevent reflected light from reaching the seed module.

[0057] As depicted in Figures 130 and 140, RF power is supplied to the first AOM 106 and remains on for a period equal to the sum of the time required for the acoustic beam aperture 94 (denoted as TRISE) and the optical pulse duration (denoted as TP). After a time delay (denoted as TDELAY), RF power is supplied to the second AOM 112 as described in relation to the first AOM 106 in Figures 150 and 160.

[0058] The time delay, labeled “TP”, is introduced by the delay device 110. The delay device 110 can, for example, provide a delay of at least 300 nanoseconds. The timing of the AOM and the amount of delay introduced by the delay line vary depending on the beam diameter, the direction of acoustic wave propagation within the AOM, and the presence of beam imaging. The delay can be calculated in a variety of ways for different implementations. The following example implementation is provided as guidance on how the necessary amount of delay can be determined.

[0059] The diameter of the beam affects the amount of time required for the acoustic wave to block the beam aperture 94 by TRISE. For a beam with a diameter defined as 1 / e... 2 For a Gaussian beam of a certain size, TRISE can be approximated as the time to traverse its width. As will be apparent to those skilled in the art, for a 2.7 mm beam, TRISE is 610 nanoseconds, and for a 6.5 mm beam, TRISE is 1470 nanoseconds.

[0060] When the sound waves within the AOM propagate in the same direction, as discussed in relation to Figure 5, the minimum delay that should be provided by the delay device between the AOMs positioned in the isolation stage can be calculated as follows:

[0061] TDELAY>TRISE+TP / 2

[0062] Where TDELAY is the delay provided by delay device 110, TRISE is the time required for acoustic blocking of the beam aperture in the AOM, and TP is the duration of the optical pulse. The delay is at least a calculated time that allows the AOM to open at different times, and is the time difference between when the respective gates are open long enough to ensure that the two combined AOMs are completely or substantially closed when the reflected light reaches the isolation stage. As will be apparent to those skilled in the art based on this disclosure, the upper limit of the time delay is defined by the properties of delay device 110, including but not limited to the length, volume, and losses of delay device 110.

[0063] In instances where the corresponding sound waves in the AOM propagate in opposite directions, the AOM is referred to as cross-started. Cross-starting of the AOM is accomplished by starting sound waves at a first end in the first AOM and at the opposite end in the second AOM. Because the sound waves travel in opposite directions when the AOM is cross-started, the minimum delay provided by the delay device between the AOMs positioned in the isolation stage can be calculated as follows:

[0064] TDELAY>(TRISE+TP) / 2

[0065] In some instances, such as those depicted in Figure 170, beam imaging can be observed. As explained above, beam imaging can occur when the acoustic wave partially overlaps with the beam aperture on the AOM. Figure 6 As depicted, the beam imaging phenomenon can also be used to reduce the amount of delay introduced by the delay device, such that the first portion of the reflected light is redirected at the second AOM 112 and the remaining portion of the light is redirected by the first AOM 106. Since the AOM only needs to be partially closed to redirect a portion of the reflected light, the delay introduced by the delay device 110 can be shortened according to the same equation described above for the AOM used for cross-starting.

[0066] Figure 7 This is a flowchart of one embodiment of a method 200 for redirecting reflected light using an isolation level. The operation of method 200 can be performed during overlapping time points as described herein.

[0067] In operation 202, the laser pulse is optionally passed through a first gain element. The first gain element may be, for example, Figure 3 Preamplifiers such as the 74 preamplifier.

[0068] Next, in operation 204, a first AOM (such as first AOM 106 in Figure 5) is transformed to deliver laser pulses onto an optical path (e.g., optical path 104 in Figure 5). As discussed above, the first AOM is transformed by creating an acoustic wave that propagates across the AOM to overlap with the beam aperture (e.g., beam aperture 94 in Figure 5).

[0069] Next, in operation 206, the laser pulse is passed through a delay device (e.g., delay device 110 of FIG5). The delay device increases the travel time between the first AOM and the second AOM in the isolation stage by an amount.

[0070] Next, in operation 208, the second AOM (e.g., the second AOM 112 in Figure 5) is switched to deliver the laser pulse to an optional second gain element (e.g., optical path 104) on the optical path. Figure 3(Preamplifier 84). The second AOM is similarly transformed when the sound wave propagates through the beam aperture in the AOM.

[0071] Next, in operation 210, the first AOM is shifted to redirect the reflected light that has passed through the second AOM and the delay device. The first AOM is shifted as the sound wave propagates through the beam aperture in the AOM. In practice, operation 210 preferably follows operation 204 and overlaps with operations 206 and 208.

[0072] Next, in operation 212, a second AOM transition is performed to redirect reflected light from further components in the LPP EUV system. In this operation, operation 212 preferably follows operation 208 and overlaps with operation 210.

[0073] The isolation stage described in this paper allows the pulse to travel along the optical path within the seed pulse generation system while preventing reflected light traveling in the opposite direction along the optical path from reaching sensitive and fragile components upstream of the isolation stage. The isolation stage introduces a delay between the two AOMs within the system. This delay can be reduced by cross-starting the AOMs or when beam imaging is observed.

[0074] The disclosed methods and apparatus have been described above with reference to several embodiments. In view of this disclosure, other embodiments will be apparent to those skilled in the art. Certain aspects of the described methods and apparatus may be readily implemented using configurations different from those described in the foregoing embodiments or in combination with elements different from those described above. For example, different algorithms and / or logic circuits may be used, perhaps more complex than those described herein, and may be different types of driving lasers and / or focusing lenses.

[0075] Note that, as used herein, the term "optical component" and its derivatives include, but are not limited to, one or more components that reflect and / or transmit and / or manipulate incident light, and include, but are not limited to, one or more lenses, windows, filters, wedges, prisms, prisms, gradations, transmission fibers, etalons, diffusers, homogenizers, detectors and other instrument components, apertures, axial prisms, and mirrors including multilayer mirrors, near-normal incident mirrors, grazing incident mirrors, specular reflectors, diffuse reflectors, and combinations thereof. Furthermore, unless otherwise specified, the terms "optical device," "optical component," and their derivatives as used herein are not intended to limit to components that operate individually or advantageously within one or more specific wavelength ranges, such as at EUV output light wavelengths, irradiated laser wavelengths, suitable wavelengths for measurement, or some other wavelength.

[0076] As this article points out, various variations are possible. In some cases, a single seed laser can be used instead of... Figure 2The diagram shows two seed lasers. A common isolation level can protect both seed lasers, or either or both seed lasers can have their own isolation level for protection. The isolation level can be located elsewhere in the seed generation system 60, such as after the preamplifier 84. In some cases, a single Bragg AOM can be used, or more than two Bragg AOMs can be used to protect a single seed laser if desired. Other types of AOMs can also be used.

[0077] It should also be understood that the described methods and apparatus can be implemented in various ways, including as processes, devices, or systems. The methods described herein can be implemented by program instructions for instructing a processor to perform such methods, and such instructions can be recorded on a computer-readable storage medium such as a hard disk, floppy disk, optical disc such as a CD or DVD, flash memory, etc., or transmitted via a computer network, whereby the program instructions are transmitted through an optical or electronic communication link. Such program instructions can be executed by means of a processor or controller, or can be incorporated into fixed logic elements. It should be noted that the order of the steps of the methods described herein can be changed and remains within the scope of this disclosure.

[0078] These and other variations regarding the embodiments are intended to be covered by this disclosure, which is limited only by the appended claims.

Claims

1. An optical system, the system comprising: The first gain element is positioned along the optical path of the laser beam; The second gain element is positioned after the first gain element along the optical path; as well as An isolation stage, positioned along the optical path between the first gain element and the second gain element, is configured to redirect light that has been reflected back along the optical path by the second gain element. The isolation stage includes: A first acousto-optic modulator is configured to transition between a first state in which light is guided along the optical path and a second state in which light is not guided along the optical path during a first time period, the first time period being based on the diameter of the laser beam. A second acousto-optic modulator is configured to transition between a first state in which light is guided along the optical path and a second state in which light is not guided along the optical path during a second time period, the second time period being based on the diameter of the laser beam, and the transition of the second acousto-optic modulator occurring at a time after the transition of the first acousto-optic modulator; and A delay device, positioned between the first and second acousto-optic modulators, includes a beam-folding optical arrangement comprising optical components configured to delay the transmission of light between the first and second acousto-optic modulators by a time determined based on a first and second transition times of the acousto-optic modulators, such that any light reflected back along the optical path after passing through the second acousto-optic modulator will not pass through the first acousto-optic modulator. The delay mentioned above is further based on the occurrence of beam imaging. If beam imaging occurs, the delay is further determined such that a first portion of the laser beam is redirected by the second acousto-optic modulator, and the remaining portion of the laser beam is redirected by the first acousto-optic modulator.

2. The system of claim 1, further comprising one or more other elements located outside the second gain element.

3. The system of claim 2, wherein one or more of the other elements comprise an extreme ultraviolet (EUV) plasma chamber.

4. The system of claim 2, wherein one or more of the other components include a power amplifier.

5. The system of claim 1, wherein the first gain element and the second gain element comprise a preamplifier.

6. The system of claim 1, further comprising a second isolation stage positioned outside the second gain element along the optical path.

7. The system of claim 1, further comprising a second isolation stage positioned along the optical path between the first gain element and the module generating the laser beam.

8. The system of claim 1, wherein the isolation stage is further configured to prevent spontaneous lasing in the first gain element by redirecting the reflected light.

9. The system of claim 1, wherein the first acousto-optic modulator and the second acousto-optic modulator are cross-activated.

10. A method for redirecting reflected light, the method comprising: The laser pulse generated from the laser beam is passed through a first gain element positioned along the optical path; The laser pulse is passed through an isolation stage positioned along the optical path after the first gain element, the isolation stage being configured to deflect light reflected back along the optical path from any element located outside the isolation stage, the isolation stage comprising: A first acousto-optic modulator is configured to transition between a first state in which light is guided along the optical path and a second state in which light is not guided along the optical path during a first time period, the first time period being based on the diameter of the laser beam. A second acousto-optic modulator is configured to transition between a first state in which light is guided along the optical path and a second state in which light is not guided along the optical path during a second time period, the second time period being based on the diameter of the laser beam, and the transition of the second acousto-optic modulator occurring at a time after the transition of the first acousto-optic modulator; and A delay device, positioned between the first and second acousto-optic modulators, includes a beam-folding optical arrangement comprising optical components configured to delay the transmission of light between the first and second acousto-optic modulators by a time determined based on a first and second transition times of the acousto-optic modulators, such that any light reflected back along the optical path after passing through the second acousto-optic modulator will not pass through the first acousto-optic modulator; and The laser pulse is passed through a second gain element positioned after the isolation stage along the optical path. The delay mentioned above is further based on the occurrence of beam imaging. If beam imaging occurs, the delay is further determined such that a first portion of the laser beam is redirected by the second acousto-optic modulator, and the remaining portion of the laser beam is redirected by the first acousto-optic modulator.

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