Optical pulse generation for extreme ultraviolet light source

Abstract

To modify characteristics of a part of an optical pulse in generating an optical pulse of an extreme ultraviolet light source.SOLUTION: An optical pulse for an extreme ultraviolet (EUV) light source may be formed by illuminating a semiconductor material of a modulation system with a first light beam having a first wavelength; applying a voltage to the semiconductor material for a time duration, the applied voltage being sufficient to modify an index of refraction of the semiconductor material such that a polarization state of a light beam having a second wavelength passing through the semiconductor material is modified to pass through at least one polarization-based optical element of the modulation system; and forming an optical pulse by passing a second light beam having the second wavelength through the semiconductor material during the time duration.SELECTED DRAWING: Figure 6

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Application No. 15 / 469,267, filed March 24, 2017, which is incorporated herein by reference in its entirety.

[0002]

[0002] This disclosure relates to generating optical pulses for extreme ultraviolet light sources. [Background technology]

[0003]

[0003] In photolithography processes, electromagnetic radiation including extreme ultraviolet (EUV) light, for example light having a wavelength of about 13 nm or less (sometimes referred to as soft x-ray), can be used to create extremely small features in a substrate, for example a silicon wafer.

[0004]

[0004] Methods of generating EUV light include, but are not necessarily limited to, converting materials with elements having emission lines in the EUV region, such as xenon, lithium, or tin, into a plasma state. In one such method, often referred to as laser-produced plasma ("LPP"), the required plasma can be generated by irradiating a target material, for example in the form of a droplet, plate, tape, stream, or cluster of material, with an amplified light beam, which may be referred to as a drive laser. For this process, the plasma is typically generated in a closed vessel, such as a vacuum chamber, and monitored using various types of metrology equipment. Summary of the Invention

[0005] In one general aspect, a method of forming a light pulse of an extreme ultraviolet (EUV) light source includes illuminating a semiconductor material of a modulation system with a first light beam having a first wavelength, applying a voltage to the semiconductor material for a duration sufficient to modify the refractive index of the semiconductor material such that a polarization state of a light beam having a second wavelength passing through the semiconductor material is modified to pass through at least one polarization-based optical element of the modulation system, and forming a light pulse by passing a second light beam having the second wavelength through the semiconductor material for the duration. The light pulse formed includes a first portion and a second portion, the first portion and the second portion being consecutive in time, the first portion occurring before the second portion, and one or more characteristics of the first portion of the light pulse formed are modified by illuminating the semiconductor material of the modulation system with the first light beam.

[0006]

[0006] Implementations may include one or more of the following features: The one or more characteristics of the first portion of the light pulse may include one or more of an average intensity, a maximum intensity, and a duration. The pulse formed may be capable of propagating toward a target region, and the first portion of the pulse formed may have a maximum intensity that is less than a maximum intensity of a second portion of the pulse formed. The maximum intensity of the second portion may be sufficient to convert a target material of a target in the target region into a plasma that emits EUV light.

[0007]

[0007] The second wavelength may be at least three times as large as the first wavelength.

[0008]

[0008] The semiconductor material may be associated with a spectral transmission characteristic, the spectral transmission characteristic including a transmission region and an absorption edge wavelength, the absorption edge wavelength being the lowest wavelength of the transmission region, the first wavelength being between the absorption edge wavelength and a second wavelength, the second wavelength being a wavelength within the transmission region, the second wavelength being no more than 3.5 times as large as the first wavelength.

[0009]

[0009] The semiconductor material may be associated with a band gap energy, the band gap energy being the energy difference between the valence band of the semiconductor material and the conduction band of the semiconductor material, and the photon energy of the first wavelength may be less than the band gap energy. The semiconductor material may include defects, which create deep level traps having energy levels between the valence band and the conduction band, and the photon energy of the first wavelength may be equal to or greater than the energy difference between at least one energy level of the deep level traps and the conduction band or between at least one energy level of the deep level traps and the valence band.

[0010]

[0010] The second wavelength may include 10.6 μm and the semiconductor material may be one of cadmium zinc telluride (CdZnTe), cadmium telluride (CdTe), zinc telluride (ZnTe), and gallium arsenide (GaAs). The first wavelength may be a wavelength between 0.75 microns (μm) and 3.5 μm and the second wavelength may include a wavelength between 9 μm and 11 μm.

[0011]

[0011] The first light beam and the second light beam may follow the same spatial path through the semiconductor material. The first light beam and the second light beam may be in the semiconductor material at the same time.

[0012]

[0012] The characteristics of the first light beam may be adjusted to adjust one or more characteristics of the first portion of the light pulse. Adjusting the characteristics of the first light beam may include increasing the intensity of the first light beam to reduce a maximum or average intensity of the first portion of the light pulse.

[0013]

[0013] In another general aspect, an extreme ultraviolet (EUV) light source system includes a modulation system including a semiconductor material having one or more types of defects, the semiconductor material having a refractive index that changes in response to an application of a voltage; a first light source configured to generate a first light beam having a first wavelength, where illuminating the semiconductor material with light having a first wavelength increases a leakage current of the semiconductor material; and a control system coupled to the modulation system, and a control system configured to cause a voltage to be applied to the semiconductor material while a second light beam having a second wavelength propagates through the semiconductor material to form light pulses from the second light beam, the light pulses being configured to convert at least some of the target material into a plasma that emitting EUV light.

[0014]

[0016] Implementations may include one or more of the following features: The pulse may include a first portion and a second portion, the first portion and the second portion being consecutive in time, the first portion occurring before the second portion.

[0015]

[0017] The modulation system may also include at least one polarization-based optical element. The semiconductor material may be quartz. The second light beam may be a continuous light beam. The defect types may include one or more of precipitates, inclusions, twins, and slip planes.

[0016]

[0018] The second light source may include a pulsed light source, and the control system may be coupled to the modulation system and the second light source, and the control system may be configured to control the second light source to emit pulses of light. The control system may also be configured to control the second light source to direct pulses of the second light beam toward the semiconductor material while a voltage is applied to the semiconductor material and while the first light beam is directed toward the semiconductor material. At least one pulse of the first light beam and the second light beam may be in the semiconductor material simultaneously.

[0017]

[0019] The control system may also be configured to control the first light source, thereby controlling one or more characteristics of the first portion of the light pulse. The one or more characteristics of the first portion may include one or more of an average intensity, a maximum intensity, and a duration. The control system may be configured to control the intensity of the first portion of the pulse by controlling the first light source.

[0018]

[0020] The EUV light source system may also include an optical beam delivery system between the first light source and the semiconductor material, the optical beam delivery system configured to direct the first light beam to a specific location on the semiconductor material.

[0019]

[0021] In another general aspect, a method of modifying an acoustic effect in an electro-optic modulator includes applying a voltage to a semiconductor of the electro-optic modulator for a first time period, the application of the voltage generating an acoustic effect in the semiconductor comprising an oscillatory acoustic wave, and illuminating the semiconductor with a seed light beam having a wavelength having a photon energy less than a band gap energy of the semiconductor, thereby modifying one or more of an amplitude and a frequency of the acoustic wave.

[0020]

[0022] Implementations may include one or more of the following features: A continuous wave light beam may be directed toward a semiconductor of an electro-optic modulator, where a first quantity of the continuous wave light beam passes through the electro-optic modulator in a first polarization state for a first period of time when a voltage is applied to the semiconductor; A second quantity of the continuous wave light beam passes through the electro-optic modulator at a time other than the first time period when no voltage is applied to the semiconductor and an acoustic effect is present in the semiconductor, and illuminating the semiconductor with a seed light beam varies the second quantity of the light beam passing through the electro-optic modulator.

[0021]

[0024] Pulses of the pulsed light beam may be directed towards an electro-optic modulator, where a first amount of the pulsed light passes through the semiconductor for a first time period when a voltage is applied to the semiconductor and a second amount of the pulsed light passes through the semiconductor at times other than the first time period when no voltage is applied to the semiconductor, where the second amount of light is altered by illuminating the semiconductor with a seed light beam.

[0022]

[0025] Implementations of any of the above-mentioned technologies may include an EUV light source, a system, a method, a process, a device, or an apparatus. Details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims. [Brief description of the drawings]

[0023] [Figure 1] FIG. 1 is a block diagram of an example of an extreme ultraviolet (EUV) lithography system including an EUV light source. [Figure 2A] FIG. 2 is a block diagram of a modulation module used in the EUV light source of FIG. [Figure 2B]

[0028] FIG. [Figure 3A] FIG. 2B is a plot of the light leakage of the modulation module of FIG. 2A and two pulses formed by the modulator of FIG. 2A. [Figure 3B] FIG. 3B is an example of a timing diagram of the applications to a modulation module to generate the pulses shown in FIG. 3A. [Figure 4A] FIG. 2B is an example of an energy diagram of the semiconductors of the modulation module of FIG. 2A. [Figure 4B]

[0032] FIG. 2B is an example of a spectral transmittance characteristic of a semiconductor of the modulation module of FIG. 2A. [Figure 5A] FIG. 2B is a plot of two optical pulses that may be generated by the modulation module of FIG. 2A. [Figure 5B] FIG. 5B is an example of a timing diagram of the applications to a modulation module to generate the pulses shown in FIG. 5A. [Figure 6] 1 is a flow chart of an example process for forming a light pulse. [Figure 7]

[0036] An example of experimental results. [Figure 8]

[0036] An example of experimental results. [Figure 9]

[0036] An example of experimental results. [Figure 10]

[0036] An example of experimental results. [Figure 11] FIG. 2 is a perspective view of a pulse generation system that may be used in the EUV light source of FIG. [Figure 12] FIG. 2 is a perspective view of a pulse generation system that may be used in the EUV light source of FIG. [Figure 13A] FIG. 1 is a block diagram of an example of an EUV light source. [Figure 13B] FIG. 1 is a block diagram of an example of an EUV light source. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024]

[0039] A technique for forming light pulses in an extreme ultraviolet (EUV) light source is described. An electro-optic modulator used to form the light pulses releases trapped charge within a semiconductor of the electro-optic modulator when illuminated with a seed light beam. The charge is trapped due to defects present in the semiconductor, and the trapped charge increases light leakage of the modulator. The light leakage contributes additional or spurious light to the pulse formed by the modulator. Some of this spurious light forms a "pedestal" at the beginning of the formed pulse. By releasing the trapped charge, the light leakage is reduced and / or controlled, and the pedestal may also be reduced and / or controlled.

[0025]

[0040] Referring to FIG. 1, a block diagram of a system 100 is shown. System 100 is an example of an EUV lithography system. System 100 includes an EUV light source 101, which provides EUV light 196 to a lithography tool 195. The lithography tool 195 exposes a wafer (e.g., a silicon wafer) to the EUV light 196 to form electronic features on the wafer. The EUV light 196 is emitted from a plasma formed by irradiating a target material of a target 118 with a light pulse 107. The target material is any material (e.g., tin) that emits EUV light in a plasma state.

[0026]

[0041] The EUV light source 101 includes a pulse generation system 104 that generates pulses 107. The pulse generation system 104 includes a light source 105, which may be, for example, a pulsed (e.g., Q-switched) laser or a continuous wave carbon dioxide (CO2) laser. The light source 105 generates a light beam 106, which may be a pulsed train of light or a continuous light beam. The light source 105 emits the light beam 106 towards a modulation module 120 that includes a semiconductor 122.

[0027]

[0042] The modulation module 120 is an electro-optic modulator that modulates the incident light beam based on the electro-optic effect. The electro-optic effect describes the change in the refractive index of a material (semiconductor 122) caused by the application of a direct current (DC) or low frequency electric field. The modulation module 120 is controlled by a control system 175 to form a pulse 107 from the light beam 106 or from a pulse of light in the light beam 106. The pulse 107 propagates on a path 111 toward a vacuum vessel 180 that contains a target 118. The pulse 107 and the target 118 interact at a target region 115 within the vacuum vessel 180, and the interaction converts at least some of the target material of the target 118 into a plasma that emits EUV light 196.

[0028]

[0043] The EUV light source 101 also includes a seed light source 110. The seed light source 110 emits a seed light beam 114 that illuminates the semiconductor 122. The semiconductor 122 includes defects that may trap charge. The seed light beam 114 has a wavelength associated with sufficient photon energy to excite the trapped charge and move the trapped charge into the conduction band of the semiconductor 122, increasing the conductivity of the semiconductor 122. As described in more detail below with respect to Figures 2-12, removing or reducing the trapped charge improves the ability of the modulation module 120 to form the pulse 107.

[0029]

[0044] The control system 175 exchanges data and / or information with the pulse generation system 104 and / or any of the components of the pulse generation system 104 via a communication interface 176. For example, in some implementations, the control system 175 may provide trigger signals to operate the modulation module 120 and / or the light source 105. The control system 175 includes an electronic processor 177, electronic storage 178, and an input / output (I / O) interface 179. The electronic processor 177 may comprise one or more processors suitable for executing computer programs, such as general purpose or special purpose microprocessors, as well as any one or more processors of any type of digital computer. Generally, the electronic processor receives instructions and data from a read-only memory, a random access memory, or both. The electronic processor 177 may be any type of electronic processor.

[0030]

[0045] Electronic storage 178 may be volatile memory, such as RAM, or may be non-volatile memory. In some implementations, electronic storage 178 includes non-volatile and volatile portions or components. Electronic storage 178 may store data and information used in the operation of control system 175 and / or components of control system 175.

[0031]

[0046] The electronic storage 178 may also store instructions, possibly as a computer program, that when executed cause the processor 177 to communicate with components in the control system 175, the modulation module 120, and / or the light source 105. For example, in an implementation in which the light source 105 is a pulsed light source, the instructions may be to cause the electronic processor 177 to generate a signal that causes the light source 105 to emit a pulse of light.

[0032]

[0047] I / O interface 179 is any type of electronic interface that allows control system 175 to receive and / or provide data and signals to an operator, modulation module 120, and / or light source 105, and / or an automated process running on another electronic device. For example, I / O interface 179 may include one or more of a visual display, a keyboard, and a communication interface.

[0033]

[0048] 2A is a block diagram of modulation module 120. Modulation module 120 includes semiconductor 122, which is positioned between electrodes 123a and 123b. Electrodes 123a and 123b are controllable to create an electric field between electrodes 123a and 123b. For example, control system 175 may cause electrode 123a to be held at a higher voltage than electrode 123b, thereby creating an electric field or potential difference (V) across semiconductor 122.

[0034]

[0049] The modulation module 120 also includes one or more polarization-based optical elements 124. In the example of FIG. 2A, only one polarization-based optical element 124 is shown. However, in other implementations, additional polarization-based optical elements 124 may be included. For example, a second polarization-based optical element 124 may be on the side of the modulation module 120 that receives the light beam 106.

[0035]

[0050] The polarization-based optical element 124 is any optical element that interacts with light based on its polarization state. For example, the polarization-based optical element 124 may be a linear polarizer that transmits horizontally polarized light and blocks vertically polarized light, or vice versa. The polarization-based optical element 124 may be a polarizing beam splitter that transmits horizontally polarized light and reflects vertically polarized light. The polarization-based optical element 124 may be an optical element that absorbs all light except light having a particular polarization state. In some implementations, the polarization-based optical element 124 may include a quarter wave plate. At least one polarization-based optical element 124 is positioned to receive light passing through the semiconductor 122 and to direct light of a certain polarization state onto the beam path 111.

[0036]

[0051] The semiconductor 122 can be any material that is transparent to one or more wavelengths of the light beam 106. The semiconductor 122 also exhibits anisotropy that can be modified by application of a controllable external force (such as a potential difference (V)). Control of the anisotropy allows for control of the refractive index of various polarization components of light propagating through the semiconductor 122. For implementations in which the light beam 106 includes light at a wavelength of 10.6 microns (μm), the semiconductor 122 can be, for example, cadmium zinc telluride (CdZnTe or CZT), cadmium telluride (CdTe), zinc telluride (ZnTe), and / or gallium arsenide (GaAs). Other materials can be used at other wavelengths. For example, the semiconductor 122 may be potassium phosphate monobasic (KDP), ammonium dihydrogen phosphate (ADP), quartz, cuprous chloride (CuCl), zinc sulfide (ZnS), zinc selenide (ZnSe), lithium niobate (LiNbO3), gallium phosphide (GaP), lithium tantalate (LiTaO3), or barium titanate (BaTiO3). Other materials that are transparent to one or more wavelengths of the light beam 106 and that exhibit birefringence in response to the application of an external force may also be used.

[0037]

[0052] The polarization state of the light passing through the semiconductor 122 can be controlled by controlling the potential difference (V) between the electrodes 123a, 123b. Under ideal operation, the modulation module 120 emits light only when the potential difference V applied to the semiconductor 122 causes the polarization state of the light passing through the semiconductor 122 to match the polarization-based optical element 124. For example, if the polarization-based optical element 124 is a linear polarizer positioned to transmit horizontally polarized light onto the beam path 111, and the light beam 106 is vertically polarized upon initial incidence on the semiconductor 122, then the pulse 107 will be formed only when the potential difference V applied to the semiconductor 122 changes the polarization state of the light beam 106 such that it is horizontally polarized.

[0038]

[0053] However, in actual operation, modulation module 120 also emits unwanted light (light leakage). This light leakage is present throughout the operation of modulation module 120 and causes additional amounts of light to pass through modulation module 120, including times when light should not pass through modulation module 120. For example, this additional light provides an additional or extra amount of light at the beginning of pulse 107, and this additional light forms the pedestal portion.

[0039]

[0054] Referring also to Figure 2B, a diagram of a pulse 107 having a pedestal 125 is shown. Figure 2B shows the intensity of the pulse 107 as a function of time. The pedestal 125 occurs within a window labeled 121. The pedestal 125 also occurs earlier in time than the remainder of the pulse 107. The portion of the pulse 107 that is not the pedestal 125 is referred to as the main portion 168. Both the pedestal 125 and the main portion 168 are part of the pulse 107, with the pedestal 125 being connected in time to the main portion 168.

[0040]

[0055] The average and maximum intensity and light energy of the pedestal 125 is less than the average and maximum intensity and light energy of the main portion 168. The main portion 168 has enough intensity or energy to convert at least some of the target material of the target 118 into a plasma that emits EUV light. The pedestal 125 does not have as much energy and would not have enough energy to convert the target material into a plasma. However, the light of the pedestal 125 may reflect off the target 118 and evaporate material from the surface of the target 118 and / or remove part of the target 118.

[0041]

[0056] The pedestal 125 originates before the main portion 168 and reaches the target 118 before the main portion 168. The pedestal 125 may interfere with plasma formation by altering the target 118 before the main portion 168 reaches it and / or cause undesirable reflections that propagate back on the path 111. Therefore, it is desirable to control the amount of light in the pedestal 125. Illuminating the semiconductor 122 with the seed light beam 114 also allows one or more characteristics of the pedestal 125 to be altered, controlled, reduced, or eliminated. For example, the average amount of light and / or the maximum amount of light in the pedestal 125 may be reduced.

[0042]

[0057] FIG. 3A shows a plot of light leakage of an optical modulator as a function of time without the benefit of illumination by a seed light beam 114. The example shown in FIG. 3A is described with respect to a modulation module 120 and a semiconductor 122. Pulses 307_1 and 307_2 (FIG. 3A) are generated by applying a potential difference to the semiconductor 122 of the modulation module 120 (FIG. 3B). Light leakage is the percentage of light incident on the modulation module 120 that passes through the modulation module 120 even when no potential difference is applied to the semiconductor 122.

[0043]

[0058] In the example of Figure 3A, a continuous wave light beam is incident on semiconductor 122. Referring also to Figure 3B, a timing diagram 300B is shown. Timing diagram 300B is an example of a voltage applied to semiconductor 122 during the time period shown in Figure 3A. When a potential difference V is applied to semiconductor 122 and the polarization of the light exiting semiconductor 122 matches polarization-based optical element 124, a relatively high percentage of the incident light passes through modulation module 120. Pulses 307_1 and 307_2 are formed from the light passing through modulation module 120 while potential difference V is applied to semiconductor 122.

[0044]

[0059] Timing diagram 300B shows four periods 333-336. Period 333 includes time (t=0) to time (t=t2). During this period 333, potential difference V is not applied to semiconductor 122. Period 334 begins at time (t=t2) and ends at time (t=t3). During this period 334, potential difference V is applied to semiconductor 122, forming pulse 307_1. Period 335 begins at time (t=t3) and ends at time (t=t5). During this period 335, potential difference V is not applied to semiconductor 122. Potential difference V is again applied to semiconductor 122 during period 336 (starting at t=t5 and ending at t=t6), forming pulse 307_2.

[0045]

[0060] The duration of the periods 333-336 depends on the characteristics of the modulation module 120 and the operating parameters of the EUV light source 101. For example, the rise time of the modulation module 120 (the time for the semiconductor 122 to react to the applied potential difference) may determine the minimum duration of the periods 334 and 336. The desired duration of the pulses 307_1 and 307_2 may also determine the duration of the periods 334 and 336. In some implementations, the periods 334 and 336 may be, for example, 50-300 nanoseconds (ns), 50-1000 ns, or 50-100 ns. The period 335 may be, for example, 20,000 ns. The example of FIG. 3A shows two pulses 307_1 and 307_2. However, additional pulses may be formed by applying the potential difference V again at a later time. For example, pulses may be generated at 50 kHz by applying the potential difference V to the semiconductor 122 every 20,000 ns.

[0046]

[0061] In addition to pulses 307_1 and 307_2, at other times, unwanted light (e.g., optical leakage) exits modulation module 120 due to defects and other effects in semiconductor 122. Optical leakage has several components, all of which are types of optical leakage: static leakage 328, acoustic leakage 329, and dynamic leakage offset (DLO) 330. Static optical leakage 328 exists whether or not a potential difference V is applied. Acoustic leakage 329 and DLO 330 arise due to the application of a potential difference V. Acoustic leakage 329 and DLO 330 together may be considered as dynamic optical leakage.

[0047]

[0062] Static light leakage 328 is the percentage of incident light that passes through modulation module 120 regardless of whether or not a potential difference V is applied across semiconductor 122. In the example of Figure 3B, line 327 shows static light leakage 328 as a function of time.

[0048]

[0063] Dynamic light leakage also includes acoustic leakage 329. The application of the potential difference V affects the piezoelectric properties of the semiconductor 122 and generates acoustic waves that propagate through the semiconductor 122. The acoustic waves persist after the potential difference V is removed. The presence of the acoustic waves in the semiconductor 122 changes the refractive index of the semiconductor 122, which can result in unintended transmission of light through the modulation module 120 during a period 335. The portion or percentage of the incident light that is emitted as acoustic leakage 329 fluctuates over time between a minimum acoustic leakage 326 and a peak acoustic leakage 329p. The acoustic leakage 329 also decays over the period 335 (the amount of acoustic leakage decreases from the peak 329p with each subsequent dip that occurs during the period 335). The decay characteristics depend on the properties of the semiconductor, and the dip will not decay completely until the next application of the potential difference V. For example, the dip will not decay completely until about 100 μs after the application of the potential difference V, which period 335 may be, for example, about 20 μs. Thus, the perturbation may still be present in the semiconductor 122 when the potential difference V is applied again.

[0049]

[0064] Dynamic light leakage also includes DLO 330. DLO 330 is believed to be primarily caused by the presence of charges (e.g., electrons) trapped in defects in the semiconductor 122. The trapped charges create an electric field within the semiconductor 122 that prevents an applied potential difference V from being uniform across the entire semiconductor 122.

[0050]

[0065] Thus, during periods 334 and 336, a potential difference V is applied to the semiconductor to form each pulse 307_1 and 307_2, but due to the presence of light leakage, the pulses 307_1 and 307_2 contain light from the unwanted light source. For example, the pulses 307_1 and 307_2 each contain a pedestal portion 325a, 325b. The pedestal portion 325a, 325b occurs before the remaining portion of each pulse 307_1, 307_2. In the example of FIG. 3A, the pedestal portion 325a, 325b is shown as a percentage of the transmission of the average total light leakage within a 400 nanosecond (ns) window. The windows are labeled 321a and 321b. Window 321a is from time=t1 to time=t2, and window 321b is from time=t4 to time=t5. The pedestal portions 325 a and 325 b are generated when no potential difference V is applied to the semiconductor 122 .

[0051]

[0066] In the example of FIG. 3A, the DLO 330 is the largest component of the total light leakage, and most of the pedestal portions 325a, 325b can be eliminated by reducing or eliminating the DLO 330. For example, the DLO 330 can account for 33% to 66% of the total light leakage. In one example, the total light leakage is 0.3%, the static leakage 328 is 0.07%, the acoustic leakage 329 is 0.07%, and the DLO 330 accounts for 0.16%. However, other examples may have different parameters. For example, CZT (which may be used as the semiconductor 122) may have a DLO of 2%. In another example, the light leakage in a modulation module using CdTe as the semiconductor 122 may have a relatively small DLO component and a more dominant acoustic component. In these implementations, reducing the acoustic component of the light leakage has a greater impact than reducing or eliminating the pedestal portion. As described with respect to FIGS. 4A and 4B, the DLO 330 and acoustic leakage 329 are reduced by illuminating the semiconductor 122 with a seed light beam 114.

[0052]

[0067] Referring to FIG. 4A, a diagram of the electronic band structure 440 (energy versus carrier momentum) of the semiconductor 122 is shown. The electronic band structure 440 represents the range of energies that electrons can have in the semiconductor 122. The semiconductor 122 has a band gap energy (Eg) 441, which is the energy difference (in electron volts or eV) between the conduction band 442 and the valence band 443. An electron (e.g., electron 445) can move from the valence band 443 to the conduction band 442 when the electron gains enough energy to move into the conduction band 442. The electron may gain energy by absorbing phonons (heat) or photons (light). Electrons in the conduction band 442 flow as an electric current.

[0053]

[0068] The semiconductor 122 includes defects, such as precipitates, inclusions, twins, and / or slip planes. The defects act as sites where charge (e.g., electrons 446) can be trapped. In the example of FIG. 4A, deep level traps 444 are formed from the defects. The deep level traps 444 are associated with a trap gap energy 447, which is the energy difference between the deep level traps 444 and the conduction band 442. The trap gap energy 447 is less than the band gap energy 441.

[0054]

[0069] 4B, there is shown a spectral transmission characteristic 450 of semiconductor 122. Spectral transmission characteristic 450 represents the percentage transmission as a function of the wavelength of the incident light. In spectral transmission characteristic 450, the lowest wavelength (i.e., shortest wavelength) is at the far left of the horizontal axis, and the wavelengths increase (i.e., get longer) as the x-axis increases to the right.

[0055]

[0070] The spectral transmission characteristic 450 includes a transmissive region 452, an electro-absorptive region 451, and an absorption edge 453. The absorption edge 453 is the interface or boundary between the transmissive region 452 and the electro-absorptive region 451. The wavelength at the absorption edge 453 has a photon energy that corresponds to the band gap energy 441. A photon having a wavelength (λ) has a photon energy (E) given by Equation 1:

number

[0056]

[0073] The semiconductor 122 may be CdZnTe. In this example, the transmission region 452 is about 0.8 microns (μm) to 20 (μm) and the absorption edge is about 730 nanometers (nm). For light having a wavelength in the transmission region, about 60% of the incident light is transmitted. In another example, the semiconductor 122 is CdTe. In this example, the transmission region 452 is about 0.9 μm to 20 μm and the absorption edge is about 861 nanometers (nm). For light having a wavelength in the transmission region, about 65% of the incident light is transmitted. Light having a wavelength in the electroabsorption region 451 has photon energy greater than the band gap energy 441. Such photons are easily absorbed by the semiconductor 122.

[0057]

[0074] The light beam 106 used to form the pulse 107 has a wavelength in the transparent region 452 and is therefore transmitted through the semiconductor 122. The light beam 114 used to illuminate the semiconductor 122 may have a broad spectrum and may comprise many wavelengths (and therefore many different photon energies), including wavelengths with photon energies less than the band gap energy 441. Light with wavelengths greater than the absorption edge 453 (or with photon energies less than the band gap energy 441) may travel through the semiconductor 122 and excite charges trapped in deep level traps formed by defects, such as the electron 446 in trap 444 in FIG. 4A. The photons of the light beam 114 transfer energy to the trapped charges, and the transferred energy may be sufficient to move the trapped charges into the conduction band 442, where the charges flow as a current. For example, these photons may increase the thermal energy of the trapped charges so that the charges can move into the conduction band 442. Additionally or alternatively, such photons may provide energy to charges stored in the valence band 443, causing those charges to move into traps 444. Other photons from the beam 114 may further excite those charges, causing them to move from the traps 444 into the conduction band 442 and flow as electric current.

[0058]

[0075] Thus, illuminating the semiconductor 122 with the light beam 114 may release trapped charges (such as electrons 446 in FIG. 4A) and excite these charges to the conduction band 442. Illuminating the semiconductor 122 with the light beam 114 may also increase the amount of electrons in the valence band 443 that move to the conduction band 442. Once in the conduction band 442, the charges flow as a current, thereby increasing the conductivity and decreasing the resistivity of the semiconductor 122. The higher the conductivity, the lower the DLO 330. Furthermore, removing or reducing the trapped charges makes the electric field in the semiconductor 122 more uniform when a potential difference V is applied. Illuminating the semiconductor 122 with the seed light beam 114 may also reduce acoustic leakage 329 and modify the nature of the acoustic effect of the semiconductor 122. For example, one or more of the frequency and amplitude of the acoustic effect (or acoustic leakage) may be changed. Thus, illuminating the semiconductor 122 with the seed light beam 114 reduces light leakage and reduces the amount of light in the pedestal portions 325a, 325b.

[0059]

[0076] Figures 3A and 3B show examples where the light beam 106 is a continuous wave light beam, although in some implementations the light beam 106 is a pulsed light beam including a train of time separated pulses of light, synchronized to the potential V applied to the crystal. The modulation module 120 may be used to form a light pulse from any of the pulses of the pulsed light beam. Figures 5A and 5B relate to a light pulse 507 formed from the pulses of light.

[0060]

[0077] In implementations in which light beam 106 is a pulsed light beam, control system 175 (FIGS. 1 and 2A) may provide a trigger to light source 105 such that a pulse of light is emitted while the potential difference is applied across semiconductor 122. For example, light source 105 may be a Q-switched CO2 laser that is controlled to emit pulses by receiving a trigger from control system 175. In some implementations, light source 105 emits pulses periodically without receiving a trigger from control system 175.

[0061]

[0078] FIG 5A is a plot of light transmitted by optical modulator 120 as a function of time. The vertical axis of FIG 5A is a logarithmic scale. FIG 5B is a plot showing the voltage applied to semiconductor 122 during the time period shown in FIG 5A. Optical pulse 507 is formed by applying a potential difference V to semiconductor 122 while all or a portion of the pulse of optical beam 106 propagates through semiconductor 122. Applying a potential difference to semiconductor 122 while the pulse propagates through semiconductor 122 allows the polarization of the pulse to be changed such that polarization-based optical element 124 (FIG 2A) directs the light exiting semiconductor 122 onto beam path 111 (FIG 2A).

[0062]

[0079] In the example of Figures 5A and 5B, the potential difference V is applied to the semiconductor 122 for a period 537 beginning at time t = t8 and ending at time t = t9. The potential difference V is not applied at any other time shown in Figure 5B. When the potential difference V is applied at time t = t8, the refractive index of the semiconductor 122 changes instantaneously such that the polarization of the light passing through the semiconductor 122 matches the polarization-based optical element 124 and exits the light modulator 120. Between times t7 and t8, light from the pulse of the light beam 106 passes through the modulation module 120 due to optical leakage, forming a pedestal portion 525. The optical leakage may include acoustic leakage 329 caused by the DLO 330 and / or acoustic waves formed in the semiconductor during the previous application of the potential difference V. By illuminating the semiconductor 122 with the seed light beam 114, the maximum amount of light or the average amount of light of the pedestal 525 may be altered, reduced, or eliminated. In the example of FIG. 5A, window 521 shows a portion of pedestal 525 of pulse 507.

[0063]

[0080] Referring to Figure 6, a flow chart of a process 600 is shown. Process 600 is an example of a process for generating a light pulse, such as light pulse 107 (Figures 1 and 2B), for use in EUV light source 101 of Figure 1 or any other EUV light source. Process 600 may be used to control or modify one or more characteristics of a pedestal portion of a light pulse. For example, process 600 may be used to reduce the average intensity or maximum intensity of the pedestal portion of a generated light pulse. Process 600 will be described with respect to EUV light source 101 and modulation module 120 of Figure 1 (Figures 1 and 2A).

[0064]

[0081] The semiconductor 122 of the modulation module 120 is illuminated (610) with a first light beam having a first wavelength. The first light beam may be the seed light beam 114. The first wavelength is any wavelength that can release charge trapped in the semiconductor 122. Defects in the semiconductor 122 can act as sites where charge is trapped, such as deep level traps 444 (FIG. 4A). The trapped charge results in lower conductivity and higher resistivity compared to an ideal version of the semiconductor 122 without defects and trapped charge. The traps formed by the defects have a band gap energy smaller than the band gap energy 441 of the semiconductor 122. Thus, the trapped charge can be released from the defect traps by exciting the charge with light having a wavelength associated with a photon energy smaller than the band gap energy 441.

[0065]

[0082] For example, the semiconductor 122 may be CZT with a band gap energy corresponding to a wavelength of 730 nm, or CdTe with a band gap energy corresponding to a wavelength of 861 nm. In these examples, the seed light source 110 may be any light source that generates near infrared (NIR) light, including light with wavelengths having photon energy less than the band gap energy of the semiconductor 122. NIR light includes wavelengths between 750 nm and 3.5 microns (μm). For example, a diode laser with a peak wavelength of 905.7 nm and a full width at half maximum (FWHM) of 1 nm, or a light emitting diode with a peak wavelength of 873 nm and a FWHM of 70 nm may be used as the seed light source 110. In another example, the seed light beam 114 may include one or more wavelengths between 838 nm and 988 nm. The first wavelength may be associated with a photon energy greater than the trapping gap energy 447. As described above, the trapping gap energy 447 is less than the band gap energy 441.

[0066]

[0083] A voltage is applied to the semiconductor 122 for a duration (620). During this duration, a light pulse (such as light pulse 107) is formed (630) by passing a second light beam having a second wavelength through the semiconductor. The second light beam may be, for example, light beam 106 of FIG. 1. Light beam 106 may be a continuous wave or a beam including one or more pulses of light.

[0067]

[0084] The second light beam includes at least one wavelength having a photon energy far below the band gap 441 of the semiconductor 122. Thus, the second light beam is transmitted through the semiconductor and used to form the pulse 107. The light beam 106 includes one or more wavelengths different from the first wavelength. The light beam 106 may include a wavelength that is not more than 3.5 times the peak wavelength of the first light beam. In some implementations, the light beam 106 includes a wavelength that is not more than 10 times the peak wavelength of the seed light beam 114. Continuing with the above example, the semiconductor 122 may be CZT with a band gap of 730 nm or CdTe with a band gap of 861 nm. In these examples, the second light beam may be generated by a CO2 laser and may include one or more wavelengths between 8 μm and 15 μm or one or more wavelengths between 9 μm and 11 μm. The light beam 106 and the light beam 114 may propagate through the semiconductor 122 simultaneously. Also, the light beam 106 and the light beam 114 may follow the same path through the semiconductor 122. The effect of illuminating the semiconductor 122 with the seed light beam 114 may persist even after the seed light beam 114 no longer illuminates the semiconductor 122. For example, the electrical properties (such as leakage current and resistivity) may be more stable (have less variation over time) after illumination with the seed light beam 114.

[0068]

[0085] Illuminating the semiconductor 122 with the seed light beam 114 releases the trapped charge in the semiconductor 122, increasing the conductivity of the semiconductor 122 and reducing the light leakage of the modulation module 120. Reducing the light leakage also reduces the amount of light in the pedestal portion of the pulse 107. Figures 7 to 10 show experimental results obtained by illuminating the semiconductor 122 with the seed light beam 114.

[0069]

[0086] FIG. 7 includes current-voltage (IV) plots 701 and 702 showing the measured current (in microamps) versus the voltage (in kilovolts) applied to the semiconductor 122 without (IV plot 701) and with (IV plot 702) illumination by the seed light beam 114. In the example of FIG. 7, CZT was used as the semiconductor 122. The applied voltage was varied from 0V to 5000V (5kV) by a direct current (DC) power supply. A 2 megaohm (MΩ) sampling resistor was connected to the semiconductor 122, and the leakage current was measured with a high voltage probe across the 2MΩ resistor. The leakage current was first measured without illuminating the semiconductor 122. This measured current is included in plot 701. A NIR source was used as the seed light source 110. The leakage current was measured while the semiconductor 122 was illuminated with the seed light beam 114. This measured leakage current is included in plot 702.

[0070]

[0087] 7, illuminating the semiconductor 122 with the seed light beam 114 increased the leakage current for all applied voltages greater than zero. The increase in leakage current indicates that trapped charges were released upon illumination with the seed light beam 114. This increase in leakage current upon illumination with the seed laser beam can also be considered as an increase in the photoconductivity of the semiconductor.

[0071]

[0088] FIG. 8 includes a plot 800 of total light leakage (%) as a function of resistivity (in ohm-centimeters) of the semiconductor 122 (at 1 kV-DC). In the plot 800, the semiconductor 122 was CZT. A seed light beam 114 illuminated the semiconductor 122 while data was collected. The seed light beam 114 was a NIR beam, and the intensity of the beam was increased from zero to a maximum of 1 microwatt (μW) during data collection and coupled into the semiconductor. As shown, the total light leakage decreases as the resistivity of the semiconductor 122 decreases and the intensity of the seed light beam 114 increases. Since resistivity is inversely proportional to conductivity, the plot 800 shows that light leakage decreases with increasing conductivity and increasing intensity of the seed light beam 114. The increased conductivity corresponds to the release of trapped charge through excitation by the seed light beam 114. The plot 800 also shows that different light leakage can be obtained by varying the intensity of the seed light beam 114. Therefore, by varying the intensity of the seed light beam 114, in addition to reducing the amount of light leakage, the amount of light leakage (and thus the amount of light in the pedestal of the pulse 107) can be controlled or altered.

[0072]

[0089] FIG. 9 shows experimentally measured light leakage as a function of time. Plot 901 shows the light leakage when the semiconductor 122 is not illuminated with the seed light beam 114, and plot 902 shows the light leakage when the semiconductor 122 is illuminated with the seed light beam 114. In the example of FIG. 9, the light beam 106 was a continuous wave light beam generated by a CO2 laser. The seed light beam 114 was a continuous wave NIR light beam. Pulses 907_1 and 907_2 were formed using the modulation module 120.

[0073]

[0090] As shown by comparing plots 901 and 902 in FIG. 9, illuminating the semiconductor 122 with the seed light beam 114 reduces the light leakage. In this particular example, the maximum light leakage in the pedestal region 921 is reduced from about 1.1% (without the seed light beam 114) to about 0.05% (with the seed light beam). The light leakage (including the component of leakage due to acoustic leakage) occurring between pulses 907_1 and 907_2 is also reduced from a maximum of about 1.4% (without the seed light beam 114) to a maximum of about 0.3% (with the seed light beam 114). Considering only acoustic leakage, excluding the DLO, the difference between the maximum and minimum values ​​is about 0.6% without illumination and about 0.2% with illumination. Thus, in this example, illuminating the semiconductor 122 with the seed light beam reduced the acoustic leakage by about a factor of three. In other examples, further reductions in acoustic leakage (e.g., to a factor of six) may be achieved. Illuminating the semiconductor 122 with the seed light beam 114 does not eliminate or substantially alter the static leakage, shown as line 927 in FIG.

[0074]

[0091] FIG. 10 shows pulses 1007_a and 1007_b as measured sensor voltage versus time. Pulse 1007_a was formed by the optical modulator 120 without illuminating the semiconductor 122 with the seed light beam 114. Pulse 1007_b was formed by the optical modulator 120 while illuminating the semiconductor 122 with the seed light beam 114. In the example of FIG. 10, the light beam 106 was a pulsed light beam generated by a CO2 laser. Pulses 1007_a and 1007_b were each formed from separate pulses of the pulsed light beam, but are shown on the same time scale for comparison. The light passing through the modulation module 120 was measured by a sensor that generated a voltage in response to detecting the light. The voltage generated by the sensor was measured over time and used to generate the plot shown in FIG. 10.

[0075]

[0092] 1007_a and 1007_b each have a pedestal portion 1025a, 1025b that occurs over a time window 1021. In this example, the maximum voltage measured by the sensor for the pedestal 1025a was approximately 0.8 millivolts (mV). The maximum voltage measured by the sensor for the pedestal 1025b was approximately less than 0.1 mV. Thus, by illuminating the semiconductor 122 with the seed light beam 114, the amount of light in the pedestal was reduced. The maximum voltage measured by the sensor for the pedestal portion of the pulse generated while the semiconductor 122 is illuminated may be 10 to 20 times lower compared to the pedestal portion of the pulse generated while the semiconductor 122 is not illuminated with the seed light beam. For example, during the first 1 to 2 hours of use of the system using the modulation module 120, the semiconductor 122 is heated from an initial unheated state by the light beam 106 and the application of the potential difference V. The reduction in the pedestal portion will be approximately a factor of 20 when the semiconductor 122 is in an unheated state. As the semiconductor 122 warms up due to the light beam 106 and the application of the potential difference V, the reduction in the pedestal portion will be on the order of a factor of 10.

[0076]

[0093] The data shown in Figures 8 through 10 was measured with the same voltage sensor, a PEM sensor available from Boston Electronics, but the data was normalized to the peak intensity of the pulse generated.

[0077]

[0094] 11 and 12 show perspective views of pulse generation systems 1104 (FIG. 11) and 1204 (FIG. 12). Pulse generation systems 1104 and 1204 are examples of implementations of pulse generation system 104 that may be used in EUV light source 101 (FIG. 1).

[0078]

[0095] In the pulse generation system 1104, the light beam 106 and the seed light beam 114 are combined by a dichroic optical element 1161. The dichroic optical element 1161 may be any optical element that interacts with light based on the wavelength of the light. For example, the dichroic optical element 1161 may be a dichroic mirror that transmits a first wavelength (the wavelength of the light beam 106) and reflects a second wavelength (the wavelength of the seed light beam 114).

[0079]

[0096] After interacting with the dichroic optical element 1161, the light beam 106 and the seed light beam 114 interact with the semiconductor 122 of the light modulator 120 (FIGS. 1 and 2A). The light beam 106 and the seed light beam 114 follow the same path through the semiconductor 122. The light beam 106 and the seed light beam 114 may also propagate through the semiconductor 122 simultaneously. A portion of the light beam 106 is extracted by applying a potential difference V to the semiconductor 122 to form a pulse 107. A portion of the seed light beam 114 may also pass through the light modulator 120.

[0080]

[0097] In the pulse generation system 1204, the light beam 106 enters the semiconductor 122 at side 1262, and the seed light beam 114 enters the semiconductor 122 at side 1263. Sides 1262 and 1263 are different sides, and the seed light beam 114 and the light beam 106 propagate in different directions through the semiconductor 122. In the implementation of FIG. 12, the seed light beam 114 is diffused or branched such that the seed light beam 114 illuminates more than one portion of the semiconductor simultaneously. The seed light beam 114 may be diffused by a diffuser element (not shown), such as a polytetrafluoroethylene (PTFE) diffuser. The diffuser element is placed between the seed light source 110 and the semiconductor 122.

[0081]

[0098] 13A, an LPP EUV light source 1300 is shown. Pulse generation systems 104, 1104, and 1204 can be part of an EUV light source such as light source 1300. Pulse generation systems 104, 1104, and 1204 are not shown in FIG. 13A. However, pulse generation systems 104, 1104, and 1204 can be located within light source 1300, for example, as part of beam delivery system 1320. In these implementations, light source 105 of systems 104, 1104, and 1204 is part of drive laser 1315, and control system 175 can be part of master controller 1355, can be one of the components of master controller 1355, or can be implemented as a separate control system.

[0082]

[0099] The LPP EUV light source 1300 is formed by illuminating a target mixture 1314 in a target area 1305 with an amplified light beam 1310 traveling along a beam path toward the target mixture 1314. The target material of the target 118 may be or may include the target mixture 1314. The target area 1305 is in an interior 1307 of a vacuum chamber 1330. When the amplified light beam 1310 strikes the target mixture 1314, the target material in the target mixture 1314 is converted to a plasma state having elements with emission lines in the EUV region. The generated plasma has certain characteristics that depend on the composition of the target material in the target material 1314. These characteristics may include the wavelength of the EUV light generated by the plasma and the type and amount of debris emitted from the plasma.

[0083] [000100] The light source 1300 also includes a target material delivery system 1325 that delivers, controls, and guides a target material 1314 in the form of liquid droplets, liquid streams, solid particles or clusters, solid particles contained in liquid droplets, or solid particles contained in a liquid stream. The target mixture 1314 includes a target material such as, for example, water, tin, lithium, xenon, or any material that has an emission line in the EUV region when converted to a plasma state. For example, elemental tin can be used as pure tin (Sn), as a tin compound such as SnBr4, SnBr2, SnH4, or as a tin alloy such as a tin-gallium alloy, a tin-indium alloy, or a tin-indium-gallium alloy, or any combination of these alloys. The target mixture 1314 can also include impurities such as non-target particles. Thus, in the absence of impurities, the target mixture 1314 consists only of the target material. The target mixture 1314 is delivered by a target material delivery system 1325 to the interior 1307 of the chamber 1330 and to the target area 1305 .

[0084] [000101] The light source 1300 includes a drive laser system 1315 that generates an amplified light beam 1310 by population inversion in a gain medium or medium of the laser system 1315. The light source 1300 includes a beam delivery system between the laser system 1315 and a target area 1305, the beam delivery system including a beam delivery system 1320 and a focusing assembly 1322. The beam delivery system 1320 receives the amplified light beam 1310 from the laser system 1315, directs and modifies the amplified light beam 1310 as necessary, and outputs the amplified light beam 1310 to the focusing assembly 1322. The focusing assembly 1322 receives the amplified light beam 1310 and focuses the beam 1310 onto the target area 1305.

[0085] [000102] In some implementations, the laser system 1315 may include one or more optical amplifiers, lasers, and / or lamps to provide one or more main pulses and possibly one or more pre-pulses. Each optical amplifier includes a gain medium capable of optically amplifying a desired wavelength with high gain, a pump source, and internal optics. The optical amplifiers may or may not have laser mirrors or other feedback devices forming a laser cavity. Thus, the laser system 1315 generates the amplified light beam 1310 by population inversion in the gain medium of the laser amplifier even in the absence of a laser cavity. Furthermore, the laser system 1315 may generate an amplified light beam 1310 that is a coherent laser beam if there is a laser cavity to provide sufficient feedback to the laser system 1315. The term "amplified light beam" includes one or more of light from the laser system 1315 that is merely amplified but not necessarily coherent lasing and light from the laser system 1315 that is amplified and coherent lasing.

[0086] [000103] The optical amplifiers of the laser system 1315 may include a gas filling including CO2 as a gain medium and may amplify light at wavelengths between about 9100 and about 11000 nm, particularly at about 10600 nm, with a gain of 800 times or more. Amplifiers and lasers suitable for use in the laser system 1315 may include pulsed laser devices, such as pulsed gas discharge CO2 laser devices, generating radiation at about 9300 nm or about 10600 nm, for example by DC or RF excitation, and operating at relatively high powers, for example 10 kW or more, and high pulse repetition rates, for example 40 kHz or more. The pulse repetition rate may be, for example, 50 kHz. The optical amplifiers of the laser system 1315 may also include a cooling system, such as water, that may be used when the laser system 1315 is operated at higher powers.

[0087] 13B shows a block diagram of drive laser system 1380. Drive laser system 1380 may be used as part of drive laser system 1315 of light source 1300. Drive laser system 1380 includes three (or more) power amplifiers 1381, 1382, and 1383. Any or all of power amplifiers 1381, 1382, and 1383 may include internal optical elements (not shown).

[0088] [000105] Light 1384 exits power amplifier 1381 through output window 1385 and is reflected off curved mirror 1386. After reflection, light 1384 passes through spatial filter 1387, is reflected off curved mirror 1388, and enters power amplifier 1382 through input window 1389. Light 1384 is amplified and redirected in power amplifier 1382 to exit power amplifier 1382 through output window 1390 as light 1391. Light 1391 is directed by folding mirror 1392 towards amplifier 1383 and enters amplifier 1383 through input window 1393. Amplifier 1383 amplifies light 1391 and directs light 1391 to exit amplifier 1383 through output window 1394 as output beam 1395. A fold mirror 1396 directs the output beam 1395 upward (out of the page) toward beam delivery system 1320 (FIG. 13A).

[0089] [000106] Referring again to Figure 13B, spatial filter 1387 defines aperture 1397, which may be, for example, a circle having a diameter of about 2.2 mm to 3 mm. Curved mirrors 1386 and 1388 may be, for example, off-axis parabolic mirrors having focal lengths of about 1.7 m and 2.3 m, respectively. Spatial filter 1387 may be positioned such that aperture 1397 is coincident with the focal point of drive laser system 1380.

[0090] [000107] Referring again to Figure 13A, the light source 1300 includes a collector mirror 1335 having an aperture 1340 that allows the amplified light beam 1310 to pass through and reach the target area 1305. The collector mirror 1335 may be, for example, an ellipsoidal mirror, and has a first focus at the target area 1305 and a second focus (also referred to as an intermediate focus) at an intermediate location 1345 where the EUV light may be output from the light source 1300 and input, for example, to an integrated circuit lithography tool (not shown). The light source 1300 may also include a hollow, open-ended, conical shroud 1350 (e.g., a gas cone). The shroud tapers from the collector mirror 1335 to the target area 1305 to reduce the amount of plasma generated debris entering the focusing assembly 1322 and / or the beam delivery system 1320 while still allowing the amplified light beam 1310 to reach the target area 1305. To this end, a gas flow may be provided within the shroud that is directed towards the target area 1305 .

[0091] [000108] The light source 1300 may also include a master controller 1355 connected to a droplet position detection feedback system 1356, a laser control system 1357, and a beam control system 1358. The light source 1300 may include one or more target or droplet imagers 1360, which provide an output representative of the position of the droplets, e.g., relative to the target area 1305, and provide this output to the droplet position detection feedback system 1356. The droplet position detection feedback system may, e.g., calculate the position and trajectory of the droplets, from which a droplet position error may be calculated on a droplet-by-droplet or average basis. The droplet position detection feedback system 1356 thus provides the droplet position error as an input to the master controller 1355. Thus, the master controller 1355 can provide laser position, direction, and timing correction signals to a laser control system 1357 that can be used, for example, to control laser timing circuitry, and / or to a beam control system 1358 to control the position and shaping of the amplified light beam of the beam delivery system 1320 to vary the location and / or focusing power of the beam focus within the chamber 1330.

[0092] [000109] The target material delivery system 1325 includes a target material delivery control system 1326 which is operable in response to signals from the master controller 1355 to, for example, modify the ejection points of droplets ejected by the target material supply 1327 to correct for errors in the droplets arriving at the desired target area 1305.

[0093] [000110] Additionally, the light source 1300 may include light source detectors 1365 and 1370. These light source detectors measure one or more EUV light parameters, including but not limited to pulse energy, energy distribution as a function of wavelength, energy within a particular band of wavelengths, energy outside a particular band of wavelengths, and angular distribution of EUV intensity and / or average power. The light source detector 1365 generates a feedback signal that is used by the master controller 1355. The feedback signal represents errors in parameters such as the timing and focus of the laser pulse to properly intercept the droplets at the right place and time for effective and efficient EUV light generation.

[0094] [000111] The light source 1300 may also include a guide laser 1375 that may be used to align various portions of the light source 1300 or to help direct the amplified light beam 1310 to the target area 1305. In association with this guide laser 1375, the light source 1300 includes a metrology system 1324 located in the focusing assembly 1322 to sample a portion of the light from the guide laser 1375 and the amplified light beam 1310. In other implementations, the metrology system 1324 is located in the beam delivery system 1320. The metrology system 1324 may also include optical elements that sample or redirect a subset of the light. Such optical elements are made of any material that can withstand the power of the guide laser beam and the amplified light beam 1310. A beam analysis system is formed from the metrology system 1324 and the master controller 1355. This is because the master controller 1355 analyzes the sampled light from the guide laser 1375 and uses this information to adjust components in the focusing assembly 1322 through the beam control system 1358.

[0095] [000112] Thus, in summary, light source 1300 generates an amplified light beam 1310 that is directed along a beam path to irradiate a target mixture 1314 at a target region 1305 to convert the target material in the mixture 1314 into a plasma that emits light in the EUV range. The amplified light beam 1310 operates at a particular wavelength (also referred to as the drive laser wavelength) that is determined based on the design and characteristics of a laser system 1315. The amplified light beam 1310 may also be a laser beam when the target material provides sufficient feedback to the laser system 1315 to generate coherent laser light or if the drive laser system 1315 includes suitable optical feedback to form a laser cavity.

[0096] [000113] Other implementations are within the scope of the following claims. Other aspects of the invention are described in the following numbered clauses.

[0097] Clause 1. A method for forming a light pulse of an extreme ultraviolet (EUV) light source, comprising: Illuminating a semiconductor material of the modulation system with a first light beam having a first wavelength; applying a voltage to the semiconductor material for a duration sufficient to modify the refractive index of the semiconductor material such that a polarization state of a light beam having a second wavelength passing through the semiconductor material is modified to pass through at least one polarization-based optical element of the modulation system; forming a light pulse by passing a second light beam having a second wavelength through the semiconductor material for a duration; Equipped with the optical pulse formed comprises a first portion and a second portion, the first portion and the second portion being consecutive in time, the first portion occurring before the second portion; A method in which one or more characteristics of a first portion of a formed light pulse are modified by illuminating a semiconductor material of a modulation system with a first light beam.

[0098] Clause 2. The method of clause 1, wherein the one or more characteristics of the first portion of the light pulse comprise one or more of average intensity, maximum intensity, and duration.

[0099] Clause 3. The method of clause 1, further comprising allowing the formed pulse to propagate toward a target region, wherein a first portion of the formed pulse has a maximum intensity that is less than a maximum intensity of a second portion of the formed pulse.

[0100] Clause 4. The method of clause 3, wherein a maximum intensity of the second portion is sufficient to convert a target material of the target at the target region into a plasma that emits EUV light.

[0101] Clause 5. The semiconductor material is associated with a spectral transmittance characteristic, the spectral transmittance characteristic having a transmission region and an absorption edge wavelength, the absorption edge wavelength being the lowest wavelength in the transmission region; the first wavelength is between the absorption edge wavelength and the second wavelength; 2. The method of claim 1, wherein the second wavelength is a wavelength within the transmission region.

[0102] Clause 6. The method of clause 1, wherein the semiconductor material is associated with a band gap energy, the band gap energy being an energy difference between a valence band of the semiconductor material and a conduction band of the semiconductor material, and the photon energy of the first wavelength is less than the band gap energy.

[0103] Clause 7. The method of clause 6, wherein the semiconductor material comprises defects, the defects creating deep level traps having energy levels between a valence band and a conduction band, and the photon energy of the first wavelength is equal to or greater than an energy difference between at least one energy level of the deep level traps and the conduction band or between at least one energy level of the deep level traps and the valence band.

[0104] Clause 8. The method of clause 1, wherein the second wavelength comprises 10.6 μm and the semiconductor material comprises one of cadmium zinc telluride (CdZnTe), cadmium telluride (CdTe), zinc telluride (ZnTe), and gallium arsenide (GaAs).

[0105] Clause 9. The method of clause 1, wherein the first wavelength comprises a wavelength between 0.75 microns (μm) and 3.5 μm, and the second wavelength comprises a wavelength between 9 μm and 11 μm.

[0106] Clause 10. The method of clause 1, wherein the first light beam and the second light beam follow the same spatial path through the semiconductor material.

[0107] Clause 11. The method of clause 10, wherein the first light beam and the second light beam are simultaneously in the semiconductor material.

[0108] Clause 12. The method of clause 1, further comprising adjusting a characteristic of the first light beam to adjust one or more characteristics of the first portion of the light pulse.

[0109] Clause 13. The method of clause 12, wherein adjusting the characteristics of the first light beam comprises increasing an intensity of the first light beam to reduce a maximum intensity or an average intensity of the first portion of the light pulse.

[0110] Clause 14. A system of extreme ultraviolet (EUV) light sources, comprising: a modulation system comprising a semiconductor material having one or more types of defects, the semiconductor material having a refractive index that changes in response to the application of a voltage; a first light source configured to generate a first beam of light having a first wavelength, where illuminating the semiconductor material with light having the first wavelength increases a leakage current of the semiconductor material; a control system coupled to the modulation system, a control system configured to apply a voltage to the semiconductor material while a second light beam having a second wavelength propagates through the semiconductor material to form light pulses from the second light beam, the light pulses configured to convert at least some of the target material into a plasma that emits EUV light; and Including, the system.

[0111] Clause 15. The system of clause 14, wherein the pulse comprises a first portion and a second portion, the first portion and the second portion being consecutive in time, and the first portion occurring before the second portion.

[0112] Clause 16. The second light source comprises a pulsed light source; The control system is coupled to the modulation system and to the second light source; 15. The system of claim 14, wherein the control system is configured to control the second light source to emit pulses of light.

[0113] Clause 17. The system of clause 16, wherein at least one pulse of the first light beam and the second light beam are simultaneously within the semiconductor material.

[0114] Clause 18. The system of clause 15, wherein the control system is further configured to control the first light source, thereby controlling one or more characteristics of the first portion of the light pulse.

[0115] Clause 19. The system of clause 18, wherein the one or more characteristics of the first portion comprise one or more of average intensity, maximum intensity, and duration.

[0116] Clause 20. The system of clause 19, wherein the control system is configured to control the intensity of the first portion of the pulse by controlling the first light source.

[0117] Clause 21. The system of clause 14, wherein the second light beam comprises a continuous light beam.

[0118] Clause 22. A method for modifying an acoustic effect in an electro-optic modulator, comprising the steps of: applying a voltage to a semiconductor of the electro-optic modulator for a first time period, the application of the voltage causing an acoustic effect in the semiconductor, the acoustic effect including a vibrational acoustic wave; illuminating the semiconductor with a seed light beam having a wavelength with a photon energy less than the band gap energy of the semiconductor, thereby modifying one or more of the amplitude and frequency of the acoustic wave; A method comprising:

[0119] Clause 23. The method further comprises directing a continuous wave light beam toward a semiconductor of an electro-optic modulator; a first quantity of the continuous wave light beam passes through the electro-optic modulator in a first polarization state for a first time period when a voltage is applied to the semiconductor; a second quantity of the continuous wave light beam passes through the electro-optic modulator at a time other than the first time period when no voltage is applied to the semiconductor and an acoustic effect exists in the semiconductor; 23. The method of claim 22, wherein illuminating the semiconductor with a seed light beam varies a second amount of the light beam passing through the electro-optic modulator.

[0120] Clause 24. The method further comprises directing a pulse of the pulsed light beam toward an electro-optic modulator, wherein a first amount of the pulsed light passes through the semiconductor for a first period of time when a voltage is applied to the semiconductor, and a second amount of the pulsed light passes through the semiconductor at a time not during the first period of time when no voltage is applied to the semiconductor; 23. The method of claim 22, wherein the second amount of light is varied by illuminating the semiconductor with a seed light beam.

Claims

1. 1. A method for forming a light pulse of an extreme ultraviolet (EUV) light source, comprising: Illuminating a semiconductor material of the modulation system with a first light beam having a first wavelength; applying a voltage to the semiconductor material for a duration sufficient to modify the refractive index of the semiconductor material such that a polarization state of a light beam having a second wavelength passing through the semiconductor material is modified upon passing through at least one polarization-based optical element of the modulation system; forming a light pulse by passing a second light beam having the second wavelength through the semiconductor material for the duration; Equipped with the formed light pulse comprises a first portion and a second portion, the first portion and the second portion being consecutive in time, the first portion occurring before the second portion; A method in which one or more characteristics of the first portion of the formed light pulse are modified by illuminating the semiconductor material of the modulation system with the first light beam.

2. The method of claim 1 , wherein the one or more characteristics of the first portion of the light pulse comprise one or more of an average intensity, a maximum intensity, and a duration.

3. 2. The method of claim 1, further comprising: allowing the formed pulse to propagate toward a target area, the first portion of the formed pulse having a maximum intensity that is less than a maximum intensity of the second portion of the formed pulse.

4. The method of claim 3 , wherein the maximum intensity of the second portion is sufficient to convert a target material of a target in the target region into a plasma that emits EUV light.

5. the semiconductor material is associated with a spectral transmission characteristic, the spectral transmission characteristic comprising a transmission region and an absorption edge wavelength, the absorption edge wavelength being the lowest wavelength of the transmission region; the first wavelength is between the absorption edge wavelength and the second wavelength, The method of claim 1 , wherein the second wavelength is a wavelength within the transmission region.

6. 2. The method of claim 1 , wherein the semiconductor material is associated with a band gap energy, the band gap energy being the energy difference between a valence band of the semiconductor material and a conduction band of the semiconductor material, and wherein photon energy of the first wavelength is less than the band gap energy.

7. 7. The method of claim 6, wherein the semiconductor material comprises defects creating deep level traps having energy levels between the valence band and the conduction band, and the photon energy of the first wavelength is equal to or greater than an energy difference between at least one energy level of the deep level traps and the conduction band or between at least one energy level of the deep level traps and the valence band.

8. 2. The method of claim 1, wherein the second wavelength comprises 10.6 μm and the semiconductor material comprises one of cadmium zinc telluride (CdZnTe), cadmium telluride (CdTe), zinc telluride (ZnTe), and gallium arsenide (GaAs).

9. 2. The method of claim 1, wherein the first wavelength comprises a wavelength between 0.75 microns (μm) and 3.5 μm, and the second wavelength comprises a wavelength between 9 μm and 11 μm.

10. The method of claim 1 , wherein the first light beam and the second light beam follow the same spatial path through the semiconductor material.

11. The method of claim 10 , wherein the first light beam and the second light beam are in the semiconductor material simultaneously.

12. The method of claim 1 , further comprising adjusting a characteristic of the first light beam to adjust one or more of the characteristics of the first portion of the light pulse.

13. 13. The method of claim 12, wherein adjusting the characteristics of the first light beam comprises increasing an intensity of the first light beam and reducing a maximum intensity or an average intensity of the first portion of the light pulse.

14. 1. A system for an extreme ultraviolet (EUV) light source, comprising: a modulation system comprising a semiconductor material having one or more types of defects, the semiconductor material having a refractive index that changes in response to the application of a voltage; a first light source configured to generate a first beam of light having a first wavelength, where illuminating the semiconductor material with light having a first wavelength increases a leakage current of the semiconductor material; a control system coupled to the modulation system, a control system configured to apply a voltage to the semiconductor material while a second light beam having a second wavelength propagates through the semiconductor material to form a light pulse from the second light beam, the light pulse configured to convert at least some of the target material into a plasma that emits EUV light; and Including, the system.

15. 15. The system of claim 14, wherein the pulse comprises a first portion and a second portion, the first portion and the second portion being consecutive in time, and the first portion occurring before the second portion.

16. the second light source comprises a pulsed light source; the control system is coupled to the modulation system and to the second light source; The system of claim 14 , wherein the control system is configured to control the second light source to emit pulses of light.

17. 20. The system of claim 16, wherein at least one pulse of the first light beam and the second light beam are in the semiconductor material simultaneously.

18. The system of claim 15 , wherein the control system is further configured to control the first light source, thereby controlling one or more characteristics of the first portion of the light pulse.

19. 20. The system of claim 18, wherein the one or more characteristics of the first portion comprise one or more of an average intensity, a maximum intensity, and a duration.

20. 20. The system of claim 19, wherein the control system is configured to control an intensity of the first portion of the pulse by controlling the first light source.

21. The system of claim 14 , wherein the second light beam comprises a continuous light beam.

22. 1. A method for modifying an acoustic effect in an electro-optic modulator, comprising: applying a voltage to a semiconductor of the electro-optic modulator for a first time period, the application of the voltage generating an acoustic effect in the semiconductor, the acoustic effect comprising a vibrational sound wave; illuminating the semiconductor with a seed light beam having a wavelength with a photon energy less than the band gap energy of the semiconductor, thereby modifying one or more of the amplitude and frequency of the acoustic wave; A method comprising:

23. and directing a continuous wave light beam toward the electro-optic modulator semiconductor. a first amount of the continuous wave light beam passes through the electro-optic modulator in a first polarization state for the first time period when the voltage is applied to the semiconductor; a second quantity of the continuous wave light beam passes through the electro-optic modulator at a time other than the first time period when the voltage is not applied to the semiconductor and the acoustic effect is present in the semiconductor; 23. The method of claim 22, wherein illuminating the semiconductor with the seed light beam changes the second amount of the light beam passing through the electro-optic modulator.

24. directing a pulse of a pulsed light beam towards the electro-optic modulator, a first amount of light of the pulse passing through the semiconductor during the first time period when the voltage is applied to the semiconductor, and a second amount of light of the pulse passing through the semiconductor at a time not during the first time period when the voltage is not applied to the semiconductor; 23. The method of claim 22, wherein illuminating the semiconductor with the seed light beam varies the second amount of light.

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