CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to U.S. Application No. 62/747,518, filed October 18, 2018 and titled Control of an Optical Modulator, which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

[0002] This disclosure relates to control of an optical modulator. For example, optical leakage in an optical modulator may be controlled. The optical modulator may be part of an extreme ultraviolet (EUV) light source and/or lithography system.

BACKGROUND

[0003] Extreme ultraviolet (“EUV”) light, for example, electromagnetic radiation having wavelengths of 100 nanometers (nm) or less (also sometimes referred to as soft x-rays), and including light at a wavelength of, for example, 20 nm or less, between 5 and 20 nm, or between 13 and 14 nm, may be used in photolithography processes to produce extremely small features in substrates, for example, silicon wafers, by initiating polymerization in a resist layer.

[0004] Methods to produce EUV light include, but are not necessarily limited to, converting a material that includes an element, for example, xenon, lithium, or tin, with an emission line in the EUV range in a plasma state. In one such method, often termed laser produced plasma (“LPP”), the required plasma may be produced 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 that may be referred to as a drive laser. For this process, the plasma is typically produced in a sealed vessel, for example, a vacuum chamber, and monitored using various types of metrology equipment.

SUMMARY

[0005] In one general aspect, an apparatus for an extreme ultraviolet (EUV) light source includes an optical modulation system including an electro-optic material, the optical modulation system configured to receive a pulsed light beam that includes a plurality of pulses of light separated from each other in time; and a control system configured to control an electrical source such that a first electrical pulse is applied to the electro-optic material while a first pulse of light is incident on the electro-optic material, a second electrical pulse is applied to the electro-optic material while a second pulse of light is incident on the electro optic modulator, and an intermediate electrical pulse is applied to the electro-optic material after the first pulse of light is incident on the electro-optic material and before the second pulse of light is incident on the electro-optic material.

[0006] Implementations may include one or of the following features. The first electrical pulse applied to the electro-optic material may cause a physical effect in the electro-optic material, and the physical effect may be present in the electro-optic material when the intermediate electrical pulse is applied to the electro-optic material. The physical effect may include acoustic waves that travel in the electro-optic material and/or mechanical strain. Applying intermediate electrical pulse to the electro-optic material may reduce the physical effect.

[0007] The first pulse of light and the second pulse of light may be consecutive pulses of light in the pulsed light beam.

[0008] The control system may be configured to control an amount of time between the first electrical pulse and the intermediate electrical pulse.

[0009] The electro-optic material may include a semiconductor.

[0010] The electro-optic material may include an insulator.

[0011] The electro-optic material may include an electro-optic crystal.

[0012] The apparatus also may include at least one polarization-based optical element.

[0013] In another general aspect, an apparatus for forming optical pulses includes an optical modulation system including an electro-optic material, the optical modulation system configured to transmit light in an ON state and to block light in an OFF state, and the optical modulation system is configured to receive a pulsed light beam that includes at least a first light pulse and a second light pulse separated from each other in time. A control system may be coupled to a voltage source, the control system configured to: generate a first formed optical pulse by causing the voltage source to apply the first voltage pulse to the electro-optic modulator while the first light pulse is incident on the electro-optic modulator, the first voltage pulse being configured to switch the electro-optic modulator into the ON state; apply an intermediate voltage pulse to the electro-optic material; and generate a second formed optical pulse by applying a second voltage pulse to the electro-optic material after applying the first voltage pulse and the intermediate voltage pulse and while the second light pulse is incident on the electro-optic material. The second voltage pulse is configured to switch the electro-optic modulator into the ON state, and a property of the second formed optical pulse is controlled by the application of the intermediate voltage pulse to the electro-optic material.

[0014] Implementations may include one or more of the following features. The second formed optical pulse may include a pedestal portion and a main portion, and the property of the second formed optical pulse may include a property of the pedestal such that a property of the pedestal portion is controlled by the application of the intermediate voltage pulse to the electro-optic material. The pedestal portion and the main portion may be temporally contiguous. The property of the pedestal portion may be a temporal duration, a maximum intensity, and/or an average intensity of the pedestal portion.

[0015] The application of the intermediate voltage pulse to the electro-optic material may modify an amount of optical leakage light transmitted by the optical modulation system in the OFF state. The application of the intermediate voltage pulse to the electro-optic material may reduce the amount of optical leakage light transmitted by the optical modulation system in the OFF state.

[0016] In some implementations, the control system causes the first voltage pulse to be applied to the electro-optic material at a first time, the control system causes the intermediate voltage pulse to be applied to the electro-optic material at a second time that is after the first time, the second time and the first time are separated in time by a delay time, and the control system is further configured to adjust the delay time to thereby control a property of the second formed optical pulse.

[0017] The control system also may be configured to control an amplitude, a temporal duration, and/or a phase of the intermediate voltage pulse.

[0018] The control system may be further configured to: receive an indication of a measured property of the pedestal portion, and adjust a property of the intermediate voltage pulse based on the received indication.

[0019] The control system may be further configured to: receive an indication of an amount of extreme ultraviolet (EUV) light produced by a plasma, and adjust a property of the intermediate voltage pulse based on the received indication of the amount of EUV light. The control system being configured to adjust a property of the intermediate voltage pulse may include the control system being configured to adjust an amplitude of the intermediate voltage pulse, a temporal duration of the intermediate voltage pulse, a phase of the intermediate voltage pulse, and/or a second time, the second time being a time at which the intermediate voltage pulse is applied to the electro-optic material.

[0020] In another general aspect, a method of adjusting a property of an optical pulse includes: forming a first optical pulse by applying a first voltage pulse to an electro-optic material of an optical modulation system while light is incident on the optical modulation system; applying an intermediate voltage pulse to the electro-optic material after applying the first voltage pulse; and forming a second optical pulse by applying a second voltage pulse to the electro-optic material after the first voltage pulse and the intermediate voltage pulse and while light is incident on the electro-optic material. A property of the optical pulse is adjusted based on the application of the intermediate voltage pulse.

[0021] Implementations may include one or more of the following features. The first optical pulse may be amplified to form an amplified first optical pulse; an indication of an amount of extreme ultraviolet (EUV) light emitted from a plasma produced by interacting the amplified first optical pulse with target material may be received; and at least one property of the intermediate voltage pulse may be determined based on the received indication of the amount of EUV light emitted from the plasma. The at least one property of the intermediate voltage pulse may include a time delay after the application of the first voltage pulse, and determining at least one property of the intermediate voltage pulse may include determining the time delay based on the received indication of the amount of EUV light emitted from the plasma. The at least one property of the intermediate voltage pulse may include an amplitude and/or a duration of the intermediate voltage pulse, and determining at least one property of the intermediate voltage pulse may include determining the amplitude and/or the duration of the intermediate voltage pulse.

[0022] In some implementations, the second optical pulse includes a pedestal portion and a main portion, and a property of the pedestal portion is adjusted based on the application of the intermediate voltage pulse. The pedestal portion may be temporally contiguous with the main portion.

[0023] In another general, an extreme ultraviolet (EUV) light source includes a vessel; a target material supply apparatus configured to couple to the vessel; an optical modulation system configured to be positioned to receive a pulsed light beam, the optical modulation system including an electro-optic material; and a control system coupled to a voltage source, the control system configured to: cause the voltage source to apply a plurality of formation voltage pulses to the electro-optic material, each of the plurality of formation voltage pulses being applied to the electro-optic material at a different time, and cause the voltage source to apply at least one intermediate voltage pulse to the electro-optic material, the at least one intermediate voltage pulse being applied to the electro-optic material between two of the plurality of formation voltage pulses.

[0024] Implementations may include one or more of the following features. The target material supply apparatus may be configured to provide a plurality of target material droplets to a target region in the vessel, the target material droplets arriving at the target region at a target delivery rate, and the control system applies the formation voltage pulses to the electro optic material at a formation rate that depends on the target delivery rate.

[0025] The characteristics of the intermediate voltage pulse may include an amplitude and/or a phase, and the control system may be further configured to: access an amplitude and/or a phase stored in association with the formation rate, and cause the voltage source to produce the intermediate voltage pulse with the accessed amplitude and/or phase. The control system may be further configured to control a time delay between an application of one of the formation voltage pulses and one of the intermediate voltage pulses.

[0026] The EUV light source also may include an optical amplifier. An optical pulse may be formed each time a formation voltage pulse is applied to the electro-optic material; the formed optical pulse may be amplified by the optical amplifier to form an amplified optical pulse; the control system may be further configured to couple to a metrology system that is configured to measure an amount of EUV light produced by a plasma in the vessel, the plasma may be formed by irradiating the target material with the formed amplified optical pulse, the control system may be configured to receive the measured amount of EUV light from the metrology system; and the control system may be configured to modify one or more characteristics of the intermediate voltage pulse based on the measured amount of EUV light. The one or more characteristics of the intermediate voltage pulse may include an amplitude of the intermediate voltage pulse, a temporal duration of the intermediate voltage pulse, a phase of the intermediate voltage pulse, and/or a delay time after application of a most recent formation voltage pulse.

[0027] Implementations of any of the techniques described above may include an EUV light source, a system, a method, a process, a device, or an apparatus. The details of one or more implementations are set forth in the accompanying drawings and the description below.

Other features will be apparent from the description and drawings, and from the claims. DESCRIPTION OF THE DRAWINGS

[0028] FIG. 1 is a block diagram of an example of an EUV lithography system.

[0029] FIG. 2A is a block diagram of an example of a modulation system.

[0030] FIG. 2B is an illustration of an example of an optical pulse.

[0031] FIG. 2C is an illustration of an example of a modified optical pulse formed from the optical pulse of FIG. 2B.

[0032] FIG. 3A is an illustration of an example of an optical beam as a function of time.

[0033] FIG. 3B is an illustration of an example of an electrical signal as a function of time.

[0034] FIG. 4 is a flow chart of an example of a process for controlling a property of an irradiating optical pulse.

[0035] FIGS. 5 and 6 are block diagrams of an example of a lithographic apparatus.

[0036] FIG. 7 is a block diagram of an example of an EUV light source.

DETAILED DESCRIPTION

[0037] Techniques for controlling optical leakage of an electro-optic modulator are described. The electro-optic modulator is used to modulate an initial light beam to form a modified optical pulse. The electro-optic modulator includes an electro-optic material. An electrical signal (for example, a voltage pulse having a finite duration) is applied to the electro-optic material to change the index of refraction of the electro-optic material such that the initial light beam is modulated and the modified optical pulse is formed. An intermediate electrical signal is applied to the electro-optic material after the application of the electrical signal. As discussed in greater detail below, application of the intermediate electrical signal to the electro-optic material allows the optical leakage of the electro-optical modulator to be controlled. The intermediate electrical signal mitigates, changes, or controls acoustic waves generated by the electrical signal. Controlling the acoustic waves also controls the amount of optical leakage of the electro-optic modulator, thereby allowing characteristics or properties of a subsequently formed (or later-formed) modified optical pulse to be controlled.

[0038] Referring to FIG. 1, a block diagram of a system 100 is shown. The system 100 is an example of an EUV lithography system. The system 100 includes an EUV light source 101, which provides EUV light 196 to a lithography apparatus 195. The lithography apparatus 195 exposes a wafer (for example, a silicon wafer) with the EUV light 196 to form electronic features on the wafer. The EUV light 196 is emitted from a plasma 197 that is formed by irradiating target material in a target 118 with an irradiating optical pulse 108. The target material is any material (for example, tin) that emits EUV light in a plasma state.

[0039] The EUV light source 101 includes an optical pulse generating system 104, which produces an amplified optical pulse 108 from a modified optical pulse 107. The optical pulse generating system 104 includes a light source 105, which may be, for example, a pulsed (for example, a Q-switched) or continuous -wave carbon dioxide (CO2) laser or a solid-state laser (for example, Nd:YAG laser or an erbium-doped fiber (Er:glass) laser). The light source 105 produces an optical beam 106 (or light beam 106), which may be a train of pulses of light or a continuous light beam. The light source 105 emits the optical beam 106 onto a path 111 toward a modulation system 120 that includes an electro-optic material 122. The electro optic material 122 is on the path 111, and the optical beam 106 is incident on the electro-optic material 122.

[0040] The modulation system 120 is an electro-optic modulator that modulates the optical beam 106 based on the electro-optic effect. The electro-optic effect describes the change in the refractive index of the electro-optic material 122 that results from the application of a direct-current (DC) or low-frequency electric field 124 generated by an electrical source 123. The electrical source 123 may be, for example, a voltage source, a function generator, or a power supply. By controlling the electro-optic effect in the electro-optic material 122 while the optical beam 106 is incident on the electro-optic material 122, the modulation system 120 modulates the phase, polarization, or amplitude of the optical beam 106 to form the pulse 107.

[0041] The electric field 124 may be used to control whether or not the modulation system 120 transmits light. The electric field 124 may be used to control the electro-optic material 122 such that only a certain portion or portions of the optical beam 106 pass through the electro-optic material 122. In this way, the modulation system 120 forms the pulse 107 from a portion of the optical beam 106.

[0042] The optical pulse generating system 104 also includes one or more optical amplifiers 130, each of which includes a gain medium 132 on the path 111. The gain medium 132 receives energy through pumping and provides the energy to the pulse 107 such that the pulse 107 is amplified into the amplified or irradiating optical pulse 108. The amount of amplification of the pulse 107 is determined by the gain of the amplifier 130 and the gain medium 132. The gain is an amount or factor of increase in energy that the amplifier 130 provides to an input light beam. [0043] The pulse 108 propagates on the path 111 toward a vacuum vessel 180, which receives the target 118. The pulse 108 and the target 118 interact at a target region 115 in the vacuum vessel 180, and the interaction converts at least some of the target material in the target 118 into plasma 197 that emits the EUV light 196.

[0044] The application of the electric field 124 to the electro-optic material 122 creates acoustic waves in the material 122. The acoustic waves cause strain in the material 122 and can persist even after the electric field 124 is no longer being applied to the material 122 and/or after a property of the electric field 124 is changed. The strain caused by the acoustic waves changes the index of refraction of the material 122 even during periods of time during which the index of refraction of the material 122 is not expected to change. These changes in the index of refraction may lead to optical leakage. Optical leakage is light that passes through the modulator 120 when the modulation system 120 is in a state in which light should not pass through the modulation system 120. As discussed below, the electric field 124 includes components (for example, pulses) that are applied to the material 122 to mitigate and/or control the optical leakage by mitigating and/or controlling the residual acoustic waves in the material 122.

[0045] The EUV light source 101 also includes a metrology system 182 positioned relative to the target region 115. The metrology system 182 includes one or more sensors 184 configured to sense the EUV light 196. The metrology system 182 generates a representation of an amount of EUV light 196 in the vacuum chamber 180 (for example, at the target region 115). The metrology system 182 provides data that represents the amount of measured EUV light to a control system 175 via a communications link 183.

[0046] In some implementations, the metrology system 182 also includes an optical sensing system 185 that includes one or more optical sensors that are configured to measure properties of the pulse 107 and/or the amplified pulse 108. The optical sensing system 185 may include any sensor that is able to detect the wavelength or wavelengths in the pulse 107 and/or the pulse 108 such that the sensing system 185 is able to determine properties of the pulse 107 and/or the amplified pulse 108 (for example, properties of a pedestal portion). In the example of FIG. 1, the optical sensing system 185 is part of the metrology system 182, however, the optical sensing system 185 may be separate from the metrology system 182.

For example, the optical sensing system 185 may be positioned to receive a sample of the optical pulse 107 between the modulation system 120 and the optical amplifier 130. The optical sensing system 185 also may provide data related to measurements of the optical pulse 107 and/or the pulse 108 to the control system 175. In some implementations, information from the EUV sensor 184 and/or the optical sensing system 185 are used by the control system 175 to set and/or change parameters of the electric field 124.

[0047] In addition to receiving data from the metrology system 182, the control system 175 exchanges data and/or information with the pulse generating system 104 and/or any of the components of the pulse generating system 104 via a communications interface 176. For example, in some implementations, the control system 175 may provide trigger signals to operate the modulation system 120 and/or the light source 105. In another example, the control system 175 may receive measured amounts of EUV light from the EUV sensor 184 for many interactions between an instance of the irradiating optical pulse 108 and an instance of the target 118 to determine which of many possible settings of a particular parameter or property of the electric field 124 results in the generation of the highest amount of EUV light. In yet another example, the control system 175 provides data and/or information to the electrical source 123, which generates the electric field 124. In this example, the data provided by the control system 175 determines various properties of the electric field 124 such as, for example, amplitude and/or time delay between two pulses of the electric field 124.

[0048] The control system 175 includes an electronic processor 177, an electronic storage 178, and an input/output (I/O) interface 179. The electronic processor 177 includes one or more processors suitable for the execution of a computer program such as a general or special purpose microprocessor, and any one or more processors of any kind of digital computer. Generally, an 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.

[0049] The electronic storage 178 may be volatile memory, such as RAM, or non-volatile memory. In some implementations, and the electronic storage 178 includes non-volatile and volatile portions or components. The electronic storage 178 may store data and information that is used in the operation of the control system 175 and/or components of the control system 175.

[0050] The electronic storage 178 also may store instructions, perhaps as a computer program, that, when executed, cause the processor 177 to communicate with components in the control system 175, the modulation system 120, and/or the light source 105. For example, in implementations in which the source 105 is a pulsed source, the instructions may be instructions that cause the electronic processor 177 to generate a signal that results in the light source 105 emitting an optical pulse.

[0051] The I/O interface 179 is any kind of electronic interface that allows the control system 175 to receive and/or provide data and signals with an operator, the modulation system 120, and/or the light source 105, and/or an automated process running on another electronic device. For example, the I/O interface 179 may include one or more of a visual display, a keyboard, and a communications interface.

[0052] FIG. 2A is a block diagram of a modulation system 220. The modulation system 220 is an example of an implementation of the modulation system 120 (FIG. 1). The modulation system 220 includes the electro-optic material 122, which, in the implementation shown in FIG. 2A, is positioned between electrodes 223a, 223b. The electrodes 223a, 223b are controllable to form an electric field between the electrode 223a and 223b. For example, the control system 175 may cause a voltage source 223 to provide a voltage signal 214 to the electrode 223b such that the electrode 223b is held at a different voltage than the electrode 223a, thus creating an electric field or a potential difference (V) across the electro-optic material 122.

[0053] Moreover, the modulation system 220 may include more than one instance of the electro-optic material 122. For example, the modulation system 220 may include two electro-optic materials 122, three electro-optic materials 122, or any number that is suitable for the application, may be placed on the beam path 111. Each instance of the electro-optic material 122 also includes respective electrodes 223 a, 223b, which are controllable to apply an electric field to that electro-optic material 122. In implementations that include more than one instance of the electro-optic material 122, the electric field applied to each electro-optic material 122 may be the same, or at least some of the electro-optic fields may be different. The electro-optic materials 122 may be controlled by the control system 175 as a group, or the various electric fields may be individually controllable by respective instances of the control system 175.

[0054] The modulation system 220 also includes one or more polarization-based optical elements 224. In the example of FIG. 2A, only one polarization-based optical element 224 is shown. However, in other implementations, additional polarization-based optical elements 224 may be included. For example, a second polarization-based optical element 224 may be on a side of the modulation system 220 that receives the optical beam 106. Furthermore, the polarization-based optical element 224 is shown as being physically separated from the electro-optic material 122, but other implementations are possible. For example, the polarization-based optical element 224 may be a film that is formed on the electro-optic material 122 such that the optical element 224 and the electro-optic material 122 are in contact with each other.

[0055] The polarization-based optical element 224 is any optical element that interacts with light based on the polarization state of the light. For example, polarization-based optical element 224 may be a linear polarizer that transmits horizontally polarized light and blocks vertically polarized light, or vice versa. The polarization-based optical element 224 may be a polarizing beam splitter that transmits horizontally polarized light and reflects vertically polarized light. The polarization-based optical element 224 may be an optical element that absorbs all light except for light having a particular polarization state. In some

implementations, the polarization-based optical element 224 may include a quarter- wave plate. At least one polarization-based optical element 224 is positioned to receive light that passes through the electro-optic material 122 and to direct light of a certain polarization state onto the beam path 111.

[0056] As discussed above, although one electro-optic material 122 and one polarization- based optical element 224 are shown in FIG. 2, more than one of either or both of these components may be in series with each other on the beam path 111 and included in the modulation system 120. For example, the modulation system may include three polarization- based optical elements 224 and two electro-optic materials 122 in series on the beam path 111, with each of the electro-optic materials 122 being between two of the three polarization- based optical elements 224.

[0057] The electro-optic material 122 may be any material that transmits one of more wavelengths of the optical beam 106. For implementations in which the optical beam 106 includes light of a wavelength of 10.6 microns (pm), the material 122 may be, for example, cadmium zinc telluride (CdZnTe or CZT), cadmium telluride (CdTe), zinc telluride (ZnTe), and/or gallium arsenide (GaAs). Other materials may be used at other wavelengths. For example, the material 122 may be monopotassium phosphate (KDP), ammonium dihydrogen phosphate (ADP), quartz, cuprous chloride (CuCl), zinc sulphide (ZnS), zinc selenide (ZnSe), lithium niobate (LiNbOri, gallium phosphide (GaP), lithium tantalate (LiTaOp, or barium titanate ( BaTiOq. Other materials that transmit one or more wavelengths of the optical beam 106 and exhibit birefringence in response to application of an external force also may be used as the material 122. For example, quartz may be used as the material 122. [0058] The electro-optic material 122 also exhibits anisotropy. In a material that exhibits anisotropy, the properties of the material (such as the index of refraction) are spatially non- uniform. Thus, the properties of the material 122 may be modified along a particular direction or directions by application of a controllable external force (such as the potential difference (V)). For example, the indices of refraction for different polarization components of light propagating through the material 122 may be controlled through application of the external force. Thus, the polarization state of the light that passes through the material 122 may be controlled by controlling the potential difference (V) between the electrodes 223a, 223b.

[0059] Under ideal operation, the modulation system 220 only transmits light when the potential difference V applied to the material 122 causes the polarization state of the light passing through the material 122 to match the polarization conditions of the polarization- based optical element 224. For example, if the polarization-based optical element 224 is a linear polarizer positioned to transmit horizontally polarized light onto the beam path 111, and the optical beam 106 is vertically polarized when initially incident on the material 122, the pulse 107 is only formed when the potential difference V applied to the material 122 changes the polarization state of the optical beam 106 such that the optical beam 106 becomes horizontally polarized prior to interacting with the polarization-based optical element 224.

[0060] The modulation system 220 is considered to be in an ON state anytime the modulation system 220 is controlled to intentionally transmit light. For example, when the applied potential difference V is such that the polarization state of the optical beam 106 is matched to the polarization-based optical element 224, the optical modulation system 220 is considered to be in the ON state and the pulse 107 is formed. When the applied potential difference V is such that the polarization state of the optical beam 106 is expected to be orthogonal to the polarization-based optical element 224, the optical modulation system 220 is in the OFF state. Under ideal conditions, the optical beam 106 does not pass through the modulation system 120 when the optical modulation system 220 is in the OFF state.

[0061] However, applying the potential difference V to the material 122 causes acoustic waves to propagate in the material 122. These acoustic waves may persist after the potential difference V is removed from the material 122. Additionally, the acoustic waves cause strain in the material 122 that change the optical properties of the material 122 and allow incident light to pass through the modulation system 220 (as optical leakage) even when the potential difference V is not applied. Thus, in actual operation, the modulation system 220 may transmit spurious light (optical leakage) even when the polarization condition of the polarization-based optical element 224 is such that light incident on the material 122 should not pass through the modulation system 220. For example, when the optical leakage is present just prior to the formation of an irradiating optical pulse, the optical leakage forms a pedestal portion on the irradiating optical pulse.

[0062] Referring also to FIGS. 2B and 2C, an illustration of an example of the optical pulse 206 (FIG. 2B) and an example of a modified optical pulse 207 (FIG. 2C) formed from the optical pulse 206 are shown. The pulse 207 include a pedestal portion 225 and a main portion 268. FIG. 2B shows the intensity of the pulse 206 as a function of time, and FIG. 2C shows the intensity of the pulse 207 as a function of time.

[0063] The pulse 206 has a temporal profile (intensity versus time) that is approximately Gaussian. The pulse 206 interacts with the modulation system 220 to form the pulse 207.

The control system 175 controls the modulation system 220 to select or extract a particular portion 267 of the pulse 206. In the example of FIG. 2B, at a time t=ta, the modulation system 220 is set to transmit light and at the time t=tb, the modulation system 220 is set to block light. In other words, the optical modulation system 220 is only intended to transmit the light in the portion 267 (which is the light in the pulse 206 between time ta and time tb). For example, the control system 175 may control the modulation system 220 to transmit light at the time ta by applying the voltage signal 214 such that light passing through the electro optic material 122 has a polarization that matches the polarization of the polarization-based optical element 224. The modulation system 220 may be controlled to stop transmitting light at the time tb by removing the voltage signal 214.

[0064] However, due to acoustic waves in the electro-optic material 122 (or other disturbances such as unexpected motion of the polarization-based optical element 224), optical leakage may be transmitted by the modulation system 220 at times before ta and/or at times after time tb. In the example of FIG. 2B, leakage light 266 is optical leakage that occurs just prior to the time ta. The leakage light 266 passes through the modulation module 120 just prior to the portion 267.

[0065] Referring to FIG. 2C, the leakage light 266 forms the pedestal portion 225. In the example shown, the pedestal portion 225 occurs during a window labeled as 221, and the pedestal portion 225 occurs earlier in time than the rest of the pulse 207. The portions of the optical pulse 207 that are not the pedestal portion 225 are referred to as the main portion 268. The pedestal portion 225 and the main portion 268 are both part of the optical pulse 207, and the pedestal portion 225 is temporally connected to the main portion 268. In other words, there is no period without light between the pedestal portion 225 and the main portion 268.

[0066] The pedestal portion 225 has a different temporal profile (intensity as a function of time) than the main portion 268. For example, the average and maximum intensity and optical energy of the pedestal portion 225 are less than the average and maximum intensity and optical energy of main portion 268. Moreover, the shape of the pedestal portion 225 is different from the shape of the main portion 268. Further, the characteristics (for example, intensity, temporal profile, and/or duration) of the pedestal portion 225 are different from the characteristics of the early part of a pulse formed without any optical leakage.

[0067] The modified pulse 207 is amplified by the amplifier 130 to form an amplified pulse 208, which propagates to the target region 115. The amplified pulse 208 includes the pedestal portion 225 and the main portion 268, with each portion 225, 268 of the amplified pulse 208 having a greater intensity than the corresponding portion of the modified pulse 207. In the example of FIG. 2C, the pedestal portion 225 occurs before the main portion 268 and reaches the target 118 before the main portion 268. In some implementations, the main portion 268 has an intensity or energy sufficient to convert at least some of the target material in the target 118 into plasma that emits EUV light. The pedestal portion 225 does not have as much energy as the main portion 268, and may or may not have sufficient energy to convert the target material into plasma. However, the light in the pedestal portion 225 may reflect off of the target 118, evaporate material from the surface of the target 118, and/or break off parts of the target 118. The pedestal portion 225 may interfere with the plasma formation by altering the target 118 before the main portion 268 reaches the target 118 and/or cause undesirable reflections that propagate back on the path 111.

[0068] On the other hand, the pedestal portion 225 may condition the target 118 such that the properties (for example, density, shape, and/or size) are more favorable to plasma production. As such, it is desirable to control the amount of light in the pedestal portion 225 by controlling the amount of optical leakage. The control system 175 controls the amount of optical leakage (the leakage light 266 in this example) with an intermediate electrical signal that is applied to the electro-optic material 122 prior to formation of the pulse 207.

[0069] The pulse 207 discussed with respect to in FIGS. 2B and 2C is provided as one example of a modified optical pulse 207. The pulse 207 may have other forms. For example, the leakage light 266 may occur before the time ta such that the pedestal portion 225 is separate from the main portion 268. In these implementations, there is a period without light between the pedestal portion 225 and the main portion 268. Furthermore, the leakage light 266 may occur after the time tb such that the pedestal portion 225 occurs after the main portion 268. In these implementations, the pedestal portion 225 reaches the target region 115 after the main portion 268. In some implementations, the leakage light 266 occurs before the time ta and after the time tb such that there is a pedestal portion 225 on each side of the main portion 268.

[0070] FIG. 3A is a plot of intensity of an optical beam 306 as a function of time. FIG. 3B is a plot of voltage of an electrical signal 324 as a function of time. The same time scale is used in FIG. 3 A and FIG. 3B. The electrical signal 324 is an example of an electrical signal that may be produced by a function generator controlled by the control system 175 and applied to the electro-optic material 122 (FIGS. 1 and 2A). The electrical signal 324 is discussed with respect to the modulation system 220 (FIG. 2A). The optical beam 306 is an example of an optical beam (or light beam) that may be incident on the electro-optic material 122.

[0071] The optical beam 306 includes two initial optical pulses of light, a first initial optical pulse 306_l that is incident on the material 122 and a second initial optical pulse 306_2. The first initial optical pulse 306_l is incident on the material 122 before the second initial pulse 306_2 is incident on the material 122. The first optical pulse 306_l and the second optical pulse 306_2 are separate optical pulses that are separated from each other in time. The optical beam 306 may include optical pulses in addition to the initial optical pulses 306_l and 306_2.

[0072] The electrical signal 324 includes a first electrical pulse 325a_l, which is applied to the material 122 beginning at the time tl, and a second electrical pulse 325a_2, which is applied to the material 122 beginning at the time t2. The electrical pulses 325a_l, 325a_2 are voltage pulses that have an amplitude of A volts for a finite temporal duration 33 l_l, 331 _2 , respectively. Thus, the application of the electrical pulses 325a_l and 325a_2 results in a voltage of A being applied to the material 122 for the temporal durations 331_1, 331 _2 , respectively.

[0073] The voltage A is a voltage that is sufficient to place the modulation system 220 in the ON state. Thus, while the electrical pulses 325a_l and 325a_2 are applied to the material 122, light that is incident on the material 122 is transmitted though the optical modulation system 220. After the electrical pulses 325a_l and 325a_2 end, the modulation system 220 returns to the OFF state. [0074] The temporal durations 331 _ 1 and 33l_2 may be the same or different. In the implementation of FIG. 3B, the first and second electrical pulses 325a_l, 325a_2 have the same voltage amplitude (A). However, in other implementations, the electrical pulses 325a_l, 325a_2 have different voltage amplitudes, with both of the electrical pulses 325a_l, 325a_2 having a voltage that is sufficient to transition the modulation system 220 to the ON state.

[0075] The electrical signal 324 also includes an intermediate electrical pulse 325b_l, which is applied to the material 122 at a time ti. The time ti occurs after the time tl and before the time t2. The time ti is separated from the time at which the first electrical pulse 325a_l ends by a delay time 330. The intermediate electrical pulse 325b_l has an amplitude B for a temporal duration or width 332.

[0076] At the time 0, the modulation system 220 is in the OFF state. The application of the first electrical pulse 325a_l at the time tl transitions the modulation system 220 to the ON state. At the time tl, the first optical pulse 306_l is incident on the material 122. The temporal duration of the first electrical pulse 325a_l is shorter than the temporal duration of the first optical pulse 306_l. Thus, only the portion of the first optical pulse 306_l that is incident on the material 122 during the duration 33 l_l passes through the material 122. The portion of the first optical pulse 306_l that passes through the material forms a modified optical pulse (such as the modified optical pulse 207 of FIG. 2C). The application of the first electrical pulse 325a_l to the material 122 creates acoustic waves in the material 122. The acoustic waves cause strain and change the optical properties of the material 122. The acoustic waves continue to propagate in the material 122 even after the first electrical signal 325a_l ends and no voltage is applied to the material 122. The acoustic waves caused by application of the first electrical signal 325a_l may be present when the second optical pulse 306_2 is incident on the material 122 but before the second electrical signal 325a_2 is applied to the material 122. In these circumstances, light may pass through the modulation system 220 even though the modulation system 220 is in the OFF state. Such light is optical leakage and forms a pedestal on an optical pulse later formed by the modulation system 220.

[0077] FIG. 4 is a flow chart of an example of a process 400 for controlling a property of an irradiating optical pulse. The irradiating optical pulse may include a pedestal portion. The process 400 is discussed with respect to the EUV light source 101 and control system 175 (FIG. 1), the modulation system 220 (FIG. 2A), the optical beam 306 (FIG. 3A), and the electrical signal 324 (FIG. 3B). However, the process 400 may be performed by other EUV light sources, other optical beams, other electrical signals, and/or other electro-optic modulation systems.

[0078] A first modified optical pulse is formed using the modulation system 220 (410). The first modified optical pulse is formed by causing the first optical pulse 306_l to be incident on the material 122 and applying the first voltage pulse 325a_l to the material 122 at the time tl. At the time tl, the first optical pulse 306_l is incident on the material 122 and the modulation system 220 is in the ON state. Thus, the portion of the first optical pulse 306_l that begins at the time tl through the end of the duration 33 l_l passes through the material 122 becomes the first modified optical pulse. Additionally, application of the first voltage pulse 325a_l causes acoustic waves to propagate in the material 122. The acoustic waves are referred to as first acoustic waves and may continue to propagate in the material 122 after the first voltage pulse 325a_l has ended and the modulation system 220 is in the OFF state.

[0079] The intermediate voltage pulse 325b_l is applied to the material 122 (420). The application of the intermediate voltage 325b_l to the material 122 also causes acoustic waves (referred to as second acoustic waves) to propagate in the material 122. The second acoustic waves interfere with the first acoustic waves. Constructive interference increases the amplitude of the acoustic waves and destructive interference decreases the amplitude of the acoustic waves. The amplitude and/or duration 332 of the intermediate voltage pulse 325b_l determines the amplitude of the second acoustic waves. The delay 330 determines the phase of the second acoustic waves relative to the first acoustic waves. Thus, by controlling the delay 330 and/or the amplitude B, the nature of the interactions between the first and second acoustic waves may be controlled. For example, when the second acoustic waves have the same amplitude and the opposite phase as the first acoustic waves, the first and second acoustic waves interfere such that no acoustic waves propagate in the material 122 after the intermediate electrical pulse 325b_l is applied.

[0080] A second modified optical pulse is formed using the modulation system 220 (430). The second modified optical pulse is formed after applying the intermediate voltage pulse 325b_l to the material 122. The second optical pulse 306_2 is incident on the material 122. At the time t2, the second voltage pulse 325a_2 is applied to the material 122 such that the modulation system 220 is transitioned to the ON state. The portion of the second optical pulse 306_2 beginning at the time t2 through the duration 331_2 is transmitted through the material 122. [0081] As discussed above, the first acoustic waves and the second acoustic waves interfere, and the characteristics of the acoustic waves in the material 122 depend on the nature of the interference. The acoustic waves cause strain in the material 122 and change the index of refraction of the material 122, and these changes in the index of refraction may allow light to pass through the modulation system 220 as optical leakage when the modulation system 220 is in the OFF state. The intermediate voltage pulse 325b_l is used to modify one or more properties of the second optical pulse 306_2 in a desired manner. For example, the intermediate voltage pulse 325b_l may be used to modify the maximum or average intensity, temporal duration, and/or the temporal profile of the second optical pulse 306_2. In some implementations, the intermediate voltage pulse 325b_l is used to control and/or form a pedestal portion.

[0082] For example, depending on the characteristics of the acoustic waves in the material 122, some of the light in the second optical pulse 306_2 that is incident on the material 122 before the time t2 or a time after the voltage pulse 325a_2 is no longer being applied (when the modulation system 220 is in the OFF state) also may be transmitted through the modulation system 220 as optical leakage to form a pedestal portion on the second modified optical pulse. The intensity, duration, and other properties of the pedestal depend on the amount of optical leakage, which is controllable by adjusting the acoustic waves in the material 122. The acoustic waves in the material 122 may be controlled, adjusted, or mitigated by applying the intermediate voltage pulse 325b_l to the material 122. Thus, one or more properties of the pedestal portion is controlled or adjusted by applying the intermediate voltage pulse 325b_l . Moreover, one or more properties of the main portion of the optical pulse 306_2 may be controlled with the intermediate voltage pulse 325b_l.

[0083] The intermediate voltage pulse 325b_l may be used to control one or more properties of the second optical pulse 306_2 in other ways. For example, as discussed above, the pedestal portion may be temporally separate from the main portion of the modified pulse. In these implementations, the intermediate voltage pulse 325b_l may be used to modify the separate pedestal portion and/or the main portion.

[0084] Furthermore, the optical beam 306 may include additional optical pulses and the electrical signal 324 may include additional voltage pulses. In some implementations, the control system 175 receives an indication of an amount of EUV light generated by an interaction between an modified optical pulse and the target material 118 as measured by the EUV sensor 184 (FIG. 1). For example, the measured amount of EUV light is tracked over two or more interactions while the properties of the intermediate voltage pulse are changed to determine the optimal settings for the intermediate voltage pulse. The delay 330 may be changed for each of several interactions to determine which delay 330 produces the most EUV light. In another example, the amplitude B of the intermediate voltage pulse 325b_l is changed to determine the amplitude B that produces the most EUV light.

[0085] In some implementations, a look up table or database that correlates optimal values of the amplitude B, width 332, and/or the delay 330 with the repetition rate of the pulses in the optical beam 306 is stored on the electronic storage 178 such that the amplitude B and/or delay 330 may be changed if the repetition rate of the optical beam 306 changes.

[0086] FIGS. 5 and 6 relate to a lithographic apparatus in which a modulation system such as the systems 120 and 22 may be used. FIG. 5 is a block diagram of a lithographic apparatus 500 that includes a source collector module SO. The lithographic apparatus 500 includes:

• an illumination system (illuminator) IL configured to condition a

radiation beam B (for example, EUV radiation).

• a support structure (for example, a mask table) MT constructed to

support a patterning device (for example, a mask or a reticle) MA and connected to a first positioner PM configured to accurately position the patterning device;

• a substrate table (for example, a wafer table) WT constructed to hold a substrate (for example, a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate; and

• a projection system (for example, a reflective projection system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (for example, including one or more dies) of the substrate W.

[0087] The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

[0088] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure may use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system.

[0089] The term“patterning device” should be broadly interpreted as referring to any device that may be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. The pattern imparted to the radiation beam may correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

[0090] The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase- shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix.

[0091] The projection system PS, like the illumination system IL, may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of a vacuum. It may be desired to use a vacuum for EUV radiation since other gases may absorb too much radiation. A vacuum environment may therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

[0092] In the example of FIGS. 5 and 6, the apparatus is of a reflective type (for example, employing a reflective mask). The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more patterning device tables). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.

[0093] Referring to FIG. 5, the illuminator IL receives an extreme ultraviolet radiation beam from the source collector module SO. Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has at least one element, for example, xenon, lithium or tin, with one or more emission lines in the EUV range. In one such method, often termed laser produced plasma (“LPP”) the required plasma is produced by irradiating a fuel, such as a droplet, stream or cluster of material having the required line- emitting element, with a laser beam. The source collector module SO may be part of an EUV radiation system including a laser, not shown in FIG. 5, for providing the laser beam exciting the fuel. The resulting plasma emits output radiation, for example, EUV radiation, which is collected using a radiation collector, disposed in the source collector module. The laser and the source collector module may be separate entities, for example when a carbon dioxide (CO2) laser is used to provide the laser beam for fuel excitation.

[0094] In such cases, the laser is not considered to form part of the lithographic apparatus and the radiation beam is passed from the laser to the source collector module with the aid of a beam delivery system including, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the source collector module, for example when the source is a discharge produced plasma EUV generator, often termed as a DPP source.

[0095] The illuminator IL may comprise an adjuster for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as s-outer and s-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may comprise various other components, such as facetted field and pupil mirror devices. The illuminator IL may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section.

[0096] The radiation beam B is incident on the patterning device (for example, mask) MA, which is held on the support structure (for example, mask table) MT, and is patterned by the patterning device. After being reflected from the patterning device (for example, mask) MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor PS2 (for example, an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, for example, so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor PS 1 can be used to accurately position the patterning device (for example mask) MA with respect to the path of the radiation beam B. Patterning device (for example mask) MA and substrate W may be aligned using patterning device alignment marks Ml, M2 and substrate alignment marks Pl, P2.

[0097] The depicted apparatus may be used in at least one of the following modes:

1. In step mode, the support structure (for example, mask table) MT and the

substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (that is, a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed.

2. In scan mode, the support structure (for example, mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (that is, a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure (for example, mask table) MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS.

3. In another mode, the support structure (for example, mask table) MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.

[0098] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0099] FIG. 6 shows an implementation of the lithographic apparatus 500 in more detail, including the source collector module SO, the illumination system IL, and the projection system PS. The source collector module SO is constructed and arranged such that a vacuum environment can be maintained in an enclosing structure 620 of the source collector module SO. The systems IL and PS are likewise contained within vacuum environments of their own. An EUV radiation emitting plasma 2 may be formed by a laser produced LPP plasma source. The function of source collector module SO is to deliver EUV radiation beam 20 from the plasma 2 such that it is focused in a virtual source point. The virtual source point is commonly referred to as the intermediate focus (IF), and the source collector module is arranged such that the intermediate focus IF is located at or near an aperture 621 in the enclosing structure 620. The virtual source point IF is an image of the radiation emitting plasma 2.

[0100] From the aperture 621 at the intermediate focus IF, the radiation traverses the illumination system IL, which in this example includes a facetted field mirror device 22 and a facetted pupil mirror device 24. These devices form a so-called“fly’s eye” illuminator, which is arranged to provide a desired angular distribution of the radiation beam 21, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA (as shown by reference 660). Upon reflection of the beam 21 at the patterning device MA, held by the support structure (mask table) MT, a patterned beam 26 is formed and the patterned beam 26 is imaged by the projection system PS via reflective elements 28, 30 onto a substrate W held by the substrate table WT. To expose a target portion C on substrate W, pulses of radiation are generated while substrate table WT and patterning device table MT perform synchronized movements to scan the pattern on patterning device MA through the slit of illumination.

[0101] Each system IL and PS is arranged within its own vacuum or near-vacuum

environment, defined by enclosing structures similar to enclosing structure 620. More elements than shown may generally be present in illumination system IL and projection system PS. Further, there may be more mirrors present than those shown. For example there may be one to six additional reflective elements present in the illumination system IL and/or the projection system PS, besides those shown in FIG. 6.

[0102] Considering source collector module SO in more detail, a laser energy source including a laser 623 is arranged to deposit laser energy 624 into a fuel that includes a target material. The target material may be any material that emits EUV radiation in a plasma state, such as xenon (Xe), tin (Sn), or lithium (Li). The plasma 2 is a highly ionized plasma with electron temperatures of several lO's of electron volts (eV). Higher energy EUV radiation may be generated with other fuel materials, for example, terbium (Tb) and gadolinium (Gd). The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma, collected by a near-normal incidence collector 3 and focused on the aperture 621. The plasma 2 and the aperture 621 are located at first and second focal points of collector CO, respectively.

[0103] Although the collector 3 shown in FIG. 6 is a single curved mirror, the collector may take other forms. For example, the collector may be a Schwarzschild collector having two radiation collecting surfaces. In an embodiment, the collector may be a grazing incidence collector which comprises a plurality of substantially cylindrical reflectors nested within one another.

[0104] To deliver the fuel, which, for example, is liquid tin, a droplet generator 626 is arranged within the enclosing structure 620, arranged to fire a high frequency stream 628 of droplets towards the desired location of plasma 2. The droplet generator 626 may be the target formation apparatus 216 and/or includes an adhesive such as the adhesive 234. In operation, laser energy 624 is delivered in a synchronism with the operation of droplet generator 626, to deliver impulses of radiation to turn each fuel droplet into a plasma 2. The frequency of delivery of droplets may be several kilohertz, for example 50 kHz. In practice, laser energy 624 is delivered in at least two pulses: a pre pulse with limited energy is delivered to the droplet before it reaches the plasma location, in order to vaporize the fuel material into a small cloud, and then a main pulse of laser energy 624 is delivered to the cloud at the desired location, to generate the plasma 2. A trap 630 is provided on the opposite side of the enclosing structure 620, to capture fuel that is not, for whatever reason, turned into plasma.

[0105] The droplet generator 626 comprises a reservoir 601 which contains the fuel liquid (for example, molten tin) and a filter 669 and a nozzle 602. The nozzle 602 is configured to eject droplets of the fuel liquid towards the plasma 2 formation location. The droplets of fuel liquid may be ejected from the nozzle 602 by a combination of pressure within the reservoir 601 and a vibration applied to the nozzle by a piezoelectric actuator (not shown).

[0106] As the skilled reader will know, reference axes X, Y, and Z may be defined for measuring and describing the geometry and behavior of the apparatus, its various components, and the radiation beams 20, 21, 26. At each part of the apparatus, a local reference frame of X, Y and Z axes may be defined. In the example of FIG. 6, the Z axis broadly coincides with the direction optical axis O at a given point in the system, and is generally normal to the plane of a patterning device (reticle) MA and normal to the plane of substrate W. In the source collector module, the X axis coincides broadly with the direction of fuel stream 628, while the Y axis is orthogonal to that, pointing out of the page as indicated in Figure 6. On the other hand, in the vicinity of the support structure MT that holds the reticle MA, the X axis is generally transverse to a scanning direction aligned with the Y axis. For convenience, in this area of the schematic diagram FIG. 6, the X axis points out of the page, again as marked. These designations are conventional in the art and will be adopted herein for convenience. In principle, any reference frame can be chosen to describe the apparatus and its behavior.

[0107] Numerous additional components used in the operation of the source collector module and the lithographic apparatus 500 as a whole are present in a typical apparatus, though not illustrated here. These include arrangements for reducing or mitigating the effects of contamination within the enclosed vacuum, for example to prevent deposits of fuel material damaging or impairing the performance of collector 3 and other optics. Other features present but not described in detail are all the sensors, controllers and actuators involved in controlling of the various components and sub-systems of the lithographic apparatus 500.

[0108] Referring to FIG. 7, an implementation of an LPP EUV light source 700 is shown.

The light source 700 may be used as the source collector module SO in the lithographic apparatus 500. Furthermore, the optical pulse generating system 104 of FIG. 1 may be part of the drive laser 715. The drive laser 715 may be used as the laser 623 (FIG. 6).

[0109] The LPP EUV light source 700 is formed by irradiating a target mixture 714 at a plasma formation location 705 with an amplified light beam 710 that travels along a beam path toward the target mixture 714. The target material discussed with respect to FIGS. 1,

2 A, 2B, and 3, and the targets in the stream 121 discussed with respect to FIG. 1 may be or include the target mixture 714. The plasma formation location 705 is within an interior 707 of a vacuum chamber 730. When the amplified light beam 710 strikes the target mixture 714, a target material within the target mixture 714 is converted into a plasma state that has an element with an emission line in the EUV range. The created plasma has certain

characteristics that depend on the composition of the target material within the target mixture 714. These characteristics may include the wavelength of the EUV light produced by the plasma and the type and amount of debris released from the plasma.

[0110] The light source 700 also includes the supply system 725 that delivers, controls, and directs the target mixture 714 in the form of liquid droplets, a liquid stream, solid particles or clusters, solid particles contained within liquid droplets or solid particles contained within a liquid stream. The target mixture 714 includes the target material such as, for example, water, tin, lithium, xenon, or any material that, when converted to a plasma state, has an emission line in the EUV range. For example, the element tin may be used as pure tin (Sn); as a tin compound, for example, SnBr ₄ , SnBn, SnH ₄ ; as a tin alloy, for example, tin-gallium alloys, tin-indium alloys, tin-indium-gallium alloys, or any combination of these alloys. The target mixture 714 may also include impurities such as non-target particles. Thus, in the situation in which there are no impurities, the target mixture 714 is made up of only the target material. The target mixture 714 is delivered by the supply system 725 into the interior 707 of the chamber 730 and to the plasma formation location 705.

[0111] The light source 700 includes a drive laser system 715 that produces the amplified light beam 710 due to a population inversion within the gain medium or mediums of the laser system 715. The light source 700 includes a beam delivery system between the laser system 715 and the plasma formation location 705, the beam delivery system including a beam transport system 720 and a focus assembly 722. The beam transport system 720 receives the amplified light beam 710 from the laser system 715, and steers and modifies the amplified light beam 710 as needed and outputs the amplified light beam 710 to the focus assembly 722. The focus assembly 722 receives the amplified light beam 710 and focuses the beam 710 to the plasma formation location 705.

[0112] In some implementations, the laser system 715 may include one or more optical amplifiers, lasers, and/or lamps for providing one or more main pulses and, in some cases, one or more pre -pulses. In implementations that include one or more pre-pulses, an optical pulse generating system such as the optical pulse generating system 104 may be placed in the path of one or more of the pre-pulses. Each optical amplifier includes a gain medium capable of optically amplifying the desired wavelength at a high gain, an excitation source, and internal optics. The optical amplifier may or may not have laser mirrors or other feedback devices that form a laser cavity. Thus, the laser system 715 produces an amplified light beam 710 due to the population inversion in the gain media of the laser amplifiers even if there is no laser cavity. Moreover, the laser system 715 may produce an amplified light beam 710 that is a coherent laser beam if there is a laser cavity to provide enough feedback to the laser system 715. The term“amplified light beam” encompasses one or more of: light from the laser system 715 that is merely amplified but not necessarily a coherent laser oscillation and light from the laser system 715 that is amplified and is also a coherent laser oscillation. [0113] The optical amplifiers in the laser system 715 may include as a gain medium a filling gas that includes C0 ₂ and may amplify light at a wavelength of between about 9100 and about 11000 nm, and in particular, at about 10600 nm, at a gain greater than or equal to 800 times. Suitable amplifiers and lasers for use in the laser system 715 may include a pulsed laser device, for example, a pulsed, gas-discharge CO2 laser device producing radiation at about 9300 nm or about 10600 nm, for example, with DC or RF excitation, operating at relatively high power, for example, lOkW or higher and high pulse repetition rate, for example, 40 kHz or more. The pulse repetition rate may be, for example, 50 kHz. The optical amplifiers in the laser system 715 may also include a cooling system such as water that may be used when operating the laser system 715 at higher powers.

[0114] The light source 700 includes a collector mirror 735 having an aperture 740 to allow the amplified light beam 710 to pass through and reach the plasma formation location 705. The collector mirror 735 may be, for example, an ellipsoidal mirror that has a primary focus at the plasma formation location 705 and a secondary focus at an intermediate location 745 (also called an intermediate focus) where the EUV light may be output from the light source 700 and may be input to, for example, an integrated circuit lithography tool (not shown). The light source 700 may also include an open-ended, hollow conical shroud 750 (for example, a gas cone) that tapers toward the plasma formation location 705 from the collector mirror 735 to reduce the amount of plasma-generated debris that enters the focus assembly 722 and/or the beam transport system 720 while allowing the amplified light beam 710 to reach the plasma formation location 705. For this purpose, a gas flow may be provided in the shroud that is directed toward the plasma formation location 705.

[0115] The light source 700 may also include a master controller 755 that is connected to a droplet position detection feedback system 756, a laser control system 757, and a beam control system 758. The light source 700 may include one or more target or droplet imagers 760 that provide an output indicative of the position of a droplet, for example, relative to the plasma formation location 705 and provide this output to the droplet position detection feedback system 756, which may, for example, compute a droplet position and trajectory from which a droplet position error may be computed either on a droplet by droplet basis or on average. The droplet position detection feedback system 756 thus provides the droplet position error as an input to the master controller 755. The master controller 755 may therefore provide a laser position, direction, and timing correction signal, for example, to the laser control system 757 that may be used, for example, to control the laser timing circuit and/or to the beam control system 758 to control an amplified light beam position and shaping of the beam transport system 720 to change the location and/or focal power of the beam focal spot within the chamber 730.

[0116] The supply system 725 includes a target material delivery control system 726 that is operable, in response to a signal from the master controller 755, for example, to modify the release point of the droplets as released by a target material supply apparatus 727 to correct for errors in the droplets arriving at the desired plasma formation location 705. The target material supply apparatus 727 includes a target formation apparatus that employs an adhesive such as the adhesive 234.

[0117] Additionally, the light source 700 may include light source detectors 765 and 770 that measures 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 of a particular band of wavelengths, and angular distribution of EUV intensity and/or average power. The light source detector 765 generates a feedback signal for use by the master controller 755. The feedback signal may be, for example, indicative of the errors in parameters such as the timing and focus of the laser pulses to properly intercept the droplets in the right place and time for effective and efficient EUV light production.

[0118] The light source 700 may also include a guide laser 775 that may be used to align various sections of the light source 700 or to assist in steering the amplified light beam 710 to the plasma formation location 705. In connection with the guide laser 775, the light source 700 includes a metrology system 724 that is placed within the focus assembly 722 to sample a portion of light from the guide laser 775 and the amplified light beam 710. In other implementations, the metrology system 724 is placed within the beam transport system 720. The metrology system 724 may include an optical element that samples or re-directs a subset of the light, such optical element being made out of any material that may withstand the powers of the guide laser beam and the amplified light beam 710. A beam analysis system is formed from the metrology system 724 and the master controller 755 since the master controller 755 analyzes the sampled light from the guide laser 775 and uses this information to adjust components within the focus assembly 722 through the beam control system 758.

[0119] Thus, in summary, the light source 700 produces an amplified light beam 710 that is directed along the beam path to irradiate the target mixture 714 at the plasma formation location 705 to convert the target material within the mixture 714 into plasma that emits light in the EUV range. The amplified light beam 710 operates at a particular wavelength (that is also referred to as a drive laser wavelength) that is determined based on the design and properties of the laser system 715. Additionally, the amplified light beam 710 may be a laser beam when the target material provides enough feedback back into the laser system 715 to produce coherent laser light or if the drive laser system 715 includes suitable optical feedback to form a laser cavity.

[0120] The implementations may further be described by the following clauses:

1. An apparatus for an extreme ultraviolet (EUV) light source, the apparatus comprising: an optical modulation system comprising an electro-optic material, the optical modulation system configured to receive a pulsed light beam that comprises a plurality of pulses of light separated from each other in time; and

a control system configured to control an electrical source such that a first electrical pulse is applied to the electro-optic material while a first pulse of light is incident on the electro-optic modulator, a second electrical pulse is applied to the electro-optic material while a second pulse of light is incident on the electro-optic material, and an intermediate electrical pulse is applied to the electro-optic material after the first pulse of light is incident on the electro optic material and before the second pulse of light is incident on the electro-optic material.

2. The apparatus of clause 1 , wherein applying the first electrical pulse to the electro optic material causes a physical effect in the electro-optic material, and the physical effect is present in the electro-optic material when the intermediate electrical pulse is applied to the electro-optic material.

3. The apparatus of clause 2, wherein the physical effect comprises acoustic waves that travel in the electro-optic material and/or mechanical strain.

4. The apparatus of clause 2, wherein applying the intermediate electrical pulse to the electro-optic material reduces the physical effect.

5. The apparatus of clause 1, wherein the first pulse of light and the second pulse of light are consecutive pulses of light in the pulsed light beam.

6. The apparatus of clause 1, wherein the control system is configured to control an amount of time between the first electrical pulse and the intermediate electrical pulse.

7. The apparatus of clause 1, wherein the electro-optic material comprises a

semiconductor.

8. The apparatus of clause 1, wherein the electro-optic material comprises an insulator. 9. The apparatus of clause 1, wherein the electro-optic material comprises an electro optic crystal.

10. The apparatus of clause 1, further comprising at least one polarization-based optical element.

11. The apparatus of clause 1, wherein the intermediate electrical pulse creates an acoustic disturbance that interferes with an acoustic disturbance caused by the first electrical pulse.

12. An apparatus for forming optical pulses, the apparatus comprising:

an optical modulation system comprising an electro-optic material, the optical modulation system configured to transmit light in an ON state and to block light in an OFF state, and the optical modulation system configured to receive a pulsed light beam that comprises at least a first light pulse and a second light pulse separated from each other in time; and

a control system coupled to a voltage source, the control system configured to:

generate a first formed optical pulse by causing the voltage source to apply a first voltage pulse to the electro-optic modulator while the first light pulse is incident on the electro-optic modulator, the first voltage pulse being configured to switch the electro-optic modulator into the ON state;

apply an intermediate voltage pulse to the electro-optic material; and

generate a second formed optical pulse by applying a second voltage pulse to the electro optic material after applying the first voltage pulse and the intermediate voltage pulse and while the second light pulse is incident on the electro-optic material, wherein the second voltage pulse is configured to switch the electro-optic modulator into the ON state, and a property of the second formed optical pulse is controlled by the application of the intermediate voltage pulse to the electro-optic material.

13. The apparatus of clause 12, wherein the second formed optical pulse comprises a pedestal portion and a main portion, and the property of the second formed optical pulse comprises a property of the pedestal such that a property of the pedestal portion is controlled by the application of the intermediate voltage pulse to the electro-optic material.

14. The apparatus of clause 13, wherein the pedestal portion and the main portion are temporally contiguous.

15. The apparatus of clause 13, wherein the property of the pedestal portion comprises a temporal duration, a maximum intensity, and/or an average intensity of the pedestal portion. 16. The apparatus of clause 12, wherein the application of the intermediate voltage pulse to the electro-optic material modifies an amount of optical leakage light transmitted by the optical modulation system in the OFF state.

17. The apparatus of clause 16, wherein the application of the intermediate voltage pulse to the electro-optic material reduces optical leakage light transmitted by the optical modulation system in the OFF state.

18. The apparatus of clause 12, wherein

the control system causes the first voltage pulse to be applied to the electro-optic material at a first time,

the control system causes the intermediate voltage pulse to be applied to the electro-optic material at a second time that is after the first time,

the second time and the first time are separated in time by a delay time, and

the control system is further configured to adjust the delay time to thereby control a property of the second formed optical pulse.

19. The apparatus of clause 18, wherein control system is further configured to control at least one of an amplitude, a temporal duration, and a phase of the intermediate voltage pulse.

20. The apparatus of clause 13, wherein the control system is further configured to: receive an indication of a measured property of the pedestal portion, and

adjust a property of the intermediate voltage pulse based on the received indication.

21. The apparatus of clause 12, wherein the control system is further configured to: receive an indication of an amount of extreme ultraviolet (EUV) light produced by a plasma, and

adjust a property of the intermediate voltage pulse based on the received indication of the amount of EUV light.

22. The apparatus of clause 21, wherein the control system being configured to adjust a property of the intermediate voltage pulse comprises the control system being configured to adjust an amplitude of the intermediate voltage pulse, a temporal duration of the intermediate voltage pulse, a phase of the intermediate voltage pulse, and/or a second time, the second time being a time at which the intermediate voltage pulse is applied to the electro-optic material.

23. A method of adjusting a property of a pedestal of an optical pulse, the method comprising: forming a first optical pulse by applying a first voltage pulse to an electro-optic material of an optical modulation system while light is incident on the optical modulation system, the first optical pulse comprising a first pedestal portion and a first main portion;

applying an intermediate voltage pulse to the electro-optic material after applying the first voltage pulse; and

forming a second optical pulse by applying a second voltage pulse to the electro-optic material after the first voltage pulse and the intermediate voltage pulse and while light is incident on the electro-optic material, wherein a property of the second pedestal portion is adjusted based on the application of the intermediate voltage pulse.

24. The method of clause 23, further comprising:

amplifying the first optical pulse to form an amplified first optical pulse;

receiving an indication of an amount of extreme ultraviolet (EUV) light emitted from a plasma produced by interacting the amplified first optical pulse with target material; and determining at least one property of the intermediate voltage pulse based on the received indication of the amount of EUV light emitted from the plasma.

25. The method of clause 24, wherein the at least one property of the intermediate voltage pulse comprises a time delay after the application of the first voltage pulse, and determining at least one property of the intermediate voltage pulse comprises determining the time delay based on the received indication of the amount of EUV light emitted from the plasma.

26. The method of clause 24, wherein the at least one property of the intermediate voltage pulse comprises an amplitude and/or a duration of the intermediate voltage pulse, and determining at least one property of the intermediate voltage pulse comprises determining the amplitude and/or the duration of the intermediate voltage pulse.

27. The method of clause 23, wherein the second optical pulse comprises a pedestal portion and a main portion, and a property of the pedestal portion is adjusted based on the application of the intermediate voltage pulse.

28. The method of clause 27, wherein the pedestal portion is temporally contiguous with the main portion.

29. An extreme ultraviolet (EUV) light source comprising:

a vessel;

a target material supply apparatus configured to be coupled to the vessel;

an optical modulation system configured to be positioned to receive a pulsed light beam, the optical modulation system comprising an electro-optic material; and a control system coupled to a voltage source, the control system configured to:

cause the voltage source to apply a plurality of formation voltage pulses to the electro-optic material, each of the plurality of formation voltage pulses being applied to the electro-optic material at a different time, and

cause the voltage source to apply at least one intermediate voltage pulse to the electro-optic material, the at least one intermediate voltage pulse being applied to the electro-optic material between two consecutive formation voltage pulses among the plurality of formation voltage pulses.

30. The EUV light source of clause 29, wherein the target material supply apparatus is configured to provide a plurality of target material droplets to a target region in the vessel, the target material droplets arriving at the target region at a target delivery rate, and the control system applies the formation voltage pulses to the electro-optic material at a formation rate that depends on the target delivery rate.

31. The EUV light source of clause 28, wherein the characteristics of the intermediate voltage pulse comprise an amplitude and/or a phase, and

the control system is further configured to:

access an amplitude and/or a phase stored in association with the formation rate, and cause the voltage source to produce the intermediate voltage pulse with the accessed amplitude and/or phase.

32. The EUV light source of clause 29, wherein the control system is further configured to control a time delay between an application of one of the formation voltage pulses and one of the intermediate voltage pulses.

33. The EUV light source of clause 29, further comprising an optical amplifier, and wherein

an optical pulse is formed each time a formation voltage pulse is applied to the electro-optic material;

the formed optical pulse is amplified by the optical amplifier to form an amplified optical pulse;

the control system is further configured to couple to a metrology system that is configured to measure an amount of EUV light produced by a plasma in the vessel,

the plasma is formed by irradiating the target material with the formed amplified optical pulse, the control system is configured to receive the measured amount of EUV light from the metrology system; and

the control system is configured to modify one or more characteristics of the intermediate voltage pulse based on the measured amount of EUV light.

34. The EUV light source of clause 33, wherein the one or more characteristics of the intermediate voltage pulse comprise an amplitude of the intermediate voltage pulse, a temporal duration of the intermediate voltage pulse, a phase of the intermediate voltage pulse, and/or a delay time after application of a most recent formation voltage pulse.

[0121] Other implementations are within the scope of the claims.