Control of optical modulators

Abstract

A device for an extreme ultraviolet (EUV) light source, comprising an optical modulation system, the optical modulation system including an electro-optical material, the optical modulation system being configured to receive a pulsed light beam, the pulsed light beam including a plurality of light pulses separated from each other in time; and a control system, the control system being configured to control a power supply so that a first electric pulse is applied to the electro-optical material while a first light pulse is incident on the electro-optical material, a second electric pulse is applied to the electro-optical material while a second light pulse is incident on the electro-optical modulator, and an intermediate electric pulse is applied to the electro-optical material after the first light pulse is incident on the electro-optical material and before the second light pulse is incident on the electro-optical material.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. Application No. 62 / 747,518, filed on October 18, 2018, entitled “Control of an Optical Modulator,” the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present disclosure relates to the control of optical modulators. For example, optical leakage in an optical modulator can be controlled. The optical modulator can be part of an extreme ultraviolet (EUV) light source and / or a lithography system. Background Art

[0004] Extreme ultraviolet (“EUV”) light (e.g., electromagnetic radiation having a wavelength of 100 nanometers (nm) or less (sometimes also referred to as soft X-rays) and including light having a wavelength of, for example, 20 nm or less, between 5 nm and 20 nm, or between 13 nm and 14 nm) can be used in photolithography processes to create extremely small features in a substrate of, for example, a silicon wafer by inducing polymerization in a resist layer.

[0005] Methods of generating EUV light include, but are not limited to, converting a material including an element such as xenon, lithium, or tin that has emission lines in the EUV range in a plasma state. In one such method, often referred to as laser produced plasma ("LPP"), the desired plasma may be generated by irradiating a target material in the form of, for example, a droplet, sheet, ribbon, stream, or cluster of material with an amplified beam of light, which may be referred to as a drive laser. For this process, the plasma is typically generated in a sealed container (e.g., a vacuum chamber) and monitored using various types of metrology equipment. Summary of the invention

[0006] In one general aspect, an apparatus for an extreme ultraviolet (EUV) light source includes an optical modulation system including an electro-optical material, the optical modulation system being configured to receive a pulsed light beam including a plurality of light pulses separated in time from one another; and a control system configured to control a power supply so that: a first electric pulse is applied to the electro-optical material while a first light pulse is incident on the electro-optical material, a second electric pulse is applied to the electro-optical material while a second light pulse is incident on the electro-optical material, and an intermediate electric pulse is applied to the electro-optical material after the first light pulse is incident on the electro-optical material and before the second light pulse is incident on the electro-optical material.

[0007] Implementations may include one or more of the following features. A first electrical pulse applied to the electro-optic material may induce 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 and / or mechanical strain traveling in the electro-optic material. Applying the intermediate electrical pulse to the electro-optic material may reduce the physical effect.

[0008] The first light pulse and the second light pulse may be consecutive light pulses in a pulsed light beam.

[0009] The control system may be configured to control the amount of time between the first electrical pulse and the intermediate electrical pulse.

[0010] The electro-optic material may include a semiconductor.

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

[0012] The electro-optic material may comprise an electro-optic crystal.

[0013] The apparatus may also include at least one polarization-based optical element.

[0014] In another general aspect, an apparatus for forming an optical pulse includes an optical modulation system including an electro-optic material, the optical modulation system being configured to transmit light in an on state and block light in an off state; and the modulation system being configured to receive a pulsed light beam including at least a first light pulse and a second light pulse separated in time from each other. A control system may be coupled to a voltage source, the control system being 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 to an 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 to an on state, and a characteristic of the second formed optical pulse is controlled by applying the intermediate voltage pulse to the electro-optic material.

[0015] 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 a characteristic of the second formed optical pulse may include a characteristic of the pedestal, such that the characteristic of the pedestal portion is controlled by applying an intermediate voltage pulse to the electro-optical material. The pedestal portion and the main portion may be continuous in time. The characteristic of the pedestal portion may be a duration, a maximum intensity, and / or an average intensity of the pedestal portion.

[0016] Applying an intermediate voltage pulse to the electro-optic material may modify the amount of optical leakage light transmitted by the optical modulation system in the off state. Applying an 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.

[0017] In some embodiments, the control system causes a first voltage pulse to be applied to the electro-optical material at a first time, the control system causes an intermediate voltage pulse to be applied to the electro-optical material at a second time after the first time, the second time and the first time are separated in time by a delay time, and the control system is also configured to adjust the delay time to control the characteristics of the second formed optical pulse.

[0018] The control system may also be configured to control the amplitude, duration and / or phase of the intermediate voltage pulses.

[0019] The control system may be further configured to receive an indication of a measured characteristic of the base portion and to adjust a characteristic of the intermediate voltage pulse based on the received indication.

[0020] The control system may also be configured to receive an indication of an amount of extreme ultraviolet (EUV) light generated by the plasma, and adjust a characteristic of the intermediate voltage pulse based on the indication of the amount of EUV light received. The control system being configured to adjust the characteristic of the intermediate voltage pulse may include the control system being configured to adjust an amplitude of the intermediate voltage pulse, a duration of the intermediate voltage pulse, a phase of the intermediate voltage pulse, and / or a second time, the second time being a time of applying the intermediate voltage pulse to the electro-optical material.

[0021] In another general aspect, a method of adjusting a characteristic of an optical pulse includes: forming a first light 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 light 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. The characteristic of the optical pulse is adjusted based on the application of the intermediate voltage pulse.

[0022] 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 created by interacting the amplified first optical pulse with a target material may be received; and at least one characteristic 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 characteristic of the intermediate voltage pulse may include a time delay after applying the first voltage pulse, and determining the at least one characteristic 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 characteristic of the intermediate voltage pulse may include an amplitude and / or a duration of the intermediate voltage pulse, and determining the at least one characteristic of the intermediate voltage pulse may include determining the amplitude and / or the duration of the intermediate voltage pulse.

[0023] In some implementations, the second optical pulse includes a pedestal portion and a main portion, and a characteristic of the pedestal portion is adjusted based on applying the intermediate voltage pulse.The pedestal portion can be continuous in time with the main portion.

[0024] In another general aspect, an extreme ultraviolet (EUV) light source includes a container; a target material supply device configured to be coupled to the container; an optical modulation system configured to be positioned to receive a pulsed light beam, the optical modulation system including an electro-optical material; and a control system, the coupling system being coupled to a voltage source, the control system being configured to: cause the voltage source to apply a plurality of forming voltage pulses to the electro-optical material, each of the plurality of forming voltage pulses being applied to the electro-optical material at a different time; and cause the voltage source to apply at least one intermediate voltage pulse to the electro-optical material, the at least one intermediate voltage pulse being applied to the electro-optical material between two of the plurality of forming voltage pulses.

[0025] Implementations include one or more of the following features: The target material supply device can be configured to provide a plurality of target material droplets to a target area in the container, the target material droplets reaching the target area at a target delivery rate, and the control system applies a forming voltage pulse to the electro-optical material at a forming rate dependent on the target delivery rate.

[0026] The characteristics of the intermediate voltage pulses may include amplitude and / or phase, and the control system may be further configured to: access the amplitude and / or phase stored in association with the forming rate, and cause the voltage source to generate the intermediate voltage pulses having the accessed amplitude and / or phase. The control system may also be configured to control a time delay between applying one of the forming voltage pulses and applying one of the intermediate voltage pulses.

[0027] The EUV light source may further include an optical amplifier. Whenever a forming voltage pulse is applied to the electro-optical material, an optical pulse may be formed; the formed optical pulse may be amplified by the optical amplifier to form an amplified optical pulse; the control system may further be configured to couple to a metrology system configured to measure the amount of EUV light generated by a plasma in a container, the plasma may be formed by irradiating a 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 pulses based on the measured amount of EUV light. The one or more characteristics of the intermediate voltage pulses may include an amplitude of the intermediate voltage pulse, a duration of the intermediate voltage pulse, a phase of the intermediate voltage pulse, and / or a delay time after the application of the most recent forming voltage pulse.

[0028] Implementations of any of the techniques described above may include EUV light sources, systems, methods, processes, devices, or apparatuses. Details of one or more implementations are set forth in the accompanying drawings and the following description. Other features will be apparent from the description and drawings and from the claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 is a block diagram of an example of an EUV lithography system.

[0030] Figure 2A is a block diagram of an example of a modulation system.

[0031] Figure 2B is a diagram of an example of an optical pulse.

[0032] Figure 2C Is Figure 2B Illustration of an example of a modified optical pulse formed by optical pulse formation.

[0033] Figure 3A is a graphical representation of an example of a beam over time.

[0034] Figure 3B is a graphical representation of an example of an electrical signal over time.

[0035] Figure 4 is a flow chart of an example of a process for controlling characteristics of an illuminating optical pulse.

[0036] Figure 5 and Figure 6 is a block diagram of an example of a lithographic apparatus.

[0037] Figure 7 is a block diagram of an example of an EUV light source. DETAILED DESCRIPTION

[0038] Techniques for controlling optical leakage of an electro-optic modulator are described. An 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 (e.g., a voltage pulse having a finite duration) is applied to the electro-optic material to change the refractive index of the electro-optic material so that the initial light beam is modulated and a modified optical pulse is formed. After applying the electrical signal, an intermediate electrical signal is applied to the electro-optic material. As discussed in more detail below, applying the intermediate electrical signal to the electro-optic material allows for controlling the optical leakage of the electro-optic modulator. The intermediate electrical signal mitigates, changes, or controls an acoustic wave generated by the electrical signal. Controlling the acoustic wave also controls the amount of optical leakage of the electro-optic modulator, thereby allowing for control of the characteristics or properties of a subsequently formed (or later formed) modified optical pulse.

[0039] refer to Figure 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 that provides EUV light 196 to a lithography apparatus 195. The lithography apparatus 195 uses the EUV light 196 to expose a wafer (e.g., a silicon wafer) to form electronic features on the wafer. The EUV light 196 is emitted from a plasma 197 that is formed by irradiating a target material in a target 118 using an irradiation optical pulse 108. The target material is any material that emits EUV light in a plasma state (e.g., tin).

[0040] The EUV light source 101 includes an optical pulse generation system 104 that generates an amplified optical pulse 108 from a modified optical pulse 107. The optical pulse generation system 104 includes a light source 105, which can be, for example, a pulsed (e.g., Q-switched) or continuous wave carbon dioxide (CO2) laser or a solid-state laser (e.g., a Nd:YAG laser or an erbium-doped fiber (Er:glass) laser). The light source 105 generates an optical beam 106 (or light beam 106), which can be a train of light pulses or a continuous light beam. The light source 105 emits the light beam 106 onto a path 111 to a modulation system 120 including an electro-optical material 122. The electro-optical material 122 is located on the path 111, and the light beam 106 is incident on the electro-optical material 122.

[0041] The modulation system 120 is an electro-optic modulator that modulates the light beam 106 based on the electro-optic effect. The electro-optic effect describes a change in the refractive index of the electro-optic material 122, which is generated by applying a direct current (DC) or low-frequency electric field 124 generated by a power supply 123. The power supply 123 can 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 light beam 106 is incident on the electro-optic material 122, the modulation system 120 modulates the phase, polarization or amplitude of the light beam 106 to form a pulse 107.

[0042] Electric field 124 can be used to control whether modulation system 120 transmits light. Electric field 124 can be used to control electro-optic material 122 so that only one or more portions of light beam 106 pass through electro-optic material 122. In this way, modulation system 120 forms pulse 107 from a portion of light beam 106.

[0043] The optical pulse generation 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 energy to the pulse 107, so that the pulse 107 is amplified into an amplified or illuminated optical pulse 108. The amount of amplification of the pulse 107 is determined by the gain of the amplifier 130 and the gain of the medium 132. The gain is the amount of increase or increase factor of energy that the amplifier 130 provides to the input light beam.

[0044] Pulse 108 propagates on path 111 to vacuum vessel 180 where it receives target 118. Pulse 108 and target 118 interact at target region 115 in vacuum vessel 180 and the interaction converts at least some of the target material in target 118 into plasma 197 that emits EUV light 196.

[0045] Applying an electric field 124 to the electro-optic material 122 generates acoustic waves in the material 122. The acoustic waves induce strain in the material 122, and the acoustic waves may persist even after the electric field 124 is no longer applied to the material 122 and / or after the characteristics of the electric field 124 have changed. The strain caused by the acoustic waves may change the refractive index of the material 122 even during time periods when the refractive index of the material 122 is not expected to change. These changes in the refractive index may cause optical leakage. Optical leakage is light that passes through the modulator 120 when the modulation system 120 is in a state where light should not pass through the modulation system 120. As discussed below, the electric field 124 includes components (e.g., pulses) that mitigate and / or control optical leakage by mitigating and / or controlling residual acoustic waves in the material 122.

[0046] The EUV light source 101 also includes a metrology system 182 positioned relative to the target area 115. The metrology system 182 includes one or more sensors 184 configured to sense EUV light 196. The metrology system 182 generates a representation of the amount of EUV light 196 in the vacuum chamber 180 (e.g., at the target area 115). The metrology system 182 provides data representing the measured amount of EUV light to the control system 175 via a communication link 183.

[0047] In some implementations, the metrology system 182 also includes an optical sensing system 185 that includes one or more optical sensors configured to measure characteristics of the pulse 107 and / or the amplified pulse 108. The optical sensing system 185 may include any sensor that is capable of detecting one or more wavelengths in the pulse 107 and / or the pulse 108 such that the sensing system 185 is able to determine characteristics of the pulse 107 and / or the amplified pulse 108 (e.g., characteristics of the pedestal portion). Figure 1 In the example of FIG. 1 , the optical sensing system 185 is part of the metrology system 182, however, the optical sensing system 185 can be separate from the metrology system 182. For example, the optical sensing system 185 can be positioned to receive samples of the optical pulses 107 between the modulation system 120 and the optical amplifier 130. The optical sensing system 185 can also provide data related to the measurement of the optical pulses 107 and / or pulses 108 to the control system 175. In some implementations, the control system 175 uses information from the EUV sensor 184 and / or the optical sensing system 185 to set and / or change parameters of the electric field 124.

[0048] In addition to receiving data from the metrology system 182, 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 the communication interface 176. For example, in some implementations, the control system 175 may provide a trigger signal 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 instances of the radiated optical pulse 108 and instances of the target 118 to determine which of many possible settings of a particular parameter or characteristic of the electric field 124 results in the generation of the greatest amount of EUV light. In yet another example, the control system 175 provides data and / or information to the power source 123, which generates the electric field 124. In this example, the data provided by the control system 175 determines various characteristics of the electric field 124, such as, for example, the amplitude and / or time delay between two pulses of the electric field 124.

[0049] Control system 175 includes electronic processor 177, electronic storage device 178, and input / output (I / O) interface 179. Electronic processor 177 includes one or more processors adapted to execute a computer program such as a general purpose processor or a special purpose microprocessor; and any one or more processors in any type of digital computer. Typically, the electronic processor receives instructions and data from a read-only memory, a random access memory, or both. Electronic processor 177 can be any type of electronic processor.

[0050] Electronic storage 178 may be volatile memory such as RAM or non-volatile memory. In some implementations, electronic storage 178 includes non-volatile portions or components and volatile portions or components. Electronic storage 178 may store data and information used to control the operation of system 175 and / or components of system 175.

[0051] Electronic storage 178 may also store instructions, possibly as a computer program, that, when executed, cause processor 177 to communicate with components in control system 175, modulation system 120, and / or light source 105. For example, in an implementation where source 105 is a pulsed source, the instructions may be instructions that cause electronic processor 177 to generate a signal that causes light source 105 to emit optical pulses.

[0052] The I / O interface 179 is any type of electronic interface that allows the control system 175 to receive and / or provide data and signals (which utilize 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 the following: a visual display, a keyboard, and a communication interface.

[0053] Figure 2A 2 is a block diagram of a modulation system 220. The modulation system 220 is a block diagram of a modulation system 120 ( Figure 1 ). The modulation system 220 includes an electro-optic material 122, which is Figure 2A In the implementation shown, the electrode 223a and the electrode 223b are located between the electrodes 223a and 223b. The electrodes 223a and 223b are controllable to form an electric field between the electrodes 223a and 223b. For example, the control system 175 can cause the voltage source 223 to provide a voltage signal 214 to the electrode 223b so that the electrode 223b remains at a voltage different from the voltage of the electrode 223a, thereby generating a potential difference (V) or an electric field across the electro-optical material 122.

[0054] 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 of electro-optic materials that may be placed on the beam path 111 as appropriate for the application. Each instance of the electro-optic material 122 also includes a corresponding electrode 223a, 223b that is controllable to apply an electric field to the electro-optic material 122. In an implementation that includes 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 as a group by the control system 175, or various electric fields may be individually controlled by corresponding instances of the control system 175.

[0055] The modulation system 220 also includes one or more polarization-based optical elements 224. Figure 2A In the example of FIG. 2 , 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 located on one side of the received light beam 106 of the modulation system 220. Further, 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 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.

[0056] The polarization-based optical element 224 is any optical element that interacts with light based on the polarization state of the light. For example, the polarization-based optical element 224 can be a linear polarizer that transmits horizontally polarized light and blocks vertically polarized light, or vice versa. The polarization-based optical element 224 can be a polarization beam splitter that transmits horizontally polarized light and reflects vertically polarized light. The polarization-based optical element 224 can be an optical element that absorbs all light except light with a specific polarization state. In some implementations, the polarization-based optical element 224 can include a quarter wave plate. At least one polarization-based optical element 224 is positioned to receive light that passes through the electro-optical material 122 and directs light of a specific polarization state onto the beam path 111.

[0057] 2 shows one electro-optic material 122 and one polarization-based optical element 224, more than one - one or both of these components - may be connected 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 connected in series on the beam path 111, wherein each of the electro-optic materials 122 is located between two of the three polarization-based optical elements 224.

[0058] Electro-optic material 122 can be any material that transmits one of the multiple wavelengths of light beam 106. For implementations in which light beam 106 includes light having a wavelength of 10.6 micrometers (μm), material 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, material 122 can be monopotassium phosphate (KDP), diammonium 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 transmit one or more wavelengths of light beam 106 and exhibit birefringence in response to the application of an external force can also be used as material 122. For example, quartz can be used as material 122.

[0059] The electro-optic material 122 also exhibits anisotropy. In a material that exhibits anisotropy, the material's properties, such as the refractive index, are spatially non-uniform. Therefore, the properties of the material 122 can be modified along one or more specific directions by applying a controllable external force, such as a potential difference (V). For example, the refractive index of different polarization components of light propagating through the material 122 can be controlled by applying an external force. Therefore, the polarization state of light passing through the material 122 can be controlled by controlling the potential difference (V) between the electrodes 223a, 223b.

[0060] Under ideal operation, the modulation system 220 transmits light only 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 condition 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 light beam 106 is vertically polarized when initially incident on the material 122, then the pulse 107 is formed only when the potential difference V applied to the material 122 changes the polarization state of the light beam 106 such that the light beam 106 becomes horizontally polarized before interacting with the polarization-based optical element 224.

[0061] The modulation system 220 is considered to be in the on state any time the modulation system 220 is controlled to intentionally transmit light. For example, when the applied potential difference V causes the polarization state of the light beam 106 to match 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 causes the polarization state of the light beam 106 to be expected to be orthogonal to the polarization-based optical element 224, the optical modulation system 220 is in the off state. Under ideal conditions, when the optical modulation system 220 is in the off state, the light beam 106 does not pass through the modulation system 120.

[0062] 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 generate strain in the material 122, which changes the optical properties of the material 122 and allows incident light to pass through the modulation system 220 (as optical leakage) even when the potential difference V is not applied. Therefore, in actual operation, the modulation system 220 may transmit stray 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 optical leakage is present just before the radiated optical pulse is formed, the optical leakage forms a pedestal portion on the radiated optical pulse.

[0063] Also refer to Figure 2B and Figure 2C , showing an optical pulse 206 ( Figure 2B ) and a modified optical pulse 207 ( Figure 2C ). Pulse 207 includes a base portion 225 and a main portion 268. Figure 2B shows the intensity of pulse 206 over time, and Figure 2C The intensity of pulse 207 is shown over time.

[0064] Pulse 206 has an approximately Gaussian temporal distribution (intensity versus time). Pulse 206 interacts with modulation system 220 to form pulse 207. Control system 175 controls modulation system 220 to select or extract a specific portion 267 of pulse 206. Figure 2BIn the example of FIG. 1 , at time t=ta, the modulation system 220 is set to transmit light, and at time t=tb, the modulation system 220 is set to block light. In other words, the optical modulation system 220 is intended to transmit light only in portion 267 (which is the light in pulse 206 between time ta and time tb). For example, the control system 175 can control the modulation system 220 to transmit light at time ta by applying a voltage signal 214 so that the polarization of the light passing through the electro-optic material 122 matches the polarization of the polarization-based optical element. By removing the voltage signal 214, the modulation system 220 can be controlled to stop transmitting light at time tb.

[0065] However, due to acoustic waves in the electro-optic material 122 (or other disturbances, such as undesired motion of the polarization-based optical element 224), the modulation system 220 may transmit optical leakage at a time before ta and / or at a time after tb. Figure 2B In the example of , the leakage light 266 is an optical leakage that occurs just before time ta. The leakage light 266 passes through the modulation module 120 just before the portion 267.

[0066] refer to Figure 2C , the leakage light 266 forms the base portion 225. In the example shown, the base portion 225 occurs during the window labeled 221, and the base portion 225 occurs earlier in time than the rest of the pulse 207. The portion of the optical pulse 207 that is not the base portion 225 is called the main portion 268. Both the base portion 225 and the main portion 268 are part of the optical pulse 207, and the base portion 225 is connected to the main portion 268 in time. In other words, there is no period of time without light between the base portion 225 and the main portion 268.

[0067] The pedestal portion 225 has a different temporal distribution (intensity versus 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 the main portion 268. Also, the shape of the pedestal portion 225 is different from the shape of the main portion 268. Further, the characteristics (e.g., intensity, temporal distribution, and / or duration) of the pedestal portion 225 are different from the characteristics of the early portion of the pulse formed without any optical leakage.

[0068] The modified pulse 207 is amplified by the amplifier 130 to form an amplified pulse 208, which propagates to the target area 115. The amplified pulse 208 includes a base portion 225 and a main portion 268, wherein the intensity of each portion 225, 268 of the amplified pulse 208 is greater than the intensity of the corresponding portion of the modified pulse 207. Figure 2CIn the example of , pedestal portion 225 occurs before main portion 268 and reaches target 118 before main portion 268. In some implementations, the intensity or energy of main portion 268 is sufficient to convert at least some of the target material in target 118 into a plasma that emits EUV light. Pedestal portion 225 does not have as much energy as main portion 268, and the energy may or may not be sufficient to convert the target material into a plasma. However, light in pedestal portion 225 may reflect off target 118, evaporate material from the surface of target 118, and / or break off portions of target 118. Pedestal portion 225 may interfere with the formation of a plasma by altering target 118 before main portion 268 reaches target 118, and / or cause undesirable reflections that propagate back on path 111.

[0069] On the other hand, the base portion 225 can adjust the target 118 so that its characteristics (e.g., density, shape, and / or size) are more conducive to plasma generation. In this way, it is desirable to control the amount of light in the base portion 225 by controlling the amount of optical leakage. The control system 175 uses the intermediate electrical signal applied to the electro-optic material 122 before forming the pulse 207 to control the amount of optical leakage (in this example, the leakage light 266).

[0070] about Figure 2B to Figure 2C The pulse 207 discussed is provided as an example of a modified optical pulse 207. The pulse 207 may have other forms. For example, the leakage light 266 may occur before time ta, so that the base portion 225 is separated from the main portion 268. In these implementations, there is a period of no light between the base portion 225 and the main portion 268. Further, the leakage light 266 may occur after time tb, so that the base portion 225 occurs after the main portion 268. In these implementations, the base portion 225 reaches the target area 115 after the main portion 268. In some implementations, the leakage light 266 occurs before time ta and after time tb, so that there is a base portion 225 on each side of the main portion 268.

[0071] Figure 3A is a plot of the intensity of the light beam 306 over time. Figure 3B is a plot of the voltage of the electrical signal 324 over time. Figure 3A and Figure 3B The same time scale is used in all cases. The electrical signal 324 is generated by a function generator that can be controlled by the control system 175 and applied to the electro-optic material 122 ( Figure 1 and Figure 2A ) of an electrical signal. Figure 2A) is discussed with respect to electrical signal 324. Light beam 306 is an example of an optical beam (or light beams) that may be incident on electro-optic material 122.

[0072] The light beam 306 includes two initial optical pulses of light, namely, a first initial light pulse 306_1 incident on the material 122, and a second initial light pulse 306_2. The first initial optical pulse 306_1 is incident on the material 122 before the second initial pulse 306_2 is incident on the material 122. The first optical pulse 306_1 and the second optical pulse 306_2 are separate optical pulses separated from each other in time. The light beam 306 may also include optical pulses other than the initial optical pulses 306_1 and 306_2.

[0073] The electrical signal 324 includes a first electrical pulse 325a_1, which is applied to the material 122 starting at time t1; and a second electrical pulse 325a_2, which is applied to the material 122 starting at time t2. The electrical pulses 325a_1, 325a_2 are voltage pulses having an amplitude of A volts for a limited duration 331_1, 331_2, respectively. Therefore, applying the electrical pulses 325a_1 and 325a_2 results in applying a voltage of A volts to the material 122 for a duration 331_1, 331_2, respectively.

[0074] Voltage A is a voltage sufficient to turn on modulation system 220. Therefore, light incident on material 122 is transmitted through optical modulation system 220 while electrical pulses 325a_1 and 325a_2 are applied to material 122. After electrical pulses 325a_1 and 325a_2 end, modulation system 220 returns to the off state.

[0075] Durations 331_1 and 331_2 may be the same or different. Figure 3B In an implementation of the present invention, the first electrical pulse 325a_1 and the second electrical pulse 325a_2 have the same voltage amplitude (A). However, in other implementations, the electrical pulses 325a_1 and 325a_2 have different voltage amplitudes, wherein the voltages of both electrical pulses 325a_1 and 325a_2 are sufficient to turn the modulation system 220 into an on state.

[0076] The electrical signal 324 also includes an intermediate electrical pulse 325b_1, which is applied to the material 122 at time ti. Time ti occurs after time t1 and before time t2. Time ti is separated from the end of the first electrical pulse 325a_1 by a delay time 330. The intermediate electrical pulse 325b_1 has an amplitude B within a duration or width 332.

[0077] At time 0, the modulation system 220 is in the off state. Applying the first electrical pulse 325a_1 at time t1 turns the modulation system 220 to the on state. At time t1, the first optical pulse 306_1 is incident on the material 122. The duration of the first electrical pulse 325a_1 is shorter than the duration of the first optical pulse 306_1. Therefore, only a portion of the first optical pulse 306_1 that is incident on the material 122 during the duration 331_1 passes through the material 122. The portion of the first optical pulse 306_1 that passes through the material forms a modified optical pulse (such as Figure 2C Modified optical pulse 207). Applying the first electrical pulse 325a_1 to material 122 generates acoustic waves in material 122. The acoustic waves induce strain and change the optical properties of material 122. Even after the first electrical signal 325a_1 ends and no voltage is applied to material 122, the acoustic waves continue to propagate in material 122. The acoustic waves induced by applying the first electrical signal 325a_1 may exist when the second optical pulse 306_2 is incident on material 122 but before the second electrical signal 325a_2 is applied to material 122. In these cases, light may pass through modulation system 220 even if modulation system 220 is in an off state. This light is optical leakage and forms a pedestal on optical pulses formed later by modulation system 220.

[0078] Figure 4 4 is a flow chart of an example of a process 400 for controlling characteristics of an irradiation optical pulse. The irradiation optical pulse may include a pedestal portion. Figure 1 )、Modulation system 220( Figure 2A ), beam 306( Figure 3A ) and electrical signal 324( Figure 3B ) is discussed with respect to process 400. However, process 400 may be performed by other EUV light sources, other light beams, other electrical signals, and / or other electro-optical modulation systems.

[0079] A first modified optical pulse is formed (410) using the modulation system 220. The first modified optical pulse is formed by causing the first optical pulse 306_1 to be incident on the material 122 at time t1 and applying the first voltage pulse 325a_1 to the material 122. At time t1, the first optical pulse 306_1 is incident on the material 122 and the modulation system 220 is in an on state. Therefore, a portion of the first optical pulse 306_1 starting at time t1 and ending at the duration 331_1 passes through the material 122 and becomes the first modified optical pulse. Additionally, applying the first voltage pulse 325a_1 causes an acoustic wave to propagate in the material 122. The acoustic wave is referred to as a first acoustic wave and may continue to propagate in the material 122 after the first voltage pulse 325a_1 has ended and the modulation system 220 is in an off state.

[0080] An intermediate voltage pulse 325b_1 is applied to material 122 (420). Applying the intermediate voltage 325b_1 to material 122 also causes an acoustic wave (referred to as a second acoustic wave) to propagate in material 122. The second acoustic wave interferes with the first acoustic wave. Constructive interference increases the amplitude of an acoustic wave, while destructive interference decreases the amplitude of an acoustic wave. The amplitude and / or duration 332 of the intermediate voltage pulse 325b_1 determines the amplitude of the second acoustic wave. The delay 330 determines the phase of the second acoustic wave relative to the first acoustic wave. Therefore, by controlling the delay 330 and / or the amplitude B, the nature of the interaction between the first acoustic wave and the second acoustic wave can be controlled. For example, when the second acoustic wave has the same amplitude and opposite phase as the first acoustic wave, the first acoustic wave and the second acoustic wave interfere so that no acoustic wave propagates in material 122 after the intermediate electrical pulse 325b_1 is applied.

[0081] A second modified optical pulse is formed (430) using modulation system 220. The second modified optical pulse is formed after applying intermediate voltage pulse 325b_1 to material 122. Second optical pulse 306_2 is incident on material 122. At time t2, second voltage pulse 325a_2 is applied to material 122, causing modulation system 220 to transition to an on state. A portion of second optical pulse 306_2 starting at time t2 and up to duration 331_2 is transmitted through material 122.

[0082] As discussed above, the first acoustic wave and the second acoustic wave interfere, and the characteristics of the acoustic waves in the material 122 depend on the nature of the interference. The acoustic waves induce strain in the material 122 and change the refractive index of the material 122, and these changes in the refractive index can allow light to pass through the modulation system 220 as optical leakage when the modulation system 220 is in a closed state. The intermediate voltage pulse 325b_1 is used to modify one or more characteristics of the second optical pulse 306_2 in a desired manner. For example, the intermediate voltage pulse 325b_1 can be used to modify the maximum intensity or average intensity, duration, and / or time distribution of the second optical pulse 306_2. In some implementations, the intermediate voltage pulse 325b_1 is used to control and / or form a base portion.

[0083] 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 time t2 or at a time after the voltage pulse 325a_2 is no longer applied (when the modulation system 220 is in the off state) may also 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 characteristics of the pedestal depend on the amount of optical leakage, which can be controlled by adjusting the acoustic waves in the material 122. The acoustic waves in the material 122 can be controlled, adjusted, or mitigated by applying the intermediate voltage pulse 325b_1 to the material 122. Thus, one or more characteristics of the pedestal portion are controlled or adjusted by applying the intermediate voltage pulse 325b_1. Moreover, the intermediate voltage pulse 325b_1 can be used to control one or more characteristics of the main portion of the optical pulse 306_2.

[0084] The intermediate voltage pulse 325b_1 can be used to control one or more characteristics of the second optical pulse 306_2 in other ways. For example, as discussed above, the pedestal portion can be separated in time from the main portion of the modified pulse. In these implementations, the intermediate voltage pulse 325b_1 can be used to modify the separated pedestal portion and / or main portion.

[0085] Furthermore, the light 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 the signal as received by the EUV sensor 184 ( Figure 1) is an indication of the amount of EUV light generated by the interaction between the modified optical pulse and the target material 118 measured. For example, the amount of EUV light measured is tracked through two or more interactions while changing the characteristics of the intermediate voltage pulse to determine the optimal setting of the intermediate voltage pulse. For each of several interactions, the delay 330 can be changed to determine which delay 330 produces the most EUV light. In another example, the amplitude B of the intermediate voltage pulse 325b_1 is changed to determine the amplitude B that produces the most EUV light.

[0086] In some implementations, a lookup table or database associating optimal values ​​for amplitude B, width 332, and / or delay 330 with the repetition rate of pulses in beam 306 is stored on electronic storage device 178 such that amplitude B and / or delay 330 can be changed if the repetition rate of beam 306 changes.

[0087] Figure 5 and Figure 6 Concerning lithographic apparatus in which modulation systems such as systems 120 and 22 may be used. Figure 5 is a block diagram of a lithographic apparatus 500 including a source collector module SO. The lithographic apparatus 500 includes:

[0088] • An illumination system (illuminator) IL configured to condition a radiation beam B (eg EUV radiation).

[0089] a support structure (e.g., mask table) MT constructed to support a patterning device (e.g., mask or reticle) MA and connected to a first positioner PM configured to accurately position the patterning device;

[0090] a substrate table (e.g., wafer stage) WT configured to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate; and

[0091] • A projection system (eg, a reflective projection system) PS configured to project the pattern imparted to the radiation beam B by the patterning device MA onto a target portion C of the substrate W (eg, comprising one or more dies).

[0092] 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.

[0093] The support structure supports the patterning device 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 the patterning device is held in a vacuum environment. The support structure can use mechanical, vacuum, electrostatic, or other clamping techniques to hold the patterning device. The support structure can, for example, be a frame or a stage, which can be fixed or movable, for example, as desired. The support structure can ensure that the patterning device is in a desired position, for example, relative to the projection system.

[0094] The term "patterning device" should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section so as to form a pattern in a target portion of a substrate. The pattern imparted to the radiation beam may correspond to a specific functional layer in a device produced in the target portion such as an integrated circuit.

[0095] The patterning device may be a transmissive patterning device or a reflective patterning device. 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 to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in the radiation beam that is reflected by the matrix of mirrors.

[0096] Depending on the exposure radiation being used or other factors such as the use of a vacuum, like the illumination system IL, the projection system PS may include various types of optical components, such as refractive components, reflective components, magnetic components, electromagnetic components, electrostatic components, or other types of optical components, or any combination thereof. It may be desirable to use a vacuum for EUV radiation, as other gases may absorb too much radiation. Therefore, a vacuum environment may be provided to the entire optical path with the aid of a vacuum wall and a vacuum pump.

[0097] exist Figure 5 and Figure 6 In the example of , the apparatus is a reflective apparatus (e.g. 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 a "multi-stage" machine, additional tables may be used in parallel, or preparatory steps may be carried out at one or more tables while one or more other tables are being used for exposure.

[0098] Reference Figure 5, the illuminator IL receives a beam of extreme ultraviolet radiation from a source collector module SO. Methods of generating EUV light include, but are not limited to, converting a material into a plasma state having at least one element (e.g., xenon, lithium, or tin) wherein one or more emission lines are in the EUV range. In one such method, often referred to as laser produced plasma ("LPP"), the desired plasma is generated by irradiating a fuel (such as droplets, streams, or clusters of a material having the desired line-emitting element) with a laser beam. The source collector module SO may be a laser ( Figure 5 The source collector module is a part of an EUV radiation system (not shown) for providing a laser beam for exciting the fuel. The resulting plasma emits output radiation, such as EUV radiation, which is collected using a radiation collector disposed in a source collector module. The laser and the source collector module can be separate entities, such as when a carbon dioxide (CO2) laser is used to provide a laser beam for fuel excitation.

[0099] In this case, the laser is not considered to form part of the lithographic apparatus, and the radiation beam is delivered from the laser to the source collector module by means of a beam delivery system, which includes, for example, suitable directional 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 (commonly referred to as a DPP source).

[0100] The illuminator IL may include an adjuster for adjusting the angular intensity distribution of the radiation beam. Typically, at least the outer radial extent and / or the inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator may be adjusted. In addition, the illuminator IL may include various other components, such as a faceted field and a faceted pupil mirror arrangement. The illuminator IL may be used to adjust the radiation beam to have a desired uniformity and intensity distribution in its cross section.

[0101] The radiation beam B is incident on a patterning device (e.g., mask) MA, which is held on a support structure (e.g., mask table) MT, and is patterned by the patterning device. After reflection from the patterning device (e.g., mask) MA, the radiation beam B passes through a projection system PS, which focuses the beam onto a target portion C of a substrate W. With the aid of a second positioner PW and a position sensor PS2 (e.g., an interferometer device, a linear encoder, or a capacitive sensor), the substrate table WT can be precisely moved, for example, so as to position a different target portion C in the path of the radiation beam B. Similarly, a first positioner PM and a further position sensor PS1 can be used to precisely position the patterning device (e.g., mask) MA relative to the path of the radiation beam B. The patterning device (e.g., mask) MA and the substrate W can be aligned using patterning device alignment marks M1, M2 and substrate alignment marks P1, P2.

[0102] The depicted device can be used in at least one of the following modes:

[0103] 1. In step mode, the support structure (e.g. mask table) MT and the substrate table WT remain substantially stationary while an entire pattern imparted to the radiation beam is projected at one time onto a target portion C (that is, a single static exposure). The substrate table WT is then shifted in the X-direction and / or Y-direction so that a different target portion C can be exposed.

[0104] 2. In scan mode, the support structure (e.g. 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 (i.e. a single dynamic exposure). The speed and direction of the substrate table WT relative to the support structure (e.g. mask table) MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS.

[0105] 3. In another mode, the support structure (e.g. mask table) MT is kept substantially stationary, thereby holding the 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, a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or between successive radiation pulses during a scan. This mode of operation may be readily applicable to maskless lithography utilizing a programmable patterning device, such as a programmable mirror array of the type mentioned above.

[0106] Combinations and / or variations of the above-described modes of use or entirely different modes of use may also be employed.

[0107] Figure 6The implementation of the lithographic apparatus 500 is shown in more detail, and the lithographic apparatus 500 includes a source collector module SO, an illumination system IL, and a projection system PS. The source collector module SO is constructed and arranged so that a vacuum environment can be maintained in the enclosing structure 620 of the source collector module SO. The systems IL and PS are also housed in their own vacuum environments. The EUV radiation of the emitting plasma 2 can be formed by a laser-generated LPP plasma source. The function of the source collector module SO is to transfer the EUV radiation beam 20 from the plasma 2 so that it is focused in a virtual source point. The virtual source point is usually referred to as an intermediate focus (IF), and the source collector module is arranged so that the intermediate focus IF is located at or near the hole 621 in the enclosing structure 620. The virtual source point IF is an image of the radiation of the emitting plasma 2.

[0108] From the aperture 621 at the intermediate focus IF, the radiation traverses an illumination system IL which, in this example, comprises a faceted field mirror arrangement 22 and a faceted pupil mirror arrangement 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 and a desired uniformity of radiation intensity at the patterning device MA (as indicated by reference numeral 660). The beam 21 forms a patterned beam 26 after reflection at the patterning device MA held by the support structure (mask table) MT, and the projection system PS images the patterned beam 26 via reflective elements 28, 30 onto a substrate W held by a substrate table WT. In order to expose a target portion C on the substrate W, radiation pulses are generated while the substrate table WT and the patterning device table MT perform synchronized movements to scan a pattern on the patterning device MA through an illumination slit.

[0109] Each system IL and PS is arranged in its own vacuum environment or near-vacuum environment, which is defined by an enclosing structure similar to enclosing structure 620. There may generally be more elements in illumination system IL and projection system PS than those shown. Further, there may be more mirrors than those shown. For example, in addition to Figure 6 In addition to the reflective elements shown, one to six additional reflective elements may be present in the illumination system IL and / or the projection system PS.

[0110] Considering the 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 including a target material. The target material can be any material that emits EUV radiation in a plasma state, such as xenon (Xe), tin (Sn) or lithium (Li). Plasma 2 is a highly ionized plasma with an electron temperature of tens of electron volts (eV). Higher energy EUV radiation may be generated by other fuel materials (e.g., terbium (Tb) and gadolinium (Gd). The energy radiation generated during the de-excitation and recombination of these ions is emitted from the plasma, collected by the near normal incidence collector 3 and focused on the aperture 621. Plasma 2 and aperture 621 are located at the first and second focal points of collector CO, respectively.

[0111] although Figure 6 The collector 3 shown is a single curved mirror, but the collector may take other forms. For example, the collector may be a Schwarzschild collector having two radiation collecting surfaces. In one embodiment, the collector may be a grazing incidence collector comprising a plurality of substantially cylindrical reflectors nested within each other.

[0112] To deliver a fuel, such as liquid tin, a droplet generator 626 is disposed within the enclosure 620, the droplet generator 626 being arranged to eject a high frequency stream 628 of droplets toward a desired location of the plasma 2. The droplet generator 626 may be a target forming device 216 and / or include an adhesive such as an adhesive 234. In operation, laser energy 624 is delivered in synchronization with the operation of the droplet generator 626 to deliver a shock of radiation to convert each fuel droplet into a plasma 2. The delivery frequency of the droplets 2 may be several kilohertz, for example, 50 kHz. In practice, the laser energy 624 is delivered in at least two pulses: a pre-pulse with limited energy is delivered to the droplet before reaching 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 enclosure 620 to capture fuel that is not converted into plasma for whatever reason.

[0113] The droplet generator 626 includes an accumulator 601 containing a fuel liquid (e.g., molten tin) and a filter 669, and a nozzle 602. The nozzle 602 is configured to eject droplets of the fuel liquid toward a location where the plasma 2 is formed. The droplets of the fuel liquid may be ejected from the nozzle 602 by a combination of pressure within the accumulator 601 and vibration applied to the nozzle by a piezoelectric actuator (not shown).

[0114] As will be known to those skilled in the art, in order to measure and describe the geometry and behavior of the device, its various components and the radiation beams 20, 21 and 26, reference axes X, Y and Z are defined. At each part of the device, a local reference system of the X, Y, Z axes can be defined. Figure 6 In the example of FIG. 1 , the Z axis is broadly consistent with the directional optical axis O at a given point in the system and is approximately perpendicular to the plane of the patterning device (mask) MA and to the plane of the substrate W. In the source collector module, the X axis is broadly consistent with the direction of the fuel flow 628, and the Y axis is generally consistent with the direction of the fuel flow 628. Figure 6 On the other hand, near the support structure MT holding the mask MA, the X-axis is generally transverse to the scanning direction aligned with the Y-axis. For convenience, Figure 6 In this region of the schematic view of the X-axis points out of the paper, as also indicated by the marker. These designs are conventional in the art and will be adopted herein for convenience. In principle, any reference frame can be selected to describe the device and its behavior.

[0115] Although not illustrated herein, many additional components used in the operation of the lithographic apparatus 500 and source collector modules are integrally present for a typical apparatus. These include arrangements for reducing or mitigating the effects of contamination within the enclosed vacuum, for example, to prevent deposition of fuel material from damaging or impairing the performance of the collector 3 and other optical devices. Other features present but not described in detail are all sensors, controllers, and actuators involved in controlling the various components and subsystems of the lithographic apparatus 500.

[0116] refer to Figure 7 , shows an implementation of the LPP EUV light source 700. The light source 700 can be used as a source collector module SO in the lithography apparatus 500. Furthermore, Figure 1 The optical pulse generation system 104 can be part of the drive laser 715. The drive laser 715 can be used as the laser 623 ( Figure 6 ).

[0117] The LPP EUV light source 700 is formed by irradiating a target mixture 714 at a plasma formation location 705 using an amplified light beam 710 traveling along a beam path toward the target mixture 714. Figure 1 , Figure 2A , Figure 2B and Figure 3 discussed the target material and about Figure 1The target in the stream 121 in question may be or include a target mixture 714. The plasma formation location 705 is within the interior 707 of the vacuum chamber 730. When the amplified light beam 710 strikes the target mixture 714, the target material within the target mixture 714 is converted to a plasma state having elements with emission lines in the EUV range. The generated 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 EUV light generated by the plasma and the type and amount of debris released from the plasma.

[0118] The light source 700 also includes a supply system 725 that delivers, controls, and directs a target mixture 714 in the form of liquid droplets, liquid streams, solid particles or clusters, solid particles contained in liquid droplets, or solid particles contained in liquid streams. The target mixture 714 includes a target material, such as, for example, water, tin, lithium, xenon, or any material having an emission line in the EUV range 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; as a tin alloy, such as a tin-gallium alloy, a tin-indium alloy, a tin-indium-gallium alloy, or any combination of these alloys. The target mixture 714 may also include impurities, such as non-target particles. Therefore, in the absence of impurities, the target mixture 714 consists only of the target material. The target mixture 714 is delivered by the supply system 725 into the interior 707 of the chamber 730 and reaches the plasma formation position 705.

[0119] The light source 700 includes a drive laser system 715 that generates an amplified light beam 710 due to a population inversion within one or more gain media 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 transmission system 720 and a focusing assembly 722. The beam transmission 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 focusing assembly 722. The focusing assembly 722 receives the amplified light beam 710 and focuses the light beam 710 to the plasma formation location 705.

[0120] 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 generation system such as the optical pulse generation 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 high gain, an excitation source, and internal optical devices. The optical amplifier may or may not have a laser reflector or other feedback device that forms a laser cavity. Therefore, even if there is no laser cavity, the laser system 715 will produce an amplified light beam 710 due to the inversion of the number of particles in the gain medium of the laser amplifier. Moreover, if there is a laser cavity to provide sufficient feedback to the laser system 715, the laser system 715 can produce an amplified light beam 710 as a coherent laser beam. The term "amplified light beam" covers one or more of the following: light from the laser system 715 that is only amplified and not necessarily coherent laser oscillation and light from the laser system 715 that is amplified and is also coherent laser oscillation.

[0121] The optical amplifier in the laser system 715 may include a fill gas as a gain medium, the fill gas including CO2, and may amplify light at a wavelength between about 9100 nm and about 11000 nm (particularly at about 10600 nm) with a gain greater than or equal to 800 times. Suitable amplifiers and lasers for the laser system 715 may include pulsed laser devices, such as pulsed gas discharge CO2 laser devices, which operate at relatively high powers (e.g., 10 kW or more) and high pulse repetition rates (e.g., 40 kHz or more) by DC or RF excitation, producing radiation at about 9300 nm or about 10600 nm. The pulse repetition rate may be, for example, 50 kHz. The optical amplifier in the laser system 715 may also include a cooling system such as water, which may be used when the laser system 715 is operated at higher powers.

[0122] 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 elliptical mirror having a primary focus at the plasma formation location 705 and a secondary focus (also referred to as an intermediate focus) at an intermediate location 745, where EUV light may be output from the light source 700 and may be input into, for example, an integrated circuit lithography tool (not shown). The light source 700 may also include an open-ended hollow conical shield 750 (e.g., a gas cone) that tapers from the collector mirror 735 toward the plasma formation location 705 to reduce the amount of plasma-generated debris that enters the focusing assembly 722 and / or the beam delivery system 720 while allowing the amplified light beam 710 to reach the plasma formation location 705. To this end, a gas flow directed toward the plasma formation location 705 may be provided in the shield.

[0123] The light source 700 may also include a main controller 755 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 indicating the position of a droplet, for example, relative to the plasma formation location 705, and provide the output to the droplet position detection feedback system 756, which may, for example, calculate the droplet position and trajectory, from which the droplet position error may be calculated dropwise or on an average basis. Thus, the droplet position detection feedback system 756 provides the droplet position error as an input to the main controller 755. Thus, the main controller 755 can provide laser position, direction, and timing correction signals to, for example, a laser control system 757, which can be used, for example, to control a laser timing circuit, and / or a beam control system 758, which is used to control the position and shape of the amplified beam of the beam delivery system 720 to change the position and / or focusing capability of the beam focal spot within the chamber 730.

[0124] The supply system 725 includes a target material delivery control system 726 that is operable in response to signals from the main controller 755 to, for example, modify the release point of droplets as released by the target material supply 727 to correct for errors in droplets reaching the desired plasma formation location 705. The target material supply 727 includes a target formation device that employs an adhesive such as the adhesive 234.

[0125] Additionally, the light source 700 may include light source detectors 765 and 770 that measure one or more EUV light parameters including, but not limited to, pulse energy, distribution of energy with wavelength, energy within a particular wavelength band, energy outside a particular wavelength band, and angular distribution of EUV intensity and / or average power. The light source detector 765 generates a feedback signal for use by the main controller 755. The feedback signal may, for example, indicate errors in parameters such as timing and focusing of laser pulses to properly intercept droplets at the right place and time for effective and efficient production of EUV light.

[0126] The light source 700 may also include a guide laser 775, which may be used to align various segments of the light source 700 or to help steer the amplified light beam 710 to the plasma formation location 705. In conjunction with the guide laser 775, the light source 700 includes a metrology system 724 placed within the focusing assembly 722 to sample a portion of the light from the guide laser 775 and the amplified light beam 710. In other implementations, the metrology system 724 is placed within the beam delivery system 720. The metrology system 724 may include an optical element that samples or redirects a subset of the light, such optical element being made of any material that can withstand the power of the guide laser beam and the amplified light beam 710. Since the main controller 755 analyzes the sampled light from the guide laser 775 and uses this information to adjust components within the focusing assembly 722 through the beam control system 758, a beam analysis system is formed by the metrology system 724 and the main controller 755.

[0127] Thus, in summary, the light source 700 generates an amplified light beam 710 directed along a beam path to irradiate a target mixture 714 at a plasma formation location 705 to convert a target material within the mixture 714 into a plasma that emits light in the EUV range. The amplified light beam 710 operates at a specific wavelength (also referred to as a drive laser wavelength) determined based on the design and characteristics of the laser system 715. Additionally, the amplified light beam 710 can be a laser beam when the feedback provided by the target material is sufficient to return 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.

[0128] These implementations can also be described in the following terms:

[0129] 1. An apparatus for an extreme ultraviolet (EUV) light source, the apparatus comprising:

[0130] an optical modulation system comprising an electro-optic material, the optical modulation system being configured to receive a pulsed light beam comprising a plurality of light pulses separated in time from one another; and

[0131] A control system is configured to control a power supply so that: a first electric pulse is applied to the electro-optical material while a first light pulse is incident on the electro-optical material, a second electric pulse is applied to the electro-optical material while a second light pulse is incident on the electro-optical material, and an intermediate electric pulse is applied to the electro-optical material after the first light pulse is incident on the electro-optical material and before the second light pulse is incident on the electro-optical material.

[0132] 2. A device according to clause 1, wherein applying the first electrical pulse to the electro-optic material induces a physical effect in the electro-optic material, and the physical effect exists in the electro-optic material when the intermediate electrical pulse is applied to the electro-optic material.

[0133] 3. A device according to clause 2, wherein the physical effect comprises acoustic waves and / or mechanical strain travelling in the electro-optical material.

[0134] 4. A device according to clause 2, wherein applying an intermediate electrical pulse to the electro-optical material reduces the physical effect.

[0135] 5. The apparatus of clause 1, wherein the first light pulse and the second light pulse are consecutive light pulses in a pulsed light beam.

[0136] 6. An apparatus according to clause 1, wherein the control system is configured to control the amount of time between the first electrical pulse and the intermediate electrical pulse.

[0137] 7. A device according to clause 1, wherein the electro-optical material comprises a semiconductor.

[0138] 8. The device of clause 1, wherein the electro-optic material comprises an insulator.

[0139] 9. The device of clause 1, wherein the electro-optic material comprises an electro-optic crystal.

[0140] 10. The apparatus of clause 1, further comprising at least one polarization-based optical element.

[0141] 11. The device of clause 1, wherein the intermediate electrical pulse generates an acoustic disturbance that interferes with the acoustic disturbance caused by the first electrical pulse.

[0142] 12. An apparatus for forming an optical pulse, the apparatus comprising:

[0143] an optical modulation system comprising an electro-optic material, the optical modulation system being configured to transmit light in an on state and to block light in an off state, and the optical modulation system being configured to receive a pulsed light beam comprising at least a first optical pulse and a second optical pulse separated in time from each other; and

[0144] A control system is coupled to the voltage source, the control system being configured to:

[0145] generating a first formed optical pulse by causing a voltage source to apply a first voltage pulse to the electro-optic modulator while the first optical pulse is incident on the electro-optic modulator, the first voltage pulse being configured to switch the electro-optic modulator to an on state;

[0146] applying an intermediate voltage pulse to the electro-optic material; and

[0147] A second formed optical pulse is generated by applying a second voltage pulse to the electro-optical material after applying the first voltage pulse and the intermediate voltage pulse and while the second optical pulse is incident on the electro-optical material, wherein the second voltage pulse is configured to switch the electro-optic modulator to an on state, and the characteristics of the second formed optical pulse are controlled by applying the intermediate voltage pulse to the electro-optical material.

[0148] 13. An apparatus according to clause 12, wherein the second formed optical pulse comprises a pedestal portion and a main portion, and the characteristics of the second formed optical pulse comprise characteristics of the pedestal, such that the characteristics of the pedestal portion are controlled by applying an intermediate voltage pulse to the electro-optical material.

[0149] 14. An apparatus according to clause 13, wherein the base part and the main part are continuous in time.

[0150] 15. The device of clause 13, wherein the characteristics of the base portion include duration, maximum intensity and / or average intensity of the base portion.

[0151] 16. The apparatus of clause 12, wherein applying the intermediate voltage pulse to the electro-optic material modifies the amount of optically leaked light transmitted by the optical modulation system in an off state.

[0152] 17. The apparatus of clause 16, wherein applying the intermediate voltage pulse to the electro-optic material reduces optical leakage light transmitted by the optical modulation system in an off state.

[0153] 18. The device according to clause 12, wherein

[0154] The control system causes a first voltage pulse to be applied to the electro-optic material at a first time;

[0155] a control system causing the intermediate voltage pulse to be applied to the electro-optical material at a second time after the first time,

[0156] The second time is separated in time from the first time by a delay time; and

[0157] The control system is further configured to adjust the delay time to control characteristics of the second formed optical pulse.

[0158] 19. The apparatus of clause 18, wherein the control system is further configured to control at least one of the amplitude, duration and / or phase of the intermediate voltage pulses.

[0159] 20. The apparatus of clause 13, wherein the control system is further configured to:

[0160] receiving an indication of a measured characteristic of the base portion; and

[0161] A characteristic of the intermediate voltage pulse is adjusted based on the received indication.

[0162] 21. The apparatus of clause 12, wherein the control system is further configured to:

[0163] receiving an indication of an amount of extreme ultraviolet (EUV) light generated by the plasma; and

[0164] A characteristic of the intermediate voltage pulse is adjusted based on the indication of the amount of EUV light received.

[0165] 22. An apparatus according to claim 21, wherein the control system is configured to adjust the characteristics of the intermediate voltage pulse including: the control system is configured to adjust the amplitude of the intermediate voltage pulse, the duration of the intermediate voltage pulse, the phase of the intermediate voltage pulse, and / or a second time, the second time being the time when the intermediate voltage pulse is applied to the electro-optical material.

[0166] 23. A method of adjusting a characteristic of a pedestal of an optical pulse, the method comprising:

[0167] forming a first optical pulse by applying a first voltage pulse to an electro-optical material of the optical modulation system while light is incident on the optical modulation system; the first pulse portion comprising a first base portion and a first main portion;

[0168] applying an intermediate voltage pulse to the electro-optical material after applying the first voltage pulse; and

[0169] A second optical pulse is formed 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 characteristic of the second pedestal portion is adjusted based on applying the intermediate voltage pulse.

[0170] 24. The method according to clause 23, further comprising:

[0171] amplifying the first optical pulse to form an amplified first optical pulse;

[0172] receiving an indication of an amount of extreme ultraviolet (EUV) light emitted from a plasma created by interacting the amplified first optical pulse with a target material; and

[0173] At least one characteristic of the intermediate voltage pulse is determined based on the received indication of an amount of EUV light emitted from the plasma.

[0174] 25. The method of clause 24, wherein the at least one characteristic of the intermediate voltage pulse comprises a time delay after applying the first voltage pulse, and determining the at least one characteristic of the intermediate voltage pulse comprises determining the time delay based on a received indication of an amount of EUV light emitted from the plasma.

[0175] 26. The method of clause 24, wherein the at least one characteristic of the intermediate voltage pulse comprises an amplitude and / or a duration of the intermediate voltage pulse, and determining the at least one characteristic of the intermediate voltage pulse comprises determining the amplitude and / or the duration of the intermediate voltage pulse.

[0176] 27. The method of clause 23, wherein the second optical pulse comprises a pedestal portion and a main portion, and a characteristic of the pedestal portion is adjusted based on applying the intermediate voltage pulse.

[0177] 28. A method according to clause 27, wherein the base portion is continuous in time with the main portion.

[0178] 29. An extreme ultraviolet (EUV) light source, comprising:

[0179] container;

[0180] a target material supply configured to be coupled to the container;

[0181] an optical modulation system configured and positioned to receive the pulsed light beam, the optical modulation system comprising an electro-optic material; and

[0182] A control system is coupled to the voltage source, the control system being configured to:

[0183] causing the voltage source to apply a plurality of forming voltage pulses to the electro-optic material, each forming voltage pulse of the plurality of forming voltage pulses being applied to the electro-optic material at a different time; and

[0184] The voltage source is caused 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 forming voltage pulses of the plurality of forming voltage pulses.

[0185] 30. An EUV light source according to clause 29, wherein the target material supply device is configured to provide a plurality of target material droplets to a target area in the container, the target material droplets reaching the target area at a target delivery rate, and the control system applies a forming voltage pulse to the electro-optical material at a forming rate depending on the target delivery rate.

[0186] 31. An EUV light source according to clause 28, wherein the characteristics of the intermediate voltage pulses include amplitude and / or phase, and

[0187] The control system is also configured to:

[0188] accessing the magnitude and / or phase stored in association with the formation rate, and

[0189] The voltage source is caused to generate an intermediate voltage pulse having the accessed amplitude and / or phase.

[0190] 32. The EUV light source of clause 29, wherein the control system is further configured to control a time delay between applying one of the forming voltage pulses and applying one of the intermediate voltage pulses.

[0191] 33. The EUV light source according to clause 29, further comprising an optical amplifier, and

[0192] in

[0193] Whenever a forming voltage pulse is applied to the electro-optical material, an optical pulse is formed;

[0194] The formed optical pulse is amplified by an optical amplifier to form an amplified optical pulse;

[0195] The control system is further configured to be coupled to a metrology system configured to measure an amount of EUV light generated by the plasma in the vessel,

[0196] The plasma is formed by irradiating the target material with the formed amplified optical pulses,

[0197] The control system is configured to receive the measured amount of EUV light from the metrology system; and

[0198] The control system is configured to modify one or more characteristics of the intermediate voltage pulse based on the measured amount of EUV light.

[0199] 34. The EUV light source of clause 33, wherein one or more characteristics of the intermediate voltage pulses include an amplitude of the intermediate voltage pulses, a duration of the intermediate voltage pulses, a phase of the intermediate voltage pulses, and / or a delay time after application of a most recent forming voltage pulse.

[0200] Other implementations are within the scope of the following claims.

Claims

1. A device for an extreme ultraviolet (EUV) light source, the device comprising: an optical modulation system including an electro-optic material, the optical modulation system being configured to receive a pulsed light beam including a plurality of light pulses separated in time from one another; as well as A control system is configured to control a power supply so that: a first electric pulse is applied to the electro-optical material while a first light pulse is incident on the electro-optical material, a second electric pulse is applied to the electro-optical material while a second light pulse is incident on the electro-optical material, and an intermediate electric pulse is applied to the electro-optical material after the first light pulse is incident on the electro-optical material and before the second light pulse is incident on the electro-optical material.

2. The apparatus of claim 1, wherein applying the first electrical pulse to the electro-optical material induces a physical effect in the electro-optical material, and the physical effect exists in the electro-optical material when the intermediate electrical pulse is applied to the electro-optical material.

3. A device according to claim 2, wherein the physical effect comprises acoustic waves and / or mechanical strain travelling in the electro-optical material.

4. The device of claim 2, wherein applying the intermediate electrical pulse to the electro-optical material reduces the physical effect. 5 . The apparatus of claim 1 , wherein the first light pulse and the second light pulse are consecutive light pulses in the pulsed light beam.

6. The apparatus of claim 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 device of claim 1, wherein the electro-optic material comprises a semiconductor.

8. The device of claim 1, wherein the electro-optic material comprises an insulator.

9. The device of claim 1, wherein the electro-optic material comprises an electro-optic crystal.

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

11. The device of claim 1, wherein the intermediate electrical pulse generates an acoustic disturbance that interferes with the acoustic disturbance caused by the first electrical pulse.

12. An apparatus for forming an optical pulse, the apparatus comprising: an optical modulation system comprising an electro-optic material, the optical modulation system being configured to transmit light in an on state and to block light in an off state, and the optical modulation system being configured to receive a pulsed light beam comprising at least a first optical pulse and a second optical pulse separated in time from each other; as well as A control system is coupled to the voltage source, the control system being configured to: generating a first formed optical pulse by causing the voltage source to apply a first voltage pulse to the electro-optic material while the first optical pulse is incident on the electro-optic material, the first voltage pulse being configured to switch the electro-optic material to the on-state; applying an intermediate voltage pulse to the electro-optical material; as well as A second formed optical pulse is generated by applying a second voltage pulse to the electro-optical material after applying the first voltage pulse and the intermediate voltage pulse and while the second optical pulse is incident on the electro-optical material, wherein the second voltage pulse is configured to switch the electro-optical material to the on state, and a characteristic of the second formed optical pulse is controlled by applying the intermediate voltage pulse to the electro-optical material.

13. The apparatus of claim 12, wherein the second formed optical pulse comprises a base portion and a main portion, and the characteristics of the second formed optical pulse comprise characteristics of the base, such that the characteristics of the base portion are controlled by applying the intermediate voltage pulse to the electro-optical material.

14. The device of claim 13, wherein the base portion and the main portion are continuous in time.

15. The device of claim 13, wherein the characteristics of the base portion include duration, maximum intensity and / or average intensity of the base portion.

16. The apparatus of claim 12, wherein applying the intermediate voltage pulse to the electro-optic material modifies an amount of optically leaked light transmitted by the optical modulation system in the off state.

17. The apparatus of claim 16, wherein applying 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 device according to claim 12, wherein The control system causes the first voltage pulse to be applied to the electro-optical material at a first time; the control system causes the intermediate voltage pulse to be applied to the electro-optical material at a second time 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 control a characteristic of the second formed optical pulse.

19. The apparatus of claim 18, wherein the control system is further configured to control at least one of the amplitude, duration and / or phase of the intermediate voltage pulses.

20. The apparatus of claim 13, wherein the control system is further configured to: receiving an indication of a measured characteristic of the base portion; and A characteristic of the intermediate voltage pulse is adjusted based on the received indication.

21. The apparatus of claim 12, wherein the control system is further configured to: receiving an indication of an amount of extreme ultraviolet (EUV) light generated by the plasma; and A characteristic of the intermediate voltage pulse is adjusted based on the indication of the amount of EUV light received.

22. The apparatus of claim 21 , wherein the control system is configured to adjust a characteristic of the intermediate voltage pulse comprising: The control system is configured to adjust an amplitude of the intermediate voltage pulse, a duration of the intermediate voltage pulse, a phase of the intermediate voltage pulse and / or a second time when the intermediate voltage pulse is applied to the electro-optical material.

23. A method of adjusting a characteristic 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 electro-optic material of the optical modulation system; applying an intermediate voltage pulse to the electro-optical material after applying the first voltage pulse; as well as A second optical pulse is formed 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 characteristic of the second optical pulse is adjusted based on the application of the intermediate voltage pulse.

24. The method according to claim 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 created by interacting the amplified first optical pulse with a target material; as well as At least one characteristic of the intermediate voltage pulse is determined based on a received indication of an amount of the EUV light emitted from the plasma.

25. The method of claim 24, wherein the at least one characteristic of the intermediate voltage pulse comprises a time delay after applying the first voltage pulse, and determining the at least one characteristic of the intermediate voltage pulse comprises: The time delay is determined based on a received indication of an amount of the EUV light emitted from the plasma.

26. The method of claim 24, wherein the at least one characteristic of the intermediate voltage pulse comprises an amplitude and / or a duration of the intermediate voltage pulse, and determining the at least one characteristic of the intermediate voltage pulse comprises: The amplitude and / or the duration of the intermediate voltage pulse is determined.

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

28. The method of claim 27, wherein the base portion is continuous in time with the main portion.

29. An extreme ultraviolet (EUV) light source, comprising: container; a target material supply configured to be coupled to the container; an optical modulation system configured and positioned to receive the pulsed light beam, the optical modulation system comprising an electro-optic material; as well as A control system is coupled to the voltage source, the control system being configured to: causing the voltage source to apply a plurality of forming voltage pulses to the electro-optical material, each forming voltage pulse of the plurality of forming voltage pulses being applied to the electro-optical material at a different time; as well as The voltage source is caused to apply at least one intermediate voltage pulse to the electro-optical material, the at least one intermediate voltage pulse being applied to the electro-optical material between two consecutive forming voltage pulses of the plurality of forming voltage pulses.

30. The EUV light source of claim 29, wherein the target material supply device is configured to provide a plurality of target material droplets to a target area in the container, the target material droplets arriving at the target area at a target delivery rate, and the control system applies the formation voltage pulses to the electro-optical material at a formation rate that depends on the target delivery rate.

31. The EUV light source of claim 30, wherein the characteristics of the intermediate voltage pulses include amplitude and / or phase, and The control system is also configured to: accessing the magnitude and / or phase stored in association with the formation rate, and The voltage source is caused to generate the intermediate voltage pulse having the accessed amplitude and / or phase.

32. The EUV light source of claim 29, wherein the control system is further configured to control a time delay between applying one of the forming voltage pulses and applying one of the intermediate voltage pulses.

33. The EUV light source of claim 29, further comprising an optical amplifier, and in forming an optical pulse whenever a forming voltage pulse is applied to the electro-optical material; the formed optical pulse is amplified by the optical amplifier to form an amplified optical pulse; The control system is further configured to be coupled to a metrology system configured to measure an amount of EUV light generated by the plasma in the vessel, The plasma is formed by irradiating the target material with the formed amplified optical pulses; The control system is configured to receive the measured amount of EUV light from the metrology system; as well as 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 claim 33, wherein one or more characteristics of the intermediate voltage pulses include an amplitude of the intermediate voltage pulses, a duration of the intermediate voltage pulses, a phase of the intermediate voltage pulses, and / or a delay time after application of a most recent forming voltage pulse.

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