WO2020169349A1

WO2020169349A1 - Dose control for an extreme ultraviolet optical lithography system

Info

Publication number: WO2020169349A1
Application number: PCT/EP2020/052939
Authority: WO
Inventors: Spencer Rich; Michael Anthony PURVIS
Original assignee: Asml Netherlands B.V.
Priority date: 2019-02-19
Filing date: 2020-02-06
Publication date: 2020-08-27
Also published as: TW202036174A; NL2024823A

Abstract

An apparatus for an extreme ultraviolet (EUV) lithography system includes an optical modulation system including at least one optical element configured for placement on a beam path between the optical modulation system and a plasma formation region configured to receive target material that emits EUV light in a plasma state; and a control system configured to be coupled to the optical modulation system and to receive a signal including an indication of a characteristic of exposure light impinging on a wafer in a scanning system of the lithography system. The control system is further configured to adjust the optical modulation system based on the indication of the characteristic of the exposure light to thereby control a property of a first portion of a light beam based on the characteristic of the exposure light, the light beam including at least the first portion and a second portion.

Description

DOSE CONTROL FOR AN EXTREME ULTRAVIOLET OPTICAL

LITHOGRAPHY SYSTEM

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to U.S. Application No. 62/807,568, filed February 19, 2019 and titled DOSE CONTROL FOR AN EXTREME ULTRAVIOLET OPTICAL LITHOGRAPHY SYSTEM, and which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

[0002] This disclosure relates to dose control for an extreme ultraviolet (EUV) optical lithography apparatus.

BACKGROUND

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

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

SUMMARY

[0005] In one aspect, an apparatus for an extreme ultraviolet (EUV) lithography system includes an optical modulation system including at least one optical element configured for placement on a beam path between the optical modulation system and a plasma formation region configured to receive target material that emits EUV light in a plasma state; and a control system configured to be coupled to the optical modulation system and to receive a signal including an indication of a characteristic of exposure light impinging on a wafer in a scanning system of the lithography system. The control system is further configured to adjust the optical modulation system based on the indication of the characteristic of the exposure light to thereby control a property of a first portion of a light beam based on the characteristic of the exposure light, the light beam including at least the first portion and a second portion.

[0006] Implementations may include one or more of the following features.

[0007] The apparatus also may include a dose sensor coupled to the control system, the dose sensor being configured to: sense the exposure light at the wafer and to provide the signal including the indication of the characteristic of the exposure light to the control system. The dose sensor may be positioned relative to the wafer in the scanning system of the lithography system.

[0008] The optical modulation system may be configured to produce a modified optical pulse including a pedestal portion and a main portion, and the first portion of the light beam may include the pedestal portion and the second portion of the light beam may include the main portion such that the control system is configured to adjust the modulation system to thereby control at least one property of the pedestal portion based on the characteristic of the exposure light. The at least one property of the pedestal portion may be a temporal duration, an average intensity, and/or a maximum intensity. The main portion of the modified pulse may have an energy sufficient to convert at least some of the target material to the plasma that emits EUV light. The optical modulation system may include an electro-optic modulator (EOM), and at least one optical element may include an electro-optic material. The EOM may include a first electrode, a second electrode, and the electro-optic material is between the first electrode and the second electrode. The control system being configured to adjust the at least one optical element may include the control system being configured to: adjust an amount of voltage applied to the electro-optic material by the first and second electrodes and/or adjust a time at which a voltage is applied to the electro-optic material by the first and second electrodes. The at least one optical element of the modulation system further includes at least one polarization element configured for placement on the beam path, and the control system being configured to adjust the at least one optical element may include the control system being configured to move the polarization element. The control system being configured to adjust the at least one optical element also may include the control system being configured to: adjust an amount of voltage applied to the electro-optic material by the first and second electrodes and/or adjust a time at which a voltage is applied to the electro optic material by the first and second electrodes. The optical system may include more than one EOM, and each EOM is between two polarization elements. [0009] In some implementations, the control system is further configured to: during an inactive period, cause the first and second electrodes to apply a non-zero bias voltage to the electro-optic material, and, during an active period, the control system is configured to cause the first and second electrodes to apply a second voltage that is greater than the bias voltage to the electro-optic material, an amplitude of the bias voltage is based on the indication of the characteristic of the exposure light, and the property of the pedestal is at least partially determined by the amplitude of the bias. The at least one optical element of the optical modulation system also may include an acousto-optic modulator (AOM), the AOM is between the optical modulation system and the plasma formation region, and the AOM is configured to determine a temporal duration of the pedestal.

[0010] In some implementations, the light beam includes a plurality of pulses, the first portion of the light beam includes a first one of the plurality of pulses, and the second portion of the light beam includes a second one of the plurality of pulses such that the control system is configured to adjust the modulation system to thereby control at least one property of the first one of the plurality of pulses based on the characteristic of the exposure light. All of the plurality of pulses may propagate on the beam path. The first portion of the light beam and the second portion of the light beam may be generated by different light sources.

[0011] The indication of a characteristic of the exposure light may include an indication of a dose of EUV light at a particular portion the wafer, the dose of EUV light at the wafer including a total amount of EUV light at the particular portion of the wafer over a pre determined amount of time. The control system may be further configured to: analyze the indication to determine whether a threshold dose of EUV light has been delivered to the particular portion of the wafer, and, if the threshold dose has been delivered to the particular portion of the wafer, issue a command to cause the wafer to move relative to the EUV light such that a different portion of the wafer receives the EUV light.

[0012] The apparatus also may include a light generation-module configured to produce the light beam, the light generation module including a gain medium and an energy source configured to excite the gain medium. In these implementations, the control system is coupled to the light- generation module, the control system is further configured to: control the energy source such that an energy of the light beam is substantially the same over a temporal window, and the control system being configured to adjust the modulation system includes the control system being configured to adjust the modulation system such that an energy of the first portion of the light beam is substantially the same over the temporal window. [0013] In another aspect, an extreme ultraviolet (EUV) light source includes: a vessel configured to form an evacuated space; a target supply apparatus configured to provide a target to a plasma production region in the vessel; an optical apparatus including one or more optical elements configured to be placed on a beam path between the light generation module and the plasma production region; and a control system coupled to the optical apparatus, the control system configured to control the optical apparatus to thereby control one or more properties of first portion of a light beam, the light beam including at least the first portion and a second portion. In operational use, the vessel is configured to provide EUV light to a lithography apparatus that is configured to direct the EUV light toward a wafer, and the control of the one or more properties of the first portion is based on a characteristic of the EUV light at the wafer.

[0014] Implementations may include one or more of the following features. The EUV light source also may include a light-generation module, the light-generation module including a gain medium and an energy source configured to excite the gain medium. The energy source may include a plurality of electrodes configured to be driven by a radio-frequency (RF) power source, and the light generation module includes a chamber that houses the plurality of electrodes, and the control system controls the one or more properties of the first portion of the light beam without adjusting any properties of the RF power source.

[0015] The optical apparatus may include an optical modulation system configured to produce a modified optical pulse, the modified pulse may include a pedestal portion and a main portion, and the first portion of the light beam may include the pedestal portion and the second portion of the light beam includes the main portion, and the control system may be configured to control the optical apparatus to thereby control one or more properties of the pedestal portion.

[0016] The EUV light source also may include a light-generation module, and the light- generation module may be configured to produce a pulsed light beam that includes a plurality of pulses, the first portion of the light beam may include a first one of the plurality of pulses, and the second portion of the light beam may include a second one of the plurality of pulses, and the control system may be configured to adjust the optical apparatus to thereby control at least one property of the first one of the plurality of pulses based on the characteristic of EUV light at the wafer.

[0017] The EUV light source also may include a light-generation module, and the light- generation module may include at least a first optical source and a second optical source, the first portion of the light beam may be an optical pulse produced by the first optical source, and the second portion of the light beam may be an optical pulse produced by the second optical source.

[0018] The EUV light source also may include a dose sensor coupled to the control system, the dose sensor being configured to sense the characteristic of the EUV light at the wafer and to produce the indication of the characteristic of the EUV light at the wafer.

[0019] In another aspect, an extreme ultraviolet (EUV) lithography system includes: a vessel configured to form an evacuated space; a target supply apparatus configured to provide a target to a plasma production region in the vessel; an optical apparatus including one or more optical elements configured to be placed on a beam path between the light generation module and the plasma production region; a lithography apparatus configured to receive EUV light from the vessel and to direct the EUV light toward a wafer; and a control system coupled to the optical apparatus, the control system configured to control the optical apparatus to thereby control one or more properties of first portion of a light beam, the light beam including at least the first portion and a second portion. In operational use, the control of the one or more properties of the first portion is based on a characteristic of the EUV light at the wafer.

[0020] Implementations of any of the techniques described above may include an EUV light source, a lithography system, a system, a method, a process, a device, or an apparatus. The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims.

DRAWING DESCRIPTION

[0021] FIGS. 1 and 2A-2C are block diagrams of examples of EUV lithography systems.

[0022] FIG. 3A is a block diagram of an example of an optical modulation system.

[0023] FIG. 3B is an example of an optical pulse.

[0024] FIG. 3C is an example of a modified optical pulse.

[0025] FIG. 3D is an example of experimental data.

[0026] FIG. 4A is a plot of example voltage signals that may be applied to an optical modulation system.

[0027] FIGS. 4B and 5 are block diagrams of examples of optical modulation systems.

[0028] FIG. 6 is a flow chart of an example process for controlling a property of a light beam.

[0029] FIGS. 7A and 7B are block diagrams, each showing an example of a lithography system. [0030] FIG. 8 is a block diagram of an example of an EUV light source.

DF.T ATT FD DESCRIPTION

[0031] FIG. 1 is a block diagram of an EUV lithography system 100. The lithography system 100 includes an EUV light source 101, which provides EUV light 197 to a lithography apparatus 180. The lithography apparatus 180 shapes, controls, directs, and/or focuses the EUV light 197 into exposure beam 191. The exposure beam 191 impinges on a substrate 192 to form microelectronic features at the substrate 192. The system 100 also includes a control system 150. The control system 150 controls a modulation system 140 based on an indication 154 of a characteristic of the exposure beam 191. In FIG. 1, lines having the dash-dot-dash line style represent data links over which data, commands, and/or information flow as, for example, electrical signals or as optical signals that are encoded with information. The data link may be any type of medium that is capable of carrying information. For example, the data link may be an electrical cable, optical fiber, and/or a wireless connection.

[0032] The modulation system 140 includes an optical element 142 that interacts with a light beam 107 produced by an optical source 104 to modify a property (for example, an intensity) of the light beam 107 and produce a modified light beam 108. The modified light beam 108 propagates on a path 106 to a plasma formation region 123 to interact with a target 121 that includes a target material. The interaction converts at least some of the target material in the target 121 to a plasma 196 that emits EUV light 197.

[0033] Because the EUV light 197 is formed by an interaction between the light beam 107 and the target 121, controlling a property of the light beam 107 also allows control of characteristics of the EUV light 197. Moreover, because the exposure beam 191 is based on the portion of the EUV light 197 that is directed into the lithography apparatus 180, controlling the property of the light beam 107 also allows characteristics of the exposure beam 191 to be controlled. For example, the dose of the exposure beam 191, which is an amount of optical energy delivered to the substrate 192 over a period of time, may be controlled by controlling properties of the light beam 107. As discussed below, the control system 150 implements a technique in which a single parameter of the light beam 107 is modulated by the modulation system 140 to control the EUV light 197 and the characteristics of the exposure beam 191.

[0034] Other techniques for controlling an amount of the EUV light 197 and the dose in an EUV light source are known. For example, the optical source 104 includes a gain medium 102 that is excited by an energy source 103 to produce the light beam 107. The energy source 103 may be, for example, a pair of electrodes on two sides of the gain medium. In these implementations, the electrodes are driven by a radio-frequency (RF) power source that energizes the electrodes to form a discharge that excites the gain medium. Some prior systems modulate the energy source 103 (for example, by periodically increasing a voltage applied to the electrodes) to modulate the energy of the light beam 107 and to make a corresponding change to the amount of EUV light 197 and the dose. However, modulating the energy source 103 also changes many properties of the light beam 107. For example, modulating the energy source 103 may change the energy, the pointing, and the beam size of the light beam 107. All of these properties are correlated to the production of the EUV light 197. As a result, it may be challenging to stabilize the amount of EUV light produced by modulating the energy source 103. Although the effects of unintentionally modulating various properties of the beam 107 may be mitigated through additional feedback loops, such feedback loops add complexity and may reduce reliability.

[0035] On the other hand, the control system 150 implements a technique that controls the amount of generated EUV light 197 by modulating one or more properties of the light beam 107 with the modulation system 140 and without relying on modulation of the energy source 103. Thus, the actuation or modulation of the parameter is done in a manner that is independent of the other parameters of the light beam 107 that are correlated to EUV production. As such, the approach implemented by the control system 150 does not require separate control loops to control unwanted variations in other parameters of the light beam 107.

[0036] Although the control system 150 implements an approach that does not require such separate control loops to correct for unintended or collateral variations in EUV production, the control system 150 may allow the use of separate and/or additional control loops that may be challenging or impossible to implement using the approach of the prior systems. For example, prior systems may lack a pulse-to-pulse actuator to control properties of the light beam 107 or properties of a portion of the light beam 107. The portion of the light beam 107 may be a pulse of the light beam 107 or a pedestal portion of a pulse such as the pedestal portion 366 (FIG. 3C). The control system 150 allows pulse-to-pulse actuation of the portion of the light beam 107, which allows use of control loops (such as control of the pedestal portion based on dose, total energy in a pulse of the light beam 108 that are not possible in the prior systems. [0037] Furthermore, the technique implemented by the control system 150 has very little effect on the thermal state of the optical source 104. An approach that relies on modulating the energy source 103 results in the output energy of the optical source 104 varying by a relatively large amount, for example, about 100 milliJoules (mJ). The large variations in the output energy impact the thermal state of the optical source 104 and optical elements on the path 106. On the other hand, because the control system 150 implements an approach that modulates the light beam 107, the amount of generated EUV light 197 may be varied by modulating the intensity of the light beam 107 by a relatively small amount, for example, by less than about 5 mJ. This results in greater thermal stability.

[0038] Additionally, the control system 150 is able to modulate the light beam 107 faster with the modulation system 140 than with the energy source 103. For example, in an implementation in which the optical source 104 produces optical pulses at a repetition rate of 50 kilohertz (kHz), the energy source 103 may be modulated once every ten optical pulses.

In contrast, the modulation system 140 may be modulated at 50 kHz and thus may modulate a property on each pulse produced by the optical source 104. Thus, the control system 150 is able to more finely and accurately control the modulation of the light beam 107 and this may lead to an overall improvement in performance of the system.

[0039] FIGS. 2A and 2B are block diagrams of lithography systems 200A and 200B, respectively. In FIGS. 2A and 2B, lines having the dash-dot-dash line style represent data links over which data, commands, and/or information flow as, for example, electrical signals or as optical signals that are encoded with information. The data link may be any type of medium that is capable of carrying information. For example, the data link may be an electrical cable, optical fiber, and/or a wireless connection.

[0040] Referring to FIG. 2A, the lithography system 200A is an example of an

implementation of the lithography system 100 (FIG. 1). The lithography system 200A includes an EUV light source 201A, which provides the EUV light 197 to a lithography apparatus 280. The lithography apparatus 280 exposes a substrate 192 with the exposure beam 191.

[0041] The lithography system 200A includes an optical source 204, which produces a light beam 207. The optical source 204 includes a gain medium 202 and an energy source 203 that excites the gain medium 202 to produce the light beam 207. The light beam 207 may be a pulsed light beam that includes a plurality of optical pulses that are separated from each other in time or a continuous wave (CW) beam. The optical source 204 may be, for example, a pulsed (for example, a Q-switched) or continuous-wave carbon dioxide (CO2) laser or a solid- state laser (for example, Nd:YAG laser or an erbium-doped fiber (Er:glass) laser).

[0042] The lithography system 200A also includes a modulation system 240. The modulation system 240 is any type of device that is able to vary or modulate a property of the light beam 207. For example, the modulation system 240 may be an electro-optic modulator (EOM), an acousto-optic modulator (AOM), or a combination of such devices. The modulation system 240 includes an optical element 242. The light beam 207 is incident on the optical element 242. An interaction between the optical element 242 and the light beam 207 modifies a property of the light beam 207 to produce a modified beam 208. Examples of the modulation system 240 are discussed with respect to FIGS. 3 A, 4, and 5. The EUV light source 201 A also includes an optical amplifier system 230 between the optical modulation system 240 and a plasma formation region 223. The optical amplifier system 230 includes one or more optical amplifiers on the beam path 206. Each optical amplifier includes a gain medium that amplifies the wavelengths of the modified light beam 208. The modified light beam 208 propagates through the optical amplifier system 230 to the plasma formation region 223.

[0043] The EUV light source 201 A includes a supply system 220 that produces a stream 222 of targets. The targets in the stream 222 travel in a vacuum chamber 211 toward the plasma formation region 223. In the example of FIG. 2A, a target 221 (which is part of the stream 222) is in the plasma formation region 223. Each target in the stream 222 includes target material, which is any material that emits EUV light when in a plasma state. For example, the target material may include water, tin, lithium, and/or xenon. Other materials may be used as the target material. For example, the element tin may be used as pure tin (Sn); as a tin compound, for example, SnBr4, SnBr2, SnFB; as a tin alloy, for example, tin-gallium alloys, tin-indium alloys, tin-indium-gallium alloys, or any combination of these alloys. Moreover, the target material may be a target mixture that includes impurities that do not emit EUV light in a plasma state, such as non-target particles or inclusion particles. The non target particles or inclusion particles may be, for example, particles of tin oxide (S11O2) or particles of tungsten (W).

[0044] An interaction between the modified light beam 208 and the target 221 produces the plasma 196, which emits the EUV light 197. The EUV light 197 interacts with an optical element 213, which directs at least some of the EUV light 197 to the lithography apparatus 280. The optical element 213 may be a collector mirror that has an aperture through which the modified light beam 208 propagates and a curved reflective surface faces the plasma formation region 223 and reflects and focuses wavelengths in the EUV range.

[0045] The lithography apparatus 280 includes a plurality of reflective optical elements 281 and 282, a mask 284, and a slit 283, all of which are in an enclosure 286. The enclosure 286 is a housing, tank, or other structure that is capable of supporting the reflective optical elements 281 and 282, the mask 284, and the slit 283, and is also capable of maintaining an evacuated space within the enclosure 286.

[0046] The EUY light 197 enters the enclosure 286 and is reflected by the optical element 281 through the slit 283 toward the mask 284. The slit 283 is the shape of the distributed light used to scan a wafer in a lithography process. The size of the slit 283 is a physical quantity. The dose delivered to the substrate 192 or the number of photons delivered to the substrate 192 depends on the size of the slit 283 and the speed at which the slit 283 is scanned.

[0047] The mask 284 also may be referred to as a reticle or patterning device. The mask 284 includes a spatial pattern that represents the electronic features that are to be formed on a substrate 192. The EUY light 197 interacts with the mask 284. The interaction between the EUV light 197 and the mask 284 results in the pattern of the mask 284 being imparted onto the EUV light 197 to form the exposure beam 191. The exposure beam 191 passes through the slit 283 and is directed to the substrate 192 by the optical elements 282. An interaction between the substrate 192 and the exposure beam 191 exposes the pattern of the mask 284 onto the substrate 192, and the electronic features are thereby formed at the substrate 192.

The substrate 192 includes a plurality of portions 293 (for example, dies). The area of each portion 293 in the Y-Z plane is less than the area of the entire substrate 192 in the Y-Z plane. Each portion 193 may be exposed by the exposure beam 191 to include a copy of the mask 284 such that each portion 193 includes the electronic features indicated by the pattern on the mask 284.

[0048] The lithography system 200A also includes a metrology system 260. The metrology system 260 includes a sensor system 262. The sensor system 262 includes one or more sensors. The sensor system 262 also includes an electronics module 264 that is coupled to the sensor system 262. The sensor system 262 may include a camera, sensor, detector, or any combination of such devices that are sensitive to the EUV wavelengths in the exposure beam 191. The sensor system 262 may include a sensor at the substrate 191, located such that it may monitor light impinging on the substrate or that it may monitor light correlated with light impinging on the substrate. The sensor system 262 may include a sensor configured to measure the EUV light 197 in the vacuum chamber 211 or at the entrance of the enclosure 286 (for example, at the optical element 281).

[0049] The electronics module 264 includes electronic components for the operation of the sensor system 262. For example, the electronics module 264 may include an electronic processor capable of driving the sensor system 262 to perform certain actions and to obtain data from the sensor system 262. Additionally, the electronics module 264 produces an indication or representation 254 of one or more characteristics (for example, intensity or energy) of the exposure beam 191. The indication 254 may include, for example, numerical data that represents an intensity or optical energy at the substrate 192. Moreover, the indication 254 also may include data that describes such information. For example, the indication 254 may include a time period over which the information was collected.

[0050] The metrology system 260 is coupled to a control system 250. The control system 250 exchanges data and/or information with the metrology system 260 and the modulation system 240 via a communications interface 253. For example, the control system 250 receives the indication 254 and provides trigger or command signals to the modulation system 240 based on the indication 254. Furthermore, although the control system 250 does not rely on modulation of the energy source 203 to modulate the properties of the light beam 207, the control system 250 may exchange data and/or information with the optical source 204. In implementations in which the control system 250 interacts with the optical source 204, a data link exists between the optical source 204 and the control system 250.

[0051] The control system 250 includes an electronic processor 251, an electronic storage 252, and the communications interface 253. The electronic processor 251 includes one or more processors suitable for the execution of a computer program such as a general or special purpose microprocessor, and any one or more processors of any kind of digital computer. Generally, an electronic processor receives instructions and data from a read-only memory, a random access memory, or both. The electronic processor 251 may be any type of electronic processor.

[0052] The electronic storage 252 may be volatile memory, such as RAM, or non-volatile memory. In some implementations, and the electronic storage 252 includes non-volatile and volatile portions or components. The electronic storage 252 may store data and information that is used in the operation of the control system 250 and/or components of the control system 250. For example, the electronic storage 252 may store a dose specification that indicates the acceptable dose for one portion 293. The dose specification may be a single numerical value or a range of values. The electronic storage 252 also may store instructions (for example, a sequence of instmctions that together form a computer program, software module, or callable function), that, when executed, cause the processor 251 to communicate with components in the control system 250, the modulation system 240, the metrology system 260, and/or the lithography apparatus 280. Moreover, the electronic storage may storage 252 may store instructions that specify data processing techniques for analyzing the indication 254.

[0053] The communications interface 253 is any kind of electronic interface that allows the control system 250 to receive and/or provide data and signals with an operator, the modulation system 240, the optical source 204, the metrology system 260, and/or an automated process running on another electronic device. For example, the communications interface 253 may include a visual display, a keyboard, a network connection (for example, an Ethernet connection), and/or a device that is capable of receiving audible commands and/or producing audio output.

[0054] The metrology system 260 and the control system 250 are shown as separate items in the example of FIG. 2A. However, all or part of the metrology system 260 may be implemented as part of the control system 250. For example, the electronics module 264 may be implemented by the electronic processor 251 by instmctions stored on the electronic storage 252.

[0055] FIG. 2B is a block diagram of the lithography system 200B. The lithography system 200B is the same as the system 200A (FIG. 2A), except the lithography system 200B includes an optical source 204B that includes a first optical source 204_1, which emits a first light beam 207_1, and a second optical source 204_2, which emits a second light beam 207_2. The light beams 207_1 and 207_2 may be pulsed light beams. A pulse of the light beam 207_2 may be referred to as a pre-pulse, and a pulse of the light beam 207_1 may be referred to as a main pulse. The light beams 207_1 and 207_2 interact with modulation systems 240_1 and 240_2, respectively. The modulation systems 240_1 and 240_2 are part of a modulation system 240B.

[0056] The light beam 207_2 propagates to an initial target region 223_2 in the vacuum chamber 211. The initial target region 223_2 is displaced in the -x direction relative to the plasma formation region 223 and is between the plasma formation region 223 and the supply system 220. The initial target region 223_2 receives one of the targets in the stream 222. In the example of FIG. 2B, the target in the initial target region 223_2 is labeled 221i and is referred to as the initial target 22 li. [0057] The light beam 207_2 interacts with a modulation system 240_2 to form a modified light beam 208_2. The modified light beam 208_2 interacts with the initial target 22 li at the initial target region 223_2 to condition the target 221i and form a modified target 221m. The modified target 221m drifts to the plasma formation region 223 and is irradiated by the light beam 207_1 to form the plasma 196. As compared to an unconditioned target, the modified target 221m more readily absorbs optical energy and a higher portion of target material in the modified target 221m is converted into the plasma 196. For example, the interaction between the light beam 207_2 and the target 22 li may change the shape, volume, and/or size of the distribution of the target material in the initial target 22 li and/or may reduce the density gradient of the target material along the direction of propagation of the light beam 207_1. All of these changes enhance the ability of the modified target 221m to absorb optical energy from the light beam 207_1 and increases the amount of target material converted into the plasma 196. The modified target 221m may be, for example, a disk-shaped distribution of target material that has a larger volume (and thus a lower density) than the target 22 li. The decreased density results in a higher portion of the target material in the 221m being converted to the plasma 196 and thus results in a larger amount of EUV light.

[0058] The control system 250 is coupled to the modulation system 240B. In this way, the control system 250 may be used to control the parameters of the conditioning of the initial target 221i. For example, the control system 250 is able to control properties of the modified light beam 208_2 such as the intensity and/or temporal duration.

[0059] The optical sources 204_1 and 204_2 may be, for example, two lasers. For example, the optical sources 204_1, 204_2 may be two carbon dioxide (CO2) lasers. In other implementations, the optical sources 204_1, 204_2 may be different types of lasers. For example, the optical source 204_2 may be a solid state laser, and the optical source 204_1 may be a CO2 laser. The light beams 207_1, 207_2 may have different wavelengths. For example, in implementations in which the optical sources 204_1, 204_2 include two CO2 lasers, the wavelength of the first light beam 207_1 may be about 10.26 micrometers (pm) and the wavelength of the second light beam 207_2 may be between 10.18 pm and 10.26 pm. The wavelength of the second light beam 207_2 may be about 10.59 pm. In these implementations, the light beams 207_1, 207_2 are generated from different lines of the CO2 laser, resulting in the light beams 207_1, 207_2 having different wavelengths even though both beams are generated from the same type of source.

[0060] The light beams 207_1 and 207_2 have different energies and may have different durations. For example, a pulse of the light beam 207_2 (or a pre-pulse) may have a duration of at least 1 nanosecond (ns), for example, the pre-pulse may have a duration of 1-100 ns and a wavelength of 1 pm or 10 pm. In one example, the pre-pulse of radiation is a laser pulse that has energy of 15-60 mJ, a pulse duration of 20-70 ns, and a wavelength of 1-10 pm. In some examples, the pre-pulse may have a duration of less than 1 ns. For example, the pre pulse may have a duration of 300 picoseconds (ps) or less, 100 ps or less, between 100-300 ps, or between 10-100 ps.

[0061] In the example of FIG. 2B, the first light beam 207_1 and the second light beam 207_2 interact with separate modulation systems and travel on separate optical paths.

However, in other implementations, the first light beam 207_1 and the second beam 207_2 may share all or part of the same optical path and also may share the same beam delivery system.

[0062] Referring also to FIG. 2C, the optical source 204, the optical source 204_1, and/or the optical source 204_2 may include more than source of light. FIG. 2C is a block diagram of a lithography system 200C, which includes an EUV light source 201C and an optical source 204C. The optical source 204C may be used as the optical source 204_1, and/or the optical source 204_2. The EUV light source 201C may be the EUV light source 201A (FIG. 2A) or the EUV light source 201B (FIG. 2B).

[0063] The optical source 204C includes a first optical source 204_1C and a second optical source 204C_2, and a beam combiner 205 that directs a light beam 207C_1 emitted by the first optical source 204C_1 and a light beam light 207C_2 emitted by the second optical source 204C_2 toward the optical modulation system 240. The beam combiner 205 may be, for example, one or more refractive and/or reflective devices (such as mirrors, lenses, and/or prisms). The light beams 207C_1 and 207C_2 interact with the modulation system 240 to form a modified light beam 208C. The light 207C_1 and 207C_2 are pulsed light beams.

[0064] The optical sources 204C_1 and 204C_2 may be identical to each other or may be different. The optical sources 204C_1 and 204C_2 may be, for example, two lasers. For example, the optical sources 204C_1, 204C_2 may be two carbon dioxide (CO2) lasers. In other implementations, the optical sources 204C_1, 204C_2 may be different types of lasers. For example, the optical source 204C_2 may be a solid state laser, and the optical source 204_1 may be a CO2 laser. The light beams 207C_1, 207C_2 may have different wavelengths. For example, in implementations in which the optical sources 204C_1, 204C_2 include two CO2 lasers, the wavelength of the first light beam 207C_1 may be about 10.26 micrometers (pm) and the wavelength of the second light beam 207C_2 may be between 10.18 pm and 10.26 pm. The wavelength of the second light beam 207C_2 may be about 10.59 mhi. In these implementations, the light beams 207C_1, 207C_2 are generated from different lines of the CO2 laser, resulting in the light beams 207C_1, 207C_2 having different wavelengths even though both beams are generated from the same type of source.

[0065] In the example shown, the light beams 207C_1 and 207C_2 are directed onto substantially the same beam path by the beam combiner 205. However, the optical source 204C may be implemented without the beam combiner 205. In these implementations, the the light beams 207C_1 and 207C_2 propagate on separate paths.

[0066] As noted above, the optical source 204C may be used as the optical source 204,

204_1, and/or the optical source 204_2. Thus, the light beam 207 for the EUV light source 201 A (FIG. 2 A) may be made from light that comes from two separate light sources.

Moreover, the light beam 207_1 and/or the light beam 207_2 (FIG. 2B) may be made from light that comes from two separate light sources. As discussed below, the light generated by the separate light source in these implementations may be considered a pedestal portion.

[0067] FIG. 3A shows a block diagram of a modulation system 340. The modulation system 340 is an example of a modulation system that may be used as the modulation system 140, 240, 240_1, or 240_2. The modulation system 340 includes an electro-optic modulator that modulates a light beam 307 based on the electro-optic effect to produce a modified light beam 308. The light beam 307 may be the light beam 107, 207, 207_1, or 207_2, and the modified light beam 308 may be the modified light beam 108, 208, 208_1, or 208_2, respectively.

[0068] The modulation system 340 includes an optical element 342 between electrodes 344a, 344b. The optical element 342 is any material that experiences the electro-optic effect. The electro-optic effect describes the change in the refractive index of the electro-optic material 342 that results from the application of a direct-current (DC) or low-frequency electric field or potential difference 343 across the electro-optic material 342. The electrodes are coupled to an electrical source 345, which creates the potential difference 343 by holding the electrodes 344a and 344b at different voltages. The electrical source 345 may be, for example, a voltage source, a function generator, or a power supply. The electrodes 344a, 344b are controllable to form the electric field 343. For example, the control system 150 may cause the electrical source 345 to provide a voltage signal 347 to the electrode 344a such that the electrode 344a is held at a different voltage than the electrode 344b, thus creating the electric field or a potential difference (V) 343 across the electro-optic material 342.

[0069] By controlling the electro-optic effect in the electro-optic material 342 while the light beam 307 is incident on the electro-optic material 342, the modulation system 340 modulates the phase, polarization, or amplitude of the light beam 307 to form the modified light beam 308. The potential difference 343 may be used to control whether or not the modulation system 340 transmits light. The electric field 343 may be used to control the electro-optic material 342 such that only a certain portion or portions of the light beam 107 pass through the electro-optic material 342. In this way, the modulation system 340 forms a pulse 308 (a modified light beam) from a portion of the light beam 107. The pulse 308 propagates to the vacuum chamber 211, which receives the stream 122 of targets. The pulse 308 and a target interact, and the interaction converts at least some of the target material in the target into the plasma 196 that emits the EUV light 197.

[0070] The modulation system 340 also includes one or more polarization-based optical elements 346. In the example of FIG. 3A, only one polarization-based optical element 346 is shown. However, in other implementations, additional polarization-based optical elements 346 may be included. For example, a second polarization-based optical element 346 may be on a side of the modulation system 340 that receives the light beam 107. Furthermore, the polarization-based optical element 346 is shown as being physically separated from the electro-optic material 342, but other implementations are possible. For example, the polarization-based optical element 346 may be a film that is formed on the electro-optic material 342 such that the polarization-based optical element 346 and the electro-optic material 342 are in contact with each other.

[0071] The polarization-based optical element 346 is any optical element that interacts with light based on the polarization state of the light. For example, polarization-based optical element 346 may be a linear polarizer that transmits horizontally polarized light and blocks vertically polarized light, or vice versa. The polarization-based optical element 346 may be a polarizing beam splitter that transmits horizontally polarized light and reflects vertically polarized light. The polarization-based optical element 346 may be an optical element that absorbs all light except for light having a particular polarization state. In some

implementations, the polarization-based optical element 346 may include a quarter-wave plate. At least one polarization-based optical element 346 is positioned to receive light that passes through the electro-optic material 342 and to direct light of a certain polarization state onto the beam path 106.

[0072] The electro-optic material 342 may be any material that transmits one of more wavelengths of the light beam 307. For implementations in which the light beam 307 includes light of a wavelength of 10.6 microns (qm), the electro-optic material 342 may be, for example, cadmium zinc telluride (CdZnTe or CZT), cadmium telluride (CdTe), zinc telluride (ZnTe), and/or gallium arsenide (GaAs). Other materials may be used at other wavelengths. For example, the material 342 may be monopotassium phosphate (KDP), ammonium dihydrogen phosphate (ADP), quartz, cuprous chloride (CuCl), zinc sulphide (ZnS), zinc selenide (ZnSe), lithium niobate (LiNbOa), gallium phosphide (GaP), lithium tantalate (LiTaOa), or barium titanate (BaTiOa ). Other materials that transmit one or more wavelengths of the light beam 307 and exhibit birefringence in response to application of an external force also may be used as the electro-optic material 342. For example, quartz may be used as the electro-optic material 342.

[0073] The electro-optic material 342 also exhibits anisotropy. In a material that exhibits anisotropy, the properties of the material (such as the index of refraction) are spatially non- uniform. Thus, the properties of the electro-optic material 342 may be modified along a particular direction or directions by application of a controllable external force (such as the potential difference (343)). For example, the indices of refraction for different polarization components of light propagating through the material 342 may be controlled through application of the external force. Thus, the polarization state of the light that passes through the material 342 may be controlled by controlling the potential difference (Y) between the electrodes 344a, 344b.

[0074] Under ideal operation, the modulation system 340 only transmits light when the potential difference 343 applied to the electro-optic material 342 causes the polarization state of the light passing through the electro-optic material 342 to atch the polarization conditions of the polarization-based optical element 346. For example, if the polarization- based optical element 346 is a linear polarizer positioned to transmit horizontally polarized light onto the beam path 106, and the light beam 307 is vertically polarized when initially incident on the electro-optic material 342, the pulse 308 is only formed when the potential difference 343 applied to the electro-optic material 342 changes the polarization state of the light beam 307 such that the light beam 307 becomes horizontally polarized prior to interacting with the polarization-based optical element 346.

[0075] The modulation system 340 is considered to be in an ON state or activated anytime the modulation system 340 is controlled to intentionally transmit light. For example, when the applied potential difference 343 is such that the polarization state of the light beam 307 is rotated to be matched to the polarization-based optical element 346, the optical modulation system 340 is considered to be in the ON state and the pulse 308 is formed. When the applied potential difference 343 is such that the polarization state of the light beam 307 is expected to be orthogonal to the polarization-based optical element 346, the optical modulation system 340 is in the OFF state or is not activated. Under ideal conditions, the light beam 307 does not pass through the modulation system 340 when the optical modulation system 340 is in the OFF state.

[0076] However, applying the potential difference 343 to the electro-optic material 342 causes acoustic waves to propagate in the electro-optic material 342. These acoustic waves may persist after the potential difference 343 is removed from the electro-optic material 342. Additionally, the acoustic waves cause strain in the electro-optic material 342 that change the optical properties of the electro-optic material 342 and allow incident light to pass through the modulation system 342 (as optical leakage) even when the potential difference 343 is not applied. Thus, in actual operation, the modulation system 340 may transmit spurious light (optical leakage) even when the polarization condition of the polarization-based optical element 346 is such that light incident on the electro-optic material 342 should not pass through the modulation system 340. For example, when the optical leakage is present just prior to the formation of the pulse 308, the optical leakage forms a pedestal portion on the pulse 308.

[0077] Referring also to FIGS. 3B and 3C, an illustration of an example of a pulse of the light beam 307 and an example of the modified optical pulse 308 formed by an interaction with the optical modulator 340 is shown. FIG. 3B shows the intensity of the pulse 307 as a function of time, and FIG. 3C shows the intensity of the pulse 308 as a function of time.

Note that time scale of FIG. 3B is stretched in comparison with the time scale of FIG. 3 A.) The pulse 308 includes a pedestal portion 367 and a main portion 368.

[0078] The pulse 307 has a temporal profile (intensity versus time) that is approximately Gaussian. The pulse 307 interacts with the modulation system 340 to form the pulse 308.

The control system 150 controls the modulation system 340 to extract a particular portion 365 of the pulse 307. In the example of FIG. 3B, at a time t=ta, the modulation system 340 is set to transmit light and at the time t=tb, the modulation system 340 is set to block light. In other words, the optical modulation system 340 is only intended to transmit the light in the portion 365 (which is the light in the pulse 307 between time ta and time tb). For example, the control system 250 may control the modulation system 340 to transmit light at the time ta by applying the voltage signal 347 such that light passing through the electro-optic material 342 has a polarization that matches the polarization of the polarization-based optical element 346. The modulation system 340 may be controlled to stop transmitting light at the time tb by removing the voltage signal 347. [0079] However, due to acoustic waves in the electro-optic material 342 (or other disturbances such as unexpected motion of the polarization-based optical element 346), optical leakage may be transmitted by the modulation system 340 at times before ta and/or at times after time tb. In the example of FIG. 3B, leakage light 364 is optical leakage that occurs just prior to the time ta. The leakage light 364 passes through the modulation system 340 just prior to the portion 365.

[0080] Referring to FIG. 3C, the leakage light 364 forms the pedestal portion 366. In the example shown, the pedestal portion 366 occurs during a window labeled as 367, and the pedestal portion 366 occurs earlier in time than the rest of the pulse 308. The portions of the optical pulse 308 that are not the pedestal portion 366 are referred to as the main portion 368. The pedestal portion 366 and the main portion 368 are both part of the optical pulse 308, and the pedestal portion 366 is temporally connected to the main portion 368. In other words, in the example of FIG. 3C, there is no period without light between the pedestal portion 366 and the main portion 368.

[0081] The pedestal portion 366 has a different temporal profile (intensity as a function of time) than the main portion 368. For example, the average and maximum intensity and optical energy of the pedestal portion 366 are less than the average and maximum intensity and optical energy of main portion 368. Moreover, the shape of the pedestal portion 366 is different from the shape of the main portion 368. Further, the characteristics (for example, intensity, temporal profile, and/or duration) of the pedestal portion 366 are different from the characteristics of the early part of a pulse formed without any optical leakage.

[0082] The modified pulse 308 is amplified by the amplifier system 230 to form an amplified modified pulse. The amplified modified pulse includes the pedestal portion 366 and the main portion 368, with each portion 366, 368 of the amplified modified pulse having a greater intensity than the corresponding portion of the modified pulse 308. In the example of FIG. 3C, the pedestal portion 366 occurs before the main portion 368 and reaches the target 221 before the main portion 368. In some implementations, the main portion 368 has an intensity or energy sufficient to convert at least some of the target material in the target 221 into the plasma 196 that emits the EUV light 197.

[0083] The pedestal portion 366 does not have as much energy as the main portion 368, and may or may not have sufficient energy to convert the target material into plasma. However, the light in the pedestal portion 366 may evaporate material from the surface of the target 221, break off parts of the target 221, and condition the target 221 such that the properties (for example, density, shape, and/or size) are more favorable to plasma production. As such, the properties of the pedestal portion 366 allow control over the amount of EUV light produced and the characteristics of the exposure beam 191. The properties of the pedestal portion 366 may be controlled by controlling the amount of optical leakage. The control system 250 controls the amount of optical leakage (the leakage light 364 in this example) in various ways and based on the indication 154.

[0084] The pulse 308 discussed with respect to FIGS. 3B and 3C is provided as one example of a modified optical pulse 308. The pulse 308 may have other forms. For example, the leakage light 364 may occur before the time ta such that the pedestal portion 366 is separate from the main portion 368. In these implementations, there is a period without light between the pedestal portion 366 and the main portion 368. Furthermore, the leakage light 364 may occur after the time tb such that the pedestal portion 366 occurs after the main portion 368.

In these implementations, the pedestal portion 366 reaches the target 221 after the main portion 368. In some implementations, the leakage light 364 occurs before the time ta and after the time tb such that there is a pedestal portion 366 on each side of the main portion 368.

[0085] Moreover, the pedestal portion 366 may be separated in time relative to the main portion 368, and the pedestal portion 366 may include wavelengths that are not included in the main portion 368. In some implementations, the pedestal portion 366 is a pulse of light that is generated by a separate light source. For example, the pedestal portion 366 may be generated by a separate light source when an optical source such as the optical source 207C (FIG. 2C) is used to form the light beam 307. In implementations in which the pedestal portion 366 is generated by a separate light source, the pedestal portion 366 interacts with the optical modulator 340, and the properties of the pedestal 366 are adjusted by controlling the optical modulator 340 based on the indication 254.

[0086] FIG. 3D shows examples of experimental data 300D. The x-axis shows the total intensity of a pedestal portion (such as the pedestal 366 of FIG. 3C) normalized to a percentage of total pedestal intensity required to achieve the maximum conversion efficiency (CE). The conversion efficiency is an indication of the portion of target material that is converted to the plasma 196. The total intensity of the pedestal 366 is the intensity in the entire pedestal 366. The maximum CE depends on various properties of the EUY light source. In the example of FIG. 3D, the maximum normalized CE for a light source is represented on the y-axis as 100%. However, the maximum true (non-normalized) CE for the light source is generally less than 100%. The optimal total intensity of the pedestal is represented as 100 on the x-axis of FIG. 3D. [0087] The experimental data 300D includes data obtained on two different EUV sources: EUV Source 1 (represented with the X symbol) and EUV Source 2 (represented by the open circle symbols). As shown by the experimental data 300D, the CE may be varied by about 13% by varying the total intensity of the pedestal 366. Properties of the pedestal 366 that determine the total intensity of the pedestal 366 (such as, for example, the maximum or average intensity or the temporal duration) may be controlled by the modulation system 240. The amount of the EUV light 197 depends on the CE. Thus, characteristics of the exposure beam 191 and/or the EUV light 197 may be controlled by controlling properties of the pedestal 366.

[0088] The control system 250 may control properties of the light beam 207 by controlling the intensity and/or temporal duration of the pedestal 366 by controlling leakage light via the voltage signal 347. The control system 250 controls the electrical source 345 to generate the voltage signal 347 and to apply the voltage signal 347 to the electrodes 344a, 344b. FIG. 4A shows plots of voltage signals 347_1 and 347_2 as a function of time. The voltage signals 347_1 and 347_2 are examples of the voltage signal 347.

[0089] The voltage signal 347_1 has an amplitude of 0 V when the modulation system 240 is in the OFF state. The modulation system 240 is in the OFF state until the time ta. At the time ta, the amplitude of the voltage signal 347_1 increases to a voltage 349. The voltage 349 is a voltage that is greater than zero and a voltage that is sufficient to change the index of refraction of the electro-optic material 342. At the time tb, the modulation system 240 returns to the OFF state, and the amplitude of the voltage signal 347_2 returns to 0 V. The pedestal 366 is formed by the leakage light 364 as discussed above.

[0090] The voltage signal 347_2 includes a bias voltage 348. The bias voltage 348 is greater than zero. When the voltage signal 347_2 is used as the voltage signal 347, the bias voltage 348 is applied to the electrodes 344a, 344b such that the potential difference 343 across the electro-optic material 242 when the modulation system 240 in in the OFF state. In some implementations, the bias voltage 348 is applied to the electrodes 344a, 344b at all times.

The bias voltage 348 is a constant voltage (a direct current or DC voltage) that is greater than zero and less than the voltage 349. At the time ta, the modulation system 240 is in the ON state, and the magnitude of the voltage signal 347_2 increases from the bias voltage 348 to the voltage 349. At the time tb, the modulation system 240 is in the OFF state, and the magnitude of the voltage signal 347_2 returns to the bias voltage 348.

[0091] When the voltage signal 347_2 is used as the voltage signal 347, leakage light is present when the bias voltage is applied to the electro-optic material 342. Thus, the pedestal 366 may have a longer temporal duration than a pedestal formed from the leakage light 364. Furthermore, when the pedestal 366 is formed by applying the bias voltage 348 to the electro optic material 342, the pedestal 366 may occur before and/or after the main portion 368. In these implementations, the modulation system 340 may include additional components that modify or reduce the temporal duration of the pedestal portion 366. For example, and referring also to FIG. 4B, the modified light beam 308 may interact with an acousto-optic modulator 444 to remove some of the pedestal portion 366.

[0092] The control system 250 also may control the leakage light through manipulation of a polarization element in the modulation system 240. FIG. 5 is a block diagram of an optical modulation system 540. The modulation system 540 may be used in place of the modulation system 140 (FIG. 1), the modulation system 240 (FIG. 2A), the modulation system 240_1 (FIG. 2B), or the modulation system 240_2 (FIG. 2B).

[0093] The modulation system 540 includes first, second, and third polarization elements 546_1, 546_2, and 546_3. The modulation system 540 also includes a first EOM 540_1 and a second EOM 540_2. The first EOM 540_1 is between the first polarization element 546_1 and the second polarization element 546_2, and the second EOM 540_2 is between the second polarization element 546_2 and the third polarization element 546_3. In the example of FIG. 5, the first polarization element 546_1 and the third polarization element 546_3 transmit light that is linearly polarized in the Y direction, and the polarization element 546_2 transmits light that is linearly polarized in the X direction. The EOM 540_1 and 540_2 modulate incident light based on an applied voltage as discussed with respect to the EOM 340 (FIG. 3A).

[0094] In the example of FIG. 5, the modulation system 540 transmits light and is in an ON state when the EOM 540_1 rotates the polarization of incident light from the Y direction to the X direction and when the EOM 540_2 rotates the polarization of incident light from the X direction to the Y direction. The modulation system 540 is in an OFF state when the EOM 540_2 and EOM 540_1 do not rotate the polarization of incident light. Thus, the modulation system 540 is OFF when no voltage is applied to the EOM 540_1 and/or the EOM 540_2 or when a voltage that is not sufficient to change the polarization state of incident light is applied to the EOM 540_1 and/or the EOM 540_2.

[0095] The optical modulation system 540 modulates the light beam 307 to produce the modified light beam 308. The modified light beam 308 may have the pedestal portion 366. The temporal duration and/or intensity of the pedestal portion 366 may be controlled by controlling the amount of leakage light. To vary the leakage light, the control system 250 may issue a command to control a mechanical mount 541 coupled to the third polarization element 546_3. The command causes the mechanical mount 541 to rotate the third polarization element 546_3 relative to the Y axis such that light is allowed through the element 546_3 even when the EOM 540_2 does not rotate the polarization of incident light from a state that is linearly polarized along the X direction to a state that is linearly polarized along the Y direction. For example, the EOM 540_2 may transmit light that is hnearly polarized in the X direction while the control system 250 has commanded the third polarization element 546_3 to be rotated a few degrees from the Y axis. In this example, the third polarization element transmits leakage light. Subsequently, the voltage signal 347 is applied to the EOM 540_2, and the EOM 540_2 rotates the polarization of incident light to be linearly polarized along the Y direction and the main portion 368 is formed. In this example, the main portion 368 has a pedestal that is based on the leakage light.

[0096] Similarly, the polarization element 346 (FIG. 3A) may be placed on a mechanical mount and rotated based on a command from the control system 250 to control the amount and temporal duration of leakage light.

[0097] FIG. 6 is a flow chart of a process 600. The process 600 is an example of a process for controlling a property of a light beam in an EUV light source based on an indication of a characteristic of an exposure beam. The process 600 may be performed by one or more of the electronic processors 251 of the control system 250. The process 600 is discussed with respect to the modulation systems 240, 340, and 540. However, the process 600 may be performed by other control systems and may be used with other modulation systems.

[0098] The indication 254 is received (610). The indication 254 includes information that relates to a characteristic of the exposure beam 191. For example, the indication 254 may include data that expresses a dose received at a portion 293 of the substrate 192. The dose is an amount of optical energy received over a period of time. In these implementations, the indication 254 is produced by a sensor that senses EUV light at the substrate 192. The sensor is part of the sensor system 262. Other implementations are possible. For example, the indication 254 may be an indication of an amount of the EUV light 197 produced at the plasma formation region 223 (FIG. 2A). In these implementations, the indication may be produced by a sensor that is in the vacuum chamber 211 or by a sensor that is in the lithography apparatus 280 and is positioned to measure the EUV light 197 prior to interaction with the mask 284.

[0099] The indication 254 is analyzed (620). In implementations in which the indication includes a measured dose, the analysis includes comparing the measured dose to a dose specification. The dose specification is a value or a range of values that specify an acceptable dose for a portion 293 of the substrate 192. If the measured dose is below the specification, too little EUV light reached the portion 293, and the electronic features may be partially formed or otherwise incomplete. If the measured dose is above the specification, too much EUV light reached the portion 293, and the portion is overexposed and the electronic features are again improperly formed. If the dose is within the specification or equal to the specification, then the correct amount of light reached the portion 293 and the electronic features are likely properly and completely formed.

[0100] The control system 250 uses the information about the measured dose to control one or more properties of the light beam 207 (630). For example, if the measured dose indicates that too little light is reaching the portion 293, the control system 250 controls the modulation system 240 in a manner that causes more of the EUV light to be produced. For example, the modulation system 240 may be implemented as the EOM 340. In these implementations, the control system 250 controls the voltage signal 347 and/or manipulates the polarization element 346 such that a pedestal 366 is formed to condition the target 221 so that more of the EUV light 197 is produced.

[0101] If the measured dose is within the dose specification or exceeds the dose specification, the control system 250 issues a command to the lithography apparatus 280 that indicates that the portion 293 should not be exposed by the exposure beam 191 further. In response, the lithography apparatus 280 moves the substrate 192 and/or the exposure beam 191 such that a different portion 293 is exposed.

[0102] Moreover, if the measured dose indicates that too much light is reaching the portion 293, then the control system 250 may control the optical modulation system 240 such that a reduced dose of light reaches the next portion 293. For example, the control system 250 may reduce the total amount of EUV light that reaches the substrate 192 by blocking some portions of the light beam 207 to thereby reduce the amount of the EUV light 197 produced over a temporal window. For example, the light beam 207 may be a pulsed light beam, and the control system 250 may activate the modulation system 240 to block or greatly reduce the intensity of every third or fourth pulse such that less of the EUV light 197 is produced and the dose is reduced.

[0103] Analyzing the indication 254 (620) may take other forms. For example, the indication 254 may be an indication of an amount of measured EUV light (for example, an intensity of EUV light) at an entrance to the lithography apparatus 280 instead of dose. In these implementations, the amount of measured EUV light is compared to a target amount or a specification. If the amount of measured EUY light is below the specification, then the control system 250 acts on the modulation system 240 in a manner that increases the amount of the EUV light 197 produced. If the amount of measured EUV light is above the specification, then the control system 250 issues an indication to the lithography apparatus 280 that the exposure of the portion 293 is complete. The control system 250 also may act on the modulation system 240 in a manner that decreases the amount of the EUV light produced.

[0104] In implementations in which the indication 254 is an indication of a measured amount of EUV light, the control system 250 may apply the measured amount of EUV light to a model of the slit 283 and the mask 284 to estimate a dose that corresponds to the measured amount of EUV light. The model of the slit 283 may be, for example, a low pass filter that simulates the slit 283 and thus estimates the amount of light delivered to the substrate 191. In these implementations, the estimated dose may be compared to the dose specification.

[0105] FIG. 7A is a block diagram of a lithography system 700 that includes a source collector module SO. The lithography system 700 is an example of the lithography system 100. The lithography system 700 also includes: an illumination system IL configured to condition a radiation beam B. The radiation beam B may be an EUV light beam emitted from the source collector module SO. The lithography system 700 also includes a support structure MT constructed to support a patterning device MA. The support structure MT may be, for example, a mask table, and the patterning device MA may be, for example, a mask or reticle. When the radiation beam B interacts with the patterning device MA, a spatial pattern associated with the patterning device MA is imparted onto the radiation beam B. The support structure MT is coupled to a first positioner PM that is configured to position the patterning device MA. Further, the apparatus 700 includes a substrate table WT constructed to hold a substrate W, which may be, for example, a resist-coated wafer. The substrate table WT is connected to a second positioner PW that is configured to position the substrate W. The system 700 also includes a projection system PS that is configured to project the patterned radiation beam E (also referred to as exposure light E or an exposure beam E) onto a target portion C of the substrate W. The target portion C may be any portion of the substrate W. In the example of FIG. 7A, the substrate W includes a plurality of dies D, and the target portion C includes more than one of the dies D.

[0106] The illumination system IL includes optical components for directing, shaping, and/or controlling the radiation beam B and the exposure light E. The optical components may include refractive, reflective, magnetic, electromagnetic, electrostatic, or any other type of optical components. [0107] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device MA, the design of the lithography system 700, and/or other conditions, such as, for example, whether or not the patterning device MA is held in a vacuum environment. The support structure MT may use mechanical, vacuum, electrostatic and/or other clamping techniques to hold the patterning device MA. The support structure MT may be a frame or a table, for example, which may be fixed or movable. The support structure MT may ensure that the patterning device MA is at a desired position, for example with respect to the projection system PS.

[0108] The patterning device MA is any device that may be used to impart a pattern onto the radiation beam B. The patterning device MA may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. In implementations in which the patterning device MA is a mask, the patterning device MA may be, for example, binary mask, an alternating phase-shift mask, or an attenuated phase-shift, or a hybrid mask type. In implementations in which the patterning device MA is a programmable mirror array, the patterning device MA includes a matrix arrangement of mirrors, each of which may be individually tilted so that each of the mirrors is capable of reflecting the radiation beam B in a different direction that does not depend on the direction in which the radiation beam B is reflected by the other mirrors in the matrix. The pattern that is imparted onto incident light is determined by the position of the various mirrors in the matrix. The pattern may correspond to a particular functional layer in a device being created in the target portion C of the substrate W. For example, the pattern may correspond to electronic features that together form an integrated circuit.

[0109] The projection system PS includes optical components that direct the exposure light E to the target portion C. The optical components of the projection system PS may be refractive, reflective, magnetic, electromagnetic, electrostatic, and/or other types of optical components, as appropriate for the exposure radiation being used, or for other factors such as the use of a vacuum. Furthermore, it may be desired to use a vacuum for EUV radiation because gases may absorb EUV radiation. A vacuum environment may therefore be provided with the aid of a vacuum wall and vacuum pumps.

[0110] In the example of FIGS. 7A and 7B, the apparatus 700 is a reflective type that includes reflective optical components and a reflective patterning device MA. The lithography system 700 may be of a type having two (dual stage) or more substrate tables (and/or two or more patterning device tables). In such multiple stage machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.

[0111] The illumination system IL receives an extreme ultraviolet radiation beam B from the source collector module SO. The EUV light sources 101 (FIG. 1), 201A (FIG. 2A), and 201B (FIG. 2B), and S00 (FIG. 8) are examples of the source collector module SO.

[0112] The illumination system IF may include an adjuster for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as s-outer and s-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illumination system IF may include various other components, such as facetted field and pupil mirror devices. The illumination system IL may be used to condition the radiation beam B to have a desired uniformity and intensity distribution in its cross-section.

[0113] The radiation beam B interacts with the patterning device MA such that the pattern is imparted onto the radiation beam B. The radiation beam B is reflected from the patterning device MA with the pattern imparted as the exposure light E. The exposure light E passes through the projection system PS, which focuses the beam onto the target portion C of the substrate W. With the aid of the second positioner PW and a second position sensor PS2, the substrate table WT can be moved accurately, for example, so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor PS1 can be used to accurately position the patterning device (for example mask) MA with respect to the path of the radiation beam B. The positioning sensors PS1 and PS2 may be, for example, interferometric devices, linear encoders, and/or capacitive sensors. The patterning device MA and the substrate W may be aligned using patterning device alignment marks Ml, M2 and substrate alignment marks PI, P2.

[0114] The lithography system 700 may be used in at least one of the following modes: (1) a step mode, (2) a scan mode, or (3) a third or other mode. In the step mode, the support structure MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam B is projected onto a target portion C at one time (that is, a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed. In scan mode, the support structure MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam B is projected onto a target portion C (that is, a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS. In the third or other mode, the support structure MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above. Combinations and/or variations on these three modes of use and/or entirely different modes of use may also be employed.

[0115] FIG. 7B shows an implementation of the lithography system 700 in more detail, including the source collector module SO, the illumination system IL, and the projection system PS. The source collector module SO includes a vacuum environment. Each of the systems IL and PS also include a vacuum environment. An EUV radiation emitting plasma is formed within the source collector module SO. The source collector module SO focuses the EUV radiation emitted from a plasma to an intermediate focus IF such that the radiation beam B (760) is provided to the illumination system IL.

[0116] The radiation beam B traverses the illumination system IL, which in the example of FIG. 7B includes a facetted field mirror device 22 and a facetted pupil mirror device 24. These devices form a so-called“fly’s eye” illuminator, which is arranged to provide a desired angular distribution of the radiation beam 21 at the patterning device MA and maintains a uniformity of radiation intensity at the patterning device MA. Upon reflection of the beam B at the patterning device MA, the exposure light E (the patterned beam B) is formed and the exposure light E (26) is imaged by the projection system PS via reflective elements 28, 30 onto the substrate W. Additionally, the exposure light E interacts with a slit that shapes the exposure light E such that the exposure light E has a rectangular cross-section in a plane that is perpendicular to the direction of propagation. To expose the target portion C on substrate W, the source collector module SO generates pulses of radiation to form the radiation beam B while the substrate table WT and patterning device table MT perform synchronized movements to scan the pattern on the patterning device MA through the rectangular exposure light E.

[0117] Each system IL and PS is arranged within its own vacuum or near-vacuum environment, defined by enclosing structures. More elements than shown may generally be present in illumination system IL and projection system PS. Further, there may be more mirrors present than those shown. For example there may be one to six additional reflective elements present in the illumination system IL and/or the projection system PS, besides those shown in FIG. 7B.

[0118] Numerous additional components used in the operation of the source collector module and the lithography system 700 as a whole are present in a typical apparatus, though not illustrated here. These include arrangements for reducing or mitigating the effects of contamination within the enclosed vacuum, for example to prevent deposits of fuel material damaging or impairing the performance of collector 3 and other optics. Other features present but not described in detail are all the sensors, controllers and actuators involved in controlling of the various components and sub-systems of the lithography system 700.

[0119] Referring to FIG. 8, an implementation of an LPP EUV light source 800 is shown.

The light source 800 may be used as the source collector module SO in the lithography system 700. Furthermore, the optical source 104 of FIG. 1, the optical source 204 (FIG. 2A), and the optical source 204B (FIG. 2B) may be part of the drive laser 815.

[0120] The LPP EUV light source 800 is formed by irradiating a target mixture 814 at a plasma formation region 805 with an amplified light beam 810 that travels along a beam path toward the target mixture 814. The target material discussed with respect to FIG. 1 and the targets in the stream 222 discussed with respect to FIGS. 2A and 2B may be or include the target mixture 814. The plasma formation region 805 is within an interior 807 of a vacuum chamber 830. When the amplified light beam 810 strikes the target mixture 814, a target material within the target mixture 814 is converted into a plasma state that has an element with an emission line in the EUV range. The created plasma has certain characteristics that depend on the composition of the target material within the target mixture 814. These characteristics may include the wavelength of the EUV light produced by the plasma and the type and amount of debris released from the plasma.

[0121] The light source 800 includes a drive laser system 815 that produces the amplified light beam 810 due to a population inversion within the gain medium or mediums of the laser system 815. The light source 800 includes a beam delivery system between the laser system 815 and the plasma formation region 805, the beam delivery system including a beam transport system 820 and a focus assembly 822. The beam transport system 820 receives the amplified light beam 810 from the laser system 815, and steers and modifies the amplified light beam 810 as needed and outputs the amplified light beam 810 to the focus assembly 822. The focus assembly 822 receives the amplified light beam 810 and focuses the beam 810 to the plasma formation region 805. [0122] In some implementations, the laser system 815 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. Each optical amplifier includes a gain medium capable of optically amplifying the desired wavelength at a high gain, an excitation source, and internal optics. The optical amplifier may or may not have laser mirrors or other feedback devices that form a laser cavity. Thus, the laser system 815 produces an amplified light beam 810 due to the population inversion in the gain media of the laser amplifiers even if there is no laser cavity. Moreover, the laser system 815 may produce an amplified light beam 810 that is a coherent laser beam if there is a laser cavity to provide enough feedback to the laser system 815. The term“amplified light beam” encompasses one or more of: light from the laser system 815 that is merely amplified but not necessarily a coherent laser oscillation and light from the laser system 815 that is amplified and is also a coherent laser oscillation.

[0123] The optical amplifiers in the laser system 815 may include as a gain medium a filling gas that includes CO2 and may amplify light at a wavelength of between about 9100 and about 11000 nm, and in particular, at about 10600 nm, at a gain greater than or equal to 800 times. Suitable amplifiers and lasers for use in the laser system 815 may include a pulsed laser device, for example, a pulsed, gas-discharge CO2 laser device producing radiation at about 9300 nm or about 10600 nm, for example, with DC or RF excitation, operating at relatively high power, for example, lOkW or higher and high pulse repetition rate, for example, 40 kHz or more. The pulse repetition rate may be, for example, 50 kHz. The optical amplifiers in the laser system 815 may also include a cooling system such as water that may be used when operating the laser system 815 at higher powers.

[0124] The light source 800 includes a collector mirror 835 having an aperture 840 to allow the amplified light beam 810 to pass through and reach the plasma formation region 805.

The collector irror 835 may be, for example, an ellipsoidal mirror that has a primary focus at the plasma formation region 805 and a secondary focus at an intermediate location 845 (also called an intermediate focus) where the EUV light may be output from the light source 800 and may be input to, for example, an integrated circuit lithography tool (not shown). The light source 800 may also include an open-ended, hollow conical shroud 850 (for example, a gas cone) that tapers toward the plasma formation region 805 from the collector mirror 835 to reduce the amount of plasma-generated debris that enters the focus assembly 822 and/or the beam transport system 820 while allowing the amplified light beam 810 to reach the plasma formation region 805. For this purpose, a gas flow may be provided in the shroud that is directed toward the plasma formation region 805. [0125] The light source 800 may also include a master controller 855 that is connected to a droplet position detection feedback system 856, a laser control system 857, and a beam control system 858. The light source 800 may include one or more target or droplet imagers 860 that provide an output indicative of the position of a droplet, for example, relative to the plasma formation region 805 and provide this output to the droplet position detection feedback system 856, which may, for example, compute a droplet position and trajectory from which a droplet position error may be computed either on a droplet by droplet basis or on average. The droplet position detection feedback system 856 thus provides the droplet position error as an input to the master controller 855. The master controller 855 may therefore provide a laser position, direction, and timing correction signal, for example, to the laser control system 857 that may be used, for example, to control the laser timing circuit and/or to the beam control system 858 to control an amplified light beam position and shaping of the beam transport system 820 to change the location and/or focal power of the beam focal spot within the chamber 830.

[0126] The supply system 825 includes a target material delivery control system 826 that is operable, in response to a signal from the master controller 855, for example, to modify the release point of the droplets as released by a target material supply apparatus 827 to correct for errors in the droplets arriving at the desired plasma formation region 805.

[0127] Additionally, the light source 800 may include light source detectors 865 and 870 that measures one or more EUY light parameters, including but not limited to, pulse energy, energy distribution as a function of wavelength, energy within a particular band of wavelengths, energy outside of a particular band of wavelengths, and angular distribution of EUV intensity and/or average power. The light source detector 865 generates a feedback signal for use by the master controller 855. The feedback signal may be, for example, indicative of the errors in parameters such as the timing and focus of the laser pulses to properly intercept the droplets in the right place and time for effective and efficient EUV light production.

[0128] The light source 800 may also include a guide laser 875 that may be used to align various sections of the light source 800 or to assist in steering the amplified light beam 810 to the plasma formation region 705. In connection with the guide laser 875, the light source 800 includes a metrology system 824 that is placed within the focus assembly 822 to sample a portion of light from the guide laser 875 and the amplified light beam 810. In other implementations, the metrology system 824 is placed within the beam transport system 820. The metrology system 824 may include an optical element that samples or re-directs a subset of the light, such optical element being made out of any material that may withstand the powers of the guide laser beam and the amplified light beam 810. A beam analysis system is formed from the metrology system 824 and the master controller 855 since the master controller 855 analyzes the sampled light from the guide laser 875 and uses this information to adjust components within the focus assembly 822 through the beam control system 858.

[0129] Thus, in summary, the light source 800 produces an amplified light beam 810 that is directed along the beam path to irradiate the target mixture 814 at the plasma formation region 805 to convert the target material within the mixture 814 into plasma that emits light in the EUV range. The amplified light beam 810 operates at a particular wavelength (that is also referred to as a drive laser wavelength) that is determined based on the design and properties of the laser system 815. Additionally, the amplified light beam 810 may be a laser beam when the target material provides enough feedback back into the laser system 815 to produce coherent laser light or if the drive laser system 815 includes suitable optical feedback to form a laser cavity.

[0130] Other aspects of the invention are set out in the following numbered clauses.

1. An apparatus for an extreme ultraviolet (EUV) lithography system, the apparatus comprising:

an optical modulation system comprising at least one optical element configured for placement on a beam path between the optical modulation system and a plasma formation region configured to receive target material that emits EUV light in a plasma state; and a control system configured to be coupled to the optical modulation system and to receive a signal comprising an indication of a characteristic of exposure light impinging on a wafer in a scanning system of the lithography system, wherein

the control system is further configured to adjust the optical modulation system based on the indication of the characteristic of the exposure light to thereby control a property of a first portion of a light beam based on the characteristic of the exposure light, the light beam comprising at least the first portion and a second portion.

2. The apparatus of clause 1, further comprising a dose sensor coupled to the control system, the dose sensor being configured to: sense the exposure light at the wafer and to provide the signal comprising the indication of the characteristic of the exposure light to the control system.

3. The apparatus of clause 2, wherein the dose sensor is positioned relative to the wafer in the scanning system of the lithography system. 4. The apparatus of clause 1, wherein the optical modulation system is configured to produce a modified optical pulse comprising a pedestal portion and a main portion, and the first portion of the light beam comprises the pedestal portion and the second portion of the light beam comprises the main portion such that the control system is configured to adjust the modulation system to thereby control at least one property of the pedestal portion based on the characteristic of the exposure light.

5. The apparatus of clause 4, wherein the at least one property of the pedestal portion comprises a temporal duration, an average intensity, and/or a maximum intensity.

6. The apparatus of clause 4, wherein the main portion of the modified pulse has an energy sufficient to convert at least some of the target material to the plasma that emits EUV light.

7. The apparatus of clause 4, wherein the optical modulation system comprises an electro-optic modulator (EOM), and at least one optical element comprises an electro-optic material.

8. The apparatus of clause 7, wherein the EOM comprises a first electrode, a second electrode, and the electro-optic material is between the first electrode and the second electrode.

9. The apparatus of clause 8, wherein the control system being configured to adjust the at least one optical element comprises the control system being configured to: adjust an amount of voltage applied to the electro-optic material by the first and second electrodes and/or adjust a time at which a voltage is applied to the electro-optic material by the first and second electrodes.

10. The apparatus of clause 8, wherein the at least one optical element of the modulation system further comprises at least one polarization element configured for placement on the beam path, and the control system being configured to adjust the at least one optical element comprises the control system being configured to move the polarization element.

11. The apparatus of clause 10, wherein the control system being configured to adjust the at least one optical element further comprises the control system being configured to: adjust an amount of voltage applied to the electro-optic material by the first and second electrodes and/or adjust a time at which a voltage is applied to the electro-optic material by the first and second electrodes.

12. The apparatus of clause 11, wherein the optical system comprises more than one EOM, and each EOM is between two polarization elements.

13. The apparatus of clause 9, wherein the control system is further configured to: during an inactive period, cause the first and second electrodes to apply a non-zero bias voltage to the electro-optic material, and, during an active period, the control system is configured to cause the first and second electrodes to apply a second voltage that is greater than the bias voltage to the electro-optic material, an amplitude of the bias voltage is based on the indication of the characteristic of the exposure light,

and the property of the pedestal is at least partially determined by the amplitude of the bias.

14. The apparatus of clause 13, wherein the at least one optical element of the optical modulation system further comprises an acousto-optic modulator (AOM), the AOM is between the optical modulation system and the plasma formation region, and the AOM is configured to determine a temporal duration of the pedestal.

15. The apparatus of clause 1, wherein the light beam comprises a plurality of pulses, the first portion of the light beam comprises a first one of the plurality of pulses, and the second portion of the light beam comprises a second one of the plurality of pulses such that the control system is configured to adjust the modulation system to thereby control at least one property of the first one of the plurality of pulses based on the characteristic of the exposure light.

16. The apparatus of clause 15, wherein all of the plurality of pulses propagate on the beam path.

17. The apparatus of clause 15, wherein the first portion of the light beam and the second portion of the light beam are generated by different light sources.

18. The apparatus of clause 1, wherein the indication of a characteristic of the exposure light comprises an indication of a dose of EUV light at a particular portion the wafer, the dose of EUV light at the wafer comprising a total amount of EUV light at the particular portion of the wafer over a pre-determined amount of time.

19. The apparatus of clause 18, wherein the control system is further configured to: analyze the indication to determine whether a threshold dose of EUV light has been delivered to the particular portion of the wafer, and, if the threshold dose has been delivered to the particular portion of the wafer, issue a command to cause the wafer to move relative to the EUV light such that a different portion of the wafer receives the EUV light.

20. The apparatus of clause 1, further comprising: a light generation- module configured to produce the light beam, the light generation module comprising a gain medium and an energy source configured to excite the gain medium, and wherein

the control system is coupled to the light-generation module, the control system is further configured to: control the energy source such that an energy of the light beam is substantially the same over a temporal window, and

the control system being configured to adjust the modulation system comprises the control system being configured to adjust the modulation system such that an energy of the first portion of the light beam is substantially the same over the temporal window.

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

a vessel configured to form an evacuated space;

a target supply apparatus configured to provide a target to a plasma production region in the vessel;

an optical apparatus comprising one or more optical elements configured to be placed on a beam path between a light generation module and the plasma production region; and a control system coupled to the optical apparatus, the control system configured to control the optical apparatus to thereby control one or more properties of a first portion of a light beam, the light beam comprising at least the first portion and a second portion, wherein, in operational use, the vessel is configured to provide EUV light to a lithography apparatus that is configured to direct the EUV light toward a wafer, and the control of the one or more properties of the first portion is based on a characteristic of the EUV light at the wafer.

22. The EUV light source of clause 21, further comprising a light-generation module, the light-generation module comprising a gain medium and an energy source configured to excite the gain medium.

23. The EUV light source of clause 22, wherein the energy source comprises a plurality of electrodes configured to be driven by a radio-frequency (RF) power source, and the light generation module comprises a chamber that houses the plurality of electrodes, and the control system controls the one or more properties of the first portion of the light beam without adjusting any properties of the RF power source.

24. The EUV light source of clause 21, wherein the optical apparatus comprises an optical modulation system configured to produce a modified optical pulse, the modified pulse comprising a pedestal portion and a main portion, and the first portion of the light beam comprises the pedestal portion and the second portion of the light beam comprises the main portion, and

the control system is configured to control the optical apparatus to thereby control one or more properties of the pedestal portion.

25. The EUV light source of clause 21, further comprising a light-generation module, and wherein the light-generation module is configured to produce a pulsed light beam that comprises a plurality of pulses, the first portion of the light beam comprises a first one of the plurality of pulses, and the second portion of the light beam comprises a second one of the plurality of pulses, and

the control system is configured to adjust the optical apparatus to thereby control at least one property of the first one of the plurality of pulses based on the characteristic of EUV light at the wafer.

26. The EUV light source of clause 21, further comprising a light-generation module, and wherein the light-generation module comprises at least a first optical source and a second optical source, the first portion of the light beam is an optical pulse produced by the first optical source, and the second portion of the light beam is an optical pulse produced by the second optical source.

27. The EUV light source of clause 21, further comprising a dose sensor coupled to the control system, the dose sensor being configured to sense the characteristic of the EUV light at the wafer and to produce the indication of the characteristic of the EUV light at the wafer.

28. An extreme ultraviolet (EUV) lithography system comprising:

a vessel configured to form an evacuated space;

an optical apparatus comprising one or more optical elements configured to be placed on a beam path between the light generation module and the plasma production region;

a lithography apparatus configured to receive EUV light from the vessel and to direct the EUV light toward a wafer; and

a control system coupled to the optical apparatus, the control system configured to control the optical apparatus to thereby control one or more properties of first portion of a light beam, the light beam comprising at least the first portion and a second portion, wherein, in operational use, the control of the one or more properties of the first portion is based on a characteristic of the EUV light at the wafer.

Claims

WHAT IS CLAIMED IS:

2. The apparatus of claim 1, further comprising a dose sensor coupled to the control system, the dose sensor being configured to: sense the exposure light at the wafer and to provide the signal comprising the indication of the characteristic of the exposure light to the control system.

3. The apparatus of claim 2, wherein the dose sensor is positioned relative to the wafer in the scanning system of the lithography system.

4. The apparatus of claim 1, wherein the optical modulation system is configured to produce a modified optical pulse comprising a pedestal portion and a main portion, and the first portion of the light beam comprises the pedestal portion and the second portion of the light beam comprises the main portion such that the control system is configured to adjust the modulation system to thereby control at least one property of the pedestal portion based on the characteristic of the exposure light.

5. The apparatus of claim 4, wherein the at least one property of the pedestal portion comprises a temporal duration, an average intensity, and/or a maximum intensity.

6. The apparatus of claim 4, wherein the main portion of the modified pulse has an energy sufficient to convert at least some of the target material to the plasma that emits EUV light.

7. The apparatus of claim 4, wherein the optical modulation system comprises an electro-optic modulator (EOM), and at least one optical element comprises an electro-optic material.

8. The apparatus of claim 7, wherein the EOM comprises a first electrode, a second electrode, and the electro-optic material is between the first electrode and the second electrode.

9. The apparatus of claim 8, wherein the control system being configured to adjust the at least one optical element comprises the control system being configured to: adjust an amount of voltage applied to the electro-optic material by the first and second electrodes and/or adjust a time at which a voltage is applied to the electro-optic material by the first and second electrodes.

10. The apparatus of claim 8, wherein the at least one optical element of the modulation system further comprises at least one polarization element configured for placement on the beam path, and the control system being configured to adjust the at least one optical element comprises the control system being configured to move the polarization element.

11. The apparatus of claim 10, wherein the control system being configured to adjust the at least one optical element further comprises the control system being configured to: adjust an amount of voltage applied to the electro-optic material by the first and second electrodes and/or adjust a time at which a voltage is applied to the electro-optic material by the first and second electrodes.

12. The apparatus of claim 11, wherein the optical system comprises more than one EOM, and each EOM is between two polarization elements.

13. The apparatus of claim 9, wherein the control system is further configured to: during an inactive period, cause the first and second electrodes to apply a non-zero bias voltage to the electro-optic material, and, during an active period, the control system is configured to cause the first and second electrodes to apply a second voltage that is greater than the bias voltage to the electro-optic material,

an amplitude of the bias voltage is based on the indication of the characteristic of the exposure light,

14. The apparatus of claim 13, wherein the at least one optical element of the optical modulation system further comprises an acousto-optic modulator (AOM), the AOM is between the optical modulation system and the plasma formation region, and the AOM is configured to determine a temporal duration of the pedestal.

15. The apparatus of claim 1, wherein the light beam comprises a plurality of pulses, the first portion of the light beam comprises a first one of the plurality of pulses, and the second portion of the light beam comprises a second one of the plurality of pulses such that the control system is configured to adjust the modulation system to thereby control at least one property of the first one of the plurality of pulses based on the characteristic of the exposure light.

16. The apparatus of claim 15, wherein all of the plurality of pulses propagate on the beam path.

17. The apparatus of claim 15, wherein the first portion of the light beam and the second portion of the light beam are generated by different light sources.

18. The apparatus of claim 1, wherein the indication of a characteristic of the exposure light comprises an indication of a dose of EUV light at a particular portion the wafer, the dose of EUV light at the wafer comprising a total amount of EUV light at the particular portion of the wafer over a pre-determined amount of time.

19. The apparatus of claim 18, wherein the control system is further configured to:

analyze the indication to determine whether a threshold dose of EUV light has been delivered to the particular portion of the wafer, and, if the threshold dose has been delivered to the particular portion of the wafer, issue a command to cause the wafer to move relative to the EUV light such that a different portion of the wafer receives the EUV light.

20. The apparatus of claim 1, further comprising: a light generation-module configured to produce the light beam, the light generation module comprising a gain medium and an energy source configured to excite the gain medium, and wherein

the control system is coupled to the light-generation module,

the control system is further configured to: control the energy source such that an energy of the light beam is substantially the same over a temporal window, and

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

a vessel configured to form an evacuated space;

22. The EUV light source of claim 21, further comprising a light-generation module, the light-generation module comprising a gain medium and an energy source configured to excite the gain medium.

23. The EUV light source of claim 22, wherein the energy source comprises a plurality of electrodes configured to be driven by a radio-frequency (RF) power source, and the light generation module comprises a chamber that houses the plurality of electrodes, and the control system controls the one or more properties of the first portion of the light beam without adjusting any properties of the RF power source.

24. The EUV light source of claim 21, wherein the optical apparatus comprises an optical modulation system configured to produce a modified optical pulse, the modified pulse comprising a pedestal portion and a main portion, and the first portion of the light beam comprises the pedestal portion and the second portion of the light beam comprises the main portion, and

25. The EUV light source of claim 21, further comprising a light-generation module, and wherein the light-generation module is configured to produce a pulsed light beam that comprises a plurality of pulses, the first portion of the light beam comprises a first one of the plurality of pulses, and the second portion of the light beam comprises a second one of the plurality of pulses, and

26. The EUV light source of claim 21, further comprising a light-generation module, and wherein the light-generation module comprises at least a first optical source and a second optical source, the first portion of the light beam is an optical pulse produced by the first optical source, and the second portion of the light beam is an optical pulse produced by the second optical source.

27. The EUV light source of claim 21, further comprising a dose sensor coupled to the control system, the dose sensor being configured to sense the characteristic of the EUV light at the wafer and to produce the indication of the characteristic of the EUV light at the wafer.

28. An extreme ultraviolet (EUV) lithography system comprising: a vessel configured to form an evacuated space;

an optical apparatus comprising one or more optical elements configured to be placed on a beam path between the light generation module and the plasma production region; a lithography apparatus configured to receive EUV light from the vessel and to direct the EUV light toward a wafer; and