CN108353489B

CN108353489B - Target expansion rate control in extreme ultraviolet light sources

Info

Publication number: CN108353489B
Application number: CN201680047368.0A
Authority: CN
Inventors: R·J·拉法克; D·J·里格斯
Original assignee: ASML Holding NV
Current assignee: ASML Holding NV
Priority date: 2015-08-12
Filing date: 2016-08-10
Publication date: 2021-11-19
Anticipated expiration: 2036-08-10
Also published as: TWI739755B; JP6952844B2; TWI788998B; KR20180038543A; JP2018532138A; TW202210958A; KR20240015174A; CN113966061A; TW201729480A; JP2022008595A; JP7241143B2; JP2020194178A; WO2017027566A1; JP6744397B2; KR102631831B1; CN108353489A

Abstract

A method, comprising: providing a target material comprising a component that Emits Ultraviolet (EUV) light when converted to plasma; directing a first beam of radiation toward a target material to impart energy to the target material to modify a geometric distribution of the target material to form a modified target; directing a second beam of radiation toward the modified target, the second beam of radiation converting at least a portion of the modified target into a plasma that emits EUV light; measuring one or more characteristics associated with one or more of the target material and the modified target relative to the first beam of radiation; and controlling the amount of radiant exposure delivered from the first beam of radiation to the target material to be within a predetermined energy range based on the measured one or more characteristics.

Description

Target expansion rate control in extreme ultraviolet light sources

Cross Reference to Related Applications

This application claims the benefit of U.S. Serial No. 14/824,141 entitled "TARGET EXPANSION RATE CONTROL IN AN EXTREME ULTRAVIOLET LIGHT SOURCE" filed on 8/12/2015 and U.S. Serial No. 14/824,147 entitled "STABILIZING EUV LIGHT POWER AN EXTREME ULTRAVIOLET LIGHT SOURCE" filed on 8/12/2015, both of which are incorporated herein by reference.

Technical Field

The disclosed subject matter relates to controlling the rate of expansion of a target material for a laser produced plasma extreme ultraviolet light source.

Background

Extreme Ultraviolet (EUV) light, e.g., electromagnetic radiation having a wavelength of about 50nm or less (sometimes also referred to as soft x-rays), and including light having a wavelength of about 13nm, can be used in a lithographic process to produce very small features in a substrate, such as a silicon wafer.

Methods of generating EUV light include, but are not necessarily limited to, converting a material having an element such as xenon, lithium, or tin using an emission spectrum line in the EUV range in a plasma state. In one such method, commonly referred to as laser produced plasma ("LPP"), the required plasma may be produced by irradiating the target material (e.g., in the form of droplets, slabs, ribbons, streams or clusters of material) with an amplified beam, which may be referred to as a drive laser. For this process, plasma is typically generated in a sealed container (e.g., a vacuum chamber) and monitored using various types of metrology equipment.

Disclosure of Invention

In some general aspects, a method includes: providing a target material comprising a component that Emits Ultraviolet (EUV) light when converted to plasma; directing a first beam of radiation toward a target material to impart energy to the target material to modify a geometric distribution of the target material to form a modified target; directing a second beam of radiation toward the modified target, the second beam of radiation converting at least a portion of the modified target into a plasma that emits EUV light; measuring one or more characteristics associated with one or more of the target material and the modified target relative to the first beam of radiation; and controlling an exposure of radiation delivered from the first beam of radiation to the target material to be within a predetermined energy range based on the measured one or more characteristics.

Implementations may include one or more of the following features. For example, one or more characteristics associated with one or more of the target material and the modified target may be measured by measuring the energy of the first beam of radiation. The energy of the first radiation beam may be measured by measuring the energy of the first radiation beam reflected from the optically reflective surface of the target material. The energy of the first beam of radiation may be measured by measuring the energy of the first beam of radiation directed towards the target material. The energy of the first radiation beam may be measured by measuring the spatially integrated energy in a direction perpendicular to the direction of propagation of the first radiation beam.

The first radiation beam may be directed towards the target material by overlapping the target material with a region of the first radiation beam encompassing its confocal parameter. The confocal parameter may be greater than 1.5 mm.

One or more characteristics associated with one or more of the target material and the modified target may be measured by measuring a position of the target material relative to a target location. The target position may coincide with a beam waist of the first radiation beam. The first beam of radiation may be directed along a first beam axis and the position of the target material may be measured along a direction parallel to the first beam axis. The target position may be measured relative to a primary focus of a collector device collecting the emitted EUV light. The position of the target material may be measured by measuring the position of the target material along two or more non-parallel directions.

One or more characteristics associated with one or more of the target material and the modified target may be measured by detecting dimensions of the modified target before the second beam of radiation converts at least a portion of the modified target into plasma. One or more characteristics associated with one or more of the target material and the modified target may be measured by estimating a rate of expansion of the modified target.

The radiant exposure delivered from the first beam of radiation to the target material can be controlled by controlling the rate of expansion of the modified target.

The amount of radiant exposure delivered from the first beam of radiation to the target material can be controlled by determining whether a characteristic of the first beam of radiation should be adjusted based on the measured one or more characteristics. The determination that the characteristics of the first beam of radiation should be adjusted may be performed while measuring one or more characteristics.

If it is determined that the characteristic of the first beam of radiation should be adjusted, one or more of the following may be adjusted: the energy content of the pulses of the first radiation beam and the region of the first radiation beam that interacts with the target material. The energy content of the pulses of the first radiation beam may be adjusted by adjusting one or more of: a pulse width of the first radiation beam; a duration of a pulse of the first radiation beam; and an average power within the pulses of the first radiation beam.

The first beam of radiation may be directed toward the target material by directing a first pulse of radiation toward the target material; one or more characteristics may be measured by measuring one or more characteristics of each first radiation pulse; and it may be determined whether the characteristic of the first beam of radiation should be adjusted by determining whether the characteristic should be adjusted for each first pulse of radiation.

The amount of radiant exposure delivered from the first beam of radiation to the target material may be controlled by controlling the amount of radiant exposure delivered from the first beam of radiation to the target material while at least a portion of the emitted EUV light exposes the wafer.

The target material may be provided by providing droplets of the target material; the geometric distribution of the target material can be modified by converting droplets of the target material into disk-shaped volumes of molten metal. The droplets of target material may be converted to disk-like volumes according to the rate of expansion.

The method may further comprise collecting at least a portion of the emitted EUV light; and directing the collected EUV light toward the wafer to expose the wafer to the EUV light.

The one or more characteristics may be measured by measuring at least one characteristic of each pulse of the first beam of radiation directed towards the target material.

The first beam of radiation may be directed towards the target material such that a portion of the target material is converted to EUV light-emitting plasma, and the converted plasma from the target material emits less EUV light than EUV light emitted from the modified target converted plasma, and the predominant effect on the target material is a modification of the geometric distribution of the target material to form the modified target.

The geometric distribution of the target material may be modified by transforming the shape of the target material into a modified target, including expanding the modified target along at least one axis according to an expansion rate. The radiant exposure delivered to the target material can be controlled by controlling the rate of expansion of the target material to the modified target.

The modified target may extend along at least one axis that is non-parallel to the optical axis of the second beam of radiation.

One or more characteristics associated with one or more of the target material and the modified target may be measured by measuring the number of photons reflected from the modified target. The number of photons reflected from the modified target may be measured by measuring the number of photons reflected from the modified target as a function of how many photons strike the target material.

The first beam of radiation may be directed toward the target material by directing a first pulse of radiation toward the target material; and the second beam of radiation may be directed towards the modified target by directing a second pulse of radiation towards the modified target.

The first beam of radiation may be directed by directing the first beam of radiation through a first group of one or more optical amplifiers; and the second beam of radiation may be directed by directing the second beam of radiation through a second group of one or more optical amplifiers; wherein at least one optical amplifier in the first group is located in the second group.

One or more characteristics associated with one or more of the target material and the modified target may be measured by measuring an energy of a first beam of radiation directed toward the target material; and the amount of radiant exposure delivered to the target material may be controlled by adjusting the amount of energy directed from the first beam of radiation to the target material based on the measured energy. Directing the first radiation beam towards the target material by overlapping the target material with a region of the first radiation beam encompassing its confocal parameter; and the confocal parameter may be less than or equal to 2 mm.

The amount of energy directed from the first beam of radiation to the target material may be adjusted by adjusting a property of the first beam of radiation.

The amount of radiant exposure delivered from the first beam of radiation to the target material may be controlled by adjusting one or more of: an energy of the first radiation beam immediately prior to the first radiation beam delivering energy to the target material; the location of the target material; and a region where the target material interacts with the first beam of radiation.

The first radiation beam may be directed by directing the first radiation beam through a first set of optical components including one or more first optical amplifiers; and the second radiation beam may be directed by directing the second radiation beam through a second set of optical components including one or more second optical amplifiers; wherein the first set of optical components is different from and separate from the second set of optical components.

In other general aspects, an apparatus includes a chamber defining an initial target location to receive a first beam of radiation and a target location to receive a second beam of radiation; a target material delivery system configured to provide a target material to an initial target location, the target material comprising a material that Emits Ultraviolet (EUV) light when converted to plasma; a light source configured to generate a first beam of radiation and a second beam of radiation; and an optical turning system. The optical steering system is configured to: the method includes directing a first beam of radiation toward an initial target location to transfer energy to a target material to modify a geometric distribution of the target material to form a modified target, and directing a second beam of radiation toward the target location to convert at least a portion of the modified target to a plasma that emits EUV light. The device includes: a measurement system to measure one or more characteristics associated with one or more of the target material and the modified target relative to the first beam of radiation; and a control system connected to the target material delivery system, the light source, the optical turning system, and the measurement system. The control system is configured to receive the measured one or more characteristics from the measurement system and send one or more signals to the light source to control a radiant exposure delivered from the first beam of radiation to the target material based on the measured one or more characteristics.

Implementations may include one or more of the following features. For example, the optical steering system may comprise a focusing device configured to focus the first radiation beam at or near an initial target position and to focus the second radiation beam at or near a target position.

The apparatus may include a beam adjustment system, wherein the beam adjustment system is connected to the light source and the control system, and the control system is configured to send one or more signals to the light source to control the amount of energy delivered to the target material by sending the one or more signals to the beam adjustment system, the beam adjustment system configured to adjust one or more characteristics of the light source to maintain the amount of energy delivered to the target material. The beam conditioning system can include a pulse width modulation system coupled to the first beam of radiation, the pulse width modulation system configured to modulate a pulse width of pulses of the first beam of radiation. The pulse width modulation system may comprise an electro-optic modulator.

The beam conditioning system can include a pulse power conditioning system coupled to the first beam of radiation, the pulse power conditioning system configured to adjust an average power within pulses of the first beam of radiation. The pulse power modulation system may include an acousto-optic modulator.

The beam adjustment system may be configured to send one or more signals to the light source to control the amount of energy directed to the target material by sending one or more signals to the beam adjustment system, the beam adjustment system configured to adjust one or more characteristics of the light source to control the amount of energy directed to the target material.

The optical source may comprise a first group of one or more optical amplifiers through which the first beam of radiation passes; and a second group of one or more optical amplifiers through which the second radiation beam passes, at least one optical amplifier in the first group being in the second group. The measurement system may measure an energy of the first radiation beam as the first radiation beam is directed toward the initial target position; and the control system may be configured to receive the measured energy from the measurement system and send one or more signals to the light source to control an amount of energy directed from the first beam of radiation to the target material based on the measured energy.

In some general aspects, a method includes: providing a target material comprising a component that Emits Ultraviolet (EUV) light when converted to plasma; directing a first beam of radiation toward a target material to impart energy to the target material to modify a geometric distribution of the target material to form a modified target; directing a second beam of radiation toward the modified target, the second beam of radiation converting at least a portion of the modified target into a plasma that emits EUV light; controlling a radiation exposure delivered from the first radiation beam to the target material to be within a predetermined radiation exposure range; and stabilizing the power of the EUV light emitted from the plasma by controlling the amount of radiation exposure delivered from the first beam of radiation to the target material to be within a predetermined range of radiation exposure.

Implementations may include one or more of the following features. For example, the first radiation beam may be directed by directing the first radiation beam through a first set of optical components including one or more first optical amplifiers; and the second radiation beam may be directed by directing the second radiation beam through a second set of optical components including one or more second optical amplifiers. The first set of optical components may be different from and separate from the second set of optical components.

The first radiation beam may be directed by directing the first radiation beam through a first set of one or more optical amplifiers; and the second radiation beam may be directed by directing the second radiation beam through a second set of one or more optical amplifiers; wherein at least one optical amplifier in the first group is in the second group.

The target material may be provided by providing droplets of the target material; and the geometric distribution of the target material can be altered by transforming droplets of the target material into a disk-like volume of molten metal having a substantially flat surface.

The target material may be provided by providing droplets of the target material; and the geometric distribution of the target material can be changed by transforming the droplets of the target material into a mist volume of molten metal particles.

The target material may be converted to a modified target based on the rate of expansion.

The amount of radiant exposure delivered from the first beam of radiation to the target material may be controlled by: measuring one or more characteristics associated with one or more of the target material and the modified target relative to the first beam of radiation; and maintaining the amount of radiant exposure delivered from the first beam of radiation to the target material within a predetermined range of radiant exposures based on the measured one or more characteristics.

The radiant exposure delivered from the first beam of radiation to the target material can be controlled by estimating the rate of expansion of the modified target. The radiant exposure delivered from the first beam of radiation to the target material can be controlled by maintaining a modified expansion rate of the target.

The radiant exposure delivered from the first radiation beam to the target material can be controlled by determining whether a characteristic of the first radiation beam should be adjusted. The amount of radiant exposure delivered from the first beam of radiation to the target material may be controlled by: the characteristic of the first radiation beam is adjusted by adjusting one or more of an energy content of each pulse of the first radiation beam and a region of the first radiation beam that interacts with the target material. The energy content of each pulse of the first beam of radiation may be adjusted by adjusting one or more of: a width of each pulse of the first beam of radiation, a duration of each pulse of the first beam of radiation, and a power of each pulse of the first beam of radiation.

The power of the EUV light emitted from the plasma may be stabilized by stabilizing the power of the EUV light while at least a portion of the EUV light emitted from the plasma exposes the wafer.

The geometric distribution of the target material may be modified by transforming the shape of the target material into a modified target, including expanding the modified target along at least one axis according to an expansion rate.

The amount of radiant exposure delivered from the first beam of radiation to the target material can be controlled by adjusting a property of the first beam of radiation. The property of the first beam of radiation may be modulated by modulating the energy of the first beam of radiation.

In other general aspects, an apparatus includes a chamber defining an initial target location to receive a first beam of radiation and a target location to receive a second beam of radiation; a target material delivery system configured to provide a target material to an initial target location, the target material comprising a material that Emits Ultraviolet (EUV) light when converted to plasma; a light source configured to generate a first beam of radiation and a second beam of radiation; and an optical turning system. The optical steering system is configured to: the method includes directing a first beam of radiation toward an initial target location to transfer energy to a target material to modify a geometric distribution of the target material to form a modified target, and directing a second beam of radiation toward the target location to convert at least a portion of the modified target to a plasma that emits EUV light. The apparatus includes a control system connected to the target material delivery system, the light source, and the optical steering system and configured to send one or more signals to the light source to control a radiation exposure delivered from the first beam of radiation to the target material to be within a predetermined range of radiation exposures to stabilize the power of EUV light emitted from the plasma.

Implementations may include one or more of the following features. For example, the apparatus may further include a measurement system that measures one or more characteristics associated with one or more of the target material and the modified target relative to the first beam of radiation; wherein the control system is connected to the measurement system.

The apparatus may further include a beam adjustment system, wherein the beam adjustment system is coupled to the light source and the control system, and the control system is configured to send one or more signals to the light source to control the amount of radiant exposure delivered to the target material by sending the one or more signals to the beam adjustment system, the beam adjustment system being configured to adjust one or more characteristics of the light source to control the amount of radiant exposure delivered to the target material.

Drawings

FIG. 1 is a block diagram of a laser-produced plasma extreme ultraviolet light source including a light source that produces a first beam of radiation directed toward a target material and a second beam of radiation directed toward a modified target to convert a portion of the modified target to a plasma that emits EUV light;

FIG. 2 is a schematic diagram showing a first radiation beam directed towards a first target location and a second radiation beam directed towards a second target location;

FIG. 3A is a block diagram of an exemplary light source for use in the light source of FIG. 1;

FIGS. 3B and 3C are block diagrams of an exemplary beam path combiner and an exemplary beam path splitter, respectively, that may be used in the light source of FIG. 1;

FIGS. 4A and 4B are block diagrams of exemplary optical amplifier systems that may be used in the light source of FIG. 3A;

FIG. 5 is a block diagram of an exemplary optical amplifier system that may be used in the light source of FIG. 3A;

FIG. 6 is a schematic diagram illustrating another implementation of a first beam of radiation directed toward a first target location and a second beam of radiation directed toward a second target location;

FIGS. 7A and 7B are schematic diagrams illustrating an implementation of a first radiation beam directed towards a first target location;

8A-8C and 9A-9C illustrate schematic diagrams of various implementations of a measurement system that measures at least one characteristic associated with any one or more of a target material, a modified target, and a first radiation beam;

FIG. 10 is a block diagram of an exemplary control system for the light source of FIG. 1;

FIG. 11 is a flow chart of an exemplary process performed by a light source (under control of a control system) for maintaining or controlling an Extension Rate (ER) of a modified target to increase conversion efficiency of the light source;

FIG. 12 is a flow chart of an exemplary process performed by a light source for stabilizing the power of EUV light emitted from a plasma by controlling the radiant exposure delivered from a first beam of radiation to a target material; and

FIG. 13 is a block diagram of an exemplary light source producing first and second beams of radiation and an exemplary beam delivery system modifying and focusing the first and second beams of radiation to respective first and second target locations.

Detailed Description

Techniques for improving the conversion efficiency of Extreme Ultraviolet (EUV) light production are disclosed. Referring to FIG. 1, as discussed in more detail below, the interaction between target material 120 and first radiation beam 110 causes the target material to deform and geometrically expand to form modified target 121. The geometric expansion of the modified target 121 is controlled in a manner that increases the amount of usable EUV light 130 converted from plasma due to interaction between the modified target 121 and the second beam of radiation 115. The amount of EUV light 130 available is the amount of EUV light 130 available for utilization at optics 145. Thus, the amount of EUV light 130 available may depend on aspects such as the bandwidth or center wavelength of the optical components used to utilize the EUV light 130.

Control of the geometric expansion rate of the modified target 121 enables control of the size or geometry of the modified target 121 when interacting with the second radiation beam 115. For example, adjustment of the geometric expansion rate of the modified target 121 may adjust the density of the modified target 121 when interacting with the second radiation beam 115; since the density of the modified target 121 affects the total amount of radiation absorbed by the modified target 121 and the range over which such radiation is absorbed as the modified target 121 interacts with the second beam of radiation 115. As the density of the modified target 121 increases, at some point, the EUV light 130 will not be able to escape from the modified target 121 and thus the amount of usable EUV light 130 may decrease. As another example, the adjustment of the geometric expansion rate of the modified target 121 may adjust the surface area of the modified target 121 as the modified target 121 interacts with the second radiation beam 115.

In this way, the total amount of usable EUV light 130 produced may be increased or controlled by controlling the rate of expansion of the modified target 121. In particular, the size of the modified target 121 and its rate of expansion depend on the amount of radiation exposure applied from the first beam of radiation 110 to the target material 120, which is the amount of energy delivered by the first beam of radiation 110 to a region of the target material 120. Thus, the rate of expansion of the modified target 121 may be maintained or controlled by maintaining or controlling the amount of energy delivered to the target material 120 per unit area. The amount of energy delivered to target material 120 depends on the energy of first radiation beam 110 immediately before first radiation beam 110 strikes the surface of the target material.

The energy of the pulses in first radiation beam 110 may be determined by integrating the laser pulse signal measured by the fast photodetector. The detector may be a photo-electromagnetic (PEM) detector suitable for Long Wavelength Infrared (LWIR) radiation, an InGaAs diode for measuring near Infrared (IR) radiation, or a silicon diode for visible or near IR radiation.

The rate of expansion of the modified target 121 depends at least in part on the amount of energy in the pulses of the first beam of radiation 110 intercepted by the target material 120. In the assumed baseline design, target material 120 is assumed to always have the same dimensions and be placed in the waist of focused first radiation beam 110. In practice, however, target material 120 may have a small, but mostly constant, axial position offset with respect to the beam waist of first radiation beam 110. If all of these factors are held constant, one factor controlling the rate of expansion of modified target 121 is the pulse energy of first radiation beam 110 for pulses of the first radiation beam having a duration of a few ns to 100 ns. Another factor that may control the rate of expansion of modified target 121 is the instantaneous peak power of first beam of radiation 110 if the pulses of first beam of radiation 110 have a duration equal to or below 100 ns. Other factors may control the rate of expansion of modified target 121 if the pulses of first beam of radiation 110 have a shorter duration, for example, on the order of picoseconds (ps), as described below.

As shown in fig. 1, a light source 105 (also referred to as a drive source or drive laser) is used to drive a Laser Produced Plasma (LPP) Extreme Ultraviolet (EUV) light source 100. Light source 105 produces a first beam of radiation 110 that is provided to a first target location 111 and a second beam of radiation 115 that is provided to a second target location 116. First radiation beam 110 and second radiation beam 115 may be pulsed amplified beams.

The first target location 111 receives a target material 120, such as tin, from a target material supply system 125. The interaction between first beam of radiation 110 and target material 120 imparts energy to target material 120 to modify or change (e.g., deform) its shape such that the geometric distribution of target material 120 is deformed into modified target 121. The target material 120 is generally directed from the target material supply system 125 along the-X direction or along the direction in which the target material 120 is placed within the first target location 111. After first beam of radiation 110 imparts energy to target material 120 to deform it into modified target 121, modified target 121 may continue to move along the-X direction in addition to moving along another direction, such as a direction parallel to the Z direction. As the modified target 121 moves away from the first target location 111, its geometric distribution continues to deform until the modified target 121 reaches the second target location 116. The interaction between the second beam of radiation 115 and the modified target 121 (at the second target location 116) converts at least a portion of the modified target 121 into a plasma 129 that emits EUV light or radiation 130. A light collector system (or light collector) 135 collects EUV light 130 and directs the EUV light 130 as collected EUV light 140 toward an optical device 145, such as a lithography tool. The first and

second target locations

111, 116 and the light collector 135 may be housed within a chamber 165 that provides a controlled environment suitable for producing EUV light 140.

Some target material 120 may be converted to a plasma upon interaction with first beam of radiation 110, and such a plasma may therefore emit EUV radiation. However, the properties of first radiation beam 110 are selected and controlled such that the dominant effect of first radiation beam 110 on target material 120 is a deformation or modification of the geometric distribution of target material 120 to form modified target 121.

Each of first radiation beam 110 and second radiation beam 115 is directed by beam delivery system 150 towards a

respective target location

111, 116. Beam delivery system 150 may include an optical turning component 152 and a focusing assembly 156, focusing assembly 156 focusing first beam of radiation 110 or second beam of radiation 115 to respective first and second focal regions. The first and second focus regions may overlap with first and

second target locations

111 and 116, respectively. Optical component 152 may include optical elements, such as lenses and/or mirrors, that direct beams of

radiation

110, 115 by refraction and/or reflection. The beam delivery system 150 may also include elements to control and/or move the optical components 152. For example, beam delivery system 150 may include an actuator controllable to cause movement of an optical element within optical component 152.

Referring also to FIG. 2, focusing assembly 156 focuses first radiation beam 110 such that diameter D1 of first radiation beam 110 is at a minimum in first focal region 210. In other words, focusing assembly 156 causes first beam of radiation 110 to converge as first beam of radiation 110 propagates in a first axial direction 212 toward first focal region 210, first axial direction 212 being the general direction of propagation of first beam of radiation 110. The first axial direction 212 extends along a plane defined by the XZ axis. In this example, the first axial direction 212 is parallel or nearly parallel to the Z-direction, but it may be at an angle relative to the Z-direction. In the absence of target material 120, first beam of radiation 110 diverges as first beam of radiation 110 propagates away from first focal region 210 in first axial direction 212.

In addition, the focusing assembly 156 focuses the second radiation beam 115 such that the diameter D2 of the second radiation beam 115 is smallest in the second focal region 215. Thus, the focusing assembly causes the second beam of radiation 115 to converge as the second beam of radiation 115 propagates in a second axial direction 217 toward the second focal region 215, the second axial direction 217 being the general direction of propagation of the second beam of radiation 115. The second axial direction 217 also extends along a plane defined by the XZ axis, and in this example, the second axial direction 217 is parallel or nearly parallel to the Z direction. In the absence of the modified target 121, the second radiation beam 115 diverges as the second radiation beam 115 propagates away from the second focal region 215 in the second axial direction 217.

The EUV light source 100 also includes one or more measurement systems 155, a control system 160, and a beam conditioning system 180, as described below. The control system 160 is connected to other components within the light source 100, such as, for example, the measurement system 155, the beam delivery system 150, the target material supply system 125, the beam conditioning system 180, and the light source 105. The measurement system 155 may measure one or more characteristics within the light source 100. For example, the one or more characteristics may be characteristics associated with target material 120 or modified target 121 relative to first beam of radiation 110. As another example, the one or more characteristics may be a pulse energy of first beam of radiation 110 directed towards target material 120. These examples are discussed in more detail below. Control system 160 is configured to receive the measured one or more characteristics from the measurement system so that it can control how first beam of radiation 110 interacts with target material 120. For example, control system 160 may be configured to maintain the amount of energy delivered from first beam of radiation 110 to target material 120 within a predetermined energy range. As another example, control system 160 may be configured to control the amount of energy directed from first beam of radiation 110 toward target material 120. Beam conditioning system 180 is a system that includes components within optical source 105 or that conditions components within optical source 105 to control a property of first beam of radiation 110, such as pulse width, pulse energy, instantaneous power within a pulse, or average power within a pulse.

Referring to fig. 3A, in some implementations, light source 105 includes a first optical amplifier system 300 and a second optical amplifier system 305, first optical amplifier system 300 including a series of one or more optical amplifiers through which first radiation beam 110 passes, and second optical amplifier system 305 including a series of one or more optical amplifiers through which second radiation beam 115 passes. One or more amplifiers from the first system 300 may be located in the second system 305; or one or more amplifiers in the second system 305 may be located in the first system 300. Alternatively, the first optical amplifier system 300 may be completely separate from the second optical amplifier system 305.

Additionally, although not required, the light source 105 can include a first light generator 310 that generates a first pulsed light beam 311 and a second light generator 315 that generates a second pulsed light beam 316. For example, the

light generators

310, 315 may each be a laser, a seed laser such as a master oscillator, or a lamp. An exemplary light generator that may be used as the

light generators

310, 315 is a Q-switched Radio Frequency (RF) pumped axial flow carbon dioxide (CO) that may be operated at a repetition frequency of, for example, 100kHz₂) An oscillator.

The optical amplifiers within the

optical amplifier systems

300, 305 each contain a gain medium in a respective beam path along which the light beams 311, 316 of the respective

light generators

310, 315 propagate. When the gain medium of the optical amplifier is activated, the gain medium provides photons to the beam, amplifying the

beams

311, 316 to produce amplified beams forming the first radiation beam 110 or the second radiation beam 115.

The wavelengths of

beams

311, 316 or

beams

110, 115 may be different from each other, so that

beams

110, 115 may also be separated from each other if combined at any point within source 105. If radiation beams 110, 115 consist of CO₂The amplifier produces that first radiation beam 110 may have a wavelength of 10.26 microns (μm) or 10.207 μm, and second radiation beam 115 may have a wavelength of 10.59 μm. The wavelengths are selected to more easily separate the two

radiation beams

110, 115 using dispersive optics or dichroic mirrors or beam splitter coatings. In the case where the two

radiation beams

110, 115 propagate together in the same amplifier chain (e.g., in the case where some amplifiers of optical amplifier system 300 are in optical amplifier system 305), the different wavelengths may be used to adjust the relative gain between the two

radiation beams

110, 115 even though they are passing through the same amplifier.

For example, once separated, the beams of

radiation

110, 115 may be steered or focused to two separate locations (such as first target location 111 and second target location 116, respectively) within chamber 165. In particular, the separation of

beams

110, 115 also enables modified target 121 to expand after interacting with first beam 110 as it travels from first target position 111 to second target position 116.

Light source 105 may include a beam path combiner 325, beam path combiner 325 superimposing first radiation beam 110 and second radiation beam 115 and placing

radiation beams

110, 115 on the same optical path at least some distance between light source 105 and beam delivery system 150. An exemplary beam path combiner 325 is shown in fig. 3B. Beam path combiner 325 includes a pair of

dichroic beam splitters

340, 342 and a pair of

mirrors

344, 346. Dichroic beamsplitter 340 enables first beam of radiation 110 to pass along a first path to dichroic beamsplitter 342. The dichroic beam splitter 340 reflects the second radiation beam 115 along a second path in which the second radiation beam 115 is reflected from

mirrors

344, 346, the

mirrors

344, 346 redirecting the second radiation beam 115 towards the dichroic beam splitter 342. The first radiation beam 110 freely passes through the dichroic beam splitter 342 to the output path, while the second radiation beam 115 is reflected from the dichroic beam splitter 342 onto the output path, such that both the first radiation beam 110 and the second radiation beam 115 are superimposed on the output path.

Additionally, light source 105 may include a beam path splitter 326, beam path splitter 326 separating first radiation beam 110 from second radiation beam 115, such that both

radiation beams

110, 115 may be individually diverted and focused within chamber 165. An exemplary beam path splitter 326 is shown in fig. 3C. The beam path splitter 326 includes a pair of

dichroic beam splitters

350, 352 and a pair of

mirrors

354, 356. The dichroic beam splitter 350 receives the superimposed pair of

radiation beams

110, 115, reflects the second radiation beam 115 along the second path, and transmits the first radiation beam 110 along the first path towards the dichroic beam splitter 352. First radiation beam 110 is free to pass through dichroic beam splitter 352 along a first path. The second radiation beam 115 reflects from

mirrors

354, 356 and returns to the dichroic beam splitter 352 where it is reflected onto a second path, different from the first path.

In addition, first beam of radiation 110 can be configured to have a pulse energy that is less than a pulse energy of second beam of radiation 115. This is because the first radiation beam 110 is used to modify the geometry of the target material 120, while the second radiation beam 115 is used to convert the modified target 121 into plasma 129. For example, the pulse energy of first radiation beam 110 may be 1/5-1/100 as the pulse energy of second radiation beam 115.

In some implementations, as shown in fig. 4A and 4B, the

optical amplifier system

300 or 305 includes a set of three

optical amplifiers

401, 402, 403 and 406, 407, 408, respectively, although only one amplifier or more than three amplifiers may be used. In some implementations, each of the

optical amplifiers

406, 407, 408 includes a gain medium including CO₂And light having a wavelength between about 9.1 and about 11.0 μm, and particularly about 10.6 μm, can be amplified with a gain exceeding 1000. The

optical amplifiers

401, 402, 403 may operate similarly or at different wavelengths. Suitable amplifiers and lasers for use in the

optical amplifier systems

300, 305 may include pulsed laser devices, such as pulsed gas discharge CO₂An amplifier that generates about 9.3 μm or about 10.6 μm of radiation, for example using DC or RF excitation, and operates at relatively high power (e.g., 10kW or more) and high pulse repetition rate (e.g., 50kHz or more). Exemplary

optical amplifiers

401, 402, 403 or 406, 407, 408 are axial flow high power CO with abrasion free gas circulation and capacitive RF excitation₂Lasers, such as TruFlow CO produced by TRUMPF of famington, Connecticut₂A laser.

Additionally, although not required, one or more of

optical amplifier systems

300 and 305 may include a first amplifier that serves as

pre-amplifiers

411, 421, respectively. The preamplifiers 411, 421 (if present) may be diffusion cooled CO₂Laser systems, such as TruCoax CO produced by TRUMPF of famedington, Connecticut₂A laser system.

The

optical amplifier systems

300, 305 may include optical elements not shown in fig. 4A and 4B for directing and shaping the respective

optical beams

311, 316. For example, the

optical amplifier systems

300, 305 may include reflective optics such as mirrors, partially transmissive optics such as beam splitters or partially transmissive mirrors, and dichroic beam splitters.

The light source 105 also includes an optical system 320, and the optical system 320 may include one or more optics (such as reflective optics such as mirrors, partially reflective and partially transmissive optics such as beam splitters, refractive optics such as prisms or lenses, passive optics, active optics, etc.) for directing the light beams 311, 316 through the light source 105.

Although

optical amplifiers

401, 402, 403 and 406, 407, 408 are shown as separate blocks, at least one of

amplifiers

401, 402, 403 may be located in optical amplifier system 305 and at least one of

amplifiers

406, 407, 408 may be located in optical amplifier system 300. For example, as shown in FIG. 5,

amplifiers

402, 403 correspond to

respective amplifiers

407, 408, and

optical amplifier systems

300, 305 include additional optical elements 500 (such as beam path combiner 325) for combining the two beams output from

amplifiers

401, 406 into a single path through amplifier 402/407 and amplifier 403/408. In such systems where at least some of the amplifiers and optics overlap between

optical amplifier systems

300, 305, first radiation beam 110 and second radiation beam 115 may be coupled together such that a change in one or more characteristics of first radiation beam 110 may cause a change in one or more characteristics of second radiation beam 115, and vice versa. Therefore, it becomes more important to control energy within the system, such as the energy of first beam of radiation 110 or the energy delivered to target material 120. In addition, the

optical amplifier system

300, 305 also includes an optical element 505 (such as a beam path splitter 326) for splitting the two

beams

110, 15 output from the amplifier 403/408 so that the two

beams

110, 115 can be directed towards

respective target locations

111, 116.

The target material 120 may be any material including a target material that emits EUV light when converted into plasma. The target material 120 may be a target mixture including a target substance and impurities such as non-target particles. The target substance is a substance that can be converted into a plasma state having an emission line in the EUV range. For example, the target substance may be a droplet of liquid or molten metal, a portion of a liquid stream, a solid particle or cluster, a solid particle contained within a droplet, a foam of the target material, or a solid particle contained in a portion of a liquid stream. For example, the target substance may be water, tin, lithium, xenon or any material having an emission line in the EUV range when converted into a plasma state. For example, the target substance may be elemental tin, which may be used as pure tin (Sn); as the tin compound, for example, SnBr4, SnBr2, SnH 4; as a tin alloy, for example a tin-gallium alloy, a tin-indium-gallium alloy or any combination of these alloys. Also, in the absence of impurities, the target material contains only the target substance. The following discussion provides examples in which the target material 120 is a droplet made of a molten metal such as tin. However, the target material 120 may take other forms.

The target material 120 may be provided to the first target location 111 by passing the molten target material through a nozzle of the target material supply device 125 and allowing the target material 120 to drift into the first target location 111. In some implementations, the target material 120 may be directed toward the first target location 111 by a force.

By irradiating target material 120 with pulses of radiation from first beam of radiation 110, the shape of target material 120 is changed or modified (e.g., deformed) before reaching second target location 116.

The interaction between the first beam of radiation 110 and the target material 120 causes ablation of material from the surface of the target material 120 (and the modified target 121), and this ablation provides a force that deforms the target material 120 into the modified target 121 having a shape different from that of the target material 120. For example, the target material 120 may have a shape similar to a droplet, while the shape of the modified target 121 is deformed such that its shape is closer to the shape of a disc (such as a flat shape) when it reaches the second target location 116. The modified target 121 may be a material that is not ionized (non-plasma material) or a material that is minimally ionized. For example, the modified target material 121 may be a disc of liquid or molten metal, a continuous portion of target material without voids or significant gaps, a mist of micro-or nanoparticles, or a cloud of atomic vapor. For example, as shown in FIG. 2, the modified target material 121 expands into a disk-shaped piece of molten metal 121 within the second target location 116 after about time T2-T1 (which may be on the order of microseconds (μ s)).

Furthermore, the interaction between first beam of radiation 110 and target material 120 that causes ablation of material from the surface of target material 120 (and modified target 121) may provide a force that can cause modified target 121 to gain some propulsion or velocity in the Z direction. The spread of modified target 121 in the X-direction and the obtained velocity in the Z-direction depends on the energy of first radiation beam 110, and in particular on the energy delivered to (i.e. intercepted by) target material 120.

For example, for a constant target material 120 size and a long pulse of first radiation beam 110 (a long pulse is a pulse with a duration between a few nanoseconds (ns) to 100 ns), the expansion rate is related to the energy per unit area of first radiation beam 110 (joules/cm)²) Is linearly proportional. The energy per unit area is also referred to as the radiation exposure or energy density. The radiant exposure is the radiant energy received by the surface of the target material 120 per unit area, or corresponds to the irradiance of the surface of the target material 120 integrated over the time the target material 120 is illuminated.

As another example, the relationship between the spread rate and the energy of first radiation beam 110 may be different for constant target material 120 size and short pulses (pulses with a duration less than a few hundred picoseconds (ps)). In this case, the shorter pulse duration is associated with an increase in the intensity of first beam of radiation 110 interacting with target material 120, and first beam of radiation 110 appears as a shockwave. In this case, the rate of expansion depends primarily on the intensity I of first radiation beam 110, and the intensity is equal to the energy E of the first radiation beam divided by the spot size (cross-sectional area a) and pulse duration (τ) of first radiation beam 110 interacting with target material 120, or I ═ E/(a · τ). In this ps pulse duration state, the modified target 121 expands to form a fog.

In addition, the angular orientation of the disk shape of modified target 121 (the angle relative to the Z-direction or X-direction) depends on the position of first radiation beam 110 when it strikes target material 120. Thus, if first beam of radiation 110 impinges on target material 120 such that first beam of radiation 110 surrounds the target material and the beam waist of first beam of radiation 110 is centered on target material 120, the disk shape of modified target 121 will be more likely to be aligned with its major axis 230 parallel to the X-direction and its minor axis 235 parallel to the Z-direction.

First beam of radiation 110 is comprised of pulses of radiation, and each pulse may have a duration. Similarly, the second beam of radiation 115 consists of pulses of radiation, and each pulse may have a duration. The pulse duration may be expressed in terms of the full width at a certain percentage (e.g., half) of the maximum value, i.e., the amount of time that the intensity of the pulse is at least that percentage of the maximum intensity of the pulse. However, other metrics may be used to determine pulse duration. For example, the pulse duration of pulses within first beam of radiation 110 may be 30 nanoseconds (ns), 60ns, 130ns, 50-250ns, 10-200 picoseconds (ps), or less than 1 ns. For example, the energy of first radiation beam 110 may be 1-100 millijoules (mJ). For example, the wavelength of first radiation beam 110 may be 1.06 μm, 1-10.6 μm, 10.59 μm, or 10.26 μm.

As described above, the rate of expansion of modified target 121 depends on the radiant exposure (energy per unit area) of first beam of radiation 110 that intercepts target material 120. Thus, for a pulse of first radiation beam 110 having a duration of about 60ns and an energy of about 50mJ, the actual radiation exposure depends on how tightly first radiation beam 110 is focused in first focus region 210. In some examples, the radiation exposure may be about 400-700 joules/cm at the target material 120². However, the amount of radiation exposure is very sensitive to the position of target material 120 relative to first beam of radiation 110.

The second radiation beam 115 may be referred to as the main beam and consists of pulses released at a certain repetition rate. The second beam of radiation 115 has sufficient energy to convert target species within the modified target 121 into a plasma that emits EUV light 130. The pulses of first radiation beam 110 and second radiation beam 115 are separated in time by a delay time such as, for example, 1-3 microseconds (μ s), 1.3 μ s, 1-2.7 μ s, 3-4 μ s, or any amount of time that allows the modified target 121 to be expanded into the disk shape with the desired dimensions shown in FIG. 2. Thus, as the modified target 121 expands and elongates in the XY plane, the modified target 121 undergoes two-dimensional expansion.

The second radiation beam 115 may be configured such that it is slightly defocused when it strikes the modified target 121. Such a defocus scheme is shown in fig. 2. In this case, the second focus region 215 is at a different location along the Z-axis direction than the long axis 230 of the modified target 121; further, the second focus region 215 is outside the second target location 116. In this scenario, the second focal region 215 is placed in front of the modified target 121 along the Z-direction. That is, the second radiation beam 115 reaches the focal point (or beam waist) before the second radiation beam 115 impinges the modified target 121. Other defocus schemes are possible. For example, as shown in fig. 6, a second focal region 215 is placed behind the modified target material 121 along the Z-direction. In this manner, the second radiation beam 115 reaches the focal point (or beam waist) after the second radiation beam 115 strikes the modified target 121.

Referring again to fig. 2, when the modified target 121 moves (e.g., drifts) from the first target position 111 to the second target position 116, the rate of expansion of the modified target 121 may be referred to as the rate of Expansion (ER). At first target location 111, immediately after target material 120 is impinged by first radiation beam 110 at time T1, modified target 121 has an extent (or length) S1 taken along major axis 230. When the modified target 121 reaches the second target position 116 at time T2, the modified target 121 has a range S2 taken along the major axis 230. The spreading rate is the difference in the range of the modified target 121 taken along the major axis 230 (S2-S1) divided by the difference in time (T2-T1), so:

although the modified target 121 expands along the major axis 230, the modified target 121 may also compress or thin along the minor axis 235.

The two-stage method discussed above yields a conversion efficiency of about 3-4%, where the modified target 121 is formed by interacting the first beam of radiation 110 with the target material 120 and then the modified target 121 is converted to a plasma by interacting the modified target 121 with the second beam of radiation 115. Generally, it is desirable to increase the conversion of light from the light source 105 to EUV radiation 130, because too low a conversion efficiency may require an increase in the amount of power that the light source 105 needs to deliver, which increases the cost for operating the light source 105 and also increases the thermal load on all components within the light source 100, and may result in an increase in the generation of debris within the chamber housing the first and

second target locations

111, 116. The increase in conversion efficiency can help meet the requirements of high volume manufacturing tools while maintaining the light source power requirements within acceptable ranges. Various parameters affect the conversion efficiency, such as, for example, the wavelength of first and second beams of

radiation

110, 115, target material 120, and the pulse shape, energy, power, and intensity of beams of

radiation

110, 115. Conversion efficiency may be defined as the EUV energy produced by EUV light 130 to 2 pi steradians and within a 2% bandwidth around the center wavelength of the reflectivity curve of the (multi-layer) mirror used in either or both of the light collector system 135 and the illumination and projection optics in optics 145 divided by the energy of the irradiation pulses of the second beam of radiation 115. In one example, the center wavelength of the reflectance curve is 13.5 nanometers (nm).

One way to increase, maintain or optimize the conversion efficiency is to control or stabilize the energy of the EUV light 130, and for this reason it is important to maintain the expansion rate, etc. parameters of the modified target 121 within acceptable values. By maintaining the amount of radiant exposure on target material 120 from first beam of radiation 110, the rate of expansion of modified target 121 is maintained within an acceptable range of values. Also, the radiant exposure may be maintained based on one or more characteristics associated with target material 120 or modified target 121 measured relative to first radiation beam 110. The radiant exposure is the radiant energy received by the surface of the target material 120 per unit area. Thus, if the area of target material 120 remains constant from pulse to pulse, the radiant exposure may be estimated or approximated as the amount of energy directed toward the surface of target material 120.

There are different methods or techniques to maintain the rate of expansion of the modified target 121 within an acceptable range of values. Also, the method or technique used may depend on certain properties associated with first beam of radiation 110. The conversion efficiency is also affected by other parameters, such as the size or thickness of the target material 120, the position of the target material 120 relative to the first focal region 210, or the angle of the target material 120 relative to the xy-plane.

One property that may affect how the radiation exposure is maintained is the confocal parameter of first radiation beam 110. The confocal parameter of the radiation beam is twice the rayleigh length of the radiation beam, and the rayleigh length is the distance from the waist to where the cross-sectional area is doubled along the propagation direction of the radiation beam. Referring to FIG. 2, for radiation beam 110, the Rayleigh length is the distance along the direction of propagation 212 of first radiation beam 110 from its waist (i.e., D1/2) to where the cross-section of the first beam is doubled.

For example, as shown in FIG. 7A, the confocal parameter of first radiation beam 110 is so long that the beam waist (D1/2) tends to encompass target material 120, and the area of the surface of target material 120 intercepted by first radiation beam 110 (measured in the X direction) remains relatively constant, even if the position of target material 120 deviates from the position of beam waist D1/2. For example, the area of the surface of target material 120 intercepted by first radiation beam 110 at location L1 is within 20% of the area of the surface of target material 120 intercepted by first radiation beam 110 at location L2. In a first scenario (as compared to a second scenario described below) in which the area of the surface of target material 120 intercepted by first radiation beam 110 is unlikely to deviate from the average, the amount of radiant exposure, and therefore the rate of expansion, may be maintained or controlled by maintaining the amount of energy directed from first radiation beam 110 toward target material 120 (without having to account for the surface area of target material 120 exposed by first radiation beam 110).

As another example, as shown in FIG. 7B, the confocal parameter of first radiation beam 110 is so short that the beam waist (D1/2) does not encompass target material 120, and if the position of target material 120 deviates from position L1 of beam waist D1/2, the area of the surface of target material 120 intercepted by first radiation beam 110 deviates from the average. For example, the area of the surface of target material 120 intercepted by first radiation beam 110 at location L1 is significantly different from the area of the surface of target material 120 intercepted by first radiation beam 110 at location L2. In a second scenario (as compared to the first scenario) in which the area of the surface of target material 120 intercepted by first radiation beam 110 is more likely to deviate from the average, the radiant exposure, and thus the rate of expansion, may be maintained or controlled by controlling the amount of energy transferred from first radiation beam 110 to target material 120. To control the amount of radiant exposure, the radiant energy per unit area of first beam of radiation 110 received by the surface of target material 120 is controlled. It is therefore important to control the energy of the pulses of first beam of radiation 110 and the area in which target material 120 intercepts first beam of radiation 110 of first beam of radiation 110. The area of target material 120 that intercepts first radiation beam 110 of first radiation beam 110 is associated with the surface of target material 120 that is intercepted by first radiation beam 110. Another factor that may affect the area in which target material 120 intercepts first radiation beam 110 of first radiation beam 110 is the stability of the position and size of the beam waist D1/2 of first radiation beam 110. For example, if the waist size and position of first radiation beam 110 is constant, the position of target material 120 relative to beam waist D1/2 may be controlled. The size and position of the waist of first radiation beam 110 may vary due to, for example, thermal effects in light source 105. In general, it becomes important to maintain a constant energy of the pulses in first beam of radiation 110 and also control other aspects of light source 105 so that target material 120 reaches a known axial (Z-direction) position with respect to beam waist D1/2 without much variation around this position.

All described methods for maintaining or controlling the rate of expansion of the modified target 121 within an acceptable range of values employ the use of the measurement system 155 described below.

Referring again to FIG. 1, measurement system 155 measures at least one characteristic associated with any one or more of target material 120, modified target 121, and first radiation beam 110. For example, measurement system 155 may measure the energy of first radiation beam 110. As shown in fig. 8A, exemplary measurement system 855A measures the energy of first beam of radiation 110 directed toward target material 120.

As shown in fig. 8B, exemplary measurement system 855B measures the energy of radiation 860 reflected from target material 120 after first beam of radiation 110 interacts with target material 120. The reflection of radiation 860 off of target material 120 may be used to determine the position of target material 120 relative to the actual position of first beam of radiation 110.

In some implementations, as shown in fig. 8C, an exemplary measurement system 855B can be placed within the optical amplifier system 300 of the light source 105. In this example, measurement system 855B can be positioned to measure the amount of energy in reflected radiation 860 that impinges on or reflects from one of the optical elements (e.g., a thin film polarizer) within optical amplifier system 300. The amount of radiation 860 reflected from the target material 120 is proportional to the amount of energy transferred to the target material 120; thus, by measuring the reflected radiation 860, the amount of energy delivered to the target material 120 may be controlled or maintained. In addition, the amount of energy measured in first radiation beam 110 or reflected radiation 860 is related to the number of photons in the beam. Thus, it can be said that

measurement system

855A or 855B measures the number of photons in the respective beam. Additionally, measurement system 855B can be considered to measure the number of photons reflected from target material 120 (which becomes modified target 121 once struck by first radiation beam 110) as a function of how many photons strike target material 120.

The

measurement system

855A or 855B may be a photosensor such as an array of photoelectric cells (e.g., a 2 × 2 array or a 3 × 3 array). The photocell has sensitivity to the wavelength of light to be measured and has sufficient speed or bandwidth to be suitable for the duration of the light pulse to be measured.

In general,

measurement system

855A or 855B may measure the energy of beam 110 by measuring the spatially integrated energy in a direction perpendicular to the direction of propagation of first beam 110. Because the measurement of the energy of the beam can be performed quickly, a measurement can be made for each pulse emitted in first beam of radiation 110, and thus the measurement and control can be on a pulse-by-pulse basis.

The

measurement systems

855A, 855B may be fast photodetectors such as photo-electromagnetic (PEM) detectors suitable for Long Wavelength Infrared (LWIR) radiation. The PEM detector may be a silicon diode for measuring near-infrared or visible radiation or an InGaAs diode for measuring near-infrared radiation. The energy of the pulses in first radiation beam 110 may be determined by integrating the laser pulse signals measured by

measurement systems

855A, 855B.

Referring to fig. 9A, the measurement system 155 may be an exemplary measurement system 955A that measures the position Tpos of the target material 120 relative to the target position. The target position may be at the beam waist of first radiation beam 110. The position of target material 120 may be measured along a direction parallel to a beam axis of first beam of radiation 110, such as first axial direction 212.

Referring to fig. 9B, the measurement system 155 may be an exemplary measurement system 955B that measures the position Tpos of the target material 120 relative to the primary focus 990 of the light collector 135. Such a measurement system 955B may include a laser and/or camera that reflects off of the target material 120 as the target material 120 approaches to measure the location of the target material 120 and the time of arrival of the target material 120 relative to a coordinate system within the chamber 165.

Referring to fig. 9C, the measurement system 155 may be an exemplary measurement system 955C that measures the dimensions of the modified target 121 at a particular location before the modified target 121 interacts with the second radiation beam 115. For example, the measurement system 955C may be configured to measure the dimension Smt of the modified target 121 when the modified target 121 is within the second target location 116 but before the modified target 121 is impinged by the second beam of radiation 115. The measurement system 955C may also determine the orientation of the modified target 121. Measurement system 955C may use shadow mapping techniques for a pulsed backlight illuminator and a camera, such as a charge coupled device camera.

The measurement system 155 may include a set of measurement subsystems, each designed to measure a particular characteristic at a different speed or sampling interval. Such a set of subsystems may work together to provide a clear picture of how first beam of radiation 110 interacts with target material 120 to form modified target 121.

The measurement system 155 may include a plurality of EUV sensors within the chamber 165 for detecting EUV energy emitted from a plasma produced by the modified target 121 after interaction of the modified target 121 with the second beam of radiation 115. By detecting the emitted EUV energy, information about the angle of the modified target 121 or the lateral offset of the second beam relative to the second radiation beam 115 may be obtained.

The beam conditioning system 180 is employed under the control of the control system 160 to achieve control of the amount of energy (radiant exposure) delivered toward the target material 120. If it can be assumed that the area of first beam of radiation 110 at the location where first beam of radiation 110 interacts with target material 120 is constant, the amount of radiant exposure can be controlled by controlling the amount of energy within first beam of radiation 110. The beam conditioning system 180 receives one or more signals from the control system 160. The beam adjustment system 180 is configured to adjust one or more characteristics of the light source 105 to maintain the amount of energy delivered toward the target material 120 (i.e., the radiant exposure) or to control the amount of energy directed toward the target material 120. Accordingly, the beam adjustment system 180 may include one or more actuators that control characteristics of the light source 105, which may be mechanical, electrical, optical, electromagnetic, or any suitable force device for causing characteristics of the light source 105 to be modified.

In some implementations, beam conditioning system 180 includes a pulse width modulation system coupled to first beam of radiation 110. The pulse width modulation system is configured to modulate a pulse width of first beam of radiation 110. In this implementation, the pulse width modulation system may include an electro-optic modulator, such as, for example, a pockels cell. For example, a pockels cell is disposed within the light generator 310, and the pulses sent by the pockels cell (and thus the pulses emitted from the light generator 310) may be adjusted to be shorter or longer by opening the pockels cell for shorter or longer periods of time.

In other implementations, beam conditioning system 180 includes a pulse power conditioning system coupled to first beam of radiation 110. The pulse power adjustment system is configured to adjust the power of each pulse, e.g. by adjusting the average power within each pulse of first beam of radiation 110. In this implementation, the pulse power modulation system may include an acousto-optic modulator. The acousto-optic modulator may be arranged such that a change in the RF signal applied to the piezoelectric transducer at the edge of the modulator may be altered, thereby altering the power of the pulse diffracted from the acousto-optic modulator.

In some implementations, beam conditioning system 180 includes an energy conditioning system coupled to first beam of radiation 110. The energy modulation system is configured to modulate the energy of first beam of radiation 110. For example, the energy conditioning system may be an electrically variable attenuator (such as a pockels cell that varies between 0V to a half-wave voltage, or an external acousto-optic modulator).

In some implementations, the change in position or angle of the target material 120 relative to the beam waist D1/2 is so great that the beam conditioning system 180 includes a means of controlling the position or angle of the beam waist D1/2 relative to the first target location 111 or relative to another location within the chamber 165 in the coordinate system of the chamber 165. The apparatus may be part of the focusing assembly 156 and may be used to move the beam waist in the Z-direction or in a direction transverse to the Z-direction (e.g., along a plane defined by the X and Y directions).

As described above, control system 160 analyzes the information received from measurement system 155 and determines how to adjust one or more properties of first beam of radiation 110 to control and maintain the rate of expansion of modified target 121. Referring to fig. 10, control system 160 may include one or more sub-controllers 1000, 1005, 1010, 1015 interfaced with other portions of light source 100, such as sub-controller 1000 specifically configured to interface with (receive information from and transmit information to) light source 105, sub-controller 1005 specifically configured to interface with measurement system 155, sub-controller 1010 configured to interface with light beam delivery system 150, and sub-controller 1015 configured to interface with target material supply system 125. The light source 100 may include other components not shown in fig. 1 and 10 but that may interact with the control system 160. For example, the light source 100 may include a diagnostic system, such as a drop position detection feedback system and one or more target or drop imagers. The target imager provides an output indicative of the location of the drop, for example, relative to a particular location, such as primary focus 990 of light collector 135, and this output is provided to a drop position detection feedback system, which can calculate the drop position and trajectory, for example, from which drop position errors can be calculated on a drop-by-drop basis or on average. Thus, the drop position detection feedback system provides drop position error as an input to a sub-controller of the control system 160. Control system 160 may provide laser position, direction, and timing correction signals, for example, to a laser control system within light source 105 that may be used, for example, to control laser timing circuitry and/or to a beam control system to control the position of the amplified beam and the shaping of the beam delivery system to change the position and/or focus power of the focal plane of first radiation beam 110 or second radiation beam 115.

Target material delivery system 125 includes a target material delivery control system operable to, in response to a signal from control system 160, modify the release point of a drop of target material 120 released by the internal transport mechanism to correct for errors in the drop reaching desired target location 111, for example.

The control system 160 typically includes one or more of digital electronic circuitry, computer hardware, firmware, and software. Control system 160 may also include suitable input and output devices 1020, one or more programmable processors 1025, and one or more computer program products 1030 tangibly embodied in a machine-readable storage device for execution by the programmable processors. Moreover, each sub-controller, such as sub-controllers 1000, 1005, 1010, 1015, may include their own appropriate input and output devices, one or more programmable processors, and one or more computer program products tangibly embodied in a machine-readable storage device for execution by the programmable processors.

One or more programmable processors can each execute a program of instructions to perform a desired function by operating on input data and generating appropriate output. Generally, a processor will receive instructions and data from a read-only memory and/or a random access memory. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including by way of example semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; and a CD-ROM disk. Any of the foregoing may be supplemented by, or incorporated in, specially designed ASICs (application-specific integrated circuits).

To this end, the control system 160 includes an analysis program 1040 that receives measurement data from one or more measurement systems 155. In general, the parser 1040 performs all of the analysis needed to do the following: it is determined how to modify or control the energy delivered from first beam of radiation 110 to target material 120 or to modify or control the energy of first beam of radiation 110, and if measurement data is obtained on a pulse-by-pulse basis, such analysis may be performed on a pulse-by-pulse basis.

Referring to fig. 11, the light source 100 (under control of the control system 160) performs a process 1100 for maintaining or controlling the Extension Rate (ER) of the modified target 121 to thereby improve the conversion efficiency of the light source 100. The light source 100 provides the target material 120 (1105). For example, the target material supply system 125 (under control of the control system 160) may deliver the target material 120 to the first target location 111. The target material supply system 125 may include its own actuation system (connected to the control system 160) and a nozzle through which the target material is forced, wherein the actuation system controls the amount of target material directed through the nozzle to produce a stream of droplets directed towards the first target location 111.

Next, light source 100 directs first beam of radiation 110 toward target material 120 to impart energy to target material 120 to modify the geometric distribution of target material 120 to form modified target 121 (1110). In particular, first beam of radiation 110 is directed through a first set 300 of one or more optical amplifiers toward target material 120. For example, light source 105 may be activated by control system 160 to generate first beam of radiation 110 (in pulses), which may be directed toward target material 120 within target location 111, as shown in FIG. 2. The focal plane of the first light beam 110 (which is at beam waist D1/2) may be configured to span the target location 111. Furthermore, in some implementations, the focal plane may overlap target material 120, or an edge of target material 120 facing first beam of radiation 110. First beam of radiation 110(1110) may be directed toward target material 120 by, for example, directing first beam of radiation 110 through beam delivery system 150, where various optics may be used to modify the direction or shape or divergence of radiation 110 so that it may interact with target material 120.

First beam of radiation 110(1110) may be directed toward target material 120 by overlapping target material 120 with a region of first beam of radiation 110 encompassing its confocal parameter. In some implementations, the confocal parameters of first radiation beam 110 may be so long that the beam waist (D1/2) easily encompasses target material 120, and the area of the surface of target material 120 intercepted by first radiation beam 110 (measured in the X direction) remains relatively constant, even if the position of target material 120 deviates from the position of beam waist D1/2 (as shown in FIG. 7A). For example, the confocal parameter of first radiation beam 110 may be greater than 1.5 mm. In other implementations, the confocal parameter of first radiation beam 110 is so short that the beam waist (D1/2) does not encompass target material 120, and if the position of target material 120 deviates from position L1 of beam waist D1/2, the area of the surface of target material 120 intercepted by first radiation beam 110 deviates considerably (as shown in FIG. 7B). For example, the confocal parameter may be less than or equal to 2 mm.

The modified target material 121 changes its shape from that of target material 120 to an expanded shape immediately after being impinged by first beam of radiation 110, and this expanded shape continues to deform as it moves from first target location 111 toward second target location 116. The modified target 121 may have a geometric distribution of molten metal that is deformed from the shape of the target material into a disk-like volume having a substantially flat surface (such as shown in fig. 1 and 2). The modified target 121 is converted into a disk-shaped volume according to the expansion rate. The modified target 121 is transformed by expanding the modified target 121 along at least one axis according to an expansion rate. For example, as shown in FIG. 2, the modified target 121 is expanded at least along a major axis 230 that is substantially parallel to the X-direction. The modified target 121 is expanded along at least one axis that is non-parallel to the optical axis of the second radiation beam 115 (which is the second axial direction 217).

Although first beam of radiation 110 interacts with target material 120 primarily by changing the shape of target material 120, first beam of radiation 110 may interact with target material 120 in other ways; for example, first beam of radiation 110 may convert a portion of target material 120 into a plasma that emits EUV light. However, less EUV light is emitted from the plasma generated by the target material 120 than from the plasma generated by the modified target 121 (due to subsequent interaction between the modified target 121 and the second beam of radiation 115), and the predominant effect on the target material 120 from the first beam of radiation 110 is a modification of the geometric distribution of the target material 120 to form the modified target 121.

The light source 100 directs a second beam of radiation 115 towards the modified target 121 such that the second beam of radiation converts at least a portion of the modified target 121 into a plasma 129(1115) that emits EUV light. In particular, the light source 100 directs the second beam of radiation 115 through the second set 305 of one or more optical amplifiers toward the modified target 121. For example, the light source 105 may be activated by the control system 160 to generate a second beam of radiation 115 (in pulses) that may be directed towards the modified target 121 within the second target position 116, as shown in fig. 2. At least one optical amplifier in the first group 300 may be in a second group 305, such as the example shown in fig. 5.

Light source 100 measures one or more characteristics (e.g., energy) associated with one or more of target material 120 and modified target 121 relative to first beam of radiation 110 (1120). For example, the measurement system 155 measures characteristics under the control of the control system 160, and the control system 160 receives measurement data from the measurement system 155. Light source 100 controls the amount of radiant exposure at target material 120 from first beam of radiation 110 based on one or more characteristics (1125). As described above, the radiant exposure is the amount of radiant energy transferred from first radiation beam 110 to target material 120 per unit area, in other words, the radiant energy received by the surface of target material 120 per unit area.

In some implementations, the characteristic that may be measured 1120 is the energy of first radiation beam 110. In other general implementations, a characteristic that may be measured 1120 is a position of target material 120 relative to a position of first radiation beam 110 (e.g., relative to a beam waist of first radiation beam 110), such position may be determined in a longitudinal (Z) direction or a direction transverse to the longitudinal direction (e.g., in an XY plane).

The energy of first radiation beam 110 may be measured by measuring the energy of radiation 860 reflected from an optically reflective surface of target material 120 (such as shown in fig. 8B and 8C). The energy of the radiation 860 reflected from the optically reflective surface of the target material 120 may be measured by measuring the total intensity of the radiation 860 over four individual photovoltaic cells.

The total energy content of the retro-reflected radiation 860 may be used in combination with other information about first radiation beam 110 to determine the relative position between target material 120 and the beam waist of first radiation beam 110 along the Z-direction or a direction transverse to the Z-direction, such as in an XY-plane. Alternatively, the total energy content of the retro-reflected radiation 860 (along with other information) may be used to determine the relative position along the Z-direction between the target material 120 and the beam waist of the first radiation beam.

The energy of first beam of radiation 110 may be measured by measuring the energy of first beam of radiation 110 directed towards target material 120 (such as shown in figure 8A). The energy of first radiation beam 110 may be measured by measuring the spatially integrated energy in a direction perpendicular to the direction of propagation of first radiation beam 110 (first axial direction 212).

In some implementations, the characteristic that may be measured 1120 is the pointing direction or direction of first beam of radiation 110 as it travels toward target material 120 (as shown in FIG. 8A). This information about the pointing direction can be used to determine the overlap error between the position of target material 120 and the axis of first beam of radiation 110.

In some implementations, the characteristic that may be measured (1120) is the position of the target material 120 relative to the target location. The target position may be at the beam waist (D1/2) of first radiation beam 110 along the Z-direction. The position of target material 120 may be measured along a direction parallel to first axial direction 212. The target position may be measured relative to the primary focus 990 of the light collector 135. The position of the target material 120 may be measured along two or more non-parallel directions.

In some implementations, the characteristic that can be measured (1120) is a size of the modified target before the second beam of radiation converts at least a portion of the modified target to plasma.

In some implementations, the characteristic that may be measured (1120) corresponds to an estimate of the spreading rate of the modified target.

In some implementations, the characteristic that may be measured (1120) corresponds to a spatial characteristic of radiation 860 reflected from an optically reflective surface of the target material 120 (such as shown in fig. 8B and 8C). Such information may be used to determine the relative position (e.g., along the Z-direction) between target material 120 and the beam waist of first beam of radiation 110. This spatial characteristic may be determined or measured using an astigmatic imaging system placed in the path of the reflected radiation 860.

In some implementations, the characteristic that can be measured (1120) corresponds to the angle at which radiation 860 is directed relative to the angle of first radiation beam 110. This measured angle may be used to determine the distance between target material 120 and the beam axis of first beam of radiation 110 along a direction transverse to the Z-direction.

In other implementations, the characteristic that may be measured (1120) corresponds to a spatial aspect of the modified target 121 formed after the first beam of radiation 110 interacts with the target material 120. For example, the angle of the modified target 121 may be measured relative to a particular direction, such as a direction in the XY plane that is transverse to the Z direction. Such information about the angle of modified target 121 may be used to determine the distance between target material 120 and the axis of first beam of radiation 110 along a direction transverse to the Z-direction. As another example, the size or rate of expansion of modified target 121 may be measured after a predetermined or set time after it is first formed from the interaction between target material 120 and first beam of radiation 110. Such information about the size or rate of expansion of modified target 121 may be used to determine the distance along the longitudinal direction (Z-direction) between target material 120 and the beam waist of first radiation beam 110, if the energy of first radiation beam 110 is known to be constant.

This characteristic may be measured 1120 as fast as for each pulse of first beam of radiation 110. For example, if the measurement system 155 includes a PEM or four cells (an arrangement of 4 PEMs), the measurement rate may be as fast as pulse by pulse.

On the other hand, for a measurement system 155 that measures a characteristic such as the size or rate of expansion of the target material 120 or modified target 121, a camera may be used for the measurement system 155, but the camera is typically much slower, e.g., the camera may measure at a rate of about 1Hz to about 200 Hz.

In some implementations, the radiant exposure delivered from first beam of radiation 110 to target material 120 can be controlled (1125), thereby controlling or maintaining the modified target's rate of expansion. In other implementations, the radiant exposure delivered from first beam of radiation 110 to target material 120 can be controlled (1125) by determining whether a characteristic of first beam of radiation 110 should be adjusted based on the measured one or more characteristics. Thus, if it is determined that a characteristic of first radiation beam 110 should be adjusted, for example, the energy content of the pulses of first radiation beam 110 may be adjusted, or the region of first radiation beam 110 at the location of target material 120 may be adjusted. The energy content of the pulses of first radiation beam 110 may be adjusted by adjusting one or more of: the pulse width of first beam of radiation 110, the pulse duration of first beam of radiation 110, and the average or instantaneous power of first beam of radiation 110. The region of interaction of first beam of radiation 110 with target material 120 may be adjusted by adjusting the relative axial (along the Z-direction) position between target material 120 and the beam waist of first beam of radiation 110.

In some implementations, one or more characteristics may be measured (1120) for each pulse of first beam of radiation 110. In this way, it may be determined whether a characteristic of first radiation beam 110 should be adjusted for each pulse of first radiation beam 110.

In some implementations, the radiant exposure may be controlled by controlling the radiant exposure (e.g., within an acceptable radiant exposure range) delivered from first beam of radiation 110 to target material 120 while at least a portion of the emitted and collected EUV light 140 is exposing a wafer of a lithography tool.

The process 1100 may also include: collecting at least a portion of the EUV light 130 emitted from the plasma (using a light collector 135); and directing the collected EUV light 140 toward the wafer to expose the wafer to the EUV light 140.

In some implementations, the measured one or more characteristics (1120) include a number of photons reflected from the modified target 121. The number of photons reflected from the modified target 121 may be measured in terms of how many photons strike the target material 120.

As described above, process 1100 includes controlling the amount of radiant exposure at target material 120 from first beam of radiation 110 based on one or more characteristics (1125). For example, the radiant exposure may be controlled 1125 such that it is maintained within a predetermined range of radiant exposures. The radiant exposure is the amount of radiant energy transferred per unit area from first beam of radiation 110 to target material 120. In other words, it is the radiant energy received by the surface of the target material 120 per unit area. If the unit area of the surface of target material 120 exposed to or intercepted by first beam of radiation 110 is controlled (or maintained within an acceptable range), this factor of the amount of radiant exposure remains relatively constant, and the amount of radiant exposure can be controlled or maintained at target material 120 by maintaining the energy of first beam of radiation 110 within an acceptable energy range (1125). There are various ways to maintain a unit area of the surface of target material 120 exposed to first beam of radiation 110 within an acceptable area range. These are discussed next.

The radiant exposure (1125) at the target material 120 from the first beam of radiation 110 can be controlled such that the energy of the pulses of the first beam of radiation 110 (by feedback control using the measured characteristic 1120) is maintained at a constant level or within an acceptable range of values, despite disturbances that may lead to energy fluctuations.

In other aspects, the radiant exposure at target material 120 from first beam of radiation 110 can be controlled (1125) such that the energy of the pulses of first beam of radiation 110 is adjusted (e.g., increased or decreased) by feedback control using measured characteristic 1120 to compensate for errors in the placement of the position of target material 120 relative to the longitudinal (Z direction) of the beam waist of first beam of radiation 110.

First beam of radiation 110 may be a pulsed beam of radiation such that pulses of light are directed towards target material 120 (1110). Similarly, second radiation beam 115 may be a pulsed radiation beam such that light pulses are directed towards modified target 121 (1115).

The target material 120 may be droplets of the target material 120 generated from the target material supply system 125. In this way, the geometric distribution of the target material 120 may be modified into a modified target 121, the modified target 121 being transformed into a disk-shaped volume of molten metal having a substantially flat surface. The target material drop is transformed into a disk-like volume according to the rate of expansion.

Referring to fig. 12, a process 1200 is performed by the light source 100 (under control of the control system 160) to stabilize EUV light energy generated by the plasma 129 formed by the interaction between the modified target 121 and the second beam of radiation 115. Similar to process 1100 described above, light source 100 provides target material 120 (1205); light source 100 directs first beam of radiation 110 toward target material 120 to deliver energy to target material 120 to modify the geometric distribution of target material 120 to form modified target 121 (1210); and the light source 100 directs a second beam of radiation 115 towards the modified target 121 such that the second beam of radiation converts at least a portion of the modified target 121 into a plasma 129(1215) that emits EUV light. Light source 100 uses process 1110 to control the amount of radiant exposure applied to target material 120 from first beam of radiation 110 (1220).

The power or energy of the EUV light 130 is stabilized by controlling the radiant exposure (1225). The EUV energy (or power) generated by the plasma 129 depends on at least two functions, the first being the conversion efficiency CE and the second being the energy of the second radiation beam 115. The conversion efficiency is the percentage of the modified target 121 that is converted to plasma 129 by the second beam of radiation 115. The conversion efficiency depends on several variables, including the peak power of the second radiation beam 115, the size of the modified target 121 when interacting with the second radiation beam 115, the position of the modified target 121 relative to the desired position, the lateral area or size of the second radiation beam 115 when interacting with the modified target 121. Since the position of modified target 121 and the size of modified target 121 depend on how target material 120 interacts with first beam of radiation 110, by controlling the amount of radiant exposure applied from first beam of radiation 110 to target material 120, the rate of expansion of modified target 121, and thus both factors, can be controlled. In this way, the conversion efficiency can be stabilized or controlled by controlling the radiant exposure (1220), which consequently stabilizes the EUV energy (1225) produced by the plasma 129.

Referring also to FIG. 13, in some implementations, first beam of radiation 110 may be generated by a dedicated subsystem 1305A within optical source 105, and second beam of radiation 115 may be generated by a dedicated and separate subsystem 1305B within optical source 105, such that beams of

radiation

110, 115 follow two separate paths en route to respective first and

second target locations

111, 116. In this manner, each of

beams

110, 115 travels through a respective subsystem of beam delivery system 150, and thus through respective separate optical control components 1352A, 1352B and focusing

assemblies

1356A, 1356B.

For example, subsystem 1305A may be a solid state gain medium based system, while subsystem 1305B may be based on a signal such as CO₂A system of gaseous gain media produced by an amplifier. Example of a subsystem 1305A that may be usedThe solid state gain medium includes erbium doped fiber lasers and neodymium doped yttrium aluminum garnet (Nd: YAG) lasers. In this example, the wavelength of first radiation beam 110 may be different from the wavelength of second radiation beam 115. For example, the wavelength of the first radiation beam 110 using a solid gain medium may be about 1 μm (e.g., about 1.06 μm), and the wavelength of the second radiation beam 115 using a gaseous medium may be about 10.6 μm.

Other implementations are within the scope of the following claims.

Claims

1. A method for generating extreme ultraviolet light, comprising:

providing a target material comprising a component that emits ultraviolet EUV light when converted to plasma;

directing a first beam of radiation toward the target material to impart energy to the target material to modify a geometric distribution of the target material to form a modified target;

directing a second beam of radiation toward the modified target, the second beam of radiation converting at least a portion of the modified target into a plasma that emits EUV light;

measuring one or more characteristics associated with one or more of the target material and the modified target; and

controlling a radiant exposure delivered from the first beam of radiation to the target material to be within a predetermined energy range based on the one or more measured characteristics.

2. The method of claim 1, wherein measuring one or more characteristics associated with one or more of the target material and the modified target comprises measuring an energy of the first beam of radiation.

3. The method of claim 2, wherein measuring the energy of the first beam of radiation comprises:

measuring the energy of the first radiation beam reflected from the optically reflective surface of the target material, or

Measuring an energy of the first beam of radiation directed toward the target material.

4. The method of claim 2, wherein measuring the energy of the first beam of radiation comprises measuring a spatially integrated energy in a direction perpendicular to a direction of propagation of the first beam of radiation.

5. The method of claim 4, wherein directing the first beam of radiation toward the target material comprises overlapping the target material with a region of the first beam of radiation encompassing its confocal parameter.

6. The method of claim 1, wherein measuring one or more characteristics associated with one or more of the target material and the modified target comprises measuring a position of the target material relative to a target position.

7. The method of claim 6, wherein the first beam of radiation is directed along a first beam axis and the position of the target material is measured along a direction parallel to the first beam axis.

8. The method of claim 6, wherein measuring the location of the target material comprises measuring the location of the target material along two or more non-parallel directions.

9. The method of claim 1, wherein measuring one or more characteristics associated with one or more of the target material and the modified target comprises one or more of:

detecting a size of the modified target before the second beam of radiation converts at least a portion of the modified target into plasma; and

estimating a spreading rate of the modified target.

10. The method of claim 1, wherein controlling the radiant exposure delivered from the first beam of radiation to the target material based on the one or more measured characteristics comprises controlling a rate of expansion of the modified target.

11. The method of claim 1, wherein controlling the radiant exposure delivered from the first beam of radiation to the target material based on the one or more measured characteristics comprises determining whether a characteristic of the first beam of radiation should be adjusted based on the one or more measured characteristics.

12. The method of claim 11, wherein if it is determined that the characteristic of the first beam of radiation should be adjusted, adjusting one or more of: an energy content of the pulses of the first radiation beam and a region of the first radiation beam that interacts with the target material.

13. The method of claim 12, wherein adjusting the energy content of the pulses of the first radiation beam comprises one or more of:

adjusting a width of a pulse of the first beam of radiation;

adjusting a duration of a pulse of the first beam of radiation; and

adjusting an average power within pulses of the first beam of radiation.

14. The method of claim 11, wherein:

directing the first beam of radiation toward the target material includes directing a first pulse of radiation toward the target material;

measuring the one or more characteristics comprises measuring the one or more characteristics for each first radiation pulse; and

determining whether a characteristic of the first beam of radiation should be adjusted includes determining, for each first pulse of radiation, whether the characteristic should be adjusted.

15. The method of claim 1, wherein:

providing the target material comprises providing droplets of the target material;

modifying the geometric distribution of the target material includes converting droplets of the target material into a disk-like volume of molten metal; and

the target material droplet is converted into the disk-like volume according to a rate of expansion.

16. The method of claim 1, wherein directing the first beam of radiation toward the target material also converts a portion of the target material into a plasma that emits EUV light, wherein less EUV light is emitted from the plasma into which the target material is converted than EUV light emitted from the plasma into which the modified target is converted, and the predominant effect on the target material is a modification of a geometric distribution of the target material to form the modified target.

17. The method of claim 1, wherein:

modifying the geometric distribution of the target material comprises converting the shape of the target material to the modified target, including expanding the modified target along at least one axis according to an expansion rate; and

controlling the amount of radiant exposure delivered to the target material includes controlling the rate of expansion of the target material to the modified target.

18. The method of claim 17, wherein the modified target expands along the at least one axis that is non-parallel to an optical axis of the second beam of radiation.

19. The method of claim 1, wherein:

measuring one or more characteristics associated with one or more of the target material and the modified target includes measuring an energy of the first beam of radiation directed toward the target material;

controlling the amount of radiant exposure delivered to the target material includes adjusting an amount of energy directed from the first beam of radiation to the target material based on the measured energy; and

directing the first beam of radiation toward the target material includes overlapping the target material with a region of the first beam of radiation encompassing its confocal parameter.

20. The method of claim 19, wherein adjusting the amount of energy directed from the first beam of radiation to the target material comprises adjusting a property of the first beam of radiation.

21. The method of claim 1, wherein controlling the amount of radiant exposure delivered from the first beam of radiation to the target material comprises one or more of:

adjusting an energy of the first beam of radiation immediately prior to the first beam of radiation delivering energy to the target material;

adjusting the position of the target material; and

adjusting a region of the target material that interacts with the first beam of radiation.

22. An apparatus for generating extreme ultraviolet light, comprising:

a chamber defining an initial target position to receive a first beam of radiation and a target position to receive a second beam of radiation;

a target material delivery system configured to provide a target material to the initial target location, the target material comprising a material that emits ultraviolet EUV light when converted to plasma;

a light source configured to generate the first radiation beam and the second radiation beam;

an optical steering system configured to:

directing the first beam of radiation toward the initial target location to impart energy to the target material to modify a geometric distribution of the target material to form a modified target, an

Directing the second beam of radiation toward the target location to convert at least a portion of the modified target to a plasma that emits EUV light;

a measurement system to measure one or more characteristics associated with one or more of the target material and the modified target; and

a control system connected to the target material delivery system, the light source, the optical steering system, and the measurement system and configured to receive the measured one or more characteristics from the measurement system and send one or more signals to the light source to control a radiant exposure delivered from the first beam of radiation to the target material based on the measured one or more characteristics.

23. The device of claim 22, wherein the optical steering system comprises a focusing device configured to focus the first beam of radiation at or near the initial target position and to focus the second beam of radiation at or near the initial target position.

24. The apparatus of claim 22, further comprising a beam conditioning system, wherein the beam conditioning system is connected to the light source and the control system, and the control system is configured to send one or more signals to the beam conditioning system to control the amount of energy delivered to the target material, the beam conditioning system configured to adjust one or more characteristics of the light source to maintain the amount of energy delivered to the target material.

25. The apparatus of claim 24, wherein the beam adjustment system comprises a pulse width adjustment system coupled to the first beam of radiation, the pulse width adjustment system configured to adjust a pulse width of pulses of the first beam of radiation.

26. The apparatus of claim 25, wherein the pulse width modulation system comprises an electro-optic modulator.

27. The apparatus of claim 24, wherein the beam conditioning system comprises a pulse power conditioning system coupled to the first beam of radiation, the pulse power conditioning system configured to adjust an average power within a pulse of the first beam of radiation.

28. The apparatus of claim 27, wherein the pulse power modulation system comprises an acousto-optic modulator.

29. The apparatus of claim 24, wherein the beam adjustment system is configured to send one or more signals to the light source to control the amount of energy directed to the target material, the beam adjustment system configured to adjust one or more characteristics of the light source to control the amount of energy directed to the target material.

30. The apparatus of claim 22, wherein the light source comprises:

a first set of optical components comprising a first set of one or more optical amplifiers through which the first beam of radiation passes; and

a second set of optical components comprising a second set of one or more optical amplifiers through which the second beam of radiation passes.

31. The apparatus of claim 30, wherein at least one of the optical amplifiers in the first set is in the second set.

32. The device of claim 30, wherein the first set of optical components is different from and separate from the second set of optical components.

33. The apparatus of claim 30, wherein:

the measurement system measures an energy of the first radiation beam as the first radiation beam is directed toward the initial target position; and

the control system is configured to receive the measured energy from the measurement system and send one or more signals to the light source to control an amount of energy directed from the first beam of radiation to the target material based on the measured energy.