Target expansion rate control in extreme ultraviolet light sources

Abstract

The present disclosure relates to target expansion rate control in an extreme ultraviolet light source. A method includes: providing a target material, the target material including a component that emits extreme ultraviolet (EUV) light when converted to plasma; directing a first radiation beam toward the target material to transfer energy to the target material to modify the geometric distribution of the target material to form a modified target; directing a second radiation beam toward the modified target, the second radiation beam 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 radiation beam; and controlling the radiation exposure delivered from the first radiation beam to the target material within a predetermined energy range based on the measured one or more characteristics.

Description

[0001] This application is a divisional application of the Chinese invention patent application with the application date of August 10, 2016, application number 201680047368.0, and invention name “Target Expansion Rate Control in Extreme Ultraviolet Light Source”.

[0002] CROSS-REFERENCE TO RELATED APPLICATIONS

[0003] This application claims the benefit of U.S. Serial No. 14 / 824,141, filed on August 12, 2015, entitled “TARGET EXPANSION RATE CONTROL IN AN EXTREME ULTRAVIOLET LIGHT SOURCE,” and U.S. Serial No. 14 / 824,147, filed on August 12, 2015, entitled “STABILIZING EUV LIGHT POWER IN AN EXTREME ULTRAVIOLET LIGHT SOURCE,” both of which are incorporated herein by reference. Technical Field

[0004] The disclosed subject matter relates to controlling the expansion rate of target materials for laser-produced plasma extreme ultraviolet light sources. Background Art

[0005] Extreme ultraviolet (EUV) light (e.g., electromagnetic radiation having a wavelength of approximately 50 nm or less (sometimes also called soft x-rays) and including light with a wavelength of approximately 13 nm) can be used in photolithography processes to create very small features in substrates such as silicon wafers.

[0006] Methods of generating EUV light include, but are not necessarily limited to, converting materials having elements such as xenon, lithium, or tin using emission lines in the EUV range in a plasma state. In one such method, often referred to as laser produced plasma ("LPP"), the desired plasma can be generated by irradiating a target material (e.g., in the form of droplets, slabs, ribbons, streams, or clusters of the material) with an amplified beam of light, which can be referred to as a drive laser. For this process, the plasma is typically generated in a sealed container (e.g., a vacuum chamber) and monitored using various types of metrology equipment. Summary of the invention

[0007] In some general aspects, a method includes providing a target material, the target material including a component that emits extreme ultraviolet (EUV) light when converted to a plasma; directing a first radiation beam toward the target material to deliver energy to the target material to modify a geometric distribution of the target material to form a modified target; directing a second radiation beam toward the modified target, the second radiation beam 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 radiation beam; and controlling an amount of radiation exposure delivered from the first radiation beam to the target material within a predetermined energy range based on the measured one or more characteristics.

[0008] Implementations may include one or more of the following features. For example, one or more characteristics associated with one or more of a target material and a modified target may be measured by measuring the energy of a first radiation beam. The energy of the first radiation beam may be measured by measuring the energy of the first radiation beam reflected from an optically reflective surface of the target material. The energy of the first radiation beam may be measured by measuring the energy of the first radiation beam directed toward 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 propagation direction of the first radiation beam.

[0009] The first radiation beam may be directed towards the target material by causing the target material to overlap with a region of the first radiation beam surrounding its confocal parameter.The confocal parameter may be greater than 1.5 mm.

[0010] One or more properties associated with one or more of the target material and the modified target may be measured by measuring the position of the target material relative to the target position. The target position may coincide with a beam waist of the first radiation beam. The first radiation beam 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 that collects 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.

[0011] One or more characteristics associated with one or more of the target material and the modified target may be measured by detecting a size of the modified target before the second radiation beam 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 an expansion rate of the modified target.

[0012] The amount of radiation exposure delivered to the target material from the first radiation beam may be controlled by controlling the rate of expansion of the modified target.

[0013] The radiation exposure delivered to the target material from the first radiation beam may be controlled by determining whether a characteristic of the first radiation beam should be adjusted based on the measured one or more characteristics.Determining whether a characteristic of the first radiation beam should be adjusted may be performed while measuring the one or more characteristics.

[0014] If it is determined that the characteristics of the first radiation beam 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 area 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 the following: the pulse width of the first radiation beam; the duration of the pulses of the first radiation beam; and the average power within the pulses of the first radiation beam.

[0015] A first radiation beam can be directed toward a target material by directing a first radiation pulse toward the target material; one or more characteristics can be measured by measuring one or more characteristics of each first radiation pulse; and it can be determined whether a characteristic of the first radiation beam should be adjusted by determining whether the characteristic should be adjusted for each first radiation pulse.

[0016] The amount of radiation exposure delivered to the target material from the first radiation beam may be controlled by controlling the amount of radiation exposure delivered to the target material from the first radiation beam while at least a portion of the emitted EUV light exposes the wafer.

[0017] The target material may be provided by providing droplets of the target material; the geometric distribution of the target material may be modified by converting the droplets of the target material into a disk-shaped volume of molten metal. The droplets of the target material may be converted into the disk-shaped volume according to a spreading rate.

[0018] The method may also include 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.

[0019] The one or more characteristics may be measured by measuring at least one characteristic of each pulse of the first radiation beam directed towards the target material.

[0020] The first radiation beam can be directed toward a target material such that a portion of the target material is converted into a plasma that emits EUV light, and the plasma converted from the target material emits less EUV light than the EUV light emitted by the plasma converted from the modified target, and the dominant effect on the target material is the modification of the geometric distribution of the target material to form the modified target.

[0021] The geometric distribution of the target material can 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 radiation exposure delivered to the target material can be controlled by controlling the expansion rate of the target material into the modified target.

[0022] The modified target may extend along at least one axis which is non-parallel to the optical axis of the second radiation beam.

[0023] One or more properties associated with one or more of the target material and the modified target can be measured by measuring the number of photons reflected from the modified target. The number of photons reflected from the modified target can be measured by measuring the number of photons reflected from the modified target as a function of how many photons strike the target material.

[0024] A first radiation beam may be directed toward a target material by directing a first radiation pulse toward the target material; and a second radiation beam may be directed toward the modified target by directing a second radiation pulse toward the modified target.

[0025] The first radiation beam may be directed by directing it through a first group of one or more optical amplifiers; and the second radiation beam may be directed by directing it 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.

[0026] 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 radiation beam directed toward the target material; and an amount of radiation exposure delivered to the target material may be controlled by adjusting an amount of energy directed from the first radiation beam toward the target material based on the measured energy. The first radiation beam may be directed toward the target material by overlapping the target material with a region of the first radiation beam surrounding its confocal parameter; and the confocal parameter may be less than or equal to 2 mm.

[0027] The amount of energy directed toward the target material from the first radiation beam may be adjusted by adjusting a property of the first radiation beam.

[0028] The amount of radiation exposure delivered to the target material from the first radiation beam can be controlled by adjusting one or more of: the energy of the first radiation beam immediately before the first radiation beam delivers energy to the target material; the position of the target material; and the area where the target material interacts with the first radiation beam.

[0029] The first radiation beam can be directed by directing it through a first set of optical components comprising one or more first optical amplifiers; and the second radiation beam can be directed by directing it through a second set of optical components comprising 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.

[0030] In other general aspects, an apparatus includes a chamber defining an initial target location receiving a first radiation beam and a target location receiving a second radiation beam; a target material delivery system configured to provide a target material to the initial target location, the target material including a material that emits extreme ultraviolet (EUV) light when converted to a plasma; a light source configured to generate the first radiation beam and the second radiation beam; and an optical steering system. The optical steering system is configured to: direct the first radiation beam toward the initial target location to transfer energy to the target material to modify the geometric distribution of the target material to form a modified target, and direct the second radiation beam toward the target location to convert at least a portion of the modified target into a plasma that emits EUV light. The apparatus includes: 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 radiation beam; and a control system connected to the target material delivery system, the light source, the optical steering 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 the radiation exposure delivered from the first radiation beam to the target material based on the measured one or more characteristics.

[0031] Implementations may include one or more of the following features.For example, the optical steering system may include a focusing device configured to focus the first radiation beam at or near an initial target location and to focus the second radiation beam at or near a target location.

[0032] The apparatus may include 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 light source to control the amount of energy delivered to the target material by sending one or more signals to the beam conditioning system, the beam conditioning system being 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 may include a pulse width conditioning system coupled to the first radiation beam, the pulse width conditioning system being configured to adjust the pulse width of the pulses of the first radiation beam. The pulse width conditioning system may include an electro-optic modulator.

[0033] The beam conditioning system may include a pulse power conditioning system coupled to the first radiation beam, the pulse power conditioning system configured to adjust the average power within pulses of the first radiation beam. The pulse power conditioning system may include an acousto-optic modulator.

[0034] The beam adjustment system can 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, and the beam adjustment system is configured to adjust one or more characteristics of the light source to control the amount of energy directed to the target material.

[0035] The light source may include a first group of one or more optical amplifiers through which the first radiation beam passes; and a second group of one or more optical amplifiers through which the second radiation beam passes, at least one of the optical amplifiers in the first group being in the second group. The measurement system may measure an energy of the first radiation beam as it is directed toward an initial target location; 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 radiation beam toward the target material based on the measured energy.

[0036] In some general aspects, a method includes providing a target material, the target material including a component that emits extreme ultraviolet (EUV) light when converted to a plasma; directing a first radiation beam toward the target material to transfer energy to the target material to modify a geometric distribution of the target material to form a modified target; directing a second radiation beam toward the modified target, the second radiation beam converting at least a portion of the modified target into a plasma that emits EUV light; controlling a radiation exposure delivered to the target material from the first radiation beam within a predetermined radiation exposure range; and stabilizing the power of EUV light emitted from the plasma by controlling the radiation exposure delivered to the target material from the first radiation beam within the predetermined radiation exposure range.

[0037] Implementations may include one or more of the following features. For example, a 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 a 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 distinct from and separate from the second set of optical components.

[0038] The first radiation beam may be directed by directing it through a first set of one or more optical amplifiers; and the second radiation beam may be directed by directing it through a second set of one or more optical amplifiers; wherein at least one optical amplifier in the first set is in the second set.

[0039] The target material may be provided by providing droplets of the target material; and the geometric distribution of the target material may be changed by transforming the droplets of the target material into a disk-shaped volume of molten metal having a substantially flat surface.

[0040] The target material may be provided by providing droplets of the target material; and the geometric distribution of the target material may be changed by transforming the droplets of the target material into a mist-like volume of molten metal particles.

[0041] The target material may be converted to a modified target according to the expansion rate.

[0042] The radiation exposure delivered from the first radiation beam to the target material can be controlled by: measuring one or more characteristics associated with the target material and one or more of the modified target relative to the first radiation beam; and maintaining the radiation exposure delivered from the first radiation beam to the target material within a predetermined radiation exposure range based on the measured one or more characteristics.

[0043] The radiation exposure delivered to the target material from the first radiation beam may be controlled by estimating the expansion rate of the modified target.The radiation exposure delivered to the target material from the first radiation beam may be controlled by maintaining the expansion rate of the modified target.

[0044] The amount of radiation exposure delivered to the target material from the first radiation beam may be controlled by determining whether a characteristic of the first radiation beam should be adjusted. The amount of radiation exposure delivered to the target material from the first radiation beam may be controlled by adjusting the characteristic of the first radiation beam by adjusting one or more of an energy content of each pulse of the first radiation beam and an area of ​​the first radiation beam that interacts with the target material. The energy content of each pulse of the first radiation beam may be adjusted by adjusting one or more of a width of each pulse of the first radiation beam, a duration of each pulse of the first radiation beam, and a power of each pulse of the first radiation beam.

[0045] The power of 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.

[0046] The method may also include 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.

[0047] 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.

[0048] The amount of radiation exposure delivered to the target material from the first radiation beam may be controlled by adjusting a property of the first radiation beam.The property of the first radiation beam may be adjusted by adjusting the energy of the first radiation beam.

[0049] In other general aspects, an apparatus includes a chamber defining an initial target location for receiving a first radiation beam and a target location for receiving a second radiation beam; a target material delivery system configured to provide a target material to the initial target location, the target material including a material that emits extreme ultraviolet (EUV) light when converted to a plasma; a light source configured to generate the first radiation beam and the second radiation beam; and an optical steering system. The optical steering system is configured to: direct the first radiation beam toward the initial target location to transfer energy to the target material to modify the geometric distribution of the target material to form a modified target, and direct the second radiation beam 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 the radiation exposure delivered from the first radiation beam to the target material within a predetermined radiation exposure range to stabilize the power of EUV light emitted from the plasma.

[0050] 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 properties associated with one or more of the target material and the modified target relative to the first radiation beam; wherein the control system is connected to the measurement system.

[0051] The apparatus may further 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 radiation exposure amount delivered to the target material by sending one or more signals to the beam adjustment system, and the beam adjustment system is configured to adjust one or more characteristics of the light source to control the radiation exposure amount delivered to the target material. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 is a block diagram of a laser produced plasma extreme ultraviolet light source including a light source that produces a first radiation beam directed toward a target material and a second radiation beam directed toward a modified target to convert a portion of the modified target into a plasma that emits EUV light;

[0053] Figure 2 is a schematic diagram showing a first radiation beam directed toward a first target location and a second radiation beam directed toward a second target location;

[0054] Figure 3A is used in Figure 1 A block diagram of an exemplary light source for use in a light source of;

[0055] Figure 3B and Figure 3C They can be Figure 1A block diagram of an exemplary beam path combiner and an exemplary beam path splitter for use in a light source;

[0056] Figure 4A and Figure 4B Yes, you can Figure 3A A block diagram of an exemplary optical amplifier system for use in a light source;

[0057] Figure 5 Yes, you can Figure 3A A block diagram of an exemplary optical amplifier system for use in a light source;

[0058] Figure 6 is a schematic diagram illustrating another implementation of a first radiation beam directed toward a first target location and a second radiation beam directed toward a second target location;

[0059] Fig. 7A and Figure 7B is a schematic diagram illustrating implementation of a first radiation beam directed toward a first target location;

[0060] Figure 8A-8C and Figure 9A-9C Schematic diagrams showing various implementations of a measurement system for measuring at least one property associated with any one or more of a target material, a modified target, and a first radiation beam;

[0061] Fig.10 yes Figure 1 A block diagram of an exemplary control system for a light source;

[0062] 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 a modified target expansion rate (ER) to thereby increase the conversion efficiency of the light source;

[0063] 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 amount of radiation exposure delivered to a target material from a first radiation beam; and

[0064] Fig.13 is a block diagram of an exemplary light source that generates first and second radiation beams and an exemplary beam delivery system that modifies and focuses the first and second radiation beams to respective first and second target locations. DETAILED DESCRIPTION

[0065] Techniques for improving the conversion efficiency of extreme ultraviolet (EUV) light generation are disclosed. Figure 1As discussed in more detail below, the interaction between the target material 120 and the first radiation beam 110 causes the target material to deform and geometrically expand to form a 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 the plasma due to the interaction between the modified target 121 and the second radiation beam 115. The amount of usable EUV light 130 is the amount of EUV light 130 that is available for utilization at the optical device 145. Therefore, the amount of usable EUV light 130 may depend on aspects such as the bandwidth or central wavelength of the optical components used to utilize the EUV light 130.

[0066] 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 can adjust the density of the modified target 121 when interacting with the second radiation beam 115; because the density of the modified target 121 affects the total amount of radiation absorbed by the modified target 121 and the range of such absorption when the modified target 121 interacts with the second radiation beam 115. As the density of the modified target 121 increases, at some point, EUV light 130 will not be able to escape from the modified target 121 and thus the amount of available EUV light 130 can decrease. As another example, adjustment of the geometric expansion rate of the modified target 121 can adjust the surface area of ​​the modified target 121 when the modified target 121 interacts with the second radiation beam 115.

[0067] In this way, the total amount of usable EUV light 130 produced can be increased or controlled by controlling the expansion rate of the modified target 121. In particular, the size of the modified target 121, as well as its expansion rate, depends on the radiation exposure applied to the target material 120 from the first radiation beam 110, which is the amount of energy delivered to an area of ​​the target material 120 by the first radiation beam 110. Therefore, the expansion rate of the modified target 121 can 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 the target material 120 depends on the energy of the first radiation beam 110 immediately before the first radiation beam 110 strikes the surface of the target material.

[0068] The energy of the pulses in the first radiation beam 110 can be determined by integrating the laser pulse signal measured by a fast photodetector. The detector can be a photoelectromagnetic (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.

[0069] 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 radiation beam 110 that are intercepted by the target material 120. In a hypothetical baseline design, the target material 120 is assumed to always have the same size and be placed in the waist of the focused first radiation beam 110. However, in practice, the target material 120 may have a small but mostly constant axial position offset relative to the waist of the first radiation beam 110. If all of these factors remain constant, one factor that controls the rate of expansion of the modified target 121 is the pulse energy of the first radiation beam 110 for pulses of the first radiation beam having a duration of a few ns to 100 ns. If the pulses of the first radiation beam 110 have a duration equal to or lower than 100 ns, another factor that may control the rate of expansion of the modified target 121 is the instantaneous peak power of the first radiation beam 110. If the pulses of the first radiation beam 110 have a shorter duration, such as on the order of picoseconds (ps), other factors may control the rate of expansion of the modified target 121, as described below.

[0070] like Figure 1 As shown, a laser produced plasma (LPP) extreme ultraviolet (EUV) light source 100 is driven using a light source 105 (also referred to as a driving source or a driving laser). The light source 105 generates a first radiation beam 110 provided to a first target location 111 and a second radiation beam 115 provided to a second target location 116. The first radiation beam 110 and the second radiation beam 115 may be pulsed amplified beams.

[0071] The first target location 111 receives a target material 120, such as tin, from a target material supply system 125. The interaction between the first radiation beam 110 and the target material 120 transfers energy to the target material 120 to modify or change (e.g., deform) its shape, such that the geometric distribution of the target material 120 is deformed into a 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 the first radiation beam 110 transfers energy to the target material 120 to deform it into the modified target 121, the modified target 121 may continue to move along the -X direction in addition to moving in 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 radiation beam 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 the EUV light 130 and directs the EUV light 130 toward an optical device 145, such as a lithography tool, as collected EUV light 140. 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 generating the EUV light 140.

[0072] Some of the target material 120 may be converted to plasma upon interaction with the first radiation beam 110, and thus such plasma may emit EUV radiation. However, the properties of the first radiation beam 110 are selected and controlled such that the dominant effect of the first radiation beam 110 on the target material 120 is a deformation or modification of the geometric distribution of the target material 120 to form a modified target 121.

[0073] Each of the first radiation beam 110 and the second radiation beam 115 is directed toward a corresponding target position 111, 116 by a beam delivery system 150. The beam delivery system 150 may include an optical steering component 152 and a focusing assembly 156, which focuses the first radiation beam 110 or the second radiation beam 115 to a corresponding first focal region and a second focal region. The first focal region and the second focal region may overlap with the first target position 111 and the second target position 116, respectively. The optical component 152 may include an optical element, such as a lens and / or a reflector, that guides the radiation beams 110, 115 by refraction and / or reflection. The beam delivery system 150 may also include an element that controls and / or moves the optical component 152. For example, the beam delivery system 150 may include an actuator that can be controlled to cause an optical element within the optical component 152 to move.

[0074] Also refer to Figure 2 , the focusing assembly 156 focuses the first radiation beam 110 so that the diameter D1 of the first radiation beam 110 is minimum in the first focal region 210. In other words, the focusing assembly 156 causes the first radiation beam 110 to converge as the first radiation beam 110 propagates toward the first focal region 210 in the first axial direction 212, the first axial direction 212 being the general propagation direction of the first radiation beam 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 the target material 120, the first radiation beam 110 diverges as the first radiation beam 110 propagates away from the first focal region 210 in the first axial direction 212.

[0075] In addition, the focusing assembly 156 focuses the second radiation beam 115 such that the diameter D2 of the second radiation beam 115 is minimum in the second focal region 215. Thus, the focusing assembly causes the second radiation beam 115 to converge as the second radiation beam 115 propagates toward the second focal region 215 in the second axial direction 217, which is the general direction of propagation of the second radiation beam 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.

[0076] As described below, the EUV light source 100 also includes one or more measurement systems 155, a control system 160, and a beam conditioning system 180. 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 can measure one or more characteristics within the light source 100. For example, the one or more characteristics can be characteristics associated with the target material 120 or the modified target 121 relative to the first radiation beam 110. As another example, the one or more characteristics can be the pulse energy of the first radiation beam 110 directed toward the target material 120. These examples will be discussed in more detail below. The control system 160 is configured to receive the measured one or more characteristics from the measurement system so that it can control how the first radiation beam 110 interacts with the target material 120. For example, the control system 160 can be configured to maintain the amount of energy transferred from the first radiation beam 110 to the target material 120 within a predetermined energy range. As another example, the control system 160 can be configured to control the amount of energy directed from the first radiation beam 110 toward the target material 120. The beam conditioning system 180 is a system that includes components within the light source 105 or that adjusts components within the light source 105 to control properties of the first radiation beam 110, such as pulse width, pulse energy, instantaneous power within a pulse, or average power within a pulse.

[0077] refer to Figure 3A In some implementations, the light source 105 includes a first optical amplifier system 300 including a series of one or more optical amplifiers through which the first radiation beam 110 passes, and a second optical amplifier system 305 including a series of one or more optical amplifiers through which the second radiation beam 115 passes. One or more amplifiers from the first system 300 can be located in the second system 305; or one or more amplifiers in the second system 305 can be located in the first system 300. Alternatively, the first optical amplifier system 300 can be completely separate from the second optical amplifier system 305.

[0078] In addition, although not required, the light source 105 may 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 (CO2) oscillator that may be operated at a repetition rate of, for example, 100 kHz.

[0079] The optical amplifiers in the optical amplifier systems 300, 305 each include a gain medium on a corresponding beam path, along which the light beams 311, 316 of the corresponding light generators 310, 315 propagate. When the gain medium of the optical amplifier is excited, the gain medium provides photons to the light beam, amplifying the light beams 311, 316 to generate an amplified light beam forming the first radiation beam 110 or the second radiation beam 115.

[0080] The wavelengths of the light beams 311, 316 or radiation beams 110, 115 can be different from each other so that the radiation beams 110, 115 can also be separated from each other if they are combined at any point within the light source 105. If the radiation beams 110, 115 are generated by a CO2 amplifier, the first radiation beam 110 can have a wavelength of 10.26 micrometers (μm) or 10.207 μm, and the second radiation beam 115 can have a wavelength of 10.59 μm. The wavelengths are selected to make it easier to 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 (for example, in the case where some amplifiers of the optical amplifier system 300 are in the optical amplifier system 305), the different wavelengths can be used to adjust the relative gain between the two radiation beams 110, 115, even if they are passing through the same amplifier.

[0081] For example, once separated, the radiation beams 110, 115 can be steered or focused to two separate locations (such as the first target location 111 and the second target location 116, respectively) within the chamber 165. Specifically, the separation of the radiation beams 110, 115 also enables the modified target 121 to expand after interacting with the first radiation beam 110 when traveling from the first target location 111 to the second target location 116.

[0082] The light source 105 may include a beam path combiner 325 that superimposes the first radiation beam 110 and the second radiation beam 115 and places the radiation beams 110 , 115 on the same optical path for at least some distance between the light source 105 and the beam delivery system 150 . Figure 3B346. An exemplary beam path combiner 325 is shown in FIG. The beam path combiner 325 includes a pair of dichroic beam splitters 340, 342 and a pair of mirrors 344, 346. The dichroic beam splitter 340 enables the first radiation beam 110 to pass along a first path leading to the dichroic beam splitter 342. The dichroic beam splitter 340 reflects the second radiation beam 115 along a second path where the second radiation beam 115 is reflected from the mirrors 344, 346 which redirect the second radiation beam 115 toward the dichroic beam splitter 342. The first radiation beam 110 freely passes through the dichroic beam splitter 342 to an 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.

[0083] Additionally, the light source 105 may include a beam path splitter 326 that separates the first radiation beam 110 from the second radiation beam 115 so that the two radiation beams 110, 115 may be independently steered and focused within the chamber 165. Figure 3C 3. An exemplary beam path splitter 326 is shown in FIG. 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 a second path, and transmits the first radiation beam 110 along the first path toward the dichroic beam splitter 352. The first radiation beam 110 freely passes through the dichroic beam splitter 352 along the first path. The second radiation beam 115 reflects from the mirrors 354, 356 and returns to the dichroic beam splitter 352, where it is reflected onto a second path different from the first path.

[0084] In addition, the first radiation beam 110 may be configured to have a pulse energy less than a pulse energy of the second radiation beam 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 the plasma 129. For example, the pulse energy of the first radiation beam 110 may be 1 / 5-1 / 100 of the pulse energy of the second radiation beam 115.

[0085] In some implementations, such as Figure 4A and 4BAs shown, 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 that includes CO2 and can amplify light having a wavelength between about 9.1 and about 11.0 μm and particularly about 10.6 μm with a gain in excess of 1000. The optical amplifiers 401, 402, 403 can 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 CO2 amplifiers, which, for example, generate radiation at about 9.3 μm or about 10.6 μm using DC or RF excitation and operate at relatively high powers (e.g., 10 kW or more) and high pulse repetition rates (e.g., 50 kHz or more). An exemplary optical amplifier 401, 402, 403 or 406, 407, 408 is an axial flow high power CO2 laser with wear-free gas circulation and capacitive RF excitation, such as a TruFlow CO2 laser produced by TRUMPF, Inc. of Farmington, Connecticut.

[0086] Additionally, although not required, one or more of the optical amplifier systems 300 and 305 may include a first amplifier that functions as a preamplifier 411, 421, respectively. The preamplifiers 411, 421, if present, may be diffusion cooled CO2 laser systems, such as the TruCoax CO2 laser systems manufactured by TRUMPF, Inc. of Farmington, Connecticut.

[0087] The optical amplifier system 300, 305 may include Figure 4A and Figure 4B Not shown are optical elements for directing and shaping the respective light 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.

[0088] The light source 105 also includes an optical system 320, which may include one or more optical devices for guiding the light beams 311, 316 through the light source 105 (such as, reflective optical devices such as mirrors, partially reflective and partially transmissive optical devices such as beam splitters, refractive optical devices such as prisms or lenses, passive optical devices, active optical devices, etc.).

[0089] 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, Figure 5 As shown, 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 amplifiers 402 / 407 and amplifiers 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 the first radiation beam 110 may cause a change in one or more characteristics of the second radiation beam 115, and vice versa. As a result, it becomes more important to control energy within the system, such as the energy of the first radiation beam 110 or the energy delivered to the target material 120. In addition, the optical amplifier system 300 , 305 further includes an optical element 505 (such as a beam path splitter 326 ) for separating the two light beams 110 , 15 output from the amplifier 403 / 408 so that the two light beams 110 , 115 can be directed toward corresponding target locations 111 , 116 .

[0090] The target material 120 may be any material including a target material that emits EUV light when converted into a 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 a liquid or molten metal, a portion of a liquid stream, a solid particle or cluster, a solid particle contained in a droplet, a foam of a 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 a tin compound such as SnBr4, SnBr2, SnH4; as a tin alloy such as a tin-gallium alloy, a tin-indium alloy, a tin-indium-gallium alloy, or any combination of these alloys. Moreover, in the absence of impurities, the target material contains only the target substance. The following discussion provides an example 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.

[0091] 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 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.

[0092] By irradiating the target material 120 with the radiation pulses from the first radiation beam 110 , the shape of the target material 120 is changed or modified (eg, deformed) before reaching the second target location 116 .

[0093] The interaction between the first radiation beam 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 a 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 so that when it reaches the second target location 116, its shape is closer to the shape of a disk (such as a flat shape). The modified target 121 may be a material that is not ionized (a non-plasma material) or a material that is minimally ionized. For example, the modified target material 121 may be a disk of liquid or molten metal, a continuous portion of the target material without voids or significant gaps, a mist of micro- or nano-particles, or a cloud of atomic vapor. For example, as Figure 2 As shown, the modified target material 121 expands into a disk-shaped molten metal sheet 121 within the second target location 116 after approximately time T2 - T1 , which may be on the order of microseconds (μs).

[0094] Furthermore, the interaction between the first radiation beam 110 and the target material 120 that causes ablation of material from the surface of the target material 120 (and the modified target 121) can provide a force that can cause the modified target 121 to gain some propulsion or velocity along the Z direction. The expansion of the modified target 121 in the X direction and the resulting velocity in the Z direction depends on the energy of the first radiation beam 110, and specifically on the energy delivered to (i.e., intercepted by) the target material 120.

[0095] For example, for a constant target material 120 size and a long pulse of the first radiation beam 110 (a long pulse is a pulse with a duration between a few nanoseconds (ns) and 100 ns), the expansion rate is a function of the energy per unit area (joules / cm2) of the first radiation beam 110. 2) is linearly proportional. The energy per unit area is also referred to as radiation exposure or energy density. The radiation exposure is the radiation energy received by the surface of the target material 120 per unit area, or is equivalent to the irradiance of the surface of the target material 120 integrated over the time the target material 120 is irradiated.

[0096] As another example, for a constant target material 120 size and short pulses (pulses with a duration less than a few hundred picoseconds (ps)), the relationship between the expansion rate and the energy of the first radiation beam 110 may be different. In this case, the shorter pulse duration is associated with an increase in the intensity of the first radiation beam 110 interacting with the target material 120, and the first radiation beam 110 behaves as a shock wave. In this case, the expansion rate depends primarily on the intensity I of the 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) of the first radiation beam 110 interacting with the target material 120 and the pulse duration (τ), or I=E / (A·τ). In this ps pulse duration regime, the modified target 121 expands to form a fog.

[0097] Additionally, the angular orientation (angle relative to the Z-direction or the X-direction) of the disk shape of the modified target 121 depends on the position of the first radiation beam 110 when it strikes the target material 120. Thus, if the first radiation beam 110 strikes the target material 120 such that the first radiation beam 110 surrounds the target material and the beam waist of the first radiation beam 110 is centered on the target material 120, then the disk shape of the modified target 121 will more likely be aligned with its major axis 230 parallel to the X-direction and its minor axis 235 parallel to the Z-direction.

[0098] The first radiation beam 110 is composed of radiation pulses, and each pulse may have a duration. Similarly, the second radiation beam 115 is composed of radiation pulses, and each pulse may have a duration. The pulse duration can be represented by the full width at a certain percentage (e.g., half) of the maximum value, that is, the amount of time when the intensity of the pulse is at least the percentage of the maximum intensity of the pulse. However, other metrics can also be used to determine the pulse duration. For example, the pulse duration of the pulse in the first radiation beam 110 can be 30 nanoseconds (ns), 60ns, 130ns, 50-250ns, 10-200 picoseconds (ps), or less than 1ns. For example, the energy of the first radiation beam 110 can be 1-100 millijoules (mJ). For example, the wavelength of the first radiation beam 110 can be 1.06μm, 1-10.6μm, 10.59μm, or 10.26μm.

[0099] As described above, the rate of expansion of the modified target 121 depends on the radiation exposure (energy per unit area) of the first radiation beam 110 that intercepts the target material 120. Thus, for a pulse of the first radiation beam 110 having a duration of about 60 ns and an energy of about 50 mJ, the actual radiation exposure depends on how tightly the first radiation beam 110 is focused at the first focal region 210. In some examples, the radiation exposure may be about 400-700 Joules / cm at the target material 120. 2 However, the radiation exposure is very sensitive to the position of the target material 120 relative to the first radiation beam 110 .

[0100] The second radiation beam 115 may be referred to as a primary beam and is composed of pulses released at a specific repetition rate. The second radiation beam 115 has sufficient energy to convert target material within the modified target 121 into a plasma that emits EUV light 130. The pulses of the first radiation beam 110 and the pulses of the second radiation beam 115 are separated in time, such as, for example, by a delay time of 1-3 microseconds (μs), 1.3 μs, 1-2.7 μs, 3-4 μs, or 1-2.7 μs to allow the modified target 121 to be extended to a plasma that emits EUV light 130. Figure 2 Thus, as modified target 121 expands and elongates in the XY plane, modified target 121 undergoes two-dimensional expansion.

[0101] The second radiation beam 115 may be configured such that it is slightly defocused when it strikes the modified target 121 . Figure 2 Such a defocusing scheme is shown in FIG. In this case, the second focal region 215 is at a different position from the long axis 230 of the modified target 121 along the Z-axis direction; in addition, the second focal region 215 is outside the second target position 116. In this scheme, 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 focus (or beam waist) before the second radiation beam 115 hits the modified target 121. Other defocusing schemes are possible. For example, as Figure 6 As shown, the second focal region 215 is positioned after 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 .

[0102] Reference again Figure 2, the rate of expansion of the modified target 121 as it moves (e.g., drifts) from the first target location 111 to the second target location 116 may be referred to as the expansion rate (ER). At the first target location 111, immediately after the target material 120 is struck by the first radiation beam 110 at time T1, the modified target 121 has an extent (or length) S1 obtained along the major axis 230. When the modified target 121 reaches the second target location 116 at time T2, the modified target 121 has an extent S2 obtained along the major axis 230. The expansion rate is the difference in the extent of the modified target 121 obtained along the major axis 230 (S2-S1) divided by the difference in time (T2-T1), thus:

[0103]

[0104] Although the modified target 121 is expanded along the major axis 230 , the modified target 121 may also be compressed or thinned along the minor axis 235 .

[0105] The two-stage method discussed above produces a conversion efficiency of approximately 3-4%, wherein the modified target 121 is formed by interacting the first radiation beam 110 with the target material 120 and then the modified target 121 is converted to plasma by interacting the modified target 121 with the second radiation beam 115. In general, 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 heat load on all components within the light source 100, and may result in an increase in debris generation within the chamber housing the first target location 111 and the second target location 116. An increase in conversion efficiency can help meet the requirements of high-volume manufacturing tools while keeping the light source power requirements within an acceptable range. Various parameters affect the conversion efficiency, such as, for example, the wavelength of the first radiation beam 110 and the second radiation beam 115, the target material 120, and the pulse shape, energy, power, and intensity of the radiation beams 110, 115. The conversion efficiency may be defined as the EUV energy generated by the EUV light 130 into a 2% bandwidth around the center wavelength of the reflectivity curve of the (multilayer) mirrors used in either or both of the illumination and projection optics in the light collector system 135 and the optical device 145 divided by the energy of the irradiation pulse of the second radiation beam 115. In one example, the center wavelength of the reflectivity curve is 13.5 nanometers (nm).

[0106] One way to increase, maintain or optimize conversion efficiency is to control or stabilize the energy of EUV light 130, and to do this, it is important to maintain parameters such as the expansion rate of the modified target 121 within an acceptable range of values. By maintaining the radiation exposure on the target material 120 from the first radiation beam 110, the expansion rate of the modified target 121 is maintained within an acceptable range of values. And, the radiation exposure can be maintained based on one or more characteristics associated with the target material 120 or the modified target 121 measured relative to the first radiation beam 110. The radiation exposure is the radiation energy received by the surface of the target material 120 per unit area. Therefore, if the area of ​​the target material 120 is maintained constant from pulse to pulse, the radiation exposure can be estimated or approximated as the amount of energy directed toward the surface of the target material 120.

[0107] There are different methods or techniques to maintain the expansion rate of the modified target 121 within an acceptable range of values. Also, the method or technique used may depend on certain properties associated with the first radiation beam 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.

[0108] One property that may affect how the radiation exposure is maintained is the confocal parameter of the 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 along the propagation direction of the radiation beam from the waist to where the cross-sectional area doubles. Figure 2 , for the radiation beam 110, the Rayleigh length is the distance along the propagation direction 212 of the first radiation beam 110 from its waist (ie D1 / 2) to the point where the cross-section of the first beam doubles.

[0109] For example, Fig. 7AAs shown, the confocal parameter of the first radiation beam 110 is so long that the beam waist (D1 / 2) easily surrounds the target material 120, and the area of ​​the surface of the target material 120 intercepted by the first radiation beam 110 (measured in the X direction) remains relatively constant, even if the position of the target material 120 deviates from the position of the beam waist D1 / 2. For example, the area of ​​the surface of the target material 120 intercepted by the first radiation beam 110 at position L1 is within 20% of the area of ​​the surface of the target material 120 intercepted by the first radiation beam 110 at position L2. In the first scenario (compared to the second scenario described below) in which the area of ​​the surface of the target material 120 intercepted by the first radiation beam 110 is less likely to deviate from the average, the radiation exposure, and therefore the rate of expansion, can be maintained or controlled by maintaining the amount of energy directed from the first radiation beam 110 to the target material 120 (without regard to the surface area of ​​the target material 120 exposed by the first radiation beam 110).

[0110] As another example, Figure 7BAs shown, the confocal parameter of the first radiation beam 110 is so short that the beam waist (D1 / 2) does not surround the target material 120, and if the position of the target material 120 deviates from the position L1 of the beam waist D1 / 2, the area of ​​the surface of the target material 120 intercepted by the first radiation beam 110 deviates from the average value. For example, the area of ​​the surface of the target material 120 intercepted by the first radiation beam 110 at the position L1 is significantly different from the area of ​​the surface of the target material 120 intercepted by the first radiation beam 110 at the position L2. In the second scenario (compared to the first scenario) in which the area of ​​the surface of the target material 120 intercepted by the first radiation beam 110 is more likely to deviate from the average value, the radiation exposure and therefore the expansion rate can be maintained or controlled by controlling the amount of energy transferred from the first radiation beam 110 to the target material 120. In order to control the radiation exposure, the radiation energy of the first radiation beam 110 received by the surface of the target material 120 per unit area is controlled. Therefore, it is important to control the energy of the pulses of the first radiation beam 110 and the area of ​​the first radiation beam 110 where the target material 120 intercepts the first radiation beam 110. The area of ​​the first radiation beam 110 where the target material 120 intercepts the first radiation beam 110 is related to the surface of the target material 120 intercepted by the first radiation beam 110. Another factor that can affect the area of ​​the first radiation beam 110 where the target material 120 intercepts the first radiation beam 110 is the stability of the position and size of the beam waist D1 / 2 of the first radiation beam 110. For example, if the waist size and position of the first radiation beam 110 are constant, the position of the target material 120 relative to the beam waist D1 / 2 can be controlled. The waist size and position of the first radiation beam 110 may vary due to, for example, thermal effects in the light source 105. Generally, it becomes important to maintain a constant energy of the pulses in the first radiation beam 110 and also control other aspects of the light source 105 so that the target material 120 arrives at a known axial (Z-direction) position relative to the beam waist D1 / 2 without much variation around this position.

[0111] All described methods for maintaining or controlling the expansion rate of modified target 121 within an acceptable range of values ​​employ the use of measurement system 155 described below.

[0112] Reference again Figure 1 , the measurement system 155 measures at least one characteristic associated with any one or more of the target material 120, the modified target 121, and the first radiation beam 110. For example, the measurement system 155 may measure the energy of the first radiation beam 110. Fig. 8A As shown, exemplary measurement system 855A measures the energy of first radiation beam 110 directed toward target material 120 .

[0113] like Figure 8BAs shown, exemplary measurement system 855B measures the energy of radiation 860 reflected from target material 120 after first radiation beam 110 interacts with target material 120. Reflection of radiation 860 off target material 120 can be used to determine the position of target material 120 relative to the actual position of first radiation beam 110.

[0114] In some implementations, such as Figure 8C As shown, an exemplary measurement system 855B can be placed within the optical amplifier system 300 of the light source 105. In this example, the measurement system 855B can be placed 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 the optical amplifier system 300. The amount of radiation 860 reflected from the target material 120 is proportional to the amount of energy delivered to the target material 120; therefore, by measuring the reflected radiation 860, the amount of energy delivered to the target material 120 can be controlled or maintained. In addition, the amount of energy measured in the first radiation beam 110 or the reflected radiation 860 is related to the number of photons in the beam. Therefore, it can be said that the measurement system 855A or 855B measures the number of photons in the corresponding beam. In addition, the measurement system 855B can be considered to measure the number of photons reflected from the target material 120 (which once struck by the first radiation beam 110 becomes the modified target 121) as a function of how many photons strike the target material 120.

[0115] Measurement system 855A or 855B may be a photoelectric sensor such as an array (eg, a 2x2 array or a 3x3 array) of photoelectric cells having sensitivity to the wavelength of light to be measured and sufficient speed or bandwidth for the duration of the light pulse to be measured.

[0116] Typically, the measurement system 855A or 855B can measure the energy of the radiation beam 110 by measuring the spatially integrated energy in a direction perpendicular to the propagation direction of the first radiation beam 110. Because the measurement of the energy of the beam can be performed rapidly, the measurement can be made for each pulse emitted in the first radiation beam 110, and thus measurement and control can be on a pulse-by-pulse basis.

[0117] The measurement systems 855A, 855B may be fast photodetectors, such as photoelectromagnetic (PEM) detectors suitable for long wavelength infrared (LWIR) radiation. The PEM detectors may be silicon diodes for measuring near infrared or visible radiation or InGaAs diodes for measuring near infrared radiation. The energy of the pulses in the first radiation beam 110 may be determined by integrating the laser pulse signals measured by the measurement systems 855A, 855B.

[0118] refer to Fig. 9A, the measurement system 155 may be an exemplary measurement system 955A that measures a position Tpos of a target material 120 relative to a target position. The target position may be at the beam waist of the first radiation beam 110. The position of the target material 120 may be measured along a direction parallel to the beam axis of the first radiation beam 110, such as the first axial direction 212.

[0119] refer 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 a camera that reflects off the target material 120 as the target material 120 approaches to measure the position of the target material 120 and the arrival time of the target material 120 relative to a coordinate system within the chamber 165.

[0120] refer to Fig. 9C , the measurement system 155 may be an exemplary measurement system 955C that measures a dimension 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 a 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 struck by the second radiation beam 115. The measurement system 955C may also determine an orientation of the modified target 121. The measurement system 955C may use a shadowgraph technique of a pulsed backlight illuminator and a camera, such as a charge coupled device camera.

[0121] The measurement system 155 may include a set of measurement subsystems, each designed to measure a specific characteristic at a different speed or sampling interval. Such a set of subsystems can work together to provide a clear picture of how the first radiation beam 110 interacts with the target material 120 to form the modified target 121.

[0122] The measurement system 155 may include a plurality of EUV sensors within the chamber 165 for detecting EUV energy emitted from a plasma generated by the modified target 121 after the modified target 121 interacts with the second radiation beam 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.

[0123] The beam adjustment system 180 is employed under the control of the control system 160 to achieve control over the amount of energy delivered toward the target material 120 (radiation exposure). If the area of ​​the first radiation beam 110 at the location where the first radiation beam 110 interacts with the target material 120 can be assumed to be constant, the radiation exposure can be controlled by controlling the amount of energy within the first radiation beam 110. The beam adjustment 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., radiation exposure) or to control the amount of energy directed toward the target material 120. Thus, 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.

[0124] In some implementations, the beam conditioning system 180 includes a pulse width modulation system coupled to the first radiation beam 110. The pulse width modulation system is configured to adjust the pulse width of the first radiation beam 110. In this implementation, the pulse width modulation system can include an electro-optic modulator, such as, for example, a Pockels cell. For example, the Pockels cell is arranged within the light generator 310, and by turning on the Pockels cell for a shorter or longer period of time, the pulses sent by the Pockels cell (and therefore the pulses emitted from the light generator 310) can be adjusted to be shorter or longer.

[0125] In other implementations, the beam conditioning system 180 includes a pulse power conditioning system coupled to the first radiation beam 110. The pulse power conditioning system is configured to adjust the power of each pulse, for example by adjusting the average power within each pulse of the first radiation beam 110. In this implementation, the pulse power conditioning system may include an acousto-optic modulator. The acousto-optic modulator may be arranged so that changes in the RF signal applied to the piezoelectric transducer at the edge of the modulator may be changed, thereby changing the power of the pulses diffracted from the acousto-optic modulator.

[0126] In some implementations, the beam conditioning system 180 includes an energy conditioning system coupled to the first radiation beam 110. The energy conditioning system is configured to adjust the energy of the first radiation beam 110. For example, the energy conditioning system can be an electrically variable attenuator (such as a Pockels cell that varies between 0 V and a half-wave voltage, or an external acousto-optic modulator).

[0127] In some implementations, the position or angle of the target material 120 relative to the beam waist D1 / 2 varies so much that the beam conditioning system 180 includes a device for controlling the position or angle of the beam waist D1 / 2 in the coordinate system of the chamber 165 relative to the first target location 111 or relative to another location within the chamber 165. The device can be part of the focusing assembly 156 and can be used to move the beam waist along the Z direction or along a direction transverse to the Z direction (e.g., along a plane defined by the X and Y directions).

[0128] As described above, the control system 160 analyzes the information received from the measurement system 155 and determines how to adjust one or more properties of the first radiation beam 110 to control and maintain the expansion rate of the modified target 121. Fig.10 , the control system 160 may include one or more sub-controllers 1000, 1005, 1010, 1015 that interface with other parts of the light source 100, such as a sub-controller 1000 specifically configured to interface with the light source 105 (receive information from it and send information to it), a sub-controller 1005 specifically configured to interface with the measurement system 155, a sub-controller 1010 configured to interface with the beam delivery system 150, and a sub-controller 1015 configured to interface with the target material supply system 125. The light source 100 may include Figure 1 and 10 Other components not shown in the drawings but which may interact with the control system 160. For example, the light source 100 may include a diagnostic system, such as a droplet position detection feedback system and one or more target or droplet imagers. The target imager provides an output indicating the position of a droplet, for example, relative to a particular location (such as the primary focus 990 of the light collector 135), and provides this output to the droplet position detection feedback system, which can, for example, calculate the droplet position and trajectory, from which the droplet position error can be calculated on a droplet-by-droplet basis or on an average. Thus, the droplet position detection feedback system provides the droplet position error as an input to a sub-controller of the control system 160. The control system 160 can provide laser position, direction, and timing correction signals, for example, to a laser control system within the light source 105 that can be used, for example, to control a laser timing circuit and / or to a beam control system to control the position of an amplified beam and the shaping of a beam delivery system to change the position of a focal plane and / or a focused power of the first radiation beam 110 or the second radiation beam 115.

[0129] The target material delivery system 125 includes a target material delivery control system operable to respond to signals from the control system 160 , for example, to modify a release point of a drop of target material 120 released by an internal delivery mechanism to correct for errors in the drop reaching a desired target location 111 .

[0130] The control system 160 typically includes one or more of digital electronic circuitry, computer hardware, firmware, and software. The 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 implemented in a machine-readable storage device for execution by the programmable processor. Moreover, each sub-controller, such as sub-controllers 1000, 1005, 1010, 1015, etc., may include their own suitable input and output devices, one or more programmable processors, and one or more computer program products tangibly implemented in a machine-readable storage device for execution by the programmable processor.

[0131] 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. Typically, the processor receives instructions and data from a read-only memory and / or a random access memory. Storage devices suitable for tangibly implementing computer program instructions and data include all forms of non-volatile memory, including, for 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 CD-ROM disks. Any of the foregoing may be supplemented or incorporated by specially designed ASICs (application-specific integrated circuits).

[0132] To this end, the control system 160 includes an analysis program 1040 that receives measurement data from the one or more measurement systems 155. Generally, the analysis program 1040 performs all analysis necessary to determine how to modify or control the energy delivered from the first radiation beam 110 to the target material 120 or to modify or control the energy of the first radiation beam 110, and such analysis may be performed on a pulse-by-pulse basis if the measurement data is obtained on a pulse-by-pulse basis.

[0133] refer to Fig.11 , the light source 100 (under the control of the control system 160) performs a process 1100 for maintaining or controlling the expansion rate (ER) of the modified target 121 to improve the conversion efficiency of the light source 100. The light source 100 provides a target material 120 (1105). For example, the target material supply system 125 (under the control of the control system 160) can 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 the target material that is directed through the nozzle to produce a stream of droplets directed toward the first target location 111.

[0134] Next, the light source 100 directs the first radiation beam 110 toward the target material 120 to transfer energy to the target material 120 to modify the geometric distribution of the target material 120 to form a modified target 121 (1110). In particular, the first radiation beam 110 is directed toward the target material 120 through the first set 300 of one or more optical amplifiers. For example, the light source 105 can be activated by the control system 160 to generate the first radiation beam 110 (in the form of pulses), which can be directed toward the target material 120 within the target location 111, such as Figure 2 As shown. The focal plane of the first beam 110 (which is at the beam waist D1 / 2) can be configured to span the target location 111. Moreover, in some implementations, the focal plane can overlap the target material 120, or overlap an edge of the target material 120 facing the first radiation beam 110. The first radiation beam 110 (1110) can be directed toward the target material 120 by, for example, directing the first radiation beam 110 through a beam delivery system 150, wherein various optical devices can be used to modify the direction or shape or divergence of the radiation 110 so that it can interact with the target material 120.

[0135] The first radiation beam 110 (1110) can be directed toward the target material 120 by overlapping the target material 120 with a region of the first radiation beam 110 that encompasses its confocal parameter. In some implementations, the confocal parameter of the first radiation beam 110 can be so long that the beam waist (D1 / 2) easily encompasses the target material 120, and the area of ​​the surface of the target material 120 intercepted by the first radiation beam 110 (measured in the X direction) remains relatively constant, even if the position of the target material 120 deviates from the position of the beam waist D1 / 2 (e.g., Fig. 7A For example, the confocal parameter of the first radiation beam 110 may be greater than 1.5 mm. In other implementations, the confocal parameter of the first radiation beam 110 is so short that the beam waist (D1 / 2) does not surround the target material 120, and if the position of the target material 120 deviates from the position L1 of the beam waist D1 / 2, the area of ​​the surface of the target material 120 intercepted by the first radiation beam 110 deviates considerably (e.g., Figure 7B For example, the confocal parameter can be less than or equal to 2 mm.

[0136] The modified target material 121 changes its shape from the shape of the target material 120 to an expanded shape immediately after being struck by the first radiation beam 110, and this expanded shape continues to deform as it moves from the first target location 111 toward the second target location 116. The modified target 121 can have a geometric distribution (such as a molten metal volume) that is deformed from the shape of the target material to a disk-shaped volume with a substantially flat surface. Figure 1 and Figure 2The 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 the expansion rate. For example, Figure 2 As shown, the modified target 121 is extended along at least a major axis 230 that is substantially parallel to the X direction. The modified target 121 is extended 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).

[0137] Although first radiation beam 110 interacts with target material 120 primarily by changing the shape of target material 120, first radiation beam 110 may also interact with target material 120 in other ways; for example, first radiation beam 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 created by target material 120 than from the plasma created by modified target 121 (due to subsequent interaction between modified target 121 and second radiation beam 115), and the dominant effect from first radiation beam 110 on target material 120 is the modification of the geometric distribution of target material 120 to form modified target 121.

[0138] Light source 100 directs second radiation beam 115 toward modified target 121 such that second radiation beam converts at least a portion of modified target 121 into plasma 129 that emits EUV light (1115). In particular, light source 100 directs second radiation beam 115 toward modified target 121 through second set 305 of one or more optical amplifiers. For example, light source 105 may be activated by control system 160 to generate second radiation beam 115 (in the form of pulses) that may be directed toward modified target 121 within second target location 116, such as Figure 2 At least one optical amplifier in the first group 300 may be in the second group 305, such as Figure 5 Example shown.

[0139] The light source 100 measures one or more characteristics (e.g., energy) associated with one or more of the target material 120 and the modified target 121 relative to the first radiation beam 110 (1120). For example, the measurement system 155 measures the characteristics under the control of the control system 160, and the control system 160 receives the measurement data from the measurement system 155. The light source 100 controls the radiation exposure at the target material 120 from the first radiation beam 110 based on the one or more characteristics (1125). As described above, the radiation exposure is the amount of radiation energy transferred from the first radiation beam 110 to the target material 120 per unit area, in other words, the radiation energy received by the surface of the target material 120 per unit area.

[0140] In some implementations, the characteristic that can be measured (1120) is the energy of the first radiation beam 110. In other general implementations, the characteristic that can be measured (1120) is the position of the target material 120 relative to the position of the first radiation beam 110 (e.g., relative to the beam waist of the first radiation beam 110), such position can be determined in the longitudinal (Z) direction or in a direction transverse to the longitudinal direction (e.g., in the XY plane).

[0141] The energy of the first radiation beam 110 may be measured by measuring the energy of the radiation 860 reflected from the optically reflective surface of the target material 120 (such as Figure 8B and 8C The energy of radiation 860 reflected from the optically reflective surface of target material 120 can be measured by measuring the total intensity of radiation 860 on four individual photovoltaic cells.

[0142] The total energy content of the retroreflected radiation 860 can be used in conjunction with other information about the first radiation beam 110 to determine the relative position of the target material 120 along the Z direction or in a direction transverse to the Z direction (such as in the XY plane) with respect to the beam waist of the first radiation beam 110. Alternatively, the total energy content of the retroreflected radiation 860 (along with other information) can be used to determine the relative position of the target material 120 along the Z direction with respect to the beam waist of the first radiation beam.

[0143] The energy of the first radiation beam 110 may be measured by measuring the energy of the first radiation beam 110 directed toward the target material 120 (such as Fig. 8A The energy of the first radiation beam 110 may be measured by measuring the spatially integrated energy in a direction perpendicular to the propagation direction of the first radiation beam 110 (the first axial direction 212 ).

[0144] In some implementations, the characteristic that can be measured (1120) is the orientation or direction of the first radiation beam 110 as it travels toward the target material 120 (e.g., Fig. 8AThis information about the pointing direction can be used to determine the overlay error between the position of the target material 120 and the axis of the first radiation beam 110.

[0145] In some implementations, the property that can be measured (1120) is the position of the target material 120 relative to the target position. The target position can be at the beam waist (D1 / 2) of the first radiation beam 110 along the Z direction. The position of the target material 120 can be measured along a direction parallel to the first axial direction 212. The target position can be measured relative to the primary focus 990 of the light collector 135. The position of the target material 120 can be measured along two or more non-parallel directions.

[0146] In some implementations, the characteristic that can be measured (1120) is a size of the modified target before the second radiation beam converts at least a portion of the modified target into plasma.

[0147] In some implementations, the characteristic that can be measured ( 1120 ) corresponds to an estimate of the rate of expansion of the modified target.

[0148] In some implementations, the characteristic that can be measured (1120) corresponds to a spatial characteristic of radiation 860 reflected from an optically reflective surface of target material 120 (such as Figure 8B and 8C ). Such information can be used to determine the relative position (e.g., along the Z direction) between the target material 120 and the beam waist of the first radiation beam 110. This spatial characteristic can be determined or measured using an astigmatic imaging system placed in the path of the reflected radiation 860.

[0149] 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 can be used to determine the distance between target material 120 and the beam axis of first radiation beam 110 along a direction transverse to the Z direction.

[0150] In other implementations, the characteristics that may be measured (1120) correspond to spatial aspects of the modified target 121 formed after the first radiation beam 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 transverse to the Z direction. Such information regarding the angle of the modified target 121 may be used to determine the distance between the target material 120 and the axis of the first radiation beam 110 along a direction transverse to the Z direction. As another example, the size or expansion rate of the modified target 121 may be measured after a predetermined or set time after it is first formed from the interaction between the target material 120 and the first radiation beam 110. If it is known that the energy of the first radiation beam 110 is constant, such information regarding the size or expansion rate of the modified target 121 may be used to determine the distance along the longitudinal direction (Z direction) between the target material 120 and the beam waist of the first radiation beam 110.

[0151] This characteristic may be measured 1120 as fast as for each pulse of the first radiation beam 110. For example, if the measurement system 155 comprises a PEM or four cells (an arrangement of 4 PEMs), the measurement rate may be as fast as pulse by pulse.

[0152] On the other hand, for a measurement system 155 that measures properties such as the size or expansion rate of a target material 120 or a modified target 121, a camera can be used in the measurement system 155, but cameras are typically much slower, for example, a camera can measure at a rate of about 1 Hz to about 200 Hz.

[0153] In some implementations, the radiation exposure delivered from the first radiation beam 110 to the target material 120 can be controlled (1125) to control or maintain the expansion rate of the modified target. In other implementations, the radiation exposure delivered from the first radiation beam 110 to the target material 120 can be controlled (1125) by determining whether a characteristic of the first radiation beam 110 should be adjusted based on the measured one or more characteristics. Thus, if it is determined that a characteristic of the first radiation beam 110 should be adjusted, for example, the energy content of the pulses of the first radiation beam 110 can be adjusted, or the area of ​​the first radiation beam 110 at the location of the target material 120 can be adjusted. The energy content of the pulses of the first radiation beam 110 can be adjusted by adjusting one or more of the following: the pulse width of the first radiation beam 110, the pulse duration of the first radiation beam 110, and the average or instantaneous power of the first radiation beam 110. The area where the first radiation beam 110 interacts with the target material 120 can be adjusted by adjusting the relative axial (along the Z direction) position between the target material 120 and the beam waist of the first radiation beam 110.

[0154] In some implementations, one or more characteristics may be measured 1120 for each pulse of the first radiation beam 110. In this manner, it may be determined whether a characteristic of the first radiation beam 110 should be adjusted for each pulse of the first radiation beam 110.

[0155] In some implementations, radiation exposure can be controlled by controlling the radiation exposure delivered from the first radiation beam 110 to the target material 120 (e.g., within an acceptable radiation exposure range) while at least a portion of the emitted and collected EUV light 140 is exposing a wafer of a lithography tool.

[0156] Process 1100 may also include collecting at least a portion of EUV light 130 emitted from the plasma (using light collector 135 ); and directing the collected EUV light 140 toward the wafer to expose the wafer to the EUV light 140 .

[0157] In some implementations, the one or more characteristics measured (1120) include the number of photons reflected from the modified target 121. The number of photons reflected from the modified target 121 can be measured based on how many photons strike the target material 120.

[0158] As described above, process 1100 includes controlling the radiation exposure (1125) at the target material 120 from the first radiation beam 110 based on one or more characteristics. For example, the radiation exposure can be controlled 1125 so that it is maintained within a predetermined radiation exposure range. The radiation exposure is the amount of radiation energy delivered to the target material 120 from the first radiation beam 110 per unit area. In other words, it is the radiation energy received by the surface of the target material 120 per unit area. If the unit area of ​​the surface of the target material 120 exposed to the first radiation beam 110 or intercepted by the first radiation beam 110 is controlled (or maintained within an acceptable range), then this factor of the radiation exposure remains relatively constant, and the radiation exposure (1125) at the target material 120 can be controlled or maintained by maintaining the energy of the first radiation beam 110 within an acceptable energy range. There are various ways to maintain the unit area of ​​the surface of the target material 120 exposed to the first radiation beam 110 within an acceptable area range. These are discussed next.

[0159] The radiation exposure (1125) from the first radiation beam 110 at the target material 120 can be controlled so that the energy of the pulses of the first radiation beam 110 is maintained at a constant level or within an acceptable range of values ​​(by feedback control using the measured characteristic 1120) regardless of disturbances that may cause energy fluctuations.

[0160] In other aspects, the radiation exposure (1125) from the first radiation beam 110 at the target material 120 can be controlled such that the energy of the pulses of the first radiation beam 110 is adjusted (e.g., increased or decreased) through feedback control using the measured characteristic 1120 to compensate for errors in the longitudinal (Z direction) placement of the target material 120 relative to the beam waist of the first radiation beam 110.

[0161] The first radiation beam 110 may be a pulsed radiation beam such that pulses of light are directed toward the target material 120 (1110). Similarly, the second radiation beam 115 may be a pulsed radiation beam such that pulses of light are directed toward the modified target 121 (1115).

[0162] The target material 120 may be a droplet of the target material 120 generated from the target material supply system 125. In this manner, the geometric distribution of the target material 120 may be modified into a modified target 121, which is transformed into a disk-shaped volume of molten metal having a substantially flat surface. The target material droplet is transformed into the disk-shaped volume according to the expansion rate.

[0163] refer to Fig.12 , process 1200 is performed by light source 100 (under control of control system 160) to stabilize EUV light energy generated by plasma 129 formed by interaction between modified target 121 and second radiation beam 115. Similar to process 1100 described above, light source 100 provides target material 120 (1205); light source 100 directs first radiation beam 110 toward target material 120 to transfer energy to target material 120 to modify the geometric distribution of target material 120 to form modified target 121 (1210); and light source 100 directs second radiation beam 115 toward modified target 121 so that the second radiation beam converts at least a portion of modified target 121 into plasma 129 that emits EUV light (1215). Light source 100 uses process 1110 to control the amount of radiation exposure applied from first radiation beam 110 to target material 120 (1220).

[0164] The power or energy of the EUV light 130 is stabilized (1225) by controlling the radiation exposure. 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 into the plasma 129 by the second radiation beam 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, and the lateral area or size of the second radiation beam 115 when interacting with the modified target 121. Since the position of the modified target 121 and the size of the modified target 121 depend on how the target material 120 interacts with the first radiation beam 110, by controlling the radiation exposure applied from the first radiation beam 110 to the target material 120, the expansion rate of the modified target 121 can be controlled, and thus both factors can be controlled. In this manner, the conversion efficiency may be stabilized or controlled by controlling the radiation exposure (1220), which thereby stabilizes the EUV energy (1225) generated by the plasma 129.

[0165] Also refer to Fig.13 In some implementations, the first radiation beam 110 can be generated by a dedicated subsystem 1305A within the light source 105, and the second radiation beam 115 can be generated by a dedicated and separate subsystem 1305B within the light source 105, such that the radiation beams 110, 115 follow two separate paths on their way to the respective first target location 111 and second target location 116. In this manner, each of the radiation beams 110, 115 travels through a respective subsystem of the beam delivery system 150, and thus travels through respective separate optical control components 1352A, 1352B and focusing assemblies 1356A, 1356B.

[0166] For example, subsystem 1305A can be a system based on a solid-state gain medium, while subsystem 1305B can be a system based on a gas gain medium such as produced by a CO2 amplifier. Exemplary solid-state gain media that can be used as subsystem 1305A include erbium-doped fiber lasers and neodymium-doped yttrium aluminum garnet (Nd:YAG) lasers. In this example, the wavelength of the first radiation beam 110 can be different from the wavelength of the second radiation beam 115. For example, the wavelength of the first radiation beam 110 using the solid-state gain medium can be about 1 μm (e.g., about 1.06 μm), and the wavelength of the second radiation beam 115 using the gas medium can be about 10.6 μm.

[0167] Other implementations are within the scope of the following claims.

Claims

1. A device for an extreme ultraviolet (EUV) light source, the device comprising: an optical device configured to cause the first radiation beam to interact with the target material within the chamber to form a modified target; a first measurement system including a sensor configured to receive radiation reflected from the target material when the target material interacts with the first radiation beam; a second measurement system comprising a plurality of measurement subsystems, each subsystem configured to measure a spatial aspect of the modified target formed from an interaction between the first radiation beam and the target material; as well as A control system is in communication with the first and second measurement systems, the control system being configured to analyze outputs from the first and second measurement systems and to send information based on the analysis to the optical device.

2. The apparatus of claim 1, wherein the control system is configured to send information based on the analysis to the optical device so that the optical device adjusts one or more properties of the first radiation beam.

3. The apparatus of claim 1, wherein the control system is configured to send information based on the analysis to the optical device comprising: The energy of the first radiation beam is maintained within an acceptable energy range.

4. The apparatus of claim 1 , wherein the control system is configured to analyze outputs from the first measurement system and the second measurement system comprises: Based on the analysis, it is determined whether a characteristic of the first radiation beam should be adjusted.

5. The apparatus of claim 1 , wherein the optical device is configured to cause the first radiation beam to interact with the target material to form the modified target comprises: Modifying the geometric distribution of the target material.

6. The apparatus of claim 1, wherein each measurement subsystem is configured to measure a spatial aspect of the modified object by measuring one or more of a size, a position, and an orientation of the modified object.

7. The apparatus of claim 6, wherein each measurement subsystem comprises a backlight illuminator and a camera.

8. The apparatus of claim 1, wherein the optical device is further configured to cause a second radiation beam to interact with the modified target to convert at least a portion of the modified target into a plasma that emits EUV light.

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