Target expansion rate control in extreme ultraviolet light sources

Abstract

The invention relates to target expansion rate control in an extreme ultraviolet light source. A method comprising: providing a target material comprising 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 beam of radiation toward the modified target, the second beam of radiation converting at least a portion of the modified target into a plasma emitting 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.

Description

[0001] This application is a divisional application of a Chinese patent application with an application date of August 10, 2016, an application number of 202111336436.4, and an invention title of "Target Expansion Rate Control in Extreme Ultraviolet Light Sources".

[0002] Cross - Reference to Related Applications

[0003] This application claims the benefit of U.S. Serial No. 14 / 824,141, filed August 12, 2015, entitled "TARGET EXPANSION RATE CONTROL IN AN EXTREME ULTRAVIOLET LIGHT SOURCE" and U.S. Serial No. 14 / 824,147, filed 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 a target material for a laser-produced plasma extreme ultraviolet light source. BACKGROUND OF THE INVENTION

[0005] Extreme ultraviolet (EUV) light (e.g., electromagnetic radiation having a wavelength of about 50 nm or less (sometimes also referred to as soft x-rays) and including light having a wavelength of about 13 nm) can be used in a lithography process to create very small features in a substrate (e.g., a silicon wafer).

[0006] Methods for generating EUV light include, but are not necessarily limited to, converting a material having an element such as xenon, lithium, or tin using emission spectral lines in the EUV range in a plasma state. In one such method, commonly referred to as laser-produced plasma ("LPP"), the required plasma can be generated by irradiating a target material (e.g., in the form of droplets, plates, ribbons, streams, or clusters of the material) with an amplified light beam, 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 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 to 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 a radiation exposure amount transferred from the first radiation beam to the target material within a predetermined energy range based on the one or more measured characteristics.

[0008] The implementation may include one or more of the following features. For example, one or more characteristics associated with one or more of the target material and the modified target may be measured by measuring an energy of the first radiation beam. The energy of the first radiation beam may be measured by measuring an 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 an energy of the first radiation beam directed toward the target material. The energy of the first radiation beam may be measured by measuring a spatially integrated energy in a direction perpendicular to a propagation direction of the first radiation beam.

[0009] The first radiation beam may be directed toward the target material by overlapping the target material with a region of the first radiation beam that encloses its confocal parameter. The confocal parameter may be greater than 1.5 mm.

[0010] One or more characteristics associated with one or more of the target material and the modified target may be measured by measuring a position of the target material relative to a target 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 main 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 to a plasma. One or more characteristics associated with one or more of the target material and the modified target may be measured by estimating a rate of expansion of the modified target.

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

[0013] The amount of radiation exposure transferred from the first radiation beam to the target material can be controlled by determining whether the characteristics of the first radiation beam should be adjusted based on one or more measured characteristics. The determination of whether the characteristics of the first radiation beam should be adjusted can 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 can be adjusted: the energy content of the pulses of the first radiation beam and the region of interaction of the first radiation beam with the target material. The energy content of the pulses of the first radiation beam can 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] The first radiation beam can be directed toward the target material by directing first radiation pulses toward the target material; the one or more characteristics can be measured by measuring one or more characteristics of each first radiation pulse; and whether the characteristics of the first radiation beam should be adjusted can be determined by determining whether the characteristics should be adjusted for each first radiation pulse.

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

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

[0018] The method can further 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 can be measured by measuring at least one characteristic of each pulse of the first radiation beam directed toward the target material.

[0020] The first radiation beam can be directed toward the 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 compared to the EUV light emitted from 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 a modified target.

[0021] The geometric distribution of a 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 amount of 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 can be expanded along at least one axis that is not parallel to the optical axis of the second radiation beam.

[0023] One or more characteristics 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 impinge on the target material.

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

[0025] The first radiation beam can be directed by directing the first radiation beam through a first set of one or more optical amplifiers; and the second radiation beam can be directed by directing the second radiation beam through a second set of one or more optical amplifiers; wherein at least one optical amplifier in the first set is located in the second set.

[0026] One or more characteristics associated with one or more of the target material and the modified target can be measured by measuring the energy of the first radiation beam directed toward the target material; and the amount of radiation exposure delivered to the target material can be controlled by adjusting the amount of energy directed from the first radiation beam to the target material based on the measured energy. The first radiation beam can be directed toward the target material by overlapping the target material with a region of the first radiation beam that encloses its confocal parameter; and the confocal parameter can be less than or equal to 2 mm.

[0027] The amount of energy directed from the first radiation beam to the target material can be adjusted by adjusting the properties of the first radiation beam.

[0028] The amount of radiation exposure delivered from the first radiation beam to the target material can be controlled by adjusting one or more of the following: 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 region 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 including one or more first optical amplifiers; and the second radiation beam can be directed by directing it through a second set of optical components including one or more second optical amplifiers; wherein the first set of optical components is different from and separated from the second set of optical components.

[0030] In other general aspects, an apparatus includes a chamber defining an initial target position for receiving a first radiation beam and a target position for receiving a second radiation beam; a target material delivery system configured to supply a target material to the initial target position, 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 position 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 position to convert at least a portion of the modified target into a plasma that emits EUV light. The apparatus includes: a measurement system configured to measure 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 coupled 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 amount of radiation exposure transferred from the first radiation beam to the target material based on the measured one or more characteristics.

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

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

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

[0034] The beam conditioning system can be configured to send one or more signals to a light source to control the amount of energy directed to a target material by sending one or more signals to the beam conditioning system, and the beam conditioning 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 can include a first set of one or more optical amplifiers through which a first radiation beam passes; and a second set of one or more optical amplifiers through which a second radiation beam passes, with at least one optical amplifier in the first set being in the second set. The measurement system can measure the energy of the first radiation beam when the first radiation beam is directed toward an initial target position; and the control system can be configured to receive the measured energy from the measurement system and send one or more signals to the light source to control the amount of energy directed from the first radiation beam to the target material based on the measured energy.

[0036] In some general aspects, a method includes: providing a 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 the geometric profile 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 to a plasma that emits EUV light; controlling the amount of radiation exposure transferred from the first radiation beam to the target material within a predetermined range of radiation exposure; and stabilizing the power of the EUV light emitted from the plasma by controlling the amount of radiation exposure transferred from the first radiation beam to the target material within a predetermined range of radiation exposure.

[0037] Implementations can include one or more of the following features. For example, the first radiation beam can be directed by directing the first radiation beam through a first set of optical components including one or more first optical amplifiers; and the second radiation beam can 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 can be different from and separated from the second set of optical components.

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

[0039] The target material can be provided by providing droplets of the target material; and the geometric profile of the target material can 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 can be provided by supplying droplets of the target material; and the geometric distribution of the target material can be changed by transforming the droplets of the target material into a fog-like volume of molten metal particles.

[0041] The target material can be converted into a modified target according to an 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 one or more of the target material and 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 from the first radiation beam to the target material can be controlled by estimating the expansion rate of the modified target. The radiation exposure delivered from the first radiation beam to the target material can be controlled by maintaining the expansion rate of the modified target.

[0044] The radiation exposure delivered from the first radiation beam to the target material can be controlled by determining whether the characteristics of the first radiation beam should be adjusted. The radiation exposure delivered from the first radiation beam to the target material can be controlled by: adjusting the characteristics of the first radiation beam by adjusting the energy content of each pulse of the first radiation beam and one or more in the region where the first radiation beam interacts with the target material. The energy content of each pulse of the first radiation beam can be adjusted by adjusting one or more of the following: the width of each pulse of the first radiation beam, the duration of each pulse of the first radiation beam, and the power of each pulse of the first radiation beam.

[0045] While at least a portion of the EUV light emitted from the plasma exposes the wafer, the power of the EUV light emitted from the plasma can be stabilized by stabilizing the power of the EUV light.

[0046] The method can further 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 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.

[0048] The radiation exposure delivered from the first radiation beam to the target material can be controlled by adjusting the properties of the first radiation beam. The properties of the first radiation beam can 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 position for receiving a first radiation beam and a target position for receiving a second radiation beam; a target material delivery system configured to provide a target material to the initial target position, 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 position to transfer energy to the target material to modify a geometric distribution of the target material to form a modified target, and direct the second radiation beam toward the target position to convert at least a portion of the modified target into a plasma that emits EUV light. The apparatus includes a control system connected to the target material delivery system, the light source, and the optical steering system and configured to send one or more signals to the light source to control a radiation exposure amount transferred from the first radiation beam to the target material within a predetermined radiation exposure amount range to stabilize a power of the EUV light emitted from the plasma.

[0050] The implementation may include one or more of the following features. For example, the apparatus may further include a measurement system that measures one or more characteristics associated with one or more of the target material and the modified target relative to the first radiation beam; wherein the control system is connected to the measurement system.

[0051] The apparatus may further 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 radiation exposure amount transferred to the target material by sending one or more signals to the beam conditioning system, and the beam conditioning system is configured to adjust one or more characteristics of the light source to control the radiation exposure amount transferred 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 generates 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 the first radiation beam directed toward a first target position and the second radiation beam directed toward a second target position;

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

[0055] Figure 3B and Figure 3C are respectively diagrams that can be used in Figure 1Block diagram of an exemplary beam path combiner and an exemplary beam path splitter used in a light source;

[0056] Figure 4A and Figure 4B is a block diagram of an exemplary optical amplifier system that can be used in a Figure 3A light source;

[0057] Figure 5 is a block diagram of an exemplary optical amplifier system that can be used in a Figure 3A light source;

[0058] Figure 6 is a schematic diagram showing another implementation of a first radiation beam directed towards a first target position and a second radiation beam directed towards a second target position;

[0059] Figure 7A and Figure 7B is a schematic diagram showing an implementation of a first radiation beam directed towards a first target position;

[0060] Figures 8A - 8C and Figures 9A - 9C shows schematic diagrams of 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] Figure 10 is Figure 1 a block diagram of an exemplary control system for a light source;

[0062] Figure 11 is a flowchart of an exemplary process performed by a light source (under the control of a control system) for maintaining or controlling the expansion rate (ER) of a modified target to improve the conversion efficiency of the light source;

[0063] Figure 12 is a flowchart 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 transferred from a first radiation beam to a target material; and

[0064] Figure 13 is a block diagram of an exemplary light source for generating first and second radiation beams and an exemplary beam delivery system for modifying and focusing the first and second radiation beams to respective first and second target positions. DETAILED DESCRIPTION

[0065] Techniques for improving the conversion efficiency of extreme ultraviolet (EUV) light generation are disclosed. Referring to Figure 1, as 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 such a way that it increases the amount of available 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 available EUV light 130 is the amount of EUV light 130 that can be utilized at the optical device 145. Thus, the amount of available EUV light 130 can depend on aspects such as the bandwidth or central wavelength of the optical components used to utilize the EUV light 130.

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

[0067] In this way, the total amount of available EUV light 130 generated can be increased or controlled by controlling the expansion rate of the modified target 121. In particular, the size of the modified target 121 and its expansion rate depend on the radiation exposure amount applied from the first radiation beam 110 to the target material 120, and the radiation exposure amount is the amount of energy transferred by the first radiation beam 110 to the region of the target material 120. Thus, the expansion rate of the modified target 121 can be maintained or controlled by maintaining or controlling the amount of energy transferred per unit area to the target material 120. The amount of energy transferred to the target material 120 depends on the energy of the first radiation beam 110 immediately before it impinges on 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 photo-electromagnetic (PEM) detector suitable for long-wavelength infrared (LWIR) radiation, an InGaAs diode for measuring near-infrared (IR) radiation, or a silicon diode for visible or near-IR radiation.

[0069] The expansion rate of the modified target 121 depends at least in part on the amount of energy in the pulse of the first radiation beam 110 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 reality, the target material 120 may have a small but mostly constant axial position offset relative to the beam waist of the first radiation beam 110. If all these factors remain constant, one factor that controls the expansion rate of the modified target 121 is the pulse energy of the first radiation beam 110 in a pulse of the first radiation beam having a duration of a few ns to 100 ns. If the pulse of the first radiation beam 110 has a duration equal to or less than 100 ns, another factor that can control the expansion rate of the modified target 121 is the instantaneous peak power of the first radiation beam 110. If the pulse of the first radiation beam 110 has a shorter duration, on the order of picoseconds (ps) for example, other factors can control the expansion rate of the modified target 121, as described below.

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

[0071] A first target position 111 receives a target material 120, such as tin, from a target material supply system 125. The interaction between a 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 typically 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 position 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 can continue to move along the -X direction in addition to moving in another direction, such as along a direction parallel to the Z direction. As the modified target 121 moves away from the first target position 111, its geometric distribution continues to deform until the modified target 121 reaches a second target position 116. The interaction between a second radiation beam 115 and the modified target 121 (at the second target position 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 as collected EUV light 140 toward an optical device 145, such as a lithography tool. The first target position 111 and the second target position 116, as well as the light collector 135, can 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 into a plasma when interacting with the first radiation beam 110, and thus such a plasma may emit EUV radiation. However, the nature of the first radiation beam 110 is selected and controlled such that the dominant effect of the first radiation beam 110 on the target material 120 is the deformation or modification of the geometric distribution of the target material 120 to form the modified target 121.

[0073] Each of the first radiation beam 110 and the second radiation beam 115 is directed toward the respective target positions 111, 116 by a beam delivery system 150. The beam delivery system 150 can include an optical steering component 152 and a focusing assembly 156 that focuses the first radiation beam 110 or the second radiation beam 115 onto a respective first focusing region and second focusing region. The first focusing region and the second focusing region can overlap the first target position 111 and the second target position 116, respectively. The optical component 152 can include optical elements, such as lenses and / or mirrors, that direct the radiation beams 110, 115 through refraction and / or reflection. The beam delivery system 150 can also include elements that control and / or move the optical component 152. For example, the beam delivery system 150 can include actuators that are controllable to cause movement of the optical elements within the optical component 152.

[0074] Also refer to Figure 2 , the focusing component 156 focuses the first radiation beam 110 such that the diameter D1 of the first radiation beam 110 is minimized in the first focusing region 210. In other words, when the first radiation beam 110 propagates toward the first focusing region 210 in the first axial direction 212, the focusing component 156 causes the first radiation beam 110 to converge, and the first axial direction 212 is the general propagation direction of the first radiation beam 110. The first axial direction 212 extends along the 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 with respect to the Z direction. In the absence of the target material 120, when the first radiation beam 110 propagates away from the first focusing region 210 in the first axial direction 212, the first radiation beam 110 diverges.

[0075] In addition, the focusing component 156 focuses the second radiation beam 115 such that the diameter D2 of the second radiation beam 115 is minimized in the second focusing region 215. Thus, when the second radiation beam 115 propagates toward the second focusing region 215 in the second axial direction 217, the focusing component causes the second radiation beam 115 to converge, and the second axial direction 217 is the general propagation direction of the second radiation beam 115. The second axial direction 217 also extends along the 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, when the second radiation beam 115 propagates away from the second focusing region 215 in the second axial direction 217, the second radiation beam 115 diverges.

[0076] As described below, 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, 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, one or more characteristics can be the pulse energy of the first radiation beam 110 directed towards the target material 120. These examples will be discussed in more detail below. The control system 160 is configured to receive the one or more measured characteristics from the measurement system such 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 to the target material 120. The beam conditioning system 180 is a system that includes components within the light source 105 or modifies components within the light source 105 to control the properties of the first radiation beam 110, such as pulse width, pulse energy, instantaneous power within the pulse, or average power within the pulse).

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

[0078] Additionally, although not required, the light source 105 can include a first light generator 310 that generates a first pulsed beam 311 and a second light generator 315 that generates a second pulsed beam 316. For example, each of the light generators 310, 315 can be a laser, a seed laser such as a master oscillator, or a lamp. Exemplary light generators that can be used as the light generators 310, 315 are Q-switch radio frequency (RF) pumped axial flow carbon dioxide (CO2) oscillators that can operate at a repetition rate of, for example, 100 kHz.

[0079] The optical amplifiers within the optical amplifier systems 300, 305 each include a gain medium on a respective beam path, and the beams 311, 316 of the respective optical generators 310, 315 propagate along the respective beam paths. When the gain medium of the optical amplifier is excited, the gain medium supplies photons to the beam, amplifying the beams 311, 316 to produce amplified beams that form the first radiation beam 110 or the second radiation beam 115.

[0080] The wavelengths of the beams 311, 316 or the radiation beams 110, 115 can be different from each other such that if the radiation beams 110, 115 are combined at any point within the light source 105 they can also be separated from each other. 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 chosen to more easily separate the two radiation beams 110, 115 using dispersive optics or dichroic mirrors or beam splitter coatings. In the case where the two radiation beams 110, 115 propagate together in the same amplifier chain (e.g., where some of the amplifiers in the optical amplifier system 300 are in the optical amplifier system 305), different wavelengths can be used to adjust the relative gain between the two radiation beams 110, 115 even as they are passing through the same amplifier.

[0081] For example, once separated, the radiation beams 110, 115 can be steered or focused within the chamber 165 to two separate locations (such as the first target location 111 and the second target location 116 respectively). 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 as it travels from the first target location 111 to the second target location 116.

[0082] The light source 105 can 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 3BAn exemplary beam path combiner 325 is shown. 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 a first radiation beam 110 to pass along a first path leading to the dichroic beam splitter 342. The dichroic beam splitter 340 reflects a second radiation beam 115 along a second path in which the second radiation beam 115 is reflected from the mirrors 344, 346, and the mirrors 344, 346 redirect the second radiation beam 115 towards the dichroic beam splitter 342. The first radiation beam 110 freely passes through the dichroic beam splitter 342 to the output path, while the second radiation beam 115 is reflected from the dichroic beam splitter 342 onto the output path such that both the first radiation beam 110 and the second radiation beam 115 are superimposed on the output path.

[0083] Additionally, the light source 105 may include a beam path separator 326 that separates the first radiation beam 110 from the second radiation beam 115 such that the two radiation beams 110, 115 can be individually steered and focused within the chamber 165. In Figure 3C An exemplary beam path separator 326 is shown. The beam path separator 326 includes a pair of dichroic beam splitters 350, 352 and a pair of mirrors 354, 356. The dichroic beam splitter 350 receives a 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 a first path towards 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 is reflected 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] Additionally, the first radiation beam 110 may be configured to have less pulse energy than 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 a 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 systems 300 or 305 respectively include a set of three optical amplifiers 401, 402, 403 and 406, 407, 408, 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 approximately 9.1 and approximately 11.0 μm, and particularly approximately 10.6 μm, with a gain of more than 1000. The optical amplifiers 401, 402, 403 may operate similarly or at different wavelengths. Suitable amplifiers and lasers for use in the optical amplifier systems 300, 305 may include pulsed laser devices, such as pulsed gas discharge CO2 amplifiers, which, for example, use DC or RF excitation to produce radiation at approximately 9.3 μm or approximately 10.6 μm and operate at relatively high power (e.g., 10 kW or higher) and high pulse repetition rates (e.g., 50 kHz or higher). Exemplary optical amplifiers 401, 402, 403 or 406, 407, 408 are axial flow high power CO2 lasers with wear-free gas circulation and capacitive RF excitation, such as the 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 serves 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 system produced by TRUMPF Inc. of Farmington, Connecticut.

[0087] The optical amplifier systems 300, 305 may include Figure 4A and Figure 4B optical elements (not shown in the figure) for guiding and shaping the respective light beams 311, 316. For example, the optical amplifier systems 300, 305 may include reflective optical devices such as mirrors, partially transmissive optical devices such as beam splitters or partially transmissive mirrors, and dichroic beam splitters.

[0088] The light source 105 further includes an optical system 320, which may include one or more optical devices (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.) for guiding the light beams 311, 316 through the light source 105.

[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 within optical amplifier system 305, and at least one of amplifiers 406, 407, 408 may be located within optical amplifier system 300. For example, as Figure 5 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 combiners 325) for combining two light 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 optical devices overlap between optical amplifier systems 300, 305, first radiation beam 110 and second radiation beam 115 may be coupled together such that a change in one or more characteristics of first radiation beam 110 may cause a change in one or more characteristics of second radiation beam 115, and vice versa. Thus, it becomes more important to control energy such as the energy of first radiation beam 110 or the energy delivered to target material 120 within the system. Additionally, optical amplifier systems 300, 305 also include optical elements 505 (such as beam path separators 326) for separating two light beams 110, 15 output from amplifiers 403 / 408 such that the two light beams 110, 115 can be directed towards respective target locations 111, 116.

[0090] Target material 120 may be any material including target materials that emit EUV light when converted to a plasma. 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 to a plasma state having an emission spectral line within the EUV range. For example, the target substance may be a droplet of liquid or molten metal, a portion of a liquid stream, a solid particle or cluster, a solid particle contained within a droplet, a foam of the target material, or a solid particle contained within a portion of a liquid stream. For example, the target substance may be water, tin, lithium, xenon, or any material that has an emission spectral line within the EUV range when converted to 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 where target material 120 is a droplet made of a molten metal such as tin. However, target material 120 may take other forms.

[0091] The target material 120 can be provided to the first target position 111 by passing the molten target material through the nozzle of the target material supply device 125 and allowing the target material 120 to drift into the first target position 111. In some implementations, the target material 120 can be directed toward the first target position 111 by a force.

[0092] By irradiating the target material 120 with radiation pulses from the first radiation beam 110, the shape of the target material 120 is changed or modified (e.g., deformed) before it reaches the second target position 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 the force that deforms the target material 120 into a modified target 121 having a shape different from the shape of the target material 120. For example, the target material 120 can have a shape similar to a droplet, while the shape of the modified target 121 is deformed such that its shape is closer to the shape of a disk (such as a flat shape) when it reaches the second target position 116. The modified target 121 can be a non-ionized material (non-plasma material) or a minimally ionized material. For example, the modified target material 121 can be a disk of liquid or molten metal, a continuous portion of the target material without voids or significant gaps, a fog of micron or nanoparticle, or a cloud of atomic vapor. For example, as Figure 2 shown, the modified target material 121 expands into a disk-shaped molten metal sheet 121 within the second target position 116 after approximately the time T2 - T1, which can be on the order of microseconds (μs).

[0094] In addition, 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 acquire some propulsion or velocity along the Z direction. The expansion of the modified target 121 in the X direction and the velocity acquired in the Z direction depend on the energy of the first radiation beam 110, and specifically on the energy transferred (i.e., intercepted) to 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 related to the energy per unit area of the first radiation beam 110 (joules / cm 2)is in linear proportion. The energy per unit area is also referred to as the 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 equivalently the irradiance of the surface of the target material 120 integrated over the time during which 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 can be different. In this case, the shorter pulse duration is related to 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 mainly depends 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 mist.

[0097] In addition, the angular orientation (angle relative to the Z direction or X direction) of the disk shape of the modified target 121 depends on the position of the first radiation beam 110 when it impacts the target material 120. Thus, if the first radiation beam 110 impacts 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, the disk shape of the modified target 121 will be more likely to be aligned, where its major axis 230 is parallel to the X direction and its minor axis 235 is parallel to the Z direction.

[0098] The first radiation beam 110 consists of radiation pulses, and each pulse can have a duration. Similarly, the second radiation beam 115 consists of radiation pulses, and each pulse can have a duration. The pulse duration can be expressed as the full width at a certain percentage (e.g., half) of the maximum value, that is, the amount of time during which the intensity of the pulse is at least that 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 pulses within the first radiation beam 110 can be 30 nanoseconds (ns), 60 ns, 130 ns, 50 - 250 ns, 10 - 200 picoseconds (ps), or less than 1 ns. 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 1.06 μm, 1 - 10.6 μm, 10.59 μm, or 10.26 μm.

[0099] As described above, the expansion rate 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 approximately 60 ns and an energy of approximately 50 mJ, the actual radiation exposure depends on how tightly the first radiation beam 110 is focused in the first focal region 210. In some examples, the radiation exposure at the target material 120 can be approximately 400 - 700 joules / cm 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 can be referred to as the main beam and is composed of pulses released at a specific repetition rate. The second radiation beam 115 has sufficient energy to convert the 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 by a delay time such as, for example, 1 - 3 microseconds (μs), 1.3 μs, 1 - 2.7 μs, 3 - 4 μs, or any amount of time that allows the modified target 121 to expand into Figure 2 the disk shape shown with the desired dimensions. Thus, as the modified target 121 expands and elongates in the XY plane, the modified target 121 undergoes two-dimensional expansion.

[0101] The second radiation beam 115 can be configured such that it is slightly defocused when it impinges on the modified target 121. Figure 2 Such a defocusing scheme is shown in. In this case, the second focal region 215 is at a different position along the Z-axis direction from the long axis 230 of the modified target 121; furthermore, the second focal region 215 is outside the second target position 116. In this scheme, the second focal region 215 is placed along the Z direction in front of the modified target 121. That is, the second radiation beam 115 reaches the focus (or beam waist) before the second radiation beam 115 impinges on the modified target 121. Other defocusing schemes are possible. For example, as Figure 6 shown, the second focal region 215 is placed along the Z direction behind the modified target material 121. In this way, the second radiation beam 115 reaches the focus (or beam waist) after the second radiation beam 115 impinges on the modified target 121.

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

[0103]

[0104] Although the modified target 121 expands along the major axis 230, the modified target 121 can also compress or thin along the minor axis 235.

[0105] The two-stage method discussed above yields a conversion efficiency of approximately 3 - 4%, where 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 a plasma by interacting the modified target 121 with the second radiation beam 115. Generally, it is desirable to increase the conversion of light from the light source 105 to EUV radiation 130 because too low a conversion efficiency may require an increase in the amount of power that the light source 105 needs to deliver, which increases the cost of operating the light source 105 and also increases the thermal load on all components within the light source 100, and may lead to an increase in debris generation within the chamber that houses the first target position 111 and the second target position 116. An increase in conversion efficiency can help meet the requirements of high-volume manufacturing tools and at the same time keep the light source power requirements within an acceptable range. Various parameters affect the conversion efficiency, such as for example the wavelengths 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 can be defined as the EUV energy generated within 2% bandwidth near the center wavelength of the reflectivity curve of the EUV light 130 into 2π steradians and used in either or both of the illumination and projection optics in the light collector system 135 and the (multilayer) mirrors in 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 for this, it is important to maintain parameters such as the expansion rate of the modified target 121 within an acceptable value range. 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 value range. 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 per unit area by the surface of the target material 120. Thus, if the area of the target material 120 is maintained constant between pulses, the radiation exposure can be estimated or approximated as the amount of energy directed towards 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 value range. And the method or technique used can 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 focusing region 210, or the angle of the target material 120 relative to the xy plane.

[0108] One property that can affect how the radiation exposure is maintained is the confocal parameter of the first radiation beam 110. The confocal parameter of a 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 the point where the cross-sectional area doubles. Referring to 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 (i.e., D1 / 2) to the point where the cross-section of the first light beam doubles.

[0109] For example, as Figure 7AAs shown, the confocal parameter of the first radiation beam 110 is 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 when the position of the target material 120 is offset 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 a first scenario (compared to a second scenario described below) where the area of the surface of the target material 120 intercepted by the first radiation beam 110 is not likely to deviate from the average value, the radiation exposure and thus the expansion rate can be maintained or controlled by maintaining the amount of energy directed from the first radiation beam 110 towards the target material 120 (without having to consider the surface area of the target material 120 exposed by the first radiation beam 110).

[0110] As another example, as Figure 7BAs shown, the confocal parameter of the first radiation beam 110 is so short that the beam waist (D1 / 2) does not enclose 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 a second scenario (compared to the first scenario) where 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 thus 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. 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 such that the target material 120 reaches a known axial (Z - direction) position relative to the beam waist D1 / 2, with not much variation near this position.

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

[0112] Referring again to 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 can measure the energy of the first radiation beam 110. As Figure 8A shown, the exemplary measurement system 855A measures the energy of the first radiation beam 110 directed towards the target material 120.

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

[0114] In some implementations, as Figure 8C shown, the 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 the reflected radiation 860 that is incident on or reflected 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; thus, by measuring the reflected radiation 860, the amount of energy delivered to the target material 120 can be controlled or maintained. Additionally, 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 light beam. Thus, it can be said that the measurement system 855A or 855B measures the number of photons in the corresponding light beam. Additionally, the measurement system 855B can be considered to measure the number of photons reflected from the target material 120 (which becomes the modified target 121 once struck by the first radiation beam 110) as a function of how many photons strike the target material 120.

[0115] The measurement system 855A or 855B can be a photoelectric sensor such as an array of photoelectric cells (e.g., a 2×2 array or a 3×3 array). The photoelectric cells are sensitive to the wavelength of the light to be measured and have sufficient speed or bandwidth suitable for the duration of the light pulse to be measured.

[0116] Generally, 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. Since the measurement of the energy of the light beam can be performed quickly, each pulse emitted in the first radiation beam 110 can be measured, and thus the measurement and control can be on a pulse-by-pulse basis.

[0117] The measurement systems 855A, 855B can be fast photodetectors such as a photo-electromagnetic (PEM) detector suitable for long-wavelength infrared (LWIR) radiation. The PEM detector can be a silicon diode for measuring near-infrared or visible light radiation or an InGaAs diode for measuring near-infrared radiation. The energy of the pulses in the first radiation beam 110 can be determined by integrating the laser pulse signals measured by the measurement systems 855A, 855B.

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

[0119] Reference Figure 9B , the measurement system 155 can be an exemplary measurement system 955B that measures the position Tpos of the target material 120 relative to the principal focus 990 of the light collector 135. Such a measurement system 955B can include a laser and / or a camera that reflects off the target material 120 when 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 the coordinate system within the chamber 165.

[0120] Reference Figure 9C , the measurement system 155 can be an exemplary measurement system 955C that measures the dimensions of the modified target 121 at a specific position before the modified target 121 interacts with the second radiation beam 115. For example, the measurement system 955C can be configured to measure the dimensions Smt of the modified target 121 when the modified target 121 is within the second target position 116 but before the modified target 121 is struck by the second radiation beam 115. The measurement system 955C can also determine the orientation of the modified target 121. The measurement system 955C can use shadowgraph techniques with a pulsed backlight illuminator and a camera (such as a charge-coupled device camera).

[0121] The measurement system 155 can 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 can include a plurality of EUV sensors within the chamber 165 for detecting EUV energy emitted from the 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 can be obtained.

[0123] The beam conditioning system 180 is employed under the control of the control system 160 to effect control of the amount of energy (radiation exposure) delivered to the target material 120. If it can be assumed that the area of the first radiation beam 110 at the location where the first radiation beam 110 interacts with the target material 120 is constant, then the radiation exposure can be controlled by controlling the amount of energy within the first radiation beam 110. The beam conditioning system 180 receives one or more signals from the control system 160. The beam conditioning system 180 is configured to adjust one or more characteristics of the light source 105 to maintain the amount of energy (i.e., radiation exposure) delivered to the target material 120 or to control the amount of energy directed to the target material 120. Accordingly, the beam conditioning system 180 can include one or more actuators that control the characteristics of the light source 105, and the actuators can be mechanical, electrical, optical, electromagnetic, or any suitable force device for causing the characteristics of the light source 105 to be modified.

[0124] In some implementations, the beam conditioning system 180 includes a pulse width adjustment system coupled to the first radiation beam 110. The pulse width adjustment system is configured to adjust the pulse width of the first radiation beam 110. In this implementation, the pulse width adjustment system can include an electro-optic modulator, such as a Pockels cell for example. For instance, the Pockels cell is disposed within the optical generator 310, and by opening the Pockels cell for a shorter or longer period of time, the pulse transmitted by the Pockels cell (and thus the pulse emitted from the optical generator 310) can be adjusted to be shorter or longer.

[0125] In other implementations, the beam conditioning system 180 includes a pulse power adjustment system coupled to the first radiation beam 110. The pulse power adjustment 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 adjustment system can include an acousto-optic modulator. The acousto-optic modulator can be arranged such that a change in the RF signal applied to the piezoelectric transducer at the edge of the modulator can be varied, thereby changing the power of the pulse diffracted from the acousto-optic modulator.

[0126] In some implementations, the beam conditioning system 180 includes an energy adjustment system coupled to the first radiation beam 110. The energy adjustment system is configured to adjust the energy of the first radiation beam 110. For example, the energy adjustment system can be an electro-optic variable attenuator (such as a Pockels cell that varies between 0V and the 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 means for controlling the position or angle of the beam waist D1 / 2 relative to the first target position 111 or relative to another position within the chamber 165 in the coordinate system of the chamber. The means 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 the 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 growth rate of the modified target 121. Referring Figure 10 to, the control system 160 can 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 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 can include Figure 1 and 10 other components not shown in but that can interact with the control system 160. For example, the light source 100 can 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 the droplet relative to a specific position (such as the main 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, whereby the droplet position error can be calculated on a droplet-by-droplet basis or on 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 to control, for example, the laser timing circuit and / or to a beam control system to control the position of the amplified beam and the shaping of the beam delivery system to change the position and / or focusing power of the focusing plane 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 that is operable to respond to a signal from the control system 160, such as modifying the release point of the droplets of the target material 120 released by an internal delivery mechanism to correct for the error of the droplets reaching the desired target position 111.

[0130] The control system 160 generally 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 embodied 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 its own appropriate input and output devices, one or more programmable processors, and one or more computer program products tangibly embodied in a machine-readable storage device for execution by the programmable processor.

[0131] Each of the one or more programmable processors may execute an instruction program to perform a desired function by operating on input data and generating appropriate output. Generally, the processor receives instructions and data from a read-only memory and / or a random-access memory. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, such as including semiconductor storage 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 a specially designed ASIC (application specific integrated circuit).

[0132] To this end, the control system 160 includes an analysis program 1040 that receives measurement data from one or more measurement systems 155. Generally, the analysis program 1040 performs all of the analysis required to determine how to modify or control the energy transferred 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] Reference Figure 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 so as to increase the conversion efficiency of the light source 100. The light source 100 provides the target material 120 (1105). For example, a target material supply system 125 (under the control of the control system 160) may convey the target material 120 to a first target location 111. The target material supply system 125 may include its own actuation system (connected to the control system 160) and a nozzle through which the target material is forced, wherein the actuation system controls the amount of target material that is directed through the nozzle to produce a stream of droplets directed towards the first target location 111.

[0134] Next, the light source 100 directs a first radiation beam 110 towards 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 towards the target material 120 through a 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 towards the target material 120 within the target location 111, as Figure 2 shown. The focal plane of the first light beam 110 (at the beam waist D1 / 2) can be configured to span the target location 111. Additionally, in some implementations, the focal plane can overlap the target material 120, or intersect the edge of the target material 120 facing the first radiation beam 110. The first radiation beam 110(1110) can be directed towards the target material 120 by, for example, directing the first radiation beam 110 through the beam delivery system 150, where various optical devices can be used to modify the direction or shape or divergence of the radiation 110 such that it can interact with the target material 120.

[0135] The first radiation beam 110(1110) can be directed towards the target material 120 by overlapping the target material 120 with the region of the first radiation beam 110 that encloses 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 encloses 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 (as Figure 7A shown). For example, the confocal parameter of the first radiation beam 110 can 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 enclose 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 (as Figure 7B shown). For example, the confocal parameter can be less than or equal to 2 mm.

[0136] The modified target material 121, immediately after being struck by the first radiation beam 110, changes its shape from the shape of the target material 120 to an expanded shape, and this expanded shape continues to deform as it moves from the first target location 111 towards the second target location 116. The modified target 121 can have a geometric distribution of molten metal that changes from the shape of the target material to a disk-like volume with a substantially flat surface (such as Figure 1 and Figure 2As shown). The modified target 121 is converted into a disk-shaped volume according to the expansion rate. The modified target 121 is transformed by expanding the modified target 121 along at least one axis according to the expansion rate. For example, as Figure 2 shown, the modified target 121 is expanded at least along the major axis 230 that is substantially parallel to the X direction. The modified target 121 is expanded along at least one axis that is not parallel to the optical axis of the second radiation beam 115 (which is the second axial direction 217).

[0137] Although the first radiation beam 110 mainly interacts with the target material 120 by changing the shape of the target material 120, the first radiation beam 110 can also interact with the target material 120 in other ways; for example, the first radiation beam 110 can convert a portion of the target material 120 into a plasma that emits EUV light. However, compared to the EUV light emitted from the plasma generated by the modified target 121, less EUV light is emitted from the plasma generated by the target material 120 (due to the subsequent interaction between the modified target 121 and the second radiation beam 115), and the dominant effect of the first radiation beam 110 on the target material 120 is the modification of the geometric distribution of the target material 120 to form the modified target 121.

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

[0139] The light source 100 measures one or more characteristics (e.g., energy) (1120) associated with one or more of the target material 120 and the modified target 121 relative to the first radiation beam 110. For example, the measurement system 155 measures the characteristics under the control of the control system 160, and the control system 160 receives 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 (1125) based on one or more characteristics. 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 first radiation beam 110 (e.g., relative to the beam waist of the first radiation beam 110), and such a position can be determined in the longitudinal (Z) direction or a direction transverse to the longitudinal direction (e.g., in the XY plane).

[0141] The energy of the first radiation beam 110 can 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 shown). The energy of the radiation 860 reflected from the optically reflective surface of the target material 120 can be measured by measuring the total intensity of the radiation 860 on four individual photoelectric units.

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

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

[0144] In some implementations, the characteristic that can be measured (1120) is the pointing or direction of the first radiation beam 110 as it travels towards the target material 120 (as Figure 8AAs shown). This information about the pointing can be used to determine the overlap error between the position of the target material 120 and the axis of the first radiation beam 110.

[0145] In some implementations, a property that can be measured (1120) is the position of the target material 120 relative to a 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 principal 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, a property that can be measured (1120) is the size of the modified target before the second radiation beam converts at least a portion of the modified target into a plasma.

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

[0148] In some implementations, a property that can be measured (1120) corresponds to the spatial properties (such as Figure 8B and 8C as shown) of the radiation 860 reflected from the optical reflective surface of the target material 120. 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 property can be determined or measured by using an astigmatic imaging system placed in the path of the reflected radiation 860.

[0149] In some implementations, a property that can be measured (1120) corresponds to the angle at which the radiation 860 is directed relative to the first radiation beam 110. This measured angle can be used to determine the distance between the target material 120 and the beam axis of the first radiation beam 110 along a direction transverse to the Z direction.

[0150] In other implementations, the property that can be measured (1120) corresponds to the spatial aspect 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 can be measured relative to a specific direction, such as a direction in the XY plane transverse to the Z direction. Such information about the angle of the modified target 121 can be used to determine the distance along a direction transverse to the Z direction between the target material 120 and the axis of the first radiation beam 110. As another example, the size or expansion rate of the modified target 121 can 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 the energy of the first radiation beam 110 is known to be constant, such information about the size or expansion rate of the modified target 121 can 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 property can be measured (1120) as fast as for each pulse of the first radiation beam 110. For example, if the measurement system 155 includes a PEM or an arrangement of four units (a 4-PEM arrangement), the measurement rate can 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 the target material 120 or the modified target 121, a camera can be used in the measurement system 155, but the camera is typically much slower. For example, the camera can make measurements 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 based on one or more measured properties whether the characteristics of the first radiation beam 110 should be adjusted. Thus, if it is determined that the characteristics 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 can be measured (1120) for each pulse of the first radiation beam 110. In this way, it can be determined whether the characteristics of the first radiation beam 110 should be adjusted for each pulse of the first radiation beam 110.

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

[0156] Process 1100 may further include: collecting at least a portion of the EUV light 130 emitted from the plasma (using the 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, one or more of the measured characteristics (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 impinge on the target material 120.

[0158] As described above, process 1100 includes controlling the radiation exposure amount (1125) at the target material 120 from the first radiation beam 110 based on one or more characteristics. For example, the radiation exposure amount 1125 can be controlled such that it is maintained within a predetermined radiation exposure amount range. The radiation exposure amount is the amount of radiation energy transferred from the first radiation beam 110 to the target material 120 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 that is exposed to or intercepted by the first radiation beam 110 is controlled (or maintained within an acceptable range), this factor of the radiation exposure amount remains relatively constant, and the radiation exposure amount 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 amount (1125) at the target material 120 from the first radiation beam 110 can be controlled such that the energy of the pulses of the first radiation beam 110 (through feedback control using the measured characteristics 1120) is maintained at a constant level or within an acceptable value range, regardless of disturbances that may cause energy fluctuations.

[0160] In other aspects, the radiation exposure (1125) of 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) using feedback control of the measured property 1120 to compensate for an error in the longitudinal (Z - direction) placement of the position of the target material 120 relative to the beam waist of the first radiation beam 110.

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

[0162] The target material 120 can be droplets of the target material 120 generated from a target material supply system 125. In this way, the geometric distribution of the target material 120 can be modified into a modified target 121, and the modified target 121 is transformed into a molten metal having a disk - shaped volume with a substantially flat surface. The target material droplets are transformed into a disk - shaped volume according to an expansion rate.

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

[0164] The power or energy of EUV light 130 is stabilized by controlling the radiation exposure (1225). The EUV energy (or power) generated by the plasma 129 depends on at least two functions, the first being the conversion efficiency CE and the second being the energy of the second radiation beam 115. The conversion efficiency is the percentage of the modified target 121 that is converted 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 and 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 these two factors can be controlled. In this way, the conversion efficiency can be stabilized or controlled by controlling the radiation exposure (1220), which thus stabilizes the EUV energy (1225) generated by the plasma 129.

[0165] Also refer to Figure 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 respective first target position 111 and second target position 116. In this way, each of the radiation beams 110, 115 travels through a corresponding subsystem of the beam delivery system 150 and thus through corresponding separate optical control components 1352A, 1352B and focusing assemblies 1356A, 1356B.

[0166] For example, the subsystem 1305A can be a solid-state gain medium-based system, while the subsystem 1305B can be a gas gain medium-based system such as generated by a CO2 amplifier. Exemplary solid-state gain media that can be used as the 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 a solid-state gain medium can be approximately 1 μm (e.g., approximately 1.06 μm), while the wavelength of the second radiation beam 115 using a gas medium can be approximately 10.6 μm.

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

Claims

1. A method, comprising: Providing a target material, the target material comprising 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; Measuring a size of the modified target; Analyzing the measured size of the modified target; And Based on an analysis of the measured size of the modified target, controlling a radiation exposure amount transferred from the first radiation beam to the target material within a predetermined radiation exposure range.

2. The method according to claim 1, wherein measuring the size of the modified target comprises using a shadowgraph technique.

3. The method according to claim 1, wherein measuring the size of the modified target comprises: Measuring the size of the modified target before the second radiation beam converts at least a portion of the modified target into a plasma.

4. The method according to claim 1, further comprising: Measuring a position of the target material.

5. The method according to claim 4, wherein the modified target has a disk shape, and an angular orientation of the disk shape of the modified target depends on a position where the first radiation beam impinges on the target material.

6. The method according to claim 1, wherein measuring the size of the modified target comprises: Measuring an extent of the modified target along a direction perpendicular to a direction of the second radiation beam.

7. An apparatus, comprising: A chamber defining an initial target position for receiving a first radiation beam and a target position for receiving a second radiation beam; A target material delivery system configured to provide a target material to the initial target position, the target material comprising a material that emits extreme ultraviolet (EUV) light when converted to a plasma; An optical steering system configured to: Direct a first radiation beam from a light source toward the initial target position to transfer energy to the target material to modify a geometric distribution of the target material to form a modified target, and Direct a second radiation beam from the light source toward the target position to convert at least a portion of the modified target into a plasma that emits EUV light; A measurement system that measures a size of the modified target; And A control system connected to the target material delivery system, the light source, the optical steering system, and the measurement system, wherein the control system is configured to: Receive the measured size from the measurement system; Analyze the received measured size; and Based on an analysis of the measured size, send one or more signals to the light source to control a radiation exposure amount transferred from the first radiation beam to the target material.

8. The apparatus according to claim 7, wherein the measurement system comprises a pulsed backlight illuminator and a camera.

9. The apparatus according to claim 8, wherein the measurement system employs a shadowgraph technique.

10. The apparatus according to claim 7, further comprising another measurement system configured to measure a position of the target material relative to the target position.

11. The apparatus according to claim 10, wherein the control system is configured to receive the measured position from the other measurement system; analyze the received measured position; and send one or more signals to the light source to control the amount of radiation exposure transferred from the first radiation beam to the target material based on the analysis of the measured dimensions and the analysis of the measured position.

12. An apparatus comprising: a chamber defining an initial target position for receiving a first radiation beam and a target position for receiving a second radiation beam; a target material delivery system configured to provide a target material to the initial target position, the target material comprising 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; an optical steering system configured to: direct the first radiation beam toward the initial target position 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 position to convert at least a portion of the modified target into a plasma that emits EUV light; a measurement system including measurement subsystems, each measurement subsystem being configured to measure a property associated with one or more of the first radiation beam, the target material, and the modified target material; and a control system connected to the target material delivery system, the light source, the optical steering system, and the measurement system, wherein the control system is configured to: receive the measured properties from each of the measurement subsystems of the measurement system; analyze the received properties; and send one or more signals to the light source based on the analysis of the received measured properties to control the amount of radiation exposure transferred from the first radiation beam to the target material.

13. A method comprising: providing a target material comprising 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 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 properties associated with one or more of the first radiation beam, the target material, and the modified target; analyzing the measured one or more properties associated with one or more of the first radiation beam, the target material, and the modified target; and controlling the geometric expansion rate of the modified target based on the analysis of the measured one or more properties to increase the amount of EUV light converted from the plasma due to the interaction between the modified target and the second radiation beam.

14. A method comprising: Providing a target material, the target material including a component that emits extreme ultraviolet (EUV) light when converted to a plasma; Interacting a first radiation beam with the target material to transfer energy to the target material, including modifying a geometric distribution of the target material to form a modified target; Interacting a second radiation beam with the modified target, the second radiation beam converting at least a portion of the modified target into a plasma that emits EUV light; Measuring a spatial aspect of the modified target with a first measurement system and measuring a spatial aspect of the modified target with a second measurement system; And Controlling the radiation beams based on measurements from the first measurement system and the second measurement system.

15. The method according to claim 14, wherein controlling the radiation beam comprises: Adjusting one or more properties of the first radiation beam.

16. The method according to claim 14, wherein controlling the radiation beam comprises: Controlling a radiation exposure amount transferred from the first radiation beam to the target material.

17. The method according to claim 14, wherein controlling the radiation beam comprises: Controlling a unit area of a surface of the target material exposed to or truncated by the first radiation beam.

18. The method according to claim 14, wherein measuring the spatial aspects of the modified target comprises: Measuring a spatial aspect of the modified target before the second radiation beam interacts with the modified target.

19. An apparatus, comprising: A chamber defining an initial target position configured to receive a first radiation beam and a target position configured to receive a second radiation beam; A target material delivery system configured to provide a target material to the initial target position, the target material including a material that emits extreme ultraviolet (EUV) light when converted to a plasma; Optical means configured to: At the initial target position, interact the first radiation beam with the target material to transfer energy to the target material and modify a geometric distribution of the target material to form a modified target; And At the target position, interact the second radiation beam with the modified target to convert at least a portion of the modified target into a plasma that emits EUV light; Two measurement systems, each measurement system configured to measure a spatial aspect of the modified target; And A control system connected to the target material delivery system, the optical means, and the measurement systems, the control system configured to receive measurement data from the two measurement systems and send one or more signals to the optical means to control the radiation beams based on the received measurement data.

20. The apparatus according to claim 19, wherein each measurement system includes a backlight illuminator and a camera.

21. The apparatus according to claim 20, wherein the camera is a charge-coupled device camera.

22. The apparatus according to claim 19, further comprising: A beam conditioning system, the beam conditioning system in communication with the optical means and the control system, wherein the control system is configured to send the one or more signals to the optical means to control the radiation beams based on the received measurement data by sending one or more signals to the beam conditioning system.

23. A method, comprising: Interacting a first radiation beam with a target material within a chamber to form a modified target; Sensing radiation reflected from the target material when the target material interacts with the first radiation beam; Measure the spatial aspects of the modified target with a first measurement subsystem and measure the spatial aspects of the modified target with a second measurement subsystem; Analyze the sensed radiation and the outputs from the first measurement subsystem and the second measurement subsystem; And Control the first radiation beam based on the analysis.

24. The method according to claim 23 further comprises: Measure the angle of the modified target relative to a direction based on an analysis of the outputs from the first measurement subsystem and the second measurement subsystem.

25. The method according to claim 23, wherein causing the first radiation beam to interact with the target material to form the modified target comprises: Modify the geometric distribution of the target material to form the modified target.

26. The method according to claim 25, wherein modifying the geometric distribution of the target material to form the modified target comprises: Form a disk-shaped volume that extends at least along the major axis.

27. The method according to claim 23, further comprising: Interact a second radiation beam with the modified target, the second radiation beam converting at least a portion of the modified target into a plasma that emits EUV light.

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