阅读说明：本技术 极紫外光源 (Extreme ultraviolet light source ) 是由 陶业争 J·T·斯特瓦特四世 J·朱尔 D·布朗 J·M·亚查恩德 A·A·沙夫甘斯 M 于 2015-06-25 设计创作，主要内容包括：本公开的实施例涉及极紫外光源。生成初始辐射脉冲；提取初始辐射脉冲的一段以形成修改辐射脉冲,修改辐射脉冲包括第一部分和第二部分,第一部分在时间上连接到第二部分,并且第一部分的最大能量小于第二部分的最大能量；修改辐射脉冲的第一部分与靶材相互作用以形成修改靶；并且修改辐射脉冲的第二部分与修改靶相互作用以生成发射极紫外(EUV)光的等离子体。(Embodiments of the present disclosure relate to extreme ultraviolet light sources. Generating an initial radiation pulse; extracting a segment of the initial radiation pulse to form a modified radiation pulse, the modified radiation pulse comprising a first portion and a second portion, the first portion being temporally connected to the second portion and a maximum energy of the first portion being less than a maximum energy of the second portion; interacting a first portion of the modified radiation pulse with the target material to form a modified target; and a second portion of the modified radiation pulse interacts with the modifying target to generate a plasma of Emitter Ultraviolet (EUV) light.)

1. A method of generating Extreme Ultraviolet (EUV) light, the method comprising:

providing a target to a target location, the target comprising a target material that emits EUV light when in a plasma state, the target having a range of 200 nanometers (nm) or less in a first direction and a range of 300 micrometers (μm) or more in a second direction;

forming a main radiation pulse from an initial radiation pulse, the main radiation pulse being a single radiation pulse having a first portion and a second portion, the second portion having a temporal energy distribution based on a temporal energy distribution of the initial radiation pulse and the first portion having a temporal energy distribution different from the temporal energy distribution of the initial radiation pulse; and

directing the formed main radiation pulse toward the target location to cause the radiation to interact with the target, the second portion reaching the target location after the first portion, the formed main radiation pulse propagating in a direction parallel to the first direction at the target location, wherein:

interaction of the formed first portion of the main radiation pulse with the target forms a modified target having a lower density than the target; and

interaction of the formed second portion of the main radiation pulse with the modifying target converts at least some of the target material in the modifying target into a plasma that emits EUV light.

2. The method of claim 1, wherein the range of the target in the first direction is between 50nm and 200nm, and the range of the target in the second direction is between 300 μ ι η and 350 μ ι η.

3. The method of claim 1, wherein the range of the target in the first direction is between 50nm and 200nm, and the range of the target in the second direction is between 300 μ ι η and 500 μ ι η.

4. The method of claim 1, wherein a first portion of the radiation is associated with a first peak energy and a second portion of the radiation is associated with a second peak energy, the second peak energy being greater than the first peak energy.

5. The method of claim 4, wherein the first peak energy is 1-10% of the second peak energy.

6. The method of claim 4, wherein the first portion of the radiation has a duration of 50-150ns and the first peak energy is 5 millijoules (mJ) or less.

7. The method of claim 1, wherein the first portion of radiation is a first pulse of radiation and the second portion of radiation is a second pulse of radiation.

8. The method of claim 1, wherein providing the target to a target location comprises:

providing a droplet of target material to an initial target position; and

directing a radiation pulse toward the initial target location such that the radiation pulse and the target material droplet interact to form the target.

9. The method of claim 8, wherein the target droplets are substantially spherical and have a diameter of 17-35 μ ι η.

10. The method of claim 8, wherein the diameter is less than 30 μ ι η.

11. The method of claim 1, wherein the modified target comprises a collection of patches of target material in a volume.

12. The method of claim 11, wherein the volume is substantially ellipsoidal.

13. A method of generating Extreme Ultraviolet (EUV) light, the method comprising:

providing a target to a target location, the target comprising a target material that emits EUV light when in a plasma state, the target having a range of 200 nanometers (nm) or less in a first direction and a range of 300 micrometers (μm) or more in a second direction;

forming a main radiation pulse from an initial radiation pulse, the main radiation pulse having a first portion and a second portion, the second portion having a temporal energy distribution based on a temporal energy distribution of the initial radiation pulse and the first portion having a temporal energy distribution different from the temporal energy distribution of the initial radiation pulse; and

directing the main radiation pulse toward the target location such that the main radiation pulse interacts with the target, the interaction between the second portion of the main radiation pulse and the target converting at least some of the target material in the target into a plasma that emits EUV light.

14. The method of claim 13, further comprising:

providing an initial target to an initial target location; and

providing a radiation pulse to the initial target location, wherein an interaction between the radiation pulse and the initial target modifies a geometric distribution of a target material of the initial target such that the target is provided to the target location.

15. The method of claim 14, wherein the maximum extent of the initial target in the second direction is less than the extent of the target.

16. The method of claim 15, wherein the initial target is substantially spherical and has a diameter of 17-35 μ ι η.

17. The method of claim 13, wherein the range of the target in the second direction is between 300 μ ι η and 350 μ ι η.

18. The method of claim 13, wherein the range of the target in the second direction is between 300 μ ι η and 500 μ ι η.

19. The method of claim 17, wherein the range of the target in the first direction is between 50nm and 200 nm.

20. The method of claim 13, wherein:

a second portion of the main radiation pulse reaches the target location after the first portion,

an interaction between the first part of the main radiation pulse and the target forms a modified target, an

Interacting the second portion of the main radiation pulse with the target comprises: interacting between the second portion of the main radiation pulse and the modified target to convert at least some of the target material in the modified target into a plasma that emits EUV light.

21. The method of claim 13, wherein the target comprises a collection of patches of target material in a volume.

22. The method of claim 13, wherein forming the primary radiation pulse further comprises: controlling one or more of a peak energy of the first portion and a duration of the first portion.

23. The method of claim 22, wherein controlling the duration of the first portion comprises varying one or more of the duration of the first portion and a peak energy of the first portion.

24. The method of claim 23, wherein forming the primary radiation pulse comprises passing the initial radiation pulse through an optical modulator.

25. The method of claim 13, wherein:

the temporal energy distribution of the first portion has a first duration,

the temporal energy profile of the second portion having a second duration and a peak energy, the peak energy occurring at a time between the beginning and the end of the second duration,

the temporal energy profile of the first portion has a first energy slope over the first duration,

the temporal energy profile of the second portion has a second energy slope between the beginning of the duration and the time of the peak energy, an

The first energy slope is less than the second energy slope.

26. A method, comprising:

directing the optical pulse toward an optical element;

controlling the optical element to form a modified optical pulse from the optical pulse, the modified optical pulse comprising a first portion and a second portion, the second portion having a temporal energy distribution based on the optical pulse, the first portion having a temporal distribution different from the temporal energy distribution of the optical pulse, wherein one or more characteristics of the modified optical pulse are controlled at least in part by controlling the optical element; and

interacting the modified optical pulse with a target comprising a target material, wherein interaction between a second portion of the modified optical pulse and the target converts at least a portion of the target material into a plasma of Emitter Ultraviolet (EUV) light.

27. The method of claim 26, wherein the one or more characteristics of the modified optical pulse comprise a duration, a peak energy, and/or a temporal energy distribution of the first portion of the modified optical pulse.

28. The method of claim 27, wherein the target is a modified target and the method further comprises interacting a pre-pulsed optical pulse with an initial target to form the modified target.

29. The method of claim 28, wherein the modification target has a first extent in a first direction parallel to a direction of propagation of a second portion of the modified optical pulse, the one or more characteristics of the modified optical pulse comprise a duration, and controlling the optical element to form the modified optical pulse comprises: controlling a duration of the second portion of the modified optical pulse based on the first range of the modified target.

30. The method of claim 29, wherein a duration of the second portion of the modified target is inversely related to the first range of the modified target.

31. The method of claim 26, wherein interacting the first portion of the modified optical pulse with the target comprises a target material modifying a geometric distribution of the target material in at least one dimension.

32. The method of claim 31, wherein the geometric distribution of the target material increases in at least one of the at least two dimensions.

33. The method of claim 26, wherein an interaction between a first portion of the modified optical pulse and the target modifies a geometric distribution of the target material to form a modified target, the modified target occupies a larger volume than the target, the modified target includes at least a region having a density less than the target, and a second portion of the modified optical pulse interacts with the modified target to form an EUV light-emitting plasma.

34. The method of claim 26, wherein controlling the optical element comprises applying a voltage to an electro-optic modulator (EOM).

35. A system for an Extreme Ultraviolet (EUV) light source, the system comprising:

a light generation module;

an optical element; and

a control system configured to determine one or more characteristics of a modified optical pulse formed from an optical pulse generated by the light generation module, wherein the modified optical pulse includes a first portion and a second portion, the second portion having a temporal energy distribution based on the temporal energy distribution of the optical pulse, the first portion having a temporal distribution different from the temporal energy distribution of the optical pulse, and the second portion having an energy sufficient to convert at least some target material in a target to a plasma that emits EUV light.

36. The EUV light source of claim 35, wherein the optical element comprises an electro-optic modulator (EOM).

37. The EUV light source of claim 35, further comprising a second light generating module configured to emit a second optical pulse.

38. The EUV light source of claim 37, further comprising a beam combiner configured to interact with and direct the modified optical pulse and the second optical pulse toward a receptacle configured to receive a target.

39. An EUV light source according to claim 38, wherein the light generation module comprises carbon dioxide (CO) ₂) A laser, and the second light generation module comprises a solid state laser.

40. The EUV light source of claim 39, wherein the modified optical pulse comprises light having a wavelength of 10.6 micrometers (μm) and the second optical pulse comprises light having a wavelength of 1.06 μm.

41. The EUV light source of claim 37, wherein the modified optical pulse comprises light having a wavelength of 10.6 microns (μ ι η) and the second optical pulse comprises light having a wavelength of 1.06 μ ι η.

42. The EUV light source of claim 41, wherein the light generation module and the second light generation module are part of a single light source.

43. The EUV light source of claim 41, wherein the light generation module and the second light generation module are unused separate light sources.

44. An EUV light source comprising:

a light generation module;

an optical element;

a container configured to receive a target comprising a target material that emits EUV light when in a plasma state; and

a control system configured to determine one or more characteristics of a modified optical pulse formed by the optical element based on an optical pulse generated by the light generation module, wherein the modified optical pulse includes a first portion and a second portion, the second portion having a temporal energy distribution based on the temporal energy distribution of the optical pulse, the first portion having a temporal distribution different from the temporal energy distribution of the optical pulse, and the second portion having an energy sufficient to convert at least some target material in a target to a plasma that emits EUV light.

45. An EUV light source comprising:

a light generation module configured to emit an optical pulse;

an optical element configured to form a main light pulse from the optical pulse, the main light pulse being a single radiation pulse having a first portion and a second portion, the second portion having a temporal energy distribution based on the temporal energy distribution of the optical pulse, and the first portion having a temporal energy distribution different from the temporal energy distribution of the optical pulse; and

a container comprising a target region configured to receive a target, wherein the target comprises a target material that emits EUV light when in a plasma state, and the target has a range of 200 nanometers (nm) or less in a first direction and a range of 300 micrometers (μm) or more in a second direction.

46. The EUV light source of claim 45, further comprising:

a control system configured to control the optical element to determine one or more characteristics of the main light pulse.

47. The EUV light source of claim 45, further comprising:

a target supply system configured to supply the target to the container.

48. The EUV light source of claim 47, wherein the target supply system is configured to supply droplets of molten metal to the vessel, and the EUV light source further comprises:

a second light generation module configured to emit an initial pulse of light configured to interact with the molten metal droplet to form the target before the target is received at the target region.

49. The EUV light source of claim 48, wherein the molten metal droplets are 17-35 μm in diameter.

Technical Field

The disclosed subject matter relates to an extreme ultraviolet light source.

Background

Extreme ultraviolet ("EUV") light, such as electromagnetic radiation having a wavelength of about 50nm or less (sometimes also referred to as soft X-rays) and including light having a wavelength of about 13nm and less, such as about 6.5nm, may be used in photolithography processes to produce extremely small features in substrates, such as silicon wafers.

Methods of generating EUV light include, but are not necessarily limited to, converting a material having an element such as xenon, lithium, or tin with an emission line in the EUV range in a plasma state. In one such method, commonly referred to as laser produced plasma ("LPP"), the desired plasma may be produced by irradiating a target in the form of, for example, a droplet, plate, ribbon, stream or cluster of material with an amplified beam, which may be referred to as a driven laser. For this process, plasma is typically generated in a sealed container, such as a vacuum chamber, and monitored using various types of metrology equipment.

Disclosure of Invention

In one general aspect, a method includes: generating an initial radiation pulse; extracting a segment of the initial radiation pulse to form a modified radiation pulse, the modified radiation pulse comprising a first portion and a second portion, the first portion being temporally connected to the second portion and a maximum energy of the first portion being less than a maximum energy of the second portion; interacting a first portion of the modified radiation pulse with the target material to form a modified target; and interacting the second portion of the modified radiation pulse with the modifying target to generate a plasma of Emitter Ultraviolet (EUV) light.

Implementations may include one or more of the following features. The modified radiation pulse may pass through a gain medium to form an amplified modified radiation pulse, the gain medium amplifying a first portion of the modified radiation pulse by a greater amount than a second portion of the modified radiation pulse. The gain medium may have a small signal gain and a saturation gain, and the first portion of the modified radiation pulse may be amplified by the small signal gain and the second portion of the modified radiation pulse may be amplified by the saturation gain.

Extracting a segment of the initial radiation pulse to form a modified radiation pulse may include passing the initial radiation pulse through a gating module. The gating module may comprise an electro-optical gating module. The electro-optical gating module may include an electro-optical modulator that includes one or more polarizers.

The initial radiation pulse may comprise a light pulse. The initial radiation pulse may be pulsed carbon dioxide (CO) ₂) And (4) laser. The energy of the first part of the modified radiation pulse may increase continuously over time. The first portion of the modified radiation pulse may have a duration of 50 nanoseconds (ns) or less. The initial radiation pulse and the modified radiation pulse may each be associated with a temporal distribution characterizing the energy as a function of time, and the temporal distribution of the initial radiation pulse and the modified radiation pulse may be different.

The target may interact with the first radiation pulse to form the target material before interacting the first portion of the modified radiation pulse with the target material. The first radiation pulse may have a wavelength of 1 micrometer (μm).

In another general aspect, a method of generating Extreme Ultraviolet (EUV) light includes: providing a target to a target location, the target spatially expanding before reaching the target location; directing a radiation pulse toward the target location, the radiation pulse including a first portion and a second portion that reaches the target location after the first portion; interacting a first portion of the radiation pulse with the target to form a modified target having a different absorption than the target; and interacting the second portion of the radiation pulse with the modified target to generate the EUV light-emitting plasma.

Implementations may include one or more of the following features. A modified target having a different absorption than the target can include a modified target that absorbs a greater amount of radiation than the target. The target includes a pre-expanded target, the spatial extent of which is expanded in one dimension and reduced in a second dimension before being provided to the target location.

In another general aspect, an Extreme Ultraviolet (EUV) system includes: a light source configured to emit a light beam; a modulator configured to receive a light beam emitted from a light source and extract a portion of the light beam; and an amplifier comprising a gain medium, the amplifier being configured to receive an extracted portion of the light beam and to amplify the extracted portion with the gain medium into a pulse comprising a first portion and a second portion, the first portion and the second portion being temporally connected, the first portion being amplified by a greater amount than the second portion, and the second portion comprising energy sufficient to convert a target emitting EUV light into a plasma state when in the plasma state, wherein, in use, the target is positionable in a target position receiving the pulse, the target comprising a target emitting EUV light when in the plasma state.

Implementations may include one or more of the following features. The light source may comprise a source that generates laser light. The light source may be a pulsed carbon dioxide (CO2) laser. The modulator may be configured to extract a portion of the light beam by allowing only a portion of the light beam to pass through the modulator.

The system may further include a second light source configured to generate a radiation pulse comprising energy sufficient to spatially spread the target material droplet to form a target positionable in the target location. The second light source may emit laser pulses comprising light having a wavelength of 1.06 micrometers (μm). The light source configured to emit a beam of light may be further configured to emit a laser pulse prior to emitting the beam of light, the laser pulse comprising an energy sufficient to spatially expand the target material droplet to form a target positionable in the target location.

Implementations of any of the above techniques may include an EUV light source, a method, a process, an apparatus, executable instructions or devices stored on a computer readable medium. The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims.

Drawings

FIG. 1 is a block diagram of an exemplary laser-produced plasma Extreme Ultraviolet (EUV) source.

Fig. 2 is a flow chart of an exemplary process for generating radiation pulses.

FIG. 3A is a block diagram of an exemplary optical system for an EUV light source.

Fig. 3B is a plot of exemplary pulses emitted from a seed laser.

Fig. 3C is a plot of an exemplary pulse with a base (pedestal).

FIG. 4 is a flow chart of an exemplary process for generating EUV light.

Fig. 5A, 5C, and 6A show exemplary target locations over time.

Fig. 5B and 6B are plots of exemplary radiation pulses.

Figure 7 is a plot of an exemplary relationship between EUV power and base level.

Fig. 8 is an exemplary measured radiation pulse.

Fig. 9 is a plot of an exemplary relationship between conversion efficiency and target size.

10A-10D are exemplary shadowgraphs of target locations over time.

FIG. 11 is a top view of another laser-produced plasma Extreme Ultraviolet (EUV) light source and a lithography tool coupled to the EUV light source.

FIG. 12 is a block diagram of an exemplary laser-produced plasma Extreme Ultraviolet (EUV) source.

Detailed Description

Techniques for conditioning a target are disclosed. The target comprises a target material that Emits Ultraviolet (EUV) light when in a plasma state. As discussed in more detail below, this conditioning may enhance the ability of the target to absorb laser radiation, and thus may improve the Conversion Efficiency (CE) of an EUV light source employing this conditioning technique.

The target is conditioned with a radiation pulse comprising a first portion ("base") and a second portion (main pulse or heating pulse). The first and second portions are connected to each other in time. In other words, the first portion and the second portion are part of a single radiation pulse, and there is no gap or region between the first portion and the second portion that lacks radiation.

A first portion (or "base") of the radiation pulse interacts with the target to modify the absorption characteristics of the target. For example, by reducing the density gradient of the target and increasing the volume of the target that interacts with the radiation pulses at the surface that receives the radiation pulses, such interaction can modify the absorption characteristics, which increases the amount of radiation that the target can absorb. In this manner, the interaction between the target and the first portion of the radiation pulse modulates the target. The second portion of the radiation pulse has an energy sufficient to convert a target material in the target into a plasma that emits EUV light. Because the amount of radiation that can be absorbed by the target is increased by the conditioning of the first portion, the conditioning can result in a greater portion of the target being converted into a plasma that emits EUV light. In addition, the modulation may reduce the reflectivity of the target and, thus, the amount of back reflection into the optical source generating the radiation pulse.

As described below, the characteristics of the base, such as duration and energy, can be controlled and varied to suit a particular target.

Referring to fig. 1, an optical amplifier system 106 forms at least a portion of an optical source 105 (also referred to as a drive source or drive laser) for driving a Laser Produced Plasma (LPP) Extreme Ultraviolet (EUV) light source 100. The optical amplifier system 106 includes at least one optical amplifier such that the optical source 105 produces an amplified light beam 110 that is provided to the target location 130. Target location 130 receives target 120, such as tin, from a target supply system and the interaction between amplified beam 110 and target 120 produces a plasma that emits EUV light or radiation 150. The light collector 155 collects the EUV light 150 and directs it as collected EUV light 160 towards an optical device 165, such as a lithography tool.

The amplified light beam 110 is directed by the beam delivery system 140 to the target location 130. The beam delivery system 140 may include an optical assembly 135 and a focusing component 142 that focuses the amplified light beam 110 in a focal region 145. The assembly 135 may include optical elements, such as lenses and/or mirrors, that direct the amplified light beam 110 by refraction and/or reflection. The assembly 135 may also include elements to control and/or move the assembly 135. For example, assembly 135 may include an actuator that is controllable to move an optical element of beam delivery system 140.

The focusing element 142 focuses the amplified light beam 110 such that the diameter of the beam 110 is at a minimum in a focal region 145. In other words, the focusing element 142 causes the radiation in the amplified light beam 110 to converge as it propagates in the direction 112 toward the focal region 145. In the absence of a target, the radiation in the amplified light beam 110 diverges as the beam 110 propagates away from the focal region 145 in the direction 112.

As described below, the optical source 105 generates a pulse having a first portion and a second portion that are temporally connected. The first portion may be referred to as the "base". The first portion conditions the target 120 to more easily absorb the second portion of the pulse. The second portion of the pulse has an energy sufficient to convert the target into a plasma that emits EUV light.

In addition, the spatial distribution of target 120 may be modified to increase the size of target 120 in the direction intersecting amplified beam 110 before the first portion of the pulse interacts with target 120. For example, target 120 may be expanded from a droplet into a flat disk with a single pulse of radiation ("pre-pulse") that interacts with target 120 before the first and second portions. Increasing the size of target 120 prior to interacting with amplified light beam 110 may increase the portion of target 120 exposed to amplified light beam 110, which may increase the amount of EUV light produced for a given amount of target 120 (this is due to the increased target volume, which may more effectively absorb radiation pulses, and the larger EUV emission volume, which may generate increased amounts of EUV light).

Referring to fig. 2, a flow chart of an exemplary process 200 is shown. The process 200 may generate a radiation pulse, which may be used as the amplified beam 110 in the EUV light source 100 of fig. 1 or any other EUV light source. Process 200 is discussed with reference to fig. 3A-3C.

An initial radiation pulse (210) is generated. A segment of the initial radiation pulse is extracted to form a modified radiation pulse, the modified portion of the radiation including a first portion ("base") and a second portion (220). The first and second portions are connected in time without an intermediate region lacking radiation.

Referring also to fig. 3A, a block diagram of an exemplary system 301 that may perform process 200 is shown. The system 301 includes an optical source 305. The optical source 305 or system 301 may be used in an EUV light source rather than in the optical source 105. The optical source 305 includes a seed laser 302, a gating module 304, and an optical amplifier 306. Amplifier 306 includes a gain medium 307. The optical source 305, gating module 304, amplifier 306, and gain medium 307 are located in the beam path 309 along which the light travels. The beam path 309 intersects a target location 330 that receives a target comprising a target material.

For example, seed laser 302 may be carbon dioxide (CO) ₂) A laser that generates an amplified light beam 303 and emits the beam 303 onto a beam path 309 towards a gating module 304. Fig. 3B shows an exemplary pulse shape (energy versus time) of beam 303. The pulse shape of beam 303 shown in fig. 3B is for illustrative purposes, and beam 303 may have other pulse shapes. Referring again to FIG. 3A, the seed laser 302 may be, for example, a Master Oscillator Power Amplifier (MOPA) CO that emits laser pulses ₂A laser. In some implementations, the seed laser 302 may be a laser that emits light having a wavelength of 1 micrometer (μm), such as a neodymium-doped yttrium aluminum garnet (Nd: YAG) laser. The gating module 304 acts as a switch or filter that allows only a portion of the beam 303 to pass through. For example, gating module 304 may include an electro-optical gating module that cuts beam 303 into pulses.

In some implementations, the gating module 304 includes polarizers 304a and 304b located on the path 309. The polarizers 304a and 304b may be, for example, linear polarizers that prevent light from emerging from the gate module 304 when they are oriented with their transmission axes perpendicular to each other. In this example, when polarizers 304a and 304b are aligned with their transmission axes parallel to each other and parallel to the polarization of beam 303, the light passes through gating module 304. Thus, by controlling the relative orientations of polarizers 304a and 304b, gating module 304 can selectively pass or block beam 303 to extract particular portions of beam 303. In this manner, the gating module 304 extracts a portion of the beam 303 to form a modified pulse 315. In addition, due to the limited amount of time that the gating module 304 changes from the light-transmitting state to the light-blocking state, there may be a small amount of light leakage on either or both sides of the modification pulse 315.

The system 301 also includes a controller 317. The optical source 305 communicates with a controller 317 via link 312. The controller 317 includes an electronic processor 318 and an electronic storage device 319. Electronic storage 319 may be volatile memory, such as RAM. In some implementations, electronic storage 319 may include non-volatile and volatile portions or components. The processor 318 may be one or more processors suitable for the execution of a computer program, such as a general or special purpose microprocessor, or any one or more processors of any kind of digital computer. The electronic processor receives instructions and data from a read-only memory or a random access memory or both. The electronic processor 318 can be any type of electronic processor and can be more than one electronic processor.

Electronic storage 319 stores instructions, perhaps as a computer program, that when executed cause processor 318 to communicate with optical source 305 and/or components thereof. For example, the instructions may be instructions to generate signals that drive actuators to position polarizers 304a and 304b relative to each other to cause gating module 304 to block or transmit light. In other words, the controller 317 may be programmed or set such that a particular portion of the beam 303 is extracted.

Referring also to fig. 3B, an exemplary temporal distribution (intensity versus time) of the beam 303 emitted from the seed laser 302 and the modified pulse 315 emitted from the gating module 304 is shown. The modification pulse 315 is part of the beam 303.

In the example of fig. 3B, the beam 303 is a laser pulse with a temporal distribution that is approximately gaussian. The beam 303 is passed to a gating module 304 to form a modified pulse 315. The gating module 304 may be controlled to select or extract a particular portion of the beam 303. In the example of fig. 3B, the gating module 304 is set to emit light at time t-t 1, and the gating module 304 is set to block light at time t-t 2. As a result, the modification pulse 315 is the portion of the beam 303 between times t1 and t2, and its temporal profile is approximately equal to the temporal profile of the beam 303 between times t1 and t 2. However, as the gating module 304 switches between transmitting and/or blocking light for a finite amount of time, leakage light 311 is present at the leading edge of the modification pulse 315 (time t-t 1). The amount of leakage light 311 may be determined by the switching time of the gate module 304. In some implementations, the gating module 304 cuts the beam 303 into pulses with steep slopes (almost instantaneous transition from the blocking beam 303 to the penetrating beam 303), with pulse durations of 50-250 nanoseconds (ns).

In other examples, gating module 304 may extract different portions of beam 303. For example, the gating module 304 may be activated to transmit light for a longer period of time to generate the modified pulse 315 with a longer duration. Additionally or alternatively, the gating module 304 may be activated at different times to capture a portion of the beam 303 having a different temporal intensity profile than the portion extracted in the example of fig. 3B. Selectively capturing specific portions of the beam 303 allows the energy or intensity of the modification pulse 315 to be controlled. For example, activating the gating module 304 when the beam 303 is at its peak energy results in the energy of the modification pulse 315 being greater than the modification pulse generated by activating the gating module 304 when the beam 303 is at a lower energy.

Referring again to fig. 3A, the modified pulse 315 is input to the amplifier 306, and the amplifier 306 generates an amplified modified pulse 308. The optical amplifier 306 comprises a gain medium 307 which receives energy by pumping and provides the energy to a modification pulse 315 and converts the modification pulse 315 into an amplified modification pulse 308.

The amount of amplification of the modified pulse 315 is determined by the gain of the gain medium 307 and the amplifier 306. The gain is the amount or factor by which the energy provided to the input beam by amplifier 306 is increased. The optical amplifier 306 has a "small signal gain" and a "saturation gain", and the gain seen by a beam incident on the optical amplifier 306 depends on the energy of the beam. For toolsWith a relatively low energy beam, the gain of the optical amplifier 306 is linear, i.e., the gain is the same, regardless of changes in the energy of the input signal. The gain in this case is referred to as "small signal gain". However, for a beam of sufficient energy or intensity, the amplifier 306 may become saturated. Saturation is a form of non-linear behavior in which the energy of the beam output by the amplifier increases disproportionately compared to the input beam. The gain of amplifier 306 in saturation may be referred to as the "saturated gain". The saturation gain may be less than the small-signal gain. The small signal gain of the optical amplifier 306 may be, for example, a factor of 100,000. In some implementations, the small signal gain of the optical amplifier 306 may be, for example, a range, e.g., 10 ⁴To 10 ⁷By a factor of (c).

As described above, when the gate module 304 forms the modification pulse 315, the leakage light 311 exists at the leading edge of the modification pulse 315. The leakage light 311 has less energy than other portions of the modified pulse 315. As a result, the leakage light 311 may be amplified by the small-signal gain of the gain medium 307, and the remaining portion of the modification pulse 315 may be amplified by the saturation gain. Accordingly, the leakage light 311 at the leading edge of the modification pulse 315 may be amplified by a greater factor than the remainder of the modification pulse 315.

Referring also to fig. 3C, a time profile (energy as a function of time) of an exemplary amplified modified pulse 308 is shown. The amplified modified pulse 308 includes a first portion 308a ("base") and a second portion 308 b. The amplified modified pulse 308 is a single pulse and the first and second portions 308a, 308b are temporally connected to each other without an intervening gap or region that lacks radiation.

The first portion 308a results from an amplification of the leakage light 311. The first portion 308a has a base duration 313. The base duration 313 is the length of time between the start of the modification pulse 308 (t-t 3) and the start of the second portion 308b (t-t 4). The first portion 308a also has a base level 314. The base level 314 is the maximum energy or maximum power of the first portion 308a over the duration 313. Although the example of fig. 3C shows the base level 314 immediately adjacent to the beginning of the second portion 308b, the level 314 may occur at any time during the duration 313. The second portion 308b has a peak energy 316 sufficient to convert the target material in the target into a plasma that emits EUV light.

Referring to fig. 4, a flow chart of an exemplary process 400 is shown. The process 400 may be used to generate EUV light and may be performed, for example, with the EUV light source 100 and the system 301.

Process 400 is discussed with reference to FIGS. 5A-5C, which show an exemplary target location 530 over time period 501. Fig. 5A shows the change in the geometric distribution of the target when the target interacts with the radiation pulses 508 over the time period 501. Fig. 5B is a time distribution of the radiation pulse 508. The radiation pulse 508 includes a base portion 508a and a second portion 508b that are temporally connected. The target 521 interacts with the base 508a to form a modified target 524, and the second portion 508b illuminates the modified target 524 to convert target material in the modified target 524 into plasma that emits EUV light 550. Fig. 5C shows an exemplary beam width of base 508a and second portion 508b in target location 530 during time period 501. In the example shown, base 508a has a beam width of 511 in target location 530.

Referring again to FIG. 4, a target 521 is provided at a target location 530 (410). The target location 530 is a region of space that receives the amplified beam, which in this example is the radiation pulse 508, and the distribution of the target material. Target location 530 may be similar to target location 130 (FIG. 1) or target location 330 (FIG. 3A).

Target 521 is a geometric distribution of non-ionized target material (material that is not plasma). The target 521 may be, for example, a disk of liquid or molten metal, a droplet of liquid or molten metal, a continuous segment of target material without voids or significant gaps, a mist of micro-or nanoparticles, or an atomic vapor cloud. The dimensions of the target 521 are characterized by an extent 522 along a first direction "x" and an extent 523 in a second direction "z" perpendicular to the first direction. The range 523 is parallel to the propagation direction 512 of the radiation pulse 508.

In some implementations, the target 521 is an extended target with a range 522 greater than a range 523. For example, range 522 may be 220 micrometers (μm) and range 523 may be 370 nanometers (nm). In another example, range 522 may be 300 μm and range 523 may be 200 nm. Ranges 522 and 523 may also be larger, smaller, or between these example values. For example, the range 522 may be between 30 μm and 500 μm. The range 523 may be between 30 μm and 50nm, where the range 523 has a value at the larger end of the range (e.g., 30 μm) when the target 521 is not hit, and a value at the smaller end of the range (e.g., 50nm) when the target 521 is flattened in a direction parallel to the propagation direction 512.

Spatially expanding the target 521 can increase the amount of EUV light generated. First, because the range 522 is greater than the range 523, the extended target presents a relatively large area to the radiation pulse 508 in a direction perpendicular to the propagation direction 512. This exposes more of the target material 521 in the target to the amplified beam. In addition, the extended target may have a relatively short length along the propagation direction 512, allowing the radiation pulse 508 to penetrate deep into the target and convert a higher portion of the target material into plasma.

Second, the expanding target spatially spreads the target material, thereby minimizing the occurrence of regions of excessive material density during heating of the plasma by the second portion 508 b. If the plasma density is high in the entire region irradiated with radiation, the absorption of the radiation may be limited to the portion of the region that receives the radiation first. The heat generated by this initial absorption may be too far from the bulk target, and it is therefore difficult to maintain the evaporation and heating process of the target surface long enough to not utilize (evaporate) a meaningful amount of the bulk target during the limited duration of the second portion 508 b. In the case of a region with a high electron density, the light pulse only penetrates a part of the path to the region before reaching the "critical surface", where the electron density is so high that the light pulse is reflected. The light pulse cannot travel to those portions of the region and little EUV light is generated from the target in those regions. The region of high plasma density may also block EUV light emitted from portions of the region that do emit EUV light. Therefore, the total amount of EUV light emitted from the region is less than in the case of a portion of the region lacking high plasma density. Thus, using an extended target may result in the second portion 508b reaching more of the target material before being reflected. This may increase the amount of EUV light that is subsequently produced.

In some implementations employing a spatially extended target 521, and referring to fig. 6A and 6B, the target is a pre-extended target 621, which is a distribution of target material that is spatially extended by causing the pre-pulse 612 to interact with the target droplet 620 before reaching the target location 530. Target droplets 620 may be, for example, molten metal droplets 17-35 μm in diameter that are part of a stream of molten metal droplets released from a system, such as target feed system 115 (fig. 1). The impact force of the first pre-pulse 612 on the droplet 620 deforms the droplet 620 into a shape closer to a disk, which deforms into a disk-like piece of molten metal after about 1-3 microseconds (μ s).

Pre-pulse 612 has a duration 614 and is separated in time from radiation pulse 508 by a delay time 613, where pre-pulse 612 occurs before radiation pulse 508. Duration 614 may be measured using a suitable metric, such as foot-to-foot duration or full width at half maximum (FWHM). The duration 614 may be, for example, 20-70ns, less than 1ns, 300 picoseconds (ps) or less, 100ps or less, 100-300ps, or 10-100 ps. The wavelength of the pre-pulse 612 may be, for example, 1.06 μm or 10.6 μm. The pre-pulse 612 may have an energy of, for example, 3-60 millijoules (mJ). The pre-pulse 612 may be generated by the same secondary source as the source that generated the radiation pulse 508, or by a source separate and also different from the source that generated the radiation pulse 508.

An example of using a pre-pulse to generate an extended target is one way to provide an extended target. However, implementations in which target 521 is an extended target may involve other techniques. For example, the target 521 can be expanded during descent from a target supply system (such as the system 115 of fig. 1) to the target location 530 without interaction with the pre-pulse. In another example, the target 521 may be a preformed or machined target formed prior to reaching the target location. In some implementations, the target 521 is not an extended target.

Referring again to fig. 4, the radiation pulse 508 is directed toward the target 521 to form a modified target 524 (420). As discussed above with reference to fig. 2 and 3A-3C, radiation pulses 508 may be formed by process 200.

The radiation pulse 508 includes a first portion 508a ("base") and a second portion 508b, the second portion 508b reaching the target location 530 after the first portion 508 a. The radiation pulse 508 is a single pulse and there is no gap or temporal separation between the first portion 508a and the second portion 508 b. The first portion 508a has a duration 513 and a level 514. Duration 513 is the amount of time between the start of radiation pulse 508 and the start of second portion 508 b. The duration may be, for example, 10-150 ns. The energy of the base 508a may vary over a duration 513. Level 514 is the maximum or average energy of base 508 a. Level 514 may be, for example, 1-4 mJ. In some examples, the level 514 is represented as a percentage of the peak (maximum) energy 516 of the second portion 508 b. For example, the level 514 may be 1-10% of the peak energy 516 of the second portion 508 b.

As discussed with respect to fig. 3A, when the base 508a is generated by selectively activating the gating module 304, the base level 514 and duration 513 are set. The optimal values of base duration 513 and level 514 depend on the spatial characteristics of the target 521. For example, the target 521 may be a relatively thick target having a range 522 between about 200 μm and 220 μm and a range 523 between about 400 μm and 370 nm. In some embodiments, a thick target has a range 523 greater than 350nm, while a thin target has a range 523 less than 200 nm. For relatively thick targets, the amount of EUV light produced increases as base level 514 decreases. For such targets, the duration 513 of the base 508a may be set to, for example, 150ns or less, while the base level 514 may be set to have an energy of 1-3 mJ. In another example, the target 521 may be a relatively thin target in the range 522 of 300 μm and the range 523 of 200 nm. In this example, the base level 514 may be set to 5mJ and the duration 513 may be 50-150 ns. In this example, the level 514 may be about 1% of the maximum energy of the second portion 508 b.

Referring again to fig. 4, base 508a interacts with target 521 to form modified target 524 (430). Base 508a strikes target 521 to form modified target 524. The modified target 524 can take many forms. For example, the modified target 524 may be a pre-plasma that is spatially close to the bulk target material and is formed when the base 508a interacts with the metal in the modified target 524. A pre-plasma is a plasma used to enhance the absorption of incident light. While the pre-plasma may emit a small amount of EUV light in some cases, the emitted EUV light does not have the wavelength or amount emitted by converting the target material in the modified target 524 into plasma. In some implementations, the modified target 524 is a fog or a large amount of debris of the target material. The interaction of the base 508a with the target 521 may cause all or a portion of the target 521 to fragment, forming a fog or a large amount of debris.

The modified target 524 has different properties than the target 521. For example, the density of the modified target 524 may be different than the density of the target 521. The density of at least a portion of the modified target 524 can be less than the density of the target 521. Additionally or alternatively, the geometric distribution of the modified target 524 may be different from the geometric distribution of the target 521. For example, the modified target 524 may be larger in one or more dimensions than the target 521.

A second portion 508b of the radiation pulse 508 interacts with the modified target 524 to generate EUV light 550 (340). The second portion 508b has an energy sufficient to convert the target material in the modified target 524 into a plasma that emits EUV light.

Fig. 7, 9 and 10A-10D show exemplary measurement data obtained by interacting a target with a radiation pulse comprising a base, and fig. 8 shows an exemplary measurement radiation pulse comprising a base.

Referring to fig. 7, a plot 700 shows an exemplary measured relationship between EUV power and base level for a target having a diameter of 300 μm and a thickness of 200 nm. The diameter is measured along a direction perpendicular to the propagation direction of the illuminating radiation beam, and the thickness is measured along a direction parallel to the propagation direction.

As described above, the optimal base level may depend on the characteristics of the target. In the example shown in FIG. 7, the thickness of the target is 200nm and the EUV power is measured as a function of base level. The EUV power produced is highest for base levels between 1-2% of the peak intensity of the radiant pulse heated portion. The heating portion of the radiant pulse is the portion of the pulse having sufficient energy to convert the target material into a plasma, such as second portion 508 b. In the example shown in fig. 7, the optimal base level (the level that produces the most EUV) is about 4 mJ.

Referring to FIG. 8, drawing 800 shows a CO including a base 804a and a second portion 804b ₂An exemplary measured waveform of laser pulse 804. Base 804a conditions a target (not shown) and may be generated by a process such as process 200 discussed with reference to fig. 2. The exemplary pulse 804 is optimized for a target that extends from a 35 μm diameter molten tin droplet to 350 μm. The target of the example of fig. 8 has a larger diameter than the example of fig. 7. For this target, the optimal base level is larger, as shown in fig. 8. The optimal base level is 3% of the peak power of the second portion 804b, the base level is 10mJ, and the duration is 100 ns.

Referring to fig. 9, a graph 900 illustrates an exemplary measured Conversion Efficiency (CE) as a function of target diameter. The target diameter is the diameter of the expanding target along a direction perpendicular to the direction of propagation of the illuminating radiation beam. As shown, the CE is greater than or equal to 3.5% when the diameter of the expanded target is greater than 300 μm.

Referring to fig. 10A-10D, exemplary measured shadow maps of an extended target 1021 taken at different times relative to the main pulse arrival time are shown. Target 1021 is an extended target with extent 1022 in a direction parallel to direction "x" and extent 1023 parallel to direction "z". Range 1022 is perpendicular to direction "z", and range 1022 is greater than range 1023.

The target 1021 is at a target location that receives a pulse of radiation (not shown) that propagates in a direction parallel to the "z" direction. The radiation pulse includes a base and a second portion (or main pulse) behind the base that reaches the target location. The second portion of the radiation pulse has an energy sufficient to convert the target material in the extended target into a plasma that emits EUV light.

Fig. 10A shows the pre-expansion target 1021 at a time 200ns before the start of the second portion of the radiation pulse. FIG. 10B shows the target 1021 between 0-50ns before the second portion begins and at the time when the base interacts with the target 1021. The interaction between the base and the target 1021 forms a low density plume (plume)1005 near the target 1021. The base may make two modifications to the target 1021. First, the pedestal may homogenize the lateral and/or longitudinal density distribution of the target 1021 by generating the low-density plume 1005. This may make the target density locally more uniform, resulting in a locally smooth target. Second, the interaction between the base and the target 1021 creates a gentler plasma density scale length along the radiation incident line (direction "z"). The mild plasma density scale length is shown in plume 1005 in fig. 10B. The relatively uniform density distribution of the expanded target may allow the main pulse to penetrate the target more efficiently, interact with the upper portion of the target, and convert the upper portion of the target into a plasma. Therefore, the Conversion Efficiency (CE) can be higher. A gentle plasma density distribution may result in a larger plasma volume including more atoms or particles of the target material. This may result in higher radiation absorption due to longer plasma scale length and more EUV light. Furthermore, a gentle plasma density distribution results in a less reflective surface and may reduce the reflection of the main pulse back to the light source that generated the main pulse.

Fig. 10C shows the target 1021 during the time of main pulse strike, where the target material in the target is converted to plasma and EUV light. Fig. 10D shows the trailing plume remaining 200ns after the main pulse arrives.

Fig. 11 and 12 provide additional information about a system that can generate radiation pulses that include a base.

Referring to fig. 11, a top plan view of an exemplary optical imaging system 1100 is shown. The optical imaging system 1100 includes an LPP EUV light source 1102 that provides EUV light 1150 to a lithography tool 1170. The light source 1102 may be similar to the light source 100 of fig. 1, and/or include some or all of the components of the light source 100 of fig. 1.

The system 1100 includes optical sources such as a drive laser system 1105, an optical element 1122, a pre-pulse source 1143, a focusing component 1142, and a vacuum chamber 1140. The laser system 1105 is driven to produce an amplified beam 1110. The amplified light beam 1110 includes a base portion and a second portion, e.g., similar to the radiation pulses 308 and 508 discussed above. The amplified light beam 1110 has energy sufficient to convert the target material in the target 1120 into a plasma that emits EUV light. Any of the targets discussed above may be used as target 1120.

The pre-pulse source 1143 emits radiation pulses 1117. The radiation pulse may be used as a pre-pulse 612 (fig. 6B). For example, pre-pulse source 1143 may be a Q-switched Nd: YAG laser operating at a 50kHz repetition rate, and radiation pulses 1117 may be pulses from the Nd: YAG laser having a wavelength of 1.06 μm. The repetition rate of pre-pulse source 1143 indicates the frequency at which pre-pulse source 1143 generates radiation pulses. For the example of a pre-pulse source 1143 having a 50kHz repetition rate, radiation pulses 1117 are emitted every 20 microseconds (μ s).

Other sources may be used as pre-pulse source 1143. For example, pre-pulse source 1143 can be any rare earth doped solid state laser other than Nd: YAG, such as an erbium doped fiber (Er: glass) laser. In another example, the pre-pulse source may be a carbon dioxide laser that generates pulses having a wavelength of 10.6 μm. The pre-pulse source 1143 may be any other radiation or light source that generates a pulse of light having the energy and wavelength used for the pre-pulse described above.

The optical element 1122 directs the amplified light beam 1110 and the radiation pulse 1117 from the pre-pulse source 1143 into the chamber 1140. Optical element 1122 is any element that can direct amplified light beam 1110 and radiation pulse 1117 along similar or identical paths. In the example shown in fig. 11, optical element 1122 is a dichroic beam splitter that receives amplified light beam 1110 and reflects it toward chamber 1140. Optical element 1122 receives pulse 1117 of radiation and transmits the pulse toward chamber 1140. The dichroic beamsplitter has a coating that reflects the wavelength(s) of the amplified light beam 1110 and transmits the wavelength(s) of the radiation pulse 1117. The dichroic beam splitter may be made of diamond, for example.

In other implementations, optical element 1122 is a mirror that defines an aperture (not shown). In this implementation, the amplified light beam 1110 reflects from a mirror surface and is directed to the chamber 1140, and the radiation pulse travels through an aperture and toward the chamber 1140.

In still other implementations, the amplified light beam 1110 and the pre-pulse 1117 may be separated into different angles by their wavelengths using wedge optics (e.g., a prism). A wedge-shaped optical device may be used in addition to optical element 1122, or it may be used as optical element 1122. The wedge optics may be positioned just upstream (in the "-z" direction) of the focusing component 1142.

Additionally, pulse 1117 may be delivered to chamber 1140 in other manners. For example, the pulse 1117 may pass through an optical fiber that delivers the pulse 1117 to the chamber 1140 and/or the focusing component 1142 without using the optical element 1122 or other directing elements. In these implementations, the fiber directs the radiation pulse 1117 directly into the interior of the chamber 1140 through an opening formed in a wall of the chamber 1140.

The amplified light beam 1110 reflects from the optical element 1122 and propagates through the focusing component 1142. Focusing component 1142 focuses amplified light beam 1110 at a focal plane 1146, which focal plane 1146 may or may not coincide with target location 1130. Pulsed radiation 1117 passes through optical element 1122 and is directed into chamber 1140 by focusing component 1142. The amplified light beam 1110 and the radiation pulse 1117 are directed to different locations in the chamber 1140 along the "x" direction and arrive at the chamber 1140 at different times.

In the example shown in fig. 11, a single box represents pre-pulse source 1143. However, pre-pulse source 1143 may be a single light source or multiple light sources. For example, two separate sources may be used to generate the plurality of pre-pulses. The two separate sources may be different types of sources that produce radiation pulses having different wavelengths and energies. For example, a pre-pulse may have a wavelength of 10.6 μm and be formed from CO ₂The laser generates, another pre-pulse may have a wavelength of 1.06 μm and be generated by a rare earth doped solid state laser.

In some implementations, the pre-pulse 1117 and the amplified light beam 1110 can be generated by the same source. For example, the pre-pulse 1117 of radiation may be generated by driving the laser system 1105. In this example, the drive laser system may include two COs ₂A seed laser subsystem and an amplifier. One of the seed laser subsystems may produce an amplified light beam having a wavelength of 10.26 μm, while the other seed laser subsystem may produce an amplified light beam having a wavelength of 10.26 μmTo produce an amplified beam having a wavelength of 10.59 μm. The two wavelengths may be from CO ₂Different lines of lasers. In other examples, CO ₂Other lines of lasers may be used to generate the two amplified beams. The two amplified beams from the two seed laser subsystems are amplified in the same power amplifier chain and then angularly dispersed to different positions within the chamber 1140. An amplified beam having a wavelength of 10.26 μm may be used as the pre-pulse 1117, and an amplified beam having a wavelength of 10.59 μm may be used as the amplified beam 1110. In implementations employing multiple pre-pulses, three seed lasers may be used, one for generating each of the amplified light beam 1110, the first pre-pulse, and the second individual pre-pulse.

Both the amplified light beam 1110 and the pre-pulse 1117 of radiation may be amplified in the same optical amplifier. For example, three or more power amplifiers may be used to amplify the amplified light beam 1110 and the pre-pulse 1117.

Referring to fig. 12, in some implementations, the extreme ultraviolet light system 100 is part of a system that includes other components, such as a vacuum chamber 1200, one or more controllers 1280, one or more actuation systems 1281, and a guidance laser 1282.

Vacuum chamber 1200 may be a single unitary structure, or it may be provided with separate sub-chambers that house particular components. The vacuum chamber 1200 is an at least partially rigid enclosure from which air and other gases are removed by a vacuum pump, resulting in a low pressure environment within the chamber 1200. The walls of the chamber 1200 may be made of any suitable metal or alloy suitable for vacuum use (which can withstand lower pressures).

Target delivery system 115 delivers target 120 to target location 130. The target material 120 at the target location may be in the form of a liquid droplet, a liquid stream, a solid particle or cluster, a solid particle contained within a liquid droplet, or a solid particle contained within a liquid stream. For example, the target 120 may comprise water, tin, lithium, xenon, or any material that has an emission line in the EUV range when converted to a plasma state. For example, elemental tin may be used as pure tin (Sn) as a tin compound such as SnBr ₄、SnBr ₂、SnH ₄As a tin alloy, for example, a tin-gallium alloy, a tin-indium-gallium alloy, or any combination of these alloys is used. Target 120 may comprise a wire coated with one of the elements described above, such as tin. If target 120 is in a solid state, it may have any suitable shape, such as a ring, sphere, or cube. Target 120 may be delivered to the interior of chamber 1200 and target location 130 by target delivery system 115. Target location 130 is also referred to as an illumination point where target 120 optically interacts with amplified beam 110 to generate a plasma.

The drive laser system 105 may include one or more optical amplifiers, lasers, and/or lamps for providing one or more main pulses and, in some cases, one or more pre-pulses. Each optical amplifier includes a gain medium capable of optically amplifying a desired wavelength with high gain, an excitation source, and internal optics. The optical amplifier may or may not have a laser mirror or other feedback device forming a laser cavity. Thus, even without a laser cavity, the drive laser system 105 produces an amplified beam 110 due to population inversion in the gain medium of the laser amplifier. Furthermore, if there is a laser cavity that provides sufficient feedback to the drive laser system 105, the drive laser system 105 can produce an amplified beam 110 that is a coherent laser beam. The term "amplifying the light beam" includes one or more of the following: light from the drive laser system 105 that is only amplified but not necessarily coherent laser oscillation, and light from the drive laser system 105 that is amplified and is also coherent laser oscillation.

The optical amplifier in the drive laser system 105 may include a fill gas comprising CO as the gain medium ₂And light having a wavelength between about 9100 and about 11000nm, and particularly about 10600nm, can be amplified with a gain of greater than or equal to 1000. Suitable amplifiers and lasers for driving in laser system 105 may include pulsed laser devices, such as pulsed gas discharge CO that produces radiation at about 9300nm or about 10600nm ₂Laser devices, e.g. using DC or RF excitation, operating at relatively high power, e.g.Operating at a power of 10kW or more and a high pulse repetition rate of, for example, 50kHz or more. The optical amplifier in the driving laser system 105 may also include a cooling system, such as water, which may be used when operating the driving laser system 105 at higher power.

The light collector 155 can be a collector mirror 1255 having an aperture 1240 to allow the amplified light beam 110 to pass through to the focal region 145. For example, collector mirror 1255 can be an ellipsoidal mirror having a first focus at target location 130 or focal region 145 and a second focus at intermediate location 1261 (also referred to as an intermediate focus), where EUV light 160 can be output from the extreme ultraviolet light system and can be input to optics 165.

One or more controllers 1280 are connected to one or more actuation or diagnostic systems, such as a droplet position detection feedback system, a laser control system, and a beam control system, and one or more targets or droplet imagers. The target imager provides an output indicative of, for example, the location of the drop relative to the target location 130 and provides that output to a drop location detection feedback system, which can calculate, for example, drop location and trajectory, from which drop location errors can be calculated on a drop-by-drop basis or on an average basis. Thus, the drop position detection feedback system provides a drop position error as an input to the controller 1280. Thus, the controller 1280 may, for example, provide laser position, orientation, and timing correction signals to a laser control system, which may, for example, be used to control a laser timing circuit and/or a beam control system to control the amplified beam position and shaping of the beam delivery system to change the position and/or focal power of the beam focal spot within the chamber 1200.

Target delivery system 115 includes a target delivery control system operable in response to a signal from controller 1280, for example, to modify the release point of a droplet released by an internal delivery mechanism to correct for errors in the droplet reaching a desired target location 130.

Additionally, the extreme ultraviolet light system may include a light source detector that measures one or more EUV light parameters, including, but not limited to, pulse energy, energy distribution as a function of wavelength, energy within a particular wavelength band, energy outside a particular wavelength band, and angular distribution and/or average power of EUV intensity. The light source detector generates a feedback signal for use by the controller 1280. For example, the feedback signal may indicate an error in a parameter such as the timing and focus of the laser pulse for properly intercepting the droplet at the correct position and time for efficient and effective EUV light generation.

In some implementations, the drive laser system 105 has a master oscillator/power amplifier (MOPA) configuration with multi-stage amplification and with seed pulses initiated by a Q-switched Master Oscillator (MO) with low energy and high repetition rate, e.g., capable of 100kHz operation. By MO, e.g. fast axial flow CO which can be pumped using RF ₂The amplifier amplifies the laser pulses to produce an amplified beam 110 that travels along the beam path.

Although three optical amplifiers may be used, as few as one amplifier and more than three amplifiers may be used in this implementation. In some implementations, each CO ₂The amplifier may be an RF pumped axial CO with an amplifier length of 10 meters folded by internal mirrors ₂A laser cube.

At the point of illumination, the amplified beam 110 is used to create a plasma having specific characteristics, which depend on the composition of the target 120. These characteristics may include the wavelength of EUV light 160 produced by the plasma and the type and amount of debris released from the plasma. The amplified light beam 110 vaporizes the target 120 and heats the vaporized target to a critical temperature where electrons fall off (plasma state), leaving ions that are further heated until they begin to emit photons having wavelengths in the extreme ultraviolet range.

Various implementations have been described. However, other implementations are within the scope of the following claims.

Although shown as a linear path, the beam path 309 (fig. 3A) may take any form. In addition, the optical source 305 may include other components, such as lenses and/or mirrors, to steer the light along the path 309. The optical amplifier 306 (fig. 3A) is shown as a single stage, however in other implementations, the optical amplifier 306 may be a chain of multiple amplifiers. The amplifier chain may include one or more preamplifiers and one or more power amplifier stages.