CLOG-resistant droplet generators for EUV light sources

Abstract

A droplet generator for an EUV light source includes a fluid cavity within a structure, the fluid cavity having a first fluid cavity end being open or otherwise capable of receiving a fluid and a second fluid cavity end; a particle filter within the fluid cavity dividing the fluid cavity into an upstream volume between an upstream surface of the particle filter and the first fluid cavity end, a filter volume occupied by the filter, and a downstream volume downstream of a downstream surface of the filter; and a tube having a first opening positioned outside the fluid cavity, the tube extending from the first opening through the second fluid cavity end to a second opening within the downstream volume, the tube extending into the downstream volume by more than 10% of a distance from the first opening to the second opening.

Description

CLOG-RESISTANT DROPLET GENERATORS FOR EUV LIGHT SOURCESCROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Application No. 63 / 580,549, filed September 5, 2023, titled CLOG-RESISTANT DROPLET GENERATORS FOR EUV LIGHT SOURCES, which is incorporated herein in its entirety by reference.TECHNICAL FIELD

[0002] The present disclosure relates to clog-resistant droplet generators for producing droplets without clogging, or with reduced clogging, in extreme ultraviolet (EUV) light sources.BACKGROUND

[0003] EUV light that is used for semiconductor photolithography is supplied by a system referred to as an EUV light source. In EUV light sources, a droplet generator produces droplets that are irradiated by light from a light source often referred to as a source laser. The droplets are formed of or include a material or materials capable of emitting EUV radiation when in a plasma state. Energy from the irradiation of a droplet by light from the source laser creates an EUV-emitting plasma from at least some of the material(s) of the irradiated droplet. EUV light from the plasma is collected and transmitted to a lithography apparatus for use in forming patterns in or on semiconductor substrates. EUV light source uptime is highly valued by the users of EUV light sources due to the high cost of — and the high value generated by — semiconductor manufacturing processes.SUMMARY

[0004] In some general aspects, a droplet generator for an EUV light source includes a fluid cavity within a structure, the fluid cavity having a first fluid cavity end being open or otherwise capable of receiving a fluid and a second fluid cavity end; a particle filter within the fluid cavity dividing the fluid cavity into an upstream volume between an upstream surface of the particle filter and the first fluid cavity end, a filter volume occupied by the filter, and a downstream volume downstream of a downstream surface of the filter; and a tube having a first opening positioned outside the fluid cavity, the tube extending from the first opening through the second fluid cavity end to a second opening within the downstream volume, the tube extending into the downstream volume by more than 10% of a distance from the first opening to the second opening.

[0005] Implementations can include one or more of the following features.

[0006] The tube can extend into the downstream volume by more than 20% of the distance from the first opening to the second opening, or more than 30% of the distance from the first opening to the second opening. A distance from the second opening to any surface other than a surface of the tube can be at least one millimeter (1 mm), or at least 3 mm or more. The downstream volume can includeat least one dead volume. The downstream volume can include at least two dead volumes. The downstream volume can include a dead volume positioned to be gravitationally below a fluid flow path when the droplet generator is in operation. The downstream volume can include a dead volume positioned to be gravitationally above a fluid flow when the droplet generator is in operation. The downstream volume can include a dead volume positioned to be gravitationally above a fluid flow when the droplet generator is in operation.

[0007] The tube can be oriented to be essentially perpendicular to gravity when the droplet generator is in operation. The tube can have at least one bend positioned within the downstream volume.

[0008] The downstream volume can include one or more respective dead volumes, and one or more initial-fill flow paths can extend from the filter to the one or more respective dead volumes, and the one or more initial-fill flow paths can avoid the second opening of the tube so as not to encompass the second opening of the tube.

[0009] The filter can be positioned in-line between the first fluid cavity end and the second fluid cavity end. The filter can be a cylindrical filter with a central opening extending axially at least partially through the filter. The second opening of the tube can be positioned within the central opening of the filter. The filter can be a cylindrical filter with a central opening extending axially through the filter. The tube can extend completely through the central opening of the filter, with the second opening positioned outside of the central opening of the filter.

[0010] A droplet material flowing within the fluid cavity can make a turn of about 90 degrees or greater to enter the second opening of the tube when the droplet generator is in use. A droplet material flowing within the fluid cavity can make a turn of about 180 degrees to enter the second opening of the tube when the droplet generator is in use.

[0011] In additional general aspects, a droplet generator includes a filter having an inner volume, the filter configured to prevent particles in a fluid from passing through the filter to the inner volume; and a tube for delivery of droplets of the fluid by the droplet generator, the tube having an upstream tube opening extending into the inner volume of the filter, whereby the fluid flows through the filter and into the upstream tube opening and the particles are prevented from entering the upstream tube opening.

[0012] Implementations can include one or more of the following features.

[0013] The tube can extend to proximate an upstream end of the filter. The tube can further include an upstream end portion with an outer diameter that is smaller than an inner diameter of the inner volume, with the first outer diameter at least partially forming a trapping volume for particles downstream of the filter. The filter can circumferentially surround the inner volume and the fluid can enter the filter circumferentially.

[0014] The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims.DRAWING DESCRIPTION

[0015] FIG. 1 is a block diagram of an implementation of an extreme ultraviolet (EUV) light source.

[0016] FIG. 2 is a functional diagram of a clog-resistant droplet generator.

[0017] FIGS. 3A, 3B, 3C, 3D, 3E, 3F, and 3G show cross-sectional views of various implementations of various aspects of clog -resistant droplet generators.

[0018] FIG. 4 is a diagram of an EUV light source in use with a photolithography apparatus.DETAILED DESCRIPTION

[0019] Referring to FIG. 1, a block diagram of an EUV light source 100 that includes a droplet generator 110 is shown. The droplet generator 110 emits a stream of droplets 121 such that a droplet 12 Ip is delivered to a plasma formation location 123 in a vacuum chamber 109. The droplet 12 Ip includes one or more materials that emit EUV light when in a plasma state, or in one or more of multiple possible plasma states. The materials could include, for example, water, tin, lithium, and / or xenon, but tin is generally employed. The plasma formation location 123 receives a light beam 106. The light beam 106 is generated by an optical source 105 and delivered to the vacuum chamber 109 via an optical path 107. An interaction between the light beam 106 and the droplet material in the droplet 12 Ip produces a plasma 196 that emits EUV light, which is then collected and transmitted from the source 100 as transmitted EUV light 155.

[0020] The droplet generator 110 can include a tube 114 such as a capillary tube that is fluidly coupled to a reservoir 112. The tube 114 can be held by a nozzle apparatus 140. The tube 114 defines an orifice 119 through which a droplet material flows to form the stream of droplets 121. The tube 114 is mechanically coupled to an actuator 193, which is coupled to a control system 190 via a control link 192. The control system 190 may include a function generator, an electronic processor (not shown), and an electronic storage (not shown) to carry out the functions of the control system 190. The control link 192 is any type of connection capable of transferring an electronic and / or actuating signal from the control system 190 to the actuator 193. For example, the control link 192 may be a wired and / or wireless connection configured to transmit electronic and / or actuating signals and commands from the control system 190 to the actuator 193.

[0021] The control system 190 generates signals that, when applied to the actuator 193 or to an element associated with the actuator 193, cause the actuator 193 to move. For example, the actuator 193 may be a piezoelectric ceramic material that changes shape based on an applied voltage. The magnitude and / or polarity of the voltage applied to the actuator 193 is based on the signals from the control system 190. Due to the mechanical coupling between the tube 114 and the actuator 193, when the actuator 193 moves or vibrates, the tube 114 experiences a corresponding motion or vibration.More specifically, a radial contraction of the actuator 193 results in a local contraction of the tube 114 and the expansion of the actuator 193 results in the local expansion of the tube 114. This expansion and contraction produces acoustic waves at the frequency of the applied electrical signal in droplet material that is inside the tube.

[0022] The reservoir 112 contains droplet material under pressure P. The droplet material is in a liquid state able to flow (such as molten tin), and the pressure in the vacuum chamber 109 is lower than the pressure P. Thus, the droplet material flows through the tube 114 and is emitted into the chamber 109 through the orifice 119. The droplet material exits the orifice 119 as a jet 124 or continuous stream of droplet material. The jet 124 of droplet material breaks up into individual droplets. The break-up of the jet 124 may be controlled such that the individual droplets coalesce into larger droplets that arrive at the plasma formation location 123 at a desired rate by vibrating the tube 114 and creating acoustic waves inside of the tube 114.

[0023] For example, the control system 190 may provide a signal that has at least a first frequency and a second frequency via the control link 192 to thereby drive the actuator 193 to vibrate at the first and second frequencies. The first frequency can be in the megahertz (MHz) range. Vibrating the tube 114 at the first frequency causes the jet 124 to break into relatively small droplets of desired sizes. The second frequency is lower than the first frequency. For example, the second frequency can be in the kilohertz (kHz) range. The second frequency is used to modulate the velocity of the droplets in the stream and to encourage droplet coalescence. Driving the tube 114 at the second frequency causes groups of droplets to form in which the droplets travel at different velocities. The droplets with higher velocities can coalesce with the droplets with lower velocities to form larger coalesced droplets that then make up the stream of droplets 121 for the EUV source. After coalescence, these resulting larger droplets are separated from each other by a larger distance than the smaller non-coalesced droplets. The larger separation helps to mitigate the influence of the plasma formed from one droplet on the subsequent droplet(s) in the droplet stream. The droplets in the stream of droplets 121 may be approximately spherical, with a diameter in of about 20 or 35 pm, for example.

[0024] By causing the tube 114 to vibrate in this manner, the final droplets may be generated at frequencies of, for example, between 40 to 300 kHz or more and may travel toward the plasma formation location 123 at a velocity of, for example, between 40 and 120 meters per second (m / s) or up to 500 m / s or more. The spatial separation between two adjacent droplets in the stream of droplets 121 may be, for example, between 1 and 3 millimeters (mm). Between 50 and 300 initial droplets (also called Rayleigh droplets) may coalesce to form a single larger droplet.

[0025] FIG. 2 is a functional diagram of a clog-resistant droplet generator 210 that can be, for example, the droplet generator 110 in FIG. 1. Similarly to the droplet generator 110 of FIG. 1, a pressurized reservoir 212 contains droplet material under pressure in liquid form. Droplet material is delivered from the reservoir 212 into a first fluid cavity end 241-fe of a fluid cavity 241. The fluid cavity 241 can be defined within a structure 242 (a portion of which is shown in partial cross sectionin FIG. 2) that can be, or form part of, a nozzle assembly 240, such as nozzle assembly 140 of FIG. 1. A filter 244 is provided in or along the fluid cavity 241 for filtering particles that can or may be entrained in the droplet material. At the second fluid cavity end 241-se a tube 214, such as a capillary tube, is connected to the fluid cavity 241, with the tube 214 extending from a first opening 214-01 or orifice 219 of the tube 214 to a second opening 214-02, through a seal 245 at the second fluid cavity end 241-se. The tube can be a glass tube, such as a borosilicate glass tube, or an aluminosilicate glass tube, or a quartz tube or a ceramic tube such as a silicon carbide or other ceramic material resistant to corrosion by liquid tin, which is a typical main component of the droplet material.

[0026] A dead volume 236 is provided in or along the fluid cavity 241, after the filter 244 and before the tube 214. A dead volume such as the dead volume 236 can be defined as a volume that, in use, will be occupied by the fluid droplet material, during an initial fill of the fluid cavity 241, before the fluid droplet material enters the tube 214, and in which the fluid droplet material substantially changes flow direction before it enters the tube 214. The dead volume accordingly is not within a flow pattern or flow path (such as a flow path beginning from the first fluid cavity end 241-fe toward a second fluid cavity end 241-se) once the dead volume is filled; little or no pressure-driven flow occurs within the dead volume after the initial fill.

[0027] Similarly to FIG. 1, the tube 214 is mechanically coupled to an actuator 293, which is coupled to a control system 290 via a control link 292. Similarly to the operation described above with respect to FIG. 1, as a stream of droplet material, such as a stream of liquid tin, is ejected under pressure from the first opening 214-01 of the tube 214, and modulation of the actuator 293 causes the droplet material to coalesce into droplets 221 travelling along a direction A. Note that the direction A of droplet travel can be oriented in any direction with respect to gravity, that is, gravity can be but need not be downward in the plane of the page of FIG.2.

[0028] The tube 214 extends into the fluid cavity 241 (beyond the seal 245) by a length L. The length L is greater than 10% of the overall length of the tube 214 (the distance from the first opening 214-01 to the second opening 214-02), and can be greater than 15%, greater than 20%, greater than 30%, or even greater than 40% of the length of the tube 214. Alternatively, the length L can be greater than or equal to 1 millimeter (mm), 2 mm, 3 mm, 4 mm, 5 mm, 7 mm, 10 mm, 15 mm, or even 20 mm. A distance D between the second opening 214-02 and the nearest solid surface inside the fluid cavity 241 (other than a surface of the tube 214) can be greater than or equal to 0.5 mm, 1 mm, 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, 8 mm, or even 10 mm. The length L and the distance D, considered together, effectively position the second opening 241-02 away from solid surfaces or walls within the fluid cavity 241 (aside from the tube 214 itself). Because the material of the tube 214 is relatively inert, even in the presence of liquid tin, for example, contamination in the form of reaction products of tin is generally not formed on or near the tube 214. By positioning the second opening 214-02 away from other surfaces, any reaction products which may or can occasionally form at such other surfaces are unlikely or less likely to flow into the second opening 214-02 during operation of the clog-resistant droplet generator 210, reducing the likelihood of the tube 214 clogging with reaction products of droplet material.

[0029] Positioning a dead volume 236 between the fdter 244 and the second fluid cavity end 241-se or between the filter 244 and the second opening 214-02 of the tube 214 allows particles that can or may initially exist within the filter 244, or on or near a downstream surface of the filter 244, to pass into and remain in the dead volume 236 upon initial filling of the dead volume 236, further preventing or reducing opportunities for clogging of the tube 214.

[0030] FIGS. 3A-3G are cross-sectional views of various implementations of various aspects of clog-resistant droplet generators, with reservoirs, such as reservoirs 112 and 212 of FIGS 1 and 2, and actuators, such as actuators 193 and 293 of FIGS. 1 and 2, omitted for ease of illustration.

[0031] With reference to FIGS. 3A-3G, a clog-resistant droplet generator 310 includes a fluid cavity 341 within a structure 342. The fluid cavity 341 has a second cavity end 341-se, and a first end 341-fe that is open or otherwise capable of receiving a fluid such as a fluid droplet material such as liquid tin. The structure 342 may include or be formed of tungsten, molybdenum, tantalum, titanium, rhenium, or alloys of these; ceramics such as alumina or silicon carbide; or other material or materials resistant to corrosion or other unwanted interaction with liquid tin.

[0032] A tube 314 such as a capillary tube extends from a first opening 314-01 A outside the structure 342 through a seal 345 to a second opening 314-02 inside the fluid cavity 341. The seal 345 can, for example, be a compression seal utilizing high temperature compressible materials such as high temperature polymers, or a glass-to-metal fusion seal with fusion to the tube 314, or combinations of these, such as with glass-to-metal fusion to a metal ring or collar plus a compression seal of the metal ring or collar to the structure 342.

[0033] A particle filter 344 within the fluid cavity 341 divides the fluid cavity 341 into (1) an upstream volume VU between an upstream surface US of the particle filter 344 and the first fluid cavity end 341-fe, a filter volume VF occupied by the filter, and a downstream volume VD downstream of a downstream surface DS of the filter 344.

[0034] The first opening 341-01 of the tube 314 is positioned within the downstream volume VD, with the tube 314 extending into the downstream volume VD by a length L of more than 10% of a distance from the first opening 314-01 to the second opening 314-02, or greater than 15%, greater than 20%, greater than 30%, or even greater than 40% of the length of distance from the first opening 314-01 to the second opening 314-02. Alternatively, the length L can be greater than or equal to 1 millimeter (mm), 2 mm, 3 mm, 4 mm, 5 mm, 7 mm, 10 mm, 15 mm, or even 20 mm. A distance D between the second opening 314-02 and the nearest solid surface inside the fluid cavity 341 can be greater than or equal to 0.5 mm, 1 mm, 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, 8 mm, or even 10 mm.

[0035] The downstream volume VD includes at least one dead volume 336 (FIGS. 3A, 3D, 3F), or dead volume 336a (FIGS. 3B, 3C, 3E, 3G) with the term “dead volume” understood as described above with respect to FIG. 2. As noted above, the length L and the distance D position the secondopening 341-02 away from solid surfaces or walls within the fluid cavity 341, such that any reaction products which may or can occasionally form at such surfaces are unlikely or less likely to flow into the second opening 314-02 during operation of the clog-resistant droplet generator 310, reducing the likelihood of the tube 314 clogging with droplet-material reaction products. Also as noted above, a dead volume 346 between the fdter 344 and the second fluid cavity end 341-se or between the fdter 344 and the second opening 314-02 of the tube 314 allows particles that can or may initially exist within the fdter 344, or on or near a downstream surface of the fdter 344, to pass into and remain in the dead volume 346 upon initial fdling of the dead volume 346, further reducing clogging of the tube 314.

[0036] With specific reference to FIG. 3 A, the dead volume 336 can be in the form of a gravitationally downward dead volume 337, namely, a dead volume positioned gravitationally below a fluid flow path P (in the case that gravity is oriented as shown by the arrow G in FIG. 3A) from the fdter 344 to the second opening 314-02 of the tube 314. (Note that the orientation shown is optional, and that clog -resistant droplet generator 310 of FIG. 3A can be operated in other orientations than the one shown.) With the gravitationally downward dead volume 337, when the clog-resistant droplet generator 301 is in operation, any contamination that is heavier than the droplet fluid in the fluid cavity 341 positioned between the downstream surface DS and the dead volume 337 will tend to fall into and remain in the dead volume 337, even after the dead volume 336, 337 is initially filled. Thus relatively heavy particles, such as particles of (momentarily) solidified tin and / or others, can be prevented from clogging the tube 314, at least in part.

[0037] With specific reference to FIG. 3B, a clog -resistant droplet generator 310 is shown having many features in common with the clog-resistant droplet generator 310 of 3 A, with some differences highlighted below.

[0038] In the implementation of FIG. 3B, the downstream volume VD of the fluid cavity 341 includes three dead volumes: two relatively large dead volumes 336a, 336b, and one smaller dead volume 336c. The downstream volume VD includes two gravitationally downward dead volumes 337al, 337a2, that are positioned gravitationally below a flow path P from the filter VF to the second opening 314-02 of the tube 314, when the clog-resistant droplet generator 310 is in operation with gravity in the (optional) orientation shown. The downstream volume VD also includes a gravitationally upward dead volume 337b that is positioned gravitationally above the flow path P from the filter VF to the second opening 314-02 of the tube 314. Similarly to the implementation of FIG. 3 A, when the clog -resistant droplet generator 310 is in operation, any contamination that is heavier than the droplet fluid in the downstream volume VD of the fluid cavity 341 will tend to fall into and remain in one of the two the dead volumes 337al and 337a2, even after they are initially filled, preventing, or reducing heavy particles clogging the tube 314. In addition, if relatively light particle contaminants are present in the droplet material in the downstream volume VD of the fluidcavity 341, they will tend to accumulate in the gravitationally upward dead volume 337b, preventing or reducing light particles clogging the tube 314.

[0039] Because of the relative positioning of the fdter 344, of the dead volumes 336a and 366b, and of the second opening 314-02 of the tube 314, the respective dead volumes 336a and 336b each have corresponding respective initial -fdl flow paths IFP-1 and IFP-2 that extend from the fdter 344 to the to the dead volumes 336a, 336, which initial-fdl flow paths IFP-1 and IFP-2 do not encompass or directly pass over or directly adjacent to the second opening 314-02 of the tube 341. Thus as the fluid cavity 341 is initially fdled, any initial particles arising at or from the fdter 344 or the downstream surface DS thereof can pass into the dead volumes 336a and 336b without necessarily coming close to the second opening 314-02.

[0040] With specific reference to the implementation shown in FIG. 3C, a clog-resistant droplet generator 310 is shown having many features in common with the clog-resistant droplet generators 310 of FIGS 3A and 3B, with some differences highlighted below.

[0041] In the implementation of FIG. 3C, the clog-resistant droplet generator 310 can be (optionally) oriented with gravity as shown by the arrow G, with the upper volume VU of the fluid cavity 341 and the tube 314 oriented parallel to each other and at an angle relative to gravity. The fluid cavity 341 includes three dead volumes 336a, 336b, and 336c in the form of gravitationally upper and gravitationally lower dead volumes 337a and 337b (upper and lower relative to the path P), as well as a dead volume at an angle to gravity 337c near the tube 314. As in the implementation of FIG. 3B, because of the relative positioning of the fdter 344, the dead volumes 336a and 366b, and second opening 314-02 of the tube 314, the respective dead volumes 336a and 336b each have corresponding respective initial-fdl flow paths IFP-1 and IFP-2 that extend from the fdter 344 to the to the dead volumes 336a, 336, which initial-fdl flow paths IFP-1 and IFP-2 do not encompass or directly pass over or directly adjacent to the second opening 314-02 of the tube 341.

[0042] In the implementation of FIG. 3C, the tube 314 includes a bend 314b near the second opening 314-02. The bend 314b allows the second opening 314-02 to be perpendicular to gravity (or in other words, a cross section of the second opening 314-02 to be parallel to gravity) such that any particles moving upward or downward relative to gravity will tend to pass by the second opening 312-02. Implementations with no bends in the tube can have the entire tube perpendicular to gravity to achieve the same effect. Other bends or curves can also be used in this or other implementations, including snorkel-shaped bends and the like.

[0043] As shown in FIG. 3C, with the bend 314b present in the tube 314, the distance D is measured at the closest approach of an edge of the cross section of the second opening 314-02 to any solid surface (other than a surface of the tube 314 itself), and the length L is measured parallel to the unbent portions of the tube 314 from the seal 345, or any other structure defining the fluid cavity 341 at the second end 341-se thereof, to the closest edge of the cross section of the second opening 314-02.

[0044] With specific reference to FIG. 3D, a cross section of another implementation of a clogresistant droplet generator 310 is shown, in which the filter 344 is positioned in-line between the first fluid cavity end 341-fe and the second fluid cavity end 341-se. The clog-resistant droplet generator 310 can be oriented, during operation, with gravity perpendicular to the tube 314, as shown by the arrow G, though other orientations can also be used. In this implementation a dead volume 336 is present surrounding the tube 314 inside the downstream volume VD of the fluid cavity 341.

[0045] With specific reference to FIG. 3E, a cross section of yet another implementation of a clogresistant droplet generator 310 is shown. As in the implementation of FIG. 3D, the filter 344 is, overall, positioned in-line between the first fluid cavity end 341-fe and the second fluid cavity end 341-se, but in this implementation is in the form of a cylindrical filter 344a having a central opening extending axially at least partially through the filter 344, resulting in an interior open volume 344iv of the filter 344a. The upstream surface US of the filter 344 is thus oriented vertically in the figure, and the downstream surface DS of the filter 344 likewise is oriented vertically in the figure. As in the implementation of FIG. 3F, the clog -resistant droplet generator 310 of FIG. 3E can be oriented, during operation, with gravity perpendicular to the tube 314, as shown by the arrow G, which results in the second opening 314-02 being perpendicular to the direction of gravity. Other orientations can also be used.

[0046] In the implementation of FIG. 3E, the tube 314 extends into the central opening or interior open volume 344iv of the filter 344a and the second opening of the tube 314-02 positioned within the central opening or interior open volume 344iv of the filter 344a. The tube 324 can extend to proximate an upstream end of the filter 344a, thus placing second opening 314-02 proximate the upstream end of the filter 344a. Because an upstream end portion of the tube 314 within the open inner volume 344iv of the filter 344a has an outer diameter that is smaller than a diameter of the open inner volume 344iv (and in this implementation also smaller than an inner diameter of the fluid cavity 341 in the downstream volume VD), the first outer diameter of the tube 314 assists in at least partially forming a dead volume 336 or “trapping volume” for particles downstream of the filter 344a. As seen in FIG. 3E, the filter 344a can circumferentially surround the interior open volume 344iv such that, in use, the droplet material in the form of a fluid enters the filter 344a circumferentially. The clogresistant droplet generator 310 of FIG. 3D can be operated in an orientation relative to gravity with gravity in the direction shown by the arrow G, which results in the second opening 314-02 being perpendicular to the direction of gravity. Other orientations can be used if desired. For example, if during operation of the clog -resistant droplet generator 310, gravity is downward in the plane of the figure, then dead volume 336 would also serve as a gravitationally downward dead volume. Note that this occurs also in the implementation of FIG. 3D discussed above and FIG. 3F discussed below.

[0047] With specific reference to FIG. 3F, a cross section of still another implementation of a clogresistant droplet generator 310 is shown, with the filter 344 again in the form of a cylindrical filter 344a, positioned in-line between the first fluid cavity end 341-fe and the second fluid cavity end 341-se. The cylindrical filter 344b has a central opening or inner open volume 344iv extending axially completely through the filter 344b, with the tube 314 extending completely through the central opening or inner open volume 344iv and the second opening 314-02 positioned outside of the central opening or inner open volume 344iv, effectively between the filter 344b and the first end 341-fe of the fluid cavity 341. As an alternative, the second opening 314-02 could also be positioned inside of the filter 344b.

[0048] With the tube 314 extending completely through the central opening of the filter 344b and the second opening 314-02 being positioned outside of the central opening of the filter 344b, an initial flow path IFP exists (with an initial portion of the path indicated by the arrow in the figure), extending from the filter 344b or the downstream surface DS thereof to the to the dead volume 336 without encompassing or passing across or near to the second opening 314-02.

[0049] The clog -resistant droplet generator 310 of FIG. 3F can be operated in an orientation relative to gravity with gravity in the direction shown by the arrow G, which results in the second opening 314-02 being perpendicular to the direction of gravity. Other orientations can also be used if desired.

[0050] In the implementation of FIG. 3F, droplet material flowing from the filter 344 to the second opening 314-02 of the tube 314 is forced to make a relatively sharp turn into the second opening 314- 02 of approximately 180 degrees, or in other words, when the clog -resistant droplet generator 310 is in use, a droplet material flowing within the fluid cavity makes a turn of about 180 degrees to enter the second opening 314-02 of the tube 314. Having the second opening 314-02 positioned to require the droplet material to make a relatively sharp turn can provide some additional protection against clogging by causing particles with sufficient momentum to effectively “miss the turn” and not go into the second opening 314-02. Relatively sharp turns of less than about 180 degrees, such as turns of about 90 degrees or greater, 120 degrees or greater, 135 degrees or greater, or 150 degrees or greater, for example, imposed on the droplet material entering the second opening 314-02 in other implementations, can provide at least some of the same effect.

[0051] FIG. 3G is a cross section of one more implementation of a clog-resistant droplet generator 310, again with the filter 344 in the form of a cylindrical filter 344a with a central opening extending axially (completely) through the filter 344a. The filter 344a is positioned in-line with the first fluid cavity end 341-fe, but the second fluid cavity end 341-se is not positioned in-line with the filter 341a and the first fluid cavity end 341-fe, but is positioned after a bend in the fluid cavity 341, at the lower right of FIG. 3G, allowing for a first dead volume 336a in the form of a gravitationally downward dead volume 337a (when the clog-resistant droplet generator 310 is operated with orientation relative to gravity as shown) extending below the bend in the fluid cavity 341 at the bottom of the figure. A second dead volume 336b in the form of a gravitationally upward dead volume 337b is provided above the filter 344a (that is, between the filter 344a and the first fluid cavity end 314-fe). A first initial flow path IFP-1 exists from the filter 344a to the first dead volume 336a (gravitationally downward dead volume 337a) without encompassing or passing across or near to the second opening314-02. Similarly, a second initial flow path IFP-2 exists from the filter 344a to the second dead volume 336b (gravitationally upward dead volume 337b) without encompassing or passing across or near to the second opening 314-02. A third dead volume 336c surrounds the portion of the tube 314 that extends into the downstream volume VD.

[0052] FIG. 4 is a diagram of an EUV light source 400 in use with a photolithography apparatus 471. The EUV light source 400 can be an EUV light source such as EUV light source 100 of FIG. 1, or any other EUV source, utilizing a clog-resistant droplet generator (not shown) such as any of the implementations of the clog resistant droplet generator 310 of FIGS. 3A-3G. The lithography exposure apparatus 471 receives EUV light 455 produced by the EUV light source 400 and reflects it in one or more illumination mirrors 472 so as to illuminate a reflective pattern or reticle 473. EUV light reflected from the pattern or reticle 473 is further reflected and reduced by one or more reducing mirrors 474 and irradiated on a substrate or wafer 475 (or on one or more photosensitive layers on the substrate or wafer 475, not shown) to allow the formation of patterned structures in or on the substrate or wafer 475. The use of a clog-resistant droplet generator in the EUV source 400 allows for increased uptime in the EUV source 400 and the associated photolithography apparatus 471.

[0053] Implementations can be further described in the following numbered clauses:1. A droplet generator for an EUV light source includes a fluid cavity within a structure, the fluid cavity having a first fluid cavity end being open or otherwise capable of receiving a fluid and a second fluid cavity end; a particle filter within the fluid cavity dividing the fluid cavity into an upstream volume between an upstream surface of the particle filter and the first fluid cavity end, a filter volume occupied by the filter, and a downstream volume downstream of a downstream surface of the filter; and a tube having a first opening positioned outside the fluid cavity, the tube extending from the first opening through the second fluid cavity end to a second opening within the downstream volume, the tube extending into the downstream volume by more than 10% of a distance from the first opening to the second opening.2. The droplet generator of clause 1, wherein the tube extends into the downstream volume by more than 20% of the distance from the first opening to the second opening.3. The droplet generator of clause 1, wherein the tube extends into the downstream volume by more than 30% of the distance from the first opening to the second opening.4. The droplet generator of clause 1, wherein a distance from the second opening to any surface other than a surface of the tube is at least one millimeter (1 mm).5. The droplet generator of clause 1, wherein a distance from the second opening to any surface other than a surface of the tube is at least 3 mm.6. The droplet generator of clause 1, wherein the downstream volume includes at least one dead volume.7. The droplet generator of clause 6, wherein the downstream volume includes at least two dead volumes.8. The droplet generator of clause 1, wherein the downstream volume includes a dead volume positioned to be gravitationally below a fluid flow path when the droplet generator is in operation.9. The droplet generator of clause 8, wherein the downstream volume includes a dead volume positioned to be gravitationally above a fluid flow when the droplet generator is in operation.10. The droplet generator of clause 1, wherein the downstream volume includes a dead volume positioned to be gravitationally above a fluid flow when the droplet generator is in operation.11. The droplet generator of clause 1, wherein the second opening of the tube is oriented to be essentially perpendicular to gravity when the droplet generator is in operation.12. The droplet generator of clause 11, wherein the tube has at least one bend positioned within the downstream volume.13. The droplet generator of clause 1, wherein the tube has at least one bend positioned within the downstream volume.14. The droplet generator of clause 1, wherein the downstream volume includes one or more respective dead volumes, and wherein one or more initial-fill flow paths extend from the filter to the one or more respective dead volumes, and wherein the one or more initial-fill flow paths do not encompass the second opening of the tube.15. The droplet generator of clause 1, wherein the filter is positioned in-line between the first fluid cavity end and the second fluid cavity end.16. The droplet generator of clause 1, wherein the filter is a cylindrical filter with a central opening extending axially at least partially through the filter.17. The droplet generator of clause 16, wherein the second opening of the tube is positioned within the central opening of the filter.18. The droplet generator of clause 1, wherein the filter is a cylindrical filter with a central opening extending axially through the filter.19. The droplet generator of clause 18, wherein the second opening of the tube is positioned within the central opening of the filter.20. The droplet generator of clause 18, wherein the tube extends completely through the central opening of the filter, with the second opening positioned outside of the central opening of the filter.21. The droplet generator of clause 18, wherein when in use a droplet material flowing within the fluid cavity makes a turn of about 90 degrees or greater to enter the second opening of the tube.22. The droplet generator of clause 18, wherein when in use a droplet material flowing within the fluid cavity makes a turn of about 180 degrees or to enter the second opening of the tube.23. A droplet generator for an EUV light source includes a filter having an inner volume, the filter configured to prevent particles in a fluid from passing through the filter to the inner volume; and a tube for delivery of droplets of the fluid by the droplet generator, the tube having an upstream tube opening extending into the inner volume of the filter, whereby the fluid flows through the filter andinto the upstream tube opening and the particles are prevented from entering the upstream tube opening.24. The droplet generator of clause 23, wherein the tube extends to proximate an upstream end of the fdter. 25. The droplet generator of clause 23, the tube further including an upstream end portion with an outer diameter that is smaller than an inner diameter of the inner volume, the outer diameter at least partially forming a trapping volume for particles downstream of the fdter.26. The droplet generator of clause 23, wherein the fdter circumferentially surrounds the inner volume and the fluid enters the fdter circumferentially.

[0054] The above-described implementations and other implementations are within the scope of the following claims.

Claims

CLAIMS1. A droplet generator for an EUV light source, the droplet generator comprising: a fluid cavity within a structure, the fluid cavity having a first fluid cavity end being open or otherwise capable of receiving a fluid and a second fluid cavity end; a particle filter within the fluid cavity dividing the fluid cavity into an upstream volume between an upstream surface of the particle filter and the first fluid cavity end, a filter volume occupied by the filter, and a downstream volume downstream of a downstream surface of the filter; and a tube having a first opening positioned outside the fluid cavity, the tube extending from the first opening through the second fluid cavity end to a second opening within the downstream volume, the tube extending into the downstream volume by more than 10% of a distance from the first opening to the second opening.

2. The droplet generator of claim 1, wherein the tube extends into the downstream volume by more than 20% of the distance from the first opening to the second opening.

3. The droplet generator of claim 1, wherein the tube extends into the downstream volume by more than 30% of the distance from the first opening to the second opening.

4. The droplet generator of claim 1, wherein a distance from the second opening to any surface other than a surface of the tube is at least one millimeter (1 mm).

5. The droplet generator of claim 1, wherein a distance from the second opening to any surface other than a surface of the tube is at least 3 mm.

6. The droplet generator of claim 1, wherein the downstream volume includes at least one dead volume.

7. The droplet generator of claim 6, wherein the downstream volume includes at least two dead volumes.

8. The droplet generator of claim 1, wherein the downstream volume includes a dead volume positioned to be gravitationally below a fluid flow path when the droplet generator is in operation.

9. The droplet generator of claim 8, wherein the downstream volume includes a dead volume positioned to be gravitationally above a fluid flow when the droplet generator is in operation.

10. The droplet generator of claim 1, wherein the downstream volume includes a dead volume positioned to be gravitationally above a fluid flow when the droplet generator is in operation.

11. The droplet generator of claim 1, wherein the second opening of the tube is oriented to be essentially perpendicular to gravity when the droplet generator is in operation.

12. The droplet generator of claim 11, wherein the tube has at least one bend positioned within the downstream volume.

13. The droplet generator of claim 1, wherein the tube has at least one bend positioned within the downstream volume.

14. The droplet generator of claim 1, wherein the downstream volume includes one or more respective dead volumes, and wherein one or more initial-fdl flow paths extend from the fdter to the one or more respective dead volumes, and wherein the one or more initial-fdl flow paths do not encompass the second opening of the tube.

15. The droplet generator of claim 1, wherein the fdter is positioned in-line between the first fluid cavity end and the second fluid cavity end.

16. The droplet generator of claim 1, wherein the fdter is a cylindrical fdter with a central opening extending axially at least partially through the fdter.

17. The droplet generator of claim 16, wherein the second opening of the tube is positioned within the central opening of the fdter.

18. The droplet generator of claim 1, wherein the fdter is a cylindrical fdter with a central opening extending axially through the fdter.

19. The droplet generator of claim 18, wherein the second opening of the tube is positioned within the central opening of the fdter.

20. The droplet generator of claim 18, wherein the tube extends completely through the central opening of the fdter, with the second opening positioned outside of the central opening of the fdter.

21. The droplet generator of claim 18, wherein when in use a droplet material flowing within the fluid cavity makes a turn of about 90 degrees or greater to enter the second opening of the tube.

22. The droplet generator of claim 18, wherein when in use a droplet material flowing within the fluid cavity makes a turn of about 180 degrees or to enter the second opening of the tube.

23. A droplet generator for an EUV light source, the droplet generator comprising: a fdter having an inner volume, the fdter configured to prevent particles in a fluid from passing through the filter to the inner volume; and a tube for delivery of droplets of the fluid by the droplet generator, the tube having an upstream tube opening extending into the inner volume of the filter, whereby the fluid flows through the filter and into the upstream tube opening and the particles are prevented from entering the upstream tube opening.

24. The droplet generator of claim 23, wherein the tube extends to proximate an upstream end of the filter.

25. The droplet generator of claim 23, the tube further comprising an upstream end portion with an outer diameter that is smaller than an inner diameter of the inner volume, the outer diameter at least partially forming a trapping volume for particles downstream of the filter.

26. The droplet generator of claim 23, wherein the filter circumferentially surrounds the inner volume and the fluid enters the filter circumferentially.

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