INTRODUCTION OF EUV TARGET MATERIAL INTO AN EUY CHAMBER

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of U.S. Application No. 62/736.651 which was filed on September 26, 2018 and titled Appar atus for and Method of Controlling Introduction of EUV Target Material into an EUV Chamber, and U.S. Application No. 62/901,340, filed September 17, 2019 and titled Apparatus for and Method of Controlling Introduction of EUV Target Material into an EUV Chamber, both of which are incorporated herein in their entireties by reference.

FIELD

[0002] The present application relates to extreme ultraviolet (“EUV” ) light sources and their methods of operation. These light sources provide EUV light by creating plasma from a source or target material. In one application, the EUV light may be collected and used in a photolithography process to produce semiconductor integrated circuits.

BACKGROUND

[0003] A patterned beam of EUV light can be used to expose a resist coated substrate, such as a silicon wafer, to produce extremely small features in the substrate. EUV light (also sometimes referred to as soft x-rays) is generally defined as electromagnetic radiation having wavelengths in the range of about 5 nm to about 100 nm. One particular wavelength of interest for photolithography is 13.5 nm.

[0004] Methods to produce EUV light include, but are not necessarily limited to, converting a source material into a plasma state that has a chemical element with an emission line in the EUV range. These elements can include, but are not limited to, xenon, lithium and tin.

[0005] In one such method, often termed laser produced plasma (“LPP”), the desired plasma can be produced by irradiating a source material, for example, in the form of a droplet, stream, or wire, with a laser beam. In another method, often termed discharge produced plasma (“DPP”), the required plasma can be generated by positioning source material having an appropriate emission line between a pair of electrodes and causing an electrical discharge to occur between the electrodes. [0006] One technique for generating droplets involves melting a target material, also sometimes referred to as a source material, such as tin and then forcing it under high pressure through a relatively small diameter orifice in a droplet generator, such as an orifice in a nozzle of a droplet generator having a diameter of about 0.1 pm to about 30 pm, to produce a fluid jet. Under most conditions, the jet will break up into droplets due to a hydrodynamic instability commonly known as the Rayleigh-Plateau instability. These droplets may have varying velocities and may combine with each other to coalesce into larger droplets.

[0007] There may be issues with frequently stopping and restarting a droplet generator including nozzle clogging and droplet instability on start up. Thus, typically, the droplet generator is configured to generate droplets continuously. There are, however, times when the droplet generator is generating superfluous droplets that are not needed for EUV production. Times when droplets may be generated but not used include when the droplet generator is starting up or shutting down. Droplet generation in this time can result in deposition of tin (“tin writing") on the walls of the EUV vessel. Another time when unused droplets may be generated is during the tuning of the droplet generator. Droplets may be generated but not used during source idle time, that is, when the laser used to convert the target material is not being fired on the droplets. Unused droplets may also be generated during source operation when not producing plasma (e.g., between bursts, between lots and between wafers). In all, there are circumstances in which it is possible that a majority of the droplets generated by the droplet generator are not needed or used to generate a plasma.

[0008] Ideally the unused droplets traverse the vessel and land in a receptacle called a tin catcher that catches the droplets without permitting any backsplash. During droplet generator startup or shutdown however, droplets may miss the tin catcher entrance and splash on the walls that are close to the optics in the EUV chamber such as the collector, thus causing collector degradation. Also, after an extended period of operation the tin catcher may degrade, permitting significant backsplash to occur, further contributing to collector degradation. Thus, despite countermeasures, unused droplets permitted to enter the EUV chamber in an uncontrolled fashion have the potential to contaminate surfaces in the EUV vessel and of the collector. There thus remains a need to abate this problem.

SUMMARY

[0009] The following presents a simplified summary of one or more embodiments in order to provide a basic understanding of the embodiments. This summary is not an extensive overview of all contemplated embodiments, and is not intended to identify key or critical elements of all embodiments nor delineate the scope of any or all embodiments. Its sole purpose is to present some concepts of one or more embodiments in a simplified form as a prelude to the more detailed description that is presented later.

[0010] According to one aspect of an embodiment, target material contamination issues are alleviated by diverting droplets which will not be used for EUV production from entering the EUV vessel at all.

[0011] According to one aspect of an embodiment there is disclosed an apparatus comprising a vacuum chamber, an optical element positioned within the vacuum chamber, the optical element having a primary focus within the vacuum chamber, a target material dispenser for dispensing a stream of target material to an irradiation site at the primary focus in the vacuum chamber, a target material diverter positioned inside or outside of the vacuum chamber and arranged to seleetably divert selected portions of target material so that the selected portions of target material dispensed by the target material dispenser are diverted from entering the vacuum chamber. The optical element may be a collector mirror. The target material dispenser may have a nozzle and be for providing target material to the irradiation site in the form of stream of droplets released by the nozzle. The target material diverter may be arranged to divert a portion of the stream at a position between a release point of the stream from the nozzle and an entry point of the stream into the vacuum chamber. The target material diverter may be arranged to expel a jet of gas in a direction transverse to a direction of travel of the stream. The gas may be any one of a number of gases, including, e.g., hydrogen and the jet may be subsonic or supersonic. The target material diverter may comprise a deflector seleetably positionable in a path of the stream. The target material diverter may comprise a cond uctive element seleetably connected to an electrical source and arranged to place an electrical charge on droplets in the stream when connected to the electrical source. The target material diverter may comprise a laser arranged to divert the selected droplets by partial or complete ablation. The target material diverter may be part of the target material dispenser. The stream of target material may be partially a substream of uncoalesced droplets and the target material diverter may be arranged to seleetably divert selected portions of target material in the substream.

[0012] According to another aspect of an embodiment there is disclosed an apparatus comprising a receptacle for holding EUV target material, a nozzle in fluid communication with the receptacle for dispensing the target material along a first path, and a target material diverter arranged adjacent to the nozzle to seleetably divert selected portions of target material dispensed through the nozzle so that the selected portions of target material are diverted along a second path. The target material diverter may be arranged to expel a jet of gas in a direction transverse to a direction of travel of the stream. The gas may be hydrogen. The jet may be subsonic or supersonic. The target material diverter may comprise a deflector selectably positionable in a path of the stream. The target material diverter may comprise a conductive element selectably connected to an electrical source and arranged to place an electrical charge on droplets in the stream when connected to the electrical source. The target material diverter may comprise a laser arranged to deflect or vaporize selected droplets. The diverter may comprise of a plate or plates with an electrical charge that divert the droplet flow. The plate charge may be quickly controlled on and off or variably so that the droplets may be diverted on and off or be slowly steered away.

[0013] According to another aspect of an embodiment there is disclosed a method of using a target material deflector to control introduction of target material into a vacuum chamber by a target material dispenser comprising the steps of enabling the target material deflector, starting the target material dispenser to dispense target material, the dispensed target material being deflected by the deflector so that the dispensed target material does not enter the vacuum chamber, and disabling the target material deflector so that dispensed target material enters the vacuum chamber. The step of enabling the target material deflector may comprise causing the target material deflector to expel a jet of gas in a direction transverse to a direction of travel of the target material. The gas may be hydrogen. The jet may be subsonic or supersonic. The step of enabling the target material deflector may comprise moving a deflector into a path of the target material. The step of enabling the target material deflector may comprise placing an electrical charge on droplets in the stream. The step of enabling the target material deflector may comprise vaporizing the droplets.

[0014] According to another aspect of an embodiment there is disclosed an apparatus comprising a vacuum chamber, an optical element positioned within the vacuum chamber, the optical element having a primary focus within the vacuum chamber, a target material dispenser for dispensing a stream of target material, a target material aperture system comprising structure defining an exit aperture and a diverting structure, and at least one actuator coupled to the target material dispenser and arranged to have a first position in which the stream of target material passes through the exit aperture towards the irradiation region and a second position in which the stream of target material passes through the diverting structure so that the stream of target material is diverted from entering the irradiation region. The at least one actuator may comprise a piezo electric actuator. The target material dispenser may have a nozzle and the target material aperture system may be positioned between a release point of the stream from the nozzle and an entry point of the stream into the vacuum chamber. The apparatus may further comprise a target material collection receptacle arranged to collect target material passing through the diverting aperture.

[0015] According to another aspect of an embodiment there is disclosed an apparatus comprising a source of EUV target material, a nozzle in fluid communication with the source for dispensing a stream of droplets of the EUV target material, a target material aperture system comprising structure defining an exit aperture and a diverting aperture, and a target material steering system arranged to steer the nozzle so that the stream travels along a first path through the exit aperture or a second path through the diverting aperture. The target material steering system may comprise at least one actuator. The at least one actuator may comprises a piezo electric actuator. The target material aperture system may be positioned between a release point of the stream from the nozzle and an entry point of the stream into the irradiation region. The apparatus may further comprise a target material collection receptacle arranged to collect target material passing through the diverting aperture.

[0016] According to another aspect of an embodiment there is disclosed a method of using a target material deflector to control introduction of target material into an irradiation region by a target material dispenser, the method comprising the steps of providing a target material aperture system comprising structure defining an exit aperture and a diverting aperture, and steering the target material dispenser to direct target material along a first path through the exit aperture or a second path through the diverting aperture. The step of steering the target material dispenser may comprise controlling at least one actuator coupled to the target material dispenser. The step of steering the target material dispenser may comprise controlling at least one piezo electric actuator coupled to the target material dispenser. The target material dispenser may comprise a nozzle and the step of providing a target material aperture system may comprise positioning the target material aperture system between a release point of the nozzle and an entry point of the target material into the irradiation region. The method may further comprise a step of collecting target material that has passed through the exit aperture in a target material collection receptacle.

[0017] Further embodiments, features, and advantages of the present invention, as well as the structure and operation of the various embodiments are described in detail below with reference to accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the methods and systems of embodiments of the invention by way of example, and not by way of limitation. Together with the detailed description, the drawings further serve to explain the principles of and to enable a person skilled in the relevant art(s) to make and use the methods and systems presented herein. In the drawings, like reference numbers indicate identical or functionally similar elements. The drawings are not necessarily to- scale.

[0019] FIG. l i a simplified schematic view' of an EUV light source coupled with an exposure device.

[0020] FIG. 1A is a simplified, schematic diagram of an apparatus including an EUV light source having an LPP EUV light radiator.

[0021] FIG. 2 is a schematic diagram of a droplet generation subsystem for an EUV light source.

[0022] FIGS. 3, 3A, and 3B illustrate several different techniques for coupling one or more electro-actuatable element(s) with a fluid to create a disturbance in a stream exiting an orifice;

[0023] FIG. 4 is a not-to-scale diagram of an arrangement for controlling introduction of EUV target material into an EUV chamber according to one aspect of an embodiment.

[0024] FIGS. 5 and 5 A are not-to-scale diagrams of an arrangement for controlling introduction of EUV target material into an EUV chamber according to one aspect of an embodiment.

[0025] FIGS. 6 and 6A are not-to-scale diagrams of an arrangement for controlling introduction of EUV target material into an EUV chamber according to one aspect of an embodiment.

[0026] FIG. 7 is a not-to-scale diagram of an arrangement for controlling introduction of EUV target material into an EUV chamber according to one aspect of an embodiment.

[0027] FIGS. BA and SB are flowcharts of methods of controlling introduction of EUV target material into an EUV chamber according to one aspect of an embodiment.

[0028] FIGS. 9 and 9A are not-to-scale diagrams of operation of an arrangement for controlling introduction of EUV target material into an EUV chamber according to one aspect of an embodiment. [0029] Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. It is noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art based on the teachings contained herein.

DF.TAIT .F.D DESCRIPTION

[0030] Various embodiments are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to promote a thorough understanding of one or more embodiments. It may be evident in some or all instances, however, that any embodiment described below can be practiced without adopting the specific design details described below. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate description of one or more embodiments. The following presents is not an extensive overview of all contemplated embodiments, and is not intended to identify key or critical elements of all embodiments nor delineate the scope of any or all embodiments.

[0031] With initial reference to FIG. 1, there is shown a simplified, schematic, sectional view of selected portions of one example of an EUV photolithography apparatus, generally designated 10". The apparatus 10" may be used, for example, to expose a substrate 11 such as a resist coated wafer with a patterned beam of EUV light. For the apparatus 10", an exposure device 12" utilizing EUV light, (e.g., an integrated circuit lithography tool such as a stepper, scanner, step and scan system, direct write system, device using a contact and/or proximity mask, etc.), may be provided having one or more optics 13a, 13b, for example, to illuminate a patterning optic 13c with a beam of EUV light, such as a reticle, to produce a patterned beam, and one or more reduction projection optic(s) 13d, 13e, for projecting the patterned beam onto the substrate 11. A mechanical assembly (not shown) may be provided for generating a controlled relative movement between the substrate 11 and patterning means 13c. As further shown in FIG. 1, the apparatus 10" may include an EUV light source 20" including an EUV light radiator 22 emitting EUV light in a chamber 26" that is reflected by optic 24 along a path into the exposure device 12" to irradiate the substrate 1 l .The illumination system may include various types of optical components, such as refractive, reflective, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

[0032] As used herein, the term“optic'’ and its derivatives is meant to be broadly construed to include, and not necessarily be limited to, one or more components which reflect and/or transmit and/or operate on incident light, and includes, but is not limited to, one or more lenses, windows, filters, wedges, prisms, grisms, gratings, transmission fibers, etalons, diffusers, homogenizers, detectors and other instrument components, apertures, axicons and mirrors including multi-layer mirrors, near-normal incidence mirrors, grazing incidence mirrors, specular reflectors, diffuse reflectors and combinations thereof. Moreover, unless otherwise specified, neither the term“optic” nor its derivatives, as used herein, are meant to be limited to components which operate solely or to advantage within one or more specific wavelength range(s) such as at the EUV output light wavelength, the irradiation laser wavelength, a wavelength suitable for metrology or any other specific wavelength.

[0033] FIG. 1A illustrates a specific example of an apparatus 10” including an EUV light source 20 having an LPP EUV light radiator. As shown, the EUV light source 20 may include a system 21 for generating a train of light pulses and delivering the light pulses into a light source chamber 26. For the apparatus 10, the light pulses may travel along one or more beam paths from the system 21 and into the chamber 26 to illuminate source material at an irradiation region 48 to produce an EUV light output for substrate exposure in the exposure device 12.

[0034] Suitable lasers for use in the system 21 shown in FIG. 1 A, may include a pulsed laser device, e.g., a pulsed gas discharge CO2 laser device producing radiation at 9.3 pm or 10.6 pm, e.g., with DC or RF excitation, operating at relatively high power, e.g., 10 kW or higher and high pulse repetition rate, e.g., 50 kHz or more. In one particular implementation, the laser may be an axial-flow RF-pumped CO2 laser having an oscillator-amplifier configuration (e.g., master osci I la tor/power amplifier (MOP A) or pow¾r oscillator/power amplifier (POPA)) with multiple stages of amplification and having a seed pulse that is initiated by a Q-switched oscillator with relatively low energy and high repetition rate, e.g., capable of 100 kHz operation. From the oscillator, the laser pulse may then be amplified, shaped and/or focused before reaching the irradiation region 48. Continuously pumped CO2 amplifiers may be used for the laser system 21 in some embodiments. Alternatively, the laser may be configured as a so-called“self-targeting” laser system in which the droplet serves as one mirror of the optical cavity. [0035] Depending on the application, other types of lasers may also be suitable, e.g., an excimer or molecular fluorine laser operating at high power and high pulse repetition rate. Other examples include a solid state laser, e.g., having a fiber, rod, slab, or disk-shaped active media, other laser architectures having one or more chambers, e.g., an oscillator chamber and one or more amplifying chambers (with the amplifying chambers in parallel or in series), a master oscillator/power oscillator (MOPO) arrangement, a master oscillator/power ring amplifier (MOPRA) arrangement, or a solid state laser that seeds one or more excimer, molecular fluorine or CO? amplifier or oscillator chambers, may be suitable. Other designs may be suitable.

[0036] In some instances, a source material may first be irradiated by a pre-pulse and thereafter irradiated by a main pulse. Pre-pulse and main pulse seeds may be generated by a single oscillator or two separate oscillators. In some setups, one or more common amplifiers may be used to amplify both the pre-pulse seed and main pulse seed. For other arrangements, separate amplifiers may be used to amplify the pre-pulse and main pulse seeds.

[0037] FIG. 1A also shows that the apparatus 10 may include a beam conditioning unit 50 having one or more optics for beam conditioning such as expanding, steering, and/or focusing the beam between the laser source system 21 and irradiation site 48. For example, a steering system, which may include one or more mirrors, prisms, lenses, etc., may be provided and arranged to steer the laser focal spot to different locations in the chamber 26. For example, the steering system may include a first flat mirror mounted on a tip-tilt actuator which may move the first mirror independently in two dimensions, and a second flat mirror mounted on a tip-tilt actuator which may move the second mirror independently in two dimensions. With this arrangement, the steering system may controllably move the focal spot in directions substantially orthogonal to the direction of beam propagation (beam axis).

[0038] The beam conditioning unit 50 may include a focusing assembly to focus the beam to the irradiation site 48 and adjust the position of the focal spot along the beam axis. For the focusing assembly, an optic, such as a focusing lens or mirror, may be used that is coupled to an actuator for movement in a direction along the beam axis to move the focal spot along the beam axis.

[0039] As further shown in FIG. 1A, the EUV light source 20 may also include a source material delivery system 90, e.g., delivering target or source material, such as tin droplets, into the interior of chamber 26 to an irradiation region or primary focus 48, where the droplets will interact with light pulses from the system 21, to ultimately produce plasma and generate an EUV emission to expose a substrate such as a resist coated wafer in the exposure device 12, More details regarding various droplet dispenser configurations and their relative advantages may be found for example in U.S. Pat. No. 7,872,245, issued on January 18, 201 1 , titled “Systems and Methods for Target Material Delivery in a Laser Produced Plasma EUV Light Source”, U.S. Pat. No. 7,405,416, issued on July 29, 2008, titled“Method and Apparatus For EUV Plasma Source Target Delivery”, and U.S. Pat. No. 7,372,056, issued on May 13, 2008, titled“LPP EUV Plasma Source Material Target Delivery System”, the contents of each of which are hereby incorporated by reference in their entirety.

[0040] The source material for producing an EUV light output for substrate exposure may include, but is not necessarily limited to, a material that includes tin, lithium, xenon or combinations thereof. The EUV emitting element, e.g., tin, lithium, xenon, etc., may be in the form of liquid droplets and/or solid particles contained within liquid droplets. For example, the element tin may be used as pure tin, as a tin compound, e.g., SnBq SnBr?, SnlD, as a tin alloy, e.g., tin-gallium alloys, tin-indium alloys, tin-indium-gallium alloys, or a combination thereof. Depending on the material used, the source material may be presented to the irradiation region at various temperatures including room temperature or near room temperature (e.g., tin alloys, SnBr.f). at an elevated temperature, (e.g., pure tin) or at temperatures below' room temperature, (e.g., S11H4), and in some cases, the material can be relatively volatile, e.g., SnBr4.

[0041] Continuing with reference to FIG. 1A, the apparatus 10 may also include an EUV controller 60, which may also include a drive laser control system 65 for controlling devices in the system 21 to thereby generate light pulses for delivery into the chamber 26, and/or for controlling movement of optics in the beam conditioning unit 50. The apparatus 10 may also include a droplet position detection system which may include one or more droplet imagers 70 that provide an output indicative of the position of one or more droplets, e.g., relative to the irradiation region 48. The imager(s) 70 may provide this output to a droplet position detection feedback system 62, which can, e.g., compute a droplet position and trajectory, from which a droplet error can be computed, e.g., on a droplet-by-droplet basis, or on average. The droplet error may then be provided as an input to the controller 60, which can, for example, provide a position, direction and/or timing correction signal to the system 21 to control laser trigger timing and/or to control movement of optics in the beam conditioning unit 50, e.g., to change the location and/or focal power of the light pulses being delivered to the irradiation region 48 in the chamber 26. Also for the EUV light source 20, the source material delivery system 90 may have a control system operable in response to a signal (which in some implementations may include the droplet error described above, or some quantity derived therefrom) from the controller 60, to e.g., modify the release point, initial droplet stream direction, droplet release timing and/or droplet modulation to correct for errors in the droplets arriving at the desired irradiation region 48.

[0042] Continuing with FIG. 1A, the apparatus 10 may also include an optic 24" such as a near-normal incidence collector mirror having a reflective surface in the form of a prolate spheroid (i.e., an ellipse rotated about its major axis) having, e.g., a graded multi-layer coating with alternating layers of molybdenum and silicon, and in some cases, one or more high temperature diffusion barrier layers, smoothing layers, capping layers and/or etch stop layers. FIG. 1A show's that the optic 24" may be formed with an aperture to allow' the light pulses generated by the system 21 to pass through and reach the irradiation region 48. As shown the optic 24" may be, e.g., a prolate spheroid mirror that has a first or primary focus PF within or near the irradiation region 48 and a second focus at a so-called intermediate region IF 40, where the EUV light may be output from the EUV light source 20 and input to an exposure device 12 utilizing EUV light, e.g., an integrated circuit lithography tool. It is to be appreciated that other optics may be used in place of the prolate spheroid mirror for collecting and directing light to an intermediate location for subsequent delivery to a device utilizing EUV light.

[0043] A buffer gas such as hydrogen, helium, argon or combinations thereof, may be introduced into, replenished and/or removed from the chamber 26. The buffer gas may be present in the chamber 26 during plasma discharge and may act to slow plasma created ions to reduce optic degradation and/or increase plasma efficiency. Alternatively, a magnetic field and/or electric field (not shown) may be used alone, or in combination with a buffer gas, to reduce fast ion damage.

[0044] FIG. 2 illustrates the droplet generation system in more detail. The source material delivery system 90 delivers droplets to an irradiation site / primary focus 48 within chamber 26. A waveform generator 230 provides a drive waveform to an eiectro-actuatabie element in the droplet generator 90 which induces a velocity perturbation into the droplet stream. Hie waveform generator operates under the control of a controller 250 least partially on the basis of data from a data processing module 252. The data processing module receives data from one or more detectors. In the example shown, the detectors include a camera 254 and a photodiode 256. The droplets are illuminated by one or more lasers 258. In this typical arrangement, the detectors detect / image droplets at a point in the stream where coalescence is expected to have occurred. Also, the detectors and lasers are arranged outside of the vacuum chamber 26 and view the stream through windows in the walls of vacuum chamber 26.

[0045] FIG. 3 illustrates the components of a simplified droplet source 92 in schematic format. As shown there, the droplet source 92 may include a reservoir 94 holding a fluid, e.g. molten tin, under pressure. Also shown, the reservoir 94 may be formed with an orifice 98 allowing the pressurized fluid 96 to flow through the orifice establishing a continuous stream 100 which subsequently breaks into a plurality of droplets 102a, b.

[0046] FIG. 3 shows a possible configuration for a droplet source 92 as part of the droplet generator 90. Droplet source 92 further includes a sub-system producing a disturbance in the fluid having an electro-actuatable element 104 that is operably coupled with the fluid 96 and a signal generator 106 driving the electro-actuatable element 104. FIGS. 3A-3B show various ways in which one or more electro-actuatable element(s) may be operably coupled with the fluid to create droplets. Beginning with FIG. 3A, an arrangement is shown in which the fluid is forced to flow from a reservoir 108 under pressure through a tube 1 10, e.g., capillary tube, having an inside diameter between about 0.2 mm to about 0.8 mm, and a length of about 10 mm to about 50 m, creating a continuous stream 112 exiting an orifice 114 of the tube 110 which subsequently breaks up into droplets 1 16a,b. It should be understood that the features will have other dimensions in other embodiments. As shown, an electro-actuatable element 1 18 may be coupled to the tube. For example, an electro-actuatable element may be coupled to the tube 1 10 to deflect the tube 110 and disturb the stream 112. FIG. 3B shows a similar arrangement having a reservoir 120, tube 122 and a pair of electro-actuatable elements 124, 126, each coupled to the tube 122 to deflect the tube 122 at a respective frequency. These and other arrangements are described in U.S. Patent No. 8,513,629, issued August 20, 2013, and incorporated by reference herein in its entirety.

[0047] The overall droplet coalescence process may be regarded as a succession of multiple subcoalescence steps or regimes evolving as a function of distance from the nozzle. For example, in a first regime, that is, when the target material first exits the orifice or nozzle, the target material is in the form of a velocity-perturbed laminar fluid jet. In a second regime, the fluid jet breaks up into a series of microdroplets having varying velocities. In the third regime, measured either in time of flight or by distance from the nozzle, the microdroplets coalesce into droplets of an intermediate size, referred to as subcoalesced droplets, having varying velocities with respect to one another. In the fourth regime the subcoalesced droplets coalesce into droplets having the desired final size. The number of subcoalescence steps can vary. The distance from the nozzle to the point at which the droplets reach their final coalesced state is the coalescence distance. Ideally, the coalescence distance of the droplets is as short as possible. When the droplets have coalesced into bigger droplets, they are less sensitive for source conditions such as hydrogen flow and ion impact.

[0048] As mentioned, not all of the droplets generated by the droplet generator are destined to be used in the production of EUV light. Some of the droplets that enter the EUV vessel, won Id not be converted. Droplets may be generated but not used when the droplet generator is starting up, during the tuning of the droplet generator, during source idle time, between bursts, between lots and between wafers, and at droplet generator shut down. These periods of generation of unused drops have different timescales. For example, idle times may last for time scales on the order of hours, whereas the time between bursts may be measured in milliseconds. For the reasons mentioned, measures must be taken to prevent the target material in these unused droplets from depositing in places inside the vessel w'here it can cause problems. For the most part, prior solutions have entailed diverting the target material away from the primary focus of the collector and catching it in a tin catcher. The present invention, however, provides for diverting the unused target material from entering the EUV vessel at all.

[0049] Superfluous droplets can be prevented from entering the vessel any one or combination of a number of different ways that will be explained in more detail below. As an example, the stream of superfluous droplets could be subjected to a transverse stream of gas that will move the stream laterally to a path on which they will not enter the vessel but will instead be caught outside of the vessel. Also, a deflection plate can be moved into the path of the superfluous droplets to deflect them to where they can be caught. The superfluous droplets may be charged using an electric field or by causing the droplets to fly through a plasma. Only droplets that are not going to be entering the vessel will be charged. The charge can be applied with a conductive ring around the target material stream. If the ring is charged, it causes the droplets that are generated to be charged. Microdroplets that are breaking up from the fluid jet that is still in contact with the droplet generator are charged because the electric field generated by the charged ring is attracting or repelling the surface charge on the tin jet that is still in contact with the nozzle. The charged droplets are later deflected by an electric field. The superfluous droplets may also be converted (i.e., deflected or evaporated) using a laser so that they do not enter the chamber.

[0050] Whatever arrangement is used, it is advantageous to carry out deflection close to (within a few' millimeters of) the droplet generator nozzle , w'here the droplets are still small and have not coalesced into larger droplets. These small droplets are more susceptible to perturbations. Also small droplets cause less splashing, so it is easier to catch them. Depending on the physical arrangement, the required lateral deflection of the path of the droplet from the target path should be in the range of about 3 mm to about 80 mm.

[0051] An arrangement such as that described herein has several advantages. There is no tin writing. Substantially less tin is introduced into the EUV vessel, which greatly reduces the requirements for tin contamination abatement within the vessel. For example, it decreases the operational requirements the tin catcher within the chamber must satisfy. In fact, it is possible that the need for a tin catcher within the vessel can be avoided entirely.

[0052] Describing these arrangements in more detail, FIG. 4 show's a droplet generator 92 including a nozzle 200 which emits a stream of droplets 210. The portion of the stream before it breaks up into droplets is not shown in this and subsequent figures for ease of illustration and comprehension. The stream of droplets 210 passes from the droplet generator 92 and through an aperture in a wall 28 of an EUV chamber or vessel 26. It will be understood that both the interior of the chamber 26 and the environment traversed by the droplets 210 within the droplet generator 92 are in a vacuum.

[0053] If it is desired that droplets 210 released by the nozzle 200 no longer reach the irradiation region in the interior of the chamber 26, a jet 220 expels a stream of gas in the direction of arrow 230 under the control of valve 240. The gas, for example, may be hydrogen. The jet 220 may be subsonic or supersonic. The gas causes the droplets 210 to be deflected so that they no longer pass into the interior portion of the chamber 26. Instead, the droplets become deflected droplets 250 which are caught in a tin catcher 260 which is then drained to a removed location through a drain 270. Both the droplet generator and the tin catcher inside the droplet generator must be inside the vacuum although not necessarily inside the vessel, that is, the conical volume through which the EUV light travels from the PF to the IF.

[0054] FIG. 5 show's an alternative embodiment in which a deflection plate is moved into the path of the droplets in order to prevent them from entering the chamber in an uncontrolled manner . More specifically, as shown in FIG. 5, the nozzle 200 releases the stream of droplets 210 which exit the droplet generator 92 and enter chamber 26. When it is desired to prevent the droplets 210 from entering the interior of the chamber 26, a deflection plate 280 is moved to block the stream of droplets 210 as shown in FIG. A. This causes the droplets 210 to be deflected and collected in a tin catcher 260 as described above. Also shown in FIGS. 5 and 5A are a droplet generator catcher 290 which can be moved from the position shown in FIG. 5 to the position shown in FIG. 5A to catch backsplattered tin. The droplet generator catcher 290 can be connected to a heater block (not shown) in droplet generator 92.

[0055] FIGS. 6 and 6A show an arrangement in which droplets are electrostatically deflected instead of mechanically deflected as in the above example. As shown in FIG. 6, a ring 300 is positioned around the stream of droplets 210. When it is desired that the droplets 210 reach the interior of chamber 26, the ring 300 is not connected to any source of charge. When it is desired, however, that the droplets 210 do not reach the interior of chamber 26, then the ring 300 is connected to a source of electrical charge 310 through a switch 320. This causes the droplets 210 passing through the ring 300 to acquire a charge. Then, when the droplets 210 pass through a standing electrical field produced by plates 330 they are deflected toward receptacle 260 as set forth above. The plates 330 could alternatively be used to generate a plasma which would place a charge on the droplets. As mentioned, microdroplets that are separating from the fluid jet that is still in electrical contact with the droplet generator are charged because the electric field generated by the charged ring attracts or repels the surface charge on the fluid jet. The charged droplets are deflected by the electric field.

[0056] FIG. 7 show's an arrangement in which a laser beam 400 is used to ablate or deflect, i.e„ disturb forward propagation of, droplets which are not needed in chamber 26. As above, a receptacle 260 can be positioned to catch products of the ablation or deflection which can be drained away through drain 270 to a removed location. The superfluous droplets may also be converted (i.e., deflected or evaporated) using the laser so that they do not enter into the chamber. This causes the droplet 210 to be deflected and collected in a tin catcher 260 as described above.

[0057] FIG. 8A is a flowchart showing a process by w'hich superfluous droplets are prevented from reaching the interior of the EUV chamber 26 in a droplet generator initiation protocol. In a first step S500 the deflector is activated. In a second step 510 droplet generation is started. Then, after an interval during which operation of the droplet generator become stable, the deflector is deactivated in step S520. Of course, if it is desired to prevent droplets from reaching the interior of the chamber 26 w hile the droplet generator 92 is in operation, then the procedure would be to activate the deflector and deactivate the deflector depending on whether droplets are needed in the chamber 26.

[0058] FIG. 8B is a flowchart showing a process by which superfluous droplets are prevented from reaching the interior of the EUV chamber 26 in a droplet generator depressurization / repressurization protocol. In a first step S550 the deflector is activated. In a second step S560 the droplet generator is depressurized. Then, at a later time the droplet generator is repressurized in step S570. Then, after an interval during which operation of the droplet generator become stable, the deflector is deactivated in step S580.

[0059] As noted above, unless the target material, still using tin as an example, is properly handled there is a possibility of“writing” the tin on the collector or of spraying the collector with low velocity tin. Such occurrences are not readily detectable within some systems. In other words, in some systems it is not readily ascertainable whether target material has been safely captured in the catch reservoir designed for that purpose, i.e. the tin catch, or whether tin is hitting the collector. Rather, operation proceeds on the assumption that reducing the target material pressure in the droplet generator stops the tin stream, and cameras in some systems can determine only whether or not the tin stream is present at the focus of the irradiating energy.

[0060] It is thus potentially advantageous in some applications to have the ability to positively cut off the tin stream at the droplet generator. This can be achieved, for example, through the use of a mechanical actuator such as a piezo electric actuator. The cutoff may be positioned in such a way as to physically block the tin stream after the actuator has redirected the tin stream. When deployed in conjunction with additional measures such as a droplet generator tin catch, one or more freeze valves, a plasma capillary cleaning system, and a piezo electric steering system, the stream of droplets may be stopped before pressure is reduced, and allows rapid resumption of the tin stream after pressure again reaches its nominal value. This can be accomplished in a few microseconds with very high precision.

[0061] FIGS. 9 and 9A show a system implementing these concepts in accordance with one aspect of an embodiment. In FIG. 9 a droplet generator nozzle and capillary 600 is arranged to generate a stream 610 of droplets of target material. The direction in which the droplet generator nozzle and capillary' 600 directs the steam 610 is determined by the actuation state of actuators 630 and 640, which may be piezo electric actuators, and are used to modulate, steer, and divert the stream 610. In the actuation state shown in FIG. 9 the actuators 630 and 640 are in a position in which the stream 610 is directed through the droplet generator exit aperture 620 and on to the irradiation region, and tin is in general not directed into the droplet generator tin catch 650. The pressure of the target material in the droplet generator nozzle and capillary' 600 is at its nominal operational value (full pressure). The actuators 630 and 640 may be controlled by a control signal C from, for example, EUV controller 60 (FIG. 1A).

[0062] If, however, it is desired that tin not be introduced into the irradiation region then one or both of the piezo electric actuators 630 and 640 can be actuated to aim the droplet generator nozzle and capillary 600 away from the droplet generator exit aperture 620. This is shown in FIG. 9A. As depicted, the actuators 630 and 640 are actuated to cause the tin stream 610 to enter a droplet generator diverting aperture 660 instead of the droplet generator exit aperture 620. After entering the droplet generator exit aperture 620 the tin accumulates in the droplet generator tin catch 650 as a mass 670 of collected tin. The pressure of the target material in the droplet generator nozzle and capillary 6(X) is permitted to reduce slowly.

[0063] Thus, in a system such as that just described, the introduction of superfluous tin into the irradiation region at times when tin that is not needed for plasma generation is avoided, thus reducing the possibility that such unneeded tin will contaminate surfaces inside the chamber including surfaces of the collector. At the same time, the droplet generator can be kept in a state of relative readiness from which it can rapidly transition to a fully operational state.

[0064] The foregoing discussion is in terms of using tin as a target material. It will, of course, be obvious to one of ordinary skill in the art that other target materials can be used and that tin is simply being used as an example.

[0065] The present invention has been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed.

[0066] The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying knowledge within the skill of the ait, readily modify and/or adapt for various applications such specific embodiments, without undue experimentation, without departing from the general concept of the present invention. Therefore such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by the skilled artisan in light of the teachings and guidance. The breadth and scope of the present invention should not be limited by any of the above- described exemplary embodiments, but should be defined only in accordance writh the following claims and their equivalents.

[0067] The implementations may further be described using the following clauses:

1. Apparatus comprising: a vacuum chamber;

an optical element positioned within the vacuum chamber, the optical element having a primary focus within the vacuum chamber;

a target material dispenser positioned outside of the vacuum chamber for dispensing a stream of target material to an irradiation site at the primary focus in the vacuum chamber; and a target material diverter arranged to selectably divert selected portions of target material so that the selected portions of target material dispensed by the target material dispenser are diverted from entering the vacuum chamber.

2. Apparatus as in clause 1 wherein the optical element is a collector mirror.

3. Apparatus as in clause 1 wherein the target material dispenser has a nozzle for providing the target material to the irradiation site in the form of a stream of droplets released by the nozzle.

4. Apparatus as in clause 1 wherein the target material diverter is arranged to divert a portion of the stream at a position between a release point of the stream from the nozzle and an entry point of the stream into irradiation site.

5. Apparatus as in clause 1 wherein the target material diverter is arranged to expel a jet of gas in a direction transverse to a direction of travel of the stream.

6. Apparatus as in clause 5 wherein the gas is hydrogen.

7. Apparatus as in clause 5 wherein the jet is subsonic.

8. Apparatus as in clause 5 wherein the jet is supersonic.

9. Apparatus as in clause 1 wherein the target material diverter comprises a deflector selectably positionable in a path of the stream.

10. Apparatus as in clause 1 wherein the target material diverter comprises a conductive element selectably connected to a source of charge and arranged to place an electrical charge on droplets in the stream when connected to the source.

1 1. Apparatus as in clause 1 wherein the target material diverter comprises a plasma and is arranged to place an electrical charge on droplets in the stream when the droplets in the stream pass through the plasma.

12. Apparatus as in clause 1 wherein the target material diverter comprises a laser arranged to vaporize selected droplets.

13. Apparatus as in clause 1 wherein the target material diverter comprises a laser arranged to deflect selected droplets.

14. Apparatus as in clause 1 wherein the target material diverter is part of the target material dispenser.

15. Apparatus as in clause 1 wherein the stream of target material is partially a substream of uncoalesced droplets and wherein the target material diverter is arranged to selectably divert selected portions of target material in the substream.

16. Apparatus comprising:

a receptacle for holding EUV target material;

nozzle in fluid communication with the receptacle for dispensing the target material along a first path; and

a target material diverter arranged adjacent the nozzle to selectably divert selected portions of target material dispensed through the nozzle so that the selected portions of target material are diverted along a second path.

17. Apparatus as in clause 16 wherein the target material travels in a stream along the first path and the target material diverter is arranged to expel a jet of gas in a direction transverse to a direction of travel of the stream.

18. Apparatus as in clause 17 wherein the gas is hydrogen.

19. Apparatus as in clause 17 wherein the jet is subsonic.

20. Apparatus as in clause 17 wherein the jet is supersonic.

21. Apparatus as in clause 16 wherein the target material diverter comprises a deflector selectably positionable in a path of the stream.

22. Apparatus as in clause 16 wherein the target material travels in a stream along the first path and the target material diverter comprises a conductive element selectably connected to an electrical source and arranged to place an electrical charge on droplets in the stream when connected to the electrical source.

23. Apparatus as in clause 16 wherein the target material diverter comprises a laser arranged to vaporize selected droplets.

24. Apparatus as in clause 16 wherein the target material diverter comprises a laser arranged to deflect selected droplets.

25. A method of using a target material deflector to control introduction of target material to an irradiation region by a target material dispenser, the method comprising the steps of: enabling the target material deflector;

starting the target material dispenser to dispense target material in a stream, the dispensed target material being deflected by the deflector so that the dispensed target material does not enter the irradiation region; and disabling the target material deflector so that dispensed target material enters the vacuum chamber.

26. A method as in clause 25 wherein the step of enabling the target material deflector comprises causing the target material deflector to expel a jet of gas in a direction transverse to a direction of travel of the target material.

27. A method as in clause 26 wherein the gas is hydrogen.

28. A method as in clause 26 wherein the jet is subsonic.

29. A method as in clause 26 wherein the jet is supersonic.

30. A method as in clause 25 wherein the step of enabling the target material deflector comprises moving a deflector into a path of the target material.

31. A method as in clause 25 wherein the step of enabling the target material deflector comprises placing an electrical charge on droplets in the stream.

32. A method as in clause 25 wherein the step of enabling the target material deflector comprises using a laser to vaporize the droplets.

33. A method as in clause 25 wherein the step of enabling the target material deflector comprises using a laser to deflect the droplets.

34. Apparatus comprising:

a vacuum chamber;

an optical element positioned within the vacuum chamber, the optical element having a primary focus within the vacuum chamber;

a target material dispenser for dispensing a stream of target material;

a target material aperture system comprising structure defining an exit aperture and a diverting structure; and

at least one actuator coupled to the target material dispenser and arranged to have a first position in which the stream of target material passes through the exit aperture towards the irradiation region and a second position in which the stream of target material passes through the diverting structure so that the stream of target material is diverted from entering the irradiation region.

35. Apparatus as in clause 34 wherein the at least one actuator comprises a piezo electric actuator.

36. Apparatus as in clause 34 wherein the target material dispenser has a nozzle and wherein the target material aperture system is positioned between a release point of the stream from the nozzle and an entry point of the stream into the vacuum chamber.

37. Apparatus as in clause 34 further comprising a target material collection receptacle arranged to collect target material passing through the diverting aperture.

38. Apparatus comprising:

a source of EUV target material;

a nozzle in fluid communication with the source for dispensing a stream of droplets of the EUV target material;

a target material aperture system comprising structure defining an exit aperture and a diverting aperture; and

a target material steering system arranged to steer the nozzle so that the stream travels along a first path through the exit aperture or a second path through the diverting aperture.

39. Apparatus as in clause 38 wherein the target material steering system comprises at least one actuator.

40. Apparatus as in clause 39 wherein the at least one actuator comprises a piezo electric actuator.

41. Apparatus as in clause 38 wherein the target material aperture system is positioned between a release point of the stream from the nozzle and an entry point of the stream into the irradiation region.

42. Apparatus as in clause 38 further comprising a target material collection receptacle arranged to collect target material passing through the diverting aperture.

43. A method of using a target material deflector to control introduction of target material into an irradiation region by a target material dispenser, the method comprising the steps of: providing a target material aperture system comprising structure defining an exit aperture and a diverting aperture; and

steering the target material dispenser to direct target material along a first path through the exit aperture or a second path through the diverting aperture.

44. A method as in clause 43 wherein the step of steering the target material dispenser comprises controlling at least one actuator coupled to the target material dispenser.

45. A method as in clause 43 wherein the step of steering the target material dispenser comprises controlling at least one piezo electric actuator coupled to the target material dispenser.

46. A method as in clause 43 wherein the target material dispenser comprises a nozzle and wherein providing a target material aperture system comprises positioning the target material aperture system between a release point of the nozzle and an entry point of the target material into the irradiation region. 77

47. A method as in clause 43 further comprising a step of collecting target material that has passed through the exit aperture in a target material collection receptacle.