Controlled droplet generator nozzle environment for improved reliability

Abstract

Disclosed are apparatus and methods for controlling the environment surrounding a nozzle orifice of a droplet generator in a radiation source system for an extreme ultraviolet (EUV) radiation source, wherein the amount of oxygen-containing gas in the nozzle orifice environment is controlled to reduce the formation of oxides that can potentially interfere with the operation of the droplet generator.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Application No. 63 / 431,481, entitled "Controlled Droplet Generator Nozzle Environment for Improved Reliability," filed December 9, 2022, the entire contents of which are incorporated herein by reference.

[0002]

[0002] The present disclosure relates to extreme ultraviolet generators that produce light by excitation of a radiation source material, and in particular to devices and systems and methods for distributing the radiation source material in such extreme ultraviolet generators. [Background technology]

[0003]

[0003] Extreme ultraviolet ("EUV") radiation, for example, electromagnetic radiation having a wavelength of approximately 50 nm or less (sometimes called soft x-rays) and including light with a wavelength of about 13 nm, is used in photolithography processes to create extremely small features in and on substrates, such as silicon wafers.

[0004]

[0004] Methods for generating EUV radiation include, but are not limited to, converting the physical state of a source material to a plasma state. The source material includes elements with emission lines in the EUV range, such as xenon, lithium, or tin. In one such method, often referred to as laser-produced plasma ("LPP"), the required plasma is generated by irradiating the source material, for example in the form of droplets or clusters, with an amplified laser beam, which may be referred to as a drive laser beam. For this process, the plasma is typically generated in a sealed vessel, such as a vacuum chamber, and monitored using various types of metrology equipment.

[0005]

[0005] EUV radiation can be produced in a process involving multiple steps in which a target (e.g., a droplet of radiation source material) is impinged by one or more conditioning pulses of radiation that condition or prepare the target before it reaches the emission site for an eventual phase transformation at the emission site. Conditioning in this context can include changing the shape of the droplets (e.g., flattening the droplets), or changing the distribution of the droplets (e.g., at least partially dispersing some of the droplets as a mist), or even a partial phase change. Pulses preceding the main heating pulse are called conditioning pulses and include pre-pulses, rarefaction pulses, and pedestal pulses, regardless of whether they are generated by the main drive laser or another laser.

[0006] One technique for generating droplets involves melting a target or source material, such as tin, and then forcing the liquid tin under high pressure through a relatively small orifice, such as one having a diameter of about 0.5 μm to about 30 μm, to generate a stream of droplets. Under most conditions, naturally occurring instabilities (e.g., noise) in the stream exiting the orifice will cause the stream to break up into tiny droplets in a process called Rayleigh breakup. These droplets may have different velocities and may combine with each other as they move through the stream, coalescing into larger droplets. For purposes of generating EUV light, the frequency of the droplets is controlled to match the frequency of the driving laser.

[0007]

[0007] A droplet generator places droplets to be irradiated for EUV generation at the primary focus of the collector mirror. The droplets must arrive at the primary focus within certain spatial and temporal stability criteria, i.e., at positions and timings that are repeatable within acceptable limits. They must also arrive at a given frequency and velocity. Furthermore, the droplets must be fully coalesced, which means that they must be monodisperse (uniform size) and reach a given drive frequency.

[0008]

[0008] The tiny orifices of the droplet generator nozzles can easily become clogged by by-products of the source material. For example, when tin is used as the source material, oxidation contaminants such as O2 gas and H2O vapor, or those from the native oxides on the wetted surface of the tin, can reach the tin inside the nozzle and form tin oxide (SnO X ) particles can form. Oxidation of the free surface of the tin can occur inside the nozzle (especially during cold restarts when the tin retreats due to contraction) or right at the orifice. These tin oxides do not melt at the operating temperatures of the droplet generator and can clog the nozzle. This can result in droplet generator start-up failures (failure to operate initially) and droplet generator cold restart failures (failure to restart the droplet generator after an idle period). Therefore, for at least some applications, it is advantageous to control the moisture and oxygen levels around the nozzle orifice.

[0009]

[0009] It is in these circumstances that the need for the presently disclosed subject matter arises. Summary of the Invention

[0010] The following is a brief summary intended to provide a basic understanding of embodiments. This summary is not an exhaustive overview of all contemplated embodiments, and is not intended to identify key or critical elements of any embodiment or to limit the scope of any embodiment. Its sole purpose is to present some concepts related to one or more embodiments as a prelude to the more detailed description that is presented later.

[0011] According to one aspect of an embodiment, a droplet generator system for an extreme ultraviolet (EUV) radiation source is disclosed, the droplet generator system including: a nozzle housing; a nozzle having an exit orifice positioned within the housing, the exit orifice being in selectable fluid communication with a source of liquid EUV radiation source material and adapted to deliver the EUV radiation source material into a chamber of the EUV radiation source; and a valve in fluid communication with an interior of the housing and the chamber, the valve having a first state in which the interior of the housing is in fluid communication with the chamber and a second state in which the interior of the housing is not in fluid communication with the chamber. The nozzle may be adapted to deliver the EUV radiation source material into the chamber along a path from the exit orifice through the valve, the valve being adapted to maintain the path in an open state when the valve is in the first state and to maintain the path in a closed state when the valve is in the second state.

[0012] The valve may be a gate valve. The valve may be adapted to transition between a first state and a second state in response to a control signal. The droplet generator system may further include a sensor positioned to detect whether the valve is in the first state or the second state. The droplet generator system may further include an inlet in fluid communication with the interior of the housing and adapted to be connected to a source of purge gas.

[0013]

[0013] The purge gas may comprise Ar+H2. The purge gas may comprise an inert gas. The inert gas may comprise at least one of nitrogen and argon. The droplet generator system may further comprise a purge gas purifier arranged to purify the purge gas.

[0014]

[0014] According to another aspect of the embodiment, a droplet generator for an extreme ultraviolet (EUV) radiation source is disclosed, comprising: a nozzle housing; a nozzle having an exit orifice positioned within the housing, the exit orifice being selectably fluidly connected to a source of liquid EUV radiation source material and adapted to transport the EUV radiation source material into a chamber of the EUV radiation source; a valve in fluid communication with the interior of the housing and the chamber, the valve having a first state in which the interior of the housing is fluidly connected to the chamber and a second state in which the interior of the housing is not fluidly connected to the chamber; a source of purge gas selectably fluidly connected to the interior of the housing; and a vacuum pump selectably fluidly connected to the interior of the housing.

[0015]

[0015] The vacuum pump may be a turbomolecular vacuum pump or a scroll vacuum pump. The droplet generator system may further include a high conductance conduit arranged to connect the interior of the housing to the turbomolecular vacuum pump.

[0016]

[0016] According to another aspect of the embodiment, an assembly for purging the environment of a nozzle outlet of a droplet generator in an extreme ultraviolet (EUV) radiation source is disclosed, the assembly including: a housing positioned around and surrounding the nozzle outlet except for an opening in a droplet path that transports droplets to a chamber of the EUV radiation source; a source of purge gas that is selectably fluidly connected to the interior of the housing; and a pump that is selectably fluidly connected to the interior of the housing and adapted to evacuate the housing.

[0017]

[0017] The purge gas may include Ar+H2. The purge gas may include an inert gas. The purge gas may include at least one of nitrogen and argon. The assembly may further include a purge gas purifier arranged to purify the purge gas.

[0018]

[0018] The pump may include a roughing pump. The pump may include a turbomolecular vacuum pump or a scroll vacuum pump. The pump may be fluidly connected to the interior of the housing by a high conductance conduit.

[0019] The assembly further includes a valve disposed in the droplet path, the valve having a first state in which the interior of the housing is fluidly connected with the chamber and a second state in which the interior of the housing is not fluidly connected with the chamber, and the purge gas source and pump are adapted to perform periodic purging including at least supplying purge gas to the interior of the housing when the droplet generator is depressurized and the valve is in the second state, and to perform continuous purging by supplying purge gas to the interior of the housing when the droplet generator is depressurized and the valve is in the first state. The valve may be a gate valve.

[0020]

[0020] According to another aspect of the embodiment, a method for controlling the environment of a nozzle of a droplet generator in an extreme ultraviolet (EUV) radiation source is disclosed, the method including: positioning the nozzle in a housing; determining whether the droplet generator is depressurized; and, when it is determined that the droplet generator is depressurized, sending a signal to activate a valve positioned to isolate the nozzle from a chamber of the EUV radiation source.

[0021] The method may further include, after sending a signal to actuate a valve to isolate the nozzle from a chamber of the EUV radiation source, determining whether the valve operated to isolate the nozzle from the chamber in response to the signal. If it is determined that the valve did not operate to isolate the nozzle from the chamber in response to the signal, the method may further include periodically purging the housing by alternating between supplying a purge gas to the housing and evacuating the housing.

[0022] The purge gas may include Ar+H2. The purge gas may include an inert gas. The purge gas may include at least one of nitrogen and argon. The method may further include purifying the purge gas before supplying it to the housing. The method may further include, upon determining that the valve did not operate to isolate the nozzle from the chamber in response to the signal, continuously supplying the purge gas to the housing to continuously purge the housing.

[0023] The purge gas may include Ar+H2. The purge gas may include an inert gas. The purge gas may include at least one of nitrogen and argon. The method may further include purifying the purge gas before supplying it to the housing. The method may further include evacuating the housing if it is determined that the valve did not operate to isolate the nozzle from the chamber in response to the signal. Evacuating the housing may be performed using a turbomolecular vacuum pump or a scroll vacuum pump.

[0024] The method may further include evacuating the housing after sending a signal to activate a valve positioned to isolate the nozzle from a chamber of the EUV radiation source.

[0025] Further embodiments, features, and advantages of the subject matter of the present disclosure, as well as the structure and operation of the various embodiments, are described in detail below with reference to the accompanying drawings. [Brief explanation of the drawings]

[0026]

[0026] The accompanying drawings, which are incorporated in and form a part of this specification, illustrate the presently disclosed subject matter and, together with the description, serve to further explain the principles of the presently disclosed subject matter and to enable those skilled in the art to make and use the presently disclosed subject matter.

[0027] [Figure 1]

[0027] FIG. 1 is a partially schematic functional block diagram illustrating the overall broad concept of a laser-produced plasma EUV radiation source system in which an aspect of an embodiment may be implemented. [Figure 2A]

[0028] FIG. 1 is a schematic diagram of a system for controlling the environment surrounding a nozzle orifice of a drop generator in accordance with an aspect of an embodiment. [Figure 2B]

[0029] FIG. 1 is a schematic diagram of a system for controlling the environment surrounding a nozzle orifice of a drop generator in accordance with an aspect of an embodiment. [Figure 3A]

[0030] FIG. 1 is a schematic diagram of a valve that may be used in a system for controlling the environment surrounding a nozzle orifice of a drop generator in accordance with an aspect of an embodiment. [Figure 3B]

[0030] A schematic diagram of a valve that can be used in a system for controlling the environment around a nozzle orifice of a droplet generator in accordance with one aspect of one embodiment. [Figure 4]

[0031] 1 is a flowchart illustrating steps in a process for controlling the environment surrounding a nozzle orifice of a drop generator in accordance with an aspect of an embodiment. [Figure 5]

[0032] 1 is a flowchart illustrating steps in a process for controlling the environment surrounding a nozzle orifice of a drop generator in accordance with an aspect of an embodiment. [Figure 6]

[0033] 1 is a flowchart illustrating steps in a process for controlling the environment surrounding a nozzle orifice of a drop generator in accordance with an aspect of an embodiment. [Figure 7]

[0034] 1 is a flowchart illustrating steps in a process for controlling the environment surrounding a nozzle orifice of a drop generator in accordance with an aspect of an embodiment. [Figure 8]

[0035] 1 is a flowchart illustrating steps in a process for controlling the environment surrounding a nozzle orifice of a drop generator in accordance with an aspect of an embodiment. [Figure 9]

[0036] 1 is a flowchart illustrating steps in a process for controlling the environment surrounding a nozzle orifice of a drop generator in accordance with an aspect of an embodiment.

[0028]

[0037] Further features and advantages of the disclosed apparatus and method, as well as the structure and operation of various embodiments of the disclosed apparatus, are described in detail below with reference to the accompanying drawings. The disclosed apparatus and method are not limited to the specific embodiments described herein. Such embodiments are shown herein for illustrative purposes only. Additional embodiments will be apparent to those skilled in the art based on the teachings herein. DETAILED DESCRIPTION OF THE INVENTION

[0029]

[0038] Various embodiments will now be described with reference to the drawings, wherein like reference numerals are used to refer to like components throughout the specification. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the embodiments. However, it will be apparent that in some or all cases, any of the embodiments described below can be practiced without employing the specific design details described below.

[0030]

[0039] 1 is a schematic diagram of an example of an EUV radiation source, e.g., a laser-produced plasma EUV radiation source 10. As shown, EUV radiation source 10 can include a pulsed or continuous laser source 22, such as a pulsed gas discharge CO2 laser source that produces a beam 22b of pulses of radiation, typically at a wavelength less than 20 μm, e.g., in the range of about 11 μm to about 9 μm or less. The pulsed gas discharge CO2 laser source can have DC or RF excitation operating at high power and high pulse repetition rates.

[0031]

[0040] The EUV radiation source 10 also includes a source material delivery system 24 for delivering the source material in the form of droplets or a continuous liquid stream. In this example, the source material is a liquid, but it could also be a solid, for example. The source material may be comprised of tin or a tin compound, although other materials could also be used. In the illustrated system, the source material delivery system 24 introduces droplets 14 of the source material to an irradiation region 28 within a vacuum chamber 26, where the droplets 14 can be irradiated to generate a plasma. Note that, as used herein, an irradiation region refers to an area where irradiation of the source material occurs, even if no irradiation is actually occurring. The EUV light source 10 also includes a beam focusing and steering system 32.

[0032]

[0041] In the illustrated example, the components are arranged so that the droplet 14 moves substantially horizontally. The direction from the laser source 22 toward the irradiation region 28, i.e., the nominal direction of propagation of the beam 22b, may be considered the Z-axis. The path that the droplet 14 follows from the source material delivery system 24 to the irradiation region 28 may be considered the X-axis. The perspective of FIG. 1 is therefore perpendicular to the XZ plane. Also, although a system is depicted in which the droplet 14 moves substantially horizontally, one skilled in the art will understand that other arrangements in which the droplet 14 moves vertically, or at an angle between 90 degrees (horizontal) and 0 degrees (vertical) with respect to gravity, may also be used.

[0033]

[0042] EUV radiation source 10 may also include an EUV light source controller system 60 and a laser firing control system 65, along with beam steering system 32. EUV radiation source 10 may also include a detector, such as a target position detection system, which may include, for example, one or more droplet imagers 70 that generate an output indicative of the absolute or relative position of the target droplets with respect to irradiation area 28 and provide this output to target position detection feedback system 62. Target position detection feedback system 62 may use the output of droplet imager 70 to calculate target position and target trajectory, from which target error can be calculated.

[0034]

[0043] The EUV radiation source 10 shown in FIG. 1 may also include a conditioning laser 23 for generating a conditioning beam 23b. This conditioning beam 23b is composed of pulses that condition the target for subsequent heating by the main drive pulse. The conditioning pulses can change the shape and distribution of the target. Such conditioning pulses include various pulses called pre-pulses, pedestal pulses, and rarefaction pulses. It is necessary for the conditioning pulses from the conditioning laser 23 to impinge on the target in an optimal manner. To this end, a laser beam steering system 32 functions to steer the conditioning beam 23b generated by the conditioning laser 23.

[0035]

[0044] As shown in FIG. 1 , the source material transport system 24 may include a target transport control system 90. The target transport control system 90 operates in response to a signal, e.g., a target error or some quantity derived from the target error, provided by the system controller 60, to adjust the path of the target 14 through the irradiation region 28. This may be achieved, for example, by repositioning the point at which the target transport mechanism 92 releases droplets 14. The droplet release point may be repositioned, for example, by tilting or moving the target transport mechanism 92. The target transport mechanism 92 preferably extends into the chamber 26, and the source material and gas are preferably supplied externally so that the source material within the target transport mechanism 92 is under pressure. The EUV radiation source 10 also includes a source material receiver 80 that captures and retains unvaporized source material to limit contamination from such source material.

[0036]

[0045] Continuing with reference to FIG. 1 , EUV radiation source 10 may also include one or more optical elements. In the following description, collector 30 is used as an example of such an optical element, but the description also applies to other optical elements. Collector 30 may be a normal-incidence reflector implemented, for example, as a multi-layer mirror (“MLM”) with an additional thin barrier layer, such as B4C, ZrC, Si3N4, or C, deposited at each layer interface to effectively block thermally induced interlayer diffusion. Collector 30 may take the form of an oblong ellipsoid with a central opening to allow beam 22b and conditioned beam 23b to pass through and reach illumination region 28. Collector 30 has a first focus at illumination region 28 and a second focus at a so-called midpoint 40 (also referred to as intermediate focus 40), where EUV radiation may be output from EUV radiation source 10 and input, for example, to an integrated circuit lithography scanner or stepper 50. An integrated circuit lithography scanner or stepper 50 uses this radiation to process a silicon wafer workpiece 52 in a known manner, for example, using a reticle or mask 54. The silicon wafer workpiece 52 is then further processed in a known manner to yield an integrated circuit device.

[0037]

[0046] Maintenance of the EUV radiation source 10 may require performing service operations during which the droplet generator is shut down, i.e., the droplet generator is not required to emit droplets of source material. These service operations include, for example, replacing the collector 30 in the EUV chamber 26, draining the source material receiver 80, or updating software or firmware. Another example of such a service operation is replacing the droplet generator. During such a replacement, all surfaces close to the nozzle are exposed to air and may re-adsorb water. Additionally, the droplet generator has numerous vacuum seals. The leak rate, outgassing (in the case of polymer seals), and oxygen permeability of these seals affect the partial pressure of water vapor and oxygen gas around the nozzle, i.e., in the nozzle environment. Other sources of oxygen-containing gases may also be present within the enclosure, such as tin catch seals.

[0038]

[0047] Therefore, during these service operations, a problem may arise in that oxygen in the nozzle orifice environment oxidizes the tin in and around the nozzle orifice to form tin oxides. The oxygen may be present as molecular oxygen or as part of an oxygen-containing gas such as water vapor. These tin oxides have melting points higher than the operating temperature of the droplet generator and therefore remain solid, which creates a risk that the tin oxides will clog the nozzle or interfere with the operation of the droplet generator.

[0039]

[0048] To mitigate this risk, according to one aspect of the embodiment, the droplet generator is maintained in a "hot isolation" state during periods when the droplet generator is not operating. In this state, the droplet generator is maintained at a reduced pressure but at or near its operating temperature, and the nozzle orifice environment is isolated from the main chamber where EUV radiation is generated. The droplet generator is maintained at an elevated temperature ("hot") to avoid oxide precipitation in the tin. Isolating the hot isolation state can be achieved, for example, by placing a valve, such as a gate valve, in the path of the droplets from the nozzle orifice to the main chamber. A gate valve, also known as a sluice valve, is a valve that opens by lifting a barrier (gate) out of the fluid path. Furthermore, the nozzle environment is contained within a sealed enclosure, which is sealed to allow the environment, or a combination of these means, to be purged or evacuated to reduce the amount of oxygen in the nozzle orifice environment. Here and elsewhere, "purging" means introducing a second gas to replace a first gas from the nozzle orifice environment, and "evacuating" means removing a gas from the nozzle orifice environment.

[0040]

[0049] It will be appreciated that a gate valve is only one example of a type of valve that can be used as a route valve as disclosed herein, and other types of valves, such as, for example, large diameter diaphragm valves, may also be used.

[0041]

[0050] For example, in some embodiments, a gate valve separating the droplet generator from the main EUV source vessel is closed, and the environment around the nozzle orifice is periodically purged. As used herein, "periodically purged" means that the system is configured to cycle the nozzle orifice environment between stages. These stages may include (1) a low-pressure vacuum stage and (2) a high-pressure purge stage, in which a purge gas is introduced to dilute water and oxygen that enters the nozzle environment from sources such as outgassing and vacuum leaks. In some embodiments, the pressure of the high-pressure phase is greater than atmospheric pressure. Those skilled in the art will appreciate that the terms "vacuum state" and "vacuum" refer to a partial vacuum (very low pressure), since absolute vacuum is unattainable. The periodic purge may include three stages: a pump-down stage, a purge stage, and an evacuation stage.

[0042]

[0051] The droplet generator can be safely maintained in this high temperature, isolated condition for extended periods of time, greatly reducing the risk of SnOx particles forming in the nozzle.

[0043]

[0052] The operating parameters of the cyclic purge, e.g., the minimum and maximum pressures during various stages such as the purge stage of the cycle, the pump down stage of the cycle, and the evacuation stage of the cycle, as well as the number of cycles per unit time, if used, and the relative duration of each stage of the cycle, are generally selected to optimize (e.g., minimize) the partial pressures of HO and O.

[0044]

[0053] In some embodiments, the nozzle environment is enclosed in a housing that may be part of a structure already in place for another purpose, such as a flight tube assembly provided to protect target material ejected from the nozzle orifice. As used herein, terms such as "enclosed" and "sealed" mean that the nozzle environment is effectively isolated, except for openings that accommodate the droplet exit path from the housing, except for inlets and outlets, as described below, and except for unavoidable leakage. The flight tube assembly or other portions of the droplet generator assembly may be provided with dedicated features to direct the flow of purge gas and facilitate efficient purging within the volume surrounding the nozzle orifice.

[0045]

[0054] The purge gas may be any gas that displaces and dilutes the oxygen-containing gas in the nozzle environment without adversely interacting with the condition or operation of components within that environment. For example, the purge gas may be an inert gas such as nitrogen or argon, or it may be hydrogen. The purge gas may also be a mixture of an inert gas such as argon and molecular hydrogen, i.e., Ar+H. The advantage of this gas mixture is that, in some systems, it may already be used for other purposes within the droplet generator. Using partial hydrogen rather than pure hydrogen reduces oxides without the complexity and cost of installing hydrogen safety equipment. According to one aspect of the embodiment, the purge gas is purified before use.

[0046]

[0055] Isolation valves in the droplet path can become partially clogged and unable to close completely. This can occur, for example, when a phenomenon known as "tin writing" occurs, in which tin accumulates in or near an isolation valve, preventing its operation. To address this, in some embodiments, whenever the droplet generator is depressurized and the isolation valve becomes clogged and unable to close, the nozzle orifice environment is controlled by a continuous purge instead of a periodic purge. In other words, the purge pressure is maintained and is not alternated with intervening evacuation steps. This continuous purge is maintained until droplets are needed, at which point the droplet generator transitions from an idle state to an operational state.

[0047]

[0056] In some embodiments, a system for purging the nozzle environment can be integrated with a gas nozzle used for droplet acceleration. Gas acceleration of droplets generated by a droplet generator has been considered as a way to increase droplet velocity without increasing the driving gas pressure. For example, U.S. Patent No. 8,598,551, entitled "EUV Radiation Source and Lithography Apparatus Including a Droplet Accelerator," issued December 3, 2013, to Mestrom et al., discloses an EUV radiation source including a droplet accelerator configured to accelerate droplets using a gas. International Patent Publication No. 2022 / 002662, entitled "Apparatus and Method for Accelerating Droplets in a Droplet Generator of an EUV Source," published January 6, 2022, to Ershov et al., discloses an arrangement in which droplets are exposed to a gas flow that entrains and accelerates the droplets.

[0048]

[0057] All patent applications, patents, and publications cited herein are incorporated by reference in their entirety, except for definitions, disclaimers, or disclaimers of subject matter, and except to the extent that the incorporated material contradicts an express disclosure herein. In the case of a conflict, the language of the disclosure herein will control.

[0049]

[0058] According to another aspect of the embodiment, instead of periodically or continuously purging the isolated nozzle environment, a pumping system may be used to create a vacuum inside the isolated housing, which can very efficiently remove water vapor and oxygen from the nozzle environment when the isolation valve is closed and the droplet generator is at reduced pressure (idle) at high operating temperatures. This can be achieved by providing a pumping system connected to the housing interior via a conduit. The pumping system may include a roughing pump, such as a scroll pump, if large volumes of gas need to be pumped, or a high-capacity turbomolecular vacuum pump or another vacuum pump with high pumping speed to achieve an ultra-high vacuum, or a combination of a roughing pump and a high-performance pump.

[0050]

[0059] In some embodiments, the conduit connecting the isolated nozzle environment to the pumping system may be a high conductance conduit. Flow resistance typically arises as a result of external friction between the gas molecules and the wall and internal friction (viscosity) between the gas molecules themselves. In vacuum technology, it is common to use conductance C, which is the reciprocal of flow resistance. Conductance has the dimension of volumetric flow rate and is typically expressed in standard liters per second. Herein, the hydraulic diameter of the high conductance conduit is selected to be large compared to its length in order to obtain a very high conductance.

[0051]

[0060] FIG. 2A is a schematic diagram of a system 100 for controlling the environment of a droplet generator nozzle orifice according to one aspect of an embodiment. The system shown in FIG. 2A includes a nozzle 110 having a nozzle orifice 120. The nozzle 110 and the nozzle orifice 120 are positioned within a housing 140. As mentioned above, the housing 140 may be part of an existing structure, such as a flight tube. The housing 140 may also be part of an arrangement for supplying a gas that accelerates the droplet stream 14 emitted from the nozzle orifice 120. The droplet stream 14 passes through a valve 150, which may be a gate valve, into the EUV radiation source chamber 26.

[0052]

[0061] 2A also shows pump 180 fluidly connected (i.e., in fluid communication) with interior 145 of housing 140 by conduit 185. As used herein, the terms "fluidly connected" and "in fluid communication" are intended to imply that a path exists for a fluid to pass through. Pump 180 may be a roughing pump, a high-volume pump such as a turbomolecular pump, or a combination of both types of pumps. Conduit 185 may be configured as a high-conductance conduit.

[0053]

[0062] 2A also shows a purge gas supply 160 in fluid communication with a purge gas control unit 165, which supplies purge gas to a purge gas inlet 170. The purge gas is then conveyed to the housing interior 145 by a purge gas conduit 175. The purge gas can be a mixture of an inert gas, such as argon, and molecular hydrogen.

[0054]

[0063] The configuration shown in FIG. 2A includes a control line 157 that carries a signal to control whether the valve 150 is in a first state in which the housing interior 145 is in fluid communication with the EUV radiation source chamber 26 (i.e., open, allowing the passage of droplet stream 14 emitted from the nozzle orifice 120), or in a second state in which the housing interior 145 is not in fluid communication with the EUV radiation source chamber 26 (i.e., closed). Also shown is a data line 155 that carries a signal from the valve 150 indicating whether the valve 150 is open or closed. This may be a simple binary “open / close” signal, or a signal indicating degrees of a partially open or partially closed position. The signal carried by the data line 155 is useful for determining whether the gate 150 is fully open or only partially open in response to an “open” control signal carried by the control line 157.

[0055]

[0064] 2A also includes a control line 167 that controls purge gas control 165 to determine whether purge gas from purge gas source 160 is supplied to purge gas inlet 170. Thus, housing interior 145 is selectably fluidly connected to purge gas source 160 in the sense that the "open / closed" state of the connection is selectable.

[0056]

[0065] 2B illustrates system 105 in which pump 180 and purge gas source 160 share a common inlet to the interior of housing 140. Specifically, pump 180 and purge gas source 160 are selectably connected to inlet 192 via valve 195. Valve 195 may be a three-way valve that is also connected to atmospheric outlet 190. Inlet 192 communicates with the interior of housing 140 via conduit 197. For example, system 105 can be used when cyclic purging has three stages: a pump-down stage, a purge stage, and a vacuum stage.

[0057]

[0066] 2B also includes a purge gas purifier 162 positioned to purify the purge gas before it advances to the interior of the housing 140. Inclusion of this purge gas purifier 162 in the system 105 can ensure that the purified purge gas maintains low concentrations of oxygen-containing gases, such as water vapor, in the purge gas by purifying the purge gas as close in time and physically as possible to its use.

[0058]

[0067] 3A and 3B illustrate the operation of valve 150 according to one aspect of the embodiment. In FIG. 3A, valve 150 is configured as a gate valve having gate 200. In the state shown in FIG. 3A, valve 150 is open, with gate 200 removed from the path of droplet stream 14 traveling from housing interior 145 into chamber 26. Also shown in FIG. 3A is actuator 210, which moves gate 200 into and out of a closed position, and sensor 220, which may be configured as a valve position sensor, which detects whether gate 200 is in the closed or open position, or an intermediate position. In FIG. 3B, gate 200 has been moved to a position isolating housing interior 145 from chamber 26 by a control signal applied to actuator 210 via control line 157.

[0059]

[0068] Sensor 220 provides a valve status signal on data line 155 indicating whether valve 150 is open or closed. According to one embodiment, the valve status signal is used to determine whether a command to close valve 150 was successfully executed. For example, tin contamination on gate 200 may prevent gate 200 from assuming the fully closed position shown in FIG. 3B. This valve status information can be used to determine the characteristics of the nozzle orifice environmental control action being taken.

[0060]

[0069] FIG. 4 is a flowchart illustrating one operational mode of a system for controlling the nozzle orifice environment according to one aspect of the embodiment. In step S10, a determination is made as to whether the droplet generator, abbreviated as "DGEN" in the figure, is idle by determining whether the droplet generator has been depressurized. Typically, liquid target material is supplied to the droplet generator under pressure. This pressure is removed during periods when the droplet generator is idle, i.e., when it is not being requested to deliver droplets. If the determination in step S10 is negative, no action is taken. If the determination in step S10 is positive, a valve is closed in step S20, thereby isolating the interior of the droplet generator near the nozzle orifice from the EUV radiation source chamber. Next, in step S30, a periodic purge of the droplet generator nozzle orifice environment is performed. As described above, the periodic purge is performed using multiple cycles, each cycle including evacuating the housing using a vacuum pump followed by supplying a purge gas to the housing.

[0061]

[0070] FIG. 5 is a flowchart illustrating another operational mode of a system for controlling the nozzle orifice environment according to one aspect of the embodiment. Again, in step S10, it is determined whether the droplet generator has been depressurized. If the determination in step S10 is negative, no action is taken. If it is determined in step S10 that the droplet generator has been depressurized, it is determined in step S40 whether the valve can be closed. This step S40 can be performed, for example, by providing a signal to close the valve and then determining whether the valve has closed in response to the signal. If the valve cannot be closed because it is clogged, for example, a continuous purge is applied to the droplet generator nozzle environment in step S50. Here, "continuously" means that purge gas is continuously supplied to the interior of the housing without an intervening evacuation phase. This continuous purge in step S50 continues until the droplet generator is again called upon to generate droplets, and begins generating droplets in step S60. If it is determined in step S60 that droplets have not yet been generated, the continuous purge continues in step S50. However, if it is determined in step S60 that droplets are being generated, then in step S70 the continuous purging of the droplet generator nozzle environment is stopped.

[0062]

[0071] 6 is a flow chart illustrating another mode of operation of a system for controlling the environment of a nozzle orifice according to one aspect of an embodiment. Again, step S10 determines whether the droplet generator has been depressurized. If the determination in step S10 is negative, no action is taken. If it is determined in step S10 that the droplet generator has been depressurized, step S40 determines whether the valve can be closed. If the valve cannot be closed, step S50 applies a continuous purge to the droplet generator nozzle environment. This continuous purge in step S50 continues until the droplet generator is again requested to generate droplets in step S60 and begins to generate droplets. If it is determined in step S60 that droplets have not yet been generated, the continuous purge continues in step S50. On the other hand, if it is determined in step S60 that droplets are being generated, the continuous purge of the droplet generator nozzle environment is stopped in step S70.

[0063]

[0072] If it is determined in step S40 that the valve can be closed, then in step S20 the valve is closed, thereby isolating the interior of the droplet generator near the nozzle orifice from the EUV radiation source chamber. Next, in step S30, a periodic purge of the droplet generator nozzle orifice environment is performed. Next, in step S80, it is determined whether droplets are needed, i.e., whether the droplet generator is to be restarted. If it is determined in step S80 that droplets are not yet needed, then the periodic purge continues in step S30. On the other hand, if it is determined in step S80 that droplets are needed, then the periodic purge of the droplet generator nozzle environment is stopped and the valve is opened in step S90.

[0064]

[0073] 7 is a flowchart illustrating another mode of operation of a system for controlling the environment of a nozzle orifice according to an aspect of an embodiment. In step S10, it is determined whether the droplet generator is idle by determining whether the droplet generator has been depressurized. If the determination in step S10 is affirmative, in step S120, a valve is closed, thereby isolating the interior of the droplet generator near the nozzle orifice from the EUV radiation source chamber. Next, in step S130, the droplet generator nozzle environment is evacuated. This process can be modified by adding a step to verify that the valve is closed in step S120 and by not performing the evacuation step S130 if the valve is not verified to be closed.

[0065]

[0074] 8 is a flowchart illustrating another operational mode of a system for controlling the environment of a nozzle orifice according to an aspect of an embodiment. In step S10, it is determined whether the droplet generator is idle by determining whether the droplet generator has been depressurized. If the determination in step S10 is affirmative, in step S200, the droplet generator nozzle environment is evacuated. In step S210, it is determined whether a valve isolating the droplet generator nozzle environment from the chamber interior can be closed. If the valve can be closed, in step S220, the valve is closed, thereby isolating the interior of the droplet generator near the nozzle orifice from the EUV radiation source chamber. Next, in step S230, a periodic purge of the droplet generator nozzle orifice environment is performed. Next, in step S240, it is determined whether droplets are needed, i.e., whether the droplet generator is operating. If it is determined in step S240 that droplets are not yet needed, the periodic purge continues in step S230. On the other hand, if a drop is needed as determined in step S240, the periodic purge of the drop generator nozzle environment is stopped and the valve is opened in step S250.

[0066]

[0075] If it is determined in step S210 that the valve cannot be closed, then evacuation of the droplet generator nozzle environment continues in step S260. This evacuation in step S260 continues until it is determined in step S270 that the droplet generator is again requested to generate droplets. If it is determined in step S270 that a droplet is not yet needed, then evacuation continues in step S260. However, if it is determined in step S270 that a droplet is needed, then evacuation of the droplet generator nozzle environment is stopped in step S280.

[0067]

[0076] In some embodiments, before initiating evacuation of the droplet generator nozzle environment, a step of determining whether a valve can be closed can be performed. FIG. 9 is a flowchart illustrating such a process. In step S10, it is determined whether the droplet generator is idle by determining whether the droplet generator has been depressurized. If the determination in step S10 is affirmative, it is determined in step S210 whether a valve isolating the droplet generator nozzle environment from the chamber interior can be closed. If this valve can be closed, in step S220, the valve is closed, thereby isolating the interior of the droplet generator near the nozzle orifice from the EUV radiation source chamber. Next, in step S230, a periodic purge of the droplet generator nozzle orifice environment is performed. Next, in step S240, it is determined whether droplets are needed, i.e., whether the droplet generator is operating. If it is determined in step S240 that droplets are not yet needed, the periodic purge continues in step S230. On the other hand, if it is determined in step S240 that a drop is needed, the periodic purge of the drop generator nozzle environment is stopped and the valve is opened in step S250.

[0068]

[0077] If it is determined in step S210 that the valve cannot be closed, evacuation of the droplet generator nozzle environment is initiated in step S270. This evacuation in step S270 continues until it is determined in step S280 that the droplet generator needs to generate droplets. If it is determined in step S280 that droplets are not yet needed, evacuation continues in step S270. However, if it is determined in step S280 that droplets are needed, evacuation of the droplet generator nozzle environment is stopped in step S290.

[0069]

[0078] The present disclosure is presented using functional building blocks that illustrate the implementation of certain functions and their relationships. The boundaries of these functional building blocks are arbitrarily defined herein for the convenience of description. Alternative boundaries may be defined so long as the certain functions and their relationships are appropriately implemented. For example, the functionality of a control module may be divided into several systems or may be performed at least in part by an overall control system.

[0070]

[0079] The above description includes examples of one or more embodiments. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the above-described embodiments, and one of ordinary skill in the art will recognize that many additional combinations and permutations of the various embodiments are possible. Accordingly, the described embodiments are intended to embrace all such modifications, improvements, and variations that fall within the spirit and scope of the appended claims. Furthermore, where used in the detailed description or claims, the term "includes" is intended to be inclusive, similar to the interpretation of the term "comprising" when employed as a transitional term in the claims. Furthermore, although elements of the described aspects and / or embodiments may be described or claimed in the singular, the plural is also intended unless the singular is clearly stated. Furthermore, all or a portion of any aspect and / or embodiment can be utilized with all or a portion of any other aspect and / or embodiment, unless expressly stated otherwise.

[0071]

[0080] The embodiments can be further described using the following clauses. 1. A droplet generator system for an extreme ultraviolet (EUV) radiation source, comprising: A nozzle housing; 1. A droplet generator system comprising: a nozzle having an exit orifice positioned within a housing, the exit orifice being in selectable fluid communication with a source of liquid EUV radiation source material and adapted to transport the EUV radiation source material into a chamber of the EUV radiation source; and a valve in fluid communication with an interior of the housing and the chamber, the valve having a first state in which the interior of the housing is in fluid communication with the chamber and a second state in which the interior of the housing is not in fluid communication with the chamber. 2. A droplet generator system as described in clause 1, wherein the nozzle is adapted to transport the EUV radiation source material into the chamber along a path from the exit orifice through a valve, the valve being adapted to maintain the path in an open state when the valve is in a first state and to maintain the path in a closed state when the valve is in a second state. 3. The droplet generator system of clause 1, wherein the valve is a gate valve. 4. The droplet generator system of clause 1, wherein the valve is adapted to transition between a first state and a second state in response to a control signal. 5. The droplet generator system of clause 1, further comprising a sensor positioned to detect whether the valve is in the first state or the second state. 6. The droplet generator system of clause 1, further comprising an inlet in fluid communication with the interior of the housing and adapted to be connected to a source of purge gas. 7. The droplet generator system of clause 6, wherein the purge gas comprises Ar+H2. 8. The droplet generator system of clause 6, wherein the purge gas comprises an inert gas. 9. The droplet generator system of clause 8, wherein the inert gas comprises at least one of nitrogen and argon. 10. The droplet generator system of clause 6, further comprising a purge gas purifier arranged to purify the purge gas. 11. A droplet generator system for an extreme ultraviolet (EUV) radiation source, comprising: A nozzle housing; a nozzle having an exit orifice positioned within the housing, the exit orifice in selectable fluid communication with a source of liquid EUV radiation source material and adapted to deliver the EUV radiation source material into a chamber of the EUV radiation source; a valve in fluid communication with an interior of a housing and a chamber, the valve having a first state in which the interior of the housing is in fluid communication with the chamber and a second state in which the interior of the housing is not in fluid communication with the chamber; a source of purge gas in selectable fluid communication with the interior of the housing; a vacuum pump in selectable fluid communication with the interior of the housing; 1. A droplet generator system comprising: 12. The droplet generator system of clause 11, wherein the vacuum pump is a turbomolecular vacuum pump or a scroll vacuum pump. 13. The droplet generator system of clause 11, wherein the vacuum pump is a turbomolecular vacuum pump, and wherein the droplet generator system further comprises a high conductance conduit positioned to connect the interior of the housing to the turbomolecular vacuum pump. 14. An assembly for purging an environment at a nozzle outlet of a droplet generator in an extreme ultraviolet (EUV) radiation source, comprising: a housing disposed around and surrounding the nozzle outlet, except for an opening of a droplet path that carries the droplets to a chamber of an EUV radiation source; a purge gas source in selectable fluid communication with the interior of the housing; a pump in selectable fluid communication with the interior of the housing and adapted to evacuate the housing; Including the assembly. 15. The assembly of clause 14, wherein the purge gas comprises Ar+H2. 16. The assembly of clause 14, wherein the purge gas comprises an inert gas. 17. The assembly of clause 14, wherein the purge gas comprises at least one of nitrogen and argon. 18. The assembly of clause 14, further comprising a purge gas purifier arranged to purify the purge gas. 19. The assembly of clause 14, wherein the pump includes a roughing pump. 20. The assembly of clause 14, wherein the pump comprises a turbomolecular vacuum pump or a scroll vacuum pump. 21. The assembly of clause 14, wherein the pump is fluidly connected to the interior of the housing by a high conductance conduit. 22. The method further includes a valve disposed in the droplet path, the valve having a first state in which the interior of the housing is in fluid communication with the chamber and a second state in which the interior of the housing is not in fluid communication with the chamber, wherein the purge gas source and pump: performing a periodic purge that includes at least supplying a purge gas to the interior of the housing when the droplet generator is depressurized and the valve is in the second state; and and supplying a purge gas to the interior of the housing when the droplet generator is depressurized and the valve is in a first state to provide continuous purging. 15. An assembly as described in clause 14, adapted 23. The assembly according to clause 22, wherein the valve is a gate valve. 24. A method of controlling the environment of a nozzle of a droplet generator in an extreme ultraviolet (EUV) radiation source, comprising: disposing a nozzle within a housing; determining whether the droplet generator is depressurized; sending a signal to actuate a valve positioned to isolate the nozzle from a chamber of the EUV radiation source when it is determined that the droplet generator is depressurized; A method comprising: 25. The method of clause 24, further comprising, after transmitting a signal to actuate the valve to isolate the nozzle from the chamber of the EUV radiation source, determining whether the valve operated to isolate the nozzle from the chamber in response to the signal. 26. The method of clause 25, further comprising, upon determining that the valve did not operate to isolate the nozzle from the chamber in response to the signal, periodically purging the housing by alternating between supplying a purge gas to the housing and evacuating the housing. 27. The method of claim 26, wherein the purge gas comprises Ar+H2. 28. The method of clause 26, wherein the purge gas comprises an inert gas. 29. The method of clause 26, wherein the purge gas comprises at least one of nitrogen and argon. 30. The method of clause 26, further comprising purifying the purge gas before supplying it to the housing. 31. The method of clause 25, further comprising, upon determining that the valve did not operate to isolate the nozzle from the chamber in response to the signal, continuously purging the housing by continuously supplying purge gas to the housing. 32. The method of clause 31, wherein the purge gas comprises Ar+H2. 33. The method of clause 31, wherein the purge gas comprises an inert gas. 34. The method of clause 31, wherein the purge gas comprises at least one of nitrogen and argon. 35. The method of clause 31, further comprising purifying the purge gas before supplying it to the housing. 36. The method of clause 25, further comprising evacuating the housing if it is determined that the valve did not operate to isolate the nozzle from the chamber in response to the signal. 37. The method of clause 36, wherein evacuating the housing is performed using a turbomolecular vacuum pump or a scroll vacuum pump. 38. The method of clause 24, further comprising evacuating the housing after sending a signal to actuate a valve positioned to isolate the nozzle from a chamber of the EUV radiation source.

[0072]

[0081] These and other implementations are within the scope of the following claims.

Claims

1. 1. A droplet generator system for an extreme ultraviolet (EUV) radiation source, comprising: A nozzle housing; a nozzle having an exit orifice positioned within the housing, the exit orifice in selectable fluid communication with a source of liquid EUV radiation source material and adapted to deliver the EUV radiation source material into a chamber of the EUV radiation source; a valve in fluid communication with an interior of the housing and the chamber, the valve having a first state in which the interior of the housing is in fluid communication with the chamber and a second state in which the interior of the housing is not in fluid communication with the chamber; 1. A droplet generator system comprising:

2. 2. The droplet generator system of claim 1, wherein the nozzle is adapted to transport the EUV radiation source material into the chamber along a path from the exit orifice through the valve, the valve being adapted to maintain the path in an open state when the valve is in the first state and to maintain the path in a closed state when the valve is in the second state.

3. The droplet generator system of claim 1 , wherein the valve is a gate valve.

4. The droplet generator system of claim 1 , wherein the valve is adapted to transition between the first state and the second state in response to a control signal.

5. The droplet generator system of claim 1 , further comprising a sensor positioned to detect whether the valve is in the first state or the second state.

6. The droplet generator system of claim 1 , further comprising an inlet in fluid communication with the interior of the housing and adapted to be connected to a source of purge gas.

7. The droplet generator system of claim 6 , wherein the purge gas comprises an inert gas, the inert gas comprising at least one of nitrogen and argon.

8. The droplet generator system of claim 6 , further comprising a purge gas purifier positioned to purify the purge gas.

9. 1. A droplet generator system for an extreme ultraviolet (EUV) radiation source, comprising: A nozzle housing; a nozzle having an exit orifice positioned within the housing, the exit orifice in selectable fluid communication with a source of liquid EUV radiation source material and adapted to deliver the EUV radiation source material into a chamber of the EUV radiation source; a valve in fluid communication with an interior of the housing and the chamber, the valve having a first state in which the interior of the housing is in fluid communication with the chamber and a second state in which the interior of the housing is not in fluid communication with the chamber; a source of purge gas in selectable fluid communication with the interior of the housing; a vacuum pump in selectable fluid communication with the interior of the housing; 1. A droplet generator system comprising:

10. The droplet generator system of claim 9 , wherein the vacuum pump is a turbomolecular vacuum pump or a scroll vacuum pump.

11. 10. The droplet generator system of claim 9, wherein the vacuum pump is a turbomolecular vacuum pump, and the droplet generator system further comprises a high conductance conduit positioned to connect the interior of the housing to the turbomolecular vacuum pump.

12. 1. A method for controlling an environment of a nozzle of a droplet generator in an extreme ultraviolet (EUV) radiation source, comprising: disposing a nozzle within a housing; determining whether the droplet generator is depressurized; sending a signal to actuate a valve positioned to isolate the nozzle from a chamber of the EUV radiation source when it is determined that the droplet generator is depressurized; A method comprising:

13. 13. The method of claim 12, further comprising, after sending the signal to actuate the valve to isolate the nozzle from the chamber of the EUV radiation source, determining whether the valve was operated to isolate the nozzle from the chamber in response to the signal.

14. 14. The method of claim 13, further comprising, if it is determined that the valve did not operate to isolate the nozzle from the chamber in response to the signal, periodically purging the housing by alternating between supplying a purge gas to the housing and evacuating the housing.

15. The method of claim 14 , wherein the purge gas comprises an inert gas and the purge gas comprises at least one of nitrogen and argon.

16. The method of claim 14 further comprising purifying the purge gas before supplying the purge gas to the housing.

17. 14. The method of claim 13, further comprising, upon determining that the valve did not operate to isolate the nozzle from the chamber in response to the signal, continuously purging the housing by continuously supplying a purge gas to the housing.

18. The method of claim 13 , further comprising evacuating the housing if it is determined that the valve did not operate to isolate the nozzle from the chamber in response to the signal.

19. 20. The method of claim 18, wherein evacuating the housing is performed using a turbomolecular vacuum pump or a scroll vacuum pump.

20. 13. The method of claim 12, further comprising evacuating the housing after sending a signal to actuate a valve positioned to isolate the nozzle from a chamber of the EUV radiation source.

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