CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Application No. 63/009,127, filed April 13, 2020 and titled APPARATUS FOR AND METHOD OF CONTROLLING GAS FLOW IN AN EUV LIGHT SOURCE, and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present disclosure relates to an apparatus for and methods of generating extreme ultraviolet (“EUV”) radiation from a plasma created through discharge or laser ablation of a source or target material in a vessel. In such applications optical elements are used, for example, to collect and direct the radiation for use in semiconductor photolithography and inspection.

BACKGROUND

Extreme ultraviolet radiation, e.g., electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including radiation at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates such as silicon wafers.

[0003] Methods for generating EUV radiation include converting a target material to a plasma state. The target material preferably includes at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV portion of the electromagnetic spectrum. The target material can be solid, liquid, or gas. In one such method, often termed laser produced plasma (“LPP”), the required plasma can be produced by using a laser beam to irradiate a target material having the required line-emitting element.

[0004] One LPP technique involves generating a stream of target material droplets and irradiating at least some of the droplets with one or more laser radiation pulses. Such LPP sources generate EUV radiation by coupling laser energy into a target material having at least one EUV emitting element, creating a highly ionized plasma.

[0005] For this process, the plasma is typically produced in a sealed vessel, e.g., a vacuum chamber, and the resultant EUV radiation is monitored using various types of metrology equipment. In addition to generating EUV radiation, the processes used to generate plasma also typically generate undesirable by-products in the plasma chamber which can include out-of-band radiation, high energy ions, and debris, e.g., atoms and or clumps/microdroplets of residual target material.

[0006] The energetic radiation is emitted from the plasma in all directions. In one common arrangement, a near-normal-incidence mirror (often termed a “collector mirror” or simply a “collector”) is positioned to collect, direct, and, in some arrangements, focus at least a portion of the radiation to an intermediate location. The collected radiation may then be relayed from the intermediate location to a set of optics, a reticle, detectors and ultimately to a silicon wafer.

[0007] In the EUV portion of the spectrum it is generally regarded as necessary to use reflective optics for the optical elements in the system including the collector, illuminator, and projection optics box. These reflective optics may be implemented as normal incidence optics as mentioned or as grazing incidence optics. At the wavelengths involved, the collector is advantageously implemented as a multi layer mirror (“MLM”). As its name implies, this MLM is generally made up of alternating layers of material (the MLM stack) over a foundation or substrate. System optics may also be configured as coated optical elements even if they are not implemented as an MLM.

[0008] The optical elements and, in particular, the collector must be placed within the vessel with the plasma to collect and redirect the EUV radiation. The environment within the chamber is inimical to the optical elements and so limits their useful lifetime, for example, by degrading reflectivity. An optical element within the environment may be exposed to high energy ions or particles of target material. The particles of target material, which are essentially debris from the laser vaporization process, can contaminate the optical element’s exposed surface. Particles of target material can also cause physical damage to and localized heating of the MLM surface.

[0009] In some systems ¾ gas at pressures in the range of about 0.5 to about 3 mbar is used in the vacuum chamber as a buffer gas for debris mitigation. In the absence of a gas, at vacuum pressure, it would be difficult to protect the collector adequately from target material debris ejected from the irradiation region. Hydrogen is relatively transparent to EUV radiation having a wavelength of about 13.5 nm and so is preferred to other candidate gases such as He, Ar, or other gases which exhibit a higher absorption at about 13.5 nm.

[0010] ¾ gas is introduced into the vacuum chamber to slow down the energetic debris (ions, atoms, and clusters) of target material created by the plasma. The debris is slowed down by collisions with the gas molecules. Lor this purpose a flow of ¾ gas is used which may also be counter to the debris trajectory and away from the collector. This serves to reduce the damage of deposition, implantation, and sputtering target material on the optical coating of the collector.

[0011] Thus, the process of transforming the target material creates particles and deposits residual target material on surfaces where there is an unobstructed path between the irradiation site and the surface as well as in the exhaust path of gases that entrain residual target material. Lor example, if this gas is pumped across the top of vanes present in the chamber and to the mechanical pumps then soon the material is deposited on all of the cold metal parts. If the target material is tin, then this can lead to the growth of tin wool which can drop onto the collector optics and clog the exhaust paths.

[0012] When target material such as tin is illuminated with laser radiation to produce plasma, a certain portion of the target material becomes debris. Lor example, target material debris may include Sn vapor, SnH4 vapor, Sn atoms, Sn ions, Sn clusters, Sn microparticles, Sn nanoparticles, and Sn deposits. When Sn debris accumulates on an EUV collector or on one or more inner vessel walls of the EUV vessel, the EUV collector efficiency, lifetime, and availability may be reduced.

[0013] Tin debris from the source vessel can pass through intermediate focus from an EUV source to the scanner, which can cause contamination of, for example, an illuminator in scanner, an expensive optical element whose lifetime is critical to EUV system productivity and cost of ownership. As described above, one form of tin contamination is the ejection or “spitting” of molten tin from walls near the intermediate focus in the source vessel. One technique used to prevent tin debris from reaching the scanner involves applying a dynamic gas lock at the intermediate focus to suppress tin contamination as disclosed in U.S. Patent No. 9,606,445 issued March 28, 2017 and titled “Lithographic Apparatus and Method of Manufacturing a Device,” the entire contents of which are hereby incorporated by reference.

[0014] The process of generating EUV light may also cause target material to be deposited on the walls of the vessel. Controlling target material deposition on the vessel walls is important for achieving an acceptably long lifetime of EUV sources placed in production. Also, managing target material flux from the irradiation site is important for ensuring that the waste target material mitigation system works as intended.

SUMMARY

[0015] The following presents a 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 set limits on 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. [0016] According to one aspect of an embodiment, there is disclosed a system for optimizing process windows by allowing for dynamic changes in the characteristics of gas flow into the chamber enclosing an EUV source.

[0017] According to another aspect of an embodiment there is disclosed an apparatus for generating EUV radiation by laser irradiation of droplets of a target material, the apparatus comprising a vessel, inlet structure defining at least one inlet path adapted and arranged to connect a source of a gas to an interior of the vessel to add the gas to the vessel in a flow along the inlet path, outlet structure defining at least one outlet path adapted and arranged to be connected to an interior of the vessel to permit gas in the vessel to flow out of the vessel along the outlet path, a variable flow regulator selectably arranged in one of the inlet path and the outlet path and adapted to regulate a characteristic of the flow of the gas into or out of the vessel based at least in part on a mode in which the apparatus is operating, and a controller arranged to control operation of the flow controller. The controller may be adapted to operate using a look-ahead control process. The apparatus may further comprise a second variable flow regulator selectably arranged in the other of the inlet path and the outlet path and adapted to regulate a characteristic of the flow of the gas into or out of the vessel based at least in part on a mode in which the apparatus is operating.

[0018] The apparatus may have an on-droplet mode of operation in which droplets generate EUV radiation when irradiated by a laser when the apparatus is in one mode and an off-droplet mode of operation in which the droplets are not used to generate EUV radiation when not irradiated by a laser in another mode. The variable flow regulator may be selectably arranged in the inlet path, partially or wholly or not at all, and adapted to regulate a characteristic of the flow of the gas into the vessel based at least in part on the mode in which the apparatus is operating. The characteristic may be any one or combination of flow rate, flow velocity, flow profile, and flow composition. The flow composition may be such that it does not contain an active gas during the on-droplet mode and does contain an active gas during the off-droplet mode. The active gas may comprise oxygen. The inlet structure may comprise a collector cone.

[0019] The variable flow regulator may comprise a flow obstruction and a motor mechanically coupled to the flow obstruction and adapted to move the flow obstruction at least partially into the flow path. “Motor” here and elsewhere includes any device for generating a motive force. The motor may comprise a linear motor. The motor may comprise a solenoid. The flow obstruction when placed in the flow path may present a solid cross section to the flow or a cross section having at least one aperture. The flow obstruction may have an open tubular shape and when placed in the flow path be oriented so that the flow obstruction redirects a portion of the gas. The flow obstruction may have an aerodynamic shape. The variable flow regulator may comprise a mass flow controller.

[0020] The variable flow regulator may comprise a valve adapted to be in fluid communication with the gas source, and a manifold comprising plurality of fluid conduits respectively connecting the valve to the inlet, each of the plurality of fluid conduits having a respective flow restrictor restricting a flow rate through the respective conduit to a respective value, the valve being arranged to permit the gas to flow through one of the plurality of flow conduits.

[0021] According to another aspect of an embodiment there is disclosed an apparatus for generating EUV radiation by laser irradiation of droplets of a target material, the apparatus comprising a vessel having at least one inlet adapted to be connected to a source of a gas and to add the gas to the vessel in a flow along a flow path, a droplet generator arranged to introduce the droplets into the vessel to an irradiation site within the vessel where the droplets are used to generate EUV radiation when irradiated by a laser when the apparatus is in an on-droplet mode and where the droplets are not used to generate EUV radiation when not irradiated by a laser when the apparatus is in an off-droplet mode, and a variable flow regulator selectably arranged in the flow path and adapted to regulate a characteristic of the flow of the gas into the vessel based at least in part on whether the apparatus is in the on-droplet mode or the off-droplet mode.

[0022] According to another aspect of an embodiment there is disclosed a flow regulator for regulating a characteristic of the flow of a gas from a gas source into a vessel in an apparatus for generating EUV radiation, the flow regulator comprising an inlet adapted to be in fluid communication with the gas source, an outlet adapted to be in fluid communication with an inlet to the vessel, and a flow restrictor selectably impeding a flow of the gas through the regulator along a flow path from the inlet to the outlet based at least in part on an operational mode of the apparatus. The flow restrictor may comprise a flow obstruction and a motor mechanically coupled to the flow obstruction and adapted to move the flow obstruction to a position completely out of the flow path, completely within the flow path, or partially in the flow path. The flow restrictor may comprise a valve adapted to be in fluid communication with the gas source and a manifold comprising plurality of fluid conduits respectively connecting the valve to the inlet, each of the plurality of fluid conduits having a respective flow restrictor restricting a flow rate through the respective conduit to respective value, the valve being arranged to permit the gas to flow through one of the plurality of flow conduits.

[0023] According to another aspect of an embodiment there is disclosed a method of controlling operation of an apparatus for generating EUV radiation by laser irradiation of droplets of a target material in a vessel, the method comprising operating the apparatus in an off-droplet mode in which the droplets are not used to generate EUV radiation, coincident with the operating step, regulating a characteristic of a flow of a gas at least one of into and out of the vessel based at least in part on the apparatus operating in the off-droplet mode, switching to operating the apparatus in an on-droplet mode in which the droplets are used to generate EUV radiation, and coincident with the switching step, regulating a characteristic of a flow of a gas at least one of into and out of the vessel based at least in part on the apparatus operating in the on-droplet mode. The method may be carried out under control of a controller operating in accordance with a look-ahead process. The characteristic may be any one or combination of flow rate, flow velocity, flow profile, and flow composition. The flow composition may be one that does not contain an active gas during the on-droplet mode and does contain an active gas during the off-droplet mode. The active gas may comprise oxygen. The apparatus may comprise a flow obstruction and a motor for moving the flow obstruction and the step of regulating a characteristic of a flow of a gas into the vessel based at least in part on the apparatus operating in the off-droplet mode comprises moving the flow obstruction at least partially into a flow path of the gas into the vessel. The apparatus may comprise a valve adapted to be in fluid communication with a source of the gas and a manifold comprising plurality of fluid conduits respectively connecting the valve to the vessel, each of the plurality of fluid conduits having a respective flow restrictor restricting a flow rate through the respective conduit to respective value, the valve being arranged to permit the gas to flow through one of the plurality of flow conduits, and the step of regulating a characteristic of a flow of a gas into the vessel based at least in part on the apparatus operating in the off-droplet mode comprises operating the valve to place a selected one of the plurality of conduits in fluid communication with the source of the gas.

[0024] 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

[0025] FIG. 1 is a schematic, not-to-scale view of an overall broad conception for a laser-produced plasma EUV radiation source system according to an aspect of an embodiment.

[0026] FIG. 2 is a not-to-scale diagram showing a possible arrangement of a vessel and exhaust systems used in a laser-produced plasma EUV radiation source system.

[0027] FIG. 3A is a not-to-scale cutaway schematic diagram of a possible arrangement of a system for introducing gas into a vessel according to an aspect of an embodiment.

[0028] FIG. 3B is a not-to-scale cutaway schematic diagram of a possible arrangement of a system for controlling a flow of gas into and/or out of a vessel according to an aspect of an embodiment.

[0029] FIG. 4A is a not-to-scale cutaway schematic diagram of a possible arrangement of a vessel and a gas inlet according to an aspect of an embodiment.

[0030] FIG. 4B is a not-to-scale cutaway schematic diagram of a possible arrangement of a vessel and a gas inlet according to an aspect of an embodiment.

[0031] FIG. 5 is a not-to-scale cutaway schematic diagram of a possible arrangement of a vessel and a gas inlet according to an aspect of an embodiment.

[0032] FIG. 6 is a not-to-scale cutaway schematic diagram of a possible arrangement of a vessel and a gas inlet according to an aspect of an embodiment.

[0033] FIG. 7 is a not-to-scale cutaway schematic diagram of a possible arrangement of a system for introducing gas into a vessel according to an aspect of an embodiment.

[0034] FIG. 8 is a flowchart of a process for introducing gas into a vessel according to an aspect of an embodiment.

[0035] FIG. 9 is a flowchart of a process for introducing gas into a vessel according to another aspect of an embodiment.

[0036] 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. DETAILED DESCRIPTION

[0037] 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.

[0038] Before describing such embodiments in more detail, however, it is instructive to present an example environment in which embodiments of the present invention may be implemented. In the description that follows and in the claims the terms “up,” “down,” “top,” “bottom,” “vertical,” “horizontal,” and like terms may be employed. These terms are intended to show relative orientation only and not any orientation with respect to gravity unless otherwise indicated.

[0039] With initial reference to FIG. 1 there is shown a schematic view of an exemplary EUV radiation source, e.g., a laser produced plasma EUV radiation source 10 according to one aspect of an embodiment of the present invention. As shown, the EUV radiation source 10 may include a pulsed or continuous laser source 22, which may for example be a pulsed gas discharge CO2 laser source producing a beam 12 of radiation at 10.6 pm or 1 pm focusing down to a primary focus PF. The pulsed gas discharge CO2 laser source may have DC or RF excitation operating at high power and at a high pulse repetition rate.

[0040] The EUV radiation source 10 also includes a target delivery system 24 for delivering target material in the form of liquid droplets or a continuous liquid stream. In this example, the target material is a liquid, but it could also be a solid or gas. The target material may be made up of tin or a tin compound, although other materials could be used. In the system depicted the target material delivery system 24 introduces droplets 14 of the target material into the interior of a vacuum chamber 26 to an irradiation region at the PF of the collector 30 where the target material may be irradiated to produce plasma. The vacuum chamber 26 may be provided with a liner. In some cases, an electrical charge is placed on the target material to permit the target material to be steered toward or away from the irradiation region. It should be noted that as used herein an irradiation region is a region where target material irradiation may or is intended to occur, and is an irradiation region even at times when no irradiation is actually occurring. The EUV light source may also include a beam steering system 32. [0041] In the system shown, the components are arranged so that the droplets 14 travel substantially horizontally. The direction from the laser source 22 towards the irradiation region, that is, the nominal direction of propagation of the beam 12, may be taken as the Z axis. The path the droplets 14 take from the target material delivery system 24 to the irradiation region may be taken as the X axis. The view of FIG. 1 is thus normal to the XZ plane. The orientation of the EUV radiation source 10 is preferably rotated with respect to gravity as shown, with the arrow G showing the preferred orientation with respect to gravitationally down. This orientation applies to the EUV source but not necessarily to optically downstream components such as a scanner and the like. Also, while a system in which the droplets 14 travel substantially horizontally is depicted, it will be understood by one having ordinary skill in the art the other arrangements can be used in which the droplets travel vertically or at some angle with respect to gravity between and including 90 degrees (horizontal) and 0 degrees (vertical).

[0042] The EUV radiation source 10 may also include an EUV light source controller system 60, a laser firing control system 65, along with the beam steering system 32. The EUV radiation source 10 may also include a detector such as a target position detection system which may include one or more droplet imagers 70 that generate an output indicative of the absolute or relative position of a target droplet, e.g., relative to the irradiation region, and provide this output to a target position detection feedback system 62.

[0043] As shown in FIG. 1, the target material delivery system 24 may include a target delivery control system 90. The target delivery control system 90 is operable in response to a signal, for example, the target error described above, or some quantity derived from the target error provided by the system controller 60, to adjust paths of the target droplets 14 through the irradiation region. This may be accomplished, for example, by repositioning the point at which a target delivery mechanism 92 releases the target droplets 14. The droplet release point may be repositioned, for example, by tilting the target delivery mechanism 92 or by laterally translating the target delivery mechanism 92. The target delivery mechanism 92 extends into the chamber 26 and is preferably externally supplied with target material and a gas source to place the target material in the target delivery mechanism 92 under pressure. [0044] Continuing with FIG. 1, the radiation source 10 may also include one or more optical elements. In the following discussion, a collector 30 is used as an example of such an optical element, but the discussion applies to other optical elements as well. The collector 30 may be a normal incidence reflector, for example, implemented as an MLM with additional thin barrier layers, for example B ₄ C, ZrC, S13N4 or C, deposited at each interface to effectively block thermally-induced interlayer diffusion. Other substrate materials, such as aluminum (Al) or silicon (Si), can also be used. The collector 30 may be in the form of a prolate ellipsoid, with a central aperture to allow the laser radiation 12 to pass through and reach the irradiation region. The collector 30 may be, e.g., in the shape of a ellipsoid that as mentioned has a primary focus PF at the irradiation region and an intermediate focus IF on the optical axis OA of the collector 30 where the EUV radiation may be output from the EUV radiation source 10 and input to, e.g., an integrated circuit lithography scanner 50 which uses the radiation, for example, to process a silicon wafer workpiece 52 in a known manner using a reticle or mask 54. The silicon wafer workpiece 52 is then additionally processed in a known manner to obtain an integrated circuit device. [0045] The solid double arrow in FIG. 2 shows the direction of debris propagation. The outline arrows show an advantageous arrangement for ¾ flow. Outlets 42 function as exhaust ports through which the ¾ exits the chamber 26. Arrow G indicates the direction of gravity in one embodiment.

[0046] FIG. 3A is a schematic representation of an arrangement producing such flows. As shown in FIG. 3A, hydrogen flows into the chamber 26 through a cone-shaped inlet 44 positioned in a central aperture of the collector 30 (cone flow) as well as from the top of the chamber 26 from a position near the intermediate focus IF. Also shown is the position of the primary focus PF of the collector 30 on its optical axis OA. Hydrogen flows away from the collector 30 and through outlets 42. Hydrogen entering from the top of the chamber 26 also flows through outlets 42. Also shown in FIG. 3A as a fan module 46 for forcing the gas into the chamber 26. The fan module 46 which may be a fan filter unit is connected to a gas source 48 through a conduit 56. In addition a showerhead that includes a plurality of nozzles that introduce gas into the vessel may be disposed along at least a portion of the inner vessel wall. See International Application Publication No. WO 2018/127565 filed January 5, 2018 and published July 12, 2018, titled “Guiding Device and Associated System,” the specification of which is incorporated herein in its entirety by reference.

[0047] As can be appreciated, EUV sources as described above line rely on hydrogen flows for protection of the collector, metrology optics, and vessel interior surfaces from target material debris. As such systems are currently configured, the hydrogen flow recipe (including, for example, the specific choice of flow rate for collector cone flow, collector perimeter flow, shower flows on liner etc.,) is static in the sense that the recipe is not changed during operation.

[0048] As a consequence of this, the flow recipe may not be optimized for changes in EUV power or changes in exposure pattern. This can lead to an undesirably small process window. For example, high collector cone flow rates can be beneficial for collector protection to better overcome the momentum transfer from ions leaving the plasma and therefore also improve droplet/plasma stability. The downside of higher collector cone flow rates is that entrained tin overshoots the exhaust resulting in high deposition rates inside the vessel which impacts vessel and collector lifetime.

[0049] A static flow recipe can become especially problematic for ion distributions which are anisotropic. In these cases, the flow recipe requires a rebalancing of the total flow to increase flow to the collector cone to protect the collector and vessel walls from excess tin deposition. The required cone flow may be so high that in the absence of the plasma (i.e., off droplet) the flow overshoots the exhaust (asymmetric exhaust in this example) and recirculates back into the vessel causing droplets to be unstable. This is an example of a situation in which there may be no single, static process window that simultaneously maintains tin deposition below acceptable limits while maintaining satisfactory droplet stability.

[0050] Thus, according to an aspect of an embodiment, the process window for the EUV source is optimized by varying the hydrogen flow rates based on the particular use case. For example, it is contemplated that for some future applications the angular ion distribution is quite anisotropic for some plasma recipes. These plasma recipes may be employed at current and higher EUV power levels. For some ion distributions, no process window exists for both on- and off-droplet requirements. According to an aspect of an embodiment, flow rates may be varied to optimize the process window. More specifically, the collector cone flow may be varied. This can be achieved by the use of a fast actuation throttling element to control the rate of the cone flow.

[0051] An increased ion momentum toward the collector may require a rebalancing of the flows to meet collector and vessel protection requirements. In particular, it may be that increased cone flow is required. Cone flow, however, cannot be increased without limit because when the plasma is not present, for example before or after an EUV exposure, excessive cone flow may result in recirculating flows and unstable droplets.

[0052] Thus, based on the total hydrogen flow available it may not be possible with a single flow setting to simultaneously satisfy the droplet stability requirement off-droplet while also satisfying the tin deposition restriction on the vessel and collector. In essence, the center of the process window shifts based on the presence or absence of plasma. According to an aspect of an embodiment issues associated with the shifting process window problem by dynamically adjusting the collector cone flow setting lower while off droplet and higher while on droplet. The timeframe for any such adjustment in the cone flow needs to be on order of the flow-reordering timescale in the vessel. Based on droplet pushout measurements, the timescale for flow reordering is on the order of about 20 ms. Therefore, an actuator for a flow governor must be capable of operating with at least similar response times / bandwidths. [0053] Referring now to FIG. 3B, according to one aspect of an embodiment an arrangement for controlling flow characteristics such as flow rate and composition of gas entering the chamber 26 and/or leaving the chamber 26 can include a fan module 46 which includes a variable flow regulator such as will be described below in conjunction with FIGS. 4A, 4B, 5, and 6. The variable flow regulator in fan module 46 operates under the control of the controller 47 which may be a dedicated hardware controller or maybe a control system distributed across several components and made up of both hardware and software. The controller 47 also controls a controllable mixing valve 64 which is in controllable fluid communication with a first gas source 48 and a second gas source 49. The controllable mixing valve 64 may be used to control the composition of the gas flowing into the chamber 26 through the fan module 46. The composition of the gas flowing into the chamber 26 may be changed depending on the mode of operation of the system. For example, there may be one type of gas, which may be a single species of gas or a mixture of two or more gases, from the first gas source 48 flowing into the chamber 26 during on-droplet operation and a second type of gas introduced from the second gas source 49 into the chamber 26 during off droplet operation. For example, an active gas such as oxygen, which may produce unwanted side effects during plasma generation, may be introduced turn off -droplet operation for purposes such as collector surface repair. [0054] Also shown in FIG. 3B is an exhaust flow regulator 66 positioned in an outlet flow path from one of the outlets 42. The exhaust flow regulator 66 is shown as being arranged in one of the outlet flow paths it will be apparent to one of ordinary skill in the art that the exhaust flow regulator 66 could be placed in additional a outflow paths as well. The exhaust flow regulator 66 operates under control of the controller 47 to change the rate at which gas leaves the chamber 26 through the outlet port 42. The variable flow of structure can be used, for example, to troll vessel pressure. For example, vessel pressure may vary undesirably during an on-off droplet transition. Control of the exhaust flow regulator 66 can be used to compensate for such pressure variation and so contribute to process stability.

[0055] Also, in general hydrogen flows are configured for use cases in which the EUV source is operating at full power. There may be applications, however where it would be beneficial to operate the source at less than full power. At low dose targets the gas flows are contaminated with target material such as tin but there is insufficient momentum transfer from the ions to sufficiently decelerate the cone flow so that the gas in the cone flow vents into the exhaust. Therefore, tin contamination occurs in the vessel above the exhaust. This can be mitigated by reducing the collector cone flow. Decreased collector cone flow is acceptable for collector protection because the power load from the ions is much decreased. When low dose targets are requested by the scanner then the flow setting of the cone flow can be reduced in an automated fashion.

[0056] In some cases it may also be of potential benefit to vary flows during droplet generator startup and shutdown. It may also be useful to vary with time the flow of gas across the face of the collector (“umbrella flow) to prevent stagnation zones from forming. As mentioned, in addition to controlling the amount and sweep of the flow itself, it may be advantageous to modify the shape of the flow with a mechanical means.

[0057] As mentioned, during on-droplet operation, that is, operation when plasma is generated, the plasma behaves in a manner similar to a physical element impeding gas flow, essentially as a rock in a stream of water, diverting the flow and reducing the velocity, especially at the center of the plume. To control for this effect, an obstruction in the form of, for example, a mechanical flow block can be moved into the center of the cone flow between the collector mirror and the bottom of the vessel in a position in the fan filter unit (FFU). This mechanical block may be constructed so that it is not so massive that it cannot be moved move quickly using known technology, such as the type of linear motor used to move hard disk read-write heads, be able to actuate at frequencies up to 50 - 80 kHz. Typical voice coil linear motors of this kind with heads are rated to full range actuation times of 15-20 msec. Enterprise duty motors are even faster.

[0058] The material choice and construction techniques of the obstruction and its support can be chosen to be very light. A minimum of embossing of the support arm and block can make them sufficiently stiff enough that they will not be subject to excessive deflection when placed in the cone flow. [0059] The obstruction can be of any one of a variety of shapes. For example, it may be solid so as to present a solid face to the flow, and aerodynamically shaped to tailor the redirection in flow to achieve the desired results. Additional shapes of the flow block can be used, such as a thin hollow shaped block, obstructions with hollow centers, and shapes that divert the flow or portions of the flow to a specific position. As another option an iris or knife edge may be used to limit flows in or near the vessel. As another option the flow rate may be maintained constant but the gas velocity may be modified by changing the flow pattern, for example, from a narrow jet to a wider flow.

[0060] FIG. 4 A shows an arrangement in which a flow obstruction 100 is placed into the flow path feeding gas to collector cone 44. The obstruction 100 may be placed in the fan module 46. The obstruction 100 is attached by an arm 110 that is in turn attached to a motor 120 which can move the obstruction 100 in and out of the flow path. It will be understood that the obstruction can be made to move fully of the flow path, partially into the flow path, or completely into the flow path. As used here and elsewhere, the term “motor” is used to mean any device giving, imparting, or producing motion. In the example shown, the motor 120 is a voice coil linear motor. In addition to the mechanism for moving the obstruction 100 described above, other mechanisms for moving the obstruction 100 into and out of the cone flow stream can be used, such as solenoids, brush and brushless motors, pneumatic actuators, piezoelectric elements and the like. Further, all such mechanisms can be tuned by the use of mass, springs, dampers, and proportional, integral and derivative (“PID”) parameters to have dynamic properties that enhance the response time and overshoot specific to the frequencies needed for a given application.

[0061] FIG. 4B shows an arrangement similar to that in FIG. 4A except that the obstruction of 100 has an aerodynamic shape. FIG. 5 shows an arrangement similar to that of FIG. 4A except that the obstruction 100 has a tubular shape and is oriented so as to be able to direct the flow to a specific location. The obstruction 100 can have other shapes as well depending on the particular use case. Such an embodiment could have additional benefits in that it could be used to reduce or eliminate overshoot in a more effective manner rather than simply diverting the flow upwards. FIG. 6 shows an arrangement in which a mass flow controller 130 is used as the variable flow regulator. Sufficiently rapid flow controllers may be implemented using any one several possible arrangements. As one example, a rapid mass flow controller (MFC) may be placed as a position that is closer to the source than it would be in a conventional arrangement. MFCs are commercially available that have response times as low as 25 ms.

[0062] In another arrangement flow control is achieved by switching between two or more preset orifices or flow restrictor to quickly transition between two or more flow rates which modulate the chamber flow rate. This multi-orifice arrangement may be placed next to the vessel to ensure rapid change of the flows delivered to the EUV volume. FIG. 7 shows an arrangement in which flow to the chamber 26 is regulated by a switching valve 150 selectably connected to one of flow restrictors 170, 180, and 190. More specifically, gas from a mass flow controller 160 flows in conduit 56 to a switch or valve 150 which selectively connects the conduit 56 to a one of the flow controllers 170, 180, and 190 in a manifold. The valve 150 may be operated under control of a signal generated elsewhere in the system, for example, in the scanner. The flow restrictors 170, 180, 190, may advantageously have different flow impedances. The flow rate along conduit 200 into the chamber 26 will thus depend on which of the flow restrictors the valve 150 connects to the conduit 56. In this manner, the flow rate may be changed swiftly. In an alternative arrangement, the switching valve 150 can be connected to separate sources of gas which each of which is under a different pressure. The valve 150 would selectively connect one of the sources through the conduit 200 to the chamber 26.

[0063] Thus, according to aspects of an embodiment, the use of time- varying hydrogen flow rates can be used to optimize the process window of tin management performance, droplet stability and plasma stability, according to the particular use case of power and/or exposure pattern. The use of time-varying hydrogen flows within a burst to satisfy both on- and off-droplet process window requirements of tin management performance, droplet stability, and plasma stability. A mechanical flow block positioned inside of the center cone flow may be used to modify the flow for off droplet times to limit or eliminate the difference between on and off droplet H2 flows inside of the chamber. The flow block can also be used to physically redirect a portion of the center cone flow to enhance the tin mitigation effects of the liner flow by modifying the direction and location of additional flows inside of the module generating liner flow. The mechanical block may be used to modify the flows emanating from the center cone to reduce or eliminate droplet instability between on and off droplet conditions. A mechanical flow block can also be used to reduce or eliminate the amount of tin driven into the chamber by off-droplet flows of target material.

[0064] FIG. 8 shows a process according to an aspect of an embodiment in which dynamic flow control could be used in the presence of anisotropic ions. The process would start in an initial state S 10 in which the source is operating off droplet, that is, no EUV plasma is being generated. In a step S20 the cone flow can be set at a value, e.g., 100 slm, which maintains the stability of the droplets. In a step S30 EUV exposure is initiated, i.e., the source starts operating in an on-droplet mode. In a step S40, which can be simultaneous with step S30, or even started before S30 if the system is using feed-forward control, the cone flow is set to a value optimized not for droplet stability but instead to minimize accretion of tin debris, for example, 120 slm. This operation continues until the cessation of EUV exposure in step S50. Then, in a step S60, which can be simultaneous with step S50, or even started before a stop determination is made in step S50 if the system is using feed-forward control, the cone flow is set to a value optimized for droplet stability instead of minimization of accretion of tin debris [0065] FIG. 9 is a flowchart showing a process for changing gas composition depending on a mode of operation of the system. It will be understood that the process of FIG. 9 may be used by itself or in coordination with the process described in connection with FIG. 8. In step S10, the system is initiated with off-droplet operation. During off-droplet operation, in step S70, the flow composition is set to include an active gas such as oxygen. This may be type of gas that during on-droplet operation would interfere with plasma generation. In step S30 it is determined whether the system is switching to on- droplet operation. If the system as switching to on-droplet operation, then, in step S80, flow composition is established as being one that does not include the active gas. In in step S50 it is determined whether the system is switching back to off-droplet operation. If not, then the gas composition which does not include the active gas is maintained step in S60. Otherwise, the system reverts to off-droplet operation in step S10.

[0066] The description above is primarily in terms of controlling the cone flow but it will be apparent that the principles are applicable to controlling the flow of gas into the chamber through other inlets. [0067] Embodiments thus have the potential to provide several benefits, including reduction of the amount of tin deposited on the vessel walls, reduction of the amount of tin deposited on the collector, increase in the size of the process window for plasma controls, and increase in the size of the process window for droplet stability.

[0068] 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 aforementioned embodiments, but one of ordinary skill in the art may recognize that many further combinations and permutations of various embodiments are possible. Accordingly, the described embodiments are intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims. Furthermore, to the extent that the term “includes” is used in either the detailed description or the claims, such term is intended to be inclusive in a manner similar to the term “comprising” as “comprising” is construed when employed as a transitional word in a claim. Furthermore, although elements of the described aspects and/or embodiments may be described or claimed in the singular, the plural is contemplated unless limitation to the singular is explicitly stated. Additionally, all or a portion of any aspect and or embodiment may be utilized with all or a portion of any other aspect and/or embodiment, unless stated otherwise.

[0069] Other aspects of the invention are set out in the following numbered clauses.

1. Apparatus for generating EUV radiation by laser irradiation of droplets of a target material, the apparatus comprising: a vessel; an inlet structure defining at least one inlet path adapted and arranged to connect a source of a gas to an interior of the vessel to add the gas to the vessel in a flow along the inlet path; an outlet structure defining at least one outlet path adapted and arranged to be connected to an interior of the vessel to permit gas in the vessel to flow out of the vessel along the outlet path; a variable flow regulator selectably arranged in one of the inlet path and the outlet path and adapted to regulate a characteristic of the flow of the gas into or out of the vessel based at least in part on a mode in which the apparatus is operating; and a controller arranged to control operation of the flow controller.

2. Apparatus as in clause 1 wherein the controller is adapted to operate using a look-ahead control process.

3. Apparatus as in clause 1 further comprising a second variable flow regulator selectably arranged in the other of the inlet path and the outlet path and adapted to regulate a characteristic of the flow of the gas into or out of the vessel based at least in part on a mode in which the apparatus is operating.

4. Apparatus as in clause 1 wherein the apparatus has an on-droplet mode of operation in which droplets generate EUV radiation when irradiated by a laser when the apparatus is in one mode and an off-droplet mode of operation in which the droplets are not used to generate EUV radiation when not irradiated by a laser in another mode.

5. Apparatus as in clause 1 wherein the variable flow regulator is selectably arranged in the inlet path and adapted to regulate a characteristic of the flow of the gas into the vessel based at least in part on the mode in which the apparatus is operating.

6. Apparatus as in clause 5 wherein the characteristic is a flow rate.

7. Apparatus as in clause 5 wherein the characteristic is a flow velocity.

8. Apparatus as in clause 5 wherein the characteristic is a flow profile.

9. Apparatus as in clause 5 wherein the characteristic is a flow composition.

10. Apparatus as in clause 9 further comprising a mixing valve, a source of first gas in fluid communication with the mixing valve, and a source of second gas in fluid communication with the mixing valve, the mixing valve being arranged in fluid communication with the inlet structure and operating under control of the controller to provide one of the first gas, the second gas, and a mixture of the first gas and the second gas to the inlet structure.

11. Apparatus as in clause 9 wherein the flow composition does not contain an active gas during the on-droplet mode and does contain an active gas during the off-droplet mode.

12. Apparatus as in clause 11 wherein the active gas comprises oxygen.

13. Apparatus as in clause 1 wherein the inlet structure comprises a collector cone.

14. Apparatus as in clause 1 wherein the variable flow regulator comprises a flow obstruction and a motor mechanically coupled to the flow obstruction and adapted to move the flow obstruction at least partially into the flow path.

15. Apparatus as in clause 14 wherein the motor comprises a linear motor.

16. Apparatus as in clause 14 wherein the motor comprises a solenoid.

17. Apparatus as in clause 14 wherein the flow obstruction when placed in the flow path presents a solid cross section to the flow. 18. Apparatus as in clause 14 wherein the flow obstruction when placed in the flow path presents to the flow a cross section having at least one aperture.

19. Apparatus as in clause 14 wherein the flow obstruction has an open tubular shape and when placed in the flow path is oriented so that the flow obstruction redirects a portion of the gas.

20. Apparatus as in clause 14 wherein the flow obstruction has an aerodynamic shape.

21. Apparatus as in clause 1 wherein the variable flow regulator comprises a mass flow controller.

22. Apparatus as in clause 1 wherein the variable flow regulator comprises a valve adapted to be in fluid communication with the gas source, and a manifold comprising plurality of fluid conduits respectively connecting the valve to the inlet, each of the plurality of fluid conduits having a respective flow restrictor restricting a flow rate through the respective conduit to a respective value, the valve being arranged to permit the gas to flow through one of the plurality of flow conduits.

23. Apparatus for generating EUV radiation by laser irradiation of droplets of a target material, the apparatus comprising: a vessel having at least one inlet adapted to be connected to a source of a gas and to add the gas to the vessel in a flow along a flow path; a droplet generator arranged to introduce the droplets into the vessel to an irradiation site within the vessel where the droplets are used to generate EUV radiation when irradiated by a laser when the apparatus is in an on-droplet mode and where the droplets are not used to generate EUV radiation when not irradiated by a laser when the apparatus is in an off-droplet mode; and a variable flow regulator selectably arranged in the flow path and adapted to regulate a characteristic of the flow of the gas into the vessel based at least in part on whether the apparatus is in the on-droplet mode or the off-droplet mode.

24. A flow regulator for regulating a characteristic of the flow of a gas from a gas source into a vessel in an apparatus for generating EUV radiation, the flow regulator comprising: an inlet adapted to be in fluid communication with the gas source; an outlet adapted to be in fluid communication with an inlet to the vessel; and a flow restrictor selectably impeding a flow of the gas through the regulator along a flow path from the inlet to the outlet based at least in part on an operational mode of the apparatus.

25. A flow regulator as in clause 24 wherein the flow restrictor comprises a flow obstruction and a motor mechanically coupled to the flow obstruction and adapted to move the flow obstruction to a position completely out of the flow path, completely within the flow path, or partially in the flow path.

26. A flow regulator as in clause 24 wherein the flow restrictor comprises a valve adapted to be in fluid communication with the gas source, and a manifold comprising plurality of fluid conduits respectively connecting the valve to the inlet, each of the plurality of fluid conduits having a respective flow restrictor restricting a flow rate through the respective conduit to respective value, the valve being arranged to permit the gas to flow through one of the plurality of flow conduits.

27. A method of controlling operation of an apparatus for generating EUV radiation by laser irradiation of droplets of a target material in a vessel, the method comprising: operating the apparatus in an off-droplet mode in which the droplets are not used to generate EUV radiation; coincident with the operating step, regulating a characteristic of a flow of a gas at least one of into and out of the vessel based at least in part on the apparatus operating in the off-droplet mode; switching to operating the apparatus in an on-droplet mode in which the droplets are used to generate EUV radiation; and coincident with the switching step, regulating a characteristic of a flow of a gas at least one of into and out of the vessel based at least in part on the apparatus operating in the on-droplet mode.

28. A method as in clause 27 wherein the method is carried out under control of a controller operating in accordance with a look-ahead process.

29. A method as in clause 27 wherein the characteristic is a flow rate.

30. A method as in clause 27 wherein the characteristic is a flow velocity.

31. A method as in clause 27 wherein the characteristic is a flow profile.

32. A method as in clause 27 wherein the characteristic is a flow composition.

33. A method as in clause 32 wherein the flow composition does not contain an active gas during the on-droplet mode and does contain an active gas during the off-droplet mode.

34. A method as in clause 33 wherein the active gas comprises oxygen.

35. A method as in clause 27 wherein the apparatus comprises a flow obstruction and a motor for moving the flow obstruction and wherein the step of regulating a characteristic of a flow of a gas into the vessel based at least in part on the apparatus operating in the off-droplet mode comprises moving the flow obstruction at least partially into a flow path of the gas into the vessel.

36. A method as in clause 27 wherein the apparatus comprises a valve adapted to be in fluid communication with a source of the gas and a manifold comprising plurality of fluid conduits respectively connecting the valve to the vessel, each of the plurality of fluid conduits having a respective flow restrictor restricting a flow rate through the respective conduit to respective value, the valve being arranged to permit the gas to flow through one of the plurality of flow conduits, and wherein the step of regulating a characteristic of a flow of a gas into the vessel based at least in part on the apparatus operating in the off-droplet mode comprises operating the valve to place a selected one of the plurality of conduits in fluid communication with the source of the gas.

[0070] Other implementations are within the scope of the claims.