CN115380626A

CN115380626A - Apparatus and method for controlling gas flow in an EUV light source

Info

Publication number: CN115380626A
Application number: CN202180028172.8A
Authority: CN
Inventors: J·T·斯特瓦特四世; M·G·兰格洛斯; 马悦
Original assignee: ASML Holding NV
Current assignee: ASML Holding NV
Priority date: 2020-04-13
Filing date: 2021-03-16
Publication date: 2022-11-22
Also published as: KR20220167375A; TW202215906A; WO2021209214A1

Abstract

A source and method for generating extreme ultraviolet radiation is disclosed in which the flow characteristics of gas introduced into or exhausted from the source vary at least in part according to the prevailing mode of operation of the source, such as whether the source is in a droplet-on or droplet-off mode of operation.

Description

Apparatus and method for controlling gas flow in an EUV light source

Cross Reference to Related Applications

The present application claims priority from U.S. application No. 63/009,127, entitled APPARATUS FOR AND METHOD OF CONTROLLING GAS FLOW IN AN EUV LIGHT SOURCE, filed on 13/4/2020, which is incorporated herein by reference IN its entirety.

Technical Field

The present disclosure relates to an apparatus and method for generating extreme ultraviolet ("EUV") radiation from a plasma produced by electrical discharge or laser ablation of a source or target material in a container. In such applications, optical elements are used, for example, to collect and direct radiation for semiconductor lithography and inspection.

Background

Extreme ultraviolet radiation (e.g., electromagnetic radiation having a wavelength of about 50nm or less (sometimes also referred to as soft x-rays), and including radiation having a wavelength of about 13.5 nm) may be used in a lithographic process to produce extremely small features in a substrate such as a silicon wafer.

A method for generating EUV radiation includes converting a target material to a plasma state. The target material preferably comprises at least one element, such as xenon, lithium or tin, having one or more emission lines in the EUV part of the electromagnetic spectrum. The target material may be a solid, liquid or gas. In one such method, commonly referred to as laser produced plasma ("LPP"), the desired plasma may be produced by irradiating a target material having the desired line-emitting element with a laser beam.

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

For this process, the plasma is typically generated in a sealed vessel, such as a vacuum chamber, and the resulting EUV radiation is monitored using various types of metrology equipment. In addition to generating EUV radiation, the process used to generate the plasma typically generates undesirable byproducts in the plasma chamber, which may include out-of-band radiation, energetic ions, and debris, such as atoms and/or lumps/droplets of residual target material.

The energetic radiation is emitted from the plasma in all directions. In one common arrangement, a near-normal incidence mirror (often referred to as a "collector mirror" or simply "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 site to a collection of optics, reticles, detectors, and ultimately to the silicon wafer.

In the EUV portion of the spectrum, it is generally considered necessary to use reflective optics for the optical elements in the system, including collectors, illuminators and projection optics boxes. These reflective optics may be implemented as normal incidence optics or grazing incidence optics as mentioned. At the wavelengths of interest, the collector is advantageously implemented as a multilayer mirror ("MLM"). As the name implies, the MLM is typically composed of alternating layers of material (MLM stack) on a base or substrate. The system optics may also be configured as coated optical elements, even if they are not implemented as MLMs.

The optical element, in particular the collector, has to be placed inside a container with plasma to collect and redirect the EUV radiation. The environment within the chamber is detrimental to the optical components and therefore can limit their useful life, for example by reducing reflectivity. Optical elements within the environment may be exposed to high energy ions or particles of the target material. Particles of the target material (essentially debris from the laser vaporization process) may contaminate the exposed surfaces of the optical element. Particles of the target material may also cause physical damage and localized heating to the MLM surface.

In some systems, the pressure is in the range of about 0.5 to about 3 mbar H ₂ The gas is used as a buffer gas in the vacuum chamber to reduce debris. Without the gas, it is difficult to adequately protect the collector from target material fragments ejected from the irradiation region under vacuum pressure. Hydrogen is relatively transparent to EUV radiation having a wavelength of about 13.5nm and is therefore superior to other candidate gases, such as He, ar, or other gases that exhibit higher absorption at about 13.5 nm.

H ₂ Gas is introduced into the vacuum chamber to slow down energetic fragments (ions, atoms, and clusters) of the target material generated by the plasma. Debris passing collision with gas moleculesAnd then decelerates due to collision. For this purpose, H ₂ The gas stream is used, the H ₂ The air flow may also be opposite the debris trajectory and away from the collector. This serves to reduce damage to the collector's optical coating from deposition, injection and sputtering of target materials.

Thus, the process of converting the target material generates particles and deposits residual target material on the surface, with unobstructed paths between the irradiation station and the surface and in the exhaust path of the gas entraining the residual target material. For example, if the gas is pumped through the blade tips present in the chamber and to the mechanical pump, material is quickly deposited on all cold metal parts. If the target material is tin, this may result in the growth of tin wool, which may fall onto the collector optics and clog the discharge path.

When a target material such as tin is irradiated with laser radiation to generate plasma, a specific portion of the target material becomes fragmented. For example, the target material fragments may include Sn vapor, snH4 vapor, sn atoms, sn ions, sn clusters, sn particles, sn nanoparticles, and Sn deposits. The efficiency, lifetime and usability of an EUV collector may be reduced when Sn debris accumulates on the EUV collector or on one or more inner container walls of the EUV container.

Tin debris from the source vessel may pass through the intermediate focus from the EUV source to the scanner, which may lead to contamination of expensive optical elements, e.g. the illuminator in the scanner, lifetime is critical for the productivity and ownership costs of the EUV system. As mentioned above, one form of tin contamination is the ejection or "jetting" of molten tin from the walls in the source vessel near the intermediate focus. One technique for preventing tin debris from reaching the scanner involves applying a dynamic airlock at the intermediate focus to inhibit tin contamination, as disclosed in U.S. patent No. 9,606,445 entitled "Lithographic Apparatus and Method of Manufacturing a Device," issued 3, 28, 2017, the entire contents of which are incorporated herein by reference.

The process of generating EUV light may also result in target material being deposited on the walls of the container. Controlling the deposition of target materials on the container walls is important to achieve an acceptable long-life EUV source placed in production. Furthermore, managing the target material flux from the irradiation site is important to ensure that the waste target material mitigation system operates as intended.

Disclosure of Invention

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 intended to neither identify key or critical elements of all embodiments nor delineate the scope of any or all embodiments. Its sole purpose is to present some concepts of one or more embodiments in a simplified form as a prelude to the more detailed description that is presented later.

According to one aspect of an embodiment, a system for optimizing a process window by allowing dynamic variation of characteristics of a gas flow into a chamber surrounding an EUV source is disclosed.

According to another aspect of the embodiments, there is disclosed a device for generating EUV radiation by laser irradiation of droplets of a target material, the device comprising a container, an inlet structure (defining at least one inlet path adapted and arranged to connect a gas source to the interior of the container to add gas to the container in the form of a stream along the inlet path), an outlet structure (defining at least one outlet path adapted and arranged to be connected to the interior of the container to allow gas in the container to flow out of the container along the outlet path), a variable flow regulator (selectably arranged in one of the inlet and outlet paths and adapted to regulate a characteristic of the gas flow into or out of the container based at least in part on the mode in which the device is operating), and a controller (arranged to control the operation of the controller). The controller may be adapted to operate using a look-ahead control process. The device may also include a second variable flow regulator selectively disposed in the other of the inlet path and the outlet path and adapted to regulate a characteristic of gas flow into or out of the container based at least in part on a mode in which the device is operating.

The device may have a droplet-on mode of operation when the device is in one mode in which the droplet generates EUV radiation when irradiated by the laser, and a droplet-off mode of operation when the device is in another mode in which the droplet is not used to generate EUV radiation when not irradiated by the laser. The variable flow regulator may be selectively disposed in the inlet path, partially or wholly, or not at all, and is adapted to regulate a characteristic of the gas flow into the vessel based at least in part on the mode in which the device is operating. The characteristic may be any one or combination of flow, velocity, flow profile and flow composition. The stream composition may be such that it does not contain reactive gas during the droplet-on mode and contains reactive gas during the droplet-off mode. The reactive gas may include oxygen. The inlet structure may comprise a collector cone.

The variable flow regulator includes 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" herein and elsewhere includes any device used to generate power. The motor may comprise a linear motor. The motor may comprise a solenoid. When placed in the flow path, the flow obstruction may convectively assume a solid cross-section or a cross-section with at least one aperture. The flow obstruction may have an open tubular shape and be oriented such that the flow obstruction redirects a portion of the gas when placed in the flow path. The flow obstruction may have an aerodynamic shape. The variable flow regulator may include a mass flow controller.

The variable flow regulator may include a valve adapted to be in fluid communication with a gas source and a manifold including a plurality of fluid conduits respectively connecting the valve to the inlet, each of the plurality of fluid conduits having a respective flow restrictor restricting flow through the respective conduit to a respective value, the valve being arranged to allow gas to flow through one of the plurality of flow conduits.

According to another aspect of the embodiments, there is disclosed a device for generating EUV radiation by laser radiation of droplets of a target material, the device comprising: a container having at least one inlet adapted to be connected to a gas source and to add gas to the container in a stream along a flow path; a droplet generator arranged to introduce droplets into the container to a radiation station within the container, the droplets being used to generate EUV radiation when irradiated by the laser when the device is in a droplet-on mode and not being used to generate EUV radiation when not irradiated by the laser when the device is in a droplet-off mode; and a variable flow regulator selectably disposed in the flow path and adapted to regulate a characteristic of the flow of gas into the vessel based at least in part on whether the device is in a droplet-on mode or a droplet-off mode.

According to another aspect of an embodiment, a flow regulator is disclosed for regulating flow characteristics of a gas from a gas source into a container in a device for generating EUV radiation, the flow regulator comprising: an inlet adapted to be in fluid communication with a gas source; an outlet adapted to be in fluid communication with an inlet of the vessel; and a flow restrictor selectively preventing gas from flowing through the regulator along a flow path from the inlet to the outlet based at least in part on an operating mode of the device. The flow restrictor may include a flow obstruction and a motor mechanically coupled to the flow obstruction and adapted to move the flow obstruction to a position entirely outside of the flow path, entirely within the flow path, or partially within the flow path. The flow restrictor may comprise a valve adapted for fluid communication with a gas source and a manifold comprising a plurality of fluid conduits respectively connecting the valve to the inlet, each of the plurality of fluid conduits having a respective flow restrictor restricting the flow through the respective conduit to a respective value, the valve being arranged to allow gas to flow through one of the plurality of flow conduits.

According to another aspect of the embodiments, there is disclosed a method of controlling operation of an apparatus for generating EUV radiation by laser radiation of droplets of a target material in a container, the method comprising: operating the device in a droplet off mode in which the droplets are not used to generate EUV radiation; simultaneously with the operating step, adjusting a characteristic of the flow of gas into and out of at least one of the containers based at least in part on the device operating in the droplet-off mode; switching to operating the device in a droplet-on mode, wherein a droplet is used to generate EUV radiation; and adjusting, concurrently with the switching step, a characteristic of the flow of gas into and out of at least one of the containers based at least in part on the device operating in the droplet-on mode. The method may be performed under control of a controller operating according to a look-ahead process. The characteristic may be any one or combination of flow, velocity, flow profile and flow composition. The stream composition may be such that it does not contain a reactive gas during the droplet-on mode and contains a reactive gas during the droplet-off mode. The reactive gas may include oxygen. The apparatus may include a flow barrier and a motor for moving the flow barrier, and the step of adjusting a characteristic of the gas flow into the container based at least in part on the apparatus operating in the droplet-off mode comprises: the flow barrier is moved at least partially into the flow path of the gas into the container. The apparatus may include: a valve adapted to be in fluid communication with a source of gas; and a manifold comprising a plurality of fluid conduits respectively connecting the valves to the containers, each of the plurality of fluid conduits having a respective flow restrictor restricting flow through the respective conduit to a respective value, the valves being arranged to allow gas to flow through one of the plurality of flow conduits, and the step of adjusting the characteristics of the gas flow into the containers based at least in part on the apparatus operating in the droplet-closing mode comprises: the valve is operated to place a selected one of the plurality of conduits in fluid communication with a source of gas.

Further embodiments, features, and advantages of the present inventions, as well as the structure and operation of the various embodiments, are described in detail below with reference to the accompanying drawings.

Drawings

FIG. 1 is a schematic, not-to-scale view of the general broad concept of a laser produced plasma EUV radiation source system in accordance with an aspect of an embodiment.

Figure 2 is a diagram, not to scale, showing a possible arrangement of a receptacle and exhaust system for use in a laser produced plasma EUV radiation source system.

Fig. 3A is a cross-sectional schematic view, not to scale, of a possible arrangement of a system for introducing gas into a vessel in accordance with an aspect of an embodiment.

Fig. 3B is a cross-sectional schematic view, not to scale, of a possible arrangement of a system for controlling gas flow into and/or out of a vessel in accordance with an aspect of an embodiment.

Fig. 4A is a cross-sectional schematic view, not to scale, of a possible arrangement of a vessel and a gas inlet according to an aspect of an embodiment.

Fig. 4B is a cross-sectional schematic view, not to scale, of a possible arrangement of a vessel and a gas inlet according to an aspect of an embodiment.

Fig. 5 is a cross-sectional schematic view, not to scale, of a possible arrangement of a vessel and a gas inlet according to an aspect of an embodiment.

Fig. 6 is a cross-sectional schematic view, not to scale, of a possible arrangement of a vessel and a gas inlet in accordance with an aspect of an embodiment.

Fig. 7 is a cross-sectional schematic view, not to scale, of a possible arrangement of a system for introducing gas into a vessel in accordance with an aspect of an embodiment.

Fig. 8 is a flow diagram of a process for introducing gas into a vessel in accordance with an aspect of an embodiment.

FIG. 9 is a flow diagram of a process for introducing gas into a vessel in accordance with another aspect of the embodiments.

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 present 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(s) based on the teachings contained herein.

Detailed Description

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 provide a thorough understanding of one or more embodiments. However, it may be evident in some or all examples that any of the embodiments described below may be practiced without the specific design details described below. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate describing one or more embodiments.

Before describing such embodiments in more detail, however, it is helpful to set forth an example environment in which embodiments of the invention may be implemented. In the following description and claims, the terms "upward," "downward," "top," "bottom," "vertical," "horizontal," and the like may be used. Unless otherwise indicated, these terms are intended to show relative orientation only, and not any orientation with respect to gravity.

Referring initially to FIG. 1, a schematic diagram of an exemplary EUV radiation source (e.g., a laser-produced plasma EUV radiation source 10) is shown in accordance with an 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 be, for example, a pulsed gas discharge CO ₂ A laser source producing a 10.6 μm or 1 μm radiation beam 12 focused down to a main focus PF. Pulsed gas discharge of CO ₂ The laser source may have DC or RF excitation operating at high power and high pulse repetition rate.

The EUV radiation source 10 also includes a target delivery system 24 for delivering the target material in the form of liquid droplets or a continuous liquid stream. In this example, the target material is a liquid, but may also be a solid or a gas. The target material may be made of tin or a tin compound, but other materials may also be used. In the depicted system, target material delivery system 24 introduces droplets 14 of target material into the interior of vacuum chamber 26 to a radiation region at the PF of collector 30, where the target material can be radiated to generate a plasma. The vacuum chamber 26 may be provided with a liner. In some cases, an electrical charge is placed on the target material to allow the target material to be diverted or away from the irradiation region. It should be noted that as used herein, a radiation region is a region where target material radiation may or is intended to occur, and is a radiation region even when radiation is not actually occurring. The EUV light source may also include a beam steering system 32.

In the system shown, the components are arranged such that the droplet 14 travels substantially horizontally. The direction from the laser source 22 towards the irradiation region (i.e. the nominal direction of propagation of the beam 12) may be considered the Z-axis. The path taken by droplet 14 from target material delivery system 24 to the irradiation region may be considered the X-axis. Thus, the view of FIG. 1 is normal to the XZ plane. The orientation of the EUV radiation source 10 is preferably rotated with respect to gravity as shown, with arrow G showing the preferred orientation downward with respect to gravity. This orientation is suitable for EUV sources, but not necessarily for optical downstream components such as scanners and the like. Also, while a system in which droplets 14 travel substantially horizontally is depicted, one of ordinary skill in the art will appreciate that other arrangements in which droplets travel vertically or at some angle relative to gravity (between and including 90 degrees (horizontal) and 0 degrees (vertical)) may be used.

The EUV radiation source 10 may also include an EUV light source controller system 60, a laser firing control system 65, and a 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 the target droplet, for example, with respect to the radiation region, and provide the output to the target position detection feedback system 62.

As shown in fig. 1, the targeted material delivery system 24 may include a targeted delivery control system 90. The target delivery control system 90 is operable in response to a signal, such as the target error described above or some quantity derived from the target error provided by the system controller 60, to adjust the path of the target droplet 14 through the irradiation region. This may be accomplished, for example, by repositioning the point at which target delivery mechanism 92 releases target droplet 14. The droplet release point may be repositioned, for example, by tilting the target delivery mechanism 92 or by translating the target delivery mechanism 92 laterally. A target delivery mechanism 92 extends into the chamber 26 and the target material and gas source are preferably supplied externally to place the target material under pressure in the target delivery mechanism 92.

Continuing with FIG. 1, radiation source 10 may also include one or more optical elements. In the following discussion, collector 30 is used as an example of such an optical element, but the discussion is also applicable to other optical elements. Collector 30 may be a normal incidence reflector, for example implemented as an MLM, e.g., B ₄ C、ZrC、Si ₃ N ₄ Or C, additional thin barriers are deposited at each interface to effectively block thermally induced interlayer diffusion. Other substrate materials may also be used, such as aluminum (Al) or silicon (Si). The collector 30 may be in the form of an elongated ellipsoid with a central aperture to allow the laser radiation 12 to pass through and reach the radiation area. The collector 30 may be, for example, ellipsoidal in shape, which, as mentioned, has a primary focus PF at the radiation area and an intermediate focus IF on the optical axis OA of the collector 30, EUV radiation may be output from the EUV radiation source 10 and input to, for example, an integrated circuit lithography scanner 50, which integrated circuit lithography scanner 50 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.

The solid double arrows in fig. 2 show the direction of debris propagation. Outline arrow shows H ₂ Advantageous arrangement of the flows. The outlet 42 serves as H ₂ An exhaust port exiting the chamber 26. Arrow G indicates the direction of gravity in one embodiment.

Fig. 3A is a schematic representation of an arrangement for generating such a flow. As shown in fig. 3A, hydrogen gas flows into the chamber 26 from the top of the chamber 26 through a tapered inlet 44 (conical flow) located in the central aperture of the collector 30 and from a location near the intermediate focus IF. The position of the primary focus PF of the collector 30 on its optical axis OA is also shown. Hydrogen flows from the accumulator 30 and through the outlet 42. Hydrogen gas entering from the top of the chamber 26 also flows through the outlet 42. Also shown in fig. 3A is a fan module 46 for forcing gas into the chamber 26. Fan module 46, which may be a fan filter unit, is connected to air supply 48 by conduit 56. In addition, a showerhead comprising a plurality of nozzles that introduce gas into the vessel may be disposed along at least a portion of the interior wall of the vessel. See international application publication No. WO 2018/127565 entitled "Guiding Device and Associated System" filed on 5.1.2018 and published on 12.7.7.2018, the specification of which is incorporated herein by reference in its entirety.

It can be appreciated that the EUV sources described above rely on a hydrogen gas flow to protect the collector, metrology optics and interior surfaces of the vessel from target material debris. As such systems are currently being configured, the hydrogen gas flow formulation (including specific flow options such as collector cone flow, collector perimeter flow, shower flow on the pad, etc.) is static in the sense that the formulation is not altered during operation.

Therefore, the flow recipe may not be optimized for changes in EUV power or changes in exposure mode. This may result in an undesirably small process window. For example, a high collector cone flow may be beneficial for collector protection to better overcome momentum transfer of ions out of the plasma, thereby improving droplet/plasma stability. A disadvantage of a higher collector cone flow is that entrained tin can exceed the venting, resulting in a high deposition rate within the vessel, thereby affecting the useful life of the vessel and collector.

Static flow recipes can become particularly problematic for anisotropic ion distributions. In these cases, the flow distributor needs to rebalance the total flow to increase the flow to the collector cone to protect the collector and the container walls from excessive tin deposition. The required cone flow may be so high that in the absence of plasma (i.e. droplet shut-off) the flow would exceed the exhaust (asymmetric exhaust in this example) and recirculate back into the vessel, causing droplet instability. This is an example of a situation where there may not be a single static process window that can simultaneously maintain tin deposition below acceptable limits while maintaining satisfactory droplet stability.

Thus, according to an aspect of an embodiment, the process window of the EUV source is optimized by varying the hydrogen flow rate based on the specific use case. For example, it is envisaged that for some future applications, for some plasma recipes, the angular ion distribution is rather anisotropic. These plasma recipes can be used for current and higher EUV power levels. For some ion distributions, there is no process window for droplet-on and droplet-off requirements. According to an aspect of an embodiment, the flow may be changed to optimize the process window. More specifically, the collector cone flow may be varied. This may be accomplished by controlling the rate of cone flow using a rapidly actuated throttling element.

The increased ion momentum toward the collector may require rebalancing of the flow to meet collector and container protection requirements. In particular, it may be desirable to increase the cone flow. However, the cone flow cannot be increased without limitation because too much cone flow may result in recirculation flow and unstable droplets when plasma is not present, e.g. before or after EUV exposure.

Thus, it may not be possible to simultaneously meet droplet stability requirements for droplet closure while also meeting tin deposition limits on the container and collector using a single stream setup based on the total hydrogen flow available. In essence, the center of the process window will shift based on the presence or absence of plasma. According to an aspect of the embodiments, the problem associated with the shifted process window problem is dynamically adjusted by the collector cone stream settings to be lower when the droplet is off and higher when the droplet is on. The time frame for any such adjustment to the pyramidal stream needs to be the same as the flow reordering time scale in the container. The time scale for flow reordering based on droplet ejection measurements is about 20ms. Therefore, the actuators of the flow conditioner must be able to operate with at least similar response times/bandwidths.

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 chamber 26 and/or exiting chamber 26) may include a fan module 46, the fan module 46 including a variable flow regulator, such as will be described below in connection with fig. 4A, 4B, 5, and 6. The variable flow regulators in fan module 46 operate under the control of controller 47, which controller 47 may be a dedicated hardware controller or may be a control system distributed over multiple components and composed of hardware and software. Controller 47 also controls a controllable mixing valve 64 in controllable fluid communication with first gas source 48 and second gas source 49. A controllable mixing valve 64 may be used to control the composition of the gas flowing into chamber 26 through fan module 46. The composition of the gas flowing into the chamber 26 may vary depending on the operating mode 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, flowing into the chamber 26 from the first gas source 48 during a droplet-on operation, and a second type of gas introduced into the chamber 26 from the second gas source 49 during a droplet-off operation. For example, reactive gases (such as oxygen) that may produce undesirable side effects during plasma generation may be introduced to shut down droplet operations for purposes such as collector surface remediation.

Also shown in fig. 3B is an exhaust flow conditioner 66 located in the outlet flow path from one of the outlets 42. The exhaust flow conditioner 66 is shown disposed in one of the outlet flow paths, and it will be apparent to those of ordinary skill in the art that the exhaust flow conditioner 66 may also be placed in the outlet flow path in addition. The exhaust flow regulator 66 operates under the control of the controller 47 to vary the rate at which gas exits the chamber 26 through the outlet 42. The variable flow structure may be used, for example, to control vessel pressure. For example, the container pressure may undesirably change during a switching droplet transition. Control of the exhaust flow regulator 66 may be used to compensate for such pressure variations and thus contribute to process stability.

Also, hydrogen gas flows are typically configured for use cases where the EUV source is operating at full power. There may be applications, but it may be beneficial to operate the source at less than full power. At low dose targets, the gas flow is contaminated with target material (such as tin), but the momentum transfer from the ions is not sufficient to slow the cone flow sufficiently so that the gas in the cone flow is vented into the exhaust. Thus, tin contamination occurs in the container above the vent. This can be mitigated by reducing the collector cone flow. For collector protection, reducing collector cone flow is acceptable because the power load from the ions is greatly reduced. When the scanner requests a low dose target, then the flow setting of the cone flow may be reduced in an automatic manner.

In some cases, changing the flow during drop generator start-up and shut-down may also have potential benefits. It may also be useful to vary the flow of air over the collector face over time ("umbrella flow") to prevent stagnant zones from forming. As mentioned, in addition to controlling the amount and sweep of the flow itself, it may be advantageous to modify the flow shape with mechanical components.

As mentioned, during droplet-on operation, i.e. operation when the plasma is generated, the plasma behaves in a similar manner to the physical element that impedes the gas flow, essentially like a rock in a water flow, diverting the flow and reducing the velocity, especially at the center of the plume. To control this effect, obstructions, for example in the form of mechanical flow blocks, may be moved into the cone flow center between the collector mirror and the bottom of the vessel, in locations in the Fan Filter Unit (FFU). Such a mechanical block may be constructed so as not to be too large to be moved quickly using known techniques, such as a linear motor type for moving a hard disk read-write head, capable of actuation at frequencies up to 50 to 80 kHz. The nominal full range actuation time for such a typical voice coil linear motor with heads is 15 to 20 milliseconds. Enterprise work motors are even faster.

The material selection and construction technique of the obstacle and its support can be chosen to be very light. The minimal embossing of the support arms and blocks may make them sufficiently stiff that they are not excessively deflected when placed in the cone flow.

The obstruction may be any of a variety of shapes. For example, it may be solid to present a solid face to the flow, and it may be aerodynamically shaped to adjust the redirection in the flow to achieve the desired result. Additional shapes of the flow block may be used, such as a thin hollow shaped block, an obstruction with a hollow center, and a shape that diverts the flow or part of the flow to a specific location. As another option, an iris or knife edge may be used to restrict flow within or near the container. 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 stream.

Fig. 4A shows an arrangement in which a flow obstruction 100 is placed in the flow path feeding gas to the collector cone 44. The barrier 100 may be placed in the fan module 46. The barrier 100 is attached by an arm 110, the arm 110 in turn being attached to a motor 120, which motor 120 can move the barrier 100 into and out of the flow path. It is to be understood that the obstruction may be moved fully into the flow path, partially into the flow path, or fully into the flow path. The term "motor" as used herein and elsewhere is used to refer to any device that imparts, imparts or generates motion. In the example shown, the motor 120 is a voice coil linear motor. In addition to the mechanisms described above for moving the obstacle 100, other mechanisms for moving the obstacle 100 into and out of the stream of cones may be used, such as solenoids, brushed and brushless motors, pneumatic actuators, piezoelectric elements, and the like. Further, all such mechanisms can be tuned to have dynamic characteristics by using mass, springs, dampers, and proportional, integral, and derivative ("PID") parameters to improve response time and overshoot specific to the frequency required for a given application.

Fig. 4B shows an arrangement similar to that of fig. 4A, except that the obstacle of 100 has an aerodynamic shape. Fig. 5 shows an arrangement similar to that of fig. 4A, except that the barrier 100 has a tubular shape and is oriented so as to be able to direct the flow to a specific location. The obstacle 100 may also have other shapes depending on the particular use case. Such an embodiment may have additional benefits in that it may be used to reduce or eliminate overshoot in a more efficient manner, rather than simply diverting the flow upward. Fig. 6 shows an arrangement in which the mass flow controller 130 is used as a variable flow regulator. A sufficiently fast flow controller may be implemented using any of several possible arrangements. As one example, a fast Mass Flow Controller (MFC) may be placed closer to the source than it would in a conventional arrangement. MFC is commercially available with response times as low as 25ms.

In another arrangement, flow control is achieved by switching between two or more preset orifices or restrictors to quickly transition between two or more of the modulation chamber flows. Such a multi-orifice structure may be placed beside the container to ensure a fast change of the flow delivered to the EUV volume. Fig. 7 shows an arrangement in which flow to the chamber 26 is regulated by a switching valve 150, the switching valve 150 being selectively connected to one of the

restrictors

170, 180 and 190. More specifically, gas from the mass flow controller 160 flows in the conduit 56 to a switch or valve 150 that selectively connects the conduit 56 to one of the

flow controllers

170, 180, and 190 in the manifold. The valve 150 may be operated under control of a signal generated elsewhere in the system (e.g., a scanner). The flow restrictors 170, 180, 190 may advantageously have different flow impedances. The flow rate into the chamber 26 along the conduit 200 will therefore depend on which restrictor of the conduit 56 the valve 150 is connected to. In this way, the flow rate can be changed rapidly. In an alternative arrangement, the switching valve 150 may be connected to separate gas sources, each at a different pressure. The valve 150 will selectively connect one of the sources to the chamber 26 through conduit 200.

Thus, according to various aspects of the embodiments, the use of a time-varying hydrogen flow rate may be used to optimize the process window for tin management performance, droplet stability, and plasma stability according to specific use cases of power and/or exposure modes. Hydrogen gas is flowed within the burst using time to meet the droplet-on and droplet-off process window requirements for tin management performance, droplet stability, and plasma stability. A mechanical flow block located inside the central cone stream can be used to modify the flow of droplet closing times to limit or eliminate the difference between droplet on and droplet off H2 flow inside the chamber. The flow block may also be used to physically redirect a portion of the core cone flow to enhance the tin reduction effect of the pad flow by modifying the direction and location of additional flows that generate the pad flow inside the module. The mechanical block may be used to modify the flow emanating from the central cone to reduce or eliminate droplet instability between droplet-on and droplet-off conditions. Mechanical flow blocks may also be used to reduce or eliminate the amount of tin driven into the chamber by the droplet-closing stream of target material.

Fig. 8 illustrates a process in accordance with an aspect of an embodiment, wherein dynamic flow control can be used in the presence of anisotropic ions. The process will start in an initial state S10, where the source is operated with the droplet off, i.e. no EUV plasma is generated. In step S20, the cone flow may be set to a value, e.g. 100slm, which maintains the stability of the droplet. In step S30, an EUV exposure is initiated, i.e. the source starts to operate in droplet-on mode. In step S40, which may be performed simultaneously with step S30, or if the system uses feed forward control, even starting before S30, the cone flow is set to a value that is not optimized for droplet stability, but for minimizing the build-up of tin fragments, e.g. 120slm. This operation continues until the EUV exposure is stopped in step S50. Then, in step S60, which may be simultaneous with step S50, or if the system is using feed forward control, even starting before the stop determination is made in step S50, the cone flow is set to a value optimized for droplet stability, rather than minimizing the build-up of tin debris.

Fig. 9 is a flow chart illustrating a process of changing the gas composition depending on the operation mode of the system. It is to be understood that the process of fig. 9 may be used alone or in cooperation with the process described in conjunction with fig. 8. In step S10, the system is initiated with a droplet shutdown operation. During droplet closing operations, in step S70, the flow composition is set to include a reactive gas such as oxygen. This may be the type of gas that may interfere with plasma generation during droplet-on operations. In step S30, it is determined whether the system is switching to droplet-on operation. If the system switches to droplet-on operation, then in step S80, the stream composition is established as a composition that does not include the reactive gas. In step S50, it is determined whether the system is switching back to droplet closing operation. If not, the gas composition excluding the reactive gas is maintained in step S60. Otherwise, the system reverts to droplet shutdown operation in step S10.

The above description is primarily concerned with controlling the cone flow, but it will be apparent that these principles apply to controlling the entry of gas into the chamber through other inlets.

Embodiments thus have the potential to provide several benefits, including reducing the amount of tin deposited on the vessel walls, reducing the amount of tin deposited on the collector, increasing the size of the process window for plasma control, and increasing the size of the process window for droplet stability.

What has been described above 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 interpreted 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.

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

1. A device for generating EUV radiation by laser irradiation of droplets of a target material, the device comprising:

a container;

an inlet arrangement defining at least one inlet path adapted and arranged to connect a gas source to the interior of the container to add gas to the container in a stream along the inlet path;

an outlet structure defining at least one outlet path adapted and arranged to be connected to the interior of the vessel to allow gas in the vessel to flow out of the vessel along the outlet path;

a variable flow regulator selectively disposed in one of the inlet and outlet paths and adapted to regulate a characteristic of the flow of gas into or out of the vessel based at least in part on a mode in which the device is operating; and

a controller arranged to control operation of the flow controller.

2. The apparatus according to clause 1, wherein the controller is adapted to operate using a look-ahead control process.

3. The device according to clause 1, further comprising a second variable flow regulator selectively disposed in the other of the inlet path and the outlet path and adapted to regulate a characteristic of the flow of gas into or out of the container based at least in part on the mode in which the device is operating.

4. The device according to clause 1, wherein the device has a droplet-on mode of operation when the device is in one mode in which a droplet generates EUV radiation when irradiated by the laser, and a droplet-off mode of operation when in another mode in which a droplet is not used to generate EUV radiation when not irradiated by the laser.

5. The device according to clause 1, wherein the variable flow regulator is selectively disposed in the inlet path and adapted to regulate the characteristics of the flow of gas into the vessel based at least in part on the mode in which the device is operating.

6. The apparatus according to clause 5, wherein the characteristic is flow rate.

7. The apparatus according to clause 5, wherein the characteristic is a flow rate.

8. The apparatus according to clause 5, wherein the characteristic is a flow profile.

9. The apparatus according to clause 5, wherein the characteristic is stream composition.

10. The apparatus according to clause 9, further comprising a mixing valve, a first gas source in fluid communication with the mixing valve, and a second gas source in fluid communication with the mixing valve, the mixing valve being arranged in fluid communication with the inlet structure and operating under the 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. The device according to clause 9, wherein the stream composition does not contain a reactive gas during the droplet-on mode and contains a reactive gas during the droplet-off mode.

12. The apparatus according to clause 11, wherein the reactive gas comprises oxygen.

13. The apparatus according to clause 1, wherein the inlet structure comprises a collector cone.

14. The device according to 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. The apparatus of clause 14, wherein the motor comprises a linear motor.

16. The apparatus of clause 14, wherein the motor comprises a solenoid.

17. The apparatus according to clause 14, wherein the flow obstruction, when placed in the flow path, presents a solid cross-section to the flow.

18. The apparatus according to clause 14, wherein the flow obstruction, when placed in the flow path, presents a cross-section having at least one aperture.

19. The apparatus according to clause 14, wherein the flow barrier has an open tubular shape and, when placed in the flow path, is oriented such that the flow barrier redirects a portion of the gas.

20. The device according to clause 14, wherein the flow obstacle has an aerodynamic shape.

21. The device according to clause 1, wherein the variable flow regulator comprises a mass flow controller.

22. The device according to clause 1, wherein the variable flow regulator comprises

A valve adapted to be in fluid communication with a gas source, an

A manifold comprising a plurality of fluid conduits respectively connecting the valves to the inlets, each fluid conduit of the plurality of fluid conduits having a respective flow restrictor that restricts flow through the respective conduit to a respective value,

the valve is arranged to allow gas to flow through one of the plurality of flow conduits.

23. A device for generating EUV radiation by laser irradiation of droplets of a target material, the device comprising:

a container having at least one inlet adapted to be connected to a source of gas and adapted to add gas to the container in a stream along a flow path;

a droplet generator arranged to introduce droplets into the container to a radiation site within the container, wherein when the device is in a droplet-on mode, the droplets are used to generate EUV radiation when irradiated by the laser, and wherein when the device is in a droplet-off mode, the droplets are not used to generate EUV radiation when not irradiated by the laser; and

a variable flow regulator selectively disposed in the flow path and adapted to regulate a characteristic of the flow of gas into the container based at least in part on whether the device is in a droplet-on mode or a droplet-off mode.

24. A flow regulator for regulating characteristics of a flow of gas from a gas source into a container in a device for generating EUV radiation, the flow regulator comprising:

an inlet adapted to be in fluid communication with a gas source;

an outlet adapted to be in fluid communication with an inlet of the vessel; and

a flow restrictor selectively prevents a flow of gas from flowing through the regulator along a flow path from the inlet to the outlet based at least in part on an operating mode of the device.

25. The flow regulator of 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 entirely outside the flow path, entirely within the flow path, or partially within the flow path.

26. The flow regulator of clause 24, wherein the flow restrictor comprises

A valve adapted to be in fluid communication with a gas source, an

27. A method of controlling the operation of a device for generating EUV radiation by laser irradiation of droplets of a target material in a container, the method comprising:

operating the device in a droplet off mode in which the droplet is not used to generate EUV radiation;

simultaneously with the operating step, adjusting a characteristic of a flow of gas into at least one of the container and out of the container based at least in part on the device operating in a droplet-off mode;

switching to operating the device in a droplet-on mode in which the droplet is used to generate EUV radiation; and

simultaneously with the switching step, adjusting a characteristic of the flow of gas into at least one of the container and out of the container based at least in part on the device operating in the droplet on mode.

28. The method of clause 27, wherein the method is performed under the control of a controller operating according to a look-ahead process.

29. The method according to clause 27, wherein the characteristic is flow.

30. The method of clause 27, wherein the characteristic is flow rate.

31. The method according to clause 27, wherein the characteristic is flow profile.

32. The method of clause 27, wherein the characteristic is stream composition.

33. The method according to clause 32, wherein the stream composition does not contain a reactive gas during the droplet-on mode and contains a reactive gas during the droplet-off mode.

34. The method of clause 33, wherein the reactive gas comprises oxygen.

35. The method according to clause 27, wherein the device comprises a flow barrier and a motor for moving the flow barrier, and wherein the step of adjusting the characteristics of the flow of gas into the container based at least in part on the device operating in the droplet-off mode comprises: the flow barrier is moved at least partially into the flow path of the gas into the container.

36. The method of clause 27, wherein the device comprises: a valve adapted to be in fluid communication with a source of gas; and a manifold comprising a plurality of fluid conduits respectively connecting the valves to the containers, each of the plurality of fluid conduits having a respective flow restrictor restricting the flow through the respective conduit to a respective value, the valves being arranged to allow gas to flow through one of the plurality of flow conduits, and wherein the step of adjusting the characteristics of the flow of gas into the containers based at least in part on the apparatus operating in the droplet-closing mode comprises: the valve is operated to place a selected one of the plurality of conduits in fluid communication with a source of gas.

Other implementations are within the scope of the following claims.

Claims

a container;

an inlet arrangement defining at least one inlet path adapted and arranged to connect a gas source to the interior of the container to add the gas to the container in a stream along the inlet path;

a variable flow regulator selectably disposed in one of said inlet path and said outlet path and adapted to regulate a characteristic of said flow of said gas into or out of said container based at least in part on a mode in which said device is operating; and

a controller arranged to control operation of the flow controller.

2. The apparatus of claim 1, wherein the controller is adapted to operate using a look-ahead control process.

3. The device of claim 1, further comprising a second variable flow regulator selectably disposed 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 container based at least in part on a mode in which the device is operating.

4. The device of claim 1, wherein the device has a droplet-on mode of operation when the device is in one mode in which the droplet generates EUV radiation when irradiated by the laser, and a droplet-off mode of operation when in another mode in which the droplet is not used to generate EUV radiation when not irradiated by the laser.

5. The device of claim 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 container based at least in part on a mode in which the device is operating.

6. The apparatus of claim 5, wherein the characteristic is flow rate.

7. The device of claim 5, wherein the characteristic is a flow rate.

8. The device of claim 5, wherein the characteristic is a flow profile.

9. The apparatus of claim 5, wherein the characteristic is a stream composition.

10. The apparatus of claim 9, further comprising a mixing valve, a first gas source in fluid communication with the mixing valve, and a second gas source in fluid communication with the mixing valve, the mixing valve being arranged in fluid communication with the inlet arrangement 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 arrangement.

11. The device of claim 9, wherein the stream composition contains no active gas during the droplet-on mode and contains active gas during the droplet-off mode.

12. The apparatus of claim 11, wherein the reactive gas comprises oxygen.

13. The device of claim 1, wherein the inlet structure comprises a collector cone.

14. The device of claim 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. The apparatus of claim 14, wherein the motor comprises a linear motor.

16. The apparatus of claim 14, wherein the motor comprises a solenoid.

17. The apparatus of claim 14, wherein the flow obstruction, when placed in the flow path, presents a solid cross-section to the flow.

18. The apparatus of claim 14, wherein the flow obstruction, when placed in the flow path, presents a cross-section to the flow having at least one aperture.

19. The apparatus of claim 14, wherein the flow obstruction has an open tubular shape and is oriented such that the flow obstruction redirects a portion of the gas when placed in the flow path.

20. The device of claim 14, wherein the flow obstruction has an aerodynamic shape.

21. The device of claim 1, wherein the variable flow regulator comprises a mass flow controller.

22. The device of claim 1, wherein the variable flow regulator comprises

A valve adapted to be in fluid communication with the gas source, an

the valve is arranged to allow the gas to flow through one of the plurality of flow conduits.

a container having at least one inlet adapted to be connected to a source of gas and adapted to add the gas to the container in a stream along a flow path;

a droplet generator arranged to introduce the droplet into the container to a radiation site within the container, wherein when the device is in a droplet-on mode the droplet is used to generate EUV radiation when irradiated by the laser, and wherein when the device is in a droplet-off mode the droplet is not used to generate EUV radiation when not irradiated by the laser; and

a variable flow regulator selectably disposed in the flow path and adapted to regulate a characteristic of the flow of the gas into the container based at least in part on whether the device is in the droplet-on mode or the droplet-off mode.

an inlet adapted to be in fluid communication with the gas source;

an outlet adapted to be in fluid communication with an inlet of the vessel; and

a flow restrictor selectively preventing flow of the gas through the regulator along a flow path from the inlet to the outlet based at least in part on an operating mode of the device.

25. The flow conditioner as recited in claim 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 entirely outside the flow path, entirely within the flow path, or partially within the flow path.

26. The flow regulator of claim 24, wherein the flow restrictor comprises:

a valve adapted to be in fluid communication with the gas source, an

27. A method of controlling operation of a device for generating EUV radiation by laser irradiation of droplets of a target material in a container, the method comprising:

simultaneously with the operating step, adjusting a characteristic of a flow of gas into at least one of the container and out of the container based at least in part on the device operating in the droplet-off mode;

simultaneously with the switching step, adjusting a characteristic of a flow of gas into at least one of the container and out of the container based, at least in part, on the device operating in the droplet-on mode.

28. The method of claim 27, wherein the method is performed under control of a controller operating according to a look-ahead process.

29. The method of claim 27, wherein the characteristic is flow rate.

30. The method of claim 27, wherein the characteristic is a flow rate.

31. The method of claim 27, wherein the characteristic is a flow profile.

32. The method of claim 27, wherein the characteristic is a stream composition.

33. The method of claim 32, wherein the stream composition does not contain a reactive gas during the droplet-on mode and contains a reactive gas during the droplet-off mode.

34. The method of claim 33, wherein the reactive gas comprises oxygen.

35. The method of claim 27, wherein the device comprises a flow barrier and a motor for moving the flow barrier, and wherein the step of adjusting a characteristic of the flow of gas into the container based at least in part on the device operating in the droplet-closing mode comprises: moving the flow barrier at least partially into a flow path of the gas into the container.

36. The method of claim 27, wherein the device comprises: a valve adapted to be in fluid communication with a source of the gas; and a manifold comprising a plurality of fluid conduits respectively connecting the valve to the container, each of the plurality of fluid conduits having a respective flow restrictor that restricts flow through the respective conduit to a respective value, the valve being arranged to allow the gas to flow through one of the plurality of flow conduits, and wherein the step of adjusting the characteristics of the flow of gas into the container based at least in part on the apparatus operating in the droplet-closing mode comprises: operating the valve to place a selected one of the plurality of conduits in fluid communication with the source of the gas.