Optical element lifetime extension in EUV lithography systems

Abstract

To extend the life of an optical element in an EUV lithography system.SOLUTION: Degradation of the reflectivity of one or more reflective optical elements in a system (SO) for generating EUV radiation is reduced by controlling the introduction of gas into a vacuum chamber (26) housing the optical elements. The gas may be added to the flow of another gas, such as hydrogen, or may be alternated with the introduction of hydrogen radicals.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Application No. 62 / 803,057, filed February 8, 2019, U.S. Application No. 62 / 736,108, filed September 25, 2018, and U.S. Application No. 62 / 638,778, filed March 5, 2018, the contents of each of which are incorporated herein by reference in their entirety.

[0002]

[0002] The present disclosure relates to systems for the production of extreme ultraviolet radiation. Such systems typically use reflective optical elements located within a vacuum environment. The process of generating and controlling EUV radiation in such systems tends to degrade the reflectivity of these optical elements over time. One example of such an environment is the vacuum chamber of an EUV source, within which a plasma is created by electrical discharge or laser ablation of target or source material. In this application, optical elements are used to collect and direct radiation, for example, for use outside the vacuum chamber, for example, for semiconductor photolithography. Another example is a vacuum chamber housing the optical components of the projection system of such an apparatus. [Background technology]

[0003]

[0003] EUV radiation, for example, electromagnetic radiation having a wavelength of approximately 50 nm or less (sometimes referred to as soft X-rays) and including radiation with a wavelength of about 13.5 nm, can be used in photolithography processes to create extremely small features in substrates such as silicon wafers.

[0004]

[0004] Some methods of producing EUV radiation involve converting a target material (also called a source material) from a liquid state to a plasma state. The target material preferably contains at least one element that has one or more emission lines in the EUV region, such as xenon, lithium, or tin. In one such method, laser-produced plasma ("LPP"), the required plasma can be generated by irradiating a target material with the required line-emitting element with a laser beam.

[0005]

[0005] One LPP technique involves generating a stream of target material droplets and irradiating at least some of the droplets with pulses of laser radiation. More theoretically, an LPP source generates EUV radiation by depositing laser energy into a target material having at least one EUV-emitting element, such as xenon (Xe), tin (Sn), or lithium (Li), creating a highly ionized plasma with an electron temperature of tens of electron volts.

[0006]

[0006] Energetic radiation produced during the de-excitation and recombination of these ions is emitted from the plasma in all directions. In one common arrangement, a near-normal incidence mirror (often called a "collector mirror" or simply "collector") is positioned to collect, direct, and, in some arrangements, focus the radiation to an intermediate location. From the intermediate location, the collected radiation can then be relayed to a set of scanner optics and ultimately to the wafer.

[0007]

[0007] In the EUV portion of the spectrum, it is generally considered necessary to use reflective optics for the collector and other EUV optics. At the wavelengths of interest, the collector is advantageously implemented as a multi-layer mirror ("MLM"). As the name implies, the MLM generally consists of alternating layers of materials on a base or substrate.

[0008]

[0008] The optical element must be placed in a vacuum chamber with the plasma to collect and redirect EUV radiation. The environment within the chamber is harmful to the optical element, thus limiting its useful life by, for example, degrading its reflectivity through any one or a combination of several mechanisms. The optical element in the environment may be exposed to high-energy ions or particles of the target material, which can contaminate the exposed surface of the optical element. The particles of the target material can also cause physical damage and localized heating of the MLM surface. The target material may be particularly reactive with the material that constitutes at least one layer of the optical element surface. Even in the case of less reactive target materials, such as tin, indium, or xenon, issues of temperature stability, ion implantation, and diffusion must be addressed. Blistering of the MLM coating must also be avoided. The target material may also deposit on the surface of the optical element.

[0009] Another significant contributor to the collector degradation rate is the formation of local stains due to the deposition of tin debris, for example around the central aperture of the collector. Depending on the EUV source operating vessel pressure, the contribution of stains to the collector degradation rate can be between about 30% and about 60% of the overall degradation rate.

[0010]

[0010] There are techniques that can be employed to extend the life of optical elements despite these harsh conditions. For example, a capping layer can be placed on the optical element to protect its surface. To make it more reflective, the capping layer may have multiple layers spaced apart to increase the reflectivity at the wavelength of the radiation to be reflected. However, such capping layers are themselves susceptible to damage through mechanisms such as reduction, hydrogen diffusion, and blistering.

[0011] In some systems, H2 gas at a pressure in the range of about 0.5 to about 3 mbar is used in the vacuum chamber for debris mitigation. Hydrogen is relatively transparent to EUV radiation, which has a wavelength of about 13.5 nm. H2 gas is introduced into the vacuum chamber to decelerate energetic debris (ions, atoms, and clusters) of the target material created by the plasma through collisions with gas molecules. For this purpose, a flow of H2 gas is used that can oppose the debris trajectory. This helps reduce damage caused by deposition, implantation, and target material sputtering on the collector optical coating.

[0012] Another reason for introducing H2 gas into the vacuum chamber is to facilitate cleaning of the collector surface. The EUV radiation generated by the plasma creates hydrogen radicals (represented herein by the notation H*) by dissociating H2 molecules. The hydrogen radicals then serve to clean deposits of target material from the collector surface. For example, if tin is the target material, the hydrogen radicals participate in a reaction on the collector surface that leads to the formation of volatile gaseous stannane (SnH4), which can be removed by pumping. For this chemical pathway to be efficient, it is preferable that there be a slow rate of H recombination (back to H2 molecules) on the collector surface so that the hydrogen radicals are available to participate in the cleaning reaction.

[0013] Another cause of collector mirror degradation in EUV sources is the instability of the capping layer. If the capping layer is, for example, oxidizing, i.e., made of an oxide material, reduction of the oxide material to a metal-rich sub-oxide material, or even to a metal, can cause severe tin contamination because the capping layer then becomes catalytically active, promoting both atomic hydrogen recombination (thus reducing the cleaning rate) and redeposition of SnHx to form the EUV-blocking Sn layer. As another example, if the capping layer is made of a nitride material, reduction of the nitride material to a metal-rich sub-nitride material, or even to a metal, can also cause severe tin contamination because the capping layer then becomes catalytically active. Summary of the Invention

[0014] The following presents a simplified summary of one or more embodiments to provide a basic understanding of those embodiments. This summary is not an extensive overview of all possible embodiments, and is not intended to identify key or critical elements of all embodiments or to limit 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.

[0015] According to one aspect of an embodiment, a reflective element is provided having a capping layer made of an insulator. The capping layer may be, for example, a nitride or oxide, which has high resistance to deposition of target material, good energy reduction of incident ions, and low secondary electron yield. A gas control system is provided that introduces a flow of a relaxation gas, such as a gas containing oxygen or nitrogen, across at least a portion of the capping layer to promote cleaning and stability of the capping layer. The relaxation gas may also be introduced into other parts of the source to control the effect of source material or other vacuum environments of the system where EUV radiation tends to degrade reflectivity.

[0016] According to another aspect of an embodiment, an apparatus is disclosed that includes a vacuum chamber, a reflective optical element disposed within the vacuum chamber and including a capping layer, a gas distribution system for adding gas to the vacuum chamber, a source of relaxation gas in selective fluid communication with the gas distribution system, and a gas control system for controlling the gas distribution system, the gas control system having a state in which the relaxation gas from the source of gas is regulated by the gas distribution system and introduced into the interior of the vacuum chamber. The relaxation gas may be an oxygen-containing gas or a nitrogen-containing gas. The oxygen-containing gas may include any one or combination of O2, H2O, H2O2, O3, CO2, and air. The capping layer may include an insulator, and the insulator may include an oxide, and the oxide may include a metal oxide. The insulator may include a nitride. The insulator may include a carbide.

[0017]

[0017] The relaxation gas may be flowed over at least a portion of the capping layer. The gas distribution system may introduce the relaxation gas from the gas source into the vacuum chamber by adding a quantity of the relaxation gas to the flow of the hydrogen-containing gas. The gas distribution system may be configured to add the relaxation gas to the hydrogen-containing gas by introducing the relaxation gas into the flow of the hydrogen-containing gas. The relaxation gas may be mixed with an inert gas to obtain a mixture containing at least 0.1% of the relaxation gas. The relaxation gas may be O2 and may be mixed with an inert gas. The relaxation gas may be N2 and may be mixed with an inert gas. The inert gas may be argon. The relaxation gas may be H2O and may be mixed with an inert gas. The relaxation gas may be CO2 and may be mixed with an inert gas. The relaxation gas may be O3 and may be mixed with an inert gas. The relaxation gas may be air and may be mixed with an inert gas. The relaxation gas may be water vapor and may be accompanied by an inert gas. The relaxation gas may be 10 -5 mbar of inert gas and 10 -7 The relaxation gas may be mixed with an inert gas at 1000 mbar.

[0018] The gas distribution system may have at least one outlet positioned to generate a flow of gas containing the relaxation gas across at least a portion of the capping layer. The gas control system may add the relaxation gas continuously. The gas control system may add the relaxation gas in-line while the plasma is being generated. The gas control system may add a dose of the relaxation gas intermittently.

[0019] The apparatus may further include a sensor positioned to sense a reflectivity of the optical element and generate a signal representative of the reflectivity, and the gas control system may receive the signal and control at least one of the relaxation gas concentration and the relaxation gas composition based at least in part on the reflectivity. The sensor may be positioned to sense a relaxation gas concentration adjacent to the capping layer and generate a signal representative of the relaxation gas concentration, and the gas control system may receive the signal and control at least one of the relaxation gas concentration and the relaxation gas composition based at least in part on the relaxation gas concentration adjacent to the capping layer. The gas control system may control at least one of the relaxation gas concentration and the relaxation gas composition based on a duration of operation of an EUV source including the optical element. The gas control system may control at least one of the relaxation gas concentration and the relaxation gas composition based on a number of pulses of EUV radiation generated by an EUV source including the optical element.

[0020]

[0020] The apparatus may further include a plurality of structures located within the vacuum chamber for directing the flow of gas within the vacuum chamber, and may further include a gas distribution system arranged to flow the relaxation gas over the structures.

[0021]

[0021] According to another aspect of one embodiment, an apparatus is disclosed that includes a vacuum chamber, a reflective optical element disposed within the vacuum chamber and having a capping layer, a primary gas distribution system that adds hydrogen gas to the vacuum chamber, a secondary gas distribution system that adds a mitigation gas to the vacuum chamber, and a gas control system that controls the primary gas distribution system and the secondary gas distribution system, wherein the mitigation gas is regulated by the secondary gas distribution system and introduced into the interior of the vacuum chamber.

[0022] According to another aspect of an embodiment, a method is disclosed that includes measuring operating parameters of an EUV light source that includes a vacuum chamber and a reflective optical element located within the chamber, and controlling addition of a relaxation gas to the vacuum chamber based at least in part on the measured operating parameters. Controlling the addition of the relaxation gas may include controlling a concentration of the relaxation gas. Controlling the addition of the relaxation gas may include controlling a flow rate of the relaxation gas. Controlling the addition of the relaxation gas may include controlling a composition of the relaxation gas. The operating parameter may be a reflectivity of the optical element, a concentration of the relaxation gas adjacent to the optical element, a duration of operation of the EUV light source, or a number of EUV pulses generated by the EUV light source from a predetermined start time.

[0023] According to another aspect of an embodiment, an apparatus is also disclosed that includes a vacuum chamber, a reflective optical element disposed within the vacuum chamber, a gas distribution system having an inlet selectively permitting a flow of gas into the vacuum chamber, a source of relaxation gas in selective fluid communication with the inlet, a source of radical-containing gas in selective fluid communication with the inlet, and a gas control system configured to control the gas distribution system, the gas control system having a first state in which the gas control system causes the relaxation gas to be introduced into the vacuum chamber through the inlet and a second state in which the gas control system causes the radical-containing gas to be introduced into the vacuum chamber through the inlet. The source of radical-containing gas may include at least one heating element in thermal communication with the inlet and positioned to heat gas flowing through the inlet to a temperature sufficient to create radicals from at least a small amount of gas in the gas flow. The source of radical-containing gas may include a plasma source positioned to generate radicals in the gas before the gas reaches the inlet.

[0024]

[0024] According to another aspect of one embodiment, a method for reducing target material contamination of a reflective surface of a reflective optical element in a vacuum chamber of an extreme ultraviolet light source is also disclosed, the method comprising the steps of (a) starting a flow of a relaxation gas into the vacuum chamber, (b) stopping the flow of the relaxation gas into the vacuum chamber, (c) starting a flow of a radical-containing gas into the vacuum chamber, and (d) stopping the flow of the radical-containing gas into the vacuum chamber, wherein steps (a) to (d) are repeated sequentially multiple times.

[0025] According to another aspect of an embodiment, an apparatus is disclosed that includes a vacuum chamber, a reflective optical element disposed within the vacuum chamber, a gas supply system operable to supply at least a first gas and a second gas to the vacuum chamber, the second gas comprising oxygen, and a control system operable to control the gas supply system to supply the second gas based at least in part on a partial pressure of at least one of the first gas and the second gas detected in the vacuum chamber. The reflective optical element may include a capping layer, and the capping layer may comprise an oxide. The oxide may comprise a metal oxide. The control system may be operable to control the gas supply system to supply the second gas based at least in part on a partial pressure of the second gas detected in the vacuum chamber. The control system may be operable to control the gas supply system to supply the second gas based at least in part on a partial pressure of the first gas detected in the vacuum chamber. The second gas may comprise O2. The second gas may comprise H2O. The second gas may comprise H2O2. The second gas may comprise O3. The second gas may comprise CO2. The second gas may comprise air. The second gas may comprise an inert gas. The inert gas may comprise argon. The inert gas may comprise helium. The second gas may comprise an inert gas mixed with at least 0.1% of an oxygen-containing gas. The apparatus may further comprise a gas pressure sensor positioned to sense a partial pressure of the second gas in the vacuum chamber and to generate a first signal to a control system representative of the partial pressure, the control system controlling the supply of the second gas based at least in part on the first signal. The gas pressure sensor may directly sense the partial pressure of the second gas. The gas pressure sensor may indirectly sense the partial pressure of the oxygen-containing gas by sensing the partial pressure of at least one gas other than the second gas.

[0026] According to another aspect of an embodiment, an apparatus is disclosed that includes a vacuum chamber, a reflective optical element disposed within the vacuum chamber, a gas distribution system for adding gas to the vacuum chamber, the gas distribution system having an interface for coupling with a source of oxygen-containing gas in selective fluid communication with the gas distribution system, and a gas control system configured to control the gas distribution system to supply the oxygen-containing gas to the vacuum chamber until a partial pressure of the oxygen-containing gas reaches a first value, discontinue supplying the oxygen-containing gas to the vacuum chamber until the partial pressure of the oxygen-containing gas reaches a second value less than the first value, and resume supplying the oxygen-containing gas to the vacuum chamber until the partial pressure of the oxygen-containing gas reaches a third value greater than the second value. The reflective optical element may comprise a multi-layer mirror including a capping layer and a plurality of foundation layers, the capping layer constructed and arranged to protect the foundation layers from damage, the capping layer comprising an oxide. The oxide may comprise a metal oxide. The third value may be substantially equal to the first value. The oxygen-containing gas may comprise O2. The oxygen-containing gas may comprise H2O. The oxygen-containing gas may comprise H2O2. The oxygen-containing gas may comprise O3. The oxygen-containing gas may comprise CO2. The oxygen-containing gas may comprise air. The second gas may comprise an inert gas. The inert gas may comprise argon. The inert gas may comprise helium. The second gas may comprise an inert gas mixed with at least 0.1% of the oxygen-containing gas. The apparatus may also include a gas pressure sensor positioned to sense a partial pressure of the oxygen-containing gas in the vacuum chamber and to generate a first signal to a gas control system representative of the partial pressure, the gas control system controlling the supply of the oxygen-containing gas based at least in part on the first signal. The gas pressure sensor may directly sense the partial pressure of the oxygen-containing gas. The gas pressure sensor may indirectly sense the partial pressure of the oxygen-containing gas by sensing the partial pressure of at least one gas other than the oxygen-containing gas. The gas pressure sensor may be positioned to sense the partial pressure of the oxygen-containing gas proximate to the reflective optical element.

[0027] According to another aspect of an embodiment, a method for extending the operational lifetime of a reflective surface in a vacuum chamber of an EUV source is disclosed, the method comprising: providing a gas supply system operable to supply at least a first gas and a second gas to the vacuum chamber, the second gas comprising oxygen; sensing a partial pressure in the vacuum chamber of at least one of the first gas and the second gas; and controlling the gas supply system to supply the second gas based at least in part on the sensed partial pressure. The sensing step may comprise sensing a partial pressure of the second gas. The sensing step may comprise sensing a partial pressure of the first gas. The second gas may comprise O2. The second gas may comprise H2O. The second gas may comprise H2O2. The second gas may comprise O3. The second gas may comprise CO2. The second gas may comprise air. The second gas may comprise an inert gas. The inert gas may comprise argon. The inert gas may comprise helium. The second gas may comprise an inert gas mixed with at least 0.1% of an oxygen-containing gas.

[0028] According to another aspect of an embodiment, a method for extending the operational lifetime of a reflective surface in a vacuum chamber of an EUV source is disclosed, the method comprising: (a) supplying an oxygen-containing gas to the vacuum chamber; (b) ceasing the supply of the oxygen-containing gas to the vacuum chamber when a partial pressure of the oxygen-containing gas reaches a first value; (c) supplying more oxygen-containing gas to the vacuum chamber when the partial pressure of the oxygen-containing gas reaches a second value less than the first value; (d) ceasing the supply of the oxygen-containing gas to the vacuum chamber when the partial pressure of the oxygen-containing gas reaches a third value greater than the second value; and (e) repeating steps (c) and (d) to maintain the partial pressure of the oxygen-containing gas between the first value and the second value. The third value may be substantially equal to the first value. The oxygen-containing gas may comprise O2. The oxygen-containing gas may comprise H2O. The oxygen-containing gas may comprise H2O2. The oxygen-containing gas may comprise O3. The oxygen-containing gas may comprise CO2. The oxygen-containing gas may comprise air. The second gas may comprise an inert gas. The inert gas may comprise argon. The gas may comprise helium. The second gas may comprise an inert gas mixed with at least 0.1% of the oxygen-containing gas. Steps (b), (c), and (d) may each comprise sensing a partial pressure of the oxygen-containing gas in the vacuum chamber. Steps (b), (c), and (d) may each comprise directly sensing the partial pressure of the oxygen-containing gas in the vacuum chamber. Steps (b), (c), and (d) may each comprise indirectly sensing the partial pressure of the oxygen-containing gas in the vacuum chamber by sensing the partial pressure of at least one gas other than the oxygen-containing gas. Steps (b), (c), and (d) may each comprise sensing the partial pressure of the oxygen-containing gas proximate a reflective surface in the vacuum chamber.

[0029] 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 the accompanying drawings. [Brief explanation of the drawings]

[0030]

[0030] The accompanying drawings, which are incorporated in and form a part of this specification, illustrate by way of example, and not by way of limitation, methods and systems of embodiments of the present invention. The drawings, together with the detailed description, further serve to explain the principles of the methods and systems presented herein and to enable one skilled in the art to make and use the methods and systems. In the drawings, like reference numbers represent identical or functionally similar elements.

[0031] [Figure 1]

[0031] A schematic diagram of the overall broad concept of an EUV lithography system is shown, not to scale. [Figure 2]

[0032] 1 shows a schematic diagram, not to scale, of an overall broad concept of a laser-produced plasma EUV radiation source system according to an aspect of an embodiment of the present invention; [Figure 3]

[0033] 1 is a cross-sectional view of an EUV optical element according to an aspect of an embodiment of the present invention. [Figure 4]

[0034] 1 is a cross-sectional view of gas flow over an EUV optical element according to an aspect of an embodiment of the present invention. [Figure 5]

[0035] FIG. 2 is a functional block diagram of a system for adding gas to one of multiple vacuum chambers of an EUV lithography system in accordance with an aspect of an embodiment of the present invention. [Figure 6]

[0036] FIG. 1 is a perspective view of a flow cone according to an aspect of one embodiment of the present invention. [Figure 7]

[0037] FIG. 1 is a plan view of a flow cone heating element according to an aspect of an embodiment of the present invention. [Figure 8A]

[0038] FIG. 1 is a plan view of a collector with a flow cone according to an aspect of an embodiment of the present invention; [Figure 8B]

[0038] A side view of a collector with a flow cone according to one aspect of one embodiment of the present invention. [Figure 8C]

[0038] A perspective view of a collector with a flow cone according to one aspect of one embodiment of the present invention. [Figure 9]

[0039] 1 is a partially perspective and partially schematic view of a flow cone and gas distribution and control system according to an aspect of an embodiment of the present invention; [Figure 10]

[0040] FIG. 2 is a timing diagram illustrating certain principles of operation of a gas distribution and control system in accordance with an aspect of an embodiment of the present invention. [Figure 11]

[0041] 4 is a flow chart illustrating a particular procedure for using a gas distribution and control system in accordance with an aspect of an embodiment of the present invention. [Figure 12]

[0042] 1 is a partial schematic functional block diagram of a system for adding an oxygen-containing gas to one of a plurality of vacuum chambers of an EUV lithography system in accordance with an aspect of an embodiment of the present invention; [Figure 13]

[0043] FIG. 2 is a timing diagram illustrating certain principles of operation of a gas distribution and control system in accordance with an aspect of an embodiment of the present invention. [Figure 14]

[0044] 4 is a flow chart illustrating a particular procedure for using a gas distribution and control system in accordance with an aspect of an embodiment of the present invention. [Figure 15]

[0045] FIG. 2 is a timing diagram illustrating certain principles of operation of a gas distribution and control system in accordance with an aspect of an embodiment of the present invention. [Figure 16]

[0046] 4 is a flow chart illustrating a particular procedure for using a gas distribution and control system in accordance with an aspect of an embodiment of the present invention.

[0032]

[0047] Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. It should be 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 those skilled in the art based on the teachings contained herein. DETAILED DESCRIPTION OF THE INVENTION

[0033]

[0048] Various embodiments will now be described with reference to the drawings. 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 facilitate a thorough understanding of one or more embodiments. It will be apparent, however, that in some or all cases, any of the embodiments described below can be practiced without employing 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.

[0034]

[0049] Figure 1 schematically depicts a lithographic apparatus according to one embodiment of the present invention. The apparatus comprises an illumination system IL configured to condition a radiation beam B. The apparatus also includes a support structure (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask or reticle) MA and connected to a first positioner PM configured to accurately position the patterning device according to certain parameters, a substrate table (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate according to certain parameters, and a projection system (e.g., a refractive or reflective projection lens system) PS, also referred to as a projection light box or POB, configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0035]

[0050] The illumination system IL may include various types of optical components, such as refractive, reflective, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping or controlling radiation.

[0036]

[0051] The support structure MT holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure MT may use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure MT may be, for example, a frame or a table, which may be fixed or movable as required. The support structure MT may ensure that the patterning device is at a desired position, for example with respect to the projection system.

[0037]

[0052] Referring to Figure 1, the illumination system IL receives a radiation beam from a radiation source SO. The radiation source SO and the illumination system IL, together with the beam delivery system if required, may be referred to as a radiation system. The illumination system IL may comprise an adjuster for adjusting the angular intensity distribution of the radiation beam. Typically, at least the outer and / or inner radial extent of the intensity distribution in a pupil plane of the illumination system may be adjusted. The illumination system IL may also comprise various other components, such as integrators and condensers. The illumination system may be used to condition the radiation beam to have a desired uniformity and intensity distribution across its cross-section.

[0038]

[0053] The radiation beam B is incident on a patterning device (e.g., mask) MA, which is held on a support structure (e.g., mask table) MT, and is patterned by the patterning device. Having traversed the patterning device MA, the radiation beam B passes through a projection system PS, which focuses the beam onto a target portion C of a substrate W. With the aid of a second positioner PW and a position sensor IF2 (e.g., an interferometric device, a linear encoder, or a capacitive sensor), the substrate table WT can be precisely moved, for example to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor IF1 can be used to accurately position the patterning device MA with respect to the path of the radiation beam B, for example after mechanical retrieval from a mask library or during a scan.

[0039]

[0054] 2 is a schematic diagram of an exemplary EUV radiation source SO, e.g., a laser-produced plasma EUV radiation source, in accordance with an aspect of an embodiment of the present invention. As shown, the EUV radiation source SO may include a pulsed or continuous laser source 22, which may be, for example, a pulsed gas discharge CO2 laser source producing radiation at 10.6 μm. The pulsed gas discharge CO2 laser source may have DC or RF excitation operating at high power and high pulse repetition rate.

[0040]

[0055] The EUV radiation source 20 also includes a target delivery system 24 that delivers the target material in the form of liquid droplets or a continuous liquid stream. The target material may be comprised of tin or a tin compound, although other materials may be used. The target material delivery system 24 introduces the target material to an irradiation region 28 within the chamber 26, where the target material may be irradiated to generate a plasma. In some cases, an electric charge is placed on the target material to enable it to be steered toward or away from the irradiation region 28. Note that, as used herein, an irradiation region is an area where irradiation of the target material may occur, even when irradiation is not actually occurring. As shown, the target material delivery system 24 may include a target delivery control system 90 and a target material dispenser 92.

[0041]

[0056] Continuing with reference to FIG. 2 , the radiation source SO 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 also applies to other optical elements. Collector 30 may be a normal-incidence reflector implemented, for example, as an MLM, i.e., a silicon carbide (SiC) substrate coated with a molybdenum / silicon (Mo / Si) multilayer, with an additional thin barrier layer, e.g., B4C, ZrC, Si3N4, or C, deposited at each interface to effectively block thermally induced interlayer diffusion. Other substrate materials, such as aluminum (Al) or silicon (Si), may also be used. Collector 30 may be oblong in shape and include an aperture that allows laser radiation to pass through and reach illumination region 28. Collector 30 may, for example, be in the shape of an ellipsoid having a first main focus at irradiation area 28 and a second focus at a so-called midpoint 40 (also referred to as intermediate focus 40) at which EUV radiation may be output from EUV radiation source SO and input to illumination system IL shown in Figure 1. Chamber 26 may also be provided with additional structures such as vanes 94 for controlling the flow of gas within chamber 26.

[0042]

[0057] 1 , the projection system PS (which may be referred to as a projection light box or “POB”) comprises a vacuum chamber that houses a number of reflective EUV optical elements that direct radiation from the patterning device MA onto the wafer W. The projection system PS may, for example, house focusing mirrors (not shown) for focusing the patterned radiation onto the wafer W. These reflective EUV optical elements may include so-called grazing incidence mirrors.

[0043]

[0058] As mentioned above, one of the technical challenges in designing optical elements used in EUV systems is extending their lifetime. One approach to extending optical element lifetime involves protecting the optical element from damage by using an outermost capping layer. An example of an EUV optical element with such a capping layer is shown in FIG. 3, which is a cross-section through a portion of such a collector 30. As seen in the figure, collector 30 includes a substrate 100. Located on substrate 100 is a multilayer coating 110. Multilayer coating 110 is constructed in a known manner from alternating layers of materials, such as molybdenum and silicon. Located on top of multilayer coating 110 is a capping layer 120, which may consist of an outermost layer and a series of repeating bilayers. As used herein, a capping layer is a layer added to protect collector 30 without unduly reducing the overall reflectivity of collector 30 at wavelengths of interest, e.g., 13.5 nm. It is advantageous to provide a capping layer system that protects the surface of collector 30 against target material (e.g., tin) deposition, hydrogen ion penetration, hydrogen diffusion, and hydrogen or oxygen induced blistering. It is also advantageous to select a material for capping layer 120 that will resist blistering.

[0044]

[0059] Referring again to FIG. 3, the capping layer 120 (or the top layer of the capping layer 120 if the capping layer 120 is a multi-layer cap) may be an insulating material such as a ceramic or a nitride or oxide that has a high resistance to deposition of the target material. In other words, they may be materials that have a low recombination rate for atomic hydrogen to allow for a high formation rate of stannane. These are typically about 10 -4 ~about 10 -3 The preferred material will have a hydrogen recombination coefficient in the range of 0.01 to 0.01. This effectively means that the preferred material will exhibit a good tin cleaning rate because H can react with Sn before recombining to form H. The preferred material for the top layer 130 of the capping layer 120 also preferably exhibits good energy reduction of incident ions and low secondary electron yield. Examples of suitable materials include insulators such as ZrN, TiO, Ta, and ZrO. Generally, metals and ceramic compounds that can be oxidized to a predetermined thickness without affecting the underlying multilayer coating can also be used. The insulator may be selected to minimize hydrogen radical recombination and decomposition of SnH and SnH.

[0045]

[0060] In general, the vacuum quality of an EUV source is intended to be as clean as possible (lowest possible levels of O, N, HO, etc.) to prevent unwanted chemical reactions with tin or the plasma that could risk oxidation or degradation of the collector. Oxygen, for example, can also undesirably cause oxidation of other components within the vacuum chamber, such as the nozzle of the droplet generator. However, it has been found that the presence of low levels of gases that would otherwise be considered impurities can have a positive effect on maintaining or improving the reflectivity of optical elements within the system. For collectors within a source, one mechanism that can lead to increased deposition of tin on the capping layer is oxygen depletion from the capping layer.

[0046]

[0061] It is therefore advantageous to intentionally provide a controllable amount of such a gas or mixture of such gases into, for example, a vacuum chamber of the light source SO or projection system PS. Such non-hydrogen gases or mixtures of gases including non-hydrogen gases, when intentionally introduced into a vacuum chamber, are referred to herein as relaxation gases because their intentional introduction aids in relaxation through chemical and / or physical reactions and processes that would otherwise reduce reflectivity. These relaxation gases may, for example, be provided in an air mixture (e.g., ultra-clean dry air, or XCDA) or may be mixed with a noble or inert gas (e.g., Ar).

[0047]

[0062] One way to add a relaxation gas to the vacuum chamber is to incorporate it into the flow of hydrogen gas into the chamber. For example, an oxygen / water-containing gas can be added to the vacuum chamber by adding it to the hydrogen flow passing near the collector to increase the local concentration of the relaxation gas to a level that stabilizes the capping layer without consuming significant amounts of H* in the water-gas reaction. Thus, the relaxation gas can be, for example, any one of O2, HO, HO2, O3, and N2, or a mixture of these in relative proportions that suppress unwanted reactions and effects while promoting desired reactions and effects. Note that water photodecomposes into H2 and O2 under EUV. For example, a mixture of argon and oxygen can be added. A mixture of argon and O2 can be added at a rate of 10 -5 mbar of Ar and 10 -7 The relaxation gas concentration can be, for example, 2E-5 mbar AR and 4E-7 mbar O. The mixture to be used may be determined based on reflectance measurements. The upper limit of the relaxation gas concentration is determined in part by the deep oxidation below the capping layer. The exact limit depends on the plasma load driven by the EUV light.

[0048]

[0063] There are engineering risks associated with introducing oxygen-containing gases. Oxygen-containing gases are used in reference to gases containing O or oxygen compounds such as HO. As used herein, the phrase "hydrogen-containing gas" also refers to gases containing O or oxygen compounds such as HO. For example, the presence of oxygen-containing gases can cause oxidation of the droplet generator nozzles. Such risks can be mitigated by switching off the addition of oxygen-containing gas upon droplet generator startup and monitoring the success rate of droplet generator restarts.

[0049]

[0064] The above discussion primarily relates to reflective optical elements located in the source vacuum chamber. The beneficial effects of adding a relaxation gas to the gas mixture in the vacuum chamber extend to reflective EUV optical elements located in other parts of the system, such as reflective optical elements in the POB. For example, a relaxation gas can be added to the gas mixture in the POB vacuum chamber to suppress etching of the surfaces of reflective optical elements located there and to mitigate SiH4 formation. The pressure of the relaxation gas at the relaxation gas inlet to this vacuum environment can be on the order of 1E-8 mbar. As another example, the patterning device MA also resides in a vacuum environment, sometimes referred to as the reticle stage or reticle microenvironment. Again, a low concentration of relaxation gas can be added here to mitigate SiH4 formation. The relaxation gas can be oxygen or mixed with nitrogen. The pressure of the relaxation gas at the relaxation gas inlet to this vacuum environment can be on the order of 4E-8 mbar. For both of these environments, the lower end of the relaxation gas concentration range is primarily determined by the minimum amount required to obtain the benefits of SiOx mitigation. The lower end of the range of relaxation gas concentrations is determined primarily by the need to avoid deep oxidation, and the desired concentration of relaxation gas is determined in part by the ion flux to which the SiOx-containing surfaces of the reflective EUV optics are exposed, with lower ion fluxes lowering the demand for the presence of relaxation gas.

[0050]

[0065] As a specific example, a metal oxide may be selected as the collector cap material due to its limited H* recombination and SnH4 redeposition. Reduction of the metal oxide to metal (or suboxide or subnitride) would undesirably promote H* recombination and SnH4 redeposition on the surface of the capping layer. Flowing a gas containing oxygen or nitrogen over the capping layer stabilizes the metal oxide or nitride capping layer, providing further tin cleaning. The oxygen or nitrogen concentration may be selected to promote this effect while not consuming excessive amounts of H* (to form water), thereby leaving less H* available for the tin cleaning reaction. At intermediate levels of oxygen, tin can be oxidized, which requires more H* for cleaning but reduces SnH4 redeposition. The goal is to achieve tin cleaning (Sn+xH* → SnH) while suppressing unwanted reactions such as reduction ((metal)Ox + 2xH* → metal + H2O), water formation (2H* + O* → H2O), H* recombination (2H** → H2), and SnH4 redeposition (SnHx → Sn+1 / 2xH2). x ) and metal oxidation (metal + 1 / 2xO2 → (metal)O x The same applies to nitrogen.

[0051]

[0066] Other candidate materials for the insulator include nitrides, which can be stabilized, for example, by adding nitrogen or ammonia mixtures as mitigation gases. They can also be stabilized with oxygen, since oxygen oxidizes nitrogen-depleted areas of the capping layer, thus restoring conditions for suppressing unwanted reactions and promoting desired reactions. Further candidate materials for the insulator include carbides, which can be stabilized, for example, by adding CH4 and / or light hydrocarbons.

[0052]

[0067] At the source, a flow of the gas mixture is desirably established to flow across (i.e., with velocity components adjacent to and parallel to) at least a portion of the surface of the EUV-reflective optical element. This can be achieved, for example, by adding an outlet to the system for flowing hydrogen into the chamber and then adding a relaxation gas to the hydrogen flow. As previously mentioned, the collector 30 may be oblong in shape and includes a central aperture 306 that allows the laser radiation to pass through and reach the irradiation region 28. A collector and gas flow assembly 302 is shown in FIG. 4 . The central aperture 306 extends through the reflective surface 304 of the collector mirror 30. The central aperture 306 is located on the axial symmetry line AX of the collector 30. The central aperture 306 has an edge 308. The assembly 302 includes a tubular body 310 (also referred to as a flow cone 310) that extends through the central aperture 306. Tubular body 310 has inner surfaces 312 and 318 and an outer surface 314 .

[0053]

[0068] The term "tubular" is understood by those skilled in the art as a broad term that can encompass or be synonymous with a variety of different structures. For example, a tubular body may not have parallel walls, but instead may be conical or flared. A tubular body may be any structure that forms a conduit from one side of a collector (e.g., the non-collecting side) to the other, opposite side of the collector (e.g., the collecting side). A tubular body may have a circular cross-section, or an elliptical cross-section, or any other suitable cross-section.

[0054]

[0069] In use, a primary gas flow GF is directed through the tubular body 310, which in this embodiment is constructed and arranged to direct the gas flow GF in a direction transverse to the reflective surface 304. For example, the inner surface 312 of the tubular body 310 is constructed and arranged to direct the gas flow GF in a direction transverse to the reflective surface 304. Typically, the gas flow GF is directed toward one or both of a first focal point and a second focal point. An opening 316 may be located between the outer surface 314 of the tubular body 310 and the edge 308 of the central aperture 306. In the embodiment of FIG. 4, the outer surface 314 and the edge 308 form or define the opening 316.

[0055]

[0070] The opening 316 (including one or more structures forming at least a portion of the opening 316, such as the outer surface 314 and edge 308 of the tubular body 310) may be positioned to direct the additional gas flow GF' away from the tubular body 310 substantially along the reflective surface 304 as shown. The nozzles 320 and 322 may be positioned to direct the additional gas flow GF' from substantially the periphery of the collector 30 toward the tubular body 310 along the reflective surface 304 as shown. If there is radial flow from both the center and the periphery, the nozzles can be positioned so that the flows meet but still flow substantially adjacent to the surface of the collector 30 other than where they meet.

[0056]

[0071] The flow through the tubular body 310 may be referred to as a cone flow. A flow from the center to the periphery along the surface of the collector 30 may be referred to as an umbrella flow. A flow from the periphery to the center along the surface of the collector 30 may be referred to as a peripheral flow. The cone flow rate of the hydrogen / relaxation gas mixture may be in the range of about 50 SLM to about 165 SLM. The umbrella flow rate of the hydrogen / relaxation gas mixture may be in the range of about 40 SLM to about 90 SLM. The peripheral flow rate of the hydrogen / relaxation gas mixture may be in the range of about 60 SLM to about 160 SLM. Under these types of H2 flows, the flow rate of the pure relaxation gas alone is about 0.002 to about 0.8 SCCM. If a carrier gas, such as an inert gas, is present, the flow rates are adjusted (the flow rates scale). For example, if the relaxation gas mixture is 2% relaxation gas and 98% inert gas, the total flow rate is about 0.1 to about 40 SCCM. Ultimately, it is the relaxation gas partial pressure that is important, which depends on the ratio of relaxation gas to H2 (the dominant flow in the vessel) and the differential pumping speed of these two gases, and the total vessel pressure can be in the range of about 1.2 mbar to about 2.2 mbar.

[0057]

[0072] Alternatively, there may be a gas manifold system that is specialized for flowing relaxation gas across the surface of the EUV reflective optical element, in which case only the relaxation gas is supplied to apertures / nozzles that are directed to flow gas along the surface of the local surface of the collector 30 (with a velocity component that is substantially locally parallel to that local surface).

[0058]

[0073] FIG. 5 illustrates a system for controlling the introduction of a relaxation gas into one or more vacuum chambers of a system for producing and utilizing EUV radiation. As shown in FIG. 5, the vacuum chamber 26 of the EUV source SO is connected to a source 400 of source gas, such as hydrogen. The conduit relaying the source gas to the vacuum chamber 26 has a mixing node 410 under the control of a gas control system 420. The mixing node 410 is also connected to a source 430 of relaxation gas. The gas control system 420 controls whether the relaxation gas is added to the flow of source gas entering the chamber 26. The gas control system 420 also controls the mixing ratio of the relaxation gas to the source gas. Alternatively, or in addition, the relaxation gas can flow directly into the chamber 26 under the control of the gas control system 420.

[0059]

[0074] FIG. 5 also shows a system for controlling the introduction of relaxation gas into the vacuum chamber 440 of the POB. As shown in FIG. 5, the vacuum chamber 440 of the POB is connected to a source of gas 450. The conduit relaying the gas to the vacuum chamber 26 has a mixing node 460 under the control of a gas control system 420. The mixing node 460 is also connected to a source of relaxation gas 470. The gas control system 420 controls whether the relaxation gas is added to the flow of gas entering the chamber 440. The gas control system 420 also controls the mixing ratio of the relaxation gas to the gas. Alternatively or additionally, gas can flow directly into the chamber 440 and under the control of the gas control system 420.

[0060]

[0075] The gas control system can be configured to add relaxation gas continuously in-line while the source is operating. The gas control system can be configured to add relaxation gas intermittently. The gas control system can be configured to add relaxation gas while the source is offline. The gas control system can be configured to add relaxation gas and / or control the concentration and / or composition of the relaxation gas based on a measurement of the reflectivity of an EUV reflective element in the chamber measured by sensor 480 or 490. The gas control system can be configured to add relaxation gas and / or control the concentration and / or composition of the relaxation gas based on a measurement of the relaxation gas concentration adjacent to the EUV reflective element measured by sensor 480 or 490. The gas control system can be configured to add relaxation gas and / or control the concentration and / or composition of the relaxation gas based on a measurement of the amount of time the chamber has been operated or the number of pulses generated by the source.

[0061]

[0076] An umbrella flow cone 310, in the form of a circumferential gap between the interior and exterior of the tubular body, provides an umbrella flow of hydrogen across the reflective surface 304 of the collector 30. This is shown in Figures 6 and 8A-8C.

[0062]

[0077] 9, a source 430 of relaxation gas, e.g., oxygen, can be connected to the umbrella flow cone 310 through a mixing node 410, e.g., a valve or controllable inlet to the flow cone 310, to provide a predetermined amount, e.g., a trace amount, of relaxation gas to the umbrella flow from the gas source 400. When the valve 490 or inlet is open, a controlled amount of relaxation gas is mixed with the hydrogen gas in the umbrella flow. When the inlet is closed, only H flows through the umbrella flow cone 310. The valve 490 or inlet is controlled by a control signal from the gas control system 420.

[0063]

[0078] According to another aspect of an embodiment, the tubular body 310 includes one or more heating elements 500. Referring to FIGS. 6 and 7, these heating elements 500 may be disposed around the exterior of the tubular body 310. Each of the heating elements 500 may comprise one or more filaments and may be selectively energized by the gas control system 420. When energized, the heaters 500 generate hydrogen radicals in the hydrogen flow of the umbrella flow cone 310. The gas control system 420 is configured to provide energy to the heaters so that they reach a temperature sufficient to achieve a desired dissociation rate of hydrogen in the flow, e.g., greater than 1300°C. This increases the concentration of H* in the H flow around the umbrella cone 310, which in turn enhances Sn etching from the reflective surface 304 and the formation of gaseous stannane by reaction. Sn(s)+4H*(g) → SnH4(g)

[0064]

[0079] According to another embodiment, this increase in the concentration of H* in the H flow alternates with the introduction of a moderating gas such as O. When O is introduced, it inhibits the autocatalytic decomposition of stannane, thereby reducing the reverse reaction (conversion of gaseous stannane to solid tin) and preventing additional Sn formation. The alternating hydrogen radical generation and oxygen addition results in the overall removal of tin staining on the reflective surface 304.

[0065]

[0080] In the arrangement just described, hydrogen radicals are generated using a heated filament provided above the tubular body, but hydrogen radicals can be introduced from other or additional sources. For example, H* could be delivered to the tubular body 310 via a remote source, such as the remote plasma source 315 shown in Figure 8B below, to achieve the same effect.

[0066]

[0081] Figure 8A is a front elevation view of collector 30 and its reflective surface 304 and central aperture 306. An umbrella flow cone 310 is defined by the interior and exterior of tubular body 310. Figure 8B is a side view of the arrangement of Figure 8A, and Figure 8C is a perspective view of the arrangement of Figure 8A, also showing circumferential support 530 of collector 30 with circumferential gas vents 540 that may be provided to cooperate with umbrella flow cone 310 to establish umbrella flow over reflective surface 304 of collector 30.

[0067]

[0082] 9 and 10, the gas control system 420 can be configured to alternate between (1) energizing the heating element 500 to generate hydrogen radicals and (2) injecting a relaxation gas. As shown in FIG. 10, there are alternating intervals 580 when the heater is energized and intervals 560 when the relaxation gas is added to the umbrella flow.

[0068]

[0083] 11 is a flowchart describing this procedure. In step S10, hydrogen is supplied to the umbrella flow cone to establish an umbrella flow of hydrogen. In step S20, gas control system 420 energizes heater 500 to convert a small amount of hydrogen in the umbrella flow to radicals over interval A. After interval A ends, in step S30, the gas control system opens valve 490 to add a predetermined amount of relaxation gas, e.g., oxygen, to the umbrella flow over interval B. The process is then repeated as desired to maintain or improve the reflectivity of a reflective optical element in the same environment as flow cone 310.

[0069]

[0084] Additionally, when the source is operating, molten tin debris continuously arrives on surfaces within the chamber 26, such as the surfaces of the vanes 94. Hydrogen radicals generated during source operation interact with the molten tin, causing small particles of tin to be ejected from the surface of the molten tin. These particles are typically about 1 micron in size and leave the surface at velocities of several meters per second. This phenomenon is called "tin spitting." Particles generated by tin spitting can also end up on the collector and contribute to the loss of EUV reflectivity of the collector mirror.

[0070]

[0085] To reduce or even completely eliminate tin spitting, the tin reaching the vanes can be oxidized by periodically adding a mitigating gas, such as oxygen as described above. Oxygen concentrations on the order of 10E-5 mbar would be sufficient for this purpose. Oxygen exposure can be performed, for example, at intervals of about every 1-2 hours during high-duty-cycle operation of the source. To mitigate the negative impact of oxygen exposure on Sn self-cleaning of the collector surface, this oxygen addition can occur offline, i.e., when no plasma is being generated. Therefore, the laser is switched to a mode of operation in which droplets are not generated and a small amount of oxygen can be introduced into the chamber. This can be done, for example, through a separate gas supply containing a mixture of 2% oxygen and 98% Ar. The presence of molecular hydrogen does not interfere with oxygen spitting suppression, so the normal hydrogen flow can continue uninterrupted. The gas mixture can be a 1 slm O2 / Ar mixture and a normal flow of hydrogen (e.g., about 200 slm). Larger or smaller amounts can also be used. Also, the hydrogen supply may be switched off during the procedure. The duration of the oxygen exposure may be on the order of a few minutes, for example about 5 minutes, but this may also be adjustable. After the exposure, normal operation of the source may be resumed.

[0071]

[0086] The oxygen-containing gas may be added to the vessel continuously or intermittently. However, continuous addition of oxygen-containing gas may result in oxidation of the Sn layer present on the collector surface. In that case, a SnOx layer may form on the collector surface, which may lead to a loss of reflectivity. Continuous addition of oxygen-containing gas may also result in oxidation of the silicon layer of the multilayer mirror collector. In that case, an oxide layer may form on the collector surface, which may also lead to a loss of reflectivity. Furthermore, continuous addition of oxygen-containing gas may significantly increase splashback from the tin catcher, which contaminates the collector. Therefore, for some applications, it may be desirable to add oxygen-containing gas to the vessel intermittently rather than continuously, regardless of whether the introduction is alternating with another gas.

[0072]

[0087] Thus, according to one aspect of an embodiment, and referring to FIG. 12 , a partial pressure measuring device 610 reads parameters of source operating conditions within the vessel 440, such as the partial pressure of O, N, and HO, when the reflectivity of the collector reflective surface 304 is within defined specifications. These parameters may then be set and recorded as thresholds. The measuring device 610 may be implemented, for example, by a residual gas mass spectrometer. Preferably, the measuring device 610 will provide measurements continuously. It may also be advantageous to have the measuring device 610 sufficiently close to the reflective surface 304 so that the partial pressure values ​​measured by the measuring device 610 represent the partial pressures adjacent the reflective surface 304.

[0073]

[0088] Once an in-specification value is determined, the source is operated and the operating value of at least one of the parameters, e.g., the partial pressure of the oxygen-containing gas, is monitored. If it is determined that the partial pressure of the oxygen-containing gas has dropped below a threshold, the control system 420 switches on the supply of oxygen-containing gas from the gas source 430 by switching on the dosing valve 600. The supply of oxygen-containing gas remains on until it is determined that the partial pressure of the oxygen-containing gas has exceeded the threshold, at which point the control system 420 switches off the supply of oxygen-containing gas.

[0074]

[0089] This process is illustrated graphically in FIG. 13. In FIG. 13, the partial pressure of the oxygen-containing gas, represented by line 650, is increased to an initial level above threshold level 660 during time interval T1. After time interval T1, the supply of oxygen-containing gas to the container is shut off. The partial pressure of the oxygen-containing gas then gradually decreases during time interval T2 as the oxygen-containing gas is depleted in the container. At the end of interval T2, the level has decreased sufficiently to fall below threshold level 660. At this time, control system 420 resumes supplying oxygen-containing gas to the container until the level of oxygen-containing gas exceeds threshold level 660. The process continues in this manner. Note that gas flow does not necessarily have to be immediately stopped when the threshold is met, and gas flow does not necessarily have to be immediately resumed when the partial pressure of the oxygen-containing gas drops below the threshold.

[0075]

[0090] During time interval T1, the flow of oxygen-containing gas can continue, for example, for about 30 minutes, thereby continuing to oxidize the collector surface. Time interval T2 can last, for example, for a time frame of 3 to 8 hours, during which time there is no excess oxygen-containing gas in the vessel. However, a gradual depletion of oxygen-containing gas exists until its partial pressure drops below a threshold value. The threshold value will generally be set depending on the application, but an exemplary value can be on the order of 4E-8 mbar.

[0076]

[0091] Figure 14 is a flowchart describing this process. In step S100, a partial pressure value that results in reflectivity within the collector specification is determined. In step S110, it is determined whether the partial pressure of the oxygen-containing gas is less than a threshold value. If the determination in step S110 is negative, dosing is shut off in step S120, i.e., the flow of oxygen-containing gas into the vessel is stopped, and the process returns to determining the partial pressure of the oxygen-containing gas in step S110. If the determination in step S110 is positive, then in step S130, oxygen-containing gas is supplied to reach the threshold value, i.e., dosing is turned on.

[0077]

[0092] According to another embodiment, as shown in FIG. 15, the system may operate to maintain the partial pressure of the oxygen-containing gas within a given range rather than above a single threshold. In such an embodiment, an upper threshold TU and a lower threshold TL for in-specification performance of the collector mirror are determined. The source is operated, and the operating value of a source parameter, such as the partial pressure of the oxygen-containing gas, is monitored. If the partial pressure of the oxygen-containing gas is determined to be below the lower threshold, the control device switches on the supply of oxygen-containing gas, which occurs at the end of time interval T2. The supply of oxygen-containing gas remains on until the partial pressure of the oxygen-containing gas is determined to meet or exceed the upper threshold, which occurs at the end of time interval T3. At that point, the control system switches off the supply of oxygen-containing gas.

[0078]

[0093] Figure 16 is a flowchart describing this process. In step S150, the partial pressure range that results in reflectance within specification is determined. In step S160, it is determined whether the partial pressure of the oxygen-containing gas is within the range. If so, then in step S170, dosing is turned off, i.e., the supply of oxygen-containing gas to the container is stopped, and the process returns to step S160. If the determination step and S160 are negative, dosing is turned on in step S180. As already mentioned, the partial pressure of the oxygen-containing gas may be measured directly or may be estimated by measuring the partial pressures of other gases in the container.

[0079]

[0094] According to another aspect, a control system may monitor the partial pressure of the oxygen-containing gas. If the partial pressure of the oxygen-containing gas drops below a threshold, the control system may turn on the supply of oxygen-containing gas to the vessel for a period of time known a priori to be sufficient to raise the partial pressure of the oxygen-containing gas above the threshold. The control system may then shut off the gas supply for a predetermined amount of time known a priori to be the amount of time until depletion causes the partial pressure of the oxygen-containing gas to drop below the predetermined threshold.

[0080]

[0095] As previously mentioned, the reflective element may have a capping layer, which may include an oxide such as a metal oxide. The oxygen-containing gas may include any one or combination of the oxygen-containing gases listed above, i.e., O2, HO, HO2, O3, CO2, and air. The oxygen-containing gas may be mixed with an inert gas such as argon or helium. For example, the added gas may be an inert gas mixed with at least 0.1% of the oxygen-containing gas.

[0081]

[0096] While the above examples refer to oxygen, it will be understood that other gases, such as nitrogen, may be used alone or in combination with oxygen. Also, the direction of flow across the face of the collector may be in either direction. An alternative implementation to achieve the same effect is to deliver the relaxation gas and / or radicals from the collector rim towards the center rather than from the center to the rim, or from both locations, or alternatively, for example, first from the rim to the center and then from the center to the rim, for the same effect.

[0082]

[0097] The present disclosure is made with the aid of functional building blocks that illustrate implementations of specific functions and relationships thereof. The boundaries of these functional building blocks are arbitrarily defined herein for the convenience of description. Alternative boundaries may be defined so long as the specific functions and relationships thereof are appropriately implemented.

[0083]

[0098] The above description includes examples of one or more embodiments. Of course, it is not possible to describe every conceivable combination of elements or methodologies for purposes of describing the above-described embodiments, and those skilled in the art will recognize that many additional 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, the term "comprising" is intended to be a transitional term when used in the claims; therefore, to the extent the term "includes" is used in either the detailed description or the claims, such term is intended to be as inclusive as "comprising." Furthermore, although multiple elements of the described aspects and / or embodiments may be described or claimed in the singular, they are considered in the plural unless limitation to the singular is expressly stated. Furthermore, 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 otherwise specified.

[0084]

[0099] Other aspects of the invention are described in the following numbered clauses: 1. A vacuum chamber; a reflective optical element disposed within the vacuum chamber; a gas distribution system for adding gas to the vacuum chamber, the gas distribution system having an interface for coupling with a source of a mitigation gas that mitigates contamination of the reflective optical elements by chemical reaction; a gas control system for controlling the gas distribution system, the gas control system having a state in which a relaxation gas from a gas source is regulated by the gas distribution system and introduced into the interior of the vacuum chamber; An apparatus comprising: 2. The apparatus of clause 1, wherein the mitigation gas comprises an oxygen-containing gas. 3. The apparatus of clause 2, wherein the oxygen-containing gas comprises O2. 4. The apparatus of clause 2, wherein the oxygen-containing gas comprises H2O. 5. The apparatus of clause 2, wherein the oxygen-containing gas comprises O3. 6. The apparatus of clause 2, wherein the oxygen-containing gas comprises CO2. 7. The apparatus of clause 2, wherein the oxygen-containing gas comprises air. 8. The apparatus of clause 1, wherein the reflective optical element comprises a multi-layer mirror comprising a capping layer and a plurality of base layers, the capping layer being positioned to protect the base layers from damage. 9. The apparatus of clause 8, wherein the capping layer comprises an oxide. 10. The apparatus of clause 9, wherein the oxide comprises a metal oxide. 11. The apparatus of clause 8, wherein the relaxation gas comprises an oxygen-containing gas and the insulator comprises a nitride. 12. The apparatus of clause 8, wherein the insulator comprises carbide. 13. The apparatus of clause 1, wherein the gas distribution system is configured to direct a flow of the mitigation gas across at least a portion of the capping layer. 14. The apparatus of clause 1, wherein the gas distribution system is configured to add a relaxation gas from the gas source into the vacuum chamber by adding a quantity of the relaxation gas to the flow of hydrogen-containing gas. 15. The apparatus of clause 1, wherein the gas distribution system is configured to add the mitigation gas to the hydrogen-containing gas by entraining the mitigation gas in the flow of the hydrogen-containing gas. 16. The apparatus described in clause 1, wherein the gas distribution system is configured to mix the mitigation gas with the inert gas to obtain a mixture containing at least 0.1% of the mitigation gas. 17. The apparatus described in clause 1, wherein the gas distribution system is configured to mix the mitigation gas with the inert gas. 18. The apparatus of clause 17, wherein the relaxation gas comprises O2 and the inert gas comprises argon. 19. The apparatus of clause 17, wherein the mitigation gas comprises H2O. 20. The apparatus of clause 17, wherein the mitigation gas comprises CO2. 21. The apparatus of clause 17, wherein the mitigation gas comprises O3. 22. The apparatus of clause 17, wherein the mitigation gas comprises air. 23. The apparatus of clause 1, wherein the mitigation gas comprises water vapor. 24. The gas distribution system mixes the mitigation gas with the inert gas to -5 mbar of inert gas and 10 -7 10. The apparatus of claim 1, configured to form a mixture comprising: a relaxation gas at 1000 mbar; 25. The apparatus described in clause 1, wherein the reflective optical element has a capping layer and the gas distribution system has at least one outlet positioned to generate a flow of gas containing a relaxation gas across at least a portion of the capping layer. 26. The apparatus described in clause 1, wherein the gas control system is configured to continuously add the mitigation gas. 27. The apparatus of clause 1, wherein the gas control system is configured to add the relaxation gas in-line while the plasma is being generated. 28. The apparatus of clause 1, wherein the gas control system is configured to intermittently add a dose of mitigation gas. 29. The apparatus of clause 1, further comprising a sensor arranged to sense the reflectivity of the optical element and generate a signal representative of the reflectivity, wherein the gas control system is arranged to receive the signal and configured to control at least one of the relaxation gas concentration and the relaxation gas composition based at least in part on the sensed reflectivity. 30. The apparatus of clause 1, further comprising a sensor positioned to sense a relaxation gas concentration adjacent the reflective optical element and generate a signal representative of the relaxation gas concentration, wherein the gas control system is configured to receive the signal and to control at least one of the relaxation gas concentration and the relaxation gas composition based at least in part on the relaxation gas concentration adjacent the reflective optical element. 31. The apparatus of clause 1, wherein the reflective optical element is part of an EUV source of EUV radiation, and the gas control system is configured to control at least one of the relaxation gas concentration and the relaxation gas composition based on the duration of operation of the EUV source. 32. The apparatus described in clause 1, wherein the reflective optical element is part of an EUV source of EUV radiation, and the gas control system is operable to control at least one of the relaxation gas concentration and the relaxation gas composition based on the number of pulses of EUV radiation generated by the EUV source. 33. The apparatus of clause 1, further comprising a plurality of structures located within the vacuum chamber for directing the flow of gas within the vacuum chamber, and further comprising a second gas distribution system arranged to flow a mitigation gas over the structures. 34. A vacuum chamber; a reflective optical element disposed within the vacuum chamber; a primary gas distribution system for adding hydrogen gas to the vacuum chamber; a secondary gas distribution system for adding a mitigation gas to the vacuum chamber that mitigates contamination of the reflective optical elements by chemical reaction; a gas control system for controlling the primary gas distribution system and the secondary gas distribution system, the gas control system having a state in which the mitigation gas is regulated by the secondary gas distribution system and introduced into the interior of the vacuum chamber; An apparatus comprising: 35. Measuring operating parameters of an EUV light source comprising a vacuum chamber and a reflective optical element located within the chamber; controlling the addition of a mitigation gas to the vacuum chamber that mitigates contamination of the reflective optical elements by chemical reaction based at least in part on the measured operating parameters; A method for providing 36. The method of clause 35, wherein the step of controlling the addition of the mitigation gas comprises controlling the concentration of the mitigation gas. 37. The method of clause 35, wherein the step of controlling the addition of the mitigation gas comprises controlling the flow rate of the mitigation gas. 38. The method of clause 35, wherein the step of controlling the addition of the relaxation gas comprises controlling the composition of the relaxation gas. 39. The method of clause 35, wherein the operating parameter is the reflectivity of the optical element. 40. The method of clause 35, wherein the operating parameter is the concentration of the relaxation gas adjacent to the optical element. 41. The method of clause 35, wherein the operating parameter is the duration of operation of the EUV light source. 42. The method of clause 35, wherein the operating parameter is the number of EUV pulses generated by the EUV light source from a predetermined start time. 43. A vacuum chamber; a reflective optical element disposed within the vacuum chamber; a gas distribution system having an inlet that selectively allows gas to enter the vacuum chamber; a source of a mitigation gas in selective fluid communication with the inlet for mitigating contamination of the reflective optical element by chemical reaction; at least one heating element in thermal communication with the inlet and positioned to heat gas flowing through the inlet to a temperature sufficient to generate radicals in at least a small amount of gas in the gas flow; a gas control system configured to control the gas distribution system and the at least one heating element, the gas control system having a first state in which the gas control system causes introduction of a relaxation gas into the vacuum chamber through the inlet and a second state in which the gas control system causes energization of the at least one heating element; An apparatus comprising: 44. The device of clause 43, wherein at least one heating element comprises at least one filament. 45. The apparatus of clause 43, wherein the mitigation gas comprises an oxygen-containing gas. 46. ​​The apparatus of clause 45, wherein the oxygen-containing gas comprises O2. 47. The apparatus of clause 45, wherein the oxygen-containing gas comprises H2O. 48. The apparatus of clause 45, wherein the oxygen-containing gas comprises O3. 49. The apparatus of clause 45, wherein the oxygen-containing gas comprises CO2. 50. The apparatus of clause 45, wherein the oxygen-containing gas comprises air. 51. The apparatus of clause 43, wherein the relaxation gas is operable to flow across at least a portion of the reflective surface of the reflective optical element. 52. The apparatus of clause 43, wherein the gas distribution system is operable to introduce relaxation gas from the gas source into the vacuum chamber by adding a quantity of relaxation gas to the flow of hydrogen-containing gas. 53. The apparatus of clause 43, wherein the gas distribution system is configured to introduce the relaxation gas from the gas source into the vacuum chamber by entraining the relaxation gas in the flow of hydrogen-containing gas. 54. Apparatus according to clause 43, wherein the inlet comprises a conical element arranged coaxially with and around the central aperture of the reflective optical element. 55. The apparatus of clause 54, further comprising a plurality of heating elements circumferentially disposed around the outer surface of the conical element. 56. The apparatus of clause 54, wherein each of the plurality of heating elements comprises at least one filament. 57. A method of reducing target material contamination of a reflective surface of a reflective optical element of an extreme ultraviolet light source, comprising: establishing an umbrella flow of hydrogen across a reflective surface; ionizing at least a small amount of hydrogen in the umbrella flow for a first interval; adding a predetermined amount of relaxation gas to the umbrella flow for a second interval different from the first interval; repeating the ionizing step and the doping step alternately multiple times; A method for providing 58. A vacuum chamber; a reflective optical element disposed within the vacuum chamber; a gas distribution system having an inlet that selectively allows gas to enter the vacuum chamber; a source of a mitigation gas in selective fluid communication with the inlet for mitigating contamination of the reflective optical element by chemical reaction; a gas distribution system configured to place a source of radical-containing gas in selective fluid communication with the inlet; a gas control system configured to control the gas distribution system, the gas control system having a first state in which the gas control system causes a relaxation gas to be introduced into the vacuum chamber through the inlet and a second state in which the gas control system causes a radical-containing gas to be introduced into the vacuum chamber through the inlet; An apparatus comprising: 59. The apparatus of clause 58, wherein the source of radical-containing gas comprises at least one heating element in thermal communication with the inlet and positioned to heat the flow of gas flowing through the inlet to a temperature sufficient to generate radicals in at least a small amount of gas in the flow of gas to generate the radical-containing gas. 60. An apparatus as described in clause 58, wherein the source of radical-containing gas comprises a plasma source arranged to create radicals in the gas before the gas reaches the inlet. 61. A method of reducing target material contamination of a reflective surface of a reflective optical element in a vacuum chamber of an extreme ultraviolet light source, comprising: (a) initiating a flow of a relaxation gas into a vacuum chamber; (b) stopping the flow of relaxation gas into the vacuum chamber; (c) initiating a flow of radical-containing gas into the vacuum chamber; (d) stopping the flow of radical-containing gas into the vacuum chamber; wherein steps (a) through (d) are repeated in sequence multiple times. 62. A vacuum chamber; a reflective optical element disposed within the vacuum chamber; a gas supply system operable to supply at least a first gas and a second gas to the vacuum chamber, the second gas comprising oxygen; a control system operable to control the gas supply system to supply the second gas based at least in part on a partial pressure of at least one of the first gas and the second gas sensed in the vacuum chamber; An apparatus comprising: 63. The apparatus of clause 62, wherein the reflective optical element comprises a capping layer comprising an oxide. 64. The apparatus of clause 63, wherein the oxide comprises a metal oxide. 65. The apparatus of clause 62, wherein the control system is operable to control the gas supply system to supply the second gas based at least in part on a partial pressure of the second gas sensed in the vacuum chamber. 66. The apparatus of clause 62, wherein the control system is operable to control the gas supply system to supply the second gas based at least in part on the partial pressure of the first gas sensed in the vacuum chamber. 67. The apparatus of clause 62, wherein the second gas comprises O2. 68. The apparatus of clause 62, wherein the second gas comprises H2O. 69. The apparatus of clause 62, wherein the second gas comprises H2O2. 70. The apparatus of clause 62, wherein the second gas comprises O3. 71. The apparatus of clause 62, wherein the second gas comprises CO2. 72. The apparatus of clause 62, wherein the second gas comprises air. 73. The apparatus of any one of clauses 62 to 72, wherein the second gas comprises an inert gas. 74. The apparatus of clause 73, wherein the inert gas comprises argon. 75. The apparatus of clause 73, wherein the inert gas comprises helium. 76. The apparatus of any one of clauses 62 to 72, wherein the second gas comprises an inert gas mixed with at least 0.1% of an oxygen-containing gas. 77. The apparatus of clause 62, further comprising a gas pressure sensor positioned to sense a partial pressure of the second gas in the vacuum chamber and to generate a first signal to the control system representative of the partial pressure, the control system controlling the supply of the second gas based at least in part on the first signal. 78. The apparatus of clause 77, wherein the gas pressure sensor directly senses the partial pressure of the second gas. 79. The apparatus of clause 77, wherein the gas pressure sensor indirectly senses the partial pressure of the oxygen-containing gas by sensing the partial pressure of at least one gas other than the second gas. 80. A vacuum chamber; a reflective optical element disposed within the vacuum chamber; a gas distribution system for adding gas to the vacuum chamber, the gas distribution system including an interface for coupling with a source of oxygen-containing gas; a gas control system configured to control the gas distribution system to supply the oxygen-containing gas to the vacuum chamber until a partial pressure of the oxygen-containing gas reaches a first value, to discontinue supplying the oxygen-containing gas to the vacuum chamber until the partial pressure of the oxygen-containing gas reaches a second value less than the first value, and to resume supplying the oxygen-containing gas to the vacuum chamber until the partial pressure of the oxygen-containing gas reaches a third value greater than the second value; An apparatus comprising: 81. The apparatus of clause 80, wherein the reflective optical element comprises a multi-layer mirror comprising a capping layer and a plurality of underlying layers, the capping layer constructed and arranged to protect the underlying layers from damage, and the capping layer comprising an oxide. 82. The apparatus of clause 81, wherein the oxide comprises a metal oxide. 83. The apparatus of clause 80, wherein the third value is substantially equal to the first value. 84. The apparatus of clause 80, wherein the oxygen-containing gas comprises O2. 85. The apparatus of clause 80, wherein the oxygen-containing gas comprises H2O. 86. The apparatus of clause 80, wherein the oxygen-containing gas comprises H2O2. 87. The apparatus of clause 80, wherein the oxygen-containing gas comprises O3. 88. The apparatus of clause 80, wherein the oxygen-containing gas comprises CO2. 89. The apparatus of clause 80, wherein the oxygen-containing gas comprises air. 90. The apparatus of any one of clauses 80 to 89, wherein the second gas comprises an inert gas. 91. The apparatus of clause 90, wherein the inert gas comprises argon. 92. The apparatus of clause 90, wherein the inert gas comprises helium. 93. The apparatus of any one of clauses 80 to 92, wherein the second gas comprises an inert gas mixed with at least 0.1% of an oxygen-containing gas. 94. The apparatus of clause 80, further comprising a gas pressure sensor positioned to sense a partial pressure of the oxygen-containing gas in the vacuum chamber and to generate a first signal to a gas control system representative of the partial pressure, the gas control system controlling the supply of the oxygen-containing gas based at least in part on the first signal. 95. The apparatus of clause 94, wherein the gas pressure sensor directly senses the partial pressure of the oxygen-containing gas. 96. The apparatus of clause 94, wherein the gas pressure sensor indirectly senses the partial pressure of the oxygen-containing gas by sensing the partial pressure of at least one gas other than the oxygen-containing gas. 97. The apparatus of clause 94, wherein the gas pressure sensor is positioned to sense the partial pressure of the oxygen-containing gas adjacent the reflective optical element. 98. A method of extending the operational life of a reflective surface in a vacuum chamber of an EUV source, comprising: providing a gas supply system operable to supply at least a first gas and a second gas to the vacuum chamber, the second gas comprising oxygen; sensing a partial pressure of at least one of a first gas and a second gas in the vacuum chamber; controlling the gas supply system to supply a second gas based at least in part on the sensed partial pressure; A method for providing 99. The method of clause 98, wherein the sensing step comprises sensing a partial pressure of the second gas. 100. The method of clause 98, wherein the sensing step comprises sensing a partial pressure of the first gas. 101. The method of clause 98, wherein the second gas comprises O2. 102. The method of clause 98, wherein the second gas comprises H2O. 103. The method of clause 98, wherein the second gas comprises H2O2. 104. The method of clause 98, wherein the second gas comprises O3. 105. The method of clause 98, wherein the second gas comprises CO2. 106. The method of clause 98, wherein the second gas comprises air. 107. The method of any one of clauses 98 to 106, wherein the second gas comprises an inert gas. 108. The method of clause 107, wherein the inert gas comprises argon. 109. The method of clause 107, wherein the inert gas comprises helium. 110. The method of any one of clauses 98 to 109, wherein the second gas comprises an inert gas mixed with at least 0.1% of an oxygen-containing gas. 111. A method for extending the operational life of a reflective surface in a vacuum chamber of an EUV source, comprising: (a) supplying an oxygen-containing gas to a vacuum chamber; (b) ceasing the supply of the oxygen-containing gas to the vacuum chamber when the partial pressure of the oxygen-containing gas reaches a first value; (c) supplying more oxygen-containing gas to the vacuum chamber when the partial pressure of the oxygen-containing gas reaches a second value less than the first value; (d) ceasing the supply of the oxygen-containing gas to the vacuum chamber when the partial pressure of the oxygen-containing gas reaches a third value greater than the second value; (e) repeating steps (c) and (d) to maintain the partial pressure of the oxygen-containing gas between the first value and the second value; A method comprising: 112. The method of clause 111, wherein the third value is substantially equal to the first value. 113. The method of clause 111, wherein the oxygen-containing gas comprises O2. 114. The method of clause 111, wherein the oxygen-containing gas comprises H2O. 115. The method of clause 111, wherein the oxygen-containing gas comprises H2O2. 116. The method of clause 111, wherein the oxygen-containing gas comprises O3. 117. The method of clause 111, wherein the oxygen-containing gas comprises CO2. 118. The method of clause 111, wherein the oxygen-containing gas comprises air. 119. The method of any one of clauses 111 to 118, wherein the second gas comprises an inert gas. 120. The method of clause 119, wherein the inert gas comprises argon. 121. The method of clause 119, wherein the inert gas comprises helium. 122. The method of any one of clauses 111 to 119, wherein the second gas comprises an inert gas mixed with at least 0.1% of an oxygen-containing gas. 123. The method of clause 111, wherein steps (b), (c), and (d) each comprise sensing the partial pressure of the oxygen-containing gas in the vacuum chamber. 124. The method of clause 111, wherein steps (b), (c), and (d) each comprise directly sensing the partial pressure of the oxygen-containing gas in the vacuum chamber. 125. The method of clause 111, wherein steps (b), (c), and (d) each comprise indirectly sensing the partial pressure of the oxygen-containing gas in the vacuum chamber by sensing the partial pressure of at least one gas other than the oxygen-containing gas. 126. The method of clause 111, wherein steps (b), (c), and (d) each comprise sensing the partial pressure of an oxygen-containing gas adjacent to the reflective surface in the vacuum chamber.

Claims

1. a vacuum chamber; a reflective optical element disposed within the vacuum chamber; a gas distribution system for adding gas to the vacuum chamber, the gas distribution system having an interface for coupling with a source of a mitigation gas that mitigates contamination of the reflective optical elements by chemical reaction; a gas control system for controlling the gas distribution system, the gas control system having a state in which the relaxation gas from the gas source is regulated by the gas distribution system and introduced into the interior of the vacuum chamber; An apparatus comprising:

2. The apparatus of claim 1 , wherein the relaxation gas comprises an oxygen-containing gas.

3. The apparatus of claim 2 , wherein the oxygen-containing gas comprises O 2 .

4. 3. The apparatus of claim 2, wherein the oxygen-containing gas comprises H2O.

5. 3. The apparatus of claim 2, wherein the oxygen-containing gas comprises O3.

6. The apparatus of claim 2 , wherein the oxygen-containing gas comprises CO 2 .

7. The apparatus of claim 2 , wherein the oxygen-containing gas comprises air.

8. the reflective optical element comprises a multi-layer mirror comprising a capping layer and a plurality of underlayers; The apparatus of claim 1 , wherein the capping layer is positioned to protect the underlying layer from damage.

9. The device of claim 8 , wherein the capping layer comprises an oxide.

10. The device of claim 9 , wherein the oxide comprises a metal oxide.

11. the mitigation gas comprises an oxygen-containing gas; The device of claim 8 , wherein the insulator comprises a nitride.

12. The apparatus of claim 8 , wherein the insulator comprises a carbide.

13. The apparatus of claim 1 , wherein the gas distribution system is configured to direct a flow of the mitigation gas across at least a portion of the capping layer.

14. 10. The apparatus of claim 1, wherein the gas distribution system is configured to add the relaxation gas from the gas source into the vacuum chamber by adding a quantity of the relaxation gas to a flow of hydrogen-containing gas.

15. The apparatus of claim 1 , wherein the gas distribution system is configured to add the relaxation gas to the hydrogen-containing gas by entraining the relaxation gas in the flow of the hydrogen-containing gas.

16. 10. The apparatus of claim 1, wherein the gas distribution system is configured to mix the mitigation gas with an inert gas to obtain a mixture containing at least 0.1% mitigation gas.

17. The apparatus of claim 1 , wherein the gas distribution system is configured to mix the mitigation gas with an inert gas.

18. the mitigation gas comprises O2; 20. The apparatus of claim 17, wherein the inert gas comprises argon.

19. 20. The apparatus of claim 17, wherein the mitigation gas comprises H2O.

20. 20. The apparatus of claim 17, wherein the mitigation gas comprises CO2.

21. 20. The apparatus of claim 17, wherein the mitigation gas comprises O3.

22. 20. The apparatus of claim 17, wherein the mitigation gas comprises air.

23. The apparatus of claim 1 , wherein the mitigation gas comprises water vapor.

24. The gas distribution system mixes the mitigation gas with an inert gas to －５ Inert gas at about 10 mbar －７ 10. The apparatus of claim 1, configured to form a mixture comprising: a relaxation gas; and a relaxation gas of 0.1 mbar.

25. the reflective optical element has a capping layer; The apparatus of claim 1 , wherein the gas distribution system comprises at least one outlet positioned to generate a flow of gas containing the mitigation gas across at least a portion of the capping layer.

26. The apparatus of claim 1 , wherein the gas control system is configured to add the mitigation gas continuously.

27. The apparatus of claim 1 , wherein the gas control system is configured to add the relaxation gas in-line while a plasma is being generated.

28. The apparatus of claim 1 , wherein the gas control system is configured to add a dose of the relaxation gas intermittently.

29. a sensor positioned to sense the reflectivity of the optical element and to generate a signal representative of the reflectivity; 10. The apparatus of claim 1, wherein the gas control system is disposed to receive the signal and configured to control at least one of a relaxation gas concentration and a relaxation gas composition based at least in part on the detected reflectance.

30. a sensor positioned to sense a relaxation gas concentration adjacent the reflective optical element and to generate a signal representative of the relaxation gas concentration; 10. The apparatus of claim 1, wherein the gas control system is configured to receive the signal and to control at least one of a relaxation gas concentration and a relaxation gas composition based at least in part on the relaxation gas concentration adjacent the reflective optical element.

31. the reflective optical element is part of an EUV source of EUV radiation; 10. The apparatus of claim 1, wherein the gas control system is configured to control at least one of a relaxation gas concentration and a relaxation gas composition based on a duration of operation of the EUV source.

32. the reflective optical element is part of an EUV source of EUV radiation; 10. The apparatus of claim 1, wherein the gas control system is operable to control at least one of a relaxation gas concentration and a relaxation gas composition based on a number of pulses of EUV radiation produced by the EUV source.

33. 10. The apparatus of claim 1, further comprising a plurality of structures located within the vacuum chamber that direct the flow of gas within the vacuum chamber, and further comprising a second gas distribution system positioned to flow the mitigation gas over the structures.

34. a vacuum chamber; a reflective optical element disposed within the vacuum chamber; a primary gas distribution system for adding hydrogen gas to the vacuum chamber; a secondary gas distribution system for adding a mitigation gas to the vacuum chamber that mitigates contamination of the reflective optical elements by chemical reaction; a gas control system for controlling the primary gas distribution system and the secondary gas distribution system, the gas control system having a state in which the mitigation gas is regulated by the secondary gas distribution system and introduced into the interior of the vacuum chamber; An apparatus comprising:

35. measuring operating parameters of an EUV light source comprising a vacuum chamber and a reflective optical element located within said chamber; controlling the addition of a mitigation gas to the vacuum chamber that mitigates contamination of the reflective optical elements by chemical reaction based at least in part on the measured operating parameters; A method comprising:

36. 36. The method of claim 35, wherein controlling the addition of the relaxation gas comprises controlling a concentration of the relaxation gas.

37. 36. The method of claim 35, wherein controlling the addition of the relaxation gas comprises controlling a flow rate of the relaxation gas.

38. 36. The method of claim 35, wherein controlling the addition of the relaxation gas comprises controlling the composition of the relaxation gas.

39. 36. The method of claim 35, wherein the operating parameter is the reflectivity of the optical element.

40. 36. The method of claim 35, wherein the operating parameter is a concentration of the relaxation gas adjacent to the optical element.

41. 36. The method of claim 35, wherein the operating parameter is a duration of operation of the EUV light source.

42. 36. The method of claim 35, wherein the operating parameter is a number of EUV pulses generated by the EUV light source from a predetermined start time.

43. a vacuum chamber; a reflective optical element disposed within the vacuum chamber; a gas distribution system having an inlet that selectively allows gas to enter the vacuum chamber; a source of a mitigation gas in selective fluid communication with the inlet for mitigating contamination of the reflective optical element by chemical reaction; at least one heating element in thermal communication with the inlet and positioned to heat gas flowing through the inlet to a temperature sufficient to generate radicals in at least a small amount of the gas in the flow of gas; a gas control system configured to control the gas distribution system and the at least one heating element, the gas control system having a first state in which the gas control system introduces the mitigation gas into the vacuum chamber through the inlet and a second state in which the gas control system energizes the at least one heating element; An apparatus comprising:

44. 44. The apparatus of claim 43, wherein the at least one heating element comprises at least one filament.

45. 44. The apparatus of claim 43, wherein the mitigation gas comprises an oxygen-containing gas.

46. 46. ​​The apparatus of claim 45, wherein the oxygen-containing gas comprises O2.

47. 46. ​​The apparatus of claim 45, wherein the oxygen-containing gas comprises H2O.

48. 46. ​​The apparatus of claim 45, wherein the oxygen-containing gas comprises O3.

49. 46. ​​The apparatus of claim 45, wherein the oxygen-containing gas comprises CO2.

50. 46. ​​The apparatus of claim 45, wherein the oxygen-containing gas comprises air.

51. 44. The apparatus of claim 43, wherein the relaxation gas is operable to flow across at least a portion of a reflective surface of the reflective optical element.

52. 44. The apparatus of claim 43, wherein the gas distribution system is operable to introduce relaxation gas from the gas source into the vacuum chamber by adding a quantity of the relaxation gas to a flow of hydrogen-containing gas.

53. 44. The apparatus of claim 43, wherein the gas distribution system is configured to introduce the relaxation gas from the gas source into the vacuum chamber by entraining the relaxation gas in the flow of the hydrogen-containing gas.

54. 44. The apparatus of claim 43, wherein the inlet comprises a conical element disposed coaxially about a central aperture of the reflective optical element.

55. 55. The apparatus of claim 54, further comprising a plurality of heating elements circumferentially disposed around an outer surface of the conical element.

56. 55. The apparatus of claim 54, wherein each of the plurality of heating elements comprises at least one filament.

57. 1. A method for reducing target material contamination of a reflective surface of a reflective optical element of an extreme ultraviolet light source, comprising: establishing an umbrella flow of hydrogen across the reflective surface; ionizing at least a small amount of the hydrogen in the umbrella flow for a first interval; adding a predetermined amount of relaxation gas to the umbrella flow for a second interval different from the first interval; repeating the ionizing step and the adding step alternately multiple times; A method comprising:

58. a vacuum chamber; a reflective optical element disposed within the vacuum chamber; a gas distribution system having an inlet that selectively allows gas to enter the vacuum chamber; a source of a mitigation gas in selective fluid communication with the inlet for mitigating contamination of the reflective optical element by chemical reaction; the gas distribution system configured to place a source of radical-containing gas in selective fluid communication with the inlet; a gas control system configured to control the gas distribution system, the gas control system having a first state in which the gas control system causes the relaxation gas to be introduced into the vacuum chamber through the inlet and a second state in which the gas control system causes the radical-containing gas to be introduced into the vacuum chamber through the inlet; An apparatus comprising:

59. 59. The apparatus of claim 58, wherein the source of radical-containing gas comprises at least one heating element in thermal communication with the inlet and positioned to heat a flow of gas flowing through the inlet to a temperature sufficient to generate radicals in at least a small amount of the gas in the flow of gas to produce the radical-containing gas.

60. 60. The apparatus of claim 58, wherein the source of radical-containing gas comprises a plasma source positioned to create radicals in the gas before the gas reaches the inlet.

61. 1. A method for reducing target material contamination of reflective surfaces of reflective optical elements in a vacuum chamber of an extreme ultraviolet light source, comprising: (a) initiating a flow of a relaxation gas into the vacuum chamber; (b) stopping the flow of the relaxation gas into the vacuum chamber; (c) initiating a flow of radical-containing gas into the vacuum chamber; (d) stopping the flow of the radical-containing gas into the vacuum chamber; The method wherein steps (a) through (d) are repeated multiple times in sequence.

62. a vacuum chamber; a reflective optical element disposed within the vacuum chamber; a gas supply system operable to supply at least a first gas and a second gas to the vacuum chamber, the second gas comprising oxygen; a control system operable to control the gas supply system to supply the second gas based at least in part on a partial pressure of at least one of the first gas and the second gas sensed in the vacuum chamber; An apparatus comprising:

63. 63. The apparatus of claim 62, wherein the reflective optical element comprises a capping layer comprising an oxide.

64. 64. The apparatus of claim 63, wherein the oxide comprises a metal oxide.

65. 63. The apparatus of claim 62, wherein the control system is operable to control the gas supply system to supply the second gas based at least in part on a partial pressure of the second gas sensed in the vacuum chamber.

66. 63. The apparatus of claim 62, wherein the control system is operable to control the gas supply system to supply the second gas based at least in part on a partial pressure of the first gas sensed in the vacuum chamber.

67. 63. The apparatus of claim 62, wherein the second gas comprises O2.

68. 63. The apparatus of claim 62, wherein the second gas comprises H2O.

69. 63. The apparatus of claim 62, wherein the second gas comprises H2O2.

70. 63. The apparatus of claim 62, wherein the second gas comprises O3.

71. 63. The apparatus of claim 62, wherein the second gas comprises CO2.

72. 63. The apparatus of claim 62, wherein the second gas comprises air.

73. 73. The apparatus of any one of claims 62 to 72, wherein the second gas comprises an inert gas.

74. 74. The apparatus of claim 73, wherein the inert gas comprises argon.

75. 74. The apparatus of claim 73, wherein the inert gas comprises helium.

76. 73. The apparatus of any one of claims 62 to 72, wherein the second gas comprises an inert gas mixed with at least 0.1% of an oxygen-containing gas.

77. a gas pressure sensor positioned to sense a partial pressure of the second gas in the vacuum chamber and to generate a first signal to the control system representative of the partial pressure; 63. The apparatus of claim 62, wherein the control system controls the supply of the second gas based at least in part on the first signal.

78. 78. The apparatus of claim 77, wherein the gas pressure sensor directly senses the partial pressure of the second gas.

79. 78. The apparatus of claim 77, wherein the gas pressure sensor indirectly senses the partial pressure of the oxygen-containing gas by sensing the partial pressure of at least one gas other than the second gas.

80. a vacuum chamber; a reflective optical element disposed within the vacuum chamber; a gas distribution system for adding gas to the vacuum chamber, the gas distribution system including an interface for coupling with a source of oxygen-containing gas; a gas control system configured to control the gas delivery system to supply the oxygen-containing gas to the vacuum chamber until a partial pressure of the oxygen-containing gas reaches a first value, to discontinue supplying the oxygen-containing gas to the vacuum chamber until the partial pressure of the oxygen-containing gas reaches a second value less than the first value, and to resume supplying the oxygen-containing gas to the vacuum chamber until the partial pressure of the oxygen-containing gas reaches a third value greater than the second value; An apparatus comprising:

81. the reflective optical element comprises a multi-layer mirror comprising a capping layer and a plurality of underlayers; the capping layer is constructed and arranged to protect the underlying layer from damage; 81. The apparatus of claim 80, wherein the capping layer comprises an oxide.

82. 82. The apparatus of claim 81 , wherein the oxide comprises a metal oxide.

83. 81. The apparatus of claim 80, wherein the third value is substantially equal to the first value.

84. 81. The apparatus of claim 80, wherein the oxygen-containing gas comprises O2.

85. 81. The apparatus of claim 80, wherein the oxygen-containing gas comprises H2O.

86. 81. The apparatus of claim 80, wherein the oxygen-containing gas comprises H2O2.

87. 81. The apparatus of claim 80, wherein the oxygen-containing gas comprises O3.

88. 81. The apparatus of claim 80, wherein the oxygen-containing gas comprises CO2.

89. 81. The apparatus of claim 80, wherein the oxygen-containing gas comprises air.

90. 90. The apparatus of any one of claims 80 to 89, wherein the second gas comprises an inert gas.

91. 91. The apparatus of claim 90, wherein the inert gas comprises argon.

92. 91. The apparatus of claim 90, wherein the inert gas comprises helium.

93. 93. The apparatus of any one of claims 80 to 92, wherein the second gas comprises an inert gas mixed with at least 0.1% of an oxygen-containing gas.

94. a gas pressure sensor positioned to sense a partial pressure of the oxygen-containing gas in the vacuum chamber and to generate a first signal to the gas control system representative of the partial pressure; 81. The apparatus of claim 80, wherein the gas control system controls the supply of the oxygen-containing gas based at least in part on the first signal.

95. 95. The apparatus of claim 94, wherein the gas pressure sensor directly senses the partial pressure of the oxygen-containing gas.

96. 95. The apparatus of claim 94, wherein the gas pressure sensor indirectly senses the partial pressure of the oxygen-containing gas by sensing the partial pressure of at least one gas other than the oxygen-containing gas.

97. 95. The apparatus of claim 94, wherein the gas pressure sensor is positioned to sense the partial pressure of the oxygen-containing gas proximate the reflective optical element.

98. 1. A method for extending the operational lifetime of a reflective surface in a vacuum chamber of an EUV source, comprising: providing a gas supply system operable to supply at least a first gas and a second gas to the vacuum chamber, the second gas comprising oxygen; sensing a partial pressure of at least one of the first gas and the second gas in the vacuum chamber; controlling the gas supply system to supply the second gas based at least in part on the sensed partial pressure; A method comprising:

99. 99. The method of claim 98, wherein the sensing step comprises sensing a partial pressure of the second gas.

100. 99. The method of claim 98, wherein the sensing step comprises sensing a partial pressure of the first gas.

101. 99. The method of claim 98, wherein the second gas comprises O2.

102. 99. The method of claim 98, wherein the second gas comprises H2O.

103. 99. The method of claim 98, wherein the second gas comprises H2O2.

104. 99. The method of claim 98, wherein the second gas comprises O3.

105. 99. The method of claim 98, wherein the second gas comprises CO2.

106. 99. The method of claim 98, wherein the second gas comprises air.

107. 107. The method of any one of claims 98 to 106, wherein the second gas comprises an inert gas.

108. 108. The method of claim 107, wherein the inert gas comprises argon.

109. 108. The method of claim 107, wherein the inert gas comprises helium.

110. 110. The method of any one of claims 98 to 109, wherein the second gas comprises an inert gas mixed with at least 0.1% of an oxygen-containing gas.

111. 1. A method for extending the operational lifetime of a reflective surface in a vacuum chamber of an EUV source, comprising: (a) supplying an oxygen-containing gas into the vacuum chamber; (b) ceasing the supply of the oxygen-containing gas to the vacuum chamber when the partial pressure of the oxygen-containing gas reaches a first value; (c) supplying more of the oxygen-containing gas to the vacuum chamber when the partial pressure of the oxygen-containing gas reaches a second value less than the first value; (d) ceasing the supply of the oxygen-containing gas to the vacuum chamber when the partial pressure of the oxygen-containing gas reaches a third value greater than the second value; (e) repeating steps (c) and (d) to maintain the partial pressure of the oxygen-containing gas between the first value and the second value; A method comprising:

112. 112. The method of claim 111, wherein the third value is substantially equal to the first value.

113. 112. The method of claim 111, wherein the oxygen-containing gas comprises O2.

114. 112. The method of claim 111, wherein the oxygen-containing gas comprises H2O.

115. 112. The method of claim 111, wherein the oxygen-containing gas comprises H2O2.

116. 112. The method of claim 111, wherein the oxygen-containing gas comprises O3.

117. 112. The method of claim 111, wherein the oxygen-containing gas comprises CO2.

118. 112. The method of claim 111, wherein the oxygen-containing gas comprises air.

119. 119. The method of any one of claims 111 to 118, wherein the second gas comprises an inert gas.

120. 120. The method of claim 119, wherein the inert gas comprises argon.

121. 120. The method of claim 119, wherein the inert gas comprises helium.

122. 120. The method of any one of claims 111 to 119, wherein the second gas comprises an inert gas mixed with at least 0.1% of an oxygen-containing gas.

123. 112. The method of claim 111, wherein steps (b), (c), and (d) each comprise sensing the partial pressure of the oxygen-containing gas in the vacuum chamber.

124. 112. The method of claim 111, wherein steps (b), (c), and (d) each comprise directly sensing the partial pressure of the oxygen-containing gas in the vacuum chamber.

125. 112. The method of claim 111, wherein each of steps (b), (c), and (d) comprises indirectly sensing the partial pressure of the oxygen-containing gas in the vacuum chamber by sensing the partial pressure of at least one gas other than the oxygen-containing gas.

126. 112. The method of claim 111, wherein steps (b), (c), and (d) each comprise sensing the partial pressure of the oxygen-containing gas proximate the reflective surface in the vacuum chamber.

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