Extending service life of optical elements in EUV lithography systems

Abstract

Degradation of reflectivity of one or more reflective optical elements in a system (SO) for generating EUV radiation is reduced by controlled introduction of gas into a vacuum chamber (26) containing the optical elements. The gas may be added to another gas stream, such as hydrogen, or alternated with the introduction of hydrogen radicals.

Description

[0001] Description of the case

[0002] This application is a divisional application of the Chinese invention patent application with application number 201980017251.1 filed on February 28, 2019 and entitled “Extending the service life of optical elements in EUV lithography systems”.

[0003] CROSS-REFERENCE TO RELATED APPLICATIONS

[0004] This application claims priority to U.S. Application No. 62 / 803,057 filed on February 8, 2019, U.S. Application No. 62 / 736,108 filed on September 25, 2018, and U.S. Application No. 62 / 638,778 filed on March 5, 2018, the entire contents of which are incorporated herein by reference. Technical Field

[0005] The present disclosure relates to systems for generating extreme ultraviolet radiation. Such systems typically use reflective optical elements arranged in a vacuum environment. The process of generating and controlling EUV radiation in such systems is susceptible to degradation of the reflectivity of these optical elements over time. An example of such an environment is a vacuum chamber of an EUV source, in which a plasma is generated by discharge or laser ablation of a target or source material. In the present application, the optical elements are used, for example, to collect and direct radiation for use outside the vacuum chamber, such as for semiconductor lithography. Another example is a vacuum chamber housing optical elements of a projection system for such an apparatus. Background Art

[0006] EUV radiation (eg, electromagnetic radiation having a wavelength of about 50 nm or less (sometimes also called soft X-rays), and including radiation having a wavelength of about 13.5 nm) can be used in photolithography processes to form extremely small features in substrates such as silicon wafers.

[0007] Some methods for generating EUV radiation include converting a target material (also referred to as a source material) from a liquid state to a plasma state. The target material preferably includes at least one element, such as xenon, lithium, or tin, having one or more emission lines in the EUV range. In one such method, laser produced plasma ("LPP"), the desired plasma can be produced by irradiating a target material having the desired line-emitting element with a laser beam.

[0008] One LPP technique involves generating a stream of droplets of a target material 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), thereby producing a highly ionized plasma with an electron temperature of tens of electron volts.

[0009] The high energy radiation generated during the deexcitation and recombination of these ions is emitted from the plasma in all directions. In one common arrangement, a near normal incidence mirror (often referred to as a "collector mirror" or simply a "collector") is positioned to collect, direct, and in some arrangements focus the radiation to an intermediate location. The collected radiation can then be transferred from the intermediate location to a set of scanner optics and ultimately to the wafer.

[0010] In the EUV part of the spectrum, it is generally considered necessary to use reflective optical elements for the collector and other EUV optical elements. At the wavelengths involved, the collector is advantageously implemented as a multilayer mirror ("MLM"). As the name implies, such an MLM is generally composed of alternating layers of material on a base or substrate.

[0011] The optical element must be placed in a vacuum chamber with the plasma to collect and redirect EUV radiation. The environment inside the chamber is not friendly to the optical element and therefore limits its service life, for example by degrading its reflectivity using 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 may contaminate the exposed surface of the optical element. The particles of the target material can also cause physical damage and local heating of the MLM surface. In particular, the target material can react with the material of at least one layer constituting the surface of the optical element. Even in the case of less reactive target materials (e.g., tin, indium, or xenon), temperature stability, ion implantation, and diffusion issues may need to be addressed. Blistering of the MLM coating must also be avoided. The target material may also be deposited on the surface of the optical element.

[0012] Another major contributor to the collector degradation rate is localized contamination formation due to tin debris deposition, for example around the central hole of the collector. Depending on the vessel pressure at which the EUV source operates, the contribution of contamination to the collector degradation rate can be between about 30% and about 60% of the total rate.

[0013] Despite these harsh conditions, there are some techniques that can be used to increase the life of optical elements. For example, a cover layer can be placed on the optical element to protect the surface of the optical element. In order to make the cover layer more reflective, the cover layer can also have multiple layers spaced apart to increase the reflectivity at the wavelength of the radiation to be reflected. However, such a cover layer itself is susceptible to damage by mechanisms such as reduction, hydrogen diffusion and blistering.

[0014] In some systems, H2 gas is used in the vacuum chamber at a pressure in the range of about 0.5 to about 3 mbar to reduce debris. Hydrogen is relatively transparent to EUV radiation with a wavelength of about 13.5 nm. H2 gas is introduced into the vacuum chamber to slow down the high-energy debris (ions, atoms and clusters) of the target material generated by the collision of the plasma with the gas molecules. To this end, a flow of H2 is used, which can also move in the opposite direction to the debris track. This is used to reduce damage to the deposited, implanted and sputtered target material on the optical coating of the collector.

[0015] 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 produces hydrogen radicals (represented in this article as H*) by dissociating H2 molecules. The hydrogen radicals in turn help to remove target material deposits from the collector surface. For example, in the case of tin as the target material, the hydrogen radicals will participate in the reaction on the collector surface, resulting in the formation of volatile gaseous stanane (SnH4), which can be pumped away. In order to make this chemical pathway effective, it is preferred to have a low H recombination rate (reduced to H2 molecules) on the collector surface so that the hydrogen radicals can be used to participate in the cleaning reaction.

[0016] Another reason for collector mirror degradation in EUV sources is instability of the capping layer. For example, if the capping layer is oxidized, i.e., made of an oxide material, the reduction of the oxide material to a metal-rich suboxide material or even to a metal can lead to severe tin contamination, since the capping layer will then be catalytically active, both promoting the recombination of atomic hydrogen (thus reducing the cleaning rate) and promoting SnH x As another example, if the capping layer is made of a nitride material, reducing the nitride material to a metal-rich low-valent nitride material or even to a metal may also result in severe tin contamination because the capping layer then becomes catalytically active. Summary of the invention

[0017] A simplified overview of one or more embodiments is given below to provide a basic understanding of these embodiments. This overview is not an exhaustive overview of all contemplated embodiments, and is not intended to identify key or important elements of all embodiments, nor is it intended to set limitations on the scope of any or all embodiments. Its sole purpose is to present some concepts of one or more embodiments in a simplified form as a prelude to a more detailed description presented later.

[0018] According to one aspect of an embodiment, a reflective element is provided, the reflective element having a cap layer made of an insulator. The cap layer can be, for example, a nitride or oxide that has high resistance to the deposition of the target material, has a good energy reduction effect on the incident ions, and has a low secondary electron yield. In order to promote the cleanliness and stability of the cap layer, a gas control system is provided that introduces a gas flow containing a reducing gas (e.g., oxygen or nitrogen) across at least a portion of the cap layer. The reducing gas can also be introduced into other parts of the source to control the impact of the source material, or used in other vacuum environments of the system where EUV radiation tends to degrade the reflectivity.

[0019] According to another aspect of the embodiment, a device is disclosed, which includes a vacuum chamber, a reflective optical element arranged in the vacuum chamber and including a cover layer, a gas distribution system for adding gas to the vacuum chamber, a source of reducing gas selectively fluidly connected to 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 reducing gas from the source of gas is introduced into the interior of the vacuum in a regulated manner through the gas distribution system. The reducing gas can be an oxygen-containing gas or a nitrogen-containing gas. The oxygen-containing gas can include any one or a combination of O2, H2O, H2O2, O3, CO2 and air. The cover layer can include an insulator, which can include an oxide, which can include a metal oxide. The insulator can include a nitride. The insulator can include a carbide.

[0020] A reducing gas may be caused to flow over at least a portion of the cap layer. The gas distribution system may introduce the reducing gas from a source of gas into the vacuum chamber by adding a certain amount of the reducing gas to the hydrogen-containing gas flow. The gas distribution system may be configured to add the reducing gas to the hydrogen-containing gas by causing the reducing gas to permeate into the hydrogen-containing gas flow. The reducing gas may be mixed with an inert gas to obtain a mixture containing at least 0.1% of the reducing gas. The reducing gas may be O2 and may be mixed with an inert gas. The reducing gas may be N2 and may be mixed with an inert gas. The inert gas may be argon. The reducing gas may be H2O and mixed with an inert gas. The reducing gas may be CO2 and mixed with an inert gas. The reducing gas may be O3 and mixed with an inert gas. The reducing gas may be air and mixed with an inert gas. The reducing gas may be water vapor and mixed with an inert gas. The reducing gas may be mixed with an inert gas, wherein the inert gas is 10 - 5 mbar level and reduce the gas to 10 -7 mbar level.

[0021] The gas distribution system may have at least one outlet arranged to generate a gas flow comprising a reducing gas across at least a portion of the cap layer. The gas control system may continuously add the reducing gas. The gas control system may add the reducing gas in-line while the plasma is being generated. The gas control system may intermittently add a dose of the reducing gas.

[0022] The apparatus may further comprise a sensor arranged to sense a reflectivity of the optical element and to generate a signal indicative of the reflectivity, and the gas control system may receive the signal and control at least one of reducing the gas concentration and reducing the gas composition based at least in part on the reflectivity. The sensor may be arranged to sense a reduced gas concentration near the cover layer and to generate a signal indicative of the reduced gas concentration, and the gas control system may receive the signal and control at least one of reducing the gas concentration and reducing the gas composition based at least in part on the reduced gas concentration near the cover layer. The gas control system may control at least one of reducing the gas concentration and reducing the 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 reducing the gas concentration and reducing the gas composition based on a number of pulses of EUV radiation generated by an EUV source including the optical element.

[0023] The apparatus may further comprise a plurality of structures within the vacuum chamber for directing a flow of gas within the vacuum chamber, and a gas distribution system arranged to flow the reducing gas over the structures.

[0024] According to another aspect of the embodiment, a device is disclosed, which includes a vacuum chamber, a reflective optical element arranged in the vacuum chamber and including a cover layer, a main gas distribution system for adding hydrogen to the vacuum chamber, an auxiliary gas distribution system for adding reducing gas to the vacuum chamber, and a gas control system for controlling the main gas distribution system and the auxiliary gas distribution system, the gas control system having a state in which the reducing gas is introduced into the interior of the vacuum chamber in a regulated manner through the auxiliary gas distribution system.

[0025] According to another aspect of the embodiment, a method is disclosed, the method comprising the steps of measuring an operating parameter of an EUV light source, the EUV light source comprising a vacuum chamber and a reflective optical element located within the vacuum chamber; and controlling the addition of a reducing gas to the vacuum chamber based at least in part on the measured operating parameter. The step of controlling the addition of the reducing gas may include controlling the concentration of the reducing gas. The step of controlling the addition of the reducing gas may include controlling the flow rate of the reducing gas. The step of controlling the addition of the reducing gas may include controlling the composition of the reducing gas. The operating parameter may be a reflectivity of the optical element, a concentration of the reducing 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 since a predefined start-up time.

[0026] According to another aspect of the embodiment, a device is also disclosed, which includes a vacuum chamber, a reflective optical element arranged in the vacuum chamber, a gas distribution system having an inlet for selectively allowing a gas flow into the vacuum chamber, a source of a reduction gas selectively fluidly connected to the inlet, a source of a gas containing free radicals selectively fluidly connected to 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 reduction gas to be introduced into the vacuum chamber through the inlet and a second state in which the gas control system causes the gas containing free radicals to be introduced into the vacuum chamber through the inlet. The source of the gas containing free radicals may include at least one heating element, which is thermally connected to the inlet and is arranged to heat the gas flowing through the inlet to a temperature sufficient to generate free radicals from at least a portion of the gas in the gas flow. The source of the gas containing free radicals may include a plasma source, which is arranged to generate free radicals in the gas before the gas reaches the inlet.

[0027] According to another aspect of the embodiment, a method for reducing target material contamination of a reflective surface of a reflective optical element in an extreme ultraviolet light source is also disclosed, the method comprising the following steps: (a) starting a reduced gas flow flowing into a vacuum chamber; (b) stopping the reduced gas flow flowing into the vacuum chamber; (c) starting a gas flow containing free radicals flowing into the vacuum chamber; (d) stopping the gas flow containing free radicals flowing into the vacuum chamber; wherein steps (a)-(d) are repeated multiple times in sequence.

[0028] According to another aspect of the embodiment, a device is disclosed, which includes a vacuum chamber, a reflective optical element arranged in the vacuum chamber, a gas supply system operating to supply at least a first gas and a second gas including oxygen to the vacuum chamber, and a control system operating to control the gas supply system to supply the second gas based at least in part on the partial pressure of at least one of the first gas and the second gas sensed in the vacuum chamber. The reflective optical element may include a cap layer, and the cap layer may include an oxide. The oxide may include a metal oxide. The control system operates to control the gas supply system to supply the second gas based at least in part on the partial pressure of the second gas sensed in the vacuum chamber. The control system operates 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. The second gas may include O2. The second gas may include H2O. The second gas may include H2O2. The second gas may include O3. The second gas may include CO2. The second gas may include air. The second gas may include an inert gas. The inert gas may include argon. The inert gas may include helium. The second gas may include an inert gas mixed with at least 0.1% of an oxygen-containing gas. The apparatus may further comprise a gas pressure sensor arranged to sense a partial pressure of the second gas in the vacuum chamber and to generate a first signal indicative of the partial pressure to a control system, and wherein the control system controls 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.

[0029] According to another aspect of the embodiment, a device is disclosed, the device comprising a vacuum chamber, a reflective optical element arranged in the vacuum chamber, a gas distribution system for adding gas to the vacuum chamber and having an interface for connecting to a source of oxygen-containing gas that is selectively fluidly connected to the gas distribution system, and a gas control system, the gas control system being configured to control the gas distribution system to supply oxygen-containing gas to the vacuum chamber until the partial pressure of the oxygen-containing gas reaches a first value, thereby stopping the supply of 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 resuming the supply of 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 include a multilayer reflector, the multilayer reflector including a cap layer and a plurality of lower layers, the cap layer being composed and arranged to protect the lower layers from damage, the cap layer including an oxide. The oxide may include a metal oxide. The third value may be substantially equal to the first value. The oxygen-containing gas may include O2. The oxygen-containing gas may include H2O. The oxygen-containing gas may include H2O2. The oxygen-containing gas may include O3. The oxygen-containing gas may include CO2. The oxygen-containing gas may include air. The second gas may include an inert gas. The inert gas may include argon. The inert gas may include helium. The second gas may include an inert gas mixed with at least 0.1% of an oxygen-containing gas. The device may also include an air pressure sensor arranged to sense the partial pressure of the oxygen-containing gas in the vacuum chamber and generate a first signal indicating the partial pressure to a gas control system, and wherein the gas control system controls the supply of the oxygen-containing gas based at least in part on the first signal. The air pressure sensor may directly sense the partial pressure of the oxygen-containing gas. The air 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 air pressure sensor may be arranged to sense the partial pressure of the oxygen-containing gas close to the reflective optical element.

[0030] According to another aspect of the embodiment, a method for extending the useful life of a reflective surface in a vacuum chamber of an EUV source is disclosed, the method comprising providing a gas supply system operating to supply at least a first gas and a second gas including oxygen to the vacuum chamber, sensing a partial pressure of at least one of the first gas and the second gas in the vacuum chamber, 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 include sensing the partial pressure of the second gas. The sensing step may include sensing the partial pressure of the first gas. The second gas may include O2. The second gas may include H2O. The second gas may include H2O2. The second gas may include O3. The second gas may include CO2. The second gas may include air. The second gas may include an inert gas. The inert gas may include argon. The inert gas may include helium. The second gas may include an inert gas mixed with at least 0.1% of an oxygen-containing gas.

[0031] According to another aspect of the embodiment, a method for extending the service life of a reflective surface in a vacuum chamber of an EUV source is disclosed, the method comprising the following steps: (a) supplying an oxygen-containing gas to the vacuum chamber; (b) stopping 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) stopping 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 include O2. The oxygen-containing gas may include H2O. The oxygen-containing gas may include H2O2. The oxygen-containing gas may include O3. The oxygen-containing gas may include CO2. The oxygen-containing gas may include air. The second gas may include an inert gas. The inert gas may include argon. The gas may include helium. The second gas may include an inert gas mixed with at least 0.1% of the oxygen-containing gas. Each of steps (b), (c) and (d) may include the partial pressure of the oxygen-containing gas in the vacuum chamber. Steps (b), (c) and (d) may include directly sensing the partial pressure of the oxygen-containing gas in the vacuum chamber. Each of steps (b), (c) and (d) may include 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. Each of steps (b), (c) and (d) may include sensing the partial pressure of the oxygen-containing gas in the vacuum chamber near the reflective surface.

[0032] 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 DESCRIPTION OF THE DRAWINGS

[0033] The accompanying drawings, which are incorporated herein and constitute a part of the specification, illustrate the methods and systems of embodiments of the present invention by way of example and not limitation. Together with the detailed description, the accompanying drawings are also used to explain the principles and enable those skilled in the relevant art to make and use the methods and systems proposed herein. In the accompanying drawings, the same reference numerals represent the same or functionally similar elements.

[0034] Figure 1 A schematic, not-to-scale view of the overall general concept of an EUV lithography system is shown.

[0035] Figure 2 A schematic, not-to-scale view of the overall broad concept of a laser produced plasma EUV radiation source system according to an aspect of an embodiment of the present invention is shown.

[0036] Figure 3 is a cross-sectional view of an EUV optical element according to an aspect of an embodiment of the present invention.

[0037] Figure 4 is a cross-sectional view of gas flow over an EUV optical element according to an aspect of an embodiment of the present invention.

[0038] Figure 5 is a functional block diagram of a system for adding gas to one of a plurality of vacuum chambers in an EUV lithography system according to an aspect of an embodiment of the present invention.

[0039] Figure 6 is a perspective view of a flow cone according to an aspect of an embodiment of the present invention.

[0040] Figure 7 is a plan view of a flow cone heating element according to one aspect of an embodiment of the present invention.

[0041] Figures 8A-8C 3 are a plan view, a side view and a perspective view, respectively, of a collector with a flow cone according to an aspect of an embodiment of the present invention.

[0042] Fig. 9 is a partially schematic, partial perspective view of a flow cone with a gas distribution and control system according to an aspect of an embodiment of the present invention.

[0043] Fig.10 is a timing diagram illustrating certain operating principles of a gas distribution and control system according to one aspect of an embodiment of the present invention.

[0044] Fig.11 is a flow chart illustrating certain processes for using a gas distribution and control system according to one aspect of an embodiment of the present invention.

[0045] Fig.12 is a partially schematic functional block diagram of a system for adding oxygen-containing gas into one of a plurality of vacuum chambers in an EUV lithography system according to an aspect of an embodiment of the present invention.

[0046] Fig.13 is a timing diagram illustrating certain operating principles of a gas distribution and control system according to one aspect of an embodiment of the present invention.

[0047] Fig.14 is a flow chart illustrating certain processes for using a gas distribution and control system according to one aspect of an embodiment of the present invention.

[0048] Fig.15 is a timing diagram illustrating certain operating principles of a gas distribution and control system according to one aspect of an embodiment of the present invention.

[0049] Fig.16 is a flow chart illustrating certain processes for using a gas distribution and control system according to one aspect of an embodiment of the present invention.

[0050] Other features and advantages of the present invention and the structure and operation of various embodiments of the present invention are described in detail below with reference to the accompanying drawings. Note that the present invention is not limited to the specific embodiments described herein. Such embodiments are proposed herein for illustrative purposes only. Based on the teachings contained herein, other embodiments will be clear to those skilled in the relevant art. DETAILED DESCRIPTION

[0051] Various embodiments are now described with reference to the accompanying drawings, wherein throughout the text, similar reference numerals are used to refer to similar elements. In the following description, for the purpose of explanation, many specific details are set forth to facilitate a thorough understanding of one or more embodiments. However, it may be clear in some or all cases that any of the embodiments described below can be practiced without adopting the specific design details described below. In other examples, known structures and devices are shown in block diagram form to facilitate description of one or more embodiments.

[0052] Figure 1 A lithographic apparatus according to an embodiment of the invention is schematically depicted. The apparatus comprises an illumination system IL configured to condition a radiation beam B of radiation. The apparatus further comprises: a support structure (e.g., a mask stage) MT, which is constructed to support a pattern forming device (e.g., a mask or reticle) MA and is connected to a first positioner PM configured to accurately position the pattern forming device according to certain parameters; a substrate stage (e.g., a wafer stage) WT, which is constructed to hold a substrate (e.g., a wafer coated with a resist) W and is 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 optical element box or POB), which is configured to project a pattern imparted to the radiation beam B by the pattern forming device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0053] 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 the radiation.

[0054] 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 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 in a desired position, for example relative to the projection system.

[0055] refer to Figure 1 , the illumination system IL receives a radiation beam from a radiation source SO. The source SO and the illumination system IL, and if necessary, the beam delivery system, may be referred to as a radiation system. The illumination system IL may include 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. In addition, the illumination system IL may include various other components, such as an integrator and a condenser. The illumination system may be used to adjust the radiation beam to have a desired uniformity and intensity distribution in its cross-section.

[0056] The radiation beam B is incident on a patterning device (e.g. a mask) MA, which is held on a support structure (e.g. a mask table) MT, and is patterned by the patterning device. After passing through the patterning device MA, the radiation beam B passes through a projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of a second positioner PW and a position sensor IF2 (e.g. an interferometer, a linear encoder or a capacitive sensor), the substrate table WT can be precisely moved, for example, in order to position a different target portion C in the path of the radiation beam B. Similarly, a first positioner PM and a further position sensor IF1 can be used to precisely 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.

[0057] Figure 2 is a schematic diagram of an exemplary EUV radiation source SO (e.g., a laser-generated plasma EUV radiation source) according to one 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 that produces 10.6 μm radiation. The pulsed gas discharge CO2 laser source may have a DC or RF excitation that operates at high power and high pulse repetition frequency.

[0058] The EUV radiation source 20 also includes a target delivery system 24 for delivering a target material in the form of droplets or a continuous liquid stream. The target material can be composed of tin or a tin compound, but other materials can also be used. The target material delivery system 24 introduces the target material into the interior of the chamber 26 to the irradiation area 28, where the target material can be irradiated to produce a plasma. In some cases, an electric charge is placed on the target material to allow the target material to turn toward or away from the irradiation area 28. It should be noted that, as used herein, the irradiation area is the area where irradiation of the target material occurs, and it is also an irradiation area even when irradiation does not actually occur. As shown, the target material delivery system 24 may include a target delivery control system 90 and a target material dispenser 92.

[0059] continue Figure 2 , the radiation source SO may also include one or more optical elements. In the following discussion, the collector 30 is used as an example of such an optical element, but the discussion is also applicable to other optical elements. The collector 30 may be a normal incidence reflector, for example implemented 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 may also be used, such as aluminum (Al) or silicon (Si). The collector 30 may be in the form of an oblate ellipsoid having a hole that allows the laser radiation to pass through and reach the irradiation area 28. The collector 30 may, for example, be in the shape of an ellipsoid having a first principal focus at the irradiation area 28 and a second focus at a so-called intermediate point 40 (also referred to as intermediate focus 40), at which the EUV radiation may be output from the EUV radiation source SO and input to the irradiation system IL, as Figure 1 The chamber 26 may also be provided with additional structures for controlling the airflow within the chamber 26 , such as vanes 94 .

[0060] Reference again Figure 1 The projection system PS (which may be referred to as a projection optics box or "POB") comprises a vacuum chamber containing a plurality of reflective EUV optical elements that direct radiation from the patterning device MA onto the wafer W. The projection system PS may include, for example, 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.

[0061] As mentioned above, one of the technical difficulties in the design of optical elements used in EUV systems is to extend their lifetime. One way to extend the lifetime of an optical element involves protecting it from damage by using an outermost cap layer. Examples of EUV optical elements with such a cap layer are shown in Figure 3 It is shown in Figure 3It is a cross section through a portion of such a collector 30. As shown, the collector 30 includes a substrate 100. A multilayer coating 110 is located on the substrate 100. The multilayer coating 110 is composed of alternating layers of materials (e.g., molybdenum and silicon) in a known manner. A cap layer 120 is located on the multilayer coating 110, and the cap layer 120 can be composed of an outermost layer and a series of repeated double layers. As used herein, a cap layer is added to protect the collector 30 without excessively reducing the total reflectivity of the collector 30 at a wavelength of interest (e.g., 13.5nm). It is advantageous to provide a cap layer system that protects the surface of the collector 30 from the effects of target material (e.g., tin) deposition, hydrogen ion penetration, hydrogen diffusion, and blistering caused by hydrogen or oxygen. It is also advantageous to select a material that will resist blistering for the cap layer 120.

[0062] Reference again Figure 3 The cap layer 120 (or, if the cap layer 120 is a multi-layer cap layer, the uppermost layer of the cap layer 120) may be an insulating material that is highly resistant to the deposition of the target material, such as a ceramic or a nitride or an oxide. In other words, these materials may be materials that have a low recombination rate for atomic hydrogen to achieve a high stannane formation rate. These materials are typically materials with a hydrogen recombination coefficient of about 10 -4 to about 10 -3 range. Effectively, this means that the preferred material exhibits a good tin cleaning rate because H can react with Sn before recombination to H2. The preferred material for the topmost layer 130 of the cap layer 120 also preferably exhibits good energy reduction for incident ions and a low secondary electron yield. Examples of suitable materials include insulators such as ZrN, TiO2, Ta2O5, and ZrO2. In general, metals and ceramic compounds that can be oxidized to a determined thickness without affecting the underlying multilayer coating can also be used. The insulator can be selected to minimize the recombination of hydrogen radicals and the decomposition of SnH3 and SnH4.

[0063] Generally, it is desirable that the EUV source vacuum be as clean as possible (lowest levels of O2, N2, H2O, etc.) to prevent any risk of oxidation or deleterious chemical reactions with the tin or plasma, as this could result in collector degradation. Additionally, oxygen can undesirably cause oxidation of other components within the vacuum chamber, such as the nozzles of the droplet generator, for example. However, it has been determined that the presence of small amounts of gases that would otherwise be considered impurities can have a positive effect on maintaining or enhancing the reflectivity of optical elements in the system. For the collector within the source, one mechanism is the depletion of oxygen from the cap layer, which results in increased accumulation of tin on the cap layer.

[0064] It is therefore advantageous to intentionally supply a controlled amount of such a gas or a mixture of such gases to the vacuum chamber, e.g., of the source SO or projection system PS. Such non-hydrogen gases or gas mixtures comprising non-hydrogen gases will be referred to herein as reducing gases when intentionally introduced into the vacuum chamber, since their intentional introduction is used to reduce and degrade the reflectivity by chemical and / or physical reactions. These reducing gases may be supplied, for example, in the form of an air mixture (ultra-clean dry air or XCDA) or mixed with a rare or inert gas (e.g., Ar).

[0065] One method of adding a reducing gas to a vacuum chamber is to infiltrate the reducing gas into the hydrogen flow entering the vacuum chamber. For example, oxygen / water-containing gas can be added to the vacuum chamber in the following manner: oxygen / water-containing gas is added to the hydrogen flow passing near the collector to increase the local concentration of the reducing gas to a level that stabilizes the cap layer without consuming a large amount of H* in the water-gas reaction. Therefore, the reducing gas can be, for example, any one of O2, H2O, H2O2, O3 and N2 or a mixture thereof in relative proportions to suppress unwanted reactions and effects and promote desired reactions and effects. It should be noted that water will photodecompose into H2 and O2 under EUV. For example, a mixture of argon and oxygen can be added. In a mixture of argon and O2, Ar is 10 -5 mbar level and O2 is 10 -7 mbar order, for example, 2E-5 mbar for AR and 4E-7 mbar for O2. The mixture to be used can be determined based on reflectivity measurements. The upper limit of the reduced gas concentration depends in part on the deep oxidation below the cap layer. The exact limit depends on the plasma load driven by the EUV light.

[0066] There are engineering risks associated with the introduction of oxygen-containing gas, which is used to refer to a gas containing O2 or an oxygen compound such as H2O. As used herein, the phrase "a gas comprising hydrogen" also means a gas containing O2 or an oxygen compound such as H2O. For example, the presence of oxygen-containing gas may cause oxidation of the nozzle of a droplet generator. Such risks can be reduced by shutting off the addition of oxygen-containing gas during the start-up of the droplet generator and by monitoring the success rate of restarting the droplet generator.

[0067] The above discussion is mainly about the reflective optical elements located in the source vacuum chamber. The beneficial effects of adding reducing gas to the gas mixture in the vacuum chamber also extend to reflective EUV optical elements located in other parts of the system, such as reflective optical elements in the POB. For example, reducing gas can be added to the gas mixture in the vacuum chamber of the POB to inhibit etching of the surface of the reflective optical element located there and reduce the formation of SiH4. The pressure of the reducing gas at the reducing gas inlet of the vacuum environment can be on the order of 1E-8mbar. As another example, the pattern forming device MA is also in a vacuum environment, sometimes referred to as a mask stage or a mask microenvironment. Similarly, a low concentration of reducing gas can be added here to reduce the formation of SiH4. The reducing gas can be oxygen and can be mixed with nitrogen. The pressure of the reducing gas at the reducing gas inlet of the vacuum environment can be on the order of 4E-8mbar. For these two environments, the lower limit of the reducing gas concentration range is mainly determined by the minimum amount required to obtain the benefits of SiOx reduction. The lower limit of the reducing gas concentration range is mainly determined by the need to avoid deep oxidation. The desired concentration of the reducing gas depends to some extent on the ion flux to which the SiOx-containing surface of the reflective EUV optic is subjected, with lower ion fluxes being less demanding for the presence of the reducing gas.

[0068] As a specific example, a metal oxide can be selected as a collector cap material with limited H* recombination and SnH4 redeposition. Reducing the metal oxide to metal (or suboxide or subnitride) can undesirably cause the surface of the cap layer to promote H* recombination and SnH4 redeposition. Flowing a gas containing oxygen or nitrogen over the cap layer can stabilize the metal oxide or nitride cap layer, thereby achieving more tin cleaning. The concentration of oxygen or nitrogen can be selected to promote this effect while not consuming excessive H* (generating water), which would leave less H* available for tin cleaning reactions. At intermediate levels of oxygen, tin will oxidize, which requires more H* to remove, but will reduce SnH4 redeposition. The goal is to promote reactions such as tin cleaning (Sn+xH*→SnH x ) and metal oxidation (metal + 1 / 2xO2 → (metal)O x ), while inhibiting reactions such as reduction ((metal)Ox + 2xH* → metal + H2O), water formation (2H* + O* = H2O), H* recombination (2H** = H2), and SnH4 redeposition (SnH x =Sn+1 / 2xH2) and other unwanted reactions. The same is true for nitrogen.

[0069] Other candidate materials for insulators include nitrides. These can be stabilized, for example, by adding nitrogen or ammonia mixtures as reducing gases. They can also be stabilized with oxygen, since oxygen will oxidize areas of the cap layer that have been depleted of nitrogen, thereby restoring conditions that inhibit unwanted reactions and promote desired reactions. Additional candidate materials for insulators include carbides. These can be stabilized, for example, by adding CH4 and / or light hydrocarbons.

[0070] In the source, it is desirable to establish a flow of the gas mixture flowing over at least a portion of the surface of the EUV reflective optical element (i.e., adjacent and with a parallel velocity component). This can be achieved, for example, by adding to the system an outlet for flowing hydrogen gas within the chamber and then adding the reducing gas to the hydrogen flow. As described above, the collector 30 can be in the form of an oblong with a central hole 306 to allow the laser radiation to pass through and reach the irradiation area 28. In Figure 4 304. A collector and airflow component 302 is shown in FIG. A central hole 306 extends through the reflective surface 304 of the collector mirror 30. The central hole 306 is located on the axisymmetric line AX of the collector 30. The central hole 306 has an edge 308. Component 302 is provided with a tubular body 310 (also referred to as a flow cone 310) extending through the central hole 306. The tubular body 310 has inner surfaces 312 and 318 and an outer surface 314.

[0071] The term "tubular or tubular" will be understood by those skilled in the art as a broad term that may include or be synonymous with a variety of different structures. For example, the tubular body may not have parallel walls, but may be tapered or trumpet-shaped. The tubular body may be any structure that forms a conduit from one side of the collector (e.g., a non-collecting surface) to the other opposite side of the collector (e.g., a collecting surface). The tubular body may have a circular cross-section, or an elliptical cross-section, or any other suitable cross-section.

[0072] In use, the 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 perpendicular to the reflective surface 304. The inner surface 312 of the tubular body 310 is constructed and arranged to direct the gas flow GF in a direction perpendicular to the reflective surface 304. Typically, the gas flow GF is directed toward one or both of the first focal point and the second focal point. The opening 316 may be located between the outer surface 314 of the tubular body 310 and the edge 308 of the central hole 306. Figure 4 In the embodiment of FIG. 3 , the outer surface 314 and the edge 308 form or define an opening 316 .

[0073] The opening 316 (which includes 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) can be arranged to guide another airflow GF' substantially along the reflective surface 304 and away from the tubular body 310, as shown. As shown, the nozzles 320 and 322 can be arranged to guide another airflow GF' substantially from the periphery of the collector 30 along the reflective surface 304 toward the tubular body 310. If radial flows from both the center and the circumference exist, the nozzles can be arranged so that these flows merge, but nevertheless, except for the merge, these flows flow substantially adjacent to the surface of the collector 30.

[0074] The flow through the tubular body 310 can be called a conical flow. The flow from the center to the circumference along the surface of the collector 30 can be called an umbrella flow. The flow from the circumference along the surface of the collector 30 toward the center can be called a circumferential flow. The flow rate of the conical flow of the hydrogen / reducing gas mixture can be in the range of about 50SLM to about 165SLM. The flow rate of the umbrella flow of the hydrogen / reducing gas mixture can be in the range of about 40SLM to about 90SLM. The flow rate of the circumferential flow of the hydrogen / reducing gas mixture can be in the range of about 60SLM to about 160SLM. Under these types of H2 flows, the pure reducing gas flow is only about 0.002 to 0.8SCCM. If there is a carrier gas (such as an inert gas), the flow rate will scale. For example, if the reducing gas mixture is 2% reducing gas and 98% inert gas, the total flow rate is about 0.1 to 40SCCM. Ultimately, what is important is the reduced gas partial pressure, which depends on the ratio of reduced gas to H2 (mainstream in the vessel) and the difference in pumping speeds of the two gases. Note that the total pressure of the vessel can be in the range of about 1.2 mbar to about 2.2 mbar.

[0075] Alternatively, there may be a gas manifold system dedicated to flowing abatement gas over the surface of the EUV reflective optic, wherein holes / nozzles directed to flow gas along a local surface of the collector 30 (with a velocity component parallel to the local surface) are supplied with abatement gas only.

[0076] Figure 5 A system is shown for controlling the introduction of a reducing gas into one or more vacuum chambers for use in a system for generating and utilizing EUV radiation. Figure 5As shown, the vacuum chamber 26 of the EUV source SO is connected to a gas source 400 of a source gas such as hydrogen. The conduit that relays 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 a reducing gas. The gas control system 420 controls whether to add a reducing gas to the flow of the source gas entering the chamber 26. The gas control system 420 also controls the mixing ratio of the reducing gas to the source gas. Alternatively or additionally, the reducing gas can flow directly to the chamber 26 under the control of the gas control system 420.

[0077] Figure 5 Also shown is a system for controlling the introduction of reducing gas into the vacuum chamber 440 of the POB. Figure 5 As shown, the vacuum chamber 440 of the POB is connected to a source 450 of gas. The conduit that relays the gas to the vacuum chamber 26 has a mixing node 460 under the control of the gas control system 420. The mixing node 460 is also connected to a source 470 of reducing gas. The gas control system 420 controls whether the reducing gas is added to the gas flow entering the chamber 440. The gas control system 420 also controls the mixing ratio of the reducing gas to the gas. Alternatively or additionally, the gas can flow directly to the chamber 440 under the control of the gas control system 420.

[0078] The gas control system may be configured to add reducing gas continuously online while the source is operating. The gas control system may be configured to add reducing gas intermittently. The gas control system may be configured to add reducing gas while the source is offline. The gas control system may be configured to add reducing gas and / or control the concentration and / or composition of reducing gas based on the measurement of the reflectivity of the EUV reflective element in the chamber measured by the sensor 480 or 490. The gas control system may be configured to add reducing gas and / or control the concentration and / or composition of reducing gas based on the measurement of the reducing gas concentration near the EUV reflective element measured by the sensor 480 or 490. The gas control system may be configured to add reducing gas and / or control the concentration and / or composition of reducing gas based on the measurement of the amount of time the chamber has been running or the number of pulses that the source has generated.

[0079] An umbrella-shaped flow cone 310 in the form of a circumferential gap between the interior and exterior of the tubular body provides an umbrella-shaped flow of hydrogen gas across the reflective surface 304 of the collector 30. Figure 6 and Figures 8A-8C Shown in.

[0080] like Fig. 9As shown, a reducing gas (e.g., oxygen) source 430 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 reducing gas from the gas source 400 to the umbrella flow. When the valve 490 or inlet is open, a controlled amount of reducing gas is mixed into the hydrogen in the umbrella flow. When the inlet is closed, only H2 flows through the umbrella flow cone 310. The valve 490 or inlet is controlled by a control signal from the gas control system 420.

[0081] According to another aspect of the embodiment, the tubular body 310 is provided with one or more heating elements 500. Figure 6 and 7 , these heating elements 500 can be arranged around the circumference of the outside of the tubular body 310. Each heating element 500 can be composed of one or more heating wires 510 and can be selectively powered by the gas control system 420. The heater 500 generates hydrogen radicals in the hydrogen flow in the umbrella-shaped flow cone 310 when it is excited. The gas control system 420 is arranged to supply energy to the heater so that the heater reaches a temperature sufficient to achieve the desired dissociation fraction of the hydrogen in the flow, for example, above 1300°C. This increases the concentration of H* in the H2 flow around the umbrella-shaped cone 310. In turn, this in turn enhances the Sn etching from the reflective surface 304 and the formation of gaseous stannane (SnH4) according to the following reaction:

[0082] Sn(s)+4H*(g)→SnH4(g)

[0083] According to another aspect, this increase in H* concentration in the H2 stream is alternated with the introduction of a reducing gas such as O2. After the O2 is introduced, it reduces the reverse reaction (conversion of gaseous stannane to solid tin) by poisoning the autocatalytic decomposition of stannane, thereby preventing additional Sn formation. Alternating the generation of hydrogen radicals with the addition of oxygen results in the overall removal of tin contamination on the reflective surface 304.

[0084] Although in the arrangement just described, hydrogen radicals are generated by using a heated heating filament disposed on the tubular body, hydrogen radicals may also be introduced from other or additional sources. Figure 8B As shown, H* may also be delivered to the tubular body 310 via a remote source such as remote plasma source 315 to achieve the same effect.

[0085] Fig. 8A 30 is a front plan view of the collector 30 with its reflective surface 304 and central aperture 306. An umbrella-shaped flow cone 310 is defined by the interior and exterior of the tubular body 310. Figure 8B yes Fig. 8A A side view of the arrangement of Figure 8C yes Fig. 8AThe perspective view of the arrangement also shows a circumferential support 530 for the collector 30, which has a circumferential exhaust hole 540 that can be arranged to cooperate with the umbrella flow cone 310 to establish an umbrella flow on the reflective surface 304 of the collector 30.

[0086] Therefore, if Fig. 9 and 10 As shown, the gas control system 420 can be configured to alternately (1) energize the heating element 500 to generate hydrogen radicals and (2) inject the reducing gas. Fig.10 As shown, when the heater is energized, there is a gap 580, while when reducing gas is added to the umbrella flow, there is a gap 560.

[0087] Fig.11 is a flow chart describing the process. In step S10, hydrogen is supplied to the umbrella flow cone to establish a hydrogen umbrella flow. In step S20, the gas control system 420 activates the heater 500 to convert a portion of the hydrogen in the umbrella flow into free radicals in interval A. After the interval A ends, in step S30, the gas control system opens the valve 490 to add a predetermined amount of reducing gas (e.g., oxygen) to the umbrella flow in interval B. The process is then repeated as needed to maintain or improve the reflectivity of the reflective optical element in the same environment as the flow cone 310.

[0088] In addition, when the source is running, molten tin chips continuously reach surfaces within chamber 26, such as the surface of blades 94. Hydrogen radicals generated during operation of the source interact with the molten tin, resulting in small particles of tin being ejected from the surface of the molten tin. The typical size of these particles is about microns, and they leave the surface at a speed of several meters per second. This phenomenon is called "tin splashing". Particles or particles generated by tin splashing may eventually fall on the collector, which also causes EUV reflectivity loss of the collector mirror.

[0089] In order to reduce or even completely eliminate the splashing of tin, the regular addition of a reducing gas (e.g., the above-mentioned oxygen) can oxidize the tin that reaches the blade. For this purpose, an oxygen concentration of the order of 10E-5mbar may be sufficient. Oxygen exposure may be performed at specific intervals, for example, about every 1-2 hours of high duty cycle operation of the source. In order to reduce the negative impact of oxygen exposure on the self-cleaning of Sn on the collector surface, this addition of oxygen may be performed offline, that is, oxygen is added when no plasma is generated. Therefore, the laser is switched to an operating mode in which no droplets are produced and a small amount of oxygen can be introduced into the container. For example, this can be accomplished by a separate gas supply containing a mixture of 2% oxygen and 98% Ar. Since the presence of molecular hydrogen does not interfere with the splash suppression effect of oxygen, a regular hydrogen flow can be allowed to continue to flow uninterruptedly. The gas mixture may be an O2 / Ar mixture of 1slm and a normal hydrogen flow (e.g., may be about 200slm). Larger or smaller quantities may also be used. Similarly, the hydrogen supply may be turned off during the process. The duration of the oxygen exposure time may be on the order of a few minutes, for example, about 5 minutes, although it may also be adjusted. After exposure, normal operation of the light source may be resumed.

[0090] The oxygen-containing gas can be added to the container continuously or intermittently. However, a continuous supply of oxygen-containing gas may lead to oxidation of the Sn layer present on the collector surface. In this case, a layer of SnOx is formed on the collector surface, which may lead to reflection losses. A continuous supply of oxygen-containing gas may also lead to oxidation of the silicon layer of the multilayer reflector collector. In this case, a layer of oxide is formed on the collector surface, which also leads to reflection losses. Similarly, a continuous supply of oxygen-containing gas greatly improves splash protection against tin traps that contaminate the collector. Therefore, for some applications, it may be desirable to add the oxygen-containing gas to the container intermittently, rather than continuously, regardless of whether the introduction is alternated with another gas.

[0091] Therefore, according to one aspect of an embodiment, referring to Fig.12 , when the reflectivity of the collector reflective surface 304 is within the defined specification range, the partial pressure measuring device 610 reads the parameters of the source operating conditions in the container 440, such as, for example, the partial pressure of O2, the partial pressure of N2, and the partial pressure of H2O. These parameters can then be set and recorded as thresholds. The measuring device 610 can be implemented, for example, by a residual gas mass spectrometer. Preferably, the measuring device 610 will continuously provide measured values. It is also advantageous to make the measuring device 610 close enough to the reflective surface 304 so that the partial pressure value measured by the measuring device 610 represents the partial pressure of the adjacent reflective surface 304.

[0092] Once the specification value is determined, the source is operated and the operating value of at least one parameter, such as 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 the threshold, the control system 420 turns on the supply of oxygen-containing gas from the gas source 430 by opening the metering valve 600. Before it is determined that the partial pressure of the oxygen-containing gas has exceeded the threshold, the supply of oxygen-containing gas will remain open. At this point, the control system 420 turns off the supply of oxygen-containing gas.

[0093] The process Fig.13 This is shown graphically in Fig.13 660. In the embodiment of the present invention, during the time interval T1, the partial pressure of the oxygen-containing gas as shown in line 650 is established as an initial level higher than the threshold level 660. After the time interval T1, the supply of oxygen-containing gas to the container is stopped. Then, during the time interval T2, as the oxygen-containing gas decreases, the partial pressure of the oxygen-containing gas in the container gradually decreases. At the end of the time interval T2, the level has dropped sufficiently to drop below the threshold 660. At this point, the control system 420 resumes supplying oxygen-containing gas to the container until the level of the oxygen-containing gas is higher than the threshold level 660. The process continues in this manner. It should be noted that when the threshold is reached, the gas flow does not have to be immediately cut off, and when the partial pressure of the oxygen-containing gas is below the threshold, the gas flow does not have to be immediately restored.

[0094] During time interval T1, the oxygen-containing gas flow may last, for example, for about 30 minutes. This keeps the collector surface oxidized. During time interval T2, which may last, for example, for a time range of 3 to 8 hours, there is no excess oxygen-containing gas in the container. However, the oxygen-containing gas will gradually be consumed / reduced until the partial pressure drops below a threshold. The threshold will typically be set according to the application, but an exemplary value may be of the order of 4E-8 mbar.

[0095] Fig.14 is a flow chart describing the process. In step S100, a partial pressure value that produces a standardized internal reflectivity of the collector 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 of step S110 is negative, then in step S120, the dosing is suspended, that is, the flow of oxygen-containing gas to the container is stopped, and the process returns to determining the partial pressure of the oxygen-containing gas in step S110. If the determination of step S110 is positive, then in step S130, the oxygen-containing gas is supplied to reach the value threshold, that is, the feed is turned on.

[0096] On the other hand, Fig.15As shown, the system can be operated 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 are determined for the performance within the specifications of the collector reflector. The gas source is operated, and the operating value of the source parameter, such as the partial pressure of the oxygen-containing gas, is monitored. If it is determined that the partial pressure of the oxygen-containing gas is below the lower threshold, the control device turns on the supply of the oxygen-containing gas, such as at the end of the time interval T2. The supply of oxygen-containing gas remains open until it is determined that the partial pressure of the oxygen-containing gas has reached or exceeded an upper threshold, such as occurs at the end of the time interval T3. At this point, the control system will shut off the supply of the oxygen-containing gas.

[0097] Fig.16 is a flow chart describing the process. At step S150, a partial pressure range that produces a reflectivity within the specification is determined. At step S160, it is determined whether the partial pressure of the oxygen-containing gas is within the range. If so, then at step S170, the feed is turned off, i.e., the supply of oxygen-containing gas to the container is cut off, and the process returns to step S160. If the determination at step S160 is negative, then at step S180, the batching is turned on. As described above, the partial pressure of the oxygen-containing gas can be measured directly or can be inferred by measuring the partial pressures of other gases in the container.

[0098] According to another aspect, the control system can monitor the partial pressure of the oxygen-containing gas. Once the partial pressure of the oxygen-containing gas drops below a certain threshold, the control system can turn on the supply of oxygen-containing gas to the container for a period of time that is known in advance and sufficient to increase the partial pressure of the oxygen-containing gas to exceed the certain threshold. The control system can then turn off the gas supply for a predetermined period of time that is a known amount of time before a reduction that will cause the partial pressure of the oxygen-containing gas to drop below the predetermined threshold.

[0099] As described above, 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 a combination of the oxygen-containing gases listed above, i.e., O2, H2O, H2O2, 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.

[0100] Although the above examples are about oxygen, it should be understood that other gases such as nitrogen can be used alone or in combination with oxygen. Likewise, the direction of flow over the surface of the collector can be in any direction. An alternative implementation for achieving the same effect is to deliver the reducing gas and / or free radicals from the edge of the collector to the center rather than from the center to the edge to achieve the same effect, or from both locations or alternately (e.g., first from the edge to the center and then from the center to the edge).

[0101] The present disclosure is accomplished by means of functional building blocks that illustrate the implementation of specific functions and their relationships. For ease of description, the boundaries of these functional building blocks have been arbitrarily defined herein. Other boundaries may be defined as long as the specified functions and their relationships are properly performed.

[0102] The above description includes examples of one or more embodiments. Of course, it is not possible to describe every possible combination of components or methods for the purpose of describing the aforementioned embodiments, but it will be appreciated by those of ordinary skill in the art that many other combinations and permutations of various embodiments are possible. Therefore, the described embodiments are intended to cover all such changes, modifications and variations that fall within the spirit and scope of the appended claims. In addition, to the extent that the term "includes" is used in the detailed description or claims, such terms are intended to be inclusive in a manner similar to the term "comprising" because when used as a transitional term in the claims, it is interpreted as "comprising". In addition, although the elements of the described aspects and / or embodiments may be described or claimed in the singular, the plural form is contemplated unless the limitation to the singular form is explicitly stated. In addition, unless otherwise stated, all or part of any aspect and / or embodiment may be used together with all or part of any other aspect and / or embodiment.

[0103] Further aspects of the invention are set out in the following numbered clauses.

[0104] 1. A device comprising:

[0105] Vacuum chamber;

[0106] a reflective optical element disposed in the vacuum chamber;

[0107] a gas distribution system for adding gas to the vacuum chamber, the gas distribution system having an interface for connecting to a source of reducing gas that reduces contamination of the reflective optical element through a chemical reaction; and

[0108] A gas control system for controlling the gas distribution system has a state for introducing the reduced gas from the source of the gas through the gas distribution system into the interior of the vacuum chamber in a regulated manner.

[0109] 2. An apparatus according to clause 1, wherein the reducing gas comprises an oxygen-containing gas.

[0110] 3. An apparatus according to clause 2, wherein the oxygen-containing gas comprises O2.

[0111] 4. An apparatus according to clause 2, wherein the oxygen-containing gas comprises H2O.

[0112] 5. An apparatus according to clause 2, wherein the oxygen-containing gas comprises O3.

[0113] 6. An apparatus according to clause 2, wherein the oxygen-containing gas comprises CO2.

[0114] 7. The apparatus of clause 2, wherein the oxygen-containing gas comprises air.

[0115] 8. The device of clause 1, wherein the reflective optical element comprises a multilayer mirror comprising a cap layer and a plurality of underlying layers, the cap layer being arranged to protect the underlying layers from damage.

[0116] 9. The device of clause 8, wherein the cap layer comprises an oxide.

[0117] 10. The device of clause 9, wherein the oxide comprises a metal oxide.

[0118] 11. The apparatus of clause 8, wherein the reducing gas comprises an oxygen-containing gas and the cap layer comprises a nitride.

[0119] 12. The device of clause 8, wherein the cap layer comprises a carbide.

[0120] 13. The apparatus of clause 1, wherein the gas distribution system is configured to direct a flow of the reducing gas across at least a portion of the cap layer.

[0121] 14. The apparatus of clause 1, wherein the gas distribution system is configured to add the reducing gas from a source of the gas into the vacuum chamber by adding an amount of the reducing gas to a flow of hydrogen-containing gas.

[0122] 15. The apparatus of clause 1, wherein the gas distribution system is configured to add the reducing gas to the hydrogen-containing gas by permeating the reducing gas into the hydrogen-containing gas flow.

[0123] 16. The apparatus according to clause 1, wherein the gas distribution system is configured to mix the reducing gas with an inert gas to obtain a mixture comprising at least 0.1% of the reducing gas.

[0124] 17. The apparatus of clause 1, wherein the gas distribution system is configured to mix the reducing gas with an inert gas.

[0125] 18. The apparatus of clause 17, wherein the reducing gas comprises O2 and the inert gas comprises argon.

[0126] 19. The apparatus of clause 17, wherein the reducing gas comprises H2O.

[0127] 20. The apparatus of clause 17, wherein the reducing gas comprises CO2.

[0128] 21. The apparatus of clause 17, wherein the reducing gas comprises O3.

[0129] 22. The apparatus of clause 17, wherein the reducing gas comprises air.

[0130] 23. The apparatus of clause 1, wherein the reducing gas comprises water vapor.

[0131] 24. The apparatus of clause 1, wherein the gas distribution system is configured to mix the reducing gas with an inert gas to form a gas comprising 10 -5 mbar level inert gas and 10 -7 mbar level reducing gas mixture.

[0132] 25. The apparatus of clause 1, wherein the reflective optical element has a cover layer and the gas distribution system has at least one outlet arranged to generate a flow of gas comprising a reducing gas across at least a portion of the cover layer.

[0133] 26. The apparatus of clause 1, wherein the gas control system is configured to continuously add the reducing gas.

[0134] 27. The apparatus of clause 1, wherein the gas control system is configured to add the reducing gas in-line while plasma is being generated.

[0135] 28. The apparatus of clause 1, wherein the gas control system is configured to intermittently add a dose of the reducing gas.

[0136] 29. The apparatus of clause 1, further comprising a sensor arranged to sense a reflectivity of the optical element and generate a signal indicative of the reflectivity, and wherein the gas control system is arranged to receive the signal and is configured to control at least one of reducing gas concentration and reducing gas composition based at least in part on the sensed reflectivity.

[0137] 30. The apparatus of clause 1, further comprising a sensor arranged to sense a reduced gas concentration adjacent to the reflective optical element and to generate a signal indicative of the reduced gas concentration, and wherein the gas control system is arranged to receive the signal and to control at least one of the reduced gas concentration and the reduced gas composition based at least in part on the reduced gas concentration adjacent to the reflective optical element.

[0138] 31. An apparatus according to clause 1, wherein the reflective optical element is part of an EUV source of EUV radiation, and wherein the gas control system is configured to control at least one of reducing gas concentration and reducing gas composition based on a duration of operation of the EUV source.

[0139] 32. An apparatus according to clause 1, wherein the reflective optical element is part of an EUV source of EUV radiation, and the gas control system operates to control at least one of reducing gas concentration and reducing gas composition based on the number of pulses of EUV radiation generated by the EUV source.

[0140] 33. The apparatus of clause 1, further comprising a plurality of structures within the vacuum chamber for directing a flow of a gas within the vacuum chamber, and further comprising a second gas distribution system arranged to flow the reducing gas over the structures.

[0141] 34. An apparatus comprising:

[0142] Vacuum chamber;

[0143] a reflective optical element disposed in the vacuum chamber;

[0144] a main gas distribution system for adding hydrogen gas to the vacuum chamber;

[0145] an auxiliary gas distribution system for adding a reducing gas to the vacuum chamber, the reducing gas reducing contamination of the reflective optical element through a chemical reaction; and

[0146] 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 reducing gas is introduced into the interior of the vacuum chamber in a regulated manner through the secondary gas distribution system.

[0147] 35. A method comprising the steps of:

[0148] measuring an operating parameter of an EUV light source, the EUV light source comprising a vacuum chamber and a reflective optical element within the vacuum chamber; and

[0149] Addition of a reducing gas to the vacuum chamber is controlled based at least in part on the measured operating parameter, the reducing gas reducing contamination of the reflective optical element through a chemical reaction.

[0150] 36. The method of clause 35, wherein the step of controlling the addition of the reducing gas comprises controlling the concentration of the reducing gas.

[0151] 37. The method of clause 35, wherein the step of controlling the addition of the abatement gas comprises controlling a flow rate of the abatement gas.

[0152] 38. The method of clause 35, wherein the step of controlling the addition of the reducing gas comprises controlling the composition of the reducing gas.

[0153] 39. A method according to clause 35, wherein the operating parameter is the reflectivity of the optical element.

[0154] 40. The method of clause 35, wherein the operating parameter is a concentration of the reducing gas adjacent the optical element.

[0155] 41. The method of clause 35, wherein the operating parameter is a duration of operation of the EUV light source.

[0156] 42. The method of clause 35, wherein the operating parameter is a number of EUV pulses generated by the EUV light source since a predefined start-up time.

[0157] 43. An apparatus comprising:

[0158] Vacuum chamber;

[0159] a reflective optical element disposed in the vacuum chamber;

[0160] a gas distribution system having an inlet for selectively allowing a flow of gas into the vacuum chamber;

[0161] reducing a source of gas to reduce contamination of the reflective optical element in selectable fluid communication with the inlet by chemical reaction;

[0162] at least one heating element in thermal communication with the inlet and arranged to heat gas flowing through the inlet to a temperature sufficient to generate free radicals in at least a portion of the gas in the gas flow; and

[0163] 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 the reducing gas to be introduced into the vacuum chamber through the inlet and a second state in which the gas control system causes the at least one heating element to be energized.

[0164] 44. The device of clause 43, wherein the at least one heating element comprises at least one heating wire.

[0165] 45. The apparatus of clause 43, wherein the reducing gas comprises an oxygen-containing gas.

[0166] 46. ​​An apparatus according to clause 45, wherein the oxygen-containing gas comprises O2.

[0167] 47. The apparatus of clause 45, wherein the oxygen-containing gas comprises H2O.

[0168] 48. An apparatus according to clause 45, wherein the oxygen-containing gas comprises O3.

[0169] 49. An apparatus according to clause 45, wherein the oxygen-containing gas comprises CO2.

[0170] 50. The apparatus of clause 45, wherein the oxygen-containing gas comprises air.

[0171] 51. An apparatus as recited in clause 43, wherein the reducing gas operates to flow across at least a portion of a reflective surface of the reflective optical element.

[0172] 52. The apparatus of clause 43, wherein the gas distribution system operates to introduce reducing gas from a source of the gas into the vacuum chamber by adding an amount of the reducing gas to a flow of hydrogen-containing gas.

[0173] 53. The apparatus of clause 43, wherein the gas distribution system is configured to introduce reducing gas from a source of the gas into the vacuum chamber by permeating the reducing gas into the hydrogen-containing gas flow.

[0174] 54. An apparatus according to clause 43, wherein the inlet comprises a conical element coaxially with and arranged around the central hole of the reflective optical element.

[0175] 55. The device of clause 54, further comprising a plurality of heating elements arranged circumferentially around the outer surface of the conical element.

[0176] 56. The device of clause 54, wherein each heating element of the plurality of heating elements comprises at least one heating wire.

[0177] 57. A method of reducing target material contamination of a reflective surface of a reflective optical element in an extreme ultraviolet light source, the method comprising the steps of:

[0178] forming an umbrella-like flow of hydrogen gas across the reflective surface;

[0179] ionizing at least a portion of the hydrogen gas in the umbrella stream during the first interval;

[0180] adding a predetermined amount of reducing gas to the umbrella flow during a second interval different from the first interval; and

[0181] The ionization step and the addition step are repeated alternately a plurality of times.

[0182] 58. An apparatus comprising:

[0183] Vacuum chamber;

[0184] a reflective optical element disposed in the vacuum chamber;

[0185] a gas distribution system having an inlet for selectively allowing a flow of gas into the vacuum chamber;

[0186] a source of a reducing gas that chemically reacts to reduce contamination of the reflective optical element in selectable fluid communication with the inlet;

[0187] The gas distribution system is configured to place a source of a gas containing free radicals in selective fluid communication with the inlet; and

[0188] 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 reducing 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.

[0189] 59. An apparatus according to clause 58, wherein the source of the gas containing free radicals includes at least one heating element, which is thermally connected to the inlet and is arranged to heat the gas flow flowing through the inlet to a temperature sufficient to generate free radicals in at least a portion of the gas in the gas flow to produce the gas containing free radicals.

[0190] 60. Apparatus according to clause 58, wherein the source of the gas containing free radicals comprises a plasma source arranged to generate free radicals in the gas before the gas reaches the inlet.

[0191] 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, the method comprising the steps of:

[0192] (a) initiating a reduced gas flow into the vacuum chamber;

[0193] (b) stopping the reduced gas flow into the vacuum chamber;

[0194] (c) starting a flow of a gas containing free radicals into the vacuum chamber;

[0195] (d) stopping the flow of the gas containing free radicals into the vacuum chamber;

[0196] The steps (a)-(d) are repeated multiple times in sequence.

[0197] 62. An apparatus comprising:

[0198] Vacuum chamber;

[0199] a reflective optical element disposed in the vacuum chamber;

[0200] a gas supply system operative to supply at least a first gas and a second gas to the vacuum chamber, the second gas comprising oxygen; and

[0201] A control system operates 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.

[0202] 63. The device of clause 62, wherein the reflective optical element comprises a cap layer comprising an oxide.

[0203] 64. The device of clause 63, wherein the oxide comprises a metal oxide.

[0204] 65. The apparatus of clause 62, wherein the control system operates 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.

[0205] 66. The apparatus of clause 62, wherein the control system operates 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.

[0206] 67. An apparatus according to clause 62, wherein the second gas comprises O2.

[0207] 68. The apparatus of clause 62, wherein the second gas comprises H2O.

[0208] 69. The apparatus of clause 62, wherein the second gas comprises H2O2.

[0209] 70. The apparatus of clause 62, wherein the second gas comprises O3.

[0210] 71. An apparatus according to clause 62, wherein the second gas comprises CO2.

[0211] 72. The device of clause 62, wherein the second gas comprises air.

[0212] 73. An apparatus according to any one of clauses 62 to 72, wherein the second gas comprises an inert gas.

[0213] 74. The apparatus of clause 73, wherein the inert gas comprises argon.

[0214] 75. The apparatus of clause 73, wherein the inert gas comprises helium.

[0215] 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.

[0216] 77. The apparatus of clause 62 further comprises a gas pressure sensor arranged to sense the partial pressure of the second gas in the vacuum chamber and to generate a first signal indicative of the partial pressure to the control system, and wherein the control system controls the supply of the second gas at least in part based on the first signal.

[0217] 78. The device of clause 77, wherein the air pressure sensor directly senses the partial pressure of the second gas.

[0218] 79. The apparatus of clause 77, wherein the gas pressure sensor senses the partial pressure of the oxygen-containing gas indirectly by sensing the partial pressure of at least one gas other than the second gas.

[0219] 80. An apparatus comprising:

[0220] Vacuum chamber;

[0221] a reflective optical element disposed in the vacuum chamber;

[0222] a gas distribution system for adding gas to the vacuum chamber, the gas distribution system comprising an interface for connecting to a source of oxygen-containing gas; and

[0223] a gas control system configured to control the gas distribution system to supply the oxygen-containing gas to the vacuum chamber until the partial pressure of the oxygen-containing gas reaches a first value, thereby stopping the supply of 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 resuming the supply of 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.

[0224] 81. A device according to clause 80, wherein the reflective optical element comprises a multilayer reflector, the multilayer reflector comprising a cap layer and a plurality of underlying layers, the cap layer being composed and arranged to protect the underlying layers from damage, the cap layer comprising an oxide.

[0225] 82. The device of clause 81, wherein the oxide comprises a metal oxide.

[0226] 83. An apparatus as described in clause 80, wherein the third value is substantially equal to the first value.

[0227] 84. An apparatus according to clause 80, wherein the oxygen-containing gas comprises O2.

[0228] 85. An apparatus according to clause 80, wherein the oxygen-containing gas comprises H2O.

[0229] 86. An apparatus according to clause 80, wherein the oxygen-containing gas comprises H2O2.

[0230] 87. An apparatus according to clause 80, wherein the oxygen-containing gas comprises O3.

[0231] 88. An apparatus according to clause 80, wherein the oxygen-containing gas comprises CO2.

[0232] 89. An apparatus according to clause 80, wherein the oxygen-containing gas comprises air.

[0233] 90. The apparatus of any one of clauses 80 to 89, wherein the second gas comprises an inert gas.

[0234] 91. The apparatus of clause 90, wherein the inert gas comprises argon.

[0235] 92. The apparatus of clause 90, wherein the inert gas comprises helium.

[0236] 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.

[0237] 94. The apparatus of clause 80, further comprising a gas pressure sensor arranged to sense the partial pressure of the oxygen-containing gas in the vacuum chamber and to generate a first signal indicative of the partial pressure to the gas control system, and wherein the gas control system controls the supply of the oxygen-containing gas at least in part based on the first signal.

[0238] 95. The device of clause 94, wherein the air pressure sensor directly senses the partial pressure of the oxygen-containing gas.

[0239] 96. The apparatus of clause 94, wherein the gas pressure sensor senses the partial pressure of the oxygen-containing gas indirectly by sensing the partial pressure of at least one gas other than the oxygen-containing gas.

[0240] 97. An apparatus according to clause 94, wherein the air pressure sensor is arranged to sense the partial pressure of the oxygen-containing gas proximate to the reflective optical element.

[0241] 98. A method for extending the useful life of a reflective surface in a vacuum chamber of an EUV source, the method comprising the steps of:

[0242] providing a gas supply system operative to supply at least a first gas and a second gas to the vacuum chamber, the second gas comprising oxygen;

[0243] sensing a partial pressure of at least one of the first gas and the second gas in the vacuum chamber;

[0244] The gas supply system is controlled to supply the second gas based at least in part on the sensed partial pressure.

[0245] 99. The method of clause 98, wherein the sensing step comprises sensing a partial pressure of the second gas.

[0246] 100. The method of clause 98, wherein the sensing step comprises sensing a partial pressure of the first gas.

[0247] 101. The method of clause 98, wherein the second gas comprises O2.

[0248] 102. The method of clause 98, wherein the second gas comprises H2O.

[0249] 103. The method of clause 98, wherein the second gas comprises H2O2.

[0250] 104. The method of clause 98, wherein the second gas comprises O3.

[0251] 105. The method of clause 98, wherein the second gas comprises CO2.

[0252] 106. The method of clause 98, wherein the second gas comprises air.

[0253] 107. The method of any one of clauses 98 to 106, wherein the second gas comprises an inert gas.

[0254] 108. The method of clause 107, wherein the inert gas comprises argon.

[0255] 109. The method of clause 107, wherein the inert gas comprises helium.

[0256] 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.

[0257] 111. A method for extending the useful life of a reflective surface in a vacuum chamber of an EUV source, the method comprising the steps of:

[0258] (a) supplying an oxygen-containing gas to the vacuum chamber;

[0259] (b) when the partial pressure of the oxygen-containing gas reaches a first value, stopping the supply of the oxygen-containing gas to the vacuum chamber;

[0260] (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;

[0261] (d) when the partial pressure of the oxygen-containing gas reaches a third value greater than the second value, stopping the supply of the oxygen-containing gas to the vacuum chamber; and

[0262] (e) repeating steps (c) and (d) to maintain the partial pressure of the oxygen-containing gas between the first value and the second value.

[0263] 112. The method of clause 111, wherein the third value is substantially equal to the first value.

[0264] 113. A method according to clause 111, wherein the oxygen-containing gas comprises O2.

[0265] 114. The method of clause 111, wherein the oxygen-containing gas comprises H2O.

[0266] 115. A method according to clause 111, wherein the oxygen-containing gas comprises H2O2.

[0267] 116. A method according to clause 111, wherein the oxygen-containing gas comprises O3.

[0268] 117. A method according to clause 111, wherein the oxygen-containing gas comprises CO2.

[0269] 118. The method of clause 111, wherein the oxygen-containing gas comprises air.

[0270] 119. The method of any one of clauses 111 to 118, wherein the second gas comprises an inert gas.

[0271] 120. The method of clause 119, wherein the inert gas comprises argon.

[0272] 121. The method of clause 119, wherein the inert gas comprises helium.

[0273] 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.

[0274] 123. The method of clause 111, wherein each of steps (b), (c) and (d) comprises sensing the partial pressure of the oxygen-containing gas in the vacuum chamber.

[0275] 124. The method of clause 111, wherein each of steps (b), (c) and (d) comprises directly sensing the partial pressure of the oxygen-containing gas in the vacuum chamber.

[0276] 125. The method of clause 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.

[0277] 126. The method of clause 111, wherein each of steps (b), (c) and (d) comprises sensing a partial pressure of the oxygen-containing gas in the vacuum chamber proximate the reflective surface.

Claims

1. A radiation source, comprising: container; a reflective optical element disposed at one end of the container, wherein the reflective optical element includes a focal point positioned at another end of the container; a gas distribution system configured to introduce a gas into the vessel, wherein the gas distribution system comprises a hydrogen-containing gas source, an oxygen-containing gas source, and a valve coupled to the oxygen-containing gas source; and A gas control system is coupled to the oxygen-containing gas source.

2. The radiation source of claim 1, further comprising: A gas flow assembly is coupled to the gas distribution system, wherein the gas flow assembly includes a tubular body aligned with a central hole of the reflective optical element, an opening between an outer surface of the tubular body and an edge of the central hole, and at least one exhaust hole arranged along a circumference of the reflective optical element.

3. The radiation source of claim 2, wherein the hydrogen-containing gas source and the oxygen-containing gas source are fluidly coupled to the opening and the at least one exhaust port.

4. The radiation source of claim 2, wherein the gas flow assembly further comprises a plurality of heating elements positioned around a circumference of the tubular body.

5. The radiation source of claim 2, wherein the gas flow assembly is coupled to a plasma source.

6. A radiation source according to claim 2, wherein the opening is configured to introduce a flow from the center of the reflective optical element, along the surface of the reflective optical element toward the circumference of the reflective optical element, and the at least one exhaust hole is configured to introduce a flow from the circumference of the reflective optical element, along the surface of the reflective optical element toward the center of the reflective optical element.

7. The radiation source of claim 1, wherein a controlled amount of oxygen-containing gas from the oxygen-containing gas source is mixed with hydrogen-containing gas from the hydrogen-containing gas source prior to introducing the gas into the vessel.

8. The radiation source of claim 1, wherein the top layer of the reflective optical element comprises an oxygen-containing material.

9. The radiation source of claim 1, wherein the gas introduced into the container comprises hydrogen mixed with at least 0.1% of an oxygen-containing gas.

10. The radiation source of claim 1, further comprising a measurement device coupled to the gas control system, wherein the measurement device is configured to read at least one of a partial pressure of an oxygen-containing gas and a surface reflectivity of the reflective optical element.

11. A method for reducing contamination in a radiation source, comprising: introducing a hydrogen-containing gas into the source container, wherein a reflective optical element is positioned at one end of the radiation source container; introducing an oxygen-containing gas into the source container through a valve; measuring a parameter in the source container; determining whether the parameter is less than a first threshold; and If the determination is negative, the introduction of the oxygen-containing gas into the source container is stopped via a gas control system.

12. The method according to claim 11, further comprising: If the determination is yes, then the oxygen-containing gas continues to be introduced into the source vessel via the gas control system.

13. The method according to claim 11, further comprising: determining whether the parameter is greater than a second threshold; as well as If the determination is negative, the introduction of the oxygen-containing gas into the source vessel is stopped by the gas control system.

14. The method of claim 11, wherein introducing the oxygen-containing gas is configured to stabilize a cap layer of the reflective optical element.

15. The method of claim 14, wherein stabilizing the cap layer of the reflective optical element comprises oxidizing a depletion region of the cap layer.

16. The method of claim 11, wherein the introducing the oxygen-containing gas is performed while plasma is generated in the source container.

17. The method of claim 11, wherein the introducing the oxygen-containing gas is performed when no plasma is generated in the source container.

18. The method according to claim 11, further comprising: Produce hydrogen free radicals; reacting the hydrogen radicals with a target material in the radiation source container to form a volatile gas species; as well as The volatile gaseous substances are evacuated.

19. The method of claim 18, wherein generating hydrogen radicals comprises energizing at least one heating element to convert a portion of the hydrogen-containing gas into the hydrogen radicals.

20. The method of claim 18, wherein generating hydrogen radicals comprises providing a plasma source to convert a portion of the hydrogen-containing gas into the hydrogen radicals.

21. The method of claim 18, further comprising increasing the concentration of hydrogen radicals and introducing the oxygen-containing gas in an alternating manner.

22. The method of claim 11, wherein the central hole of the reflective optical element is coupled to a tubular body, and wherein introducing the oxygen-containing gas into the source container comprises: A source of oxygen-containing gas is fluidly coupled to an opening positioned between an outer surface of the tubular body and an edge of the central aperture of the reflective optical element, and the source of oxygen-containing gas is fluidly coupled to at least one exhaust hole disposed along a circumference of the reflective optical element.

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