WO2020231746A1

WO2020231746A1 - Long life laser chamber electrode

Info

Publication number: WO2020231746A1
Application number: PCT/US2020/031906
Authority: WO
Inventors: Andrew Jay EFFENBERGER, Jr.
Original assignee: Cymer, Llc
Priority date: 2019-05-10
Filing date: 2020-05-07
Publication date: 2020-11-19
Also published as: TW202400821A; JP2022533528A; JP7369205B2; CN113875101A; TW202108791A

Abstract

Disclosed is an apparatus and method for creating a protective layer on at least one electrode in a laser chamber in which a layer-forming gas is added to the laser chamber and then the electrode is used to generate a plasma in the laser chamber causing formation of the protective layer.

Description

LONG LIFE LASER CHAMBER ELECTRODE

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to U.S. Application No. 62/845,926, filed May 10, 2019 and titled LONG LIFE LASER CHAMBER ELECTRODE, and which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

[0002] The present disclosed subject matter relates to laser-generated light sources such as are used for integrated circuit photolithographic manufacturing processes.

BACKGROUND

[0001] In laser discharge chambers such as an ArF power ring amplifier excimer discharge chamber (“PRA”) or a KrF excimer discharge chamber, electrode erosion imposes significant limits on the useful lifetime of the chamber module. One measure to extend the useful lifetime of a KrF excimer discharge chamber module involves making the anode of a material which resists wear. Information on materials suitable for use as anode materials can be found, for example, in U.S. Patent No. 7,301,980, issued November 27, 2007 and U.S. Patent No. 6,690,706 issued February 10, 2004, both of which are assigned to the assignee of the present application and both of which are hereby incorporated by reference in their entirety.

[0002] Fluorine-containing plasmas are highly corrosive to metals and can therefore result in electrode corrosion and erosion during operation of the chamber. For example, nucleation and growth of localized zones of corrosion product build-up on the surface of the anode may occur. This leads to non-uniformity in discharges between the electrodes and down- stream arcing. Erosion leads to both an increase in the width of the discharge gap and broadening of the discharge. Both of these phenomena lead to lower energy density in the discharge which in turn drives a need to increase the voltage differential across the electrodes necessary to maintain energy output. In addition, discharge broadening reduces the clearing ratio of the gas flow leading to increased downstream arcing leading to energy dropouts and resultant dose errors. Once the dose error rate increases above a predetermined threshold the chamber is deemed to have reached the end of its useful life and must be replaced. [0003] One or more metal oxide layers or metal oxy-nitride layers may serve as a protective layer to the surface of the electrodes. For example, the formation of CuO or ZnO can protect the electrode material, e.g., brass, from fluoridation. This is also true by the formation of metal oxynitrides, which have superior compression strength, flexural strength, fracture toughness, Knoop hardness, and shear modulus, as well as being very resistant to fluoridation. Having such layers could improve the lifetime of the electrodes. Techniques to make metal oxides on electrodes, however, presently involve heating the electrodes in a furnace in a bath of oxygen gas. These techniques warp, shrink, and otherwise deform the electrodes. In addition, they typically result in the entire electrode being covered with the protective layer which is undesirable because these methods are not in situ and if the entire electrode is coated it is then difficult if not impossible to mount the coated electrode in the chamber.

SUMMARY

[0004] The following presents a simplified summary of one or more embodiments in order to provide a basic understanding of the present invention. This summary is not an extensive overview of all contemplated embodiments, and is not intended to identify key or critical elements of all embodiments nor delineate the scope of any or all embodiments. Its sole purpose is to present some concepts of one or more embodiments in a simplified form as a prelude to the more detailed description that is presented later.

[0005] According to one aspect of an embodiment, there is disclosed a laser chamber having an electrode, the laser chamber being configured to expose the electrode to layer-forming gas while a plasma is generated in the laser chamber to grow a protective metal oxide layer or metal oxynitride layer on the electrode. Thus the layers are grown in the chamber, i.e., in situ, during plasma discharge. This provides better spatial control of the layers and does not deform the electrodes.

[0006] Further embodiments, features, and advantages of the present invention, as well as the structure and operation of the various embodiments are described in detail below with reference to accompanying drawings. BRIEF DESCRIPTION OF THE DRAWING

[0007] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the methods and systems of embodiments of the invention by way of example, and not by way of limitation. Together with the detailed description, the drawings further serve to explain the principles of and to enable a person skilled in the relevant art(s) to make and use the methods and systems presented herein. In the drawings, like reference numbers indicate identical or functionally similar elements.

[0008] FIG. 1 shows a schematic, not to scale, view of an overall broad conception of a photolithography system according to an aspect of the disclosed subject matter.

[0009] FIG. 2 shows a schematic, not to scale, view of an overall broad conception of an illumination system according to an aspect of the disclosed subject matter.

[0010] FIG. 3 is a diagrammatic cross section, not to scale, of a discharge chamber for an excimer laser according to aspects of the disclosed subject matter.

[0011] FIG. 4 is a diagrammatic cross section, not to scale, of a discharge chamber for an excimer laser according to aspects of the disclosed subject matter.

[0012] FIG. 5 is a cross-sectional view of an electrode with a protective layer according to aspects of the disclosed subject matter.

[0013] FIG. 6 is a flowchart showing a method according to aspect of the disclosed subject matter.

[0014] Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. It is noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art based on the teachings contained herein.

DETAILED DESCRIPTION

[0015] Various embodiments are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to promote a thorough understanding of one or more embodiments. It may be evident in some or all instances, however, that any embodiment described below can be practiced without adopting the specific design details described below. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate description of one or more embodiments. The following presents a simplified summary of one or more embodiments in order to provide a basic understanding of the embodiments. This summary is not an extensive overview of all contemplated embodiments, and is not intended to identify key or critical elements of all embodiments nor delineate the scope of any or all embodiments.

[0016] FIG. 1 shows a photolithography system 100 that includes an illumination system 105. As described more fully below, the illumination system 105 includes a light source that produces a pulsed light beam 110 and directs it to a photolithography exposure apparatus or scanner 115 that patterns microelectronic features on a wafer 120. The wafer 120 is placed on a wafer table 125 constructed to hold the wafer 120 and connected to a positioner configured to accurately position the wafer 120 in accordance with certain parameters.

[0017] The photolithography system 100 uses a light beam 110 having a wavelength in the deep ultraviolet (DUV) range, for example, with wavelengths of 248 nanometers (nm) or 193 nm. The minimum size of the microelectronic features that can be patterned on the wafer 120 depends on the wavelength of the light beam 110, with a lower wavelength resulting in a smaller minimum feature size. When the wavelength of the light beam 110 is 248 nm or 193 nm, the minimum size of the microelectronic features can be, for example, 50 nm or less but other wavelengths of light and other minimum feature sizes can be produced according to other embodiments. The bandwidth of the light beam 110 can be the actual, instantaneous bandwidth of its optical spectrum (or emission spectrum), which contains information on how the optical energy of the light beam 110 is distributed over different wavelengths. The photolithography exposure apparatus or scanner 115 includes an optical arrangement having, for example, one or more condenser lenses, a mask, and an objective arrangement. The mask is movable along one or more directions, such as along an optical axis of the light beam 110 or in a plane that is perpendicular to the optical axis. The objective arrangement includes a projection lens and enables the image transfer to occur from the mask to photoresist on the wafer 120. The illumination system 105 adjusts the range of angles for the light beam 110 impinging on the mask. The illumination system 105 also homogenizes (makes uniform) the intensity distribution of the light beam 110 across the mask. [0018] The scanner 115 can include, among other features, a lithography controller 130, air conditioning devices, and power supplies for the various electrical components. The lithography controller 130 controls how layers are printed on the wafer 120. The lithography controller 130 includes a memory that stores information such as process recipes. A process program or recipe determines the length of the exposure on the wafer 120 based on, for example, the mask used, as well as other factors that affect the exposure. During lithography, a plurality of pulses of the light beam 110 illuminates the same area of the wafer 120 to constitute an illumination dose.

[0019] The photolithography system 100 may also advantageously include a control system 135. In general, the control system 135 includes one or more of digital electronic circuitry, computer hardware, firmware, and software. The control system 135 also includes memory which can be read-only memory and/or random access memory. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including, by way of example, semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto- optical disks; and CD-ROM disks.

[0020] The control system 135 can also include one or more input devices (such as a keyboard, touch screen, microphone, mouse, hand-held input device, etc.) and one or more output devices (such as a speaker or a monitor). The control system 135 also includes one or more programmable processors, and one or more computer program products tangibly embodied in a machine-readable storage device for execution by one or more programmable processors. The one or more programmable processors can each execute a program of instructions to perform desired functions by operating on input data and generating appropriate output. Generally, the processors receive instructions and data from the memory. Any of the foregoing may be supplemented by, or incorporated in, specially designed ASICs (application-specific integrated circuits). The control system 135 can be centralized or be partially or wholly distributed throughout the photolithography system 100.

[0021] Referring to FIG. 2, an exemplary illumination system 105 is a pulsed laser source that produces a pulsed laser beam as the light beam 110. FIG. 2 shows illustratively and in block diagram a gas discharge laser system according to an embodiment of certain aspects of the disclosed subject matter. The gas discharge laser system may include, e.g., a solid state or gas discharge seed laser system 140, an amplification stage, e.g., a power ring amplifier (“PR A”) stage 145, relay optics 150 and a laser system output subsystem 160. The seed system 140 may include, e.g., a master oscillator (“MO”) chamber 165.

[0022] The seed laser system 140 may also include a master oscillator output coupler (“MO OC”) 175, which may include a partially reflective mirror, forming with a reflective grating (not shown) in a line narrowing module (“LNM”) 170, an oscillator cavity in which the seed laser 140 oscillates to form the seed laser output pulse, i.e., forming a master oscillator (“MO”). The system may also include a line-center analysis module (“LAM”) 180. The LAM 180 may include an etalon spectrometer for fine wavelength measurement and a coarser resolution grating spectrometer. A MO wavefront engineering box (“WEB”) 185 may serve to redirect the output of the MO seed laser system 140 toward the amplification stage 145, and may include, e.g., beam expansion with, e.g., a multi prism beam expander (not shown) and coherence busting, e.g., in the form of an optical delay path (not shown).

[0023] The amplification stage 145 may include, e.g., a PRA lasing chamber 200, which may also be an oscillator, e.g., formed by seed beam injection and output coupling optics (not shown) that may be incorporated into a PRA WEB 210 and may be redirected back through the gain medium in the chamber 200 by a beam reverser 220. The PRA WEB 210 may incorporate a partially reflective input/output coupler (not shown) and a maximally reflective mirror for the nominal operating wavelength (e.g., at around 193 nm for an ArF system) and one or more prisms.

[0024] A bandwidth analysis module (“BAM”) 230 at the output of the amplification stage 145 may receive the output laser light beam of pulses from the amplification stage and pick off a portion of the light beam for metrology purposes, e.g., to measure the output bandwidth and pulse energy. The laser output light beam of pulses then passes through an optical pulse stretcher (“OPuS”) 240 and an output combined autoshutter metrology module (“CASMM”) 250, which may also be the location of a pulse energy meter. One purpose of the OPuS 240 may be, e.g., to convert a single output laser pulse into a pulse train. Secondary pulses created from the original single output pulse may be delayed with respect to each other. By distributing the original laser pulse energy into a train of secondary pulses, the effective pulse length of the laser can be expanded and at the same time the peak pulse intensity reduced. The OPuS 240 can thus receive the laser beam from the PRA WEB 210 via the BAM 230 and direct the output of the OPuS 240 to the CASMM 250. [0025] The PR A lasing chamber 200 and the MO 165 are configured as chambers in which electrical discharges between electrodes may cause lasing gas discharges in a lasing gas to create an inverted population of high energy molecules, including, e.g., Ar, Kr, and/or Xe, to produce relatively broad band radiation that may be line narrowed to a relatively very narrow bandwidth and center wavelength selected in a line narrowing module (‘‘LNM’’) 170, as is known in the art.

[0026] A configuration for such a chamber 300 is shown in FIG. 3, which is a highly stylized cross-sectional diagram of a discharge chamber. A chamber 300 includes an upper electrode 310 acting as a cathode and a lower electrode 320 acting as an anode. One or both of the lower electrode 300 and the upper electrode 310 may be entirely contained in the pressure envelope of chamber 300 defined by the chamber wall 305 or one of the electrodes may not be so contained. Lasing gas discharges occur between these two electrodes in a gap A. Also shown in FIG. 3 are an upper insulator 315 and a lower insulator 325. The lower electrode 320 is electrically connected to the wall 305 of the chamber 300. For safety reasons it is desirable to maintain the chamber wall 305 and also the lower electrode 320, at ground potential. In the embodiment shown in FIG. 3, the upper electrode 310 is driven by a voltage supply 340 at a voltage which is negative with respect to the lower electrode 320.

[0027] As mentioned, also shown in FIG. 3 is a voltage supply 340 which establishes a voltage gradient across cathode 310 and anode 320. While the notation (-) is shown for the polarity of the output of the voltage supply 340 it will be understood that this is a relative rather than absolute polarity, that is, relative to the polarity of the lower electrode 320, which will generally be in electrical contact with the body of the chamber 300 and must remain held at a ground (0) potential. The upper electrode (cathode 310) is charged to a large (~20 kV) negative voltage.

[0028] The electrodes in the chamber 300 have been known to erode. The erosion may be as a result of fluorine reactions with the electrode materials according to embodiments in which ArF or KrF, for example, are used, or the erosion may be due to any of various other erosion mechanisms. According to one aspect of an embodiment, a layer-forming gas is introduced into the chamber 300 and then a plasma is struck in the chamber 300 to promote the formation of a protective layer on the electrodes. This is shown in FIG. 4. In FIG.4 there is a gas inlet 400 for introducing a layer forming gas into the chamber 300. The inlet 400 is in fluid communication with a valve 410 which operates under control of a control unit 430 to selectably connect the inlet 400 to at least one source 420 of layer forming gas. Once the partial pressure of layer forming gas in the chamber 300 has reached a desired value, a plasma is struck in the chamber 300 by establishing an appropriate voltage differential between the electrodes 310, 320. After a predetermined interval, the voltage differential is removed and the layer forming gas is evacuated, but a protective layer 510 has been formed on the electrodes 310, 320 as shown in FIG. 5.

[0029] Plasma may be used to assist in the growth of metal oxides, metal nitrides, and metal oxy-nitrides on surfaces. The discharge chamber is inherently a plasma source, so under the proper conditions it is possible to grow the protective layer in situ. As an example, a layer-forming gas containing oxygen and/or nitrogen may be introduced to the chamber. The chamber will then be operated in a fashion that is similar to how it is normally operated to function as a laser chamber. The plasma along with the oxygen and/or nitrogen will create protective layers.

[0030] The layer-forming gas may be, for example, an oxygen-containing gas if it is desired that the protective layer be a metal oxide. Examples of oxygen-containing gases include 02, H20, H202, 03, nitrous oxides (NO_x), and air. The layer-forming gas may be, for example, a nitrogen- containing gas if it is desired that the protective layer be a nitride. Examples of nitrogen-containing gases include N2, NH3, nitrous oxides (NOx), and air. The layer-forming gas may be, for example, a gas containing or which is a mixture of nitrogen and oxygen if it is desired that the protective layer be a metal oxynitride. Examples of such gases include nitrous oxides (NOx), mixtures of the oxygen-containing gasses and nitrogen-containing gasses mentioned above, and air. These are all only examples and it will be apparent to one having ordinary skill in the art that other gasses may be used.

[0031] The concentration / pressure of layer-forming gas for the formation of the protective layer may advantageously lie within the range of about parts per million levels to about 38 kPa or in the range of about parts per million to about 4kPa. Because the total fill pressure of the chamber is on the order of 380 kPa of laser gas, this corresponds to concentrations on the order of parts per million to about 1% of layer forming gas.

[0032] Under these conditions, a plasma may be struck in the chamber 300 by applying a voltage differential in the range of about 17kV to about 28 kV as an example, although other voltage differentials could be used.

[0033] As shown in FIG. 5, the electrode 500, which could be either electrode 310 or 320, will be formed of a bulk material such as brass which is an alloy of copper and zinc. As a result of being exposed to a plasma and layer-forming gas, a protective layer 510 will form on the exposed surface of the electrode 500. The composition of the protective layer 510 will in general depend on the electrode material and the layer- forming gas being used. An oxygen-containing gas may be used to create a protective layer 510 made of a mixture of CuO or ZnO on a brass electrode. A nitrogen-containing gas may be used to create a protective layer 510 of copper nitride (Cu3N) or zinc nitride (Zn3N2) with a brass electrode. A nitrogen-containing and oxygen-containing gas may be used to create a protective layer 510 of copper oxynitride (Cu_xO_yN_z) or zinc oxynitride (Zn_xO_yN_z) with a brass electrode. It will be apparent to one having ordinary skill in the art that other combinations are possible. The thickness of the protective layer 510 is generally in the range of on the order of nanometers to on the order of 10 microns.

[0034] The thickness of the protective layer 510 will in general be a function of formation rate and time of formation. The formation rate will depend on the chemistry of the layer formation and the characteristics of the plasma.

[0035] The protective layer formed on top of the electrode surface as the result of in situ layer reduces erosion of the electrode. In many embodiments, the protective layer formed on top of the electrode surface as the result of in situ layer formation plays important role in reducing the fluorine reaction with the bulk material of the electrodes. The more dense and uniform the protective layer is, the more it can be expected that the erosion rate will decrease.

[0036] FIG. 6 is a flowchart describing a process for in-situ formation of the protective layer on an electrode in accordance with one aspect of an embodiment. In a step S10 a layer- forming gas is introduced to a desired partial pressure to the chamber containing the electrode. In a step S20 a plasma is struck by applying a voltage to the electrode for a predetermined period of time to enable the layer-forming gas to form a protective layer on the electrode surface. In a step S30 the plasma is quenched by removing the voltage. In a step S40 the layer-forming gas is evacuated from the chamber. As a result, a protective layer is formed on the electrode.

[0037] These steps can be repeated periodically to re- grow the layers. Alternatively the protective layer may be grown continuously by controlled introduction of dilute mixtures containing oxygen, nitrogen, or both.

[0038] One advantage of the process just described is that layer growth is confined to the discharge region of the electrode where the plasma is present so there is better spatial control of the layer growth. Also, heating of the electrodes is no different than what the electrode typically experiences during the normal operation of the chamber so that there is less potential for electrode deformation. The potential to repeat the growth cycle at any time also contributes to longer overall electrode life.

[0039] The above description includes examples of multiple embodiments. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the aforementioned embodiments, but one of ordinary skill in the art may recognize that many further combinations and permutations of various embodiments are possible. Accordingly, the described embodiments are intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims. Furthermore, to the extent that the term“includes” is used in either the detailed description or the claims, such term is intended to be inclusive in a manner similar to the term“comprising” as “comprising” is construed when employed as a transitional word in a claim. Furthermore, although elements of the described aspects and/or embodiments may be described or claimed in the singular, the plural is contemplated unless limitation to the singular is explicitly stated. Additionally, all or a portion of any aspect and/or embodiment may be utilized with all or a portion of any other aspect and/or embodiment, unless stated otherwise.

[0040] Other aspects of the invention are set out in the following numbered clauses.

1. Apparatus comprising:

a laser chamber;

an electrode positioned at least partially within laser chamber;

a source of a layer-forming gas connectable to the laser chamber; and

a voltage supply electrically connected to the electrode and configured to supply voltage to the electrode to generate a plasma at a surface of the electrode in the presence of the layer-forming gas to form a protective layer on the electrode.

2. Apparatus as in clause 1 wherein the layer-forming gas comprises an oxygen-containing gas.

3. Apparatus as in clause 2 wherein the oxygen-containing gas comprises 02.

4. Apparatus as in clause 2 wherein the oxygen-containing gas comprises H20.

5. Apparatus as in clause 2 wherein the oxygen-containing gas comprises H202.

6. Apparatus as in clause 2 wherein the oxygen-containing gas comprises 03.

7. Apparatus as in clause 2 wherein the oxygen-containing gas comprises a nitrous oxide.

8. Apparatus as in clause 2 wherein the oxygen-containing gas comprises air. 9. Apparatus as in any one of clauses 1-8 wherein the protective layer comprises a metal oxide.

10. Apparatus as in any one of clauses 2-8 wherein the electrode comprises brass and the protective layer comprises copper oxide CuO.

11. Apparatus as in in any one of clauses 2-8 wherein the electrode comprises brass and the protective layer comprises zinc oxide ZnO.

12. Apparatus as in clause 1 wherein the layer-forming gas comprises a nitrogen-containing gas.

13. Apparatus as in clause 12 wherein the nitrogen-containing gas comprises N2.

14. Apparatus as in clause 12 wherein the nitrogen-containing gas comprises NH3.

15. Apparatus as in clause 12 wherein the nitrogen-containing gas comprises a nitrous oxide.

16. Apparatus as in clause 12 wherein the nitrogen-containing gas comprises air.

17. Apparatus as in any one of clauses 12-16 wherein the protective layer comprises a metal nitride.

18. Apparatus as in any one of clauses 12-16 wherein the electrode comprises brass and the protective layer comprises copper nitride.

19. Apparatus as in any one of clauses 12-16 wherein the electrode comprises brass and the protective layer comprises zinc nitride.

20. Apparatus as in clause 1 wherein the layer-forming gas comprises a nitrogen-containing and oxygen-containing gas.

21. Apparatus as in clause 20 wherein the nitrogen-containing and oxygen-containing gas comprises a nitrous oxide.

22. Apparatus as in clause 20 wherein the nitrogen-containing and oxygen-containing gas comprises air.

23. Apparatus as in any one of clauses 20-22 wherein the protective layer comprises a metal oxynitride.

24. Apparatus as in any one of clauses 20-22 wherein the electrode comprises brass and the protective layer comprises copper oxynitride.

25. Apparatus as in any one of clauses 20-22 wherein the electrode comprises brass and the protective layer comprises zinc oxynitride.

26. A method of forming a protective layer on an electrode in a laser discharge chamber, the method comprising:

adding a layer-forming gas to the laser discharge chamber to achieve a predetermined partial pressure; and

using the electrode to generate a plasma within the laser discharge chamber for a predetermined amount of time.

27. A method as in clause 26 wherein the layer-forming gas comprises an oxygen-containing gas.

28. A method as in clause 27 wherein the oxygen-containing gas comprises 02.

29. A method as in clause 27 wherein the oxygen-containing gas comprises H20.

30. A method as in clause 27 wherein the oxygen-containing gas comprises H202.

31. A method as in clause 27 wherein the oxygen-containing gas comprises 03.

32. A method as in clause 27 wherein the oxygen-containing gas comprises a nitrous oxide.

33. A method as in clause 27 wherein the oxygen-containing gas comprises air.

34. A method as in any one of clauses 26-33 wherein the protective layer comprises a metal oxide.

35. A method as in any one of clauses 26-33 wherein the electrode comprises brass and the protective layer comprises copper oxide CuO.

36. A method as in in any one of clauses 26-33 wherein the electrode comprises brass and the protective layer comprises zinc oxide ZnO.

37. A method as in clause 26 wherein the layer-forming gas comprises a nitrogen-containing gas.

38. A method as in clause 37 wherein the nitrogen-containing gas comprises N2.

39. A method as in clause 37 wherein the nitrogen-containing gas comprises NH3.

40. A method as in clause 37 wherein the nitrogen-containing gas comprises a nitrous oxide. 41. A method as in clause 37 wherein the nitrogen-containing gas comprises air.

42. A method as in any one of clauses 37-41 wherein the protective layer comprises a metal nitride.

43. A method as in any one of clauses 37-41 wherein the electrode comprises brass and the protective layer comprises copper nitride.

44. A method as in any one of clauses 37-41 wherein the electrode comprises brass and the protective layer comprises zinc nitride. 45. A method as in clause 26 wherein the layer-forming gas comprises a nitrogen-containing and oxygen-containing gas.

46. A method as in clause 26 wherein the nitrogen-containing and oxygen-containing gas comprises a nitrous oxide.

47. A method as in clause 26 wherein the nitrogen-containing and oxygen-containing gas comprises air.

48. A method as in any one of clauses 45-47 wherein the protective layer comprises a metal oxynitride.

49. A method as in any one of clauses 45-47 wherein the electrode comprises brass and the protective layer comprises copper oxynitride.

50. A method as in any one of clauses 45-47 wherein the electrode comprises brass and the protective layer comprises zinc oxynitride.

[0041] Other aspects of the invention are set out in the following claims.

Claims

1. Apparatus comprising:

i. a laser chamber;

ii. an electrode positioned at least partially within laser chamber;

iii. a source of a layer-forming gas connectable to the laser chamber; and iv. a voltage supply electrically connected to the electrode and configured to supply voltage to the electrode to generate a plasma at a surface of the electrode in the presence of the layer-forming gas to form a protective layer on the electrode.

2. The apparatus as in claim 1 wherein the layer-forming gas comprises an oxygen-containing gas.

3. The apparatus as in claim 2 wherein the oxygen-containing gas comprises 02, H20, or H202.

4. The apparatus as in claim 2 wherein the oxygen-containing gas comprises 03.

5. The apparatus as in claim 2 wherein the oxygen-containing gas comprises a nitrous oxide or air.

6. The apparatus as in claim 1 wherein the protective layer comprises a metal oxide.

7. The apparatus as in claim 1 wherein the electrode comprises brass and the protective layer comprises copper oxide CuO or zinc oxide ZnO.

8. The apparatus as in claim 1 wherein the layer-forming gas comprises a nitrogen-containing gas.

9. The apparatus as in claim 8 wherein the nitrogen-containing gas comprises N2 or NH3.

10. The apparatus as in claim 8 wherein the protective layer comprises a metal nitride, the electrode comprises brass, the metal nitride being copper nitride or zinc nitride.

11. The apparatus as in claim 1 wherein the layer-forming gas comprises a nitrogen-containing and oxygen-containing gas.

12. The apparatus as in claim 11 wherein the protective layer comprises copper oxynitride or zinc oxynitride and the electrode comprises brass.

13. A method of forming a protective layer on an electrode in a laser discharge chamber, the method comprising:

i. adding a layer-forming gas to the laser discharge chamber to achieve a predetermined partial pressure; and

ii. using the electrode to generate a plasma within the laser discharge chamber for a predetermined amount of time.

14. The method as in claim 13 wherein the layer-forming gas comprises an oxygen-containing gas.

15. The method as in claim 14 wherein the oxygen-containing gas comprises 02, H20 or H202.

16. The method as in claim 14 wherein the oxygen-containing gas comprises 03.

17. The method as in claim 14 wherein the oxygen-containing gas comprises a nitrous oxide or air.

18. The method as in claim 13 wherein the electrode comprises brass and the protective layer comprises copper oxide CuO or zinc oxide ZnO.

19. The method as in claim 13 wherein the layer-forming gas comprises a nitrogen-containing gas.

20. The method as in claim 19 wherein the nitrogen-containing gas comprises N2 or NH3.

21. The method as in claim 19 wherein the protective layer comprises a metal nitride.

22. The method as in claim 21 wherein the electrode comprises brass and the metal nitride comprises copper nitride or zinc nitride.

23. The method as in claim 13 wherein the layer- forming gas comprises a nitrogen-containing and oxygen-containing gas.

24. The method as in claim 13 wherein the electrode comprises brass and the protective layer comprises copper oxynitride or zinc oxynitride.