阅读说明：本技术 长寿命激光器腔室电极 (Long-life laser cavity electrode ) 是由 A·J·小埃芬贝格尔 于 2020-05-07 设计创作，主要内容包括：公开了一种用于在激光器腔室中的至少一个电极上创建保护层的装置和方法,其中层形成气体被添加到激光器腔室中,然后,电极用于在激光器腔室中生成等离子体,导致保护层的形成。(An apparatus and method for creating a protective layer on at least one electrode in a laser chamber is disclosed, wherein a layer forming gas is added to the laser chamber, and then the electrode is used to generate a plasma in the laser chamber, resulting in the formation of the protective layer.)

1. An apparatus, comprising:

i. a laser chamber;

an electrode located at least partially within the laser cavity;

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

a voltage source electrically connected to the electrode and configured to supply a 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 of claim 1, wherein the layer forming gas comprises an oxygen containing gas.

3. The apparatus of claim 2, wherein the oxygen-containing gas comprises O₂、H₂O or H₂O₂。

4. The apparatus of claim 2, wherein the oxygen-containing gas comprises O₃。

5. The apparatus of claim 2, wherein the oxygen-containing gas comprises nitrogen oxides or air.

6. The device of claim 1, wherein the protective layer comprises a metal oxide.

7. The apparatus of claim 1, wherein the electrode comprises brass and the protective layer comprises copper oxide (CuO) or zinc oxide (ZnO).

8. The apparatus of claim 1, wherein the layer forming gas comprises a nitrogen containing gas.

9. The apparatus of claim 8, whereinThe nitrogen-containing gas comprises N₂Or NH₃。

10. The device of claim 8, wherein the protective layer comprises a metal nitride, the electrode comprises brass, and the metal nitride is copper nitride or zinc nitride.

11. The apparatus of claim 1, wherein the layer forming gas comprises a nitrogen-containing and an oxygen-containing gas.

12. The apparatus of 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

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

14. The method of claim 13, wherein the layer forming gas comprises an oxygen containing gas.

15. The method of claim 14, wherein the oxygen-containing gas comprises O₂、H₂O or H₂O₂。

16. The method of claim 14, wherein the oxygen-containing gas comprises O₃。

17. The method of claim 14, wherein the oxygen-containing gas comprises nitrogen oxides or air.

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

19. The method of claim 13, wherein the layer forming gas comprises a nitrogen containing gas.

20. The method of claim 19, wherein the nitrogen-containing gas comprises N₂Or NH₃。

21. The method of claim 19, wherein the protective layer comprises a metal nitride.

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

23. The method of claim 13, wherein the layer forming gas comprises nitrogen and oxygen containing gas.

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

Technical Field

The subject matter of the present disclosure relates to laser-generated light sources, such as those used in integrated circuit lithographic fabrication processes.

Background

In laser discharge chambers such as ArF power loop amplifier excimer discharge chambers ("PRAs") or KrF excimer discharge chambers, electrode erosion severely limits the useful life of the chamber module. One measure to extend the service life of KrF excimer discharge chamber modules involves the use of wear resistant materials for the anode. For example, information regarding materials suitable for use as anode materials can be found in U.S. patent No.7,301,980, granted on 27/2007 and U.S. patent No.6,690,706, granted on 2/2004 on 10/2004, both assigned to the assignee of the present application and both incorporated herein by reference in their entirety.

Fluorine-containing plasmas are highly corrosive to metals and therefore can cause electrode erosion and corrosion during operation of the chamber. For example, nucleation and growth of localized areas of corrosion product build-up may occur on the surface of the anode. This results in non-uniform discharge between the electrodes and downstream arcing. Erosion results in an increase in discharge gap width and discharge spread. Both phenomena lead to a reduction in the energy density in the discharge, which in turn drives the need to increase the voltage difference across the electrodes necessary to maintain the energy output. In addition, the discharge spread reduces the gas flow clearance rate, resulting in increased downstream arcing, which results in energy loss and resultant dose errors. Once the dose error rate increases above a predetermined threshold, the chamber is deemed to have reached its useful life and must be replaced.

One or more metal oxide layers or metal oxynitride layers may be used as a protective layer for the electrode surface. For example, the formation of CuO or ZnO may protect the electrode material (e.g., brass) from fluorination. This is also true of the formation of metal oxynitrides, which have excellent compressive strength, flexural strength, fracture toughness, knoop hardness, and shear modulus, and are very resistant to fluorination. Having such a layer may improve the lifetime of the electrode. However, current techniques for producing metal oxides on electrodes involve heating the electrode in a furnace in an oxygen bath. These techniques cause the electrodes to warp, shrink and deform. In addition, they often result in the entire electrode being covered by a protective layer, which is undesirable because these methods are not in situ (in situ), and if the entire electrode is coated, it is difficult, if not impossible, to install the coated electrode in the chamber.

Disclosure of Invention

The following presents a simplified summary of one or more embodiments in order to provide a basic understanding of the invention. This summary is not an extensive overview of all contemplated embodiments, and is intended to neither identify key or critical elements of all embodiments nor delineate the scope of any or all embodiments. Its sole purpose is to present some concepts of one or more embodiments in a simplified form as a prelude to the more detailed description that is presented later.

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

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

Drawings

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

FIG. 1 depicts a schematic, not to scale, view of the overall generalized concept of a lithography system, according to aspects of the disclosed subject matter.

FIG. 2 illustrates a schematic, not to scale, view of the overall generalized concept of an illumination system, in accordance with an aspect of the disclosed subject matter.

Fig. 3 is a schematic, not to scale, cross-sectional view of a discharge chamber for an excimer laser in accordance with aspects of the disclosed subject matter.

Fig. 4 is a schematic, not to scale, cross-sectional view of a discharge chamber for an excimer laser in accordance with aspects of the disclosed subject matter.

Fig. 5 is a cross-sectional view of an electrode with a protective layer in accordance with aspects of the disclosed subject matter.

FIG. 6 is a flow chart illustrating a method in accordance with aspects of the disclosed subject matter.

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 should be noted that the present invention is not limited to the particular embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein.

Detailed Description

Various embodiments are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of one or more embodiments. However, in some or all examples, it should be apparent that any of the embodiments described below may be practiced without employing the specific design details described below. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate describing one or more embodiments. The following presents a simplified summary of one or more embodiments in order to provide a basic understanding of such embodiments. This summary is not an extensive overview of all contemplated embodiments, and is intended to neither identify key or critical elements of all embodiments nor delineate the scope of any or all embodiments.

FIG. 1 shows a lithography system 100 including an illumination system 105. As described more fully below, the illumination system 105 includes a light source that generates and directs a pulsed light beam 110 to a lithographic exposure apparatus or scanner 115, which lithographic exposure apparatus or scanner 115 patterns microelectronic features on a wafer 120. The wafer 120 is placed on a wafer stage 125, the wafer stage 125 being configured to hold the wafer 120 and connected to a positioner configured to accurately position the wafer 120 according to certain parameters.

The lithography system 100 uses a beam 110 having a wavelength in the Deep Ultraviolet (DUV) range, e.g., having a wavelength of 248 nanometers (nm) or 193 nm. The minimum size of microelectronic features that can be patterned on the wafer 120 depends on the wavelength of the beam 110, with shorter wavelengths resulting in smaller minimum feature sizes. For example, when the wavelength of the optical beam 110 is 248nm or 193nm, the minimum size of the microelectronic features may be 50nm or less, although other wavelengths of light and other minimum feature sizes may be produced according to other embodiments. The bandwidth of the light beam 110 may be the actual instantaneous bandwidth of its spectrum (or emission spectrum) that contains information about how the light energy of the light beam 110 is distributed over different wavelengths. The lithographic exposure apparatus or scanner 115 comprises an optical arrangement with, for example, one or more condenser lenses, a mask and an objective lens arrangement. The mask may be moved in one or more directions, such as along the optical axis of the beam 110 or in a plane perpendicular to the optical axis. The objective lens arrangement comprises a projection lens and enables image transfer from the mask to the photoresist on the wafer 120 to take place. The illumination system 105 adjusts the range of angles for which the beam 110 is incident (imping) on the mask. The illumination system 105 also homogenizes (homogenizes) the intensity distribution of the beam 110 on the mask.

The scanner 115 may include, among other features, a lithography controller 130, air conditioning equipment, and power supplies for 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 a process recipe). The process recipe or recipe determines the length of the exposure on the wafer 120 based on, for example, the mask used and other factors that affect the exposure. During photolithography, multiple pulses of the beam 110 irradiate the same area of the wafer 120 to constitute an irradiation dose.

The lithography system 100 may also advantageously include a control system 135. Generally, the control system 135 includes one or more devices in digital electronic circuitry, computer hardware, firmware, and software. The control system 135 also includes memory, which may 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 a CD-ROM disk.

The control system 135 may also include one or more input devices (such as a keyboard, touch screen, microphone, mouse, handheld input device, etc.) and one or more output devices (such as a speaker or 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 the 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, a processor receives instructions and data from a memory. Any of the above may be supplemented by, or incorporated in, specially designed ASICs (application-specific integrated circuits). The control system 135 can be centralized, or can be distributed partially or entirely throughout the lithography system 100.

Referring to FIG. 2, an exemplary illumination system 105 is a pulsed laser source that generates a pulsed laser beam as a beam 110. Fig. 2 schematically illustrates, in a block diagram, a gas discharge laser system in accordance with an embodiment of certain aspects of the disclosed subject matter. The gas discharge laser system may include, for example, a solid state or gas discharge seed laser system 140, an amplification stage (e.g., a power loop amplifier ("PRA") stage 145), relay optics 150, and a laser system output subsystem 160. The seed system 140 may include, for example, a master oscillator ("MO") chamber 165.

The seed laser system 140 may further include a master oscillator output coupler ("MO OC") 175, which master oscillator output coupler 175 may include: a partial mirror formed with a reflective grating (not shown) in line width narrowing module ("LNM") 170; an oscillator cavity in which the seed laser 140 oscillates to form a seed laser output pulse, i.e., 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 measurements and a coarser resolution grating spectrometer. MO wavefront engineering box ("WEB") 185 may be used to redirect the output of MO seed laser system 140 towards amplification stage 145 and may include, for example, beam expansion using, for example, a multi-prism beam expander (not shown) and coherent blanking, for example, in the form of an optical delay path (not shown).

The amplification stage 145 may comprise, for example, a PRA lasing chamber 200, which PRA lasing chamber 200 may also be an oscillator, for example, formed by seed beam injection and out-coupling optics (not shown) that may be incorporated into the PRA WEB210 and may be redirected back through the gain medium in the chamber 200 by the beam inverter 220. The PRA WEB210 may incorporate a partially reflective input/output coupler (not shown) and a maximum mirror for a nominal operating wavelength (e.g., at approximately 193nm for an ArF system) and one or more prisms.

A bandwidth analysis module ("BAM") 230 at the output of the amplification stage 145 may receive the output laser beam pulses from the amplification stage and pick up a portion of the beam for metrology purposes, e.g., to measure the output bandwidth and pulse energy. The laser output beam pulses then pass through an optical pulse stretcher ("OPuS") 240 and an output combining automatic shutter metering module ("CASMM") 250, which may also be the location of the pulse energy meter. One purpose of OPuS 240 may be, for example, to convert a single output laser pulse into a pulse train. The secondary pulses generated from the original single output pulse may be delayed with respect to each other. By distributing the original laser pulse energy into a series of secondary pulses, the effective pulse length of the laser can be extended while reducing the peak pulse intensity. Thus, the OPuS 240 may receive the laser beam from the PRA WEB210 via the BAM 230 and direct the output of the OPuS 240 to the CASMM 250.

The PRA lasing chamber 200 and MO 165 are configured as chambers in which a discharge between electrodes may cause a lasing gas discharge in a lasing gas to create inverted populations of high energy molecules (including, for example, Ar, Kr, and/or Xe) to produce relatively broad band radiation, which may be lines narrowed to a relatively very narrow bandwidth and a center wavelength selected in a line width narrowing module ("LNM") 170, as is known in the art.

Fig. 3 shows an arrangement for such a chamber 300, fig. 3 is a highly stylized cross-sectional view of a discharge chamber. The chamber 300 includes an upper electrode 310 serving as a cathode and a lower electrode 320 serving as an anode. One or both of the lower electrode 300 and the upper electrode 310 may be completely contained within the pressure envelope of the chamber 300 defined by the chamber walls 305, or one of the electrodes may not be so contained. A lasing gas discharge occurs between the two electrodes in the gap a. Fig. 3 also shows 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 walls 305 and lower electrode 320 at ground potential. In the embodiment shown in FIG. 3, the upper electrode 310 is driven by a voltage source 340 at a negative voltage relative to the lower electrode 320.

As mentioned, a voltage source 340 is also shown in fig. 3, the voltage source 340 establishing a voltage gradient across the cathode 310 and the anode 320. While the symbol (-) is shown for the polarity of the output of the voltage source 340, it is to be understood that this is the relative polarity and not the absolute polarity, that is, the lower electrode 320 is generally in electrical contact with the body of the chamber 300 and must be held at ground (0) potential relative to the polarity of the lower electrode 320. The upper electrode (cathode 310) is charged to a large (-20 kV) negative voltage.

The electrodes in chamber 300 are known to erode. According to embodiments in which, for example, ArF or KrF is used, the erosion may be the result of reaction of fluorine with the electrode material, or the erosion may be due to any of a variety of 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 ignited (strike) 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 the layer forming gas into the chamber 300. The inlet 400 is in fluid communication with a valve 410, the valve 410 operating under the control of a control unit 430 to selectively connect the inlet 400 to at least one layer forming gas source 420. Once the partial pressure of the layer forming gas in the chamber 300 reaches a desired value, a plasma is ignited in the chamber 300 by establishing an appropriate voltage difference between the electrodes 310, 320. After a predetermined interval (interval), the voltage difference is removed and the layer forming gas is evacuated, but the protective layer 510 has been formed on the electrodes 310, 320, as shown in fig. 5.

The plasma may be used to help grow metal oxides, metal nitrides, and metal oxynitrides on the surface. The discharge chamber is essentially a plasma source, so that under the appropriate conditions, the protective layer can be grown in situ. As an example, a layer forming gas comprising oxygen and/or nitrogen may be introduced into the chamber. The chamber would then operate in a manner similar to its normal operation to function as a laser chamber. The plasma creates a protective layer along with oxygen and/or nitrogen.

If it is desired that the protective layer is a metal oxide, the layer forming gas may be, for example, an oxygen-containing gas. Examples of the oxygen-containing gas include O₂、H₂O、H₂O₂、O₃Nitrogen Oxide (NO)_x) And air. If the protective layer is desired to be a nitride, the layer forming gas may be, for example, a nitrogen-containing gas. Examples of the nitrogen-containing gas include N₂、NH₃Nitrogen Oxide (NO)_x) And air. If it is desired that the protective layer is a metal oxynitride, the layer forming gas may be, for example, a gas containing nitrogen and oxygen or a mixture of nitrogen and oxygen. Examples of such gases include Nitrogen Oxides (NO)_x) A mixture of an oxygen-containing gas and a nitrogen-containing gas as mentioned above, and air. These are examples only, and it will be apparent to one of ordinary skill in the art that other gases may be used.

The concentration/pressure of the layer forming gas used to form the protective layer may advantageously be in the range of about parts per million level to about 38kPa or in the range of about parts per million to about 4 kPa. Since the total filling pressure of the chamber is in the order of 380kPa of laser gas, this corresponds to a concentration of layer forming gas in the order of a few parts per million to about 1%.

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

As shown in fig. 5, electrode 500 (which may be electrode 310 or electrode 320) is formed from a bulk material, such as brass, which is an alloy of copper and zinc. Due to the exposure to the plasma and the layer forming gas, a protective layer 510 will be formed on the exposed surface of the electrode 500. In general, the composition of the protective layer 510 depends on the electrode material and layer forming gas being used. The oxygen-containing gas may be used to form a protective layer 510 made of a mixture of CuO or ZnO on the brass electrode. The nitrogen-containing gas may be used to create copper nitride (Cu) using a brass electrode₃N) or zinc nitride (Zn)₃N₂) The protective layer 510. Nitrogen-and oxygen-containing gases may be used to create copper oxynitride (Cu) with a brass electrode_xO_yN_z) Or zinc oxynitride (Zn)_xO_yN_z) The protective layer 510. It will be apparent to those of ordinary skill in the art that other combinations are possible. The thickness of the protective layer 510 is typically on the order of nanometersOn the order of 10 microns.

In general, the thickness of the protective layer 510 is a function of the rate of formation and the time of formation. The rate of formation depends on the chemical composition of the layer formation and the characteristics of the plasma.

The protective layer formed over the electrode surface due to the in situ layer reduces erosion of the electrode. In many embodiments, the protective layer formed over the electrode surface due to in situ formation plays an important role in reducing the reaction of fluorine with the bulk material of the electrode. The denser and more uniform the protective layer, the lower the erosion rate.

FIG. 6 is a flow chart describing a process for in situ formation of a protective layer on an electrode in accordance with an aspect of an embodiment. In step S10, a layer forming gas is introduced into the chamber containing the electrodes to a desired partial pressure. In step S20, plasma is triggered 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 step S30, the plasma is quenched by removing the voltage. In step S40, the layer forming gas is evacuated from the chamber. Thereby, a protective layer is formed on the electrode.

These steps may be repeated periodically to re-grow the layers. Alternatively, the protective layer may be grown continuously by controlling the introduction of a dilute mixture comprising oxygen, nitrogen, or both.

One advantage of the process just described is that layer growth is limited to the discharge region of the electrode where the plasma is present, thus having better spatial control of layer growth. Furthermore, the heating of the electrodes is not different from the heating that the electrodes typically experience during normal operation of the chamber, so the likelihood of electrode deformation is less. The possibility of repeating the growth cycle at any time also contributes to the overall life of the electrode.

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

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

1. An apparatus, comprising:

a laser chamber;

an electrode at least partially located within the laser cavity;

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

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

2. The apparatus of clause 1, wherein the layer forming gas comprises an oxygen-containing gas.

3. The apparatus of clause 2, wherein the oxygen-containing gas comprises O₂。

4. The apparatus of clause 2, wherein the oxygen-containing gas comprises H₂O。

5. The apparatus of clause 2, wherein the oxygen-containing gas comprises H₂O₂。

6. The apparatus of clause 2, wherein the oxygen-containing gas comprises O₃。

7. The apparatus of clause 2, wherein the oxygen-containing gas comprises nitrogen oxides.

8. The apparatus of clause 2, wherein the oxygen-containing gas comprises air.

9. The device of any of clauses 1 to 8, wherein the protective layer comprises a metal oxide.

10. The apparatus of any of clauses 2-8, wherein the electrode comprises brass and the protective layer comprises copper oxide, CuO.

11. The device of any of clauses 2 to 8, wherein the electrode comprises brass and the protective layer comprises zinc oxide (ZnO).

12. The apparatus of clause 1, wherein the layer forming gas comprises a nitrogen containing gas.

13. The apparatus of clause 12, wherein the nitrogen-containing gas comprises N₂。

14. The apparatus of clause 12, wherein the nitrogen-containing gas comprises NH₃。

15. The apparatus of clause 12, wherein the nitrogen-containing gas comprises nitrogen oxides.

16. The apparatus of clause 12, wherein the nitrogen-containing gas comprises air.

17. The apparatus of any of clauses 12 to 16, wherein the protective layer comprises a metal nitride.

18. The apparatus of any of clauses 12 to 16, wherein the electrode comprises brass and the protective layer comprises copper nitride.

19. The apparatus of any of clauses 12 to 16, wherein the electrode comprises brass and the protective layer comprises zinc nitride.

20. The apparatus of clause 1, wherein the layer forming gas comprises a nitrogen-containing and an oxygen-containing gas.

21. The apparatus of clause 20, wherein the nitrogen-containing and oxygen-containing gas comprises nitrogen oxides.

22. The apparatus of clause 20, wherein the nitrogen-containing and oxygen-containing gas comprises air.

23. The apparatus of any of clauses 20 to 22, wherein the protective layer comprises a metal oxynitride.

24. The apparatus of any of clauses 20 to 22, wherein the electrode comprises brass and the protective layer comprises copper oxynitride.

25. The apparatus of any of clauses 20 to 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

generating a plasma within the laser discharge chamber using an electrode for a predetermined amount of time.

27. The method of clause 26, wherein the layer forming gas comprises an oxygen-containing gas.

28. The method of clause 27, wherein the oxygen-containing gas comprises O₂。

29. The method of clause 27, wherein the oxygen-containing gas comprises H₂O。

30. The method of clause 27, wherein the oxygen-containing gas comprises H₂O₂。

31. The method of clause 27, wherein the oxygen-containing gas comprises O₃。

32. The method of clause 27, wherein the oxygen-containing gas comprises nitrogen oxides.

33. The method of clause 27, wherein the oxygen-containing gas comprises air.

34. The method of any of clauses 26 to 33, wherein the protective layer comprises a metal oxide.

35. The method of any of clauses 26 to 33, wherein the electrode comprises brass and the protective layer comprises copper oxide, CuO.

36. The method of any of clauses 26 to 33, wherein the electrode comprises brass and the protective layer comprises zinc oxide (ZnO).

37. The method of clause 26, wherein the layer forming gas comprises a nitrogen containing gas.

38. The method of clause 37, wherein the nitrogen-containing gas comprises N₂。

39. The method of clause 37, wherein the nitrogen-containing gas comprises NH₃。

40. The method of clause 37, wherein the nitrogen-containing gas comprises nitrogen oxides.

41. The method of clause 37, wherein the nitrogen-containing gas comprises air.

42. The method of any of clauses 37 to 41, wherein the protective layer comprises a metal nitride.

43. The method of any of clauses 37 to 41, wherein the electrode comprises brass and the protective layer comprises copper nitride.

44. The method of any of clauses 37 to 41, wherein the electrode comprises brass and the protective layer comprises zinc nitride.

45. The method of clause 26, wherein the layer forming gas comprises a nitrogen-containing and an oxygen-containing gas.

46. The method of clause 26, wherein the nitrogen-containing and oxygen-containing gas comprises nitrogen oxides.

47. The method of clause 26, wherein the nitrogen-containing gas and oxygen-containing gas comprise air.

48. The method of any of clauses 45 to 47, wherein the protective layer comprises a metal oxynitride.

49. The method of any of clauses 45 to 47, wherein the electrode comprises brass and the protective layer comprises copper oxynitride.

50. The method of any of clauses 45 to 47, wherein the electrode comprises brass and the protective layer comprises zinc oxynitride.

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