JP3971382B2 - High repetition rate laser with improved electrode - Google Patents

Description

  The present invention relates to a discharge laser, and more particularly to a laser having a chamber with a long-life electrode. The present invention relates to U.S. Patent Application Serial No. 09 / 590,958 filed June 9, 2000, U.S. Patent Application Serial No. 09 / 590,961, filed June 9, 2000, November 2000. US patent application serial number 09 / 703,697 filed on 1 day, US patent application serial number 09 / 742,485 filed on December 20, 2000, US patent application serial filed on January 23, 2001 No. 09 / 768,753, U.S. Patent Application Serial No. 09 / 776,044 filed on Feb. 1, 2001, U.S. Patent Application Serial No. 09 / 953,026 filed on Sep. 13, 2001 US patent application serial number 10 / 081,589 filed February 21, 2002, and US patent application serial number filed March 22, 2002 Claims priority to No. 10 / 104,502.

KrF Excimer Laser The main components of a prior art KrF excimer laser system are shown in FIGS. This laser system is used as a light source for integrated circuit lithography. These components include a laser chamber housing 2. The housing is preceded by two electrodes 84 and 83, about 50 cm long and spaced by about 20 mm, and the next following pulse with a pulse repetition rate in the range of 1000 Hz to 4000 Hz or more. A fan 4 that circulates the laser gas between the electrodes at a rate fast enough to remove debris from the two pulses (from the discharge region between the two electrodes) and removes the heat applied to the laser gas by the fan and by the discharge between the electrodes. One or more water-cooled finned heat exchangers 6. The term “debris” is used herein to define any physical state of the gas after the laser pulse, which is different from the state of the gas before the pulse. The chamber can also include baffles and vanes that improve the aerodynamic shape of the chamber. The laser gas consists of a mixed gas of about 0.1 percent fluorine, about 1.0 percent krypton, and the remainder neon. As shown in the electrical circuit of FIG. 3, each pulse is generated by applying a very high voltage potential between the electrodes using a pulse power system 8, which results in a discharge (between the electrodes) that lasts about 30 nanoseconds. ) Produces a gain region about 20 mm high, about 3 mm wide and about 500 mm long. Each discharge drops about 2.5 J of energy into the gain region. As shown in FIG. 2, the lasing is generated in a resonant cavity, which is a grating-based narrow line that includes an output coupler 2A and a three-prism beam expander, a tuning mirror, and a Littrow-type array of diffraction gratings. Formed by a narrowing unit (shown largely unbalanced and also referred to as a narrowed package, or LNP) 2B. The energy of the output pulse 3 of this prior art KrF lithography laser is typically about 10 mJ.

  This KrF laser light source generates an ultraviolet light beam that has been narrow-band pulsed at a wavelength of about 248 nm. These lasers usually operate in a so-called “burst mode” consisting of pulse bursts with a pulse repetition rate in the range of about 1000 to 4000 Hz. Each burst consists of, for example, a pulse number of about 80 to 300 pulses, each burst being a wafer divided by bursts divided by a fraction of a second that the lithographic machine shifts the emitted beam between die sections. Irradiate the upper die section. Apart from this, there is a long downtime of a few seconds when a new wafer is loaded. Thus, during production, a 2000 Hz KrF excimer laser, for example, can operate at a current rate of about 30 percent. Operation is 24 hours a day, 7 days a week, 52 weeks a year. A laser operating at 2000 Hz with a 30 percent duty cycle and "24 hours a day" accumulates over 1.5 billion pulses every month. Any interruption in production can be extremely costly. For these reasons, prior art excimer lasers designed for the lithography industry are modular. In general, modules can be replaced within seconds so that downtime due to maintenance is minimized. The laser utilization of these lasers is generally over 99 percent.

  Maintaining the high quality laser beams produced by these lasers is currently required for lithography systems in which these laser sources are used to create integrated circuits with features less than 0.25 microns. Furthermore, since the characteristic dimension is getting smaller year by year, it is very important. As a result, the specifications set for the laser beam limit variations in individual pulse energy, variation in the integrated energy of a series of pulses, variation in laser wavelength, and the magnitude of the spectral bandwidth of the laser beam.

Prior Art Electrodes The prior art electrodes of the gas discharge laser described above are generally about 50 cm long, but can have a width of about 3 cm and a cross-sectional shape similar to 83 and 84 in FIG. The actual discharge between the electrodes generally needs to be a few mm wide (eg 3-4 mm), and this requirement determines the shape of the electrodes. The two electrodes shown generate a relatively high electrode field across a 3-4 mm wide region in the central region of both electrodes (referred to herein as the discharge footprint, or discharge surface), and the electrode spacing Produces a substantially rectangular discharge with a width of about 3-4 mm and a length of the discharge area of about 50 cm . One problem with these prior art electrodes is that the erosion of the approximately 3-4 mm discharge footprint port of both electrodes over billions of pulses causes changes in the cross-sectional shape of the electrodes, which in turn affects the discharge footprint. The discharge shape is no longer uniform because of the change in the electric field applied, and substantially expands, narrows, splits or distorts, thereby adversely affecting the laser beam quality and reducing the laser efficiency.

  Electrode designs have been proposed that are intended to minimize the effects of erosion by providing protruding portions on the electrode that have the same width as the discharge. Several embodiments are described in Japanese Patent No. 2631607. However, these designs are detrimental to gas flow when the protrusion is large, and eroded relatively quickly when the protrusion is small.

Other Lithographic Lasers Other gas discharge lasers used as lithography light sources, very similar to KrF lasers, are ArF (argon fluorine) laser and F ₂ (fluorine molecular laser). In the ArF laser, the active gas is mainly a mixed gas of argon and fluorine and neon as a buffer gas, and the wavelength of the output beam is in the range of about 193 nm. Although these ArF lasers are currently being used to a significant extent for integrated circuit manufacturing, the use of these lasers is expected to expand rapidly. In F ₂ lasers that are expected to be widely used in the future for integrated circuit manufacturing, the active gas is F ₂ and the buffer gas can be neon or helium, or a compound of neon and helium. All of these gas discharge lithography lasers utilize similar electrodes, but the spacing between them can be slightly different.

Japanese Patent No. 2631607

  What is needed is a gas discharge laser having electrodes that can withstand billions of discharges without adversely affecting the gas flow and without substantially adversely affecting the quality of the laser beam.

  The present invention provides a gas discharge laser having at least one long-lived elongated electrode for generating at least 12 billion high voltage discharges in a fluorine-containing laser gas. In a preferred embodiment, at least one of the electrodes comprises a first material having a relatively low anodic erosion rate and a second anodic material having a relatively high anodic erosion rate. The first anode material is disposed in the desired anode discharge region of the electrode. The second anode material is placed adjacent to the first anode material along at least two long sides of the first material. While the laser is operating, erosion occurs in both materials, but the higher erosion rate of the second material is sufficient to stop any expansion of the discharge in the second material. Ensuring that the second material erodes away as quickly as possible. In a preferred embodiment, the anode is as described above and the cathode is a two-material electrode having a first material in the discharge region that is C26000 brass and a second material that is C36000 brass. The pulse power system supplies electrical pulses at a repetition rate of at least 1 KHz. A blower circulates the laser gas between the electrodes at a speed of at least 5 m / s, and a heat exchanger is provided to remove the heat generated by the blower and the discharge.

In a preferred embodiment, the two-material electrode is the anode of a fluorine-containing gas discharge laser. The portion of the anode disposed on the discharge surface of the anode is comprised of an anode material containing lead along with other metals selected to produce a porous insulating layer that covers the discharge surface of the anode during operation. The layer is produced by fluorine ion sputtering of the anode surface, which in part produces an insulating layer made of lead fluoride together with other metal fluorides. In a particularly preferred embodiment, the anode is made of two parts and the second part has a generally shaped prior art anode with a groove-type cavity on the top surface. A material such as C26000 brass in this portion becomes eroded when exposed to a discharge in a gas environment of a normal discharge laser. A first portion of brass having a lead content greater than 3% is soldered into the groove and protrudes about 0.2 mm above the surface. The anode is attached to the laser. When exposed to a pulsed discharge in a fluorine-containing laser gas environment, an insulating layer made of porous lead fluoride is formed on the surface of the first portion to protect the anode from significant erosion. Applicants' computer electric field model has shown that this insulating layer does not significantly affect the electric field between the cathode and the anode. The overall electrode shape is such that there is no significant discharge from the second part at the start of operation by the electrode. As long as discharge occurs from the second portion, erosion occurs in the discharge region, and the height of the anode in the discharge region decreases. This erosion has the effect of reducing the discharge from the second portion . Approximately 50,000 small holes develop in the insulating layer on the first part, allowing electrons to flow freely back and forth on the metal surface of the anode. However, after the development of the insulating layer, fluorine ion sputtering on the metal surface of the anode is rather limited. Applicants believe that the reduction in fluorine ion sputtering is due to a decrease in the number of fluorine ions reaching the metal surface and a decrease in ion energy reaching the metal surface.

  Tests by the applicant have shown that a porous insulating layer that substantially covers all of the anode discharge surface does not significantly disturb the electric field between the electrodes and discharges over the lifetime of the chamber as compared to prior art anode designs. It is shown that it helps to control the discharge shape which makes the space more uniform. Improved uniformity in the discharge shape will greatly improve the quality of the laser pulse over the lifetime of the chamber. A better discharge shape also minimizes the adverse effects of acoustic disturbances in the chamber resulting from reflected sound waves from one pulse that reflects to the discharge area during the immediately following pulse.

  Embodiments of the present invention provide reduced burn-in time, extended operating life, and improved laser beam quality, as well as beam stability.

  Preferred embodiments of the present invention can be described with reference to the drawings.

Pulse Power Supply System The main components of an electric circuit 8 for supplying pulsed power for generating discharge in a gas discharge laser are shown in FIG. The pulsed power system operates with a standard 208 volt three-phase power supply. A power supply using rectifier 22, inverter 24, transformer 26, and rectifier 30 charges charging capacitor C ₀ 42 to a voltage level of approximately 500-1200 volts in accordance with instructions from a laser control processor (not shown). The laser control processor, when a pulse is required, to instruct the closing of the IGBT switches 46, thereby the energy of C ₀ is discharged to the subsequent portion of the pulse power system. The charge on C ₀ continues through inductor 48 to capacitor bank C ₁ 52, then through saturable inductor 54, through voltage transformer 56 to capacitor bank C _p-1 62, and then into saturable inductor 64. And is moved to peaking capacitor bank C _p 82. As shown in FIG. 3, the peaking capacitor bank C _p is electrically connected to the electrodes 84 and 83 in parallel.

FIG. 4A shows the potentials of capacitor banks C ₀ , C ₁ , C _p−1 and C _p as a function of time during the subsequent 9 microseconds starting with switch 42 closed. FIG. 4B shows an 800 nanosecond time slice immediately before and after discharge. Note that the peaking capacitor bank C _p is charged to approximately −15,000 volts just prior to discharge. The discharge lasts about 30 nanoseconds. During discharge, electrons first flow from the cathode 84 of the upper electrode to the anode 83 of the lower ground electrode. As all shown in FIG. 4B, the “overshoot” of the current charges C _p to a positive value of about +6,000 volts, at which point the downward flow of electrons is reversed, after which the last of the discharge For about 15 nanoseconds, electrons flow from the lower ground electrode to the upper electrode.

New Electrode The surface of a newly manufactured prior art brass electrode of the type shown in FIG. 1 is very smooth. However, when viewed under a high magnification microscope, the actual surface consists of a series of alternating ridges and valleys spanning the length of the electrode separated by about 1 to 2 microns, the bottom of which is the ridge. About 1 to 2 microns below the summit. The surface under the microscope looks like a long and narrow cultivated field as a result of machining operations.

Burned-in Electrodes A common prior art method for assembling a new laser system or reconstructing a laser chamber is to expose the chamber to a “burn-in” phase in which the chamber is operated with about 500 million pulses. is there. This takes about 72 hours at 2000 Hz. During this period, significant sputtering occurs on the discharge surface of each electrode. The discharge surface is about 3.5 mm wide and about 54.5 cm long on each electrode. Sputtering generated on the discharge surface of the electrode and the discharge between the electrodes considerably changes the electrode surface on the discharge portion of the surface. The “cultivated rows” are no longer seen after “burn-in”, but are mostly due to relatively irregularly spaced, shallow spot-like depressions, generally about 5 microns deep and about 3 to 10 microns wide. Replaced. These spot-like depressions or craters are placed on the cathode in close proximity (or slightly overlapping). Since these are generally somewhat further apart on the anode, they are about 4 times more per unit area on the cathode than on the anode.

Erosion The Applicant has found that electrode erosion occurs on both electrodes, but the erosion rate of the ground electrode (anode 83) is about four times that of the high voltage negative electrode (cathode 84). In almost all other gas discharge devices where electrode erosion is a problem, for example flash lamps, most of the erosion occurs at the cathode. Anodic erosion is not usually seen. When the laser is operated using a brass electrode, an insulating layer of metal fluoride is deposited very slowly on the anode part. The Applicant has found that the degree of fluoride formed is related to the lead content of the brass anode. For example, an anode made of C26,000 brass with a lead content of less than 1% does not produce a significant fluoride layer. However, an anode made of C36,000 brass with a lead content of 3 to 4% produces a relatively uniform fluoride layer covering the entire discharge surface with a thickness of about 100 to 200 microns. In the area covered by the fluoride layer, the discharge current flows through a small hole that tends to have a generally circular cross-section, generally having a diameter of about 20 to 150 microns. Surfaces covered with a fluoride layer will not receive any more significant erosion, but if the fluoride layer is not uniform, the erosion rate will be covered, especially if the uncovered surface area is reduced. Not on the discharge surface. Although there appears to be some erosion on the covered surface at the location of the stoma, this erosion may be at least an order of magnitude less than the erosion rate of the brass surface covered by the fluoride layer and two orders of magnitude less is there.

Erosion Rate In an embodiment of the present invention, the electrode is made of two different materials with different erosion rates. The material having a relatively low erosion rate is disposed at the position of the discharge surface of the electrode, which is a long and thin surface of about 3.5 mm × 545 mm, for example. The higher erosion rate material is placed along both the longer sides of the discharge area.

  In this specification and claims, when the applicant compares the erosion rates of two materials used for a particular electrode, this comparison is based on materials that are under equal conditions such as electric field and current. When a low erosion rate material is exposed to a higher electric field and discharge current than the low erosion rate material, the actual erosion rate over a period of time is higher for a lower erosion rate material than for a higher erosion rate material. Note that it will be higher. However, with the electrode design described herein, any larger erosion of the low erosion rate material will cause the electric field in the region of the low erosion rate material relative to the electric field of the surrounding high erosion rate material. Can be slightly reduced.

  However, no matter how the electric field pattern shifts from the desired electric field pattern towards the surrounding higher erosion rate material, it will increase the erosion rate of the surrounding material, which tends to restore the desired electric field pattern. It is in. Thus, the electrode design described herein results in significant erosion over billions of pulses without any significant change in the electric field pattern and discharge current distribution.

Sputtered metal ions In order to create a good laser active medium, a uniform discharge plasma must be created between the electrodes. First, the gas in the gap between the electrodes is preionized by the preionizer 12 shown in FIG. As the voltage rises on the electrode, ion sputtering generates a plasma in a region near the electrode surface. The metal atoms sputtered from the electrode are mostly in the vapor state, and a significant portion of the metal atoms are ionized, helping to form a positive ion cathode “falling” region in the immediate vicinity of the cathode surface, It creates an extremely large electric field that contributes to the flow of electrons from the cathode and accelerates the electrons leaving the cathode. This treatment is first applied to the cathode 84 during the first portion of each pulse. However, as shown in FIG. 4B, since the polarity of the electrode switches at approximately the center of the entire pulse, this effect also occurs at the anode 83 which functions as the cathode (ie, negative electrode) at that time. Both during and after the pulse, metal ions can be pulled back to the electrode in response to rapidly changing electric field conditions, but some of the released electrode material is carried across the gas flow boundary layer. In addition, most of the metal ions are blown away by the circulating laser gas. Applicants have found that when the anode is highly positively charged, significant sputtering of copper from the anode is generated by negative fluorine ions during the first part of each discharge.

Fluoride layers on brass electrodes The Applicant has conducted extensive testing on various electrode materials in an attempt to improve electrode life from 10 to 13 billion pulses. For brass electrodes, erosion at the discharge surface of the anode is always a major limiting factor in electrode life. The erosion changes the electrode shape from its optimal shape and consequently adversely affects the quality of the laser beam. Applicant's tests on these brass electrodes have shown that the longest lifetime is obtained when a material that produces a uniform and stable fluoride layer on the discharge surface of the anode is used. In particular, in one embodiment, an anode made of C36,000 brass (61.5% copper, 35.5% zinc, 3% lead) produces 13 billion pulses without any degradation in laser performance. did. (The typical useful operating life of these electrodes is about 5 to 6 billion pulses.) Testing of the anode after 13 billion pulses shows that a fluoride layer about 100 microns thick is about 2 cm long. It was revealed that the entire discharge surface was covered except for the region. This uncovered area faced the portion of the cathode that was heavily eroded. Applicants have found that the erosion of the cathode in the area where this erosion was significant produced a very high electric field, resulting in a very hot discharge that burned out the 2 cm missing portion of the anode, resulting in 13 billion pulses. I guess that led to the life of the cathode. The fluoride layer is mostly composed of copper and zinc fluoride, but appears to contain another material from the anode containing lead. The measured value of the electrical resistance of the layer by the applicant confirms that the layer is highly insulating, and the measured resistance value is infinite with a handheld ohmmeter.

  The insulating layer includes thousands of small holes with a width of about 20 to 150 microns that reach the bottom on the metal surface of the anode. These holes are arranged at intervals of about 20 to 30 per square mm on the discharge surface of the anode. The total number of 3.5 mm × 545 mm discharge surface holes is estimated by the Applicant to be about 50,000, which represents about 5% to 10% of the discharge area. The remaining 90% to 95% of the discharge area is made of an insulating dielectric material capable of repelling negatively charged fluorine ions by rapid accumulation of negative electron surface charges.

  Applicants have also tested several other types of brass, such as C26,000 brass (69.7% copper, 29.6% zinc, and less than 0.7% lead) to reduce low It has been confirmed that lead brass generally does not form a large fluoride layer in the discharge region of the anode. Applicant's conclusion is that a lead concentration greater than 1% is required to produce a stable fluoride layer on the anode.

First Preferred Embodiment A first preferred embodiment of the present invention is a gas discharge laser such as KrF, ArF, or F ₂ and has an elongated anode with a cross section shown in FIG. The anode consists of two types of brass, and the body 40 of the anode 83 is C26000 brass (having a lead content of less than 1%) with a length of 600 mm. This anode is a modification of the prior art anode widely used in these gas discharge lasers. The prior art anode 83 has a cross section as shown in FIG. The bottom width is 1.2 inches. The height to the center tip is 0.380 inches. The tip radius is 0.5 inches. The shoulder height from the bottom surface is 0.13 inches. The inclined surface is a flat surface having an angle of 27.67 degrees with respect to the bottom surface. Applicants have demonstrated that, over many years of laser operation, this common anode configuration provides excellent electric field characteristics and excellent discharge performance, along with very good laser gas flow compatibility. In the improved electrode shown in FIG. 6, a long groove type cavity is cut into the upper surface of the anode 83. The cavity is 545 mm long, 3 mm wide at the top, 2.5 mm deep, and 1.7 mm wide at the bottom. The cavity is made of C36,000 brass (having a lead content of about 3-4%), a second brass that is cut to fit exactly into the cavity and extends by about 0.2 mm above the surface Filled with portion 42. The second brass portion can be coupled to the cavity by lead / tin solder.

The anode is placed in a laser as shown in FIG. 1 using a laser gas consisting of, for example, 1% krypton, 0.1% F ₂ and the remainder neon. A porous fluoride layer comprising copper fluoride, zinc fluoride and lead fluoride is formed on the upper surface of the second brass portion 42 shown in FIG. 6 by operating a laser of approximately 500 million pulses. . This takes about 3 days at 2000 pulses per second. This evolving porous insulation layer reduces the erosion of the discharge surface and allows the anode to maintain this very suitable shape during billions of discharges. Electrons flow easily through about 50,000 small holes that develop within an area of about 1,855 mm ² (3.5 mm × 530 mm) of the lead fluoride layer. (This is about 30 holes per mm ^2. ) Meanwhile, individual fluorine ions, which are much heavier than electrons, pass through the hole to the underlying brass with sufficient energy to cause sputtering. The probability of doing is low. In one of the parent applications of this application, Applicants have enabled the present invention to at least double or triple the anode lifetime so that anode erosion no longer limits the lifetime of the laser chamber. I guess it will be. Subsequent proof tests by the applicant have confirmed these expectations. These tests are quite time consuming because the laser available for electrode testing can only produce about 2500 pulses per second. To accumulate 13 billion pulses at 2500 pulses per second, a test period of approximately 60 days is required. At the time of filing this application document, the laser chamber with this initial shape anode shown in FIG. 6 is more than 13.5 billion pulses without significant degradation in the quality of the laser pulses when removed for inspection. Accumulated. The aging of the prior art chamber with the prior art electrode as shown in FIG. 1 reduces the laser efficiency, gradually increases the fluorine concentration in the laser gas, or increases the standard discharge voltage to provide a constant It is necessary to maintain the pulse energy output. The standard method is to set the F ₂ concentration for optimum beam quality and increase the operating voltage to compensate for the reduced laser efficiency. The chamber reaches the end of life when the laser beam quality falls below acceptable levels or when the fluorine concentration and discharge voltage reach design limits.

  FIG. 6C is a graph representing the power supply voltage (approximately proportional to the discharge voltage) as a function of chamber life for the test chamber life of the first prototype of the anode of FIG. A similar graph for a similar chamber with a prior art electrode of the type shown in FIG. 1 is also shown in FIG. 6C. As shown in FIG. 6C, the lifetime of the prototype of FIG. 6 is already more than twice the expected lifetime of the prior art anode, and further, based on fluorine and voltage values, according to the applicant, the anode of this design Is expected to be able to continue excellent performance up to at least about 20 billion pulses. Applicants expect that the useful life of the anode itself can be well over 20 billion pulses, as the apparent efficiency drop in the graph of FIG.

  Reusing anodes taken from a chamber that has reached the end of life in a new chamber can be very suitable.

  During the electrode life test described above, the Applicant was unable to perform a detailed inspection of the electrode. However, Applicants periodically observed the electrode by removing the LNP and viewing the electrode through the chamber window. The electrode can be observed while it is discharged at 2500 Hz. Applicants' eyes are protected by a shield that transmits visible light and blocks ultraviolet light. The discharge is described as “beautiful” by the applicant and is perfect even after 13 billion pulses. Since the porous fluoride layer is initially formed with about 500 million pulses, the discharge surface appears to be essentially unchanged. The very shallow grooves of the C26000 brass portion are aligned as shown in FIG. 6A. A protective porous fluoride coating layered on the surface of portion 42 is shown at 42A in FIG. 6A. There is no insulating layer layered over the anode portion 40. Therefore, when the discharge is expanded to the portion 40, the discharge erodes the groove at the edge of the portion 40, and the discharge from this region stops. This limits the discharge to the anode portion 42.

FIG. 6B is a reproduction of a photograph showing a cross section of the discharge surface of the prototype anode described in detail above. This photo was taken immediately after the anode was removed from the chamber. The photograph shows a discharge surface with a width of 3.5 mm covered with an insulating surface of porous fluoride. In addition, the two solder joints can be clearly seen in the photograph. The photograph shows that some fluoride material is deposited on portion 1 downstream of the electrode. This deposition is very thin and does not affect the electrode performance.

Importance of lead The applicant's test is very advantageous if the copper-based electrode material contains a small amount of lead in order to produce a good and stable porous fluoride layer on the discharge surface of the anode. Prove that there is. C36000 brass is a three-phase alloy containing copper-zinc α-phase, β-phase and separated pure lead clusters. Lead atoms on the surface form fluoride by contact with fluorine in the laser gas. Applicants speculate that lead fluoride clusters form nucleation points where copper and zinc fluoride are deposited. Lead fluoride is a very stable compound and is more stable than copper fluoride and zinc fluoride. Although the Applicant has no adequate explanation as to why thousands of small, nearly circular holes grow and survive, these holes clearly grow and survive, significantly reducing anode erosion. In this state, approximately 2.5 Joules of electrical energy can flow through the hole with each of the billions of pulses.

  Applicants have shown that superior performance is achieved with about 3-4% lead. A lead content less than 1% does not produce a stable fluoride layer. The applicant expects that good results will be obtained up to a lead content of up to about 8%, but at present there is no valid test data to support this expectation.

Second Preferred Embodiment In the second preferred embodiment of the present invention, the anode is as described above, and the cathode is also the first cathode with low cathode erosion rate that is placed in the desired cathode discharge region. It consists of two materials: a material and a second cathode material with a relatively high cathode erosion rate disposed along two long sides of the first cathode material. Applicants have concluded from years of experimentation with brass electrodes that C36000 brass erodes about twice as fast as C26000 brass when used as the cathode electrode in a fluorine-containing gas discharge laser. . Cross sections of the cathode and anode of this embodiment are shown in FIGS. 7C and 7D, respectively. This is in comparison to the single material cathode and anode design shown in FIGS. 7A and 7B, pre-initialized in a laser chamber designed and constructed by the applicant and its collaborators.

  In the cathode, the first material 90 placed in the desired cathode discharge area is C26000 brass and the second material 92 forms the rest of the electrode. When used as a cathode, none of these brasses form the porous insulating layer described above, however, C26000 brass erodes at about half the rate of C36000 brass. Therefore, whatever the tendency of the discharge to expand on the C36000 portion of the cathode, the C36000 brass is rapidly eroded away in the expanded region and the expansion stops. As explained above, the first material 42 at the discharge region relative to the anode is C36000 brass and the remaining portion 40 of the anode is C26000 brass.

Types of Annealing Applicants have concluded through experimentation that annealing of the brass electrode material has a significant effect on the cathodic erosion rate. In general, Applicants have found that the erosion rate is approximately proportional to grain size over a wide range of grain sizes. Annealing reduces grain size, so annealing the material can reduce cathodic erosion. Thus, another cathode design can utilize annealed brass as the first material 90 and unannealed brass as the second material 92. For example, the grain size of the second material is reduced to about one-fourth of the grain size of the first material so that the first material is 54 microns and the second material is 13 microns. It is preferable to perform annealing.

Discharged surface anodized layer A cross section of the anode of the second preferred embodiment is shown in FIG. 7E . A porous layer of insulating material 46 is placed over the discharge surface of the prior art anode before the anode is incorporated into the laser chamber. The base 44 anode is C26000 brass containing less than 1% lead. Therefore, the insulating layer is not generated by any discharge that expands beyond the critical porous layer 46 as described above, and in fact, any discharge that expands in that region tends to erode C26000 brass. The discharge is terminated in the region, and the discharge is stopped within the porous insulating surface 46. A preferred porous insulating surface can be provided using an anodization treatment, as described with respect to the next two embodiments.

  Porous oxide grows on aluminum in a process called anodization. High purity aluminum foil is deposited on the brass electrode. The brass electrode functions as the anode of the electrochemical cell. In general, the purpose of anodic oxidation is to produce a uniform protective alumina film on the anode. Using the appropriate electrolyte and operating voltage, etching results in the creation of a self-organized porous structure. Holes having a diameter of 10 to several hundred microns can be created by changing the anodization parameters. The thickness of the layer can be up to several hundred microns, but a thickness of about 100 to 1000 microns is preferred in this application. This layer can be used as layer 46 as shown in FIG. 7E. Another method is to use aluminum as the electrode base material instead of brass. This simplifies the anodization process.

Test Results Using Anodized Strips Applicant's experiment using alumina anodized on brass electrode shows that in a high voltage discharge environment with F ₂ gas, oxygen in the anodized aluminum is C36000 brass. It shows that a porous layer similar to the porous layer formed on the electrode can be replaced. The Applicant is therefore convinced that this anodized aluminum surface produces an excellent long-life discharge surface material. The material on either side of the 3.5 mm wide discharge region is preferably not anodized to erode in the discharge region that is intended to spread beyond the desired 3.5 mm.
Since it is well known to the Applicant that the aluminum exposed as the anode of the laser erodes at about twice the erosion rate of the C26000 brass anode, an aluminum electrode treated with a 3.5 mm wide anodized strip is excellent. An inexpensive anode should be formed.

C36000 Brass Insert Porous Alumina In a third preferred embodiment, as shown in FIG. 9, the top surface 44 of the portion 42 that is C36000 brass shown in FIG. 6 is in the cavity of the portion 40 where the portion 42 is C26000 brass. Before being soldered, it is coated with porous alumina. In this embodiment, C36000 brass forms a protective fluoride layer in cross section when the porous alumina is eroded away.

Plasma Spray Composite Coating In another preferred embodiment, a protective coating is sprayed onto the anode in the discharge area. A preferred method is to use a standard spray torch (available from Hobart Torch, Inc.) to provide the alumina coating. It is preferable to mix conductive metal particles with 99% alumina. The metal proportion is preferably between 5% and 50%. A ratio of about 25% is recommended. The metal powder may be Cu, Ni, Al, PA, or Mg. This method can also be used to coat the discharge surface of the cathode. A preferred base electrode material is C36000 or C26000 brass.

Another method is to texture the discharge surface of a prior art brass electrode, then plasma spray the surface with an insulating coating such as alumina, and then polish the coating enough to expose the underlying high portion of brass. It is to be. Texturing can be any of machining, knurling, or spraying. The coating may be CVD, Al ₂ O ₃ , AIN, MgO, MgF or CaF.

Micro-insulating particles In a fourth preferred embodiment, the micro-insulating particles 66, which are approximately sand-grained in the size of about 100 to 300 microns, are made of a prior art brass electrode having the cross-sectional shape shown in FIGS. 10A and 10B. Brazed to discharge surface. In this embodiment, the width of the discharge is 3.5 mm. The particles preferably cover about 95% of the surface area of the discharge surface, as shown in FIG.

In a similar embodiment, micro-insulating particles such as Al ₂ O ₃ are mixed with molten brass such as C26000 brass, and the mixture is formed into the shape of portion 42 as shown in FIG. The resulting part is machined to fit exactly into a part, such as part 40 shown in FIG. Preferably, the particle size is 20 to 150 microns, and the particles should make up about 80-90% of the volume of the mixture. After several days of operation, the surface brass will sputter away, leaving an insulating layer on the surface, but the material will remain conductive below the surface. 8A and 8B are diagrams showing the electrode surface after the brass on the surface has been sputtered away. The particles must be stable at the melting point of brass and resistant to fluorine chemical reactions. Good choices are Al ₂ O ₃ , CaF ₂ and MgF ₂ . The composite material shown in FIG. 8A can also be formed using powder metallurgy techniques such as hot pressing or cold pressing.

Other Fluoride Layers As explained above, Applicants have produced anodes (with C26000 brass and C36000 brass discharge inserts) that provide a surprisingly long discharge laser lifetime. This embodiment increases the lifetime of the anode so that the lifetime of the anode no longer affects the chamber lifetime. Currently, other components such as blower bearings affect chamber life. In the future, if these other components are modified to extend their lifetime, further improvements in anode lifetime may be required. It is difficult to prove whether a change in electrode design improves lifetime, since the only reliable evidence of improved lifetime is a lifetime test that requires several months of expensive laser operation. This is an expensive operation.

  Applicants believe that there may be alloys other than the combination of C26000 and C36000 that produce anodes that are even better than those described above. It is also possible to create a better porous insulation layer using a different gas mixture instead of the laser working gas mixture in which the electrodes are used. Thus, embodiments of the present invention are specific techniques for generating passivation in gas discharge laser electrodes.

Electrode passivation layer technology The fifth embodiment of the present invention requires a special chamber configuration for electrode passivation. Preferably this chamber can be a laser chamber specially adapted, used or modified to form an electrode with a passivation layer. Alternatively, larger chambers can be provided with means to simultaneously passivate several electrodes. In order to determine whether there are better alloy combinations than those disclosed above, it is necessary to experiment with alloys that combine various elements. For example, combinations of brass alloys with various contents of copper, zinc and lead need to be tested. Other elements such as tin also need to be tested. In one embodiment, a single electrode comprising several segments, each having a different alloy combination, can be tested to determine which produces the best passivation layer. By adjusting the electrode composition, microstructure (Pb separation), chamber fluorine concentration, electrode potential, and current density, the growth rate, thickness, and porosity of the passivation coating can be manipulated. Passivation can also be performed in and out of the laser chamber using custom equipment or the like. In the past, porous fluoride insulation layers were sometimes formed or not formed. By creating individual alloy compositions, the experimenter can promote the growth of fluoride layers in a standard way. This is possible by adjusting both the metallurgical factors and the material composition (when the current conditions are constant). So far, Applicants' test data has shown that copper, zinc, and lead are important for formation, resulting in a passivated "reef" structure formed during fluorine erosion. . By increasing the lead content of the alloy, reef formation can be promoted. This may be due to the increased number of nucleation points for PbF ₂ growth, which the applicant believes is the mechanism of reef nucleation. Zinc does not form gas phase by-products when eroded by fluorine, and therefore may play a role in increasing reef volume, but preferentially fluorinates over copper. Chemical analysis of the reef reveals that the reef consists mostly of copper and zinc. More specifically, it is a nucleation point of CuF ₂ , ZnF ₂ , and PbF ₄ . Since tin forms many stable gas phase fluorides, it is possible to adjust the porosity (electrical impedance) of the reef, possibly by changing the tin content of the original alloy. Furthermore, reef formation kinetics can be altered by changing the structure of the metal particles through annealing. Applicants have shown that during annealing, Pb can separate in high lead copper alloys and produce large nucleation points in the reef. It is said that there is an interaction with the grain size of the original material, the Pb content, and the annealing state during the growth of the passivation reef. By using the statistical optimization software package, the volume, porosity, and surface coverage of the anode passivation reef can be optimized. The trade-off here is erosion protection against the electrical impedance of the passivation layer. In addition, this impedance can have a strong impact on reef growth, as fluorine movement through the cover regulates the growth rate at least during the early stages of reef formation. Applicants have observed this in experiments and recognize that current density (higher is better) also affects reef formation.

The variables for plasma anodization include:
A. Lead content: reef thickness, nucleation / effective range, and morphology (roughness)
B. Tin content: Reef porosity C.I. Zinc content: reef thickness and morphology Copper content: Reef thickness E. Grain size of the original material: reef morphology, nucleation / range of coverage I. Ion current: The higher the reef, the thicker the reef. System voltage Improvement of photon generation and fluoride generation F2 concentration: The relationship needs to be determined. Original metal surface roughness (nucleation depends on absorption F2)

Use of Alumina to Promote Reef Formation Applicants have shown that the pattern of alumina deposited on the discharge surface of the anode promotes the growth of excellent porous passivation. Applicants are confident that alumina exhibits a nucleation point at which the COF ₂ layer grows. A preferred method of applying an alumina pattern on the discharge surface is as follows. The discharge surface (width: about 3.5 mm) of the copper electrode (C11000) was knurled to produce a regular parallelogram knurl pattern that was slightly raised (about 1.5 mm).

  Next, the surface was coated with alumina by plasma spraying. The discharge surface was then polished to expose about 90% of the underlying copper, leaving about 10% of the parallelogram patterned surface coated with alumina. The electrode was then operated with a KrF laser to form an excellent porous copper fluoride layer over the discharge surface that produced a well-patterned discharge during approximately 2 days of operation (200 million pulses). . Applicants speculate that the lifetime of electrodes passivated by this process has a very long lifetime exceeding 10 billion pulses.

In addition to alumina, other materials such as CaF2 or MgO may also be used in place of Al ₂ O _3.

Plasma Electrode FIGS. 13A, 13B, 13C, and 14 show an embodiment of the present invention in which the plasma on the insulator surface functions as an electrode discharge surface. Such electrodes are expected to exhibit a virtually infinite lifetime in these laser chambers. 13A, 13B, and 13C show a plasma cathode. Both the cathode unit 84B and the anode unit 83B have the general electrode shape shown in FIGS. 7A and 7B. However, the cathode 84B is longitudinally divided into two parts 84B1 and 84B2, separated by an insulator 87, preferably alumina. Portion 84B1 is connected to the high voltage side of capacitor bank Cp ₁ which supplies a 30 nanosecond electrical pulse with a peak potential of up to about 24,000 volts during normal discharge. As the voltage of Cp ₁ is increased, a plasma arc (also known as corona discharge) is formed under the cathodes 84B, as shown in FIG. 13B, the flow of electrons through the plasma arc, about the Cp ₁ capacitor The capacitor bank Cp ₂ having a small capacitance of 10% will be charged. When the potential of the cathode 84B reaches a discharge potential in the range of about 20,000 volts, the discharge shown in FIG. 13C (between the cathode 84B and the anode 83B) starts, and both Cp ₁ and Cp ₂ are discharged as shown in FIG. 13C. Continue until In this embodiment, the anode is of the type described above and shown in FIG. 7D.

  FIG. 14 shows an embodiment in which both cathode 84B and anode 83C are designed as plasma electrodes. These have the electrode shapes shown in FIGS. 7A and 7B, each having an alumina discharge surface. In this case, pulse transformer 56A is substantially the same as that shown at 56 in FIG. 3 so that a negative high voltage is applied to cathode portion 84B1 and a positive high voltage is applied to 83C1. has been edited.

Plasma is generated on the surface of both electrodes, and as described above, the discharge between the cathode and the anode occurs when a sufficient potential difference occurs between the two electrodes.
In both of these embodiments, a preionizer such as that shown at 12 in FIG. 1 is not required because the plasma on the electrode surface generates sufficient ultraviolet radiation to preionize the discharge region between the electrodes. . Note that special capacitor banks Cp ₂ and Cp ₃ are not required because portions 84B2 and 83C2 provide sufficient self-capacitance to generate plasma on the two electrode surfaces.

  FIG. 15 shows another concept of the plasma electrode. Here, the cathode 84D is grounded, and the anode is energized with a high voltage positive pulse. The insulator portion 12B has a grounding rod 81B inside and is disposed in the discharge region of the anode. While each electrical pulse is applied, plasma accumulates on the surface of the insulator portion 81B, and electrons can flow from the insulator portion of the cathode to the anode during discharge. Since the corona region growing on the surface of the insulator 12B produces ultraviolet radiation, a separate preionizer is not necessary.

Blade Electrode with Porous Dielectric Discharge Surface and Dielectric Flow Spacer FIG. 7F shows another long-life electrode embodiment. In this case, the electrode configuration consists of a conductive element having a cross section in the shape of a blunt blade 10A3 and a flow-type dielectric spacer 10A5 arranged on both sides of the conductive element. The discharge surface of the blade element 10A3 is preferably covered with a porous insulating layer. This layer can be generated “in place” by selecting a material for the blade element 10A3 (such as C36000 brass) that produces a porous insulating layer in the presence of a discharge in the F ₂ laser gas. Alternatively, the porous insulating layer can be applied onto the blade element using one of the methods described above, such as anodizing. A variation on the design of FIG. 7F is to extend the sides of the spacer as shown by dashed line 10A3 in FIG. 7F. This further improves the gas flow in the discharge area.

In one variation of this design, a relatively thick insulating layer is applied to the discharge surface of the metal electrode, and many small holes are drilled through the insulating layer to the conductive metal under the insulating layer. This method can be used for the anode and / or the cathode. In the electrode of FIG. 7F, insulating spacers are placed along the sides of the discharge surface, but another method uses a conventional electrode as shown in FIG. 1 to cover the entire surface of the metal electrode, Make holes only on the discharge surface. As another modification, the general shape shown in FIG. 1 is used, but a small hole is formed in the insulating surface after applying the insulating layer only to the discharge surface. It is preferable to make a hole with an excimer laser. No matter how the discharge expands on the adjacent uncoated metal surface, the metal in the adjacent region is gradually eroded away, resulting in the stop of the discharge expansion. Preferred hole major cross-sectional dimensions (such as the diameter of a generally circular hole) range from a few microns, such as from about 10 microns to about 150 microns, with the optimum range being from about 30 to 80 microns. If the hole is too large, the fluoride or electron flow through the hole will become congested, resulting in plugging of the hole, resulting in excessive concentration of erosion and hot spots. If the hole is too small, there will be insufficient current through the hole. Holes in the range of about 200 microns appear to grow a microscopic volcanic appearance due to unwanted excess fluoride accumulated around the holes in some cases. A preferred hole spacing in the discharge area is about 5 to 50 per mm ² . The proportion of the surface covering the discharge area composed of holes is about 5 to 10%.

Long-life electrode design that reduces acoustic effects Each discharge of a laser generates a shock wave that travels at nearly the speed of sound through the laser gas. At a repetition rate of 4000 Hz, the time between pulses is 0.25 milliseconds. The shock wave travels about 5.8 cm during this time interval. The Applicant has found that it is important to remove as much as possible the reflective surfaces arranged at a distance equal to one-half of the distance traveled by the shock wave between the pulses. This is due to the fact that shock waves that reflect to the discharge region within 30 to 50 nanoseconds of the next discharge from one discharge typically adversely affect the quality of the resulting beam. It is impossible to completely remove all the impact reflecting surfaces. Accordingly, Applicants have developed several methods that minimize the adverse effects of reflected shock waves. One way is to minimize the symmetry associated with shock waves reflected in the longitudinal direction of the laser. This means that the shock wave forward of the laser is reflected to the discharge region at a different time than the reflected wave behind the chamber. Another method is to design the reflective surface to disperse shock waves at various angles with respect to the longitudinal direction of the electrode.

  Another method is to provide a discharge shape that varies in the longitudinal direction of the discharge region. For example, FIG. 7G shows the proposed discharge surface for each of the cathode and anode. Arrow 7G1 indicates the discharge shape at the first position in the discharge region, and arrow 7G2 indicates the second discharge shape in the second discharge region. Thus, the reflected shock waves resulting from each discharge will eventually be dispersed, reducing the impact on beam quality. A similar method is to slightly offset the anode with respect to the cathode so as to produce a discharge deviated by a slight angle, such as about 5 to 10 degrees from the vertical direction. This also has the effect of reducing the influence of reflected sound waves.

Solution to Excessive Erosion of Electrode End Applicants have found that the life of most electrode sets is due to electrode erosion occurring over 2 inches at one or both ends of the electrode set. Excessive erosion usually occurs at both the anode and cathode. The Applicant believes that this higher than average erosion is caused, at least in part, by a slightly higher electric field than the average at the electrode ends. Another factor that can result in a relatively high erosion rate at the electrode ends is that the circulating gas velocity between the electrodes at the electrode ends is slightly lower than in the central portion.

  Applicants have made several laser chamber improvements to improve this situation. One solution is to provide an additional ramp at the end of the electrode. Applicants' previous designs have used a 0.75 inch radius at the end of the electrode. A gentle slope starting about 2 inches from the end of the electrode reduces the electric field at the end.

Another solution to this problem is to change the orientation of one or both electrodes at both ends, as shown in FIG. 16B, so that the spacing between the electrodes in the discharge region remains constant with respect to the ends. In one embodiment, one of the electrodes has the shape shown in FIG. 16B and the other has a standard shape as shown in FIG. 16A. In another embodiment, both ends of both electrodes are oriented as shown in FIG. 16B. The cathode and anode are preferably oriented in opposite directions at each end of the chamber. Yet another solution to the end erosion problem is to remove one or more current feedback device “ribs” at each end of the electrode. In the prior art design, current feedback device ribs (generally shaped like whale bones) as shown at 10A8 in FIG. 7F are evenly arranged along the entire length of the electrode. In one prior art design, the current feedback device structure consisted of 27 ribs spaced 1 inch apart. Applicants have cut away ribs 2, 3, and 4, and 24, 25, and 26 in the end region of the current feedback device structure. This is expected to provide a significant improvement in the energy distribution in the discharge region and greatly extend the electrode life. Similar results could be obtained by removing a through rod that conducted current through the insulator to the cathode 84 from the pulse power system as shown in FIG. The laser typical prior art, are arranged in _^1/2-inch intervals along the through rod length of the cathode of about 15. A preferred way to reduce excessive end erosion is to reduce the number of through rods at each end by one to three. A second advantage of reducing through rods in these end regions is that the seals associated with these rods tend to leak in some cases due to thermal expansion differences between the insulator and the top wall of the chamber. That is.

Tungsten-based electrodes with cold traps Tungsten is known as an excellent electrode material for numerous applications, but has previously been avoided as an electrode for fluorine-containing gas discharge lasers. This is because tungsten and fluorine combine to form a gas called WF ₆ , which absorbs laser energy. Also, tungsten can plate the chamber window due to photodissociation of WF ₆ .

Embodiments of the present invention use tungsten, tungsten alloys, or tungsten composites for the electrodes. For example, the portion 42 shown in FIG. 6 may be a very low erosion rate tungsten composite. In order to eliminate or minimize the adverse effects of WF ₆ , Applicants have added a WF ₆ purification loop to the basic laser chamber design shown in FIG. The prior art chamber extracts a small percentage of the circulating gas stream that passes through an electrostatic filter (not shown) on the high pressure side of the blower (portion 56), where the metal fluoride particles are removed. The purified gas flows from the two sides of the filter to the housings of both laser chamber windows and slightly pressurizes the window area (relative to the discharge area) with clean gas, thereby removing the gas containing debris. Keep away from the window. (For details, see US Pat. No. 5,018,162, incorporated herein by reference.) A cooling trap is preferably disposed within the filter loop. Only a small portion (about 5 to 10%) of the filter stream is preferably directed to the cold trap and cooled to the liquid nitrogen temperature. WF ₆ is condensed at a temperature of about 17 ° C. and is thereby completely removed from the portion of the gas stream that passes through the filter. A heater is preferably included in the cold trap so that WF ₆ can be vaporized and removed from the chamber during gas exchange.

Improved Preionizer Typically, prior art preionizers used for gas discharge lasers of the type described above are basically one or two cylindrical alumina tubes (only one is used in the embodiment of FIG. 2). [ Shown in FIG. 17 ]), comprising a grounding rod disposed along the axis of the tube (or tubes). A thin flexible conductor called a shim (not shown) that contacts the cathode 84 and is at the same potential as the cathode presses the outer surface of the cylindrical tube. Near the beginning of each pulse, a corona discharge occurs along the outer surface of the preionization tube, which generates ultraviolet light that pre-ionizes the discharge region, producing a stable and predictable discharge between the electrodes. To date, the applicant has experienced several problems with poor contact between the shim and the tube surface.

  In an embodiment of the invention, a conductive coating such as Pt, Cu, Ni, or An is permanently applied to a narrow line (about 1 mm wide) along the length of about 50 cm of the active portion of the preionization tube. A flexible conductive shim that is rigidly connected to the cathode presses the surface of the preionization tube against a thin conductive coating. The shim is preferably divided into segments approximately 2 to 3 inches in length to improve the mechanical contact between the shim and the conductive coating.

FIG. 17 shows an apparatus comprising a cathode 83E, a preionizer 12, a grounding rod 12A, an alumina tube 12B, a conductive laser 12C, a flexible conductive shim 12D, and a main insulator 13. The preionization tube is attached in place by three positioning members (not shown here), as described in US Pat. No. 5,771,258, incorporated herein by reference.
In another design, conductor portion 12D is made as part of cathode 83E during machining of cathode 83E such that cathode 83E and conductor 12D are an integral part. The shim can be machined extremely thin (particularly near the edge that contacts the preionization tube 12) to provide some flexibility. Alternatively, the edge of the conductor portion 12D can be machined into a concave shape to match the surface of the preionization tube 12. In this case, it is preferable that the flexible force is applied to the opposite side of the preliminary ionization tube 12, and the tube 12 is held in a state of being pressed against the conductor portion 12D.

Gas Protected from Anode-F ₂ Another embodiment of the present invention that provides a long-life anode is shown in FIG. In this case, the anode is made of a porous sintered metal such as C26000 brass. In a KrF laser, a mixture of 1% krypton and 99% neon is emitted from the central anode so that the discharge surface is always protected by a gas layer without F ₂ . This prevents anodic fluorine sputtering. Applicants have concluded that an F ₂ -free layer with a thickness of about ¼ micron is sufficient to prevent fluorine sputtering. The addition of small quantities of krypton and neon, resulted in a decrease in F ₂ concentration, which adds a laser gas containing a relatively large amount of _{F 2 (1.0% F 2,} 99% Kr, etc. 1.0% Ne) that it is necessary to compensate, the existing gas control because already available to compensate for the loss of F ₂ through F ₂ chemical reactions in the chamber, thereby does not occur any problem.
Also, as in the above embodiment, portion 42 can be made from sintered C36000 brass and the remainder of the electrode can be made from C26000 brass. Therefore, any exposed portion of portion 42 improves the insulating fluoride level. In another design similar to this design, the porous sintered brass is used in the portion 42 shown in FIG. 6A and a gas flow without F ₂ is supplied to the portion 42.

Flow Shaping In these gas discharge lasers, it is necessary to provide sufficient laser gas circulation to remove substantially all of the debris generated during the discharge from the discharge prior to the next subsequent pulse. Currently manufactured lasers operate at a pulse repetition rate of 4000 Hz, which means that a discharge area of about 4 mm in width must be cleaned during 1/4000 seconds (0.25 milliseconds) between pulses. It means not to be. This requires an interelectrode gas velocity of at least 16 m / sec (about 58 km / h). Future plans include lasers from 6,000 Hz to 10,000 Hz. These speeds in the range of 100 km / h require discharge areas that are highly aerodynamically designed. FIG. 12A shows a design with improved flow shaping, with the discharge region components slightly modified to provide a substantial improvement in aerodynamic parameters. In this case, the preionizer 12A has a non-cylindrical shape, and the grounding rod 12A1 is arranged to promote the accumulation of electrons on the lower surface. FIG. 12B shows another aerodynamic design, where a preionizer is incorporated into the main insulator to greatly improve aerodynamics. The ground bar 81B is inserted into the main insulator 82 and runs in parallel along the entire length of the electrode. Preionization is caused by high-energy electrons that track from the base of cathode 84A along the surface of insulator 82 at the beginning of the pulse and attempt to reach ground bar 81B. The tracking electrons and associated plasma generate high energy ultraviolet photons, which ionize the gas in the discharge region to promote early discharge in the electrical pulse period.

Current Feedback Device In another preferred embodiment, a laser current feedback device is made in the shape shown in FIG. In this case, the central portion 76 of the current feedback device has a cross section similar to the prior art anode cross section so as to generate a very high electric field along the center of the structure. This very high electric field is about 3.5 mm wide and defines a discharge area that is about 3.5 mm wide, and this electric field rapidly decreases on both sides of the discharge area. That is, the anode generates a high electric field over a width of about 3.5 mm that forms a discharge region of the anode along the center line of the anode, and has a sharp drop in the electric field on both sides of the anode discharge region. Having a cross section selected. The porous insulating layer 78 is generated so as to cover the discharge region. This layer can be generated using any of the methods described above. For example, the current feedback device can be machined from C26000 brass with a C36000 insert in the discharge region shown in FIG. 6A. In a preferred embodiment, the current feedback device has about 40 whale skeletons 80 on each side. The top of the current feedback device is bolted to the top of the chamber and the electrode portion can be bolted to a rigid electrode support. With respect to other embodiments of the invention, the current feedback device material is selected such that the material on either side of the 3.5 mm wide discharge region is a material that erodes faster than the material that forms the discharge surface.

Porous coating of the cathode discharge surface So far, the cathode erosion of these gas discharge lasers has not been a problem because the anode has been eroded at a rate about 4 times the rate of cathode erosion. In a preferred embodiment, the cathode discharge surface is also covered with a porous insulating material. It should be understood that the lead fluoride layer does not spontaneously develop on the cathode because the cathode repels negatively charged fluorine ions during the major portion of the discharge pulse time. However, it is possible to produce a cathode coated in an F ₂ environment with a cathode operating as an anode. Also, other methods described above that provide a porous insulating layer for the anode can be used to produce a cathode having a porous insulating layer covering the discharge region. These layers protect the cathode from positive ion bombardment in the same way that the anode protective layer shields the anode from negative fluorine ion bombardment. As mentioned above, the material on both sides of the discharge surface erodes faster than the material on the discharge surface.

While the invention has been described above with particularity in terms of preferred embodiments, it should be understood and appreciated that many modifications and changes can be made without departing from the spirit of the invention. . The two electrode materials can be selected such that the erosion rate of the first material is about ¼ to ½ of the erosion rate of the second electrode material, but very much compared to the first material. A second material having a high (eg, 10 to 20 times higher) erosion rate may be used. This ensures that any tendency for the beam to spread is quickly removed. In order to clean the discharge debris gap before the next pulse, it is important to maintain good flow in the gap between the electrodes. The width of the porous insulating layer should preferably match the width of the discharge surface, which is preferably equal to or slightly larger than the desired width of the laser beam. The thickness of the insulating layer should preferably be between about 20 microns and 300 microns, with the most preferred range being about 50 to 150 microns. However, some of Applicants' test anode thicknesses were up to about 1 mm, but no serious problems occurred. Two grooves can be provided along both edges of the discharge surface during electrode fabrication. This prevents the grooves from forming naturally during laser operation due to erosion. Although not explained above, another advantage of the two-material electrode can be a cost-saving idea in that the amount of the first (low erosion rate) material can be significantly reduced. . This makes it possible to economically use a very expensive low erosion rate material at the discharge surface, and without having to use an expensive material for the remainder of the electrode. Several good methods can be used to attach the first material (eg, 42 in FIG. 6) to the second material structure 40. For example, shrink fitting, welding, brazing, or fastening with a small screw can be performed. Portions 42 can be cut from about 35 stacks of thin sheets (eg, 0.1 mm) of insulating material such as alumina (each sheet is coated with a very thin layer of conductive material such as copper). . (The laminate can be heat treated to melt the layers together before cutting the portion 42.) The portion 42 is about 50 cm long, about 5 mm high, and about 3.5 mm wide. Disconnected. This part is then inserted into the part 40 as shown in FIG. 6, in which case the section of the part 42 is rectangular. In operation, current flows through the copper layer between the alumina sheets into the conductor material of the portion 40. The electrodes used in some situations of the prior art had a circular or other arcuate surface in the discharge area. These arcuate surfaces tend to flatten by erosion after millions of pulses. This usually occurs during the burn-in period in Applicant's laser. The Applicant has found that this burn-in period can be shortened when the electrode on the arcuate surface is planarized during electrode fabrication. The preionization tube shown in FIG. 12A can be provided with a flat portion that coincides with a correspondingly shaped holder incorporated in the main insulator, thereby preventing any rotation of the preionizer. This ensures that there is always contact between the shim and the coating in the case of a conductive coating. The matching flat surface can also prevent rotational movement of the preionizer that can cause shim wear. Accordingly, the scope of the invention can be determined by the appended claims and their legal equivalents.

1 shows a cross-sectional view of a chamber of a prior art gas discharge laser. Fig. 2 shows another feature of a prior art laser. The main features of the prior art gas discharge laser pulse power system are shown. FIG. 4 shows the voltage applied to each capacitor bank of FIG. FIG. 4 shows the voltage applied to each capacitor bank of FIG. It is sectional drawing of the anode of a prior art. A cross-sectional view of a preferred anode is shown. A cross-sectional view of a preferred anode is shown. A cross-sectional view of a preferred cathode is shown. A cross-sectional view of a preferred anode is shown. A cross-sectional view of a preferred cathode is shown. A cross-sectional view of a preferred anode is shown. A cross-sectional view of a preferred anode is shown. 1 shows a preferred embodiment of the present invention. 1 shows a preferred embodiment of the present invention. A cross-sectional view of a preferred anode is shown. A cross-sectional view of a preferred anode is shown. FIG. 10B is a plan view of the anode of FIG. 10A. The anode unit of a current feedback device is shown. Figure 2 shows a cross-sectional view of an aerodynamically designed chamber. Figure 2 shows a cross-sectional view of an aerodynamically designed chamber. A plasma electrode is shown. A plasma electrode is shown. A plasma electrode is shown. A plasma electrode is shown. A plasma electrode is shown. 2 shows a comparison between a prior art electrode and a preferred embodiment. 2 shows a comparison between a prior art electrode and a preferred embodiment. Figure 2 shows an improved electrode configuration for better preionization. A method for reducing fluorine that causes anodic erosion is shown.

Explanation of symbols

40 Anode portion 42 Second brass portion 42A Protective porous fluoride coating

Claims (20)

A) a laser chamber containing a laser gas containing fluorine;
B) two long-lived elongated electrode means spaced apart to define a discharge region to produce a discharge;
C) a pulsed power system that supplies electrical pulses to generate the discharge;
D) a blower system for circulating the laser gas between the two electrodes;
E) a heat exchanger that removes heat generated by the blower system and the discharge from the laser gas;
A gas discharge laser comprising:
The electrode means includes two long-lived elongated electrode elements defining a cathode and an anode, each of the cathode and anode having a predetermined width selected to define the discharge width between the electrode elements Having a narrow discharge area, the anode is
a) a first anode material disposed in a long and narrow discharge region of the anode in the discharge region defining a first anode erosion rate and forming two long edges;
b) a second anode material disposed along at least two sides of the anode's long and narrow discharge region along the two long edges and adjacent to the anode's long and narrow discharge region;
Wherein the first and second anode materials are selected such that a second anode erosion rate is higher than the first anode erosion rate, and the high erosion rate of the second anode material is A gas discharge laser characterized in that it prevents any expansion of the discharge width over a considerable long period.   The first anode material is selected from a group of materials each known to produce a porous insulating layer when exposed to a discharge from a cathode in a fluorine-containing gas. The laser according to claim 1.   The first anode material is selected from a group of materials each known to produce a porous fluoride layer when exposed to a discharge from the cathode in a fluorine-containing gas. The laser according to claim 1.   The laser of claim 1, wherein the first anode material includes a porous insulating layer.   The anode generates a high electric field over a width of about 3.5 mm forming a discharge region of the anode along the center line of the anode and has a sharp drop in the electric field on both sides of the anode discharge region The laser of claim 1 having a selected cross section.   The laser of claim 1, wherein at least one of the electrode elements defines a discharge surface and includes a groove running longitudinally along two sides of the discharge surface.   The laser according to claim 2, wherein the porous insulating layer is made of insulating particles embedded in a metal. A method for producing an elongated electrode for use in a laser comprising:
a) making an elongated electrode structure composed of one or more electrically conductive materials;
b) generating a porous insulating layer in a portion defining the discharge region of the elongated electrode;
Wherein the one or more electrically conductive materials includes a lead rich brass having a lead content greater than 1 percent, and the step of generating the porous insulating layer comprises porous insulation on the lead rich brass Activating the electrode in a fluorine-containing laser gas such that the layer accumulates. A method for producing an elongated electrode for use in a laser comprising:
a) making an elongated electrode structure composed of one or more electrically conductive materials;
b) generating a porous insulating layer in a portion defining the discharge region of the elongated electrode;
The method of generating the porous insulating layer comprises diffusing insulating particles into a discharge region of the elongated electrode structure. A method for producing an elongated electrode for use in a laser comprising:
a) making an elongated electrode structure composed of one or more electrically conductive materials;
b) generating a porous insulating layer in a portion defining the discharge region of the elongated electrode;
The step of generating the porous insulating layer comprises:
a) mixing insulating particles into the molten metal to create a discharge zone comprising filled metal and the insulating particles at the elongated electrode;
b) operating the elongated electrode in a fluorine-containing laser gas environment such that a portion of the fill metal is sputtered away leaving a porous insulating layer covering the discharge region;
A method comprising the steps of: A method for producing an elongated electrode for use in a laser comprising:
a) making an elongated electrode structure composed of one or more electrically conductive materials;
b) generating a porous insulating layer in a portion defining the discharge region of the elongated electrode;
Said step of producing a porous insulating layer comprises:
a) generating a plurality of nucleation points in the discharge region;
b) operating the electrode in a fluorine gas-containing laser such that the porous insulating layer grows on the discharge region;
A method comprising the steps of: The cathode is
a) defining a first cathode erosion rate and, in the discharge region forming two long edges, a first cathode material placed in the long and narrow discharge region of the cathode;
b) a second cathode erosion rate is defined and is disposed along at least two sides of the cathode long and narrow discharge area along the two long edges and adjacent to the cathode long and narrow discharge area; A cathode material of
And wherein the first and second cathode materials are selected such that the second cathode erosion rate is higher than the first cathode erosion rate, so that during the laser operation, the second cathode erosion rate is higher than the first cathode erosion rate. The laser of claim 1, wherein a high erosion rate of the cathode material prevents any expansion of the discharge over a substantial period of time.   The laser of claim 1, wherein the electrode means includes at least one plasma electrode.   A gas that provides a substantially fluorine-free gas flow through the porous electrode to supply a gas layer that is substantially free of fluorine on the discharge surface of the porous electrode, the electrode means comprising a porous electrode; The laser according to claim 1, further comprising supply means.   The laser of claim 1, wherein the electrode means includes at least one electrode, the electrode having an elongated portion defining an electrode direction and an end portion extending at an angle from the electrode direction.   The laser according to claim 1, wherein the electrode means includes two electrodes, each having an end portion extending at an angle from the electrode direction.   The electrode means comprises an anode having an insulator and an elongated cathode at ground potential, the anode having an elongated insulator portion, an elongated conductor portion being disposed on the insulator portion, and the pulse power system comprising: A corona plasma is generated on the surface of the elongated insulator portion and in the discharge between the elongated anode and the elongated cathode, configured to supply a high voltage positive electrical pulse to the elongated conductor portion of the anode. The laser according to claim 1. The laser of claim 1 wherein said elongate electrode means comprises tungsten, said laser is characterized in that it further comprises a cold trap for confining WF ₆ gas.   2. The electrode means comprising at least one electrode comprising a conductor shim machined as part of the electrode, the electrode being mounted so that the shim contacts a preionization tube. The laser described.   The laser of claim 1, further comprising a preionization tube having a non-circular cross section.

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