Source: http://www.google.com/patents/US7132123?dq=6,587,403
Timestamp: 2017-09-26 11:14:36
Document Index: 192530152

Matched Legal Cases: ['art 42', 'art 42', 'art 56', 'art 42', 'art 42', 'art 42', 'art 40', 'art 42', 'art 40']

Patent US7132123 - High rep-rate laser with improved electrodes - Google Patents
The present invention provides a gas discharge laser having at least one long-life elongated electrode for producing at least 12 billion high voltage electric discharges in a fluorine containing laser gas. In a preferred embodiment at least one of the electrodes is comprised of a first material having...http://www.google.com/patents/US7132123?utm_source=gb-gplus-sharePatent US7132123 - High rep-rate laser with improved electrodes
Publication number US7132123 B2
Application number US 10/629,364
Also published as US6937635, US20040022292, US20040037338
Publication number 10629364, 629364, US 7132123 B2, US 7132123B2, US-B2-7132123, US7132123 B2, US7132123B2
Inventors Richard G. Morton, Timothy S. Dyer, Thomas D. Steiger, Richard C. Ujazdowski, Tom A. Watson, Bryan Moosman, Alex P. Ivaschenko, Walter Gillespie, Curtis Rettig
Patent Citations (25), Non-Patent Citations (1), Referenced by (3), Classifications (45), Legal Events (2)
US 7132123 B2
In embodiments of this invention electrodes are comprised of two different materials having erosion rates different from each other. The relatively lower erosion rate material is located at the location of the discharge surface of the electrode which is a long thin surface, for example about 3.5 mm×545 mm. The higher erosion rate material is located along both of the longer sides of the discharge region.
The insulating layer contains thousands of tiny holes with widths of from about 20 to 150 microns which bottom out on the metal surface of the anode. The holes are spaced at about 20 to 30 holes per square mm on the discharge surface of the anode. The total number of holes in the 3.5 mm×545 mm discharge surface was estimated by Applicants to be about 50,000, and the holes represent about 5% to 10% of the discharge surface area. The other 90% to 95% of the discharge area is comprised of an insulating, dielectric material that can repel negatively charged fluorine ions due to rapid accumulation of negative electronic surface charge.
The anode is installed in a laser such as the one shown in FIG. 1 with, for example, a laser gas consisting of 1% krypton, 0.1% F2 and the rest neon. A porous fluoride layer, comprising copper fluoride, zinc fluoride and lead fluoride, is created on the top surface of the second brass part 42 shown in FIG. 6 by operation of the laser for about 500 million pulses. At 2000 pulse per second this requires about 3 days. This porous insulating layer which develops, retards erosion of the discharge surfaces which allows the anode to maintain this extremely good shape for may billions of discharges. Electrons flow easily through the approximately 50,000 small holes which develop in the approximately 1,855 square millimeter area (3.5 mm×530 mm) of the lead fluoride layer. (This works out to about 30 holes per square millimeter.) On the other hand, individual fluorine ions, which are far more massive than the electrons, have a low probability of passing through the holes to the underlying brass with sufficient energy to cause sputtering. In one of the parents of this application, Applicants estimate that the present invention will permit at least a doubling or tripling of anode life, so that anode erosion no longer limits laser chamber life. Applicants' subsequent proof testing has supported these predictions. These tests are very time consuming since lasers available for electrode testing produce only about 2500 pulses per second. To accumulate 13 billion pulses at 2500 pulses per second requires a test period of about 60 days. As of the filing of this application a laser chamber with an anode with this initial shape shown in FIG. 6 had accumulated more than 13.5 billion pulses with no significant deterioration of laser pulse quality when it was removed for inspection. Aging of a prior art chamber with a prior art electrode as shown in FIG. 1, causes a reduction in laser efficiency and requires a gradual increase in the fluorine concentration in the laser gas or an increase in the normal discharge voltage to maintain a consistent pulse energy output. Normal practice is to set F2 concentration for optimum beam quality and to increase operating voltage to compensate for reduced laser efficiency. Chamber lifetime is reached when laser beam quality deteriorates below acceptable levels or when the fluorine concentration and discharge voltage reach design limits.
Applicants have determined through experiments that annealing of the brass electrode material can substantially effect cathode erosion rate. In general Applicants have discovered that erosion rate is roughly inversely proportional to grain size over a large range of grain sizes. Since annealing reduces grain size, cathode erosion can be reduced by annealing the material. Therefore, an alternative cathode design would utilize annealed brass as the first material 90 and non-annealed brass as the second material 92. Preferably, sufficient annealing should be provided to reduce the grain size of the second material to about ¼ the grain size of the first material, for example, 54 microns for the first material and 13 for the second material.
C. Zn content: Reef thickness and morphology.
In another solution to the problem, either one or both of the electrodes are turned at both ends as shown in FIG. 16B so that the distance between the electrodes in the discharge region remains constant to both ends. In one embodiment one of the electrodes has the shape shown in FIG. 16B and the other one has the standard shape as shown in FIG. 16A. In another version both ends of both electrodes are turned as shown in FIG. 16B. Preferably the cathode and anode turn in opposite directions at each end of the chamber. Still another solution to the end erosion problem is to remove one or more of the current return “ribs” at each end of the electrodes. In prior art design current return ribs (which have the general whale-bone shape) as shown at 10A8 in FIG. 7F are distributed evenly along the entire length of the electrodes. In one prior art design the current return structures comprised 27 ribs spaced at 1-inch intervals. Applicants cut out ribs 2, 3 and 4 and 24, 25 and 26 in the end regions of the current return structure. This produced a significant improvement in energy distribution in the discharge region and is expected to substantially increase electrode life. Similar results could be obtained by eliminating feedthrough rods which conduct current from the pulse power system through the insulator to the cathode 84 as shows in FIG. 1. In typical prior art lasers there are about 15 feedthrough rods spaced at 1½-inch intervals along the length of the cathode. A preferred technique to reduce excess end erosion is to reduce by 1 to 3 the number of feedthrough rods at each end. A second advantage of reducing the feedthrough rods in these end regions is that the seals associated with these rods have a tendency to leak in some cases because of differential thermal expansion between the insulator and the chamber upper wall.
Embodiments of the present invention utilize tungsten, tungsten alloys or tungsten composites in the electrodes. For example, part 42 shows in FIG. 6 could be a tungsten composite with a very low rate of erosion. To avoid or minimize any adverse effects of WF6, Applicants have added a WF6 purification loop to the basic laser chamber design shown in FIG. 1. The prior art chamber extracts a small percentage of the circulating gas flow at the high-pressure side of blower (part 56) which passes through an electrostatic filter (not shown) where it is cleaned of metal fluoride particles. The cleaned gas flows from two sides of the filter to both laser chamber window housings to slightly pressurize the window region (relative to the discharge region) with clean gas thereby keeping debris laden gas away from the window. (For details see U.S. Pat. No. 5,018,162, incorporated by reference herein). Preferably the cold trap is located in the filter loop. Preferably, only a small portion (about 5–10%) of the filter flow is directed to the cold trap which is cooled to liquid nitrogen temperature. WF6 condenses at a temperature of about 17° C. and is therefore completely removed from the portion of the gas flow passing through the filter. Preferably a heater is included in the cold trap so the WF6 can be evaporated and removed from the chamber at the time of gas replacement.
Another embodiment of the present invention for providing a long life anode is shown in FIG. 18. In this case the anode is made of a porous sintered metal such as C26000 brass. For a KrF laser, a 1% Kr, 99% Ne mixture is forced through the centered anode so that the discharge surface us always protected by a layer of F2 free gas. This avoids fluorine sputtering of the anode. Applicants have determined that a F2 clean layer of about ¼ micron thick should be large enough to avoid fluorine sputtering. The addition of the small amount of krypton and neon will produce a reduction of F2 concentration which will have to be made up with the addition of laser gas relatively rich in F2 (such as 1.0% F2 99% Kr and 1.0% Ne) but this will create no problem because existing gas controls are already available to make up for the loss of F2 through F2 chemical reactions in the chamber.
While the invention has been described above with specificity in terms of preferred embodiments, the reader should understand and recognize that many changes and alterations could be made without deviating from the spirit of the invention. As indicated above the two electrode materials may be selected such that erosion rates of the first material is about ¼ to ½ that of the second electrode material but second materials with erosion rates very high (such as 10 to 20 times higher) compared to the first material could be used. This would assure that any tendency of the beam to spread would be quickly eliminated. It is important to maintain good flow conditions in the gap between the electrodes to clear the gap of discharge debris prior to the next pulse. The width of the porous insulating layer should preferably correspond to the width of the discharge surface which preferably is about equal to the desired width of the laser beam or slightly larger than the beam width. The thickness of the insulating layer should preferably be between about 20 microns and 300 microns with a most preferred range of about 50 to 150 microns. However, the thickness of some of Applicants' test anodes have ranged up to about 1 mm without causing serious problems. Two trenches could be provided along both edges of the discharge surface when the electrodes are fabricated. This avoids having the trenches develop naturally during operation of the laser due to erosion. An additional advantage of the two-material electrodes, not described above, is that it could be a cost saving idea in that it would allow a major reduction in the quantity of the first (low erosion) material. This would permit economical use of very expansive low erosion material at the discharge surface and much less costly material as the remainder of the electrode. Several good techniques are available for fixing the first material (e.g., 42 in FIG. 6) into the second material structure 40. For example, it could be shrunk fit, welded, braised or held in with small screws. Part 42 could be cut from a stack of about 35 thin (such as 0.1 mm) sheets of an insulator material such as alumina (each sheet having deposited on it a very thin layer of a conducting material such as copper). (The stacks could be heat treated to fuse the layers together prior to cutting out part 42). Part 42 would be cut so that it is about 50 cm long, about 5 mm high and about 3.5 mm wide. The part is then inserted into part 40 as shown in FIG. 6, but in this case the cross-section of part 42 is rectangular. In operation current flows through the copper layers between the sheets of alumina to the conductor material in Part 40. Electrodes utilized in some prior art situations have had rounded or other arched surfaces at discharge regions. These arched surfaces tend to become flattened due to erosion after a few million pulses. This occurs typically in Applicants' lasers during a burn-in period. Applicants have discovered that this burn-in period can be shortened if the arched surface electrode of this is flattened when the electrode is fabricated. The pre-ionizer tube shown in FIG. 12A could be provided with a flattened portion which would be matched with a correspondingly shaped holder built into the main insulator which would prevent any rotation of said pre-ionizer. This would assure in the case of the conductive coating that there always is contact between the shin and the coating. Matching flat surfaces also prevent rotational movement of the pre-ionizer which could otherwise cause wearing of the shim. Therefore, the scope of the invention should be determined by the appended claims and their legal equivalents.
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U.S. Classification 427/58, 427/384, 427/372.2, 427/180, 427/195, 427/201
International Classification G03F7/20, B05D5/12, H01S3/041, H01S3/0979, H01S3/038, H01S3/223, B05D3/02, H01S3/22, H01S3/036, H01S3/225, H01S3/097
Cooperative Classification H01S3/097, H01S3/0382, H01S3/0381, H01S3/038, G03F7/70933, H01S3/0385, H01S3/0979, G03F7/70575, H01S3/0388, G03F7/70025, H01S3/041, H01S3/225, H01S3/0387, H01S3/22, H01S3/09702, H01S3/2207, H01S3/036, H01S3/223, G03F7/70041
European Classification G03F7/70B8, G03F7/70L4F, G03F7/70B4, G03F7/70P8F, H01S3/036, H01S3/223, H01S3/225, H01S3/038, H01S3/038G