Abstract:
A laser chamber with a circuitous gas return path dissipates shock waves. In one embodiment, the laser chamber includes a heat exchanger with a large surface area that defines an aerodynamic passage through which gas circulates in the laser chamber. The passage through which the gas circulates directs shock waves away from the discharge region so that the shock waves may dissipate elsewhere in the laser chamber. In addition, the large surface area of the heat exchanger efficiently cools the thermally energetic gas within the laser chamber. In another embodiment, ancillary chambers that are fluidically coupled to the main laser chamber are provided to permit shock waves to be directed away from the discharge area and to be dissipated within the ancillary chambers. Openings to the ancillary chambers are positioned such that shock waves generated by the electrode structure of the laser chamber may propagate directly into the ancillary chamber, where the shock waves then dissipate. Flow guides, such as blowers or flow vanes, may be provided in the ancillary chambers to generate a circulation of gas within the ancillary chambers that will support the laser chamber&#39;s flow of gas at the openings to the ancillary chambers. Thus, the circulating gas within the laser chamber remains uniform and stable.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a laser chamber, such as that used with a pulsed energy laser, and in particular the present invention relates to laser chambers having shock wave dissipation properties. 
     BACKGROUND 
     Pulsed laser systems, such as excimer lasers, are well known. FIG. 1 is a side view of a laser chamber  10  used in a pulsed laser system. Laser chamber  10  includes an electrode structure  12 , a blower  14 , windows  16 ,  18 , a laser beam  20 . Between electrode structure  12  is the laser discharge region  24 . 
     FIG. 2 is a front view of laser chamber  10 . As shown in FIG. 2, laser chamber  10  additionally includes heat exchanger  26 , a pre-ionizer  28 , baffles  30  and a current return  32 , which is used to connect the lower of electrodes  12  to ground. 
     As well known by those skilled in the art, a pulsed laser system, such as an excimer laser, produces high energy, high frequency pulses in a gas that is between electrodes  12  in laser chamber  10 . The gas, which may contain krypton and fluorine, is maintained at high pressure, for example 3 atm. Pre-ionizer  28  first floods the gas within discharge area  24  with free electrons (10 6  to 10 7  per cm 3 ). Once the gas within discharge area  24  is conditioned with a sufficiently increased electron density, electrodes  12  produce a high energy discharge, which may be for example 15-50 kV. The lasing action from the high energy discharge occurs within 100 nsec from the time of discharge. 
     The high energy discharge in discharge area  24  produces a large amount of local heating and pressure disturbances in the gas. The thermal and pressure disturbances change the index of refraction of the gas, which has a deleterious effect on the energy efficiency of the laser system. The high energy discharge of the gas does not affect the lasing action from the pulse that caused it because the lasing action occurs within a short amount of time after the high energy discharge, approximately 100 nsec. However, subsequent high energy discharges, which occur at a frequency of approximately 1 KHz, will be produced in the highly disturbed, thermally energetic gas unless the gas is circulated within laser chamber  10 . Thus, blower  14  is used to circulate the gas within laser chamber  10 . Heat exchanger  26  is placed in the path of the gas flow to cool the gas as it circulates. Typically, the gas in laser chamber  10  is circulated at a speed of 20-30 meters per second through discharge region  24 , however, this speed is dictated by the frequency of the pulsed laser system. 
     It is desirable for the circulating gas within laser chamber  10  to be as uniform and as stable as possible, i.e., thermally, optically, and kinetically stable, because a stable gas maximizes the energy output of the laser system. One cause of disturbance in the gas is shock waves generated from the high energy discharge from electrodes  12 . Shock waves from the high energy discharge are reflected by the walls of laser chamber  10 , as well as from heat exchanger  26  and other components, back into discharge area  24  where the shock waves interfere with the energy output of the pulsed laser system. 
     Another cause of disturbance in the gas that is circulating within laser chamber  10  is heat exchanger  26 . Although heat exchanger  26  is necessary to cool the thermally excited gas, heat exchanger  26  acts as a choke to the gas flow within laser chamber  10 . Consequently, blower  14  is required to overcome the impedance of heat exchanger  26 . Further, the position and configuration of heat exchanger  26  disturbs the uniformity of the circulating gas. Fins (not shown) on heat exchanger  26  are conventionally used to assist in heat exchange. However, fins, which are typically one inch high and 0.1 inch apart, further impede the flow of the circulating gas. 
     In addition, laser chamber  10  fails to circulate the entire volume of gas. The flow of the gas in laser chamber  10  is illustrated by arrows, as shown in FIG.  2 . Baffles  30  are used in conjunction with blower  14  to guide the gas flow around laser chamber  10 , nevertheless, there are typically dead areas within laser chamber  10  where the gas fails to circulate properly. For instance, laser chamber  10 , as shown in FIG. 2, has a dead area in the center of laser chamber  10  where the gas circulates in a small area, i.e., an eddy, and thus fails to circulate throughout laser chamber  10 . 
     SUMMARY 
     A laser chamber in accordance with an embodiment of the present invention redirects the shock waves away from the discharge area and into other areas of the laser chamber where the shock waves can be dissipated. In conventional systems, the walls of the laser chamber, the heat exchanger and/or other components within the laser chamber provide surfaces for shock waves to be deflected back into the discharge region, thereby disturbing the energy stability of subsequent pulses. Thus, a laser chamber that redirects the shock waves away from the discharge area to be dissipated elsewhere advantageously maintains stability of the gas within the discharge area during pulsing. 
     One embodiment of the laser chamber has an electrode structure that defines a laser discharge area, a blower that circulates gas within the laser chamber and a heat exchanger with a surface area that defines a passage for the gas circulating within the laser chamber. The circuitous path defined by the heat exchanger allows shock waves to be directed away from the discharge region and dissipated in other areas of the laser chamber. Further, the additional surface area of the heat exchanger efficiently cools the thermally excited gas. In some embodiments, the heat exchanger is curved to create an inner surface area defining a space, and an outer surface. A protrusion from the side wall of the laser chamber extends into the space defined by the heat exchanger thereby lengthening the passage for the circulating gas. 
     In another embodiment, the working volume of the laser chamber is increased through the addition of ancillary chambers. The ancillary chambers are fluidically coupled to the laser chamber and are positioned such that shock waves generated by high energy discharges of the electrodes propagate directly into the openings of the ancillary chambers. The shock waves may then dissipate within the ancillary chambers rather than being reflected back to the laser discharge area. Flow guides, such as blowers or flow vanes, may be included within the ancillary chambers. The flow guides within the ancillary chambers generate a circulation of gas within the ancillary chambers that supports the circulating gas within the laser chamber at the openings of the ancillary chambers. The flow guides (or other baffles) within the ancillary chambers also act to trap the shock waves within the ancillary chamber, allowing the shock waves to dissipate through the action of multiple reflections within the ancillary chambers. Thus, the gas flow within the laser chamber remains stable and uniform. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying figures, where: 
     FIG. 1 is a side view of a conventional laser chamber used in a pulsed laser system; 
     FIG. 2 is a front view of a conventional laser chamber used in a pulsed laser system; 
     FIG. 2A is a front view of a state of the art laser chamber. 
     FIG. 3 is a front view of a laser chamber with an expanded working volume and a heat exchanger defining a path for the circulating gas; 
     FIG. 4 is a side view of the laser chamber from FIG. 3; 
     FIG. 5 shows a front view of a laser chamber with an oval heat exchanger that defines a path for the circulating gas; 
     FIG. 6 is a front view of a laser chamber with ancillary chambers providing additional working volume; 
     FIG. 7 shows a side view of the laser chamber from FIG. 6; 
     FIG. 8 shows a front view of a laser chamber with ancillary chambers and shows the direction of gas flow within the laser chamber and ancillary chambers; 
     FIG. 9 shows a front view of the laser chamber shown in FIG. 3 with the addition of ancillary chambers; and 
     FIG. 10 shows a front view of a laser chamber with the addition of a middle section. 
    
    
     DETAILED DESCRIPTION 
     FIG. 2A is the front view of a laser chamber of a state-of-the-art excimer laser. The drawing shows electrodes  12 , blower  14 , heat exchanger  26 , pre-ionizers  28 , baffles  30  and current return  32 . 
     FIG. 3 is a front view of a laser chamber  100  with an expanded working volume in which shock waves may be dissipated. Laser chamber  100  includes electrodes  102 ,  104 , a heat exchanger  106  with a large surface area that acts as a flow vane for the circulating gas, and a blower  108 . FIG. 3 also shows the back window  110  of laser chamber  100 . 
     FIG. 4 is a side view of laser chamber  100 . FIG. 4 shows electrodes  102 ,  104  with electrode  104  positioned adjacent to heat exchanger  106  and laser discharge zone  103  between electrodes  102  and  104 . FIG. 4 also shows blower  108 , back window  110 , and front window  112 . 
     It will be understood by those skilled in the art that electrodes  102 ,  104  include conventionally used devices, such as a pre-ionizer, current return and insulator, however, these devices are not shown in FIGS. 3 and 4 for the sake of clarity. It should be understood that electrodes  102 ,  104  and blower  108  may be of the type conventionally used in a pulsed laser system. Electrodes  102 ,  104 , for example, may be the type used in Excimer Laser Model 5000, produced by CYMER, Inc. in San Diego, Calif. Additionally, blower  108  may be supported by either a mechanical bearing or a magnetic bearing. 
     Laser chamber  100 , including electrodes  102 ,  104 , heat exchanger  106  and blower  108  are manufactured out of materials compatible with the specific gases used in laser chamber  100 , e.g., fluorine and krypton. For example, electrodes  102  and  104  may be manufactured from brass and insulated with ceramic, while other devices within laser chamber  100 , such as heat exchanger  106 , blower  108 , and the walls of laser chamber  100  may be manufactured with nickel plated aluminum or steel. Of course, other materials that are compatible with the specific gas mixture may be used in laser chamber  100 . 
     The outside dimension of laser chamber  100 , as shown in FIG. 3, is approximately 325 mm in the Z direction, 350 mm in the X directions, and 725 mm in the Y dimension, as shown in FIG.  4 . The inner dimensions of laser chamber  100  are approximately 275 mm in the Z direction, 300 mm in the X direction, and 675 mm in the Y direction. 
     The working volume of laser chamber  100  shown in FIG. 3, is approximately 45,000 cm 3 , which is two to three times larger than working volumes used in conventional laser chambers. 
     Heat exchanger  106  in laser chamber  100 , as shown in FIG. 3, is curved 180 degrees into a sideways U shaped configuration. The curvature of heat exchanger  106  forms an inner surface area  105 , which defines a space  109  and an outer surface area  107 . A protrusion  116  of the wall  117  of laser chamber  100  extends into space  109  defined by inner surface area  105  of heat exchanger  106 . Consequently, a long continuous path is created by the surface of heat exchanger  106  and protrusion  116 , through which the gas within laser chamber  100  circulates. The circuitous path also permits shock waves to be directed away from the discharge region and dissipated elsewhere in laser chamber  100 . Further, heat exchanger  106  does not provide a deflective surface to redirect the shock waves back into the discharge region. 
     The performance of heat exchanger  106  is improved because the thermally excited gas is exposed to a greater surface area of heat exchanger  106 . Consequently, heat exchanger  106  removes heat from the circulating gas more efficiently than with heat exchangers in conventional laser chambers. Due to the greater surface area of heat exchanger  106 , smaller fins (not shown) on heat exchanger  106  may be used. For example, fins may be used with heat exchanger  106  that are parallel to the flow of the gas, and that extend approximately a quarter inch into the gas flow and are spaced a quarter inch apart. Thus, fins used with heat exchanger  106  generate less disturbance in the gas flow than found in conventional systems. 
     Heat exchanger  106  includes several tube like voids  114  through which flows the cooling medium. Of course the particular number of voids  114  carrying cooling medium may vary as desired to efficiently cool the thermally excited gas in laser chamber  100 . The cooling medium, water for example, flows through heat exchanger  106  at approximately two gallons per minute, and is at a temperature of approximately 18 to 25 degrees Celsius. Other flow rates and temperatures may be used as desired to control the temperature of the gas within laser chamber  100 . 
     The voids  114  in heat exchanger  106  may carry the cooling medium in a serial or parallel fashion. For example, voids  114  may include several tubes that run independently through heat exchanger  106  or may be only one tube that winds throughout heat exchanger  106 . Heat exchanger  106  may be connected to both the front end and the back end of laser chamber if, for example, voids  114  have a parallel configuration through heat exchanger  106 , or heat exchanger may be connected to only one end, as shown in FIG. 4, where voids  114  have a serial configuration. Of course, other configurations of voids  114  are possible, such as having multiple serial voids  114  running through heat exchanger  106 . 
     Heat exchanger  106  also serves as a flow vane, defining the circulation path of the gas within laser chamber  100 . Thus, heat exchanger  106  is not an obstruction to the flow of gas as found in conventional systems, but rather produces an aerodynamic path through which the gas may circulate. 
     As will be understood by those skilled in the art, other heat exchanger  106  configurations are possible. For example, the curvature of heat exchanger  106  may be more obtuse with the surface of protrusion  116  following the curve of heat exchanger  106 . 
     FIG. 5 shows a laser chamber  120  with an oval heat exchanger  122  in the center. Similar to laser chamber  100 , laser chamber  120  improves shock wave dissipation by providing a circuitous path defined by heat exchanger  122  to permit shock waves to be directed away from the discharge area and dissipated elsewhere. Heat exchanger  122  is similar in manufacture and operation to heat exchanger  106 , shown in FIG.  3 . Heat exchanger  122  has an increased surface area relative to heat exchangers in conventional laser chambers. Further, heat exchanger  122  generates a gas circulation path in which, unlike heat exchanger  106 , there are no 180 degree turns. Thus, with the use of heat exchanger  122 , blower  108  is not required to push the gas around sharp 180 degree curves. However, the circulation path defined by heat exchanger  122  is shorter than that created by heat exchanger  106 . Moreover, the shape of heat exchanger  122  defines a large dead space in the center of laser chamber  100 . Thus, there is a decreased working volume with the use of heat exchanger  122  relative to heat exchanger  106 . 
     FIG. 6 is a side view of another embodiment of a laser chamber  200  with shock dissipating properties. As shown in FIG. 6, laser chamber  200  includes electrodes  202 ,  204 , blower  206 , heat exchanger  208 , and two ancillary chambers  210  and  212 . As shown in FIG. 6, laser chamber  200  also includes a current return  214 , back window  216 , and baffles  218 . Laser chamber  200  may be similar in size and manufacture as laser chamber  10  shown in FIGS. 1 and 2, however, ancillary chambers  210  and  212  permit shock waves to be directed away from the discharge area and to be dissipated elsewhere, i.e., within ancillary chambers  210  and  212 . 
     The components used in laser chamber  200 , such as electrodes  202 ,  204 , which include the pre-ionizers  215 , current return  214 , and heat exchanger  208  may be conventional components, for example the type that are used in Excimer Laser Model 5000, by CYMER, Inc. Blower  206  may be a conventional mechanical bearing blower, such as the type used in the Model 5000 or a magnetic bearing, such as the type described in reference to FIG.  3 . 
     Ancillary chambers  210  and  212  are fluidically connected to laser chamber  200  via converging openings  211  and  213 . Ancillary chambers  210  and  212  are approximately 100 mm in the direction labeled A, 150 mm in the direction labeled B, and are substantially the length of laser chamber  200 , illustrated as direction C in FIG.  7 . FIGS. 7 and 8 show a respective side view and front view of laser chamber  200 . Ancillary chambers  210  and  212  may extend to the ends of laser chamber  200  or may be short of the ends, for example, by two inches on either side. Openings  211  and  213 , however, extend the entire length of electrodes  202  and  204 . It is understood that ancillary chambers  210  and  212  may differ in shape and size than that shown in FIG.  6 . For example, ancillary chambers  210  and  212  may be round, square, or rectangular. 
     Openings  211  and  213  are positioned in line with electrodes  202 ,  204  so that shock waves generated by the high energy discharge of electrodes  202 ,  204  propagate directly into ancillary chambers  210  and  212 . Openings  211  and  213  are shown as converging which assists in the propagation of shock waves into ancillary chambers  210  and  212 . 
     Ancillary chambers  210  and  212  create additional volume to laser chamber  200  in which shock waves are permitted to dissipate. Moreover, ancillary chambers  210  and  212  are configured such that once shock waves enter openings  211  and  213  the shock waves are reflected within ancillary chambers  210  and  212  rather than being reflected directly back towards electrodes  202 ,  204 . Thus, the shock waves are substantially dissipated within ancillary chambers  210  and  212  thereby avoiding interference with subsequent high energy discharges produced by electrodes  202 ,  204 . A parabolic or oval configuration, such as the type shown in FIG. 6, may be used to maintain the reflected shock waves within ancillary chambers  210  and  212 . In other embodiments, ancillary chambers  210  and  212  may include baffles (not shown) to guide shock waves within ancillary chambers  210  and  212  and to prevent shock waves from exiting ancillary chambers  210  and  212  prior to being substantially dissipated. 
     As shown in FIG. 8, ancillary chambers  210  and  212  may each contain a flow guide, such as a blower  220 ,  222  and/or flow vanes (not shown), for generating a flow of gas within ancillary chambers  210  and  212 . The gas within ancillary chambers  210  and  212  circulates in a direction opposite the circulation direction within the main laser chamber  200 , as shown in FIG.  8 . Because the circulation directions are opposite, at openings  211  and  213  the gas within main chamber  200  is flowing in the same direction as the gas in ancillary chambers  210  and  212 . Thus, the flow of gas in ancillary chambers  210  and  212  supports the flow of gas within main chamber  200 . The inter-supported gas permits the circulating gas within main chamber  200  to maintain its proper direction and integrity. Further, the inter-supported gas at openings  211  and  213  permits shock waves to pass from main chamber  200  to ancillary chambers  210  and  212  because the velocity of gas at openings  211  and  213  is much slower than the velocity of the shock waves, by approximately a factor of  10 . Consequently, the shock waves may pass into ancillary chambers  210  and  212  were they are dissipated. Further, due to the controlled flow of gas within ancillary chambers  210  and  212 , the circulation of gas within main chamber  200  remains uniform and stable. 
     FIG. 9 shows a front view of laser chamber  300 , which is similar to laser chamber  100  shown in FIG. 3, like designated elements being the same. However, laser chamber  300  includes ancillary chambers  310  and  312 , as shown in FIG. 6, fluidically coupled to laser chamber  300 . Ancillary chambers  310  and  312  are similar in manufacture and use to ancillary chambers  210  and  212  shown in FIG.  6 . Consequently, laser chamber  300  has the advantages of the ancillary chamber embodiment as well as the advantages found in laser chamber  100 . Laser chamber  300  may also use differently configured heat exchangers  106 , such as heat exchanger  122 , shown in FIG.  5 . Also, laser chamber  300  may have blowers or flow vanes within ancillary chambers  310  and  312 . 
     FIG. 10 shows a front view of laser chamber  400 , which is similar to laser chamber  120  shown in FIG. 5, like designated elements being the same. Although laser chamber  400  includes a middle section  402  that extends the gas flow path through the chamber cavity, laser chamber  400  uses a conventional heat exchanger  404 , for example the type that is used in Excimer Laser Model 5000, by CYMER, Inc. Laser chamber  400  advantageously has a circuitous return path defined by middle section  402 , which along with baffles  406  and  408  direct shock waves away from the discharge region so that the shock waves may be dissipated elsewhere. 
     The added middle section  402  and its accompanying set of baffles also act to block the shock waves from directly traveling from the discharge region to the blower  108  and/or other internal components, as well as blocking their reflection directly back into the discharge region, as is possible with chamber  10  in FIG.  2 . 
     Although embodiments of the present invention have been described in considerable detail with reference to certain variations thereof, other embodiments are possible. For example, different blowers or electrodes may be used within the scope of the invention. Further, additional components, such as baffles, current returns and pre-ionizers may be included in a laser chamber. Moreover, additional configurations may be used for heat exchanger  106  to provide an aerodynamic passage for the circulating gas as well to take advantage of the additional surface area. Additional heat exchangers may also be used, for example protrusion  116  may serve as a heat exchanger. Therefore, the spirit and scope of the appended claims should not be limited to the description of the versions depicted in the figures.