Abstract:
A gas discharge modular laser with beam train isolation between laser chamber module and front and rear optics which define the laser resonant cavity. Beam train isolation units isolates the beam train from atmospheric air while permitting quick and easy removal of the laser chamber without disturbing the optics of the resonant cavity. In preferred embodiments, metal bellows units are bolted at only side so that the chamber module can be removed and replaced without unbolting the bellows unit.

Description:
BACKGROUND OF THE INVENTION 
     Ultraviolet lasers are widely used in industry. Important examples are current use of KrF and ArF excimer lasers (and the currently scheduled use of F 2  excimer lasers) for lithographic fabrication of integrated circuits. These lasers typically operate 24 hours a day, 7 days per week 365 days per year with only short down times for maintenance. 
     Ultraviolet light generated in these lasers can damage sensitive optical components in the presence of oxygen or a wide variety of other chemicals or chemical compounds. Also, oxygen is significantly absorptive of the ArF laser beam and very absorptive of the F 2  laser beam. For these reasons, a common practice is to purge sensitive optical components of these lasers with nitrogen or helium. Another known practice for reducing optical damage is to minimize the use of components or materials which out-gas chemical vapors during laser operation. 
     A well used technique used in the construction of these lasers is to group components into modules which can be quickly and easily replaced as a part of a maintenance program. 
     The path of the laser beam through a laser system is referred to as a “beam train”. Attempts have been made to seal the beam trains from the outside environment. These attempts especially attempts to seal the sections of the beam train in between modules, have often made module replacement much more difficult. Also, seals between modules may permit unwanted vibration produced in one module to be transferred to another module where the vibration adversely affects performance. This is an especially serious concern for the modules containing the optical components which form the resonant cavity of the laser and the component (sometimes called a “wavemeter”) that measures beam parameters such as wavelength and bandwidth. 
     FIG. 1 is a drawing of a prior art KrF laser system with the front doors of the laser cabinet removed. The drawing shows chamber  156 , line narrowing module  120  output coupler module  130  and wavemeter  140 . The direction of the output laser beam is shown at  142 . Chamber  156  weighs about 200 pounds but is fitted with wheels and can be replaced quickly and easily by disconnecting two gas lines and rolling the old chamber out and rolling a new chamber in on rails as shown in FIGS. 8,  8 A,  9  and  9 B. In this prior art KrF laser, the portions of the beam train between the chamber and the output coupler end and between the chamber and the LNP are not sealed so problems associated with transmittal of vibration through seals and seal interference with chamber removal does not exist. 
     What is needed is an effective method for protecting the portion of the beam train between a laser chamber and optical equipment forming the resonant cavity of the laser while permitting easy replacement of the laser chamber. 
     SUMMARY OF THE INVENTION 
     The present invention provides beam train isolation between a gas discharge laser chamber of a modular laser system and front and rear optics defining the laser resonant cavity while permitting quick and easy removal of the laser chamber without disturbing the optics of the resonant cavity. In preferred embodiments, metal bellows units are bolted at only one side so that the chamber can be removed and replaced without unbolting the bellows unit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a modular prior art laser system. 
     FIG. 2 is an exploded view showing features of a first embodiment present invention. 
     FIG. 3 is an enlarged view of a portion of FIG.  2 . 
     FIG. 4 is a top view of the FIG. 2 embodiment. 
     FIGS. 5A, B and C show how the present invention works. 
     FIGS. 6A,  6 B,  6 C and  7  show features of a second embodiment of the present invention. 
     FIGS. 8,  8 A,  9  and  9 A show how a chamber rolls into position. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Preferred Embodiment 
     A first embodiment of the present invention is shown in FIGS. 2,  3 ,  4  and  5 A, B and C. In this embodiment a special beam sealing bellows unit is used on both the LNP side of the chamber and the output coupler side of the chamber. These seal units: 
     1) contain no elastomers 
     2) provide vibration isolation for the LNP and the OC from chamber vibration 
     3) provide beam train isolation from atmospheric gases 
     4) permit unrestricted replacement of the chamber without disturbance of the LNP and the output coupler. 
     An exploded perspective view of the LNP, chamber and output coupler portions of an ArF laser system demonstrating this preferred embodiment is shown in FIG. 2. A bellows system between the LNP and the back chamber window unit is shown at  12 A and an identical bellows system between the output coupler and the front chamber window unit is shown at  12 B. An enlarged exploded view of the LNP-chamber interface is shown in FIG. 3 and a top exploded view is shown in FIG.  4 . 
     As has been done in prior art designs, the LNP  120  and the output coupler  130  are rigidly connected together using a structure called a three-bar mount and this structure is kinematically mounted on the laser frame separate from the chamber  156  using a bearing system which avoids any potential distortion of the structure. The three-bar mount comprises output coupler support frame  176  and LNP support frame  178  which are connected to each other with three cross braces  174 , each of which are comprised of bars having almost zero coefficient of thermal expansion all as described in U.S. Pat. No. 6,109,574 which is incorporated herein by reference. It is important that the optical components of the LNP and the output coupler be isolated as much as feasible from the laser chamber because the chamber is subject to relatively wide swings in temperature during normal operation. Also, the chamber fan and fan motor generate substantial vibrations. Vibrations are also generated in the chamber by electric discharges which occur at rates in the range of 1000 hz to 4000 hz. 
     Bellows Details 
     The bellows systems shown at  12 A and  12 B are identical. FIGS. 5A,  5 B and  5 C show the basic details of both bellows design using the LNP bellows as an example. These figures also show how the chamber is installed without any assembly or disassembly of the two bellows systems. The principal components of each bellows system are a bellows unit  13  and flexible alignment bracket  14 . The bellows unit  13  comprises a flexible metal bellows  13 A with accordion-like cylindrically-shaped walls, back base plate  13 B and front base plate  13 C. Back base plate  13 B is bolted to the front plate  178  of the LNP. Both surfaces of back base plate  13 B and the mating surface of LNP front plate  178  are very smooth and flat so that when bolted together they form a very tight fit. Alignment bracket  14  comprises four springy metal legs which springingly deform when force is applied. The bracket is attached to the LNP frame with four small bolts at the locations shown at  15  on FIG.  3 . The corresponding bolt slots in bracket  14  are oval shaped and the bolts are designed to permit alignment bracket to spread out when a force is applied to it in the direction of the LNP. FIGS. 5A,  5 B and  5 C show three views of the LNP bellows system as a chamber  156  is being installed in laser cabinet. In FIG. 5A a chamber  156  having chamber window block  156 A is being rolled into position and in this view is about one inch from its final installed position. In FIG. 5B the chamber has been rolled in closer and begins compressing flexible alignment bracket  14  which in turn compresses bellows unit  13 . In FIG. 5C the chamber is fully installed and both bellows units and alignment bracket  14  are compressed into their normal operational position. In this configuration, bellows unit  13  is not in contact with alignment bracket  14  so that all of the compression force generation by the deformation of bellows unit  13  is applied between chamber window block  156 A and LNP front plate  178 A. In this preferred embodiment, this compressive force is about one to two pounds which effectively isolates the beam train at this location from atmospheric air. However, Applicants have shown that this force could be reduced substantially down to about 0.1 pound without significantly reducing the quality of the seal. The reader should note that a tighter seal can be provided by using a bellows having a larger compressive force applied at this junction. However, a greater force applied in this manner would increase the vibrational coupling between the chamber and the LNP. Applicants have determined that forces in the range of 1 to 2 pounds force is a reasonable compromise. Another consideration in the design of this beam isolation unit is that very low force bellows are difficult to fabricate and tend to be very expensive. Also, these bellows are more subject to damage during use and handling. 
     As indicated above, the bellows system between the chamber and the output coupler is substantially identical to the one described above so that the chamber can be moved into and out of position without the necessity of manually connecting or disconnecting any beam train isolation components. 
     Preferably, both the LNP and the output coupler are purged at least during laser operation with N 2  or another appropriate purge gas at a flow rate such that the LNP and the output coupler are pressurized very slightly above atmospheric pressure. Applicants&#39; test have shown that with the above configuration the oxygen content inside the LNP and output coupler is reduced to less than 100 parts per million. 
     The reader should also note that this design does not include any elastomer seals so that out-gassing from such seals is not a problem. 
     Second Preferred Embodiment 
     A second preferred embodiment of the present invention is shown in FIGS. 6A,  6 B and  7 . 
     FIG. 6A is a cross sectional drawing showing important features of this second preferred embodiment. This embodiment provides a substantially tighter seal at both the LNP and the output coupler sides of the chamber but also provides ease of chamber replacement minimal vibration transfer with no elastomer seals. 
     The bellows structure  19  is a flexible unit comprised of a chamber window block mounting flange  20  a clampable flange  22 , a rigid cylinder  24  machined from 304SST and two accordion-type bellows assemblies  26 A and  26 B made from Perkin Elmer Fluid Sciences AM 350 bellows material. The chamber window unit includes two metal c-seals which fit at locations  28  and  30  as shown in FIG. 6A to seal purge gas from contamination. Flange  20  attaches to the window assembly by four 10×32 cap screws at location  32  in FIG.  6 A. Flange  22  attaches to LNP frame  178  using a V-clamp unit  31  shown in FIG. 6B which is seal mounted on the LNP frame  178 . V-clamp mechanism  31  is a mechanical device which captures clampable flange  22  when the chamber is rolled into place. The components of the v-clamp includes a bracket  36  which is machined from solid AL 6061. Two cams  38  made from free cutting brass (C36000) located on precision ground (303 stainless steel) cam shaft  40  and fixed to position by dowel pins  42 . Activation handle  44  made from AL 6061 is located on cam shaft  40  and fixed in position by dowel pin  42 . This handle activates yoke-like lever  46  which is attached to shaft  48  which is made from 303 stainless steel and pivots about the axis of shaft  48 . 
     The V-clamp works as follows. The V-clamp shown in FIG. 6 is mounted on LNP frame  178  with bolts at  50 . Torsion spring  52  holds the front edge  47  of yoke-like lever  46  about 1 cm off the surface of LNP frame  178  (not shown). As chamber  156  is rolled into position, clampable flange  22  passes very close to the surface of LNP frame  178  until the outer edge  22 A of clampable flange  22  is positioned between yoke-like lever  46  and the surface of LNP frame  178 . 
     When chamber  156  is in its proper position between LNP  120  and output coupler  130 , clampable flange  22  is clamped into position by rotating activation handle  44  90° to 180° (into the page in the FIG. 6B drawing). Cams  38  being offset from the axis of shaft  40  applies a force out of the page (in the FIG. 6B drawing) against the underside of extensions  45  of yoke-like lever  46  which forces the  45 B portion of lever  46  downward clamping clampable flange  22  into position. A metal c-seal in slot  30  is compressed by the clamping force providing an air-tight seal between the bellows structure  19  and LNP frame  178 . FIG. 6C shows the operation of the V-clamp unit. 
     FIG. 7 shows the bellows unit in place sealing the chamber-LNP interface. This is a cross-sectional top view. Shown on the drawing are metal c-seals at  54  and  56 , chamber window block  156 A, purge vent hole  58 , chamber window  60  with seal  60 A. Arrow  62  shows where the outer edge  22 A of clampable flange  22  is clamped against LNP frame  178  by yoke-like lever  46 . 
     A similar bellows structure is utilized at the interface of the chamber  156  and output coupler  130 . When the chamber is to be removed, actuation handles are pivoted 90° to 180° in the direction opposite the clamping direction and this separates the bellows structures from LNP frame  174  and output coupler  164  permitting the chamber to be rolled out for replacement without any disturbance of the resonant cavity optics. 
     Although this invention has been described in detail with reference to specific preferred embodiment, the reader should understand that many variations of the above embodiments are possible. Therefore the reader should understand that the scope of the invention should be determined by the appended claims and their legal equivalents.