Abstract:
A laser and method for operating a laser and disclosed. The laser may include a first discharge electrode and a second discharge electrode positioned at a distance from the first electrode. A laserable gas may be provided together with a circulation system to flow the gas into a space between the electrodes. A voltage source may be connected to the electrodes for creating a discharge in the space; and a flow guide having a guide surface in contact with the gas may be provided that is guide selectively moveable relative to the first electrode to guide a flow of gas into the space. In one implementation, the flow guide directs flow toward an extremity of the first electrode and is adjustable to direct flow toward the extremity as the first electrode wears.

Description:
FIELD OF THE INVENTION 
     The present invention relates to pulsed, gas discharge lasers. The present invention is particularly, but not exclusively useful as a gas discharge laser having one or more discharge electrodes that wear down (e.g. erode) over the lifetime of the laser. 
     BACKGROUND OF THE INVENTION 
     Modern pulsed, high repetition rate, gas discharge lasers such as ArF XeF and Krf excimer lasers and molecular fluorine lasers generally employ a pair of spaced apart, elongated (e.g. 30 cm long) main discharge electrodes to initiate lasing in a gaseous material. For example, each pulse may be produced by applying a very high, voltage potential across the electrodes with a power supply which causes a discharge between the electrodes having a duration of about 30 nanoseconds. A typical discharge may deposit about 2.5 J of energy into a gain region that is about 20 mm high, 3 mm wide and 500 mm long. 
     Each discharge alters the physical condition of the gas in the discharge region rendering the gas unsuitable for use in the next pulse. For this reason, a circulation system, which may include a high-speed cross-flow type fan, is typically provided to quickly exhaust “spent” gas from the discharge region immediately after a pulse and present a fresh portion of gas to the electrodes for the next pulse. Thus, at a pulse repetition rate in the range of 1000 Hz or greater, relatively high gas velocities are required to completely clear spent gas and debris prior to the next pulse. 
     In the absence of suitable precautions, the relatively high gas velocities that are required to exhaust all of the “spent” discharge gas are capable of creating turbulent flow. In particular, turbulence can develop near the discharge electrodes and cause undesirable arcing between the electrodes. This arcing, in turn, may result in poor laser performance, including, but not limited to, lowered pulse energy and lowered pulse-to-pulse energy stability. Heretofore, discharge chambers have been disclosed which include various permanently installed, non-adjustable, baffles, vanes and/or fairings to improve the aerodynamic geometry of the chamber and to reduce turbulence in the flow of laser gas. These features have also included non-adjustable fairings to minimize turbulent flow in and around the discharge electrodes, for example see U.S. Pat. No. 6,914,919, issued on Jul. 5, 2005 and titled, “Six To Ten KHZ, Or Greater Gas Discharge Laser System”. 
     For the above-described arrangement (i.e. high voltage, high repetition rate gas discharge laser), erosion and/or other wear mechanisms that are operable during electrode discharge may cause one or both of the electrodes to lose mass and physically shorten over the life of a laser. Indeed, currently available gas discharge lasers may have chambers having a useful life of 12 billion pulses or more. The significant shortening of the electrode (which may occur after only a half billion pulses or less) may create a geometrical mis-match between the electrode and any fairing or other surface feature that is provided to minimize turbulence in the flow of laser gas passing by the electrode. Although a small mis-match may be tolerable, larger mis-matches between the electrode and fairing can result in undesirable turbulent flow, arcing, and a corresponding reduction in laser performance. 
     With the above considerations in mind, Applicants disclose a laser and methods for operating a laser that adjust laser gas flow over the life of the laser to accommodate electrode shortening due to erosion. 
     SUMMARY OF THE INVENTION 
     A laser and method for operating a laser are disclosed. The laser may include a first discharge electrode and a second discharge electrode positioned at a distance from the first electrode. A laserable gas may be provided together with a circulation system to flow the gas into a space between the electrodes. A voltage source may be connected to the electrodes for creating a discharge in the space; and a flow guide having a guide surface in contact with the gas may be provided that is selectively moveable relative to the first electrode to guide a flow of gas into the space. 
     In one implementation, the flow guide directs flow toward an extremity of the first electrode and is adjustable to direct flow toward the extremity as the first electrode shortens due to erosion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an exploded, perspective view of a gas discharge laser chamber; 
         FIG. 2  shows a cross-sectional view of the gas discharge laser chamber of  FIG. 1 , as seen in the direction of line  2 — 2  in  FIG. 1 ; 
         FIG. 3  shows a cross-sectional view of a portion of the gas discharge laser chamber of  FIG. 1 , as seen in the direction of line  3 — 3  in  FIG. 2 ; 
         FIG. 4  shows a perspective, top view of an elongated electrode and adjustable flow guides; and 
         FIG. 5  shows a cross-sectional view as in  FIG. 3  of an embodiment having adjustable field shaping elements to accommodate electrode erosion. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring initially to  FIG. 1 , a chamber for a pulsed, gas discharge laser, such as an excimer laser (e.g. KrF, ArF, XeF, XeCl, etc.) or molecular fluorine laser, is shown and generally designated  10 . As shown, the chamber  10  typically includes a two-part chamber housing  12   a,b  that may be made of a relatively strong, corrosion resistant material, e.g. nickel plated aluminum, and is generally rectangular in construction with closed ends. With this structure, the housing components  12   a,b  and seal  13  may surround and enclose a volume  14  which may hold a laserable gas. 
       FIG. 2  shows that the chamber  10  may also include a gas discharge sub-system having two spaced apart electrodes  16 ,  18 , one of which may be designated a cathode and the other an anode. With this arrangement, a gas discharge region  20  is established in the space between the electrodes  16 ,  18  which includes the laser&#39;s beam axis  22  (axis  22  shown in  FIG. 1 ). Each electrode  16 ,  18  may be elongated, for example, to a length of about 40–80 cm and aligned in a direction generally parallel to the axis  22 . In addition, one of the electrodes, (in this case electrode  18 ) may be slightly longer than the other (i.e. electrode  16 ). Thus, the gas discharge region  20  for the chamber  10  shown in  FIGS. 1–3  is an elongated volume having a length approximately equal to the length of the shorter electrode (i.e. electrode  18  and a somewhat rectangular cross section, which may be for example, 3 mm wide by about 12 mm in the direction of electrode spacing for an ArF excimer laser. 
       FIG. 2  shows that electrical contact between the electrode  16  and housing  12  may be prevented by the use of an insulator  23 , sometimes referred to as a main insulator, which may be made from a dielectric, e.g. ceramic material. In one implementation, electrode  18  may be maintained at a constant reference potential, e.g. ground potential, and electrode  16  may be biased relative to the reference potential to initiate an electric discharge in the gas discharge region  20 . It is to be appreciated that other biasing schemes are possible. 
       FIGS. 1 and 3  also show that the laser can include a gas circulation system, which may include a high-speed cross-flow type fan  24  that is sufficient to quickly exhaust “spent” gas from the discharge region  20  immediately after a pulse and present a fresh portion of gas to the electrodes  16 ,  18  for the next pulse.  FIG. 3  shows that the fan  24  directs gas in the direction of arrow  26 . Further details regarding a suitable gas circulation system for use in a high repetition rate, discharge laser are disclosed in U.S. Pat. No. 6,914,919, which is hereby incorporated by reference herein. 
     Continuing with  FIG. 3 , it can be seen that the electrode  18  is mounted on a support member  28  and extends therefrom to an electrode extremity  30 . It is contemplated that the electrode  18  may have an initial height, H, as shown and may shorten due to erosion and/or other operable wear mechanisms, to a subsequent height, h.  FIGS. 3 and 4  also show that the laser may include a ceramic flow guide  32  which may have, for example, a triangular cross section normal to the direction of the laser axis  22 . As best seen in  FIG. 4 , for the chamber  10 , the flow guide  32  may be elongated and extend the length of the electrode  18 . 
       FIG. 3  illustrates one possible adjustment mechanism for moving the flow guide  34  relative to the electrode  18 . As shown there, the flow guide  34  may have an end  34  and an end  36 . For the mechanism shown, the end  34  may be pivotally attached to the support member  28  allowing the flow guide  32  to rotate about the end  34  at the pivot relative to the support member  28 . For example, a pivot joint may be established by forming a hole (not shown) in the flow guide  32  extending parallel to the laser axis  22  and inserting into the hole an appropriately sized pin (not shown) that is fixedly attached to the support member  28  allowing the flow guide  32  to rotate about the pin. Other arrangements are possible. 
       FIG. 3  further shows that the flow guide  32  may include an operative guide surface  38  that is in contact with and may direct the gas flow from the fan  26 . For the chamber  10 , the flow guide  32  may be oriented such that the guide surface  38  directs flow toward and past the extremity  30  of electrode  18  to minimize turbulent flow near the electrode  18 . As indicated above, turbulent flow near an electrode may cause unwanted arcing during an electrode discharge. Functionally, the adjustment mechanism may be provided to move the flow guide  34  for the purpose of maintaining a flow that directed toward and past the extremity  30  of the electrode  18  as the electrode  18  shortens due to erosion and/or other wear mechanisms. 
     In greater structural detail, the adjustment mechanism shown in  FIG. 3  may include a pull rod  40  that passes through a hole formed in the support member  28 . One end of the pull rod  40  may be attached to the flow guide  32  at the end  36  and the other end of the pull rod  40  may be attached to a flange  42  for movement therewith. With this cooperation of structure, spring  44  may be interposed between flange  42  and support member  28 . With this arrangement, end  36  of flow guide  32  is biased toward the support member  28 .  FIGS. 2 and 3  show that a cam operable mechanism  46  having cam surface  48  may be provided to selectively move flange  42  against the biasing force of the spring  44 . In particular, rotation of the mechanism  46  moves flange  42 , which in turn moves rod  40  and the end  36  of flow guide  32 . One end of the cam operable mechanism may extend through the housing  12  through bellows  49  as shown in  FIG. 2 . 
       FIGS. 3 and 4  show that a flow guide  50  may be positioned downstream of the electrode  18  and adjusted to minimize turbulent flow near electrode  18 . As shown, flow guide  50  may be adjusted using the same flange  42  and cam operable mechanism  46  used to adjust flow guide  32 . In particular, rod  52  connects flange  42  to flow guide  50  and spring  54  biases flange  42  away from support member  28 .  FIG. 3  also shows that stationary flow guides  56 ,  58  may be used to reduce turbulent flow near electrode  16 . As shown in  FIG. 2 , it may be beneficial to locate the adjustment mechanism near the end of the electrode  18  where little or no discharge occurs (note: electrode  16  is shorter than electrode  18 ). Also, a second adjustment mechanism (not shown) can be located at the other end of the chamber and operated by the same rod  46  (see  FIG. 2 ). 
     OPERATION 
     For the chamber  10 , it is contemplated that the electrode  18  may shorten over the life of the laser due erosion and other operable wear mechanisms. For example, in one application, the electrode  18  may be an anode of an anode-cathode pair and may be held at a reference potential, e.g. ground while the cathode is biased to create a discharge. For this arrangement, in some types of gas discharge systems, the anode may shorten considerable more than the cathode. Thus, an adjustable flow guide arrangement may be used for the anode in combination with stationary flow guides for the cathode. However, it is to be appreciated that the adjustable flow guides may be used on the anode, the cathode or both. 
     Adjustment of the flow guide(s) can be performed in response to one or more measured laser parameters, and may be performed either continuously or intermittently. Moreover, flow guide adjustment can be made manually, e.g. by service personnel, or automatically as part of a feedback control loop. In either case, adjustment can be made in response to a laser performance parameter including, but not necessarily limited to, pulse energy, pulse energy stability, pulse count (i.e. number of pulses since last adjustment or total number of pulses), or after a visual inspection of the electrode height or a visual indication that arcing is occurring. 
       FIG. 5  illustrates another embodiment which field shaping elements  52 ,  54  may be adjustable positioned using positioning mechanism  56  to accommodate electrode wear. These elements  52 ,  54  may be made of a metal or ceramic and function to effect the shape of the electric field generated between the electrodes  16 ′,  18 ′ in a manner known in the pertinent art. 
     While the particular aspects of embodiment(s) described and illustrated in this patent application in the detail required to satisfy 35 U.S.C. §112 is fully capable of attaining any above-described purposes for, problems to be solved by or any other reasons for or objects of the aspects of an embodiment(s) above described, it is to be understood by those skilled in the art that it is the presently described aspects of the described embodiment(s) of the present invention are merely exemplary, illustrative and representative of the subject matter which is broadly contemplated by the present invention. The scope of the presently described and claimed aspects of embodiments fully encompasses other embodiments which may now be or may become obvious to those skilled in the art based on the teachings of the Specification. The scope of the present invention is solely and completely limited by only the appended claims and nothing beyond the recitations of the appended claims. Reference to an element in such claims in the singular is not intended to mean nor shall it mean in interpreting such claim element “one and only one” unless explicitly so stated, but rather “one or more”. All structural and functional equivalents to any of the elements of the above-described aspects of an embodiment(s) that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Any term used in the specification and/or in the claims and expressly given a meaning in the Specification and/or claims in the present application shall have that meaning, regardless of any dictionary or other commonly used meaning for such a term. It is not intended or necessary for a device or method discussed in the Specification as any aspect of an embodiment to address each and every problem sought to be solved by the aspects of embodiments disclosed in this application, for it to be encompassed by the present claims. No element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element in the appended claims is to be construed under the provisions of 35 U.S.C. §112, sixth paragraph, unless the element is expressly recited using the phrase “means for” or, in the case of a method claim, the element is recited as a “step” instead of an “act”. 
     It will be understood by those skilled in the art that the aspects of embodiments of the present invention disclosed above are intended to be preferred embodiments only and not to limit the disclosure of the present invention(s) in any way and particularly not to a specific preferred embodiment alone. Many changes and modification can be made to the disclosed aspects of embodiments of the disclosed invention(s) that will be understood and appreciated by those skilled in the art. The appended claims are intended in scope and meaning to cover not only the disclosed aspects of embodiments of the present invention(s) but also such equivalents and other modifications and changes that would be apparent to those skilled in the art.