Abstract:
A cross-flow fan impeller for circulating gas in a transversely excited, pulsed, gas discharge laser is disclosed and may comprise a plurality of hubs, the hubs spaced apart along the impeller&#39;s rotation axis and establishing at least two impeller segments wherein a first segment has an output flow within 80-120% of a second segment and the first and second segment having differing blade pass frequencies. In some embodiments of this aspect, the first segment may have n number of blades the second segment m number of blades, and m≢n. In one embodiment, the impeller may be configured with n=29 and m=23 and in another embodiment, the impeller may be configured with n=23 and m=19. The impeller may be configured wherein n and m are prime numbers.

Description:
[0001]    The present application is related to U.S. Pat. No. 6,023,486, which issued on Feb. 8, 2000, titled SOLDERED FAN ASSEMBLY FOR ELECTRIC DISCHARGE LASER, Attorney Docket Number 1999-0014-01, U.S. Pat. No. 6,034,984, which issued on Mar. 7, 2000, titled TANGENTIAL FAN WITH CUTOFF ASSEMBLY AND VIBRATION CONTROL FOR ELECTRIC DISCHARGE LASER, Attorney Docket Number 1998-0029-01, U.S. Pat. No. 6,061,376, which issued on May 9, 2000, titled TANGENTIAL FAN FOR EXCIMER LASER, Attorney Docket Number 1998-0024-01, U.S. Pat. No. 6,195,378 which issued on Feb. 27, 2001, titled, TWISTED BLADE TANGENTIAL FAN FOR EXCIMER LASER, Attorney Docket Number 1997-0029-01, U.S. Pat. No. 6,144,686 which issued on Nov. 7, 2000, titled TANGENTIAL FAN WITH CUTOFF ASSEMBLY AND VIBRATION CONTROL FOR ELECTRIC DISCHARGE LASER, Attorney Docket Number 1999-0048-01 and U.S. Pat. No. 6,765,946 which issued on Jul. 20, 2004, titled FAN FOR GAS DISCHARGE LASER, Attorney Docket Number 2001-0095-04, the entire contents of each of which are hereby incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to pulsed, gas discharge lasers. The present invention is particularly, but not exclusively useful as a cross-flow fan impeller for a transversely excited, pulsed, gas discharge laser. 
       BACKGROUND OF THE INVENTION 
       [0003]    Gas discharge lasers such as excimer lasers are well known light sources useful for integrated circuit lithography. These lasers typically include two elongated discharge electrodes (for example, about 30 cm in length) that are separated by about 5-20 mm to establish a discharge region between the electrodes. A high voltage pulse power source provides high voltage electrical pulses to produce discharges between the electrodes to create a gain region in a laser gas. A laser gas circulation system is generally employed to produce sufficient laser gas flow between the electrodes to remove from the discharge region substantially all of the heated, ionized gas volume and erosion debris particles produced by each discharge prior to the next succeeding discharge. For this purpose, it is typically desirable to establish a gas flow through the discharge region that is relatively uniform along the length of the electrodes. For this reason, cross-flow fans (also referred to in the art as tangential fans) have been used. For example, co-owned U.S. Pat. Nos. 6,023,486, 6,034,984, 6,061,376, 6,195,378, 6,144,686 and 6,765,946 disclose several cross-flow fan designs, each of which is hereby incorporated by reference herein. 
         [0004]    Structurally, cross-flow fans include an elongated, somewhat cylindrical, impeller (also sometimes referred to as a squirrel cage rotor) which is rotated about a longitudinal axis by one or more motors. For example, the impeller may include a plurality of annularly shaped hubs that are spaced apart along the rotation axis and oriented orthogonal to (and substantially centered on) the rotation axis. For the impeller, each hub-pair may constitute an. impeller segment and a number of blades may be provided connecting the hubs together at or near the periphery of each segment. In this manner, the blades surround a somewhat cylindrical internal impeller volume. In use, the impeller is typically disposed within and rotates relative to a flow guiding structure which establishes an intake side and a discharge side of the impeller. For some arrangements, this flow guiding structure may include one or more so-called flow cutoff members. When the fan impeller is rotated, laser gas is drawn through the blades into the internal cylindrical volume over the entire length of the fan impeller. Inside the impeller, the laser gas flow is diverted and accelerated by a vortex that is created by the rotation of the impeller. The laser gas then exits over the entire length of the impeller on the discharge side. 
         [0005]    As the impeller blades pass the director(s), e.g. flow cutoff member, they may adversely affect laser performance in two ways. First, a mechanical vibration may be produced in the cut-off member structure that may be transmitted to the optical components defining the laser cavity. Second, the flow produced by the impeller may not be smooth, but instead, may consist of many small pressure pulses. Some of these pressure pulses may reach the gain volume where they may perturb the gain media&#39;s index of refraction. This perturbation, in turn, may result in an undesirable deterioration of one or more laser performance parameters such as spectral bandwidth, divergence, pulse-to-pulse energy stability, etc. In general, laser performance deterioration is more pronounced at discharge repetition rates which corresponding to the impeller&#39;s blade pass frequency and its sub-harmonics (each of which is a function of the impeller rotation speed and the number of blades distributed around the impeller&#39;s periphery). As used herein, the term “blade pass frequency” and its derivatives means the reciprocal of the time duration between successive passes of a blade by a stationary point during an impeller rotation. Applicant&#39;s have found that an impeller with an even number of blades may generate more undesirable sub-harmonics than an impeller with an odd number of blades, and moreover, impellers having a prime number of blades may generate fewer sub-harmonics than impellers having a non-prime number of blades. 
         [0006]    With the above considerations in mind, Applicants disclose a cross-flow fan impeller and cross-flow fan system for a gas discharge laser. 
       SUMMARY OF THE INVENTION 
       [0007]    In a first aspect, a cross-flow fan impeller for circulating gas in a transversely excited, pulsed, gas discharge laser is disclosed. For this aspect, the impeller may comprise at least three hubs, the hubs spaced apart along an impeller rotation axis with each pair of adjacent hubs establishing an impeller segment and each segment having a peripheral region. One of the segments may have n number of blades located at the segment&#39;s peripheral region and another segment may have m number of blades located at the segment&#39;s peripheral region, with m≢n, and each segment may generate a flow speed in a range of 40 to 60 m/s at a fan rotation speed of about 3500 rpm, and in specific cases may generate a flow speed in a range of 45 to 55 m/s at a fan rotation speed of about 3500 rpm. In one embodiment, the impeller may be configured with n=29 and m=23 and in another embodiment, the impeller may be configured with n=23 and m=19. 
         [0008]    The impeller may be configured wherein n and m are prime numbers. Each blade may be aligned parallel to the rotation axis or an impeller may be configured wherein one or more blades are aligned nonparallel to the rotation axis. In one implementation, the segment having n number of blades may be adjacent to the segment having m number of blades. In one arrangement the impeller may comprises more than 15 segments. In one setup, blades in each segment may be azimuthally offset from blades in adjacent segments. Each blade may have a length and a curved cross-section normal to it&#39;s length. The blades in each segment may be nonuniformly spaced around the peripheral region. In some arrangements, each hub may be annularly shaped. 
         [0009]    In another aspect, a cross-flow fan impeller for circulating gas in a transversely excited, pulsed, gas discharge laser may comprise a plurality of hubs, the hubs spaced apart along the impeller&#39;s rotation axis and establishing at least two impeller segments wherein a first segment has an output flow within 80-120% of a second segment and the first and second segment having differing blade pass frequencies, and in specific cases the first segment may have an output flow within 90-110% of the second segment, the first and second segment having differing blade pass frequencies. In some embodiments of this aspect, the first segment may have n number of blades, the second segment m number of blades, with m≢n. In one embodiment, the impeller may be configured with n=29 and m=23 and in another embodiment, the impeller may be configured with n=23 and m=19. The impeller may be configured wherein n and m are prime numbers. 
         [0010]    Another aspect is disclosed in which a cross-flow fan system for circulating gas in a transversely excited, pulsed, gas discharge laser may comprise an impeller defining a rotation axis, the impeller comprising a plurality of hubs, the hubs spaced apart along the axis and establishing at least two impeller segments wherein a first segment has an output flow within 80-120% of a second segment and the first and second segment having differing blade pass frequencies, and in specific cases the first segment may have an output flow within 90-110% of the second segment, the first and second segment having differing blade pass frequencies. For this aspect, the fan system may further comprise at least one motor for rotating the impeller about the axis; and a flow guiding structure which establishes an intake side and a discharge side of the impeller. In some embodiments of this aspect, the first segment may have n number of blades the second segment m number of blades, and m≢n. In one embodiment, the impeller may be configured with n=29 and m=23 and in another embodiment, the impeller may be configured with n=23 and m=19. The impeller may be configured wherein n and m are prime numbers. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  shows a simplified, perspective, partially exploded view of a transverse discharge gas laser; 
           [0012]      FIG. 2  shows a simplified sectional view of a laser chamber as seen along line  2 - 2  in  FIG. 1 ; 
           [0013]      FIG. 3  shows a perspective view of a cross-flow fan impeller having eighteen segments; 
           [0014]      FIG. 4  shows a sectional view of the impeller shown in  FIG. 3  as seen along line  4 - 4  in  FIG. 3  showing a segment having 19 blades; 
           [0015]      FIG. 5  shows a sectional view of the impeller shown in  FIG. 3  as seen along line  5 - 5  in  FIG. 3  showing a segment having 23 blades; 
           [0016]      FIG. 6  shows measured flow performances (fan speed in RPM v. flow speed in m/s) for several fan configurations; 
           [0017]      FIG. 7  shows measured flow performances (motor power in kW v. flow speed in m/s) for several fan configurations; and 
           [0018]      FIG. 8  shows measured flow performances (motor current in A v. flow speed in m/s) for several fan configurations. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0019]    Referring initially to  FIG. 1 , a simplified, partially exploded view of portions of a transverse discharge gas laser device are shown and generally designated  20 . For example, the device  20  may be a KrF excimer laser, an XeF excimer laser, an XeCl excimer laser, an ArF excimer laser, a molecular fluorine laser or any other type of transverse discharge gas laser known in the pertinent art. As shown, the device  20  may include a two-part chamber housing  22   a,b  being formed of a chamber wall that may be made of a conductive, corrosion resistant material, e.g., nickel-plated aluminum. As further shown in  FIG. 1 , window assemblies  24   a,b  may be provided at each end of the chamber housing  22   a,b  to allow light to enter, exit and pass through the chamber housing  22   a,b  along a common beam path. With this structure, the hollow chamber housing  22   a,b  and window assemblies  24   a,b  may surround a volume which holds a gas medium under pressure together with other components suitable to create a discharge in the medium. These other components may include, for example, a pair of discharge electrodes (not shown in  FIG. 1 ), a fan system to circulate the gas (not shown in  FIG. 1 ), heat exchangers to cool the gas (not shown in  FIG. 1 ), etc. It is to be appreciated that the chamber housing  22   a,b  may also be formed with a number of sealed inlets/outlets (not shown in  FIG. 1 ), to allow gas to be introduced/extracted from the chamber, to allow conductors  26  to deliver an excitation voltage to the electrodes, etc. 
         [0020]    In addition to the chamber,  FIG. 1  shows that the device  20  may also include a beam reverser  28  and output coupler  30  cooperatively arranged to form an optical cavity. For the device  20 , the beam reverser  28  may be as simple as a flat, fully reflective mirror or as complex as a grating-based line-narrowing unit. Continuing with  FIG. 1 , the device  20  may also include a pulse power system  32  delivering electrical pulses to electrodes located within the chamber housing  22   a,b  via conductors  26 .  FIG. 1  further illustrates that during operation of the device  20 , a laser beam  34  is created which exits the optical cavity via the output coupler  30 . 
         [0021]    It is to be appreciated that the use of the fan impeller and fan system described infra is not limited to the stable, standing wave cavity alluded to above, rather, the fan system and fan impeller described herein may be employed within other optical arrangements such as a one-pass amplifier, multi-pass amplifier, traveling wave amplifier such as a ring amplifier, unstable cavities, etc. 
         [0022]      FIG. 2  shows the inside of the chamber in greater detail. As seen there, chamber components may include chamber housing upper  22   a  and lower  22   b , cathode  36  and anode  38  (which are spaced apart to establish a discharge region  40 ), preionizer tube  42 , anode support flow shaping structure  44 , flow shaping structure  46 , fan impeller  48 , and four water cooled heat exchanger units  50   a - d.    
         [0023]    Cross referencing  FIGS. 2 and 3 , is can be seen that the device  20  includes a cross-flow fan system for circulating laser gas through the chamber. As shown, the fan system may include an impeller  48  defining a rotation axis  52 , a motor  54  (note: in some cases, two motors may be used with one on each end of the impeller) for rotating the impeller  48  about the axis  52 ; and a flow guiding structure which establishes an intake side and a discharge side of the impeller  48 . For the arrangement shown in  FIG. 2 , flow guiding structure includes directors  56   a,b  with director  56   a  (sometimes referred to as a so-called flow cutoff member) extending from the anode support flow shaping structure  44  to a location proximate the impeller  48  and director  56   b  attached to the housing wall and extending therefrom to a location proximate the impeller  48 , as shown. Typically, each director  56   a,b  is configured to extend along the length of the impeller  48 . With the arrangement shown in  FIG. 2 , gas from the discharge region  40  flows in the direction of arrow  58  and passes through the heat exchanger units  50   a - d  where it is drawn into the central volume  60  of the impeller  48 . Gas is then discharged from the impeller  48  and directed to the discharge region  40 . 
         [0024]    As best seen in  FIG. 3 , the impeller may include a plurality of annularly shaped hubs (of which hubs  62   a - c  have been identified with reference numerals). Although an impeller  48  with nineteen hubs is shown in  FIG. 3 , it is to be appreciated that more than nineteen and as few as three hubs may be used. Also shown, the hubs  62   a - c  may be spaced apart along the axis  52 , centered on the axis  52  and oriented orthogonal to (and centered on) the rotation axis  52 , with each adjacent hub pair establishing an impeller segment (of which segments  64   a - c  have been identified with reference numerals). Although an impeller  48  with eighteen segments is shown in  FIG. 3 , it is to be appreciated that more than eighteen and as few as two segments may be used. 
         [0025]    Cross referencing  FIGS. 3 and 4 , it can be seen that segment  64   a  may include a plurality of blades  66  (of which blades  66   a - c  have been identified with reference numerals in  FIG. 4 ) connecting hub  62   a  to hub  62   b  at or near the periphery of the segment  64   a . As best seen in  FIG. 4 , segment  64   a  includes nineteen blades  66 , with each blade  66  having a curved cross section normal to the rotation axis  52 . Although the blades  66  are shown uniformly distributed around the periphery of the segment in  FIG. 4 , in other embodiments, the spacing between adjacent blades within a segment may vary around the segment periphery.  FIG. 3  also shows that the blades  66  may be relatively straight along their length and aligned substantially parallel with the rotation axis  52 . 
         [0026]    Cross referencing  FIGS. 3 and 5 , it can be seen that segment  64   b  may include a plurality of blades  68  (of which blades  68   a - c  have been identified with reference numerals in  FIG. 5 ) connecting hub  62   c  to hub  62   b  at or near the periphery of the segment  64   b . As best seen in  FIG. 5 , segment  64   b  includes twenty three blades  68 , with each blade  68  having a curved cross section normal to the rotation axis  52 . Although the blades  68  are shown uniformly distributed around the periphery of the segment in  FIG. 5 , in other embodiments, the spacing between adjacent blades within a segment may vary around the segment periphery.  FIG. 3  also shows that the blades  68  may be relatively straight along their length and aligned substantially parallel with the rotation axis  52 . 
         [0027]    With the above-described cooperation of structure, it can be seen that adjacent impeller segments may have a different number of blades. Thus, for example, the impeller may be configured having nine segments with nineteen blades and nine segments with twenty-three blades, the dissimilar segments alternating along the length of the impeller. Alternatively, several adjacent segments may have the same number of blades.  FIG. 3  also shows that a blade in one segment may be azimuthally offset from a corresponding blade in an adjacent segment. 
         [0028]      FIG. 6  shows measured flow performances (fan speed in RPM v. flow speed in m/s) for fans having 23 blades in all segments and 0.65, 0.76 and 0.85 inside diameter to outside diameter ratio (ID/OD), fans having 19 blades in all segments and 0.65 and 0.76 ID/OD and a fan having 29 blades in all segments and 0.85 ID/OD.  FIG. 7  shows measured flow performances (motor power in kW v. flow speed in m/s) for fans having 23 blades in all segments and 0.65, 0.76 and 0.85 ID/OD, fans having 19 blades in all segments and 0.65 and 0.76 ID/OD and a fan having 29 blades in all segments and 0.85 ID/OD.  FIG. 8  shows measured flow performances (motor current in A v. flow speed in n/s) for fans having 23 blades in all segments and 0.65, 0.76 and 0.85 ID/OD, fans having 19 blades in all segments and 0.65 and 0.76 ID/OD and a fan having 29 blades in all segments and 0.85 ID/OD. 
         [0029]      FIGS. 6-8  illustrate and support the proposition that an impeller having segments with different numbers of blades can be configured having an acceptably uniform flow speed along the length of the impeller. 
         [0030]    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”. 
         [0031]    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.