RELATED APPLICATION 
     This application claims benefit of U.S. Provisional Patent Application No. 60/742,162, filed Dec. 1, 2005. 
    
    
     COPYRIGHT NOTICE 
     © 2006 Electro Scientific Industries, Inc. A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 37 CFR §1.71(d). 
     TECHNICAL FIELD 
     The present disclosure relates to laser-based workpiece processing systems and, in particular, to optical component cleanliness and debris management in laser micromachining applications performed in such systems. 
     BACKGROUND INFORMATION 
     The state of the art for sealing laser beam paths in semiconductor processing equipment entails enclosing the entire optics volume with a cabinet style enclosure. Some designs incorporate a purging system using some sort of clean dry air or inert gas. Beam tubes are also used in other laser applications outside of micromachining, such as integrated circuit error correction. Most recent designs of UV optic rails and beam paths use covers to protect the optical components. Neither purge gas inside the enclosed volume nor beam tubes with purge gas have been used in laser micromachining applications. 
     It is well known in the laser industry that UV wavelength laser light can be very damaging to system optical components. The photon energy given by E=hν(where h=Planck&#39;s constant and ν=optical frequency) for UV light is sufficient to break and reform bonds in many common airborne molecular contaminants (AMCs). In this process known as photopolymerization, polymers are formed on optical surfaces that intersect the laser beam. The polymers cloud the lenses and mirrors, reducing optical transmittance of the system, and causing beam distortion that degrades performance. Similar problems may occur in the presence of particulate contamination. Particulates can become vaporized, and in turn, polymerized onto optical surfaces. Additionally, in the presence of high instantaneous energy pulsed beams, an acoustic “shock” wave may be formed as a particulate is ablated. This acoustic shock wave may damage optical coatings, substrates, or both, as it propagates into an optical component. 
     Currently available pulsed lasers with nanosecond, picosecond, or femtosecond pulse widths suffer optical degradation resulting from the high peak powers incident upon their optical components. Often, based upon the application, the laser may deliver excess output energy that must be attenuated. Currently available attenuators, often composed of a half-wave plate and polarizer combination (or a variation of this theme), are inserted into the path of the laser beam to attenuate the laser beam by manipulating its polarization state. Although the technique of using a half-wave plate and polarizer offers the ability to adjust the level of attenuation, the attenuator assembly usually must be placed after several optical components “downstream” from the laser output. The reason is that the half-wave plate and polarizers work best when collimated or nearly collimated light is incident upon them. In addition, the half-wave plate, in the case of a sealed laser rail, would not make a very good window into the sealed portion because waveplates are prone to contamination, are fragile, and are temperature sensitive. 
     A laser rail, forming part of a laser optical system and sealed from the outside environment, uses input and output windows of the optical system to allow the beam to pass into and out of the sealed portion of the laser rail. Moreover, it is desirable to decrease the amount of laser light incident on all optical components because the intensity of the laser light (in W/cm 2 , peak W/cm 2 , or J/cm 2 ) is proportional to the age of the optics. Therefore, in an ideal laser system that produces excessive laser power, the very first component in the optical system would be an attenuator of some type. In summary, it would be desirable to provide the same optical element(s) functioning as an input window and an attenuator. 
     Laser optical systems include laser shutters that can be divided into two different categories. They include modulation, exposure, and pulse gating shutters and safety interlock and process control shutters. Safety interlock shutters, which are of interest here, intermittently block the laser beam by means of a material that is opaque to the laser wavelength and is caused to move selectively in and out of the line of propagation of the laser beam. The blocked laser beam is reflected into or onto a laser beam “block” or “dump,” which serves to absorb and attenuate the blocked beam. Shutter actuation devices include, but are not limited to, electro-mechanical (solenoid), electrical, and magnetic devices. 
     A shutter operating as a safety (rather than a modulation) device opens and closes at a low frequency of repeated operation (&lt;&lt;1 Hz). The open and closed positions are sensed and fed back to the operating system. A properly designed laser shutter blocks laser emission and does not cause it to reflect back into the lasing cavity. Shutter construction materials should be free from components that are likely to contaminate the optical system. 
     SUMMARY OF THE DISCLOSURE 
     Preferred embodiments of a purge gas port, laser beam attenuating input window, and laser shutter constitute subsystems of a UV laser optical system in which the light beam path is completely enclosed to reduce contamination of the optical system components. Purge gas is injected through multiple, e.g., nine, different locations in a beam tube assembly to make certain that the optical component surfaces sensitive to contamination are in the flow path of the purge gas. Clean, dry air is preferred over an inert purge gas because of the availability of compressed air in semiconductor device fabrication facilities and the absence of ozone formation at the preferred 355 nm operating wavelength. All of the beam tube assembly components are preferably made from nickel plated aluminum, stainless steel, brass, and Teflon® materials because of their outgasing properties. The flow level of the purge gas is determined by two criteria: (1) maximize gas flow without generating turbulence in the beam spot on the work surface, and (2) provide a minimum number of air volume exchanges each hour. Five air volume exchanges each hour are chosen as a minimum for the embodiment described. Porous diffusers are used at the purge gas injection points to reduce turbulence-induced beam motion. Purge gas flow in a controlled environment containing the laser beam path allows the laser rail to last longer than one that is not equipped with purge gas flow through a beam tube assembly. 
     An input window functioning as a fixed level attenuator is formed of a transparent optical material including, for example, but not limited to, fused silica with plano-plano surfaces oriented at 45 degrees with respect to the incident laser beam for S-polarization. Since this optical component acts as the input window assembly into a sealed optical assembly, it is desirable to have a mounting system that is resistant to perturbations from mechanical components sealing the optical system (in this case, the covers) that would in turn cause potential misalignment of the laser beam. Preferred mounting hardware is, therefore, resistant to any stresses caused by the covers as it accomplishes beam attenuation and damage prevention. Lastly, since it sees the highest W/cm 2 , and is exposed to the external environment, the input window has the highest probability of damage resulting from external contaminants polymerizing or burning onto the outer surface. The input window is, therefore, slightly offset from the centerline of the laser beam and is of sufficient size so that, if a burn occurred, the input window could be rotated to a new “clean” portion of the optical surface. When the input window is rotated in response to damage to the external optical surface, no additional beam steering occurs. When the covers are installed to seal the optical assembly, no additional beam steering occurs from the stresses of the cover fitted on the opto-mechanical assembly. 
     A laser shutter composed of pneumatic cylinder retracts and extends to position a metal shutter blade out of and into the path of the laser beam. A magnetic reed switch senses the position of the cylinder and feeds a position signal back to a system control computer. In the blocked state, the shutter blade reflects the laser beam into a feature of a structural gusset to safely attenuate the beam. The shutter mechanism is partly covered and shares a clean air purge source with the bulk of the laser optical assembly. Three laser shutter design features include (1) portions of the mechanism exposed to laser light are made from non-outgasing materials to limit optical contamination; (2) the pneumatic cylinder imparts a motive force to the shutter blade that does not contain materials likely to cause contamination of the optical components and does not generate heat and thereby ensures that beam steering resulting from the heat generation is kept to a minimum; and (3) features containing and attenuating the reflected/blocked beam are integral to the assembly and require no external components. 
     The shutter blocks the laser radiation directly after it emanates from the output window of the laser head. This occurs automatically upon disruption of the laser safety-interlock circuits or through manual command of the system control computer. Waste heat generated by the shutter actuator and the presence of materials known to cause optical component contamination are eliminated through the use of this device. 
     Additional objects and advantages relating to this disclosure will be apparent from the following detailed description of preferred embodiments thereof which proceeds with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of a laser optical system with its housing cover removed. 
         FIGS. 2A and 2B  are isometric views of opposite sides of the laser optical system of  FIG. 1 , showing where a laser head and a spatial filter, respectively, are located. 
         FIGS. 3A ,  3 B,  3 C, and  3 D are, respectively, side elevation, plan, side cross-sectional, and plan cross-sectional views of a mirror mount assembly used in the laser optical system of  FIG. 1 . 
         FIG. 4  is a simplified diagram showing a path of light propagating through a beam attenuating input window incorporated in the laser optical system of  FIG. 1 . 
         FIGS. 5 ,  6 , and  7  are, respectively, cross-sectional, rear side elevation, and exploded views of a preferred implementation of a light beam attenuating input window set in a window bulkhead of the laser optical system of  FIG. 1 . 
         FIG. 8  is an isometric view of a laser shutter assembly that is installed adjacent to an exit window of a laser head incorporated in the laser optical system of  FIG. 1 . 
         FIGS. 9A and 9B  are isometric views of a laser shutter assembly mounted in a structural gusset and shown with a shutter blade in, respectively, extended (light blocking) and retracted (light transmitting) positions. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The laser optical system in which preferred embodiments of the above-summarized three subsystems are included is shown in  FIGS. 1 ,  2 A, and  2 B.  FIG. 1  is a plan view of a laser optical system  10  with its cover removed, and  FIGS. 2A and 2B  are isometric views of opposite sides of the laser optical system  10  of  FIG. 1 , showing where a laser head and a spatial filter, respectively, are located.  FIGS. 1 ,  2 A, and  2 B show the laser rail assembly without the outer covers that seal the entire laser optical system  10 . All of the beam tube sets  18  are visible in these three drawing figures and are specifically identified in  FIG. 1 . 
     With reference to  FIGS. 1 ,  2 A, and  2 B, the output beam of a laser head  12  (housed within a separate cover) propagates along a beam path in the general shape of an “S” through an exit window  16  of laser optical system  10 . The output beam propagates through the interior regions of multiple beam tube sets  18  to and from enclosed optical components positioned along the laser beam path. The beam tube set assembly confines the laser beam in an atmospherically controlled environment within laser optical system  10 . 
     The output beam of laser head  12  propagates through a laser shutter  20  to a light beam attenuating input window  24  and its associated support member or bulkhead  26 . Laser shutter  20  and beam attenuating input window  24  are two subsystems described in greater detail below. The attenuated laser beam propagates through a pre-expander assembly  30  to a first turn mirror assembly  34  and a second turn mirror assembly  38 . Turn mirror assemblies  34  and  38  cooperate to reverse the direction of the laser beam path. The laser beam reflects off the turn mirror of second turn mirror assembly  38  and propagates through a first manual attenuator  44  and an acousto-optic modulator (AOM)  46  to a third turn mirror assembly  50  and a fourth turn mirror assembly  54 . Turn mirror assemblies  50  and  54  cooperate to reverse the laser beam path to its original propagation direction. The laser beam reflects off the turn mirror of turn mirror assembly  54  and propagates through a spatial filter  58  (housed within a separate cover), a second manual attenuator  60 , and a variable beam expander assembly  64  to exit window  16 . 
     Each of turn mirror assemblies  33 ,  38 ,  50 , and  54 ; manual attenuators  44  and  60 ; and the input and output of beam attenuating input window  24  is equipped with a gas purge port  68  for injecting purge gas flow through the beam tube assembly. (Laser head  12  also includes a gas purge port, which is not shown.) Purge gas injection in a structure integrated with the enclosed laser beam tube assembly is a subsystem described in greater detail below. 
       FIGS. 3A ,  3 B,  3 C, and  3 D are, respectively, side elevation, plan, side cross-sectional, and plan cross-sectional views of an optical component or mirror mount assembly  72  for first turn mirror assembly  34 . (Turn mirror assembly  34  is used by way of example; mirror mount assembly  72  can be used with any of the other turn mirror assemblies  38 ,  50 , and  54 .)  FIG. 3C  shows the integrated purge features of mirror mount assembly  72 , in which the placement of a purge gas inlet port  74  minimizes the ability of a purge hose  76  to deflect the mirror mount. The laser beam propagating through the interior of light beam path directing assembly  18  enters mirror mount assembly  72  and is incident on an exterior light beam-receiving or optical surface  78 . Purge gas entering through inlet port  74  makes a 90-degree bend, and flows through a gas diffuser  80  and upward across optical surface  78  to prevent accumulation of contaminants on it. A preferred gas diffuser  80  is a Model Series 4450K bronze/steel exhaust muffler/filter, which is manufactured by McMaster-Carr, Los Angeles, Calif., and which operates at 300 psi (2.1 MPa) maximum pressure and exhibits 40 μm filtration. 
     Purge rates for the laser rail are preferably determined by increasing the flow rate for a particular inlet port  74  until the laser beam becomes unstable, as measured by photodiode position detectors at the beam spot. When the gas flow deflects the laser beam, the flow rate is noted and reduced by 20 percent. This flow reduction eliminates any gas turbulence that caused the beam to become unstable. 
     The housings for attenuators  44  and  60  and the mount for beam attenuating input window  24  are also configured with the integrated purge features described and shown for mirror mount assembly  72 . Gas flow into the attenuators is determined by the degree of change in laser power reduction. The purge gas flow removes the humidity in the cavity where the optical component is contained and thereby changes its attenuation properties. 
     The hardware described above has the benefit of controlling the flow of the purge gas so that fresh purge gas is constantly introduced to the beam path and optical surfaces where it is needed. In a sealed assembly without purge, residual contaminants would eventually lead to degradation of the optical surfaces even if at very low concentrations. By constantly diluting and removing existing contaminants with a flow of purge gas, the chance of those contaminants coming into contact with optical surfaces is greatly reduced. If the purge gas were introduced into a large sealed assembly without the benefit of tubes, purge ports, or both, localized at the optical surfaces, controlling flow to individual optical components would be impossible. Stagnation zones would likely exist that could give contaminants a chance to accumulate. 
     The sealed outer cover (not shown) fitted against a gasket  82  ( FIGS. 2A and 2B ) affords an added benefit of a second barrier with predetermined exhaust outlets for the purge effluent. The sealed outer cover helps reduce or eliminate the possibility of contaminants outside of the optical rail from migrating into the assembly and onto optical surfaces. 
       FIG. 4  is a simplified theoretical diagram showing the path of light propagating through beam attenuating input window  24 . With reference to  FIG. 4 , S-polarized input light beam  100  is incident, at a 45° angle, on an entrance surface  102  of a fused silica window  24 . An S-polarized low intensity light beam  104  reflects off entrance surface  102  as the remainder of light beam  100  enters and propagates through the interior of window  24 . An S-polarized low intensity light beam  106  reflects off an exit surface  108 , propagates back through the interior of window  24 , and exits entrance surface  102 . An S-polarized output light beam  110  refracted and attenuated by window  24  propagates through exit window  108  along a beam path that differs from the beam path of input light beam  100 . 
     The following expressions for Fresnel reflections are used to calculate light reflection as a function of angle: 
               P   ⁢     -     ⁢   Polarization   ⁢     :     ⁢           ⁢     R   P       =         tan   2     ⁡     (       θ   i     -     θ   t       )           tan   2     ⁡     (       θ   i     +     θ   t       )                       S   ⁢     -     ⁢   Polarization   ⁢     :     ⁢           ⁢     R   S       =         sin   2     ⁡     (       θ   i     -     θ   t       )           sin   2     ⁡     (       θ   i     +     θ   t       )               
where θ i  is the incident angle and θ t  is the transmitted angle in the glass. The angles are found per Snell&#39;s Law: n i  sin θ i =n t  sin θ t , where n i  is the index of refraction of air and n t  is the index of refraction in the glass. An example of reflection loss calculations for fused silica at 1024 nanometers for n=1.45 is given on  FIG. 4 . Beam attenuating input window  24  by intentional design introduces, therefore, light loss at a fixed value established in accordance with the expressions set forth above.
 
       FIGS. 5 ,  6 , and  7  are, respectively, cross-sectional, rear side elevation, and exploded views of an actual implementation of a light beam attenuating input window  24  set in a window bulkhead  26  of laser optical system  10 . 
     With reference to  FIG. 5 , input light beam  100  is processed and output light beam  110  is formed as described with reference to  FIG. 4 . Low intensity reflected light beams  104  and  106  propagate to a beam dump  122 , which absorbs them. A second beam attenuating window  24   a  is positioned in window bulkhead  26  to receive output light beam  110  propagating through a sealed beam tube  18 . Beam attenuating window  24   a  is preferably made of the same optically transparent material as that of beam attenuating input window  24  and is set at an angle to provide added attenuation and an output light beam  124  propagating through an exit surface  126  and along the same beam path as that of input light beam  100 . Low intensity reflected light beams  128  and  130  analogous to the respective low intensity reflected light beams  104  and  106  propagate to a beam dump  132 . Skilled persons will appreciate that low intensity internal reflections within attenuating windows  24  and  24   a  propagate through exit surfaces  108  and  126  and may be blocked by an aperture (not shown) positioned to allow passage of output light beam  124 . 
     With reference to  FIGS. 6 and 7 , beam attenuating input window  24  is set in and releasably coupled to window bulkhead  26  by a window mounting assembly  140 . Window mounting assembly  140  permits manual rotation of attenuating input window  24  to move a damaged optical surface away from the laser beam path. Window mounting assembly  140  includes an annular holder  142  that is sized to rest on a support surface in the form of an annular shoulder  144  of a stepped opening  146  in window bulkhead  26 . 
     Annular holder  142  includes a shallow recess  148  terminated by a smooth annular flange surface  150  against which attenuating input window  24  rests. Three spring clips  152  fixed by bolts  154  in window bulkhead  26  press against attenuating input window  24  to secure it in place in annular holder  142 . Annular holder  142  includes around its periphery multiple angularly spaced apart wrench holes  156 . Loosening bolts  154  and placement of a tool in one of the exposed wrench holes  156  allow a user to accomplish manual rotation of annular holder  142  along annular shoulder  144  to present a different optical surface region to the incident laser beam. 
     Thus, this opto-mechanical design achieves with a single optical assembly an input window that is positioned in the sealed portion of the optical system and a fixed level of attenuation that is set into the optical system. The mounting system provides a quick solution, if a burn were to occur, by rotating the window about the laser beam so as to reduce end user down time. 
       FIG. 8  is an isometric view of a laser shutter assembly  20 , which is installed in laser optical system  10  adjacent the exit window of laser head  12  ( FIG. 1 ). Laser shutter assembly  20  includes a shutter blade  160  formed of an electroless nickel-plated aluminum cylinder that is mounted to a free end of a normally extended, nonrotating pneumatic cylinder  162 .  FIGS. 9A and 9B  are isometric views of laser shutter assembly  20  mounted in a structural gusset  164  and shown with shutter blade  160  in, respectively, extended (light blocking) and retracted positions. Structural gusset  164  is mounted to laser shutter assembly  20  in position for connection to the beam tube set  18  that is sealed against the exit window of laser head  12  ( FIG. 1 ). 
     In an unpressurized state, the extended pneumatic cylinder  162  positions shutter blade  160  such that it blocks the laser beam. The blocked beam reflects off the angled face  166  of blade  160  and is directed down a hole  168  drilled in gusset  164  and functioning as a beam dump. The reflected beam undergoes subsequent reflections off the curved rough surface of the interior surface of beam dump  168 . The structural components serve as adequate thermal mass for absorbing the reflected energy, and the numerous internal diffuse reflections ensure that there is no collimated retro-reflection of the blocked beam back into laser head  12 . In a pressurized state, pneumatic cylinder  162  retracts and thereby removes shutter blade  160  from the laser beam path. The extended and retracted positions are sensed with a magnetic reed switch  170  and fed back to the system control computer (not shown). 
     The use of a pneumatic actuator minimizes waste-heat generation, and the incorporation of an integral beam dump provides a compact design. The laser shutter mechanism incorporates no materials that, when exposed to the laser radiation during normal use, outgas contaminants detrimental to the optical components in laser optical system  10 . The laser shutter is operated completely manually during laser rail assembly and alignment. Easy manual operation during assembly is a benefit achieved with this laser shutter design. 
     It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments without departing from the underlying principles of the invention. The scope of the present invention should, therefore, be determined only by the following claims.
Preferred embodiments of a purge gas port, laser beam attenuating input window, and laser shutter constitute subsystems of a UV laser optical system in which a laser beam is completely enclosed to reduce contamination of the optical system components. Purge gas is injected through multiple locations in a beam tube assembly to ensure that the optical component surfaces sensitive to contamination are in the flow path of the purge gas. The input window functions as a fixed level attenuator to limit photopolymerization of airborne molecules and particles. Periodically rotating optical elements asymmetrically in their holders reduces burn damage to the optics.