Patent Document

RELATED APPLICATIONS 
     This is a continuation-in-part of U.S. patent application Ser. No. 11/676,937, filed Feb. 20, 2007, now U.S. Pat. No. 7,760,331, and claims benefit of U.S. Provisional Patent Application No. 60/890,807, filed Feb. 20, 2007. 
    
    
     COPYRIGHT NOTICE 
     ©2007 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 specimen processing systems and, in particular, to achieving operational efficiency in performing specimen processing and inspection. 
     BACKGROUND INFORMATION 
     Wafer transport systems configured for use in semiconductor wafer-level processing typically include a stage having a chuck that secures the wafer for processing. Sometimes the stage is stationary, and sometimes it is moveable. Some applications require that the stage move linearly in one, two, or three Cartesian dimensions, with or without rotation. The speed of the stage motion can dictate the throughput of the entire wafer processing platform if a significant amount of the total process time is spent aligning and transporting the wafer. 
     Some systems have the flexibility to move the processing and inspection devices to the wafer as well as move the wafer to the devices. This can eliminate wafer alignment steps and thereby save time. For applications including laser processing, a moveable optics assembly can be mounted above the wafer surface, thereby minimizing the wafer transport distances required. The chuck holding the wafer, or specimen, to be processed may be mounted to a major axis stage for movement in the primary direction of stage motion, a minor axis stage for movement in a direction perpendicular to the primary direction of stage motion, or in a stationary position below the major and minor axes. The major axis stage may support the minor axis stage, or they may be independent of each other. 
     Stage design of such optical systems is becoming more critical as electrical circuit dimensions shrink. One stage design consideration is the impact of process quality stemming from vibrational and thermal stability of the wafer chuck and optics assembly. In the case in which the laser beam position is continually adjusted, state-of-the-art structures supporting the laser assembly are too flexible to maintain the required level of precision. Moreover, as circuit dimensions shrink, particle contamination becomes of greater concern. 
     In semiconductor wafer fabrication, many wafer processing operations are followed by an inspection to ensure that an operation was successful before moving a wafer on to a next processing step. The inspection may be conducted with use of a separate piece of equipment, usually a powerful optical microscope or an electron microscope. Or, the inspection equipment may be directly built into a processing system platform, eliminating overhead associated with delivery of wafers to an additional station. Transporting a heavy processing or inspection device unnecessarily is also undesirable, because the accuracy and stability of a moving structure is optimized when its mass is low. In addition, thermal dissipation increases with motor size and, therefore, with payload mass. 
     SUMMARY OF THE DISCLOSURE 
     The present disclosure relates to a laser processing system in which a positioning system is designed to support a specimen undergoing one or more processing operations. In one embodiment, for example, the positioning system is used in “scribing and dicing” finished electronic devices patterned on semiconductor wafers. Wafer scribing entails a laser beam traversing boundaries between integrated circuit chips patterned on a silicon wafer and ablating upper dielectric and metal layers along the boundaries. Wafer dicing entails a laser beam traversing boundaries between integrated circuit chips patterned on a silicon wafer and separating adjacent chips from each other. 
     A feature of a preferred embodiment of the positioning system is a rigid stone slab substrate that provides a vibration-free platform on which to mount processing equipment and a specimen stage. This stable platform also provides an attractive foundation for integrating post-processing inspection equipment. 
     A “split axis stage” architecture is implemented in a preferred embodiment, supporting a laser optics assembly and a workpiece having a surface on which a laser beam is incident for laser processing. The multiple stage positioning system is capable of vibrationally and thermally stable material transport at high speed and rates of acceleration. The split axis design decouples driven stage motion along two perpendicular axes lying in separate, parallel planes. In a preferred embodiment motion in the horizontal plane is split between a specimen (major axis or lower) stage and a scan optics assembly (minor axis or upper) stage that move orthogonally relative to each other. 
     The dimensionally stable substrate, or slab, is used as the base for the lower and upper stages. The massive and structurally stiff substrate isolates and stabilizes the motions of the laser optics assembly and the specimen, absorbs vibrations, and allows for smoother acceleration and deceleration because the supporting structure is inherently rigid. The substrate also provides thermal stability by acting as a heat sink. Moreover, because it is designed in a compact configuration, the system is composed of less material and is, therefore, less susceptible to expansion when it undergoes heating. The substrate is precisely cut (“lapped”) such that portions of its upper and lower stage surfaces are flat and parallel to each other. The slab and the stages are preferably fabricated from materials with similar coefficients of thermal expansion to cause the system to advantageously react to temperature changes in a coherent fashion. 
     In a preferred embodiment, a lower guide track assembly that guides a lower stage carrying a specimen-holding chuck is coupled to a flat lower surface of the substrate. An upper guide track assembly that guides an upper stage carrying a laser beam focal region control subsystem is coupled to a flat upper surface of the substrate. The specimen stage carrying the wafer and the process stage carrying the optics glide along low-friction or frictionless guide rails. Linear motors positioned along adjacent rails of the guide track assemblies control the movements of the lower and upper stages. A laser beam focal region control subsystem is supported above the lower stage and includes a vertically adjustable optics assembly positioned within a rigid air bearing sleeve mounted to the upper stage by a support structure. An oval slot cut out of the middle of the substrate exposes the specimen below to the laser beam and allows for vertical motion of the laser optics assembly through the substrate. Otherwise, the specimen is shielded by the substrate from particles generated by overhead motion, except for the localized region undergoing laser processing. 
     When laser processing of the specimen is complete, the result is inspected, typically under a high-power microscope outfitted with a camera. On-board inspections maximize efficiency of both production and process development activities. Furthermore, there is a significant stability advantage in supporting the inspection equipment, which is made massive by heavy zoom lenses, on the same rigid substrate that supports the lighter weight processing equipment. Instead of mounting heavy inspection equipment on the processing stage along with the optics assembly, the inspection equipment is mounted on a third, separate, non-motorized or passive specimen inspection stage. While laser processing is ongoing, the specimen inspection stage remains parked in its home position, located at one end of the guide rail. When it is time for the post-processing inspection, the processing stage, which is equipped with a stage coupling and decoupling mechanism, is used to transport the specimen inspection stage to and from the wafer position. 
     Additional aspects and advantages will be apparent from the following detailed description of preferred embodiments, which proceeds with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an isometric view of a decoupled, multiple stage positioning system. 
         FIG. 2  is a partly exploded isometric view of the positioning system of  FIG. 1 , showing upper and lower stages that, when the system is assembled, are mounted to a dimensionally stable substrate such as a stone slab. 
         FIG. 3  is an isometric view of the positioning system of  FIG. 1 , showing the upper stage supporting a scan lens and upper stage drive components. 
         FIG. 4  is an isometric view of the positioning system of  FIG. 1 , showing the lower stage supporting a specimen-holding chuck and lower stage drive components. 
         FIG. 5  is a fragmentary isometric view of the back end of a specimen inspection stage parked at its home position on the upper surface of a substrate slab. 
         FIG. 6  is an enlarged fragmentary isometric view of a coupling mechanism joining the specimen inspection stage with a specimen processing stage. 
         FIG. 7  is a fragmentary isometric view of the top surface of the specimen inspection stage alone in its home position shown in  FIG. 5 . 
         FIG. 8  is an isometric view of the specimen inspection stage coupled to the specimen process stage. 
         FIG. 9  is an isometric view of the top surface of the specimen inspection stage, shown with its microscope and camera system in place, coupled to the specimen processing stage, shown with a laser optics assembly in place. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIGS. 1 and 2  show a decoupled, multiple stage positioning system  10 , which, in a preferred embodiment, supports components of a laser processing system through which a laser beam propagates for incidence on a target specimen. Positioning system  10  includes a dimensionally stable substrate  12  made of a stone slab, preferably formed of granite, or a slab of ceramic material, cast iron, or polymer composite material such as Anocast™. Substrate  12  has a first or upper flat major surface  14  and a second or lower flat major surface  16  that has a stepped recess  18 . Major surfaces  14  and  16  include surface portions that are plane parallel to each other and conditioned to exhibit flatness and parallelism within about a ten micron tolerance. 
     A surface portion of upper major surface  14  and a first guide track assembly  20  are coupled to guide movement of a laser optics assembly stage  22  along a first axis, and a surface portion of lower major surface  16  and a second guide track assembly  24  are coupled to guide movement of a specimen stage  26  along a second axis that is transverse to the first axis. Optics assembly stage  22  supports a laser beam focal region control subsystem  28 , which includes a scan lens  30  that depends downwardly below lower major surface  16  of substrate  12 . Specimen stage  26  supports a specimen-holding chuck  32 . The guided motions of stages  22  and  26  move scan lens  30  relative to laser beam processing locations on a surface of a specimen (not shown) held by chuck  32 . 
     In a preferred implementation, substrate  12  is set in place so that major surfaces  14  and  16  define spaced-apart horizontal planes and guide track assemblies  20  and  24  are positioned so that the first and second axes are perpendicular to each other and thereby define respective Y- and X-axes. This split axis architecture decouples motion along the X- and Y-axes, simplifying control of positioning the laser beam and chuck  32 , with fewer degrees of freedom allowed. 
       FIG. 3  shows in detail optics assembly stage  22 , which operates with first guide track assembly  20  shown in  FIG. 2 . First guide track assembly  20  includes two spaced-apart guide rails  40  secured to support portions of upper major surface  14  and two U-shaped guide blocks  42  supported on a bottom surface  44  of optics assembly stage  22 . Each one of guide blocks  42  fits over and slides along a corresponding one of rails  40  in response to an applied motive force. Each rail guide  40 —guide block  42  pair of first guide track assembly  20  shown in  FIG. 2  is a rolling element bearing assembly. A motor drive for optics assembly stage  22  includes a linear motor  46  that is mounted on upper major surface  14  and along the length of each guide rail  40 . Linear motor  46  imparts the motive force to propel its corresponding guide block  42  for sliding movement along its corresponding guide rail  40 . Each linear motor  46  includes a U-channel magnet track  48  that holds spaced-apart linear arrays of multiple magnets  50  arranged along the length of guide rail  40 . A forcer coil assembly  52  positioned between the linear arrays of magnets  50  is connected to bottom surface  44  of optics assembly stage  22  and constitutes the movable member of linear motor  46  that moves optics assembly stage  22 . A suitable linear motor  46  is a Model MTH480, available from Aerotech, Inc., Pittsburgh, Pa. 
     A pair of encoder heads  60  secured to bottom surface  44  of optics assembly stage  22  and positioned adjacent different ones of guide blocks  42  includes position sensors that measure yaw angle and distance traveled of optics assembly stage  22 . Placement of the position sensors in proximity to guide rails  40 , guide blocks  42 , and linear motors  46  driving each of stages  22  and  26  ensures efficient, closed-loop feedback control with minimal resonance effects. A pair of stop members  62  limits the travel distance of guide blocks  42  in response to limit switches included in encoder heads  60  that are tripped by a magnet (not shown) attached to substrate  12 . A pair of dashpots  64  dampen and stop the motion of optics assembly stage  22  to prevent it from overtravel movement off of guide rails  40 . 
     An oval slot  66  formed in substrate  12  between and along the lengths of guide rails  40  provides an opening within which scan lens  30  can travel as optics assembly stage  22  moves along guide rails  40 . A pair of through holes  68  formed in the region of stepped recess  18  in substrate  12  provides operator service access from upper surface  14  to encoder heads  60  to maintain their alignment. 
     Processing equipment mounted to optics assembly stage  22  in  FIG. 3  comprising laser beam control subsystem  28  includes an air bearing assembly  202 , a lens forcer assembly  210 , and a yoke assembly  212 . Elements of yoke assembly  212 , forming a supporting structure for the processing equipment, and therefore prominently evident in  FIGS. 1 ,  2 ,  3 ,  8 , and  9 , include a voice coil bridge  236 , side members  238 , uprights  240 , yoke side plates  300 , and a yoke mount  310 . Visible components of the processing equipment thus supported include an encoder  328  and a beam deflection device  346 . A detailed description of preferred optical processing equipment is set forth in copending U.S. patent application Ser. No. 11/676,937, of which the present patent application is a continuation-in-part. 
       FIG. 4  shows in detail specimen stage  26  in operative association with second guide track assembly  24  of  FIG. 2 . Second guide track assembly  24  includes guide rails, U-shaped guide blocks, linear motors, U-channel magnet tracks, magnets, forcer coil assemblies, and encoder heads that correspond to and are identified by the same reference numerals as those described above in connection with first guide track assembly  20 . Linear motors  46  and the components of and components supported by second guide track assembly  24  are mounted on a surface  70  of a specimen stage bed  72 . 
     The mechanical arrangement of stages  22  and  26  and motors  46  results in reduced pitch and roll of stages  22  and  26 , and enhances accuracy of high velocity motion. Symmetric placement of motors  46  on opposite sides of stages  22  and  26  improves control of yaw. The placement of motors  46  along the sides of stages  22  and  26 , as opposed to underneath them, minimizes thermal disturbance of critical components and position sensors. 
     Second guide track assembly  24  and specimen stage  26  supporting chuck  32  fits into and is secured within stepped recess  18 . Surface  70  of specimen stage bed  72  is secured against a surface portion  74  of lower major surface  16  adjacent the wider, lower portion of stepped recess  18 , and chuck  32  is positioned below the innermost portion of stepped recess  18  of lower major surface  16  and moves beneath it in response to the motive force imparted by linear motors  46  moving specimen stage  26  along second guide track assembly  24 . A pair of stop members  76  limits the travel distance of guide blocks  42  in response to limit switches included in encoder heads  60  that are tripped by a magnet (not shown) attached to substrate  12 . A pair of dashpots  78  dampen and stop the motion of specimen stage  26  to prevent it from overtravel movement off of guide rails  40 . 
     Laser optics assembly stage  22  has an opening  200  that receives control subsystem  28 , which includes an air bearing assembly  202  containing scan lens  30 . Control subsystem  28  controls the axial position of a laser beam focal region formed by scan lens  30  as the laser beam propagates generally along a beam axis  206 , which is the optic axis of scan lens  30 , and through scan lens  30  for incidence on a work surface of a target specimen supported on specimen stage  26 . 
     The following description is directed to a specimen inspection stage and stage-coupling apparatus shown in  FIGS. 5-9 . The inspection equipment along with its stage is an optional sub-assembly that can be included in, but is not required by, the overall wafer processing system.  FIG. 5  shows the back end of a preferred specimen inspection stage  400  to which inspection equipment and three coupling devices are mounted. Specimen inspection stage  400  rests parked at its home position located at the end of its travel along guide rails  40 , at the edge of substrate  12 . Specimen inspection stage  400  includes a monolithic aluminum carriage  404  that has four buttresses  406  forming a support structure to stabilize inspection equipment  408  being transported. Inspection equipment  408  includes a microscope  410  and a specialized camera  412  mounted at the upper end of a microscope column  414 . In this embodiment, microscope  410  includes a 6.5× ultra zoom lens  415  and two objective lenses  416  with motorized zoom and focus adjustment control ( FIG. 6 ). A suitable ultra-zoom lens  415  and suitable objective lenses  416  are, respectively, Part Nos. 1-62638 and 1-60228, both available from Navitar, Inc., Rochester, N.Y. A suitable digital microscope camera  412  is a Model No. FLEA-HICOL-CS, available from Point Grey Research, Vancouver, British Columbia. 
     Three guided cylinder blocks are attached to specimen inspection stage  400 , and they include two guided cylinder blocks  418  positioned on either side, and a guided cylinder block  420  positioned at the rear, of specimen inspection stage  400 . Guided cylinder block  420 , located at the rear, locks specimen inspection stage  400  into its home position via a V-groove mount  422 , which is bolted to the surface of substrate  12 . Guided cylinder blocks  418 , located at the sides of specimen inspection stage  400 , attach inspection stage  400  to a specimen processing and transporting stage, which, in this embodiment, is optical assembly stage  22  ( FIG. 8 ). Suitable guided cylinder blocks  418  and  420  are each a Part No. SGDAQ-12X20 ZE 155A1, available from Koganei Corporation, Kalamazoo, Mich. Each of guided cylinder blocks  418  and  420  houses a central pneumatic cylinder  424  and two guide rods  425  fitted in guide bushing assemblies. Pneumatic cylinder  424  and guide rods  425  extend and retract as a unit into and out of the housing, along the z-axis. Pneumatic cylinder  424  is attached to a lower actuation plate  426 , the underside of which is fitted with an adaptor  428 . 
       FIG. 6  presents an enlarged view of guided cylinder blocks  418 , each of which functions as a component of a coupling device  429  that releasably couples inspection stage  400  to optical assembly stage  22 . Each coupling device  429  employs a kinematic mount to ensure repeatable positioning. Adaptor  428  bonds actuation plate  426  to a hemispherical coupler  430 , which fits snugly into V-groove  432  when cylinders  424  are lowered. The vertical cylinder position is sensed by a photodetector  434  when a flag  436  that extends along the y-axis intercepts a light beam (not shown) that propagates along the x-axis, as defined by the coordinate system in the diagram. The light beam propagates from a light source  438  mounted inside the front of photodetector  434  to a sensor  440  mounted inside the back of photodetector  434 . Interruption of the light beam by flag  436  indicates that specimen inspection stage  400  is coupled to optical assembly stage  22 . A flange  442  extending from a V-groove member  444  is bolted to, and thereby causes specimen inspection stage  400  to move in response to movement of, optical assembly stage  22  when they are coupled. When coupling is complete, U-shaped guide block  42  associated with optical assembly stage  22  is positioned adjacent to U-shaped guide block  42  associated with specimen inspection stage  400 , so that both stages glide together along fixed guide rail  40 . An L-shaped bracket  446  bolted to stage  400  opposite adaptor  428  serves as a mounting surface for guided cylinder blocks  418 . 
       FIG. 7  shows a mechanism providing vertical travel for inspection equipment assembly  408 . Microscope  410  features internal LED coaxial illumination that enables use of camera  412 . To dissipate energy, the LEDs require a large heat sink  448 , which is shown mounted to the front of microscope column  414 . The back of microscope column  414  is attached to a paddle-shaped bracket  450 , which is in turn attached to linear motor-driven cross-roller stage  452 , such as Part No. PRC43AL0025C D3 H2 L1E7, available from Primatics, Inc., Tangent, Oreg. Cross-roller stage  452  raises and lowers microscope  410  along the z-axis. This z-axis motion is counter-balanced by compression springs  454  (only one shown) that are contained in a compression spring housing  456  having slots  457  into which the free ends of a U-shaped spring bar  458  are inserted to constrain its movement. Compression springs  454  resist motion by pushing upward against U-shaped spring bar  458 , which is attached to cross-roller stage  452 . Counter-balancing the mass of cross roller stage  452  aids in positioning and establishing a rest position for cross roller stage  452 , and thereby prevents microscope  410  from striking specimen inspection stage  400  in the event of a power failure. 
     A harness connector  460  mounted on the top end of cross-roller stage  452  receives at one end a cable terminator  462 . Cable terminator  462  indicates where power supply and data transmission wiring that feeds inspection equipment  408  plugs into harness connector  460 . 
       FIG. 8  shows specimen inspection stage  400  coupled to optical assembly stage  22 , which accommodates inspection equipment  408  via a rectangular cutout  464 .  FIG. 9  shows a similar view to that of  FIG. 8 , with laser beam focal region control subsystem  28  in place. 
     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.

Technology Category: b