Patent Abstract:
A split axis stage architecture is implemented as a multiple stage positioning system that is capable of vibrationally and thermally stable material transport at high speed and rates of acceleration. A split axis design decouples stage motion along two perpendicular axes lying in separate, parallel planes. A dimensionally stable substrate in the form of a granite, or other stone slab, or of ceramic material or cast iron, is used as the base for lower and upper stages. The substrate is precisely cut (“lapped”) such that its upper and lower stage surface portions are flat and parallel to each other.

Full Description:
COPYRIGHT NOTICE 
       [0001]    ©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  
       [0002]    The present disclosure relates to specimen processing systems and, in particular, to stage architecture for control of two- or three-dimensional positioning of a processing device relative to a target specimen. 
       BACKGROUND INFORMATION  
       [0003]    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. 
         [0004]    For applications including optical processing, a moveable optics assembly can be mounted above the wafer surface, thereby minimizing the wafer transport distances required. The primary direction of stage motion is referred to as the “major axis,” and the direction of stage motion perpendicular to the primary direction is referred to as the “minor axis.” The chuck holding the wafer, or specimen, to be processed may be mounted to a major axis stage for movement along the major axis, a minor axis stage for movement along the minor axis, or in 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. 
         [0005]    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. 
       SUMMARY OF THE DISCLOSURE  
       [0006]    A “split axis stage” architecture is implemented as a multiple stage positioning system that, in a preferred embodiment, supports 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. A “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. 
         [0007]    A dimensionally stable substrate in the form of a granite, or other stone slab, or a slab of ceramic material, cast iron, or polymer composite material such as Anocast™, is used as the base for the lower and upper stages. 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. The substrate is precisely cut (“lapped”) such that portions of its upper and lower stage surfaces are flat and parallel to each other. In a preferred embodiment, a lower guide track assembly that guides a lower stage carrying a specimen-holding chuck is coupled to a 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 an upper surface of the substrate. Linear motors positioned along adjacent rails of the guide track assemblies control the movements of the lower and upper stages. 
         [0008]    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 stiffness of the substrate and close separation of the stage motion axes result in higher frequency resonances, and less error in motion along all three axes. 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. 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. 
         [0009]    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. The rigidity of the support structure allows for faster and more accurate vertical motion along the beam axis. The inner surface of the sleeve acts as an outer race, and the outer surface of the lens acts as an inner race, thus forming an air bearing guiding the vertical motion of the focal region of the laser beam. Vertical motion is initiated by a lens forcer residing at the top end of the sleeve, which imparts a motive force to the optics assembly to adjust its height relative to the workpiece on the lower chuck, and in so doing, adjusts the focal region of the laser relative to the work surface. An isolation flexure device, rigid along the beam axis and compliant in the horizontal plane, buffers excess motion of the lens forcer from the optics assembly. 
         [0010]    The split axis stage design is applicable to many platforms used in semiconductor processing including dicing, component trim, fuse processing, inking, printed wire board (PWB) via drilling, routing, inspection, and metrology. The advantages afforded by such a design are also of benefit to a whole class of mechanical machining tools. 
         [0011]    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  
         [0012]      FIG. 1  is an isometric view of a decoupled, multiple stage positioning system. 
           [0013]      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. 
           [0014]      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. 
           [0015]      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. 
           [0016]      FIGS. 5A ,  5 B, and  5 C are diagrams showing alternative guide track assembly configurations for moving one or both of the upper and lower stages of the positioning system of  FIGS. 1-4 . 
           [0017]      FIG. 6  is an exploded view of a preferred embodiment of a laser beam focal region control subsystem that includes an air bearing sleeve assembly housing a scan lens and guiding its vertical (Z-axis) motion. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0018]      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. 
         [0019]    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 . 
         [0020]    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. 
         [0021]      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. 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. 
         [0022]    Each rail guide  40 —guide block  42  pair of first guide track assembly  20  shown in  FIG. 2  is a rolling element bearing assembly. Alternatives for guide track assembly  20  include a flat air bearing or a vacuum preloaded air bearing. Use of either type of air bearing entails removal of each guide rail  40 , exposing the surface portions of upper surface  14  to form guide surfaces, and substitution for each guide block  42  the guide surface or bearing face of the bearing, which is attached to bottom surface  44  of laser optics assembly stage  22 . Vacuum preloaded air bearings, which have a pressure port and a vacuum port, hold themselves down and lift themselves off the guide surface at the same time. Use of vacuum preloaded air bearings needs only one flat guide surface; whereas use of opposed bearing preloading needs two flat, parallel guide surfaces. Suitable air bearings are available from New Way Machine Components, Inc., Aston, Pa. Thus, depending on the type of guide track assembly used, surface portions of upper major surface  14  may represent a guide rail mounting contact surface or a bearing face noncontacting guide surface. 
         [0023]    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 . 
         [0024]    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. 
         [0025]      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 . 
         [0026]    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. 
         [0027]    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 . 
         [0028]    A first alternative to guide track assembly  24  is a magnetic preloaded air bearing using specimen stage bed  72  as a bearing land or guideway. Use of a magnetic preloaded air bearing entails removal of each guide rail  40 , exposing the surface portions of specimen stage bed  72 , and the removal of each guide block  42 , providing on the bottom surface of specimen stage  26  space for mounting the air bearing with its (porous) bearing face positioned opposite the exposed surface portion. 
         [0029]      FIG. 5A  is a schematic diagram showing the placement of two magnetic preloaded air bearings  100  in the this first alternative arrangement. A steel plate, or steel laminate structure  102 , is fixed on surface  70  in the space between and along the lengths of forcer coil assemblies  52 . Two spaced-apart flat air bearings  100  are fixed to corresponding surface portions  104  of a bottom surface  106  of specimen stage  26  and run along the lengths of linear motors  46 . A suitable air bearing is a silicon carbide porous media flat bearing series Part No. S1xxxxx, available from New Way Machine Components, Inc., Aston, Pa. A sheet magnet  108  is positioned in the space between air bearings  100  on bottom surface  106  of specimen stage  26  and spatially aligned with steel plate  102  so that the exposed surfaces of magnet  108  and steel plate  102  confront each other. The magnetic force of attraction urges sheet magnet  108  downwardly toward steel plate or steel laminate  102  as indicated by the downward pointing arrow in  FIG. 5A , and the net force of air bearings  100  urges specimen stage  26  upwardly away from surface  70  from specimen stage bed  72 , as indicated by two parallel upward pointing arrows in  FIG. 5A . The simultaneous application of opposed magnetic force and pressurized air creates a thin film of air in spaces  110  between (porous) bearing faces  112  of air bearings  100  and bearing guideways  114  on surface  70 . The lift force of air bearings  100  equals twice the sum of the weight of specimen stage  26  and the magnetic force of magnet  108 . Linear motors  46  impart the motive force that results in nearly zero friction motion of specimen stage  26  along the lengths of bearing guideways  114 . 
         [0030]    A second alternative to guide track assembly  24  is a vacuum preloaded air bearing using specimen stage bed  72  as a bearing land or guideway. Similar to the above-described first alternative to guide track assembly  20 , use of a vacuum preloaded air bearing entails removal of each guide rail  40 , exposing surface portion  114  of specimen stage bed  72 , and the removal of each guide block  42 , providing on bottom surface  106  of specimen stage  26  space for mounting the vacuum loaded air bearing, with its pressure land positioned opposite exposed surface portion  114 . 
         [0031]      FIG. 5B  is a schematic diagram showing the placement of two vacuum preloaded air bearings  120  in the second alternative arrangement. Two spaced-apart vacuum preloaded air bearings  120  are fixed to corresponding surface portions  104  of bottom surface  106  of specimen stage  26  and run along the lengths of linear motors  46 . A suitable air bearing is a vacuum preloaded air bearing series Part No. S20xxxx, available from New Way Machine Components, Inc., Aston, Pa. Vacuum preloaded bearings  120  simultaneously hold themselves down and lift themselves off bearing guideways  114  on surface  70 . Each vacuum preloaded bearing  120  has a pressure land that is divided into spaced-apart land portions  122   a  and  122   b . A vacuum area  124  is located between land portions  122   a  and  122   b . The simultaneous application and distribution of air pressure and vacuum pressure creates a thin film of air in spaces  126  between pressure land portions  122   a  and  122   b  of vacuum preloaded air bearings  120  and bearing guideways  114  on surface  70 . Linear motors  46  impart the motive force that results in nearly zero friction motion of specimen stage  26  along the lengths of the bearing guideways  114 . 
         [0032]    A third alternative to guide track assembly  24  entails the use of either a magnetic preloaded air bearing of the first alternative, or a vacuum preloaded air bearing of the second alternative in the absence of specimen stage bed  72 , as well as each guide rail  40  and each guide block  42 . 
         [0033]      FIG. 5C  is a schematic diagram showing specimen stage  26  riding on magnetic preloaded air bearings or vacuum preloaded air bearings  140  along bottom surface  142  of substrate  12 . When substrate  12  is in a horizontal disposition, magnetic preloaded or vacuum preloaded air bearings  140  develop sufficient force to overcome the gravitational force on specimen stage  26  as it rides along bottom surface  142 . Skilled persons will appreciate that laser optics assembly stage  22  can similarly be adapted to ride on magnetic preloaded air bearings or vacuum preloaded air bearings along upper major surface  14  of substrate  12 . The stage configuration can use mechanical linear guides in place of the air bearings described above. Other devices for measuring position, such as interferometers, can be implemented in this positioning system design. 
         [0034]    The mass of substrate  12  is sufficient to decouple the mass of optics assembly stage  22  and the mass of specimen stage  26 , including the specimen mounted on it, so that the guided motion of one of stages  22  and  26  contributes a negligible motive force to the other one of them. The masses of stages  22  and  26  moving along the X- and Y-axes are low, and thereby allow high acceleration and high velocity processing and limit heat generation in linear motors  46 . Because the center of mass of the laser beam focal region control subsystem  28  is aligned with the center of mass of optics assembly stage  22 , perturbations in the motion of optics assembly stage  22  are minimized. 
         [0035]    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 . 
         [0036]      FIG. 6  shows in greater detail the components of control subsystem  28  and its mounting on laser optics assembly stage  22 . With reference to  FIG. 6 , control subsystem  28  includes a lens forcer assembly  210  that is coupled by a yoke assembly  212  to scan lens  30  contained in the interior of an air bushing  214  of air bearing assembly  202 . A suitable air bushing is Part No. S307501, available from New Way Machine Components, Inc., Aston, Pa. Lens forcer assembly  210 , which is preferably a voice coil actuator, imparts by way of yoke assembly  212  a motive force that moves scan lens  30  and thereby the focal region of the laser beam to selected positions along beam axis  206 . 
         [0037]    Voice coil actuator  210  includes a generally cylindrical housing  230  and an annular coil  232  formed of a magnetic core around which copper wire is wound. Cylindrical housing  230  and annular coil  232  are coaxially aligned, and annular coil  232  moves axially in and out of housing  230  in response to control signals (not shown) applied to voice coil actuator  210 . A preferred voice coil device  210  is an Actuator No. LA 28-22-006 Z, available from BEI Kimco Magnetics, Vista, Calif. 
         [0038]    Annular coil  232  extends through a generally circular opening  234  in a voice coil bridge  236  having opposite side members  238  that rest on uprights  240  ( FIG. 1 ) mounted on laser optics assembly stage  22  to provide support for laser beam focal region control subsystem  28 . Voice coil bridge  236  includes in each of two opposite side projections  242  a hole  244  containing a tubular housing  250  through which passes a rod  252  extending from an upper surface  254  of a guiding mount  256 . Each rod  252  has a free end  258 . Guiding mount  256  has on its upper surface  254  an annular pedestal  260  on which annular coil  232  rests. Two stacked, axially aligned linear ball bushings  264  fit in tubular housing  250  contained in each hole  244  of side projections  242  of voice coil bridge  236 . Free ends  258  of rods  252  passing through ball bushings  264  are capped by rod clamps  266  to provide a hard stop of lower travel limit of annular coil  232  along beam axis  206 . 
         [0039]    Housing  230  has a circular opening  270  that is positioned in coaxial alignment with the center of annular coil  232 , opening  234  of voice coil bridge  236 , and the center of annular pedestal  260  of guiding mount  256 . A hollow steel shaft  272  extends through opening  270  of housing  230 , and a hexagonal nut  274  connects in axial alignment hollow steel shaft  272  and a flexible tubular steel member  276 , which is coupled to yoke assembly  212  as further described below. Hexagonal nut  274  is positioned in contact with a lower surface  278  of annular coil  232  to drive flexible steel member  276  along a drive or Z-axis  280  in response to the in-and-out axial movement of annular coil  232 . Hollow steel shaft  272  passes through the center and along the axis of a coil spring  282 , which is confined between a top surface  284  of housing  230  and a cylindrical spring retainer  286  fixed at a free end  290  of hollow steel shaft  272 . Coil spring  282  biases annular coil  232  to a mid-point of its stroke along Z-axis  280  in the absence of a control signal applied to voice coil actuator  210 . 
         [0040]    Yoke assembly  212  includes opposed yoke side plates  300  (only one shown) secured at one end  302  to a surface  304  of a yoke ring  306  and at the other end  308  to a multilevel rectangular yoke mount  310 . Scan lens  30  formed with a cylindrical periphery  312  and having an annular top flange  314  fits in yoke assembly  212  so that top flange  314  rests on surface  304  of yoke ring  306 . Scan lens  30  contained in the interior of air bushing  214  forms the inner race of air bearing assembly  202 , and an inner surface  316  of air bushing  214  forms the outer race of air bearing assembly  202 . The implementation of air bearing assembly  202  increases the rigidity of scan lens  30  in the X-Y plane but allows scan lens  30  to move along the Z-axis in a very smooth, controlled manner. 
         [0041]    Flexible steel member  276  has a free end  320  that fits in a recess  322  in an upper surface  324  of yoke mount  310  to move it along Z-axis  280  and thereby move scan lens  30  along beam axis  206 . An encoder head mount  326  holding an encoder  328  and attached to voice coil bridge  236  cooperates with an encoder body mount  330  holding an encoder scale and attached to guiding mount  256  to measure, using light diffraction principles, the displacement of guiding mount  256  relative to voice coil bridge  236  in response to the movement of annular coil  232 . Because flexible tubular steel member  276  is attached to annular coil  232 , the displacement measured represents the position of scan lens  30  along beam axis  206 . 
         [0042]    A quarter-waveplate  340  secured in place on a mounting ring  342  is positioned between a lower surface  344  of rectangular yoke mount  310  and top flange  314  of scan lens  30 . A beam deflection device  346 , such as a piezoelectric fast steering mirror, attached to optics assembly stage  22  ( FIG. 3 ) is positioned between rectangular yoke mount  310  and quarter-waveplate  340 . Fast steering mirror  346  receives an incoming laser beam  348  propagating along beam axis  206  and directs laser beam  348  through quarter-waveplate  340  and scan lens  30 . Quarter-waveplate  340  imparts circular polarization to the incoming linearly polarized laser beam, and fast steering mirror  346  directs the circularly polarized laser beam for incidence on selected locations of the work surface of a target specimen supported on specimen stage  26 . When fast steering mirror  346  is in its neutral position, Z-axis  280 , beam axis  206 , and the propagation path of laser beam  348  are collinear. When fast steering mirror  346  is in operation, the propagation path of laser beam  348  is generally aligned with beam axis  206 . 
         [0043]    Flexible steel member  276  is rigid in the Z-axis direction but is compliant in the X-Y plane. These properties of flexible steel member  276  enable it to function as a buffer, isolating the guiding action of air bearing assembly  202  containing scan lens  30  from the guiding action of lens forcer assembly  210  that moves scan lens  30 . 
         [0044]    Lens forcer assembly  210  and air bearing assembly  202  have centers of gravity and are positioned along the Z-axis. Voice coil bridge  236  of lens forcer assembly  210  has two depressions  350 , the depths and cross sectional areas of which can be sized to achieve the axial alignment of the two centers of gravity. Such center of gravity alignment eliminates moment arms in control system  28  and thereby helps reduce propensity of low resonant frequency vibrations present in prior art cantilever beam designs. 
         [0045]    Several examples of possible types of laser processing systems in which positioning system  10  can be installed include semiconductor wafer or other specimen micromachining, dicing, and fuse processing systems. In a wafer dicing system, laser beam  348  moves along scribe locations on the wafer surface. In a wafer fuse processing system, a pulsed laser beam  348  moves relative to wafer surface locations of fuses to irradiate them such that the laser pulses either partly or completely remove fuse material. 
         [0046]    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 Classification (CPC): 1