Abstract:
Laser beam positioners ( 300, 340 ) employ a steering mirror ( 236, 306 ) that performs small-angle deflection of a laser beam ( 270 ) to compensate for cross-axis ( 110 ) settling errors of a positioner stage ( 302 ). A two-axis mirror is preferred because either axis of the positioner stages may be used for performing work. In one embodiment, the steering mirror is used for error correction only without necessarily requiring coordination with the positioner stage position commands. A fast steering mirror employing a flexure mechanism and piezoelectric actuators to tip and tilt the mirror is preferred in semiconductor link processing (“SLP”) applications. This invention compensates for cross-axis settling time, resulting in increased SLP system throughput and accuracy while simplifying complexity of the positioner stages because the steering mirror corrections relax the positioner stage servo driving requirements.

Description:
RELATED APPLICATIONS  
       [0001]    This application claims priority from U.S. Provisional Application No. 60/269,646, filed Feb. 16, 2001. 
     
    
     
       TECHNICAL FIELD  
         [0002]    This invention relates to laser processing of circuit links and, in particular, to a laser system and method employing a laser beam and substrate positioning system that incorporates a steering mirror to compensate for stage positioning errors and enhance link severing throughput.  
         BACKGROUND OF THE INVENTION  
         [0003]    Yields in integrated circuit (“IC”) device fabrication processes often incur defects resulting from alignment variations of subsurface layers or patterns or particulate contaminants. FIGS. 1, 2A, and  2 B show repetitive electronic circuits  10  of an IC device or workpiece  12  that are typically fabricated in rows or columns to include multiple iterations of redundant circuit elements  14 , such as spare rows  16  and columns  18  of memory cells  20 . With reference to FIGS. 1, 2A, and  2 B, circuits  10  are also designed to include particular laser severable circuit links  22  between electrical contacts  24  that can be removed to disconnect a defective memory cell  20 , for example, and substitute a replacement redundant cell  26  in a memory device such as a DRAM, an SRAM, or an embedded memory. Similar techniques are also used to sever links to program a logic product, gate arrays, or ASICs.  
           [0004]    Links  22  are designed with conventional link widths  28  of about 2.5 microns, link lengths  30 , and element-to-element pitches (center-to-center spacings)  32  of about 8 microns from adjacent circuit structures or elements  34 , such as link structures  36 . Although the most prevalent link materials have been polysilicon and like compositions, memory manufacturers have more recently adopted a variety of more conductive metallic link materials that may include, but are not limited to, aluminum, copper, gold nickel, titanium, tungsten, platinum, as well as other metals, metal alloys such as nickel chromide, metal nitrides such as titanium or tantalum nitride, metal suicides such as tungsten silicide, or other metal-like materials.  
           [0005]    Circuits  10 , circuit elements  14 , or cells  20  are tested for defects. The links to be severed for correcting the defects are determined from device test data, and the locations of these links are mapped into a database or program. Laser pulses have been employed for more than 20 years to sever circuit links  22 . FIGS. 2A and 2B show a laser spot  38  of spot size diameter  40  impinging a link structure  36  composed of a link  22  positioned above a silicon substrate  42  and between component layers of a passivation layer stack including an overlying passivation layer  44  (shown in FIG. 2A but not in FIG. 2B) and an underlying passivation layer  46  (shown in FIG. 2B but not in FIG. 2A). FIG. 2C is a fragmentary cross-sectional side view of the link structure of FIG. 2B after the link  22  is removed by the laser pulse.  
           [0006]    [0006]FIG. 3 is a plan view of a beam positioner travel path  50  performed by a traditional link processing positioning system. Because links  22  are typically arranged in rows  16  and columns  18  (representative ones shown in dashed lines), the beam position and hence the laser spots  38  are scanned over link positions along an axis in a first travel direction  52 , moved to a different row  16  or column  18 , and then scanned over link positions along an axis in a second travel direction  54 . Skilled persons will appreciate that scanning may include moving the workpiece  12 , moving the laser spot  38 , or moving the workpiece  12  and the laser spot  38 .  
           [0007]    Traditional positioning systems are characterized by X-Y translation tables in which the workpiece  12  is secured to an upper stage that moves along a first axis and is supported by a lower stage that moves along a second axis that is perpendicular to the first axis. Such systems typically move the workpiece relative to a fixed beam position or laser spot  38  and are commonly referred to as stacked stage positioning systems because the lower stage supports the inertial mass of the upper stage which supports workpiece  12 . These positioning systems have excellent positioning accuracy because interferometers are typically used along each axis to determine the absolute position of each stage. This level of accuracy is preferred for link processing because the laser spot size  40  is typically only a little bigger than link width  28 , so even a small discrepancy between the position of laser spot  38  and link  22  can result in incomplete link severing. In addition, the high density of features on semiconductor wafers results in small positioning errors potentially causing laser damage to nearby structures. Stacked stage positioning systems are, however, relatively slow because the starting, stopping, and change of direction of the inertial mass of the stages increase the time required for the laser tool to process all the designated links  22  on workpiece  12 .  
           [0008]    In split-axis positioning systems, the upper stage is not supported by, and moves independently from, the lower stage and the workpiece is carried on a first axis or stage while the tool, such as a fixed reflecting mirror and focusing lens, is carried on the second axis or stage. Split-axis positioning systems are becoming advantageous as the overall size and weight of workpieces  12  increase, utilizing longer and hence more massive stages.  
           [0009]    More recently, planar positioning systems have been employed in which the workpiece is carried on a single stage that is movable by two or more actuators while the tool remains in a substantially fixed position. These systems translate the workpiece in two dimensions by coordinating the efforts of the actuators. Some planar positioning systems may also be capable of rotating the workpiece.  
           [0010]    Semiconductor Link processing (“SLP”) systems built by Electro Scientific Industries, Inc. (“ESI”) of Portland, Oregon employ on-the-fly (“OTF”) link processing to achieve both accuracy and high throughput. During OTF processing, the laser beam is pulsed as a linear stage beam positioner passes designated links  12  under the beam position. The stage typically moves along a single axis at a time and does not stop at each link position. The on-axis position of beam spot  38  in the direction travel  52  does not have to be accurately controlled; rather, its position is accurately sensed to trigger laser spot  38  to hit link  22  accurately.  
           [0011]    In contrast and with reference again to FIG. 3, the position of beam spot  38  along cross-axes  56  or  58  is controlled within specified accuracy as the beam positioner passes over each link  22 . Due to the inertial mass of the stage or stages, a set-up move to start an OTF run produces ringing in the cross-axis position, and the first link  22  in an OTF run cannot be processed until the cross-axis position has settled properly. The settling delay or setting distance  60  reduces processing throughput. Without a settling delay (or, equivalently, a buffer zone of settling distance  60 ) inserted before the first laser pulse, several links  22  would be processed with serious cross-axis errors.  
           [0012]    Although OTF speed has been improved by accelerating over gaps in the link runs, one limiting factor on the effectiveness of this “gap profiling” is still the requirement for the cross axis to settle within its specified accuracy. At the same time, feature sizes, such as link length  30  and link pitch  32 , are continuing to decrease, causing the need for dimensional precision to increase. Efforts to further increase the performance of the stage or stages substantially increase the costs of the positioning system.  
           [0013]    The traditional way to provide for two-axis deflection of a laser beam employs a high-speed short-movement positioner (“fast positioner”)  62 , such as a pair of galvanometer driven mirrors  64  and  66  shown in FIG. 4. FIG. 4 is a simplified depiction of a galvanometer-driven X-axis mirror  64  and a galvanometer-driven Y-axis mirror  66  positioned along an optical path  70  between a fixed mirror  72  and focusing optics  78 . Each galvanometer-driven mirror deflects the laser beam along a single axis. U.S. Pat. No. 4,532,402 of Overbeck discloses a stacked stage beam positioning system that employs such a fast positioner, and U.S. Pat. Nos. 5,751,585 and 5,847,960 of Cutler et al. disclose split-axis beam positioning systems in which the upper stage(s) carry at least one fast positioner. Systems employing such fast positioners are used for nonlink blowing processes, such as via drilling, because they cannot currently deliver the beam as accurately as “fixed” laser head positioners.  
           [0014]    The split-axis nature of such positioners may introduce rotational Abbe errors, and the galvanometers may introduce additional positioning errors. In addition, because there must be separation between the two galvanometer-controlled mirrors, the mirrors cannot both be located near the entrance pupil to the focusing optics. This separation results in an offset of the beam that can degrade the quality of the focused spot. Moreover, two-mirror configurations constrain the entrance pupil to be displaced farther from the focusing optics, resulting in an increased complexity and limited numerical aperture of the focusing optics, therefore limiting the smallest achievable spot size. Even assuming such positioners could be used for link-severing, the above-described spot quality degradation would cause poor quality link-severing or incomplete link-severing and result in low open resistance across severed links  22 .  
           [0015]    What is still needed, therefore, is a system and method for achieving higher link-processing throughput while maintaining focused spot quality.  
         SUMMARY OF THE INVENTION  
         [0016]    An object of the invention is, therefore, to provide a system and/or method for achieving higher link-processing throughput while maintaining focused spot quality.  
           [0017]    Another object of the invention is to employ a two-axis steering mirror to correct for linear stage settling errors.  
           [0018]    Yet another object of the invention is to provide a positioner system employing coordinated motion for semiconductor link processing applications.  
           [0019]    This invention preferably employs a two-axis steering mirror, pivotally mounted at the entrance pupil of the focusing lens, to perform small-angle motions that deflect the laser beam enough to compensate for cross-axis settling errors on the order of tens of microns. Although the settling errors occur in both axes, one embodiment of this invention is concerned primarily with correcting settling errors in a cross-axis direction to the OTF direction of linear stage travel. A two-axis steering mirror is employed for these corrections because either axis of the linear stage may be used as the OTF axis. The beam steering mirror is preferably used for error correction only and does not require coordination with or modification of the linear stage position commands, although such coordination is possible.  
           [0020]    At least three technologies can be used to tilt a mirror in two axes about a single pivot point. These technologies include fast steering mirrors (“FSMs”) that employ a flexure mechanism and voice coil actuators to tilt the mirror, piezoelectric actuators that rely upon deformation of piezoelectric materials to tilt a mirror, and deformable mirrors that employ piezoelectric or electrostrictive actuators to deform the surface of the mirror. Piezoelectric actuators are preferred.  
           [0021]    Advantages of the invention include the elimination of cross-axis settling time, resulting in increased throughput particularly for SLP systems. The invention also facilitates improved manufacturability of the main positioning stage(s) due to relaxed servo performance requirements because the steering mirror can correct for linear stage errors.  
           [0022]    Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof which proceed with reference to the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    [0023]FIG. 1 is a schematic diagram of a portion of a DRAM showing the redundant layout of and programmable links in a spare row of generic circuit cells.  
         [0024]    [0024]FIG. 2A is a fragmentary cross-sectional side view of a conventional, large semiconductor link structure receiving a laser pulse characterized by a prior art pulse parameters.  
         [0025]    [0025]FIG. 2B is a fragmentary top view of the link structure and the laser pulse of FIG. 2A, together with an adjacent circuit structure.  
         [0026]    [0026]FIG. 2C is a fragmentary cross-sectional side view of the link structure of FIG. 2B after the link is removed by the prior art laser pulse.  
         [0027]    [0027]FIG. 3 is a plan view of a prior art beam travel path.  
         [0028]    [0028]FIG. 4 is a simplified side view of a prior art fast positioner employing a pair of galvanometer-driven mirrors that deflect the laser beam along different respective single axes.  
         [0029]    [0029]FIG. 5 schematically illustrates a side sectional view of a preferred two-axis mirror employed in the practice of the invention.  
         [0030]    [0030]FIG. 6 schematically illustrate a partial front view of a preferred two-axis mirror employed in the practice of the invention.  
         [0031]    [0031]FIG. 7 illustrates the effect of the steering mirror during the OTF run.  
         [0032]    [0032]FIG. 8 illustrates an exemplary multi-row, cross-axis dithering (“MRCAD”) work path.  
         [0033]    [0033]FIG. 9 is a side sectional view of a representative two-axis steering mirror.  
         [0034]    [0034]FIG. 10 is a simplified plan view of a representative two-axis steering mirror.  
         [0035]    [0035]FIG. 11 is a simplified schematic block diagram of an exemplary positioner control system for coordinating the stage positioning and the steering mirror for error correction.  
         [0036]    [0036]FIG. 12 is a simplified schematic block diagram of an exemplary positioner control system for coordinating the stage positioning and the steering mirror for beam-to-work scans and error correction. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0037]    One embodiment of a representative beam positioning system is described in detail in U.S. Pat. No. 4,532,402 of Overbeck, which is assigned to the assignee of this application. A preferred X-Y stage is a “Dynamix” model available from Newport Corporation of Irvine, Calif.  
         [0038]    The beam positioning system preferably employs a laser controller that controls a stacked, split-axis, or planar positioner system and coordinates with reflectors to target and focus laser system output to a desired laser link  22  on IC device or workpiece  12 . The beam positioning system permits quick movement between links  22  on the same or different workpieces  12  to effect unique link-severing operations based on provided test or design data. The beam positioning system may alternatively or additionally employ the improvements or beam positioners or coordinated motion schemes described in U.S. Pat. Nos. 5,751,585, 5,798,927, and 5,847,960 of Cutler et al., which are assigned to the assignee of this application. Other fixed head or linear motor driven conventional positioning systems could also be employed, as well as the systems employed in the 9000, 9800, and 1225 model series manufactured by ESI of Portland, Oreg., the assignee of this application.  
         [0039]    With reference to FIGS. 5 and 6 and with respect to this invention, the final turn mirror of a fixed head system or alternatively fast positioner  66  (FIG. 4) is preferably replaced a single high-speed, high-accuracy two-axis steering mirror system  100  that includes a mirror  102  capable of actuation with at least two degrees of freedom. Mirror  102  has a centrally positioned pivot point  104  that preferably coincides with an entrance pupil  106  of a focusing lens  108 . Two-axis steering mirror system  100  is preferably used for error correction, although it may be employed for beam steering because either axis of the linear stage may be used as the OTF axis.  
         [0040]    Because the beam is focused to a very fine spot size for SLP applications, the mechanism directing mirror system  100  preferably pivots the mirror  102  along at least two axes about pivot point  104 , which is located at or near the entrance pupil of focusing optics or lens  108 . Small angle perturbations in the position of mirror  102  deflect the beam enough to correct for linear stage settling errors at the work surface, and because mirror  102  is located at or near the entrance pupil of focusing lens  108 , the beam is shifted without distorting the focused spot, allowing delivery of a small, high quality spot.  
         [0041]    In one embodiment, settling errors in a cross-axis direction  110  are corrected by mirror  102 , while motion in an on-axis direction  112  is not corrected. This single axis correction allows the linear stage interferometer feedback to be the sole source of laser pulse triggering. However, with proper coordination, on-axis direction  112  steering mirror  102  motion is possible, although it complicates the design and introduces additional error sources that can degrade on-axis direction  112  accuracy if such errors are not addressed.  
         [0042]    Motion in each axis of mirror  102  exhibits scale factor and offset errors, noise, and cross-axis coupling. These error source can be well-controlled and calibrated out in the system, with noise and temperature stability effects controlled through conventional design techniques.  
         [0043]    Calibration of mirror system  100  through beam-to-work (“BTW”) alignments can correct for any non-linearity and alignment errors in steering mirror  102 . Traditionally, the term beam-to-work is used as nomenclature for the process of scanning the linear stage back and forth, while directing the laser beam spot at low power at an alignment target on the wafer or workpiece  12  (FIG. 1). Optical measurements of the reflection off the target are used to precisely determine target and hence wafer location. By scanning several targets with BTW scans, the offset and rotation of the wafer relative to the beam spot can be ascertained. It is also possible to map out other effects such as axis orthogonality and positional distortions.  
         [0044]    After mirror system  100  is added to the laser system, traditional BTW type scans can be used to map out any inaccuracies/nonlinearities in steering mirror  102  response. This is accomplished by doing a BTW scan with mirror  102  in the nominal zero offset (in either axis) position. Then mirror  102  is tilted, and another BTW scan is performed to determine how much lateral offset of the laser beam spot is imparted by the tilt. By measuring the offset caused by numerous mirror tilts in the U and V axes, mirror system  100  can be fully characterized.  
         [0045]    Once the response of mirror system  100  is determined to sufficiently fine precision, then instead of moving the linear stage back and forth, it is possible to use mirror system  100  for subsequent BTW type alignment scans.  
         [0046]    [0046]FIG. 7 illustrates the corrective effect of two-axis steering mirror system  100  during an OTF run. A linear stage ringing is represented by a ringing curve  120 . Mirror  102  deflects the laser beam in cross-axis direction  110  as represented by a correction curve  122  that is the inverse of ringing curve  120 . The resulting beam position is the sum of the linear stage motion and the deflected beam position and is represented by a resulting beam path curve  124 , which is free of cross-axis error.  
         [0047]    [0047]FIG. 8 illustrates using steering mirror system  100  for MRCAD processing during boustrophedon or raster scanning in the context of link severing to further improve the speed at which links are blown. In a preferred mode of operation, MRCAD scanning is done in cross-axis direction  110  while moving along a row  130  of links  132 . MRCAD scanning employs steering mirror  102  (FIGS. 5 and 6) to direct the laser beam along a pathway  134  at links  132  and adjacent links  136  in adjacent rows  138  without needing to move the slower linear motion stage in cross-axis direction  110 . This is possible because not all the links in each row need to be blown. Link processing becomes far more efficient with MRCAD because the linear or stages do not have to be scanned or slewed down each row, so the total number of link row scans can be substantially reduced. As integration increases and link sizes, spot sizes, and pitch distance decrease, MRCAD scanning will become an even more valuable technique.  
         [0048]    In another mode, supplemental on-axis dithering (“SOAD”) uses mirror  102  to deflect the beam in on-axis direction  112  (FIGS.  5 - 7 ). In this operational mode, the beam can be quickly directed ahead in on-axis direction  112 , severing links while the linear motion stage catches up. The SOAD scan ahead or scan behind the stage feature allows the positioning system to reduce stage velocity changes or to sever several links during a single slowed movement segment.  
         [0049]    At least three technologies can be employed to tilt mirror  102  in two axes about pivot point  104 . These technologies include FSMs that employ a flexure mechanism and voice coil actuators, piezoelectric actuators that rely upon deformation of piezoelectric materials, and piezoelectric or electrostrictive actuators to deform the surface of a mirror. Suitable voice coil actuated FSMs are available from Ball Aerospace Corporation of Broomfield, Colo. and Newport Corporation of Irvine, Calif. However, the preferred actuator is a model S-330 Ultra-Fast Piezo Tip/Tilt Platform manufactured by Physik Instrumente (“PI”) GmbH &amp; Co. of Karlsruhe, Germany.  
         [0050]    Traditional galvanometers are not typically used for this application because they each only tilt a mirror about one axis and ordinarily have insufficient positioning accuracy. Moreover, a pair of physically separated galvanometer mirrors are required for two axes of actuation. This separation is incompatible with the desire that actuation occur about one pivot point located near the entrance pupil of focusing lens  108  (FIGS. 5 and 6) to maintain a high quality laser spot at the surface of workpiece  12 . Nevertheless, it is possible to employ galvanometer deflected mirrors in this invention, particularly if employed in single-axis and small deflection applications to maintain accuracy and well focused laser spots.  
         [0051]    By way of example only, FIGS. 9 and 10 show an FSM two-axis mirror system  200  in which four electrical to mechanical vibration generators or transducers are supported by a transducer support platform  220  in a quadrature relationship, whereby a set of transducers  222 ,  224 ,  226 , and  228  are positioned at 0, 90, 180, and 270 degrees with respect to a central axis  230  and are, therefore, at right angles with respect to each other. A movable mirror support member  232  has a central portion or hub  234  bearing a mirror or reflective surface  236  centered with respect to axis  230 . Mirror  236  has a diameter of about 30 mm or less to reduce its weight and facilitate high frequency response for desired beam correction. Mirror  236  is coated with conventional laser optical coatings to account for laser wavelength or design parameters.  
         [0052]    Four lightweight rigid struts or elongated members  242 ,  244 ,  246 , and  248  extend radially from hub  234  of mirror support member  232 , and have respective peripheral terminal portions  252 ,  254 ,  256 , and  258  affixed to respective transducers  222 ,  224 ,  226 , and  228 , which are electrically movable voice coils. For a further description of a suitable conventional voice coil/loudspeaker arrangement, see Van Nostrand&#39;s Scientific Encyclopedia, Sixth Edition, page 1786. The use of such conventional loudspeaker coils for the transducers to perform mechanical actuation, decreases the manufacturing cost of the apparatus. The floating mirror support  232  can beneficially be made of a lightweight material, such as metal (e.g. aluminum or beryllium) or plastic, enabling rapid response to the electrical input signals to the voice coils to be described.  
         [0053]    A tip control generator  260  is connected to transducers  224  and  228  to cause them to move in a complementary “push pull” relationship with each other. Similarly, a tilt control generator  262  is connected to transducers  222  and  226  to cause these coils to also move in a complementary push pull relationship with each other. A laser beam  270  is reflected off reflective surface  236  and a reflected beam  272  is positioned by the generators controlling the cross axis, which is perpendicular to OTF direction of travel, to compensate for cross axis errors. The pairs of signals produced by each generator assume a push-pull relationship, so that when transducer  222  is pulling upper terminal portion  252  of support member  232  to the right in FIG.  10 , lower transducer  226  is pushing terminal portion  256  to the left, to tilt reflective surface  236 , thereby deflecting reflected beam  272 . The actuation can be alternated at the beginning of an OTF run, for example, to move reflective surface  236  at a proper frequency and damped amplitude to compensate for linear stage ringing in cross-axis direction  110 , thereby eliminating the negative effects of linear stage settling time and producing a relatively straight beam path. Thus, links that would otherwise be in the conventional buffer zone can be processed accurately.  
         [0054]    Mirror systems suitable for use with this invention can be implemented with a large enough field to do MRCAD scans by providing beam deflection in a range of about 50 to 100 microns; however, such mirror systems can also be implemented for cross-axis correction only by providing beam deflection in a range of about 10 to 50 microns or as little as about 10 to 20 microns. The mirror is preferably positioned within about plus or minus 1 mm of the entrance pupil of the focusing lens. These ranges are exemplary only and can be modified to suit the system design and particular link processing applications.  
         [0055]    The preferred model S-330 Tip/Tilt Platform manufactured by PI uses piezoelectric actuators for high speed, two-dimensional mirror tilting. Strain gage sensors accurately determine mirror position and provide feedback signals to the control electronics and drive circuitry. A more complete description of the model S-330 Tip/Tilt Platform is available at the PI web site, www.physikinstrumente.com.  
         [0056]    The main advantages of the PI Piezo Tip/Tilt Platform are that the device is commercially available and has a very compact size that readily mounts in an ESI model 9820 positioning system.  
         [0057]    Disadvantages of the PI Piezo Tip/Tilt Platform are that it has insufficient beam deflection range for use in beam-to-work scanning applications even though its range is sufficient for error correction applications; and nonlinear motion, thermal drive, hysteresis, and high-voltage actuation are all inherent problems with piezoelectric actuation that have to be accounted for.  
         [0058]    Of course, other vendors or other types of mirror or actuator designs are suitable for use with this invention.  
         [0059]    In addition to all the other above-described advantages, this invention permits a relaxation on the requirements for the linear motors (jerk time, settling time) using the secondary system to correct for errors. This substantially reduces the cost of the linear motors and also reduces the dependency of the system throughput on the acceleration limit of the linear stage or stages.  
         [0060]    [0060]FIG. 11 shows an embodiment of a positioner control system  300  of this invention for coordinating the positioning of X- and Y-axis motions stages  302  and  304 , and also the positioning of a two-axis steering mirror  306  for positioning error correction. Of course, motion stages  302  and  304  may be combined into a single planar motion stage having positioning control in the X- and Y-axis directions. In a standard operational mode, two-axis steering mirror  306  is used to correct positioning errors caused by X- and Y-axis motion stages  302  and  304 .  
         [0061]    A position command generator  308  generates X- and Y-axis position command signals for delivery through summing junctions  310  and  312  to X- and Y-axis motion controllers  314  and  316  to respective X- and Y-axis motion stages  302  and  304 . The actual positions of X- and Y-axis motion stages  302  and  304  are sensed by respective X- and Y-axis position sensors  318  and  320  and signals representing the actual positions are conveyed to adders or summing junctions  310  and  312  to generate X- and Y-axis position error signals. X- and Y-axis motion controllers  314  and  316  receive the error signals and act to minimize any errors between the commanded and actual positions. For high-accuracy applications, X- and Y-axis position sensors  318  and  320  are preferably interferometers.  
         [0062]    Residual error signals, such as those generated by ringing, are conveyed through enabling gates  322  and  324  to a coordinate transformation generator  326 , which may be optional depending on whether motion stages  302  and  304  share a common coordinate system with two-axis steering mirror  306 . In either event, the residual error signals are passed through adders or summing junctions  328  and  330  to U- and V-axis steering mirror controllers  332  and  334 , which act to tip and/or tilt steering mirror  306  by controlled amounts to deflect, for example, laser beam  270  (FIG. 9) to correct for positioning errors of X- and Y-axis motion stages  302  and  304 . The actual tip and/or tilt positions of two-axis steering mirror  306  are sensed by respective tip and tilt sensors  336  and  338  and signals representing the actual tip and tilt positions are conveyed to adders or summing junctions  328  and  330  to generate tip and tilt position error signals. U- and V-axis steering mirror controllers  332  and  334  receive the error signals and act to correct any errors between the commanded and actual positions. For high-accuracy applications, two-axis steering mirror  306  is preferably a piezoelectric tilt/tip platform, and position sensors  318  and  320  are preferably strain gages. Suitable alternative sensors may include optical, capacitive, and inductive sensing techniques. In this embodiment, skilled workers will understand that U- and V-axis steering mirror controllers  332  and  334  should be adapted to provide zero to 100 volt drive signals to the piezoelectric actuators deflecting two-axis steering mirror  306 .  
         [0063]    Enabling gates  322  and  324  implement a provision in which position command generator  308  can selectively disable position error correction for either the X or the Y axis, thereby enabling error correction for the cross-axis while leaving the on-axis unaffected, or vice versa.  
         [0064]    [0064]FIG. 12 shows an embodiment of a positioner control system  340  for coordinating the positioning of X- and Y-axis motions stages  302  and  304  and, in this embodiment, FSM  236  (FIGS. 9 and 10) for MRCAD scans and positioning error correction. In an extended operational mode, the steering mirror is used for error correction and MRCAD scanning. In this mode of operation, a position command generator  342  generates X- and Y-axis positioning commands for X- and Y-axis motion stages  302  and  304  and also U- and V-axis tip and tilt commands for deflecting FSM  236 . Summing junctions  328  and  330  generate the positioning command for FSM  236  as the sum of the error signals from X- and Y- axis motion stages  302  and  304  and, in this embodiment, also the U- and V-axis tip and tilt commands.  
         [0065]    The error signals are generated in the same manner as in the standard error correction mode. The additional U- and V-axis tip and tilt commands are produced by position command generator  342  to accomplish the desired beam-to-work scanning. Because beam-to-work and MRCAD applications typically require wider ranges of mirror deflection, this embodiment of the invention preferably employs voice coil actuated FSM two-axis mirror system  200 .  
         [0066]    In typical operation, the position commands for MRCAD scanning are used to produce cross-axis motion of the laser beam without commanding cross-axis motion of the motion stages. However, other applications are envisioned that would benefit from on-axis supplemental dithering to boustrophedon scanning.  
         [0067]    The control schemes depicted in these figures are intended to illustrate the basic implementation and operation of this invention. More advanced control schemes, such as those employing feedforward commands to the motion stages and steering mirror, will be obvious to those skilled in the art.  
         [0068]    Skilled workers will appreciate that the two-axis steering mirror systems of this invention can be adapted for use in etched-circuit board via drilling, micro-machining, and laser trimming applications as well as for link severing.  
         [0069]    It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiment of this invention without departing from the underlying principles thereof. The scope of this invention should, therefore, be determined only by the following claims.