Abstract:
A method selectively processes structures on a workpiece with laser pulses. The structures are arranged in a linear pattern having approximately equal pitch. The pulses propagate along a laser beam propagation path terminating at a laser beam spot on the workpiece. The method fires a first processing pulse when the spot coincides with a first structure location, selectively blocks or clears the propagation path during the first pulse, moves the workpiece and the spot relative to one another such that the spot moves toward a second structure location at a speed less than the product of the laser&#39;s PRF and the pitch, fires a dummy pulse before the spot reaches the second structure location, blocks the propagation path during the dummy laser pulse, fires another processing pulse when the beam spot coincides with the second structure location, and selectively blocks or clears the propagation path during the second processing pulse.

Description:
RELATED APPLICATION  
       [0001]     This is a continuation of application Ser. No. 10/931,461, filed Aug. 31, 2004, entitled “Method and System for Decreasing the Effective Pulse Repetition Frequency of a Laser,” issuing as U.S. Pat. No. 7,227,098 on Jun. 5, 2007, which in turn claims priority under 35 U.S.C. § 119 to U.S. Provisional Application No. 60/599,400, entitled “Method and System for Decreasing the Effective Pulse Repetition Frequency of a Laser,” filed Aug. 6, 2004. Both of the preceding applications are incorporated by reference herein in their entireties. 
     
    
     TECHNICAL FIELD  
       [0002]     This disclosure relates generally to operation of a pulsed laser and more particularly to the use of laser beam pulses to process a semiconductor integrated circuit during its manufacturing.  
       BACKGROUND  
       [0003]     During their fabrication process, ICs (integrated circuits) often incur defects due to minor imperfections in the process or in the semiconductor substrate. For that reason, IC devices are usually designed to contain redundant circuit elements, such as spare rows and columns of memory cells in semiconductor memory devices, e.g., a DRAM (dynamic random access memory), an SRAM (static random access memory), or an embedded memory. Such devices are also designed to include particular laser-severable links between electrical contacts of the redundant circuit elements. Such links can be removed, for example, to disconnect a defective memory cell and to substitute a replacement redundant cell. Similar techniques are also used to sever links in order to program or configure logic products, such as gate arrays or ASICs (application-specific integrated circuits). After an IC has been fabricated, its circuit elements are tested for defects, and the locations of defects may be recorded in a data file or defect map. Combined with positional information regarding the layout of the IC and the location of its circuit elements, a laser-based link processing system can be employed to remove selected links so as to make the IC useful.  
         [0004]     A typical link processing system adjusts the position of the laser beam spot on a semiconductor wafer by moving the wafer in an XY plane underneath a stationary optics table, which supports a laser and other optical hardware. The wafer is moved underneath in the XY plane by placing it on a chuck that is carried by a motion stage. A typical wafer contains a number of dies, each containing an IC. Circuit elements within an IC are typically arranged in a regular geometric arrangement, as are the links between those elements. The links usually lie in regular rows in groups that are termed “link banks,” having an approximately uniform center-to-center pitch spacing. To remove selected links in a link bank, a laser beam spot (i.e., the position at which the laser beam&#39;s propagation path axis intersects the wafer workpiece) continuously advances along the link bank at an approximately uniform speed while the laser emits pulses to selectively remove links. The laser is triggered to emit a pulse and thereby to sever a link at a selected target position when the laser beam spot is on the target position. As a result, some of the links are not irradiated and left as unprocessed links, while others are irradiated to become severed. The process of progressing along a row of links and severing selected links with a laser pulse is termed a “link run.” 
         [0005]     Two of the key parameters that impact the time spent processing a link run, and thus throughput, are the maximum velocity limit of the motion stage and the desired link run velocity. Desired link run velocity is the product of the laser pulse repetition frequency (“PRF”) and the link pitch. When the desired link run velocity exceeds the maximum velocity limit of the motion stage, a method must be practiced that results in a link run at a velocity that can be accommodated by the stage. Assume, for example, that the links in a link run have a pitch spacing of 4 μm (micrometers), that the motion stage  170  can travel at a maximum speed (while processing) of 200 mm/sec (millimeters per second), and that the laser is designed to have an optimum PRF of 50 kHz (kilohertz). In that case, the system can operate at both the optimum PRF and the maximum motion stage speed. As another example, assume that the laser instead has an optimum PRF of 60 kHz. In this case, pulsing the laser at that rate on the links sequentially would require that the motion stage move at 240 mm/sec, which is faster than it is capable. A simple solution to this problem is to block every other laser pulse and to slow down the motion stage by a factor of two. The main drawback of that solution is that it significantly decreases throughput (e.g., link run velocity reduced to 120 mm/sec in this case). Thus, operation of a laser with a higher PRF can, ironically, slow down the system.  
         [0006]     Altering the PRF directly can be disadvantageous, as lasers are often optimized for a particular PRF value. More specifically, the pulse characteristics can vary considerably with changes in the PRF as the charge time of the laser is altered. That is problematic because typical link processing scenarios require fairly consistent laser pulse characteristics from link to link. In other words, the so-called “processing window” for reliably severing links is a small window that is sensitive to pulse characteristics. Characteristics that may change as the PRF of a laser is changed include pulse characteristics such as temporal shape, rise time, width, height, energy, energy stability, and beam propagation characteristics such as beam waist position, beam waist diameter, and M 2  value.  
         [0007]     U.S. Pat. No. 6,172,325, assigned to the assignee of the present invention and incorporated in its entirety herein by reference, describes laser pulse-on-position technology with a fixed laser PRF. Pulse-on-position technology is desirable because it provides very accurate placement of link blows; however, that patent does not discuss varying laser PRF.  
         [0008]     U.S. Pat. No. 6,339,604, which is also incorporated herein by reference, describes operating a laser with a fixed, predetermined charge time to stabilize pulse properties for the purpose of trimming components on a semiconductor IC. Use of a predetermined time is not possible when using the pulse-on-position approach.  
       SUMMARY  
       [0009]     According to one embodiment, a method selectively processes one or more of a set of structures on or within a workpiece by irradiating selected structures in the set with pulses from a laser. The set of structures is arranged in a linear pattern having an approximately equal pitch between adjacent structures. The pulses propagate along a laser beam propagation path terminating at a laser beam spot on the workpiece. The laser is characterized by a PRF parameter that specifies a PRF at which pulses produced by the laser have desirable pulse properties for processing the structures, The method comprises firing a first processing laser pulse when the laser beam spot coincides with a first structure location, selectively blocking or clearing the laser beam propagation path during the first laser pulse, moving the workpiece and the laser beam spot relative to one another such that the laser beam spot moves toward a second structure location at a speed less than the product of the PRF parameter and the pitch, firing a dummy laser pulse before the laser beam spot reaches the second structure location, blocking the laser beam propagation path during the dummy laser pulse, firing another processing laser pulse when the laser beam spot coincides with the second structure location, and selectively blocking or clearing the laser beam propagation path during the second processing laser pulse.  
         [0010]     According to another embodiment, a system operates a machine having a laser characterized by a PRF parameter that specifies a PRF at which pulses produced by the laser have desirable pulse properties for irradiating structures on or within a workpiece for a given purpose. The structures are arranged in on the workpiece a linear pattern having an approximately equal pitch between adjacent structures. The laser emits a laser pulse that propagates along a laser beam propagation path terminating at a laser beam spot on the workpiece. The machine is capable of moving the workpiece and the laser beam spot relative to one another at a maximum speed that is less than the product of the PRF parameter and the pitch. The machine is effective to move the laser beam spot across the structures along the linear pattern to selectively irradiate selected ones of the structures with the laser without substantially degrading the desirable pulse properties. The system comprises a means for receiving data indicating the locations on the workpiece of the structures, a means for moving the workpiece and the laser beam spot relative to one another such that the laser beam spot moves along the linear pattern across the structures at a motion speed less than the product of the PRF parameter and the pitch, a means for commencing charging of the laser approximately a desired charging time before the laser beam spot is expected to reach each structure location, wherein the desired charging time is less than the quotient of the pitch divided by the motion speed, a means for firing the laser when the laser beam spot reaches each structure location, whereby the laser emits a pulse having desired pulse properties, and a means for selectively blocking the laser beam propagation path depending upon whether the structure to which the laser beam is directed has been selected for irradiation.  
         [0011]     According to another embodiment, a system irradiates selected targets in a series of structures on or within a workpiece. The system comprises a laser, a laser beam propagation path, a motion stage, and optical shutter, and a controller. The laser is characterized by a PRF parameter that specifies a PRF at which pulses produced by the laser have desirable pulse properties. The laser beam propagation path goes from the laser to the workpiece. The path has an axis that intersects the workpiece at a laser beam spot. The motion stage moves the workpiece and the laser beam spot relative to one another. The optical shutter is disposed in the laser beam propagation path. The optical shutter is capable of selectively blocking the laser pulse from reaching the workpiece or passing the laser pulse to the workpiece. A controller is configured to receive data indicating locations on the workpiece of the structures, to cause charging of the laser to commence approximately a desired charging time before the laser beam spot is expected to reach each structure location, to cause the laser to fire when the laser beam spot reaches each structure location, whereby the laser emits a pulse having desired pulse properties, and to selectively cause the optical shutter to block or to pass the laser beam propagation path depending upon whether the structure to which the laser beam is directed has been selected for irradiation, whereby the system can move the laser beam spot across the structures along the linear pattern at a speed greater than the product of the PRF and the pitch to selectively irradiate selected ones of the structures with the laser without substantially degrading the desirable pulse properties.  
         [0012]     According to yet another embodiment, a semiconductor integrated circuit workpiece is manufactured in part according to the a method for operating a machine having a laser characterized by a PRF parameter that specifies a PRF at which pulses produced by the laser have desirable pulse properties for irradiating structures on or within a workpiece for a given purpose. The structures re arranged in on the workpiece a linear pattern having an approximately equal pitch between adjacent structures. The laser emits a laser pulse that propagates along a laser beam propagation path terminating at a laser beam spot on the workpiece, the machine being capable of moving the workpiece and the laser beam spot relative to one another at a maximum speed that is less than the product of the PRF parameter and the pitch. The method is effective to move the laser beam spot across the structures along the linear pattern to selectively irradiate selected ones of the structures with the laser without substantially degrading the desirable pulse properties. The method by which the workpiece is manufactured comprises receiving data indicating the locations on the workpiece of the structures; moving the workpiece and the laser beam spot relative to one another such that the laser beam spot moves along the linear pattern across the structures at a motion speed less than the product of the PRF parameter and the pitch; commencing charging of the laser approximately a desired charging time before the laser beam spot is expected to reach each structure location, wherein the desired charging time is less than the quotient of the pitch divided by the motion speed; firing the laser when the laser beam spot reaches each structure location, whereby the laser emits a pulse having desired pulse properties; and selectively blocking the laser beam propagation path depending upon whether the structure to which the laser beam is directed has been selected for irradiation.  
         [0013]     Details concerning the construction and operation of particular embodiments are set forth in the following sections with reference to the below-listed drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a block diagram of a link processing system according to one embodiment.  
         [0015]      FIG. 2  is a diagram of the laser in the system shown in  FIG. 1 .  
         [0016]      FIG. 3  is a chart showing a laser pulse cycle, interpulse time, charge time, no-blow zone, and acceptable pre-pulse zone, according to one embodiment.  
         [0017]      FIG. 4  is a chart showing techniques for selecting laser pre-triggering and link run velocity for the performance of link runs, according to certain embodiments.  
         [0018]      FIG. 5  is a chart showing the states of the Q switch and the AOM of  FIGS. 2 and 1 , respectively, during processing of a bank of links according to a technique of  FIG. 4 .  
         [0019]      FIG. 6  is flowchart of a method according to one embodiment.  
         [0020]      FIG. 7  is a set of plots of laser beam spot position and laser charge as functions of time according to one embodiment. 
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0021]     With reference to the above-listed drawings, this section describes particular embodiments and their detailed construction and operation. As one skilled in the art will appreciate, certain embodiments may be capable of achieving certain advantages over the known prior art, including some or all of the following: (1) more precise matching of a laser&#39;s effective PRF to the requirements of the motion stage during link processing operations without adversely affecting laser pulse properties; (2) higher throughput for link processing in some situations; (3) greater flexibility to use a laser over a range of effective PRFs; (4) more consistent laser pulse properties in cases where the PRF is inherently time-varying; and (5) improved system accuracy attained through position feedback for pulse triggering. These and other advantages of various embodiments will be apparent upon reading the following.  
         [0022]      FIG. 1  is a block diagram a link processing system  100  according to one embodiment. The system  100  comprises a pulsed laser  110 , which produces a laser beam  120 . The nominal PRF of the laser  110  is the approximately constant desired PRF, which may be selected for a number of reasons including production of desirable pulse characteristics and compatibility with temporal requirements of the link processing system  100 . The laser beam  120  propagates along a propagation path until it reaches a workpiece  130  at a laser beam spot  135 . Disposed along the propagation path are a number of optics elements, including an acoustic-optical modulator (AOM)  140 , a mirror  150 , and a focusing lens  160 . The AOM  140  is responsive to a radio frequency (RF) input, which changes the direction in which the laser beam  120  exits the AOM  140 . By selectively driving the AOM  140  with an RF signal having an appropriate amplitude, the AOM  140  can be configured to selectively block or pass the laser beam  120  to the mirror  150 , through the lens  160 , and onto the workpiece  130 . In other words, the AOM  140  behaves like a light switch or shutter in the laser beam propagation path. Any device capable of functioning as a light switch or shutter (e.g., an electro-optic modulator (EOM) or a liquid crystal modulator) can be used in place of the AOM  140 .  
         [0023]     The workpiece  130  is mounted to a motion stage  170  that moves the workpiece in an XY plane (the laser beam  120  being incident upon the workpiece in the Z direction). The firing of the laser  110 , the shuttering of the AOM  140 , and the movement of the motion stage  170  are controlled by a control computer  175 , a motion stage controller  180 , and a laser controller  185 . Together they control laser pulsing and AOM shuttering with the laser beam spot  135  relative to the workpiece  130  by monitoring the motion stage position, which is commanded by a the motion stage controller  180 , and using this position information to coordinate the laser pulsing and AOM shuttering. Triggering laser emissions based upon position ensures accurate delivery of laser pulses to target link structures. Timing-based laser trigger methods may have diminished accuracy due to errors in the XY stage velocity, causing the motion stage  170  to be in an incorrect position when the laser  110  emits a pulse.  
         [0024]     The control computer  175  accesses a target map (not shown), which contains data indicating target positions on the workpiece  130  that should be irradiated (e.g., to sever a link at that position). The target map is typically generated, for example, from a testing process that determines which circuit elements in an IC workpiece are defective. The control computer  175  generates link run commands, sending movement commands to the motion stage controller  180  and sending laser trigger position commands to the laser controller  185 . The functions of the control computer  175  may be distributed over one or more physical computers.  
         [0025]     In addition to affecting movement of the motion stage  170  via control commands, the motion stage controller  180  includes a position sensor that senses where the laser beam spot  135  is relative to the workpiece  130 . That position data can be used in a feedback control loop to accurately control the position of the laser beam spot  135  on the workpiece  130 . That measured laser beam spot position data is also forwarded to the laser controller  185 , where the position data is used to control the firing of the laser  110 . The phrase “laser beam spot” is actually a shorthand expression for the location at which the axis of the laser beam&#39;s propagation path intersects the workpiece  130 , and the surrounding area of a size and extent approximately equal to what would be illuminated by the laser beam, whether the laser beam is on or off. That spot moves along the top surface of the workpiece  130  as the motion stage  170  moves. Even though the laser beam impinges on that surface at times (when pulsed and not blocked along its propagation path) and not at other times (when not pulsed or pulsed but blocked), the axis of its propagation path is always present.  
         [0026]     The laser controller  185  comprises two position registers  192  and  194  as well as a comparator  196 . The position register  192  stores the current position of the laser beam spot  135  relative to the workpiece  130 , while the position register  194  stores the position of a target or another position of interest (e.g., a pre-charging or pre-pulsing position) as reported by the control computer  175 . The comparator  196  compares the contents of both position registers  192  and  194  to determine when they match. When the comparator  196  indicates a match, the laser controller  185  generates commands to the laser  110  and/or the AOM  140 , triggering the laser to emit a pulse and/or causing the AOM  140  to be set to a blocking or passing state.  
         [0027]     Note that it is immaterial whether the laser-optics is stationary and the workpiece moves (as assumed above for ease of comprehension), or vice versa, or some combination of movement by both bodies occurs. All that is required is the laser beam spot  135  and the workpiece  130  move relative to one another. The purpose of the laser irradiation may be a purpose other than link severing. Laser micromachining, machining, drilling, via drilling, trimming, component trimming, marking, scribing, annealing, changing states of matter, measuring, and reconnecting fuses are a few of the many alternative purposes of the laser irradiation. Nonetheless, link severing is the preferred application. Note finally that the functions of the control computer  175 , motion stage controller  180 , and laser controller  185  may be performed in fewer physical devices, even a single computer or controller.  
         [0028]      FIG. 2  is a diagram of an embodiment of the laser  110  in simplified form. The laser  110  comprises a laser power supply  204  and RF amplifier  206 . The laser  110  also comprises two reflectors  210  and  220 , which together define a laser cavity  225 , which may be a resonator. The laser  110  also comprises a pump  230 , a laser medium  240 , and a Q switch  250 . Optionally, the laser resonator  225  may also contain an intracavity second harmonic crystal (not shown) and possibly a third harmonic crystal (not shown). The pump  230  pumps energy into the laser medium  240 . The pump source  230  preferably provides continuous wave (CW) pumping of the laser medium  240 . The laser medium  240  can be, for example, Nd:YLF, Nd:YAG, or Nd:YVO 4 . Other embodiments of the laser medium  240  may be employed. The pump  230  is preferably a semiconductor laser. Alternatively, the pump  230  may be a flashlamp or arc lamp or other excitation source suitable for pumping the laser medium  240 . The pump  230  may further include lenses for efficiently coupling the pump  230  into the laser medium  240 . The action of the pump  230  produces excited ions suitable for generation of laser output through the process of stimulated emission. The pump  230  may be physically located at the end of the laser cavity  225  (as shown) or to its side. When the Q switch  250  is closed, energy is stored in the laser cavity  225  as a result of the pumping action. When the Q switch  250  is opened, the stored energy propagates out of the laser cavity  225  as a laser pulse. The opening and closing of the Q switch  250  is controlled by an RF signal RFQ from the RF amp  206 . As those skilled in the art will recognize,  FIG. 2  is not meant to illustrate the physical arrangement of the laser  110  accurately.  
         [0029]     The laser  110  typically operates in pulsed mode, in which the Q switch  250  is alternately closed then opened. During the time when the Q switch  250  is closed, energy is stored in the cavity  225 , and that energy is released as a laser pulse when the Q switch  250  is opened. When the laser  110  is repetitively Q-switched, the frequency at which the Q switch  250  is opened is the PRF. As the PRF changes, the properties of the laser pulse (e.g., pulse shape, pulse rise time, pulse width, pulse height, pulse energy, pulse energy stability, etc.) can change significantly. For that reason and because link processing systems are typically highly sensitive to variations in laser pulse properties, lasers in link processing systems typically operate at an approximately fixed nominal PRF at which the laser  110  generates pulses having suitable properties for link blowing. The selection of the nominal PRF may also take into account link processing system parameters, such as the maximum rate at which control computer  175  can process link coordinate information.  
         [0030]      FIG. 3  is a chart showing different zones of a laser pulse period. The start and end of the chart show the generation of two pulses at the nominal laser PRF or period  1 /PRF=T INTERPULSE .  FIG. 3  is not drawn to scale.  FIG. 3  depicts the pulse duration, and pulse trigger time, which is when a pulse begins.  FIG. 3  also depicts the binary states of the signal RFQ in simplified form. The standard laser charging time is T CHG ≈T INTERPULSE −T EMIT , where T EMIT  is the RF off time pre-programmed in the laser power supply  204 .  FIG. 3  also depicts two zones. A “pre-pulse” zone is shown in which the laser may be pulsed early by opening the Q switch  250 . This results in a depletion of the energy stored in the laser cavity  225 . Pulses emitted when triggered early will not have the same pulse properties as a pulse triggered with charging time T CHG . A “no-blow” zone is also depicted. The no-blow zone consists of the duration of the current pulse, plus some additional time for system components to change state. System components may include the laser controller  185 , the RF amp  204 , an AOM switch driver, and the AOM  140 . In this time zone, in a preferred embodiment, it is not possible to trigger laser pulses due to the finite RF off time T EMIT , during which the Q-switch  250  remains commanded open. A typical value of T EMIT  is in the range 1-5 μs and can be selected to prevent unwanted parasitic laser pulses from being emitted. Other appropriate values may be selected.  
         [0031]     In another embodiment, the step of pre-pulsing the laser  110  can be omitted, and the laser charge time T CHG  can be held approximately constant before each pulse. This method can be practiced by keeping the Q switch  250  open following a pulse until the laser beam spot  135  reaches the pre-pulse position or until the pre-pulse time. Then the Q switch  250  is closed, allowing the laser  110  to charge until triggered to emit a pulse or dummy pulse. In essence, the pre-pulse position and/or time is replaced by a laser charge position and/or time. Because this method ensures an approximately constant laser charge time T CHG , pulse properties are not degraded and the laser  110  is operated at an effective PRF lower than the nominal PRF.  
         [0032]      FIG. 4  is a set of diagrams depicting position-based triggering of laser emissions according to a number of different techniques applied to link banks  410 ,  420 ,  430 , and  440 , respectively.  FIG. 4A  is a diagram depicting position-based triggering of laser emissions when the link run velocity, equal to link pitch times nominal laser PRF, is less than or equal to the maximum stage velocity. Link bank  410  comprises a number of regularly spaced links, the illustrated ones in the segment being labeled  450 A- 459 A. In this mode of operation, the laser beam spot  135  moves along the link bank  410  left to right as shown. The laser beam spot  135  is shown over the link  459 A. Before reaching that position, the laser beam spot  135  moved across the links  450 A- 458 A in that order. When the position of the laser beam spot  135  coincided with the positions of each of the links  450 A- 458 A, the Q switch  250  opened to emit a pulse from the laser  110 . The AOM  140  was in a blocking state when the laser beam spot  135  was over the links  452 A,  456 A, and  458 A so that they were not severed, while the AOM  140  was in a passing state when the laser beam spot  135  was on the links  450 A and  454 A so that they were severed by the laser pulse. Because the links  452 A,  456 A, and  458 A were not severed, they are termed “dummy” targets and the pulses that were directed toward them but blocked are termed dummy pulses. Likewise, the laser beam spot  135  will sever the link  459 A if the AOM is not in a blocking state when the laser beam spot  135  is on that link.  
         [0033]     In  FIG. 4A , the time for the laser spot  135  to traverse between target links and dummy links on the wafer is approximately equal to the reciprocal of the nominal PRF; thus the laser interpulse period T INTERPULSE  and charging time T CHG  are approximately constant. Minor perturbations in the trigger time can occur because the velocity of the motion stage  170  typically cannot be exactly controlled, and the stage position sensor typically contains some sensor noise. When the stage velocity is slightly higher than the desired link run velocity, laser emission is triggered early and the interpulse period and charging time are slightly less than is desired. Likewise, when the stage velocity is slightly lower than the desired link run velocity, laser emission is triggered late and the interpulse period and charging time are greater than is desired.  
         [0034]     These minor perturbations in charging and trigger times cause minor perturbations in laser pulse properties. However, these minor perturbations in a position-based laser trigger are a tradeoff for having laser pulses accurately impinge upon the desired locations of target link structures. If laser emission was triggered based upon fixed time, stage velocity error would lead to improper pulse positioning, with pulses arriving offset from the center of the desired target links along the axis of the link run.  
         [0035]     In  FIG. 4A  the spatial distance between adjacent links (e.g., links  452 A and  454 A) is denoted d INTERPULSE . In other words, the link pitch is d INTERPULSE , which is the laser interpulse period T INTERPULSE  converted to distance units. For a constant velocity v of the motion stage  170 , d INTERPULSE  and T INTERPULSE  are related by the equation T INTERPULSE =d INTERPULSE /v or the equivalent equation v=d INTERPULSE ·PRF. Because  FIG. 4A  assumes that v can be chosen to satisfy that equation, the laser  110  can operate at its nominal PRF and therefore generate pulses having desirable properties for effective link blowing, as the laser cavity charges for approximately its natural charging time t CHG  between pulses.  
         [0036]     In  FIG. 4B , a second mode of operation is depicted that shows the insertion of pre-pulses. Pre-pulses are added because in this case, the desired link run velocity v=d*PRF exceeds the maximum permissible link run velocity. It is not possible to traverse pitch distance in the inherent time of a laser pulse period. In this case, it is undesirable to diminish throughput by dramatically reducing link run velocity and delivering every nth pulse to target link structures. It is also undesirable to operate the laser at a PRF that is lower than the nominal PRF, since laser pulse properties will be impacted. The laser cavity  225  would store more energy than a desirable pulse should have, and pulse properties such as shape and stability may be degraded.  
         [0037]     The mode of operation depicted in  FIG. 4B  results in link runs at the maximum stage velocity, and very nearly uniform pulse properties. This mode of operation involves the insertion of pre-pulses, which are triggered based upon timing or position comparison. Pre-pulses are generated to release energy from the laser cavity  225  such that pulses (labeled  530  and  535  on  FIG. 5 ) are substantially equal. By use of pre-pulses, the laser  110  will have an approximately constant interpulse period or distance d INTERPULSE , as shown between a pre-pulse position  470  and the position of a link  452 B. An approximate charging time results because the true laser trigger to emit a transmitted pulse to a link or to emit a blocked pulse toward a dummy link is generated by position comparison. The resultant behavior, is that link runs are performed at the maximum stage velocity, which is good for throughput, and the laser interpulse period T INTERPULSE  or the charge time T CHG  or both are approximately constant, which is good for pulse properties. Thus, this method reduces the effective laser PRF without an undesirable degradation of pulse properties.  
         [0038]      FIG. 4C  depicts a third mode of operation, in which the desired link run velocity is multiple times more than the maximum stage velocity. In this mode, multiple pre-pulses are generated between each actual or dummy pulse. By releasing the built-up energy in the laser cavity  225  repeatedly in this way, the workpiece  130  can be more effectively shielded from unwanted laser radiation than if the laser were allowed to charge without release up to the point of the final pre-pulse, which might then have sufficient energy to leak through the AOM  140  and reach the workpiece  130 .  
         [0039]      FIG. 4D  depicts a fourth mode of operation, in which the desired link run velocity is only slightly more than the maximum stage velocity. In this case, the insertion of pre-pulses and completion of the link run at the maximum velocity is not possible because the pre-pulses would be inserted in the no-blow region depicted in  FIG. 3 . Instead, in this mode the link run velocity is decreased below the maximum link run velocity such that it is possible to use pre-pulses. The minor decrease in throughput that results is necessary in order to achieve the desired laser pulse properties through the use of pre-pulses. However, this throughput decrease is minor in comparison with using every other pulse and dividing link run velocity in half. Thus, higher throughput and consistent pulse properties result.  
         [0040]     In all cases illustrated in  FIG. 4 , triggered laser pre-pulses and dummy pulses are blocked from reaching the workpiece  130  by the AOM  140 . Desired pulses are allowed to pass through the AOM  140  and reach target link structures.  
         [0041]     Note also that the position or timing of pre-pulses and regular (blocked or delivered) pulses may also be calculated taking into account the time delays attributable to hardware in the system  100 , e.g., in the laser  110 , AOM  140 , and laser controller  185 . Furthermore, it is possible to generate the pre-pulse and pulse signals to take into account the velocity error that exists before generation of the pre-pulse. This method involves using the measured or estimated stage velocity from the motion stage controller  180 . While this results in a more accurate estimate of what the true laser charging time will be, the actual laser charging time will still vary due to changes in the stage velocity error, and noise in the stage position sensor.  
         [0042]     Note that although the pitch spacings illustrated in  FIG. 4  are uniform in each link bank, that need not be the case. The techniques described can be applied naturally to links of non-uniform spacing. In that case, the distance from the previous event-trigger to a current pre-pulse position may vary while the distance from the current pre-pulse to its following laser triggering can be set constant by placing the pre-pulse position appropriately.  
         [0043]      FIG. 5  is a chart showing the states of the Q switch  250  and the AOM  140  during processing of the link bank  420  depicted in  FIG. 4B . The Q switch  250  alternates between a closed state  510  and an open state  520 . The Q switch  250  is set in the open state  520  where an actual pulse  530  or a dummy pulse  535  is desired as well as where or when a pre-pulse  540  is generated. As can be seen, the distance from each pre-pulse  540  to the next actual or dummy pulse  530  or  535  is the same as d INTERPULSE , which results in an interpulse period of approximately T INTERPULSE . Consequently, the charging distance d CHG  before each dummy pulse  530  or actual pulse  535  is approximately constant. The AOM  140  alternates between a passing state  550  and a blocking state  560 . The AOM  140  is set in the blocking state  560  during each pre-pulse  540  and during each dummy pulse  535 . However, the AOM  140  is set in the passing state at each actual pulse  530 . Similar charts for the other operation modes of  FIG. 4  can easily be generated using the teachings set forth herein and are therefore omitted.  
         [0044]     The foregoing makes clear that the schemes illustrated in  FIG. 4  operate the laser at a lower effective PRF while preserving desirable pulse properties of the nominal PRF by holding the period of time from the pre-pulse to the actual or dummy pulses approximately constant, consistent with the precise position accuracy of a pulse-on-position system. By adjusting the time that the laser  110  is pre-pulsed, the effective PRF can be set to precisely match the speed limit of the motion stage  170  and thus maximize throughput.  
         [0045]     A numerical example illustrates this advantage. Assume that link pitch is 4 μm and that the maximum speed achievable by the motion stage  170  is 200 mm/sec. Assume further that the nominal PRF in  FIG. 3  is 50 kHz. Thus, in these circumstances, the scheme in  FIG. 3  would be optimum in the sense that operation of the laser  110  at its nominal PRF exactly matches the maximum speed achievable by the motion stage  170 . However, assume that the laser has a nominal PRF equal to 60 kHz instead (PRF′=60 kHz). That means that the optimum interpulse period T INTERPULSE  approximately equals 16.67 μsec. However, it takes 20 μsec for the laser beam spot  135  to traverse the pitch distance d when the motion stage moves at maximum speed. The schemes illustrated in  FIG. 4B  can solve this mismatch by moving the laser beam spot  135  at its maximum speed (200 mm/sec) while pre-pulsing the laser  110  at a time or corresponding position approximately 3.33 μsec after the previous pulse. This time is approximately 16.67 μsec before reaching each link and triggering the laser  110  to emit a pulse. Approximate times result due to position-based laser pulse generation as the laser beam spot  135  reaches each link and moves toward the next link.  
         [0046]      FIG. 6  is flowchart of a method  600  according to one embodiment. The method begins by obtaining ( 610 ) link coordinates, which contains position data for targets and dummy links on the workpiece  130 . The source of that data may be, for example, a defect map of the workpiece  130 . The link coordinates are preferably sorted in a sensible order, such as by link runs (i.e., row by row) in order along each link run. If necessary, the method  600  can sort the target data into a desired order before iterating on each target datum in the subsequent steps.  
         [0047]     Starting with the first link, the method  600  computes ( 615 ) a trigger or firing position and, if necessary, a pre-pulse position. The method  600  moves ( 620 ) the laser beam spot  135  relative to the workpiece  130  toward the pre-pulse position. The method  600  tests ( 630 ) whether the sensed position of the laser beam spot  135  matches the computed pre-pulse position and continues performing the moving step  620  as necessary until they match. When that occurs, the method  600  pre-pulses ( 635 ) the laser  110  by, e.g., opening the Q switch  250  briefly to release any energy built up in the laser cavity  225  while blocking ( 640 ) the pre-pulse by, e.g., setting the AOM  140  in a blocking state. Steps  630 - 640  may be repeated if multiple pre-pulses are required between adjacent links. After the pre-pulse, the method  600  commences ( 645 ) charging the laser  110  by, e.g., closing the Q switch  250  to terminate the pre-pulse. Meanwhile the method  600  moves ( 650 ) the laser beam spot  135  toward the trigger/link position. In an ideal system, the trigger position is the link position, but owing to hardware delays, the trigger position may be offset from the link position slightly. The method  600  tests ( 655 ) whether the sensed position of the laser beam spot  135  matches the trigger position and continues to perform the moving step  650  until they match. Then, the method  600  tests ( 660 ) whether the link is a dummy link. If so, the method  600  prepares to block ( 665 ) the pulse before it reaches the dummy link, e.g., by setting the AOM  140  in a blocking state. If the link is an actual target to be irradiated, then the method  600  skips the blocking step  665 , fires ( 670 ) the laser  110  by, e.g., opening the Q switch  250 , and ends ( 675 ) the laser firing by, e.g., closing the Q switch  250 . Finally, to complete the iteration, the method  600  tests ( 680 ) whether the current link is the last one. If so, the method  600  is complete. If not, the method  600  repeats the iterative loop for the next link by returning to the obtaining step  610 . Although the motion of the laser beam spot  135  relative to the workpiece  130  could be in discrete steps, it is preferably continuous throughout the method  600 .  
         [0048]      FIGS. 7A and 7B  are plots of laser beam spot position and laser charge as functions of time according to one embodiment. In  FIG. 7A , the position of the laser beam spot  135  is plotted as a function of time, as the laser beam spot  135  moves along a bank of links. The dotted line is a straight line representing the ideal stage velocity, and the solid line represents measured stage velocity. Measured stage velocity has minor deviations from the ideal stage velocity. The “X” marks along the line represent the estimated position for firing the laser on the links. The diamond marks represent the actual firings. The circle marks represent the time/position at which the laser pre-pulse commands are issued an estimated time or distance before each firing position. The time difference between each diamond mark and its preceding circle mark is approximately the desired interpulse period. The velocity errors and hence timing difference are over-exaggerated on these figures to better illustrate the difference between estimated and actual laser firing time.  
         [0049]      FIG. 7B  shows an approximate representation of the energy in the laser cavity  225  on an identical time scale for an embodiment utilizing a Q-switch laser, such as the laser  110 . As can be seen, when the laser  110  is fired and the Q switch  250  is closed (at the circle marks) the laser energy in the cavity  225  builds up until the Q switch  250  is opened (at the diamond marks). Laser cavity energy storage is lower before pre-pulse (circle) events and actual trigger (diamond) events. Careful observation shows that when a pulse occurs early (diamond precedes X) the laser pulse may have slightly less energy and the subsequent pre-pulse may have slightly more energy. One again, though, the timing differences between estimated and actual firing time on  FIGS. 7A and 7B  are over-exaggerated, resulting in over-exaggerated representations of laser cavity energy.  
         [0050]     Other embodiments could employ alternative laser technologies. For example the laser could be a pulsed fiber laser configured in a master oscillator power amplifier configuration. In that embodiment, rather than relying on a Q-switch to gate the discharge of the laser, the semiconductor laser employed as the master oscillator to pump the gain fiber in the master oscillator power amplifier configuration can be controlled in such a fashion as to create the pre-pulse and the actual pulse at the appropriate times. In other alternate embodiments, wavelength-shifted lasers including green lasers or UV lasers, may be employed.  
         [0051]     The methods and systems illustrated and described herein can exist in a variety of forms both active and inactive. For example, they can exist as one or more software programs comprised of program instructions in source code, object code, executable code or other formats. Any of the above can be embodied on a computer-readable medium, which include storage devices and signals, in compressed or uncompressed form. Exemplary computer-readable storage devices include conventional computer system RAM (random access memory), ROM (read only memory), EPROM (erasable, programmable ROM), EEPROM (electrically erasable, programmable ROM), flash memory and magnetic or optical disks or tapes. Exemplary computer-readable signals, whether modulated using a carrier or not, are signals that a computer system hosting or running a computer program can be configured to access, including signals downloaded through the Internet or other networks. Concrete examples of the foregoing include distribution of software on a CD ROM or via Internet download. In a sense, the Internet itself, as an abstract entity, is a computer-readable medium. The same is true of computer networks in general.  
         [0052]     The terms and descriptions used herein are set forth by way of illustration only and are not meant as limitations. Those skilled in the art will recognize that many variations can be made to the details of the above-described embodiments without departing from the underlying principles of the invention. For example, certain steps of the method  600  may be performed in an order other than shown or simultaneously. The scope of the invention should therefore be determined only by the following claims (and their equivalents) in which all terms are to be understood in their broadest reasonable sense unless otherwise indicated.