Abstract:
A laser ( 126 ) and an AOM ( 10 ) are pulsed at substantially regular and substantially similar constant high repetition rates to provide working laser outputs ( 40 ) with variable nonimpingement intervals ( 50 ) without sacrificing laser pulse-to-pulse energy stability. When a working laser output ( 40 ) is demanded, an RF pulse ( 38 ) is applied to the AOM ( 10 ) in coincidence with the laser output ( 24 ) to transmit it to a target. When no working laser output ( 40 ) is demanded, an RF pulse ( 38 ) is applied to the AOM ( 10 ) in noncoincidence with the laser output ( 24 ) so it gets blocked. So the average thermal loading on the AOM ( 10 ) remains substantially constant regardless of how randomly the working laser outputs ( 40 ) are demanded. The AOM ( 10 ) can also be used to control the energy of the working laser output ( 40 ) by controlling the power of the RF pulse ( 38 ) applied. When the RF power is changed, the RF duration ( 44 ) of the RF pulse ( 38 ) is modified to maintain the constant average RF power. Consistent loading on the AOM ( 10 ) eliminates deterioration of laser beam quality and laser beam pointing accuracy associated with thermal loading variation on the AOM ( 10 ) and is advantageous for applications such as IC chip link processing where stable working laser outputs ( 40 ) with variable output intervals ( 50 ) are needed.

Description:
COPYRIGHT NOTICE 
   © 2003 Electro Scientific Industries, Inc. A portion of the disclosure of this patent document contains material which 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 
   This invention relates to lasers and, more particularly, to a method and an apparatus for providing high-repetition-rate, stable-energy laser pulses on demand with a load-controlled acousto-optic modulator (“AOM”) to minimize distortion of the quality or positional accuracy of the laser beam. 
   SUMMARY OF THE INVENTION 
   Lasers are widely employed in a variety of R &amp; D operations including spectroscopic and biotech study and industrial operations including inspecting, processing, and micromachining a variety of electronic materials and substrates. For example, to repair a dynamic random access memory (“DRAM”), laser pulses are used to sever electrically conductive links to disconnect faulty memory cells from a DRAM device, and then to activate redundant memory cells to replace the faulty memory cells. Because faulty memory cells needing link removals are randomly located, the links that need to be severed are located randomly. Thus, during the laser link repairing process, the laser pulses are fired at random pulse intervals. In other words, the laser pulses are running at a wide variable range of pulse repetition frequencies (“PRFs”), rather than at a constant PRF. For industrial processes to achieve greater production throughput, the laser pulse is fired at the target link without stopping the laser beam scanning mechanism. This production technique is referred to in the industry as “on-the-fly” (“OTF”) link processing. Other common laser applications employ laser pulses that are fired only when they are needed at random time moments. 
   However, the laser energy per pulse typically decreases with increasing PRF while laser pulse width increases with increasing PRF, characteristics that are particularly true for Q-switched, solid-state lasers. While many laser applications require randomly time-displaced laser pulses on the demand, these applications also require that the laser energy per pulse and the pulse width be kept substantially constant. For link processing on memory or other integrated circuit (“IC”) chips, an inadequate laser energy will result in incomplete link severing, while too much laser energy will cause unacceptable damage to the passivation structure or the silicon substrate. The acceptable range of laser pulse energies is often referred to as a “process window.” For many practical IC devices, the process window requires that laser pulse energy vary by less than 5% from a selected pulse energy value. 
   Skilled persons have taken various approaches for ensuring operation within a process window or for expanding the process window. For example, U.S. Pat. No. 5,590,141 for METHOD AND APPARATUS FOR GENERATING AND EMPLOYING A HIGH DENSITY OF EXCITED IONS IN A LASANT, which is assigned to the assignee of this application, describes solid-state lasers having lasants exhibiting a reduced pulse energy drop-off as a function of PRF and, therefore, a higher usable PRF. Such lasers are, therefore, capable of generating more stable pulse energy levels when operated below their maximum PRF. U.S. Pat. No. 5,265,114 for SYSTEM AND METHOD FOR SELECTIVELY LASER PROCESSING A TARGET STRUCTURE OF ONE OR MORE MATERIALS OF A MULTIMATERIAL, MULTILAYER DEVICE, which is also assigned to the assignee of this application, describes using a longer laser wavelength such as 1,320 nanometers (“nm”) to expand the link process window to permit a wider variation of the laser pulse energy during the process. U.S. Pat. No. 5,226,051 for LASER PUMP CONTROL FOR OUTPUT POWER STABILIZATION, which is assigned to Lightwave Electronics, describes a technique of equalizing the laser pulse energy by controlling the current of the pumping diodes. The technique works well in practical applications employing a laser PRF below about 25 or 30 kHz. 
   The above-described laser processing applications typically employ infrared (“IR”) lasers having wavelengths from 1,047 nm to 1,324 nm, running at a PRF not more than about 25 or 30 kHz. However, production needs are demanding much higher throughput, so lasers should be capable of operating at PRFs much higher than about 25 kHz, such as 50-60 kHz or higher. In addition, many laser processing applications are improved by employing ultraviolet (“UV”) energy wavelengths, which are typically less than about 400 nm. Such UV wavelengths may be generated by subjecting an IR laser to a harmonic generation process that stimulates the second, third, or fourth harmonics of the IR laser. Unfortunately, due to the nature of the harmonic generation, the pulse-to-pulse energy levels of such UV lasers are particularly sensitive to time variations in PRF and laser pulse interval. 
   U.S. Pat. No. 6,172,325 for LASER PROCESSING POWER OUTPUT STABILIZATION APPARATUS AND METHOD EMPLOYING PROCESSING POSITION FEEDBACK, which is also assigned to the assignee of this application, describes a technique of operating the laser at a constant high repetition rate in conjunction with a position feedback-controlled laser pulse picking or gating device to provide laser pulse picking on demand, at random a time interval that is a multiple of the laser pulse interval, with good laser pulse energy stability and high throughput. 
   A typical laser pulse picking or gating device is an acousto-optic modulator (“AOM”) or electro-optic modulator (“EOM”, also referred to as a Pockels cell). Typical EOM materials such as KD*P or KDP suffer from relatively strong absorption at the UV wavelengths, which results in a lower damage threshold of the material at the wavelength used and a local heating along the laser beam path within the device, causing changes of the half-wave-plate voltage of the device. Another disadvantage of the EOM is its questionable ability to perform well at a repetition rate over 50 kHz. AOM material is, on the other hand, quite transparent to the UV of 250 nm up to the IR of 2,000 nm, which allows the AOM to perform well throughout typical laser wavelengths within the range. An AOM can also easily accommodate the desirable gating of pulses at a repetition rate of up to a few hundred kHz. One disadvantage of the AOM is its limited diffraction efficiency of about 75-90%. 
     FIG. 1  shows a typical AOM  10  employed for a laser pulse picking or gating application, and  FIGS. 2A-2D  (collectively  FIG. 2 ) show corresponding prior art timing graphs for laser pulses  14 , AOM radio-frequency (“RF”) pulses  18 , and AOM output pulses.  FIG. 2A  shows constant repetition-rate laser pulses  14   a - 14   k  that are emitted by a laser and propagated to an AOM  10 .  FIG. 2B  demonstrates two exemplary schemes for applying RF pulses  18  to AOM  10  to select which of the laser pulses  14  are propagated toward a target. In a first scheme, a single RF pulse  18   cde  shown in dashed lines is extended to cover a time period that includes laser pulses  14   a ,  14   b , and  14   c ; and, in a second scheme, separated RF pulses  18   c ,  18   d , and  18   e  are generated to individually cover the respective time periods for laser pulses  14   a ,  14   b , and  14   c .  FIGS. 2C and 2D  show the respective first-order beam  20  and zero-order beam  16  propagated from AOM  10  as determined by the presence or absence of RF pulses  18  applied to AOM  10 . 
   With reference to  FIGS. 1 and 2 , AOM  10  is driven by an RF driver  12 . When no RF power  22  is applied to AOM  10 , the incoming laser pulses  14  pass through AOM  10  substantially along their original beam path and exit as zero-order beam  16 . When RF power  22  is applied to AOM  10 , part of the incoming laser pulse&#39;s energy is diffracted from the beam path of the zero-order beam  16  to a beam path of a first-order beam  20 . The diffraction efficiency is defined as the ratio of the laser energy in the first order beam  20  to the laser energy in the incoming beam of laser pulses  14 . Either the first-order beam  20  or the zero-order beam  16  can be used as a working beam, based on different application considerations. For simplicity, the pulses from the laser that enter AOM  10  will be referred to as “laser pulses” or “laser output”, and the pulses delivered to a target because they are picked by the AOM  10  will be referred to as “working laser pulses” or “working laser output”. 
   When the first-order beam is used as the working beam, the energy of the working laser pulses can be dynamically controlled from 100% of its maximum value down to substantially zero, as the RF power  22  changes from its maximum power to substantially zero, respectively. Because the practical limited diffraction efficiency of an AOM  10  under an allowed maximum RF power load is about 75-90%, the maximum energy value of the working laser pulses is about 75-90% of the laser pulse energy value from the laser. However, when the zero-order beam  16  is used as the working beam, the energy of the working laser pulses can be dynamically controlled from 100% of the maximum value of the laser pulse energy from the laser down to 15-20% of the maximum value, as the RF power  22  changes from substantially zero to its maximum power, respectively. For memory link processing, for example, when the working laser pulse is not on demand, no leakage of system laser pulse energy is permitted, i.e., the working laser pulse energy should be zero so that the first-order laser beam  20  is used as the working beam. 
   With reference again to  FIG. 2 , RF power  18  is applied to an AOM  10  only when a working laser pulse is demanded at random time intervals, in this case at random integral multiples of the laser pulse interval, and, therefore, results in random variable thermal loading on the AOM  10 . Thermal loading on AOM  10  causes geometric distortion and temperature gradients in the AOM  10 , which cause gradients in its refractive index. These consequences of thermal loading will distort a laser beam passing through the AOM  10 , resulting in deteriorated laser beam quality and instability in the laser beam path or poor beam positioning accuracy. These distortions could be corrected to some degree if they could be kept constant. However, when the system laser pulses are demanded randomly, such as in laser link processing, these distortions will have the same random nature and cannot be corrected practically. 
   Test results on an AOM device, such as a Model N23080-2-1.06-LTD, made by NEOS Technologies, Melbourne, Fla., showed that with only 2 W RF power, the laser beam pointing accuracy can deviate as much as 1 mrad when the RF power  22  to the AOM  10  is applied on and off randomly. This deviation is a few hundred times greater than the maximum allowed for the typical memory link processing system. Laser beam quality distortion due to the random thermal loading on the AOM  10  will also deteriorate the focusability of the laser beam, resulting in a larger laser beam spot size at the focusing point. For applications such as the memory link processing that require the laser beam spot size to be as small as possible, this distortion is very undesirable. 
   What is needed, therefore, is an apparatus and a method for randomly picking working laser pulses from a high-repetition-rate laser pulse train without causing distortion to the laser beam quality and positioning accuracy due to the random thermal loading variation on the AOM. Moreover, what is needed is an apparatus and method for generating working laser pulses having constant laser energy per pulse and constant pulse width on demand and/or on-the-fly at a high PRF and with high accuracy at vastly different pulse time intervals for a variety of laser applications such as spetroscopic, biotech, or micromachining applications, such as laser link processing on memory chips. 
   SUMMARY OF THE INVENTION 
   An object of this invention is, therefore, to provide an apparatus and a method for picking laser pulses on demand from a high-repetition-rate pulsed laser. 
   Another object of this invention is to perform such pulse picking with minimal thermal loading variation on the AOM to minimize distortion to laser beam quality and positioning accuracy. 
   Still another object of this invention is to provide an apparatus and a method for generating system laser pulses on demand, having stable pulse energies and stable pulse widths at selected wavelengths from the UV to near IR and at high PRFs for high-accuracy laser processing applications, such as memory link severing. 
   The present invention uses a laser with high-repetition-rate pulsed output in cooperation with an extra-cavity AOM device for picking or gating the laser pulses such that selected laser pulses are transmitted to the target on demand, while the rest of the laser pulses are blocked. Instead of applying the RF pulses to the AOM only when the working laser pulses are demanded (as is done in the prior art), RF pulses with substantially similar pulse interval times, such as those of the laser pulses, are applied to the AOM regardless of whether a working laser pulse is demanded. Whenever a working laser pulse is demanded, the RF pulse is applied in coincidence with the corresponding laser pulse. Whenever a working laser pulse is not demanded, an RF pulse is also applied to the AOM, but in noncoincidence with the corresponding laser pulse. The RF pulse in noncoincidence with the laser pulse preferably has the same RF power and duration time as does the RF pulse in coincidence with the laser pulse. The timing shift between noncoincident RF pulses and the laser pulses is small enough so that the time shifts are substantially negligible in terms of thermal loading on the AOM. Thus, the AOM will experience substantially no thermal loading variation, regardless of how randomly the working laser pulses are demanded. 
   In a preferred embodiment, the working laser pulses are picked or gated from laser pulses generated at a constant high repetition rate or at a constant laser pulse interval. Such working laser pulses have high stability and consistency in their energy and pulse width. 
   Similarly, the AOM is operated at a substantially constant RF power loading or constant thermal loading, regardless of how randomly the working laser pulses are demanded. So, there is substantially no adverse effect on the working laser beam quality and its pointing accuracy due to having a randomly transmissive AOM. 
   The RF pulse power can also be controlled to perform working laser pulse energy control with the same AOM device to suit application needs. To avoid an adverse effect on the working laser beam quality due to the random variation of the RF pulse power for performing laser pulse energy control, the RF pulse duration can be modulated accordingly such that the product of the RF pulse power and the RF pulse duration remains substantially constant, or an additional RF pulse can be added such that the total RF energy applied to the AOM during one laser pulse interval remains substantially constant. 
   This invention is advantageous for generating stable pulse-to-pulse working laser pulse energy for applications that ordinarily require randomly shutting the laser pulse on or off, including applications such as IC chip link severing. This invention is also advantageous for stabilizing the working laser pulse-to-pulse energy of a Q-switched solid-state laser that employs a nonlinear harmonic generation process to produce frequency-doubled, -tripled, or -quadrupled laser pulses, in which the working laser pulses are shut on and off randomly. 
   This invention is advantageous for typical AOM materials, such as fused quartz and tellurium dioxide (TeO 2 ) used in the previously mentioned AOM Model N23080-2-1.06-LTD, that are quite transparent to laser wavelengths in a broad spectrum range, from the UV spectrum all the way to near IR, such as from 250 nm to 2,000 nm. In a preferred embodiment, the first-order beam is employed as the working beam; however, for some applications, if 10-15% leakage of the laser pulse energy does not cause problems, then either the first-order beam or the zero-order beam can be used as the working beam. 
   Additional aspects and advantages of this invention will be apparent from the following detailed description of preferred embodiments, which proceeds with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a partly schematic view of a prior art AOM device and an RF driver, transmitting a zero-order beam and/or a first-order beam. 
       FIGS. 2A-2D  are corresponding prior art timing graphs of laser pulses, RF pulses, and first and zero-order AOM output laser pulses. 
       FIGS. 3A-3C  are corresponding exemplary timing graphs of laser outputs, RF pulses, and working laser outputs as employed in a preferred embodiment. 
       FIGS. 4A-4C  are alternative corresponding exemplary timing graphs of laser outputs, RF pulses, and working laser outputs that demonstrate the use of the AOM for energy control of the working laser outputs. 
       FIGS. 5A and 5B  are alternative corresponding exemplary timing graphs of RF pulses and working laser outputs that demonstrate the dynamic control range of working laser output energy afforded by the AOM. 
       FIGS. 6A-6C  are corresponding timing and beam position graphs that demonstrate how working laser outputs may be randomly demanded for an exemplary link processing application. 
       FIG. 7  is a schematic block diagram showing a preferred embodiment of an exemplary laser system employing a consistently thermally loaded AOM to provide stable pulse-to-pulse UV laser energy on demand to process unevenly spaced links selected for removal. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIGS. 3A-3C  (collectively  FIG. 3 ) show corresponding timing graphs of laser outputs  24   a - 24   k  (generically laser outputs  24 ), RF pulses  38   a - 38   k  (generically RF pulses  38 ) applied to AOM  10 , and working laser outputs  40   a ,  40   c ,  40   d ,  40   e , and  40   i  (generically working laser output  40 ). In particular,  FIG. 3A  shows laser outputs  24  that are emitted by a laser  126  ( FIG. 7 ) at a constant repetition rate and separated by substantially identical laser output intervals  26 . In typical embodiments, the laser output repetition rate may be from about 1 kHz up to about 500 kHz. Exemplary laser output repetition rates are greater than about 25 kHz, greater than about 40 kHz, or greater than about 100 kHz. For link processing embodiments, each working laser output  40  preferably includes a typical single laser pulse with a multiple-nanosecond pulse width. However, skilled persons will recognize that each working laser output  40  may include a burst of one or more laser pulses each having an ultrashort pulse width, such as disclosed in U.S. Pat. No. 6,574,250 for LASER SYSTEM AND METHOD FOR PROCESSING A MEMORY LINK WITH A BURST OF LASER PULSES HAVING ULTRASHORT PULSE WIDTHS, which is assigned to assignee of this application, or bursts of one or more pulses having pulse widths between about 10 picoseconds and about one nanosecond. 
     FIG. 3B  shows a preferred embodiment of an RF pulsing scheme  30  that employs RF pulses  38  separated by RF pulse intervals  32   a - 32   j  (generically RF pulse intervals  32 ) that are substantially regular or uniform to maintain variations of thermal loading on AOM  10  to within a preassigned operational tolerance. Such tolerance may be a specific thermal load window, but the preassigned tolerance may also or alternatively be windows of spot size or beam position accuracy. In one embodiment, the thermal loading variation is maintained within 5% and/or the beam pointing accuracy is maintained within 0.005 mrad. In a preferred embodiment, at least one RF pulse  38  is generated to correspond with each laser output  24 . 
   Whenever a working laser output  40  is demanded to impinge a target such as an electrically conductive link  60  (FIG.  6 A), an RF pulse  38  is applied to AOM  10  in coincidence with a laser output  24  such that it is transmitted through AOM  10  and becomes a working laser output  40 . 
   In  FIG. 3B , the coincident RF pulses  38  are RF pulses  38   a ,  38   c ,  38   d ,  38   e , and  30   i .  FIG. 3C  shows the resulting corresponding working laser outputs  40   a ,  40   c ,  40   d ,  40   e , and  40   i . When no working laser output is demanded to correspond with a laser output  24 , an RF pulse  38  is applied to AOM  10  in noncoincidence with laser output  24 . In  FIG. 3B , the noncoincident RF pulses  38  are RF pulses  38   b ,  38   f ,  38   g ,  38   h , and  38   j .  FIG. 3C  shows that no working laser outputs  40  correspond with the noncoincident RF pulses  38 . 
   The noncoincident RF pulses  38  are preferably offset from the initiations of respective laser outputs  40  by a time offset  42  that is longer than about 0.5 microseconds. Skilled persons will appreciate that while time offsets  42  are shown to follow laser outputs  40  in  FIG. 3B , the time offsets  42  could alternatively precede laser outputs  40  by a sufficient time period to prevent targeting of working laser output  40 . Thus, RF pulse intervals  32  surrounding a noncoincident RF pulse  38  may be shorter (such as RF pulse intervals  32   b ,  32   f , and  32   h ) than the overall average RF pulse interval  32  (such as  32   c ,  32   d ,  32   f ,  32   g , and  32   i ) or longer (such as RF pulse intervals  32   a ,  32   e , and  32   i ) than the average RF pulse intervals  32 . 
   With reference again to  FIG. 3C , the nonimpingement intervals  50   b  and  50   c  between working laser outputs  40   c  and  40   d  and between working laser outputs  40   d  and  40   e , respectively, are about the same as the laser output interval  26 . The nonimpingent intervals  50   a  and  50   d  between working laser outputs  40   a  and  40   c  and between working laser outputs  40   e  and  40   i , respectively, are roughly multiples of the laser output interval  26 . 
   Skilled persons will appreciate that even though the working laser output  40  is preferably the first-order beam  20  for most applications, such as link processing, the working laser output  40  may be the zero-order beam  16  where leakage is tolerable and higher working laser output power is desirable. 
   In a preferred embodiment, the coincident and noncoincident RF pulses  38  not only employ about the same RF energy, which is the product of an RF power value and an RF duration, but also employ about the same RF power value and about the same RF duration. 
     FIGS. 4A-4C  (collectively  FIG. 4 ) show corresponding timing graphs of laser outputs  24 , RF pulses  38  applied to AOM  10 , and working laser outputs  40  that demonstrate how AOM  10  can be additionally employed to control the output power of working laser outputs  40 .  FIG. 4A  is identical to FIG.  3 A and is shown for convenience only.  FIGS. 4B and 4C  show RF pulses  38 ′ and working laser outputs  40 ′ with the corresponding RF pulses  38  and working laser outputs  40  shown superimposed on them in dashed lines for convenience. The energy of working laser outputs  40 ′ is attenuated by employing less RF power to AOM  10  for RF pulses  38 ′ than for RF pulses  38 ; however, the RF durations  44 ′ are increased for RF pulses  38 ′ over the RF durations  44  employed for RF pulses  38  to maintain a substantially constant product of RF power value and RF duration in order to maintain substantially constant thermal loading on AOM  10 . Skilled persons will appreciate that this technique would permit on-demand selection for a continuum of output powers between working laser outputs  40  or  40 ′ without substantial variance in thermal loading on AOM  10 . Skilled persons will also appreciate that the RF power values and RF durations  44  of the noncoincident RF pulses  38  can be kept as original or can be altered to be within a specified tolerance of the RF loading variation of the coincident RF pulses  38 ′. 
   RF pulse duration  44  is preferably selected to be from about one microsecond to about a half of the laser output interval  26 , more preferably shorter than 30% of the laser output interval  26 . For example, if the laser repetition rate is 50 kHz and the laser output interval  26  is 20 microseconds, the RF pulse duration  44  can be anywhere between one and ten microseconds. The minimum RF pulse duration  44  is determined by the laser pulse jittering time and the response time of AOM  10 . For coincident RF pulse  38 , it is preferable to initiate laser output  24  at the middle point of the RF pulse  38 . For noncoincident RF pulse  38 , it is preferable for the RF offset to be delayed by about half of the minimum RF pulse duration  44  from the initiation of the corresponding laser output  24 . 
     FIGS. 5A-5B  (collectively  FIG. 5 ) show alternative corresponding timing graphs for RF pulses  38  and working laser outputs  40  that demonstrate a large dynamic control range of the working laser output energy. 
   With reference to  FIG. 5 , a very low-energy working laser output  40   a   1  can be generated by applying an RF pulse  38   a   1 , of a near minimum of RF power sufficient to permit targeted propagation of working laser output  40   a . The RF duration  44   a  coincident with laser output  24   a  may be kept short, such as the same duration as RF duration  44 , to minimize variations in RF pulse intervals  32 , and one or more additional noncoincident RF pulses  38   a   2  having higher RF power, but also a short RF duration  44   a   2 , may be applied to AOM  10  such that the sum of the RF energy loading for RF pulses  38   a   1  and  38   a   2  substantially equals that of RF pulse  38   b . In a preferred embodiment, the offset time  52  between RF pulses  38   a   1  and  38   a   2  can be from zero to a few microseconds. Skilled persons will appreciate that RF pulses  38   a , and  38   a   2  can be merged into a single RF pulse  38  that ramps up the RF power after laser output  24   a  is completed. Skilled persons will also appreciate that RF pulse  38   a   2  may precede RF pulse  38   a   1  instead of following it. Skilled persons will appreciate that due to the thermal inertia of AOM  10 , small differences in RF interval  32   a   1  and RF intervals  32  will not cause any meaningful thermal loading variation from the point of view of deterioration of the laser beam quality and pointing accuracy. Accordingly, the RF interval  32   a   1  can be kept sufficiently similar to RF intervals  32  to maintain variations in thermal loading on AOM  10  within a preassigned operational tolerance. The original noncoincident RF pulse  38   b  can be maintained at its original RF duration  44   b  and RF power value, or it can be modulated in the same manner as the set of RF pulses  38   a   1  and  38   a   2 . 
     FIGS. 6A-6C  (collectively  FIG. 6 ) show timing graphs of the target alignment position  70  (also scanning position  70 ) ( FIG. 7 ) and the working laser outputs  40  during an exemplary laser micromachining process, such as laser processing of electrically conductive links  60   a - 60   k  (generically links  60 ).  FIG. 6A  shows a typical link bank  62  having evenly spaced links  60  that are crossed in a scan direction  64  by a target alignment position  70  of a beam positioning system. Based on the results of chip testing, the positioning system is controlled to target randomly positioned links  60  that must be severed to repair an IC device or other workpiece  120  ( FIG. 7 ) while the remaining links  60  remain intact. For example, the scan speed of the beam positioning system can be set to be constant or can be controlled and variable such that the target alignment position  70  crosses over each link  60  at substantially constant positioning intervals, and the laser  126  fires laser outputs at a substantially constant interval, which equals the positioning interval. Thus, with the right timing coordination, whenever position  70  crosses over a link  60 , a laser output  24  is fired. For convenience, the links  60   a ,  60   c ,  60   d ,  60   e , and  60   i  are designated for severing such that  FIG. 6B , which depicts working laser outputs  40 , can be identical to FIG.  3 C. The working laser outputs  40   a ,  40   c ,  40   d ,  40   e , and  40   i , therefore, impinge links  60   a ,  60   c ,  60   d ,  60   e , and  60   i .  FIG. 6C  shows links  60   a ,  60   c ,  60   d ,  60   e , and  60   i  after they have been severed. The laser outputs  24  are fired in synchronization with the scanning position  70  and at the same constant interval such that each working laser output  40  would hit one link  60 . Thus, with the help of the laser pulse picking or gating AOM  10 , whenever a link  60  is selected for removal, the AOM  10  transmits the laser output  24  to sever link  60  as working laser output  40 . Whenever a link  60  is not selected, the AOM  10  does not transmit the laser output  24 , so the link  60  remains intact. In this manner, the laser  126  is running at a substantially constant repetition rate and the laser outputs  24  have a substantially constant interval  26 , but the working laser outputs  40  occur at random multiple intervals of the laser output interval  26 . 
     FIG. 7  shows, as an example, an IC chip link severing system  110  employing RF loading control on AOM  10  to provide stable pulse-to-pulse UV laser energy on demand for processing unevenly spaced links with undistorted working laser output  40 . In system  110 , a system control computer  112  and an embedded control computer  114  co-act to control a beam position controller  116  that receives position information from an X-Y positioner  118  that positions a workpiece  120  relative to a target alignment position  70  of a working laser output  40 . Working laser output  40  may propagate through various optical elements (not shown) in addition to the fold mirrors that are shown. X-Y positioner  118  may also include a Z positioner  123  that may be coupled to either the X or Y stage. X-Y positioner  118  is preferably based on a positioning system described in U.S. Pat. No. 5,751,585 for HIGH SPEED, HIGH ACCURACY MULTI-STAGE TOOL POSITIONING SYSTEM, which is assigned to the assignee of this application. 
   In one embodiment, a UV laser subsystem  124  preferably includes a Q-switched solid state IR laser  126 , such as a diode-pumped, acousto-optically Q-switched Nd:YVO 4  laser; an AOM  128  for picking or gating and amplitude-modulating the laser output of IR laser  126 ; and a frequency multiplier  130  for converting the infrared wavelength emissions from IR laser  126  into green and/or UV wavelengths by employing well-known second, third, or fourth harmonic conversion processes. AOM  10  may be alternatively positioned after frequency multiplier  130  as indicated by the position of an AOM  10   a  (generically AOM  10 ) shown in phantom lines. In either embodiment, a laser controller  134  controls the transmissivity of AOM  10  to transmit or block the laser outputs  24  from the laser  126  to propagate working laser outputs  40  on demand toward workpiece  120 . 
   System control computer  112  conveys across a bus  136  into embedded control computer  114  position coordinates of workpiece  120  locations requiring laser processing. In a typical specimen processing application, workpiece  120  includes regularly spaced-apart device structures, such as fusible links  60 , only some of which require processing. The locations requiring processing are referred to as “target locations”, and the locations not requiring processing are referred to as “intermediate locations”. Embedded control computer  114  adds to the target location coordinates intermediate location coordinates that are spaced apart to trigger IR laser  126  at nearly equal intervals  26 . Embedded control computer  114  conveys the target and intermediate position coordinates one at a time at a predetermined rate across a bus  138  to registers  140  in beam position controller  116  and simultaneously loads control data across a bus  142  to registers  144  in laser controller  134 . The predetermined rate controls the movement velocity of X-Y positioner  118 , and the control data indicate whether the coordinate location is a target location to be processed and may further include output mode, timing, and amplitude information. 
   Laser controller  134  operates timers  146  in either an autopulse mode or a pulse-on-target mode. In autopulse mode, timers  146  start in response to the control data in registers  144 ; and, in the pulse-on-target mode, timers  146  start in response to receiving a position coincidence signal  148  from a comparator  150  in beam position controller  116 . Position encoders  152  in beam position controller  116  indicate to comparator  150  the current position of X-Y positioner  118 , and when the current position matches the position coordinates stored in registers  140 , position coincidence signal  148  is generated, indicating that workpiece  120  is properly positioned over a target position or an intermediate position. Accordingly, if workpiece  120  is positioned over a target position, timers  146  simultaneously operate the Q-switch in IR laser  126  and set AOM  10  to a transmissive state by applying an RF pulse  38  with predetermined RF power and RF duration  44  to AOM  10  such that a working laser output  40  passes through AOM  10  and hits the target link  60 . If workpiece  120  is positioned over an intermediate position, timers  146  operate the Q-switch in IR laser  126  and apply an RF pulse  38  with predetermined RF power and RF duration  44  to AOM  10  only after a predetermined time offset  42  from the Q-switch operation. Thus, the RF pulse  38  is in noncoincidence with laser output  24  and no working laser output  40  is gated through. 
   Since the movement velocity of X-Y positioner  118  is preferably controlled such that the positioner  118  moves over the combination of the targets and intermediate positions at a constant rate, the laser Q-switch is fired at such a constant repetition rate, or in other words, the laser output interval  26  is made substantially equal to position move times. Therefore, the IR laser  126  is operated at a substantially constant repetition rate, or the laser output interval  26  is substantially constant so there are virtually negligible instabilities in laser output  24  and in laser pulse harmonic conversion due to the variation of the laser output interval  26 . Further details concerning on-demand triggering of AOM  10  can be found in U.S. Pat. No. 6,172,325 for LASER PROCESSING POWER OUTPUT STABILIZATION APPARATUS AND METHOD EMPLOYING PROCESSING POSITION FEEDBACK, which is herein incorporated by reference. 
   The RF loading control techniques provide nearly constant thermal loading on AOM  10  by applying an RF pulse  38  to AOM  10  in coincidence with laser output  40  when the positioner  118  is over a target or, in other words, when a working laser output  40  is demanded, and by applying an RF pulse  38  with the same RF energy to AOM  10  but in non-coincidence with the laser output  24  when the positioner  118  is over an intermediate position or, in another words, when a working laser output  40  is not demanded. Skilled persons will appreciate that with such substantially constant thermal loading on AOM  10 , there are minimal adverse effects by AOM  10  on the quality and positioning accuracy of working laser output  40 . 
   It will be further appreciated that the RF power of the RF pulse  38  on AOM  10  can be adjusted to control the energy of working laser output  40  to meet target processing needs, while the RF duration  44  of the RF pulse  38  can be controlled accordingly to maintain a substantially constant Rf energy or arithmetic product of the RF power and the RF duration  44  of the RF pulse  38 . 
   Skilled persons will recognize that portions of this invention may be implemented differently from the implementations described above for preferred embodiments. 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.