Abstract:
A method and system for increasing throughput of laser micromachining systems use more than one laser. Two or more pulsed laser beams are combined and then separated into multiple laser beams that enable the system to work simultaneously at multiple locations on the workpiece with pulse rates greater than those achievable with independently operating lasers while maintaining pulse energy equal to or greater than the pulse energy of each of the original independent laser beams. Most laser micromachining applications required multiple sequential pulses to process a workpiece. Increasing the pulse rate while maintaining pulse energy effects more rapid material removal and thereby increases throughput for a laser micromachining system.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to laser processing a workpiece and, in particular, to combining the outputs of two or more lasers to achieve at a given power level a pulse repetition frequency that is greater than the repetition frequency of either laser operating independently at the given power level.  
       BACKGROUND OF THE INVENTION  
       [0002]     Laser processing can be conducted on numerous different workpieces using various lasers effecting a variety of processes. The specific types of laser processing of interest with regard to the present invention are laser processing of a single or multilayer workpiece to effect hole and/or blind via formation and laser processing of a semiconductor wafer to effect wafer dicing or drilling. The laser processing methods described herein could also be applied to any type of laser micromachining, including but not limited to removal of semiconductor links (fuses) and thermal annealing or trimming passive thick or thin film components.  
         [0003]     Regarding laser processing of vias and/or holes in a multilayer workpiece, U.S. Pat. Nos. 5,593,606 and 5,841,099 of Owen et al. describe methods of operating an ultraviolet (UV) laser system to generate laser output pulses characterized by pulse parameters set to form in a multilayer device through-hole or blind vias in two or more layers of different material types. The laser system includes a nonexcimer laser that emits, at pulse repetition rates of greater than 200 Hz, laser output pulses having temporal pulse widths of less than 100 ns, spot areas having diameters of less than 100 μm, and average intensities or irradiance of greater than 100 mW over the spot area. The preferred nonexcimer UV laser identified is a diode-pumped, solid-state (DPSS) laser.  
         [0004]     Published U.S. Patent Application No. US/2002/0185474 of Dunsky et al. describes a method of operating a pulsed CO 2  laser system to generate laser output pulses that form blind vias in a dielectric layer of a multilayer device. The laser system emits, at pulse repetition rates of greater than 200 Hz, laser output pulses having temporal pulse widths of less than 200 ns and spot areas having diameters of between 50 μm and 300 μm.  
         [0005]     Laser ablation of a target material, particularly when a UV DPSS laser is used, relies upon directing to the target material a laser output having a fluence or energy density that is greater than the ablation threshold of the target material. A UV laser emits a laser output that can be focused to have a spot size of between about 10 μm and about 30 μm at the 1/e 2  diameter. In certain instances, this spot size is smaller than the desired via diameter, such as when the desired via diameter is between about 50 μm and 300 μm. The diameter of the spot size can be enlarged to have the same diameter as the desired diameter of the via, but such enlargement would reduce the laser output energy density to the extent that it is less than the target material ablation threshold and cannot effect target material removal. Consequently, the 10 μm to 30 μm focused spot size is used and the focused laser output is typically moved in a spiral, concentric circular, or “trepan” pattern to form a via having the desired diameter. Spiraling, trepanning, and concentric circle processing are types of so-called non-punching via formation processes. For via diameters of about 70 μm or smaller, direct punching delivers a higher via formation throughput.  
         [0006]     In contrast, the output of a pulsed CO 2  laser is typically larger than 50 μm and is capable of maintaining an energy density sufficient to effect formation of vias having diameters of 50 μm or larger on conventional target materials. Consequently, a punching process is typically employed when a CO 2  laser is used to effect via formation. However, a via having a spot area diameter of less than 50 μm cannot be formed using a CO 2  laser.  
         [0007]     The high degree of reflectivity of copper at the CO 2  wavelength makes very difficult the formation of a through-hole via using a CO 2  laser in a copper sheet having a thickness greater than about 5 microns. Thus CO 2  lasers can typically be used to form through-hole vias only in copper sheets that have thicknesses of between about 3 microns and about 5 microns or that have been surface treated to enhance the absorption of the CO 2  laser energy.  
         [0008]     The most common materials used in making multilayer structures for printed circuit board (PCB) and electronic packaging devices in which vias are formed typically include metals (e.g., copper) and dielectric materials (e.g., polymer polyimide, resin, or FR-4). Laser energy at UV wavelengths exhibits good coupling efficiency with metals and dielectric materials, so the UV laser can readily effect via formation on both copper sheets and dielectric materials. Also, UV laser processing of polymer materials is widely considered to be a combined photo-chemical and photo-thermal process, in which the UV laser output partly ablates the polymer material by disassociating its molecular bonds through a photon-excited chemical reaction, thereby producing superior process quality as compared to the photo-thermal process that occurs when the dielectric materials are exposed to longer laser wavelengths. For these reasons, solid-state UV lasers are preferred laser sources for processing these materials.  
         [0009]     CO 2  laser processing of dielectric and metal materials and UV laser processing of metals are primarily photo-thermal processes, in which the dielectric material or metal material absorbs the laser energy, causing the material to increase in temperature, soften or become molten, and eventually ablate, vaporize, or blow away. Ablation rate and via formation throughput are, for a given type of material, a function of laser energy density (laser energy (J) divided by spot size (cm 2 )), power density (laser energy density divided by pulse width (seconds)), pulse width, laser wavelength, and pulse repetition rate.  
         [0010]     Thus, laser processing throughput, such as, for example, via formation on PCB or other electronic packaging devices or hole drilling on metals or other materials, is limited by the laser power density available and pulse repetition rate, as well as the speed at which the beam positioner can move the laser output in a spiral, concentric circle, or trepan pattern and between via positions. An example of a UV DPSS laser is a Model LWE Q302 (355 nm) sold by Lightwave Electronics, Mountain View, Calif. This laser is used in a Model 5330 laser system or other systems in its series manufactured by Electro-Scientific Industries, Inc., Portland, Oreg., the assignee of the present patent application. The laser is capable of delivering 8 W of UV power at a pulse repetition rate of 30 kHz. The typical via formation throughput of this laser and system is about 600 vias each second on bare resin. An example of a pulsed CO 2  laser is a Model Q3000 (9.3 μm) sold by Coherent-DEOS, Bloomfield, Conn. This laser is used in a Model 5385 laser system or other systems in its series manufactured by Electro-Scientific Industries, Inc. The laser is capable of delivering 18 W of laser power at a pulse repetition rate of 60 kHz. The typical via formation throughput of this laser and system is about 1000 vias each second on bare resin and 250-300 vias each second on FR-4.  
         [0011]     Increased via formation throughput can be accomplished by increasing the pulse repetition rate at a pulse energy sufficient to cause ablation as described above. However, for the UV DPSS laser and the pulsed CO 2  laser, as pulse repetition rates increase, pulse energy decreases in a non-linear fashion, i.e., twice the pulse repetition rate results in less than one-half the pulse energy for each pulse. Thus for a given laser, there will be a maximum pulse repetition rate and hence maximum rate of via formation governed by the minimum pulse energy needed to cause ablation.  
         [0012]     Regarding dicing a semiconductor wafer, there are two common methods of effecting the dicing: mechanical sawing and laser dicing. Mechanical sawing typically entails using a diamond saw to dice wafers having a thickness of greater than about 150 microns to form streets having widths of greater than about 100 microns. Mechanically sawing wafers having a thickness that is less than about 100 microns results in cracking of the wafer.  
         [0013]     Laser dicing typically entails dicing the semiconductor wafer using a pulsed IR, green, or UV laser. Laser dicing offers various advantages over mechanically sawing a semiconductor wafer, such as the ability to reduce the width of the street to about 50 microns when using a UV laser, the ability to dice a wafer along a curved trajectory, and the ability to effectively dice silicon wafers thinner than those that can be diced using mechanical sawing. For example, a silicon wafer having a thickness of about 75 microns may be diced with a DPSS UV laser operated at a power of about 8 W and a repetition rate of about 30 kHz at a dicing speed of 120 mm/sec to form a kerf having a width of about 35 microns. However, one disadvantage of laser dicing semiconductor wafers is the formation of debris and slag, both of which could adhere to the wafer and are difficult to remove. Another disadvantage of laser dicing semiconductor wafers is that the workpiece throughout rate is limited by the power capabilities of the laser.  
         [0014]     What is needed, therefore, is a method of and laser system for effecting high-speed laser processing of a workpiece at a high rate of throughput to effect the formation of vias and/or holes using UV, green, IR, and CO 2  lasers and to efficiently and accurately dice semiconductor wafers using UV, green, and IR lasers.  
       SUMMARY OF THE INVENTION  
       [0015]     An object of the present invention is, therefore, to provide a method of and a laser system for improving the speed and/or efficiency of (1) laser processing vias and/or holes in single and multilayer workpieces and (2) dicing semiconductor wafers such that the rates of material removal and workpiece throughput are increased and process quality is improved.  
         [0016]     The method of the present invention effects rapid removal of material from a workpiece by maximizing the pulse repetition rate at a given power level in a dual laser system. The method entails triggering two lasers so that the individual pulses appear at different times at the outputs of the lasers. These two beams are then combined into a single beam in which the pulses of the two beams are interleaved. The single beam has a pulse repetition frequency (PRF) that is equal to the combined pulse rate of each beam, and each pulse in the combined beam has the same pulse characteristics as it had before combination. The combined beam may be subsequently divided into two beams that have the same PRF. In the divided beams, some of the pulse characteristics, such as pulse duration and overall pulse shape, will remain substantially similar to those of the undivided beam. Some of the pulse characteristics, such as pulse peak power and pulse energy, will, however, be divided between the two beams such that the linear sum of the pulse characteristics will be approximately equal to those of the undivided beam.  
         [0017]     A preferred embodiment of the method entails synchronizing two lasers to achieve alternate pulsing at the desired PRF. The two pulsed laser beams produced at the laser outputs are then collimated and directed for incidence on a beam combiner, which combines them into a single beam. The combined beam may be left in its inherent Gaussian profile or optionally shaped and/or imaged to create a desired non-Gaussian profile. The combined beam is then divided into two beams, which may be directed for incidence on different locations of the workpiece to perform micromachining. Because of the non-linear nature of the relationship between PRF and power, the combining and separating of the two beams result in greater power density at two locations on the workpiece than that which would be achievable if each laser were separately pulsed and directed to the workpiece at each of two locations at the equivalent PRF. The consequence of achieving greater power density in this manner is an increase in the throughput of the micromachining system.  
         [0018]     The advantages afforded by this invention are not limited to two lasers. Using similar techniques, three or more lasers could be combined and divided into three or more beams; however, even numbers of lasers are easier to combine and divide into similar output beams.  
         [0019]     Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof, which proceeds with reference to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  is a fragmentary view of an exemplary multilayer workpiece of the type to be processed by a laser beam formed in accordance with the method of the present invention.  
         [0021]      FIG. 2  is a simplified schematic diagram of a preferred system that combines two laser beams and later divides them in accordance with the method of the present invention in cooperation with optional beam shaping and imaging optics.  FIG. 2  also shows in phantom lines optical components that further divide the combined laser beams into optional third and fourth laser beams.  
         [0022]      FIG. 3  is a graph showing the relationship between pulse energy and PRF for an exemplary prior art laser.  
         [0023]      FIG. 4  is a graph showing the relationship between pulse energy and PRF for a two-laser system beam output formed in accordance with the present invention.  
         [0024]      FIG. 5A  is a graph showing the pulse train PRF and peak energy produced by a prior art dual laser system in which each laser operates independently.  
         [0025]      FIG. 5B  is a graph showing the pulse train PRF and peak energy of a combined laser beam produced in accordance with the present invention.  
         [0026]      FIG. 5C  is a graph showing the pulse train PRF and peak energy for a separated laser beam produced in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0027]     In a first implementation of a preferred embodiment of this invention, laser pulses generated by the invention disclosed herein form vias in single layer or multilayer workpieces by aiming a laser at least two particular areas of the workpiece with sufficient energy to cause ablation. It is assumed that a single pulse is insufficient to remove all of the desired material from a particular location on the workpiece. Multiple pulses are, therefore, directed to the workpiece to effect removal of the desired material at each specified location. The processing time and hence the system throughput is dependent upon the number of pulses delivered to the workpiece for each unit time at energies above the ablation threshold of the workpiece.  
         [0028]     Preferred single layer workpieces include thin copper sheets, polyimide sheets for use in electrical applications, and other metal pieces, such as aluminum, steel, and thermoplastics, for general industry and medical applications. Preferred multilayer workpieces include a multi-chip module (MCM), circuit board, or semiconductor microcircuit package.  FIG. 1  shows an exemplary multilayer workpiece  20  of an arbitrary type that includes layers  34 ,  36 ,  38 , and  40 . Layers  34  and  38  are preferably metal layers that each include a metal, such as, but not limited to, aluminum, copper, gold, molybdenum, nickel, palladium, platinum, silver, titanium, tungsten, a metal nitride, or a combination thereof. Metal layers  34  and  38  preferably have thicknesses that are between about 9 μm and about 36 μm, but they may be thinner than 9 μm or as thick as 72 μm.  
         [0029]     Each layer  36  preferably includes a standard organic dielectric material such as benzocyclobutane (BCB), bismaleimide triazine (BT), cardboard, a cyanate ester, an epoxy, a phenolic, a polyimide, polytetrafluorethylene (PTFE), a polymer alloy, or a combination thereof. Each organic dielectric layer  36  is typically thicker than metal layers  34  and  38 . The preferred thickness of organic dielectric layer  36  is between about 20 μm and about 400 μm, but organic dielectric layer  36  may be placed in a stack having a thickness as great as 1.6 mm.  
         [0030]     Organic dielectric layer  36  may include a thin reinforcement component layer  40 . Reinforcement component layer  40  may include fiber matte or dispersed particles of, for example, aramid fibers, ceramics, or glass that have been woven or dispersed into organic dielectric layer  36 . Reinforcement component layer  40  is typically much thinner than organic dielectric layer  36  and may have a thickness that is between about 1 μm and about 10 μm. Skilled persons will appreciate that reinforcement material may also be introduced as a powder into organic dielectric layer  36 . Reinforcement component layer  40  including this powdery reinforcement material may be noncontiguous and nonuniform.  
         [0031]     Skilled persons will appreciate that layers  34 ,  36 ,  38 , and  40  may be internally noncontiguous, nonuniform, and nonlevel. Stacks having several layers of metal, organic dielectric, and reinforcement component materials may have a total thickness that is greater than 2 mm. Although the arbitrary workpiece  20  shown as an example in  FIG. 1  has five layers, the present invention can be practiced on a workpiece having any desired number of layers, including a single layer substrate.  
         [0032]      FIG. 2  is a simplified schematic diagram of a preferred embodiment of the present invention composed of two processing lasers  50  and  52  driven by a synchronizer source  54 . Source  54  could synchronize lasers  50  and  52  by any one of a number of methods including synchronizing the trigger signals sent to illumination sources that pump energy into the lasers or possibly synchronizing Q-switches positioned inside the lasers  50  and  52  to enable them to pulse in an alternating fashion. The lasers  50  and  52  provide at their outputs respective processing beams  56  and  58 , each comprised of a laser pulse train. The lasers  50  and  52  are arranged so that the intrinsic linear polarization planes of their respective output processing beams  56  and  58  are substantially parallel. Laser beams  56  and  58  pass through respective collimators  60  and  62 , each reducing the diameter of its incident laser beam while maintaining its focus at infinity.  
         [0033]     Processing lasers  50  and  52  may be a UV laser, an IR laser, a green laser, or a CO 2  laser. A preferred processing laser output has a pulse energy that is between about 0.01 μJ and about 1.0 J. A preferred UV processing laser is a Q-switched UV DPSS laser including a solid-state lasant such as Nd:YAG, Nd:YLF, Nd:YAP, or Nd:YVO4, or a YAG crystal doped with ytterbium, holmium, or erbium. The UV laser preferably provides harmonically generated UV laser output at a wavelength such as 355 nm (frequency tripled Nd:YAG), 266 nm (frequency quadrupled Nd:YAG), or 213 nm (frequency quintupled Nd:YAG).  
         [0034]     A preferred CO 2  processing laser is a pulsed CO 2  laser operating at a wavelength of between about 9 μm and about 11 μm. An exemplary commercially available pulsed CO 2  laser is the Model Q3000 Q-switched laser (9.3 μm) manufactured by Coherent-DEOS of Bloomfield, Conn. Because CO 2  lasers are unable to effectively drill vias through metal layers  34  and  38 , multilayer workpieces  20  drilled with CO 2  processing lasers either lack metal layers  34  and  38  or are prepared such that a target location has been pre-drilled with a UV laser or pre-etched using another process such as, for example, chemical etching, to expose dielectric layer  36 .  
         [0035]     Skilled persons will appreciate that other solid-state lasants or CO 2  lasers operating at different wavelengths may be used in the laser system of the present invention. Various types of laser cavity arrangement, harmonic generation of the solid state laser, Q-switch operation for both the solid-state laser and the CO 2  laser, pumping schemes, and pulse generation methods for the CO 2  laser are well known to those skilled in the art.  
         [0036]     Laser  50  emits a processing beam  56  that reflects off a mirror/combiner  64 , which in the case of two lasers is implemented as a mirror, and subsequently encounters a first ½ wave plate  66 . The first ½ wave plate  66  is set to rotate by 90° the polarization plane of the incident laser beam  56 . The optical paths of laser beams  56  and  58  are arranged to meet at a beam combiner  68  that is constructed to transmit substantially all of laser beam  58  polarized at a first angle and reflect substantially all of laser beam  56  polarized at a second angle that is rotated 90° relative to the first angle. The optical components are arranged so that the transmitted beam  58  and the reflected beam  56  combine to form a combined coaxial beam  70  having approximately one-half of its energy polarized at a first angle and the rest of its energy polarized at a second angle rotated 90° relative to the first angle. The combined beam  70  propagating from beam combiner  68  passes through optional beam shaping optics  72 , which transform the essentially Gaussian beam profile into a more desirable beam profile. An example of a desirable beam profile is the “top hat” profile, which provides essentially even illumination. The optional beam shaping optics  72  also serves as imaging optics, which enables the beam to achieve the appropriate properties such as spot size and shape when it is projected onto the workpiece. Those skilled in the art will also recognize that similar methods could be used to combine more than two lasers to create combined beam  70  with correspondingly more power.  
         [0037]     The combined beam  70  is then directed for incidence on a second ½ wave plate  74 , which as a result of being rotated by 22.5° rotates the polarization planes of the combined beam  70  by 45 degrees providing a beam with substantially equal p (vertical) and s (horizontal) polarization components. The combined and rotated beam  71  is directed onto a Brewster polarizer beam splitter  78  with its polarization axes set 45° relative to either of the polarization planes of combined and rotated beam  71 . In the absence of the second ½ wave plate  74 , the beam splitter  78  would transmit substantially all of the portion of the combined and rotated beam  71  that was polarized parallel to the beam splitter polarization axis and reflect substantially all of the portion of the combined and rotated beam  71  that was polarized perpendicular to the beam splitter polarization axis. This would essentially separate the combined and rotated beam  71  into its constituent parts, recreating laser beams  56  and  58 . However, since the polarization of combined and rotated beam  71  has been rotated 45°, each of the orthogonally polarized components of the combined and rotated beam  71  is partly transmitted and partly reflected by the beam splitter  78 . This has the effect of mixing the two polarized components of the combined and rotated beam  71 , transmitting about one-half of the power and reflecting about one-half of the power in separated laser beams  80  and  82 . Each of these separated beams  80  and  82  is comprised of pulses from both laser beams  56  and  58  and hence has a pulse rate equal to the sum of the pulse rates of the two beams. The ratio of power in the two separated beams  80  and  82  can be adjusted by varying the angle of the ½ wave plate  74  from the nominal angle of 22.5°.  
         [0038]     The combined and rotated beam  71  can optionally be divided into four laser beams  80 ,  82 ,  88 , and  90 , each of which equal to about one-fourth of the combined power of lasers  50  and  52  and having a pulse rate equal to the sum of the pulse rates of beams  56  and  58  emitted by lasers  50  and  52 , respectively. This division is accomplished by the components shown in a dashed line enclosure and represented by phantom lines in  FIG. 2 . The combined and rotated beam  71 , which is the optional embodiment initially propagates from a ½ wave plate  92 , is divided into two approximately equal beams by optional splitter  94  to create optional beams  96  and  98 . Each of beams  96  and  98  can be directed by well-known techniques to desired locations on the workpiece by optional mirror  100 , optional ½ wave plate  102 , optional splitter  104 , and optional mirror  106  to create a total of four output beams  80 ,  82 ,  88 , and  90 . The ratio of power available to each beam can be set by adjusting ½ wave plates  74 ,  92 , and  102  as described above. Those skilled in the art will recognize that this method can be extended to create additional pairs of laser beams as desired.  
         [0039]     Graph  110  in  FIG. 3  illustrates the non-linear relationship between PRF in kHz and pulse energy in μJ for a single laser. Curved line  112  represents the peak pulse energy available as a function of PRF for a given laser. Those skilled in the art will recognize that this relationship is typical for a wide range of laser types used for micromachining applications. Straight line  114  represents the minimum peak pulse energy, about 80 μJ, required for ablation of a particular workpiece. Lines  112  and  114  intersect at a point  116  that represents the maximum PRF usable to ablate the workpiece selected, which in this case is about 62 kHz. If a system were constructed with two lasers operating independently, the maximum throughput of the system would be limited to two spots, each being ablated at 62 kHz.  
         [0040]     Graph  120  in  FIG. 4  illustrates the performance of a dual laser system constructed in accordance with the principles described herein. Two lasers with PRF/pulse energy characteristics identical to those shown in  FIG. 3  are combined as shown in  FIG. 2 . Curved line  122  in graph  120  shows the PRF/pulse energy relationship of the combined beam  70  comprised of alternating pulses from lasers  50  and  52 . Straight line  124  in graph  120  shows the minimum peak pulse energy required to ablate the selected workpiece. Since combined beam  70  is to be divided substantially equally between two beams, the peak pulse energy required is about twice the peak pulse energy shown by straight line  104  in  FIG. 3 , or about 160 μJ. Lines  122  and  124  intersect at a point  126  that represents the maximum combined PRF, about 87 kHz, usable to ablate the selected workpiece. Because of the non-linear relationship between PRF and pulse energy, this PRF is greater than the 62 kHz PRF shown in  FIG. 3  to ablate the same material. Thus, a two laser system implemented in accordance with the techniques disclosed herein would have a maximum system throughput equal to two spots being ablated at a PRF of 87 kHz. Since the maximum ablation rate and hence the system throughput is a function of the PRF, a two laser system constructed in accordance with the principles disclosed herein would have a throughput of up to 140% of that of a system constructed with each laser operating independently.  
         [0041]     In a second implementation of a preferred embodiment; the laser pulses generated by the invention disclosed herein are used to effect singulation or dicing of a wafer or substrate into multiple independent parts. It is common in electronics manufacturing to construct multiple copies of a given circuit or circuit element on a single substrate. Preferred workpieces for semiconductor dicing include silicon wafers, other silicon-based materials including silicon carbide and silicon nitride, and compounds in the III-V and II-VI groups, such as gallium arsenide upon which integrated circuits are constructed using photolithography techniques. A second example is thick film circuitry, in which circuit elements or electronic devices are screen printed on a substrate typically made of a sintered ceramic material. A third example is thin film circuitry, in which conductors and passive circuit elements are applied to a substrate made of, for instance, a semiconductor material, ceramic or other materials, by sputtering or evaporation. A fourth example would be display technology, in which the plastic films used to manufacture LCD or plasma displays can be singulated using this technology. What these applications all have in common is the desire to efficiently divide a substrate containing multiple circuits, circuit elements, or simply regions of the substrate into separate parts.  
         [0042]     The advantages of applying the invention disclosed herein to singulation are similar to the advantages described above for via drilling. Applying two or more lasers to the process can increase the throughput of a system, since multiple parallel linear cuts are typically required to singulate most substrates. Using the invention described herein will increase the throughput of the system, since the rate of singulation, like via drilling, is a function of the number of pulses at energies greater than the ablation threshold delivered for each unit time.  
         [0043]      FIGS. 5A, 5B , and  5 C illustrate this process by comparing the number of pulses delivered for each unit time by a dual laser system constructed with independent lasers and a dual laser system constructed according to the invention disclosed herein.  
         [0044]     Graph  130  in  FIG. 5A  shows the relationship between pulse energy and PRF for one of two similar exemplary lasers in a prior art system that uses two independent lasers to process two locations on a workpiece at the same time. Graph  130  shows a pulse train  132 , each pulse  134  having a pulse energy e 0 , requiring time t 0  to complete processing at a particular location on a workpiece. Interval  138  shows the time between adjacent pulses  134 , which is the reciprocal of PRF. Since it represents a two laser system, this system can process two locations on a workpiece in time to.  
         [0045]     Graph  140  in  FIG. 5B  shows the combined beam  70  comprised of a pulse train  142 . The pulse train  142  is comprised of solid line pulses  144  from laser  50  and dashed line pulses  146  from laser  52  after having been combined by beam combiner  68 . The peak energy e 1  of each pulse  144 ,  146  is equal to more than twice the peak energy e 0  of each pulse  134  of a beam delivered by a similar laser at the PRF illustrated in  FIG. 5A , while the intervals  148  between adjacent pulses  144  from laser  50  and between adjacent pulses  146  from laser  52  are each less than twice the interval  138 . This is a consequence of the non-linear relationship between pulse energy and PRF illustrated in  FIGS. 3 and 4 .  
         [0046]     Graph  150  in  FIG. 5C  shows the result of dividing pulse train  142  with beam splitter  78  to form two pulse trains, one of which is shown as pulse train  152 , comprised of solid line pulses  154  from laser  50  and dashed line pulses  156  from laser  52 . The peak energy e 2  of the divided beam  152  is equal to the peak energy e 0  of a single laser as shown in  FIG. 5A , but the inter-pulse interval  158  is less than the inter-pulse interval  138 . The PRF synthesized from two laser beams is, therefore, greater than the PRF of either of two lasers working independently. Thus the required number of pulses is delivered to the workpiece in time t 2 , which less than time t 0 . Since two pulse trains  152  are delivered to the workpiece by the divided laser beams  56  and  58 , the invention described herein can process two locations in less time than that which would be required if the lasers worked independently.  
         [0047]     Skilled persons will appreciate that for different single or multilayer workpieces composed of different materials, varying laser parameters, such as pulse repetition rate, energy per pulse, and beam spot size, can be programmed during different processing stages to effect optimal laser micromachining throughput and quality. See, e.g., U.S. Pat. No. 5,841,099 of Owen et al. and U.S. Pat. No. 6,407,363 of Dunsky et al., both of which are assigned to the assignee of the present patent application. Those skilled in the art will also appreciate that the operational parameters of the heating source, such as its power, energy distribution profile, and spot size, can be kept constant or changed during various stages of laser processing.  
         [0048]     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 of this invention without departing from the underlying principles thereof. The scope of the present invention should, therefore, be determined only by the following claims.