Abstract:
An improved method for singulation of electronic substrates into dice uses a laser to first form cuts in the substrate and then chamfers the edges of the cuts by altering the laser parameters. The chamfers increase die break strength by reducing the residual damage and removes debris caused by the initial laser cut without requiring additional process steps, additional equipment or consumable supplies.

Description:
This application is a continuation of prior application Ser. No. 12/753,367, filed Apr. 2, 2010. 
    
    
     TECHNICAL FIELD 
     The present invention relates to improved methods and apparatus for machining feature in brittle materials. In particular, it relates to improved methods and apparatus for machining of workpieces containing electronic devices in which multiple copies of a device are constructed on a common substrate. In more particular it relates to device singulation or the separation of semiconductor wafers into individual devices using a laser. 
     BACKGROUND OF THE INVENTION 
     Electronic devices are typically manufactured by producing multiple copies of the same device on a substrate or workpiece. In particular, semiconductor devices are manufactured on substrates referred to as wafers, which are thin disks of materials such as silicon, gallium arsenide or sapphire or other materials which are capable of supporting the various processes that create semiconductor devices. These devices at some point in the manufacturing process need to be separated into individual devices for subsequent packaging and use. This separation into individual devices is referred to as “singulation”. Singulation can be performed mechanically, using diamond-coated saw blades, chemically, by masking and etching, photonically by directing laser energy at the wafer or substrate, or combinations of these methods. Singulation can be accomplished by cutting completely through the wafer or substrate, or by making a partial cut or cuts into one or more surfaces of the wafer or substrate and then mechanically cleaving the wafer or substrate in to individual dice. Cutting completely through the wafer or substrate is commonly referred to as “dicing” and cutting partially through the wafer or substrate in preparation for subsequent cleaving is commonly called “scribing”. In general, devices are rectangular in shape and are laid out on a grid pattern on the wafer or substrate, allowing the devices to be separated by making a series of cuts either fully through the substrate (dicing) or partially through the substrate (scribing) between the devices in first in one direction  16  and then in a direction at 90 degrees to the first direction  14 , as illustrated in  FIG. 1 . Referring to  FIG. 1 , singulation of devices on substrates generally proceeds by forming a series of cuts in one direction, for example the X direction between each row of devices then forming a series of cuts in the perpendicular Y direction between each column of devices thereby separating each device from the other devices on the wafer. 
     Issues related to device singulation include real estate, device damage, cost, complexity and system throughput. Real estate refers to the fact that in order to permit singulation without damage to devices, room must be left between the devices to allow room for cuts to be made without having to cut through active devices. The area between the active devices is known as a “street”. Streets  14 ,  16  are shown in  FIG. 1  between active devices  12  on a wafer  10 . Since the cost of processing a wafer or substrate is generally fixed, more devices per substrate yields greater profits for the manufacturer. This places a premium on making the streets as narrow as possible to squeeze as many active devices as possible onto the wafer or substrate. Factors working against narrow streets include the size of the kerf and the width of the damaged area next to the cut. While lasers typically can cut a narrower kerf than mechanical saws or chemical etch, lasers also typically create a heat affected zone (HAZ) next to the kerf due to the heat generated by the intense amount of energy required to cut the wafer or substrate. This HAZ should not be allowed to overlap the active device area or it may either cause part failures immediately or limit the useful life of the part. The HAZ also can cause device failure from cracks or chips in the HAZ propagating from the HAZ to the active device area of the device as the device is used. 
     Another problem related to laser singulation of wafers or substrates is the re-deposit of material removed from the kerf. The laser typically removes material from the substrate or wafer to form the kerf by both ablation and thermal means. The laser beam may be energetic enough to ablate material it impinges near the center of the beam, meaning that the material is ionized and forms a plasma cloud as it exits the kerf. Nearer the periphery of the beam or directly adjacent to the beam the material of the substrate may not receive enough energy to ablate the material but rather vaporizes or melts and boils the material. In this case the material is ejected from the kerf as it vaporizes or boils. As the material is ejected from the kerf and moves away from the laser beam it cools and re-solidifies as debris near the edges of the kerf. If this debris reaches active circuit areas of the device it can cause undesirable electronic malfunctions and therefore must be removed prior to packaging the device. 
     One means for predicting the reliability of semiconductor die which have been diced or scribed from a wafer is to test die break strength. In this test, a semiconductor die is subjected to bending to the point of failure. Variations is die break strength can predict semiconductor component failures, therefore, wafer dicing or scribing methods that improve die break strength also improve component reliability. Things that are known to reduce die break strength include chips or cracks along the edge of a cut caused by mechanical or laser cutting. In addition, debris re-deposited along the edge of the cut from materials vaporized or liquefied by the laser can cause damage to the circuit and reduce reliability. 
     In particular, laser parameters that provide desired cutting speed and kerf size and shape also cause a HAZ at the top edge of the cut and also tend to create debris from material removed the kerf which is re-deposited at the edge of the cut.  FIG. 2  shows a cross-sectional view of a silicon wafer that has been cut with a laser showing the heat affected zone and debris caused by redeposit of material from the kerf.  FIG. 2  shows a cross-sectional view of a cut wafer  20 , having a top surface  22  and a bottom surface  24  and die attach film (DAF)  26 . One side of the kerf  28  is shown. Also shown are re-deposited debris  30  and the HAZ  32 . The HAZ can cause a reduction in die break strength and the re-deposited debris can cause device failure. An exemplary laser processing device for performing this singulation is the ESI Cignis Laser Singulation machine, manufactured by Electro Scientific Industries, Inc., Portland Oreg., 97229. This machine uses a picosecond laser to singulate silicon and other substrate materials. 
     Another issue with laser singulation of substrates or wafers is improving system throughput. In particular, laser parameters which provide higher cutting speed also create more HAZ and re-deposited debris, which is undesirable. It is also noted that laser parameters which provide high cutting speed also create a debris cloud at the cutting site. This debris cloud is comprised of plasma created by the laser pulses and gaseous, liquid and solid debris created by the laser pulses and ejected from the workpiece. It is known that the debris cloud created by laser pulses can absorb energy from subsequent laser pulses.  FIG. 3  is a graph showing depth of cut vs. number of pulses for an exemplary laser substrate cutting process. This shows that cutting a silicon workpiece with a 4 Watt Nd: YVO 4  laser with 10 nanosecond pulses aimed at a single point at a repetition rate of 5 KHz. As can be seen in  FIG. 3 , in this example cutting saturates at about 10 pulses. This saturation is believed to be caused by the debris cloud created by the first laser pulses. A debris cloud created in this manner not only blocks laser radiation from reaching the workpiece and prevents material removal it also absorbs laser energy heating the plasma further. As the plasma absorbs energy it heats up and due to its proximity to the workpiece transfers some of that heat energy to portions of the workpiece including sidewalls of the feature being machined. In addition vaporous, liquid or solid material in the cloud may be ejected from the cloud and deposited on the workpiece. This transferred energy and material causes cracking, deterioration of the feature sidewall and increased debris. In addition, if more energetic pulses or more pulses are directed to the workpiece in an effort to transmit more energy through the debris cloud to the workpiece and thereby continue to machine material, more energy is coupled into the debris cloud making the cracking, deterioration and debris problems worse. Even ultra fast processes which use short duration pulses in the picosecond or femtosecond range to ablate material before the material has time to transfer heat to adjacent regions cannot avoid coupling energy into the debris cloud. This energy causes consequent damage to the workpiece depending upon the material and the laser parameters used. 
     Issues related to dicing or scribing wafers or substrates have been the subject of previous work. U.S. Pat. No. 6,271,102 METHOD AND SYSTEM FOR DICING WAFERS, AND SEMICONDUCTOR STRUCTURES INCORPORATING THE PRODUCTS THEREOF, inventors Donald W. Brouillette, Robert F. Cook, Thomas G. Ference, Wayne J. Howell, Eric G. Liniger, and Ronald L. Mendelson, Aug. 7, 2001, describes cutting wafers with chamfered edges with a saw blade prior to laser cutting from both the front and backside to improve die break strength. In this case the chamfered cuts are produced instead of straight cuts with either a laser or a saw. U.S. Pat. No. 7,129,114 METHODS RELATING TO SINGULATING SEMICONDUCTOR WAFERS AND WAFER SCALE ASSEMBLIES, inventor Salman Akram, Oct. 31, 2006, tries to solve the problem of die break strength by scribing trenches next to the laser cut and coating the trenches with protective material. US patent publication No. 2006/0249480, LASER MACHINING USING AN ACTIVE ASSIST GAS, inventor Adrian Boyle, Nov. 9, 2006, discloses using an assist gas to etch away the damaged portion of the edges of the cuts to improve die break strength. WIPO patent publication No. 2008/064863 LASER MACHINING, inventors Kali Dunne and Fallon O&#39;Briain, Jun. 5, 2008 discusses using particular patterns of laser pulse spacing along the path being cut with multiple passes to avoid the debris cloud. 
     What these references have in common is a desire to improve die quality following dicing or scribing by attempting to overcome problems with die break strength and debris caused by laser cutting. These methods either require that cutting be done from both the top and the bottom of a wafer, be accomplished by a combination of laser and mechanical sawing, or require additional processing steps and equipment such as chemical etching, or require multiple passes by the laser. What is needed then is an efficient way to dice wafers with a laser that does not require additional processing steps, passes or equipment and provides a wafer with improved die break strength and reduced debris by avoiding the debris plume caused by laser machining. 
     SUMMARY OF THE INVENTION 
     This invention is an improved method and apparatus for laser machining brittle materials. Aspects of this invention include providing a laser having laser parameters and making a first cut in the workpiece with the laser using first laser parameters, then making a second cut in the workpiece with the laser using second laser parameters, the second cut being adjacent to the first cut while avoiding the debris cloud created by the first laser cut. Aspects of this invention further include moving the point at which the laser intersects the workpiece in order to avoid the debris cloud created by the previous cut and then making a cut with a laser using second laser parameters. Aspects of this invention gain the advantages of laser processing, including increased throughput, smaller street sizes and reduced consumable cost, while eliminating problems associated with lasers such as the heat affected zone and re-deposit of debris, thereby increasing die break strength. 
     Embodiments of this invention can be implemented by adapting an existing laser wafer singulation system, the ESI Cignis Wafer Singulation system, manufactured by Electro Scientific Industries, Inc., Portland Oreg. This system uses a picosecond laser to dice or scribe semiconductor wafers. In operation, the wafer is fixtured on the system and then the laser beam is moved in relation to the wafer by the motion stages while the laser is pulsed, thereby making the linear cuts required to singulate wafers. This system is adapted to accomplish aspects of this invention by altering the laser parameters used, including possibly the type of laser employed. In addition, the laser parameters need to be programmably controlled in order to change laser parameters between cuts. The system needs to be adapted to allow subsequent laser cuts to be made adjacent or close to the previous cut while avoiding the debris cloud. In this aspect of this invention the laser pulses machine a particular area with a number of pulses up to a limit predetermined by the number of pulses required to saturate the location and then move rapidly to an adjacent location away from the debris cloud to continue machining in an unsaturated location. Saturation refers to the reduction in machining efficiency of a series of laser pulses caused by absorption of laser energy by the debris cloud as a function of number of pulses. 
     Aspects of this invention singulate substrates or wafers by first making a laser cut in the substrate or wafer. Laser parameters are adjusted to provide desired cutting speed and kerf size and shape. This laser cut is made partially through the wafer or substrate. The laser parameters associated with this cut are selected so as to minimize cracking and re-deposited debris. Lower fluence laser parameters increase cutting time but since this cut is only partially through the wafer or substrate the impact on system throughput is not great. Also note that these initial laser cuts may require multiple passes of the laser beam to make the cut to the depth desired or the laser beam may be directed to impinge on the substrate of wafer at various points while cutting. 
     Following the initial cut, aspects of this invention include adjusting the laser parameters to increase the laser fluence at the surface of the wafer or substrate and then directing the laser beam to machine a through cut in the wafer or substrate adjacent to the previous cut. Laser parameters are adjusted to permit the laser beam to remove material from the wafer while minimizing additional damage to the wafer or substrate. This is because the debris cloud created by the laser pulses making the second cut has room to expand into the space made by the previous cut. In this way the debris cloud dissipates more rapidly permitting increased repetition rate pulsing with more energetic pulses which supports improved throughput while reducing cracking and re-deposited debris. This permits increased throughput with increased die break strength. Aspects of this invention can also process wafers or substrates in an iterative fashion, alternating first laser parameter cuts with second parameters cuts as the cuts go deeper into the wafer or substrate. Laser pulses are focused at increasing depths to permit machining deeper and deeper cuts into the material. 
     An advantage of processing substrates or wafers according to aspects of this invention is that the first cut processing can take place on the same laser processing system that makes the second cuts. The ESI Cignis Laser Singulation system that made the initial cuts is adapted to create the cut without causing additional damage to the substrate of wafer by modifying the laser parameters associated with the laser processing beam to permit the laser to remove material from the substrate of wafer without creating a HAZ or debris. This is possible because far less material is being removed per unit time, therefore the laser fluence can be lower. The system is also adapted to permit machining adjacent locations efficiently. 
     Another aspect of this invention singulate substrates or wafers by first machining at a location with a number of laser pulses with particular laser parameters up to the saturation limit for that particular material with those laser parameters. The laser pulses are then rapidly directed to a location adjacent or near the first location by the laser processing system and machining continued with a number of pulses with second laser parameters up to a second saturation limit. This embodiment uses compound beam positioning to direct the laser pulses to the desired locations on the workpiece to achieve the desired system throughput. 
     Advantages of these aspects of this invention are that the substrate or wafer does not have to be moved and re-aligned on a separate machine nor does it have to be flipped and put back into the machine for laser processing from a different side, nor does the machine have to be designed to process substrates or wafers from both sides. In addition, no chemicals or additional process steps such as depositing an additional layer of material on the wafer or substrate are required by aspects of this invention. While this requires at least two passes to process a single cut in a wafer or substrate, the number of passes are substantially less than approaches that skip adjacent locations rely on multiple passes. Processing wafers or substrates according to aspects of this invention improves die break strength and removes re-deposited material from singulated devices without requiring additional equipment, process steps or consumable supplies. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 . Semiconductor wafer. 
         FIG. 2 . Cross-sectional view of wafer cut. 
         FIG. 3 . Graph showing cut depth vs. number of pulses. 
         FIG. 4A . Wafer with first cut. 
         FIG. 4B . Wafer with first and second cut. 
         FIGS. 5A ,  5 B and  5 C. Three-step embodiment for machining a workpiece. 
         FIG. 6 . Graph showing cut depth vs. number of pulses. 
         FIG. 7 . Adapted laser processing system. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     This invention is an improved method for laser singulation of electronic devices fabricated on a substrate or wafer. An embodiment of this invention includes a laser processing system having a laser having laser parameters. This embodiment uses this laser to make a partial cut in the substrate or wafer using a first set of laser parameters. These parameters allow the laser to make the desired cut in the wafer or substrate with an acceptable kerf width and at an acceptable rate without creating unacceptable HAZ or re-deposited debris. This embodiment then cuts the wafer or substrate with a laser using second set of laser parameters adjacent to the first cut. Making the second cut adjacent to the first cut allows the debris cloud from the substrate or wafer to being cut to dissipate into the volume of the first cut, thereby reducing the deleterious effects associated with heating the debris cloud with subsequent laser pulses. Workpieces machined in this fashion exhibit improved die break strength and reduced debris. This embodiment gains the advantages of laser processing, including increased throughput, smaller street sizes and reduced consumable cost, while eliminating problems associated with lasers such as the heat affected zone and re-deposit of debris. 
     An embodiment of this invention singulates substrates or wafers by making laser cuts partially through the substrate or wafer in preparation for subsequent through-cutting with a second cut made adjacent to the first. Referring to  FIG. 1 , a wafer  10  has electronic devices  12  fabricated on a surface of wafer  10 . These electronic devices are separated by horizontal  14  and vertical  16  streets. Streets are areas of the wafer or substrate that has been left between the active devices to permit room for the singulation without harming the active devices. Singulation of devices on substrates generally proceeds by forming a series of cuts in one direction, for example the X direction (horizontal street  14 ) between each row of devices then forming a series of cuts in the perpendicular Y direction (vertical street  16 ) between each column of devices, thereby separating each device from the other devices on the wafer. Laser parameters are adjusted to provide desired cutting speed and kerf size and shape. These initial laser cuts may require multiple passes of the laser beam to make the cut to the depth desired or the laser beam may be directed to impinge on the substrate of wafer at various points while cutting. 
     Making the initial cut in a wafer or substrate is shown in  FIG. 4A . In  FIG. 4A , a laser beam  42  is directed at a workpiece  40  to remove material from the wafer  40  to form an initial cut  50 . Laser parameters are chosen to permit removal of material from the workpiece at an acceptable rate while creating minimal HAZ and debris. Exemplary laser parameters used by an embodiment of this invention to create the initial cut are shown in Table 1. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 First laser pulse parameters 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Laser Type 
                 DPSS Nd: YVO 4   
               
               
                   
                 Wavelength 
                 355-1064 nm 
               
               
                   
                 Pulse Duration 
                 10 ps-250 ns 
               
               
                   
                 Pulse Temporal 
                 Gaussian/square/tailored 
               
               
                   
                 Laser Power 
                 1-14 W 
               
               
                   
                 Rep Rate 
                 10 KHz-1 MHz 
               
               
                   
                 Speed 
                 50-2500 mm/s 
               
               
                   
                 Pitch 
                 8-12.5 microns 
               
               
                   
                 Spot Size 
                 6-20 microns 
               
               
                   
                 Spot Shape 
                 Gaussian/Top Hat (round 
               
               
                   
                   
                 or square) 
               
               
                   
                 Focal Height 
                 0-100 mm 
               
               
                   
                   
               
             
          
         
       
     
     Following the initial cut, an embodiment of this invention changes the laser parameters to parameters suited to laser machining through cuts with acceptable throughput rates. This process is shown in  FIG. 4B . In  FIG. 4B , the workpiece,  40 , has been laser machined by laser pulses  52  to form a through cut  44 . As the laser pulses  52  machine the through cut  44 , the debris cloud (not shown) from the through cut  44  escapes from the volume currently being machined into the cut  50  made in the previous step, thereby reducing the density and volume of the debris cloud blocking the laser pulses  52  from reaching the bottom of the cut  44  as it is being made. Exemplary second laser pulse parameters used to form the trough cut  44  on workpiece  40  are given in Table 2. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Second laser pulse parameters 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Laser Type 
                 DPSS Nd: YVO 4   
               
               
                   
                 Wavelength 
                 355-1064 nm 
               
               
                   
                 Pulse Duration 
                 10 ps-250 ns 
               
               
                   
                 Pulse Temporal 
                 Gaussian/square/tailored 
               
               
                   
                 Laser Power 
                 1 W-14 W 
               
               
                   
                 Rep Rate 
                 10 KHz-1 MHz 
               
               
                   
                 Speed 
                 1000-4000 mm/s 
               
               
                   
                 Pitch 
                 6-20 microns 
               
               
                   
                 Spot Size 
                 6-20 microns 
               
               
                   
                 Spot Shape 
                 Gaussian/Top Hat (round 
               
               
                   
                   
                 or square) 
               
               
                   
                 Focal Height 
                 0-100 mm 
               
               
                   
                   
               
             
          
         
       
     
     Second laser parameters are selected to permit the laser pulse to laser machine the through cut at acceptable throughput rates. One way an embodiment of this invention can accomplish this is by increasing the laser pulse fluence at the workpiece. Laser pulse fluence can be increased by increasing the laser pulse power, increasing the pulse duration, decreasing the spot size, changing the focal height, or decreasing the pulse repetition rate. Any or all of the changes, among others, can increase laser fluence at the workpiece and permit more rapid material removal. This is possible without increasing HAZ or debris and thereby reducing die break strength because the first cut allows the debris cloud to dissipate and reduce the tendency to block subsequent pulses from reaching the workpiece. 
     Other embodiments of this invention singulate substrates or wafers by first machining at a location with a number of laser pulses with particular laser parameters up to the saturation limit for that particular material with those laser parameters. The laser pulses are then rapidly directed to a location adjacent or near the first location by the laser processing system and machining continued with a number of pulses with second laser parameters up to a second saturation limit. This embodiment uses compound beam positioning to direct the laser pulses to the desired locations on the workpiece to achieve the desired system throughput.  FIGS. 5A ,  5 B and  5 C show a three step embodiment of this invention. In  FIG. 5A , a workpiece  60  is impinged by laser pulses  61  machining a first feature  62 . The number of laser pulses is less than the predetermined saturation limit of the material with the particular laser parameters used. Note that these laser parameters may be allowed to cause damage to the workpiece adjacent  63  to the feature  62  in order to increase the machining rate. In the second step of the process, shown in  FIG. 5B , laser pulses  64  with second laser parameters impinge workpiece  60  with a number of laser pulses less than the saturation limit machine additional features  66 ,  67  adjacent to first feature  62 . Second laser parameters are selected to remove material without causing damage to the adjacent areas. In the third step, shown in  FIG. 5C , laser pulses  68  with third laser parameters impinge workpiece  60  with laser to complete machining a through cut  69 . Note that the adjacent features  66  and  67  provide relief for the debris cloud generated by laser pulses  68 , thereby assisting the debris cloud in dissipating and thereby increasing the saturation limit. 
     These particular laser parameters are exemplary. Laser pulse parameters and can be altered to suit the particular substrate or wafer being cut. Laser Type could be any laser capable of producing pulses with the required wavelength, energy, pulse width and repetition rate. For example Nd:YAG, Nd:YVO4, Nd:YLF and fiber lasers all could be used for application such as this. These lasers can be adapted by harmonic generation where non-linear optical elements are used in the laser ( FIG. 7 ,  70 ) to generate second, third or fourth harmonics of the original laser pulses to decrease the wavelength of the laser pulses. In addition, a continuous wave (CW) laser can be adapted for this application by the use of laser beam optics ( FIG. 7 ,  74 ) which chops or otherwise shapes the CW beam into pulses. Wavelength choice depends upon the material being cut. Pulse Duration measures the length of each pulse and for basically Gaussian pulses is measured by calculating either the full width at half maximum (FWHM) or 1/e 2  width of the pulse. Long duration pulses cut material quickly but tend to couple excessive thermal energy into the wafer, thereby causing large HAZs and creating lots of debris. Short duration pulses tend to be better in this respect but take longer to cut workpieces and can require very expensive lasers. Pulse Temporal distribution can affect how the laser interacts with the material. By modifying the typical Gaussian distribution, the manner in which energy from the laser pulse is transmitted to the substrate or wafer. For example, electro-optical components can be placed in the optical path of the laser beam to temporally shape the laser pulse to provide control over pulse rise time, fall time and to possibly add one or more “power spikes”, which are parts of the laser pulse where the laser power is increased so that a narrow spike of power greater than 25% above the average power of the pulse, thereby “tailoring” the pulse. These pulses can also be chopped or otherwise modified by the laser beam optics to produce temporally “square” pulses. Non-Gaussian pulses require that Pulse Duration be measured by different measures than FWHM or 1/e 2 , for example the integral square method which involves a dividing the squared integral by the integral squared to calculate Pulse Duration. Fast rise times will couple laser energy into the workpiece and cause ablation faster than thermal transfer can transfer heat to adjacent regions. Fast rise times therefore can help to reduce HAZ. 
     Laser Power describes how much average energy can be input to the wafer or substrate to perform cutting. Typically with pulsed lasers the average power is calculated over a period of time that encompasses many pulses, for example one second, in order to accurately assess how much energy is being input to the wafer per unit time. Repetition Rate is the rate at which pulses of a given energy can be emitted by the laser on a continuous basis. Speed refers to the rate at which the laser beam is moved in relation to the wafer or substrate as the laser is pulsed. For a given laser pulse energy, pulse size and repetition rate, the speed at which the beam is moved in relation to the wafer or substrate will determine the total dose or fluence delivered to the wafer or substrate by the laser pulses. The pitch is similar to speed and is defined as the distance between successive pulses delivered to the wafer or substrate. Pitch is a function of repetition rate and speed. Spot Size is a measure of the size of the laser pulse as it impinges on the wafer or substrate. It is related to focal spot size, which is the minimum diameter of the laser beam waist measured at the focal distance of the optical system delivering the beam to the workpiece. Spot Size will differ from the focal spot size depending upon how far the laser beam focal distance is from the surface of the workpiece. As the laser beam focal distance is moved from the surface of the workpiece, the laser beam becomes unfocused and the Spot Size becomes larger and thereby reduces the laser fluence. Spot Shape is a description of the spatial shape of the laser beam. Optical elements are placed in the optical path of the laser beam to cause the beam to assume various cross-sectional distributions. For example, a diffractive optical element is used to alter the laser beam from a basically Gaussian cross-section to a “top hat” distribution where the laser energy is distributed evenly over the focal spot rather than falling off at the edges as with a Gaussian distribution. Spot Shape can also be asymmetric. One example of an asymmetric Spot Shape is an elliptical spot, wherein optical elements in the laser beam optical path cause the laser beam to assume an elliptical cross-section, where one axis is larger than another. The spot can also be apertured by the laser beam optics to cause the focal spot to assume a circular or square cross-section, for example. Focal Height is a measure of the distance between the focal distance and the surface of the workpiece. Since the laser spot goes out of focus as Focal Height differs from zero, for a given set of laser optics, Focal Height will determine the Spot Size on the workpiece. 
       FIG. 6  is a graph showing the results of using an embodiment of this invention to machine a workpiece with a laser.  FIG. 6  graphs the depth of cut in microns versus the number of pulses for a one step process (diamonds) and a two step process (squares). As can be seen from the graph, cutting a partial feature and then making a second cut adjacent to the first cut permits the laser pulses to remove material at a greater rate than is possible without making the first cut. This is shown by the single cut depth of cut saturating prior to ten pulses being directed to the workpiece, whereas the two-cut depth of cut is still increasing past 12 pulses. This shows that the efficiency of the cut is increased thereby decreasing the amount of HAZ and debris created by laser machining. 
       FIG. 7  shows a diagram of a laser processing system adapted for this application. A laser  70  emits laser pulses  72  which are shaped by laser beam optics  74  and then directed to the workpiece  78  by beam steering optics  76  and field optics  77 . The workpiece  78  is fixtured on motion stages  80 . The laser  70 , laser beam optics  74 , beam steering optics  76 , field optics  77  and motion stages  80  are all controlled by controller  82 . The laser  70  can be any one of the types of lasers discussed herein. The laser  70  can either be a continuous wave (CW) laser or a pulsed laser. In the case of a CW laser, the laser beam optics  74  are adapted to add a shutter function that effectively chops the CW laser into pulses with desired pulse duration and repetition rate. The laser beam optics  74  serve to collimate and focus the laser pulses to the correct size and shape and optionally use electro-optic components such as polarizers, electro-optical modulators or acoustic optical modulators to shape the pulses temporally. The laser beam optics  74  may also include a polarization based beam blocker to prevent reflected laser energy from being transmitted back into the laser  70  and thereby disrupt the lasing capability. The beam steering optics  76  can be electro-optical components that programmable direct the laser pulses to points on the workpiece. Exemplary beam steering components include galvanometers, piezo-electric devices, fast steering mirrors or electrical or acoustic optical modulators. The beam steering optics  76  may also include zoom optics focus height adjustments. The field optics  77  typically includes a field lens that permits the laser pulses  72  to be directed to various points on the workpiece  78  while maintaining substantially perpendicular orientation of the laser pulses  72  to the workpiece  78 . The field optics  77  may also include an aperture and attendant optics to permit the laser pulses  72  to be shaped spatially. The controller  82  directs the motion stages  80  and the beam steering optics  76  to position the laser pulses  72  in relation to the workpiece  78 . Note that embodiments of this invention can move either the laser pulses  72  or the workpiece  78  to accomplish the change in relative location. 
     This embodiment also employs compound beam positioning to increase system throughput. This increases throughput by coordinating the motion stages  80  with the beam steering optics  76  to allow the motion stages  80  to put the workpiece into motion with respect to the laser pulses  72  while the beam steering optics  76  compensate for the motion of the motion states  80  to allow the laser pulse  72  to impinge the workpiece  78  at a single location even as that location is moving relative to the laser pulses  72 . 
     Having hereby disclosed the subject matter of the present invention, it should be obvious that many modifications, substitutions, and variations of the present invention are possible in view of the teachings. It is therefore understood that the invention may be practiced other than as specifically described, and should be limited in its breadth and scope only by the following claims.