Abstract:
Disclosed are laser scribing systems for laser scribing semiconductor substrates with backside coatings. In particular these laser scribing systems laser scribe opto-electric semiconductor wafers with reflective backside coatings so as to avoid damage to the opto-electric device while maintaining efficient manufacturing. In more particular these laser scribing systems employ ultrafast pulsed lasers at wavelength in the visible region and below in multiple passes to remove the backside coatings and scribe the wafer.

Description:
TECHNICAL FIELD 
     The present invention relates to laser scribing opto-electric devices such as light-emitting diodes (LEDs) constructed on substrates such as sapphire wafers. In particular it relates to scribing opto-electric devices constructed on substrates having a backside coating such as a distributed Bragg reflector (DBR). In more particular it relates to laser scribing opto-electric devices constructed on substrates having backside coatings in multiple passes so as to prevent damage to the opto-electric devices or the substrate. 
     BACKGROUND OF THE INVENTION 
     Opto-electric devices such as light-emitting diodes are typically manufactured by making multiple identical copies of a device in parallel on a substrate. Substrates typically used to manufacture opto-electric devices include wafers of sapphire, gallium arsenide, indium phosphide, silicon, germanium, diamond, or ceramics. These substrates typically need to be singulated into individual devices. Singulation can be performed by first scribing the substrate with a diamond saw or laser. Scribing is defined as creating a modified region on the surface or within the volume of a substrate which facilitates cracking and thereby the separation of the substrate proximate to the scribe. U.S. Pat. No. 6,580,054 SCRIBING SAPPHIRE SUBSTRATES WITH A SOLID STATE UV LASER, inventors Kuo-Ching Liu, Pei Hsien Fang, Dan Dere, Jenn Liu, Jih-Chuang Huang, Antonio Lucero, Scott Pinkham, Steven Oltrogge, and Duane Middlebusher, issued Jun. 17, 2003 describes the basic process of scribing substrates to prepare them for singulation by cleaving, wherein scribed substrates are subjected to localized stress at the scribe location to create cracks in the substrate which propagate through the thickness of the substrate thereby separating the wafer along the scribe. Substrates are often adhesively attached to a die attach film or plastic sheet attached to a surrounding frame called a tape frame during singulation to keep the singulated devices in place in spite of being separated. Laser scribing is distinguished from laser dicing where the substrate is completely or nearly through cut with the laser or saw to perform singulation without the cleaving step. 
     Problems with laser scribing include selecting laser parameters to process materials with widely varying laser energy absorption characteristics, avoiding damage to substrates and devices and maintaining acceptable system throughput. Opto-electric devices are particularly difficult to laser scribe or dice because the performance of the device can be dependent upon the manner in which the device was singulated. In particular, some opto-electric devices are constructed with a backside coating which reflects stray light back out of the device thereby increasing the device output efficiency as measured in candelas per ampere. The article “Nitride-Based LEDs With a Hybrid Al Mirror+Tio2/Slo2 DBR Backside Reflector,” authors S. J. Chang, C. F. Shen, M. H. Hsieh, C. T. Kuo, T. K. Ko, W. S. Chen and S. C. Shei, describes the construction of these types of opto-electric devices. Laser scribing techniques which cause an undesirable heat affected zone adjacent to the scribes can reduce device output efficiency, as can damage which causes the reflective coating to “lift-off” or separate from the substrate. U.S. Pat. No. 7,804,043 METHOD AND APPARATUS FOR DICING OF THIN AND ULTRA THIN SEMICONDUCTOR WAFER USING ULTRAFAST PULSE LASER, inventor Tan Deshi, issued Sep. 28, 2010, describes using ultrafast lasers to scribe wafers but does not consider the need to remove coatings such as a DBR. U.S. Pat. No. 6,992,026, LASER PROCESSING METHOD AND LASER PROCESSING APPARATUS, inventors Fumitsugu Fukuyo, Keshi Fukumitsu, Maoki Uchiyama and Toshimitsu Wakuda, issued Jan. 31, 2006 discusses damage to substrates caused by pulsed lasers while scribing. This patent teaches using laser beams focused deep within the substrate to create altered zones unconnected to either the frontside or backsides of the wafer to permit cleaving while preventing damage to the substrate. U.S. Pat. No. 7,494,900, BACKSIDE WAFER DICING, inventors Richard S. Harris and Ho W. Lo, issued Feb. 24, 2009, discusses the advantages of performing backside processing of substrates but does not discuss scribing in the presence of a coating nor does it discuss methods of preventing damage to the substrate while scribing. U.S. patent application Ser. No. 2006/0220183 SEMICONDUCTOR WAFER HAVING MULTIPLE SEMICONDUCTOR ELEMENTS AND METHOD FOR DICING THE SAME, inventors Makoto Asai, Muneo Tamura, Kazuhiko Sugiura and Tetsuo Fujii, published Oct. 5, 2006 describes a two step process for scribing wafers having two layers where the layers have different refractive indices with respect to the laser wavelength being used. This application discusses only frontside scribing rather than backside scribing and does not discuss applications where the layer to be processed does not have a refractive index, but is in fact opaque to the wavelengths in question. Further this application does not discuss ways to avoid damage to the singulated devices as a result of the laser energy applied. 
     There remains a continuing need for a laser scribing process that scribes the backside opto-electric device substrates with reflective coatings in an efficient fashion without damaging the substrate using ultrafast laser pulses. 
     SUMMARY OF THE INVENTION 
     Aspects of the present invention improve the scribing of opto-electric devices on a substrate in the presence of a backside coating by first removing the backside coating with a laser in the region to be scribed and then scribing the substrate. Using multiple passes with reduced the laser fluence per unit time to remove the backside coating or scribe the substrate can avoid damage to the substrate or nearby active circuitry. Opto-electric device substrates typically have a frontside having streets and a backside having a surface coating. This coating reflects stray light emitted by the device which would otherwise be absorbed and cause decreased light output and higher operating temperatures which reduce device lifetime. Aspects of the invention include a laser scribing system with an ultrafast pulsed laser having first and second laser pulse parameters. The laser scribing system removes the coating from backside of substrate in regions generally aligned with the frontside streets of the substrate using an ultrafast pulsed laser with first laser pulse parameters without causing substantial damage to said substrate. The laser scribing system then scribes the substrate in regions generally aligned with the frontside devices using an ultrafast pulsed laser with second laser pulse parameters. 
     Opto-electric devices such as light-emitting diodes are typically manufactured by making multiple identical copies of a device in parallel on a substrate. A typical substrate containing opto-electric devices is shown in  FIG. 1 . These substrates typically need to be singulated into individual devices. Singulation can be performed by first scribing the substrate with a diamond saw or laser along the streets between the devices. Opto-electric devices are typically constructed with active circuitry on the frontside surface and a backside coating which facilitates light output of the device. Scribing the wafer from the backside permits higher fluence laser pulses and therefore permits higher throughput without causing damage to the active circuitry. However, even laser pulses directed to the backside of the substrate can penetrate the substrate and cause damage on the frontside if the laser fluence is not carefully controlled. Laser fluence is defined as Joules/cm 2  and is a measure of the total energy per unit area applied to a substrate by a laser. Desirable properties for scribed opto-electric device would be to have clean, planar sidewalls with some texture caused by the laser scribe but no debris or redeposited material. The device would have an intact, cleanly scribed backside coating evenly attached to the device and no damage to the active circuitry on the frontside. 
     The laser used to scribe these substrates is an ultrafast pulsed laser with laser pulse duration of about 100 ps or less. Laser material removal is generally a mixture of thermal processes or ablative processes. Thermal processes are those where the material is heated and either melts, vaporizes or sublimates directly to a gas. Material is ejected from the area as a vapor, a liquid, or solid particles. Ablative processes alter the material through photo-electric interactions, typically multi-photon, which disassociates the material into plasma. Ultrafast pulses couple energy into the material and create plasma faster than the heat can substantially transfer to adjacent material, thereby creating alterations in the material while minimizing thermal effects which could cause undesirable damage to the substrate or device. The alteration of material may be totally within the surface of the substrate. It is not typically necessary to remove material or make a trench or other feature in the surface of the substrate to promote cracking and subsequent singulation. The material must merely be altered enough to promote and direct stress fractures in the material reliably. 
     Substrates are laser scribed using an adapted laser scribing system. Adaptations include fitting a laser operative to create laser pulses with the desired laser pulse parameters. These laser pulse parameters include wavelength, pulse energy, pulse duration, pulse repetition rate, focal spot size, focal spot location and beam speed. The laser scribing system generates laser pulses with a laser which emits pulses with selected pulse energy and pulse duration at a selected pulse repetition rate. The laser processing system uses laser optics which select and optionally shape the laser pulses spatially and temporally. The laser pulses are focused to a very small spot size in order to concentrate sufficient laser energy in a small enough area to alter the materials in a desirable fashion. The focal spot is defined as the point in space where the laser pulse has the smallest cross-section perpendicular to the direction of propagation of the pulses. The laser scribing system has motion control stages which precisely locate the focal spot a selected height above, below or at the surface of the substrate. The laser processing system also has a combination of beam steering optics and motion control stages which move the substrate in relation to the laser pulses as the laser is pulsing thereby removing or altering material in a pattern determined by the relative motion. The rate at which the laser pulses move relative to the substrate is called beam speed and is measured in millimeters per second. An exemplary laser scribing system which may be adapted to perform aspects of this invention is the AccuScribe 2600 LED Laser Scribing System, manufactured by Electro Scientific Industries, Inc., Portland, Oreg. 97239, shown in  FIG. 2   
     Aspects of this invention scribe substrates by first attaching the substrate frontside to a tape frame with die attach film (DAF) and inserting the attached substrate into the system. The system aligns the substrate so as to be able to aim the laser to scribe the substrate on the backside while maintaining alignment with the streets on the frontside. Laser scribing systems typically process substrates by directing a pulsed laser beam to the substrate while moving the laser beam with respect to the wafer so as to cause the laser pulses to impact the substrate along a path determined by the speed and direction of the relative motion. For efficiency it is typically best to process the substrate in passes, where the system starts processing with the pulsed laser beam while moving the relative location of the laser beam with respect to the substrate in a relatively continuous fashion, typically starting once and stopping once per pass. Aspects of this invention scribe the substrate by making multiple linear passes on the backside of the substrate in orthogonal directions aligned with the frontside streets, although any pattern of curvilinear or straight lines can be scribed. 
     Aspects of this invention scribe substrates using one or more passes per scribe. In some instances, the thickness of the substrate and layout of active circuitry on the frontside permits enough laser fluence per unit time to be delivered to the substrate backside to remove the backside coating and form a scribe in a single pass. In most instances, however, applying enough laser energy to remove the backside coating and form a scribe in a single pass results in undesirable damage to the substrate or device. In this instance more than one pass is required with laser pulse parameters being altered depending upon whether the backside coating is being removed or the scribe is being formed. Aspects of this invention use one or more passes to remove the backside coating and then one or more passes to form the scribe. 
     Aspects of this invention determine laser pulse parameters which process the substrate efficiently without causing damage to the substrate or devices. Laser pulse parameters which may be determined include wavelength, pulse duration, pulse energy, laser power, pulse repetition rate, focal spot size, focal spot location and beam speed. These parameters are determined empirically for a particular substrate and device type to process the substrate with maximum throughput while avoiding damage to the substrate or devices. 
     Once the optimal laser pulse parameters are selected the number of passes is determined. Depending upon the laser energy permitted the backside coating may be removed in one pass as shown in  FIGS. 3 and 4  or two or more passes may be required to remove the backside coating as shown in  FIG. 7  or more complex patterns as shown in  FIG. 8 .  FIG. 8  shows backside coating removal using linear motion combined with beam steering optics to remove backside coating in a complex pattern. In these cases two or more passes are made horizontally adjacent or in a serpentine pattern with the focal spot located above the surface of the backside coating to defocus the laser pulses slightly to reduce the fluence and increase the area processed so that sufficient backside coating is removed to permit scribing on a subsequent pass or passes. In other cases, once the backside coating is removed, a single pass or multiple passes may be required to scribe the substrate. In this case, two or more passes may be made along the same line horizontally but the focal spot location is changed to increase the depth within the substrate to which the laser pulses are focused in order to form the scribe without damage to the substrate or devices. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  (Prior art) A substrate containing opto-electric devices 
         FIG. 2  Adapted laser scribing system 
         FIG. 3  Backside coating removal. 
         FIG. 4  Backside coating removal. 
         FIG. 5  Scribe 
         FIG. 6  Backside coating removal and scribe. 
         FIG. 7  Multi-pass backside coating removal with scribe. 
         FIG. 8  Multi-pass backside coating removal with scribe. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present invention improve the scribing of opto-electric devices on a substrate in the presence of a backside coating by first removing the backside coating with a laser in the region to be scribed and then scribing the substrate. Using multiple passes reduces the laser fluence per unit time and thereby avoids damage to the substrate or nearby active circuitry. Opto-electric device substrates typically have a frontside having streets and a backside having a surface having a coating. This coating reflects stray light emitted by the device which would otherwise be absorbed and cause decreased light output and higher operating temperatures which reduce device lifetime. Embodiments of this invention include a laser scribing system with an ultrafast pulsed laser having first and second laser pulse parameters. The laser scribing system removes the coating from backside of substrate in regions generally aligned with the frontside streets of the substrate using an ultrafast pulsed laser with first laser pulse parameters without causing substantial damage to said substrate. The laser scribing system then scribes the substrate in regions generally aligned with the frontside using an ultrafast pulsed laser with second laser pulse parameters. 
     Opto-electric devices such as light-emitting diodes are typically manufactured by making multiple identical copies of a device in parallel on a substrate. A typical substrate containing opto-electric devices is shown in  FIG. 1 . A wafer  10  has constructed on it several devices, one of which is indicated  12 . These devices are separated by horizontal  14  and vertical  16  streets. The term street refers to areas of the substrate which does not contain active device circuitry. These streets are left free of active circuitry to permit the substrate to be scribed or diced to singulate the substrate into individual devices. Singulation can be performed by first scribing the substrate with a diamond saw or laser along the streets between the devices. Opto-electric devices are typically constructed with active circuitry on the frontside surface and a backside coating which facilitates light output of the device. Scribing the wafer from the backside permits higher fluence laser pulses and therefore permits higher throughput without causing damage to the active circuitry. However, even laser pulses directed to the backside of the substrate can penetrate the substrate and cause damage on the frontside if the laser fluence is not carefully controlled. Laser fluence is defined as Joules/cm 2  and is a measure of the total energy per unit area applied to a substrate by a laser. Desirable properties for scribed opto-electric device would be to have clean, planar sidewalls with some texture caused by the laser scribe but no debris or redeposited material. The device would have an intact, cleanly scribed backside coating evenly attached to the device and no damage to the active circuitry on the frontside. 
     The laser used to scribe these substrates is an ultrafast pulsed laser with laser pulse duration of about 200 ps or less. Laser material processing is generally a mixture of thermal processes or ablative processes. Thermal processes are those where the material is heated and either melts, vaporizes or sublimates directly to a gas. Material is ejected from the area as a vapor, liquid, or solid particles. Ablative processes alter the material through photo-electric interactions, typically multi-photon, which disassociates the material into plasma. Ultrafast pulses couple energy into the material and create plasma faster than the heat can substantially transfer to adjacent material, thereby creating alterations in the material while minimizing thermal effects which could cause undesirable damage to the substrate or device. The alteration of material may be totally within the surface of the substrate. It is not typically necessary to remove material or make a trench or other feature in the surface of the substrate to promote cracking and subsequent singulation. The material must merely be altered enough to promote and direct stress fractures in the material reliably. 
     Substrates are laser scribed using an adapted laser scribing system. Adaptations include fitting a laser operative to create laser pulses with the desired laser pulse parameters. These laser pulse parameters include laser power, wavelength, pulse energy, pulse duration, pulse repetition rate, focal spot size, focal spot location and beam speed. Laser power ranges from about 0.1 W to about 20 W, more preferably in the range from about 0.5 W to about 1.5 W. Wavelengths range from about 560 nm down to about 150 nm, or more preferably in the ultraviolet range at about 355 nm. The pulse energy ranges from about 0.1 μJ to about 1.0 mJ, or more preferably in the range from 1.0 to 10 μJ. The pulse repetition rate (rep rate) ranges from about 75 kHz to about 1 MHz, or more preferable in the range from about 100 kHz to about 800 kHz. The focal spot size typically ranges from less than one μm to about 5 μm. The focal spot location can range from about 100 μm below the surface to about 100 μm above the surface, or more preferably in the range from about 30 μm below the surface to about 30 μm above the surface. Beam speed is typically in the range from about 10 mm/s to about 500 mm/s, or more preferably in the range from about 120 mm/s to about 380 mm/s. 
     Embodiments of this invention employ a laser scribing system which uses a laser to emit pulses with selected pulse energy and pulse duration at a selected pulse repetition rate. The laser processing system uses laser optics which select and optionally shape the laser pulses spatially and temporally. The laser pulses are focused to a very small spot size in order to concentrate sufficient laser energy in a small enough area to alter the materials in a desirable fashion. The focal spot is defined as the point in space where the laser pulse has the smallest cross-section perpendicular to the direction of propagation of the pulses. The laser scribing system includes a z-axis stage as part of the motion control stage which precisely locates the focal spot a selected height above or below the surface of the substrate. The laser processing system also has a combination of beam steering optics and motion control stages which move the substrate in relation to the laser pulses as the laser is pulsing thereby removing or altering material in a pattern determined by the relative motion. The rate at which the laser pulses move relative to the substrate is called beam speed and is measured in millimeters per second. 
     An exemplary laser scribing system which may be adapted to perform aspects of this invention is the AccuScribe 2600 LED Laser Scribing System, manufactured by Electro Scientific Industries, Inc., Portland, Oreg. 97239, shown in  FIG. 2 . The laser scribing system  18  includes a laser  20  which emits laser pulses  22  along a laser beam path. The laser pulses  22  are processed by laser beam optics  24  which select and shape the laser pulses  22 . The laser beam optics  24  may contain electro- or acusto-optic elements which select and direct the laser pulses. The laser beam optics may also contain optical elements that shape the laser pulses both spatially or temporally. The laser pulses  22  then are passed to the beam steering optics  26  which provide fast, limited motion steering of the laser pulses with respect to the substrate  30 . The laser pulses  22  are then typically focused on the substrate  30  by the field optics  28 . The substrate  30  is fixture on a motion control chuck  32  which holds the substrate  30  and moves it in relation to the laser pulses  22  under direction of a controller  36  which coordinates the operations of the laser  20 , laser beam optics  24 , beam steering optics  26  and the alignment camera  34 , which visually aligns the substrate  30  with respect to the laser pulses  22 . 
     One of the adaptations made is fitting a solid state IR laser model Duetto manufactured by Time-Bandwidth Products AG, CH-8005 Zurich, Switzerland. This laser emits 10 ps pulses at 1064 nm wavelength which are frequency-doubled using a solid-state harmonic generator to 532 nm wavelength and optionally frequency tripled using a solid-state harmonic generator to 355 nm wavelength. These lasers have output power of 0.1 to 1.5 Watts. 
     Aspects of this invention scribe substrates by first attaching the substrate frontside to a tape frame with die attach film (DAF) and inserting the attached substrate  30  into the system  18 . The system aligns the substrate  30  using the alignment camera  34  so as to be able to aim the laser pulses  22  to scribe the substrate  30  on the backside while maintaining alignment with the streets on the frontside. A laser scribing system  18  typically processes a substrate  30  by directing a pulsed laser beam  22  to the substrate  30  while moving the laser pulses  22  with respect to the wafer  30  using the motion control stage  32  and beam steering optics  26  so as to cause the laser pulses  22  to impact the substrate  30  along a path determined by the speed and direction of the relative motion. For efficiency it is typically best to process the substrate in passes, where the system starts processing with the pulsed laser beam while moving the relative location of the laser beam with respect to the substrate in a relatively continuous fashion, typically starting once and stopping once per pass. Aspects of this invention scribe the substrate by making multiple linear passes on the backside of the substrate in orthogonal directions aligned with the frontside streets, although any pattern of curvilinear or straight lines can be scribed. 
     Aspects of this invention scribe substrates using one or more passes per scribe. In some instances, the thickness of the substrate and layout of active circuitry on the frontside permits enough laser fluence per unit time to be delivered to the substrate backside to remove the backside coating and form a scribe in a single pass without causing damage. In most instances, however, applying enough laser energy to remove the backside coating and form a scribe in a single pass results in undesirable damage to the substrate or device. In this instance more than one pass is required with laser pulse parameters being altered depending upon whether the backside coating is being removed or the scribe is being formed. Embodiments of this invention use one or more passes to remove the backside coating and then one or more passes to form the scribe. 
     Laser pulse parameters are predetermined to process the substrate efficiently without causing damage to the substrate or devices. Laser parameters which may be predetermined include wavelength, pulse duration, pulse energy, laser power, pulse repetition rate, focal spot size, focal spot location and beam speed. These parameters are typically determined empirically for a particular substrate and device type to process the substrate with maximum throughput while avoiding damage to the substrate or devices. Along with the optimal laser pulse parameters the number of passes to be used to process the substrate is determined. The number of passes is determined by how much laser fluence is can be used per unit time without causing damage to the substrate or devices. Depending upon the laser energy permitted the backside coating may be removed in one pass as shown in  FIGS. 3 ,  4 .  FIG. 3  shows a substrate  40  with a backside coating  42 . Laser pulses  46  are focused by a lens  44  to a focal spot  48  which is directed at the surface of the backside coating  42  to remove the backside coating  42  in a particular location  50 .  FIG. 4  shows a substrate  60  with a backside coating  62 . Laser pulses  66  are focused by a lens  64  to a focal spot  68  which is directed to a location above the substrate  60  and backside coating  62  to remove backside coating  62  in a region  70 . When the laser pulses  66  are focused  68  above or below the surface of the substrate it has the effect of defocusing the laser pulses and spreading the laser pulse energy over a larger area. This not only increases the area of material removed per pulse, thereby speeding up the process but also decreases the laser fluence per unit time thereby avoiding damage.  FIG. 5  shows a substrate  80  with a backside coating  82 . Laser pulses  86  are focused by lens  84  to a focal spot  88  which is located within the substrate  80  to remove the backside coating  82  in a region  90 . As in  FIG. 4 , moving the focal spot  88  below the surface of the substrate  80  defocuses the laser pulses and spreads the energy over a larger area thereby decreasing fluence and limiting undesirable damage. 
     Table 1 shows exemplary laser parameters used by an embodiment of this invention to remove backside coating from a 140 μm thick sapphire substrate. This substrate did not have active circuitry near the region to be scribed or Gallium Nitride (GaN) layers on the frontside and therefore the backside coating could be processed with enough energy to remove the backside coating in one pass. This embodiment removes the backside coating in one pass by focusing the laser pulses 30 microns below the surface of the backside coating. 
                                   TABLE 1               Laser pulse parameters for one pass backside coating removal                                    Pulse Duration   10 ps           Laser Power   1.2 W           Stage Speed   120 mm/s           Rep Rate   200 kHz           Focal Spot Size   1-5 μm           Focal Spot Location   −30 μm           # Passes   1                        
In other cases, for example where for example GaN is present on the frontside, multiple passes may be required. Table 2 shows laser parameters used to remove backside coating on substrates with thicknesses between 120 and 140 μm. Laser power used is 0.7 W for 120 μm thick substrates and 0.8 W for 140 μm thick substrates. Stage speed is 120 mms/s for 140 μm thick substrates and 180 mm/s for 120 μm thick substrates. This embodiment removes the backside coating with three passes along the same path focused at the surface of the backside coating, 7 μm below the surface and 15 μm below the surface
 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Laser pulse parameters for vertical multi-pass backside coating removal 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Pulse Duration 
                 10 ps 
               
               
                   
                 Laser Power 
                 0.7-0.8 W 
               
               
                   
                 Stage Speed 
                 120-180 mm/s 
               
               
                   
                 Rep Rate 
                 200 kHz 
               
               
                   
                 Focal Spot Size 
                 1-5 μm 
               
               
                   
                 Focal Spot Location 
                 0, −7, −15 μm 
               
               
                   
                 # Passes 
                 3 
               
               
                   
                   
               
             
          
         
       
     
       FIG. 6  shows an embodiment of this invention which employs two laser pulse beams to process a substrate  100  in one pass. A substrate  100  with a backside coating  102 , is illuminated by two laser pulse beams  106  focused by a common lens  104  on a common optical axis. First laser beam pulses  108  are focused to a point above the substrate  112  to remove the backside coating  102  in an area  116 . The second laser pulses  110  are focused on the surface of the substrate  114  to form the scribe. When these two laser pulse beams are moved together along a path at preselected rates a substrate with a backside coating can be scribed in a single pass without causing damage to the substrate or active circuitry. 
       FIG. 7  shows backside coating removal using linear motion to remove backside coating in an area larger than the width of a single pass by laser pulses. In this case, a substrate with a backside coating has the backside coating removed along linear regions with one or more passes made at the same height with respect to the substrate. In  FIG. 7 , a substrate with a backside coating (not shown) is to be scribed along line  128 . Prior to scribing the backside coating is removed in a linear region  139  described by the overlapping laser pulses  122  opposite the frontside street (not shown) by making a pass on the substrate along dotted line  120 , impacting the coated substrate with overlapping laser pulses, one of which is indicated  122  at fluence which removes the coating without causing damage. The substrate is then indexed by the motion control stages (not shown) to permit another pass to be made along dotted line  124 , where the laser pulses again impact the coated substrate with overlapping pulses, one of which is indicated  126 . These two passes remove the backside coating without damage to the substrate or active circuitry. The substrate is then indexed to a position where the laser pulses will impact the substrate along line  128  as the substrate is moved with respect to the laser pulse beam. Laser parameter are adjusted to scribe rather than remove backside coating and a third pass is made along line  128  thereby scribing the substrate without causing damage to the underlying substrate or active circuitry. Exemplary laser pulse parameters for this embodiment are given in Tables 3 and 4. Laser power is 0.7 W for 120 μm thick substrates and 0.8 W for 140 μm thick sapphire substrate. Stage speed is 180 mm/s for 120 μm thick sapphire substrates and 120 mm/s for 140 μm thick sapphire substrates. 
                                   TABLE 3               Laser pulse parameters for horizontal multi-pass backside coating removal.                                    Pulse Duration   10 ps           Laser Power   0.7-0.8 W           Stage Speed   120-180 mm/s           Rep Rate   200 kHz           Focal Spot Size   1-5 μm           Focal Spot Location   35 μm           # Passes   2, 20 μm separation                        
Following backside coating removal in this fashion, the substrate may be scribed with a single pass with the laser pulses focused 20 microns below the surface of the substrate with the parameters as shown in Table 4. Multiple scribing passes may also be used.
 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                 Laser pulse parameters for single-pass scribing. 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Pulse Duration 
                 10 ps 
               
               
                   
                 Laser Power 
                 0.6 W 
               
               
                   
                 Stage Speed 
                 120-180 mm/s 
               
               
                   
                 Rep Rate 
                 200 kHz 
               
               
                   
                 Focal Spot Size 
                 1-5 μm 
               
               
                   
                 Focal Spot Location 
                 −15 μm 
               
               
                   
                 # Passes 
                 1 
               
               
                   
                   
               
             
          
         
       
     
     Table 5 shows laser pulse parameters for embodiments of this invention that employ multiple passes to scribe the substrate following removal of the backside coating. This is used in cases where using sufficient energy to scribe the substrate in one pass would cause damage to the substrate or active circuitry. In this case, the substrate is scribed by making two or more passes along the same path varying the focal spot location with respect to the surface of the substrate. Scribing a substrate in this fashion creates an altered region beginning at the surface of the substrate and extending into the interior which promotes and directs subsequent cleaving. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
               
                 Laser pulse parameters for multi-pass scribing. 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Pulse Duration 
                 10 ps 
               
               
                   
                 Laser Power 
                 0.6 W 
               
               
                   
                 Stage Speed 
                 120-180 mm/s 
               
               
                   
                 Rep Rate 
                 200 kHz 
               
               
                   
                 Focal Spot Size 
                 1-5 μm 
               
               
                   
                 Focal Spot Location 
                 −7, −15 μm 
               
               
                   
                 # Passes 
                 2 
               
               
                   
                   
               
             
          
         
       
     
       FIG. 8  shows an embodiment of this invention which employs fast beam steering optics to deliver laser pulses to the substrate in patterns which efficiently remove backside coatings. In this embodiment, laser pulses, one of which is shown  132  are directed to the substrate (not shown) along a dotted line  130  which is determined by the combined relative motion of the laser pulses with respect to the substrate caused by moving the substrate in a linear fashion with motion control stages while moving the laser pulses with laser beam steering optics. In  FIG. 8 , the combined motion control/beam steering optics (not shown) moves the laser in the pattern shown by dotted line  130  while delivering laser pulses  132  to the substrate. One or more passes may be required to completely remove the backside coating in the linear region  136  opposite the frontside streets described by the overlapping pulses  132  without causing damage to the substrate or active circuitry. Following the backside coating removal the substrate is scribed  134  in the region  136  where the backside coating has been removed. One or more passes may be required to scribe the substrate without causing damage to the substrate or active circuitry. 
     It will be apparent to those of ordinary 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.