Abstract:
A method and device for wafer level testing of semiconductor lasers allows probing from one side while detecting light output from the opposite side. A chuck with a transparent substrate receives the optical aperture side of a wafer of semiconductor lasers. The wafer is probed form the side opposite the side contacting the chuck and emitted light is detected on a side of the chuck opposite the side contacting the wafer.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to methods and apparatus for testing semiconductor lasers and, more specifically relates to methods and apparatus for wafer-level testing of vertical cavity surface emitting lasers (VCSELs).  
         BACKGROUND OF THE INVENTION  
         [0002]    Semiconductor lasers in use today include edge-emitting diode lasers and vertical cavity surface emitting lasers (“VCSELs”). In an edge-emitting laser, a semiconductor gain medium, for example, a quantum-well semiconductor structure, is formed on a surface of a semiconductor substrate. Once a device is detached from a wafer, cavity mirrors are formed or otherwise positioned on opposite ends of the gain medium, perpendicular to the substrate surfaces, to form a resonant cavity within which the gain medium is located. Electrical or optical pumping of the gain medium generates a laser beam which propagates in a direction along the plane of the substrate. As edge-emitting lasers generate a beam in a direction along the plane of a substrate forming the laser, these lasers can not be meaningfully tested in wafer form—that is, it is not practical to test these lasers prior to their being cleaved into individual units exposing the edges from which their beams are output.  
           [0003]    VCSELs in contrast, propagate output beams in a direction perpendicular to the plane of a substrate on which they are formed. Thus the orientation of VCSELs on a wafer substrate prior to their being separated from one another is potentially suitable for testing. Prior wafer probe methods used on VCSELs involve electrically probing the optical aperture side of a wafer and detecting light emitted from that side while shorting the opposite side of the wafer to ground. Depending on the resistance profile of the wafer, this method may stimulate emissions from a single VCSEL, or emission from other VCSELs adjacent to and even substantially separated from the VCSEL to be probed due to the low electrical resistance between adjacent ones of the VCSEL array on the probed side.  
         SUMMARY OF THE INVENTION  
         [0004]    The present invention is directed to a method for wafer level testing of semiconductor lasers comprising the steps of positioning a wafer on a chuck with a first side of the wafer contacting the chuck including optical apertures through which output beams of the lasers included therein are emitted and electrically probing individual ones of the lasers on a second side of the wafer to stimulate emission from the accessed lasers in combination with the step of detecting light from the accessed lasers after the light has passed through the chuck.  
           [0005]    The present invention is further directed to a device for wafer level testing of semiconductor lasers comprising a chuck on which a wafer including lasers to be tested is received, wherein a first side of the wafer contacting the chuck includes optical apertures through which output beams of the lasers included therein are emitted and an electrical probe accessing individual ones of the lasers on a second side of the wafer to stimulate emission from the accessed lasers in combination with a light detector receiving light from the accessed lasers after the light has passed through the chuck. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1 shows a bottom view of a wafer configured for testing in accord with the apparatus and method according to the present invention;  
         [0007]    [0007]FIG. 2, shows a side view of a portion of the wafer of FIG. 1;  
         [0008]    [0008]FIG. 3 shows a perspective view of a portion of the wafer of FIG. 1;  
         [0009]    [0009]FIG. 4 shows a chuck for use with an apparatus for wafer-level testing of semiconductor lasers according to a first embodiment of the invention;  
         [0010]    [0010]FIG. 5 shows a side view of an apparatus for wafer-level testing of semiconductor lasers according to the first embodiment of the invention;  
         [0011]    [0011]FIG. 6 shows an enlarged side view of a portion of the apparatus of FIG. 5 at the left side of FIG. 5;  
         [0012]    [0012]FIG. 7 shows an enlarged side view of a portion of for wafer-level testing of an apparatus for wafer-level testing of semiconductor lasers according to a second embodiment of the invention;  
         [0013]    [0013]FIG. 8 shows a side view of the apparatus of FIG. 7;  
         [0014]    [0014]FIG. 9 shows a perspective view of an apparatus for securing a wafer with the various parts separated from one another; and  
         [0015]    [0015]FIG. 10 shows a cross-sectional side view of the apparatus of FIG. 7 with a wafer received therein. 
     
    
     DETAILED DESCRIPTION  
       [0016]    FIGS.  1 - 3  show a wafer  10  suitable for testing in accord with the apparatus and method of the present invention. Specifically, the wafer includes a plurality of vertical cavity surface emitting lasers (VCSELs)  12  formed thereon. As shown in FIG. 2, this wafer  10  will typically include a substrate  14  with a gain region  16  formed thereon between first and second reflectors  18 ,  20 , respectively. The VCSELs  12  include first electrodes  22  mounted on a first side thereof with second electrodes  24  mounted on a second side thereof. Each first electrode  22  includes an optical aperture  26  extending therethrough with each of the optical apertures  26  being aligned with a fundamental cavity mode of a corresponding one of the VCSELs  12 . Furthermore, the first electrodes  22  are separated from one another by a series of channels  23  so that each first electrode corresponds to a respective one of the VCSELs  12 . An insulating material  30  (e.g., SiN) may be deposited and patterned (e.g., with a photolithography/etching process) to define a contact region  28  at which each second electrode  24  will contact a corresponding VCSEL  12 . A conducting material may then be deposited or otherwise mounted in electrical contact with the region defined by the insulating material  30 . The conducting material may be patterned, for example, using a photolithography/etching process to electrically separate the individual contact regions by forming a plurality of gaps  32  therebetween. The plurality of gaps  32  are formed between the second electrodes  24  with each of the gaps  32  extending through the layer of second electrodes  24  to the insulating material  30  in a corresponding recess thereby electrically isolating each of the VCSELs  12  from one another on this side of the wafer  10 . Of course those skilled in the art will recognize that the specific configuration of the VCSELs  12  on the wafer  10  and the arrangement of the various components of the VCSELs  12  on the substrate  14  may be altered in any desired manner without departing from the scope of the present invention.  
         [0017]    Although the method and apparatus of the present invention will be useful with wafers  10  including a wide variety of arrangements of lasers formed thereon, in an exemplary embodiment of this invention, an electrical resistance on an optical aperture side of the wafer  10  (the top in FIG. 2) between the VCSELs  12  is low relative to that on the opposite side of the wafer  10 . In this wafer  10 , the electrical contacts  22  of the VCSELs  12  maybe quite close together (e.g., separated by ≧100μ) so that lateral current leakage (i.e., current flow through the substrate  14 ) is more common. As will be understood by those skilled in the art, using the low resistance on the side of the optical apertures  26  allows all of the VCSELs  12  to be shorted out at once. The electrical contacts  24  opposite side of the wafer  10  are smaller and, therefore, further apart from one another. Furthermore, the electrical contacts  24  include the insulating material  30  therebetween. Thus, the electrical resistance between contacts  24  may be on the order of 100 ohms and any minimal current which may travel between VCSELs  12  probed on this side of the wafer  10  will be insufficient to cause the un-probed VCSELs  12  to emit light.  
         [0018]    As seen in FIGS.  4 - 8 , an apparatus  45  for wafer level testing of semiconductor lasers includes a chuck  50  having a rim  52  surrounding a metal film layer  54  which corresponds roughly to the surface area of the wafer  10 . The metal film layer  54  includes a series of holes  56  extending therethrough so that, when the wafer  10  is placed on the chuck  50  in a predetermined testing alignment, each of the holes  56  is substantially aligned with a corresponding one of the optical apertures  26 . As seen more clearly in FIGS. 5 and 6, the chuck  50  may be formed from an optically transparent substrate  50  on a flat surface of which a partially reflective coating  60  may be formed. The reflective layer  60  may, for example, be of the type available from CVI Laser Corp., Albuquerque, N. Mex. with a reflectivity of, for example, 70% to 90%. The metal film layer  54  is formed over the partially reflective coating  60  so that, in the view of FIG. 4, each of the holes  56  exposes a portion of the reflective coating  60 . As the wafer  10  shown in FIG. 1 is substantially circular, the chuck  50  is also shown in such a shape. However, those skilled in the art will understand that the shape of the wafer  10  and the chuck  50  may be varied in any manner so long as the optical apertures  26  substantially align with the corresponding holes  56 .  
         [0019]    The chuck  50  may be composed of a substrate  51  formed, for example, of fused silica, sapphire, or any other material which is substantially transparent at the wavelength of an output beam of the VCSELs  12  (e.g., 980 nm). The metal film layer  54  is preferably made of a material having low electrical resistance to minimize a voltage drop from a periphery of the chuck  50  at which voltage is applied to the center thereof and may consist of a plurality of sub-layers. For example, the metal film layer  54  may be formed of Ti, Au (1.5μ microns), Pt (100 A) film may be used to achieve a suitably low resistance. For example, the metal film layer  54  may be comprised of a first layer of TI approximately 200 angstroms thick adjacent to the substrate  51 , a second layer of Au approximately 1.5μ microns thick and an outer scratch resistant layer formed of Pt approximately 100 angstroms thick. The holes  56  may then be formed, for example, using a photolithograpy/etching process or a lift off process.  
         [0020]    The apparatus  45  may further include a “C” clamp  62  which, in a testing configuration, grips the chuck  50  and holds the chuck  50  in a predetermined position. When in the testing configuration, the wafer  10  is held in position on the chuck  50  with each of the optical apertures  26  in substantial alignment with a corresponding one of the holes  56  and with a plane in which the partially reflective coating  60  is formed extending substantially perpendicular to a fundamental cavity mode of each one of the VCSELs  12 . This is preferably accomplished by forming an outer surface of the first electrode  22  as a portion of a plane extending substantially perpendicular to the fundamental cavity mode of each of the VCSELs  12  of the wafer  10  and then forming an outer surface of the metal film layer  54  as a portion of a plane substantially parallel to a surface of the partially reflective coating  60  so that, when the wafer  10  is pressed against the chuck  50  (as will be described in more detail below) contact between the outer surfaces of the first electrode  22  and the metal film layer  54  bring the partially reflective coating  60  into the predetermined alignment with the fundamental cavity modes of each of the VCSELs  12  of the wafer  10 .  
         [0021]    Furthermore, as shown in FIGS. 7 and 8, when the wafer  10  is in the testing configuration with the outer surface of the first electrode  22  pressed against the outer surface of the metal film layer  54 , a light detector  64  is positioned on a side of the chuck  50  opposite the side on which the metal film layer  54  is formed. In addition, a plate  66  (e.g., a metal plate coated with an insulator) including a plurality of access holes  68  extending therethrough is pressed against an outer surface of the second electrode  24  aligned so that a portion of the second electrode  24  corresponding to one of the VCSELs  12  is exposed through a respective one of the access holes  68 . A thin layer  70  of insulating material which may, for example, be polyimide or sputtered silicon-nitride, is formed on the side of the plate  66  which will contact the second electrode  24  of the VCSELs  12 . The access holes  68  may be formed in the plate  66  by, for example, etching or cutting by wire EDM (i.e. a wire electrode with a spark gap for eroding material).  
         [0022]    Semiconductor wafers thinned to 50-100μ are also often not suitably flat to achieve suitable electrical contact between the metal film layer  54  and the first electrodes  24  of each of the VCSELs  12 . The plate  66 , when pressed against the wafer  10  flattens the wafer  10  to ensure contact between each of the VCSELs  12  and the metal film layer  54 .  
         [0023]    The chuck  50  is mounted to a support arm (not shown) which, by known mechanisms under computer control, sequentially positions the chuck  50  and the wafer  10  mounted thereon so that a desired one of the VCSELs  12  is aligned with an electrical probe  74  which is electrically coupled to a probe card  76  including known probe circuitry. When the desired one of VCSELs  12  is properly positioned, the electrical probe  74  is inserted into the corresponding one of the access holes  68  to electrically couple probe card  76  to the second electrode  24  of the desired one of the VCSELs  12  for testing. The electrical probe  74  is positioned so that, when the desired one of the VCSELs  12  is accessed by the probe  74 , the optical aperture of the VCSEL  12  and the corresponding hole  56  are aligned with the light detector  64 .  
         [0024]    As shown in FIG. 9, the wafer  10  will be clamped between a plate  66 ′ and the chuck  50  to flatten the wafer  10  and fix it in a desired position. In contrast to the plate  66  described above, the plate  66 ′ shown in FIG. 6 includes one large central opening  80  via which all of the VCSELs  12  may be accessed by the probe  74 . The opening  80  is shown as substantially square in shape, however, this opening may be arranged in any shape which corresponds to the location of the various VCSELs  12  on a wafer  10  to be tested. Alternatively, as described above a plate  66  including a plurality of access holes  68  may be used in conjunction with the apparatus and method of the invention.  
         [0025]    The wafer  10  is placed on the surface of the chuck  50  and moved so that the optical apertures  26  are in alignment with the holes  56 . For example, the chuck  50  may be sized so that it is larger than the wafer  10  to be tested. In this case, the holes  56  will extend out past the edge of the wafer  10  when the wafer  10  is received on the chuck  50 . The array of the holes  56  which are visible at the edges of the chuck  50  may then be visually aligned with the channels  32  formed between the second electrodes  24  to ensure that the wafer  10  is properly aligned on the chuck  50  with respect to the holes  56 . The chuck  50  is supported within a mounting ring  100  by an adjustable support  102  which is coupled to the mounting ring  100  via a plurality of screws (not shown) which pass through holes  106  formed in tabs  108  to enter corresponding holes  1   10  in mounting ring  100 . The distance between the adjustable support  102  and the mounting ring  100  maybe changed through adjustment of the screws  104 .  
         [0026]    The chuck  50  is sized to fit within the central hole  112  of the mounting ring  100  and the thickness of the mounting ring  100  and that of the chuck  50  are selected so that, when the adjustable support  102  is mounted to the mounting ring  100 , the wafer  10  received on the chuck  50  is substantially flush with an inner surface  114  of the mounting ring  100 . Then, the plate  66 ′ is placed on a spring which holds the plate  66 ′ above the wafer  10 . The plate  66 ′ is then maneuvered into the desired position relative to the wafer  10 . The plate  66 ′ may then be pressed down against the bias of spring and maintained in position contacting the wafer  10  by means of vacuum pressure applied through holes  113  extending through mounting ring  100 . Thus, the plate  66 ′ presses the wafer  10  flat and holds it in the desired position. Finally, a semi-circular spacer element  116  is positioned below the adjustable support  102  (and may be coupled thereto), to provide a predetermined spacing between the wafer  10  and a light detector  64  which will be position therebelow. Of course, depending on the desired distance, spacer elements  116  of various thicknesses may be employed. Furthermore, the spacer element  116  may also include tabs to maintain a constant orientation of the adjustable support  102  relative thereto.  
         [0027]    In an embodiment for the testing of VCSEL devices that employ a mirror external to the laser cavity, providing the partially reflective coating  60  on a flat surface of the chuck  50  significantly reduces the time required for testing by eliminating the need to realign a mirror relative to each VCSEL  12  to be tested—i.e., each of the VCSELs  12  is aligned with the partially reflective coating  60  (which forms the extended cavity of each VCSEL  12 ) when the wafer  10  is properly positioned on the chuck  50 .  
         [0028]    The specific embodiments described above are merely illustrative and those skilled in the art will understand that there are many variations and modifications of this intention which may be made without departing from the scope of the invention which is to be limited only by the scope of the claims appended hereto.