Abstract:
A semiconductor laser device with a first side and a second side, comprising (a) an active region, (b) a P layer, wherein the P layer contains a first contact area, (c) an N layer, wherein said N layer contains a second contact area, wherein the contact area of the first contact area of the P layer and the second contact layer of the N layer reside on the first side of the laser device.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to semiconductor optical devices, and more specifically relates to a novel flip chip construction of a semiconductor laser.  
         BACKGROUND OF THE INVENTION  
         [0002]    In recent years, considerable effort has been spent in developing low cost optical packages. Optical communication technology has increased in popularity in the data communications and telecommunications industries over the past several years. The packaging is a high cost element of producing optical devices because of the difficulties surrounding alignment and connection of optical devices and optical fibers. Alignment of laser diodes and optical fibers is a slow, labor intensive task and automation of this process is extremely difficult. As a result, the cost of using most optical packages is high.  
           [0003]    Recently, silicon optical benches have been developed to reduce the cost involved with packaging optical circuits. A silicon optical bench is a silicon platform on which a number of optical devices can be packaged. The optical devices are manufactured in a semiconductor chip, and the chip is mounted to the silicon optical bench. Using a silicon optical bench allows several optical circuits on a plurality of chips to be packaged on a single platform or bench.  
           [0004]    To mount a chip containing an optical circuit on a silicon optical bench requires contact to be made between the contacting areas on the chip and the bench itself. One side of an optical device chip normally comprises a P contact or an N contact surface, while the opposite side comprises a complimentary contact. The bottom side of the chip is soldered directly to the silicon optical bench to provide contact between the bench and one contacting layer. The chip is first placed on solder pads located on the optical bench, and then the solder is re-flowed to affix the chip to the bench. Contact is made to the complimentary contact layer using one or more bond wires.  
           [0005]    [0005]FIG. 1 illustrates an optical device chip  50  mounted to a silicon optical bench  52  in accordance with the prior art. Referring to FIG. 1, contact is made between the P contact pad  54  and the silicon optical bench  52  using a solder pad  53 . Contact is made to the N contact area  56  on the top of the chip using a bond wire  58  between the N contact area  56  on the top of the chip and an additional contact region located on the silicon optical bench (not shown).  
           [0006]    Despite the advantages of using a silicon optical bench, the mounting of optical chips remains a high cost, labor intensive process. The optical chip must be actively aligned with the desired location on the silicon optical bench during the installation process. In addition, once the chip has been soldered to the silicon optical bench, the bond wires must be individually installed to make contact to the top surface of the chip. This makes automation difficult and requires expensive, high precision equipment.  
           [0007]    In addition to manufacturing concerns, the bond wires also introduce additional parasitic parameters (e.g., capacitance, inductance) into the circuit. The bond wires also cause a higher failure rate of the circuit by providing an additional component subject to failure or defect.  
           [0008]    Non-optical device chips have been developed that have both the P contact and the N contact residing on the same side of the device. These chips are known in the art as “flip chips.” However, the fabrication of conventional flip chips requires via holes to be etched through the chip to create a contact from the top side of the chip to the bottom side of the chip. The creation of these vias add additional processing steps, which has made fabrication of optical devices in a flip chip configuration unpractical.  
           [0009]    Accordingly, there is a need for an optical chip that can be quickly and easily mounted to a silicon optical bench without requiring bond wires or extensive alignment. The present invention fulfills this need among others.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention provides for a semiconductor laser device having both the N contact area and the P contact located on the same side of the chip. Specifically, rather than having one contact area on the base of the chip connected to the optical bench while using a wire bonding technique to make contact with the second contact area located on the opposite side of the chip, the chip configuration allows both the N contact area and the P contact area to be made directly to the silicon optical bench.  
           [0011]    In addition, the semiconductor device in accordance with the present invention is passively aligned to the optical bench upon mounting. At least one solder alignment or bond pad is formed on the base of the chip and on the optical bench. Upon mounting, the solder is reflowed, and the solder bond pads on the base of the chip align with corresponding solder pads on the silicon optical bench.  
           [0012]    One aspect of the invention is a semiconductor laser device with a first side and a second side, comprising (a) an active region, (b) a P layer, wherein the P layer contains a first contact area, (c) an N layer, wherein said N layer contains a second contact area, wherein the contact area of the first contact area of the P layer and the second contact layer of the N layer reside on the first side of the laser device.  
           [0013]    An additional aspect of the invention is an optical semiconductor laser device having a plurality of solder pads formed on the first side of the chip. The solder pads passively align the device to the desired location on the silicon optical bench during solder reflow. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a cross-sectional view of an optical device mounted to a silicon optical bench in accordance with the prior art.  
         [0015]    [0015]FIG. 2 is a cross-sectional view of a device in accordance with the present invention.  
         [0016]    [0016]FIG. 3 is a top view of a device in accordance with the present invention.  
         [0017]    [0017]FIG. 4 is a cross-sectional view showing the device in accordance with the present invention mounted to a silicon optical bench. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    Referring to FIG. 2, a preferred embodiment of the semiconductor laser device  10  is shown. The illustrated device is a laser having an active region  11  for emitting light located in the horizontal center of the chip. The light emitting from the active region  11  would travel perpendicular to the cross-sectional cut of the chip as illustrated in FIG. 1 (i.e., the light would travel outward from the paper as shown in FIG. 2). The direction of the However, the invention is not limited to chips containing a single active region and may be practiced with other chip configurations.  
         [0019]    The optical chip  10  comprises: (a) an active region  11 , (b) a P layer  13  having a plurality of P-contact bond pads  20 , and (c) an N layer  17  having a plurality of N-contact bond pads  21 . The P-contact bond pads  20  and the N-contact bond pads  21  both reside on the bottom side  14  of the optical chip  10  (it should be noted that the bottom side of the optical chip  10  is the facing upwardly in FIG. 2 as FIG. 2 shows the chip prior to the “flip” installation). Referring to FIG. 2, a cross-sectional view of a single P-contact bond pad and a single N-contact bond pad is shown. FIG. 3 illustrates a plurality of P-contact and N-contact bond pads on the bottom side  14  of the chip  10 .  
         [0020]    Referring again to FIG. 2, the optical chip operates by emitting light from the active region  11 . The light emits from the edge of the chip, or upwards out of the paper when viewing FIG. 2. In order to activate the active region  11 , a current is applied to the P layer  13  through the P-contact bond pad  20 . The P layer  13  is a metal layer disposed on top of the semiconductor material. The current flows through the P layer into a contact layer  18 . The contact layer  18  is a layer of semiconductor that contacts the metal P layer  13 . This layer facilitates a highly conductive, highly stable contact. In a preferred embodiment, the contact layer  18  is approximately 0.3 microns thick.  
         [0021]    Beneath the contact layer  18  is a P-bury layer  16  approximately 2.5 microns thick. The current passes through the contact layer  18  and into the P-bury layer  16 . The current travels through the P-bury layer  23  and into the active region  11 . In a preferred embodiment, the active region  11  is approximately 0.25 microns thick and has a width approximately 1.6 microns.  
         [0022]    Current blocking regions exist on either side of the active region. In the embodiment illustrated in FIG. 2, the blocking regions comprise three layers. P type block layers  23   a ,  23   b  resides above the N side  12  of the device on both sides of the active region  11 . The P type blocking layers  23   a ,  23   b  are approximately  1  micron in thickness. N type blocking layers  25   a ,  25   b  that are approximately 1 micron thick at their widest point reside on top of the P type blocking layers  23   a ,  23   b . A P-cap layer  31   a ,  31   b  is located beneath the P-bury layer  16  and the top of the N type blocking layer on either side of the active region  11 . The use of blocking regions prevents the current from flowing laterally to ground and forces the current to flow through the active region. The use of blocking regions are well known in the art.  
         [0023]    The current that travels down through the active region  11  drives the active region  11  and causes the optical device to operate by emitting laser light. While the device shown in the illustrated embodiment is a laser, other embodiments include various devices such as modulators or amplifiers.  
         [0024]    Once the current passes through the active region  11 , it flows through the N side  12  of the device to the N contact layer  17 . The bulk of the device on the N-side comprises a semiconductor material, preferably any of the type III-V semiconductors. The current flows through the N side  12  of the device and enters the N contact layer  17  via an N contact window  24  located on the inner side of the N contact layer  17  at the bottom of a trench  29   a  formed in the bottom of the chip  10 . The N contact layer  17  is grounded by making contact through the N contact solder pads  21 .  
         [0025]    When the chip  10  is mounted on a silicon optical bench, contact to both the P layer  13  and the N layer  17  is made on the bottom or underside of the chip  10 . The P contact bond pad  20  is connected to a current source on the silicon optical bench and the N contact bond pad is connected to ground on the silicon optical bench. In a preferred embodiment, connection is made using solder; however, other techniques such as conductive adhesives or other contacting methods could be used. Both contact regions are on the bottom side of the chip (i.e., the side that contacts the silicon optical bench). As a result, there is no need to contact the top side of the chip  10 . Thus, the need for bond wires is eliminated. In addition, the need for bond pads or contact areas on the top side of the chip  10  also is eliminated. Furthermore, by configuring the chip  10  to locate the N contact window  24  at the bottom of the trench  29   a , the need to fabricate vias in the chip  10  to allow contact to the N side  12  is eliminated.  
         [0026]    A top view of the chip  10  is shown in FIG. 3. Referring to FIG. 3, the locations of a plurality of P contact bond pads  20  and a plurality of N contact bond pads  21  on the underside of the chip  10  for a preferred embodiment is shown. Surrounding the bond pads is a dielectric layer  19 . When the chip  10  is installed on the silicon optical bench, the chip is flipped over such that the side illustrated in FIG. 3 containing the P contact bond pads  20  and N contact bond pads  21  is adjacent to the surface of the silicon optical bench. When the chip  10  is placed on the silicon optical bench, the P contact bond pads  20  and N contact bond pads  21  on the chip  10  are placed in rough alignment with corresponding contact pads on the silicon optical bench. Preferably, the P contact bond pads  20  and N contact bond pads  21  are formed out of solder. During installation, the solder is re-flowed, thus forcing the chip to move into proper alignment with the silicon optical bench as a result of the surface tension present in the solder during re-flow. Alignment is controlled by configuring the shape and location of the P contact bond pads  20  and N contact bond pads  21 , as well as the composition of the solder comprising the pads. By controlling the re-flow conditions, proper alignment is assured.  
         [0027]    [0027]FIG. 4 illustrates a chip  10  in accordance with the present invention mounted to the silicon optical bench  30 . The N contact pad  21  and the P contact pad  20  are both in contact with the silicon optical bench  30 . The chip is aligned such that the active region  11  is in alignment with a core region  27  of an optical fiber  26 . The fiber  26  is positioned in a groove  28  on the silicon optical bench. The alignment of the chip  10  with respect to the silicon optical bench  30  and the optical fiber  26  is achieved during the reflow process. After alignment, the light emitting from the active region  11  of the chip  10  enters the core  27  of the fiber  26 .  
         [0028]    The solder forming the P contact bond pads  20  and N contact bond pads  21  is electrically conductive. This allows the creation of the necessary electrical contact between the contact areas on the chip  10  and the contact areas on the silicon optical bench.  
         [0029]    The chip  10  in accordance with the present invention would provide several advantages over existing opto-electrical devices. Because both the N contact and the P contact reside on the underside of the chip, direct contact can be made to the silicon optical bench. The need for bond wires is eliminated, thus reducing the cost of manufacture as well as eliminating unwanted parasitic properties introduced by bond wires. In addition, removing the bond wires removes one possible failure mechanism from the final device. Furthermore, by configuring the contact areas on the underside of the device in accordance with the present invention, complicated alignment processes currently associated with the uses of optical chips on silicon optical benches are eliminated. The chip in accordance with the present invention can be easily passively aligned during the mounting of the chip to the silicon optical bench using a solder reflow technique between the P contact bond pads  20  and N contact bond pads  21  located on the underside of the device and the corresponding contact pads on the bench.  
         [0030]    It should be understood that the foregoing is illustrative and not limiting and that obvious modifications may be made by those skilled in the art without departing from the spirit of the invention. Accordingly, the specification is intended to cover such alternatives, modifications, and equivalence as may be included within the spirit and scope of the invention as defined in the following claims.