Abstract:
A method for manufacturing a multiple-wavelength semiconductor laser comprises: forming a first bar having an array of first semiconductor chips, wherein at least two semiconductor lasers producing light of different wavelengths are monolithically formed; forming a second bar having an array of second semiconductor chips, wherein a semiconductor laser producing light having a different wavelength from the light produced by the semiconductor lasers of the first semiconductor chips is formed; forming a third bar by locating a laser-forming surface of said first bar facing a back surface of the second bar, and joining respective first semiconductor chips in the first bar to respective second semiconductor chips in the second bar; forming scribe lines by irradiating boundaries of the first semiconductor chips and boundaries of the second semiconductor chips with laser beams, and dividing the third bar along the scribe lines into respective chips.

Description:
BACKGROUND OF THE INVENTION  
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a method for manufacturing a multiple-wavelength semiconductor laser wherein two semiconductor lasers having different wavelengths are joined, and specifically, to a method for manufacturing a multiple-wavelength semiconductor laser that can accurately align two semiconductor lasers and can secure high reliability. 
         [0003]    2. Background Art 
         [0004]    Recently, optical disks including CDs, DVDs, and Blu-ray disks (BDs) are extensively used as mass storage media. The oscillation wavelengths of semiconductor lasers used in these optical disk devices become shorter in the order of CDs, DVDs, and BDs depending on the storage capacities. The oscillation wavelength of the laser for CDs is 780 nm band (infrared semiconductor laser), the oscillation wavelength of the laser for DVDs is 650 nm band (red semiconductor laser), and the oscillation wavelength of the laser for BDs is 400 nm band (blue semiconductor laser). For processing information from CDs, DVDs, and BDs in an optical disk device, three beam sources: an infrared semiconductor laser, a red semiconductor laser, and a blue semiconductor laser are required. 
         [0005]    In recent years, a two-wavelength semiconductor laser wherein an infrared semiconductor laser and a red semiconductor laser are monolithically formed in a semiconductor chip has been developed and becoming popular for downsizing and weight saving of the optical pickup device that constitutes an optical disk device. Furthermore, to correspond to BDs, a three-wavelength semiconductor laser, wherein a blue semiconductor laser and a two-wavelength semiconductor laser are combined, is being developed. 
         [0006]    The three-wavelength semiconductor laser is manufactured by stacking and joining a two-wavelength semiconductor laser and a blue semiconductor laser (for example, refer to FIG. 1 of Patent Document 1). However, there was a problem wherein the alignment of the two-wavelength semiconductor laser and a blue semiconductor laser in joining was difficult. 
         [0007]    To solve the problem, a method for dividing the two semiconductor lasers into chips using a cutting saw after joining them in a bar state is proposed (for example, refer to FIG. 1 of Patent Document 2). By this method, two semiconductor lasers can be aligned at high accuracy. In Patent Document 2, however, although the joining of the bar of the single wavelength semiconductor laser is described, the joining of the bar of the two-wavelength semiconductor laser is not described.
   [Patent Document 1] Japanese Patent No. 3486900   [Patent Document 2] Japanese Patent Application Laid-Open No. 2002-232061   
 
       SUMMARY OF THE INVENTION  
       [0010]    In the two-wavelength semiconductor laser, an infrared semiconductor laser and a red semiconductor laser are lined up on a substrate. Therefore, if the laser forming surface of the bar of the two-wavelength semiconductor laser is allowed to face the bar of the blue semiconductor laser, a large gap is produced in the joint of the bars. Therefore, when the chips are divided using a cutting saw as described in Patent Document 2, since a large force is applied to the floating bar, the cracking of the chip or the peeling of the solder portion occurs. Even in a method for dividing after the bar is scratched using a needle-like scriber, since a high pressure is applied to the bar, the similar problem occurs. 
         [0011]    When a cutting saw is used, the chip must be cut while cooling the chip and the saw with water. Therefore, moisture invades in the gap in the joint between the bars after chip dividing, causing a problem wherein the dew point is not lowered after packaging. Also when a cutting saw is used, chips produced by chip dividing fly and adhere to the electrode or the end surface of the laser to produce dirt or scratches. 
         [0012]    Therefore, since the problem as described above was caused when the method according to Patent Document 2 was used to manufacture the three-wavelength semiconductor laser according to Patent Document 1, there was a problem wherein high reliability cannot be secured. 
         [0013]    To solve the problems as described above, it is an object of the present invention to provide a method for manufacturing a multiple-wavelength semiconductor laser that can accurately align two semiconductor lasers and can secure high reliability. 
         [0014]    According to one aspect of the present invention, a method for manufacturing a multiple-wavelength semiconductor laser comprises: forming a first bar having a plurality of arrayed first semiconductor chips wherein at least two semiconductor lasers of different wavelengths are monolithically formed; forming a second bar having a plurality of arrayed second semiconductor chips wherein a semiconductor laser having a different wavelength from the semiconductor lasers of said first semiconductor chips is formed; forming a third bar by allowing a laser-forming surface of said first bar to face a back surface of said second bar, and joining respective said first semiconductor chips in said first bar to respective said second semiconductor chips in said second bar; forming scribe lines by radiating laser beams on boundaries of said first semiconductor chips and on boundaries of said second semiconductor chips, respectively and dividing said third bar along said scribe lines into each chip. 
         [0015]    Other and further objects, features and advantages of the invention will appear more fully from the following description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0016]      FIG. 1  is a sectional view showing a multiple-wavelength semiconductor laser according to the first embodiment. 
           [0017]      FIGS. 2-16  are views for explaining a method of manufacturing the multiple-wavelength semiconductor laser according to the first embodiment. 
           [0018]      FIGS. 17-18  are views for explaining a method of manufacturing a multiple-wavelength semiconductor laser according to the second embodiment. 
           [0019]      FIG. 19  is a view for explaining a method of manufacturing a multiple-wavelength semiconductor laser according to the third embodiment. 
           [0020]      FIG. 20  is a view for explaining a method of manufacturing a multiple-wavelength semiconductor laser according to the fourth embodiment. 
           [0021]      FIG. 21  is a view for explaining a method of manufacturing a multiple-wavelength semiconductor laser according to the fifth embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     First Embodiment 
     Structure of Multiple-Wavelength Semiconductor Laser According to First Embodiment  
       [0022]      FIG. 1  is a sectional view showing a multiple-wavelength semiconductor laser according to the first embodiment. The multiple-wavelength semiconductor laser is a three-wavelength semiconductor laser formed by joining a two-wavelength semiconductor laser  10  and a blue semiconductor laser  12 . The two-wavelength semiconductor laser  10  is a semiconductor laser wherein a red semiconductor laser  14  and an infrared semiconductor laser  16  are monolithically formed. 
         [0023]    The red semiconductor laser  14  is an AlGaInP-based semiconductor laser. An n-type AlGaInP clad layer  20 , an active layer  22  having an InGaP/AlGaInP multiple quantum well structure, and a p-type AlGaInP clad layer  24  are sequentially formed on a GaAs substrate  18 . A ridge  26  is formed on the p-type AlGaInP clad layer  24 . An insulating film  28  is formed on the sides of the ridge  26  and on the p-type AlGaInP clad layer  24  on the both sides of the ridge  26 . A p-electrode  30  is formed on the ridge  26 . 
         [0024]    The infrared semiconductor laser  16  is an AlGaAs semiconductor laser. An n-type AlGaAs clad layer  32 , an active layer  34  having an AlGaAs/AlGaAs multiple quantum well structure, and a p-type AlGaAs clad layer  36  are sequentially formed on the GaAs substrate  18 . A ridge  38  is formed on the p-type AlGaAs clad layer  36 . An insulating film  40  is formed on the sides of the ridge  38  and on the p-type AlGaAs clad layer  36  on the both sides of the ridge  38 . A p-electrode  42  is formed on the ridge  38 . An n-electrode  44  common to the red semiconductor laser  14  and the infrared semiconductor laser  16  is formed on the back surface of the GaAs substrate  18 . 
         [0025]    The blue semiconductor laser  12  is a gallium-nitride-based semiconductor laser. An n-type AlGaN clad layer  48 , an active layer  50  having an undoped In x Ga 1-x N/In y Ga 1-y N multiple quantum well structure, and a p-type AlGaN clad layer  52  are sequentially formed on a GaN substrate  46 . A ridge  54  is formed on the p-type AlGaN clad layer  52 . An insulating film  56  is formed on the sides of the ridge  54  and on the p-type AlGaN clad layer  52  on the both sides of the ridge  54 . A P-electrode  58  is formed on the ride  54  and an n-electrode  60  is formed on the back surface of the GaN substrate  46 . 
         [0026]    On the n-electrode  60  on the back surface of the substrate of the blue semiconductor laser  12 , a first electrode  62  is directly formed on the red semiconductor laser  14  side, and a second electrode  66  is formed via an insulating layer  64  on the infrared semiconductor laser  16  side. The p-electrode  30  of the red semiconductor laser  14  is joined to the first electrode  62  via a solder  68 , and the p-electrode  42  of the infrared semiconductor laser  16  is joined to the second electrode  66  via a solder  70 . Contrary to this example, the first electrode  62  may be formed via the insulating layer  64  on the red semiconductor laser  14  side, and the second electrode  66  may be directly formed on the infrared semiconductor laser  16  side. 
         [0027]    The three-wavelength semiconductor laser, wherein the blue semiconductor laser  12  is joined to the two-wavelength semiconductor laser  10 , is die-bonded to a sub-mount with the p-electrode  58  side of the blue semiconductor laser  12  facing down, and is mounted to a package (not shown). The first electrode  62 , the second electrode  66 , and the n-electrode  44  are wire-bonded to the electrode pins of the package. The p-electrode  58  is wire-bonded to the electrode pins of the package via a metal layer on the sub-mount (not shown). 
         [0028]    To the blue semiconductor laser  12 , a driving current is supplied via the bonding wires of the p-electrode  58  and the first electrode  62 . To the red semiconductor laser  14 , the driving current is supplied via the bonding wires of the first electrode  62  and the n-electrode  44 . To the infrared semiconductor laser  16 , the driving current is supplied via the bonding wires of the second electrode  66  and the n-electrode  44 . 
       Method for Manufacturing Multiple-Wavelength Semiconductor Laser According to First Embodiment  
       [0029]    A method for manufacturing a multiple-wavelength semiconductor laser according to the first embodiment will be described. 
       Manufacture of Two-Wavelength Semiconductor Laser  
       [0030]    First, as shown in  FIG. 2 , an n-type AlGaAs clad layer  32 , an active layer  34  having an AlGaAs/AlGaAs multiple quantum well structure, and a p-type AlGaAs clad layer  36  are sequentially formed using metal organic chemical vapor deposition (MOCVD) on a GaAs substrate  18  whose surface is previously cleaned by thermal cleaning or the like. Next, a resist is applied on the entire surface of the wafer, and a resist pattern (not shown) of a shape corresponding to the left half of the drawing is formed by lithography. The right half of the stacked layers shown in the drawing is etched off using the resist pattern as a mask. 
         [0031]    Next, as shown in  FIG. 3 , an n-type AlGaInP clad layer  20 , an active layer  22  having an InGaP/AlGaInP multiple quantum well structure, and a p-type AlGaInP clad layer  24  are sequentially formed using the MOCVD method on the GaAs substrate  18 . 
         [0032]    Next, a resist is applied on the entire surface of the wafer, and a resist pattern (not shown) of a shape corresponding to the right half of the drawing is formed by lithography. The left half of the stacked n-type AlGaInP clad layer  20 , the active layer  22 , and the p-type AlGaInP clad layer  24  as shown in  FIG. 4  is etched off using the resist pattern as a mask. 
         [0033]    Next, a resist is applied on the entire surface of the wafer, and a resist pattern (not shown) of a shape corresponding to the shape of the mesa portion is formed by lithography. The p-type AlGaAs clad layer  36  and the p-type AlGaInP clad layer  24  are etched by RIE using the resist pattern as a mask. Thereby, as shown in  FIG. 5 , ridges  26  and  38  to be a light waveguide structure are formed. 
         [0034]    Next, leaving the resist pattern (not shown) used as the mask, an insulating film composed of SiO 2  is formed on the entire surface of the substrate by, for example, CVD, vacuum vapor deposition, or sputtering, and the insulating films on the ridges  26  and  38  are removed, or lift off, at the same time of resist removal. Thereby, as shown in  FIG. 6 , insulating films  28  and  40  each having an opening are formed on the ridges  26  and  38 . 
         [0035]    Next, after sequentially forming a Ti film and an Au film on the entire surface of the wafer by, for example, vacuum vapor deposition, resist application, lithography, and wet etching or dry etching are performed to form the p-electrode  30  of the red semiconductor laser  14  and the p-electrode  42  of the infrared semiconductor laser  16  on the laser forming surface of the two-wavelength semiconductor laser  10 . Next, AuGe and Au films are sequentially formed in the back surface of the substrate by vacuum vapor deposition to form the n-electrode  44 . 
         [0036]    Through the above-described wafer processing, semiconductor chips (first semiconductor chips), wherein two-wavelength semiconductor lasers  10  having red semiconductor lasers  14  and infrared semiconductor lasers  16  having different wavelengths are formed in lines on the wafer  72 , are formed. Next, as shown in  FIG. 7 , the wafer  72  is divided by cleaving to form first bars  74  wherein a plurality of semiconductor chips of the two-wavelength semiconductor lasers  10  are arrayed. By cleaving, the end surfaces of the laser having no cracks and steps can be formed. 
         [0037]    Next, as shown in  FIG. 8 , a plurality of the first bars  74  are fixed by a jig and placed in a vacuum apparatus with the end surfaces of the laser turned up. Then, coating films are formed on the front and rear ends of the laser by vacuum vapor deposition or sputtering. 
       Manufacture of Blue Semiconductor Laser  
       [0038]    First, as shown in  FIG. 9 , an n-type AlGaN clad layer  48 , an active layer  50  having an undoped In x Ga 1-x N/In y Ga 1-y N multiple quantum well structure, and a p-type AlGaN clad layer  52  are sequentially formed on a GaN substrate  46  whose surface is previously cleaned by thermal cleaning or the like by the MOCVD method. For example, the growing temperature of the n-type AlGaN clad layer  48  is 1000° C., the growing temperature of the active layer  50  is 740° C., and the growing temperature of the p-type AlGaN clad layer  52  is 1000° C. 
         [0039]    Next, a resist is applied on the entire surface of the wafer, and a resist pattern (not shown) of a shape corresponding to the shape of the mesa portion is formed by lithography. The p-type AlGaN clad layer  52  is etched by, for example, RIE using the resist pattern as a mask. Thereby, as shown in  FIG. 10 , a ridge  54  to be a light waveguide structure is formed. 
         [0040]    Next, leaving the resist pattern (not shown) used as the mask, an insulating film composed of SiO 2  is formed again on the entire surface of the substrate by, for example, CVD, vacuum vapor deposition, or sputtering, and the insulating film on the ridge  54  is removed, or lift off, at the same time of resist removal. Thereby, as shown in  FIG. 11 , insulating film  56  having an opening is formed on the ridge  54 . 
         [0041]    Next, after sequentially forming a Pd film, a Ta film, and an Au film on the entire surface of the wafer by, for example, vacuum vapor deposition, resist application, lithography, and wet etching or dry etching are performed to form the p-electrode  58 . Next, Ti and Au films are sequentially formed in the back surface of the substrate by vacuum vapor deposition to form the n-electrode  60 . On the n-electrode  60  on the back surface of the substrate of the blue semiconductor laser  12 , the electrically isolated first electrode  62  and second electrode  66  are formed. 
         [0042]    Through the above-described wafer processing, semiconductor chips (second semiconductor chips), a blue semiconductor laser  12 , having different wavelength from red semiconductor lasers  14  and infrared semiconductor lasers  16  are formed in lines on the wafer  76 . Next, as shown in  FIG. 12 , the wafer  76  is divided by cleaving to form second bars  78  wherein a plurality of semiconductor chips of the blue semiconductor lasers  12  are arrayed. In the same manner as in the two-wavelength semiconductor lasers  10 , coating films are formed on the front and rear ends of the laser. 
       Manufacture of Three-Wavelength Semiconductor Laser  
       [0043]    First, as shown in  FIG. 13 , the first bar  74  is placed on the heating table  80  of the bonding device using vacuum contact. Next, the second bar  78  is allowed to vacuum-contact to collets  82 . Using an alignment marks  84  formed on the ends of the first bar  74  and the second bar  78 , the second bar  78  is moved onto the first bar  74  to allow the laser forming surface of the first bar  74  to face to the back surface of the substrate-of the second bar  78  so that the laser forming surface of the two-wavelength semiconductor laser  10  is aligned to the laser forming surface of the blue semiconductor laser  12  in the same plane. Then, each chip in the first bar  74  is joined to each chip in the second bar  78  to form a third bar  86 . At this time, the p-electrode  30  of the red semiconductor laser  14  and the p-electrode  42  of the infrared semiconductor laser  16  are joined to the first electrode  62  and the second electrode  66  of the blue semiconductor laser  12  with solders  68  and  70 , respectively. Alternatively, the electrode pattern of the semiconductor lasers may be used as the marks in place of the alignment marks  84 . 
         [0044]    Next, as shown in  FIG. 14 , the third bar  86  is adhered to the tape  88  with the blue semiconductor laser  12  up. Then, laser beams  90  are radiated to the boundary of chips of the blue semiconductor laser  12  to form a scribe line  92 . 
         [0045]    Next, as shown in  FIG. 15 , the third bar  86  is adhered to the tape  94  with the blue semiconductor laser  12  down, and the tape  88  is removed. Then, laser beams  90  are radiated to the boundary of chips of the two-wavelength semiconductor laser  10  (the position facing to the scribe line  92  of the blue semiconductor laser  12 ) to form a scribe line  96 . Next, as shown in  FIG. 16 , the tape  94  is expanded to divide the third bar  86  along the scribe lines  92  and  96  into chips. Finally, the chips are removed from the tape  94  to complete a three-wavelength semiconductor laser. 
       Effect of the First Embodiment  
       [0046]    In the first embodiment, after joining a blue semiconductor laser to a two-wavelength semiconductor laser in a bar state, they are divided into chips. Therefore, compared with conventional methods wherein a blue semiconductor laser in a chip state is joined to a two-wavelength semiconductor laser in a chip state, two semiconductor lasers can be precisely aligned. Since a large number of chips can be joined at once, the process can be simplified and productivity can be improved. 
         [0047]    By using a laser scriber, no pressure is applied to the bar floating in the space, compared with conventional methods for dividing chips using a cutting saw or a needle-shaped scriber, which scratches the bar. Therefore, the cracking of the chip or the peeling off of the soldered portion can be prevented. Since no cutting saw is used, there is no possibility of moisture invasion into the gap in the boundaries of bars. Furthermore, since the tape is expanded in the state wherein the bars are adhered to the tape, the dispersion of chips is little and the dirt or scratches of the electrode or the end surface of the laser is few. Therefore, according to the manufacturing method of the first embodiment, high reliability can be secured. 
       Second Embodiment 
       [0048]    In the second embodiment, optical system that can displace the focal point of laser beams in laser scribing is used. Other processes are the same as the process in the first embodiment. The laser scribing process in the second embodiment will be described. 
         [0049]    First, as shown in  FIG. 17 , the tape  94  is adhered to the third bar  86  with the blue semiconductor laser  12  up. Then, laser beams are focused on the two-wavelength semiconductor laser  10 , and the laser beams are radiated to the boundary of chips of the two-wavelength semiconductor laser  10  of the third bar  86  to form a scribe line  96 . At this time, although the blue semiconductor laser  12  is exposed to laser beams, the blue semiconductor laser  12  is not damaged because the laser beams are out of focus. 
         [0050]    Next, as shown in  FIG. 18 , without rebonding the tape  94 , the focal point of the laser beams is displaced to the blue semiconductor laser  12 , and the laser beams are radiated on the boundaries of chips of the blue semiconductor laser  12  of the third bar  86  to form the scribe line  92 . Thereafter, in the same manner as in the first embodiment, the tape  94  is expanded to divide the bar into chips. 
         [0051]    In the second embodiment, since the rebonding of the tape is not required unlike the first embodiment, the process can be simplified. 
       Third Embodiment 
       [0052]    When the first bar  74  is joined to the second bar  78 , if the pressure for bonding is adjusted only by the collet  82 , the parallelism and pressure of the both ends of the first bar  74  and the second bar  78  are deviated. In the third embodiment, therefore, spacers  98  are inserted in the both ends of the first bar  74  and the second bar  78  to maintain the distance between the first bar  74  and the second bar  78  constant as shown in  FIG. 19 . Thereby, the both ends of the first bar  74  and the second bar  78  become parallel, and the deviation of the pressure is eliminated. Other processes and effects are same as those of the first embodiment. 
       Fourth Embodiment 
       [0053]    If the cavity length of the two-wavelength semiconductor laser  10  is equalized to the cavity length of the blue semiconductor laser  12 , wire bonding to the first electrode  62  and the second electrode  66  has to be performed in the narrow gap region between the blue semiconductor laser  12  and the two-wavelength semiconductor laser  10 . In the fourth embodiment, therefore, the cavity length of the two-wavelength semiconductor laser  10  is made shorter than the cavity length of the blue semiconductor laser  12  as shown in  FIG. 20 , and a wire bonding region is provided in the substrate side of the blue semiconductor laser  12 . Then, wire bonding is performed to the first electrode  62  and the second electrode  66  of the blue semiconductor laser  12 , respectively. Thereby, wire bonding can be easily performed, and productivity is improved. If plated layers are formed on the first electrode  62  and the second electrode  66 , the current capacity is increased, and the adhesiveness of wire bonding is improved. 
       Fifth Embodiment 
       [0054]    In the fifth embodiment, as shown in  FIG. 21 , the cavity length of the two-wavelength semiconductor laser  10  is made longer than the cavity length of the blue semiconductor laser  12 , and a wire bonding region is provided in the ridge side of the two-wavelength semiconductor laser  10 . Then, wire bonding is performed to the p-electrode  30  of the red semiconductor laser  14  and the p-electrode  42  of the infrared semiconductor laser  16 , respectively. In this case, pad regions for wire bonding have to be formed on the p-electrodes  30  and  42 . Thereby, the equivalent effects as the effects of the fourth embodiment can be obtained. 
         [0055]    Although an example of a three-wavelength laser of blue, red, and infrared has been shown in the embodiments described above, the present invention can also be applied to other multiple-wavelength lasers. 
         [0056]    Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described. 
         [0057]    The entire disclosure of a Japanese Patent Application No. 2009-012995, filed on Jan. 23, 2009 including specification, claims, drawings and summary, on which the Convention priority of the present application is based, are incorporated herein by reference in its entirety.