Abstract:
In a semiconductor laser diode array, first electrodes of a laser chip are coated with an insulating substance, and contact holes are formed in the insulating substance. The laser chip is assembled by being secured on a submount while facing downward, wherein electrodes and a solder pattern are provided in the submount in a direction crossing resonators.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a semiconductor laser light source used as a light source for a laser printer or an optical disk drive, and particularly relates to a semiconductor laser diode array having plural light-emitting points provided on a single element. 
     2. Description of the Related Art 
     In order to perform face down assembly, which is advantageous in heat radiation properties, JP-A-Hei. 7-22708 discloses a semiconductor laser diode array shown in  FIG. 19 . Specifically, stripe-shaped electrodes corresponding to laser resonators provided in a semiconductor laser chip and stripe-shaped electrodes  4 ,  5 , and  6  provided on a front face of a part, which holds the laser chip and is referred to as a submount, are bonded together by means of a solder layer provided on the electrodes of the submount, to thereby electrically connects the electrodes and physical fixes the laser chip. 
     Also, JP-A-Hei. 6-97583 discloses a structure including electrodes provided in parallel to laser resonators as with the above case and submount-side electrodes provided correspondingly. Grooves are formed in the laser chip to reduce spreading of solder, which is caused when the laser chip and the submount are bonded. 
     SUMMARY OF THE INVENTION 
     In order to bond the submount and the laser chip surely, in JP-A-Hei. 7-22708 and JP-A-Hei. 97583, the solder layer has several micrometers in thickness. During the boding process, the laser chip is pressed against the submount, which is heated at temperatures higher than the melting point of the solder, to bond them. However, at this time, a positional displacement arises in the laser chip, and the fused solder spreads because the laser chip is pressed against the solder. Because of these phenomena, an interval between arrayed lasers at which it is possible to assemble the laser array with superior yield is up to 50 μm or thereabouts. 
     According to one embodiment of the invention, a semiconductor laser diode array includes a semiconductor chip, and a submount that has a plurality of submount electrodes. The semiconductor chip includes a plurality of stripe-shaped optical resonators, a plurality of first electrodes, an insulation film, and a plurality of second electrodes. The stripe-shaped optical resonators emit light beams when current flows therethrough. The optical resonators are arranged at predetermined intervals on the semiconductor chip. The first electrodes extend in a direction of stripes of the optical resonators. Each first electrode covers each optical resonator. The first electrodes are separated from each other. The insulation film covers the first electrodes. The second electrodes cover the optical resonators through the insulation film, respectively. One second electrode is electrically separated from another second electrode formed above an adjacent optical resonator. A part of the insulation film above the optical resonators defines contact holes through which the first electrodes and the second electrodes are electrically connected with each other. The contact holes are arranged so that at least a part of the contact holes are electrically connected to one of the submount electrodes. 
     Also, according to one embodiment of the invention, a semiconductor laser diode array includes a semiconductor chip, and a submount having a plurality of submount electrodes. The semiconductor chip includes a plurality of stripe-shaped optical resonators, a plurality of electrodes, an insulation film, and a low-melting glass layer. The stripe-shaped optical resonators emit light beams when current flows therethrough. The optical resonators are arranged at predetermined intervals on the semiconductor chip. The electrodes extend in a direction of stripes of the optical resonators. Each electrode covers each optical resonator. The electrodes are separated from each other. The insulation film covers the electrodes. The low-melting glass layer covers the optical resonators through the insulation film. A part of the low-melting glass layer above the optical resonators defines contact holes through which the electrodes are exposed. At least a part of the contact holes are electrically connected to one of the submount electrodes. 
     According to the structures described above, it becomes possible to assemble in a facedown manner a multi-element arrayed laser, which has three or more elements at intervals as narrow as 50 μm or less, with superior reproducibility. Also, the structure described above results in that instability of an optical output related to heat discharge characteristic of elements, such as droop characteristic or crosstalk characteristic, can be reduced to several percent or less. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view showing a manufacturing process  1  of a first embodiment; 
         FIG. 2  is a view showing a manufacturing process  2  of the first embodiment; 
         FIG. 3  is a view showing a manufacturing process  3  of the first embodiment; 
         FIG. 4  is a view showing a manufacturing process  4  of the first embodiment; 
         FIG. 5  is a lateral section view of a laser chip of the first embodiment (taken along A-A shown in  FIG. 4 ); 
         FIG. 6  is a longitudinal section view of a laser chip of the first embodiment (taken along B-B shown in  FIG. 4 ); 
         FIG. 7  is a top oblique perspective view of the laser chip of the first embodiment; 
         FIG. 8  is a block diagram of a submount; 
         FIGS. 9A and 9B  are views showing assembly processes of the first embodiment; 
         FIG. 10  is a view showing a manufacturing process  1  of a second embodiment; 
         FIG. 11  is a view showing a manufacturing process  2  of the second embodiment; 
         FIG. 12  is a view showing a manufacturing process  3  of the second embodiment; 
         FIG. 13  is a lateral section view of a laser chip of the second embodiment (taken along A-A shown in  FIG. 4 ); 
         FIGS. 14A and 14B  are views showing assembly processes of the second embodiment; 
         FIGS. 15A and 15B  are views showing assembly processes of a third embodiment; 
         FIG. 16  is a section view of a spread area according to a fourth embodiment; 
         FIG. 17  is a section view of a non-spread area of the fourth embodiment; 
         FIG. 18  is a top view of spread and non-spread areas of the fourth embodiment; 
         FIG. 19  is a block diagram of a semiconductor laser according to prior art; and 
         FIGS. 20A and 20B  are views showing assembly processes of a modification of the third embodiment; 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the invention will be described hereinbelow in detail by reference to drawings. 
     First Embodiment 
     A first embodiment of the invention will be described by reference to the drawings. First, process for fabricating a semiconductor laser chip of this embodiment will be described. In  FIGS. 1 to 6 , reference numeral  101  designates an n-type GaAs substrate. The surface orientation of this n-type GaAs substrate  101  is offset from a ( 100 ) face toward a ( 110 ) orientation by about 10 degrees. An n-type (Al 0.7 Ga 0.3 ) 0.5 In 0.5 P cladding layer  102  (Se-doped, p=1×10 18  cm −3 , 1.8 μmin thickness), a multiquantum well active layer  103 , a p-type (Al 0.7 Ga 0.3 ) 0.5 In 0.5 P cladding layer  104  (Zn-doped, p=7×10 17  cm −3 , 1.6 μm in thickness), and a p-type GaAs cap layer  105  having a thickness of about 0.2 μm (Zn-doped, p=1×10 19  cm −3 , 0.2 μm in thickness) are sequentially formed on the substrate through crystal growth. The multiquantum well active layer  103  is formed of four Ga 0.5 In 0.5  P-well layers  106  (each layer having a thickness of 7 nm) and five (Al 0.5 Ga 0.5 ) 0.5 In 0.5  P barrier layers  107  (each layer having a thickness of 4 nm), wherein each well layer  106  is sandwiched between the barrier layers  107 . The P-type GaAs cap layer  105  and the p-type (Al 0.7 Ga 0.3 ) 0.5 In 0.5 P cladding layer  104  are chemically removed to a thickness of 0.3 μm or thereabouts, except for a stripe-shaped region  108 , which has a width of 4 μm and will become a waveguide. The remaining cladding layer  104  is covered with a silicon nitride film  109  of a thickness of about 0.1 μm. In the embodiment, five stripe-shaped waveguides  108  are formed at intervals of 20 μm in a (1, −1, 0) orientation in a single semiconductor laser chip. The top silicon nitride film  109  laid on the top p-type GaAs cap layer  105  of the stripe-shaped regions  108  is removed.  FIG. 1  is a top view of a single chip in this state. 
     As shown in  FIG. 2 , first electrodes  110  having Au as a main component are formed along the respective stripe-shaped waveguides  108 . Then, an aluminum nitride film  111  of a thickness of 0.3 μm is deposited on the wafer shown in  FIG. 2 . At this time, a two-layer film consisting of a zinc oxide film (0.4 μm) and a silicon oxide film (0.05 μm) has been formed in advance in portions of the first electrodes  110  of the respective stripe-shaped waveguides. After deposition of the aluminum nitride film  111 , this two-layer film can be removed together with the aluminum nitride film deposited on the two-layer film, by means of a hydrochloric-acid-based acid. As a result, contact holes  112 , through which a part of the respective first electrodes  110  are exposed, are defined in a part of the aluminum nitride film  111 .  FIG. 3  is a top view of the wafer in this phase. Next, electrodes consisting of titanium, platinum, and gold are evaporated onto the surface of the wafer in such a state as shown in  FIG. 3 , and second electrodes  113  arranged in a matrix with five rows and three columns as shown in  FIG. 4  are formed by means of photolithography and ion milling techniques.  FIGS. 5 and 6  are section views of the stripe-shaped waveguides of the semiconductor laser fabricated through the above-described steps taken along a line perpendicular to the stripe shape waveguide and taken along a line parallel to the stripe-shaped waveguides, respectively. After the principal structure of the semiconductor laser has been formed on the GaAs substrate through the above-described processes, the substrate is rubbed to a thickness of about 100 μm and back electrodes  114  are formed. Subsequently, semiconductor laser chips  116  are cleaved to separate the laser chips  116  so that each laser chip  116  has a mirror surface  115  perpendicular to a stripe-shaped resonator. A silicon oxide film is formed on the mirror surfaces  115  for protecting end faces, to thus complete the laser chips.  FIG. 7  is a top oblique perspective view of the completed laser chip. 
     On the other hand, a submount used for fixing the semiconductor laser chip is formed by laying an electrode layer  118  consisting of titanium, platinum, and gold on an aluminum nitride substrate  117 , and further placing a solder layer  119  (an alloy consisting of gold and tin) on a part of the electrode layer  118 . The submount has a structure shown in  FIG. 8 ; namely, has five stripe-shaped electrodes  120  and bonding pads  121  used for connecting the electrodes  120  with wires. 
     The stripe-shaped electrodes  120  have a width of 80 μm and are spaced at intervals of 20 μm.  FIG. 9  shows specifications required to fix the semiconductor laser chip on the submount. The semiconductor laser chips  116  are secured to the submount while surfaces thereof (surfaces having undergone crystal growth) face downward. After determination of a position of the semiconductor laser chip  116 , the submount is heated up to the melting point of solder, thereby fixing the semiconductor laser chips onto the submount. A positional relationship among the electrodes  120  of the submount, the solder patterns  119  of the submount, the second electrodes  113  of the semiconductor laser chip  116 , and the contact holes  112  of the semiconductor chips is determined so as to have a layout shown in  FIGS. 9A and 9B . 
     Although the aluminum nitride film  111  is not fused with solder, the second electrodes  113  formed on the aluminum nitride  111  are alloyed with solder. Therefore, the semiconductor laser chips  116  are physically fixed at fifteen positions. Moreover, the second electrodes  113  have a function of dissipating the heat having developed in the stripe-shaped waveguides  108  due to current flow to the submount by way of the aluminum nitride film  111  and the silicon nitride film, both of which possess high thermal conductivity. The contact holes  112  are formed at a single position in each first electrode  110 . Current flows from the second electrodes  113  to the first electrodes  110  by way of the contact holes  112 , to thereby supply current to the overall waveguides  108 . Each of five laser resonators of the thus-fabricated semiconductor laser oscillates at a wavelength of about 650 nm, and a threshold current of the laser resonators is about 10 mA. By virtue of the heat-dissipating effect of the second electrodes, the droop characteristic and crosstalk characteristic of the semiconductor lasers show superior values of 3% or less, demonstrating that this laser is suitable for use with a laser printer or an optical disk drive. 
     Second Embodiment 
     A second embodiment of the invention will be described by reference to the drawings. First, a structure of a semiconductor laser chip according to this embodiment will be described in terms of manufacturing steps. As shown in  FIG. 13 , in this embodiment, an n-type Al 0.7 Ga 0.3 As cladding layer  202  (Se-doped, p=1×10 18  cm −3 , 1.8 μm in thickness), a multiquantum well active layer  203 , a p-type Al 0.7 Ga 0.3 As cladding layer  204  (Zn-doped, p=7×10 17  cm −3 , 1.6 μm in thickness), and a p-type GaAs cap layer  105  (Zn-doped, p=1×10 19  cm −3 , 0.2 μm in thickness) are sequentially formed on an n-type GaAs substrate  201  through crystal growth. The multiquantum well active layer  203  is formed of four Ga 0.5 Al 0.5 As well layers  205  (each layer having a thickness of 7 nm) and five Al 0.7 Ga 0.3 As barrier layers  206  (each layer having a thickness of 4 nm), wherein each well layer  205  is sandwiched between the barrier layers  206 . The P-type GaAs cap layer  105  and the p-type Al 0.7 Ga 0.3 As cladding layer  204  are chemically removed to a thickness of 0.3 μm or thereabouts except for stripe-shaped regions  108 , which have 4 μm in width and will become waveguides. The remained cladding layer  204  is covered with a silicon nitride film  207  of a thickness of about 0.1 μm. In the embodiment, the five stripe-shaped waveguides  108  are formed at intervals of 20 μm in a (1, −1, 0) orientation in a single semiconductor laser chip. The silicon oxide film  207  on the top p-type GaAs cap layer  105  of the stripe-shaped regions  108  is removed.  FIG. 10  is a top view of a single chip in this state. 
     As shown in  FIG. 11 , first electrodes  110  having Au as a main component are formed along the respective stripe-shaped waveguides  108 . A diamond film  208  of a thickness of 0.3 μm and low-melting glass  209  (boron-oxide/lead-oxide-based) having a thickness of 0.05 μm are deposited on the wafer shown in  FIG. 10 , by means of the laser abrasion method. The low-melting glass includes B 2 O 3  and PbO as main proportions, and has a softening temperature in a range of 310° C. to 500° C. At this time, a two-layer film consisting of a zinc oxide film (0.4 μm) and a silicon oxide film (0.05 μm) has been formed in advance at a part of the first electrodes  110  of the respective stripe-shaped waveguides  108 . After deposition of the diamond film  208  and the low-melting glass film  209 , the two-layer film can be removed together with the diamond film  208  and the low-melting glass film  209  deposited thereon, by means of a hydrochloric-acid-based acid. As a result, contact holes  112 , through which a part of the first electrodes  110  are exposed, are defined in a part of the diamond film  208  and low-melting glass film  209 .  FIG. 12  is a top view of the wafer in this phase.  FIG. 13  shows a section profile of the stripe-shaped waveguides  108  of the semiconductor laser fabricated through the above-described processes taken along a line perpendicular to the waveguides  108 . After the principal structure of the semiconductor laser has been formed on the GaAs substrate through the above-described processes, the GaAs substrate is rubbed to a thickness of about 100 μm. Then, form back electrodes  114  are formed. Subsequently, the semiconductor laser is cleaved to separate the semiconductor laser chips  116  so that each semiconductor laser chip  116  has a mirror surface  115  perpendicular to the stripe-shaped resonator. A silicon oxide film is formed on the mirror surfaces  115  for protecting end faces, to thus complete the laser chips  116 . 
     The semiconductor laser chips  116  are fixed to the submount having a structure analogous to that described in the first embodiment. The semiconductor laser chips  116  are secured to the submount while the surfaces thereof (the surfaces having undergone crystal growth) face downward. After determination of a position of the semiconductor laser chip  116 , the submount is heated up to the melting point of the low-melting glass  209  (about 420°), whereupon the first electrodes  110  are alloyed and fused with solder in the contact holes  112 . In the mean time, in areas where the low-melting glass  209  contacts solder, a superior mechanical and thermal junction is achieved by means of the fused solder and the low-melting glass  209 . Accordingly, an element having the same characteristics can be realized by omission of the step for forming the second electrodes  113 . 
     The five laser resonators of the thus-fabricated semiconductor laser oscillate at a wavelength of about 780 nm, and the threshold current of the laser resonators is about 10 mA. By means of the heat-dissipating effect of the low-melting glass fused with the solder layer, the droop and crosstalk characteristics of the semiconductor laser show superior values of 3% or less, demonstrating that this laser is suitable for use with a laser printer and an optical disk drive. 
     Third Embodiment 
     An a third embodiment of the invention, an example will be described in which  20  laser resonators are formed at intervals of 20 μm. The basic structure of this element is identical with that of the first embodiment. Since a plurality of array elements must be integrated, the layout of the second electrodes  113  and the contact holes  112  of the laser chip is set as shown in  FIG. 15 . On the other hand, the electrodes  118  of the submount are arranged as shown in  FIG. 15 . As a result,  20  resonators can be integrated within a chip having a width of 600 μm and a length of 300 μm. 
     In the third embodiment, the second electrodes  113  are separated from each other and are arranged in a matrix form. The third embodiment may be modified as shown in  FIG. 20 . Specifically, the second electrodes  113  may extend laterally in the same manner as patterns formed on the submount. 
     Fourth Embodiment 
     For the purpose of reducing operation current of a semiconductor laser, a fourth embodiment of the invention will be described in which a part of semiconductor resonators is formed of transparent waveguides. First, as in the case of the first embodiment, the n-type (Al 0.7 Ga 0.3 ) 0.5 In 0.5 P cladding layer  102  (Se-doped, p=1×10 18  cm −3 , 1.8 μm in thickness), the multiquantum well active layer  103 , the p-type (Al 0.7 Ga 0.3 ) 0.5 In 0.5 P cladding layer  104  (Zn-doped, p=7×10 17  cm −3 , 1.6 μm in thickness), and the p-type GaAs cap layer  105  having a thickness of about 0.2 μm (Zn-doped, p=1×10 19  cm −3 , 0.2 μm in thickness) are sequentially formed on the n-type GaAs substrate  101  through crystal growth. The multiquantum well active layer  103  is formed of the four Ga 0.5 In 0.5  P-well layers  106  (each layer having a thickness of 7 nm) and the five (Al 0.5 Ga 0.5 ) 0.5 In 0.5 P barrier layers  107  (each layer having a thickness of 4 nm), wherein each well layer  106  is sandwiched between the barrier layers  107 . A silicon oxide film is formed for forming stripe-shaped waveguides. Thereafter, zinc is diffused in an area the laser chip about 70 μm to form a transparent waveguide structure in such a way that zinc oxide  401  is used as a source. 
       FIGS. 16 and 17  show the section structure of a diffused region  402  and that of a non-diffused region  403 . In the diffused region  402 , the multiquantum well active layer  103  turns into mixed crystal as a result of diffusing zinc, thereby becoming transparent to a laser beam. After removal of zinc oxide, the P-type GaAs cap layer  105  and the p-type (Al 0.7 Ga 0.3 ) 0.5 In0.5P cladding layer  104  are removed to a thickness of 0.3 μm or thereabouts by means of chemical etching except for the stripe-shaped regions  108 , which have 4 μm in width and will become waveguide. The remained cladding layer  104  is covered with the silicon nitride film  109  of a thickness of about 0.1 μm. Then, the silicon nitride film  109  on the top p-type GaAs cap layer  105  in the stripe-shaped region  108  is removed in only the region  402  that has not been subjected to diffusion of zinc. 
       FIG. 18  is a top view of one chip in the above-described status. Fabrication processes and processes for assembling a laser chip, which are subsequent to formation of the first electrode, are made identical with those described in the first embodiment. In this embodiment, since the current injection region is limited to about half the chip length, current required to cause the active layer to generate a gain is small. In the meantime, an optical loss in the waveguides that have been made transparent is small. Hence, the semiconductor laser can oscillate at a drive current, which is about half that required to oscillate the semiconductor laser of the first embodiment. Therefore, the amount of heat developing in the overall element is small, and an element having a droop characteristic of 1% or less and a crosstalk characteristic of 1% or less can be implemented. Moreover, according to this embodiment, the number of processes can be decreased in comparison with that required in the first embodiment. Moreover, the interval between the stripes can be made smaller.