JP3945399B2 - Semiconductor laser and manufacturing method thereof - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a highly reliable semiconductor laser and a method for forming the same, and more particularly to an electrode structure that relieves thermal stress during assembly.
[0002]
[Prior art]
In recent years, there has been a remarkable spread of CDs and DVDs, and among them, the demand for recording equipment is rapidly increasing. In this case, unlike the read-only case, it is necessary to increase the output of the semiconductor laser as a light source, and in order to improve the heat dissipation characteristics of the semiconductor laser, a submount substrate using Si or SiC having excellent thermal conductivity is used. A structure in which a semiconductor laser is mounted thereon and bonded as a heat sink has been generally used (see, for example, Patent Document 1).
[0003]
However, if the size of the submount substrate is increased in order to further improve the heat dissipation characteristics, or if the bonding area between the semiconductor laser and the submount substrate is increased, the thermal stress due to the difference in thermal expansion coefficient in the heating / bonding process is increased. It is known that it is generated and applied as a residual stress to the semiconductor laser during cooling. Also, when heating and bonding the submount substrate and the semiconductor laser via an adhesive such as solder, the collet is held in pressure contact with the semiconductor laser in order to prevent misalignment. Is further added as residual stress in the cooling process after joining (see, for example, Patent Document 1).
[0004]
Therefore, Patent Document 1 describes a method of releasing the collet that holds the semiconductor laser after heating and bonding in order to remove the residual stress caused by these steps and heating again.
[0005]
[Patent Document 1]
JP 2002-217480 A (4th page, FIG. 1)
[0006]
[Problems to be solved by the invention]
When the semiconductor laser is operated with the residual stress applied, crystal dislocation may occur and the characteristics may be deteriorated. Therefore, the above method is a very effective technique for realizing a high-power semiconductor laser.
[0007]
However, in the joining process, the normal cycle of heating → cooling → heating → cooling must be carried out twice, and there is a problem that throughput is lowered.
[0008]
Also, AlGaInP semiconductor lasers emitting red light used as light sources for DVDs are more prone to crystal defects than AlGaAs semiconductor lasers emitting infrared light, and it is advantageous in terms of characteristics to join them without applying stress as much as possible. It is expected that there is a high possibility of working.
[0009]
Accordingly, in order to solve the above-described problems, the present invention provides a highly reliable semiconductor laser that has an electrode embedded with an elastic material and is less susceptible to crystal defects by absorbing and relaxing strain applied by thermal stress. The purpose is to provide.
[0010]
[Means for Solving the Problems]
The semiconductor laser of the present invention is a semiconductor laser having electrodes provided on the upper surface or lower surface, Ri structures der which the elastic material is embedded at least one of of said electrodes, said elastic material, said electrodes The first layer and the second layer are embedded between the first layer and the second layer, and the first layer and the second layer cover the entire predetermined region including the light emitting region on the surface of the semiconductor laser. The elastic material is formed and is not disposed immediately above the light emitting region .
[0011]
The elastic material is preferably arranged in the electrode in a dot shape or a lattice shape with a predetermined interval.
[0012]
The elastic material is preferably a rubber-based resist.
[0013]
With the above configuration, it is possible to relieve the mechanical stress applied from the collet during heating and bonding with the submount substrate, and to reduce the residual stress applied to the semiconductor laser. Further, the residual stress can be further reduced by absorbing the strain at the bonding interface caused by the thermal stress.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, specific embodiments of the present invention will be described.
[0015]
(Embodiment 1)
FIG. 1 shows a cross-sectional structure of a semiconductor laser according to Embodiment 1 of the present invention.
[0016]
On the n-type GaAs substrate 1, an n-type GaAs buffer layer 2, an n-type AlGaInP cladding layer 3, a GaInP / AlGaInP strained quantum well active layer 4, a p-type AlGaInP first cladding layer 5, and a p-type GaInP etching blocking layer 6 are sequentially arranged. After the p-type AlGaInP second cladding layer 7 and the p-type GaInP cap layer 8 are further grown, a silicon oxide film is deposited and processed into a stripe pattern. Using the silicon oxide film processed into the stripe pattern as a mask, the p-type AlGaInP second cladding layer 7 and the p-type GaInP cap layer 8 are etched to the p-type GaInP etching stop layer 6 to form a ridge-like stripe pattern. Next, the n-type GaAs current confinement layer 9 is selectively grown. After removing the silicon oxide film, a p-type GaAs contact layer 10 is grown on the entire surface, Au / Ge / Ni is deposited on the back surface of the n-type GaAs substrate 1 to form an n-type electrode 11, and the p-type GaAs contact layer 10 A p-type electrode 12 is formed on the upper surface.
[0017]
The present embodiment is different from the prior art in that a negative resist pattern 15 that is an elastic material is embedded in the p-type electrode 12.
[0018]
FIG. 2 shows a comparison between the conventional technique and the electrode forming flow of the present invention. Conventionally, Cr / Au / Pt / Ti / Au / Pt thin films are continuously deposited to form electrodes, and then Au plating and solder plating are formed. On the other hand, in the flow of the present invention, after depositing the Cr / Au / Pt laminated film 13, a negative resist was applied to the surface of the Pt film, exposure was performed with a mask aligner, and a resist pattern was formed through a development process. The negative resist used was OMR-83 40 cp manufactured by Tokyo Ohka Kogyo Co., Ltd., and the coating thickness was about 1 μm. In the resist pattern, 10 μmφ size dot patterns were evenly arranged at intervals of 10 μm in the entire region excluding the region immediately above the stripe portion of the strained quantum well active layer 4 and the chip end.
[0019]
FIG. 3 shows a plan layout of the resist pattern. The reason why the pattern is not formed in the region immediately above the stripe portion of the strained quantum well active layer 4 which is the light emitting region is to secure a current path injected into the active layer and not increase the element resistance. This can prevent an increase in threshold value and power consumption.
[0020]
After the pattern formation, the resist was baked by heat treatment at about 200 ° C. to evaporate excess solvent, thereby removing the influence of contamination by impurities contained in the resist. Thereafter, a Ti / Au / Pt laminated film 14 was deposited, Au plating and solder plating 16 were formed as in the conventional flow, and the heat sink and the semiconductor laser 17 were joined via the solder plating 16.
[0021]
FIG. 4 is an explanatory diagram of the bonding process between the heat sink and the semiconductor laser.
[0022]
The semiconductor laser 17 having Au plating and solder plating 16 formed on the p-type electrode 12 is adsorbed and held by the collet 18 with the p-type electrode 12 facing down (FIG. 4A). The semiconductor laser 17 is transferred onto the SiC substrate 19 serving as a heat sink while being held by the collet 18, and alignment is performed so as to come to a predetermined position of the SiC substrate 19 (FIG. 4B). Next, the collet 18 is moved to place the semiconductor laser 17 on the SiC substrate 19 through the solder plating 16. Further, the SiC substrate is heated in a state where the semiconductor laser 17 is pressed against the SiC substrate 19 by the collet 18 (FIG. 4C). Due to this heating process, the solder plating 16 begins to soften and melt, and is joined to the SiC substrate 19. Next, heating is stopped while the semiconductor laser 17 is pressed against the SiC substrate 19 to naturally cool the SiC substrate 19 and the semiconductor laser. At the same time, after the solder is cooled and solidified, the collet 18 is separated from the semiconductor laser, and the joining process is completed (FIG. 4D). The reason why the collet is pressed until the solder is solidified is to prevent the joining position of the semiconductor laser set as described above from shifting due to melting of the solder or the like.
[0023]
FIG. 5 shows a comparison of accelerated deterioration (life) test results between a conventional structure and a semiconductor laser employing the electrode structure of the present invention. The life conditions were a temperature of 70 ° C. and Po = 60 mW (pulse), and the change amount (ΔI _op ) of the laser operating current with respect to the test time was evaluated. The number of measurement chips was 100 chips each.
[0024]
As shown in FIG. 5, it was found that the semiconductor laser according to the present invention can obtain a stable result without deterioration of characteristics over a long period of time. This is because the negative resist embedded in the electrode absorbs and relaxes excessive external strain due to thermal stress applied after the laser chip is bonded to the heat sink in the assembly process, affecting the strained quantum well structure introduced in the active layer. As a result, it is considered that the initial characteristic deterioration could be suppressed.
[0025]
As a result of evaluating the distortion amount of the laser chip using an interference microscope, the conventional structure was 0.2 to 0.8 μm in the longitudinal direction of the chip, whereas the semiconductor laser employing the electrode structure of the present invention was used. It was 0.1 μm or less. Also from this result, it is considered that the negative resist absorbed and relaxed excessive strain applied to the laser chip in the assembly process.
[0026]
The negative resist pattern spacing was also examined. The examined resist pattern intervals were 10 μm, 5 μm, and 2 μm, and the number of measurement chips used in the laser life test was 100 chips each.
[0027]
FIG. 6 shows the laser life test results with respect to the negative resist pattern interval. If the interval becomes too close, the number of chips exhibiting characteristic deterioration tends to increase. This is presumed that if the interval is too close, the heat dissipation characteristics of the semiconductor laser to the heat sink deteriorate, and as a result, the life is shortened.
[0028]
From the results shown in FIG. 6, it is considered that there are optimum values for the resist pattern size and interval according to the laser chip size from the viewpoint of strain relaxation and heat dissipation.
[0029]
According to the present embodiment, the strained quantum well introduced into the active layer is formed by embedding an elastic material in the electrode and absorbing and mitigating excessive strain due to thermal stress applied after the laser chip is bonded to the heat sink. As a result of not causing excessive distortion in the structure, it is possible to suppress deterioration of initial characteristics and to provide a highly reliable semiconductor laser.
[0030]
In addition, in the process of joining the heat sink and the semiconductor laser, only one heating → cooling cycle is required, so that the manufacturing process can be shortened and the cost can be reduced without increasing the process throughput.
[0031]
Further, by using a negative resist as the elastic material, it is possible to perform pattern formation by photolithography, and the arrangement, size, and the like can be formed with good reproducibility.
[0032]
(Embodiment 2)
In this embodiment, the type of resist used in Embodiment 1 was examined. The examined resist was examined using a chemically amplified negative resist and a novolac resin positive resist in addition to the rubber negative resist used in the first embodiment. The conditions are the same as in the first embodiment except for the type of resist. The number of measurement chips is 100 chips each.
[0033]
FIG. 7 shows the laser life test results for the resist types. Except for the rubber negative resist, all the characteristics deteriorated. This is because rubber negative resists do not cure even when heat is applied and have elasticity, while other resists are thermosetting resists. This is thought to be because the function of absorbing and relaxing cannot be achieved.
[0034]
According to the present embodiment, by using a rubber-based negative resist as the elastic material, it is possible to absorb and relax excessive external strain applied by thermal stress without losing elasticity even during heating in the assembly process, Characteristic deterioration can be suppressed, and a highly reliable semiconductor laser can be provided.
[0035]
In the first and second embodiments, the example in which the elastic material is embedded in the p-type electrode has been described. However, the elastic material may be embedded in the n-type electrode.
[0036]
The elastic material may be other than a rubber-based negative resist as long as it does not lose elasticity even when heated in the assembly process.
[0037]
The shape of the resist pattern may be other than dots, for example, a lattice shape.
[0038]
Further, the types, structures, and materials of the semiconductor laser, the electrode, and the heat sink are not limited to those in the first and second embodiments.
[0039]
【The invention's effect】
According to the semiconductor laser of the present invention, the excessive external strain applied by the thermal stress in the assembly process of the heat sink and the semiconductor laser is absorbed and relaxed by the elastic material embedded in the electrode, and the strain introduced into the active layer. As a result of not affecting the quantum well structure, it is possible to suppress initial characteristic deterioration, and it is possible to provide a highly reliable semiconductor laser.
[Brief description of the drawings]
FIG. 1 is a sectional structural view of a semiconductor laser according to a first embodiment of the present invention. FIG. 2 is a diagram showing a comparison between a conventional technique and an electrode formation flow according to the first embodiment of the present invention. FIG. 4 is a plan view of a resist pattern in the first embodiment of the present invention. FIG. 4 is an explanatory diagram of a bonding process between the semiconductor laser and the heat sink in the first embodiment of the present invention. FIG. 6 is a diagram showing a result of a laser life test with respect to a negative resist pattern interval in the first embodiment of the present invention. FIG. 7 is a diagram of resist types in the second embodiment of the present invention. Of laser life test results for
1 n-type GaAs substrate 2 n-type GaAs buffer layer 3 n-type AlGaInP clad layer 4 GaInP / AlGaInP strained quantum well active layer 5 p-type AlGaInP first clad layer 6 p-type GaInP etching blocking layer 7 p-type AlGaInP second clad layer 8 p-type GaInP cap layer 9 n-type GaAs current confinement layer 10 p-type GaAs contact layer 11 n-type electrode (Au / Ge / Ni)
12 p-type electrode 13 Cr / Au / Pt laminated film 14 Ti / Au / Pt laminated film 15 negative resist pattern 16 solder plating 17 semiconductor laser 18 collet 19 SiC substrate

Claims (6)

A top or a semiconductor laser lower surface electrodes are provided, Ri least one structural der which the elastic material is embedded among the electrodes,
The elastic material is embedded between a first layer and a second layer constituting the electrode, and the first layer and the second layer are light emitting regions on the surface of the semiconductor laser. Formed over a predetermined area including directly above,
The semiconductor laser according to claim 1, wherein the elastic material is not disposed immediately above the light emitting region . 2. The semiconductor laser according to claim 1, wherein the elastic material is arranged at predetermined intervals in a dot shape or a lattice shape in the electrode. 3. The semiconductor laser according to claim 1, wherein the elastic material is a material that does not lose its elasticity upon heating in the bonding of the semiconductor laser and a heat sink. 4. The semiconductor laser according to claim 3, wherein the elastic material is a rubber-based resist. The semiconductor laser according to any one of claims 1 to 4 wherein the electrode to which the elastic material is embedded, characterized in that bonded to the heat sink via an adhesive. Depositing a first metal layer on the upper or lower surface of the semiconductor laser;
Coating an elastic material on the first metal layer;
Patterning the elastic material;
Depositing a second metal layer on the elastic material and the first metal layer to form an electrode;
Holding and moving the semiconductor laser by a collet;
Placing the semiconductor laser on the top surface of the heat sink via an adhesive with the electrode facing down;
Heating the heat sink to a temperature at which the adhesive softens with the semiconductor laser pressed against the heat sink by the collet;
A method of manufacturing a semiconductor laser, comprising: cooling the heat sink to a temperature at which the adhesive is cured while the semiconductor laser is pressed against the heat sink by the collet and bonding the semiconductor laser and the heat sink.

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