Abstract:
A method of manufacturing a semiconductor laser device includes the steps of: providing a laser chip, in which a semiconductor layer is formed on a substrate, a supporting plate which supports the laser chip, a mounting plate, a first solder film positioned between the laser chip and the mounting plate and a second solder film positioned between the mounting plate and the supporting plate to form a stacked laser chip structure; applying heat to the stacked laser chip structure sufficient to melt the first solder film and the second solder film; and, applying pressure to the stacked laser chip structure during the heating step to cause simultaneous adhering of the laser chip, the mounting plate and the supporting plate to each other.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of manufacturing a semiconductor laser device including a laser chip in which a semiconductor layer and electrode films are formed on the same surface of a substrate, and a mounting plate and a supporting plate which are used in the method of manufacturing a semiconductor laser device. 
     2. Description of the Related Art 
     In recent years, semiconductor light-emitting devices including a semiconductor laser device in which a nitride semiconductor such as GaN is employed as a light source for short-wavelength lights have been developed. 
     Generally, with the semiconductor laser device utilizing the nitride semiconductor, an n-type layer, an active layer and a p-type layer, which are made of the nitride semiconductor, are stacked in sequence on a substrate made of sapphire (Al 2 O 3 ). Among a pair of electrode films of the semiconductor laser device, a p-side electrode is formed on the p-type layer which is the most upper layer in the semiconductor layer, while an n-type electrode is formed on the n-type layer which is exposed by etching the p-type layer and the active layer. Here, the substrate, the semiconductor layer, the p-side electrode and the n-side electrode are collectively called a laser chip. 
     In general, the laser chip is supported by a supporting plate (also referred to as a heat sink) made of a metal with relatively high thermal conductivity, in order for the semiconductor layer to release generated heat effectively. Further, a mounting plate (also referred to as a sub-mount) is provided between the laser chip and the supporting plate, the mounting plate having a lead electrode on a supporting body made with an insulating material. With the conventional method of manufacturing a semiconductor laser device, a solder adheres the mounting plate to the supporting plate and then another solder adheres the mounting plate to the laser chip. 
     However, with such a conventional method of manufacturing a semiconductor laser device, two steps of adhesion process need to be performed, which creates a problem such that longer time for manufacturing a semiconductor laser device is required. Further, two steps of heating process are needed in order to melt the solder, which causes deterioration in performance of the semiconductor laser device due to repetition of heating. 
     SUMMARY OF THE INVENTION 
     The present invention has been achieved in view of the above problems. It is an object of the invention to provide a method of manufacturing a semiconductor laser device capable of reducing time required for manufacture and suppressing deterioration in performance due to heating, and a mounting plate and a supporting plate. 
     The method of manufacturing a semiconductor laser device of the present invention includes a step for simultaneously adhering a laser chip, a mounting plate and a supporting plate to each other. 
     The mounting plate of the semiconductor laser device of the present invention adheres to the laser chip, in which a semiconductor layer is formed on a substrate, for use, the mounting plate including a supporting body and a first solder film and a second solder film, each of which are formed on each of a pair of surfaces of the supporting body. 
     The supporting plate of the semiconductor laser device of the present invention supports the laser chip, which includes a semiconductor layer, having a predetermined mounting plate in between, the supporting plate including a solder film at least on one surface of the body. 
     With the method of manufacturing a semiconductor laser device of the present invention, the laser chip, the mounting plate and the supporting plate simultaneously adheres to each other, whereby two steps of adhesion process are not needed. 
     With the mounting plate of the semiconductor laser device of the present invention, the laser chip is placed on one surface of the mounting plate and the supporting plate is placed on the other surface of the mounting plate, thus the first solder film adheres the laser chip to the mounting plate, and simultaneously the second solder film adheres the mounting plate to the supporting plate. 
     With the supporting plate of the semiconductor laser device of the present invention, the mounting plate is placed on one surface of the supporting plate, whereby the solder film adheres the mounting plate to the supporting plate. 
     Other and further objects, features and advantages of the invention will appear more fully from the following description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view showing the structure of a semiconductor light-emitting device in which a semiconductor laser device of an embodiment of the present invention is employed; 
     FIG. 2 is a cross section showing the structure of the semiconductor laser device in the semiconductor light-emitting device shown in FIG. 1; 
     FIG. 3 is a cross section showing the structure of a laser chip of the semiconductor laser device shown in FIG. 1; 
     FIG. 4 is a cross section showing the structure of a mounting plate of the semiconductor laser device shown in FIG. 2; 
     FIG. 5 is a cross section showing the structure of a supporting plate of the semiconductor laser device shown in FIG. 2; 
     FIG. 6 is a cross section showing an adhesion process in the method of manufacturing a semiconductor laser device shown in FIG. 2; and 
     FIG. 7 is a cross section for describing the method of manufacturing a semiconductor laser device of a modification of the embodiment according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described hereinafter with reference to accompanying drawings. 
     First Embodiment 
     FIG. 1 shows one example of a semiconductor light-emitting device  100  in which a semiconductor laser device  1  is employed according to a first embodiment. The semiconductor light-emitting device  100  comprises the semiconductor laser device  1  and a package  10  in a predetermined shape covering the semiconductor laser device  1 . The package  10  includes a supporting disk  11  and a cylindrical lid  12  to be attached to the supporting disk  11 . One end of the lid  12  in a longitudinal direction is closed but has a window  12   a  through which a laser beam emitted from the semiconductor laser device  1  passes to the outside of the package  10 . The lid  12  is made of a metal such as copper (Cu) or iron (Fe) and the window  12   a  is made of a transparent glass or resin. The supporting disk  11  is made of a metal such as copper or iron and the semiconductor laser device  1  is formed on a surface of the supporting disk  11  (the front face in FIG.  1 ). Here, the semiconductor laser device  1  corresponds to one specific example of “semiconductor laser device” of the present invention. 
     FIG. 2 is a cross section of the structure of the semiconductor laser device  1 . The semiconductor laser device  1  comprises a laser chip  20  including a semiconductor layer, a sub-mount  30  on which the laser chip  20  is mounted, and a heat sink  40  which supports the semiconductor laser device  1  and the sub-mount  30 . The heat sink  40  is made of a metal with high thermal conductivity and releases heat generated by the laser chip  20 . The sub-mount  30  is provided between the laser chip  20  and the heat sink  40  and has lead electrode layers  32  and  33 , each of which is connected to each of electrodes  2   a  and  2   b  of the laser chip  20  to be described later. Here, the laser chip  20  corresponds to one specific example of “laser chip” of the present invention. The sub-mount  30  corresponds to one specific example of “mounting plate” of the present invention. The heat sink  40  corresponds to one specific example of “supporting plate” of the present invention. In FIG.  2  and FIGS. 3 through 7 to be described later, sizes of the laser chip  20 , the sub-mount  30  and the heat sink  40  along the thickness are emphasized. 
     FIG. 3 is a cross section showing the structure of the laser chip  20  of the semiconductor laser device  1  shown in FIG.  2 . The laser chip  20  includes a crystalline substrate  21  made of sapphire (Al 2 O 3 ). The crystalline substrate  21  may be made of spinel (MgAl 2 O 4 ), gallium nitride (GaN), silicon (Si) or silicon carbide (SiC) instead of sapphire. Formed on a surface of the crystalline substrate  21  is an n-type contact layer  22  made of n-type GaN in which n-type impurities, e.g., silicon are doped. The thickness of the n-type contact layer  22  is about 4 μm, for example. Formed on a surface of the n-type contact layer  22  is an n-type cladding layer  23  made of n-type AlGaN in which n-type impurities, e.g., silicon are doped. The thickness of the n-type cladding layer  23  is about 1.2 μm, for example. 
     An active layer  24  made of InGaN is formed on a surface of the n-type cladding layer  23 . The active layer  24  acts as a so called light-emitting layer, for example, having a light-trapping layer. Formed on a surface of the active layer  24  is a p-type cladding layer  25  made of p-type AlGaN in which p-type impurities, e.g., Mg are doped. The thickness of the p-type cladding layer  25  is about 0.8 μm, for example. Formed on a surface of the p-type cladding layer  25  is a p-type contact layer  26  made of p-type GaN in which p-type impurities, e.g., Mg are doped. The thickness of the p-type contact layer  26  is about 0.3 μm, for example. A part of the p-type cladding layer  25  and the p-type cladding layer  26  is etched. A restricting layer  27  made with an insulating film such as silicon oxide or alumina (Al 2 O 3 ) is provided so as to sandwich the p-type cladding layer  25  and the p-type contact layer  26 . 
     Formed on a surface of the p-type contact layer  26  is the p-side electrode  2   a . The p-side electrode  2   a  is alloyed by sequentially stacking a nickel (Ni) layer and a gold (Au) layer from the side of the p-type contact layer  26  and then applying heat thereon, for example. A part of the n-type contact layer  22 , the n-type cladding layer  23 , the active layer  24 , the p-type cladding layer  25  and the p-type contact layer  26  is etched and the n-type contact layer  22  is partly exposed. The n-side electrode  2   b  is formed on the exposed surface of the n-type contact layer  22 . The n-side electrode  2   b  is alloyed, for example, by sequentially stacking a titanium (Ti) layer, an aluminum (Al) layer and a gold layer from the side of the n-type contact layer  22  and then applying heat thereon. The p-side electrode  2   a  and the n-side electrode  2   b  are formed in a belt-like shape extending in a direction perpendicular to the sheet of FIG.  2 . The difference in height between the surface of the p-side electrode  2   a  and the surface of the n-side electrode  2   b  is about 3.5 μm, for example. 
     Here, the crystalline substrate  21  corresponds to one specific example of “substrate” of the present invention. The stack of layers from the n-type contact layer  22  to the p-type contact layer  26  including the restricting layer  27  corresponds to one specific example of “semiconductor layer” of the present invention. The p-side electrode  2   a  and the n-side electrode  2   b  correspond to one specific example of “a pair of electrode films” of the present invention. 
     A pair of reflecting mirror films (not shown) is formed on both ends of the laser chip  20  in a perpendicular direction to the sheet of FIG.  3 . These reflecting mirrors have the structure of alternately stacking silicon dioxide films and zirconium oxide (ZrO) films, for example. Reflectance of one reflecting mirror is lower than that of the other reflecting mirror. The light generated in the active layer  24  is amplified by reciprocating between the pair of reflecting mirror films and thus the amplified light is emitted from one reflecting mirror films as a laser beam. 
     FIG. 4 shows a cross section of the structure of the sub-mount  30 . The sub-mount  30  has a structure such that the lead electrode layers  32  and  33 , and front face solder films  3   a  and  3   b  are formed on a supporting body  31  in a rectangular plate shape. The supporting body  31  is made of insulating material with high thermal conductivity such as diamond (C), beryllium oxide (BeO), copper-tungsten alloy (CuW), aluminum nitride (AlN), cubic boron nitride (cBN), silicon (Si) or silicon carbide (SiC). The size of the supporting body  31  is 200 μm in thickness, 0.6 mm in width (length in the right-left direction in the drawing) and 1 mm in depth (length in the depth direction in the drawing), for example. 
     The supporting body  31  has an even top surface. A pair of lead electrode layers  32  and  33  of, e.g., 10 μm in thickness is formed on the even top surface of the supporting body  31 . The lead electrode layers  32  and  33  may be formed of gold, gold-tin alloy (AuSn) or the like. The lead electrode layers  32  and  33  may have the structure of subsequently stacking a titanium layer, a platinum layer and a gold layer from the side of the sub-mount  30 . As is shown in FIG. 1, the lead electrode layers  32  and  33  are electrically connected to pins  17  and  18  (FIG. 1) provided in the supporting disk  11  (FIG. 1) via wires W. respectively. Alternatively, one of the lead electrode layers  32  and  33  is electrically connected to a back face solder film  3   c  (FIG. 4) via a lead electrode (not shown). The left lead electrode layer  32  and the right lead electrode layer  33  are spaced about 50 μm. Here, the supporting body  31  corresponds to one specific example of “supporting body” of the present invention. The lead electrode layers  32  and  33  correspond to one specific example of “a pair of lead electrodes” of the present invention. 
     The front face solder films  3   a  and  3   b  are formed on surfaces of the lead electrode layers  32  and  33  in the sub-mount  30 , respectively. The front face solder films  3   a  and  3   b  are formed of a metal with a low melting point such as tin (Sn), gold-tin alloy, tin-platinum alloy (SnPt), indium-tin alloy (InSn) and indium (In). The thickness of the front face solder film  3   a  in the left in FIG. 4 is about 3.5 μm, whereas the thickness of the front face solder film  3   b  in the right in FIG. 4 is about 7 μm. That is, the difference in height between the surface of the front face solder film  3   a  and the surface of the front face solder film  3   b  is about 3.5 μm. Here, each of the front face solder films  3   a  and  3   b  correspond to one specific example of “first solder film” of the present invention. 
     The back face solder film  3   c  is formed on the back surface (the opposite surface to the surface where the lead electrode layers  32  and  33  are formed) of the supporting body  31  in the sub-mount  30 . The back face solder film  3   c  is formed of a metal with a low melting point such as tin similar to the front face solder films  3   a  and  3   b  and the thickness is about 4 μm. The back face solder film  3   c  is provided on the sub-mount  30  since it is easier than providing a solder on a structure such as the heat sink  40  (FIG.  5 ). 
     FIG. 5 is a cross section showing the structure of the heat sink  40 . The heat sink  40  has a structure such that a gold layer  42  is formed on a surface of a body  41  made of a metal with high thermal conductivity such as copper by means of plating. The body  41  in FIG. 5 is trapezoidal but may have different shapes. The gold layer  42  is provided in the heat sink  40  because the back face solder film  3   c  made of tin or the like is easily alloyed with the gold layer  42 . Further, gold is a stable metal and an unwanted oxide film is hardly formed on a surface of the gold layer  42 . 
     Method of Manufacturing a Semiconductor Laser Device 
     A method of manufacturing a semiconductor laser device of the embodiment will be described hereinafter. 
     First, the laser chip  20  shown in FIG. 3 is formed. More specifically, the n-side contact layer  22  made of n-type GaN, the n-type cladding layer  23  made of n-type AlGaN, the active layer  24  made of GaIn-N, the p-type cladding layer  25  made of p-type AlGaN, and the p-side contact layer  26  made of p-type GaN are grown in sequence on the surface of the crystalline substrate  21  made of, e.g., sapphire with a metal organic chemical vapor deposition (MOCVD) method. 
     After growing layers from the n-side contact layer  22  to the p-side contact layer  26 , the p-type contact layer  26  and the p-type cladding layer  25  are partly etched with a lithography method and the restricting layer  27  made with, e.g., an insulating material is formed thereon. Subsequently, the p-side contact layer  26 , the p-type cladding layer  25 , the active layer  24  and the n-type cladding layer  23  are selectively removed to expose the n-side contact layer  22 . Thereafter, the n-side electrode  2   b  is selectively formed on the exposed area of the n-side contact layer  22  and then the p-side electrode  2   a  is selectively formed on the p-side contact layer  26 . 
     After forming the p-side electrode  2   a  and the n-side electrode  2   b , the crystalline substrate  21  is cut vertically to the direction along the length of the p-side electrode  2   a  (in a vertical direction to the sheet of FIG. 3) with a predetermined width. After that, a pair of reflecting mirror films is formed on a pair of side surfaces of the cut semiconductor layer, respectively, and then the crystalline substrate  21  is cut parallel to the direction along the length of the p-side electrode  2   a  with a predetermined width. Thus, the laser chip  20  is formed. 
     Next, the sub-mount  30  shown in FIG. 4 is formed. More specifically, the lead electrode layers  32  and  33  are formed on the surface of the supporting body  31  made of, for example, diamond, beryllium oxide, copper-tungsten alloy, aluminum nitride, cBN, silicon or silicon carbide by means of plating, sputtering, or deposition. Subsequently, the front face solder films  3   a  and  3   b  made of a metal with a low melting point is formed on the surfaces of lead electrode layers  32  and  33  by means of deposition. The back face solder film  3   c  made of a metal with a low melting point is formed on the back surface of the supporting body  31  by means of deposition method. Accordingly, the sub-mount  30  including the front face solder films  3   a  and  3   b  and the back face solder film  3   c  is formed. 
     The heat sink  40  shown in FIG. 5 is formed. Specifically, the gold layer  42  is formed on the surface of the body  41  made of metal, for example, by means of plating. The heat sink  40  is integrally formed with the supporting disk  11  (FIG. 1) or previously mounted to the supporting disk  11 . 
     As shown in FIG. 6, the laser chip  20 , the sub-mount  30  and the heat sink  40  are put together. At this time, since the front face solder films  3   a  and  3   b  and the back face solder film  3   c  are formed on the front and back surfaces of the sub-mount  30 , the front face solder films  3   a  and  3   b  are positioned between the laser chip  20  and the sub-mount  30 , and the back face solder film  3   c  is positioned between the sub-mount  30  and the heat sink  40 . In this state, the laser chip  20 , the sub-mount  30  and the heat sink  40  are stacked and then heat and pressure are applied thereon. The weight for the application of pressure is about 5 g, for example. The pressure applied per unit area of the laser chip  20  is calculated to be about 1.2×10 −4  Pa. The temperature for the application of heat is, e.g., about 280° C. in order to melt the front face solder films  3   a  and  3   b  and the back face solder film  3   c . The process of the application of heat and pressure is preferably performed in a nitrogen gas (N 2 ) or hydrogen gas (H 2 ) or mixed gas of the nitrogen gas and hydrogen gas atmosphere in order to avoid oxidation of the solder films  3   a  and  3   b.    
     Thus, as shown in FIG. 2, the front face solder films  3   a  and  3   b  adhere the laser chip  20  to the sub-mount  30 , while the back face solder film  3   c  adheres the sub-mount  30  to the heat sink  40 . The p-side electrode  2   a  and the n-side electrode  2   b  of the laser chip  20  are electrically coupled to the lead electrode layers  3   a  and  3   b  of the sub-mount  30 , respectively. In this manner, the laser chip  20 , the sub-mount  30  and the heat sink  40  simultaneously adhere to each other and become an integral unit. Subsequently, the lead electrode layers  32  and  33  in the sub-mount  30  are connected to the pins  17  and  18  (FIG. 1) provided in the supporting disk  11  (FIG. 1) through wires (not shown), respectively. Thus, the semiconductor light-emitting device  100  shown in FIG. 1 is manufactured. 
     Effects of the Embodiment 
     The effects of the embodiment will be described herein later. As shown in FIG. 6, the laser chip  20 , the sub-mount  30  and the heat sink  40  adhere to each other simultaneously, whereby time required for manufacture is reduced as compared to the case where a process for adhesion of the laser chip  20  and the sub-mount  30  and a process for adhesion of the sub-mount  30  and the heat sink  40  are separately performed. Further, heating is performed only one time, so that deterioration in performance of the laser chip  20  due to repetition of heating is suppressed. 
     Since the front face solder films  3   a  and  3   b  and the back face solder film  3   c  are provided on the front and back surfaces of the sub-mount  30 , by only applying heat and pressure on the stacked laser chip  20 , the sub-mount  30  and the heat sink  40 , the laser chip  20 , the sub-mount  30  and the heat sink  40  become an integrated unit, whereby the manufacturing process can be made easier. Since all solder films (the front solder films  3   a  and  3   b  and the back solder film  3   c ), which are easy to oxidize, are provided on the sub-mount  30 , only the sub-mount  30  needs to be controlled for prevention of oxidation, which facilitates parts control. 
     With the embodiment, after stacking the laser chip  20 , the sub-mount  30  and the heat sink  40 , pressure, e.g., 5 g is applied thereon. The effects of application of pressure will be described below. When thermal resistance of the semiconductor laser device  1  manufactured in the embodiment was measured, the thermal resistance was 12 K/W. That is, 12 K (kelvin) increased per application of 1 W of heat. On the other hand, the semiconductor laser device was formed in such a manner that the laser chip  20 , the sub-mount  30  and the heat sink  40  adhere to each other without applying pressure. The thermal resistance was measured to be 28 K/W. Judging from the measurement results, application of pressure on the laser chip  20 , the sub-mount  30  and the heat sink  40  achieves smaller thermal resistance (that is, smaller temperature increase per application of a fixed quantity of heat) as compared to the case where no pressure is applied. As described with the method of manufacturing a semiconductor laser device of the embodiment, by achieving smaller thermal resistance, life of the semiconductor device becomes longer and further certain performance is maintained for a long period of time. 
     When life of the semiconductor laser device  1  manufactured in the embodiment and life of the semiconductor laser device manufactured without applying pressure are measured, the life of the semiconductor laser device  1  manufactured in the embodiment is 2.5 times longer than that of the semiconductor laser device manufactured without application of pressure. 
     Modification 
     FIG. 7 shows a modification of the method of manufacturing a semiconductor laser device of the embodiment. With the modification, instead of forming the solder films on the front and back surfaces of the sub-mount  30 A, solder films are formed on a surface of the sub-mount  30 A and a surface of the heat sink  40 A. The same elements as those of the first embodiment are indicated by the same reference numerals herein later and the description is omitted. 
     As shown in FIG. 7, the front face solder films  3   a  and  3   b  are formed on the surface of the supporting body  31  of the sub-mount  30 A (on the lead electrode layers  32  and  33 ), and no back face solder film  3   c  (FIG. 4) as in the first embodiment is provided on the back surface of the sub-mount  30 A. However, a heat-sink-side solder film  4   a  made of a metal with a low melting point such as tin is formed on the surface of the gold layer  42  of the heat sink  40 A. The structure of the laser chip  20  is the same as the structure of the laser chip  20  in the first embodiment. Here, the heat-sink-side solder film  4   a  corresponds to one specific example of “second solder film” of the present invention. 
     Thus structured the laser chip  20 , the sub-mount  30 A and the heat sink  40 A are stacked and heat and pressure are applied thereon similar to the first embodiment. Therefore, the front face solder film  3   a  adheres the laser chip  20  to the sub-mount  30 A, whereas the heat-sink-side solder film  4   a  adheres the sub-mount  30 A to the heat sink  40 A. Thus, with this modification similar to the first embodiment, the laser chip  20 , the sub-mount  30 A and the heat sink  40 A adhere to each other simultaneously, which reduces time required for manufacture. Moreover, repetition of heating is unnecessary with the modification, whereby deterioration in performance of the laser chip  20  due to heat is suppressed. 
     Although the present invention has been described by exemplifying the embodiment and the modification of the present invention, the present invention is not limited to the embodiment and the modification and various other modifications are possible. For example, with the first embodiment, the front face solder films  3   a  and  3   b  and the back face solder film  3   c  are formed on the front and back surfaces of the sub-mount  30 . However, the front face solder films  3   a  and  3   b  and the back face solder film  3   c  may be formed taking the shape of a foil and the foils may be inserted between the laser chip and the mounting plate and between the mounting plate and the supporting plate, respectively. Various structures of the semiconductor light-emitting device  100  are possible in addition to the one shown in FIG.  1 . 
     As described above, according to the method of manufacturing a semiconductor laser device of the invention, the laser chip, the mounting plate and the supporting plate adhere to each other simultaneously, so that the manufacturing process can be facilitated as compared to the case where the mounting plate adheres to the supporting plate and then the laser chip adheres to the mounting plate. Further, when heating is applied in the adhesion process, repetition of heating is unnecessary, whereby deterioration in performance of the laser chip due to heat is suppressed. 
     According to the method of manufacturing a semiconductor laser device of one aspect of the invention, the first solder film is provided between the laser chip and the mounting plate and the second solder film is provided between the mounting plate and the supporting plate. Thus, the laser chip, the mounting plate and the supporting plate adhere to each other simultaneously with a simple method. 
     According to the mounting plate of the semiconductor laser device of the invention, the first solder film and the second solder film are provided on the pair of surfaces of the mounting plate, respectively, so that the semiconductor laser device is easily formed by placing the laser chip on one surface of the mounting plate and placing the supporting plate on the other surface of the mounting plate. 
     According to the supporting plate of the semiconductor laser device of the invention, the solder film is provided at least on one surface of the supporting plate, whereby the semiconductor laser device is easily formed by placing the mounting plate on the supporting plate and placing the laser chip on the mounting plate (having the solder film and the like in between) and then applying heat and pressure as needed. 
     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.