Patent Publication Number: US-10763641-B2

Title: Method of manufacturing light emitting device

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     This application is a continuation application of U.S. patent Ser. No. 16/112,036, filed on Aug. 24, 2018. This application claims priority to Japanese Patent Application No. 2017-165760 filed on Aug. 30, 2017. The entire disclosures of U.S. patent Ser. No. 16/112,036 and Japanese Patent Application No. 2017-165760 are hereby incorporated herein by reference. 
    
    
     BACKGROUND 
     The present disclosure relates to a method of manufacturing a light emitting device. 
     In formation of semiconductor lasers, semiconductor layers are formed on a growth substrate. For various purposes, a technique may be employed in which a wafer in which the semiconductor layers are formed is bonded to a support substrate and then the growth substrate is removed. 
     For example, a technique in which, for the purpose of improving cleavability, semiconductor layers are bonded to a support substrate having a better cleavability and then a growth substrate is removed (e.g., JP 2002-299739 A), and a technique in which, for the purpose of preventing generation of a crack due to difference in thickness of the semiconductor layers, semiconductor layers are bonded to a support substrate with stepped portions and then a growth substrate is removed (JP 2009-123939 A) are proposed. 
     SUMMARY 
     Because the higher the output of a semiconductor laser, the greater the amount of generated heat, further improvement in heat dissipation performance is required. Further, semiconductor lasers with higher efficiency are required. 
     One object of the present invention is to provide a method of manufacturing a light emitting device that allows for achieving improvement in heat dissipation performance and increase in efficiency of the light emitting device. 
     According to certain embodiments of the present invention, a method of manufacturing a light emitting device comprising: providing an element-structure wafer having a first substrate and a laser element structure on the first substrate, the laser element structure having ridges on a side opposite to the first substrate and raising layers respectively formed above the ridges; bonding a laser element structure side of the element-structure wafer to a second substrate to obtain a bonded wafer; removing at least a portion of the first substrate to obtain a thinned bonded wafer; singulating the thinned bonded wafer to obtain a laser element with the second substrate; mounting the laser element with the second substrate on a heat dissipating member such that a laser element side of the laser element with the second substrate faces the heat dissipating member; and removing the second substrate from the laser element. 
     According to certain embodiments of the present invention, a method of manufacturing a light emitting device having higher heat dissipation performance and higher efficiency can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic cross-sectional view illustrating a method of manufacturing a light emitting device according to a first embodiment. 
         FIG. 1B  is a schematic cross-sectional view illustrating the method of manufacturing the light emitting device according to the first embodiment. 
         FIG. 1C  is a schematic cross-sectional view illustrating the method of manufacturing the light emitting device according to the first embodiment. 
         FIG. 1D  is a schematic cross-sectional view illustrating the method of manufacturing the light emitting device according to the first embodiment. 
         FIG. 1E  is a schematic cross-sectional view illustrating the method of manufacturing the light emitting device according to the first embodiment. 
         FIG. 1F  is a schematic cross-sectional view illustrating the method of manufacturing the light emitting device according to the first embodiment. 
         FIG. 1G  is a schematic plan view illustrating the method of manufacturing the light emitting device according to the first embodiment. 
         FIG. 1H  is a schematic cross-sectional view illustrating the method of manufacturing the light emitting device according to the first embodiment. 
         FIG. 1I  is a schematic cross-sectional view illustrating the method of manufacturing the light emitting device according to the first embodiment. 
         FIG. 1J  is a schematic cross-sectional view illustrating the method of manufacturing the light emitting device according to the first embodiment. 
         FIG. 2  is a graph showing a voltage-current curves of the laser device according to the first embodiment and a laser device of Comparative Example. 
         FIG. 3A  is a schematic partial cross-sectional view showing a partial modification in a structure of a laser device according to the first embodiment. 
         FIG. 3B  is a schematic cross-sectional view showing another partial modification in a structure of the laser device according to the first embodiment. 
         FIG. 3C  is a schematic plan view showing even another partial modification in a structure of the laser device according to the first embodiment. 
         FIG. 3D  is a schematic plan view showing yet another partial modification in a structure of the laser device according to the first embodiment. 
         FIG. 3E  is a schematic plan view showing still another partial modification in a structure of the laser device according to the first embodiment. 
         FIG. 3F  is a schematic plan view showing further another partial modification in a structure of the laser device according to the first embodiment. 
         FIG. 4  is a schematic cross-sectional view showing a light emitting device according to a comparative example. 
     
    
    
     DETAILED DESCRIPTION 
     The embodiments shown below are intended as illustrative to give a concrete form to technical ideas of the present invention, and the scope of the invention is not limited to those described below. The sizes and the positional relationships of the members in each of the drawings are occasionally shown exaggerated for ease of explanation. Further, in the description below, the same designations or the same reference numerals denote the same or like members and duplicative descriptions will be appropriately omitted. 
     First Embodiment 
     A method of manufacturing a light emitting device according to a first embodiment includes, as shown in  FIGS. 1A to 1J , providing an element-structure wafer including a first substrate and a laser element structure on the first substrate, bonding a laser element structure side of the element-structure wafer to a second substrate to obtain a bonded wafer, removing at least a portion of the first substrate to thin the bonded wafer, singulating the thinned bonded wafer to obtain a laser element with the second substrate, mounting the laser element with the second substrate on the heat dissipating member such that a laser element structure side of the laser element with the second substrate faces the heat dissipating member, and removing the second substrate from the laser element. 
     In this manner, the second substrate is bonded for reinforcement, which allows for thinning a wafer that includes the first substrate on which the laser element structure is formed. Further, mounting the laser element structure on the heat dissipating member allows a light-emitting portion (i.e., active layer) of the laser element, which is a heat source, to be located closer to the heat dissipating member. Thus, heat can be easily released from the laser element to the heat dissipating member, so that heat resistance of the light emitting device can be reduced. Mounting in a junction-down manner also allows a light emitting portion of a laser element to be located closer to the heat dissipating member, but when mounting in a junction-down manner, a material having a relatively high heat resistance such as an insulating layer is disposed between the light emitting portion of the laser element and the heat dissipating member. According to the method of manufacturing the light emitting device according to the first embodiment, a material having a relatively high heat resistance such as an insulating film is not necessarily disposed between the light emitting portion and the heat dissipating member, so that heat resistance of the light emitting device can be further reduced compared to the case where the laser element is mounted in a junction-down manner. 
     Further, thinning the wafer that includes the first substrate and removing the second substrate after mounting the laser element structure on the heat dissipating member allows for reducing electric resistance attributed to electric resistances of the first substrate and second substrate can be reduced. Accordingly, a driving voltage of the laser element can be reduced. Thus, further increase in efficiency of the laser element can be achieved. Further, because the second substrate is removed, an inexpensive material can be used for the second substrate irrespective of an electric resistance thereof. 
     Providing Wafer 
     The element-structure wafer including the first substrate and the laser element structure on the first substrate is provided. 
     As shown in  FIG. 1A , a semiconductor layered structure  12  is formed as a laser element structure on a first substrate  11 . 
     Examples of the first substrate  11  include a compound semiconductor substrate such as a GaN substrate, a GaAs substrate, an element semiconductor substrate such as a silicone substrate, and an insulating substrate such as a sapphire substrate. Among these, a conductive substrate is preferable, and a crystalline substrate having cleavability is more preferable. The expression “having cleavability” as used herein refers to having an easy-cleavage plane, along which cleaving can be easily performed. For example, when the first substrate  11  is a GaN substrate, the {1-100} plane, which is an m-plane, is an easy-cleavage plane. In this case, the element structure is preferably formed so that a plane perpendicular to the m-plane, that is, {0001} plane, which is a c-plane, is a main surface. At this stage, the first substrate  11  has a thickness in a range of, for example, 50 μm to 2 mm. A surface of the first substrate may include irregularities, or may have an off-angle of, for example, 1 degree or less. 
     The semiconductor layered structure  12  includes, for example, an n-side semiconductor layer, an active layer, and a p-side semiconductor layer in order from a first substrate  11  side. The n-side semiconductor layer and the p-side semiconductor layer include an n-type semiconductor layer and a p-type semiconductor layer, respectively, and each of the n-side semiconductor layer and the p-side semiconductor layer may include an undoped layer. The active layer includes, for example, a multi quantum well structure or a single quantum well structure. The semiconductor layered structure  12  can be formed by performing epitaxially growth on the first substrate  11 . Examples of a material of the semiconductor layered structure  12  include Group III-V compound semiconductor such as a GaN-based semiconductor, a GaP-based semiconductor, and a GaAs-based semiconductor. For example, a nitride semiconductor represented by a general formula In X Al Y Ga 1−X−Y N (0≤X, 0≤Y, X+Y≤1) can be used for the semiconductor layered structure  12 . 
     As shown in  FIG. 1A , in the semiconductor layered structure  12 , optical waveguides are formed using a known technique. For example, using photolithography or an etching technique, portions of an upper surface-side of the semiconductor layered structure  12 , which may be portions of the p-side semiconductor layer, are removed to form stripe-shaped ridges  13 , which serve as optical waveguides. In this manner, in the laser element structure, the ridges  13  can be formed at a side opposite to the first substrate  11 . 
     A metal layer (a first metal layer) is preferably formed on an upper surface of the laser element structure. For example, the metal layer (the first metal layer) may be disposed as an uppermost layer of each of upper-surface electrodes disposed on corresponding portions of the upper surface of the p-side semiconductor layer. The first metal layer can facilitate bonding to a metal layer  18  (second metal layer  18 ) of the second substrate, which will be described below. In order to facilitate bonding to the second substrate described below, the first metal layer is preferably made of gold. Further, after removing the second substrate described below, the first metal layer will also serve as an uppermost layer, on which wire-bonding will be performed. Accordingly, also in view of adhesion with a wire, the first metal layer is preferably made of gold. 
     The upper-surface electrodes are electrically connected to the p-side semiconductor layer. As shown in  FIG. 1B , examples of the upper-surface electrodes include structures each including a first p-electrode  14  in contact with the upper surface of the p-side semiconductor layer, and a second p-electrode  16  (i.e., the metal layer (first metal layer) of the element-structure wafer), which is a pad electrode disposed on the first p-electrode  14  for external connection. Each of the first p-electrodes  14  is preferably disposed on a portion of the p-side semiconductor layer corresponding to a respective one of the optical waveguides, and may be disposed only on an upper surface of a respective one of the ridges  13 . Each of the second p-electrodes  16  may be disposed only on a respective one of the ridges  13 , or may be disposed over the respective one of the ridges  13  and the upper surface of the p-side semiconductor layer. 
     The upper-surface electrodes can be made of, for example, a single-layer film or a layered film of a metal such as palladium, platinum, nickel, gold, titanium, tungsten, copper, silver, zinc, tin, indium, chromium, aluminum, iridium, rhodium or an alloy of two or more of these metals. Each of the upper-surface electrodes may be made of a layered film of a conductive oxide film such as ITO and a metal film layered in order from a semiconductor layered structure  12  side. 
     The first p-electrodes  14  have a thickness in a range of, for example, 10 nm to 1000 nm. The second p-electrodes  16  have a thickness in a range of, for example, 100 nm to 1 μm. 
     It is preferable that, before or after forming the upper-surface electrodes, an insulating film  15  is disposed on regions of the p-side semiconductor layer, except for regions where the upper-surface electrodes and a semiconductor layer are to be in contact with each other. For example, an insulating layer  15  is preferably disposed on portions of the upper surface of the p-side semiconductor layer, such as portions between the ridges  13  and on the lateral surfaces of ridges  13 , except for the upper surface of each of the ridges  13 . In the case where a nitride semiconductor is used for the laser element structure, forming the ridges  13  on an upper surface of the element structure and forming the insulating films  15  having a relatively low refractive index in regions except for the ridges  13  allow a light to be efficiently confined in the optical waveguide. The insulating films  15  can be made of an insulating film of an oxide, a nitride, an oxynitride, or the like of Zr, Si, V, Nb, Hf, Ta, Al, Ce, In, Sb, Zn, etc. The difference between a refractive index of the insulating films  15 , which is relatively low, and a refractive index of a semiconductor layer (e.g., GaN) is preferably greater. Examples of such a material include SiO 2 . The insulating films  15  have a thickness in a range of, for example, 100 nm to 1500 nm. 
     In a laser element using a nitride semiconductor, materials of relatively high heat resistance, such as an oxide film, may be used for electrodes and/or insulating films. When mounted in a junction-down manner, such a material with relatively high heat resistance is located between the laser element and a heat dissipating member such as a submount, impeding heat dissipation. In view of this, mounting the laser element on the heat dissipating member with a lower-surface electrode side (i.e., n-electrode side) of the laser element facing the heat dissipation member can reduce the heat resistance of the light emitting device compared to mounting in a junction-down manner. 
     As shown in  FIG. 3A , a raising layer  31  subjected to an appropriate patterning may be further formed above each of the ridges  13 . The raising layer  31  may be alternatively formed, for example, between the first p-electrode  14  and the second p-electrode  16 , as shown in  FIG. 3B . With this arrangement, an upper surface of each of the second p-electrodes  16  below which a respective one of the raising layers  31  is disposed can be located highest of the wafer, which will be bonded to the second substrate at the time of bonding to the second substrate described below. Accordingly, the bonding area between the wafer and the second substrate can be adjusted with the area of the raising layer  31  in a plan view, that is, bonding strength between the wafer and the second substrate can thereby be adjusted. In order to obtain a laser element of high output, increasing the width of each of the ridges  13  can be effective. Meanwhile, the greater the width of each of the ridges  13 , the larger the bonding area between each of the ridges  13  and the second substrate is, which may lead to difficulty in removing the second substrate in a later step. However, the bonding area can be adjusted by providing the raising layers  31 . For this purpose, in a plan view, the total area of the raising layer  31  is preferably smaller than the area of the corresponding each of the ridges  13 . The ridges  13  that are preferably provided with the raising layers  31  have a width of 30 μm or greater, and 50 μm or greater. 
     The raising layers  31  may have a plan view shape of, a rectangular shape, a circular shape, an elliptic shape, or a complex shape of two or more of these. Each of the raising layer  31  may be disposed over an entire region between a portion to be a light-emission end surface and a light-reflection end surface, or may be disposed spaced apart from the light-emission end surface and/or the light-emission end surface. Examples of the plan view shape of each of the raising layers  31  are shown in  FIGS. 3C to 3F . Regions shaded in black in  FIGS. 3C to 3F  indicate the raising layer  31   a ,  31   b ,  31   c , and  31   d , respectively. 
     Examples of a material of the raising layers  31  include an insulating material such as an oxide, and a conductive material such as a metal. When made of a conductive material, the raising layers  31  can be used as a part of a current path between the semiconductor layered structure  12  and a wire or the like, that is, can be used as a part of each of the upper-surface electrodes, increasing the degree of freedom in arrangement of the raising layers  31  at desired locations. For example, a metal with a low electric resistance such as Au can be used for a main material of the raising layers  31 . The raising layers  31  preferably have a thickness of 10 nm or more, more preferably 50 nm or more so as to allow adjustment of the bonding area. Further, for example, in the case where a plurality of raising layers is disposed above a single ridge  13 , if the raising layers have a too large thickness, dusts may remain at regions between the plurality of raising layers. In view of this, the raising layers preferably have a thickness of 200 nm or less, more preferably 100 nm or less. 
     Each of the raising layer  31  may be disposed on a respective one of the second p-electrodes  12  as shown in  FIG. 3A . In this case, a surface of each of the raising layers  31  serves as a surface bonded with the second substrate and serves as a wire-bonded surface. Accordingly, in this case, the raising layers  31  serves as the first metal layer described above. Further, each of the raising layers  31  is preferably the first metal layer made of gold. Each of the raising layers  31  may have a layered structure, and an uppermost layer of the layered structure may be the first metal layer made of gold. In the case where an area greater than an area of a surface of each of the raising layers is required for wire bonding, a portion of surface of each of the second p-electrodes exposed from the raising layer  31  can be used as a wire-bonded surface. 
     Further, even disposing each of the raising layers  31  below a respective one of the first p-electrodes  14  allows for adjusting the bonding area, but the raising layers  31  below the first p-electrodes  14  may interrupt electric current between the first p-electrodes  14  and the semiconductor layered structure  12 . Thus, the raising layers  31  are preferably disposed on or above the first p-electrodes  14 . 
     Bonding Wafer to Second Substrate 
     As shown in  FIG. 1C , a laser element structure side of the element-structure wafer is bonded to the second substrate  17 . 
     The second substrate  17  bonded to the laser element structure is preferably a substrate that can be cleaved together with the first substrate  11  and/or the laser element structure. Accordingly, the second substrate  17  is preferably made of a material having cleavability. That is the second substrate  17  is preferably a crystalline substrate including an easy-cleavage plane. As used herein, the expression “easy-cleavage plane” refers to a plane along which cleavage easily occur in a specific direction along a crystal plane. Examples of the second substrate  17  include an Si substrate and a GaAs substrate. In view of manufacturing the laser element at a low cost, an Si substrate is preferable. For example, an Si substrate having a main surface of a (100) plane or a (110) plane can be used. 
     The second substrate  17  preferably includes the second metal layer  18  on at least one surface thereof that is opposite to the laser element structure. The second metal layer  18  can be made of, for example, a single-layer film or a layered film of a metal such as palladium, platinum, nickel, gold, titanium, tungsten, copper, silver, zinc, tin, indium, chromium, aluminum, iridium, rhodium or an alloy of two or more of these metals. Among those, in order to facilitate bonding, a material used for an uppermost surface of the metal layer  18  is preferably the same material used for an uppermost surface of each of the upper-surface electrodes of the laser element structure, and is more preferably gold. The second metal layer  18  can be disposed on a main surface of a main body  171  of the second substrate  17 . In this case, the main body  171  is preferably made of a material having cleavability as described above, that is, a material such as Si or GaAs. In the present specification, the expression “second substrate  17  (or  17   a ) has cleavability” refers to that, when the second substrate  17  (or  17   a ) includes the second metal layer  18 , the main body  171  (or  171   a ) has cleavability. 
     In view of securing strength, the second substrate  17  has a thickness greater than a thickness of the first substrate after thinning the wafer as described below. The second substrate  17  preferably has a thickness that allows for reducing influence of warpage caused by removing at least a portion of the first substrate  17 . For example, the second substrate  17  has a thickness in a range of 300 μm to 5 mm, preferably in a range of 300 μm to 500 μm. 
     As shown in  FIG. 1C , the second p-electrodes  16 , each of which is a part of a respective one of the upper-surface electrodes on the upper surface of the laser element structure, are placed facing the second metal layer  18  of the second substrate  17  and the second p-electrodes  16  and the metal layer  18  are connected to each other. Thus, the laser element structure and the second substrate  17  are bonded. At this time, the first substrate  11  and the second substrate  17  are bonded to each other such that a line parallel to an easy-cleavage plane of the first substrate  11  and a line parallel to an easy-cleavage plane of the second substrate  17  are parallel to each other when viewed from a laser element structure side or a second substrate  17  side. With such a bonding, cleaving of the second substrate  17  and cleaving of the first substrate  11  can be performed at once, so that singulation can be performed even in the case where the first substrate  11  is thinned to the extent that it is difficult to form a groove or the like for cleavage, which will be described below, in the first substrate  11 . Similarly, a line parallel to an easy-cleavage plane of the laser element structure is preferably parallel to an easy-cleavage plane of the first substrate  11  and an easy-cleavage plane of the second substrate  17 . Because resonator end surfaces of a laser element are preferably formed by cleaving, alignment is preferably performed such that surfaces obtained by cleavage along an easy-cleavage plane become resonator end surfaces. For example, in the case where each of the first substrate  11  and the laser element structure is made of a GaN-based semiconductor and is cleaved along an m-plane thereof to obtain resonator end surfaces, it is preferable that the second substrate  17  is an Si substrate with a main surface of a (100) plane and bonding is performed at a position that allows a line parallel to the (−110) plane to be parallel to the m-plane of the laser element structure or the like so that cleaving can be performed along the (−110) plane. The expression “a line parallel to an easy-cleavage plane of the first substrate  11  and a line parallel to an easy-cleavage plane of the second substrate  17  are parallel to each other” refers to that the line parallel to the easy-cleavage plane of the first substrate  11  and the line parallel to the easy-cleavage plane of the second substrate  17  are parallel to each other to the extent that the second substrate  7  can be cleaved together with the first substrate  11  at once. For example, a deviation of about ±0.2 degrees is tolerated. 
     Accordingly, it is preferable that a GaN substrate is used for the first substrate  11  and an Si substrate is used for the second substrate  17 . In this case, in particular, a nitride semiconductor layer is used for the semiconductor layers of the laser element structure. 
     A solid diffusion bonding technique, a heat-pressure bonding technique, or the like, can be employed as a method of the bonding, and in consideration of physical separation to be performed later, a bonding technique that allows for maintaining a bonding interface is preferably employed. For example, a metal bonding technique in which metals are placed adjacent to each other and pressed toward the interface between the metals (bonding temperature: 200° C. to 300° C., force: 3 kN to 10 kN, duration: 5 minutes to 15 minutes) is preferably employed. At a temperature of greater than 300° C., electrodes or the like of the laser element structure may be degraded. In view of this, a bonding temperature is preferably 300° C. or less. An area for the contact surface between the first substrate  11  and the second substrate  17  for the bonding is preferably as small as possible. In the case where the element-structure wafer includes the ridges  13 , it is preferable that the element-structure wafer and the second substrate  17  are bonded at portions where the ridges  3  are formed in a plan view, and are not bonded with each other at portions other than the portions where the ridges  3  are formed in a plan view. This allows for facilitating physical separation that will be performed later. More specifically, as shown in  FIG. 1C , the element-structure wafer and the second substrate  17  are preferably connected such that, the second p-electrodes  16 , which are the uppermost surfaces of the upper-surface electrodes on the ridges  13  of the first substrate  11 , and the second metal layer  18 , which is an upper surface of the second substrate  17  are bonded, and thus gaps  19  are formed between the element-structure wafer and the second substrate  17  at other locations. 
     Meanwhile, for example, when the first metal layer is not formed as the outermost surface of the laser element structure, and/or the second metal layer  18  is not formed as the outermost surface of the second substrate  17 , a material other than a metal (for example, an insulating film, a resin layer, or the like) and a metal film or a material other than a metal are placed opposite to each other, and are bonded using a technique that allows for maintaining a bonding interface, as in the bonding between metals as described above. 
     In the laser element structure, the first substrate  11 , and the second substrate  17 , a metal layer such as an electrode is preferably absent at portions to be divided in singulation, which will be described below. In particular, the upper-surface electrodes, lower surface electrodes, which will be described below, and the metal layer are preferably absent at portions to be cut for formation of the resonator end surfaces. 
     Thinning Wafer 
     As shown in  FIG. 1D , at least a portion of the first substrate  11  is removed from a lower side to thin the bonded wafer. The first substrate  11  may be removed completely. With the second substrate  17  bonded to the element structure wafer, the bonded wafer can be thinned in this manner. It is preferable that the removing is performed by polishing and/or a dry-etching. Chemical mechanical polishing (CMP) and/or dry etching (e.g., RIE) may be performed in combination to complete a mechanical polishing. When a polishing is performed, in order to reduce influence of warpage of the wafer, it is preferable that at least a portion of the first substrate  11  is removed in a state where a second substrate  17  side of the wafer is temporally affixed to a temporal substrate having a thickness greater than a thickness of the second substrate  17 . Examples of a material of the temporal substrate include a sapphire substrate, and the temporal substrate has a thickness of, for example, approximately 2 mm. 
     By way of polishing, dry-etching, or the like, the first substrate  11  is thinned to have a thickness in a range of, for example, 0.5 μm to 8 μm, preferably to be approximately 3 μm. (Hereinafter, the first substrate  11  after thinning may be indicated with a reference numeral  11   a .) Further, the shortest distance between a lower surface of the first substrate  11   a  and the active layer (in particular, a well layer) is preferably in a range of 2 μm to 9.5 μm. With such a distance, an emitted laser light is not easily emitted to the heat dissipating member, which will be described below. 
     After reducing a thickness (i.e., thinning) of the wafer, as shown in  FIG. 1E , lower-surface electrodes  20  are preferably formed on a surface of the wafer obtained by the thinning (i.e., a surface of the first substrate  11   a  opposite to a surface thereof at the laser element structure side). The lower-surface electrodes  20  may be made of a material same as or different from that of the upper-surface electrodes, and/or have a layered structure same as or different from that of the upper-surface electrodes. The lower-surface electrodes  20  may have any appropriate thickness in accordance with, for example, a material used for lower-surface electrodes  20 . The lower-surface electrodes can be made of, for example, a single-layer film or a layered film of a metal such as palladium, platinum, nickel, gold, titanium, tungsten, copper, silver, zinc, tin, indium, chromium, aluminum, iridium, rhodium or an alloy of two or more of these metals. The lower-surface electrodes  20  have a thickness in a range of, for example, 100 nm to 5 μm. In the case of completely removing the first substrate  11 , the lower-surface electrodes  20  can be formed on a surface of the laser element structure at a side opposite to the second substrate  17 . 
     In the case where the temporal substrate has been disposed, the temporal substrate is preferably removed after the lower-surface electrodes  20  are formed. 
     Further, if necessary, as indicated by  17   a  in  FIG. 1F , the second substrate  17  may be thinned to a predetermined thickness. Such thinning of the second substrate  17  to the second substrate  17   a  can facilitate singulation of the wafer in a later step. However, excessive thinning of the second substrate  17   a  would decrease handleability. Thus, the thickness of the second substrate  17   a  is preferably greater than a thickness of the wafer, exclusive of the second substrate  17   a.    
     Singulating Wafer 
     After thinning the bonded wafer, the lower-surface electrodes  20  are optionally formed on the lower surface of the first substrate  11  or the like. Then, the bonded wafer is singulated. That is, the first substrate  11   a  (if the first substrate  11   a  is left), the laser element structure, and the second substrate  17   a  are singulated together. As shown in  FIG. 1G , the singulation generally includes dividing in a direction to create an resonator end surface (i.e., dividing in X-direction) and dividing in a direction intersecting with the resonator end surface (i.e., dividing in Y-direction). Either of the division in the X-direction and the division in the Y-direction may be performed earlier. 
     For example, grooves for division is formed at least one of the first substrate  11   a  and the second substrate  17   a . In the case of forming the grooves in both the first substrate  11   a  and the second substrate  17   a , the grooves are preferably formed so that the grooves in the first substrate  11   a  and the grooves in the second substrate  17   a  are aligned on the substantially same line in a plan view. When at least one of the first substrate  11   a  and the second substrate  12   a  contains an opaque material, it is difficult to align the grooves in the first substrate  11   a  and the grooves in second substrate  12   a  on the substantially same line in a plan view. Accordingly, it is preferable that the grooves are formed only in the second substrate  17   a . With the first substrate  11   a  and the laser element structure respectively having a small thickness, forming grooves only in the second substrate  17   a  allows for dividing the wafer. This is similar in the case where the first substrate  11  is removed completely. 
     The grooves can be formed using a laser-scribing machine. Grooves may be formed in a form of lines or broken lines. In the case where the second substrate  17   a  (i.e., the main body  171   a ) has a cleavability, the grooves are preferably in a form of broken lines along an easy-cleavage plane. In the case where the grooves in a form of lines are formed to laterally extend in the wafer, the division tends to proceed in a zigzag direction or the like if the grooves do not completely correspond to the easy-cleavage plane. On the other hand, with the grooves in a form of broken lines, cleavage occurs at gaps of the broken lines of the grooves in a direction along the easy-cleavage plane even if the grooves do not completely correspond to the easy-cleavage plane. Instead of the grooves, division-starting region may be formed by performing internal laser processing. 
     After forming the grooves, the first substrate  11   a  and the second substrate  17   a  are cleaved together. The cleaving can be performed by applying a blade or the like on a surface of the wafer opposite to the surface of the wafer in which the grooves are formed. 
     The term “(a) direction intersecting the resonator end surface(s)” can refer to a direction intersecting the resonator end surface(s) at an angle in a range of approximately 90±1 degrees. 
     By such division in the two directions, the wafer is singulated into a laser element  23  with the second substrate as shown in  FIG. 1H . 
     After forming the resonator end surfaces, reflective mirrors are preferably formed on a surface at a light-emission side and a surface at a light-reflection side, which is opposite to the light emission side, of the laser element  23  with the second substrate. For example, a first dividing step of forming the resonator end surfaces can be performed, a reflective-mirror forming step can be performed, and then a second dividing step of dividing in a direction intersecting the resonator end surfaces can be performed. 
     The reflective mirrors can be made of an oxide film, a nitride film, an oxynitride film, or a combination of two or more of these. Examples of the reflective mirrors include a dielectric multilayer film for which SiO 2 , ZrO 2 , TiO 2 , Al 2 O 3 , Nb 2 O 5 , SiN, AlN, SiON, AlON or the like is used. 
     Mounting on Heat Dissipating Member 
     In a state where the singulated laser element  23  is bonded to the second substrate  17   a , the laser element  23  is mounted to the heat dissipating member  22  such that the lower-surface electrode  20  of the laser element  23  faces the heat dissipating member  22 , as shown in  FIG. 1I . That is, the laser element  23  with the second substrate  17   a  is mounted on the heat dissipating member  22  in a junction-up manner. Examples of the heat dissipating member  22  may include a member containing SiC, AlN, or the like as a main material. 
     The mounting may be performed using, for example, using a conductive bonding material such as an AuSn-alloy solder. 
     Removing Second Substrate 
     After mounting the laser element  23  on the heat dissipating member  22 , the second substrate  17   a  is pushed using a pushing jig, so that the second substrate  17   a  is separated from the laser element  23  to be removed as shown in  FIG. 1J , and thus the laser element is obtained. In the case where the second metal layer  18  is disposed on the main body  171   a , during the removing of the second substrate  17   a , a portion or an entirety of the second metal layer  18  is removed. It is preferable that the pushing jig is applied only on the main body  171   a  of the second substrate  17   a  and is pushed. This allows for reducing damage to the laser element  23 . 
     For example, a pushing jig used for a die shear strength testing machine can be used to push the second substrate  17   a . For the pushing jig, a jig that can be brought into contact with the singulated laser element  23 , and can apply a force in a direction substantially parallel to a contact surface between the second substrate  17   a  and the laser element  23  and, when the laser element  23  includes the ridge  13  at a surface thereof, also parallel to an extending direction of the ridge  13  is preferably used. If pushing is performed to move in a width direction of the ridge  13 , the second substrate  17   a  may be inclined, which may damage the second p-electrode  16 . On the other hand, pushing to move in an extending direction of the ridge  13  allows for reducing such damage. The force applied in the pushing is, for example, in a range of 2 N to 20 N. With a bonding strength that allows for causing separation when a force in this range is applied, erroneous detachment of the second substrate  17  in the step of thinning the wafer can be prevented, and breakage of the laser element  23  at the time of removing the second substrate  17  can also be prevented. The range of the force as described above is a preferable range of the force to be applied to a single ridge  13 , and the force is preferably in a range in proportion to the number of the ridges  13  pressed at once. By such a physical separation, the upper-surface electrode (i.e., the second p-electrode  16 ) is exposed at a surface of the laser element  23  from which the second substrate  17   a  is separated. 
     In the case where the second substrate  17   a  and the laser element  23  are bonded together in a bonding manner that allows strong bonding such as a liquid phase diffusion bonding, the removal of the second substrate  17   a  can also be performed by heating or the like to reduce bonding strength of a bonded portion. However, heating at a temperature greater than 300° C. may lead to deterioration of the electrodes of the laser element  23  and the like, and accordingly a metal used for the bonded portion is limited to those having a melting point of 300° C. or less. Further, examples of such a metal having a low melting point include Sn, but if the bonded portion is made of Sn, an outermost surface of the laser element  23  after the removal is made of Sn, which requires heating again at the time of wire-bonding in order to improve adhesion between the outermost surface and a wire. 
     Accordingly, physical separation is preferably performed to remove the second substrate  17   a . In other words, the second substrate  17   a  and the laser element  23  are bonded together with a bonding strength that allows physical separation between the second substrate  17   a  and the laser element  23 . By a physical separation, the second substrate  17   a  and/or the laser element  23  can be moved in directions away from each other to remove the second substrate  17   a  from the laser element  23  without an additional step such as heating or immersing in an etching solution. This allows for preventing deterioration of the laser element  23  due to heating or the like. Such a physical separation is performed under a room temperature, for example. 
     Examples of such a physical separation include, as well as pushing with the use of the pushing jig or the like as described above, moving the second substrate  17   a  while holding the second substrate  17   a  by a clamp or the like, moving the second substrate  17   a  while suction-holding the second substrate  17   a  using an suction jig. When the clamp or the suction jig is used, the second substrate  17   a  can be lifted up, that is, the second substrate  17   a  may be moved in a direction perpendicular to a main surface of the laser element  23 . When the second substrate  17   a  is held by a clamp or the like, the second substrate  17   a  may be rotated in a plan view. 
     With the removal of the second substrate  17  in such a manner, an amount of electric resistance corresponding to the second substrate  17   a  is eliminated, so that a driving voltage for applying a predetermined current can be reduced compared to the case where the second substrate  17   a  is present. Accordingly, wall-plug efficiency (WPE), which is a ratio of light output to inputted electric power, of a light emitting device  30  to be obtained can be improved. That is, efficiency of the light emitting device  30  can be increased. Further, because the second substrate  17  is removed, an inexpensive material can be used for the second substrate  17  irrespective of an electric resistance thereof. 
     After that, wire-bonding or the like is performed in the upper-surface electrode of the laser element  23  to provide an electrical conduction path, so that a voltage can be applied to the laser element  23 . A wire having a diameter of 30 μm or greater, preferably 45 μm or greater, is used for the wire-bonding. Further, with a plurality of wires connected to a single laser element  23 , a large current can be flown. The wire preferably has a thickness that allows for performing ball-bonding to the laser element  23 . For example, the wire includes a ball portion having a diameter smaller than a width of the upper-surface electrode (i.e., a size of the second p-electrode  16  in a direction parallel to the resonator end surfaces). 
     Light Emitting Device 
     The light emitting device  30  obtained in the manner as described above includes the heat dissipating member  22  and the laser element  23 , as shown in  FIG. 1J . The laser element  23  includes the semiconductor layered structure  12 , which is a laser element structure disposed on the first substrate  11   a , an n-electrode, which is the lower-surface electrode  20  disposed on the lower surface of the first substrate  11   a , and the first p-electrode  14  and the second p-electrode  16 , which are included in the upper-surface electrode disposed on the upper surface of the semiconductor layered structure  12 . The heat dissipating member  22  faces the n-electrode, and is bonded to the laser element  23  via a bonding member  24  such as an AuSn-alloy solder. 
     Thinning or completely removing the first substrate  11   a  allows the light emitting portion of the laser element  23  to be located closer to the heat dissipating member  22 , which allows heat to be easily released to the heat dissipating member  22 , so that heat resistance of the laser element  23  can be reduced. Further, with the laser element  23  mounted on the heat dissipating member  22  so that a side opposite to the ridge  13  faces the heat dissipating member  22 , a material having a high heat resistance such as an insulating film is not necessarily disposed between the laser element  23  and a submount, so that heat resistance of the laser element  23  can be further reduced compared to the case where the laser element  23  is mounted in a junction-down. 
     Even further, with the thinned first substrate  11   a  that is remained, the light emitting portion is easily located apart from the heat dissipating member  22  compared to the case where the first substrate  11  is eliminated. Accordingly, the light emission end surface may be located outward of the heat dissipating member  22 , or a distance between the light emission surface and the heat dissipating member can be reduced. Accordingly, heat dissipation performance from the laser element  23  to the heat dissipating member  22  can be improved. 
     As described above, removing the second substrate  17   a  allows for reducing electric resistance as a whole of the light emitting device  31 , and thus driving voltage can be reduced. Accordingly, efficiency of the light emitting device can be increased. The light emitting device  30  is particularly suitable for the case where the laser element  23  has a high output, which tends to generate a large quantity of heat. The expression “high output” as used herein refers to, for example, an output in a range of 1 W to 50 W. 
     Example 1: Method of Manufacturing Light Emitting Device 
     Providing Wafer 
     As shown in  FIG. 1A , a semiconductor layered structure  12  in which an n-side semiconductor layer, an active layer, and a p-side semiconductor layer are disposed in this order was formed on a +c plane of a GaN substrate of φ50.8 mm serving as a first substrate  11 . In Example 1, a nitride semiconductor layer represented by a general formula In X Al Y Ga 1−X−Y N (0≤X, 0≤Y, X+Y≤1) was used for each of the n-side semiconductor layer, the active layer, and the p-side semiconductor layer. Subsequently, stripe-shaped ridges  13  (each having a width of 35 μm and a height of 270 nm) were formed at an upper surface side of the p-side semiconductor layer. 
     Next, as shown in  FIG. 1B , first p-electrodes  14  made of an ITO film having a width of 32 μm and a thickness of 200 nm were formed on a substantially entirety of an upper surface of a respective one of the ridges  13 . Subsequently, insulating films  15  each having a thickness of 200 nm and made of SiO 2  were formed on portions of the upper surface of the p-side semiconductor layer except for the upper surface of each of the ridges  13 . Subsequently, second p-electrodes  16  each having a layered structure of Ni/Pd/Au/Pt/Au with thicknesses of 8 nm/200 nm/400 nm/200 nm/700 nm, respectively, were formed respectively over an upper surface of a corresponding one of the first p-electrodes  14  and corresponding ones of the insulating films  15  to be electrically connected to the corresponding one of the first p-electrodes  14 . 
     Bonding Wafer to Second Substrate 
     An Si substrate with a planar orientation of (100), electrical resistivity of 0.005 Ω·cm or less, and a thickness of 400 μm was provided as a second substrate  17 . As used herein, the expression “an Si substrate with a planar orientation of (100)” as used herein refers to an Si substrate having a main surface with a planar orientation of (100). The second substrate  17  includes a Pt/Au film (having thicknesses of 200 nm/700 nm, respectively), which is formed using a sputtering device and serves as a second metal film  18 , on a surface of second substrate  17 . 
     The Au layer, which is the uppermost layer of the second metal layer  18 , and the Au layer, which is the uppermost layer of each of the second p-electrodes  16 , which is a portion of each of upper-surface electrodes of the laser element structure of the wafer, were heat-pressure bonded at 7 kN and 280° C. to form a metal bond (Au—Au heat-pressure bond). Accordingly, the wafer was bonded to the second substrate  17 . Because physical separation can be performed in a later step, in this bond, the second metal layer  18  and the second p-electrodes  16  are considered to be not melted together and an interface therebetween is considered to be not eliminated but maintained. Also, after performing physical separation in a later step, irregularities due to separation at the bonded portion were observed in an upper surface of a protruding portion of the second p-electrode  16  attributed to a shape of the ridge  13 , but portions at both sides of the protruding portions were observed to be flat. Accordingly, in a bonded state, gaps  19  are considered to be present at both sides of the protruding portion. 
     At the time of bonding, the wafer and the second substrate  17  were aligned so that a line parallel to a (−110) plane of the Si substrate, which is the second substrate  17 , when viewed from above the (−110) plane of the Si substrate and a line parallel to a (1-100) plane, which is an m-plane, of the first substrate  11  of the wafer when viewed from above the m-plane of the first substrate  11  of the wafer are parallel to each other. 
     Thinning Wafer 
     As shown in  FIG. 1D , a portion of the first substrate  11  in a thickness direction was removed to thin the wafer. In more detail, in order to reduce warpage of the wafer at the time of thinning the first substrate  11 , a second substrate  17  side of the wafer was temporally bonded to a sapphire substrate with a thickness of 2 mm using a wax, and the first substrate  11  was polished so as to have a thickness of approximately 5 μm. At this time, CMP was used to finish the polishing. An in-plane distribution of a thickness of the bonded wafer after thinning was within ±1.0 μm in a substrate of φ50.8 mm. 
     Forming Lower-Surface Electrode 
     As shown in  FIG. 1E , Ti/Pt/Au layered films (with thicknesses of 6 nm/200 nm/300 nm, respectively) were formed on a polished surface of the thinned first substrate  11   a  (at an n-side semiconductor layer side) using a sputtering apparatus, so that n-electrodes as lower surface electrodes  20  were formed. 
     Subsequently, the sapphire substrate that had been temporally bonded was removed. Next, a first substrate  11   a  side of the wafer was temporally bonded with a sapphire substrate having a thickness of 2 mm using a wax in a manner similar to that as described above, and the second substrate  17  having a thickness of 400 μm was polished to become a second substrate  17   a  having a thickness of 75 μm as shown in  FIG. 1F . By this polishing, the second substrate  17  can be thinned to have a thickness appropriate for cleavage during singulation in a later step. After that, the sapphire substrate that had been temporally bonded was removed. 
     Singulating Wafer 
     Grooves were formed in the obtained second substrate  17   a  with a bonded structure at cleavage-planned positions thereof using a laser-scribing machine. One of the cleavage-planned positions is indicated by a broken line X in  FIG. 1G . Also, grooves were formed in an end portion of the first substrate  11   a  along the cleavage-planned positions using a laser-scribing machine. 
     After forming the grooves, the first substrate  11   a  and the second substrate  17   a  were cleaved along the grooves using a breaking apparatus, so that resonator end surfaces of laser elements were formed. 
     Then, a dielectric multilayer film was formed on each of the resonator end surfaces, and a substrate with a bonded structure obtained by cleaving to form the resonator end surfaces was singulated into a plurality of chips by cutting in a direction perpendicular to the resonator end surfaces, that is, in a direction indicated by broken lines Y in  FIG. 1G , using a breaking machine. Thus, a laser element  23  was obtained as shown in  FIG. 1H . 
     Mounting on Heat Dissipating Member 
     As shown in  FIG. 1I , the singulated laser element  23  that has been bonded to the second substrate  17   a  was mounted on the heat dissipating member  22  made of mainly SiC via a bonding member  24  made of AuSn-alloy solder with a lower-surface electrode  20  side of the laser element  23  faces the heat dissipating member  22  side. The heat dissipating member  22  includes a base portion made of SiC and a conductive layer disposed on a surface of the base portion. The conductive layer and the lower-surface electrode  20  were electrically connected together via the bonding member  24 . 
     Removing Second Substrate 
     In a state where the laser element  23  was mounted on the heat dissipating member  22 , the second substrate  17   a  bonded to the laser element  23  was pushed using a pushing jig with a force of 5N in a resonator direction, so that the second substrate  17   a  was separated from the laser element  23  and removed, as shown in  FIG. 1J . Separation was performed at the interface between Au of the outermost surface of the laser element  23  (i.e., the uppermost layer of the second p-electrode  16 ) and Au of the outermost surface of the second metal layer  18  in the second substrate  17   a , so that substantially no damage was observed in the laser element  23  after separation. 
     Then, wire-bonding was performed on the second p-electrode  16  of the laser element  23  and the conductive layer of the heat dissipating member  22 , so that a light emitting device  30  to which electricity can be supplied was obtained. 
     Evaluation of Light Emitting Device: Reduction in Thermal Resistance 
     The light emitting device  30  of Example 1 was mounted on a package. For a comparison, a light emitting device  40  of Comparative Example 1 was provided as described below, and mounted on a package. Transient thermal resistances of the light emitting device  30  and the light emitting device  40  were measured using a heating method (i.e., dynamic method). The light emitting device  40  of Comparative Example 1 was provided with a structure similar to that of the light emitting device  30  of Example 1 except that a laser element of the light emitting device  40  had a substrate  41  with a thickness of 80 μm, which was greater than a thickness of the first substrate  11   a , and a laser element was mounted in a junction-down manner. As shown in  FIG. 4 , the laser element of the light emitting device  40  of Comparative Example 1 included a semiconductor layered structure  12  at a first main surface side of the substrate  41 , and had a stripe-shaped ridge  13  as in the laser element  23  of Example 1. An electrode similar to the n-electrode  20  of the laser element  23  of Example 1 was disposed at a second main surface side opposite to the first main surface side of the substrate  41 . Further, a first p-electrode  44 , insulating films  15 , and a second p-electrode  16  that were similar to those in Example 1 were disposed on a surface of the semiconductor layered structure  12 . The laser element having the configuration described above was mounted in a junction-down manner on a heat dissipating member  22  that was similar to that in Example 1 via a bonding member  24  made of an AuSn-alloy solder. 
     According to a result of measurement of the transient thermal resistance, the laser element  23  of the light emitting device  30  of Example 1 had a heat resistance approximately 0.6° C./W lower than that of the laser element of the light emitting device  40  of Comparative Example 1. 
     This is considered, in the light emitting device  40  of Comparative Example 1, a first p-electrode  14  (ITO film with a thermal conductivity of 8 W/(m·K)) and the insulating film  15  (SiO 2  film with a thermal conductivity of 1 W/(m·K)) each having a lower thermal conductivity were present between an optical waveguide part (i.e., a portion near an active layer of the ridge  13 ), which is a heat source, and a submount; meanwhile, in the laser element  23  of Example 1, the n-electrode  20  (Ti/Pt/Au, where Ti layer has a thermal conductivity of 17 W/(m·K), Pt layer has a thermal conductivity of 70 W/(m·K), and Au layer has a thermal conductivity of 320 W/(m·K)) is present between the optical waveguide part and the submount, resulting in a low heat resistance value. 
     Evaluation of Light Emitting Device: Reduction in Voltage 
     Current and voltage were measured on a light emitting device including a laser element to which a second substrate  17   a  with a thickness of 75 μm (i.e., Comparative Example 2) had been bonded and the light emitting device  30  including the laser element  23  from which the second substrate  17   a  was removed in a manner described above (i.e., Example 1) while performing continuous oscillation. A second substrate  17   a  of the light emitting device of Comparative Example 2 included a main body  171   a  that is an Si substrate with an electrical resistance of 0.005 Ω·cm or less. 
     The results are shown in  FIG. 2 . 
       FIG. 2  shows that the light emitting device  30  of Example 1, from which the second substrate  17   a  was removed, could be supplied with electricity in a state where wires were connected to a surface of the light emitting device  30  where the second substrate  17   a  had been bonded.  FIG. 2  further shows that a voltage on the light emitting device  30  of Example 1 was smaller than that on the light emitting device of Comparative Example 2 (more specifically, a voltage at 3.0 A on the light emitting device  30  was 0.11 V smaller than that on the light emitting device of Comparative Example 2). This attributed to an effect of reduction in voltage due to elimination of electric resistance caused by removal of the second substrate  17   a.    
     Example 2: Method of Manufacturing Light Emitting Device 
     In Example 2, as shown in  FIG. 3B , a raising layer  31  subjected to an appropriate patterning is further formed on each of the ridges (between a respective one of the first p-electrodes  14  and a respective one of the second p-electrodes  16 ) in the step of providing the element-structure wafer as in Example 1. 
     Accordingly, a portion of an outermost layer of the respective one of the second p-electrodes  16  under which the patterned raising layer is formed is located highest in the laser element  23 , and can be mainly mounted to the second metal layer  18  of the second substrate. Thus, a bonding area (i.e., bonding strength) can be adjusted by the patterning. 
     Further, in Example 2, the light emitting device is manufactured in a manner similar to that in Example 1 except for the step described above, so that an effect of reducing heat resistance and reducing voltage can be expected as in Example 1. 
     It is to be understood that although the present invention has been described with regard to certain embodiments thereof, various other embodiments and variants may occur to those skilled in the art, which are within the scope and spirit of the invention, and such other embodiments and variants are intended to be covered by the following claims.