Patent Publication Number: US-9431588-B2

Title: Semiconductor device and method for manufacturing the same

Description:
CROSS REFERENCE OF THE RELATED APPLICATION 
     This application is a division of and claims the benefit of priority under 35 U.S.C. §120 from U.S. Ser. No. 14,023,641, filed Sep. 11, 2013, which is a divisional of U.S. Ser. No. 12,556,134, filed Sep. 9, 2009, now U.S. Pat. No. 8,581,291, and claims the benefit of priority from Japanese Patent Application No. 2008-312453, filed on Dec. 8, 2008; the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical semiconductor device and a method for manufacturing the same. 
     2. Description of the Related Art 
     In response to the development of a high-intensity optical semiconductor element that emits blue light, there has been commercialized an optical semiconductor device that emits white light by using an optical semiconductor element that emits blue light and a phosphor capable of wavelength conversion from blue light into yellow light. Such an optical semiconductor device that emits white light has characteristics of: being small in size; emitting high-intensity light relative to a supplied power amount; having a long life; and using no toxic substance such as mercury. These characteristics allow the optical semiconductor device to be increasingly used in various fields such as application to an under-button light source or a flash light source for portable phone and application to an interior light source and an exterior light source for vehicle. 
     In terms of total flux relative to a supplied power amount, an optical semiconductor device using blue LEDs currently has an efficiency of 150 lm/W, which is larger than those of conventional light sources such as an incandescent light bulb (15 lm/W to 20 lm/W) and a fluorescent lamp (60 lm/W to 90 lm/W). However, in terms of cost required for one lm, the optical semiconductor device has a problem of requiring cost more than ten times higher than the conventional light sources which require 0.1 yen/lm to 0.2 yen/lm. As a measure to reduce the cost, there has been studied a structure of an optical semiconductor device allowing cost reduction while enhancing the luminous efficiency of an element therein. 
     The most general optical semiconductor device that has been commercialized has a structure including: an optical semiconductor element that emits blue light; an Ag-plated Cu frame formed of a white thermoplastic resin by molding, and used as a wiring board; a connection material for connecting the optical semiconductor element and the frame; a gold wire through which a current flows between the frame and a top surface electrode of the optical semiconductor element; and a silicone resin mixed with phosphor particles for wavelength conversion from blue light into yellow light and sealing in the optical semiconductor element (see JP-A No. 2000-183407 (KOKAI), for example). 
     In manufacturing this optical semiconductor device, a white thermoplastic resin is firstly formed into the Ag-plated Cu frame by molding. Then, a connection resin is applied onto a portion, on which the optical semiconductor element is to be mounted, of the frame. Thereafter, the optical semiconductor element is mounted on the portion, and the connection resin is hardened by heating in an oven. Thereby, the optical semiconductor element is connected to the frame. Then, by using a wire bonder, the electrode formed on the chip top surface of the optical semiconductor element is connected to the frame with an Au wire. Thereafter, a silicone resin with a phosphor concentration adjusted to allow the optical semiconductor device to emit white light is applied, by a dispense technique, around the portion in which the optical semiconductor element is mounted, and then hardened by heating. Lastly, a product portion including the optical semiconductor element is cut off, and the frame used as an exterior electrode is finished by a forming process. Thereby, the optical semiconductor device is completed. 
     As described above, a conventional optical semiconductor device is manufactured by incorporating a blue light semiconductor element in a structure of an optical semiconductor device that, in most cases, has been commercialized using an optical semiconductor element that emits light having a wavelength of 500 nm or more. The optical semiconductor device that emits such long-wavelength light has a high directivity and has thus been applied to devices for vehicle, display panels, amusement machines and the like. 
     Such a conventional optical semiconductor device manufactured as above has a problem of having a shorter life for reasons, such as that blue light, which has a short wavelength and a high intensity, discolors a reflector resin having benzene ring. In addition, an optical semiconductor device that emits white light commercialized in response to the development of a high-intensity optical semiconductor element that emits blue light has been increasingly applied to lighting apparatuses as well as conventionally-applied apparatuses such as devices for vehicle and display panels. With this trend, cost reduction of the optical semiconductor device has been indispensably required. However, the conventional structure of an optical semiconductor device only allows limited cost reduction, and thus the structure and the manufacturing process of the optical semiconductor device has been required to be reconsidered. 
     In addition, when an optical semiconductor device is applied to a lighting apparatus, a light source therein cannot be formed of a single optical semiconductor device. Accordingly, in order to replace a general lighting apparatus, such as a fluorescent lamp, with an apparatus using an optical semiconductor device that emits white light, the apparatus needs to include multiple optical semiconductor devices. In this case, to eliminate defects such as nonuniform intensity in a light-emitting surface, a large number of small optical semiconductor devices need to be mounted on a wiring board. Accordingly, the size reduction of the optical semiconductor device has also been required. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an optical semiconductor device that is capable of suppressing the life reduction thereof, that can be manufactured at reduced cost, and that can be miniaturized approximately to the size of an optical semiconductor element, and to provide a method for manufacturing such an optical semiconductor device. 
     A first aspect according to an embodiment of the present invention is an optical semiconductor device includes: a light-emitting layer having a first main surface, a second main surface opposed to the first main surface, a first electrode and a second electrode which are formed on the second main surface; a fluorescent layer containing phosphor particles and provided on the first main surface; a light-transmissive layer provided on the fluorescent layer and made of a light-transmissive inorganic material; a first metal post provided on the first electrode; a second metal post provided on the second electrode; a sealing layer provided on the second main surface so as to seal in the first and second metal posts with one ends of the respective first and second metal posts exposed; a first metal layer provided on the exposed end of the first metal post; and a second metal layer provided on the exposed end of the second metal post. 
     A second aspect according to an embodiment of the present invention is a method for manufacturing an optical semiconductor device, includes: manufacturing a light-emitting substrate by forming multiple groups of positive and negative electrodes on a first main surface of a light-emitting layer, the positive and negative electrodes being used to cause a current for exciting the light-emitting layer to flow through the light-emitting layer; manufacturing a fluorescent substrate by forming, on a light-transmissive inorganic film, a fluorescent layer made of a resin in which phosphor particles are dispersed; bonding the fluorescent layer of the fluorescent substrate onto a second main surface of the light-emitting layer which is opposed to the first main surface; and separating the bonded substrate into pieces each including one of the groups of the positive and negative electrodes. 
     A third aspect according to an embodiment of the present invention is a method for manufacturing an optical semiconductor device, includes: forming multiple light-emitting layers on a substrate, the light-emitting layers each having a first main surface, a second main surface opposed to the first main surface, a first electrode and a second electrode which are formed on the second main surface; forming a conductive film on the substrate, on which the multiple light-emitting layers are formed, so that the conductive film covers the multiple light-emitting layers; forming a sacrifice layer on the conductive film, the sacrifice layer having openings located respectively on the first and second electrodes of all of the light-emitting layers; forming plated layers respectively on the first and second electrodes of all of the light-emitting layers by electroplating using the conductive film as a negative electrode; removing the sacrifice layer and the conductive film from the substrate on which the plated layers are formed; forming a sealing layer on the substrate from which the sacrifice layer and the conductive film are removed, the sealing layer sealing in the plated layers of all of the light-emitting layers; exposing one ends of the respective plated layers of all of the light-emitting layers from the sealing layer; forming a fluorescent layer containing phosphor particles on a light-transmissive substrate made of a light-transmissive inorganic material; bonding the fluorescent layer formed on the light-transmissive substrate onto all of the light-emitting layers; forming metal layers respectively on the exposed ends of the plated layers of all of the light-emitting layers; and separating the resultant substrate into pieces each including one of the light-emitting layers. 
     A fourth aspect according to an embodiment of the present invention is a method for manufacturing an optical semiconductor device, includes: forming multiple light-emitting layers on a substrate, the light-emitting layers each having a first main surface, a second main surface opposed to the first main surface, a first electrode and a second electrode which are formed on the second main surface; forming a conductive film on the substrate, on which the multiple light-emitting layers are formed, so that the conductive film covers the multiple light-emitting layers; forming a sacrifice layer on the conductive film, the sacrifice layer having openings located respectively on the first and second electrodes of all of the light-emitting layers; forming plated layers respectively on the first and second electrodes of all of the light-emitting layers by electroplating using the conductive film as a negative electrode; removing the sacrifice layer and the conductive film from the substrate on which the plated layers are formed; forming a sealing layer on the substrate from which the sacrifice layer and the conductive film are removed, the sealing layer sealing in the plated layers of all of the light-emitting layers; exposing one ends of the respective plated layers of all of the light-emitting layers from the sealing layer; forming a fluorescent layer containing phosphor particles on all of the light-emitting layers; forming a light-transmissive layer made of a light-transmissive inorganic material on the fluorescent layer; forming metal layers respectively on the exposed ends of the plated layers of all of the light-emitting layers; and separating the resultant substrate into pieces each including one of the light-emitting layers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view showing a schematic structure of an optical semiconductor device according to a first embodiment of the present invention. 
         FIG. 2  is a plan view showing the lower surface of the optical semiconductor device shown in  FIG. 1 . 
         FIG. 3  is a cross-sectional view showing a schematic structure of an optical semiconductor device according to a second embodiment of the present invention. 
         FIG. 4  is a cross-sectional view showing a schematic structure of an optical semiconductor device according to a third embodiment of the present invention. 
         FIG. 5  is a plan view showing the lower surface of the optical semiconductor device shown in  FIG. 4 . 
         FIG. 6  is a cross-sectional view showing a schematic structure of an optical semiconductor device according to a fourth embodiment of the present invention. 
         FIG. 7  is a cross-sectional view showing a schematic structure of an optical semiconductor device according to a fifth embodiment of the present invention. 
         FIG. 8  is a first process cross-sectional view for illustrating a manufacturing method according to a sixth embodiment of the present invention. 
         FIG. 9  is a second process cross-sectional view. 
         FIG. 10  is a third process cross-sectional view. 
         FIG. 11  is a fourth process cross-sectional view. 
         FIG. 12  is a fifth process cross-sectional view. 
         FIG. 13  is a sixth process cross-sectional view. 
         FIG. 14  is a seventh process cross-sectional view. 
         FIG. 15  is an eighth process cross-sectional view. 
         FIG. 16  is a ninth process cross-sectional view. 
         FIG. 17  is a tenth process cross-sectional view. 
         FIG. 18  is an eleventh process cross-sectional view. 
         FIG. 19  is a twelfth process step cross-sectional view 
         FIG. 20  is a first process cross-sectional view for illustrating a manufacturing method according to a seventh embodiment of the present invention. 
         FIG. 21  is a second process cross-sectional view. 
         FIG. 22  is a third process cross-sectional view. 
         FIG. 23  is a fourth process cross-sectional view. 
         FIG. 24  is a first process cross-sectional view for illustrating a manufacturing method according to an eighth embodiment of the present invention. 
         FIG. 25  is a second process cross-sectional view. 
         FIG. 26  is a third process cross-sectional view. 
         FIG. 27  is a fourth process cross-sectional view. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     (First Embodiment) 
     With reference to  FIGS. 1 and 2 , a first embodiment of the present invention will be described. 
     As shown in  FIG. 1 , an optical semiconductor device  1 A according to the first embodiment of the present invention includes: a light-emitting layer  2 , an adhesive layer  3 , a fluorescent layer  4 , a light-transmissive layer  5 , a reflective layer  6 , a first electrode  7   a , multiple second electrodes  7   b , a first metal post  8   a , multiple second metal posts  8   b , an insulating layer  9 , a sealing layer  10 , a first metal layer  11   a  and multiple second metal layers  11   b . The light-emitting layer  2  has a first main surface M 1  and a second main surface M 2 . The adhesive layer  3 , the fluorescent layer  4  and the light-transmissive layer  5  are sequentially provided on the first main surface M 1 . The reflective layer  6  is provided on a first region of the second main surface M 2  of the light-emitting layer  2 . The first electrode  7   a  is provided on a second region of the second main surface M 2 . The second electrodes  7   b  are provided on the reflective layer  6 . The first metal post  8   a  is provided on the first electrode  7   a . The second metal posts  8   b  are provided on the respective second electrodes  7   b . The insulating layer  9  is provided on regions, other than those with the metal posts  8   a  and  8   b , of the second main surface M 2  of the light-emitting layer  2 . The sealing layer  10  is provided on the insulating layer  9  so as to seal in the metal posts  8   a  and  8   b . The first metal layer  11   a  is provided on one end of the first metal post  8   a . The second metal layers  11   b  are provided on one ends of the second metal posts  8   b , respectively. 
     The light-emitting layer  2  is formed of first and second cladding layers  2   a  and  2   b , and an active layer  2   c . The first cladding layer  2   a  is an n-type semiconductor layer. The second cladding layer  2   b  is a p-type semiconductor layer, and is smaller in area than the first cladding layer  2   a . The active layer  2   c  is held between the first and second cladding layers  2   a  and  2   b . The light-emitting layer  2  has a thickness of 5 μm, and is formed, for example, of InGaN layers that emit blue light. Note that the upper surface (in  FIG. 1 ) of the first cladding layer  2   a  serves as the first main surface M 1 , and the lower surface (in  FIG. 1 ) of the second cladding layer  2   b  and part of the lower surface (in  FIG. 1 ) of the first cladding layer  2   a  collectively serve as the second main surface M 2 , which has a step. 
     As shown in  FIG. 2 , the planar shape of the first cladding layer  2   a  is a square 550 μm on a side (see the dotted line of  FIG. 2 ). On a region, not including a corner region (a square 150 μm on a side), of the lower surface (in  FIG. 1 ) of the first cladding layer  2   a , the second cladding layer  2   b  is formed with the active layer  2   c  interposed therebetween. The active layer  2   c  has the same shape and approximately the same area as the second cladding layer  2   b.    
     The adhesive layer  3  is made of a silicone resin. The adhesive layer  3  has a thickness not more than 1 μm, for example, and bonds the fluorescent layer  4  and the first main surface M 1  of the first cladding layer  2   a  of the light-emitting layer  2  together. Specifically, as the silicone resin, methyl phenyl silicone having a refractive index of approximately 1.5 is used. As a resin for sealing in phosphor particles, methyl phenyl silicone or a silicone resin different in composition, such as dimethyl silicone, may be used. Alternatively, as long as appropriate for the intended use, another resin may be used. For example, when the optical semiconductor device  1 A has a low luminance and thus the sealing resin will not be degraded by blue light, a resin such as an epoxy resin, a hybrid resin of an epoxy resin and a silicone resin, or a urethane resin may be used as the sealing resin. 
     The fluorescent layer  4  is formed by mixing phosphor particles in a silicone resin. The phosphor particles convert blue light into long-wavelength light. The fluorescent layer  4  has a thickness of 15 μm, for example. As the silicone resin, methyl phenyl silicone having a refractive index of approximately 1.5, which is also used for the adhesive layer  3 , is used. However, the silicone resin used for the fluorescent layer  4  is not limited to this, but may be another resin. Meanwhile, as the phosphor, used is YAG:Ce, which is yttrium aluminate doped with cerium as an activator and which has a particle diameter not more than 10 μm. Alternatively, as the phosphor, (Sr, Ba) 2 SiO 4 , which is strontium-barium silicate doped with europium as an activator, Ca p (Si, Al) 12  or the like may be used according to need. Moreover, the mixed phosphor need not necessarily be made of a material having a single composition, but may be made of a mixture of two materials: one for wavelength conversion from blue light into green light; the other for wavelength conversion from blue light into red light. 
     The light-transmissive layer  5  is made of a transparent substrate made of a material such as optical glass or quartz. The light-transmissive layer  5  has a thickness of 200 μm, for example. The material of the light-transmissive layer  5  is not limited to the transparent substrate, but may be another light-transmissive material. In other words, the light-transmissive layer  5  needs only to be made of a light-transmissive substrate made of a light-transmissive inorganic material. However, in the interest of light extraction efficiency of the optical semiconductor device  1 A, a substrate having as a low refractive index as possible within a range from 1.0 to 2.0 should preferably be used as the light-transmissive substrate. This reduces a difference in refractive index between the fluorescent layer  4  and the air, and thus can improve the light extraction efficiency of the optical semiconductor device  1 A. 
     The reflective layer  6  is made of a metal such as Ag or Al, and has a thickness of 0.3 μm, for example. The reflective layer  6  is provided on the entire region (first region) of the lower surface (in  FIG. 1 ) of the second cladding layer  2   b  in the light-emitting layer  2 . Specifically, an Ni/Au contact electrode (not shown) is formed by depositing metals such as Ni and Au in a thickness of 0.1 μm/0.1 μm on the lower surface of the second cladding layer  2   b , and the reflective layer  6  having a thickness of 0.3 μm is then formed thereon. 
     The first electrode  7   a  is formed by depositing metals such as Ni and Au in a thickness of 0.1 μm/0.1 μm, and thus has a thickness of 0.2 μm. The first electrode  7   a , which is formed in a circle having a diameter of 100 μm, is provided on an exposed region (second region) of the lower surface (in  FIG. 1 ) of the first cladding layer  2   a  in the light-emitting layer  2  (see  FIG. 2 ). 
     Each second electrode  7   b  is also formed by depositing metals such as Ni and Au in a thickness of 0.1 μm/0.1 μm, and thus has a thickness of 0.2 μm. The second electrodes  7   b , each of which is formed in a circle having a diameter of 100 μm, are provided on the lower surface (in  FIG. 1 ) of the reflective layer  6  with a 200 μm pitch (see  FIG. 2 ). 
     The first metal post  8   a  is formed of a metal such as Cu to have a columnar shape. The first metal post  8   a  has a height of approximately 103 μm and a diameter of 100 μm. A current flows between the first metal post  8   a  and the first electrode  7   a . Note that the shapes of the first electrode  7   a  and the first metal post  8   a  may be changed according to need. 
     Each second metal post  8   b  is formed of a metal such as Cu to have a columnar shape. The second metal post  8   b  has a height of approximately 100 μm and a diameter of 100 μm. A current flows between the second metal post  8   b  and the corresponding second electrode  7   b . Like the second electrodes  7   b , the second metal posts  8   b  are provided with a 200 μm pitch (see  FIG. 2 ). Note that the shapes of the second electrodes  7   b  and the second metal posts  8   b  may be changed according to need. 
     The insulating layer  9  is made of SiO 2 , and functions as a passivation film (protective film). The insulating layer  9  has a thickness of 0.3 μm. The insulating layer  9  entirely covers the light-emitting layer  2  including the ends thereof, and thus prevents a current from flowing from the outside into the light-emitting layer  2  excluding the first and second electrodes  7   a  and  7   b . This can prevent defects such as short circuit attributable to the phenomenon that mounting solder creeps up. 
     The sealing layer  10  is formed of a thermosetting resin, and has a thickness of approximately 100 μm like the metal posts  8   a  and  8   b . The sealing layer  10  is provided on the entire surface of the insulating layer  9  so as to seal in the first and second metal posts  8   a  and  8   b  while leaving one ends thereof exposed. Accordingly, the circumferential surface of each of the first and second metal posts  8   a  and  8   b  is completely covered with the sealing layer  10 . 
     Note that, though provided so as to entirely cover the light-emitting layer  2  including the ends thereof, the insulating layer  9  need not necessarily be provided in this manner. Alternatively, not the insulating layer  9  but the sealing layer  10  may be provided so as to entirely cover the light-emitting layer  2  including the ends thereof. In this case as well, a current is prevented from flowing from the outside into the light-emitting layer  2  excluding the first and second electrodes  7   a  and  7   b . Accordingly, defects such as short circuit attributable to the phenomenon that mounting solder creeps up can be prevented. 
     Each of the first and second metal layers  11   a  and  11   b  is formed by depositing metals such as Ni and Au in a thickness of 0.1 μm/0.1 μm. The first metal layer  11   a  is provided on the end, that is, the exposed portion, of the first metal post  8   a . Each second metal layer  11   b  is provided on the end, that is, the exposed portion, of the corresponding second metal post  8   b . Note that the first metal layer  11   a  has the same circular planar shape as the first electrode  7   a , and each second metal layer  11   b  has the same circular planar shape as each second electrode  7   b  (see  FIG. 2 ). 
     In the optical semiconductor device  1 A as described above, when voltages are applied to the first and second metal posts  8   a  and  8   b , the resultant potential is supplied to the first cladding layer  2   a  through the first metal post  8   a , and the resultant potential is supplied to the second cladding layer  2   b  through the second metal posts  8   b . As a result, light beams are emitted from the active layer  2   c  held between the first and second cladding layers  2   a  and  2   b . Some of the emitted light beams are transmitted through the light-transmissive layer  5 , and emitted from the surface of the light-transmissive layer  5  without reflection. Some of the emitted light beams are reflected by the reflective layer  6 , then transmitted through the light-transmissive layer  5 , and emitted from the surface of the light-transmissive layer  5 . Others are incident on the phosphor particles included in the fluorescent layer  4 , so that the phosphor particles are excited to emit light beams. Some of the light beams emitted from the phosphor particles are also transmitted through the light-transmissive layer  5 , and emitted from the surface of the light-transmissive layer  5 . Others are also reflected by the reflective layer  6 , then transmitted through the light-transmissive layer  5 , and emitted from the surface of the light-transmissive layer  5 . In this way, the blue light beams emitted from the light-emitting layer  2  and the (yellow, or red and green) light beams emitted from the phosphor particles that are excited by the blue light beams are mixed into white light, which is emitted from the surface of the light-transmissive layer  5 . 
     With the above-described structure, the optical semiconductor device  1 A is simplified in configuration, and miniaturized to have a plane area as small as that of the light-emitting layer  2 . In addition, the structure eliminates the need for molding, a mounting step and a connecting step in manufacturing the optical semiconductor device  1 A. Accordingly, the optical semiconductor device  1 A can be manufactured by using normal semiconductor manufacturing equipment, and thus can be manufactured at reduced cost. Moreover, in the structure, the fluorescent layer  4  for wavelength conversion from blue light into long-wavelength light is formed on the light-emitting layer  2 , and the reflective layer  6  is formed under the lower surface of the light-emitting layer  2  (in  FIG. 1 ). Accordingly, by causing the light-emitting layer  2  to emit blue light only upward, white light can be emitted toward the top surface (in  FIG. 1 ) of the optical semiconductor device  1 A. In addition, the light-transmissive layer  5  formed on the fluorescent layer  4  reduces the refractive index difference between the fluorescent layer  4  and the air, which allows the optical semiconductor device  1 A to have improved light extraction efficiency. Furthermore, the above-mentioned structure allows the optical semiconductor device  1 A, which has a plane area as small as that of the light-emitting layer  2 , to be reliably mounted on a glass epoxy board, which is a typical wiring board. This is because the metal posts  8   a  and  8   b  reduce the linear expansion coefficient difference between the light-emitting layer  2  and the glass epoxy board. 
     As described above, according to the first embodiment of the present invention, the optical semiconductor device  1 A having the above-described structure is obtained as follows. Firstly, the fluorescent layer  4  is provided on the light-emitting layer  2 , and a light-transmissive inorganic material is then deposited on the fluorescent layer  4  so as to serve as the light-transmissive layer  5 . Thereafter, the first metal post  8   a  is provided on the first electrode  7   a  of the light-emitting layer  2 , and the second metal posts  8   b  are provided on the respective second electrodes  7   b  of the light-emitting layer  2 . Then, the sealing layer  10  is provided on the light-emitting layer  2  so as to seal in the first and second metal posts  8   a  and  8   b . In this optical semiconductor device  1 A, the light-transmissive layer  5  is made of an inorganic material, and thus prevented from being degraded by light (blue light, in particular) emitted from the light-emitting layer  2 . Accordingly, the life reduction of the optical semiconductor device  1 A is suppressed. In addition, having a simplified structure, the optical semiconductor device  1 A may be manufactured at reduced cost. Accordingly, cost reduction of the optical semiconductor device  1 A can be achieved. Moreover, having a simplified structure and a plane area as small as that of the light-emitting layer  2 , the optical semiconductor device  1 A can be miniaturized approximately to the size of a typical optical semiconductor element. 
     (Second Embodiment) 
     With reference to  FIG. 3 , a second embodiment of the present invention will be described. In the second embodiment of the present invention, only differences from the first embodiment will be described. Note that, in the second embodiment, the same parts as those in the first embodiment are denoted by the same reference numerals, and the description thereof is omitted. 
     As shown in  FIG. 3 , in an optical semiconductor device  1 B according to the second embodiment of the present invention, the first metal layer  11   a  and the second metal layers  11   b  are solder bumps. In other words, a hemispherical solder bump having a diameter of 100 μm, is formed on each of the first and second metal posts  8   a  and  8   b . The solder bump is made of a solder material used for surface mounting, such as Sn-3.0Ag-0.5Cu, Sn-0.8Cu and Sn-3.5Ag. 
     As described above, the second embodiment of the present invention can provide the same effects as the first embodiment. Moreover, since the first metal layer  11   a  and the second metal layers  11   b  are formed of solder bumps, the optical semiconductor device  1 B has a larger gap from a wiring board on which the optical semiconductor device  1 B is mounted than the optical semiconductor device  1 A according to the first embodiment. Accordingly, when the optical semiconductor device  1 B is mounted on the wiring board, a stress attributable to the linear expansion coefficient difference between the optical semiconductor device  1 B and the wiring board can be further reduced. 
     (Third Embodiment) 
     With reference to  FIGS. 4 and 5 , a third embodiment of the present invention will be described. In the third embodiment of the present invention, only differences from the first embodiment will be described. Note that, in the third embodiment, the same parts as those in the first embodiment are denoted by the same reference numerals, and the description thereof is omitted. 
     As shown in  FIGS. 4 and 5 , in an optical semiconductor device  1 C according to the third embodiment of the present invention, the first electrode  7   a  is formed as a square 100 μm on a side on the lower surface (in  FIG. 4 ) of the first cladding layer  2   a . Meanwhile, on the lower surface (in  FIG. 4 ) of the second cladding layer  2   b , the second electrode  7   b  is formed as a square 500 μm on a side with a square 150 μm on a side cut out. Specifically, the cutout portion corresponds to the corner region of the first cladding layer  2   a . The first metal post  8   a  is a rectangular column having the same planar shape as the first electrode  7   a , while the second metal post  8   b  is a rectangular column having the same planar shape as the second electrode  7   b . Moreover, the first metal layer  11   a  has the same planar shape as the first electrode  7   a , the second metal layer  11   b  has the same planar shape as the second electrode  7   b  (see  FIG. 5 ). 
     As described above, the third embodiment of the present invention can provide the same effects as the first embodiment. Moreover, in the optical semiconductor device  1 C according to the third embodiment, the plane area of each of the first and second electrodes  7   a  and  7   b  is increased, and thus the plane area of each of the first and second metal posts  8   a  and  8   b  is increased, compared to the optical semiconductor device  1 A according to the first embodiment. This expands heat dissipation paths for allowing heat generated during light emission to escape from the optical semiconductor device  1 C, and thus reduces thermal resistance thereof. This allows the optical semiconductor device  1 C to generate a reduced amount of heat during the passage of a current, and to have a greatly reduced transient thermal resistance. 
     (Fourth Embodiment) 
     With reference to  FIG. 6 , a fourth embodiment of the present invention will be described. In the fourth embodiment of the present invention, only differences from the first embodiment will be described. Note that, in the fourth embodiment, the same parts as those in the first embodiment are denoted by the same reference numerals, and the description thereof is omitted. 
     As shown in  FIG. 6 , an optical semiconductor device  1 D in according to the fourth embodiment of the present invention does not include the adhesive layer  3 , and the fluorescent layer  4  is formed directly on the first main surface M 1  of the light-emitting layer  2 . The fluorescent layer  4  has a thickness of 10 μm, and is formed on the first main surface M 1  of the light-emitting layer  2  by a method such as sputtering or chemical vapor deposition (CVD). The light-transmissive layer  5  is formed on the fluorescent layer  4  by, for example, applying a liquid glass onto the fluorescent layer  4  by spin coating, and then hardening the liquid glass. 
     As described above, the fourth embodiment of the present invention can provide the same effects as the first embodiment. Moreover, the above-described structure can eliminate, from the manufacturing process, a step of blending phosphor particles with a silicone resin and a step of bonding the fluorescent layer  4  onto the light-emitting layer  2 , both of which are needed in the manufacturing process of the optical semiconductor device  1 A according to the first embodiment. Thus, the fourth embodiment can reduce the manufacturing process time and the cost. 
     (Fifth Embodiment) 
     With reference to  FIG. 7 , a fifth embodiment of the present invention will be described. In the fifth embodiment of the present invention, only differences from the first embodiment will be described. Note that, in the fifth embodiment, the same parts as those in the first embodiment are denoted by the same reference numerals, and the description thereof is omitted. 
     As shown in  FIG. 7 , an optical semiconductor device  1 E according to the fifth embodiment of the present invention does not include the adhesive layer  3 , and the fluorescent layer  4  is formed directly on the first main surface M 1  of the light-emitting layer  2 . In this embodiment, the fluorescent layer  4  consists of two layers, namely, fluorescent layers  4   a  and  4   b  respectively made of materials mutually different in composition. Specifically, on the light-emitting layer  2 , sequentially formed are the fluorescent layer  4   a  for wavelength conversion from blue light into green light, and the fluorescent layer  4   b  for wavelength conversion from blue light into red light. Each of the fluorescent layers  4   a  and  4   b  has a thickness of 10 μm, and is formed on the first main surface M1 of the light-emitting layer  2  by a method such as sputtering or chemical vapor deposition (CVD). The light-transmissive layer  5  is formed on the fluorescent layer  4  by, for example, applying a liquid glass onto the fluorescent layer  4   b  by spin coating, and then hardening the liquid glass. 
     As described above, the fifth embodiment of the present invention can provide the same effects as the first embodiment. Moreover, the above-described structure can eliminate, from the manufacturing process, a step of blending phosphor particles with a silicone resin and a step of bonding the fluorescent layer  4  onto the light-emitting layer  2 , both of which are needed in the manufacturing process of the optical semiconductor device  1 A according to the first embodiment. Thus, the fifth embodiment can reduce the manufacturing process time and the cost. 
     (Sixth Embodiment) 
     With reference to  FIGS. 8 to 19 , a sixth embodiment of the present invention will be described. In the sixth embodiment of the present invention, a method for manufacturing the optical semiconductor device  1 A according to the first embodiment will be described. Note that this manufacturing method can be used as a method for manufacturing the optical semiconductor device  1 C according to the third embodiment. In the sixth embodiment, the same parts as those in the first embodiment are denoted by the same reference numerals, and the description thereof is omitted. 
     Firstly, as shown in  FIG. 8 , InGaN light-emitting layers  12  that emit blue light are formed on a substrate  11 , which is a sapphire wafer having a diameter of two inches and a thickness of 200 μm. Specifically, a light-emitting layer is formed by epitaxial growth, and then separated into the light-emitting layers  12  by reactive ion etching (RIE). In this way, the light-emitting layer  2  of the optical semiconductor device  1 A is formed. The light-emitting layer  2  is formed by forming the first cladding layer  2   a  on a square region 550 μm on a side, and then forming the second cladding layer  2   b  on a region, not including a corner region (a square 150 μm on a side), of the lower surface of the first cladding layer  2   a  with the active layer  2   c  interposed therebetween (see  FIGS. 1 and 2 ). 
     Then, as shown in  FIG. 9 , multi-layer films  13  are formed on the respective light-emitting layers  12  on the substrate  11 . Specifically, Ni/Au films (not shown) having a thickness of 0.1 μm/0.1 μm are firstly formed on the entire surfaces of the light-emitting layers  12  by sputtering so as to serve as contact layers of the light-emitting layers  12 , respectively. Then, metal films (not shown) made of Ag or Al and having a thickness of 0.3 μm are formed on the respective Ni/Au films by sputtering. In this way, the reflective layer  6  of the optical semiconductor device  1 A is formed. Thereafter, Ni/Au films (not shown) having a thickness of 0.1 μm/0.1 μm, which are to be formed into electrodes, are respectively formed on electrode portions of the light-emitting layers  12 . Then, SiO 2  passivation films (not shown) having a thickness of 0.3 μm are formed by sputtering on regions other than the electrode portions of the light-emitting layers  12 . In this way, the first electrode  7   a , the second electrodes  7   b  and the insulating layer  9  of the optical semiconductor device  1 A are formed. As described above, the multi-layer films  13  are formed on the respective light-emitting layers  12  on the substrate  11 . 
     Then, as shown in  FIG. 10 , a seed layer  14 , which is a conductive film serving as a power feeding layer for plating, is formed on the entire surface of the substrate  11  by a physical deposition method such as vapor deposition or sputtering. As this seed layer  14 , a multi-layer film such as a Ti/Cu film is used. When the Ti/Cu film is employed, the Ti layer, which is formed to increase adhesion strength between the seed layer  14  and a resist or pads, needs only to have a thickness as small as 0.1 μm. Meanwhile, the Cu layer, which mainly contributes to power feeding, should preferably have a thickness not smaller than 0.2 μm. 
     Thereafter, as shown in  FIG. 11 , a resist layer  15  serving as a sacrifice layer is formed on the entire surface of the substrate  11 . The resist layer  15  has openings at electrode pad portions which are to be formed into the first and second electrodes  7   a  and  7   b . As the resist, a photosensitive liquid resist or a dry film resist may be used. Specifically, the resist layer  15  is formed on the entire surface of the substrate  11  by forming a resist layer on the substrate  11 , and then forming the openings in this resist layer by exposure and development using a light shield mask. After development, the resist may be baked depending on its material. 
     Then, as shown in  FIG. 12 , plated layers  16  are formed by electroplating in the respective openings of the resist layer  15 . In this way, the metal posts  8   a  and  8   b  of the optical semiconductor device  1 A are formed. In electroplating, the substrate  11 , which is a wafer, is immersed in a plating liquid consisting of materials such as copper sulphate and sulphuric acid. Under this condition, current is caused to flow through the substrate  11  by connecting the seed layer  14  to the negative terminal of a direct-current power source, and connecting a Cu plate, serving as an anode, to the positive terminal of the direct-current power source, and thus Cu plating starts. Here, the Cu plate is placed so as to face the to-be-plated surface of the substrate  11 . Before the thickness of the plated layer, which increases with time, reaches that of the resist layer  15 , the current is stopped, and thus the plating is completed. 
     After the plating, as shown in  FIG. 13 , the resist layer  15  is peeled off the substrate  11 , and then the seed layer  14  is etch removed by acid cleaning. Thereby, the light-emitting layers  12 , the multi-layer films  13  and the plated layers  16  are exposed. 
     Then, as shown in  FIG. 14 , a thermosetting resin layer  17  to serve as a sealing layer is formed on the entire surface of the substrate  11 . Specifically, a thermosetting resin is applied by spin coating around the plated layers  16  in a thickness to allow the plated layers  16  embedded therein. Thereafter, the resultant substrate  11  is put in an oven and heated, and thus the thermosetting resin layer  17  is hardened. The resin can be hardened by being heated at 150° C. for two hours, for example. 
     After that, as shown in  FIG. 15 , the surface of the thermosetting resin layer  17  is ground off so that ends of the respective plated layers  16  can be exposed. In this way, the sealing layer  10  of the optical semiconductor device  1 A is formed. The thermosetting resin layer  17  is grounded by rotational grinding using a rotational grinding wheel, and thus the surface thereof can be planarized after being ground. After ground, the substrate  11  may be dried according to need. Note that, in the previous step, it is difficult to apply a thermosetting resin by a method such as spin coating with only the ends of the plated layers  16  exposed (It requires much time and cost). Accordingly, this grinding step is necessary for exposing the ends of the plated layers  16  after the spin coating step. 
     Then, as shown in  FIG. 16 , the light-emitting layers  12  are lifted off the substrate  11  by irradiating the interface between the substrate  11  and the light-emitting layers  12  with laser beams. Thereby, a light-emitting substrate  12 A formed of the light-emitting layers  12 , the multi-layer films  13 , the plated layers  16  and the thermosetting resin layer  17  is separated off the substrate  11 . The light-emitting layers  12  are lifted off the substrate  11  by causing an Nd:YAG third harmonic laser to irradiate the interface therebetween with laser beams having a wavelength of 355 nm through the substrate  11 . Note that the liftoff step is optional, and thus may be omitted. 
     Then, as shown in  FIG. 17 , the surface, at which the light-emitting layers  12  are exposed, of the light-emitting substrate  12 A formed by this liftoff step is bonded onto a fluorescent layer  19  that is provided on a light-transmissive substrate  18 , such as an optical glass wafer with an adhesive layer  20  interposed therebetween. This fluorescent substrate on which the light-emitting substrate  12 A is bonded is formed in another step. Specifically, a silicone resin layer mixed with phosphor particles is formed as the fluorescent layer  19  on the light-transmissive substrate  18  made of a light-transmissive inorganic material, and another silicone resin layer is formed as the adhesive layer  20  on the silicone resin layer. In this way, the light-transmissive layer  5 , the fluorescent layer  4  and the adhesive layer  3  of the optical semiconductor device  1 A are formed. 
     Here, the phosphor particles and the silicone resin are uniformly mixed using a rotary and revolutionary mixer, and then applied onto the light-transmissive substrate  18  by spin coating. The resultant light-transmissive substrate  18  is put in an oven, and the silicone resin is hardened therein. The silicone resin used here can be hardened by being heated at 150° C. for an hour, for example. In order to form the fluorescent layer  4  in a uniform thickness, after applied onto the light-transmissive substrate  18 , the silicone resin is hardened with spacers formed thereon, and with jigs having anti-sticking surfaces coated with fluorine attached thereon. Thereby, the curvature of the surface of the silicone resin film attributable to surface tension can be suppressed, and thus the silicone resin film having a uniform thickness can be formed. 
     The light-emitting layers  12  are bonded onto the fluorescent layer  19 , which is a silicone resin layer mixed with phosphor particles, as follows. Firstly, a silicone resin is applied onto the fluorescent layer  19  (or the light-emitting layers  12 ) by spraying. After that, the light-emitting substrate  12 A appropriately positioned and stacked on the light-transmissive substrate  18 . The light-emitting substrate  12 A and the light-transmissive substrate  18  thus stacked are put into an oven, and bonded together by hardening the silicone resin therein. The silicone resin can be hardened by being heated at 150° C. for an hour, for example. 
     Then, as shown in  FIG. 18 , Ni/Au layers  21  are formed by electroless plating on the respective plated layers  16  to serve as Cu electrodes. In this way, the metal layers  11   a  and  11   b  of the optical semiconductor device  1 A are formed. In Ni electroless plating, the wafer is firstly degreased by, for example, being treated with a slightly alkaline degreasing liquid for three minutes, and is then washed with running water for one minute. Thereafter, the wafer is acid cleaned, and then immersed in a nickel-phosphorus plating liquid at liquid temperature adjusted to 70° C. Thereafter, the wafer is washed with water, and thus the Ni layers are formed. In addition, in Au electroless plating, the wafer is immersed in an electroless gold plating liquid at liquid temperature adjusted to 70° C. Thereafter, the wafer is washed with water and then dried. In this way, the surfaces of the respective Cu electrodes are plated. 
     Lastly, as shown in  FIG. 19 , the resultant stack is diced into the multiple optical semiconductor devices  1 A using a dicer. In this way, the optical semiconductor device  1 A according to the first embodiment is obtained. Note that approximately the same steps as above are employed in the manufacturing process of the optical semiconductor device  1 C according to the third embodiment. By changing the size and shape of the openings in the resist layer  15 , the optical semiconductor device  1 C according to the third embodiment can be obtained. 
     As described above, according to the sixth embodiment of the present invention, the optical semiconductor device  1 A according to the first embodiment can be manufactured. Thus, the sixth embodiment can provide the same effects as the first embodiment. In addition, by changing the size and shape of the openings in the resist layer  15 , the optical semiconductor device  1 C according to the third embodiment can be manufactured. Thus, the sixth embodiment can provide the same effects as the third embodiment. Moreover, the sixth embodiment allows a large number of optical semiconductor devices  1 A or  1 C to be manufactured through a single manufacturing process, and thus allows mass production of the optical semiconductor devices  1 A or  1 C. Therefore, the sixth embodiment can suppress the costs of the optical semiconductor devices  1 A and  1 C. 
     (Seventh Embodiment) 
     With reference to  FIGS. 20 to 23 , a seventh embodiment of the present invention will be described. In the seventh embodiment of the present invention, a method for manufacturing the optical semiconductor device  1 B according to the second embodiment will be described. Note that, in the seventh embodiment, the same parts as those in the second embodiment are denoted by the same reference numerals, and the description thereof is omitted. 
     The manufacturing process according to the seventh embodiment of the present invention has the same steps as those in the sixth embodiment from the step of forming the light-emitting layers  12  shown in  FIG. 8  to the bonding step shown in  FIG. 17 . 
     After the bonding step, as shown in  FIG. 20 , contact layers  31  such as Ni/Au layers are formed on by electroless plating on the respective plated layers  16  to serve as Cu electrodes. In Ni electroless plating and Au electroless plating, the same processes are performed as those in the step of forming the Ni/Au layers  21  according to the sixth embodiment. 
     Then, as shown in  FIG. 21 , a solder paste  32  made of Sn-3.0Ag-0.5Cu is applied onto the contact layers  31  by printing. Note that the method for applying the solder paste  32  is not limited to printing. 
     Then, as shown in  FIG. 22 , the light-transmissive substrate  18 , which is a wafer, is passed through a reflow furnace. As a result, the solder is remelted and a flux residue is cleaned off. Thereby, solder bumps  33  are formed on the respective plated layers  16  to serve as Cu electrodes. In this way, the metal layers  11   a  and  11   b  of the optical semiconductor device  1 B are formed. 
     Lastly, as shown in  FIG. 23 , the resultant stack is diced into the multiple optical semiconductor devices  1 B using a dicer. In this way, the optical semiconductor device  1 B according to the second embodiment is obtained. 
     As described above, according to the seventh embodiment of the present invention, the optical semiconductor device  1 B according to the second embodiment can be manufactured. Thus, the seventh embodiment can provide the same effects as the second embodiment. Moreover, the seventh embodiment allows a large number of optical semiconductor devices  1 B to be manufactured through a single manufacturing process, and thus allows mass production of the optical semiconductor devices  1 B. Therefore, the seventh embodiment can suppress the costs of the optical semiconductor devices  1 B. 
     (Eighth Embodiment) 
     With reference to  FIGS. 24 to 27 , an eighth embodiment of the present invention will be described. In the eighth embodiment of the present invention, a method for manufacturing the optical semiconductor device  1 D according to the fourth embodiment will be described. Note that this manufacturing method can be used as a method for manufacturing the optical semiconductor device  1 E according to the fifth embodiment. In the eighth embodiment, the same parts as those in the fourth embodiment are denoted by the same reference numerals, and the description thereof is omitted. 
     The manufacturing process according to the eighth embodiment of the present invention has the same steps as those in the sixth embodiment from the step of forming the light-emitting layers  12  shown in  FIG. 8  to the liftoff step shown in  FIG. 16 . 
     After the liftoff step, as shown in  FIG. 24 , a fluorescent layer  41  is formed on the surface, at which the light-emitting layers  12  are exposed, of the light-emitting substrate  12 A, by using a sputtering apparatus. In this way, the fluorescent layer  4  of the optical semiconductor device  1 D is formed. Alternatively, the fluorescent layer  41  may be formed to have a multi-layer structure by performing sputtering multiple times. In this case, the fluorescent layer  4  of the optical semiconductor device  1 E according to the fifth embodiment can be formed. Note that the fluorescent layer  41  may alternatively be formed using a CVD apparatus. 
     Then, as shown in  FIG. 25 , a liquid glass is applied onto the fluorescent layer  41  by spin coating, and then hardened. Thereby, a light-transmissive layer  42  is formed on the fluorescent layer  41 . In this way, the light-transmissive layer  5  of the optical semiconductor device  1 D is formed. Besides spin coating, the method for applying the liquid glass may be spraying, and does not particularly limited. The glass layer can be hardened by being heated at 200° C. for an hour, for example. Besides a liquid glass, any material may be used for the light-transmissive layer  42  according to need. 
     Then, as shown in  FIG. 26 , Ni/Au layers  43  are formed on by electroless plating on the respective plated layers  16  to serve as Cu electrodes. In this way, the metal layers  11   a  and  11   b  of the optical semiconductor device  1 D are formed. In Ni electroless plating and Au electroless plating, the same processes are performed as those in the step of forming the Ni/Au layers  21  according to the sixth embodiment. 
     Lastly, as shown in  FIG. 27 , the resultant stack is diced into the multiple optical semiconductor devices  1 D using a dicer. In this way, the optical semiconductor device  1 D according to the fourth embodiment is obtained. Note that approximately the same steps as above are employed in the manufacturing process of the optical semiconductor device  1 E according to the fifth embodiment. By performing sputtering multiple times in the step of forming the fluorescent layer  41  to cause the fluorescent layer  41  to have a multi-layer structure, the optical semiconductor device  1 E according to the fifth embodiment can be obtained. 
     As described above, according to the eighth embodiment of the present invention, the optical semiconductor device  1 D according to the fourth embodiment can be manufactured. Thus, the eighth embodiment can provide the same effects as the fourth embodiment. In addition, by causing the fluorescent layer  41  to have a multi-layer structure, the optical semiconductor device  1 E according to the fifth embodiment can be manufactured. Thus, the eighth embodiment can provide the same effects as the fifth embodiment. Moreover, the sixth embodiment allows a large number of optical semiconductor devices  1 D or  1 E to be manufactured through a single manufacturing process, and thus allows mass production of the optical semiconductor devices  1 D or  1 E. Therefore, the eighth embodiment can suppress the costs of the optical semiconductor devices  1 D and  1 E. 
     (Other Embodiments) 
     Note that the present invention is not limited to the foregoing embodiments, and may be variously changed without departing from the gist of the present invention. For example, some of the components shown in the foregoing embodiments may be omitted. In addition, the components in different ones of the embodiments may be used in combination according to need. Moreover, specific values used in the foregoing embodiments are only examples, and thus the present invention is not limited to these.