Abstract:
An electrode structure includes a conductive film  24   c  formed on a base substrate  10  through an insulation film. The insulation film comprises a plurality of poles  36  of polyimide, a first film  38  formed on the side surfaces of the poles and formed of an insulation material of a high hardness than polyimide, and a second film  40  of polyimide buried among the plural poles with the first film formed on the side surfaces thereof. Because of the first film of an insulation material having high hardness formed on the side surfaces of the poles of polyimide, even when a strong force is applied upon the bonding, the poles are prevented from being distorted, and the conductive film is protected from peeling off. Because of the thick polyimide layer below the conductive film, a parasitic capacity between the conductive film and the lower layer can be small, whereby radio-frequency signals can be used.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an electrode structure, a process for fabricating the electrode structure and a semiconductor light-emitting device, more specifically an electrode structure having a parasitic capacity reduced with respect to a lower layer, a process for fabricating the electrode structure and a semiconductor light-emitting device. 
     These days, optical communication using semiconductor lasers, which enable high-speed and large-capacity information transmission, having been noted. A semiconductor laser generally has a structure including electrodes formed respectively on the upper sides and the back sides of the devices. Bonding pads are connected to the electrodes on the upper sides, and bonding wires are connected to the bonding pads. Modulation signals are supplied to the modulator region of the semiconductor laser. 
     Recently, further increase of the communication speed is required to meet larger capacities for information processing amounts. For higher communication speed it is necessary to use signal of radio-frequencies as the modulation signals. 
     However, in order that the modulation signals further have radio-frequencies, parasitic capacities between the bonding pads and the lower layer must be decreased. Delays in rises and falls of waveforms are caused corresponding to parasitic capacities between the bonding pads and the lower layer. In a case that the modulation signals have radio-frequencies, response delays due to parasitic capacities between the bonding pads and the lower layer become unnegligible. 
     In order to decrease parasitic capacities between the bonding pads and the lower layer it is proposed that the bonding pads have small areas. The bonding pads have small areas, whereby parasitic capacities between the bonding pads and the lower layer can be small. 
     However, there is a limitation to decreasing the bonding pad area. That is, the bonding pads requires a certain area for the bonding wires to be jointed to the bonding pads. When an area for the bonding is taken into account, the bonding pads cannot be made smaller than a certain area. Resultantly, parasitic capacities between the bonding pads and the lower layer cannot be decreased to about 1 pF. In a case of 1 pF, a modulation frequency could be increased to only about 2.5 GHz. Recently, the modulation speed is required to be increased to about 10 GHz. However, the modulation speed increase to about 10 GHz cannot be attained by decreasing the bonding pad area. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an electrode structure which enables decrease of parasitic capacities with respect to a lower layer, a process for fabricating the electrode structure and a semiconductor light-emitting device using radio-frequencies. 
     In order to make a parasitic capacity between the bonding pads and a lower layer small it is proposed that a thick insulation film is formed below the bonding pads. 
     However, in a case that silicon oxide film or others is formed thick below the bonding pads, the silicon oxide film or others is broken due to a force applied upon the bonding, and the bonding pad peel off. 
     Then, it is proposed that polyimide layer, which is not broken easily even by a strong force and can be formed thick is formed below the bonding pads. It is considered that the polyimide layer will not be broken by impacts applied upon the bonding because polyimide is a material having high flexibility. A semiconductor laser including a polyimide layer formed thick below the bonding pads will be explained with reference to FIG.  14 . 
     As shown in FIG. 14, a silicon nitride film  134  is formed on a substrate  110 . A thick polyimide layer  136  is formed on the silicon nitride film  134 . A silicon nitride film  138  is formed on the upper surface and the side surface of the polyimide layer  136 . 
     In the semiconductor laser shown in FIG. 14, the silicon nitride films  134 ,  138  cover the backside surface, the side surface and the upper surface of the polyimide layer  136  because the polyimide layer  136  has low adhesion to the lower layer and has high hygroscopicity. In the semiconductor laser shown in FIG. 14, the polyimide layer  136  has the back side surface, and side surface and the upper surface covered with the silicon nitride films  134 ,  138 , whereby the polyimide layer  136  can have good adhesion to the lower layer, and the polyimide layer  136  can be prohibited from absorbing water. A bonding pad  124  is formed on the silicon nitride film  138 . 
     However, in a case that the polyimide layer  136  is formed below the bonding pad  124  as shown in FIG. 14, the polyimide layer  136  is distorted due to an impact which is as large as, e.g., 500 kg/cm 2  applied to the polyimide layer  136  upon the bonding. The silicon nitride film  138  is accordingly broken. Good adhesion cannot be obtained any more between the broken silicon nitride film  138  and the bonding pad  124 . As a result, the bonding pad  124  peels off the silicon nitride film  138 . Thus, simply forming the thick polyimide layer  136  below the bonding pad  124  cannot make the semiconductor laser reliable. 
     Then, the inventors of the present application made earnest studies and have obtained an idea of art that the polyimide layer formed thick can stand impacts applied upon the bonding. 
     The above-described object is achieved by an electrode structure including a conductive film formed on a base substrate through an insulation film, the insulation film comprising a plurality of poles of polyimide, a first film formed on side surfaces of the poles and formed of an insulation material having a higher hardness than polyimide, and a second film of polyimide buried among said a plurality of poles with the first film formed on the side surfaces thereof. Because of the first film of an insulation material having high hardness formed on the side surfaces of the poles of polyimide, even when a strong force is applied upon the bonding, the poles are prevented from being distorted, and the conductive film is protected from peeling off. Because of the thick polyimide layer below the conductive film, a parasitic capacity between the conductive film and the lower layer can be small, whereby radio-frequency signals can be used. 
     The above-described object is achieved by an electrode structure including a conductive film formed on a base substrate through an insulation film, the insulation film comprising a first film of polyimide having a plurality of openings which reach the base substrate, a second film formed on inside walls of the openings and formed of an insulation material having a higher hardness than polyimide, and a plurality of poles of polyimide buried in the openings with the second film formed on the inside walls thereof. Because of the second film of an insulation material of a high hardness is formed on the inside walls of the openings formed in the first film of polyimide, even when a strong force is applied upon the bonding, the first film are prevented from being distorted, and the conductive film is protected from peeling off. Because of the thick polyimide layer below the conductive film, a parasitic capacity between the conductive film and the lower layer can be small, whereby radio-frequency signals can be used. 
     The above-described object is achieved by a semiconductor light-emitting device having an electrode structure including a conductive film formed on a base substrate through an insulation film, the insulation film comprising a plurality of poles of polyimide, a first film formed on side surfaces of the poles and formed of an insulation material having a higher hardness than polyimide, and a second film of polyimide buried among said a plurality of poles with the first film formed on side surfaces thereof. Because of the first film of an insulation material of a high hardness formed on the side surfaces of the poles of polyimide, even when a strong force is applied upon the bonding, the poles are prevented from being distorted, and the conductive film is protected from peeling off. Because of the thick polyimide layer below the conductive film, a parasitic capacity between the conductive film and the lower layer can be small, whereby radio-frequency signals can be used. 
     The above-described object is achieved by a semiconductor light-emitting device having an electrode structure including a conductive film formed on a base substrate through an insulation film, the insulation film comprising a first film of polyimide having a plurality of openings which reach the base substrate, a second film formed on inside walls of the openings and formed of an insulation material having a higher hardness than polyimide, and a plurality of poles of polyimide buried in the openings with the second film formed on the inside walls thereof. Because of the second film of an insulation material of a high hardness formed on the inside walls of the openings formed in the first film of polyimide, even when a strong force is applied upon the bonding, the first film is prevented from being distorted, and the conductive film is protected from peeling off. Because of the thick polyimide layer below the conductive film, a parasitic capacity between the conductive film and the lower layer can be small, whereby radio-frequency signals can be used. 
     The above-described object is achieved by a process for fabricating an electrode structure including a step of forming an insulation film on a base substrate, and a step of forming a conductive film on the insulation film, the step of forming the insulation film comprising a step of forming a plurality of poles of polyimide on the base substrate, a step of forming on side surface of the poles a first film of an insulation material having a higher hardness than polyimide, and a step of burying a second film of polyimide among the first film. Because of the first film of an insulation material having high hardness formed on the side surfaces of the poles of polyimide, even when a strong force is applied upon the bonding, the poles are prevented from being distorted, and the conductive film is protected from peeling off. Because of the thick polyimide layer below the conductive film, a parasitic capacity between the conductive film and the lower layer can be small, whereby radio-frequency signals can be used. 
     The above-described object is achieved by a process for fabricating an electrode structure including a step of forming an insulation film on a base substrate and a step of forming a conductive film on the insulation film, the step of forming the insulation film comprising a step of forming on a base substrate a first film of polyimide having a plurality of openings which reach the base substrate, a second step of forming on inside walls of the openings a second film of an insulation material having a higher hardness than polyimide, and a step of forming a plurality of poles of polyimide buried in the openings with the second film formed on the inside walls thereof. Because of the second film of an insulation material of high hardness is formed on the inside walls of the openings formed in the first film of polyimide, even when a strong force is applied upon the bonding, the poles are prevented from being distorted, and the conductive film is protected from peeling off. Because of the thick polyimide layer below the conductive film, a parasitic capacity between the conductive film and the lower layer can be small, whereby radio-frequency signals can be used. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of the semiconductor light-emitting device according to a first embodiment of the present invention. 
     FIG. 2A is a sectional view of the semiconductor light-emitting device according to the first embodiment of the present invention, which shows a vicinity of the bonding pad. 
     FIG. 2B is a plan view of the semiconductor light-emitting device according to the first embodiment of the present invention, which shows a vicinity of the bonding pad. 
     FIGS. 3A and 3B are sectional views of the semiconductor light-emitting device according to the first embodiment of the present invention in the steps of the process for fabricating the same, which show the process (Part  1 ). 
     FIGS. 4A and 4B are sectional views of the semiconductor light-emitting device according to the first embodiment of the present invention in the steps of the process for fabricating the same, which show the process (Part  2 ). 
     FIG. 5 is a sectional view of the semiconductor light-emitting device according to the first embodiment of the present invention in the steps of the process for fabricating the same, which shows the process (Part  3 ). 
     FIG. 6A is a sectional view of the semiconductor light-emitting device according to a first modification of the first embodiment of the present invention, which shows a vicinity of the bonding pad. 
     FIG. 6B is a plan view of the semiconductor light-emitting device according to a first modification of the first embodiment of the present invention, which shows a vicinity of the bonding pad. 
     FIG. 7A is a sectional view of the semiconductor light-emitting device according to a second modification of the first embodiment of the present invention, which shows a vicinity of the bonding pad. 
     FIG. 7B is a plan view of the semiconductor light-emitting device according to a second modification of the first embodiment of the present invention, which shows a vicinity of the bonding pad. 
     FIG. 8A is a sectional view of the semiconductor light-emitting device according to the second embodiment of the present invention, which shows a vicinity of the bonding pad. 
     FIG. 8B is a plan view of the semiconductor light-emitting device according to the second embodiment of the present invention, which shows a vicinity of the bonding pad. 
     FIGS. 9A and 9B are sectional views of the semiconductor light-emitting device according to the second embodiment of the present invention in the steps of the process for fabricating the same, which show the process (Part  1 ). 
     FIGS. 10A and 10B are sectional views of the semiconductor light-emitting device according to the second embodiment of the present invention in the steps of the process for fabricating the same, which show the process (Part  2 ). 
     FIG. 11 is sectional views of the semiconductor light-emitting device according to the second embodiment of the present invention in the steps of the process for fabricating the same, which shows the process (Part  3 ). 
     FIG. 12A is a sectional view of the semiconductor light-emitting device according to a first modification of the second embodiment of the present invention, which shows a vicinity of the bonding pad. 
     FIG. 12B is a plan view of the semiconductor light-emitting device according to a first modification of the second embodiment of the present invention, which shows a vicinity of the bonding pad. 
     FIG. 13 is a plan view of the semiconductor light-emitting device according to a second modification of the second embodiment of the present invention, which shows a vicinity of the bonding pad. 
     FIG. 14 is a conceptual view showing distortion of the polyimide layer upon the bonding. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A First Embodiment 
     The semiconductor light-emitting device according to a first embodiment of the present invention will be explained with reference to FIGS. 1 to  5 . FIG. 1 is a perspective view of the semiconductor light-emitting device according to the present embodiment. FIG. 2A is a sectional view of a vicinity of bonding pad, specifically along the line A—A′ in FIG.  1 . FIG. 2B is a plan view of the vicinity of the bonding pad. In FIG. 2B some constituent members are not shown. FIGS. 3A to  5  show sectional views of the semiconductor light-emitting device according to the present embodiment, which show the process for fabricating the semiconductor light-emitting device. 
     Semiconductor Light-emitting Device 
     First, the semiconductor light-emitting device according to the present embodiment will be explained with reference to FIG.  1 . In the present embodiment, the present invention is applied to a light-emitting device, but the present invention is applicable not only to semiconductor light-emitting devices, but also widely to all semiconductor devices in which strong forces are applied to the electrodes, such as the bonding pads, etc., when bonded. 
     As shown in FIG. 1, a guide layer  12  is formed on a substrate  10  of InP. An MQW light absorption layer  14   a  and a MQW active layer  14   b  are formed on the guide layer  12 . A clad layer  16  is formed on the MQW light absorption layer  14   a  and the MQW active layer  14   b . A clad layer  17  is formed on the clad layer  16 . A cap layer  18   a ,  18   b  is formed on the clad layer  17 . The cap layer  18   a ,  18   b  is absent in an isolation region  22  and isolated from each other in the isolation region  22 . An electrode  8  of an Au/Ge/Au film is formed on the back side of the substrate  10 . 
     The cap layer  18   a ,  18   b , the clad layers  17 ,  16 , the MQW active layer  14   b , the MQW light absorption layer  14   a , the guide layer  12  and the substrate  10  are mesa-etched, and a mesa-shaped waveguide is formed. A high-resistance buried layer  30  of InP is formed on both sides of the mesa-shaped waveguide. 
     A silicon oxide film  32  is formed on the high-resistance buried layer  30 . The silicon oxide film  32  is formed also on the clad layer  17  in the isolation region  22 . 
     Electrodes  24   a ,  24   b  of an Au/Pt/Ti film are formed respectively on the cap layer  18   a ,  18   b . The electrodes  24   a ,  24   b  are absent in the isolation region  22 . Accordingly, a modulator region  26  and a DFB (Distributed FeedBack) laser region  28  are electrically isolated from each other by the isolation region  22 . 
     In the modulator region  26  there are formed polyimide layers  36 ,  40  and a silicon nitride film  38  which will be described later. A bonding pad  24   c  is formed on the polyimide layers  36 ,  40  and the silicon nitride film  38 . The bonding pad  24   c  is connected to the electrode  24   a . Because of the polyimide layer  36 , etc., which are formed thick, a parasitic capacity between the bonding pad  24   c  and the lower layer can be small, which allows modulation radio-frequencies to be used. 
     In the DFB laser region  2 B, a bonding pad  24   d  is formed on the silicon oxide film  32 . The bonding pad  24   d  is connected to the electrode  24   b . No radio-frequency signal is used in the DFB laser region  28 , and it is not necessary to consider a parasitic capacity between the bonding pad  24   d  and the lower layer. 
     Then, a structure of the vicinity of the bonding pad  24   c  of the modulate region  26  will be explained with reference to FIG.  2 . FIG. 2A is a sectional view of the vicinity of the bonding pad  24   c  and specifically a sectional view of the vicinity along the line A—A′ in FIG.  1 . FIG. 2B is a plan view of the vicinity of the bonding pad  24   c.    
     As shown in FIG. 2A, the high-resistance buried layer  30  is formed on the substrate  10 . A 300 nm-thickness silicon oxide film  32  is formed on the high-resistance buried layer  30 . A 200 nm-thickness silicon nitride film  34  is formed on the silicon oxide film  32 . 
     The polyimide layer  36  is formed in a plurality of cylindrical patterns on the silicon nitride film  34 . The cylindrical polyimide layer  36  may have, e.g., a 2 μm-height. In the present embodiment the polyimide layer  36  is used because the polyimide layer  36  is not damaged by impacts of the bonding, etc. because of its high flexibility. In a case that a layer, as of silicon oxide film or others, having low flexibility is formed thick below the bonding pad  24   c , the silicon oxide film or others is damaged by impacts of the bonding because of its low flexibility. 
     The silicon nitride film  34  is formed in a 300 nm-thickness on the entire surface of the silicon nitride film  34  with the polyimide layer  36  formed in a plurality of cylindrical patterns formed on. The silicon nitride film  38  is formed on the sides of the cylindrical patterns of the polyimide layer  36 . The silicon nitride film  38 , the hardness of which is higher than that of polyimide, can keep the polyimide layer  36  from being distorted even when a strong force is applied to the bonding pad  24   c  by the bonding. The silicon nitride film  38 , the hardness of which is high, is formed to be as thin as 300 nm and is never damaged by impacts of the bonding. 
     The polyimide layer  40  is further formed on the silicon nitride film  38 . The polyimide layer  40  is as thin as, e.g., about 100 nm above the cylindrical polyimide layer  36 . A silicon nitride film  42  is formed on the entire surface of the polyimide layer  40 , and the bonding pad  24   c  is formed on the silicon nitride film  42 . 
     As described above, according to the present embodiment, the silicon nitride film  38 , the hardness of which is high, is formed on the sides of the polyimide layer  36  formed in a plurality of cylindrical patterns can prevent the polyimide layer  36  from being distorted even when a strong force is applied to the bonding pad  24   c  upon the bonding. Because of the polyimide layers  36 ,  40  formed thick below the bonding pad  24   c , a parasitic capacity between the bonding pad  24   c  and the lower layer can be small, which permits modulation radio-frequencies to be used, whereby according to the present embodiment, a semiconductor light-emitting device using modulation radio-frequencies can be provided. 
     Process for Fabricating the Semiconductor Light-emitting Device 
     Then, the process for fabricating the semiconductor light-emitting device according to the present embodiment will be explained with reference to FIGS. 3A to  5 . 
     First, the guide layer  12 , the MQW light absorption layer  14   a , the MQW active layer  14   b , the clad layer  16 , the clad layer  17  and the cap layer  18   a ,  18   b  are sequentially formed on a substrate  10  of InP. 
     Next, the cap layer  18   a ,  18   b , the clad layers  17 ,  16 , the MQW active layer  14   b , the MQW light absorption layer  14   a , the guide layer  12  and the substrate  10  are mesa-etched. Then, the high resistance buried layer  30  of InP is formed on both sides of the mesa. 
     Next, the cap layer  18   a ,  18   b  is patterned to be isolated by the isolation region  22 . 
     Then, the silicon oxide film  32  of a 300 nm-thickness is formed on the entire surface by CVD (Chemical Vapor Deposition). Then, the silicon nitride film  34  of a 200 nm-thickness is formed on the silicon oxide film  32  by CVD. 
     Next, the polyimide layer  36  is formed on the entire surface of the silicon nitride film  34  by spin coating. Next, the polyimide layer  36  is solidified by a heat treatment of about 400° C. Thus the polyimide layer  36  of an about 2 μm-thickness is formed (see FIG.  3 A). 
     Then, the polyimide layer  36  is patterned into cylindrical patterns by photolithography. A diameter of the cylinders may be, e.g., 5 μm , and an interval between each cylinder and its adjacent one may be, e.g., 10 μm. The polyimide layer  36  may be patterned by, dry etching using plasma discharge. An etching gas may be a mixed gas of CF 4  gas and O 2  gas. One hundred, for example, cylinders of the polyimide layer  36  are formed in a 100 μm×100 μm area near the bonding pad  24   c  (see FIG.  3 B). 
     Then, the silicon nitride film  38  is formed on the entire surface in a 300 nm-thickness by CVD (see FIG.  4 A). 
     Then, the polyimide layer  40  is formed on the entire surface by spin coating. In the present embodiment the polyimide layer  36  is formed in cylinders, which makes it difficult for the surface of the polyimide layer  40  to be uneven when the polyimide layer  40  is formed. The polyimide layer  40  can have the surface evened. Then, a heat treatment of about 400° C. to solidify the polyimide layer  40 . The polyimide layer  40  has an about 100 nm-thickness on the polyimide layer  36  in cylinders (see FIG.  4 B). 
     Then, the silicon nitride film  42  is formed on the entire surface in a 200 nm-thickness by CVD. 
     Next, an opening which reaches the cap layers  18   a ,  18   b  (see FIG. 1) are formed. The opening is for connecting the electrodes  24   a ,  24   c  to the cap layer  18   a ,  18   b.    
     Then, as shown in FIG. 5, a 100 nm-thickness Ti film, a 70 nm-thickness Pt film and a 500 nm-thickness Au film are sequentially formed on the silicon nitride film  42  by vapor deposition to form the electrodes  24   a ,  24   b  and the bonding pads  24   c ,  24   d  of the Au/Pt/Ti film. Thus, the semiconductor light-emitting device according to the present embodiment is fabricated (see FIG.  5 ). 
     (A First Modification) 
     Then, a first modification of the semiconductor light-emitting device according to the present embodiment will be explained with reference to FIGS. 6A and 6B. FIG. 6A is a sectional view of the vicinity of the bonding pad. FIG. 6B is a plan view of the vicinity of the bonding pad of the semiconductor light-emitting device according to the present modification. In FIG. 6B some constituent members are omitted. 
     As shown in FIGS. 6A and 6B, the semiconductor light-emitting device according to the present modification is characterized mainly in that the polyimide layer  36   a  is formed in quadrangular poles. 
     In the present modification, when the polyimide layer  36   a  is patterned, square patterns may be formed. In the semiconductor light-emitting device according to the present embodiment shown in FIG. 2 wherein the polyimide layer  36  is formed in cylinders, it is necessary to form circular patterns and etch the polyimide layer  36 , but simply in the present modification quadrangular patterns may be formed. In consideration of achievement of pattern drawing apparatuses it is difficult to form micronized circular patterns, but it is easy to form micronized quadrangular patterns. 
     Thus, the semiconductor light-emitting device according to the present modification can be micronized. 
     (A Second Modification) 
     Then, a second modification of the semiconductor light-emitting device according to the present embodiment will be explained with reference to FIGS. 7A and 7B. FIG. 7A is a sectional view of the vicinity of the bonding pad of the semiconductor light-emitting device according to the present modification. FIG. 7B is a plan view of the vicinity of the bonding pad of the semiconductor light-emitting device according to the present modification. In FIG. 7B some constituent members are omitted. 
     As shown in FIG. 7A, the semiconductor light-emitting device according to the present modification is characterized mainly in that the polyimide layer  36  is formed in hexagonal poles. 
     In the present modification, because the polyimide layer  36   b  is formed in hexagonal poles, the sections of the silicon nitride film  38  along the substrate  10  are hexagonal. According to the present modification, the silicon nitride film  38  can have higher resistance to a force applied by the bonding. 
     A Second Embodiment 
     The semiconductor light-emitting device according to a second embodiment of the present invention and the process for fabricating the same will be explained with reference to FIGS. 8A to  11 . FIG. 8A is a sectional view of a vicinity of bonding pad of the semiconductor light-emitting device according to the present embodiment. FIG. 8B is a plan view of a vicinity of bonding pad of the semiconductor light-emitting device according to the present embodiment. FIGS. 9A to  11  are sectional views of the semiconductor light-emitting device according to the present embodiment in the steps of the process for fabricating the same, which show the process. The same members of the present embodiment as those of the semiconductor light-emitting device according to the first embodiment and the process for fabricating the same shown in FIGS. 1 to  7 B are represented by the same reference numbers not to repeat or to simplify their explanation. 
     The semiconductor light-emitting device according to the present embodiment is characterized mainly in that silicon nitride film  38  is formed on the inside walls of openings formed in a polyimide layer  36   c.    
     As shown in FIG. 8A, a plurality of the openings  44  which reach the silicon nitride film  34  are formed in the polyimide layer  36   c . A sectional shape of the openings  44  along a substrate  10  is circular. 
     The silicon nitride film  38  is formed on the entire surface of the polyimide layer  36   c  with the openings  44  thus formed in. The silicon nitride film  38  is formed also on the inside walls of the openings  44 . Because the silicon nitride film  38  is formed on the inside walls of the openings  44 , even when a strong force is applied to the bonding pad  24   c , the polyimide layer  36   c  is prevented from being distorted. 
     A polyimide layer  40  is formed on the entire surface of the silicon nitride film  38 . The polyimide layer  40  is buried in the openings  44  with the silicon nitride film  38  formed on. On the polyimide layer  40  there are sequentially formed a silicon nitride film  42  and a bonding pad  24   c.    
     As described above, according to the present embodiment, because the silicon nitride film  38 , the hardness of which is high, is formed on the inside walls of a plurality of openings  44  formed in the polyimide layer  36   c , even when a strong force is applied to the bonding pad  24   c  by the bonding, the polyimide layer  36   c  is prevented from being distorted, whereby peeling of the bonding pad  24   c  can be prevented. Because of the thick polyimide layers  36   c ,  40  formed below the bonding pad  24   c , a parasitic capacity between the bonding pad  24   c  and the lower layer can be small, whereby radio-frequency signals can be used as the modulation signals. Thus, the semiconductor light-emitting device according to the present embodiment can have modulation radio-frequencies. 
     (Process for Fabricating the Semiconductor Light-emitting Device) 
     Then, the process for fabricating the semiconductor light-emitting device according to the present embodiment will be explained with reference to FIGS. 9A to  11 . 
     The step of forming the silicon nitride film  34  and the steps up to the silicon nitride film forming step are the same as those of the process for fabricating the semiconductor light-emitting device according to the first embodiment shown in FIG. 3A, and are not explained here. 
     Following step of forming the silicon nitride film  34 , the polyimide layer  36   c  is formed on the entire surface of the silicon nitride film  34  by spin coating. Then an about 400° C. heat treatment is performed to solidify the polyimide layer  36   c . Thus the polyimide layer  36   c  of an about 2 μm thickness is formed (see FIG.  9 A). 
     Then the openings  44  which reach the silicon nitride film  34  are formed in the polyimide layer  36   c  by photolithography. A diameter of the openings  44  may be, e.g., 5 μm and may be spaced from each other by, e.g., 15 μm. The openings  44  in the polyimide layer  36   c  may be formed by, e.g., dry etching using plasma discharges. An etching gas may be a mixed gas of CF 4  gas and O 2  gas. Thus, 49 openings, for example, are formed, e.g., a 100 μm×100 μm range near the bonding pad  24   c  (see FIG.  9 B). Preferably large openings  44  are formed in consideration of a surface tension of the polyimide so that the polyimide can intrudes without failure into the openings  44  in a later step. 
     Then, the silicon nitride film  38  is formed in a 300 nm thickness on the entire surface by CVD (see FIG.  10 A). 
     Then, the polyimide layer  40  is formed on the entire surface by spin coating, and the polyimide layer  40  is buried in the openings  44  with the silicon nitride film  38  formed in. The polyimide layer  40  on the polyimide layer  36   c  is below, e.g., 300 nm. Then, a heat treatment of about 400° C. is performed to solidify the polyimide layer  40  (FIG.  10 B). 
     Then, the silicon nitride film  42  is formed in a 200 nm thickness on the entire surface by CVD. 
     Next, an opening which reaches the cap layers  18   a ,  18   b  (see FIG. 1) is formed. The opening is for connecting electrodes  24   a ,  24   b  to the cap layer  18   a ,  18   b.    
     Then, the electrodes  24   a ,  24   b  of an Au/Pt/Ti film and the bonding pads  24   c ,  24   d  are formed on the silicon nitride film  42  by vapor deposition in the same way as in the first embodiment. Thus, the semiconductor light-emitting device according to the present embodiment is fabricated. 
     (A First Modification) 
     Then, the semiconductor light-emitting device according to a first modification of the present embodiment will be explained with reference to FIGS. 12A and 12B. FIG. 12A is a sectional view of a vicinity of the bonding pad of the semiconductor light-emitting device according to the present modification. FIG. 12B is a plan view of the vicinity of the bonding pad of the semiconductor light-emitting device according to the present modification. 
     The semiconductor light-emitting device according to the present modification is characterized in that the openings  44   a  are quadrangular. 
     In the present modification, when the polyimide layer  36   d  is patterned, quadrangular patterns may be formed. In the semiconductor light-emitting device according to the present embodiment shown in FIG. 8 wherein the polyimide layer  36   c  is formed in cylinders, it is necessary to form circular patterns and etch the polyimide layer  36   c , but simply in the present modification quadrangular patterns may be formed. In consideration of achievement of pattern drawing apparatuses it is difficult to form micronized circular patterns, but it is easy to form micronized quadrangular patterns. 
     Thus, the semiconductor light-emitting device according to the present modification can be micronized. 
     (A Second Modification) 
     Then, the semiconductor light-emitting device according to a second modification of the present embodiment will be explained with reference to FIG.  13 . FIG. 13 is a plan view of the vicinity of the bonding pad of the semiconductor light-emitting device according to the present modification. In FIG. 13 some members are not shown. 
     As shown in FIG. 13, the semiconductor light-emitting device according to the present modification is characterized mainly in that the openings  44   b  are formed in hexagonal poles. 
     In the present modification the openings  44   b  are formed in hexagonal poles, and the sections of the silicon nitride film  38  along the substrate  10  are hexagonal. Accordingly the silicon nitride film  38  can have higher resistance to a force applied by the bonding. 
     Modifications 
     The present intention is not limited to the above-described embodiment and can cover various modifications. 
     For example, in the first and the second embodiments the silicon nitride film  38  is formed on the entire surface, but the silicon nitride film  38  may be formed at least on the side walls of the polyimide layers  36  to  36   e . The silicon nitride film  38  may be formed at least on the side walls of the polyimide layers  36  to  36   e , whereby the polyimide layers  36  to  36   e  are prevented from being distorted upon the bonding. 
     A film formed on the side walls of the polyimide layers  36  to  36   b  is silicon nitride film in the first embodiment but is not essentially silicon nitride film. The film may be any film having high hardness, e.g., a silicon oxide film, an aluminum oxide film, a polysilicon film or others, as long as the film can prevent from the polyimide layers  36  to  36   b  from being distorted upon the bonding. 
     In the second embodiment, silicon nitride film is unessentially formed on the inside walls of the openings  44  to  44   b , but is not essential. Any film having high hardness, such as silicon oxide film, aluminum oxide film, polysilicon film or others, may be formed as long as the film can prevent the polyimide layers  36   c  to  36   e  from being distorted upon the bonding. 
     The first and the second embodiments have been explained by means of the semiconductor light-emitting device including the modulator region and the DFB laser region isolated from each other. The present invention may be applicable to a semiconductor light-emitting device including the modulator region which is not isolated. In such case the above-described polyimide layers are formed below the bonding pad formed in the DFB laser region. 
     In the first and the second embodiments polyimide layers are formed below the bonding pad, but the present invention is applicable to a case wherein polyimide layer is formed below electrodes for flip chip bonding in place of the bonding pad. In this case the electrodes may be formed not only on a part of the substrate but also on the entire surface, and the above-described polyimide layers may be formed below the electrode formed on the entire surface.