Abstract:
Provided is a semiconductor laser diode. The semiconductor laser diode includes a first material layer, an active layer, and a second material layer, characterized in that the semiconductor laser diode includes: a ridge waveguide, which is formed in a ridge shape over the second material layer to define a channel defined so that a top material layer of the second material layer is limitedly exposed, and in which a second electrode layer which is in contact with the top material layer of the second material layer via the channel is formed; and a first protrusion, which is positioned at one side of the ridge waveguide and has not less height than that of the ridge waveguide.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This application claims the priority of Korean Patent Application No. 2003-14614, filed on Mar. 8, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
           [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a semiconductor laser diode and more particularly, to a semiconductor laser diode having a ridge waveguide.  
           [0004]    2. Description of the Related Art  
           [0005]    As high density information recording is increasingly in demand, the need for a visible light semiconductor laser diode is increasing. Therefore, semiconductor laser diodes made of various compounds capable of emitting a visible light laser are being developed. In particular, much attention has been paid to a group III-V nitride semiconductor laser diode because its optical transition is a direct transition type that induces high frequency laser emission and because it emits a blue light laser.  
           [0006]    [0006]FIG. 1 shows a perspective view of a conventional GaN-based, group III-V nitride semiconductor laser diode having n-type and p-type electrodes, which are formed on the same side, and a ridge waveguide.  
           [0007]    Referring to FIG. 1, an n-type material layer  20 , a light emitting active layer  30 , and a p-type material layer  40  are sequentially formed on a sapphire substrate  10 . The upper surface of the p-type material layer  40  is formed with a ridge waveguide  70 . The ridge waveguide  70  is slightly protruded from the upper surface of the p-type material layer  40 . The ridge waveguide  70  comprises a channel  71  formed so that the p-type material layer  40  is exposed in a narrow stripe-type configuration, and a p-type electrode layer  50 , which is in contact with the p-type material layer  40  via the channel  71 . Strictly speaking, a reference numeral  2  is not the p-type material layer  40  but is a current restricting layer formed for defining the channel  71 .  
           [0008]    An n-type electrode layer  60  serves to feed an electric current into a bottom material layer  21  of the n-type material layer  20  and is formed on an exposed surface  22  of the bottom material layer  21  of the n-type material layer  20 .  
           [0009]    In this structure, the upper surface of the p-type electrode layer  50 , that is, the upper surface  72  of the ridge waveguide  70 , and the upper surface of the n-type electrode layer  60  are separated by a step height, h1.  
           [0010]    Generally, a temperature has an effect on a critical current and laser mode stability for laser emission of semiconductor laser diodes. As a temperature increases, both of the characteristics are lowered. Therefore, there is a need to remove heat generated in an active layer during laser emission to thereby prevent overheating of laser diodes. In the structure of the aforementioned conventional GaN-based, group III-V semiconductor laser diode, most heat is discharged only through a ridge because of very low thermal conductivity of a substrate (for a sapphire substrate, about 0.5 W/cmK). However, because heat discharge through a ridge occurs limitedly, it is difficult to carry out efficient heat discharge. Therefore, lowering of characteristics of semiconductor devices by overheating of laser diodes is not efficiently prevented.  
           [0011]    In this regard, a flip-chip bonding technology shown in FIG. 2 can be applied to the structure of a conventional semiconductor laser diode shown in FIG. 1 to discharge heat generated in an active layer.  
           [0012]    Referring to FIG. 2, a reference numeral  80  indicates a conventional GaN-based, group III-V semiconductor laser diode. A reference numeral  90  indicates a submount as a heat discharge structure, a reference numeral  91  a substrate, and reference numerals  92   a  and  92   b  first and second metal layers, respectively. Reference numerals  93   a  and  93   b  indicate first and second solder layers, which are respectively fused to an n-type electrode layer  60  and a p-type electrode layer  50  of the semiconductor laser diode  80 .  
           [0013]    By bonding the semiconductor laser diode to the submount, a separately prepared heat discharge structure, heat discharge efficiency can be increased.  
           [0014]    However, as shown in FIG. 2, the first solder layer  93   a  is thicker than the second solder layer  93   b  by the height of h1 in order to compensate for the step height, h1 between the p-type electrode  50  and the n-type electrode  60 . Due to such a thickness difference, the first and second solder layers  93   a  and  93   b  may not concurrently be molten.  
           [0015]    The first and second solder layers  93   a  and  93   b  are generally made of a metal alloy, and thus, even if the chemical composition ratios of the first and second solder layers  93   a  and  93   b  slightly differ from each other, there is a large difference between their melting temperatures. In a case wherein the first and second solder layers  93   a  and  93   b  differ in thickness in a method of manufacturing the submount, the first and second solder layers  93   a  and  93   b  must be formed under separate two processes, not under a single process. As a result, there exists a likelihood for the first and second solder layers  93   a  and  93   b  to have different chemical composition ratios.  
           [0016]    The ridge waveguide  70  is protruded from the p-type material layer  40 , and although exaggerated in FIG. 2, has a width W1 of no more than several micrometers. Therefore, when the semiconductor laser diode  80  is bonded to the submount  90 , a thermal stress may be concentrated on the ridge waveguide  70 . In addition, when the first and second solder layers  93   a  and  93   b  are not concurrently fused as mentioned above, the submount  90  may be inclined to one side. In this case, a mechanical stress may be concentrated on the narrow ridge waveguide  70 .  
           [0017]    Stresses concentrated on the ridge waveguide  70  may affect light emission in the active layer  30  below the ridge waveguide  70 .  
           [0018]    [0018]FIG. 3 shows an image plane photograph of laser light emission taken along the longitudinal direction A of the stripe-like ridge waveguide  70 . As shown in FIG. 3, light is emitted unevenly and discontinuously along the longitudinal direction A of the ridge waveguide  70 .  
         SUMMARY OF THE INVENTION  
         [0019]    The present invention has been made in view of the above problems.. The present invention provides a semiconductor laser diode with an improved structure capable of dispersing a stress concentrated on a ridge waveguide when flip-chip bonded to a submount.  
           [0020]    According to an aspect of the present invention, there is provided a semiconductor laser diode comprising first and second material layers which are multi-material layers, and an active layer interposed between the first and second material layers to emit light, characterized in that the semiconductor laser diode comprises a ridge waveguide, which is formed in a ridge shape over the second material layer to define a channel so that a top material layer of the second material layer is limitedly exposed, and in which a second electrode layer which is in contact with the top material layer of the second material layer via the channel is formed; and a first protrusion, which is positioned at one side of the ridge waveguide and has not less height than that of the ridge waveguide.  
           [0021]    According to specific embodiments of the present invention, the first protrusion may have a width wider than the width of the ridge waveguide.  
           [0022]    The semiconductor laser diode may further comprise a second protrusion having not less height than that of the ridge waveguide at the other side of the ridge waveguide. The second protrusion may have a width wider than the width of the ridge waveguide and may have the same height as the first protrusion. When the first and second protrusions differ in height, the height difference may be 0.5 μm or less.  
           [0023]    The second protrusion may be separated from the ridge waveguide by a valley portion, which is etched to expose a bottom material layer of the first material layer, and a top layer of the second protrusion may be a first electrode layer electrically connected to the bottom material layer of the first material layer.  
           [0024]    The other side of the ridge waveguide may be formed with an exposed surface of the bottom material layer of the first material layer and a first electrode layer having not less height than that of the ridge waveguide may be formed on the exposed surface. The first electrode layer serves the same function as the second protrusion. The first protrusion and the first electrode layer may have the same height. The first electrode layer may have a width wider than the width of the ridge waveguide. When the first protrusion and the first electrode layer differ in height, the height difference may be 0.5 μm or less.  
           [0025]    According to another aspect of the present invention, there is provided a semiconductor laser diode assembly comprising a semiconductor laser diode comprising first and second material layers which are multi-material layers, and an active layer interposed between the first and second material layers to emit light; and a submount flip-chip bonded to the semiconductor laser diode, wherein the semiconductor laser diode comprises a ridge waveguide, which is formed in a ridge shape over the second material layer to define a channel so that a top material layer of the second material layer is limitedly exposed, and in which a second electrode layer which is in contact with the top material layer of the second material layer via the channel is formed; a first protrusion, which is positioned at one side of the ridge waveguide and has not less height than that of the ridge waveguide; and a second protrusion, which is positioned at the other side of the ridge waveguide and has not less height than that of the ridge waveguide and of which a top material layer is a first electrode layer electrically connected to a bottom material layer of the first material layer, and wherein the submount comprises a substrate; a first solder layer bonded to the first protrusion and the ridge waveguide; and a second solder layer bonded to the second protrusion, the first and second solder layers being formed at a surface of the substrate and having substantially the same thickness.  
           [0026]    According to yet another aspect of the present invention, there is provided a semiconductor laser diode assembly comprising a semiconductor laser diode comprising first and second material layers which are multi-material layers, and an active layer interposed between the first and second material layers to emit light; and a submount flip-chip bonded to the semiconductor laser diode, wherein the semiconductor laser diode comprises a ridge waveguide, which is formed in a ridge shape over the second material layer to define a channel so that a top material layer of the second material layer is limitedly exposed, and in which a second electrode layer which is in contact with the top material layer of the second material layer via the channel is formed; a first protrusion, which is positioned at one side of the ridge waveguide and has not less height than that of the ridge waveguide; and a first electrode layer, which is positioned at the other side of the ridge waveguide, has not less height than that of the ridge waveguide, and electrically connected to a bottom material layer of the first material layer, and wherein the submount comprises a substrate; a first solder layer bonded to the first protrusion and the ridge waveguide; and a second solder layer bonded to the first electrode layer, the first and second solder layers being formed at a surface of the substrate and having substantially the same thickness. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
         [0028]    [0028]FIG. 1 is a perspective view of a conventional GaN-based, group III-V nitride semiconductor laser diode having n-type and p-type electrodes, which are formed on the same side;  
         [0029]    [0029]FIG. 2 is a sectional view of an example of a conventional semiconductor laser diode assembly with a flip-chip bonded structure of a submount and the semiconductor laser diode shown in FIG. 1;  
         [0030]    [0030]FIG. 3 is an image plane photograph of laser light emission taken along the longitudinal direction of a ridge waveguide in the semiconductor laser diode assembly shown in FIG. 2;  
         [0031]    [0031]FIG. 4 is a perspective view of a semiconductor laser diode according to an embodiment of the present invention;  
         [0032]    [0032]FIG. 5 is a sectional view of a flip-chip bonded structure of a submount and the semiconductor laser diode shown in FIG. 4 in a semiconductor laser diode assembly according to an embodiment of the present invention;  
         [0033]    [0033]FIG. 6 is a perspective view of a semiconductor laser diode according to another embodiment of the present invention;  
         [0034]    [0034]FIG. 7 is a sectional view of a flip-chip bonded structure of a submount and the semiconductor laser diode shown in FIG. 6 in a semiconductor laser diode assembly according to another embodiment of the present invention;  
         [0035]    [0035]FIG. 8 is an image plane photograph of laser light emission in the semiconductor laser diode assembly shown in FIG. 7; and  
         [0036]    [0036]FIG. 9 is a perspective view of a semiconductor laser diode according to another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0037]    Hereinafter, preferred embodiments of the present invention will be described in more detail with reference to the accompanying drawings.  
         [0038]    [0038]FIG. 4 is a perspective view of a semiconductor laser diode according to an embodiment of the present invention. The illustration of the semiconductor laser diode of FIG. 4 has been exaggerated to show characteristics of the semiconductor laser diode according to the present invention more clearly. The semiconductor laser diode according to this embodiment of the present invention is a GaN-based, group III-V nitride semiconductor laser diode.  
         [0039]    Referring to FIG. 4, the semiconductor laser diode  100   a  comprises a substrate  110 , a first material layer  120 , an active layer  130 , and a second material layer  140  that are sequentially mounted on the substrate  110 .  
         [0040]    The substrate  110  may be a GaN or SiC-based, group III-V semiconductor layer substrate or a high resistance substrate such as a sapphire substrate.  
         [0041]    The active layer  130  is a material layer of emitting light by carrier recombination of an electron and a hole. Preferably, the active layer  130  is a GaN-based, group III-V nitride semiconductor layer having a multi quantum well (MQW) and more preferably, an In x Al y Ga 1−x−y  layer where 0≦x≦1, 0≦y≦1, and x+y≦1. The active layer  130  may also be a GaN-based, group III-V nitride semiconductor layer containing a predetermined ratio of Indium (In), for example, an InGaN layer.  
         [0042]    The first material layer  120  comprises a buffer layer  121 , a first cladding layer  122 , and a first waveguide layer  123  that are sequentially mounted on the upper surface of the substrate  110 . The second material layer  140  comprises a second waveguide layer  141 , a second cladding layer  142 , and a cap layer  143  that are sequentially mounted on the upper surface of the active layer  130 . A bottom layer of the first material layer  120  is the buffer layer  121  and a top layer of the second material layer  140  is the cap layer  143 .  
         [0043]    The buffer layer  121  is an n-type material layer made of a GaN-based, group III-V nitride semiconductor or an undoped material layer. Preferably, the buffer layer  121  is an n-GaN layer.  
         [0044]    The cap layer  143  is a GaN-based, group III-V nitride semiconductor layer, preferably a p-type conductive impurity-doped direct transition layer, and more preferably a p-GaN layer. In addition, the cap layer  143  may be a GaN layer, an AlGaN layer or InGaN layer containing a predetermined ratio of Al or In, like the buffer layer  121 .  
         [0045]    The first and second waveguide layers  123  and  141  are GaN-based, group III-V nitride semiconductor layers, and preferably, an n-GaN layer and a p-GaN layer, respectively. The first and second waveguide layers  123  and  141  have a reflective index lower than the active layer  130  and higher than the first cladding layer  122  and the second cladding layer  142  as will be described later.  
         [0046]    The first cladding layer  122  is an n-AlGaN/GaN layer and the second cladding layer  142  is the same material layer as the first cladding layer  122  except that a p-type material is doped.  
         [0047]    In order to induce laser emission in the active layer  130 , the semiconductor laser diode  100   a  comprises an n-type electrode layer  190   a  and a p-type electrode layer  170  as first and second electrode layers, which are electrically connected to the buffer layer  121  and the cap layer  143 , respectively. A reference numeral  160  indicates a current restricting layer, which defines a channel  180  as a passage for limited contact between the p-type electrode layer  170  and the cap layer  143 .  
         [0048]    As shown in the left part of FIG. 4, the cap layer  143  is divided into first and second regions  143   a  and  143   b . The cap layer  143  and the partially exposed second cladding layer  142  are covered with the current restricting layer  160 . The current restricting layer  160  covering the first region  143   a  of the cap layer  143  is removed to expose the cap layer  143 . As a result, the channel  180  is formed and the p-type electrode layer  170  and the cap layer  143  are limitedly in contact with each other through the channel  180 . The channel  180  is filled with a contact layer  171  and a bonding metal layer  172  is formed on the upper surfaces of the contact layer  171  and the current restricting layer  160 . Hereinafter, the p-type electrode layer  170  is designated a layer containing the contact layer  171  and the bonding metal layer  172 .  
         [0049]    As mentioned above, when formation of the p-type electrode layer  170  is completed, a ridge waveguide  200  is formed at the first region  143   a  of the cap layer  143  and a first protrusion  210  is formed at the second region  143   b  of the cap layer  143 .  
         [0050]    Preferably, the first protrusion  210  is formed with a width wider than the width W2 of the ridge waveguide  200 . Although slightly exaggerated in FIG. 4, the ridge waveguide  200  has a width W2 of no more than several micrometers. Generally, the width W4 of the semiconductor laser diode  100   a  is about 200 μm. The first protrusion  210  may be separated from one side of the ride waveguide  200  by about 10 μm and be formed with a width W3 of about 50 to 100 μm. Preferably, the width W3 of the first protrusion  210  is formed to be wider than that W2 of the ridge waveguide  200 , but is not limited to the aforementioned range.  
         [0051]    Preferably, the upper surface  211  of the first protrusion  210  is formed to be at the same height as the upper surface  201  of the ridge waveguide  200  and more preferably, to be at a slightly higher height than the upper surface  201  of the ridge waveguide  200 . For this, for example, the upper surface of the contact layer  171  is formed to be at a height equal to or slightly lower than the upper surface of the current restricting layer  160  on the second region  143   b  of the cap layer  143  and then the bonding metal layer  172  is formed to the same thickness. In addition, various approaches such as formation of the bonding metal layer  172  at the second region  143   b  of the cap layer  143  thicker than that at the first region  143   a  of the cap layer  143  may be considered.  
         [0052]    The structural advantages of the semiconductor laser diode with the aforementioned structure will now be described.  
         [0053]    [0053]FIG. 5 is a sectional view of a flip-chip bonded structure of a submount and the semiconductor laser diode shown in FIG. 4 in a semiconductor laser diode assembly according to an embodiment of the present invention.  
         [0054]    The submount  410  is a heat discharge structure for preventing overheating of the semiconductor laser diode  100   a  by heat generated in an active layer during laser emission.  
         [0055]    Referring to FIG. 5, a reference numeral  411  indicates a substrate, reference numerals  412   a  and  412   b  indicate first and second metal layers, respectively, and reference numerals  413   a  and  413   b  indicate first and second solder layers, respectively.  
         [0056]    Preferably, the substrate  411  is made of one of AlN, SiC, GaN, and an insulating material having a heat transfer coefficient corresponding to that of one of AlN, SiC, and GaN. The first and second metal layers  412   a  and  412   b  are made of an Au/Cr alloy or a metal material corresponding to the Au/Cr alloy. The first and second solder layers  413   a  and  413   b  are made of an Au/Sn alloy or a metal material corresponding to the Au/Sn alloy.  
         [0057]    When the semiconductor laser diode  100   a  is bonded to the submount  410 , the first solder layer  413   a  is fused to the n-type electrode layer  190   a  and the second solder layer  413   b  is fused to the p-type electrode layer  170 . In the semiconductor laser diode  110   a  according to this embodiment of the present invention, the ridge waveguide  200  and the first protrusion  210  are positioned in the region of the p-type electrode layer  170 , and thus, the second solder layer  413   b  is fused to the ridge waveguide  200  and the first protrusion  210 .  
         [0058]    As mentioned above, in the case of the conventional semiconductor laser diode  80  shown in FIG. 1, only the ridge waveguide  70  with a width of no more than several micrometers is formed in the region of the p-type electrode layer  50 . As a result, when the semiconductor laser diode  80  is bonded to the submount  90 , thermal and mechanical stresses are concentrated on the protruded ridge waveguide  70 , thereby causing uneven light emission as shown in FIG. 3.  
         [0059]    In the case of the semiconductor laser diode  100   a  according to the embodiment of the present invention as shown in FIGS. 4 and 5, the first protrusion  210  with not less height than the ridge waveguide  200  is formed at one side of the ridge waveguide  200 . Therefore, when the semiconductor laser diode  100   a  is bonded to the submount  410 , the second solder layer  413   b  comes in contact with the first protrusion  210  and the ridge waveguide  200  at the same time or with first protrusion  210  first. Then, the second solder layer  413   b  is molten and spontaneously bonded to the ridge waveguide  200  and the first protrusion  210 .  
         [0060]    Because of this structural advantage in the semiconductor laser diode  100   a , a thermal stress generated during flip-chip bonding is dispersed to the first protrusion  210  adjacent to the ridge waveguide  200 . Therefore, uneven light emission due to concentration of a thermal stress on the ridge waveguide can be prevented. In addition, although the first and second solder layers  413   a  and  413   b  are not concurrently molten due to their thickness differences, a mechanical stress is dispersed to the first protrusion  210  with a width wider than the width of the ridge waveguide  200 , and thus, the concentration of a mechanical stress on the ridge waveguide  200  can be prevented.  
         [0061]    [0061]FIG. 6 is a perspective view of a semiconductor laser diode according to another embodiment of the present invention. The same reference numerals as used in FIG. 4 indicate the same constitutional elements.  
         [0062]    The semiconductor laser diode  100   b  further comprises a second protrusion  220  at the other side of the ridge waveguide  200 , that is, at the opposite side of the first protrusion  210 .  
         [0063]    Referring to FIG. 6, the second protrusion  220  is separated from the ridge waveguide  200  by a valley portion  230 , which is etched to expose the buffer layer  121 . The second protrusion  200  has a structure comprising the first material layer  120 , the active layer  130 , the second material layer  140 , and the current restricting layer  160  that are sequentially mounted on the substrate  110 , and an n-type electrode layer  190   b  mounted on the current restricting layer  160  to be electrically connected to the buffer layer  121 . The n-type electrode layer  190   b  is a top layer of the second protrusion  220  and extends to the bottom surface  231  of the valley portion  230 , to thereby be in contact with the buffer layer  121 .  
         [0064]    Preferably, the upper surface  221  of the second protrusion  220  is formed to be at the same height as the upper surface  201  of the ridge waveguide  200  and more preferably, at a slightly higher height than the upper surface  201  of the ridge waveguide  200 . As shown in FIG. 6, preferably, the top layer of the second protrusion  220  is the n-type electrode layer  190   b  electrically connected to the buffer layer  121 . In addition, the second protrusion  220  may has the same height as the first protrusion  210 . If the first and second protrusions  210  and  220  differ in height, it is preferable to limit the height difference to 0.5 μm or less.  
         [0065]    [0065]FIG. 7 is a sectional view of a flip-chip bonded structure of a submount and the semiconductor laser diode shown in FIG. 6 in a semiconductor laser diode assembly according to another embodiment of the present invention.  
         [0066]    Referring to FIG. 7, the submount  420  comprises a substrate  421 , first and second metal layers  422   a  and  422   b , and first and second solder layers  423   a  and  423   b . The submount  420  differs from the submount  410  shown in FIG. 5 in that the n-type electrode  190   b  is the top layer of the second protrusion  220  and the first and second solder layers  423   a  and  423   b  have the same thickness due to the same height of the first and second protrusions  210  and  220 . Here, it is preferable to set the thickness of the first and second solder layers  423   a  and  423   b  to be the same. Unlike the submount  90  shown in FIG. 2, the first and second solder layers  423   a  and  423   b  can be formed in a single process, and thus, can have almost the same chemical composition ratios.  
         [0067]    Because of this structural advantage, when the semiconductor laser diode  100   b  is bonded to the submount  420 , a thermal stress can be dispersed to the first protrusion  210  with a width wider than the ridge waveguide  200 . In addition, because the first and second solder layers  413   a  and  423   b  have almost the same thickness, uneven melting of the first and second solder layers  423   a  and  423   b  is less likely to occur. Therefore, a mechanical stress to be applied to the ridge waveguide  200  can be significantly reduced. Furthermore, because the first and second protrusions  210  and  220  with a width wider than the ridge waveguide  200  support the submount  420 , more stable flip-chip bonding is possible.  
         [0068]    [0068]FIG. 8 is an image plane photograph of laser light emission in the semiconductor laser diode assembly shown in FIG. 7. It can be seen from FIG. 8 that uniform and continuous light emission occurs along the ridge waveguide, unlike in FIG. 3.  
         [0069]    [0069]FIG. 9 is a perspective view of a semiconductor laser diode according to another embodiment of the present invention. The same reference numerals as used in FIGS. 4 through 7 indicate the same constitutional elements.  
         [0070]    The semiconductor laser diode  100   c  differs from the semiconductor laser diode  100   b  shown in FIG. 6 in that an n-type electrode layer  190   c  serves as the second protrusion  220 .  
         [0071]    Referring to FIG. 9, an exposed surface  240  of the buffer layer  121  is formed at the other side of the ridge waveguide  200 , that is, at the opposite side of the first protrusion  210  and the n-type electrode layer  190   c  is mounted on the exposed surface  240 . Preferably, the n-type electrode layer  190   c  has not less height than the ridge waveguide  200 . Preferably, the width W6 of the n-type electrode layer  190   c  is wider than that W2 of the ridge waveguide  200 . Preferably, the n-type electrode layer  190   c  has the same height as the first protrusion  210 .  
         [0072]    Consequently, the n-type electrode layer  190   c  serves as the second protrusion  220  of FIG. 6. Therefore, the structural advantages obtained by using the n-type electrode layer  190   c  are as described with reference to FIGS. 6 and 7.  
         [0073]    As apparent from the above description, a semiconductor laser diode and a semiconductor laser diode assembly of the present invention provide the following advantages.  
         [0074]    A thermal stress generated upon flip-chip bonding can be dispersed to the first protrusion adjacent to the ridge waveguide. In addition, the semiconductor laser diode further comprises the second protrusion, and thus, a mechanical stress generated by time difference melting of the solder layers of the submount can be effectively dispersed. Therefore, a laser diode and its assembly with uniform light emission throughout the ridge waveguide can be provided.  
         [0075]    While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.