Abstract:
A waveguide optical semiconductor device, a method of fabricating the same and an optical device module. The semiconductor device includes a substrate, a waveguide formed on the substrate, an electrode layer formed on the waveguide, and bumpers formed on the substrate. The bumpers are disposed on both side of the waveguide, and top surfaces of the bumpers are higher than a top surface of the electrode layer. The method of fabrication includes forming semiconductor layers for waveguide on a substrate, forming another semiconductor layer on the semiconductor layers, removing the another semiconductor layer and at least a part of the semiconductor layers selectively so that grooves are formed on both side of a region where the waveguide are expected to be formed, removing the another semiconductor layer at the region, remained portions of the another semiconductor layer form bumpers. The module includes a waveguide optical semiconductor device having bumpers disposed on both side of the waveguide, and a carrier having a mounting region in contact with the top surfaces of the bumpers.

Description:
FIELD OF THE INVENTION 
     The invention relates to a waveguide optical semiconductor device, a method of fabricating the same and an optical device module. 
     BACKGROUND OF THE INVENTION 
     A conventional waveguide optical semiconductor device is described, for example, in Proceedings of the 1993 Conference of the Institute of Electronics, Information and Engineers, p. 4-195. A waveguide optical semiconductor device described in this publication includes a waveguide portion and an upper electrode disposed on the waveguide portion. 
     In a conventional waveguide optical device, the waveguide portion sticks out of the surface of the device. Therefore, in manufacturing or inspecting the device, external force tends to be applied to the waveguide portion. As a result, cracks or defects are generated in the upper electrode or the waveguide portion. 
     A conventional optical device module is described, for example, in Proceedings of the 1997 Conference of the Institute of Electronics, Information and Engineers, p. 347. An optical device module described in this publication includes a waveguide optical semiconductor device and a carrier on which the optical device is mounted. 
     In a conventional optical device module, the carrier has terrace-shaped SiO 2  films on its surface for receiving the optical device. The SiO 2  films are swellings in the device mounting region of the carrier. Therefore, levelness of the device mounting region is reduced. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide a waveguide optical semiconductor device, a method of fabricating the same and an optical device module, which solve the above-described problem. According to the present invention, such a device includes a substrate, a waveguide formed on the substrate, an electrode layer formed on the waveguide, and bumpers formed on the substrate, the bumpers are disposed on both side of the waveguide, and top surfaces of the bumpers are higher than a top surface of the electrode layer. 
     Therefore, the electrode layer formed on the waveguide does not stick out of the top surface of the bumpers. As a result, it is avoided that external force is applied to the waveguide. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The objects and features of the invention will become more apparent from the consideration of the following detailed description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is an oblique view of a first embodiment according to the invention. 
     FIGS. 2 to  8  are schematic views of fabricating process of the first embodiment. 
     FIG. 9 is a graph showing etching rate of InP. 
     FIG. 10 is an oblique view of a first modification of the first embodiment. 
     FIG. 11 is an oblique view of a second modification of the first embodiment. 
     FIG. 12 is an oblique view of a third modification of the first embodiment. 
     FIG. 13 is an oblique view of a fourth modification of the first embodiment. 
     FIG. 14 is an oblique view of a second embodiment according to the invention. 
     FIG. 15 is an oblique view of a modification of the second embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The First Embodiment 
     FIG. 1 illustrates the first embodiment of the invention, an optical semiconductor device  10 . In FIG. 1, a lower clad layer  104  and an optical guide layer  106  are formed in stated order on a semiconductor  102 . A mesa stripe  42  including an upper clad layer  108  and a contact layer  110 , is formed on the optical guide layer  106 . Along with the optical guide layer  106  and the lower clad layer  104 , the mesa stripe  42  forms a waveguide  12 . 
     An both side of the mesa stripe  42 , bumpers  14  are formed on the optical guide layer  106  to be higher than the mesa stripe  42 . Each of the bumpers  14  includes an upper clad layer  108 , a contact layer  110  and a cap layer  112 . Between the mesa stripe  42  and bumpers  14 , insulating layers  122  are formed. 
     An upper electrode  124  is formed on the contact layer  110  of the mesa stripe  42  and a lower electrode  126  is formed on a bottom surface of the semiconductor substrate  102 . A part of the upper electrode  124  extends on one of the bumpers  14  in order to form a bonding pad  124   a.    
     The difference of the height between the mesa stripe  42  and the bumpers,  14  generates a hollow  20  above the mesa stripe  42 . The hollow  2 Q has enough depth so that an upper electrode  124  above the mesa stripe  42  does not stick out of the hollow  20 . Therefore, objects contact the top surfaces  16  of the bumpers  14 , but not the upper electrode  124  in the hollow  20 . Thus, it is avoided that the external force applied to the waveguide  12 . 
     When a specified electric signal is applied along with a specified voltage between the upper electrode  124  and the lower electrode  126 , an effect of confining optical wave and a specified optical function are obtained in the optical guide layer  106  of the optical waveguide  12 . Therefore, a light which propagates in the optical waveguide  12  radiates from a first facet  18   a  or a second facet  18   b  of the optical semiconductor device  10 . 
     FIGS. 2 to  8  illustrate a process of a method of fabricating the waveguide optical semiconductor device  10 . 
     In FIG. 2, a lower clad layer  104  of a first conductivity type, an optical guide layer  106 , an upper clad layer  108  of a second conductivity type, a contact layer  110  of the second conductivity type and a cap layer  112 , are formed in the stated order on a semiconductor substrate  102  of the first, conductivity type. These layers  104 - 112  are formed by, for example, a conventional epitaxial growth method. 
     For example, an n-InP crystal is grown on the substrate  102  to form the lower clad layer  104  of a thickness T 1  (1 μm). An undoped InGaAs crystal is grown on the lower clad layer  104  to form the optical guide layer  106  of a thickness T 2  (0.2 μm). A p-InP crystal is grown on the optical guide layer  106  to form the upper clad layer  108  of a thickness T 3  (1 μm). 
     Then a p + -InGaAs crystal is grown on the upper clad layer  108  to form the contact layer  110  of a thickness T 4  (0.2 μm). A p-InP crystal is grown on the contact layer  110  to form the cap layer  112  of a thickness T 5  (1 μm). 
     Then, as shown in FIG. 3, a first mask  114   a  and second masks  114   b  X are formed on the cap layer  112 . The first mask  114   a  is strip-shaped and is located to be above a waveguide portion  22 . On both side of the first mask, the second masks  114   b  are located to be above bumper portions  24 . 
     The first mask  114   a  and the second masks  114   b  are formed by patterning dielectric film, e.g. SiO 2  film, using conventional photolithography. The width Wa of the first mask  114   a  is 3 μm and the distance Ws between the first mask  114   a  and each of the second masks  114   b  is 10 μm. 
     Then, as shown in FIG. 4, the cap layer  112  and the contact layer  110  are removed selectively by anisotropic dry etching employing the first mask  114   a  and the second masks  114   b  as etching masks. The etching is kept until the grooves  116  reach the upper clad layer  108  so as to pierce the contact layer  110 . For the dry etching, for example, plasma etching using combination gas of chlorine (Cl) and argon (Ar) is applied. 
     Then, as shown in FIG. 5, the cap layer  112  is removed selectively, under the mask  114   a  and masks  114   b  by wet etching. A portion of the cap layer  12  under the mask  114   a  is etched thoroughly, and therefore, the mask  114   a  is lifted off. 
     The upper clad layer  108  is also removed selectively by this wet etching. However, the contact layer  110  and portions of the upper clad layer  108  under the contact layer  110  are not etched. Therefore, a mesa stripe  42  including the upper clad layer  108  and the contact layer  110  is formed. Simultaneously, the bumpers  14  including the layers  108 - 112  are formed on both side of the mesa stripe  42 . 
     The mesa stripe  42 , and portions of the optical guide layer  106  and the lower clad layer  104  under the mesa stripe  42 , form an optical waveguide  12 . 
     As etchani for the above wet etching, solution of hydrochloric acid (HClaq) or combination solution of hydride bromide (HBr) and acetic acid (CH 3 COOH) is used so as to etch only InP. 
     FIG. 9 shows etching rate of InP with the combination solution of hydride bromide and acetic acid. In FIG. 9, etching amount in the horizontal direction, that is amount of side etching, reaches 1.5 μm in about 1 minute and 15 seconds. Therefore, for example, when width of the cap layer  112  on the mesa stripe  42  is 3 μm, the cap layer  112  is removed through the etching in about 1 minute and 15 seconds. 
     Then, as shown in FIG. 6, after the masks  114   b  are removed, a passivation film  120  of insulating material is coated on whole exposed surface. The thickness T 6  of the passivation film  120  is 0.2 μm. Then both sides of the mesa stripe  42  are filled with the insulating layers  122  of insulating material such as a polyimide. As a result, the hollow  20  is formed above the optical waveguide  12 . 
     Then, as shown in FIG. 7, after the passivation film  120  above the mesa stripe  42 , that is at the bottom of the hollow  20 , is removed, the upper electrode  124  is formed on the contact layer  110  of the optical waveguide  12 . The thickness T 7  of the upper electrode  124  is selected not to stick out of the hollow  20 . In this embodiment, the thickness T 7  is, for example, 0.5 μm. 
     Then, as shown in FIG. 8, after the substrate  102  is ground so as to obtain specified thickness, the lower electrode  126  is formed on the bottom surface of the substrate  102 . 
     As described above, according to the invention, the optical device  10  has the bumpers  14 . Therefore, upper electrode  124  does not stick out of the hollow  20 . It is avoided that the external force is applied to the optical waveguide  12 . 
     FIG. 10 shows a first modification of the first embodiment. As shown in FIG. 10, a part of the cap layer  112  of the bumper  14  is removed so as to make a recess  30 , and the insulating layer  122  and the upper electrode  124  extend on the recess  30 . Therefore, the upper electrode  124  is formed in shape of T figure. In this modification, a bonding pad  124   a , a portion of the upper electrode  124  on the bumper  14 , does not stick out of the top surface  16  of the bumper  14 . 
     FIG. 11 shows a second modification of the first embodiment. In FIG. 11, the grooves  118  pierce the optical guide layer  106  and reach the lower clad layer  104 . In this modification, the optical guide layer  106  of the waveguide  12  is sandwiched between the insulating layers  122 . Therefore, it is improved to confine light-wave in the optical guide layer  106 . This modification has advantage when it is applied to an optical semiconductor device which has a bent waveguide. 
     FIG. 12 shows a third modification of the first embodiment. In FIG. 12, the optical device  10   c  is of buried waveguide-type and the both sides of the mesa stripe  42 ′ are buried in semiconductor layers. A portion of the cap layer  112 ′ above the optical waveguide  12  is removed so that the hollow  20  is formed above the waveguide  12 ′. In the hollow  20 , the upper electrode  124  is formed on the contact layer  110 ′ to be above the waveguide  12 ′. 
     A part of the cap layer  112 ′ of the bumper  14 ′ is removed and the upper electrode  124  extends there. The upper electrode  124  is thinner than the-cap layer  112 ′. Therefore, a bonding pad  124   a , a portion of the upper electrode  124  does not stick out of the surface  16  of the device  10   c.    
     FIG. 13 shows a fourth modification of the first embodiment. In FIG. 13, the optical device  10   d  is of another buried waveguide-type. Grooves  118 ′ are formed on both sides of the waveguide  12 ′, and are filled with insulating layers  122 ′. 
     The Second Embodiment 
     Referring to FIG. 14, in a second embodiment, an optical device module includes a waveguide optical semiconductor device  10   e  and a carrier  200   a.    
     In FIG. 14, the device  10   e  employs a second upper electrode  126 ′ instead of the lower electrode  126  of the first embodiment. The upper electrode  124  extends on the recess  30  of the one bumper  14 . The other bumper  14  has a groove  150  which reaches the lower clad layer  104 . A region of the other bumper  14  which is apart from the hollow  20  has a recess  30   a . The second upper electrode  126 ′ is formed continuously from the recess  30   a  to the bottom of the groove  150 . 
     The carrier  200   a  includes a device mounting region  206  and a fiber mounting region  208 . Electrodes  202  are formed on the device mounting region  206 . The electrodes  202  are, for example, microstrip lines or coplanar lines. A V-groove  204  is formed in the fiber mounting region  208  to hold a optical fiber. 
     The device  10   e  is mounted so that the upper electrodes  124  and  126 ′ contact the electrodes  202  of the carrier  200   a , and the top surfaces  16  of the bumpers  14  contact the surface of the device mounting region  206 . Surfaces of the upper electrodes  124  and  126 ′ at the recesses  30  and  30   a  are lower than the top surfaces  16  of the bumpers  14 . For example, the gap of the surfaces is 0.5 μm, and the thickness of the electrodes  202  of the carrier  200   a  is 0.5 μm. To strengthen the contact between the electrodes  124 ,  126 ′ and the electrodes  202 , solder may be used. 
     For coupling the bumper  14  and device mounting region  206 , metal films (not illustrated) may be formed respectively on the top surfaces  16  of the bumpers  14  and the corresponding areas of the device mounting region  206 . Then corresponding metal films of the bumpers  14  and the device mounting region  206  are coupled with the solder. 
     When amount of the solder is too much, the hollow - 20  receives the extra solder so that a short circuit between the upper electrode  124  and the second upper electrode  126 ′ is avoided. 
     FIG. 15 shows a modification of the second embodiment. In FIG. 15, carrier  200   b  has two fiber mounting regions  208  and  210  on both sides of the device mounting region  206 . 
     In the second embodiment, the device  10   e  is mounted stably by putting the surfaces  16  of the bumpers  14  to the surface of the carrier  200   a  ( 200   b ). Therefore, it is not required to make other parts on the carrier  200   a  ( 200   b ) to receive the device  10   e.    
     Moreover, morphology and levelness of the surfaces  16  of the bumpers  14  are accurate because of the crystal growth. That is advantage for aligning the device  10   e  to the optical fibers mounted on the carrier  200   a  ( 200   b ). 
     While the invention has been described with reference to two embodiments thereof, it will be understood by those skilled in the art that modifications thereof can be made without departing from the spirit and scope of the invention, and the invention includes all such modifications and variations, the scope of the invention to be limited only by the appended claims. 
     For example, the type of the waveguide in the embodiment is not limited as a ridge-type. A BH-type, a rib-type or a planer-type can be employed. Moreover, the invention is applicable to various type of optical semiconductor device such as an optical modulator, a laser diode, an optical amplifier, a wavelength transformer, an optical filter, a photo diode, a photo coupler, LED or monolithically integrated device thereof.