Optical module

Abstract: In order to position an optical semiconductor chip (1) with high accuracy and without adjustment, a recess (4) for controlling the displacement of the optical semiconductor chip (1) is formed in a position in which the optical semiconductor chip on an optical element mounting substrate (3) having an optical waveguide (2) formed in the surface thereof is fixed.

Description: BACKGROUND OF THE INVENTION The present invention relates to an improvement of the accuracy for fixedly mounting optical semiconductor elements or chips onto an optical platform in an optical semiconductor module. An optical semiconductor module is a basic device constituting an optical communication system and comprises a laser diode (LD) which is a light emitting element or chip or a photodiode (PD) which is a photodetector, an optical fiber, a lens for optically coupling therebetween, and a housing for fixedly mounting them. The optical communication system is mainly applied to a trunk line to achieve an increase of high-speed operation and transmission distance. However, the optical communication system may not only be used in a trunk line but may also be applied to a local area network (LAN) in a building and a subscriber system for use in homes. In the optical communication system used by such wider users, not only function and performance of the large-capacity trunk line but also reduction in cost of devices constituting the system are strongly desired. Accordingly, in a conventional method in which alignment of an optical axis is made and optical semiconductor chips are fixedly mounted on a mounting platform to be coupled with optical waveguides or optical fibers, since much time and labor are required in order to couple the optical semiconductor chips with the optical waveguides or the optical fibers with high accuracy, the mass productivity is deteriorated and it is difficult to achieve a cost reduction. As a structure for coupling an optical semiconductor chip with an optical waveguide or an optical fiber with high accuracy, there is a structure utilizing solder bumps as shown in Fig. 5. In the structure utilizing the solder bumps, as shown in Figs. 5(a) to 5(c), an electrode pattern 7 of metal film is formed on an optical element mounting platform 3 and another electrode pattern 6 of metal film is also formed on a rear side of an optical semiconductor chip 1 so that the electrode pattern 6 conforms to the electrode pattern 7. Thus, the optical semiconductor chip 1 is disposed on the mounting platform 3 so that the electrode patterns 6 and 7 are substantially coincident with each other, and the electrode patterns 6 and 7 are joined with solder bumps 8. At this time, by previously forming the electrode pattern 7 on the mounting platform 3 in a position where optical semiconductor chips are optically coupled with optical waveguides or optical fibers when the optical semiconductor chips 1 are joined, the self-alignment effect acts due to the surface tension of the solder bump itself upon melting of the solder bump 8 as shown in Fig. 5(b), so that the electrode patterns 6 and 7 can be positioned with high accuracy and be optically coupled with each other without any adjustment. Thus, the optical semiconductor chips can be coupled with the optical waveguides or the optical fibers without any adjustment and accordingly improvement of the productivity of the optical semiconductor device can be expected. A self-aligning technique as shown in Figure 5 is disclosed in US-A-5,283,446. This document discloses an optical semiconductor module having an optical semiconductor chip having an electrode pattern which is mounted on a substrate having a corresponding electrode pattern through a plurality of solder bumps. Each of the bumps is surrounded by a side wall lower than the predetermined height of each bump. The optical semiconductor chip is positioned on the substrate in a horizontal direction by the self-alignment effect of the molten solder and in a vertical direction with a bottom surface of the optical semiconductor chip kept in contact with an upper surface of the side walls around the solder bumps upon cooling the molten solder due to shrinkage of the solder. In the US-A-5,283,446, the optical semiconductor chip is mounted on a principal surface of the substrate. On a secondary surface elevated from the principal surface, an optical waveguide or an optical fibre are laid with a prescribed height relative to the principal surface. The Patent Abstracts of JP-A-5029387, which does not refer to an optical module, discloses a method for accurately aligning a semiconductor chip with respect to a substrate by the use of a second substrate having an opening for engaging the semiconductor chip while the solder bumps solidify. When the chip is fixed in place, the second substrate is removed. However, none of the prior art deals with the problem that during fabrication of an optical module, the optical element located in the recess of the semiconductor substrate is displaced before the solder bumps harden, resulting in the fact that the electrode pattern of the optical element and the electron pattern of the semiconductor substrate are not coupled together by the solder bumps. SUMMARY OF THE INVENTION It is the object underlying the present invention to provide an optical module and a method of fabricating such an optical module which solves the above problem and improves mass-productivity. This object is met by an optical module according to claim 1 and a method according to claim 13. Preferred embodiments are disclosed in the dependent claims. According to the present invention, it is avoided that electrode patterns are not joined to each other or electrode patterns are joined to adjacent electrode patterns or electrode patterns do not quite come into contact with solder bumps. The optical semiconductor chip is conformed on the substrate before melting of the solder bumps with a positional accuracy of about 20 µm, depending on the size and number of the electrode patterns in order to position the optical semiconductor chip with high accuracy by utilizing the self-alignment effect. According to the present invention, a recess is formed in a semiconductor substrate in the position in which an optical semiconductor element on the substrate is fixedly joined to thereby be able to fixedly join the optical semiconductor element in a desired position without adjustment. Further, since the sides of the recess are formed obliquely so that the area of the opening of the recess is larger than the area of the bottom to thereby be able to reduce the amount of reflected light which is returned to the optical semiconductor element, the characteristics of the optical semiconductor element can be prevented from being scattered due to the reflected light. In addition, the obliquely formed sides of the recess are suitable for connection of the optical semiconductor element joined on the substrate and an electrode formed externally. Furthermore, by having a depth of the recess from 10 to 50 µm, the depth of the recess is deeper than the height of the unmelted solder bump and accordingly after the optical semiconductor element is disposed in the recess, the displacement of the optical semiconductor element until the solder bump is melted can be limited. The substrate and the optical semiconductor element can be joined with high accuracy by the self-alignment effect upon melting of the solder bump. In addition, by forming an antireflection film or layer on the sides of the recess, the reflected light is not returned to the optical semiconductor element and accordingly deterioration of the characteristics of the optical semiconductor element due to the reflected light can be prevented. Further, a plurality of recesses can be formed on a wafer and an optical semiconductor element is mounted in each of the recesses. The wafer and the optical semiconductor elements are fixedly joined by solder bumps and the wafer is cut into chips to form optical modules to thereby be able to improve the mass productivity of the optical modules and attain the low-cost optical semiconductor device. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1A and 1B are schematic diagrams illustrating an optical module according to an embodiment of the present invention in which an optical semiconductor chip and an optical waveguide are coupled; Figs. 2A and 2B are schematic diagrams illustrating an optical module according to an embodiment of the present invention in which optical semiconductor chips and an optical waveguide are coupled; Figs. 3A and 3B are schematic diagrams illustrating an optical module according to an embodiment of the present invention in which an optical semiconductor chip and an optical fiber are coupled; Figs. 4A and 4B are schematic diagrams illustrating an optical module according to an embodiment of the present invention in which optical semiconductor chips and an optical fiber are coupled; Figs. 5A to 5C illustrate coupling of an optical semiconductor chip and an optical platform by means of solder bumps; Fig. 6 illustrates electrodes formed in a recess; Fig. 7 illustrates an antireflection film formed in a recess; Fig. 8 schematically illustrates an optical semiconductor device including an optical module according to an embodiment of the present invention; Figs. 9A to 9D show a process of fabricating a plurality of optical modules according to an embodiment of the present invention simultaneously; Fig. 10 illustrates a shape of a recess in an embodiment of the present invention; Fig. 11 illustrates a shape of a recess in an embodiment of the present invention; Fig. 12 illustrates a shape of a recess in an embodiment of the present invention; and Fig. 13 illustrates a shape of a recess in an embodiment of the present invention. Figs. 7 and 10-13 are schematic with respect to the inclination of the sides of the recesses so that the obliqueness of the sides is hardly visible in those figures. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figs. 1A and 1B are schematic diagrams illustrating an embodiment of an optical module according to the present invention in which an optical semiconductor element or chip is coupled with an optical waveguide, and Figs. 1A and 1B are a plan view and a sectional view taken along line A-B, respectively. In Fig. 1, an optical waveguide 2, a recess 4 and an optical guide groove 5 are formed in the surface of a semiconductor substrate 3 or optical platform made of silicon, and an optical semiconductor chip 1 is fixedly mounted in the recess 4 by means of solder bumps 8. With such a structure, since the displacement of the optical semiconductor chip 1 on the substrate 3 can be limited, there does not occur a phenomenon that the optical semiconductor chip 1 is deviated before the solder bump 8 hardens so that electrode patterns 6 and 7 are not coupled with each other upon melting of the solder bump 8 or the optical semiconductor chip 1 is deviated from a position where the optical semiconductor chip 1 is optically coupled to the optical waveguide 2 and is joined to the deviated position or the optical semiconductor 1 is not quite joined fixedly. Accordingly, the electrode patterns 6 and 7 can be coupled with each other exactly. The length and the breadth of the bottom of the recess 4 are made larger than the length and the breadth of the bottom of the optical semiconductor chip 1 in consideration of the size and the number of the solder bumps, respectively, and are made narrower than respective values obtained by adding 60 µm to the length and the breadth of the bottom of the optical semiconductor chip 1, so that the displacement of the optical semiconductor chip 1 can be reduced to 30 µm or less and a phenomenon can be avoided that the position of the optical semiconductor chip 1 is deviated too much so that the solder bumps and the electrode pattern are not coupled. In consequence, the electrode patterns 6 and 7 can be coupled with each other exactly. Further, as shown in Fig. 10, even when the recess is formed to surround all corners of the surface of the optical semiconductor chip 1 opposite to the substrate 3, the above effect can be obtained. Furthermore, as shown in Fig. 11, even when the recess is formed to surround all sides of the surface of the optical semiconductor chip 1 opposite to the substrate 3, the above effect can be also attained. In addition, as shown in Fig. 12, even when the recess is formed to surround a set of opposite corners of the surface of the optical semiconductor chip 1 opposite to the substrate 3, the above effect can be obtained. Further, as shown in Fig. 13, even when the recess is formed to surround corners at both ends of any one side of the surface of the optical semiconductor chip 1 opposite to the substrate 3 and another side of the optical semiconductor chip 1 opposite to the one side, the above effect can be obtained. Figs. 2A and 2B are schematic diagrams illustrating another embodiment of an optical module according to the present invention in which optical semiconductor chips and an optical waveguide are coupled, and Figs. 2A and 2B are a plan view and a sectional view taken along line A-B, respectively. In Fig. 2, an optical waveguide 2, recesses 4 and optical guide grooves 5 are formed in the surface of a substrate 3 made of silicon , and a light emitting chip 11 and a photodetector 12 are fixedly mounted or joined in the respective recesses 4 by means of solder bumps 8. The photodetector 12 is provided to monitor laser light emitted from the light emitting chip 11 and the reliability of the light emitting chip 11 can be compensated by monitoring the laser light by the photodetector 12. In Figs. 2A and 2B, the photodetector 12 is mounted on the substrate 3 obliquely to the light emitting chip 11, so that light reflected by an end surface of the photodetector 12 can be prevented from being returned to the light emitting chip 11. An example of a fabricating method of the optical module shown in Figs. 1 and 2 is now described. A step is first formed in the substrate 3 by means of etching, and the optical waveguide 2 is formed by means of the flame hydrolysis deposition (FHD) and dry etching. Then, etching is performed to form the recess 4 and the optical groove 5. As shown in Figs. 6 and 7, an electrode 13 is formed and patterned from the inside to the outside of the recess 4 by the evaporation method and oxide film formed on the electrode 13 is windowed by means of the CVD method to form the electrode patterns 6 and 7. Then, the optical semiconductor chip 1 or the light emitting chip 11 and the photodetector 12 having electrodes 6 formed on the rear side thereof and solder bumps 8 formed thereon are disposed in the recess 4 and the solder bumps 8 are melted in a reflow furnace, so that the optical waveguide 2 and the optical semiconductor chip 1 or the light emitting chip 11 and the photodetector 12 are joined in the position in which they are coupled optically. As described above, the optical module of the present invention is completed. In the above fabricating method, when the recess 4 is formed, silicon is subjected to anisotrophy etching while a KOH aqueous solution is used as an etchant, so that side walls are inclined to be a (111) surface as shown in Fig. 6. Accordingly, the area of an opening can be made larger than the area of the bottom. Further, since the sides of the recess 4 for fixedly mounting or joining the optical semiconductor chip 1 and the light emitting chip 11 can be inclined to reduce reflected light, deterioration of the characteristic of the optical elements such as the optical semiconductor chip 1, the light emitting chip 11 and the photodetector 12 due to reflected light can be prevented. Light emitted from the light emitting chip 11 and incident on the photodetector 12 does not reach the end surface of the photodetector 12 and accordingly reflected light which influences the characteristics of the light emitting chip 11 does not occur. Furthermore, when the recess 4 is formed by dry etching, resist used as an etching mask is previously dragged or flagged by hard baking so that side walls of the groove formed by etching are also dragged and accordingly the area of the opening can be easily made larger than that of the bottom. In addition, as shown in Fig. 7, an antireflection film or layer 15 may be formed on the sides of the recess 4 to thereby be able to reduce reflection of laser light. Accordingly, deterioration of the characteristics of the optical elements such as the optical semiconductor chip 1 and the light emitting chip 11 due to reflected light can be prevented. Figs. 3A and 3B are schematic diagrams illustrating an embodiment of an optical module according to the present invention in which an optical semiconductor chip and an optical fiber 9 are coupled, and Figs. 3A and 3B are a plan view and a sectional view taken along line A-B, respectively. A V-groove 10 for fixedly mounting or joining the optical fiber 9, a recess 4 and an optical guide groove 5 are formed in the surface of a substrate 3 made of silicon, and an optical semiconductor chip 1 is fixedly mounted in the recess 4 by means of solder bumps 8. With such a structure, since the displacement of the optical semiconductor chip 1 on the substrate 3 can be limited, there does not occur the phenomenon that the optical semiconductor chip 1 is deviated before the solder bumps 8 harden so that the electrode patterns 6 and 7 to be coupled upon melting of the solder bump 8 are not coupled or the optical semiconductor chip 1 is deviated from a position in which the optical semiconductor chip 1 is optically coupled with the optical fiber 9 and is joined to the deviated position or the optical semiconductor chip 1 is not quite joined, and accordingly the electrode patterns 6 and 7 can be coupled exactly. Figs. 4A and 4B are schematic diagrams illustrating another embodiment of an optical module according to the present invention in which optical semiconductor chips and an optical fiber 9 are coupled, and Figs. 4A and 4B are a plan view and a sectional view taken along line A-B, respectively. A V-groove 10 for fixedly mounting or joining the optical fiber 9, a recess 4 and an optical guide groove 5 are formed in the surface of a substrate 3 of silicon, and a light emitting chip 11 and a photodetector 12 are fixedly mounted in the recess 4 by means of the solder bump 8. The photodetector 12 is provided to monitor laser light emitted from the light emitting chip 11 and the reliability of the light emitting chip 11 can be compensated by monitoring the laser light by the photodetector 12. With such a structure, since the displacement of the light emitting chip 11 and the photodetector 12 on the substrate 3 can be limited, there does not occur the phenomenon that the light emitting chip 11 and the photodetector 12 are deviated before the solder bumps 8 harden so that the electrode patterns 6 and 7 to be coupled upon melting of the solder bump 8 are not coupled or the light emitting chip 11 and the photodetector 12 are deviated from a position in which the light emitting chip 11 and the photodetector 12 are optically coupled with the optical fiber 9 and joined to the deviated position or the light emitting chip 11 and the photodetector 12 are not quite joined and accordingly the electrode patterns 6 and 7 can be coupled exactly. Further, in Fig. 4, the photodetector 12 is joined on the optical platform 3 obliquely to the light emitting chip 11, so that light reflected at an end surface of the photodetector 12 can be prevented from be returned to the light emitting chip 11. An example of a fabricating method of the optical module of the present invention shown in Figs. 3 and 4 is now described. A V-groove having side walls constituted by (111) surface is first formed in the substrate 3 by anisotrophy etching of silicon. Then, etching is performed to form the recess 4 and the optical guide groove 5. As shown in Fig. 6, an electrode 13 is formed and patterned from the inside to the outside of the recess 4 by the evaporation method, and an oxide film 14 formed on the electrode 13 is windowed by means of the CVD method to form the electrode patterns 6 and 7. Then, the optical semiconductor chip 1 or the light emitting chip 11 and the photodetector 12 having electrodes 6 formed on the rear side thereof and solder bumps 8 formed thereon are disposed in the recess 4 and the solder bumps 8 are melted in a reflow furnace to join them. Then, the optical fiber 9 is fixedly mounted in the V-groove 10 with solder to thereby optically couple the optical semiconductor chip 1 with the optical fiber 9 or the light emitting chip 11 and the photodetector 12 with the optical fiber 9. As described above, the optical module of the present invention is completed. In the above fabricating method, since the recess 4 is formed to incline the sides thereof so that reflected light can be reduced, deterioration of the characteristics of the optical elements such as the optical semiconductor chip 1 and the LD chip 11 due to the reflected light can be prevented. Light emitted by the light emitting chip 11 and incident on the photodetector 12 does not reach the end surface of the photodetector 12 and accordingly reflected light which influences the characteristics of the light emitting chip 11 is not produced. Further, as shown in Fig. 7, an antireflection film or layer 15 may be formed at the sides of the recess 4 to thereby be able to reduce reflection of laser light at the sides. Accordingly, deterioration of the characteristics of the optical elements such as the optical semiconductor chip 1 and the light emitting chip 11 due to reflected light can be prevented. Fig. 8 is a schematic diagram illustrating an optical semiconductor device in which the optical module of the present invention is mounted. In Fig. 8, the substrate 3 in which the optical semiconductor chip 1 is mounted is joined to a lower package 19 to which a lead frame 17 is formed. The electrode 13 (see fig. 4A and 6) on the substrate 3 is then connected to the lead frame 17 by wire bonding. The optical fiber 9 is disposed in the V-groove on the substrate 3 and fixed therein by adhesive agent or the like. Then, the upper package 18 is closely joined to the lower package 19 by means of adhesive agent while maintaining the inside of the package to be an atmosphere of nitrogen. Thus, the optical semiconductor device having the optical module of the present invention mounted therein is completed. Figs. 9A to 9D show a process of fabricating a plurality of optical modules of the present invention simultaneously. In Fig. 9A, optical waveguides 91 and recesses 92 are formed on a semiconductor wafer 93. Then, as shown in Fig. 9B, optical semiconductor chips 94 are disposed in the recesses 92 and the semiconductor wafer 93 and the optical semiconductor chips 94 are fixedly joined by solder bumps. The semiconductor wafer 93 is cut into individual chips along broken lines shown in Fig. 9C, so that the optical module of the present invention can be obtained as shown in Fig. 9D. Since the process shown in Fig. 9 can be used to form the plurality of optical modules of the present invention simultaneously, the mass productivity can be improved. Further, since the optical semiconductor chips 94 and the optical waveguides 91 can be coupled without adjustment, the low-cost optical semiconductor device can be realized. According to the present invention, the recesses are formed in the positions on the substrate in which the optical semiconductor chips are fixedly joined to thereby be able to position the optical semiconductor chips without adjustment so that the optical semiconductor chips can be fixedly joined on the substrate platform with high accuracy. Further, according to the present invention, the sides of the recesses for fixedly joining the optical semiconductor chips are formed obliquely to thereby be able to reduce reflected light returned to the optical semiconductor chips and accordingly deterioration of the characteristics of the optical semiconductor chips due to the reflected light is prevented. In addition, according to the present invention, the depth of the recesses is made deeper than the height of the unmelted solder bumps to thereby be able to limit the displacement of the optical semiconductor chips, the light emitting chips and the photodetectors until the solder bumps are melted after the optical semiconductor chips, the light emitting chips and the photodetectors are disposed on the substrate and the optical semiconductor chips, the light emitting chips and the photodetectors can be fixedly joined to the optical platform or substrate by the self-alignment effect with high accuracy upon melting of the solder bumps.

Claims: An optical module comprising a semiconductor substrate (3), an optical waveguide (2) or an optical fibre (9) disposed on the substrate (3), a recess (4) having a first electrode pattern (6) at its bottom surface, and having slanting sidewall surfaces such that the recess (4) tapers towards its bottom, an optical semiconductor element (1) placed in the recess (4) and being held in a prealigned position by the sidewall surfaces, and having a second electrode pattern (7) connected to the first electrode pattern (6) by solder bumps (8) using the self-alignment effect, and an optical guide groove (5) for optically coupling an active portion of said semiconductor element (1) with an end surface of said waveguide (2) or fibre (9). The optical module according to claim 1, comprising a plurality of optical semiconductor elements (11, 12) fixedly mounted in corresponding recesses (4) in said semiconductor substrate (3), each recess being formed in a position in which end surfaces of active layers of said plurality of optical semiconductor elements are optically coupled with each other. The optical module according to claim 1 or 2, wherein said first electrode pattern (13) extends to the boundary of said recess (4) to be connected to a further electrode. The optical module according to any of claims 1 to 3, further comprising an antireflection film (15) formed at the sides of said recess (4). The optical module according to any of claims 1 to 4, wherein a side of said recess (4) formed in a surface of said semiconductor substrate is constituted of a {111} surface. The optical module according to any of claims 1 to 5, wherein the depth of said recess (4) is 10 to 50 µm. The optical module according to any of claims 1 to 6, wherein the length and the width of the bottom of said recess (4) are larger than the length and the width of said optical semiconductor element (1), respectively, and are narrower than respective values obtained by adding 60 µm to the length and the width of said optical semiconductor element, respectively. The optical module according to any of claims 1 to 7, wherein said recess (4) surrounds at least four corners of the bottom of said optical semiconductor element (1). The optical module according to any of claims 1 to 7, wherein said recess (14) surrounds at least four sides of the bottom of said optical semiconductor element (1). The optical module according to any one of claims 1 to 7, wherein said recess (4) surrounds at least a set of opposite corners of said optical semiconductor element (1). The optical module according to any of claims 1 to 7, wherein said recess (4) surrounds at least two corners at one side of the bottom of the optical semiconductor element (1) and another side of the bottom of said element opposite to said one side. The optical module according to any of claims 1 to 11, wherein said optical semiconductor element comprises a photodiode or a laser diode. A method of manufacturing an optical module, comprising the steps of (a) providing an optical waveguide (2) or an optical fibre (9) on a semiconductor substrate (3), (b) anisotropically etching the substrate (3) to form a recess (4) with slanting sidewall surfaces such that the recess tapers towards its bottom, and an optical guide groove (5) for providing an optical coupling path between the recess (4) and an end surface of said waveguide (2) or fibre (9), (c) forming a first electrode pattern (6) at the bottom surface of the recess (4), (d) placing an optical semiconductor element (1) with a second electrode pattern (7) at its lower surface in the recess (4) to prealign the semiconductor element (1) by the sidewall surfaces of the recess, and (e) connecting said first and second electrode patterns (6, 7) by solder bumps (8) using the self-alignment effect to align the semiconductor element (1) and the substrate (3).

Inventors: HIRATAKA TOSHINORI, KIKUCHI SATORU, HIRATAKA, TOSHINORI, KIKUCHI, SATORU
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1996
Main classification: G02B6
IPC codes: G02B6, H01L31, H01L33, H01S5