Selfaligned optical element and optical wave-guide by means of solder bumps on rectangular pads

Abstract: An optical module has an optical wave-guide structure (12) formed on a silicon substrate (11) and a laser diode array (13) solder bonded to first pads (11a-11j) patterned on the silicon substrate in a self-aligned manner by virtue of surface tension of solder during a reflow stage, and the first pads (11a-11j) are shaped into a rectangular parallelopiped configuration so as to make a ratio of a solder bump height to a pad width large without decrease of the volume of the solder bump, thereby generating large surface tension.

Description: FIELD OF THE INVENTION This invention relates to an optical module and, more particularly, to the structure of an optical module having an optical element and an optical wave-guide self-aligned with one another on a substrate and a method of assembling the optical module. DESCRIPTION OF THE RELATED ART An optical module is incorporated in an optical communication system and an optical transmission system, and includes an optical element such as a laser diode and an optical wave-guide fabricated on a silicon substrate. The optical wave-guide has a core, and the core propagates light therealong. On the other hand, the laser diode has an active layer, and laser beam is radiated from the active layer. In order to guide the laser beam along the core, it is necessary for the core to be aligned with the active layer, and the optical wave-guide and the laser diode are assembled on the silicon substrate such that the active layer is aligned with the core. The laser diode had been manually aligned with the active layer, and large amount of time and labor was consumed in the assembling work. The manually assembling work increased the cost, and the optical module was high-price. A self-alignment technology has been developed, and a typical example of the self-alignment technology is disclosed by Sasaki et. al. in "Self-aligned Assembly for Optical Devices Using AuSn Solder Bump Bonding", Technical Report of the Institute of Electronics Information and Communication Engineers, pages 61 to 66. Figures 1A, 1B and 2A and 2B illustrate the self-aligned assembly proposed by Sasaki et. al. Reference numerals 1, 2 and 3 designate a silicon substrate, an optical wave-guide and a laser diode, respectively. The three-axis reference system is represented by arrows "x", "y" and "z". The optical wave-guide 2 has a core 2a, and the optical axis of the core 2a is labeled with reference 2b. On the other hand, an active layer 3a is incorporated in the laser diode 3, and the optical axis of the active layer 3a is labeled with 3b. The optical wave-guide 2 and the laser diode 3 are aligned on the major surface of the silicon substrate 1 as follows. The optical wave-guide 2 is fabricated on the silicon substrate 1, and the core 2a and circular pads 1a, 1b, 1c and 1d are concurrently patterned on the major surface of the silicon substrate 1 by using a lithographic process. As a result, the circular pads 1a to 1d are accurately positioned with respect to the core 2a, and the error of the relative position between the core 2a and the circular pads 1a to 1d is roughly equal to the mask mis-alignment in the lithographic process. Subsequently, solder bumps 4 are provided on the circular pads 1a to 1d, respectively. Circular pads 3c, 3d, 3e ad 3f have been already formed on the back surface of the laser diode 3, and the relative position between the circular pads 3c to 3f and the active layer 3a is identical with the relative position between the circular pads 1a to 1d and the core 2a. The laser diode is put on the solder bumps 4 in such a manner as to roughly align the circular pads 1a to 1d with the circular pads 3c to 3f. However, the circular pads 3c to 3f are usually offset from the circular pads 1a to 1d, and, accordingly, the optical axis 2b of the core 2a is not aligned with the optical axis 3b of the active layer 3a as shown in figures 1A and 2A. Heat is applied to the solder bumps 4, and the solder bumps 4 are reflowed. The surface tension in the reflowed solder bumps forces the circular pads 3c to 3f to move in a direction ST, and the circular pads 1a to 1d are registry with the circular pads 3c to 3f, respectively. As described above, the relative position between the circular pads 1a to 1d and the core 2a is identical with the relative position between the circular pads 3c to 3f and the active layer 3a, and the circular pads 3c to 3f registry with the circular pads 1a to 1d automatically aligns the optical axis 3b of the active layer 3a with the optical axis 2b of the core 2a as shown in figure 2B. Thus, the prior art self-aligned assembling technology allows the manufacturer to assemble the optical modules at high speed, and the production cost is drastically decreased. However, only appropriately designed bumps generates the surface tension large enough to self-align the optical axis 3b of the active layer 3a with the optical axis 2b of the core 2a. The large surface tension requires a large ratio of the bump height to the pad diameter. In other words, the large surface tension is generated only when the bumps are formed in a large aspect ratio. If the soldering is carried out without flux, the large surface tension requires a larger aspect ratio. The present inventors evaluated various bumps formed on the circular pads, and concluded that the prior art self-aligned flux-less solder bonding in nitrogen atmosphere required the ratio of the bump height to the pad diameter equal to or greater than 0.75. The smaller the pad diameter was, the higher the alignment accuracy was. Therefore, when an optical device shaped into 300 micron cube is aligned with an optical wave-guide on a silicon substrate, it is appropriate for the circular pads to be at least 50 microns in diameter because of sufficient joint force and large heat radiation capability, and the minimum bump height for the flux-less solder bonding is equal to or greater than 38 microns. However, the core 2a of a latest optical wave-guide is spaced from the major surface of the silicon substrate 1 by only 20 microns or less, and the bumps of 38 microns high are not available for an optical module with the latest optical wave-guide spaced from the major surface by only 20 microns or less. The latest optical wave-guide requests the manufacturer to miniaturize the bumps without change of the ratio of the bump height to the pad diameter. If the latest optical wave-guide 2 is lifted up, the appropriate bumps 4 may be available without the miniaturization. However, when the bumps 4 are formed from pieces of solder provided on the circular pads through a punching, which is described hereinbelow in detail, the bump height ranges 50 microns plus minus 1.5 microns, and the fluctuation of the bump height is too large to couple the laser diode 3 to the optical wave-guide 2 in a single mode. Therefore, the miniaturization is desirable. Following problems are encountered in the miniaturization of the bumps. First, the reduction of the pad diameter results in insufficient joint force and poor heat radiation capability. This is the first problem. The second problem is that the miniature bumps are hardly formed by using an economical punching process. Au-Sn eutectic solder is usually used for the bumps 4, and the Au-Sn eutectic solder bumps are economically formed from a Au-Sn eutectic foil through a punching process disclosed in Japanese Patent Publication of Unexamined Application No. 4-152682. Figures 3A to 3C illustrate the process of forming bumps from a Au-Sn solder foil. First, the Au-Sn solder foil 5 is intermittently moved between a punch 6a and a die 6b, and a pad 7a formed on the major surface of a silicon substrate 7b is moved to an appropriate position beneath the die 6b as shown in figure 3A. The punch 6a is downwardly moved so as to cut off a piece 5a of Au-Sn solder from the Au-Sn solder foil 5, and the piece 5a of Au-Sn solder is put on the pad 7a as shown in figure 3B. Heat is applied to the piece of Au-Sn solder, and reflows the pieces of Au-Sn solder. The surface tension shapes the reflowed Au-Sn solder into a semi-spherical configuration as shown in figure 3C. The prior art process shown in figures 3A to 3C is economical, and is desirable for the optical modules. However, the volume of the piece 5a is hardly decreased to a critical value. The presently available punching technology sets limits on the size of the punch 6a, the thickness of the Au-Sn solder foil 5 and the volume of the Au-Sn solder piece 5a, and the minimum punch diameter, the minimum foil thickness and the minimum volumes are of the order of 40 microns, 20 microns and 2500 cubic microns, respectively. When the heat was applied to the minimum Au-Sn solder piece 5a, the minimum Au-Sn solder piece 5a was reflowed into the semi-spherical bump 5b. The semi-spherical bump 5b was 30 microns in height, and the bottom surface and, accordingly, the pad 7a is 30 microns in diameter. After the solder bonding, the bump height was decreased to 25 microns. However, the height was too high to the alignment with the core 2a. Of course, the Au-Sn solder bumps are formed through other processes such as a plating or an vacuum evaporation. However, the plating can not exactly control the thickness of a Au-Sn solder layer on the pad, and the vacuum evaporation is complex and expensive. A similar prior art optical module is known from document US 5,283,446. Like in the above-discussed case circular pads are provided on a substrate and on a laser diode chip, and in order to improve the desired self-alignment of the laser diode with respect to an optical waveguide during bonding, side walls are provided surrounding the solder bumps. The height of these side walls is chosen in such a way that when the solder bump shrinks during cooling, the laser diode chip will rest on a corresponding side wall and will thus be positioned at a predetermined height which assures vertical alignment with respect to the optical waveguide. The article by Deshmukh et al., "Active atmosphere solder self-alignment and bonding of optical components", International Journal of Microcircuits and Electronic Packaging, Vol. 16, No. 2, January 1, 1993, Reston, USA, discloses the possibility to align one single optoelectronic component such as an LED or a laser with one single core by means of a similar self-alignment method using rectangular pads instead of circular pads. However, although replacing circular pads by rectangular pads leads to the question how each pad is to be arranged, this document does not disclose any particular orientation of the rectangular pads. In particular no information can be found on where corresponding rectangular pads are to be provided in case an optical wave propagation means comprises a plurality of cores and an optical element comprises several ports, so that each port has to be aligned with an associated core. A similar observation holds true for prior art document WO 93/15424 which also discloses a self-alignment method based on the use of non-circular pads in order to align one single semiconductor laser with one single waveguide. As further prior art, reference is made to documents US 5,275,970, EP 0 304 118 A2, to the article by Satoh et al., "Development of a new micro-solder bonding method for VLSI's", Conference Proceedings for the Third Annual Intemational Electronic Packaging Conference, Hasca, Illinois, (1983), pages 455 to 460, and to the further publication by Satoh et al. "Optimum bonding shape control of micro-solder joint of IC and LSI", J. Japan Inst. Metals, Vol. 51 (1987), pages 553 to 560. In summary, even those documents of the prior art which propose to use non-circular pads for self-alignment do not disclose any reliable possibility to self-align an optical wave propagation means having a plurality of parallel cores for propagating light with an optical element having a plurality of ports. SUMMARY OF THE INVENTION It is therefore an important object of the present invention to provide an optical module which has an optical wave-guide having a plurality of parallel cores and an optical element having a plurality of ports aligned with one another around 20 microns in height without reduction of joint force and heat radiation capability and increase of the production cost, as well as a method of assembling such a module. According to the invention, this object is achieved by an optical module as defined in claim 1 and a method as defined in claim 10. Advantageous embodiments are defined in dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS The features and advantages of the optical module and the method of assembly according to the present invention will be more clearly understood from the following description taken in conjunction with the accompanying drawings in which: Figs. 1A and 1B are plan views showing the prior art method of assembling the optical module; Figs. 2A and 2B are cross sectional views showing the prior art method; Figs. 3A to 3C are side views showing the prior art process of forming the bumps on the pads; Figs. 4A and 4B are plan views showing a method of assembling an optical device according to the present invention; Figs. 5A and 5B are cross sectional views showing the method according to the present invention; Figs. 6A and 6B are cross sectional views showing the method according to the present invention from a different angle; Figs. 7A and 7B are plan views showing another method of assembling an optical module according to the present invention; Figs. 8A and 8B are cross sectional views showing the method according to the present invention; Fig. 9 is a plan view showing the layout of yet another optical module according to the present invention; Fig. 10 is a cross sectional view showing the structure of the optical module; and Fig. 11 is a cross sectional view showing the structure of the optical module viewed from another angle. DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment A method of assembling an optical module embodying the present invention is hereinbelow described with reference to figures 4A and 4B, figures 5A and 5B and figures 6A and 6B. The three axis reference system is indicated by arrows "x", "y" and "z", and figures 6A and 6B are seen from the right side of the structure shown in figures 4A and 4B. The method starts with preparation of a silicon substrate 11. First pads 11a, 11b, 11c, 11d, 11e, 11f, 11g, 11h, 11i and 11j are formed on a major surface of the silicon substrate 11 by using lithographic techniques. Namely, titanium/platinum/gold layers are laminated on the major surface of the silicon substrate 11, and a pattern is transferred from a reticle (not shown) to a photo-resist layer (not shown) covering the titanium/platinum/gold layers. The photo-resist layer is developed, and is patterned. Using the patterned photo-resist layer, the titanium/platinum/gold layers are partially etched away, and are patterned into the first pads 11a to 11j. Each of the first pads 11a to 11j is shaped into a generally rectangular parallelopiped configuration, and is 140 microns in length and 25 microns in width. However, the corners of each first pad 11a to 11j are rounded, and the round corners allow solder to spread over the entire upper surface of the first pads 11a to 11j. The first pads 11a, 11c, 11f and 11i have respective longitudinal axes in parallel to the y-direction, and the other first pads 11b, 11d, 11e, 11g, 11h and 11j have respective longitudinal axes in parallel to the x-direction. Thus, the first pads 11a, 11c, 11f and 11i are different from the other first pads 11b, 11d, 11e, 11g, 11h and 11j at right angle. Although the corners are rounded, the upper surface of the first pad is generally rectangular, because the side lines and the end lines are almost linear. Subsequently, an optical wave-guide structure 12 is formed on the major surface of the silicon substrate 11 by using the lithographic techniques. The optical wave-guide structure 12 has a plurality of cores 12a, 12b, 12c and 12d so as to propagate laser light therealong. The cores 12a to 12d are patterned by using a photo-resist mask (not shown), and the pattern is transferred from another reticle to a photo-resist layer. When the photo-resist layer is developed, the photo-resist layer is formed into the photo-resist mask. Thus, the first pads 11a to 11j and the cores 12a to 12d are formed by using the lithographic techniques, and are located at relative positions on the major surface of the silicon substrate 11 defined by the two reticles. The lithographic techniques accurately determine the relative positions between the first pads 11a to 11j and the cores 12a to 12d, and the accuracy of the relative positions is equivalent to the accuracy of the mask alignment in the lithographic apparatus. A laser diode array 13 has active layers 13a to 13d, and laser light beams are radiated from the active layers 13a to 13d. For this reason, each of the active layers 13a to 13d serves as a port. On the back surface of the laser diode array 13 are formed a plurality of second pads 13e, 13f, 13g, 13h, 13i, 13j, 13k, 13m, 13n and 13o which are patterned through the lithographic techniques. The lithographic techniques exactly locate the plurality of second pads 13e to 13o at relative positions with respect to the active layers 13a to 13d, and the relative positions between the second pads 13e to 13o and the active layers 13a to 13d are equivalent to the relative positions between the first pads 11a to 11j and the cores 12a to 12d. The second pads 13e, 13g, 13j and 13n have respective longitudinal axes extending in the y-direction, and are in parallel to the active layers 13a to 13d. The other second pads 13f, 13h, 13i, 13k, 13m and 13o have respective longitudinal axes extending in the x-direction, and are perpendicular to the longitudinal axes of the second pads 13e, 13g, 13j and 13n. The plurality of second pads 13e to 13o are respectively corresponding to the plurality of first pads 11a to 11j, and each of the second pads 13e to 13o is shaped into the same rectangular parallelopiped configuration as the corresponding first pad 11a to 11j. Namely, the corners of each second pad are also rounded, and the lower surface is generally rectangular. It is not necessary to prepare the optical wave-guide structure 12 and the first pads 11a to 11j before the preparation of the laser diode array 13. There is no problem if the silicon substrate 11 and the laser diode array 13 are completed before solder bumps. Subsequently, solder bumps 14 are provided on the plurality of first pads 11a to 11j, respectively. The solder bumps 14 are formed from a Au-Sn alloy foil (not shown) by using a punching, and the pieces of Au-Sn alloy are reflowed in high temperature inert atmosphere as described in conjunction with figures 3A to 3C. The pieces of Au-Sn alloy is uniform in volume, and the solder pieces keeps the height of the laser diode array 13 from the silicon substrate 11 constant. If a strictly controlled composition is not required or a cost is ignoreable, eutectic Au-Sn alloy may be plated or deposited through a vacuum evaporation onto the first pads 11a to 11j. The eutectic Au-Sn alloy is reflowed so as to form the solder bumps 14. The eutectic Au-Sn alloy may contain gold at 80 weight percent and tin at 20 weight percent. When the vacuum evaporation is used, the eutectic Au-Sn alloy is deposited to 16 microns, and the cost is increased. On the other hand, when the plating is used, the thickness is dispersed within ± 20 percent, and is undesirable for the single mode optical coupling. The amount of Au-Sn alloy on the first pad 11a to 11j is about 56550 cubic microns. Subsequently, the laser diode array 13 is put on the solder bumps 14, and the second pads 13e to 13o may be offset from the corresponding first pads 11a to 11j as shown in figures 4A, 5A and 6A. The solder bumps 14 are reflowed in nitrogen atmosphere, and no flux is used. The surface tension causes the second pads 13e to 13o to automatically register with the first pads 11a to 11j, respectively. In detail, the surface tension of the reflowed solder bumps 14 on the first pads 11b, 11d, 11e, 11g, 11h and 11j moves the laser diode array 13 in a direction labeled with ST1, and the surface tension of the reflowed solder bumps 14 on the other first pads 11a, 11c, 11f and 11i moves the laser diode array 13 in a direction labeled with ST2. These surface tensions two-dimensionally regulate the laser diode array 13 to the position where the first pads 11a to 11j are exactly overlapped with the second pads 13e to 13o as shown in figures 4B, 5B and 6B. As a result, the optical axes of the active layers 13a to 13d are aligned with the optical axes of the cores 12a to 12d as indicated by dot-and-dash lines in figure 4B. The solder is solidified, and the laser diode array 13 is bonded to the silicon substrate 11. The pieces 14 of Au-Sn alloy provided through the punching are strictly constant in volume, and the joint height is 18 microns plus minus 1 micron. As described hereinbefore, the accuracy of the alignment is dependent on the aspect ratio of the bumps 14. The present inventors studied the generally rectangular pads, and concluded that the accuracy of the alignment was dependent on the ratio of the bump height to the pad width. Using solder bumps having different ratios, the present inventors further studied the critical ratio of the bump height to the pad width allowing the first pad and the second pad to register with each other by virtue of the surface tension, and concluded that the critical ratio for the flux-less solder bump is 0.7. In other words, if the flux-less solder bumps had the ratio of the bump height to the pad width equal to or greater than 0.7, the surface tension was large enough to automatically align the optical axes with one another. When flux was used, the critical ratio of the bump height to the pad width was different from the critical ratio for the flux-less solder bumps. The above described optical module had the ratio of the bump height to the pad width equal to 0.72, and the mis-alignment was within plus minus 1 micron. When the solder bumps were provided on the circular pads, the mis-alignment was plus minus 1.5 microns. It was understood that the generally rectangular parallelopiped solder bumps 14 generated large surface tension, and achieved the accurate self-alignemnt. The joint height was 18 microns, and the optical axes of the active layers 13a to 13d were aligned with the optical axes of the cores 12a to 12d in the single mode optical coupling. As will be understood from the foregoing description, even if the pad is narrow, the long length of the rectangular parallelopiped pad maintains the volume of solder. The narrow width increases the ratio of the bump height to the pad width, and the wide bottom area does not deteriorate the joint force and the heat radiation capability. Thus, the rectangular parallelopiped pads achieve the miniaturization of the solder bumps without sacrifice of the surface tension. The large volume prevents the bump height from fluctuation, and allows a designer to decrease a margin of the bump height. The active layers 13a to 13d are accurately adjusted to a target height from the major surface of the silicon substrate 11, and are aligned with the cores of the latest optical wave-guide structure 12 in the single mode optical coupling. If the width of a pad is much less than the length thereof, it is recommendable to put a plurality of pieces of solder on the pad, because the solder is easily spread over the entire surface. Second Embodiment Another method of assembling an optical module is hereinbelow described with reference to figures 7A and 7B and figures 8A and 8B. The assembling sequence is similar to the first embodiment except for the configurations of first and second pads. For this reason, the other components of the optical module are labeled with the same references designating corresponding components of the first embodiment without detailed description. The first pads 21a, 21b, 21c and 21d are T-letter shape, and the T-letter shaped pads 21a to 21d are alternately inverted as shown in figure 7A. The T-letter shaped pad 21a/21b/21c/21d is broken down into two generally rectangular parallelopiped portions 21e and 21f, and the dimensions of the T-letter shaped pad 21d are written in figure 7A. The other T-letter shaped pads 21a to 21c are identical to the T-letter shaped pad 21d. The corners of each T-letter shaped pad 21a/21b/21c/21d are rounded so as to perfectly spread solder over the entire upper surface thereof. One of the generally rectangular parallelopiped portions 21e has a longitudinal axis in parallel to the x-direction, and the other generally rectangular parallelopiped portion 21f is elongated in the y-direction. The y-direction is in parallel to optical axes of the cores 12a to 12d. The second pads 22a, 22b, 22c and 22d are also T-letter shape. The T-letter shaped pads 22a to 22d are identical with the T-letter shaped pads 21a to 21d, respectively, and, accordingly, are broken down into two generally rectangular parallelopiped portions 22e and 22f. The corners of each T-letter shaped pad 22a/22b/22c/22d are also rounded. One of the generally rectangular parallelopiped portions 22e has a longitudinal axis extending in the x-direction, and the other generally rectangular parallelopiped portion 22f extends in the y-direction in parallel to the active layers 13a to 13d. The first T-letter shaped pads 21a to 21d are located at relative positions with respect to the cores 12a to 12d by using the lithographic techniques, and the second T-letter shaped pads 22a to 22d are also located at relative positions with respect to the active layers 13a to 13d through the lithographic process. The relative relation between the first T-letter shaped pads 21a to 21d and the cores 12a to 12d is equivalent to the relative relation between the second T-letter shaped pads 22a to 22d and the active layers 13a to 13d. When the silicon substrate 11 and the laser diode array 13 are prepared, solder bumps 23 are provided on the first T-letter shaped pads 21a to 21d, and the amount of each solder bump 23 is about 25000 cubic microns. When the solder bumps 23 are reflowed, surface tensions in each solder are exerted two directions ST3 and ST4 respectively in parallel to the x-direction and the y-direction. Thus, each of the reflowed bumps 23 two-dimensionally aligns the relative position between the first T-letter shaped pads 21a to 21d and the second T-letter shaped pads 22a to 22d as shown in figures 7B and 8B. The T-letter shaped pads 21a to 21d and 22a to 22d are less than the first and second pads 11a to 11j and 13e to 13o, and the step of forming the bumps is simpler than that of the first embodiment. The second embodiment achieves all the advantages of the first embodiment. Third Embodiment Turning to figures 9, 10 and 11 of the drawings, active layers 31a to 31d of a laser diode array 31 are aligned with cores 32a to 32d of optical fibers 32 through the self-align solder bonding according to the prevent invention. The first pads 11a to 11j are formed on a silicon substrate 33 as similar to those of the first embodiment by using the lithographic techniques, and v-shaped grooves 33a to 33d are further formed in the silicon substrate 33 by using the lithographic process followed by an anisotropic etching. The relative relation between the first pads 11a to 11j and the bottom edge lines 33e of the v-shaped grooves 32a to 32d is accurately defined by the lithographic process. The second pads 13e to 13o are patterned on the back surface of the laser diode array 31 as similar to the first embodiment. The relative relation between the second pads 13e to 13o and the active layers 31a to 31d is accurately defined by the lithographic process. The solder bumps 14 are provided on the first pads 11a to 11j, respectively, and the laser diode array 31 is put on the solder bumps 14. The solder bumps 14 are reflowed, and the surface tension causes the second pads 13e to 13o to register with the first pads 11a to 11j. This results in the self-alignment between the optical axes of the active layers 31a to 31d and the optical axes of the optical fibers 32. The solder is solidified, and the laser diode array 31 is bonded to the silicon substrate 33. The ratio of the bump height to the pad width is equal to that of the first embodiment, and the same accuracy is achieved. Although particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present invention. In the second embodiment, the first and second pads 21a to 21d and 22a to 22d are shaped into the T-letter configuration. However, any configuration is available in so far as the configuration is broken down into the generally rectangular parallelopiped portions. Such an available configurations are an L-letter shape and a shape. Solder may be PbSn alloy or AuSi alloy.

Claims: An optical module comprising: a substrate (11; 33) having an upper surface; an optical wave propagation means (12; 32) formed on a first portion of said upper surface, said means (12; 32) having a plurality of parallel cores (12a/12b/12c/12d; 32a/32b/32c/32d) for propagating light, said cores (12a/12b/12c/12d; 32a/32b/32c/32d) being arranged side by side on and extending along said upper surface and said cores (12a/12b/12c/12d; 32a/32b/32c/32d) having a centreline along their longitudinal axes; an optical element (13; 31) having a lower surface and ports (13a-13d; 31a-31d); a plurality of first pads (11a-11j; 21a-21d) formed on a second portion of said upper surface of said substrate (11; 33) and located at certain relative positions with respect to said cores (12a112b/12c/12d; 32a/32b/32c/32d); a plurality of second pads (13e-13o; 22a-22d) formed on said lower surface of said element (13; 31), said second pads (13e-13o; 22a-22d) corresponding to said first pads (11-11j; 21a-21d) and being located at certain relative positions with respect to said ports (13a-13d; 31a-31d); wherein each second pad (13e-13o; 22a-22d) is bonded through solder bumps (14; 23) to one of said corresponding first pads (11a-11j; 21a-21d) such that each of said ports (13a-13d; 31a-31d) is aligned with one of said cores (12a/12b/12c/12d; 32a/32b/32c/32d) so as to allow said light to pass therethrough; characterised in that selected ones of said plurality of corresponding first and second pads (11 a-11 j; 21 a-21 d; 13e-13o; 22a-22d) have at least a section which is generally rectangular, and are symmetric about the centreline along their longitudinal axis; and have said pad centreline lying parallel and aligned with the associated core centreline; whereby said rectangular sections of said first (11a-11j; 21a-21d) and corresponding second (13e-13o; 22a-22d) pads are overlapped. The optical module as set forth in claim 1, in which said plurality of first pads (11a-11j) are divided into first and second groups, said generally rectangular section of said upper surface of each of the first pads (11a/11c/11f/11i) of said first group having a longitudinal direction substantially in parallel to an optical axis shared between said port and said core, said generally rectangular section of said upper surface of each of the first pads (11b/11d/11e/11g/11h/11j) of said second group having a longitudinal direction substantially in perpendicular to said optical axis. The optical module as set forth in claim 1, in which each of said plurality of first pads (11a-11j) has a generally rectangular parallelepiped configuration, and each of said plurality of second pads (13e-13o) has said generally rectangular parallelepiped configuration. The optical module as set forth in claim 3, in which corners of said generally rectangular parallelepiped configuration are rounded. The optical module as set forth in claim 3, in which said plurality of first pads (11a-11j) are divided into first and second groups, said generally rectangular parallelepiped configuration of each of the first pads (11a/11c/11f/11i) of said first group having a longitudinal direction substantially in parallel to an optical axis shared between said port and said core, said generally rectangular parallelepiped configuration of each of the first pads (11b/11d/11e/11g/11h/11j) of said second group having a longitudinal direction substantially in perpendicular to said optical axis. The optical module as set forth in claim 1, in which each of said plurality of first pads (21a-21d) is a combination of generally rectangular parallelepiped portions (21e/21f), and each of said plurality of second pads (22a-22d) is said combination of said generally rectangular parallelepiped portions (22e/22f). The optical module as set forth in claim 6, in which said combination has rounded corners. The optical module as set forth in claim 6, in which said combination of said generally rectangular parallelepiped portions (21e/21f; 22e/22f) has a T-letter configuration and said plurality of first pads shaped into said T-letter configuration are alternately inverted. The optical module as set forth in claim 6, in which one (21e; 22e) of said generally rectangular parallelepiped portions forming a lateral bar of said T-letter configuration has a longitudinal axis perpendicular to an optical axis shared between said core and said port, and the other (21f; 22f) of said generally rectangular parallelepiped portions forming a vertical bar of said T-letter configuration has a longitudinal axis parallel to said optical axis. A method of assembling an optical module, comprising the steps of: providing a substrate (11; 33) having an upper surface; providing optical wave propagation means (12; 32) on a first portion of said upper surface, said means having a plurality of parallel cores (12a-12d; 32a-32d) for propagating light, said cores being arranged side by side on and extending along said upper surface and said cores having a centreline along their longitudinal axes; providing an optical element (13; 31), having a lower surface and ports (13a-13d; 31 a-31d); forming using lithographic techniques a plurality of first pads (11a-11j; 21a-21d) on a second portion of said upper surface of said substrate, said pads being located at certain relative positions with respect to said cores; forming using lithographic techniques a plurality of second pads (13e-13o; 22a-22d) on said lower surface of said optical element, said second pads corresponding to said first pads and being located at certain relative positions with respect to said ports; wherein selected ones of said plurality of corresponding first and second pads have at least a section which is generally rectangular; are symmetric about the centreline along their longitudinal axis; and selected ones of said plurality of first pads have said pad centreline lying parallel and aligned with the associated core centreline; selected ones of said plurality of second pads lying directly underneath the associated optical port; placing solder pieces on said plurality of first pads; heating said solder pieces for forming solder bumps (14; 23) on said plurality of first pads; putting said optical element onto said solder bumps in such a manner than said solder bumps are brought into contact with said plurality of second pads; reflowing said solder bumps so as to self-align said generally rectangular section of said first pads with the corresponding generally rectangular section of said second pads, thereby causing each of said ports to be aligned with said associated core; and bonding said plurality of first pads to said plurality of second pads through a solidification of said solder bumps. The method as set forth in claim 10, in which a ratio of a height of said solder bumps (14; 23) to a width of said at least one generally rectangular surface is equal to or greater than 0.7. The process as set forth in claim 10, in which each of said solder bumps is divided into a plurality of solder pieces. The process as set forth in claim 10, in which said solder pieces are formed from a solder foil by using a punching technique.

Inventors: HONMOU HIROSHI, ITOH MASATAKA, SASAKI JUN-ICHI, TORIKAI TOSHIKATA, HONMOU, HIROSHI, ITOH, MASATAKA, SASAKI, JUN-ICHI, TORIKAI, TOSHIKATA, Honmou, Hiroshi, c/o Nec Corporation, Itoh, Masataka, c/o Nec Corporation, Sasaki, Jun-ichi, c/o Nec Corporation, Torikai, Toshikata, c/o Nec Corporation
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1995
Main classification: G02B6
IPC codes: G02B6