Patent Publication Number: US-7711237-B2

Title: Optical transmission line holding member, optical module and mounting method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2007-272984, filed Oct. 19, 2007, the entire contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   Recently, the technique for reducing mounting costs is studied and developed by use of a coupling system called a direct optical coupling (butt joint) system. In the coupling system, an optical fiber and optical semiconductor element are arranged in close positions to face each other and optically coupled without using a lens. When the direct optical coupling system is used, it is important to arrange an optical semiconductor element and optical fiber in close positions and prevent light from reaching an unwanted portion. Therefore, there is proposed a method for directly forming an electrical interconnection on the main surface of an optical transmission line holding member (a so-called optical fiber ferrule) that holds the optical fiber and the like and mounting an optical semiconductor element on the main surface to set the light receiving/transmitting region thereof in opposition to the optical fiber (Jpn. Pat. Appln. KOKAI Publication No. 2001-159724). 
   In this type of method, it is desired to fill resin in between the optical semiconductor element and the optical transmission line so as to suppress reflection light caused by discontinuity of the refractive index. Specifically, resin having a role as a refractive index matching material and an under-filling function for protection of the electrical connecting portion of the optical element is filled in between the optical semiconductor element and the end face of the optical transmission line. Further, resin used to mechanically fix the optical transmission line on a ferrule is filled. 
   As a method for forming the above resins, a method for performing an under-filling process before fixing the optical transmission line and a method for fixing the optical transmission line and then performing an under-filling process are considered. When the optical transmission line is first fixed, there occurs a possibility that fixing resin flows into holding holes for the optical transmission line to reach the end face on the optical element side. Further, since the fixing resin fills in the holding holes, the holes via which air escapes when under-fill resin is injected are closed or blocked. This means that the holes via which a solvent produced at the curing time of the under-fill resin is extracted are closed. Therefore, there occurs a possibility that bubbles remain in the resin at the curing time of the under-fill resin and this may prevent the optical coupling. When the under-filling process is first performed, there occurs a problem that high mechanical precision cannot be attained. 
   BRIEF SUMMARY OF THE INVENTION 
   According to one aspect of the present invention, there is provided an optical transmission line holding member which includes a main body which mechanically positions and holds an optical transmission line, the main body including a holding hole which opens in one surface and is used to mechanically position and hold the optical transmission line, a guide hole which opens in another surface different from the one surface, has a diameter larger than that of the holding hole and is coaxially aligned with the holding hole to guide the optical transmission line into the holding hole, and a resin storage groove provided in the guide hole, and an electrical interconnection used to mount an optical element to be optically coupled with the optical transmission line, the electrical interconnection being formed on the surface in which the holding hole opens. 
   According to another aspect of the present invention, there is provided an optical module which includes an optical transmission line, a main body which mechanically positions and holds the optical transmission line, the main body including a holding hole which opens in one surface and is used to mechanically position and hold the optical transmission line, a guide hole which opens in another surface different from the one surface, has a diameter larger than that of the holding hole and is coaxially aligned with the holding hole to guide the optical transmission line into the holding hole, and a resin storage groove provided in the guide hole, an electrical interconnection formed on the surface in which the holding hole of the main body opens, an optical element mounted on the surface in which the holding hole opens, the optical element being optically coupled with the optical transmission line electrically connected to the electrical interconnection and positioned and fixed in the holding hole, and resin filled in between the optical element and the optical transmission line and in the guide hole. 
   According to still another aspect of the present invention, there is provided an optical module mounting method which includes preparing an optical transmission line holding member which includes a main body which mechanically positions and holds an optical transmission line and an electrical interconnection used to mount an optical element, the main body including a holding hole which opens in one surface and is used to mechanically position and hold the optical transmission line, a guide hole which opens in another surface different from the one surface, has a diameter larger than that of the holding hole and is coaxially aligned with the holding hole, and a resin storage groove provided in the guide hole, and the electrical interconnection being formed on the surface in which the holding hole opens, mounting an optical element on the surface in which the holding hole of the main body opens and electrically connecting the optical element to the electrical interconnection, temporarily fixing the optical transmission line on the optical transmission line holding member by filling first resin into part of a faraway portion of the guide hole far apart from the holding hole with respect to the resin storage groove after inserting the optical transmission line to a preset position of the holding hole through the guide hole, filling second resin as a refractive index matching material in between the optical element mounted and the optical transmission line temporarily fixed, and fixing the optical transmission line on the optical transmission line holding member by filling third resin into the guide hole after filling the second resin. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       FIG. 1  is a cross-sectional view showing an example of the schematic structure of an optical transmission line holding member according to a first embodiment of this invention. 
       FIGS. 2A to 2C  are side views and cross-sectional view showing portions of the optical transmission line holding member of the first embodiment. 
       FIGS. 3A to 3D  are cross-sectional views for illustrating an optical module mounting process using the optical transmission line holding member of the first embodiment. 
       FIG. 4  is a cross-sectional view showing the structure of the main portion of an optical transmission line holding member according to a second embodiment of this invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Embodiments of the present invention will be explained in detail with reference to the accompanying drawings. 
   First Embodiment 
     FIG. 1  is a cross-sectional view showing an example of the schematic structure of an optical transmission line holding member according to a first embodiment of this invention. Further,  FIG. 2A  is a side view (right-side view) as viewed in an A direction of  FIG. 1 ,  FIG. 2B  is a cross-sectional view taken along line B-B′ of  FIG. 1  and  FIG. 2C  is a side view (left-side view) as viewed in a C direction of  FIG. 1 . 
   In  FIG. 1 , a symbol  10  denotes an optical transmission line holding portion (holding member main body) used to hold an optical transmission line such as an optical fiber. The optical transmission line holding portion  10  is formed of a material obtained by mixing a filler such as silicon oxide or alumina into resin such as epoxy resin or PPS. On one side (optical element mounting surface) of the optical transmission line holding portion  10 , an electrical interconnection  11  for mounting an optical element is formed. As shown in  FIG. 1 , the electrical interconnection  11  can be realized by a structure that is formed by embedding a lead frame formed of Cu or Cu alloy into the optical element mounting surface portion and exposing the end face of the lead frame to the side surfaces (in  FIG. 1 , upper and lower surfaces) of the optical transmission line holding portion  10 . In this case, the end face of the lead frame can be used as an external connection electrode pad. Therefore, a simulated orthogonal interconnection can be formed to extend from the optical element mounting surface to another side surface without performing a special 3-dimensional process. As a result, the optical transmission line holding member can be easily mounted on the surface parallel to the optical transmission line. 
   In the optical transmission line holding portion  10 , holding holes  12  used to precisely position and hold the optical transmission line such as an optical fiber are formed. The holding holes  12  open in the surface on which the electrical interconnection  11  is formed and are arranged in a straight line as shown in  FIG. 2A . The holding hole  12  may be formed singly or a plurality of holding holes may be formed, but in this example, a plurality of holding holes are formed. 
   Further, in the optical transmission line holding portion  10 , optical transmission line guide holes  13  used to guide the optical transmission line into the holding hole  12  are formed. The guide hole  13  is coaxially aligned with the holding hole  12  and opens in the surface opposite to the holding hole  12 . As shown in  FIG. 2B , the optical transmission line guide hole  13  has a larger diameter than the holding hole  12 . Further, the connecting portion of the guide hole  13  with the holding hole  12  is formed in a tapered form so that the optical transmission line can be easily inserted and guided into the holding hole  12 . A resin storage groove portion  14  having a larger diameter is formed in an intermediate portion of the guide hole  13 . The guide hole  13  is divided into a portion  13 - 1  lying near the holding hole  12  and a portion  13 - 2  lying far apart from the holding hole  12  with respect to the resin storage groove portion  14 . The resin storage groove portion  14  may be formed of an annular groove coaxially formed with the optical transmission line guide hole  13  or may be formed by digging the optical transmission line holding portion  10  on the bottom side of the optical transmission line guide hole  13 . 
   In the optical transmission line holding portion  10 , a resin window  15  used to flow optical transmission line fixing resin is provided. The resin window  15  is connected to the optical transmission line guide hole  13  containing the resin storage groove portion  14 . By forming the structure having the resin window  15 , a groove having an opening with a large diameter can be formed by injection molding by use of a die. If the resin window  15  is not formed, for example, since it becomes necessary to coaxially form a groove having a large opening, the structure in which the die cannot be extracted is formed. Therefore, the structure may be formed by cutting or assembling divided parts and the manufacturing cost will be high. 
   In the structure of  FIG. 1 , the diameter is made larger on the optical transmission line insertion side on the rear side of the optical transmission line guide hole  13  (on the side opposite to the optical element) and, as shown in  FIG. 2C , adjacent ones of the guide holes  13  are connected to form one hole. By thus forming the guide holes  13  into one hole, the structure in which a coating of the optical transmission line can be held in the optical transmission line holding member can be attained. Thus, the effect that breakage due to bending of an optical fiber used as an optical transmission line can be alleviated can be attained. 
     FIGS. 3A to 3D  are cross-sectional views for illustrating an optical module mounting process using the optical transmission line holding member shown in  FIG. 1  and  FIGS. 2A to 2C . 
   As in  FIG. 3A , an optical semiconductor element  21  such as a light emitting element or light receiving element is electrically connected to the electrical interconnection  11  formed on the right-side surface of the optical transmission line holding portion  10  via bumps  22 . For this connection, a method such as an ultrasonic flip chip mounting method may be used. The bumps  22  for connection are formed of stud Au bumps or solder bumps, for example. 
   As shown in  FIG. 3B , an optical transmission line  23  such as an optical fiber is inserted through the opening on the left side of the optical transmission line holding member  10  and further inserted into the holding hole  12  via the optical transmission line guide hole  13 . At this time, the length of part of the optical transmission line  23  which is exposed by removing a coating portion  24  is previously determined so that part of the coating portion  24  of the optical transmission line  23  will be inserted into the optical transmission line guide hole  13 . 
   When the front end of the optical transmission line  23  reaches a preset position close to the optical semiconductor element  21 , a temporary-fixing bonding agent (first resin)  31  is coated on part of the optical transmission line guide hole  13  and the optical transmission line  23  is temporarily fixed in the guide hole  13  by curing the bonding agent  31 . At this time, the bonding agent  31  is filled on the opposite side of the holding hole  12  with respect to the resin storage groove portion  14 . That is, the bonding agent  31  is filled on the portion  13 - 2  of the optical transmission line guide hole  13  that lies far apart from the holding hole  12  with respect to the resin storage groove portion  14 . When the optical transmission line  23  is an optical fiber, the end face position of the optical transmission line  23  can be prevented from being shifted due to expansion or contraction of the coating portion  24  by fixing the optical transmission line with the coating portion  24  removed as shown in  FIG. 3B . Further, even if the bonding agent  31  tends to flow towards the holding hole  12 , the bonding agent  31  stays in the groove portion  14  due to the presence of the resin storage groove portion  14 . Therefore, the bonding agent  31  does not fill in the holding hole  12 . 
   As shown in  FIG. 3C , under-fill resin (second resin)  32  is injected to fill in between the optical semiconductor element  21  and the optical transmission line  23  and surround the bumps  22  corresponding to electrical connecting portions between the optical semiconductor element  21  and the electrical interconnection  11  and is then cured. At this time, since the holding hole  12  is not filled with the bonding agent  31 , the under-fill resin  32  is rapidly filled into the holding hole  12  without producing bubbles. 
   As shown in  FIG. 3D , resin (third resin)  33  is fully filled through the resin window  15 . Thus, the coating portion  24  is fixed in the guide hole  13  of the optical transmission line holding portion  10  and the optical transmission line  23  is fixed in the holding hole  12  and guide hole  13 . In this case, since the holding hole  12  is filled with the under-fill resin  32 , there occurs a possibility that bubbles will be produced when the resin  33  is filled into the holding hole  12 . However, since the resin  33  is used to hold and fix the optical transmission line  23 , there occurs no problem even when bubbles are produced. 
   In the step of  FIG. 3B , when the temporary-fixing bonding agent  31  flows into the holding hole  12  through the guide hole  13  and is filled into the holding hole  12 , a lowering in the light output of the optical module and an increase in variations may occur. That is, if the temporary-fixing bonding agent  31  is filled in the holding hole  12 , air is sealed in a space surrounded by the optical semiconductor element  21 , optical transmission line  23  and holding hole  12  when the under-fill agent  32  is injected. If the under-fill agent  32  is cured in this state, air remains in the resin  32  as bubbles. If the bubbles are present between the optical semiconductor element  21  and the end face of the optical transmission line  23 , the degree of optical coupling will be lowered. As a result, a lowering in the light output of the optical module and an increase in variations may occur. 
   Therefore, in the present embodiment, in order to prevent the temporary-fixing bonding agent  31  from flowing into the holding hole  12  through the guide hole  13 , the resin storage groove portion  14  is provided and the temporary-fixing bonding agent  31  is coated on the portion  13 - 2  far apart from the holding hole  12 . As a result, even if the temporary-fixing bonding agent  31  flows into the holding hole  12 , the bonding agent  31  can be stored in the groove portion  14 . Therefore, the bonding agent  31  does not reach the holding hole  12  and is not filled into the holding hole  12 . As a result, it becomes possible to prevent bubbles from occurring even when the under-fill resin  32  is coated later. 
   In the case of  FIG. 1 , the guide hole  13  is divided into the portion  13 - 1  near the holding hole  12  and the portion  13 - 2  far apart therefrom and the diameter of the nearby portion  13 - 1  is set smaller than that of the faraway portion  13 - 2 . That is, in  FIG. 1 , the height of the faraway portion  13 - 2  in a y direction is set lower than that of the nearby portion  13 - 1 . Thus, the viewing angle of the optical transmission line that permits the optical transmission line to be inserted into the portion  13 - 1  of the guide hole  13  lying near the holding hole  12  is not narrowed by the presence of the faraway portion  13 - 2 . Therefore, the effect that the optical transmission line can be mounted without causing the faraway portion  13 - 2  to act as an obstacle at the time of insertion of the optical transmission line can be attained. 
   As described above, according to the present embodiment, the optical semiconductor element  21  and optical transmission line  23  can be directly and optically coupled at a low cost by providing the holding hole  12  used to mechanically position and hold the optical transmission line  23 , the optical transmission line guide hole  13  having a diameter larger than the holding hole  12  and the resin storage groove portion  14  in the optical transmission line holding portion  10 . Further, the under-fill resin  32  can be filled into between the optical semiconductor element  21  and the optical transmission line  23  without fail with the optical transmission line  23  temporarily fixed by use of the bonding agent  31 . Therefore, production of bubbles between the optical semiconductor element  21  and the optical transmission line  23  can be suppressed and the high mechanical precision can be attained while maintaining the high optical characteristic. 
   Further, the optical semiconductor element  21  can be assembled while an optical fiber used as the optical transmission line  23  is set as a reference. Therefore, the optical transmission line can be mounted with high precision in the lateral direction by use of a normal flip chip method and the structure in which the number of parts used can be reduced and the cost can be lowered can be attained. Further, the part manufacturing cost can be greatly reduced by using resin to form the base member of the optical transmission line holding portion  10 . Also, the orthogonal transformation can be attained by forming the electrical interconnection  11  from the surface in which the holding hole  12  opens to the side surface. As a result, an advantage that the optical transmission line  23  can be prevented from rising at right angles with respect to the mounting surface while the direction in which the optical transmission line  23  extends is kept set parallel to the mounting surface can be attained. 
   Second Embodiment 
     FIG. 4  is a cross-sectional view showing the structure of the main portion of an optical transmission line holding member according to a second embodiment of this invention. Particularly,  FIG. 4  shows a cross section when viewing a faraway portion  13 - 2  of a guide hole  13  in an axial direction of optical transmission lines  23 . In  FIG. 4 , portions that are the same as those of  FIG. 1  are denoted by the same reference symbols and the detailed description thereof is omitted. 
   The adjacent faraway guide holes  13 - 2  of the optical transmission line guide holes  13  that lie far apart from holding holes  12  are connected together to form one hole. The hole has a bottom surface  40  and side surfaces  41  and is formed in a form to surround the optical transmission lines  23  in three directions. In this example, the optical transmission lines  23  are arranged at an equal interval T 1  and the sizes of the guide holes  13 - 2  are so set that the distance T 2  between one of the optical transmission lines  23  which lies on the outermost side and the side surface  41  of the guide hole  13 - 2  will be set substantially equal to T 1 . 
   With the above structure, the tensile stress in the lateral direction applied to the optical transmission lines  23  by resin contracted at the curing time after temporary-fixing resin  31  is coated can be made uniform with respect to the optical transmission lines  23  due to the effect of the presence of the sidewall  41 . That is, if a case wherein the sidewall  41  is not provided is considered as an extreme case, only stress caused by the resin between the optical transmission line  23  on the outermost side and the adjacent optical transmission line is applied to the above optical transmission line and the stress distribution becomes asymmetrical on both sides of the above optical transmission line. Therefore, there occurs a problem that asymmetrical residual stress is applied to the optical transmission line  23  and the possibility of breakage is raised. However, since the sidewall  41  is provided to set T 1 =T 2  in this embodiment, the tensile stress can be made uniform and occurrence of residual stress can be suppressed. 
   This embodiment may be advantageous in the following point in comparison with the case wherein the sidewall  41  is not provided. That is, when the temporary-fixing resin  31  is coated as shown in  FIG. 3B , an overflowing bonding agent flows into a resin storage groove portion  14 . At this time, the resin tends to flow along the sides of the groove portion  14  that are tangential lines of two walls due to the effect of surface tension. The bonding agent that reaches the bottom surface of the groove portion  14  tends to flow along the side of the bottom surface, spread on the bottom surface and flow in a direction towards the guide hole  13 - 1  along the side that vertically rises from the bottom surface of the groove portion  14  to the guide hole  13 - 1 . Therefore, in order to prevent the bonding agent from flowing into the guide hole  13 - 1 , it is effective to make the side that vertically rises from the bottom surface of the groove portion  14  to the guide hole  13 - 1  as high (long) as possible and it is effective to provide the sidewalls  41  as shown in  FIG. 4 . 
   (Modification) 
   This invention is not limited to the above embodiments. For example, the number of resin storage groove portions is not limited to one and a plurality of resin storage groove portions can be provided. In this case, the probability that the resin will flow into the holding holes can be further lowered. In the above embodiments, the cross-sectional surface of the electrical lead is set to be flush with the side surface adjacent to the optical element mounting surface, but the electrical lead may be made to project from the side surface to the exterior. Further, in the above embodiments, the optical fiber is used as the optical transmission line, but a light waveguide can be used. The material used as the base member of the optical transmission line holding member and the material of the electrical interconnection can be adequately changed according to a specification. 
   Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.