Abstract:
A set of interlocking modules supports and connects a die containing lasers, a set of precision molded lenses and a set of beam switching elements. Another embodiment of the invention is a structure for mounting a logic chip and an optical chip on a chip carrier, with the optical chip being mounted on the side of the carrier facing the system board on which the carrier is mounted, so that radiation travels in a straight path from optical sources on the optical chip into optical transmission guides on the board.

Description:
RELATED APPLICATION  
       [0001]    This is a divisional application of application Ser. No. 10/254,955, filed on Sep. 25, 2002, assigned to the assignee hereof and incorporated by reference in its entirety. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The field of the invention is that of assembling optical sources such as VCSEL arrays to substrates containing transmissions links and optical routing members.  
         BACKGROUND OF THE INVENTION  
         [0003]    As high volume digital systems expand their use of optical interconnects, the need for a manufacturable optical connection system increases.  
           [0004]    High volume systems typically have a backplane connecting a number of system boards. Such systems contain arrays of optical sources such as VCSELs that generate light that is modulated to carry data and directed to various locations where detectors such as photodiodes convert the modulated optical radiation to electronic signals that can be processed in conventional digital processing systems.  
           [0005]    A recurring problem in the field is that of misalignment in optical connector systems. Signal loss that increases the noise level or renders the signal undetectable occurs frequently. Precision alignment systems require expensive setup and constant maintenance.  
           [0006]    The art would benefit from passive alignment systems employing self-aligning structures fabricated at low cost with lithographic precision.  
         SUMMARY OF THE INVENTION  
         [0007]    The invention relates to an alignment system for assembling an array of optical sources in alignment with a set of optical transmission members on a system board.  
           [0008]    A feature of the invention is the fabrication on the optical chip containing an array of optical sources of passive alignment features.  
           [0009]    A feature of the invention is the provision of a simple unit containing an array of lenses in alignment with the array of sources and positioned with alignment members interlocking with the passive alignment features on the optical chip.  
           [0010]    Another feature of the invention is the provision of a replaceable optical switching structure fitting into a receptacle matched to the array of lenses, for receiving radiation from the VCSEL array and directing it to various locations on the system board.  
           [0011]    Another feature of the invention is a structure for mounting a logic chip and an optical chip on a chip carrier, with the optical chip being mounted on the side of the carrier facing the system board on which the carrier is mounted, so that radiation travels in a straight path from optical sources on the optical chip into optical transmission guides on the board. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIGS. 1A through 3B show layers in a first embodiment of the invention in plan and side views.  
         [0013]    [0013]FIG. 4 shows in partially pictorial, partially schematic form a second embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0014]    FIGS.  1  to  3  show in plan view and side view a three-layer module that assembles a VCSEL array, a corresponding array of lenses and another corresponding array of optical manipulators that modulate or switch the radiation. In FIGS  1 A and  1 B, M×N VCSEL array  15 - i  on die  10  generates optical radiation. A band  12  surrounding the array is illustratively composed of metallic thin film on which Sn—Pb solder preform can be applied. Fiducial marks  17  are shown as an example. Marks  17  are used to align the die in a precision stage under visual observation. Other marks may be used for self-alignment. This is shown in side view in FIG. 1B. For example, the die  10  is made of GaAs and has conventional VCSELs formed in it. Conventional electrical leads going to the VCSELs are omitted from the drawing for simplicity.  
         [0015]    [0015]FIGS. 2A and 2B show the next layer is the assembly, referred to generally as an optical transfer unit, in which a lens array  25 - i , referred to as optical transfer means, is aligned with the VCSEL array. Illustratively the material of optical unit  20  is polymer or glass, transparent to the radiation from the laser array and able to withstand the temperature of 240 degrees C. at which solder  12  is reflowed. It is molded with conventional precision techniques based on a model that is formed by semiconductor lithographic techniques. For example, the individual lenses in the array may be formed as Fresnel lenses and the projecting rim  23  on the sides in FIG. 2B is etched from silicon as a mold material. With the use of semiconductor etching and forming techniques, definition and placement of the optical lenses can readily be achieved to a tolerance of 10 microns, well within the requirements of the optical system.  
         [0016]    Strip  22  in FIG. 2A is also formed of solder electroplated on the bottom of rim  23 . On the top surface of module  20  in FIG. 2B, studs  24  (passive alignment structures) are formed by lithographic techniques and then molded. Alternatively, the studs could be etched into the material of module  20  after it is molded, aligned to fiducial marks placed in the mold when it is first made.  
         [0017]    Illustratively, alignment between die  10  and module  20  is provided by rim  23  (defining a second dimension) fitting vertical edge  13  of a step that is etched into the top of die  10 , thus defining a first dimension. Thus, the outer rim of die  10  is recessed, illustratively 10 cm, and rim  23  of module  20  surrounds the projecting center of the die, positioning the lenses with respect to the VCSEL array transversely and, since the etching of the step in die  10  and the formation of the rim in the mold for module  20  are done with high mechanical precision, the vertical distance is also set within specifications of about 50 microns. This passive self-alignment system has the advantage that pieces  10  and  20  fit together. Because of the very precise tolerances required in this application and the different coefficients of thermal expansion of the pieces being bonded, assembly is preferably done at an assembly temperature of less than 230 degrees C. so that any stresses between the pieces at room temperature are within acceptable limits.  
         [0018]    The focal length of the lenses is set such that radiation is coupled into receiving sections  25  on optical transfer member  35  of FIGS. 3A and 3B. This top unit in the stack has the function of coupling radiation emerging from lenses  25 - i  into a set of waveguides, Bragg reflectors, mirrors and other optical manipulators in member  35 . The function of member  35  is to process the radiation; e.g. to modulate it with a data bitstream. Member  35  is shown schematically as a rectangle in this figure. The radiation is then coupled from member  35  into conventional waveguides, not shown in this figure, in a system board.  
         [0019]    Shell  30  in FIG. 3 is pluggable, meaning that it can be inserted and removed by hand, for ready replacement. The individual pieces can be removed so as to provide flexibility in the design. Unit  30  is adapted for interconnection to a fiber array, or a optical routing element etc.  
         [0020]    Box  35  can be a passive optical router, such as gratings, lenses etc. It could also have liquid crystal valve arrays in an alternative embodiment,  
         [0021]    In contrast to the removable nature of connector  30 , module  20  is bonded to die  10  by solder reflow. The solder can be remelted, but the design assumption is that this is a one-time bond. Referring again to connector  30 , mounting structures  34  mate with corresponding structures  24  on module  20 . For example, plugs  24  are circular in cross section (they could alternatively be, e.g. 1 mm square) and mating sockets  34  have circular cavities that fit plugs  24  with a very low tolerance. The assembly, once made, could be kept together by epoxies or solder. The interface between units  20  and  30  is also made at a defined temperature, since the spacing between studs  24  and receptacles  34  will match at only one temperature.  
         [0022]    Referring now to FIG. 4, there is shown in cross section another embodiment of the invention in which board  110  represents a printed circuit board in a digital system or any other support for an assemblage of electronic chips. Board  110  contains optical waveguides  120 - 1  and  120 - 2 , as well as the usual electrical interconnections between chips. At the top of the Figure, chip  220  represents a number of electronic chips that process data, coupling signals and power through a set of contacts  221 . Chip  220  is bonded to a ceramic chip carrier  210 , or any other packaging unit, that contains electrical interconnects molded into it. On the side of carrier  210  facing toward the system board  120 , an optical chip  230 , connected electrically by wire bond leads  232 , emits radiation into waveguides  120 - 1  and  120 - 2 . Alternatively, chip  230  could be attached to carrier  210  using the “flip-chip” method of solder balls that provide both electrical and mechanical connection. The radiation transfer could also be two-way, with photodetectors on chip  230  receiving optical radiation from board  110 .  
         [0023]    Advantageously, the optical chip  230  contains an array of VCSELs similar to that of chip  10  in FIG. 1, with the radiation being directed at a right angle to the plane of the chip. In this embodiment, the radiation passes through an air gap between chip  230  and waveguides  120 . Those skilled in the art will be aware that lenses mirrors, etc. as shown in FIG. 1 may be placed between the laser and the waveguide to facilitate coupling.  
         [0024]    The direct transmission between source and waveguide in this embodiment facilitates assembly and replacement of the VCSEL array if required. A seal such as reflow  22  in FIG. 1A can be used, if desired.  
         [0025]    In both embodiments, the fragility of VCSEL arrays can be addressed by fabricating strengthening members using semiconductor techniques from the “back end” e.g. depositing a layer of material on the back side of the chip and etching it to form strengthening members that resist mechanical stresses that would crack the die.  
         [0026]    While the invention has been described in terms of a single preferred embodiment, those skilled in the art will recognize that the invention can be practiced in various versions within the spirit and scope of the following claims.