Patent Publication Number: US-2004052468-A1

Title: Alignable electro-optical microelectronic package and method

Description:
PRIOR APPLICATION  
     [0001] This application claims the benefit of U.S. Provisional Utility Patent Application Serial No. 60/369,592 filed Apr. 2, 2002, and Serial No. 60/407,470 filed Aug. 29, 2002. 
    
    
     
       FIELD OF THE INVENTION  
       [0002] The invention relates to microphotonic packaging and more particularly to optical alignment between an optical conduit and an electro-optical microcircuit chip in.  
       BACKGROUND OF THE INVENTION  
       [0003] Data transmission using optical media is enjoying rapid growth due to its ability to handle large amounts of data using relatively non-bulky and inexpensive transmission media such as fiber optic cable. Currently however, such optical data transmission requires the back-and-forth conversion of optical signals to electronic signals using electro-optical microelectronic converter devices. As shown in FIG. 1, such microcircuit converters are typically created on an integrated circuit chip  1  which has a wave-guide  2  for carrying an optical signal, shown as a light ray  3  to and from the end  4  of a fiber-optic cable  5 . The wave-guide typically terminates at an interface end  6  facing the cable end, and an opposite end inside the chip at a photo-electronic convertor  7  which converts received optical signals to electronic impulses which are then processed by the chip. The converter also converts electronic signals into optical signals to be transmitted to the cable.  
       [0004] For the purpose of clarity, the remainder of this specification will generally refer to an optical signal arriving at the chip from an interfacing optical cable. Those skilled in the art will readily appreciate the application of the description in terms of optical signals emanating from the chip.  
       [0005] As shown in FIG. 2, because of the need for high reliability in telecommunication systems and because the microelectronic die or chip is relatively fragile compared to its supporting circuitry, it is usually encased in a package  10  to protect it from the outside environment. This package must allow communication with the die electrically through leads  11  penetrating the package, thermally and in the present application, optically through a fiber optic cable connection  12 . These requirements have caused microcircuit packaging to become a complex science employing complex structures and manufacturing methods.  
       [0006] As shown in FIG. 3, in order to obtain maximum transmission of data, it is important that the interface  15  between the end  4  of the fiber optic cable  5  and the interface end  6  of the waveguide  2  be properly aligned. Errors in an alignment can lead to transmission loss and signal degradation across the interface. Alignment errors can be typically be characterized as an error in angle of incidence  16  of a signal carrying light ray  17  and its preferred or aligned direction  18 . Typically, the preferred or aligned direction is perpendicular to the surface of the interface end  6  of the waveguide  2 . Those skilled in the art will readily perceive that the error angle shown in FIG. 3 has been grossly emphasized for illustrative purposes.  
       [0007] Referring now to FIG. 4, it is well known that force F generated on a length L of straight wire in a uniform magnetic field B is proportional to the current i through the wire, the length of the wire, and the strength of the magnetic field according to Maxwell&#39;s equations  19 . The force direction is normal to the plane formed by the field and the current direction.  
       [0008] It is therefore desirable to provide an opto-electronic microcircuit package which provides for enhanced alignment.  
       SUMMARY OF THE INVENTION  
       [0009] The principal and secondary objects of this invention are to provide an opto-electronic microcircuit package which allows for inexpensively enhancing the alignment between an optical cable and an opto-electronic microcircuit chip.  
       [0010] These and other objects are fulfilled by an opto-electronic microcircuit package in which the microcircuit chip is mounted upon a substrate or boat which in turn is mounted within the cavity of the package upon a pool of reflowable solder. Larger chips can have backside metallization and be mounted directly upon the pool. Actuator wires connect from package pads to pads on the boat. When the solder is liquefied, the entire package is placed in a magnetic field and current runs through actuator wires in order to force rotation of the boat according to Maxwell&#39;s equations for electric current in a magnetic field. The current is adjusted to obtain the proper moment on the boat to move it into alignment with an impinging optical cable. Once in alignment, the solder is cooled to lock the boat in place and in alignment. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
     [0011]FIG. 1 is a prior art diagrammatic plan view of an optical cable interfacing with a portion of an opto-electronic microcircuit chip.  
     [0012]FIG. 2 is a prior art diagrammatic perspective view of an opto-electronic microcircuit package.  
     [0013]FIG. 3 is a prior art exaggerated diagrammatic view showing the angle of incidence of a light ray at an optical interface of an opto-electronic microcircuit chip.  
     [0014]FIG. 4 is a diagrammatic illustration of Maxwell&#39;s equations for electric current through a wire in a magnetic field.  
     [0015]FIG. 5 is a partial diagrammatic perspective view of an opto-electronic microcircuit metal package according to the invention.  
     [0016]FIG. 6 is a partial diagrammatic perspective view of an opto-electronic microcircuit ceramic package according to the invention.  
     [0017]FIG. 7 is a partial diagrammatic perspective view of a large scale opto-electronic microcircuit chip mounted directly within a ceramic package according to the invention.  
     [0018]FIG. 8 is a partial diagrammatic cross-sectional side view of the package of FIG. 5.  
     [0019]FIG. 9 is a partial diagrammatic perspective operational view of the package of FIG. 5, during the alignment process.  
     [0020]FIG. 10 is a flow chart diagram of the preferred method of aligning the optical interface of an opto-electronic microcircuit chip according to the invention.  
     [0021]FIG. 11 is a functional block diagram of a system for implementing the method of aligning the optical interface of an opto-electronic microcircuit chip according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION  
     [0022] The preferred embodiment is described in terms of a butterfly-type opto-electronic microcircuit package. However, those skilled in the art will readily appreciate the application of the invention to other opto-electronic packages and to other areas where alignment of a microcircuit chip within a package after die attach is required.  
     [0023] Referring now to the drawing, there is shown in FIGS.  5 - 7 , a portion  20  of an opto-electronic microcircuit package for packaging a pair of electronic microcircuit chips  21 , 22 . The first chip  21  is a logic or power chip which does not require special alignment. The second chip  22  is an opto-electronic chip which does require precise alignment with a fiber-optic cable  30 . The package has a base or floor portion  23  which is preferably a heat dissipating copper-tungsten base having a substantially planar gold plated upper surface  24  bounded along two opposite peripheral edges by wall portions  25  which form a portion of the package housing. Each of the wall portions is slotted to allow insertion of a ceramic feed-through  26  which carries a plurality of electrical interconnection wire bonding pads  27  which are electrically connected to leads extending from the outer surface of the package. Some of the pads are in electrical communication with the opto-electronic microcircuit chip  22  through wire bond interconnects  28 . The package encapsulates the opto-electronic die and supports a fiber-optic cable  30  which is in communication with the die  22  at an interface  31 .  
     [0024] The dies are mounted upon a substantially quadrangular substrate or boat  32  which in turn, rests upon a pool  33  of reflowable material such as solder which is held in place by a substantially circular retaining wall  34  of a tank structure.  
     [0025] The boat  32  is preferably made of a metallizable material such as silicon, ceramic, aluminum nitrite, diamond, beryllium oxide or BT resin material. As described below, the boat is metallized to form electrical bonding pads  35 , 36 , traces and to form edge electrical connection  37  to the solder pool. The undersurface of the boat can be fully metallized to form good electrical contact to the pool, and the boat may carry internal electrical vias between the pads and the undersurface.  
     [0026] Most preferably, the boat has a substantially square undersurface  38  so that its movement is restricted to rotational movement in the pool while translational movement is inhibited. In this way, the corners  39  of the boat are in close contact or held by surface tension close to the substantially circular inner surface  34 b of the retaining wall  34  of the tank.  
     [0027] The type of solder used in the pool  33  of reflowable material is selected according to its compatibility with the materials used to form adjacent structures in the package. Clearly, the solder should have a melting temperature which is below the temperature which may affect other structures in the package such as any solder joint between the chips and the boat. Possible candidates include Tin-Lead, Tin-Silver, and Tin-Silver-Copper type solders. Solder alloys having a high thermal conductivity and sharp eutectic point such as eutectic Tin-Lead, and Tin-Silver-Copper and low latent heat characteristics are preferred so that rapid onset of complete liquification is achieved and subsequently, rapid solidification when heat is removed.  
     [0028] As described above, the solder pool  33  is held in place in a substantially circularly walled tank  34  formed by a ring of non-reflowable material such as higher temperature withstanding epoxy, or more preferably electrically conductive solder having a higher melting point than the solder pool. This allows for simpler electrical interconnection with the solder pool.  
     [0029] The tank and pool structures are generally formed onto the package floor using thick film screen printing techniques well known in the art. Alternately, the tank could be machined into the floor of the metal package cavity.  
     [0030] An electrically conductive actuator wire  41  extends from a wedge bond on one of the package pads  42  to a corner pad  35  on the boat  32  carrying the microcircuit chip  22 . A similar wire  43  extends from a pad on the opposite side feed-through to a pad  44  on the diagonally opposite corner of the boat. The actuator wires are selected from a material and have a diameter which is electrically conductive enough to carry an amount of current which will result in an adequate amount of force to generate enough moment on the boat to adjust alignment. Preferably, the material has high elasticity to allow for minor bending and stretching to accommodate movement of the boat, and has high tensile strength to adequately impart the necessary mechanical force. Preferred materials are therefore gold, silver, aluminum, copper, nickel and related alloys. If gold is used, a thickness of between about 0.002 and 0.003 inch has been found to be adequate. In this way, die interconnect wire bonding an actuator wire bonding is performed in the same operation preferably using a ball/wedge-type bond configuration.  
     [0031] Each of the actuator wire bond pads  35 , 44  on the boat have an edge trace connector  37  which extends down the side of the boat and electrically interconnects with the underside metalization of the boat which contacts the solder pool  33  which itself is electrically conductive and interconnects with the conductive tank  34  which, in turn, is electrically connected to the gold plated upper surface  24  of the package cavity floor. A drain line  45  electrically connects the floor to a drain pad  46  on a feed-through  26  of the package.  
     [0032] Referring now to FIG. 7, the package is placed within a substantially uniform magnetic field indicated by upward flow lines  61 , and the solder pool  33  is heated to a degree in which it becomes liquid, thereby allowing the chip carrying boat  32  to be movable upon it through application of minor forces. A current i 1 ,i 2  is applied to the actuator wires  62 , 63  of the boat and drained through the solder pool  33  and out through the drain wires. This generates forces F 1 ,F 2  on the actuator wires proportional to the current, the length of the wires and the strength of the magnetic field according to Maxwell&#39;s equations. Because the current i 1  entering the first actuator wire  62  is oriented directly opposite in direction the current i 2  entering the second actuator wire  63 , the forces F 1 ,F 2  generated have an opposite direction. Since the wires are spaced apart by the distance  64  between the diagonal corners of the boat, the forces act in concert to create a moment M about the perpendicular axis  65  formed between the forces. This moment causes the boat to rotate. This movement, in turn, causes the die to move angularly with respect to the optical cable and thereby correct for angular error at the interface.  
     [0033] Referring now to FIG. 9, there is shown an alternate embodiment of the invention implemented in a ceramic package  90  wherein an opto-electronic chip  91  itself forms the boat mounted upon the reflowable solder pool  92 . As with the previous embodiment, the package floor has metallization printed upon it to form a circular containment pad or tank  93  and drain pads  94 . Wire bonds  95  connect the chip to the drain pads. The size of the preform is selected so that it remains contained upon the containment pad and automatically centers the chip/boat due to surface tension when the solder is molten. Bonded actuator wires  96 , 97  are used to move the chip/boat and aperture  98  into better alignment with a fiber-optic light source  99 .  
     [0034] Referring now to FIG. 10, there is shown the preferred method  100  for aligning the opto-electronic die to an impinging optical cable. First, the solder pool is heated  101  until molten and a magnetic field is applied  102 . Further, the operation of the die and source optical cable are activated  103  so that a signal may be measured which has passed through an interface between the cable and die. No particular order of the above steps is as yet preferred.  
     [0035] Once the above steps are taken, the signal is then measured  104  for its loss to see if its within an acceptable loss range and upon the results of this test  105 , the actions are taken. If the loss is not acceptable, current is increased  106  through the actuator wires and a subsequent measurement taken  104 . This feedback loop continues until the test reveals an acceptable loss, whereupon the solder pool is allowed to cool and solidify  108 , at which point the current is turned off  109 .  
     [0036] Of course, those skilled in the art will readily appreciate that care must be taken that the above-method does not exceed certain parameter ranges such as a maximum current.  
     [0037] Referring now to FIG. 11, there is shown an operational system  110  for implementing the method described above. The die package  111  is placed upon a heater  112  and within the plates  113 , 114  of a magnetic field generator  115 . Communication with the die package occurs electronically through electronic test interface circuitry  116  and optically through an optical test interface  117 . A current generator  118  provides current to the actuator wires aboard the package. An electronic system controller  119  conducts the performance of the above system components.  
     [0038] Because solder undergoes some thermal expansion, the positioning of the boat may change as the solder of the pool solidifies. This may be a known quantity which is accounted for in the adjustment routine. Also, for some applications it may be useful to support the fiber optic cable on a second boat which is mounted on a second solder pool connected to the first pool. In this way, the expansion of the solder pool is identical for both support boats.  
     EXAMPLE  
     [0039] A ceramic package similar in construction to the package shown in FIG. 8 was formed but not sealed. A film of titanium is screen printed onto the floor of the cavity of the package to form a circular pad having a diameter of approximately 0.565 inch and a thickness of between about 0.0002 and 0.0005 inch and drain lines. A 0.0005 inch thick circular preform of eutectic tin-lead or tin-gold-copper solder having a diameter slightly less than the diameter of the pad was placed on top of the pad to form the solder pool.  
     [0040] A substrate or boat was formed having an alumina-type ceramic body having a rectangular top and bottom surface measuring about 0.400 square inch and being about 0.015 inch thick. The bottom surface was metallized with a 0.0001 inch thick layer of titanium. The top surface of the boat had titanium-gold type metallization pads upon which was bonded a logic microcircuit chip and an opto-electronic chip using eutectic gold-tin bond. The top surface also had a pair of diagonally opposite metallization pads for bonding to the actuator wires. Electrical connection between the upper actuator pads and the lower metallized surface was accomplished through a titanium-gold edge connector.  
     [0041] The chip carrying boat was then soldered to the solder preform in position to allow the light source cable to communicate with the opto-electronic chip.  
     [0042] The opto-electronic chip was then wire bonded to the package body. Further, a pair of actuator wires were bonded between the actuator pads on the boat and the package body. The actuator wires were gold wire having a thickness of about 0.002 inch and a length of about 0.250 inch.  
     [0043] The package was mounted on a heatable workholder and electrically connected to test circuitry. Signal loss was measured to be about 2 dB. The heater was heated to about 225 degrees C and a uniform magnetic field of about 1000 Gauss using a magnetic field generator. A current of about 0.5 A was ramped up into each actuator wire which generated a force of about 0.03175 N which translated into a moment of about 2.016×10 −5  Nm. The heat was removed and the package cooled to about 25 degrees C within about 2 seconds, thereby causing the solder pool to solidify. The signal loss was then measured to be about 0.2 dB.  
     [0044] While the preferred embodiments of the invention have been described, modifications can be made and other embodiments may be devised without departing from the spirit of the invention.