Abstract:
A method of fabricating an integrated circuit having an optically transmissive window therein includes forming an integrated chip preform structure that includes a plurality of bonding wires connecting pads on a die structure to pads on a lead frame structure, at least some of the bonding wires having a selected portion, such as a looped portion, that defines or establishes a common mounting plane or support surface therebetween. A quantity of an uncured or partially cured optically transmissive material is deposited on the die portion of the integrated circuit preform and the window is thereafter placed on the uncured or partially cured optically transmissive material and positioned so that the window is on or in the mounting plane or support surface defined by the bonding wires. The so-assembled components are then subject to a curing step to cure the optically transmissive media and thereafter subject to an encapsulation step. If desired the curing step and the encapsulation step can be partially or fully concurrent with one another. The resulting integrated chip package utilizes the mounting or support plane defined by or established by the bonding wires to efficiently maintain the position of the window during fabrication.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of commonly owned U.S. Provisional Patent Application 60/575,096 filed by the inventor herein on May 28, 2004. This application is related to commonly owned U.S. Provisional Patent Application 60/575,101 filed May 28, 2004 by the inventor herein and U.S. Patent Application (Docket SE-2055) filed on even date herewith by the inventor herein, both entitled “Method Of Fabricating An Encapsulated Chip And Chip Produced Thereby.” 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to the fabrication of encapsulated integrated circuits and, more particularly, to a method for the fabrication of an encapsulated integrated circuit having optically active areas or devices therewith and the encapsulated integrated circuits resulting therefrom.  
         [0003]     Integrated circuit devices that include an optically active area or areas typically incorporate a window of glass, quartz, plastic, or similar material(s) that allows the transmission of optical energy therethrough to and/or from the optically active area or areas of the chip structure. Typically, the window is located on the top surface of the encapsulated chip and allows optical energy to pass to and/or from the optically active areas of the underlying die. In general, it is desirable to reduce the fabrication costs of such integrated circuits, since the placement and alignment of the window oftentimes requires the use of specially designed posts, columns, or similar structures to hold the window in place relative to the underlying die during the encapsulation process.  
       SUMMARY OF THE INVENTION  
       [0004]     An integrated circuit includes an optically transmissive window through which optical energy passes to and/or from an optically active area or areas formed in or on an underlying semiconductor die. A plurality of bonding wires connect pads on the die with corresponding contacts or pads on a surrounding lead frame structure with the various bonding wires formed with a looped portion, the uppermost reach or extent of the looped portion of at least some of the bonding wires defining a mounting plane or support surface upon which or in which the window is located.  
         [0005]     A method of fabricating an integrated circuit having an optically transmissive window therein includes forming an integrated chip preform structure that includes a plurality of bonding wires connecting pads on a die structure to pads on a lead frame structure, at least some of the bonding wires having a looped portion that defines or establishes a common mounting plane or support surface therebetween. A quantity of an uncured or partially cured optically transmissive material is deposited on the die portion of the integrated circuit preform and the window is thereafter placed on the uncured or partially cured optically transmissive material and positioned so that the window is on or in the mounting plane or support surface defined by the bonding wires. The so-assembled components are then subject to a curing step to cure the optically transmissive media and thereafter subject to an encapsulation step. If desired the curing step and the encapsulation step can be partially or fully concurrent with one another.  
         [0006]     The particular bonding wires used to define the mounting plane or support surface can be active circuit wires, or, if desired, extra “dummy” or otherwire inactive wires.  
         [0007]     The method of the present invention uses the looped portions of at least some of the bonding wires to establish the dimensional relationship and/or alignment between the window and the underlying die while the optically transmissive material is in its uncured state to thereby reduce the in-process assembly costs of the resulting package.  
         [0008]     The full scope of applicability of the present invention will become apparent from the detailed description to follow, taken in conjunction with the accompanying drawings, in which like parts are designated by like reference characters. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0009]      FIG. 1  is a side elevational view, in cross-section, of an exemplary integrated circuit chip structure in accordance with the preferred embodiment;  
         [0010]      FIG. 2  is a side elevational view of a die or chip mounted on an underlying lead frame;  
         [0011]      FIG. 3  is a side elevational view of the die of  FIG. 2  in which bonding wires electrically connect conductive pads on the die to selected leads on the lead frame;  
         [0012]      FIG. 4  is a side elevational view of the assembly of  FIG. 3  with a deposit of uncured optical coupling media deposited on the face of the die;  
         [0013]      FIG. 5  is a side elevational view of the assembly of  FIG. 4  with a window placed atop the deposit of uncured optical coupling media;  
         [0014]      FIG. 6  is a side elevational view of the assembly of  FIG. 5  with the window depressed or pressed a selected distance into the deposit of uncured optical coupling media and positioned atop or adjacent the bonding wires;  
         [0015]      FIG. 7  illustrates the optional use of an anti-flash tape placed on the window to limit or prevent encapsulation material from covering the window surface;  
         [0016]      FIG. 8  is a representative perspective view of a portion of QFN-type package in which two of the bonding wires are installed with an elevation higher then the others;  
         [0017]      FIG. 9  is a side elevational view of the assembly of  FIG. 8  with a deposit of uncured optical coupling media deposited on the face of the die;  
         [0018]      FIG. 10  is a side elevational view of the assembly of  FIG. 9  the window depressed or pressed a selected distance into the deposit of uncured optical coupling media and positioned atop or adjacent the higher elevation bonding wires  
         [0019]      FIG. 11  is a side elevational view of variant of the the assembly of  FIG. 8  in which the ‘corner’ bonding wires are at a higher elevation than the other bonding wires; and  
         [0020]      FIG. 12  is a top view of the structure of  FIG. 11 . 
     
    
     DESCRIPTION OF THE INVENTION  
       [0021]     An encapsulated semiconductor device of the type fabricated in accordance with the present invention is shown in exemplary cross-section in  FIG. 1  and is designated therein by the reference character  10 . As shown, the semiconductor device  10  includes a die pad  12  that can be part of a larger lead frame or similar structure of which leads  14  and  16  are representative. An integrated circuit die  18  is affixed to one surface of the die pad  12  by a conventional die attach adhesive or cement (unnumbered). Conductive pads or lands (not shown) on the die  18  are electrically connected to the various leads by conductors, i.e., bonding wires  20  (typically gold or aluminum or alloys thereof), that are secured in place on their respective pads or lands by suitable bonding techniques including, for example, thermocompression or thermosonic techniques or variants thereof. An optical coupling media  22  is located over and occupies a selected volume on the top surface of the die  18  and is designed, as explained more fully below, to transmit or couple optical energy to and/or from optical devices formed in or on the die  18 . A window  24  is located on or in engagement with the optical coupling media  22  and acts as the interface between the interior components of the semiconductor device  10  and the exterior thereof. Lastly, an encapsulating material  26 , such as a conventional opaque resin or epoxy material, surrounds the interior components to define the outline of the semiconductor package  10 .  
         [0022]     A first embodiment of the semiconductor device  10  of  FIG. 1  is prepared in accordance with the sequence of  FIGS. 2-7 . The embodiment of  FIGS. 2-7  is described in the context of a QFN “no-lead” type package; as can be appreciated, the invention is not so limited and can be used in the context of other types of semiconductor packages, including, for example, ball-grid arrays, pin arrays, and classic dual-in-line packages.  
         [0023]     As shown in  FIG. 2 , the circuit die  18  is attached to the die pad  12  using a conventional die attach adhesive or cement. In automated systems, the die  18  can be attached using conventional pick-and-place robotic machinery. The die  18  includes one or more optical devices or circuits formed therein or thereon. The optical devices can include devices for responding to incident optical radiation or for generating and emitting optical radiation including, for example, photoreceptive diodes/transistors and/or photoemitting photodiodes, LEDs, lasers, or the like. As used herein, optical devices are those that either respond to and/or emit radiation from and between the infrared region through the visible region and into and through the ultraviolet region of the electromagnetic spectrum.  
         [0024]     As represented in  FIG. 3  and after the placement of the die  18  on its die pad  12 , the die  18  is electrically connected to its lead frame using conventional bonding wires  20 . More specifically, individual conductive pads (not shown) on the die  18  are connected to respective leads (as represented by the leads  14  and  16 ) by wires  20  using conventional ball bond (i.e., “nail-head”) or wedge bond formations and thermocompressive, thermosonic, or equivalent bonding techniques. The bonding wires  20  are typically installed by automatic wiring machines, which, as is known in the art, can be programmed to attach the ends of the individual bonding wires  20  at precise x,y locations and control the height of the wire loop relative to some arbitrary datum. In  FIG. 3 , the height dimension “Z” is defined or established between the upmost extend or “reach” of the looped portion of the bonding wire  20  and the top surface of the die  18 , although other surfaces, such as the top or bottom surface of the die pad  12 , are equally suitable and can be used as the datum surface. As can be appreciated, the upmost extend or “reach” of the looped portion of three or more bonding wires  20  define or establish a mounting surface or plane therebetween.  
         [0025]     After the wire bonding step is completed and as shown in  FIG. 4 , a selected volume of an optical coupling media  22  is deposited on the exposed surfaces of the die  18 . The optical coupling media  22  is typically an uncured or partially cured optical material such as an epoxy, acrylate, resin, or silicone that, in its cured state, is sufficiently transparent to transmit or convey optical energy to and/or from the optical devices or circuits formed on or in the die  18 . In a preferred application of the present invention, HIPEC® Q1-4939 solventless silicone gel from the Dow Corning Corp., Midland, Mich. 48686 is used. This material, in its initial uncured state, is applied as a soft, pliable gel to the surface of the die  18  and cures into a resilient elastomeric material; the material used can be applied in its initially mixed uncured state or in a partially cured state. As is known, the as-applied viscosity of the silicone gel can be controlled in accordance with the supplier&#39;s instructions. If desired, other materials, including conventional hardenable epoxies and resins can be used as the optical coupling media  22 , provided they possess adequate optical and mechanical properties for the intended application.  
         [0026]     Once the optical coupling media  22  is deposited on the die  18  and as shown in  FIGS. 4 and 5 , the window  24  is placed on the deposited material. The window  24  can take the form of a glass, quartz, silica, or plastic material appropriately sized for the die  18  and the application. In the preferred embodiment, the window  24  is formed from conventional amorphous glass that is saw-cut from larger sheets into the desired size, which size is sufficiently large that some portion of the window  24  is located above the looped portions of at least a plurality of the bonding wires  20 . If desired, the particular material from which the window is formed can have uniform or non-uniform transmission characteristics for the wavelength or wavelengths to be transmitted to and/or from the die  18 , and, optionally, can be provided with one or more coatings to enhance or otherwise control its optical properties and/or provide physical abrasion resistance to the exposed surface of the window  24 . In automated assembly systems, the window  24  can be positioned and placed upon the optical coupling media  22  by a conventional pick-and-place robotic system.  
         [0027]     As shown in  FIG. 5 , the window  24  is placed atop the optical coupling media  22  and is pressed into or depressed into the optical coupling media  22  as part of the window placement operation until the bottom surface of the window  24  rests atop or immediately adjacent the uppermost extent or reach of at least a plurality of the bonding wires  20 . As the window  24  is pushed into the optical coupling media  22 , the media will tend to laterally displace somewhat; in general, this peripheral spreading or “bleeding” results in an acceptable lateral or peripheral expansion of the optical coupling media  22 . In general, the uppermost extent or reach of a majority of the bonding wires  20  will define a surface spaced by the dimension “Z” from the datum. Thus, by placing the window  24  on or atop the uppermost extent of reach of the bonding wires  20 , the window  24  will be thereby be positionally supported on a surface or plane defined by the uppermost extent or reach of the bonding wires. In practice, some dimensional variation exists in the looping of the bonding wires such that the uppermost extent or reach of some of the bonding wires may be less that of others. While it is desired that the uppermost extent or reach of all the bonding wires  20  participate in defining the support surface or plane upon which the window  24  is positioned, as a practical matter, at least some of the bonding wires may not extend sufficiently to support the window  24 . In general, as few as three sufficiently spaced-apart bonding wires  20  can successfully define the “Z” support surface or plane on which or in which the window  24  is positioned.  
         [0028]     While it is contemplated that the bottom surface of the window  24  contact the top of the bonding wire loops that define the “Z” surface, the volume of optical coupling media  22  applied to the die  18  and the use of higher viscosity or “stiffer” optical coupling medias may create a layer of optical coupling media that separates the bottom surface of the window  24  from the top of the bonding wire loops that define the “Z” support surface or plane. In this situation and because of the viscosity or “stiffness” of optical coupling media, the bonding wires nonetheless function to positionally define the window  24 , by virtue of this window-supporting layer, even in the absence of direct window-to-wire contact. As can be appreciated, the support surface or plane defined by at least some of the bonding wires  20  serves to positionally define or maintain the as-placed window on the uncured or partially cured optical coupling media  22 .  
         [0029]     While the window  24  can be “placed” in its supported position relative to the wires and, depending upon the viscosity/density/cure-state of the optical coupling media, allowed to ‘sink’ or settle onto those wires that define the window support plane or surface, the window  24 , if desired, can be pressed downward onto the wires or pressed downward with sufficient force to cause the wires  20  to be momentarily and resiliently depressed to increase the probability of all the wires participating in the window-support function will contact the window, or, if desired, the window  24  can be pressed downward with sufficient force to cause a small permanent deformation or yielding of the wires  20  to increase the probability of all the wires participating in the window-support function. The issue of whether the window  24  is merely placed in position on or in the wire-defined mounting surface or plane, pressed downward, resiliently pressed downward, or pressed downward to cause the wires to permanently yield is a function of the particular application.  
         [0030]     In  FIGS. 4 and 5  and as described above, the optical coupling media  22  is deposited upon the die  18  ( FIG. 4 ) and the window  24  is thereafter pressed or depressed into the so-deposited material ( FIG. 5 ). As can be appreciated, some or all of the optical coupling media can be deposited on the underside of the window  24  and the window  24  with its deposit of optical coupling media  22  can be pressed onto or assembled to the pre-form assembly of  FIG. 3 .  
         [0031]     After the window  24  is placed upon the as-applied uncured or partially cured optical coupling media  22 , the assemblage of  FIG. 5  is subject to a full or partial curing step by application of heat at a temperature and duration appropriate for the optical curing media used. For individual piece-parts and small batch quantities, curing can be accomplished in conventional “box” ovens and for large quantities, production ovens/molds can be used. In the case of the HIPEC® media mentioned above, exposure to 150° C. for about two hours is sufficient to effect a cure.  
         [0032]     After the optical coupling media is fully cured or at least sufficiently cured for the assemblage of  FIG. 5  to undergo encapsulation, the assembly of  FIG. 5  can be placed in a conventional encapsulation mold and subject to an encapsulation step by which the typically opaque encapsulating material defines the final or near final semiconductor package.  
         [0033]     It is not necessary for the curing of the optical coupling media  22  to be completed prior to the conventional encapsulation procedure. For example, the optical coupling media  22  can be subject to curing for a sufficient period of time such that the now-partially but not fully cured optical coupling media  22  will remain dimensionally stable during the subsequent encapsulation step so that the curing of the encapsulation material will concurrently “finish” the curing of the optical coupling media  22 .  
         [0034]     If desired and as shown in  FIG. 7 , a removable adhesive-backed “anti-flash” tape  30  can be provided on the exterior surface of the window  24 . This tape  30 , which is shown partially “peeled” from the window  24  in  FIG. 7 , functions to protect the surface of the window  24  during processing and functions to temporarily seal the peripheral margins of the window  24  during the encapsulation step to minimize or prevent any encapsulation material from infiltrating onto the exterior surface of the window  24 . Once the encapsulation step is completed, the tape  30  can be removed manually or by use of solvents and/or washes. Where an anti-flash tape  30  is not used, conventional flash-removing solvents, baths, and/or washes can be used.  
         [0035]     In the embodiment described above, the bonding wires  20  are nominally installed with a looped portion, the uppermost reach or extent of which defines the “Z” surface upon which or by which the window  24  is positionally supported or positionally defined. In a variation of the above-described embodiment, a sub-set of the bonding wires are formed with an uppermost reach or extent that is higher than the others. As shown in diagrammatic fashion in  FIG. 8 , two of the bonding wires, designated as  20 A, are formed with an uppermost reach or extent that is higher than that of the other bonding wires  20 . The “package” shown in  FIG. 8  is representative of QFN type packages and shows only one side of a multi-sided package. As can be appreciated, one or more bonding wires  20 A on other sides of the package can be provided so that a sub-set of the bonding wires have loops with an uppermost reach or extent that is higher than the others with that sub-set of bonding wires defining the support plane or mounting surface upon which the window  24  is positioned on or in.  
         [0036]     In  FIG. 9 , the higher elevation bonding wires  20 A (solid-line illustration) are shown relative to the lower elevation bonding wires  20  (dotted-line illustration). In the embodiment of  FIG. 9 , the window  24  is placed in the same manner as that for the embodiment of  FIGS. 1-7 , however, only a sub-set of the bonding wires, i.e., the bonding wires  20 A, serve to define or establish the support plane or surface for the window  24 . In theory, only three bonding wires  20 A, spaced-apart in a tripod “footprint” will provide adequate support, although more than three such bonding wires  20 A may be indicated. For those chips circuits in which alternate bonding wires are ground wires, these ground wires can function as the higher-elevation bonding wires  20 A.  
         [0037]     A futher variant of the present invention in shown in  FIGS. 11 and 12 ; as shown on the left in  FIG. 11 , support wires  20 A have an inverted “U” shaped relative to the bonding wires  10 . In  FIGS. 11 and 12 , the support wires  20 A are “dummy” or extra wires located and the corners of the die. More specifically and as shown in the plan view of  FIG. 12 , a support wire  20 A is located at each corner of the die and cooperate to define the window support plane or surface. In the embodiment of  FIGS. 11 and 12 , the optical coupling media, in general, is not suffused in and between the various wires  20  or  20 A and, according, the upper most reach of the loop portion of the support wires  20 A is available for contact with the underside of the window  24 .  
         [0038]     In the preferred embodiments described above, the bonding wires are described as having looped portions that define an uppermost reach or extent to define the mounting surface or support plane; these looped portions often identified in the art as a “flat loop” or a “worked loop.” As can be appreciated, the invention is not so limited can including other bonding wire configurations and organizations, including bonding wires in which each bonding wire extends from the conductive pad on the die to an attachment point in a relatively straight line so that some segment of the relatively straight bonding wires defines the support plane or mounting surface for the window. While the some of the bonding wires have been described as having an uppermost reach that defines the support plane, variants include bonding wire shapes in which a shelf or ledge is provided below the uppermost reach and which define the support plane.  
         [0039]     The present invention thus provides a method for forming a semiconductor device package of the type having an optical window therein and the product formed thereby.  
         [0040]     As will be apparent to those skilled in the art, various changes and modifications may be made to the illustrated embodiment of the present invention without departing from the spirit and scope of the invention as determined in the appended claims and their legal equivalent.