Abstract:
An apparatus and method for enabling interaction between electronic circuitry and optical signals with a package for enclosing the electronic circuitry having contacts and a window for the passage of optical signals disposed about a face of the package that becomes substantially inaccessible when the package is used.

Description:
FIELD OF THE INVENTION 
     The present invention is related to an apparatus and method for communicating optical signals to an electronic device enclosed within a flip-chip package. 
     ART BACKGROUND 
     Commonly available electronic devices exist in the form of a thin sheet of semiconductor material, or die, with electronic circuitry disposed thereon by way of various photolithographic processes. To protect the circuitry from damage, the die is often enclosed in a package designed to facilitate the attachment of the electronic device to a printed circuit board of a computer system or other electronic system. 
     Within some electronic devices are components meant to produce optical signals to be transmitted outside of the package, or to receive optical signals to be received from outside of the package. Packages have been used that provide a window through which such signals may pass. The window of such packages typically faces outwardly from the package in a direction that is generally meant to face away from the printed circuit board to which the electronic device is to be attached. One of the most common examples of such electronic devices enclosed within such a package is an ultaviolet-eraseable programmable read-only memory or UV-EPROM. 
     FIG. 1 is a cross-sectional view of a prior art package of the dual inline pin (DIP) type. The exterior or package  100  is comprised of window  110  and shell  112 . Die attach  114  attaches die  130  to an inner surface of shell  112 , thereby securing die  130  to the interior of package  100 . Bonding wires  120  provide part of the electrical connections between die  130  and solder balls  122 . Die  130  is positioned within package  100  such that the surface on which electronic circuitry (not shown) has been disposed is caused to face window  110  to allow the transmission or receipt of optical signal  132 . Pins  122  are used to attach package  100  to printed circuit board  140  through which are holes to receive the pins, and as a result, window  110  faces away from printed circuit board  140 . 
     The use of such packages, however, is based on a long-standing practice of having the surface of the die on which electronic circuitry is disposed facing away from the printed circuit board to which the electronic device is attached, thereby making this surface of the die accessible to optical signals passing through the window of the package. More recently, however, flexibility in power consumption and die size, as well as improvements in electrical signal characteristics, have been realized through the use of packages, such as the “flip-chip” or the “controlled collapsed chip connection” (C4) package, in which the surface of the die on which electronic circuitry is disposed now faces towards the printed circuit board to which the electronic device is to be attached. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The objects, features, and advantages of the present invention will be apparent to one skilled in the art in view of the following detailed description in which: 
     FIG. 1 is cross-sectional view of a prior art ball grid array package. 
     FIG. 2 cross-sectional view of one embodiment of the present invention. 
     FIG. 3 is a cross-sectional view of another embodiment of the present invention. 
     FIG. 4 is a cross-sectional view of yet another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     In the following description, for purposes of explanation, numerous details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that these specific details are not required in order to practice the present invention. In other instances, well known electrical structures and circuits are shown in block diagram form in order not to obscure the present invention unnecessarily. 
     The present invention concerns “flip-chip” or “controlled collapsed chip connection” (C4) packages for integrated circuits, wherein the surface of a die on which electronic circuitry has been disposed is positioned such that it faces towards the circuit board to which the package will be attached during use of the electronic circuitry. The present invention further concerns the addition of a window to such a package on the side of the package that faces towards the circuit board to enable the transmission of optical signals to and from the electronic circuitry. 
     FIG. 2 is a cross-sectional view of one embodiment of the present invention. The exterior of package  200  is comprised of shell  210  and substrate  212 . Die attach  214  holds die  230  in place within package  200  such that the surface of die  230  on which electronic circuitry has been disposed faces towards substrate  212 . Underfill  220  and solder balls  222  attach die  230  to substrate  212 , with solder balls  222  providing electrical connections between die  230  and substrate  212 . Substrate  212  is a printed circuit board with conductors forming electrical connections between solder balls  222  and solder balls  224 . Mounted within and through substrate  212  is window frame  219 , in which window  218  is mounted, which allows optical signal  232  to pass between the exterior of package  200  and exposed die portion  231  of die  230  where electronic circuitry capable of interacting with optical signal  232  has been disposed. 
     In one embodiment, exposed die portion  231  comprises less than all of the surface of the die that faces window  218 , while in an alternate embodiment, exposed die portion  231  comprises the entire surface of the die that faces window  218 . 
     In one embodiment, die attach  214  is a thermal grease, such as silicone, and shell  210  is a heatsink made of material capable of conducting heat away from die  230 , such as copper or aluminum. 
     In one embodiment, window  218  is made of glass, while in alternative embodiments, window  218  is made of plastic or a combination of glass and plastic. In one embodiment, the material of which window  218  is made is chosen to be transparent for optical signals of a specific frequency or frequencies, while in an alternative embodiment, the material of which window  218  is made is chosen to be transparent for a substantially wider range of frequencies. 
     In one embodiment, the external surface of window  218  is mounted flush with the external surface of window frame  219 . In an alternative embodiment, window  218  is recessed within window frame  219  to permit the insertion of an optical conductor, an assembly of lenses, an optoelectronic device, or other apparatus protruding through the opening in printed circuit board  240 . In still another alternative embodiment, a micromechanical device for manipulating optical signal  232 , such as a micromirror (not shown), is positioned within window frame  219 , either behind or in place of window  218 . 
     In one embodiment, window frame  219  is made of ceramic material having a thermal coefficient substantially similar to that of die  230 . In this embodiment, shell  210  and window frame  219  cooperate to restrict the expansion of substrate  212  in response to heat emanating from die  230  during operation of the electronic circuitry disposed onto die  230 . This ensures that substrate  212  maintains its shape and remains correctly aligned with die  230 . 
     In one embodiment, window  218 , window frame  219 , underfill  220  and exposed die portion  231  define a cavity through which optical signal  232  is transmitted. In one embodiment surface tension from the edges of the die and the inner edge of window frame  219  prevents underfill  220  from overflowing and thereby blocking exposed die portion  231 . Alternatively, exposed die portion  231  is treated with a low surface energy coating, such as a flour-carbon (CFx) deposition. Regardless of whether surface tension or a low surface energy coating is used, underfill  220  may be of a material commonly used as underfill in typical flip-chip or C4packages. 
     In one embodiment, adhesive  216  and solder balls  224  attach package  200  to printed circuit board  240 . Solder balls  224  provide electrical connections between substrate  212  and electrical contacts disposed on printed circuit board  240 . In a further embodiment, an opening is provided through printed circuit board  240  to permit optical signal  232  to pass therethrough, thereby enabling the exchange of optical signals between die  230  and other devices (not shown) positioned on the opposite side of printed circuit board  240 . 
     In one embodiment, printed circuit board  240  is a rigid laminate of electrically conductive materials and electrical insulators. In an alternative embodiment, printed circuit board  240  is a flexible laminate of such materials. In another alternative embodiment, printed circuit board  240  is replaced with solid piece of electrical insulator upon which electrical contacts are disposed for making electrical connections with solder balls  224 . 
     In one embodiment, circuitry comprising the core logic (e.g., random access memory controller, bus interface, I/O device interface, or timers) of a microcomputer system is disposed on a surface of die  230  with circuitry capable of interacting with optical signals being disposed on exposed die portion  231 . In another embodiment, circuitry comprising a central processing unit of a microcomputer system is disposed on a surface of die  230  with circuitry capable of interacting with optical signals being disposed on exposed die portion  231 . 
     In one embodiment, package  200  is assembled by first designing and fabricating package  200  with an opening in substrate  212  for window frame  219 . Then by fitting window frame  219  and window  218  into substrate  212 . Then by attaching die  230  to substrate  212  by way of solder balls  222  using a typical C4process. Then by filling the area among solder balls  222  with underfill  220  and curing underfill  220 . 
     In one embodiment, package  200  is attached to printed circuit board  240  by first attaching package  200  via solder balls  224  to printed circuit board  240  by way of typical surface mount soldering techniques. Then by applying die attach  214  to surface of die  230  that is to be attached to shell  210 . Then by applying adhesive  216  to the surface of shell  210  that is to be attached to printed circuit board  240 . Then by attaching shell  210  to both die  230  and printed circuit board  240  and curing adhesive  216 . 
     FIG. 3 is a cross-sectional view of another embodiment of the present invention. Most of the numbered elements of FIG. 3 substantially correspond to those of FIG. 2 with the exception that window frame  219  and window  218  have been replaced with window  318 . Just as window frame  219  and window  218  served to allow die  230  to interact with optical signal  232  in the embodiment depicted in FIG. 2, window  318  serves to allow die  330  to interact with optical signal  332  in the embodiment depicted in FIG.  3 . However, unlike the embodiment of FIG. 2, where window  218 , window frame  219 , underfill  220  and exposed die portion  231  defined a cavity, as earlier described, no such cavity is defined in FIG.  3 . 
     In one embodiment, window  318  is made of a material with a thermal coefficient substantially similar to that of die  330 , and window  318  cooperates with shell  310  to restrict the expansion of substrate  312  in the same way in which window frame  219  and shell  210  restricted the expansion of substrate  212  in FIG.  2 . 
     FIG. 4 is a cross-sectional view of another embodiment of the present invention. Most of the numbered elements of FIG. 4 substantially correspond to those of FIG. 2 with the exception that pins  424  have been substituted for solder balls  224  of FIG.  2 . However, unlike printed circuit board  240  of FIG. 2, additional openings (e.g., plated through-holes) have been provided in printed circuit board  440  to receive pins  424 . In one embodiment, printed circuit board  440  is a rigid laminate of electrically conductive materials and electrical insulators. In an alternative embodiment, printed circuit board  440  is a flexible laminate of such materials. In another alternative embodiment, printed circuit board  440  is replaced with solid piece of electrical insulator through which electrical contacts are disposed for making electrical connections with pins  424 . 
     Just as solder balls  224  of FIG. 2 served to attach package  200  to printed circuit board  240 , pins  424  serve to attach package  400  to printed circuit board  440 . Also, just as solder balls  224  served as electrical contacts providing a portion of the electrical connections between die  230  and electrical contacts disposed on printed circuit board  240 , pins  424  serve as electrical contacts providing a portion of the electrical connections between die  430  and electrical contacts disposed through printed circuit board  440 . 
     In the depicted embodiment, package  400  is attached directly to printed circuit board  440 . However, in an alternate embodiment, a socket (not shown) that is designed to receive pins  424  is interposed between package  400  and printed circuit board  440 . In one embodiment, this socket would, in turn, be comprised of pins by which it would be attached to printed circuit board  440 , just as package  400  would have been. 
     The invention has been described in conjunction with the preferred embodiment. It is evident that numerous alternatives, modifications, variations and uses will be apparent to those skilled in the art in light of the foregoing description. It will be understood by those skilled in the art, that the present invention may be practiced in support of other combinations of functions in a computer system. 
     The example embodiments of the present invention are described in the context of ball grid array and pin grid array packages for electronic devices that are comprised of components for sending or receiving optical signals. However, the present invention is applicable to a variety of package types and to a variety of electronic, microelectronic, optical, optoelectronic and micromechanical devices. The term optical should not be taken to be limited to encompassing only signals with frequencies within the visible spectrum of light, for the present invention is applicable to use with light of a variety of frequencies, including infrared and ultra-violet. Although the present invention is described in the context of packages attached to rigid printed circuit boards that are commonly in use, the present invention is also applicable to packages being attached to sheets of flexible material or other surfaces providing electrical connections. Also, although the present invention is described in the context of having electronic circuitry disposed onto a single surface of a die composed substantially of semiconductor material, thereby requiring that single surface to be oriented to face the window of the package, it will be understood by those skilled in the art that electronic circuitry may be positioned within a package using a variety of means and in a variety of orientations without departing from the spirit of the invention as hereinafter claimed. Furthermore, although the present invention is described in the context of packages that enclose a single die on which electronic circuitry is disposed, the present invention is applicable to packages enclosing multiple separate dies, and/or dies comprised of smaller dies.