Abstract:
One embodiment of the present invention provides a module including a primary substrate defining a base of the module, wherein the primary substrate is provided with a plurality of vias for electrical connection to a photodetector located within an interior portion of the module; a side wall member joined to the primary substrate to form side walls of the module and to define the interior portion of the module; a secondary substrate positioned within the interior portion of the module, the photodetector being mounted on the secondary substrate; an optical fiber guide extending into the interior portion of the module from outside the module, the optical fiber being arranged to receive an optical fiber and to position the optical fiber so that light emerging from the optical fiber impinges upon the photodetector; and a lid joined to the side wall member to enclose the interior portion of the module.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention pertains to hermetic packages for microelectronic circuits and methods of making them. In particular, it relates to a highly integrated optical modules for high (10 Gbps and higher) data rates. 
     2. Background of the Invention 
     Currently, modules for high frequency signals are typically configured as illustrated in FIGS. 1 and 2. FIG. 1 is a perspective view of a conventional module  110  having a “butterfly” configuration. The housing  120  for the module  110  is generally comprised of metal. Holes are drilled through the housing  120  and electrical feedthroughs  130  are inserted in through the holes. The holes are then sealed to isolate the feedthroughs from the housing by firing them at high temperature using a glass material to form a glass-to-metal seal  140  (see FIG.  2 ). Alternatively, this could be made by high-temperature co-fired ceramic typically consisting of alumina. 
     As shown in FIG. 2, which is an end-on elevation of the module  110  shown in FIG. 1 taken along line A—A of FIG. 1, a device  150  to be placed inside the module  110  is mounted on a substrate  160  and electrical interconnections  170  are made between the device  150  and substrate  160 . The substrate  160  is then positioned within the module  110 . Electrical connections  180  interconnect the electrical feedthroughs  130  with the substrate  160  and hence to the device  110 . The module  110  is then sealed with a lid  200 . Ultimately, the module  110  is placed on and mechanically fastened to a PC board  190  in a known fashion, e.g., through the use of mechanical fasteners through a flange in the module or by cutting a hole in the PC board  190 , and placing the module  110  in it. The electrical feedthroughs  130  are then soldered down to the traces on PC board  190 . Typically an angled fiber is optically coupled to the photo-detector that is positioned parallel to the angled fiber to receive the optical data stream. Standard transistor out line (TO) packages and dual in line (DIL) packages are also used. 
     While modules of the configuration shown in FIGS. 1-2 are generally suitable for their intended purpose, they are not without drawbacks. For example, the packages are bulky and cannot be surface mounted. Cavities need to be cut in the PC board to accept the module in order to bring the leads  130  in line with the board so that they can be solder connected. In packages such as DILs the leads must be formed in order to connect to the pc board. The rotational alignment of the angled fiber is cumbersome and increases assembly cost. 
     Another drawback of a module such as that depicted in FIG. 2 is that a connector such as a K connector is used. This further increases the height of the module because it should be at least as high as the connector. 
     Another drawback of a module such as that depicted in FIG. 2 is that it may necessitate the use of metal flanges with holes to permit mechanical fastening of the module to the PC board. 
     Yet another drawback is the very high cost of these packages. 
     SUMMARY OF THE INVENTION 
     One embodiment of the present invention provides a module including a primary substrate defining a base of the module, wherein the primary substrate is provided with a plurality of vias for electrical connection to a photodetector located within an interior portion of the module; a side wall member joined to the primary substrate to form side walls of the module and to define the interior portion of the module; a secondary substrate positioned within the interior portion of the module, the photodetector being mounted on the secondary substrate; an optical fiber pipe extending into the interior portion of the module from outside the module, the optical fiber pipe being arranged to receive an optical fiber and to position the optical fiber so that light emerging from the optical fiber impinges upon the photodetector; and a lid joined to the side wall member to hermetically enclose the interior portion of the module. 
     In an embodiment, the primary substrate is comprised of a ceramic material and the vias are comprised of a metallic material, such as a copper-tungsten alloy. In an embodiment, the secondary substrate is comprised of aluminum nitride. In an embodiment, the secondary substrate is placed in a cut-out region of the primary substrate and can carry circuit distribution lines. 
     In an embodiment, the module also includes an amplifier, such as a trans-impedance amplifier, mounted on the secondary substrate and electrically connected to the photodetector through circuit distribution lines on the secondary substrate. In an embodiment, the photodetector generates a current signal in response to light from the optical fiber impinging on the photodetector, and wherein the circuit distribution lines carry the current to the amplifier. 
     In an embodiment, the optical fiber pipe extends through the side wall member into the interior portion of the module from outside the module. In an embodiment, the lid comprises a ceramic material, and wherein the optical fiber pipe extends through the lid into the interior portion of the module from outside the module. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages of the invention will be readily appreciated from the following written description read in conjunction with the drawings, in which 
     FIG. 1 is a perspective view of a conventional butterfly module; 
     FIG. 2 is an end-on cut away elevation of the conventional module that uses glass-to-metal seals for electrical feedthroughs. 
     FIG. 3 is a side cut away elevation of a module for high frequency signals in accordance with an embodiment of the invention; 
     FIG. 4 is a side cut away elevation of a module for high frequency signals in accordance with an embodiment of the invention; and 
     FIG. 5 is a side cut away elevation of a module for high frequency signals in accordance with an embodiment of the invention; and 
     FIG. 6 is a cut away elevation of a module for high frequency signals in accordance with an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 3 is a side cut away elevation of a module for high frequency signals in accordance with another embodiment of the invention. The module  410  illustrated in FIG. 3 is a surface mount module were leads come from the bottom of the package with high-speed RF and other connections being made through electrically conducting vias in the ceramic substrate that forms the base of the module  410 . 
     Specifically, in FIG. 3, element  415  is a substrate. In one embodiment, the substrate  415  is a ceramic material, such as alumina. Numeral  420  designates an area where a via  420  passes through the substrate  415  to permit electrical connection to a series of components  422 ,  424 , and  426 . In one embodiment, the via is preferably made of a copper-tungsten alloy. 
     These components are all assembled on a secondary substrate  430 . In an embodiment, the secondary substrate is made of aluminum nitride. The secondary substrate  430  is placed in a cut-out region of the substrate  415  and carries the circuit distribution lines. This enables the assembly and testing of the components prior to “committing to the package,” i.e., prior to placing them in the module  410  and connecting them in. The secondary substrate  430  also provides for more efficient heat dissipation. 
     In another embodiment, the secondary substrate  430  is eliminated and all the devices and wirings are incorporated directly onto substrate  415  as shown in FIG.  6 . 
     The ceramic substrate may be manufactured in a variety of techniques known in the art. For example, general background information on substrate formation is available from U.S. Pat. No. 4,942,076, issued Jul. 17, 1990 and entitled “Ceramic Substrate With Metal Filled Via Holes For Hybrid Microcircuits And Method Of Making The Same” the disclosure of which is incorporated herein by reference. Such information is also available from U.S. Pat. No. 5,089,881, issued Feb. 18, 1992 and entitled “Fine-Pitch Chip Carrier”, the disclosure of which is also incorporated by reference. Also incorporated by reference is the disclosure of U.S. Pat. No. 5,707,575, issued Jan. 13, 1998 and entitled “Method For Filling Vias In Ceramic Substrates With Composite Metallic Paste.” 
     In the embodiment illustrated in FIG. 3, these components  422 ,  424 , and  426  are a photodetector  422 , a TIA  424 , and a limiting amplifier  426 , respectively. In one example, the photodetector  422  is a conventional PIN diode. Light falling on the photodetector  422  from an input fiber  470  with an angled end  475  causes the photodetector  422  to generate a microcurrent. This microcurrent is conducted to the TIA  424  which converts it to a voltage that is then conveyed by a wire on the substrate to the limiting amplifier  426  which produces the signal that is used as the output signal. 
     A lead frame  440  is positioned beneath the ceramic substrate  315 . Typically, the lead frame  440  is made of a metallic material. In one embodiment, it is made of an iron-based alloy with nickel and cobalt commercially known as Kovar (ASTM F15). Also, a lid  460  is provided over the enclosure  450 . In one embodiment, the enclosure  450  and the lid  460  are also made of Kovar. 
     The embodiment shown in FIG. 3 also includes an optical fiber  470 . The optical fiber  470  is brought in through a hole drilled in the enclosure  450 . A guide pipe  477  is placed in the hole and the optical fiber  470  is inserted through the guide pipe  477 . In another embodiment, this arrangement can be replaced with a ceramic lid with a hole to guide in the fiber. 
     The optical fiber  470  has an angled end  475 . Light propagating within the optical fiber  470  strikes the angled end  475  where it undergoes total internal reflection to exit the optical fiber  470  through the side and impinge upon the photodetector  422 . A support  480  supports the fiber  470 . In one embodiment, the support  480  may be made out of silicon and may be provided with a v-groove. 
     A plated, stepped metal lid  460  is then welded or soldered to the top of a side wall member of the enclosure  450  to hermetically enclose the module  410 . In another embodiment, a formed ceramic or plastic lid with a relief to include the fiber  470  can be used to enclose the assembly. 
     In one example such as that described, the vias  420  are made of tungsten copper and form the electrical connection between components external to the module  410  and the components inside the module  410 . Thus, the use of feedthroughs and glass-to-metal seals for insulating the feedthroughs from the module housing may be avoided. 
     In the embodiment employing an optical fiber  470  having an angled end  475 , light coming through the optical fiber  470  is totally internally reflected and impinges upon the photodetector  422 . In order for such an arrangement to work efficiently, it is necessary to maximize the amount of light which reaches the photodetector  422  from the fiber. This means that the optical fiber  470  must be positioned precisely above the photodetector  422 . It also means that the angled end of the fiber has to be aligned quite precisely rotationally. 
     Accordingly, the embodiment shown in FIG. 4 includes an optical fiber  570  held perpendicularly with respect to a detector  522 . More specifically, in the embodiment depicted in FIG. 4, a photodetector  522  is attached directly to a TIA  524 , using a known “flipchip” type of connection thereby eliminating bond wires, which can provide for cleaner signals. In another embodiment, the photodetector  522  can be mounted adjacent to the TIA  524 . In one example, the photodetector  522  is connected to the TIA  524  through wire or ribbon bonds. 
     Light pulses from the optical fiber  570  impinge directly on the photodetector  522  through a lens-shaped fiber  575 . The lens-shaped fiber tip  575  focuses the beam of light emerging from the optical fiber  570 . This can increase photon density and thus, signal strength. The cone-shaped or lens-shaped fiber tip  575  can reduce or eliminate the need for cumbersome rotational alignment of the optical fiber  570  with respect to the photodetector  522 . 
     In another embodiment, the end of the optical fiber  570  can be cleaved at a desired angle to focus the beam of light. 
     The embodiment shown in FIG. 4 can also incorporate a limiting amplifier within the module  510  if needed. The TIA  524  and limiting amplifier would be connected through transmission lines on the alumina substrate  530 . It is also possible to integrate the limiting amplifier into the TIA  524 . 
     Instead of a K-type connector, module  510  uses connections through vias  580  to convey signals to and from components within the module  510 . The signals and DC connections are brought to the leads through electrically conducting vias in the alumina substrate  530 . The leads, a side wall member  550 , and a fiber input pipe  590  are simultaneously brazed onto the alumina substrate  540  using a suitable alloy, for example, Cu—Ag. In an embodiment, the brazed module  510  is then plated with nickel and gold. 
     As in the previous embodiment, the TIA  524  and any limiting amplifier are mounted directly onto the secondary substrate  530  which is typically made of a ceramic material. The secondary substrate  530  acts as a heat spreader and, in an embodiment, can be made of aluminum nitride, Cu—W or Cu—Mo—Cu. The devices (photodetector  522 , TIA  524 , and any limiting amplifier) may be attached to the heat spreader/secondary substrate  530  using known attachment methods, such as epoxy or low temperature alloys. In this example, the alumina substrate  540  with the transmission lines  585  is attached to the lead frame  190  along with the side wall member  550  using a high temperature braze process. The device subassembly is then mounted vertically, preferably onto an inside wall of the module  510 , using known attachment methods such as screws, epoxy, or low temperature alloys. 
     In an embodiment, the optical fiber  570  is connected to the module  510  through a fiber input pipe  590  and aligned so that will focus on the photodetector  522  which, as mentioned, can be self standing or connected to the TIA  524  using a flip chip connection. A plated, stepped metal lid  560  is then welded or soldered to the top of the seal ring  550  to hermetically enclose the module  510 . In another embodiment, a formed ceramic or plastic lid with a relief to include the fiber  570  can be used to enclose the assembly. 
     FIG. 5 is a side cut away elevation of a module for high frequency signals in accordance with an embodiment of the invention. In the embodiment depicted in FIG. 5, a photodetector  660  is vertically mounted on a photodiode mount  670 . The photodiode mount  670  is connected to a TIA  650 , typically through wire or ribbon bonds. Light pulses from the optical fiber  680  impinge directly on the photodetector  522 , which can increase photon density and thus, signal strength. Optical fiber  680  is enclosed within fiber tube  690 . 
     The embodiment shown in FIG. 5 can also incorporate a limiting amplifier  640  within the module  610  if needed. The TIA  650  and limiting amplifier  640  would be connected through transmission lines on an alumina substrate  610 A. It is also possible to integrate the limiting amplifier  640  into the TIA  524 . 
     Instead of a K— type connector, module  510  uses connections through via  630  to convey signals to and from components within the module  610 . The signals and DC connections are brought to the leads through electrically conducting vias in the alumina substrate  610 A. The leads are brazed onto the alumina substrate  610 A using a suitable alloy, for example, Cu—Ag. In an embodiment, the brazed module is then plated with nickel and gold. 
     As in the previous embodiment, the TIA  650  and any limiting amplifier  640  are mounted directly onto the secondary substrate  620  through cavities in the secondary substrate  620 , which is typically made of a ceramic material. The secondary substrate  620  acts as a heat spreader and, in an embodiment, can be made of aluminum nitride, Cu—W or Cu—Mo—Cu. The devices (photodetector  660 , TIA  650 , and any limiting amplifier  640 ) may be attached to the heat spreader/secondary substrate  620  using known attachment methods, such as epoxy or low temperature alloys. In this example, the alumina substrate  610 A with the transmission lines  585  is also soldered or epoxy attached to the heat spreader. 
     In this embodiment, a formed ceramic lid  680 A is then welded or soldered to the top of the  630  via to hermetically enclose the module  610 . In another embodiment, the lid  680 A can comprise plastic or drawn from metal. Thus, a single part lid  680 A can enclose the assembly, rather than other coverings that might include metal Kovar rings or stepped lid enclosures. The lid can be attached using epoxy also. 
     In another embodiment, the secondary substrate  620  and the cavity in alumina substrate  610 A are eliminated. Instead, the transmission lines are placed on alumina substrate  610 A without the cavity and the devices (photodetector  660 , TIA  650  and the limiting amplifier  640 ) are placed directly over the alumina substrate  610 A or on a metal pedestal on top of alumina substrate  610 A, or a combination thereof. 
     A module of the present invention provides for a substantial size reduction better signals/noise ratio due to improve to optical efficiency as compared to conventional modules. Also, flip chip mounting the detector on the TIA provides a cleaner signal due to reduced electrical parasitics. 
     In addition, a module of the present invention allows devices within the substrate to be tested before committing to the module. Also, the embodiment of a module as that shown in FIG. 4 eliminates the need for rotational alignment of the fiber, thus facilitating assembly. Thus, a module according to the present invention is potentially scalable to 40 Gbps. 
     The invention has been described above the connection with specific embodiments for the purposes of illustration only. One of ordinary skill in the art will readily appreciate that the basic teachings of the invention can be embodied in other ways. Thus, the invention should not be considered to be limited to the specific embodiments disclosed herein, but instead should be considered to be fully commensurate in scope with the following claims.