Abstract:
A monolithic microwave integrated circuit structure having a semiconductor substrate structure with a plurality of active devices and a microwave transmission line having an input section, an output section and a interconnecting section electrically interconnecting the active devices on one surface and a metal layer on an opposite surface overlaying the interconnection section and absent from overlaying at least one of the input section and the output section.

Description:
TECHNICAL FIELD 
     This disclosure relates generally to monolithic microwave integrated circuits (MMICs) and more particularly to MMICs having heat sinks. 
     BACKGROUND AND SUMMARY 
     As is known in the art, monolithic microwave integrated circuits (MMICs) have a wide range of applications. Typically a plurality of active devices (e.g., field effect transistors (FETs)) are formed in a semiconductor substrate structure and the devices interconnected with microwave transmission lines, also formed on the substrate structure, to form, for example, a plurality of interconnected amplifiers. One type of microwave transmission line is coplanar waveguide (CPW) transmission line. 
     As is also known in the art, for certain high power applications the bottom side of the CPW MMIC is metalized so that a heat sink can be attached as shown in  FIG. 1 . We have found that this added metalized surface, in conjunction with the topside metal used for the ground plane of the CPW, forms a two-conductor, parallel plate, system that can support waveguide modes that generate feedback around the amplifiers interconnected by the CPW transmission line resulting in unwanted amplifier oscillation. If the resonance frequency associated with this mode falls within the frequency of operation of the amplifier circuit then it may hinder the proper operation of the circuit. Unwanted oscillation associated with this type of moding was experimentally verified as shown in  FIG. 2  with an unwanted oscillation at 9.187 GHz (i.e., an inherent resonance without an input signal) and was found to be a limiter for the proper operation of the circuit. In addition to the waveguide mode between the top and bottom metal surface of the MMIC, another type of moding we have found may occur if the MMIC is place in a flip-chip configuration ( FIG. 1 ) on a printed circuit board (PCB). In this case the two-conductor system is formed by the top metal surface of the MMIC and the ground plane on the PCB. This mode also may disrupt the proper circuit operation in the same way described above. 
     In one embodiment, a monolithic microwave integrated circuit structure is provided: a semiconductor substrate structure; a plurality of active devices formed in a bottom surface portion of the substrate structure; and a microwave transmission line formed on the bottom surface of the substrate structure having an input section, an output section and a interconnecting section electrically connected between the input section and the output section, such interconnecting section electrically interconnecting the active devices. The semiconductor substrate structure has: a first peripheral region disposed on the top surface thereof over the input section; a inner region disposed on the top surface thereof over the interconnecting section; and a second peripheral region disposed on the top surface thereof over the output section. A heat sink is disposed over the top surface of the substrate structure. A metal layer is disposed on the top surface of the substrate structure under the heat sink. The metal layer has an outer periphery terminating at the outer periphery of the heat sink. 
     In one embodiment, the microwave transmission line is coplanar waveguide transmission line. 
     In one embodiment, the monolithic microwave integrated circuit structure includes: a printed circuit board having: electrically conductors therein; electrically conductive bumps on an upper surface of the printed circuit board, such bumps being in electrical contact with the transmission line; and electrically conductive vias passing into the printed circuit board between the electrical conductors in the printed circuit board and the conductive bumps. 
     In one embodiment, a monolithic microwave integrated circuit structure is provided, comprising: a semiconductor substrate structure; a plurality of active devices formed in a bottom surface portion of the substrate structure; a microwave transmission line formed on the bottom surface of the substrate structure having an input section, an output section and a interconnecting section electrically connected between the input section and the output section, such interconnecting section electrically interconnecting the active devices. The semiconductor substrate structure has: a first peripheral region disposed on the top surface thereof over the input section; a inner region disposed on the top surface thereof over the interconnecting section; and a second peripheral region disposed on the top surface thereof over the output section. A thermally conductive heat sink is disposed over a top surface portion of the substrate structure, such heat sink being disposed over the interconnecting section and having an outer periphery thereof terminating at the first peripheral region and the second peripheral region of the top surface of the substrate structure. 
     In one embodiment, a metal layer is disposed on the top surface of the substrate structure under the heat sink and wherein the metal layer has an outer periphery terminating at the outer periphery of the heat sink. 
     The details of one or more embodiments of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the disclosure will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross sectional, diagrammatical sketch of a monolithic microwave integrated circuit (MMIC) structure according to the PRIOR ART; 
         FIG. 2  is a curve showing the output power vs. frequency of a power amplifier MMIC without an input signal according to the PRIOR ART; 
         FIG. 3  is an exploded, diagrammatical sketch of a monolithic microwave integrated circuit (MMIC) structure according to the disclosure; 
         FIG. 4  is a cross sectional, diagrammatical sketch of a monolithic microwave integrated circuit (MMIC) structure of  FIG. 3 ; 
         FIG. 5  is a plan view of a surface of the MMIC of  FIG. 4  looking along line  5 - 5  of  FIG. 4 ; and 
         FIG. 6  are curves showing resonance modes within the structure of  FIG. 4  for various dimensions&#39; of a metal layer and the heat sink on an MMIC of the structures are of  FIG. 4 ; one of the curves being for a metal layer shown in the PRIOR ART of  FIG. 1   
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     Referring now to  FIGS. 3 ,  4  and  5 , a monolithic microwave integrated circuit (MMIC) structure  10  is shown. The structure  10  includes an MMIC chip  12  mounted on the printed circuit board (PCB)  14  in flip-chip configuration. The MMIC structure  10  includes: a semiconductor substrate structure  12 , here for example, GaN having a plurality of active devices (e.g., transistors) formed in a bottom surface portion of the substrate structure  12 , here arranged as a plurality of, here, for example, three microwave amplifiers  16  ( FIG. 5 ); and a microwave transmission line  18 , here for example, a coplanar waveguide (CPW) microwave transmission line, formed on the bottom surface of the substrate structure  12 . As is known, the CPW has strip conductors  20  separated from a coplanar ground plane conductor  22  by portions of the semiconductor substrate  12 . The CPW has an input section  24 , an output section  28  and a interconnecting section  26  electrically connected between the input section and the output section. The interconnecting section electrically interconnects the active devices, here electrically interconnects the three microwave amplifiers  16 , as indicated in  FIG. 5 . 
     The semiconductor substrate structure  12  includes: a first peripheral region  30  disposed on the top surface thereof (prior to being flip-chip mounted to the PCB) over the input section  24 ; a inner region  32  disposed on the top surface thereof over the interconnecting section  28 ; and a second peripheral region  34  disposed on the top surface thereof over the output section  26 . 
     The MMIC structure  10  includes a thermally conductive heat sink  40  disposed over a top surface portion of the substrate structure  10 , such heat sink being disposed over the interconnecting section  26  and having an outer periphery thereof terminating at the first peripheral region  30  and the second peripheral region  34  of the top surface of the substrate structure  12 . The MMIC structure  10  includes a thermally conducing, here metal layer  42  disposed on the top surface of the substrate structure  12  under the heat sink  40 . The metal layer  42  has an outer periphery terminating at the outer periphery of the heat sink  40 . 
     It is noted that neither the heat sink  40  nor the metal layer  42  cover (i.e., are not disposed over) the input section  24  or the output section  28  of the transmission line  18 . 
     The MMIC structure  10  includes an under fill layer  50  of any suitable dielectric material having electrically conductive solder bumps  52  positioned as indicated to electrically connect the strip conductors  20  of the CPW transmissions line interconnecting the input and output sections  24 ,  28  of the CPW transmission line  18 . 
     The PCB  14  has vias  54  ( FIG. 4 ) passing from the upper surface thereof through the dielectric  55  of the PCB to electrical conductors  58  within the PCB  14 , such vias  54  being aligned to the solder bumps  52 , as indicated. The PCB has a ground plane conductor  60 . 
     To solve the moding thru the under-fill material  50 , ground bumps  59  in addition to the ground-signal-ground bumps  52  were added. These bumps connect the top metal of the MMIC to the top metal layer of the board. The additional bumps  59  connect the ground plane conductor  22  ( FIG. 5 ) of the MMIC  12  to the top metal layer  57  of the PCB board. The bumps  59  are placed strategically so that a) the solder bumps  52  do not interface with circuit operation b) where the fields associated with the mode is strong and c) symmetry is avoided. The results clearly show that the mode within the under-fill at 16.8 GHz is suppressed. The mode within the substrate is unchanged as expected. 
     With the structure  10  described above in connection with  FIGS. 3 ,  4  and  5 , because neither the heat sink  40  nor the metal layer  42  overlay (i.e., are not disposed over) the input section  24  or the output section  28  of the transmission line  18 , modes within the substrate were suppressed. It was observed that the field for this mode is strong in the input and output sections  24 ,  28  of the MMIC structure  10 . This was verified by simulation experiments using HFSS® 3D full-wave simulator.  FIG. 6  shows mode profile with different amount of back-side ground metal layer  42  being removed over the input and output sections  24 ,  28 . The results show that the isolation improves as the ground plane metal  24  is moved away from the input/output input and output sections  24 ,  28 ; more particularly: The curve labeled “FULLY METALIZED” is for the PRIOR ART structure of  FIG. 1  where the length “A” and the length “B” in  FIG. 4  are both zero (with a fully metalized case); the curve labeled “200 μm/1200 μm” are where the length “A” and the length “B” in  FIG. 4  are 200 μm and 1200 μm, respectively; the curve labeled “200 μm/1400 μm” are where the length “A” and the length “B” in  FIG. 4  are 200 μm and 1400 μm; and the curve labeled “670 μm/1400 μm” are where the length “A” and the length “B” in  FIG. 4  are 670 μm and 1400 μm, respectively. The results show a two-fold benefit: First, the reduction in coupling as the metal layer  42  and heat sink ground plane  40  is moves away from the input and output sections  24 ,  28 ; second, the reduction of the effective electrical length of the waveguide (the resonance frequency of the mode increases as a result). The combined effect is dramatic improvement of isolation within the frequency band of interest. The amount of recess is specific to a design and needs to be balanced between the isolation and heat-sinking requirement. 
     Detailed analysis of the structure  10  was conducted using a 3D full-wave EM solver. The frequency and the coupling via the waveguide modes were found to be strong functions of the following:
         1. Physical dimension (length, width) of the MMIC. The thickness of the semiconductor substrate  12  and the under-fill  50  affects the modes associated with each.   2. The relevant material properties (∈ r , σ) of the MMIC substrate  12  and the under-fill  50  material.   3. The dimension (length, width and gap) of the input and output sections  24 ,  26  CPW on the MMIC.       

     A number of embodiments of the disclosure have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. Accordingly, other embodiments are within the scope of the following claims.