Abstract:
A hybrid circuit phase shifter assembly of RF MEMS switch modules and passive phase delay shifter circuits uses a low loss, preferably flip-chip, interconnection technology. The hybrid circuit assembly approach separates the fabrication of the MEMS switch modules from the fabrication of the passive phase delay circuits thereby avoiding process incompatibilities and low yields and providing substantial production cost savings. In another aspect of the invention, the integration on a common substrate of a MEMS-based hybrid circuit phase shifter assembly behind each of a plurality of radiating elements provides a compact, low cost electronic scanning antenna array.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates generally to phase shifters utilized, for example, in electronically scanned phase array antennas, and particularly to phase shifter circuits incorporating low loss, RF microelectromechanical (MEMS) switches. 
   2. Description of the Related Art 
   The beam of a multiple element or array antenna may be propagated at a predetermined angle by inserting an appropriate phase shift in the radiated signal at each element of the array. 
     FIG. 1  is a simplified diagram of one row of a conventional phased array antenna  10  utilizing electronic beam steering, a complete planar phased array antenna having a number of such rows. The antenna  10  includes a plurality of radiating elements  12  each of which has its own phase shifter  14 . An input line  16  carrying a transmission signal is coupled to each phase shifter  14 , which imparts a respective predetermined phase shift (φ,  2 φ,  3 φ and  4 φ, respectively) to the transmission signal as it passes through that phase shifter. The phase shifted transmission signals are then coupled to respective radiating elements  12  for propagation of the beam. Various types of phase shifters  14  have been developed, including switched-line phase shifters, reflection-line phase shifters and loaded-line phase shifters. 
   An example of switched-line phase shifters is the true time delay (TTD) phase shifter circuit in which rapid phase changes for electronically scanning the beam are obtained by selectively inserting and removing discrete lengths of transmission lines by means of high speed electronic switches. For example, with a cascaded switch arrangement, a relatively small number of preselected transmission line lengths can be series-connected in various combinations to provide a substantial number of discrete delays. Thus, a cascaded four-bit switched phase shifter can insert sixteen different phase shift levels into the propagated signal. 
   By virtue of their superior isolation and insertion loss properties, RF MEMS switches are advantageous for implementing high performance, electronically scanned antennas. However, conventional MEMS-based TTD phase shifters employ monolithic architectures that present processing compatibility, cost and packaging problems. For example, although most of the monolithic die area simply comprises easily fabricated passive metal delay lines, a monolithic architecture requires processing of the entire phase shifter circuit through a series of complex, multi-level MEMS switch fabrication steps. This not only results in low yields and high product costs, but as a result of incompatibilities between the delay line and MEMS switch fabrication processes, also restricts the materials that can be used. 
   SUMMARY OF THE INVENTION 
   Broadly, the invention provides a hybrid circuit assembly of RF MEMS switch modules and passive phase delay shifter circuits using a low loss, preferably flip-chip, interconnection technology. This hybrid circuit assembly approach separates the fabrication of the MEMS switch modules from the fabrication of the passive phase delay circuits thereby avoiding process incompatibilities and low yields and providing substantial production cost savings. 
   As is known, unlike assembly techniques that rely on bonding wires or beam leads to patterns outside of the die&#39;s perimeter, flip-chip technology employs direct electrical connections between termination pads on a die face and on the substrate. These short interconnecting conductor lengths reduce losses, optimize circuit performance and permit more efficient use of the substrate area. 
   The flip-chip interconnection preferably comprises solder bumps at all of the die-bonding pad locations which are terminated simultaneously by a controlled reflow soldering operation. Alternatively, instead of solder bumps, the interconnects may comprise indium columns, plated-through holes, metal-to-metal thermocompression bonds, conductive polymers, and the like. 
   In another aspect of the invention, the integration on a common substrate of the above-described MEMS-based phase shifter circuit behind each of a plurality of radiating elements provides a compact, low cost electronic scanning antenna array. The benefits of the invention include low insertion and return losses, low power consumption, broad bandwidth and ease of integration into higher assemblies. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description when taken together with the accompanying drawings, in which: 
       FIG. 1  is a schematic representation of a conventional phased array electronic scanning antenna; 
       FIG. 2  is a schematic of one specific example of a passive phase shifter circuit that may be used in the present invention; 
       FIG. 3  is a schematic of one specific embodiment of a hybrid circuit assembly in accordance with the invention; 
       FIG. 4  is a schematic, side elevation view, partly in cross section, of the hybrid assembly of  FIG. 3  as seen along the line  4 — 4  in  FIG. 3 ; 
       FIG. 5  is a schematic of an integrated phased array electronic scanning antenna in accordance with another aspect of the present invention; and 
       FIG. 6  is a more detailed representation of the integrated electronic scanning antenna of FIG.  5 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A preferred embodiment of the present invention comprises a phased array antenna phase shifter with one or more stages, each stage comprising two or more passive phase delay circuits and utilizing switched selection of the delay circuits at each stage. The phase shifter of the invention uses low loss RF MEMS switches for selecting the desired delay circuit(s) within each stage. While a preferred embodiment described in detail herein incorporates TTD switched-line phase shifter architecture, the application of this invention to other phase shifter architectures incorporating other kinds of passive elements (such as capacitors and inductors) will be apparent to those skilled in the art. 
   A preferred embodiment shown in  FIGS. 2 and 3  comprises a hybrid phase shifter assembly  20  including a 2-bit digital delay line module  22  carrying a pair of flip-chip MEMS switch modules  24  and  26  (see  FIG. 3  ). As best seen in  FIG. 2 , the digital delay line module  22  comprises a base substrate  28  fabricated of an insulating material such as alumina, quartz, or a microwave ceramic, or a semi-insulating material such as high-resistivity silicon or GaAs. Patterned on a surface  30  of the substrate  28  are a pair of serially connected delay line stages  32  and  34  for inserting a cumulative time delay in a transmission signal, “IN” (generally the base carrier frequency of the antenna) appearing on an input line  36  coupled to the first delay line stage  32 . More stages may be used so as to provide higher beam steering resolution. 
   The first time delay stage  32  comprises two planar strip delay lines  40  and  42  patterned on the base substrate  28 . The delay line  40  has a pair of terminal pads  44  and  46 ; similarly, the delay line  42  has terminal pads  48  and  50 . The two delay lines  40  and  42  have different lengths thereby imparting different time delays to the transmission signal. The delay line  42  may interpose a reference time delay that may, for example, be substantially zero. The time delay is equivalent to the time it takes the transmission signal to transit one of the two delay lines  40  and  42  and the longer the delay line, the greater the time delay. The phase of the transmission signal is shifted in proportion to the time delay. 
   Like the first time delay stage  32 , the second time delay stage  34  comprises two delay lines  52  and  54  patterned on the base substrate  28 . The delay line  52  includes a pair of terminal pads  56  and  58 ; similarly, the delay line  54  has a pair of terminal pads  60  and  62 . In the example shown, the delay line  52  of the second stage  34  is longer than the delay line  40  of the first stage  32  while the second delay line  54  may have the same length as the delay line  42  so as to provide an identical reference time delay. 
   With reference to  FIG. 3 , one of the two delay lines  40 ,  42  in the first time delay stage  32  is activated by closing two of four MEMS input and output switches  70 - 73  to connect the selected delay line into the overall phase shifter. The input switch  70  is operable to electrically connect an input line terminal pad  76  with the terminal pad  44  of the delay line  40 ; input switch  71  electrically connects an input line terminal  78  with the pad  48  of the delay line  42 ; similarly, output switches  72  and  73  are operable to connect the terminal pads  46  and  50  with stage output terminal pads  80  and  82 , respectively. The stage output terminal pads  80  and  82  are coupled to a line  84  that interconnects the delay line stages  32  and  34 . 
   In the second time delay stage  34 , additional phase shift may be imparted to the transmission signal in the same manner as in the first time delay stage  32  by closing respective input and output switches within the second stage MEMS switch module  26 . After passing through the second time delay stage  34 , the phase-shifted signal, “OUT”, appears on an output line  86  and from there may be passed through additional time delay stages (not shown) where, for higher resolution, still additional phase shifts can be inserted by closing selected MEMS switches in the same manner as in the two previous time delay stages. 
   The RF MEMS modules  24  and  26  contain switches that are preferably of the metal-to-metal contact switches of the type disclosed, for example, in U.S. Pat. No. 5,578,976 owned by the assignee of the present invention; the &#39;976 patent is incorporated herein by reference for its teachings of the structure of such switches and methods for their fabrication. It will be evident that other MEMS switch types may be used instead. 
   A simplified cross-section of a portion of the MEMS module  24  showing switch  70  in greater detail is depicted in FIG.  4 . It will be understood that the module  24  merely typifies the MEMS modules that may be used in the invention. The switches carried by the MEMS module  24  are formed on a substrate  90  using generally known microfabrication techniques such as bulk micromachining or surface micromachining. While  FIG. 4  illustrates an example in which the MEMS module  24  contains four separate switches, it will be understood by those skilled in the art that MEMS module configurations containing one or more switches may be used. 
   Formed on an upper surface of the MEMS substrate  90  are a pair of spaced-apart, fixed metallic contacts  92  and  94  in vertical alignment with the terminal pads  44  and  76 , respectively, formed on the base substrate. The MEMS module  24  and base substrate  28  comprise a flip-chip assembly. More specifically, the contacts  92  and  94  are electrically connected to the terminal pads  44  and  76  on the base substrate by vias  96  and  98  extending through the MEMS substrate  90  and by electrical flip-chip interconnects  100  and  102  on the underside of the substrate. Although the interconnects  100  and  102  preferably comprise solder bumps, other low loss flip-chip interconnection techniques may be used, including but not limited to indium columns, plated-through holes, metal-to-metal thermocompression bonds, conductive polymer bonds, and so forth. Positioned above the fixed contacts  92  and  94  and spanning the gap therebetween is a vertically movable arm  104  carrying a metallic bridging contact  106  on a bottom surface thereof. The arm  104  may comprise a cantilevered structure of the kind that is well known in the MEMS switch art and that is typically formed of an insulating material such as silicon dioxide or silicon nitride. The movable contact  106  provides electrical continuity between the fixed contacts  92  and  94  (and hence the terminal pads  44  and  76 ) when the switch is actuated. While the MEMS switch  70  illustrated is of the ohmic contact type providing an electrically conductive path upon closure, the invention can also be implemented using capacitive switches that couple the signal through a thin insulating layer upon closure. For simplicity, the movable contact  106  is shown in  FIG. 4  directly bridging the gap between stationary contacts  92  and  94 . In an actual structure, surface conductors may be used to permit arbitrary location of the contact  92  relative to the via  96 . Further, while  FIG. 4  illustrates a face-up configuration in which the MEMS switch is on the top surface of the MEMS substrate  90  and is interconnected using through-substrate vias, the invention also encompasses face-down hybrid integration of the switch module  24  and the substrate  28 . Face-down hybrid integration obviates the need for through-substrate conductive paths such as the vias  96  and  98 . 
   The MEMS switch  70  is actuated when an appropriate stimulus is provided. For example, for an electrostatically actuated MEMS switch a drive voltage is applied between the movable and fixed contacts. The drive voltage creates an electrostatic force that attracts the movable contact  106  into engagement with the fixed contacts  92  and  94  thereby bridging the gap between the fixed contacts and providing an electrically conductive path between the contacts and hence the terminal pads  44  and  76  on the base substrate. Other switch actuation techniques may be used, including without limitation, thermal, piezoelectric, electromagnetic, gas bubble, Lorentz force, surface tension, or combinations of these. The present invention may employ MEMS switches operated by any of these methods or others known to those skilled in the art. 
     FIGS. 5 and 6  show an integrated electronic scanning array antenna  110  implementation (see  FIG. 5  ) incorporating multiple phase shifters in accordance with the present invention.  FIGS. 5 and 6  show a single package  112  (see FIG.  5 ) integrating four hybrid phase shifter assemblies  114 ,  115 ,  116  and  117  feeding time-delayed signals (φ,  2 φ,  3 φ and  4 φ, respectively, in  FIG. 5 ) to corresponding antenna elements or radiators  118 ,  119 ,  120  and  121 . The package may be hermetically sealed by a single lid or cover  122  ( FIG. 6 ) whose seal footprint does not intercept any of the elements patterned on the base substrate. Although  FIGS. 5 and 6  show four hybrid assembly phase shifters in a single package, it will evident that any number of phase shifters may be employed within a package. 
   The package of  FIGS. 5 and 6  comprises a common base substrate  124  of an insulating material such as alumina, quartz, or a microwave ceramic, or a semi-insulating material such as high resistivity silicon or GaAs. As is known in the art, the base substrate  124  may be a multi-layer microwave material with embedded conductors. The antenna elements or radiators  118 - 121  are printed onto a surface  126  of the substrate  124  or formed using an interior metal layer in a multi-layer substrate along with TTD phase shift circuit elements of the kind already described. The monolithic integration of the radiator elements and phase shifters permits compact circuit geometries and permits high physical tolerances between the phase shifter and radiator. In the example depicted, each of the four phase shifters  114 - 117  comprises a 3-bit shifter each including RF MEMS switch modules that, as already described, are coupled to the phase shifter circuit elements on the substrate by means of low loss interconnections preferably employing flip-chip technology. 
   While several illustrative embodiments of the invention have been disclosed herein, still further variations and alternative embodiments will occur to those skilled in the art. Such variations and alternative embodiments are contemplated, and can be made without departing from the spirit and scope of the invention as defined in the appended claims.