Abstract:
A flexible antenna array comprises a plurality of layers of thin metal and a flexible insulating medium arranged as a sandwich of layers. Each layer of the sandwich is patterned as needed to define: (i) antenna segments patterned in one of the metal layers, (ii) an array of metallic top elements formed in a layer spaced from the the antenna segments, the array of metallic top elements being patterned in another metal layer, (iii) a metallic ground plane formed in a layer spaced from the array of metallic top elements, the metallic ground plane having been formed from still another metal layer, and (iv) inductive elements coupling each of the top elements in the array of metallic top elements with said ground plan. An array of remotely controlled switches are provided for coupling selected ones of said antenna segments together.

Description:
FIELD OF THE INVENTION 
     This invention relates to a low-cost packaging method which utilizes a commercially available High Density Multilayer Interconnect (HDMI or sometimes simply HDI) package and multichip interconnect for the integration of a novel 2-D reconfigurable antenna array with Radio Frequency (RF) Microelectromechanical (MEM) switches on top of a high impedance surface (High-Z Surface). 
     BACKGROUND OF THE INVENTION 
     The prior art includes U.S. Pat. No. 5,541,614 to Juan F. Lam, Gregory L. Tangonan, and Richard L. Abrams, “Smart antenna system using microelectromechanically tunable dipole antennas and photoic bandgap materials”. This patent shows how to use RF MEMS switches and photonic bandgap surfaces for reconfigurable dipoles. 
     The prior art also includes RF MEMS tunable dipoles ¼ wavelength above a metallic ground plane, but this approach results in limited bandwidth and is not suspectible to convenient packaging. 
     The prior art further includes a pending application of D. Sievenpiper and E. Yablonovitch, “Circuit and Method for Eliminatig Surface Currents on Metals” U.S. provisional patent application, Ser. No. 60/079,953, filed on Mar. 30, 1998 and corresponding PCT application PCT/US99/06884, published as WO99/50929 on Oct. 7, 1999 which disclose a high impedance surface (also called a Hi-Z surface herein). 
     The present invention takes advantage of proven, low-cost, high-density, multichip module (HDMI MCM-D) packaging. Such packaging is commercially available from Raytheon of El Segundo, Calif. under name/model number HDMI. FIG. 1 illustrates a cross-section of a prior art thin film copper/polyimide multilayer HDMI MCM-D integrated structure fabricated on a silicon substrate. As is known in the art, the fabrication process involves spin or curtain coating of ˜10-μm-thick polyimide dielectric layers and sputter deposition of ˜10-μm-thick copper conductor layers in an interactive process which includes phase mask laser formation of z-axis interconnect vias and metal patterning. Using comparable processes, more than 35,000 complex 2″×4″ MCM-D modules have been built and used in airborne radar, military and commercial satellites, and space projectiles to meet demanding weight and volume requirements, with no reported field failures. 
     The substrate for this package used in the present invention is preferably either glass, quartz or silicon (Si). A Hi-Z is also provided. The dielectric for the Hi-Z surface is a polyimide layer which may have been originally used for the packaging. The antenna is placed adjacent the Hi-Z surface, and the RF MEMS switches are used to reconfigure the antenna simply by changing the dipole&#39;s length. The feed structures for the antennas and dc lines are placed behind the Hi-Z Surface, so that they do not interfere with the radiation pattern of the antenna. The whole package is environmentally protected. 
     Preferably the Hi-Z surface utilized is a Hi-Z surface with added discrete inductors. 
     There is and has been a need for a packaged device of the type described above since it has a wide variety of applications in military and commercial communications requiring small reliable high performance antennas. One reason is that RF MEMS switches offer very low insertion loss (&lt;0.2 dB) and high isolation (&gt;35 dB) over a very broad frequency range from dc to 40 GHz. Furthermore, they consume very little power (i.e. less than 200 pJ per activation). The High-Z Surface allows the antenna to be very compact. Finally, since the antenna is reconfigurable by means of the RF MEM switches, it can be made to operate at different desired frequencies. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In general terms, the present invention provides, in one aspect thereof, a method of making a thin, flexible antenna. According to this aspect of the invention, a layer of a flexible insulating medium is deposited on a substrate and patterning the layer of insulating medium to form openings therein. Thereafter, metal layers are deposited on a previously deposited insulating layer and patterned as needed and layers of a flexible insulating medium are deposited on the previously deposited metal layer and patterned as needed, the layers of metal and layers of insulating medium forming form a multilayered high impedance surface having an upper surface with antenna segments having been patterned from a metal layer previously deposited thereat, an array of metallic top elements formed in a layer spaced from the upper surface, the array of metallic top elements having been patterned from a metal layer previously deposited thereat, a metallic ground plane formed in a layer spaced from the array of metallic top elements, the metallic ground plane having been formed from a metal layer previously deposited thereat, and inductive elements coupling each of the top elements in the array of metallic top elements with the ground plane, the inductive elements having been formed from one or more metal layers previously deposited. Then optically controlled switches are disposed adjacent at least selected ones of the antenna segments for coupling the adjacent antenna segments together in response to light impinging a photovoltaic cell associated each optically controlled switch. Optic fibers are arranged on or adjacent the high impedance surface with distal ends of each optic fiber being coupled to a respective one of the optically controlled switches for coupling light carried by the optic fibre to the photovoltaic cells associated with the optically controlled switch. The multilayered high impedance surface from the substrate, the substrate simply providing a support for making the thin, flexible antenna during manufacture. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional view through a thin film copper/polyimide multilayer HDMI MCM-D integration structure fabricated upon a silicon substrate; 
     FIG. 2 depicts a HDMI decal being peeled from a reuseable quartz carrier or substrate; 
     FIG. 3 a  is a cross sectional view of an HDMI reconfigurable antenna in accordance with the present invention; 
     FIG. 3 b  is a perspective view of the HDMI reconfigurable antenna of FIG. 3 a , with the polyimide layers and the dielectric top layer omitted for clarity&#39;s sake; and 
     FIG. 4 is a top view of an optically controlled MEMS switch. 
    
    
     DETAILED DESCRIPTION 
     These HDMI fabrication processes discussed above can be used to make thin, lightweight flexible reconfigurable antennas that can assume and therefor be placed on contoured surfaces, if desired. FIG. 2 shows a 24″×24″0.007″-thick flexible multi-layer HDMI interconnection structure being removed from the reusable carrier upon which it was fabricated. 
     FIG. 3 a  shows a cross-section the reconfigurable antenna of the present invention. The first  1 , second  2 , and third  3  HDMI layers are utilized to help define a Hi-Z surface  10  and preferably a Hi-Z surface with added discrete inductors  18 . Plated through metallic vias form a plurality of pairs of studs  14   a ,  14   b , each pair connecting each metallic top element  16  of Hi-Z surface formed on the third layer  3  to a ground plane  12  formed on the first layer  1 . A plurality of discrete inductors  18  are optionally formed on the third layer with each inductor  28  of the plurality being arranged in series with each pair of studs  14   a ,  14   b  to increase the bandwidth of the Hi-Z surface. Since the studs  14   a ,  14   b  of the Hi-Z surface have some inherent inductance associated with them, those practicing the present invention may decide not to use discrete inductors  18 , in which case layers  2  and  3  can then be combined into a single layer and the plurality of pairs of studs  14   a ,  14   b  would typically then be replaced by a plurality of single studs. 
     On the third layer  3 , the top elements  16  are closely arranged to capacitively couple them to neighboring elements  16 . As illustrated, antenna dipole segments  22  and RF MEMS switches  24  are disposed above the Hi-Z surface formed on layers  1 - 3 . Indeed, the antenna dipole elements  22  are preferably formed on a layer  1  which overlays the Hi-Z surface formed on layers  1 - 3 . The antenna dipole segment feed lines  23  are preferably arranged beneath the ground plane  12  on layer  4  and are connected by studs  25  formed by metal filled via holes through layers  1 - 4  to the dipole segments  22 . The RF MEM switches  24  are preferably optically controlled. Optically controlled RF MEMS switches  24  are equipped with photovoltaic cells  16  (FIG. 4) which provide an actuation voltage for an associated cantilevered arm  28  (FIG.  4 ). 
     FIG. 3 b  is a perspective view of the HDMI reconfigurable antenna of FIG. 3 a , with the polyimide layers  1 ,  2 ,  3 , and  4  and the dielectric top layer  36  omitted for clarity&#39;s sake. In this view the top elements  16  are shown in a two dimensional array disposed over the ground plane  12 . Each top element has an associated discrete inductor  18  in this embodiment. In some embodiments the discrete inductors  18  may be omitted since there may be sufficient inductive inherent in the other structures depicted. In that case, one of the mid layers  2  or  3  may also be omitted. The inductors  18  are depicted in FIG. 3 a  are preferably coil-shaped inductors. One of these coil-shaped inductors  18 ′ is depicted as if in a perspective view in order to depict its coil shape. Since the coil-shaped inductors  18  would normally occur on a single layer of the HDMI structure, the coil shaped inductors  18  in this cross section view of FIG. 3 a  would normally appear as a simple line (as they are so depicted for five of the six inductors  18  in this view). The top elements  16  are depicted as being hexagonal in plan view (see FIG. 3 b ). The top elements can be of any convenient shape, including circular, square, rectangular, rectilinear, etc. The feed line conductors  23  are depicted over each other in FIG. 3 a , but the number of layers needed for the HDMI structure can possibly be reduced by disposing these conductors adjacent to each other instead. 
     FIG. 4 is a top view of an optically controlled MEM switch  24 . The switch  24  has a photovoltaic cell  26 , a cantilevered arm or beam  28  which is connected at one end to a pivot point  34  and has at its other end a contact or actuation pad  35  which is pulled into contact with two dipole segments, here identified as  22 - 1  and  22 - 2 . Typically a number of dipole segments  22  are arranged axially of each other and the effective length of a dipole antenna  38  formed thereby is controlled by controlling the number of segments  22  connected together by closing appropriate ones of the switches  24 . 
     It is to be appreciated that typically a large number of parallel dipole antennas, with associated feeds  23 ,  25 , would preferably be disposed in the structure of FIGS. 3 a  and  3   b . Moreover, each arm of a dipole antenna would comprise a number of segments  22  and controlling the number of segments which are connected at a given time controls the frequency at which each dipole antenna  38  is resonant. In FIGS. 3 a  and  3   b  each arm of the dipole antenna  38  is shown with two segments  22  solely for ease of representation, it being understood that typically each arm would comprise many such segments  22  and associated switches  24  and moreover that the segments  22  may have different lengths. By appropriately controlling which switches  24  are closed, the resonant frequency of the associated dipole  38  is similarly controlled. 
     For a frequency of interest, the length of a arm of a dipole is typically equal to ¼ its wavelength while the size of each top element  16  is typically about {fraction (1/10)} its wavelength. The size of the top element is its diameter (if circular viewed from the top) or the length of one of its side (if square viewed from the top) or a similar measurement of size it the top element assumes some other shape than square or circular. Indeed, the preferred shape of a top element  16  is hexagonal when viewed from the top. 
     This HDMI packaging approach enables effective integration of reconfigurable antenna, high impedance surface, and RF MEMS switch technologies as a compact ultra-lightweight antenna. The mass of commercially available seven-conductor-layer HDMI interconnection decals is approximately 506 grams/m 2 , so individual antenna can be both small and light weight. 
     Making the Hi-Z HDMI devices disclosed herein involves providing layers  1 ,  2 ,  3 ,  4  of polyimide and layers of metal which are deposited sequentially. In FIG. 3 a  conductors  23  are shown immediately adjacent a release layer  41  supported by support surface  40  and thus they would be deposited first on the release layer  41 . The use of a release layer  41  is optional. The release layer  41  facilitates removed of the fabricated Hi-Z HDMI devices from the support surface  40  used to support the device during manufacture. The support surface  40  may be a quartz substrate, particularly if the Hi-Z HDMI devices are to be removed therefrom after fabrication. Alternatively, the support surface may be a substrate  40  which becomes a part of the finished Hi-Z HDMI device if no release layer  41  is used. 
     The first layer of polyimide  4  is deposited preferably as a liquid film which can be as thin as a few microns or even thinner. The polyimide is typically thermally hardened, after which it is patterned, for example by scanning across it with a laser beam through a phase mask. The phase mask is disposed in front of the surface and it determines the pattern which is left by the laser beam. The exposed parts of the polyimide are removed with an appropriate solvent. Holes are thus formed in the polyimide and those holes define where conductive vias will occur in the layer of polyimide to form the vertically arranged feed wires and studs  14   a ,  14   b ,  25 . Metal is then deposited by evaporation or by electroplating it, filling the holes in the polyimide to form metal metal vias therein. Each metal layer is patterned, as needed, to define either the ground plane  12 , the inductors  18  or the top elements  16  using suitable a suitable etchant. 
     After patterning, an etched metal layer is typically covered by another layer of polyimide which is exposed and patterned in the same way as the prior layer, with suitable locations for the vias being defined therein and followed by another metal layer which is patterned as needed. This process is repeated building up multiple layers of etched polyimide and etched metal until a major portion of the structure depicted in FIG. 3 a  is arrived at. Thereafter, the MEM switches  24  are installed to selectively connect segments  22 . The MEM switches  24  are preferably attached with a suitable adhesive, such as epoxy, and then their contacts are wire-bonded to the antenna segments  22 . 
     In the embodiment of FIG. 3 a , the RF MEM switches  24  are preferably optically triggered. Optically triggered MEM switches, such as the MEM switch  24  depicted by FIG. 4, include an integral photovoltaic cell  26  which generates a voltage in response to light, the voltage being effective to close the switch. In FIG. 4, the MEM switch includes an actuation pad  35  disposed at the end of switch&#39;s cantilevered beam  28  which pad  35  is effective to couple the two RF lines  22 - 1  and  22 - 2  to each order in response to light impinging on the photovoltaic cell  26 . Optically controlled MEM switches are further disclosed in U.S. patent application Ser. No. 09/429,234 filed Oct. 29, 1999 and entitled “Optically Controlled MEM Switch” which is assigned to the assignee of the present application. Optically controlled MEM switches can be coupled to optic fibers  30  (see FIG. 3 a ) using the techniques disclosed in U.S. patent application Ser. No. 09/648,689 filed Aug. 25, 2000 entitled “Optical Bond Wire Interconnections” which application is assigned to the assignee of the present application, by which inclined mirrored surfaces are formed to direct light from a wave guide or an optical fiber  30  into an optically controlled MEM switch  24 . The disclosures of U.S. patent application Ser. No. 09/429,234 filed Oct. 29, 1999 entitled “Optically Controlled MEM” and U.S. patent application Ser. No. 09/648,689 filed Aug. 25, 2000 entitled “Optical Bond Wire Interconnections” are hereby incorporated herein by this reference. 
     This HDMI packaging approach can be used to form optical channels within the HDMI polyimide to provide for the optical actuation of optically activated RF MEMS switches and/or photonic distribution of signals. Thus, when optically triggered RF MEM switches are used, the present invention allows for the direct optical mixing of microwave RF signals at the antenna elements. 
     Instead of using inclined mirrored surfaces of the type disclosed in the aforementioned U.S. patent application Ser. No. 09/648,689 filed Aug. 25, 2000 entitled “Optical Bond-Wire Interconnections”, prisms may be disposed above each optically triggered MEM switch  24  to couple light from an optical wave guide, such as one of the aforementioned optical fibers  30 , into an associated optically controlled MEM switch  24 . In any case, both the prism and the inclined mirrored surface provide a reflecting surface  32  for directing the light  31  carried by a wave guide or an optical fiber  30  in a direction essentially orthogonal to the major axis of the wave guide or optical fiber  30 . 
     The optical signals can be routed to the optically activated MEM switches using planar optical wave guides, which can be printed on a dielectric substrate  36 . See the co-pending U.S. patent application Ser. No. 09/648,689 filed Aug. 1, 2000 entitled “A Reconfigurable Antenna for Multiple Band, Beam-Switching Operation” the disclosure of which is hereby incorporated herein by reference. Such wave guides  30  would typically consist of linear channels of material having a higher index of refraction provided on a substrate  36  having a lower index of refraction. This structure, when placed over the optically activated MEM switches, would radiate light in a downward direction to the optically activated MEM switches through small prisms or inclined mirrored surfaces  32 , as shown by FIG. 3 a . If prisms are used, they can be formed as molded or ground shapes disposed on glass or other optically transparent material. The substrate  36  can be glass of a lower refractive index. One material which may prove satisfactory for substrate  36  is a flexible material sold under the tradename Silastic which is a silicone-like material manufactured by Corning Glass. 
     A corresponding reflecting surface  32  is disposed above each optically triggered MEM switch  24  to couple the light from a wave guide/optical fiber  30  into the photovoltaic cell  28  associated therewith. The dipole segments are typically longer than an individual cell of the high-impedance surface which is defined size-wise by a top element  16 . The number of MEM switches utilized with depend on the capabilities of the antenna. For simply switching frequencies, only a few MEM switches  24  would be needed—typically two for each frequency band needed for each dipole  38 . For phase tuning, many switches  24  would be typically utilized-two for each phase state needed for each dipole  38 . 
     The dielectric substrate  36  is preferably patterned or formed having cavities  37  formed therein to accommodate the MEM switches  22  and to help align the reflecting surfaces  32  at the ends of the fibre optic cables  30  with the MEM switches  22 . The final package is then preferably hermetically sealed in an air-tight package which is preferably filled with an inert gas  20  such as nitrogen, argon or sulfur hexafluroide. 
     HDMI processing is well known in the art of multilayer electronic packaging and therefore the details of the HDMI processing are not spelled out here. Raytheon in Dallas, Tex. is well known in the in this field. 
     Having described the invention in connection with certain embodiments thereof, modification will now certainly suggest itself to those skilled in the art. As such, the invention is not to be limited to the disclosed embodiments except as required by the appended claims.