Abstract:
A microwave microstrip/waveguide transition structure includes a substrate, an elongated microstrip layer residing on a surface of the substrate, and an elongated integral hollow waveguide on the surface of the substrate. The microstrip layer and a side of the hollow waveguide constitute a single continuous piece of metal. The transition structure is fabricated by providing a substrate, depositing a metallic layer on the substrate, and depositing a metallic hollow housing continuous with a portion of a length of the metallic layer. The metallic hollow waveguide bounded by the metallic layer and the metallic hollow housing and having a contained volume therewithin is thereby defined.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to microwave devices, and, more particularly, to a transition structure between a microstrip and a waveguide. 
     Microwaves are high-frequency electromagnetic signals that typically have frequencies in the 0.9-120 GHz (gigahertz) range. Microwaves may be propagated in several ways, including through free space and in or along confined carriers. Examples of confined carriers are solid metallic conductors and hollow waveguides. A microwave is propagated along the surface of a solid metallic conductor. A microwave is propagated through space but within a confined volume in a waveguide. 
     The selection of the best propagation path of the microwave involves a variety of considerations. However, in many microwave systems it is necessary to perform transitions of the propagation path. For example, antennas are used to receive or send microwave signals through free space and, thence, perform the transition to or from the confined carrier. In other cases such as within microwave amplifiers or other electronic signal processing equipment, the propagation of microwave signals must undergo transitions between solid conductors and waveguides. 
     Microwave transitions between solid conductors and waveguides (either solid conductor-to-waveguide or waveguide-to-solid conductor) have historically been accomplished with a physical interpenetration of the two. For example, a solid conductor may penetrate into the interior of a waveguide perpendicular to the direction of propagation of the microwave within the waveguide. 
     For many microwave systems, such as communications satellites, it is important to reduce the size and weight of microwave systems. Microwave systems with small solid conductors, termed microstrips or striplines, have been developed to produce microwave circuitry in planar configurations and to reduce the size of the microwave electronic circuitry to a size approaching that of microelectronic devices operating at conventional frequencies. The configuring of microstrip/waveguide transitions is more difficult in microwave circuitry of this type. 
     Microwave processing circuitry and microstrip/waveguide transitions have been integrated into “micromachined” devices such as those disclosed in U.S. Pat. No. 5,608,263. The micromachined architecture, while operable, offers opportunities for improvement. These existing microelectronic transition structures are difficult to handle and are not conducive to the production of large numbers of identical devices by batch processing. They require considerable care in the alignment of matching structures. 
     There is a need for an improved approach to the fabrication of a microstrip/waveguide transition structure that overcomes the drawbacks of the existing devices, and still permits the incorporation of circuitry for microwave signal processing. The present invention fulfills this need and provides additional related advantages. 
     SUMMARY OF THE INVENTION 
     The present approach provides a microwave microstrip/waveguide transition structure and a method for making such a structure. The transition structure permits active or passive microwave devices to be incorporated into the transition structure. The microwave device is substantially planar, except for the necessary thickness to accommodate the waveguide. The fabrication technique is fully compatible with microelectronic fabrication technology and permits the use of batch processing techniques. No alignment of separate subassemblies is required. 
     In accordance with the invention, a microwave microstrip/waveguide transition structure comprises a substrate, an elongated microstrip layer residing on a surface of the substrate, and an elongated integral hollow waveguide having a side, the waveguide residing on the surface of the substrate. The microstrip layer and the side of the hollow waveguide comprise a single continuous piece of metal, which may be elongated in a common direction. 
     One embodiment of the microwave microstrip/waveguide transition structure may also be described as comprising a single substrate, a microstrip layer residing on a surface of the single substrate, and an integral hollow waveguide residing on the surface of the single substrate. The microstrip layer and the hollow waveguide comprise a single continuous piece of metal and are each elongated in a common direction. 
     In any of these embodiments, an electronic device may be affixed to the substrate and/or disposed within the interior of the waveguide. The waveguide is normally rectangular in cross section, but may be of any operable shape. One side of the waveguide contacts the substrate, and is contiguous with the microstrip layer. The microstrip layer may be of any operable thickness and width, and the width typically increases from a small value remote from the waveguide to the width of the contiguous waveguide wall as the microstrip layer transitions into the waveguide wall. 
     The materials of construction may be selected from many different operable materials. The substrate may be, for example, a ceramic or a glass. The microstrip layer and waveguide may be made of metals such as titanium-tungsten plated with gold, chromium plated with gold, or chromium-copper plated with gold. 
     A method of making a microwave microstrip/waveguide transition structure comprises the steps of providing a substrate, depositing a metallic layer on the substrate, and depositing a metallic hollow housing continuous with a portion of a length of the metallic layer, thereby defining a metallic hollow waveguide bounded by the metallic layer and the metallic hollow housing and having a contained volume therewithin. 
     The waveguide is desirably formed integral with the microstrip by depositing a layer of metal over the substrate, and then a patterned layer of photoresist material overlying a portion of the length of metal. Additional metal deposited over the photoresist forms a three-dimensional metallic structure, overlying and enclosing the photoresist core. Openings are made through the metallic structure, to permit the photoresist to be removed thermally, chemically, or otherwise. The result is the hollow, precisely dimensioned waveguide continuous with the microstrip. The transition is accomplished along the length of the transition structure. If desired, microwave processing devices may also be deposited on the substrate, either inside or outside of the interior of the waveguide, in an appropriate sequence with the formation of the hollow waveguide. 
     The microwave microstrip/waveguide transition structure of the invention thus uses a single structure to accomplish the transition in a planar, lightweight configuration. It is not required to fabricate separate parts and then register and attach the parts together, which is often difficult when the parts are very small. Large numbers of the transition structures may be fabricated in batch-processing operations. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic plan view of a microstrip/waveguide transition structure; 
     FIG. 2 is a schematic sectional view of the transition structure of FIG. 1, taken along line  2 — 2 ; 
     FIG. 3 is a schematic sectional view of the transition structure of FIG. 1, taken along line  3 — 3 ; 
     FIG. 4 is a schematic sectional view of the transition structure of FIG. 1, taken along line  4 — 4 ; 
     FIG. 5 is a block flow diagram of a fabrication method according to the invention; and 
     FIG. 6 is a schematic structural flow diagram illustrating the development of the structures at sections  2 — 2 ,  3 — 3 , and  4 — 4  of FIG. 1 during the fabrication sequence of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1-4 illustrate a microwave microstrip/waveguide transition structure  20 , which permits the transition of microwave signals from a microstrip  22  to a waveguide  24 , or from the waveguide  24  to the microstrip  22 . The microstrip  22  is an elongated strip of metal, and the waveguide  24  is an elongated hollow structure, formed of metal walls  26 . The metal walls  26  define a hollow volume  28 . The microstrip  22  and the waveguide  24  are each elongated, preferably but not necessarily parallel to a common direction of elongation  30 . 
     FIGS. 2-4 show the structure of the transition structure  20  at sections  2 — 2 ,  3 — 3 , and  4 — 4 , respectively, of FIG.  1 . Section  2 — 2  is taken at a location where the microwave signal is propagated through the microstrip  22 . Section  4 — 4  is taken at a location where the microwave signal is propagated through the waveguide  24 . Section  3 — 3  is taken at an intermediate location where the microstrip  22  and the waveguide  24  meld together in a contiguous and continuous fashion so that the microstrip  22  and the waveguide  24  are integral with each other. In each of FIGS. 2-4, arrows represent electrical field (E-field) vectors associated with the conductor of the microwave at the respective sections. 
     The transition structure  20  includes a substrate  32  upon which the microstrip  22  and the waveguide  24  reside. The substrate  32  is desirably a ceramic such as aluminum oxide, or a glass. In a typical case, the substrate  32  is about 0.010 inch thick, and of sufficient lateral and length extent to accommodate the transition structure. There is a single substrate  32 , in contrast to the structure illustrated in U.S. Pat. No. 5,608,263, which requires two overlying substrates that are fabricated separately and must be superimposed in registry during assembly of the structure. 
     In the all-microstrip region illustrated in FIG. 2, the metallic layer which forms the microstrip  22  lies on and contacts a top side  34  of the substrate  32 . A metallic ground plane  36  lies on an oppositely disposed bottom side  38  of the substrate  32 . The metal of the metallic microstrip  22  may be of any operable type, and is preferably an alloy of 10 weight percent titanium-90 weight percent tungsten alloy with an overlying gold protective layer (“TiW—Au”). Other metals such as chromium metal with a gold coating (“Cr—Au”) or chromium-copper metal with a gold coating (“CrCu—Au”) may be used for the microstrip  22 . 
     In the all-waveguide region illustrated in FIG. 4, the metallic layer is widened to define a bottom wall  40 . The microstrip  22  and the bottom wall  40  are continuous, and both reside on and contact the substrate  32 . Side walls  42  and a top wall  44  are provided. The bottom wall  40 , the side walls  42 , and the top wall  44  are integral, and together define the closed hollow volume  28 . The substrate  32  is not within this volume  28 , although the substrate  32  is contained within a separate and adjacent volume defined by the ground plane  36 , the side walls  42 , and the bottom wall  40 . 
     In the intermediate region illustrated in FIG. 3, the microstrip  22  lies on the substrate  32  and is present as a separate entity not yet joined to the side walls  42 . The side walls  42 , the top wall  44 , and the ground plane  36  are all present and define a hollow volume in which the substrate  32  and the microstrip  22  are contained. 
     An inspection of FIGS. 2-4 shows the continuous progression from the microstrip-only region of FIG. 2, through the intermediate region of FIG. 3 which is neither purely microstrip nor purely waveguide, to the waveguide-only region of FIG.  4 . 
     Other structures and/or devices may optionally be affixed to the substrate  32  as part of the transition structure  20 . FIG. 2 illustrates an “exterior” device  46  affixed to a surface of the substrate  32  so as to be exteriorly visible. FIG. 4 illustrates an “interior” device  48  fixed to the substrate  32  through the bottom wall  40 , which is not exteriorly visible. The devices  46  and  48  may be any operable type of active or passive signal processing device, such as a signal amplifier for example. The structures of such devices are known in the art. They are typically deposited onto the substrate  32  by microelectronic techniques at appropriate stages of the fabrication of the transition structure  20 . 
     FIG. 5 is a block flow diagram of a preferred approach for fabricating the transition structure  20 . FIG. 6 is a pictorial flow diagram for each of the three sections  2 — 2 ,  3 — 3 , and  4 — 4 , whose structures are developed in parallel. The corresponding structures are indicated in FIG. 6 which are associated with the various process steps in FIG.  5 . 
     The substrate  32  is provided, numeral  60 . The substrate  32  is a piece of an operable electrical nonconductor such as a ceramic or a glass, typically from about 0.01 inch to about 0.025 inch thick and sufficiently large to receive the subsequently deposited elements thereon. 
     A bottom metallization  90  is deposited on the top side  34  of the substrate  32 , numeral  62 . (It is termed a “bottom metallization” because it eventually forms the bottom of the waveguide  24 .) The bottom metallization  90  is a metal such as an alloy of titanium and tungsten, preferably having a composition of 10 weight percent titanium-90 weight percent tungsten, with a gold coating (“TiW—Au”). The bottom metallization  90  is preferably from about 1 micrometer to about 2 micrometers thick. Other metals such as chromium metal with a gold coating (“Cr—Au”) or chromium-copper metal with a gold coating (“CrCu—Au”) may be used for the bottom metallization. The bottom metallization  90  is deposited by any operable technique. It is preferably deposited by sputtering or electroplating, but other techniques may also be used. Preferably, in the same process step  62  the ground plane  36  is deposited on the opposite bottom side  38  of the substrate  32 . The ground plane  36  is preferably the same material and the same thickness as the bottom metallization  90 , and is deposited by the same technique. The bottom metallization  90  and the ground plane  36  are preferably deposited over substantially the entire top side  34  and bottom side  38  of the substrate  32 , respectively. 
     The bottom metallization is patterned, numeral  64 . The patterning accomplishes a progressive narrowing of the bottom metallization  90 , to form what ultimately becomes the microstrip  22  in section  2 — 2 , the transition microstrip  22  in section  3 — 3 , and the bottom wall  40  in section  4 — 4 . The patterning is accomplished by conventional photolithography and etching using any operable procedures. 
     Any interior device  48  that is to be within the interior of the waveguide  24  in the final transition structure  20  is optionally deposited overlying the bottom wall  40 , numeral  66 . The interior device  48 , if any, is deposited using any technique that is appropriate to the nature of the interior device  48 . The interior device  48  is not shown in the subsequent portions of FIG. 6 for clarity and because its presence is optional. 
     A thick photoresist  92  is applied over the elements previously deposited on the top side  34  of the substrate  32 , numeral  68 , typically by spin coating. The thick photoresist may be any viscous positive photoresist, for example SJR 5740. The photoresist  92  defines the interior height of the hollow volume  28  in the final transition structure  20 , and its thickness is selected accordingly. 
     The thick photoresist  92  is patterned, numeral  70 , using conventional photolithography and development techniques, as required by the selected photoresist. The photoresist is removed in the area of the pure microstrip, section  2 — 2 . The remaining photoresist in sections  3 — 3  and  4 — 4  defines the lateral position of subsequently deposited side walls and is patterned accordingly. The height and width of the hollow volume  28  is typically selected according to the wavelength of the microwaves that are to be transmitted, according to principles known in the art. For example, to transmit a microwave of a frequency of 110 GHz, the hollow volume  28  typically has an interior width of about 0.100 inch and an interior height of about 0.050 inch. 
     A top metallization  94  is deposited, numeral  72 . The top metallization serves to make the exposed portion of the microstrip  22  thicker in section  2 — 2 . However, in sections  3 — 3  and  4 — 4 , where the thick photoresist  92  is present, the top metallization  94  defines the side walls  42  and the top wall  44  of the waveguide. The top metallization  94  is preferably but not necessarily the same material used to deposit the bottom metallization  90 , and the same deposition technique may be used. 
     The top metallization  94  is patterned by conventional photolithography and etching, numeral  74 . The patterning removes excess top metallization that would produce electrical shorts between the waveguide  24  and the ground plane  36  and other structure in the final transition structure  20 . 
     At this point, the microstrip  22  has been formed in section  2 — 2 . The waveguide  24  has also been formed in section  4 — 4  continuous with the microstrip  22  through the intermediate structure of section  3 — 3 . The bottom wall  40 , side walls  42 , and top wall  44  are continuous metallic structures forming the waveguide  24 . There remains, however, the problem that the interior of the waveguide  24  is filled with the thick photoresist  92 , which must be removed to permit the waveguide to function. 
     The photoresist  92  is removed by forming a pattern of small openings  96  through the wall of the waveguide  24 , numeral  76 . The openings  96  are preferably formed in the top wall  44  of the waveguide  24 . The openings  96  are desirably much smaller in lateral extent than the wavelength of the microwave signals that are to be propagated using the transition structure  22 . The openings  96  are conveniently formed by patterning the top wall  44  using conventional photolithography and etching techniques. 
     The thick photoresist within the interior of the walls  26  is thereby exposed, and may be removed by any operable technique such as chemical dissolution (i.e., wet etching) or dry plasma etching, depending upon the nature of the thick photoresist. 
     The openings  96 , which are much smaller than the wavelength of the microwave signals, do not interfere with the propagation of the microwave signals. The openings  96  are therefore allowed to remain in the final transition structure  20 . Optionally, they could be closed off if desired. 
     Any exterior device  46  that is to be outside of the interior of the waveguide  24  in the final transition structure  20  is optionally deposited overlying the substrate  32 , numeral  78 . The exterior device  46 , if any, is deposited using any technique that is appropriate to the nature of the exterior device  46 . 
     Although a particular embodiment of the invention has been described in detail for purposes of illustration, various modifications and enhancements may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the appended claims.