Miniature microwave guide
BACKGROUND OF THE INVENTION 
     This invention pertains to a miniature microwave guide for printed circuit boards and more particularly to a guide imbedded in a board and a method of making the same. 
     Customarily, microwaves are transmitted between two devices by using metallic waveguides. The cross-sectional dimensions of the waveguides depend on the frequency of the waves and the desired transmission modes. However the waveguides required to transmit ultra high frequency signals (such as signals in the giga-hertz range) have very small wavelength (in the order of mils.) so that it is impossible to make these waveguides by standard manufacturing techniques. 
     OBJECTIVES AND SUMMARY OF THE INVENTION 
     It is a principle objective of the present invention to provide a miniature microwave guide capable of handling signals in the gigahertz range. 
     Another objective is to provide a method of making the above-mentioned waveguide. 
     A further objective is to provide a printed circuit board having a microwave guide imbedded therein. 
     The waveguide in accordance with this invention, is formed from a plurality of successive strips of a conductive material constructed and arranged to generate a preselected profile. Preferably a printed circuit board substrate is formed by overlapping layers containing said strips and a dielectric material. Appropriate probes are provided at the extreme ends of the guide for passing signals into and out of the waveguide. The probes have a cross-sectional shape which corresponds to a pre-selected mode of transmission of said signals through the guide. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric partial sectional view of the preferred embodiment of the invention; 
     FIG. 2 is an isometric partial sectional view of another embodiment of the invention; 
     FIG. 3 is a cross-sectional view of the microwave guide; and 
     FIGS. 4a-4h shows a method of making the miniature microwave guide. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In copending application, Ser. No. 671,276, filed on even date herewith, entitled &#34;MICRO-COAXIAL SUBSTRATE&#34; and incorporated herein by reference, a printed circuit board is disclosed which has a fully shielded conductor imbedded therein for interconnecting various electronic devices mounted on said board. The board comprises a substrate which is formed by overlapping a plurality of layers of dielectric and/or conducting materials to form a conductor surrounded by a shield. The method described in the above-mentioned patent application may also be used to generate a miniature waveguide in a printed circuit board substrate. 
     For example, as shown in FIG. 1, a rectangular microwave comprises a tubular member 10 made of a conductive material and having an end wall 12. A probe 14 is provided to inject the microwave signals into the waveguide. Preferably the probe is attached to an input pad 16 which is coplanar with one of the guide walls as shown for connection to a microwave signal source (not shown). At the other end of the guide 10 a similar probe and output pad is used to extract the signal from the guide. The probe shown in FIG. 1 provides a single point transmit/receive location in the center of the waveguide. This probe is suitable for the injection or extraction of E.sub.(0, 1) mode waves. 
     In the embodiment of FIG. 2, a wave guide 20 is shown with input and output probes 22, 24. These probes have a &#34;T&#34; transmitter/receiver shape suitable for injecting signals in the mode E.sub.(1,1). Probes 22 and 24 are also connected to pads 26, 28 similar to pad 16 of FIG. 1. 
     As best shown in FIG. 3, the waveguide comprises a plurality of overlapping conductive strips selected and arranged to form the desired rectangular cross-section. Obviously similar strips may be used to generate a cross-section having any other geometrical shape. The interior of the guide is preferably filled with a dielectric material 30 such as polyimide. 
     The waveguide of FIG. 1 is imbedded in the substrate of a printed circuit board as shown in FIGS. 4a-h and described below. 
     First, a layer of conductive material 32 is formed as shown in FIG. 4a. This layer forms the bottom wall of the wave guide. 
     On top of layer 32 two relatively narrow conductive strips 34, 36 are applied as shown in FIG. 4b. Then a dielectric material 30 is built up between and around conductors 34 and 36 to a level which is even with the top conductive strip as shown in FIG. 4c. In FIG. 4d, another layer of the conductive side wall is applied (34, 36) coextensive to the sidewall applied in FIG. 4b. At the same time, the tip of the probe (38) is applied in the same layer by varying the optical photomasks used in successive layer deposition. 
     In FIG. 4e, the dielectric material is again built up between and around conductors 34, 36 and 38 to a level even with the newly defined top of conductor 38. 
     The process steps of 4d and 4e are repeated to apply additional conductive and dielectric layer as shown in FIG. 4f. 
     Next, a conductive layer (40, 16) is deposited on top of the sidewalls 34 and 36 and probe 38 (FIG. 4g) to form the top wall 10 of the wave guide and the bonding pad 16. Suitable spacing is left around the probe pad such as at 42, 44 to keep the pad 16 electrically insulated from the wave guide. A final dielectric layer is applied to complete the build up as shown in FIG. 4h. 
     Typically the inner dimensions of the waveguide are in the order of 10×15 mils for a 10 gigahertz microwave. The distance between probe 16 and end wall 12 is equal to the quarter wavelength of the signal to be transmitted through the waveguide. 
     The layers of FIGS. 4a-h are superimposed by using standard photomasking techniques. Conductive layers are formed by additive processes and selectively patterned by subtractive processes. Dielectric layers are filled and leveled to the top of each successive conductor level. 
     Advantageously other elements such as a miniature shielded conductors can be formed in the substrate at the same time with the waveguide by providing an appropriate mask for each layer. 
     Obviously numerous modifications may be made to the invention without departing from its scope as defined in the attached claims.