Microwave transmission line having a 3-D shielding with a laterally separated region

A microwave transmission line having a coplanar waveguide and a pair of conductive members, each one of the pair of conductive members having a proximal end disposed on a portion of a corresponding one of a pair of ground plane conductors of the coplanar waveguide and a distal end disposed over, and vertically spaced from, a region between a center conductor of the coplanar waveguide and a corresponding one of the pair of ground plane conductors of the coplanar waveguide. The distal ends are laterally separated from each other by a region disposed over the center conductor.

TECHNICAL FIELD

This disclosure relates generally to Monolithic Microwave Integrated Circuits (MMICs) having a plurality of proximate microwave transmission lines and more particularly to MMICs having reduced electromagnetic coupling between such microwave transmission lines.

BACKGROUND OF THE INVENTION

As is known in the art, MMICs include a plurality of microwave transmission lines in close proximity one to another. A portion of one MMIC with a coplanar waveguides transmission line (CPW) and an adjacent microstrip transmission line is shown inFIGS. 1A and 1B; here shown with a scratch resistant dielectric layer over the transmission lines, as shown. More particularly, if the electrical isolation between two transmission lines is not greater than typically 20 dB more than the difference in the signal levels carried by the two transmission lines can result in unwanted feedback between the two transmission lines. One method used to solve this problem is to separate the center conductor of the microstrip transmission line from the outer edge of the proximate ground plane conductor of the CPW a distance D (FIG. 1B) sufficient to provide the requisite electrical isolation between the CPW and the microstrip transmission line. However such techniques use valuable surface area for the pair of transmission lines

SUMMARY OF THE INVENTION

In accordance with the disclosure, a microwave transmission line is provided having a coplanar waveguide (CPW) and a pair of conductive members. Each one of the pair of conductive members has a proximal end disposed on a portion of a corresponding one of the pair of ground plane conductors of the coplanar waveguide and a distal end disposed over, and vertically spaced from, a region between a center conductor of the coplanar waveguide and a corresponding one of the pair of ground plane conductors. The distal ends are laterally separated from each other by a region disposed over the center conductor.

With such an arrangement the conductive members provide shielding between the microwave transmission line and any proximate microwave transmission lines of a MMIC with less separation being required between the transmission lines than between transmission lines without such pair of conductive members thereby increasing the amount of surface area available for other devices on the MMIC.

In one embodiment, a microwave transmission line, is provided having a coplanar waveguide, comprising: a substrate; a pair of ground plane conductors disposed on a surface of the substrate; and a strip conductor disposed on the surface between the pair of ground plane conductors. A pair of conductive members is included, each one of the pair of conductive members having a proximal end disposed a portion of a corresponding one of the pair of ground plane conductors and each one of the pair of conductive members having a distal end disposed over, and vertically spaced from, a region between the center conductor and a corresponding one of the pair of ground plane conductors, the distal ends of the pair of conductive members being laterally separated from each other by a region disposed over the center conductor.

In one embodiment, the microwave transmission line includes a scratch resistant, dielectric protective coating disposed over the center conductor and over first portions of the pair of ground plane conductors, the protective coating having a pair of openings therethrough, each one of the opening exposing a second portion of a corresponding one of the pair of ground plane conductors and wherein the proximal ends of the pair of conductor members pass through the openings onto the pair of ground plane conductors.

With such an arrangement, additional patterning of existing scratch resistant protection coating dielectric material enables formation of the pair of conductive members having vertical walls with overhang to confining electric fields in the waveguide and greatly increasing isolation to surrounding circuitry. The ground plane conductors of the CPW are left exposed so that conductive material can be deposited in the opening formed in the scratch protective layer. Further, patterning of the scratch resistant protective layer enables for nation of a 3-dimensional aspect to radio frequency (RF) shielding around a signal passing through the center conductor of the CPW transmission line.

In one embodiment, a microwave transmission line is provided having a coplanar waveguide, comprising: a substrate; a pair of ground plane conductors disposed on a surface of the substrate; a strip conductor disposed on the surface between the pair of ground plane conductors; a protective coating disposed over the center conductor and over first portions of the pair of ground plane conductors, the protective coating having a pair of openings therethrough, each one of the opening exposing a second portion of a corresponding one of the pair of ground plane conductors: and a pair of conductive members, each one of the pair of conductive members having a proximal end disposed on the second portion of a corresponding one of the pair of ground plane conductors and each one of the pair of conductive members having a distal end disposed on a corresponding one of a pair of laterally spaced, upper surface portions of the protective coating, the distal ends of the pair of conductive members being laterally separated from each other by region disposed over the center conductor.

DETAILED DESCRIPTION OF THE INVENTION

Referring now toFIGS. 2A and 2B, a microwave transmission line10is shown having a coplanar waveguide (CPW)12(FIG. 2B). The coplanar waveguide12(FIG. 2B) includes: a substrate14(FIG. 2B); a pair of ground plane conductors16a,16bdisposed on an upper surface17(FIG. 2B) of the substrate14(FIG. 2B); a strip center conductor18(FIG. 2B) disposed on the surface of the substrate14(FIG. 2B) between the pair, of ground plane conductors16a,16b. Here, for example, the ground plane conductors16a,16band the center conductor18are gold having a thickness in a range from 4 to 9 um.

A dielectric scratch resistant protective coating20is disposed over the upper surface17of the substrate14over the center conductor18, over the spaces19a,19b(FIG. 2B) between the center conductor18and the ground plane conductors16a,16band over first portions22a,22bof the pair of ground plane conductors16a,16b, as shown inFIG. 2B. Here, the protective coating is Benzocyclobutene (BCB) having a thickness in, a range of 4 to 12 um. The protective coating20has a pair of openings24a,24btherethrough. Each one of the opening24a,24bexposes a second portion26a,26bof a corresponding one of the pair of ground plane conductors16a,16b, respectively as shown inFIG. 2B.

As shown inFIG. 2B, a pair of conductive members28a,28bis included. Here, for example, each one of the conductive members28a,28bis gold. Each one of the pair of conductive members28a,28bhas a proximal end30a,30b, respectively, disposed on the second portions26a,26brespectively of a corresponding one of the pair of pound plane conductors16a,16b, as shown. Each one of the pair of conductive members16a,16b, has a distal end32a,32b, respectively, disposed on a corresponding one of a pair of laterally spaced, upper surface portions33a,33bof the dielectric, scratch resistant protective coating20, as shown. The distal ends32,32bof the pair of conductive members28a,28bare laterally separated from each other by region36, here air, disposed over the center conductor18. It is noted that the region36is separated from the center conductor18by a portion20a(FIGS. 2A and 2B) of the dielectric, scratch resistant protective coating20, as shown.

Referring now toFIGS. 3A and 3B, a method used to form the microwave transmission line10(FIG. 3B) is shown; it being understood that other methods may be used. Thus, as shown inFIG. 3Bthe upper surface17of the substrate14, here for example, a dielectric or semi-insulating substrate, such as is used in a MMIC, has formed thereon, using any conventional photolithographic-etching processes, the coplanar waveguide12. Here, for example, the ground plane conductors16a,16band the strip center conductor18have thicknesses in the range of 4 to 9 um. The scratch resistant protective dielectric layer20, such as BCB having a thickness in the range of 4 to 12 um is deposited over the center conductor18, over the spaces19a,19b(FIG. 5B) between the center conductor18and the ground plane conductors16a,16band over the ground plane conductors16a, as shown inFIG. 3B. A layer38of photoresist in deposited over the scratch resistant protective dielectric layer20, as shown, here the thickness of the scratch resistant protective dielectric layer is in a range of 4 to 12 um. The layer38of photoresist is patterned using conventional lithography to form a mask39(FIG. 3B), here having a pair of windows40a,40btherein, as shown. Each one of the windows40a,40bexposes underlying portions of the scratch resistant protective dielectric layer20disposed over the second portions22a,22b, (FIG. 3B) of a corresponding one of the pair of ground plane conductors16a,16b, respectively, as shown inFIG. 3B.

Referring now also toFIGS. 4A and 4B, the upper surface of the mask39(FIG. 3B) is exposed to a suitable dry or wet etchant to remove exposed portions of scratch resistant protective dielectric layer20disposed over the second portions22a,22b(FIGS. 4A and 4B) of a corresponding one of the pair of ground plane conductors16a,16b(FIG. 4B), respectively, as shown.

The mask39(FIG. 3B) is removed and a conductive layer42, here, for example, gold, here, for example, having an upper thickness in a range of from 4 to 9 um is deposed over the upper surface of the structure; portions of the conductive layer42being deposited on the scratch resistant protective dielectric layer20and other portions passing onto the second portions22a,22bof the pair of ground plane conductors16a,16b, as shown inFIG. 5B. A layer44of photoresist is deposited over the conductive layer42and patterned into a mask46(FIG. 5B) having openings45a,45b, and45ctherein, as shown inFIGS. 5A and 5B. It is noted that the mask46(FIG. 5B) covers two regions48a,48b(FIG. 5B); one region48abeing disposed over the second portion22a, of a first one of the pair of ground plane conductors16aand extending laterally over space19abetween an adjacent outer edge of the ground plane conductors16aand an outer edge of the center conductor18adjacent ground plane conductors16a; and the other one of the two regions48bbeing disposed over the second portion22bof the ground plane conductors16band extending laterally over the space19bbetween an adjacent outer edge the ground plane conductors16band an outer edge of the center conductor18adjacent to the ground plane conductors16b, as shown inFIG. 5B. The mask46is exposed to suitable dry, or wet etch to remove the unmasked portions of the conductive layer42. The mask46(FIG. 5B) is removed forming, the microwave transmission line10shown inFIGS. 2A and 2B.

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. For example, the microwave transmission line10may be formed without scratch resistant protective; that is with the distal ends of the conductive members28a,28bsuspended over the air-filled regions60between the outer edges of the ground plane conductors62and the outer edges64of the center conductor as by etching away the solid dielectric, as shown inFIG. 6. Accordingly, other embodiments are within the scope of the following claims.