EP1077502A2

EP1077502A2 - MMIC-to-waveguide RF transition and associated method

Info

Publication number: EP1077502A2
Application number: EP00202837A
Authority: EP
Inventors: Jonathan Bruce Hacker; Emilio Sovero
Original assignee: Boeing Co
Current assignee: Boeing Co
Priority date: 1999-08-16
Filing date: 2000-08-11
Publication date: 2001-02-21
Also published as: CA2312128A1; JP2001085912A; CN1284761A; EP1077502A3; CN1192453C

Abstract

An RF transition for coupling energy propagating in a waveguide transmission line into energy propagating in a monolithic microwave integrated circuit ("MMIC") is provided. The RF transition comprises a microstrip structure that includes a MMIC substrate with backside metallization and a front side microstrip. The backside metallization defines an iris, and the microstrip includes a microstrip feed formed proximate the iris. The RF transition also includes a waveguide terminating at the metallization layer around the iris to thereby convert energy propagating in the waveguide into energy propagating in the microstrip. In one embodiment, RF signal processing circuitry is monolithically formed on the MMIC substrate. The invention enables a waveguide-to-MMIC transition to be constructed at higher RF frequencies, such as millimeter wave frequencies, even with the fragile, thinner substrates and smaller device features of higher-frequency devices. The monolithic structure avoids the use of wire bonds or ribbon welds to interconnect separate substrates, such as would be used in an MIC implementation, enabling improved RF performance at higher RF frequencies. The invention enables an RF circuit to be constructed that is adapted to communicate signals with a waveguide at higher RF frequencies, such as millimeter wave frequencies, in a rugged, producible package.

Description

BACKGROUND OF THE INVENTION

Electromagnetic systems operating at frequencies between 1 GHz and 100 GHz are employed in a wide variety of communications, radar, remote sensing and other applications. The front ends of these systems typically include RF signal processing circuitry providing various functions. This RF circuitry may be implemented in different transmission media, including rectangular waveguide, microstrip, and stripline transmission lines. Microstrip structures are widely employed in both discrete microwave integrated circuitry (MIC) and monolithic microwave integrated circuitry (MMIC).
MIC and MMIC circuitry is useful in applications demanding small size, a high level of circuit integration, and the incorporation of semiconductor control devices. MIC and MMIC circuits employ microstrip transmission lines, which typically comprise a thin conducting strip deposited on a constant-thickness MMIC substrate backed by a conductive ground plane. RF energy propagates in quasi-TEM modes in microstrip. Waveguide structures, on the other hand, are employed when low circuit loss or high power handling requirements dominate the design requirements. RF energy propagates through waveguides in TE and/or TM modes.
In many electromagnetic systems, there is a need to transition from a waveguide transmission medium to an MMIC medium. Because of the inherent difficulty in converting energy in TE or TM modes to energy in quasi-TEM modes, MMIC/waveguide transitions are not accomplished straightforwardly. The thin MMIC substrate geometries dictated by systems operating at higher RF frequencies, such as millimeter wave frequencies, often result in a fragile MMIC structure and thus further complicate the RF transition design task.
One existing approach to the design of a MMIC- or MIC- to-waveguide transition is disclosed in United States Patent No. 4,636,753 to Geller et al. ("Geller"). Geller discloses a thin metallized substrate inserted lengthwise into a rectangular waveguide in a plane parallel to the narrow walls of the waveguide. On the metallized surface of the substrate, a finline transition from the waveguide mode to a slotline mode is formed. A broadband balun is formed on the substrate to convert energy in the slotline mode into energy propagating in a microstrip formed on the substrate. MMIC or MIC components are formed or mounted, respectively, on the substrate and are fed by the microstrip. The device is symmetrical about the direction of waveguide propagation so that an MIC transition both from and to waveguide is provided on the substrate. The technique disclosed by Geller might be useful, for example, in building a waveguide amplifier by forming an MIC or MMIC amplifier on a substrate incorporating the finline transition and inserting the substrate into a section of empty rectangular waveguide parallel to the narrow wall of the waveguide. Because of tolerancing requirements and the use of wirebond connections, the Geller technique is limited to lower microwave frequency applications, however.
An MIC-to-waveguide transition is provided by U.S. Patent No. 5,414,394 to Gamand et al. ("Gamand"), which discloses a microstrip formed on one side of a substrate and a waveguide oriented perpendicular to the substrate and terminating near an end of the microstrip that acts as a field probe. The transition from waveguide to microstrip is accomplished by necking down the waveguide in the vicinity of the probe and locating the end of the waveguide cavity at a distance of one-quarter wavelength from the probe. The substrate with microstrip is dropped into a channel formed in a multi-part metal housing assembly providing conductive waveguide walls in the transition. The housing also extends over the substrate to protect circuitry formed thereon.
Another waveguide transitioning approach is to attach a MMIC circuit and a separate waveguide/microstrip transition to a common substrate in an MIC package, interconnecting the two substrates with ribbon welds or wire bonds. This common MIC technique is widely used at lower frequencies but provides poor RF performance at higher frequencies, such as millimeter wave frequencies, where ribbon weld and wire bond parasitic capacitances are significant.
The existing approaches to providing a waveguide-to-MMIC transition do not scale well to higher RF frequencies, such as millimeter wave frequencies, because the thinner substrates and smaller device features of higher-frequency devices yield more fragile devices. Moreover, the fabrication tolerances required to produce higher-frequency devices make the alignment of multiple complex housing parts more difficult, and the need to interconnect separate substrates with wire bonds or ribbon welds in an MIC implementation degrades RF performance at higher frequencies.

SUMMARY OF THE INVENTION

According to the present invention, an RF transition is provided for coupling energy propagating in a waveguide transmission line into energy propagating in a microstrip transmission line. The RF transition comprises a microstrip structure that includes a MMIC substrate with backside metallization and a front side microstrip. The backside metallization defines an iris, and the microstrip includes a microstrip feed formed proximate the iris. The RF transition of the invention also includes a waveguide terminating at the metallization layer around the iris to thereby convert energy propagating in the waveguide into energy propagating in the microstrip. As a result of its unique construction, the RF transition provides good RF performance at higher RF frequencies, such as millimeter wave frequencies. Moreover, the RF transition of the present invention enables the construction of an RF circuit that is adapted to communicate signals with a waveguide at higher RF frequencies, such as millimeter wave frequencies, in a rugged and producible package.
In one embodiment, the MMIC substrate of the RF transition is a semiconductor material, such as silicon, gallium arsenide, indium phosphide, or the like, and RF signal processing circuitry is monolithically formed on the substrate. The invention therefore provides a high performance RF transition to thin, fragile MMIC circuits that is rugged and producible.
An RF circuit adapted to interface with and communicate signals with a waveguide is also provided by the present invention. The RF circuit comprises a microstrip structure adapted to terminate the waveguide and to convert energy propagating in the waveguide into energy propagating in the microstrip. According to one advantageous embodiment, the RF circuit includes electronic circuitry, such as RF circuitry, formed as part of the microstrip structure.
The present invention further provides a method for coupling energy propagating in a waveguide into energy propagating in a microstrip. According to the invention, the method includes the steps of providing a microstrip structure and terminating a waveguide at an iris formed by the backside metallization of the microstrip structure to thereby convert energy propagating in the waveguide into energy propagating in the microstrip structure. In one advantageous embodiment, the method further includes the step of integrating the microstrip structure into an RF signal processing subsystem.
The RF transition of the present invention thus overcomes limitations inherent in prior RF transitions by providing a transition design that is rugged and producible even with the thin substrates and close dimensional tolerancing necessary for good RF performance at higher RF frequencies, such as millimeter wave frequencies. Moreover, the RF transition of the present invention is accomplished without wire bonds or ribbon welds, thereby improving RF performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a microstrip-to-waveguide RF transition according to one embodiment of the present invention.
FIG. 2 is a side view of a microstrip-to-waveguide RF transition depicting a cavity and RF circuitry formed on a MMIC substrate.
FIG. 3 is a plan view of a microstrip-to-waveguide RF transition according to one embodiment of the present invention and provides detail of the microstrip feed geometry.
FIG. 4 is a flowchart depicting a method for coupling energy from a waveguide to a microstrip according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.
A perspective view of an RF transition 20 according to one embodiment of the present invention is provided in FIG. 1. The RF transition includes a microstrip structure 22 that includes RF circuitry 24 formed on a MMIC substrate 26. The microstrip structure 22 also provides an RF transmission line with a microstrip geometry. In this regard, a thin line of metallization is deposited on a second surface of the MMIC substrate 26 to form a microstrip 28, and a metallization layer, such as a ground plane, is formed on an opposed first surface of the substrate. In operation, RF signals are carried by the microstrip structure 22 to and from the RF circuitry and other features on the microstrip structure. As known to those skilled in the art, the first surface of the MMIC substrate 26 is typically formed by the back side of the substrate upon which backside metallization is deposited in order to form a ground plane. It is likewise well known in the art for the second surface of the MMIC substrate to be formed by the front side of the substrate upon which microstrip 28 is formed.
In the RF transition 20 of the present invention, RF energy propagating in the microstrip structure 22 is coupled to a waveguide 34 via a microstrip feed 30 formed adjacent microstrip 28 through an iris 32 formed in a metallization layer on the back side of the MMIC substrate. As discussed in more detail in conjunction with FIG. 2, the waveguide 34 mates with and terminates into the backside metallization around the iris 32. The waveguide is preferably soldered or bonded with conductive epoxy to the backside metallization layer in order to provide repeatable, low-loss coupling of energy into the microstrip feed. As is known in the art, other techniques for terminating the waveguide into the backside metallization can be employed without departing from the scope of the present invention.
The geometry of the microstrip feed 30 in relation to the iris is critical to the performance of the RF transition of the present invention. The dimensions and features of the microstrip feed 30, the iris 32, and the MMIC substrate 26 determine the impedance match between the microstrip structure and the waveguide and generally determine the RF performance of the RF transition 20. It is preferable that the iris 32 be concentric with the waveguide 34 and that the microstrip feed 30 be located symmetrically with respect to both the iris 32 and the waveguide 34. Adjustment of the microstrip feed 30 features and dimensions can be used to tune the RF transition to operate over particular narrow RF frequency ranges or to broaden the band over which the RF transition 20 operates.
One advantageous embodiment of the RF transition 20 of the present invention is provided in FIG. 2, which depicts a side view of an RF transition including a cavity 40. The cavity 40 terminates the waveguide 34 and is mounted to the microstrip structure 22 adjacent the front side of the MMIC substrate 26. The cavity 40 is preferably located symmetrically with respect to the iris 32 and the waveguide 34. To optimize coupling, the dimensions of cavity 40 are adjusted so that all cavity resonance frequencies fall outside of the design bandwidth of RF transition 20. The waveguide 34 and the cavity 40 preferably extend somewhat beyond the edge of the substrate 26 as is shown in FIG. 2. The cavity 40 is therefore preferably soldered or bonded with conductive epoxy, as is known in the art, to the end of the waveguide 34 in the region beyond the substrate. The cavity 40 is also soldered or bonded with conductive epoxy to portions of the front side of the MMIC substrate 26, as is shown in FIG. 2. Cavity 40 serves to improve the coupling performance of the RF transition by more effectively terminating waveguide 34.
In one advantageous embodiment of the RF transition 20 of the present invention, the MMIC substrate 26 comprises a semiconductor material and the RF circuitry 24 is formed monolithically on the substrate, as is known by those skilled in the art. The semiconductor material may comprise silicon, gallium arsenide, indium phosphide, or other materials suitable for the monolithic formation of MMIC and electronic circuitry as is known in the art.
A plan view of a MMIC to waveguide RF transition 20 according to one advantageous embodiment of the present invention is provided in FIG. 3, where details of the microstrip feed geometry of one preferred embodiment are provided. The microstrip feed 30 preferably extends over the backside iris 32 and terminates in a microstrip radial stub 36 as shown in FIG. 3. Two opposed microstrip arms 38 preferably extend from the microstrip feed 30 adjacent the iris 32 and opposite the microstrip radial stub 36 relative to the center of the iris 32, as shown in FIG. 3. The dimensions of the microstrip feed 30, microstrip radial stub 36, and microstrip arms 38 are carefully chosen, in conjunction with the dimensions of the waveguide, iris opening, and cavity, to provide a high performance RF transition at a particular RF operating frequency. For example, in one advantageous embodiment designed to operate at a center frequency of 100 GHz, a substrate made of gallium arsenide has a thickness of 3 mils and a dielectric constant of 12.8, the inner dimensions of waveguide 34 are 10 mils by 5 mils, the inner dimensions of cavity 40 are 130 mils by 100 mils by 25 mils, the width of microstrip feed 30 is 5 mils, the length and width of microstrip arms 38 are 28 mils and 0.37 mils, respectively, and the dimensions of iris 32 are 20 mils by 50 mils, respectively. The resulting RF transition 20 has been modeled and is predicted to yield a bandwidth of 7 GHz centered at 100 GHz with a return loss better than 10 dB and an insertion loss of less than 0.25 dB over that bandwidth.
An RF circuit 48 according to one advantageous embodiment of the present invention is also illustrated in FIG. 3, which depicts the formation of electronic circuitry, such as RF circuitry 24, and microstrip transmission line structures on a single substrate. According to this advantageous embodiment, the RF circuit 48 is adapted to communicate signals with an interface waveguide to be attached to or mounted adjacent the substrate back side metallization about an iris formed by the metallization.
In one embodiment, the RF circuit 48 of the present invention includes a cavity mounted to the top side of the substrate concentrically with the backside metallization mounting location of an interface waveguide. The iris formed by the backside metallization layer is preferably symmetrical with respect to the front side microstrip feed and the mounting location for the interface waveguide. Preferably, the substrate is a semiconductor material and the electronic circuitry is formed monolithically on the semiconductor substrate, as is known in the art.
FIG. 4 illustrates a flow chart that provides a method for coupling energy from a waveguide mode to a microstrip mode according to one embodiment of the present invention. A microstrip structure is initially provided as described above according to step 50. The microstrip structure preferably comprises a MMIC substrate, a metallization layer formed on the backside of the substrate, and a microstrip formed on the front side of the substrate. The metallization layer defines an iris, and the microstrip comprises a microstrip feed located adjacent to the iris. According to the invention, the method further includes the step 58 of terminating a waveguide at the metallization layer around the iris to thereby convert energy propagating in the waveguide mode into energy propagating in a microstrip mode.
In one advantageous embodiment of the method according to the present invention, RF signal interfaces and DC power interfaces are provided to the microstrip structure according to step 52, such as via wire bonds for DC power and via ribbon welds for RF. Electronic circuitry, such as RF circuitry, is preferably formed on the MMIC substrate according to step 54 to provide signal processing functions. In one advantageous embodiment, a cavity is provided to terminate the interface waveguide adjacent the iris according to step 56. The method preferably further includes the steps 58 and 60 of terminating an interface waveguide at the iris and integrating the entire structure into an RF subsystem or system, respectively.
The RF transition of the present invention overcomes limitations inherent in prior RF transition designs. The RF transition of the present invention is rugged and producible even with the thin substrates and close dimensional tolerancing necessary for good RF performance at higher RF frequencies, such as millimeter wave frequencies. Moreover, the monolithic RF transition of the present invention is accomplished without wire bonds or ribbon welds, thereby improving RF performance.
Many modifications and other embodiments of the invention will come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

Claims

An RF transition for coupling energy propagating in a waveguide transmission line into energy propagating in a microstrip transmission line, the transition comprising:

a microstrip structure comprising a monolithic microwave integrated circuit ("MMIC") substrate, a metallization layer formed on a first surface of the MMIC substrate, and a microstrip formed on an opposed second surface of the MMIC substrate, wherein the metallization layer defines an iris, and wherein the microstrip comprises feed proximate the iris; and

a waveguide terminating at the metallization layer around the iris to thereby convert energy propagating in the waveguide into energy propagating in the microstrip.
The RF transition of Claim 1 wherein the iris is concentric with the waveguide.
The RF transition of Claim 1 or 2 further comprising a cavity positioned adjacent the second surface of the MMIC substrate concentric with the waveguide.
The RF transition of Claim 1, 2 or 3 further comprising a cavity positioned adjacent the second surface of the MMIC substrate and concentric with the waveguide;
wherein the iris is concentric with the waveguide;
wherein the microstrip feed extends over the iris and is symmetrical with respect to the center of the iris;
wherein the microstrip structure further comprises RF circuitry for processing RF signals; and
wherein the MMIC substrate comprises a semiconductor material.
An RF adapted to communicate signals with a waveguide, the circuit comprising;

a microstrip structure comprising a MMIC substrate, a metallization layer formed on a first surface of the MMIC substrate, and a microstrip formed on an opposed second surface of the MMIC substrate, wherein the metallization layer defines an iris, and wherin the microstrip comprises a microstrip feed proximate the iris;
wherein the microstrip structure is adapted to terminate a waveguide positioned at the metallization layer around the iris; and
wherein the microstrip feed is adapted to concert energy propagating in the waveguide into energy propagating in the microstrip.
The RF circuit of any of Claims 1-5 over the iris and is symmetrical with respect to the center of the iris.
The RF circuit of any of Claims 1-6 wherein the microstrip feed comprises two opposed microstrip arms.
The RF circuit of any of Claims 1-7 wherein the microstrip structure further comprises RF circuitry for processing RF signals.
The RF circuit of any of Claims 1-8 wherein the MMIC substrate comprises a semiconductor material.
The RF circuit of any of Claims 5-9 comprising a cavity positioned adjecent the second surface of the MMIC substrate.
The RF circuit of any of Claims 5-10 further comprising a cavity positioned adjacent the second surface of the MMIC substrate;
wherein the microstrip feed extends over the iris and is symmetrical with respect to the center of the iris;
wherein the microstrip structure further comprises RF circuitry for processing RF signals; and
wherein the MMIC substrate comprises a semiconductor material.
A method for coupling energy propagating in a waveguide transmission line into energy propagating in a microstrip transmission line, the method comprising the steps of:

providing a microstrip structure comprising a MMIC substrate, a metallization layer formed on a first surface of the MMIC substrate, and a microstrip formed on an opposed second surface of the MMIC substrate, wherein the metallization layer defines an iris, and wherein the microstrip comprises a microstrip feed adjacent the iris; and

terminating a waveguide at the metallization layer around the iris to thereby convert energy propagating in the waveguide into energy propagating in the microstrip.
The method of Claim 12 further comprising the step of providing a cavity positioned adjacent the second surface of the MMIC substrate and concentric with the waveguide.
The method of Claim 12 or 13 further comprising the step of forming RF circuitry on the MMIC substrate to thereby process RF signals.
The method of Claim 12, 13 or 14 further comprising the step of providing signal interfaces to the microstrip structure.
The method of any of Claims 12-15 further comprising the step of integrating the microstrip structure into an RF processing subsystem.