A Broad Bandwidth Suspended Membrane Waveguide to Thinfilm Microstrip Transition

Excellent progress in the development of Submillimeter-wave SIS and HEBmixers has been demonstrated in recent years. At frequencies below 800 GHz these mixers are typically implemented using waveguide techniques, while above 800 GHz quasi-optical (open structure) methods are often used. In many instances though, the use of waveguide components offers certain advantages. For example, broadband corrugated feed-horns with well defined on axis Gaussian beam patterns. Over the years a number of waveguide to microstrip transitions have been proposed. Most of which are implemented in reduced height waveguide with RF bandwidth less than 35%. Unfortunately reducing the height makes machining of mixer components at terahertz frequencies rather difficult. It also increases RF loss as the current density in the waveguide goes up, and surface finish is degraded. An additional disadvantage of existing high frequency waveguide mixers is the way the active device (SIS, HEB, Schottky diode) is mounted in the waveguide. Traditionally the junction, and its supporting substrate, is mounted in a narrow channel across the guide. This structure forms a partially filled dielectric waveguide, whose dimensions must kept small to prevent energy from leaking out the channel. At frequencies approaching or exceeding a terahertz this mounting scheme becomes impractical. Because of these issues, quasi-optical mixers are typically used at these small wavelength. In this paper, we propose the use of suspended silicon (Si) and silicon nitride (Si3N4) membranes with silicon micro-machined backshort and feedhorn blocks. Deposited on the membrane is a “single sided” thinfilm radial probe which extrudes partly into the waveguide. To simplify eventual assembly, the membrane based radial probe is oriented orthogonal to the E-field in the guide. The proposed waveguide to microstrip transition is a variation on the rectangular probe design that finds a wide range of use at the lower microwave and millimeter wave frequencies. Extensive 3D electromagnetic field simulations are reported in this paper. It is found that the a thinfilm radial probe on top of a suspended membrane is able to couple with 85% efficiency to an entire TE10 mode dominated waveguide bandwidth. This includes a 0.5 dB loss in the 0.035 λo “air” space above and directly below the suspended membrane. The input impedance of the probe is seen to be insensitive to airgap variation beneath or above the membrane. When combined with micromachining techniques, the discussed transition enables technology transfer to large RF bandwidth spectroscopic imaging arrays at terahertz frequencies.