Submillimeter-wave endfire slotline antennas

Abstract

A B S T R A C TEndfire slotline antennas have been integrated on a 1.75pm thick dielectric substrate. The thin substrate allows the upper frequency limit of the slotline antennas to be pushed beyond ITHz. Antennas have been fabricated and tested at 350 and SOZGHz. The patterns exhibit high directivity (15+/-ldB) and wide bandwidth. In the future, these antennas will be combined with SIS junctions for low noise astronomical receivers. I N T R O D U C T I O N A N D DESIGNEndfire slot antennas are being designed for imaging systems [l] and radio astronomical low iioisc receivers [Z]. The antennas are wideband, high gain, and can easily be arrayed in two dimensional configurations. These antennas have been measured successfully to lIOGHz, and cfforts are currently being made to increase this frequency limit. The practical upper limit of the antenna is set by the substrate thickness, and for frequencies above IlOGHz, thc substrate needs to be more thin than has been practically possible. Yngvesson et al. [3] defined an effective dielectric thickness t,,,/X, = (&-l)/Ao and experimentally determined an optimum range of &,,/A, from 0.005 to 0.03 for good radiation patterns. If the substrate is too thick, substrate mode degrade the antenna patterns, and the main lobe may be off-axis or split in the middle. If the substrate is too wide or removed entirely, the beamwidth is wide and the gain is lowered. Kollberg et al. [2] have tried to solve the substrate mode problem by sandwiching the antenna between two dielectric and substrates adding a substrate lens in front. Unfortunately, air gaps between the substates and misalignment problems have caused serious degradation of the measured radiation pattern.Traditionally, endfire slot antennas have had high cross-polarization levels on the order of -5dB in the D-plane resulting in an efficiency loss for singly polarized systems. A Broken Linearly Tapered Slotline Antenna (BLTSA) was designed at Chalmers University maintaining the high gain, wideband nature of the endfire slotline antennas while reducing the D-plane cross-polarization levels to below -10dB [Z]. This antenna was designed empirically using scale models at 43GHz with 25pm thick Icapton (e, = 3.5) substrates (Fig.1). 0.18034730-5192 $3.000 1992 IEEE. 1898The submillimeter-wave BLTSAs are integrated on 1.75pm SiOz/Si3N4 dielectric membranes. By using the thin membrane technology developed by Rebeiz [5], we have eliminated the substrate mode problem and were able to design, fabricate and test slotline antennas at 34s and 802GHz. This work is a joint effort between Chalmers University of Technology and the University of Michigan. The design and microwave modelling were performed at Chalmers, the fabrication and submillimeter-wave testing were done a t Michigan, and the 802GHz far-infrared measurements were performed at the NASA/Goddard Space Flight Center. FABRICATION AND MEASUREMENTSThe 1.75pm SiOz/Si3N4 membrane was grown on a silicon wafer using standard thermal SiOz and LPCVD processes. The silicon was then patte...

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