Micromachined filters for 38 and 77 GHz supported on thinmembranes

Abstract

This paper presents the fabrication processes formicromachined millimetre-wave devices, on two different types of semiconductorsubstrates. The first process uses micromachining on high-resistivity< 100> oriented silicon. A three-layer dielectric membrane,with a total thickness of 1.5 µm is used as support for themillimetre-wave structures. This process was used for the manufacturing oftwo coupled line filters, with central operating frequencies of 38 and77 GHz, respectively. The second process is based on GaAs micromachining.For the first time, a 2.2 µm thin GaAs/AlGaAs membrane, obtained bymolecular beam epitaxy growth and micromachining of semi-insulating< 100> GaAs, is used as a support for millimetre-wave filterstructures. Cascaded coplanar waveguide open-end series stubs filter typestructures, with central frequencies of 38 and 77 GHz, respectively, weredesigned and manufactured on a GaAs micromachined substrate. `On wafer'measurements for the filter structures were performed. Losses of less than1.5 dB at 38 GHz and less than 2 dB at 77 GHz have been obtained for boththe silicon as well as for the GaAs-based micromachinedfilters.