Micromachined filters for 38 and 77 GHz supported on thin membranes

Abstract

This paper presents the fabrication processes for micromachined millimetre-wave devices, on two different types of semiconductor substrates. 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 the millimetre-wave structures. This process was used for the manufacturing of two coupled line filters, with central operating frequencies of 38 and 77 GHz, respectively. The second process is based on GaAs micromachining. For the first time, a 2.2 µm thin GaAs/AlGaAs membrane, obtained by molecular beam epitaxy growth and micromachining of semi-insulating 100 GaAs, is used as a support for millimetre-wave filter structures. Cascaded coplanar waveguide open-end series stubs filter type structures, with central frequencies of 38 and 77 GHz, respectively, were designed and manufactured on a GaAs micromachined substrate. 'On wafer' measurements for the filter structures were performed. Losses of less than 1.5 dB at 38 GHz and less than 2 dB at 77 GHz have been obtained for both the silicon as well as for the GaAs-based micromachined filters.

Membrane Supported Transmission Lines and Filters

“…Such membrane supported transmission structures have been previously demonstrated for filters [3], a diplexer [4], antennae [5] and a receiver [6]. Membranes made of silicon nitride [3], GaAs [7], or polyimide [8] materials have been produced on silicon [3] or GaAs [7,8] substrates. A CPW line gives low radiation, and packaging is not critical.…”

Fabrication of thick silicon nitride blocks embedded in low-resistivity silicon substrates for radio frequency applications

“…Several techniques have been developed to allow the realization of low-loss RF devices on standard silicon. These techniques include the use of thick silicon dioxide layers [5,6] and dielectric layers such as polyimide [7] and benzocyclobutene [8], the use of polysilicon patterned ground shields [9], the use of silicon bulk micromachining to remove the substrate locally under the RF components [10][11][12] and the use of surface-micromachined suspended metal structures at a distance of several tens of micrometers above the silicon surface [13,14]. However, all these techniques impose restrictions on the device structures that can be realized.…”

GaAs MEMS for Millimeter Wave Communications

“…The typical thickness of the GaAs membranes for this type of sensing structures is about 1µm and the thickness of the etch-stop AlGaAs layer is about 0.2 µm. Millimeter wave filter structures supported on GaAs membranes were manufactured in the last years by groups involving the authors of this paper [9][10]. Substrateless Schottky diodes, for applications in the terahertz frequency range, were also recently reported [11][12].…”