Observation of backward-wave oscillations in silicon-MEMS beam-wave interaction circuits at 0.1 THz

We report a W-band backward-wave oscillator using micro-fabrication technologies by which a fully 3-dimensional slow-wave interaction circuit is successfully employed on multi-bonded silicon wafers. MEMS (micro-electromechanical systems) technologies such as deep RIE (reactive ion etching) and thermocompressive hermetic bonding are applied to achieve highly accurate dimensional structures for high frequency regime (>;100 GHz). A successful implementation of the slow-wave structure with high aspect ratios between in-plane and out-of-plane dimensions was confirmed by the return loss simulation and measurement. A thermionic dispenser cathode of 15 kV produced more than 91 mA electron beam current and achieved 90% beam transmission through the interaction circuit after vacuum integration. The RF measurement of the UHV-sealed interaction circuit showed backward-wave oscillations at 0.1 THz driven by an external small signal from a Gunn diode.

I. Introduction And Background

RF generation above 100 GHz in vacuum electronics has many scientific and technical issues because of the significantly increasing RF loss through dimensional tolerance, surface finish, thermal expansion, etc [1]. The electron beam conditions also have many challenges for beam transmission along the interaction circuit with high beam current density in small beam tunnels. To meet such stringent requirements, state-of-art silicon MEMS (micro-electromechanical systems) technologies can be applied to high frequency sources, e.g. beam-wave interaction circuits for backward-wave oscillations. The periodic slow-wave interaction circuit can be patterned by UV photolithography and etched by deep RIE (reactive ion etching) on the silicon wafers. Then thermocompressive hermetic bonding of the etched wafers enables 3-dimensional structures with high aspect ratio within a few microns accuracy. Previous studies have introduced good performance of the circuits by return loss measurement, etc [2], [3]. Here we report the experimental observation of backward-wave oscillations in such silicon MEMS-applied beam-wave interaction circuits at 0.1 THz.