In this paper a process for complete monolithic integration of semiconductor devices and radio-frequency micro-electro-mechanical systems (RF-MEMS) on a single substrate is presented. Our attempt was to combine RF-Schottky-Diodes to form sub-harmonic mixer and RF-MEMS-phaseshifters on a single chip.
The diodes were etched from a molecular beam epitaxy grown silicon stack using two mesa etching steps. Nickel forms a nickel-silicon alloy (nickel silicide) during a rapid thermal processing step acting as Schottky-metallisation.
On this stack, the RF-MEMS-fabrication starts with its metallisation layers as a back-end process. To insulate the relatively high actuation voltage (20-40 V) from the RF circuitry, a new concept for bias decoupling is presented.
To demonstrate the functionality of the semiconductor integration approach, a mixer for 24 GHz has been designed in coplanar waveguide technology, the local oscillator frequency is at 12 GHz. Fabricated within the same run, switched line phaseshifters are used to show the MEMS capabilities.
First tests of diodes revealed good results in their DC- and RF characteristics, the conversion loss of the subharmonic zero biased mixer reached 20 dB for 6 dBm power of the local oscillator. Fabricated teststructures of the phaseshifters achieved good results showing that
transmission losses lower than 3 dB at a phaseshift of 180° can be reached.