Klemens Brückner

Nanomechanics of single crystalline tungsten nanowires

Single crystalline tungsten nanowires were prepared from directionally solidified NiAl-W alloys by a chemical release from the resulting binary phase material. Electron back scatter diffraction (EBSD) proves that they are single crystals having identical crystallographic orientation. Mechanical investigations such as bending tests, lateral force measurements, and mechanical resonance measurements were performed on 100-300 nm diameter wires. The wires could be either directly employed using micro tweezers, as a singly clamped nanowire or in a doubly clamped nanobridge. The mechanical tests exhibit a surprisingly high flexibility for such a brittle material resulting from the small dimensions. Force displacement measurements on singly clamped W nanowires by an AFM measurement allowed the determination of a Youngs modulus of 332 GPa very close to the bulk value of 355 GPa. Doubly clamped W nanowires were employed as resonant oscillating nanowires in a magnetomotively driven resonator running at 117 kHz. The Youngs modulus determined from this setup was found to be higher 450 GPa which is likely to be an artefact resulting from the shift of the resonance frequency by an additional mass loading.

Multi-technology design of an integrated MEMS-based RF oscillator using a novel silicon-ceramic compound substrate

In this paper, an approach towards the realization of a hybrid MEMS-CMOS RF oscillator module using the novel silicon-ceramic (SiCer) compound substrate technology is described. Piezoelectric aluminium-nitride MEMS resonators with quality factors Q up to 2,200 and resonant frequencies of 240, 400 and 600 MHz have been investigated as frequency-selective elements. For RF-compatible hybrid-integrated assembly and packaging, the SiCer compound substrate has been adapted, promising an efficient integration of both, microelectronic and micromechanical devices, on a single carrier substrate. Multiphysical circuit design and simulations using parametrized behavioural MEMS models have been carried out, indicating stable oscillator operation at the design frequency. As one prospective application, such an oscillator module could form part of a compact and power-efficient reconfigurable RF transceiver frontend in SiCer technology, e.g., for mobile communications.

Gas Pressure Sensing Based on MEMS Resonators

MEMS resonators bear great potential for applications as RF sensors, filters and oscillators, e.g., in life sciences or information technology. Resonant AlN and SiC beams with operation frequencies between 0.01 and 3.4 MHz have been prepared using a semiconductor fabrication process. The metallized beams were actuated in a permanent magnetic field of about 0.5 T by the Lorentz force. The resonant response was detected in the frequency domain. Resonator geometry and material were varied to attain a generalized understanding of the RF performance in dependence of the ambient pressure. In particular the quality factor shows a high sensitivity on pressure, allowing potential application as absolute pressure sensor. Theoretical models have been applied that match well to the measurement.

Isotropic dry-etching of SiC for AlGaN/GaN MEMS fabrication

We present an isotropic fluorine based process to etch 4H-SiC substrates compatible with standard metallic etch masks and reasonable etching rates. Additionally, a new masking material has been established in order to obtain a stable protection of the layers underneath, based on a combination of sputtered AlN and Ni. The isotropic etching was achieved using a temperature assisted RF plasma etch process. The influence of gas flow, substrate temperature, RF power and working pressure on the etch rate and etch profile was analyzed. An optimized process with a lateral etch rate of 50 nm/min (RF power, working pressure, gas flow and temperature of 50 W, 0.4 mbar, 40 sccm SF6 and 425degC, respectively) was found, which enables the fabrication of novel resonant micro-electromechanical systems (MEMS) based on AlGaN/GaN heterostructures on SiC substrates.