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In this contribution fundamental design considerations for surface transverse wave devices (STW devices) as liquid sensors are presented. Detailed studies have shown that the interference of the surface transverse wave on y-rotated quartz cuts with the surface skimming bulk wave (SSBW), the triple-transit echo (TTE), and the electromagnetic crosstalk (EC) causes perturbations, which can exceed the real sensor signal caused by the STW. With an optimized sensor design, these perturbations can be suppressed and reproducible measurements become possible. The industrial use as sensors for physical liquid properties, e.g. viscosity, in rough environments demands a passivation of the interdigital transducers (IDTs) and the sensing area. Silicon carbide (SiC) has an excellent chemical and mechanical resistance and is deposited on the sensors in a PECVD process. A viscometer for Newtonian liquids, a direct immunosensor system, and a dip-stick sensor for the measurement of mineral oil viscosity are presented in this paper.

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A high performance silicon dry etch process (STS Advanced Silicon Etch ASE) which in many cases is a beneficial replacement for the usual anisotropic wet etch methods like KOH etching is presented. During fabrication of Micro-Electro-Mechanical Systems (MEMS) the patterning ofsilicon is an essential step. Conventional wet or dry etching processes used up to now cannot meet the majority of future MEMS patterning needs. The process described in this paper allows a wide range of possible geometries and freedom of design and mask layout for novel MEMS applications. The installed etch system is working with an inductively coupled plasma source (ICP) which produces high plasma densities at low pressure to achieve deep silicon etching (> 200 vim) with high etch rates up to 5 jtm/min and a high passivation layer selectivity. The new ASE process uses only fluorine based chemistry and operates at room temperature. ASE uses photoresists and silicon oxid layers as an etch passivation and allows the manufacturing of silicon structures with nearly vertical side walls in bulk and surface micromachining illustrated by several MEMS applications carried out at the Fraunhofer Institute for Solid State Technology. With depths up to 100 jim realized at the institute now and an excellent anisotropic profile control ASE is obviously the tool, useful from device development to volume production of microsystems.

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