The surface micromachining platform is based on polycrystalline Si deposited with good stress control and uniformity, and sacrificial SiO2 layers, where a typical structure thickness is around 1 µm. The platform features an etching process through porous Si. Applications include capacitive pressure sensors and micro-machined ultrasound transducers (CMUT). A complementary process using SiO2 as structural layer and Si as the sacrificial layer based on XeF2 etching is also being adopted.
Silicon-on-insulator (SOI) based MEMS uses single crystal device layer in the thickness range of several microns to tens of microns, high aspect ratio Si dry etching (DRIE) and dry-released with HF vapor. Typical applications are electrostatically actuated MEMS sensors.
CSOI & Piezo MEMS
Modern pre-etched cavity-SOI based MEMS platform uses device layer thickness in the range from several microns to tens of microns, which are released in a single high aspect ratio etch step (DRIE). Use of C-SOI allows for pristine device surface without etch holes or stiction related problems and allows thin film structures on top of the devices.
The C-SOI platform especially allows the use of piezoelectric thin film (AlN) actuation for resonant sensors and actuators requiring no DC bias or narrow actuation gaps, operating with low AC voltages (~1V). Examples of applications include timing resonators, inertial sensors and high tilt angle MEMS mirrors.