Surface Micromachining Process for the Integration of AlN Piezoelectric Microstructures

We report a novel micromachining process to fabricate AlN (Aluminum Nitride) piezoelectric microstructures for actuator applications. Piezoelectric AlN thin films can be grown with (002) preferential orientation by means of RF reactive sputtering on various substrates. For this study, AlN was deposited on doped polysilicon layers, which act as both structural, and electrode layer. A thin layer of Cr was used as top electrode and as a mask for patterning of the AlN piezoelectric layer. Silicon oxide was used as a sacrificial layer. The structures were released by a freeze drying technique. Single run deposition of AlN and Cr turns out to be necessary to ensure good adhesion of the Cr layer to the AlN thin film during the sacrificial etching process either using BHF or HF. The released beams show compressive stress due to the AlN thin film. Further process development, using Cr seed layers to reduce stress, and device testing is in progress.