Microfabrication technologies for single-crystal silicon sensors

For decades, silicon as a high quality solid material has been playing a major role in developing state-of-the-art micromechanical sensors. In this work, the performance of silicon etching processes is evaluated, and utilized in creating devices for high precision sensing of mechanical forces. Emerging methods such as atomic layer deposition (ALD) and focused ion beam (FIB) processing are combined with established technologies in order to achieve extended functionalities. The thesis is focused on the fabrication of components from single crystal silicon. In the developed processes, anisotropic tetramethylammonium hydroxide (TMAH) wet etching has a central role. The material behavior related to substrate specifications, doping level, and the effects inherent to thin films, is studied both during the fabrication stage and in the final structures. Furthermore, the intrinsic residual stress generation in mechanical devices is investigated. The results were applied in successful production of mechanical sensors with superior accuracy in acoustic wave detection. The specific features of silicon-on-insulator (SOI) technology were utilized in the fabrication of released, membrane-type cantilevers that serve as highly sensitive microphones in photoacoustic spectroscopy. Moreover, ALD thin film coatings were shown to be advantageous in manipulating the functional properties of resonant silicon sensors. A step towards further device miniaturization was taken by using FIB implantation as a wet etching mask to realize 3D nanoscale structures. Some of the developed cantilever sensors have already been implemented in commercial photoacoustic gas analyzers. In a broader perspective, the achievements related to stress control in mechanical silicon structures are of great importance in reliable production of various micro- and nanomechanical systems. All the methods employed in this work are compatible with integrated circuit (IC) processing, which enables combining the developed processes with electronic chips and the adaptation to industrial manufacturing.