Aluminum Nitride Sensors for Harsh Environments

Harsh environment applications include high temperature, pressure and mechanical shock. Aluminum nitride is a strong ceramic material with very good high temperature survivability. It also has piezoelectric properties that can be used for sensing applications and it can be deposited with good control as thin polycrystalline film for the fabrication of micro-electromechanical systems. In this dissertation, optimized deposition parameters for aluminum nitride films and characterization techniques for film stress gradients are investigated. Furthermore, two different fabrication processes are presented that can be used to build several sensors and other micromechanical elements on a single chip. The more advanced process includes a bulk-micromachining step that enables the use of the silicon substrate as large proof masses for accelerometers. At the same time, the fabrication process can be used to create membranes for devices such as pressure sensors. Several different devices are discussed from the device design and theoretical analysis to the fabrication and experimental verification. The most significant contribution is the performance improvement of several orders of magnitude that can be achieved with aluminum nitride accelerometers by switching from double-ended tuning forks to triple-beam tuning forks and by using the newly discovered sensing concept of bent-beam sensing. Additionally, rate gyroscopes were presented that use aluminum nitride as the structural material and for both actuating the drive axis and reading the sense axis. Both types of sensors have potential applications in inertial navigation at high temperature. Double-ended tuning forks and triple-beam tuning forks were tested in hot environment and it was shown that they can be used as highly sensitive strain sensors up to at least 570 ◦C. Additionally, a testing setup for high temperature testing of accelerometers was demonstrated.