This project entailed the design, fabrication, and testing of a surface micro-machined electret pressure sensor with the possible use as a microphone. The design is based on a capacitance electret microphone. This type of microphone uses a plate that has a built in charge to provide the bias of the system. This eliminates any external bias directly attached to the sensor. The surface micro-machining means that no backside etch is required to form the membrane. Without the backside etch the process can be integrated with a CMOS process much more easily. This electret pressure sensor uses a poly-silicon floating gate that has a fixed charge Q place on it to Create an internal bias between the floating gate and the upper aluminum diaphragm. This means that the device can run without a sustained external bias, instead of the constant bias that a condenser pressure sensor/microphone requires. The devices were fabricated on 6 inch wafers, using a 2 metal and floating gate process. The testing showed that the control gate was shorted to the n-well. This meant the floating gate could not be charged, thus the device could not be tested properly. The process did show the viability of the surface processed diaphragms. They were completely released from the sacrificial resist, and were shown to hold their shape.
"Surface Micro-Machined Capacitive Pressure Sensor,"
Journal of the Microelectronic Engineering Conference: Vol. 17
, Article 14.
Available at: http://scholarworks.rit.edu/ritamec/vol17/iss1/14