An active dissolved wafer process for the fabrication of integrated sensors.

Abstract

At the University of Michigan, a variety of microsensors have been successfully fabricated based on the Dissolved Wafer Process (DWP) that relies on the simple but robust heavy boron doping etch stop mechanism where layers ranging in thickness from $\sim$2-15$\mu$m may be easily achieved. However, the challenge of fabricating arbitrarily sized microstructures along with integrated circuitry cannot be supported by the DWP. This research has achieved its primary objective of directly addressing this challenge through the successful development of the Active DWP (ADWP). This new process retains DWP's qualities of simplicity, robustness and the possibility for high volume production. In addition, the ADWP incorporates the ability to fabricate a lightly-doped circuit region along with passive devices to yield an active transducer. Traditional ElectroChemical Etch (ECE) control techniques were insufficient for the ADWP so two new techniques had to be developed. The capacitive based and modified 3 terminal (3T) ECE techniques were the backbone to this effort and the capacitive version of the ADWP was demonstrated, with a simple etch control circuit, to provide integrated sensors. Etch stop uniformity of better than 1% was achieved with the capacitive technique where the monitored capacitance increased from 8nF/cm$\sp2$ to 100nF/cm$\sp2$ upon exposure of the etch stop layer to solution. In the modified 3T and other ECE schemes that rely on the constant application of potentials for control, premature passivation $\sim$2000A away from the metallurgical junction occurred consistently. Analysis of this phenomenon has not been reported previously. An accelerometer was used to demonstrate ADWP's capabilities for the fabrication of a variety of integrated sensors. The high density, multiple electrode structure achieves moderate performance levels with the possibility of force-rebalanced operation. A sensitivity of better than 1% and a bandwidth of 70Hz was achieved with a working capacitance of 2$\sim$6pF. The integration of these devices with a standard CMOS process in the ADWP was achieved in-house. Challenges included lead transfer, preservation of etch control and CMOS circuitry and judicious circuit design and layout. Working CMOS circuits and FETs along with functioning integrated accelerometers were demonstrated. Performance characteristics for both the mechanical and electrical devices achieved their design targets.