Development of a high temperature sensor suitable for post-processed integration with electronics
Integration of sensors and silicon-based electronics for harsh environment applications is driven by the automotive industry and the maturity of semiconductor processes that allow embedding sensitive elements onto the same chip without sacrificing the performance and integrity of the electronics. Sensor devices post-processed on top of electronics by surface micromachining allow the addition of extra functionality to the fabricated ICs and creating a sensor system without significant compromise of performance. Smart sensors comprised of sensing structures integrated with silicon carbide-based electronics are receiving attention from more industries, such as aerospace, defense and energy, due to their ability to operate in very demanding conditions. This thesis describes the design and implementation of a novel, integrated thin film temperature sensor that uses a half-bridge arrangement to measure thin film platinum sensitive elements. Processes have been developed to fabricate temperature insensitive thin film tantalum nitride resistors which can be combined with the platinum elements to form the temperature transducing bridge. This circuit was designed to be integrated with an existing silicon carbide-based instrumentation amplifier by post-CMOS processing and to be initially connected to the bond pads of the amplifier input and output ports. Thin films fabricated using the developed TaN and Pt processes have been characterized using resistive test structures and crystallographic measurements of blanket thin film layer samples, and the relationship between the measurement results obtained has been analyzed. An initial demonstration of temperature sensing was performed using tantalum nitride and platinum thin film resistor element chips which were fabricated on passivated silicon substrates and bonded into high temperature packages. The bridge circuit was implemented by external connections through a printed circuit board and the bridge output was connected to a discrete instrumentation amplifier to mimic the integrated amplifier. The temperature response of the circuit measured at the output of the amplifier was found to have sensitivity of 844 μV·°C–1 over the temperature range of 25 to 100 °C. Two integrated microfabrication process flows were evaluated in this work. The initial process provided a very low yield for contact resistance structures between TaN and Pt layers, which highlighted problems with the thin film platinum deposition process. Multiple improvement options have been identified among which removal of the dielectric layer separating TaN and Pt layers and thicker Pt film were considered and a redesign of both layout and the process flow has resulted in improved yield of platinum features produced directly on top of TaN features. Temperature sensitivity of the integrated sensor devices was found to depend significantly on parasitic elements produced by thin film platinum step coverage, the values of which were measured by a set of resistive test structures. A new microfabrication design has enabled the production of a group of integrated temperature sensors that had a sensitivity of 150.84 μV·°C–1 in the temperature range between 25 and 200 °C on one of the fabricated wafers while the best fabricated batch of sensors had a sensitivity of 1079.2 μV·°C–1.