This dissertation explores the development of a new stainless steel pressure sensor capable of sustaining harsh environments, including high pressures, high temperatures, and/or corrosive media. The proposed pressure sensor utilizes commercial off-the-shelf (COTS) components, adapts vacuum coupling radiation (VCR) tube fitting (Swagelok Co.) for sensor packaging, and combines micro- and conventional-machining techniques for sensor realization. Capacitive transduction is used to simplify the implementation, as well as take advantage of the high stability and low temperature drift associated with this transduction scheme. Two generations of stainless steel capacitive pressure sensors have been developed in this dissertation.
The first-generation sensor is comprised of a stainless steel diaphragm die and a stainless steel backing plate, each electrically isolated with tetraethylorthosilicate (TEOS) silicon dioxide, and packaged by a set of COTS VCR tube fitting. The pressure sensor responses show four operating regions, including stabilizing, non-touch, transition, and touch mode regions. The fully packaged pressure sensor is characterized at high pressures of up to 10,340 kPa (1,500 psi) and at high temperatures of up to 300°C. Corrosive pressure media, including potassium hydroxide (KOH) and tetramethylammonium hydroxide (TMAH) solutions, are used to demonstrate the corrosive-media compatibility of the pressure sensor. After soaking in these corrosive media and several tens of pressure cycles over a month, the fully packaged pressure sensor continues to show stable and consistent operation. Because of the anomalous stabilizing region in these sensors, sensor-to-sensor variance is very poor (i.e., 62% in full-scale (FS)).
The second-generation sensor is developed to address the shortcomings found in the first-generation pressure sensor. A hard tungsten carbide backing plate used to replace the stainless steel backing plate and a stainless steel press plate are used to eliminate the stabilizing region found in the first-generation sensor. Three typical operating regions, including non-touch mode, transition, and touch mode regions, are achieved in the sensor operation. Without the stabilizing region, the sensor-to-sensor variance is improved to 9% FS. The fully-packaged pressure sensor is operated in high pressures of up to 6,900 kPa (1,000 psi), at high temperatures of up to 320°C, and/or in corrosive media, including KOH and sodium chloride (NaCl) solutions. In addition, the pressure sensor is operated 1+ million pressure cycles in the pressure range of 0-4,830 kPa (0-700 psi) at room temperature to demonstrate its lifetime and reliability.
In addition, the relationship of deposition conditions and properties of the TEOS silicon dioxide film is investigated using analysis of variance (ANOVA). The chemical composition of TEOS oxide is investigated using X-ray photoelectron spectroscopy and etch rate experiments. Finally, metal-insulator-metal test structures are fabricated to characterize the dielectric properties of TEOS oxide film at elevated temperatures.