High-temperature giant piezoresistivity of microstructured SiOC-based strain gauges

The foundation of this work is laid out based on the efficiency of silicon oxycarbide (SiOC) as a functional material for piezoresistive device applications. The realization of a cost-efficient strain gauge which can operate at elevated temperature serves as the foremost objective of this work. This goal is fathomed based on prevailing knowledge regarding the high piezoresistivity of SiOC at the range of 10 - 102 coupled with commendable properties such as electrical conductivity, good thermal resistance, and an excellent coating material for hostile environment. An optimized process of spin coating is used to deposit ~500 nm SiOC film onto the 100-mm diameter silicon substrate with a silica layer of 500 nm. The deposition process is screened with a Taguchi design of experiment resulting into a replicable and controlled process with a crack-free and homogenous coating. An in-house piezoresistivity test setup was fabricated with considerations of minimizing the electrical contact resistances, capability to perform mechanical cyclic loads, and the ability to operate at elevated temperature until 700 °C. After the annealing process, the SiOC film manifested round-shaped segregations which were identified as carbon-rich and oxygen-depleted, evenly dispersed in an oxygen-rich matrix. Deeper investigation of the segregated area revealed 2-level hierarchical microstructure of sp2-hybridized carbon, Si3N4 and SiC. On the other hand, Raman analysis confirmed presence of sp2-hybridized carbon not just on the segregated area but also on the matrix distinctive by the difference of crystal sizes. Larger domains of carbon including tortuosity (Leq) are present on the segregation than on the matrix of the film. Kinetics study showed that the segregations area results of free carbon diffusion through the silica layer with an activation energy equal to 3.05 eV. Platinum electrodes are printed on the surface of the SiOC film via photolithography for the PZR tests. The fabricated strain gauge prototypes have high sensitivity with gauge factors (GF) in the range of 2000 – 5000 tested at 25 – 400 °C. At 500 – 700 °C, the behavior of the material shifted from semiconducting to conducting decreasing its resistance to 11 Ω, and GF of 200. This GF is still comparably larger than commercial metal- and silicon-based strain gauges. The difference of mechanical cyclic loads applied on the prototypes influenced the degree of response’ hysteresis and the linearity of the strain range. In both cases, tests under compressive load showed superiority over tensile tests. Through these results, this study provides a working strain gauge prototype based on SiOC thin film with high sensitivity, reproducibility, and robustness. The giant piezoresistivity of the fabricated strain gauge at an elevated temperature, until 700 °C, surpasses the known application of the current commercial strain gauges. Furthermore, the perceived shift on electrical behavior of the material at 460 °C broadens its applications to current-limiting devices and temperature sensors