Fabrication Methods for Environmentally Hardened Sensors

Micromachined sensors have continued to open exciting new doors in metrology. Applications in biology, pharmacology, genetics, chemistry, and other fields are driving crossdisciplinary research and development of sensors and sensing systems. The need for sensors that can function successfully in a broad range of environments is clear. Whether the objective is to produce devices for harsh chemical environments or those that will not interfere with delicate biochemical processes, developments in both materials and fabrication techniques will continue to be necessary. In the first stage, PECVD alumina, diamond-like carbon (DLC), flame-front diamond, and PECVD silicon carbide were evaluated for chemical resistance in 49% hydrofluoric acid, 4:1 sulfuric peroxide, concentrated HCL, and 25% tetramethylammonium hydroxide. Only PECVD silicon carbide demonstrated an etch rate less than 0.05 nm/min. While developing an optimized recipe for the STS 310PC PECVD reactor, bimodal behavior was discovered among the films tested; some withstood etching in 22% potassium hydroxide (KOH) at 80 °C, some etched relatively quickly. Rutherford backscattering with hydrogen forward scattering was performed to analyze the stoichiometry with a sample set of representative films. Stoichiometry did not explain the behavior. Further analysis with FTIR showed a correlation with the amount of terminal -CH3 present in the film. These results support a stoichiometric bond model which states that the etch behavior is due to a critically high level of silicon-silicon bonds within the film, which are susceptible to a attack by KOH. With this information, an optimized, etch-resistant, low-stress (