Fabrication methods for environmentally hardened sensors [electronic resource]
Keywords: Electrical Engineering

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 (< 50 MPa), CMOS-compatible recipe for PECVD silicon carbide could be selected. Deposition conditions were 1600mTorr, a methane flow rate of 1440 sccm, a 2% silane in argon flow rate of 2840 sccm, high frequency power of 100W for 4.5 s, and low frequency power of 100W for 4.5 s. The use of PECVD silicon carbide was demonstrated in several applications. The film was deposited on an off-the-shelf pressure sensor and package to improve its media compatibility. As part of this experiment, its mechanical properties were evaluated including Young's modulus (52GPa), hardness (7.3GPa), and coefficient of thermal expansion (CTE) (2.5 ppm/°C). The net effect on the parametric behavior of the pressure sensor was measured. For a 0.4 mm thick film, the offset shifted 8.6% FSO, and sensitivity was reduced approximately 25%. Both of these shifts could be accounted for by design. The addition of PECVD silicon carbide had the beneficial effect of reducing the temperature coefficient of sensitivity (TCS) by 100 ppm/°C, roughly 4%. Tests of the carbide coated pressure sensor by linear polarization and electrochemical impedance spectroscopy in sea water and nitric acid (HNO3) showed that the silicon diaphragm was protected, but the sensor failed because the PECVD failed to stick to the bond wires and package. PECVD silicon carbide was deposited on an iridium microelectrode array. The microelectrode array was designed for the measurement of heavy metal ion concentration by square wave anodic stripping voltammetry (SWASV). With the carbide coating, successful analysis of 50 ppb of Cu+2 could be performed in HF over periods of several days. A microfluidic gasket structure was designed that enabled the complete coating of a channel structure with PECVD silicon carbide. It was incorporated into a novel microfluidic total organic carbon (TOC) sensor. The principals of the TOC sensor were first tested with a benchtop version that demonstrated the capability of measuring its theoretical limit of ~10 mM. Fabricating the gasket required adapting Riston®, a dry-film printed circuit board photoresist, to wafer-level processing. When completed, extensive cracking developed in the microfluidic version. Finite element analysis supports the conclusion this was due to excessive stress from bonding around the gaskets.