ANNEALING THIN FILM PARYLENE COATINGS FOR MEDIA COMPATIBLE PRESSURE SENSORS
Original Publication Date: 1996-Aug-01
Included in the Prior Art Database: 2002-Apr-23
Parylene C is being explored as an encapsulant coating for bulk micromachined silicon piezoresistive pressure sensor devices that will be used in harsh environments, including: salt water, acids, bases, and fuels. However, coated pressure sensors have been observed to undergo change in both sensitivity (volt- age output vs. pressure slope) and zero pressure out- put (offset) following the initial excursion above the glass transition temperature for parylene C (Ta s 80 - 100°C [l]). It is believed that during the annealing process, as a result of relaxation of resid- ual stresses formed during deposition and/or a recrystallization of the polymer chains, the parylene film undergoes changes in material properties, thus causing a shift in electrical output of the coated device. Once the coated sensors have been properly annealed, subsequent heating of devices above the Ts does not shift the electrical parameters. Further- more, the magnitude of the shift in offset can be correlated to the thickness of the parylene and the 2e+7 As-Deposited easurement maximum annealing temperature. This has been shown through two different experimental methods: time-based measurement of device electrical param- eters as a hmction of film thickness and anneal tem- perature, and wafer curvature stress measurements as a function of the same parameters. Also, finite element analysis (FEA) has shown the same results, and has been used to predict the expected parametric variations caused by known process variability.