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THIN FILM ORGANIC PASSIVATION COATINGS FOR MEDIA COMPATIBLE PRESSURE SENSORS

IP.com Disclosure Number: IPCOM000007425D
Original Publication Date: 1995-Jul-01
Included in the Prior Art Database: 2002-Mar-25
Document File: 2 page(s) / 152K

Publishing Venue

Motorola

Related People

David J. Monk: AUTHOR

Abstract

Silicon piezoresistive pressure sensors have been successfidly assembled with Parylene C" in place of fluorosilicone gels. Parylene" is a conformal coating (Figure 1) that is chemically resistant to most known solvents below 175°C. Therefore, it is being used to provide media compatibility for conventional pres- sure sensors. The Parylene" coating places an addi- tional, controllable stress on the transducers. More- over, heating of this room-temperature, vacuum- deposited film, above its glass transition tempera- ture initially causes a change in the stress that is applied to the transducer. This "annealing" effect occurs only during the first heating cycle and is prob- ably the result of a reduction in residual stress and/or crystallinity formation. Parylene" thin films cause sensitivity changes in the transducer that are a func- tion of thickness. Finite element modeling has con- firmed this and is accurate to within 10% presently. Offset and linearity are not significantly affected by the addition ofthe thin film.

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8 MolylRoLA Technical Developments

THIN FILM ORGANIC PASSIVATION COATINGS FOR MEDIA COMPATIBLE PRESSURE SENSORS

by David J. Monk

  Silicon piezoresistive pressure sensors have been successfidly assembled with Parylene C" in place of fluorosilicone gels. Parylene" is a conformal coating (Figure 1) that is chemically resistant to most known solvents below 175°C. Therefore, it is being used to provide media compatibility for conventional pres- sure sensors. The Parylene" coating places an addi- tional, controllable stress on the transducers. More- over, heating of this room-temperature, vacuum- deposited film, above its glass transition tempera- ture initially causes a change in the stress that is applied to the transducer. This "annealing" effect occurs only during the first heating cycle and is prob- ably the result of a reduction in residual stress and/or crystallinity formation. Parylene" thin films cause sensitivity changes in the transducer that are a func- tion of thickness. Finite element modeling has con- firmed this and is accurate to within 10% presently. Offset and linearity are not significantly affected by the addition ofthe thin film.

  Comparative reliability (i.e., humidity) testing between the Parylene" coated and the conventional Shin-Etsu FE-S3 fluorosilicone gel-tilled XPX 2010D and XPX 4004D parts suggest that Parylene" can protect the device from electrical failure through at least 240 hours of HAST or 192 hours of autoclave (121°C 100% RH, 2 atm, no applied bias) exposure. Corrosion on the power lead was evident on Parylene" coated parts after 144 hours and gel-filled parts after 96 hours ofHAST (135°C 85% RH, 24 psig, applied bias) exposure; only one gel-tilled part was observed to fail electrically. The observed corrosion was deter- mined not to be a function of chlorine originating From the Parylene c". Non-chlorinated Parylene N" exhibited similar chlorine-driven corrosion during HAST exposure in epoxy packages. However, poly (phenylene sulfide) Parylene"-coated packages did not show any signs of corrosion. Delamination was observed with the Parylene" coated parts, but the film was still present on the silicon die aher 240 hours of HAST exposure. The work described above and the fact that the cost per part is similar to

fluorosilicone gels shows that Parylene" can be used in place of gels for non-media compatible pressure sensor applications. Surface preparation ofthe parts was observed to have a significant effect on the elec- trical response of the parts following Parylene" coat- ing and further experimentation is being performed to optimize the pre-deposition cleaning techniques.

  Superior media compatibility for Parylene"-coated pressure sensors in contrast with silicone gel-filled pressure sensors has been observed with a variety of tests described below. Comparative fuel exposure testing was performed on Shin-Etsu FE-S3 fluoro- silicone gel-filled parts and Parylene" coated XPX 4023A manifold absol...