Analytical techniques for examining reliability and failure mechanisms of barrier coating encapsulated silicon pressure sensors exposed to harsh media

摘要

Low-cost, silicon piezoresistive pressure sensors need to be compatible with a variety of chemical environments to provide pressure and liquid level sensing products for various automotive, industrial, and consumer white goods applications. Previous work has identified that the typical failure mechanism for a barrier coated device involves the delamination of the coating from the substrate followed by corrosion of exposed metal areas. This work introduces the application of known electrochemical techniques for the development of accelerated experimental test procedures for sensor exposure to harsh environments. Qualitative correlation of these results with predicted reliability lifetimes, estimated statistically from media exposure testing, is shown. Several methods are presented for assessing the quality of barrier coatings. These techniques can be used both to identify specific corrosive failure mechanisms as they are occurring during media exposure, and to make relative predictions about the reliability lifetime of barrier coated and encapsulated devices. One demonstrated method is the simple measurement of open circuit (non-biased) potential. This is envisaged to show a mixed potential between all anodic and cathodic reactions, while taking into account the resistance of the coating. The fluctuations in mean potential with time depend on variations in the activities of different sensor regions and on underfilm passivation. The standard deviation of voltage noise can be used as an indication of the quality of the coatings. The critical factor in these measurements and sensor encapsulation in general is understanding reactant diffusion through a barrier coating. In addition, polarization measurements were used to examine the rate of media diffusion through the coating and to determine the reaction mechanism.