Formation of SiO2/Si3N4/SiO2 Positive and Negative Electrets on a Silicon Substrate

A new technique is presented for creating an electret from a plasma-enhanced chemical vapor deposition (PECVD) multilayer film of SiO
₂
/Si
₃
N
₄
/SiO
₂
. The technique uses a direct contact silicon electrode during poling. This thin-film electret formation process capitalized on deep traps in the silicon nitride, which is known to develop from hydrogen interactions with silicon dangling bonds and, in some stoichiometries, nitrogen dangling bonds. The materials used are compatible with standard microfabrication processes, and the electret activation process is accomplished with a commercial anodic bonding system, traditionally used for bonding borosilicate glass to silicon wafers. Negative electrets with an effective surface voltage (ESV) up to -236 V and positive electrets with an ESV up to 195 V are produced. The lifetime of the electrets are evaluated by performing accelerated aging at elevated temperatures. Results indicate that the electrets are expected to provide a mean lifetime effective charge for decades at a temperature of 125 °C. A thin-film multilayer electret fabrication process is shown to yield electrets with a half-life 5 times greater than those values reported for other PECVD fabricated multilayer electrets.