Effects of Silicon Nitride Encapsulation on MOS Device Stability

A new threshold instability phenomenon has been observed in Sigate MOS transistors encapsulated with plasma deposited silicon nitride films. The saturation mode threshold has been observed to drift as much as ?0.5V when the MOSFET is operating with high drain voltage. Through a series of experiments it is concluded that this instability is due to a chemical effect associated with hydrogen in the nitride films. The formation of surface states and fixed charges in the channel region due to the-interaction of hot carriers with hydrogen present at the interface is proposed to be the basic mechanism. Experimental results with respect to voltage, time and device geometry, will be discussed.