TIME-DEPENDENCE OF THE CHEMICAL RESPONSE OF SILICON-NITRIDE SURFACES

Insulators used as pH-sensing surfaces in field-effect sensors all show a greater or lesser amount of hysteresis and drift, and the magnitude of these two phenomena is usually correlated. We have found that after a pH step, a silicon nitride surface has a small slow response after the immediate response. A multiple time-constant model is presented showing how this slow response causes hysteresis. Measurements of the time constants and amplitude of the slow response, and measurement of the hysteresis on the same sample were in quantitative agreement. Since the slowest time constant is long (over 3 h), the memory for previous pH changes is long and can easily be mistaken for drift. The actual long-term drift is low, about 0.1 mV/h. This slow response behavior of silicon nitride is the main characteristic which needs improving to obtain a better pH sensor. © 1990.