QUASI-STATIC AND HIGH-FREQUENCY C(V)-RESPONSE OF THIN PLATINUM METAL-OXIDE SILICON STRUCTURES TO AMMONIA

Quasi-static and high frequency (1 MHz) capacitance-voltage characteristics are reported for thin Pt (&le6 nm) MOS capacitors for exposures to ammonia in a background atmosphere of 20% O2/Ar. The ammonia-induced voltage shift of the quasi-static capacitance-voltage characteristics is always greater than or equal to the shift of the high-frequency characteristics for a given ammonia concentration. We discuss the differences in the ammonia response for the case of the two different contact metals, Al or Pd. The quasi-static results can be used to show that the decreased sensitivity with Pt film thickness may be only partly related to the high impedance of the Pt film itself. Based on these results, we argue that the intrinsic ammonia sensitivity of Pt films (less than about 20 nm thick) decreases with decreasing film thickness. We also report results for hydrogen exposure of 1.5 nm Pt films and discuss them together with complementary measurements of the film impedance. It is concluded that ammonia interacts differently with the thin Pt film compared to hydrogen. The findings in this paper support a recently proposed model for the ammonia sensitivity of thin catalytic metal films.