EFM study of the influence of humidity on charge injection and charge relaxation in silicon nitride used in electrostatically actuated MEMS

In this work, spots of charges have been injected on typical silicon nitride used for RF-MEMS switches. Several relative humidity (RH) conditions have been used for the charge injections. The evolution of these spots with time has been assessed thanks to electrostatic force microscopy (EFM) measurements. Experimental results confirm the hypothesis of surface conduction enhancement but also show a 30% increase of the density of injected charge by humidity. It has been also shown that a level of 40% RH decreases the charge relaxation time by a factor 10 to more than 100 compared to a charge relaxation process occurring under dry atmosphere. This phenomenon may be due to the ability of protons contained in the water to diffuse trough silicon nitride.