Effect of Humidity on Dielectric Charging Process in Electrostatic Capacitive RF MEMS Switches Based on Kelvin Probe For
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1222-DD02-11
Effect of Humidity on Dielectric Charging Process in Electrostatic Capacitive RF MEMS Switches Based on Kelvin Probe Force Microscopy Surface Potential Measurements U. Zaghloul1,2, G.J. Papaioannou1, 3, F. Coccetti1,4, P. Pons1 and R. Plana1,2 1 CNRS ; LAAS ; 7 avenue du colonel Roche, F-31077 Toulouse, France 2 Université de Toulouse ; UPS, INSA, INP, ISAE ; LAAS ; F-31077 Toulouse, France 3 University of Athens, Solid State Physics, Panepistimiopolis Zografos, 15784 Athens, Greece 4 Novamems, 10 av. De l’Europe, F-31520 Toulouse, France ABSTRACT In this article we investigate the effect of relative humidity on dielectric charging/discharging processes in electrostatically actuated MEMS devices. The assessment procedure is based on surface potential measurements using Kelvin Probe Force Microscopy (KPFM) and it targets in this specific work PECVD silicon nitride films in view of application in electrostatic capacitive RF MEMS switches. Charges have been injected through the AFM tip and the induced surface potential has been measured under different relative humidity levels. The impact of the charge injection duration and the bias level as well as bias polarity applied during the charge injection step, have been explored. Finally, the effect of the dielectric film thickness under different relative humidity levels has been assessed through depositing SiN films with different thicknesses. KEYWORDS Dielectric charging, Humidity, Silicon nitride, Kelvin Probe Force Microscopy INTRODUCTION A very few work has been performed in order to study the effect of humidity on the dielectric charging process in RF-MEMS switches and therefore a full understanding of this effect is still missing. Surface charging has been reported to be more detrimental to switch lifetime than bulk charging [1]. Besides, based on measuring the shift in the switch actuation voltage, bulk charging was found to dominate in dry air, while surface charging was reported to increase linearly with increasing humidity [2]. Finally, it is reported that under comparable humidity levels and electric fields, switches employing silicon dioxide dielectric are less susceptible to surface charging than switches employing silicon nitride [1]. Recently, Kelvin Probe Force Microscopy (KPFM) has proven to be a very efficient method in assessing the dielectric charging process [3-7]. For example, the surface potential distribution was found to be more confined in thinner dielectric films when the same bias is applied during charge injection, independently on the substrate nature [6]. Yet, all previous work done up to now which made use of the KPFM for dielectric charging assessment has been performed in ambient air and hence the effect of humidity and the surface charge have not been addressed. On the other sides, the removal of the water-related layer from both surfaces on the sample and the AFM tip is reported to be very important to improve the reliability of the KPFM measurements [8]. At low humidity §10% RH trapped surface charges could be detected in
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