Dielectric charging mechanisms in RF-MEMS capacitive switches George J. Papaioannou 1 and John Papapolymerou 2 1 National Kapodistrian University of Athens, Athens, Greece 2 Georgia Institute of Technology, Atlanta, GA, 30332, USA 1 gpapaioan@phys.uoa.gr 2 papapol@ece.gatech.edu Abstract — In this paper we present for the first time the simultaneous action of dipolar and space charge polarization charging mechanisms in the dielectric film of capacitive RF MEMS switches. These mechanisms charge the film surface with opposite charges. At room temperature the dominant mechanism is the space charge polarization while at higher temperatures the dipolar polarization prevails. In Si 3 N 4 the transition occurs at about 380K where the average charging is minimized, an information that can be used to engineer the dielectric properties so that the transition occurs at room temperature. I. INTRODUCTION Capacitive RF MEMS switches are one of the most promising applications in microelectromechanical systems (MEMS), but their commercialization is currently hindered by reliability problems. The most important problem is the charging of the dielectric, causing erratic device behavior [1–4]. Presently, the available models assume that the dielectric charging arises from charges distributed throughout the dielectric material [4], the presence of charges at the dielectric interface [5] and the injection of charges form the suspended bridge during ON-state [6]. So far the dielectric charging process has been investigated by recording the transient current in permanently ON switches [7-10], the transient response of the ON capacitance [11, 12] and the correlation of Poole-Frenkel current intensity to the shift of pull-out voltage [9]. These efforts, although constituted a major step towards the understanding of MEMS dielectric charging, were focused on the study of contribution of charge injection leaving out mechanisms that are related to the intrinsic polarization such as the intrinsic space charge polarization and the dipole orientation [13]. The effect of temperature on the dielectric charging has been investigated in MIM capacitors with SiO 2 dielectric [7, 9] and in MEMS switches with Si 3 N 4 dielectric [11, 12]. Finally, a process arising from contact-less charging and related to the dielectric intrinsic polarization processes has been reported recently [14, 15]. Taking all these into account it becomes clear that the simultaneous study of all charging mechanisms, of extrinsic and intrinsic origin, is of paramount importance. The aim of the present work is to demonstrate that when a RF-MEMS switch is in the ON state both the extrinsic and intrinsic charging processes occur. Moreover, that temperature plays a key issue role on the manifestation of each mechanism, so that at low temperatures the dielectric charging arises from the charge injection while at high temperatures the dominant mechanism is the dipolar polarization. II. BASIC THEORY On the time scale of interest to RF-MEMS capacitive switches response (i.e. greater than 1μsec) an electric field can interact with the dielectric film in two primary ways. These are the reorientation of defects having an electric dipole moment, such as complex defects, and the translational motion of charge carriers, which usually involve simple defects such as vacancies, ionic interstitials and defect electronic species. These processes give rise to the dipolar (P D ) and the intrinsic space charge (P SC-i ) polarization mechanisms, respectively. Moreover, when the dielectric is in contact with conducting electrodes charges are injected through the trap assisted tunneling and/or the Poole- Frenkel effect [16] giving rise to extrinsic space charge polarization (P SC-e ) whose polarity is opposite with respect to the other two cases. In RF-MEMS capacitive switches during ON state all polarization mechanisms occur simultaneously and the macroscopic polarization is given by e SC i SC D tot P P P P − − − + = (1) Now, from elementary physics it is known that the electric displacement, D, defined as the total charge density on the electrodes, will be given by P E D + = 0 ε , where E is the applied field and P the dielectric material polarization. The resulting polarization P may be further divided into two parts according to the time constant response [17]: