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Dielectric charging in silicon nitride ﬁlms for MEMS capacitive switches: Effect of ﬁlm thickness and deposition conditions U. Zaghloul a,b,*, G. Papaioannou c, F. Coccetti a, P. Pons a, R. Plana a,b a
CNRS, LAAS, 7 avenue du Colonel Roche, F-31077 Toulouse, France Université de Toulouse; UPS, INSA, INP, ISAE; LAAS; F-31077 Toulouse, France c Solid State Physics Section, University of Athens, Panepistimiopolis Zografos, Athens 15784, Greece b
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The paper presents a systematic investigation of the dielectric charging and discharging process in silicon nitride thin ﬁlms for RF-MEMS capacitive switches. The SiN ﬁlms were deposited with high frequency (HF) and low frequency (LF) PECVD method and with different thicknesses. Metal–Insulator–Metal capacitors have been chosen as test structures while the Charge/Discharge Current Transient method has been used to monitor the current transients. The investigation reveals that in LF material the stored charge increases with the ﬁlm thickness while in HF one it is not affected by the ﬁlm thickness. The dependence of stored charge on electric ﬁeld intensity was found to follow a Poole–Frenkel like law. Finally, both the relaxation time and the stored charge were found to increase with the electric ﬁeld intensity. Ó 2009 Elsevier Ltd. All rights reserved.
Article history: Received 30 June 2009 Available online 12 August 2009
1. Introduction Capacitive RF-MEMS switches are one of the most pioneering microelectromechanical systems (MEMS) devices for wireless applications. This is mainly due to their very high isolation, extremely low insertion loss and near-zero power consumption. In spite of this, their commercialization is still hindered by reliability problems nonetheless the studies on the subject carried out by several laboratories and companies worldwide. The most critical reliability issue for electrostatic capacitive MEMS switches is the charging of the dielectric . This charging process results in a change in the pull-in voltage and causes a stiction, limiting the functionality of the device. On the way to solve the problem of dielectric charging, several materials have been investigated among which the most intensively investigated ones are SiO2 and Si3N4. In contrast to the extensive knowledge gathered on their electrical properties, a comprehensive understanding of the charging process in dielectric materials used in RF-MEMS capacitive switches is still missing. In dielectrics the charging effect arises from dipoles orientation and charge injection, displacement and trapping that occurs under the strong electric ﬁeld during the down state of MEMS switches . The charge injection was reported to follow the Poole–Frenkel effect, a fact that has been supported by the shift of pull-up voltage . In the case of SiN, it has been shown that holes injection introduces metastable traps, which give rise to asymmetrical current–
voltage characteristic in symmetrical Metal–Insulator–Metal capacitors . Moreover the stored charge was found to increase with increasing the dielectric ﬁlm thickness and the nitride deposition temperature . Recently, the relaxation time determined using Kelvin Probe Microscopy was found to be larger in thinner dielectric ﬁlms . The main goal from this paper is to present for the ﬁrst time a comprehensive investigation for the charging and discharging process in SiN dielectric ﬁlms. The key issue parameters that affect the dielectric charging process that are the dielectric deposition conditions, the ﬁlm thicknesses and the applied electric ﬁeld intensity have been systematically investigated. Metal–Insulator–Metal (MIM) capacitors have been chosen as test structures while the charging and discharging currents of...