A study of deposition conditions on charging properties of PECVD silicon nitride films for MEMS capacitive switches

Keywords: RF MEMS capacitive switch Dielectric charging Silicon nitride PECVD method a b s t r a c t The present paper aims to provide a better insight to the electrical characteristics of silicon nitride films that have been deposited with PECVD method under different conditions. The effect of film thickness, substrate temperature and the frequency that produces the plasma in PECVD method on the dielectric charging phenomenon in silicon nitride films has been investigated with the aid of thermally stimulated depolarization currents (TSDC) and Kelvin Probe (KP) techniques. The results indicate that the decrease of film thickness and substrate temperature as well as the deposition at high frequency plasma (13.56 MHz) seems to produce silicon nitride films that are less prone to dielectric charging and thus better candidates for MEMS capacitive switches. Radio Frequency Micro-Electro-Mechanical System (RF-MEMS) capacitive switches have emerged as promising technology for several applications due to significant advantages such as low cost, low power consumption and high linearity [1]. The unique characteristics of RF MEMS capacitive switches make them ideal candidates for integration into passive circuits, such as phase shifters or tunable filters, and for implementation in many terrestrial and space applications, including portable telecommunication, reconfigurable antennas, wireless computer networks and others. In spite of all these, their commercialization is still hindered due to reliability issues, the most important being the effect of dielec-tric charging [2,3]. The dielectric films in RF MEMS switches are deposited on rough surfaces and at low temperatures, fact that leads to amorphous, micro/nano crystalline materials with minor or major deviation from stoichiometry [4]. In the course of improving the ON/OFF capacitance ratio in RF MEMS switches and determining the optimum electrical properties of dielectric films, several materials have been employed. Among those the most intensively investigated is the silicon nitride (Si 3 N 4). Several depo-sition conditions have been used up to now, including different substrate temperatures [5], plasma frequencies [6,7] and gas flow ratios [8]. The electrical assessment is based on charging/discharg-ing current transients (CCT/DCT) [6,7], thermally stimulated depo-larization currents (TSDC) [5,8] and Kelvin Probe Force Microscopy (KPFM) [9]. Each of these studies has been applied on limited available materials and it revealed a strong dependence of electrical properties on the deposition conditions. The aim of the present paper is to shed light on the electrical properties of PECVD silicon nitride and investigate its applicability to RF MEMS capacitive switches. …