Radiation Effect in Si3N4 Thin Films for RF-Mems Reliability Investigation
Lamhamdi, M.¹; Papandreou, E.²; Koutsoureli, M.²; Pons, P.¹; Boudou, L.³; Guastavino, J.³; Segui, Y.³; Coccetti, F.¹; Papaioannou, G.²; Plana, R.^1
1LAAS-CNRS; ²Solid State PhysicsSection, University of Athens; ³Laplace

Radio frequency Micromechanical systems (RF MEMS) are receiving increasing interest for space communication technology and for a large variety of more or less hostile terrestrial applications. The device knowledge on the sensitivity to ionizing radiation has been restricted on monitoring the shift of actuation voltage in with the cases of Si3N4 and Al2O3 dielectric upon Co60 ã-ray radiation. Ion and ã-ray radiation, in addition to ionization, introduce defects in the MEMS dielectric film which affect the dielectric charging. These defects may arise from atom displacement and/or from defect interaction in expense of preexisting ones. Therefore the defect interactions will strongly depend on the material stichiometry and the amount of incorporated hydrogen. An additional parameter that affects the defect interaction is the current induced conductivity in hydrogenated amorphous silicon-rich alloys. This is determined by the concentration of silicon dangling bond states generated by the energy released during hole-electron recombination. The principal objective of this research is to gain an understanding about the nature of the defects introduced by the space radiation (alpha particle, gamma and proton) and the resulting charging in Si3N4 thin films, typically used as dielectric layer in RF MEMS capacitive switches. The investigation has been performed with the aid of Metal-insulator-metal (MIM) capacitor (MIM capacitor represents a very good approximation of the ideal down state contact for a MEMS capacitive switch). The test structures consists of 300nm of silicon nitride PECVD deposited under three different conditions: low frequency (LF) power supply, (380 kHz); high frequency (HF) power supply (13.56 MHz); mixed frequency (MF) power supply (380 kHz/13.56MHz). The selection of these deposition methods was based on the assumtion that the stichiometry and the hydrogen concentration vary from HF to LF plasma frequency. The material stichiometry and concentration of chemical bonds were determined from RBS and FTIR measurements. The MIM capacitors structures were asymmetrical in terms of bottom and top electrodes which were Au and Ti respectively. Dielectrics charging were assessed by applying the Discharge Current Transient (DCT) and the Thermally Stimulated Depolarization Current (TSDC) methods. The first allowed the determination of charging and discharging transient time constants, while the second makes possible the determination of thermally activated charging mechanisms and the calculation of the room temperature value for very long time constants.

Following the first electrical characterization, a series of irradiations are in progress. These involve exposures to gamma-rays, proton and alpha particles. Significant changes are observed between the behavior before and after irradiation.

The electrical and FTIR results were used in order to obtain a clear view of the kinetic of charging occurring in these dielectrics under irradiation. These results will be given in the extended version of this work. Finally, optimized dielectric will be chosen and a similar investigation has been initiated on real RF-MEMS capacitive device a RF MEMS switch in order to emphasize the impact of the charging of ionizing radiation on the reliability behaviour.