PURPOSEThe objective of this work was to show that the Q-factor of MEMS resonators can be increased and stabilized with the use of getters. [ABSTRACT The need to reach high, reliable and stable values of the Q-factor is one of the key issues of resonant MEMS in order to make highperformance sensors. The Q-factor is strongly influenced by the internal environment of the MEMS packaging, by the total pressure and by the gas composition. The most experienced and technically accepted way to keep the atmosphere stable in a hermetically sealed device is the use of getters that are able to chemically absorb active gasses under vacuum or in inert gas atmosphere for the lifetime of the devices. Several types of hermetically bonded MEMS devices such as gyroscopes, accelerometers, pressure and flow sensors, IR sensors, RF-MEMS and optical mirrors require getter thin film solutions to work properly. A possible getter technical solution for wafer to wafer hermetically bonded MEMS systems is PaGeWafer. PaGeWafer is a silicon, glass or ceramic wafer ("cap wafer") with a patterned getter film, few microns thick. In this paper, a theoretical evaluation of the Q-factor of a MEMS resonant structure in presence of a getter film is investigated and compared to the results of a Residual Gas Analysis of the same MEMS resonant structure and with the conventional measurement of the Q-factor. Using getter thin film technology, total pressures down to 10 -4 mbar with corresponding high and stable Q-factors have been achieved in MEMS resonant structures. We were therefore able to confirm that a getter film can provide high Q-values, stability of the sensor signal, performances stability during the lifetime and removal of dangerous gases like H 2 and H 2 O in hermetically sealed MEMS resonant structures.

Optical properties and mechanical stress of silicon nitride films deposited at room temperature by ion-beam sputtering are studied for various compositions in the range of N/Si=0.6 to N/Si=1.4. The refractive index and mechanical stress are investigated before and after annealing by ellipsometry and the Newton ring method. The silicon-rich film extinction coefficient and stress value decrease after annealing. These property variations are explained by a high structural disorder in films deposited at room temperature which is lowered during annealing. The study of a layer deposited at high temperature (600°C) enables us to check the thermal stress effect in the film. A stress evaluation in the film and at the Si3N4-Si interface shows that sputter-deposited silicon nitride is suitable for the local oxidation of silicon.

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