Multilayer Thin Film Getter for Sustainable Vacuum in MEMS Packaging

Abstract

Some applications require MEMS devices to be packaged in a high vacuum environment, for example, high-quality factor mechanical micro-resonators or low heat loss microbolometers can only be obtained in a vacuum better than 10-3 mbar [1-2]. For this purpose, many Non-Evaporable Getter (NEG) multilayers or alloys films based on sputtered highly reactive transition metals have been investigated. For example, R.K. Sharma [3] found the Ti21Zr24V55 getter and O.B. Malyshev [4] found the Ti-Zr-Hf-V getter can be activated at 160 °C for 2 hours and 150 °C for 24 hours respectively. The getter function after its thermal activation is to absorb gases generated inside the packaging cavity by outgassing, permeation and by real and virtual leaks after packaging cap sealing by a highly hermetic bonding process. In this work we investigate Au/Zr/V/Zr getter multilayers in order to get a sustainable low pressure in a packaging cavity. All films were initially deposited by e-beam evaporation on 100 mm in diameter Si（100）wafers. The samples of Au/Zr/V/Zr were deposited in an ultra-high vacuum set-up with a background pressure of 2 10-10 mbar. To investigate their activation and oxidation by gas sorption at typical pressures found in packaging cavities, the films were annealed for 1 hour at various annealing temperatures in a rapid thermal annealing (RTA) chamber (Jipelec Jetfirst 100) at a vacuum level 10-2 mbar. Then, these films were analyzed by using four probe resistivity measurement, SEM and TEM cross section observations, EDX measurement and SIMS. All results are consistent and support that the activation temperature of zirconium under vanadium depends on vanadium layer thickness. For example, a 25nm zirconium oxide layer is found under the vanadium film after annealing at 300°C for 1h when the vanadium thickness is less than 200nm. However, for a 450nm thick vanadium layer, the 1-hour annealing temperature must reach 350°C to obtain the same thickness of zirconium oxide layer. Next, these annealed Au/Zr/V/Zr getters were exposed to air at room temperature for 7 months. After this exposure, the sheet resistance was found to increase by 6 % and 15 % for samples annealed at 250°C-1h and 300°C-1h respectively. TEM cross section observations of these samples showed that the thickness of the zirconium oxide layer under vanadium increases by about 3 nm. Finally, to evaluate its sorption ability in a real application case, the Au/Zr/V/Zr getter film was integrated in the 1.3 cmx1.3 cmx2.5 mm packaging cavity of a resonant gyroscope and activated at 300°C for 30 min before cavity sealing by Au/Sn bonding at 285°C for 30min. The time dependence of the pressure inside the packaging was evaluated from the variation of the quality factor Q of a selected mechanical resonance of the gyroscope. Quantitative values of the internal pressure were obtained owing to a previous calibration of the Q versus pressure curve that was in agreement with a squeeze film damping model. The evaluated pressure inside the cavity was 4.5 x10-3 mbar after annealing, 3x10-3 mbar after 2 weeks, and then this vacuum level still maintained after 6 months. In conclusion, this work demonstrates that Au/Zr/V/Zr getter films have a low activation temperature and suitable sorption properties to get a sustainable low pressure in a medium size vacuum package. [1] M. Elßner, Vacuum quality evaluation for uncooled micro bolometer thermal imager sensors, Microelectronics Reliability 54 (2014), 1758. [2] G.-D. Anna, Miniature and MEMS-type vacuum sensors and pumps, Vacuum 82 (2009), 1419. [3] R.K. Sharma, Jagannath, S. Bhattacharya, R.B. Tokas, K.G. Bhushan, S. Sen, S.C. Gadkari, S.K. Gupta, Journal of Alloys and Compounds 651 (2015) 375-381. [4] O.B. Malyshev, R. Valizadeh, R.M.A. Jones, A. Hannah, Vacuum 86 (2012) 2035-2039.