Low temperature sealing process for vacuum MEMS encapsulation

In this paper, CEA, LETI latest developments on thin film packaging technology are presented. Outgassing from materials used in TFP and MEMS devices become key parameters to decrease the pressure inside the package and to improve the reliability. In a first part, some outgassing of typical Thin Film Packaging (TFP) and MEMS materials are measured under different time/temperature processes. Thanks to these characterizations, an optimized outgassing baking process in term of time and thermal budget can be defined. By minimizing outgassing, materials deposited by PVD sputtering can now be implemented as sealing materials for low pressure MEMS devices. In a second part, specific low temperature Al based materials which has been developed on equipment fully compatible with front-end fab is presented. Multi-layer materials like Ti/Al based materials are compared to our single Al based material to decrease the microstructure size and to improve the sealing performances. Scanning Electronic Microscopy (SEM), Focused Ion Beam (FIB) cross section and Atomic Force Macroscopy (AFM) characterizations confirm that the grain sizes are highly impacted by sputtering process parameters. Finally, an optimized outgassing baking process for the inside cavity materials and a low temperature Al-based sealing material are performed on pressure sensitive MEMS device on 200 mm wafers. Pressure inside the cavity has been measured much smaller than 10 mbar.