Most of the time, MEMS devices require hermetic encapsulation for protection against atmosphere, moisture, particles and standard back-end manufacturing technologies. In the last few years, Wafer Level Packaging (WLP) is moving toward developments on Thin Film Packaging (TFP) in order to save footprint, to reduce chip thickness and packaging costs. In the specific case of high-vacuum MEMS encapsulation (gyro, compass), long term pressure stability is required. As the final performances of these kinds of devices are strongly dependent on the working pressure, using TFP for MEMS encapsulation with controlled vacuum becomes more challenging due to very small cavity volumes. It is then necessary to understand the outgassing phenomenon that take place during TFP encapsulation in order to reduce it.
In this paper, our 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, pressure and Residual Gas Analysis (RGA) of typical TFP and typical MEMS materials are measured under different time / temperature baking processes. Measurements show that material outgassing mainly comes from H2 and maximum pick appears in the beginning of the thermal process. Thanks to these characterizations, an optimized outgassing baking process in term of time and thermal budget is presented. By minimizing the internal 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) and Focused Ion Beam (FIB) cross section characterizations confirm that the grain sizes are highly impacted by sputtering process parameters and a compromise has to be done with MEMS outgassing.
Finally, the most suitable outgassing baking process for the inside cavity materials and various Al-based sealing materials and stacks are performed for a MEMS compass device on 200 mm wafers. Pressure inside the cavity less than 10 mbar is obtained and the TFP yield is presented on each process conditions.
These results are very promising and showed the capabilities of TFP for vacuum MEMS encapsulation applications despite very small volume cavity. Development of such technology is still under way in order to reach high vacuum level in the range of 10-1 to 10-3 mbar.
A Lateral Differential Resonant Pressure Microsensor Based on SOI-Glass Wafer-Level Vacuum Packaging