# High Vacuum Packaging of MEMS Devices Containing Heterogeneous Discrete Components
## 5 Discussion  
A systematic high vacuum packaging method for MEMS devices is proposed and introduced thoroughly in this paper, from low outgassing materials selection, prebaking, getter firing and solder reflow to inspection technologies after vacuum packaging including X-ray imaging, leak rate test and residual gas analysis. The results show that a high vacuum (about 0.1 Pa) is achieved, which is mainly caused by the ongoing desorption after vacuum packaging. The method is of high reference value for other researchers to perform vacuum packaging to their MEMS devices. Although the specific conditions are different from various kinds of MEMS devices, the method provides a general guideline for high vacuum packaging. Anyone could customize their own vacuum packaging parameters by adopting this method.  
The problem is easily located when using the method properly even if one fails to obtain the high vacuum. A high leak rate may be caused by a defect bonding and the X-ray imaging will verify this. The TGA of materials will reveal the outgassing and thermal stability under vacuum sealing. Besides, the recipe of getter could be adjusted according to the gas composition of residual gas analysis. For some applications where a lower vacuum is required, such as MEMS gyroscopes (the inner pressure is about 70 Pa) with wide bandwidth and wide linear input range [42], adhesive bonding could be used to integrate components for convenience. Adhesive bonding has the advantage of insensitivity to surface topography and the ability to join different types of materials [43]. However, the higher outgassing of adhesive will do much more harm to vacuum than eutectic solders. For other applications where a higher vacuum is required, the getter should be activated at a higher temperature to increase the gas absorption rate. The components are better to be baked at a higher temperature for a longer time to eliminate the desorption as much as possible, as long as the components have the ability to withstand a harsher baking  
Figure 12: Frequency stability of the 3 different physics packages before and after vacuum packaging. PP1B: physics package 1 before vacuum packaging; PP1A: physics package 1 after vacuum packaging; PP2B: physics package 2 before vacuum packaging; PP2A: physics package 2 after vacuum packaging; PP3B: physics package 3 before vacuum packaging; PP3A: physics package 3 after vacuum packaging.  
condition. In conclusion, the method proposed is a systematic way for MEMS devices to obtain a high vacuum.  
Conceptualization, P.G. and J.Z.; methodology, P.G. and H.M.; software, P.G. and L.D.; validation, P.G., H.M. and L.D.; formal analysis, P.G. and H.X.; investigation, P.G.; resources, H.M. and J.Z.; data curation, P.G., L.D. and J.Z.; writing--original draft preparation, P.G.; writing--review and editing, L.D., H.X. and J.Z.; visualization, H.M.; supervision, J.Z.; project administration, J.Z.; funding acquisition, H.M. and J.Z. All authors have read and agreed to the published version of the manuscript.  
This work was funded by the National Natural Science Foundation of China, Grant No. 91836301 & No. 61535001.  
Not applicable.  
Data Availability Statement: Data are contained within the article.  
Thanks to the Zhongkeqidi Optoelectronic Technology (Guangzhou) Co., Ltd. for providing us with the vacuum sealing furnace and other equipment for experiment.  
The authors declare no conflict of interest.