# High Vacuum Packaging of MEMS Devices Containing Heterogeneous Discrete Components
## 3 Vacuum Packaging
### Leak Rate Test and Residual Gas Analysis  
The leak rate of sealing depends on the quality of the eutectic bond between LCC and the ceramic shell. The inspection of sealing and detection of air voids are carried out by X-ray imaging. There are many methods used for the measurement of leak rate in packages containing small cavities, such as helium fine leak, radioisotope fine leak and optical leak [37]. The helium leak rate is measured by helium fine leak as specified in the MIL-STD 883 method 1014 [38] after the X-ray imaging showing defect-free sealing, where the helium bombs at 45 PSIA for a minimum exposure time of 2 h. The result shows that the leak rate is less than \(4\times 10^{-10}\) Pa m\({}^{3}\)/s, which is about 10 times better than the maximum leak rate (\(5\times 10^{-9}\) Pa m\({}^{3}\)/s) allowable as specified in MIL-883 method 1014 [38]. However, the testing condition of 45 PSIA helium bombing pressure is about three times the atmospheric pressure. Besides, the leakage has an inverse relationship with the relative molecular mass of gases. The relative molecular mass of He is four. Molecules with heavier relative molecular mass such as N\({}_{2}\), O\({}_{2}\) and CO\({}_{2}\) are 28, 32 and 44, which are the main constituents of air. Therefore, the leakage rate of the physics package in air will be much lower than that in the helium bomb condition. In order to get a more precise lifetime of the sealed physics package, the pressure is tested by the residual gas analysis.  
Residual gas analysis is widely used in vacuum fields to inspect the change of gases and analyze the composition of gases in MEMS devices [39]. Traditionally, the composition and partial pressure of residual gas are measured by a high sensitive quadrupole mass spectrometer (QMS) gas analyzer [40]. It is helpful to improve the performance of getter from the composition of residual gas. Table 2 shows the results of three different residual gas analyses of physics packages after vacuum packaging. The residual gas analysis is a destructive testing, where the ceramic shell is broken and the gas is analyzed by a QMS. Therefore, three different physics packages are used to test at different time to evaluate the  
Figure 11: Temperature and pressure profile during vacuum sealing.  
relationship between the inner pressure and time. Physics package 1 is tested 1 month after vacuum sealing. Physics package 2 is tested about 3 months after vacuum sealing. Physics package 3 is tested about 5 months after vacuum sealing. There is no obvious difference in the pressure of the three different physics packages in just several months. From Figure 5, the tolerable vacuum in the physics package is 1 Pa. The power consumption of the physics package at 1 Pa is almost the same with that at 0.1 Pa. The initial pressure in physics package is assumed to be 1.33 \(\times\) 10\({}^{-5}\) Pa, the same with the background pressure. It takes about 5 months to degrade from 1.33 \(\times\) 10\({}^{-5}\) Pa to 0.1 Pa. Thus, the time for degradation to 1 Pa is roughly estimated to be about 50 months (\(\sim\)4 years). If the tolerable vacuum is relaxed to 10 Pa, the power consumption of the physics package will increase about a dozen mW. Therefore, the vacuum is allowed to change over 40 years. However, oxidation will occur in the sealing and the hermeticity will be damaged. The actual time for a tolerable vacuum will be shortened.