# High Vacuum Packaging of MEMS Devices Containing Heterogeneous Discrete Components
## 3 Vacuum Packaging
### Vacuum Sealing  
The vacuum sealing process includes prebaking, in situ getter firing and solder reflow. The getter used in the physics package will be fired at 270 \({}^{\circ}\)C for 4 h in accordance with the suggestions from the supplier. However, the melting point of Sn90 Au10 is 217 \({}^{\circ}\)C. In order to protect components from thermal damage and avoid melting of Sn90 Au10 during firing of the getter, the Sn90 Au10 solder preform rim is attached to the sealing ring of LCC and the getter is deposited on the inner surface of the ceramic shell, respectively. The Sn90 Au10 solder rim is protected by a thermal shield when the getter is firing. Figure 9 shows the configuration of ceramic shell and Part C (defined in Figure 3d) during vacuum sealing. Top plate and bottom plate integrated with thermocouple are two parts of the graphite fixture for accommodating ceramic shell and Part C, respectively. Part C is placed on the bottom plate, while ceramic shell is loaded on the top plate, see Figure 9a. During the prebaking step, Part C and ceramic shell are close and are baked at the same temperature of 180 \({}^{\circ}\)C for 15 h to ensure effective desorption, as illustrated in Figure 9a,d. Getter firing follows the prebaking step. Getter requires 270 \({}^{\circ}\)C for firing while the Sn90 Au10 solder preform rim will be molten under such a high temperature. A thermal shield provided by the vacuum sealing furnace separates the ceramic shell and Part C during getter firing. It affords thermal shielding to Part C and enables firing of getter at 270 \({}^{\circ}\)C for 4 h, as illustrated in Figure 9b,e. After that, the thermal shield is removed and the ceramic shell is pushed down to the LCC of Part C. The temperature then reaches the melting point of Sn90 Au10 and remains at about 230 \({}^{\circ}\)C for a while. The solder rim melts and seals the ceramic shell and LCC together. Figure 9c,f shows the sealing of LCC and ceramic shell in solder reflow step. Figure 9: The configuration of ceramic shell and LCC during vacuum sealing. (**a**) the 3D view of Part C accommodated by bottom plate and ceramic shell loaded on the top plate during prebaking; (**b**) the 3D view of ceramic shell and Part C are separated by a thermal shield during getter firing; (**c**) the 3D view of ceramic shell is pushed down to the LCC of part C during solder reflow; (**d**) the front view of LCC accommodated by bottom plate and ceramic shell loaded on the top plate during prebaking; (**e**) the front view of configuration of ceramic shell and Part C during getter firing; (**f**) the front view of configuration of ceramic shell and Part C during solder reflow. The alignment of LCC and ceramic shell during bonding process is performed by the graphite fixture. The graphite fixture consists of the top plate, the middle plate and the bottom plate which are designed for ceramic shell load, components alignment and LCC accommodation, respectively, as shown in Figure 10. The photograph of the top plate is shown in Figure 10a. The Ceramic shell is loaded on the top plate as illustrated in Figure 10b. The bottom plate has a cavity for the LCC accommodation. The middle plate is aligned on to the bottom plate by the fixed rods. The middle plate has the locating hole for alignment of the top plate, as illustrated in Figure 10c. Figure 10d shows that an uncapped physics package (Part C) is accommodated on the bottom plate. Figure 10e shows the alignment of LCC and ceramic shell by placing the top plate into the locating hole of middle plate. Figure 10f shows the physics package after vacuum packaging. The temperature and pressure profile of vacuum sealing is illustrated in Figure 11. During the whole vacuum sealing process, the pressure in the furnace is controlled to be about 13 \(\upmu\)Pa, except that a surge of 475 \(\upmu\)Pa appears when the getter firing starts. The sudden surge in pressure is caused by the accelerated outgassing of getter due to the temperature rising of getter firing.