# High Vacuum Packaging of MEMS Devices Containing Heterogeneous Discrete Components
## 3 Vacuum Packaging
### Outgassing of Materials Used in Physics Package  
As mentioned above, outgassing of materials is unavoidable. While it is vital to slow the outgassing rate to reduce destruction to the vacuum, the outgassing rate is highly determined by the initial adsorption of gas on the surfaces and in the volume body of components. On the one hand, the surfaces of components are designed to be small and smooth enough to reduce adsorption as much as possible. The use of porous materials is avoided here. On the other hand, an in situ prebaking process is applied to all components before vacuum sealing to enhance the desorption effect. Temperature and time are the two parameters which should be determined for the prebaking process. The higher the baking temperature and the longer the baking time are, the better the desorption is. However, a very high baking temperature will cause damage to temperature sensitive components such as VCSEL. A very long baking time will increase costs. Therefore, there is a trade-off between the effect of prebaking and baking temperature and time. TGA is widely used to analyze the thermal stability of materials. Typically, TGA is a method in which the mass of a sample is measured over time as the temperature changes [35; 36]. In the physics package, the VCSEL includes As, Ga, Al and In. The thermistor is made of many kinds of materials such as platinum, Au and ceramic. However, the VCSEL and thermistor are too small compared to other components and thus thus the outgassing is neglected. The MEMS \({}^{87}\)Rb cell has a good hermeticity and thus its materials are assumed to be silicon and glass. The main material of PD is silicon. The main material of QWP and attenuator is glass. Sn90 Au10 has the lowest melting points among the four eutectic solders. Therefore, Sn90 Au10, glass and silicon are performed by typical TGA to analyze the thermal stability under vacuum sealing temperature. Besides, the desorption of materials is analyzed through constant temperature TGA. Namely, the mass of materials is measured under a constant temperature for a specific time. Thus, the temperature and time of prebaking are determined from the constant temperature TGA. At the sample preparation step, all components used in the physics package are cleaned with ultrasonic and baked before crushing into pieces. Then, the pieces are ground into powder. The powder (about 10 mg) is put into the TGA analyzer as a sample for testing. The typical TGA results of Sn90 Au10 solder, glass and silicon are shown in Figure 7. The TGA curves show the weight loss of samples with respect to the temperature increasing from ambient temperature to 300 \({}^{\circ}\)C at a rate of 10 \({}^{\circ}\)C/min in a nitrogen atmosphere. Even at 300 \({}^{\circ}\)C, the maximum weight loss is about 0.1%. Thus, decomposition of materials will not take place at the vacuum sealing temperature of 230 \({}^{\circ}\)C, which is much lower than 300 \({}^{\circ}\)C.  
Figure 6: The schematic diagram shows the hermeticity measurement of LCC and ceramic shell. Similarly, the constant temperature TGA of Sn90 Au10 solder, glass and silicon is carried out in a nitrogen atmosphere. The only difference to typical TGA lies in that the mass loss is measured at a constant temperature of 180 \({}^{\circ}\)C, which is 37 \({}^{\circ}\)C lower than the melting point of Sn90 Au10. The constant temperature TGA curves in Figure 8 show that the mass of materials remains unchanging after about 15 h. Namely, the outgassing is almost reduced by baking at 180 \({}^{\circ}\)C for 15 h. Thus, the prebaking temperature and time are set to be 180 \({}^{\circ}\)C and 15 h in the vacuum, respectively. However, outgassing is almost reduced but will not be eliminated when the mass remains unchanged because the TGA test has a resolution of 0.01 \(\upmu\)g and the outgassing will be undiscernible under such a prebaking condition. Figure 8: Constant temperature TGA at 180 \({}^{\circ}\)C of Sn90 Au10 solder, glass and silicon. Figure 7: Typical TGA results of Sn90 Au10 solder, glass and silicon.