# High Vacuum Packaging of MEMS Devices Containing Heterogeneous Discrete Components
## 2 Design and Integration of Physics Package
Figure 3a shows the explosive view of part A and the bonding pads on the surfaces of components. The bonding pads are made from gold which will not be oxidized and thus can be bonded without flux in vacuum or inert atmosphere. Before soldering, the gold pads are cleaned by plasma to remove impurities on the surfaces, which will improve the bonding even at a lower temperature. The bonding is performed by eutectic die bonders which have a pulse heat stage (PHS). The PHS provides a preheat temperature before bonding and has the ability to rise from ambient temperature to 400 \({}^{\circ}\)C in just 4 s. In addition, the eutectic die bonder is equipped with two N\({}_{2}\) nozzles. Namely, the hot N\({}_{2}\) nozzle forms the inert atmosphere around the components during the whole \begin{table}
\begin{tabular}{c c c c c} \hline
**Solder Alloy** & **Ingredients** & **Melting Point** & **Shear Strength** & **Tensile** \\  & **(Weight \%)** & **(\({}^{\circ}\)C)** & **(MPa)** & **Strength (MPa)** \\ \hline Sn90 Au10 & Sn/Au = 90/10 & 217 & 50 & 50 \\ Sn90 Sb10 & Sn/Sb = 90/10 & 250 & 44 & 44 \\ Au80 Sn20 & Au/Sn = 80/20 & 280 & 275.8 & 275.8 \\ Au8 Se12 & Au/Ge = 88/12 & 356 & 185 & 185 \\ \hline \end{tabular}
\end{table} 
Table 1: Properties of different eutectic solders. bonding process, and the cool N\({}_{2}\) nozzle cools the components down after bonding. The hot N\({}_{2}\) is used to reduce oxidation during the whole bonding process, which is a prerequisite for flux free eutectic bonding. The integration process of the physics package is divided into four steps. Firstly, the attenuator, QWP, MEMS \({}^{87}\)Rb cell and PD are bonded together as Part A by Au88 Ge12 solder as illustrated in Figure 3b. The four components are stacked vertically and aligned by the edges. They are put on the PHS and heated to 340 \({}^{\circ}\)C for about 15 s. The bonding force is adjusted to about 120 g. A higher preheat temperature, a longer preheat time and a heavier bonding force are required for the multiple surfaces bonding. Then, the temperature of PHS rises to 390 \({}^{\circ}\)C for about 10 s. After that, the cool N\({}_{2}\) cools Part A down. Secondly, The VCSEL and thermistor are bonded to Part A as Part B by Au80 Sn20 as illustrated in Figure 3c. The preheat temperature and time are set to 240 \({}^{\circ}\)C and 5 s, respectively. The bonding force is adjusted to be 25 g. Then, the temperature of PHS rises to 300 \({}^{\circ}\)C in 5 s and the cool N\({}_{2}\) cools the components down. The parameters are all lowered down due to the britteness and temperature sensitivity of the VCSEL. Thirdly, Part B is bonded to the LCC as Part C by Sn90 Sb10, as shown in Figure 3d. The preheat temperature and time are set to be 230 \({}^{\circ}\)C and 10 s, respectively. The bonding force is set to be 60 g. The temperature of PHS rises to 270 \({}^{\circ}\)C for 10 s and the cool N\({}_{2}\) cools Part C down. After that, the bonding pads on the top surface of MEMS cell and PD connect to the LCC by wire bonding. Figure 3e shows an uncompleted physics package which differs from the completed one only in that it is not vacuum sealed. The getter is deposited on the inner surfaces of the ceramic shell. Finally, at the vacuum sealing step, the ceramic shell is bonded to Part C by Sn90 Au10 solder. The completed physic package is shown in Figure 3f. The vacuum sealing step is the key to vacuum packaging, which will be discussed in detail in the next section. At the board level, a chip socket is used to connect the physics package to the control circuits for test due to the cleanliness requirement of residual gas analysis. Low melting point solders such as SnBi and SnIn are potential choices. Figure 1: Design of the physics package. Figure 2: The bottom view of the attenuator. Thermal dissipation through convection accounts for a large part of power consumption of the physics package [29]. Thus, it is necessary to study the relationship between power consumption of a physics package and vacuum.