# High Vacuum Packaging of MEMS Devices Containing Heterogeneous Discrete Components
## 2 Design and Integration of Physics Package  
The physics package is the key part of a CSAC, where the coherent population trapping (CPT) signal is produced by the interaction between the circularly polarized 795 nm laser light and the \({}^{87}\)Rb atoms, and the CPT signal is interrogated by a PD through the detection of transmission light intensity change. Generally, the components are bonded together by adhesive or epoxy. In order to avoid the deterioration to vacuum by adhesive's outgassing, we chose eutectic bonding as the integration method. Therefore, adhesive and epoxy were excluded from the physics package. Eutectic bonding is a technique in which an intermediate metal layer creates a eutectic composition with substrates [25]. Table 1 shows the melting point and other properties of different solders used in the physics package [26]. The melting points of solders are restricted to below 410 \({}^{\circ}\)C and the melting point difference should be no less than 20 \({}^{\circ}\)C. The strength should be large enough. An especially designed physics package is shown in Figure 1 with specially selected materials of low outgassing. The materials will not decompose under the vacuum sealing temperature and work well after vacuum sealing. As shown in Figure 1, the VCSEL is a die with dimensions of 160 \(\times\) 200 \(\times\) 150 \(\upmu\)m and is bonded to the bottom surface of the attenuator. The VCSEL emits 795 nm linearly polarized laser light. The direction of the linearly polarized laser light is parallel to one edge of the VCSEL pad, as shown by the red solid arrow in Figure 2. In order to sense the temperature of VCSEL precisely, the thermistor is bonded near the VCSEL. A pair of twisted gold wires work as heater to heat the physics package to 85 \({}^{\circ}\)C, which satisfies the \(-\)40-80 \({}^{\circ}\)C temperature range requirement of industrial products. Gold is one of the least reactive chemical elements and the resistance of the heater will keep constant during the physics package assembly. The thermistor, heater and control circuits form a feedback loop controlling the temperature of VCSEL to be stable at 85 \({}^{\circ}\)C, which stabilizes the VCSEL output wavelength to the \({}^{87}\)Rb D\({}_{1}\) line transition. The attenuator is used to reduce the light intensity of laser. Otherwise, the weak CPT signal will be drowned out by the strong background light. The fast axis of the QWP is parallel to one diagonal of the square cross section as illustrated by the purple solid arrow in Figure 2. The angle \(\theta\) is 45\({}^{\circ}\) in such a configuration. Thus, the linearly polarized laser light passes through the QWP, turning into a circularly polarized laser light. The circularly polarized laser light passes into the MEMS \({}^{87}\)Rb cell and interacts with \({}^{87}\)Rb atoms. The MEMS \({}^{87}\)Rb cell is fabricated by anodic bonding silicon with glass at 440 \({}^{\circ}\)C, which defines the highest soldering temperature during the physics package integration. The CPT signal is detected by a PD on the top. The leadless chip carriers (LCC) and ceramic shell are vacuum sealed together to keep air from coming into the physics package. The LCC is made from high temperature cofier ceramics (HTCC), which contains more than 91% Al\({}_{2}\)O\({}_{3}\). The ceramic shell consists of more than 95% Al\({}_{2}\)O\({}_{3}\). The ceramic is reported to be second to metals in permeability [27]. However, metals will cause an electrical short when CSACs operate. Low temperature cofierc ceramic (LTCC) is also reported to have good hermeticity [28] but it has a worse hermeticity than ceramic. Thus, ceramic is the best choice for hermetic sealing. The components are stacked vertically with each other to minimize the volume. All materials used are compliant with RoHS compliance. The integration process is shown in Figure 3. In order to minimize the influence of bonding in later steps to bonding in previous steps, the bonding temperature is set to be higher in forward steps.