# The Microfabricated Alkali Vapor Cell with High Hermeticity for Chip-Scale Atomic Clock
## 2 Fabrication and Setup
After being dried by N\({}_{2}\) flow, the chips were baked in an oven at 150 \({}^{\circ}\)C for 2 h to evaporate the residual H\({}_{2}\)O and other gasses. The sealing process of a microfabricated Alkali Vapor Cell, shown in Figure 3A, was performed using the bonding system developed by our team. The procedure mainly followed the following three steps. (1) Single-sided bonding: The anodic bonding of the silicon and glass chips were performed at an atmosphere pressure at 350 \({}^{\circ}\)C with a bonding voltage 1200 V and 4.5 kN bonding force, by the customized bonder in Figure 3B,C. The bonder was equipped with a sophisticated system for controlling the temperature, voltage, gas pressure, and bonding time. In addition, it had a flexible fixture for 0.3 to 1.5 mm thickness chips within 5 cm for the length or width. (2) Rb-dispenser packaging by second-sided bonding: The Rb-dispenser was placed into a silicon cavity with a tweezer. Then, the cavities were sealed by second-sided bonding filled with N\({}_{2}\) (50, 70, 90 Torr). Based on Paschen's law, in addition to the relative position of the electrode, the breakdown voltage was decided by the gas species and the pressure of the cavity. The electrode was discharged under N\({}_{2}\) (50, 70, and 90 Torr) at 1200 V, although a  
Figure 2: Fabrication process of the two cavities of the silicon wafer with vapor cell (**A**) mask deposition, (**B**) photolithography, (**C**) ICP-etching for Si\({}_{3}\)N\({}_{4}\) and SiO\({}_{2}\). (**D**) Two cavities by anisotropical etching. (**E**) The microchannel between two cavities. (**F**) Si\({}_{3}\)N\({}_{4}\) and SiO\({}_{2}\) removing. higher voltage means a better bonding quality, which is crucial to the leak rate and lifetime. For this second-sided anodic bonding, the voltage of the electrode was increased in steps of 100 V from 400 V to 1000 V in an N\({}_{2}\) atmosphere. The hermeticity of vapor cell was evaluated by the leakage rate. According to GJB 548B-2005-1014.2, the leakage rate was measured by the following method. For the volume of cavities, about 8 mm\({}^{3}\) (<0.05 cm\({}^{3}\)), the vapor cell was placed into a chamber, followed by pressurizing to 517 kPa of Helium (He) for 2 h. After being taken out from chamber, the vapor cell was degassed by N\({}_{2}\) blowing to removing the He attachment. Then, it was put into a mass spectrometer within 1 h to determine the diffused mass of the He. The rejection limit of this leakage test method was 5 \(\times\) 10\({}^{-8}\) mbar L/s. Finally, the vapor cell passed the test, which means the leak rate was up to 5 \(\times\) 10\({}^{-8}\) mbar L/s. (3) Dispenser activation: After the second-sided bonding, the Rb dispenser was sealed hermetically into the cavity, which was filled with N\({}_{2}\). According to the previous report, the Rb dispenser could be heated locally by using a diode laser source (808 nm) and released the Rb elemental for CPT signal detection. As Figure 4A shows, the laser beam with 3.5 W was focused on the surface of the Rb dispenser for 2 min to activate the Rb dispenser. The laser beam was transparent to the glass so as to avoid the ablation of other parts of the device. After the activating process, as shown in Figure 4B,D, the dispenser was melted to Rb, Cr\({}_{2}\)O\({}_{3}\), Al\({}_{2}\)O\({}_{3}\), and ZrO\({}_{2}\). The droplet shape Rb elemental was deposited on the inner surface of the optical cavity for CPT detection (Figure 4C). Figure 4: (**A**)The activating system with a 808 nm diode laser source; (**B**,**D**) the vapor cell after activating by laser; and (**C**) the Rb elemental on the inner-surface of the optical cavity. Figure 3: (**A**) Sealing process of vapor cell: Single-sided anodic bonding and Rb dispenser storage, Second-sided bonding, dispenser activating and (**B**) single-chip bonding by the customized bonder with electrode and pressure device, and (**C**) single-chip bonding with upper glass and lower silicon.