# The Microfabricated Alkali Vapor Cell with High Hermeticity for Chip-Scale Atomic Clock
## 2 Fabrication and Setup  
The microfabricated cell, as shown in Figure 1A, has two cavities: one is for containing the rubidium dispenser pill (SAES Getters) and the other one for the optical path. The Rb dispenser, which formed by Rb chromates and an alloy of Zr\(-\)Al was stable and nonreactive in an atmosphere environment at room temperature. The solution of single cavities is widely adopted due to its simple geometry and small size. However, in the single cavity, the solid byproducts of the oxidation materials induced by dispenser activation would affect the transmission of light. In the dual-cavity structure, the dispenser is activated by local heating using a 808 nm laser after anodic bonding at a low temperature (<500 \({}^{\circ}\)C). Rubidium atom vapor was released by the dispenser and passed through the micro-channel between the two cavities (Figure 1B). The residuals that were left in the dispenser activation cavity did not affect the interaction between incident light and atoms in the other cavity. The vapor cell was designed as a sandwich structure with one piece of silicon wafer and two pieces of borosilicate glass wafers (BF 33). The silicon wafer with a 4-inch diameter and 1 mm-thick P-type <100> was polished on both sides. The borosilicate glass, 4-inch 1 mm-thick, can be bonded to the silicon wafer by the anodic bonding method because of the similar thermal-expansion coefficients and the low surface roughness and high flatness. Two through-cavities were etched on a silicon wafer with wet etching, as shown in Figure 2. Firstly, the silicon wafer was rinsed by sufficient acetone, ethanol, and deionized water and was dried in the N\({}_{2}\) stream to remove the contaminants on the substrate. Then, the 200 nm SiO\({}_{2}\) and 300 nm Si\({}_{3}\)N\({}_{4}\) mask was deposited on the surface of the silicon by plasma enhanced chemical vapor deposition (PECVD) sequentially. After that, positive photoresist (AZ-4620) was spin-coated on the substate of Si\({}_{3}\)N\({}_{4}\) and was patterned by lithography. The exposed SiO\({}_{2}\) and Si\({}_{3}\)N\({}_{4}\) was etched by inductively coupled plasma (ICP) etching. Then, the wafer was immerged into the 40% KOH solution in a water bath of about  
Figure 1: (**A**) Alkali vapor cell and (**B**) SEM image of the channel between the two cavities. 70 \({}^{\circ}\)C to shape the two through-cavities. Finally, the Si\({}_{3}\)N\({}_{4}\) and SiO\({}_{2}\) mask was removed by a H\({}_{3}\)PO\({}_{4}\) and HF-based solution sequentially. The two cavities with 2 \(\times\) 2 mm were used as Rb-dispenser pill (SAES Getters) storage and optical CPT detection, respectively. As shown in Figure 2E, the filtration channels formed by laser etching between two cavities were used for transferring the Rb atoms. Finally, the silicon wafer with two cavities and glasses were cut into 4 \(\times\) 6 mm chips with an automatic dicing saw (Disco DAD3220). The permanent sealing of solid-state materials was mainly realized by bonding technology. Based on electric field-enhancement, anodic bonding, as the key procedure in the fabrication of vapor cell, was used for silicon to glass bonding at a low-temperature (below 500 \({}^{\circ}\)C). The permanently bonding interface with a high stability was formed by the voltage between the silicon and glass. Anodic bonding consists mainly of the following three steps: preparation of the surface for bonding, pre-bonding, final formation of bonding [20]. The residues that had a strong impact on the anodic-bonding effect were deposited on the surface of the silicon wafer during the laser etching process. Prior to the anodic-bonding process of 4 \(\times\) 6 mm silicon and glass chips, the silicon chip with two cavities was polished again by standard polishing process. After that, the cavity-etched silicon and glass chips were cleaned in acetone, ethanol by ultrasonic, and were rinsed in deionized water.