# The Microfabricated Alkali Vapor Cell with High Hermeticity for Chip-Scale Atomic Clock
## 1 Introduction  
The interaction between light and atoms in the vapor phase has become a valuable tool for precision measurement. Many physical quantities, such as frequency, temperature, and magnetic field, can be precisely determined based on the simple and well-defined quantum systems in atoms. Frequency, as the most accurately measured physical quantity, triggers the precise measurements of other physical quantities. So, the development and breakthrough of the atomic clock results in the enormous advancement of precision measurement. Vapor cell atomic clocks are under development in labs around the world, reaching rather high levels of frequency stability [1]. However, significant use as a commercial, small, and low power consuming product was not found until the chip-scale atomic clock appeared [2; 3; 4]. The chip-scale atomic clock (CSAC), combing atomic spectroscopy, silicon micromachining, and semiconductor laser technology, brings the atomic frequency standard with a high performance out of the lab and directly to the users. At the heart of the CSAC, the microfabricated cell in the physics package is the key to the performance of the CSAC. The microfabricated cell provides an isolated and stable atmosphere for the interaction between the laser and alkali metal vapor. A miniature magnetometer and gyroscope are also developed based on the microfabricated cells, which originate from the technology in CSAC [5; 6; 7; 8; 9].  
There are three prerequisites that must be met before the microfabricated vapor is applied to CSAC: high hermeticity, buffer gas, and manufacturable on a large scale. On the one hand, the high hermeticity prevents the oxidation of the alkali metal vapor. On the other hand, it also ensures the long-term stability of the CSAC is not significantly affected by the leakage of the cell itself. The buffer gas is necessary to slow down the relaxation of the atoms and reduce the linewidth of the resonance signal. Different package technologies in the wafer level, such as glass\(-\)silicon\(-\)glass bonding, which are utilized for trapping the atoms, make it possible for the mass manufacture of chip-scale atomic devices. Since the first CSAC was demonstrated by [PERSON]'s group in 2004, anodic bonding has been considered as a reliable and convenient way for vapor cell fabrication. Even after the firstcommercial CSAC appeared in 2011, NIST has never stopped research on the optimization of the microfabricated vapor cell [10,11]. The Femto-st in France developed a two-cavities solution to improve the anodic bonding quality and long-term stability [12,13]. In China, many groups, such as WIPM, NUC, and SEU [14, 15, 16], are working on the microfabricated vapor cells to develop the chip-scale atomic devices, and the prototypes of chip-scale atomic clock and magnetometers are successfully fabricated. However, hermetic MEMS cells with optimal external parameters are still strongly needed in order to achieve a reliable commercial chip scale atomic device [17,18].  
In this paper, a microfabricated vapor cell with two connected cavities based on anodic bonding is proposed and its performance is demonstrated. The breakdown voltage should be avoided when the anodic bonding in nitrogen is operated at a specific pressure, according to [PERSON]'s law [19]. The detailed structure of the cell was presented by scanning electron microscopy. The absorption spectra of the D1 line of Rubidium at 795 nm were detected by one continuous scan with a tunable diode laser system. The impacts on the CPT resonance of some critical external parameters, such as laser intensity, cell temperature, and microwave power, were studied and determined.