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Applied Sciences

The Microfabricated Alkali Vapor Cell with High Hermeticity for Chip-Scale Atomic Clock

Shuo Jia, Zhiyuan Jiang, Binbin Jiao, Xiaochi Liu, Yijie Pan, Zhenfei Song, Jifeng Qu

Published Jan 3, 2022• 20 citations
Publisher

Abstract

Herein, a microfabricated millimeter-level vapor alkali cell with a high hermeticity is fabricated through a wet etching and single-chip anodic bonding process. The vapor cell, containing Rb and N2, was investigated in a coherent population trapping (CPT) setup for the application of a chip-scale atomic clock (CSAC). The contrast of CPT resonance is up to 1.1% within the only 1 mm length of light interacting with atom. The effects of some critical external parameters on the CPT resonance, such as laser intensity, cell temperature, and buffer gas pressure, are thoroughly studied and optimized. The improved microfabricated vapor cell also exhibited great potential for other chip-scale atomic devices.

DOI
10.3390/app12010436

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Herein, a microfabricated millimeter-level vapor alkali cell with a high hermeticity is fabricated through a wet etching and single-chip anodic bonding process. The vapor cell, containing Rb and N2, was investigated in a coherent population trapping (CPT) setup for the application of a chip-scale atomic clock (CSAC). The contrast of CPT resonance is up to 1.1% within the only 1 mm length of light interacting with atom. The effects of some critical external parameters on the CPT resonance, such as laser intensity, cell temperature, and buffer gas pressure, are thoroughly studied and optimized. The improved microfabricated vapor cell also exhibited great potential for other chip-scale atomic devices.
Nowadays, the portable atomic clocks with small size and low power consumption have attracted much attention. With the development of the coherent population trapping (CPT) and the micro-electro-mechanical systems (MEMS) technologies, it is possible to fabricate the miniature atomic clocks with power consumption less than 100mW. Researchers have already made great progress in the miniaturization of VCSELs, RF circuits, and photodiodes, which are the key technologies of the chip-scale atomic clock (CSAC). However, the fabrication of micro alkali atomic vapor cells is still to be improved. In this article, we describe the fabrication of micro Rb atomic vapor cells with inner dimensions of 3 mm length and 3 mm radius using anodic bonding and chemical reaction between BaN6and RbCl. The optical absorption and CPT resonances of the cells were measured slightly above the room temperature. In this condition, the consumption in the temperature control will be very low. Moreover, the long life (more than four years) of the cells makes it possible to use them in CSACs.
This paper describes the progress made in developing wafer-scale fabrication process of alkali vapor cells for chip-scale atomic devices such as Coherent Population Trapping (CPT) clocks, optical frequency standards and Rydberg-atom-based sensors. The wafer level fabrication process uses anodic bonding and introduces alkali atoms by chemical reactions of Alkali Chloride with Barium Azide. This method is compatible to make cells with a wide range of buffer gas species and pressures, including evacuated cells. We improve the yield of the cells to above 50%. We will show the residual gas pressures for evacuated cells are mostly consistent with zero within the resolution of the saturation absorption spectroscopy measurement. The temperature coefficients for the CPT spectroscopy are improved with Ar-N2 mixture buffer gas compared with only N2 buffer gas.
We report the fabrication of a micro vapor cell which has Rb and Ne inside with internal volumes of several millimeters, which is the key component of chip-scale atomic devices like clocks and magnetometers. The cells were filled with Ne at a controlled pressure as the buffer gas by using glass-silicon anodic bonding techniques and dispenser pills of SAES Getters. And a special two-step bonding method was taken to avoid breakdown caused by the buffer gas and to secure the quality of the bonding. We also built an optical measuring system and measured the coherent population trapping (CPT) resonance of85Rb D1 line with a linewidth below 2.0 kHz and a contrast about 1.1%. Zeeman spectrum for CPT resonance was also detected in these microcells. These results show that these vapor cells are capable to be used in atomic clocks or magnetometers.
This paper describes the fabrication of chip-scale alkali atom vapor cells, for use in highly miniaturized atomic frequency references, using silicon micromachining and anodic bonding technology. The cells consist of silicon cavities with internal volume ranging from a few mm/sup 3/ to less than 1 mm/sup 3/. The cells were filled with cesium and nitrogen buffer gas either by chemical reaction of cesium chloride and barium azide, or by direct injection of elemental cesium within a controlled anaerobic environment. Cesium optical absorption spectra were obtained from the cells, and coherent population trapping resonances with linewidths of about 1 kHz were measured.
We describe the microfabrication of 85 Rb vapour cells using a glass-silicon anodic bonding technique and in situ chemical reaction between rubidium chloride and barium azide to produce Rb. Under controlled conditions, the pure metallic Rb drops and buffer gases were obtained in the cells with a few mm 3 internal volumes during the cell sealing process. At an ambient temperature of 90 °C the optical absorption resonance of 85 Rb D1 transition with proper broadening and the corresponding coherent population trapping (CPT) resonance, with a signal contrast of 1.5% and linewidth of about 1.7 kHz, have been detected. The sealing quality and the stability of the cells have also been demonstrated experimentally by using the helium leaking detection and the after-9-month optoelectronics measurement which shows a similar CPT signal as its original status. In addition, the physics package of chip-scale atomic clock (CSAC) based on the cell was realized. The measured frequency stability of the physics package can reach to 2.1 × 10 −10 at one second when the cell was heated to 100 °C which proved that the cell has the quality to be used in portable and battery-operated devices.
We describe the fabrication of Rubidium (Rb) vapor cells using glass-silicon anodic bonding techniques and chemical reaction between rubidium chloride and barium azide. The pure metallic Rb drops and buffer gases were obtained in the cells with few mm3internal volumes. The optical absorption resonance of85Rb D1 transition with proper broadening and the corresponding coherent population trapping (CPT) resonance, with a signal contrast of 1.5% and linewidth of about 1.7 KHz, have been detected at 90degC.