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IEEE International Conference on Micro Electro Mechanical Systems (MEMS)

Micromachined alkali atom vapor cells for chip-scale atomic clocks

Li‐Anne Liew, Svenja A. Knappe, John Moreland, H. G. Robinson, Leo W. Hollberg, John E. Kitching

Published Aug 2, 2004• 15 citations
Publisher

Abstract

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.

DOI
10.1109/mems.2004.1290535

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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 fabrication of chip-sized alkali atom vapor cells using silicon micromachining and anodic bonding technology. Such cells may find use in highly miniaturized atomic frequency references or magnetometers. The cells consist of cavities etched in silicon, with internal volumes as small as 1 mm3. Two techniques for introducing cesium and a buffer gas into the cells are described: one based on chemical reaction between cesium chloride and barium azide, and the other based on direct injection of elemental cesium within a controlled anaerobic environment. Cesium optical absorption and coherent population trapping resonances were measured in the cells.
The article presents the results of the development, fabrication and investigation of miniature cesium vapor cells for quantum frequency standards with optical pumping, working on the effect of coherent population trapping. Microelectromechanical systems technologies were used in the manufacture of cells. For the first time, the dry silicon etching in inductively coupled plasma of sulfur hexafluoride and freon gas in a mixed mode was used to form internal cavities of cells with a thickness of 1.5 mm. Vacuum-tight sealing of cells containing a solid-state source of cesium vapor and buffer gas was carried out by anodic bonding of glass and silicon in a neon atmosphere. The coherent population trapping signal with a linewidth of 750 Hz and a signal-to-noise ratio of 1190 in the 1-Hz bandwidth was observed. Experimental results show that these microcells are appropriate for applications to atomic clock with a relative frequency stability of6⋅10−116\cdot 10^{-11}6⋅10−11at 1 s.
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.
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.
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.
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.