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Measurement Sensors

Preparation of microfabricated alkali atomic vapor cells with good hermeticity

Yali Liu, Wei Li

Published Feb 17, 2025• 1 citation
Publisher

Abstract

The microfabricated alkali atomic vapor cells are essential for the chip scale quantum sensing instruments which provide a robust, simple platform for precision measurement for a quantum system. In order to solve the problems of difficult quantitative filling of alkali atoms and poor hermeticity in the fabricated process of the cells, the preparation of microfabricated alkali atomic vapor cells with good hermeticity is studied. The cells are fabricated by the microelectromechanical system (MEMS) technology. Rubidium (Rb) atomic vapor cells with nitrogen (N 2 ) as buffer gas are successfully obtained by photolysis of rubidium azide (RbN 3 ), which is confirmed by the presence of Rb vapor in the cells through absorption spectroscopy for the 5S 1/2 → 5P 3/2 and 5S 1/2 → 5P 1/2 transitions in Rb atoms at 780 nm and 795 nm. The influences of filling amount and baking temperature of RbN 3 solution on the hermeticity of the microfabricated atomic vapor cells are demonstrated. Also, the influences of filling amount of Rb atoms and temperature on the absorption spectrum intensities of Rb vapor in the cells are studied through a theoretical analysis. This research provides a reference for the design and preparation of high quality microfabricated alkali atomic vapor cells.

DOI
10.1016/j.measen.2025.101860

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The microfabricated alkali atomic vapor cells are essential for the chip scale quantum sensing instruments which provide a robust, simple platform for precision measurement for a quantum system. In order to solve the problems of difficult quantitative filling of alkali atoms and poor hermeticity in the fabricated process of the cells, the preparation of microfabricated alkali atomic vapor cells with good hermeticity is studied. The cells are fabricated by the microelectromechanical system (MEMS) technology. Rubidium (Rb) atomic vapor cells with nitrogen (N 2 ) as buffer gas are successfully obtained by photolysis of rubidium azide (RbN 3 ), which is confirmed by the presence of Rb vapor in the cells through absorption spectroscopy for the 5S 1/2 → 5P 3/2 and 5S 1/2 → 5P 1/2 transitions in Rb atoms at 780 nm and 795 nm. The influences of filling amount and baking temperature of RbN 3 solution on the hermeticity of the microfabricated atomic vapor cells are demonstrated. Also, the influences of filling amount of Rb atoms and temperature on the absorption spectrum intensities of Rb vapor in the cells are studied through a theoretical analysis. This research provides a reference for the design and preparation of high quality microfabricated alkali atomic vapor cells.
Abstract Chip-sized alkali atom vapor cells with high hermeticity are successfully fabricated through deep silicon etching and two anodic bonding processes. A self-built absorption spectrum testing system is used to test the absorption spectra of the rubidium atoms in alkali atom vapor cells. The influence of silicon cavity size, filling amount of rubidium atoms and temperature on the absorption spectra of rubidium atom vapor in the atom vapor cells are studied in depth through a theoretical analysis. This study provides a reference for the design and preparation of high quality chip-sized atom vapor cells.
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.
The fabrication processes of chip-scale alkali atom vapor cells are designed, which include silicon micromachining, filling arrays of miniaturized cells with rubidium azide, hermetic packaging using anodic bonding technology, and photodecomposition of rubidium azide. The improved anodic bonding technology is demonstrated and used to improve the gastight bonding of alkali atom vapor cells to the glass/silicon/glass bonding structure. After the anodic bonding of the glass and silicon wafer is finished, an Al metal film with a certain thickness is sputtered at a certain angle at the back (i.e., the glass side) and side of the glass/silicon-bonded structure. Therefore, the electrical contact is led into the silicon wafer when a second anodic bonding is performed, and a high-quality glass/silicon/glass bonding structure is realized. Such anodic structures have great bond strength and gastight bonding of as low as 2.5×10-8Pa·m3·s-1. After gastight sealing, the cells are irradiated with ultraviolet, causing the azide to photodissociate into pure rubidium and nitrogen in situ. Rubidium optical absorption spectrum indicates that rubidium atoms are effectively sealed in the cells. It is confirmed that the process for the fabrication of the chip-scale alkali atom vapor cells is simple and feasible.
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.
Micro-electro-mechanical systems (MEMS)-based (chip-scale) alkali vapor cells are key components in emerging quantum technologies, where device performance critically depends on the atomic number density. Thus, it is important to have an accurate estimate of the atomic number density in MEMS-based alkali vapor cells to optimize light-matter interactions and design efficient quantum sensing systems. Here, a quantitatively validated method is presented for determining the rubidium (Rb) atomic number density in warm vapor using Single-Pass Absorption Spectroscopy (SPAS). The absolute transmission spectra are measured and modeled using the 780.24~nm and 420.29~nm transitions in Rb-filled MEMS vapor. The theoretical model employs a density-matrix formalism within the Lindblad framework and incorporates directly measurable experimental parameters, such as laser beam power, diameter, and cell temperature. The model explicitly accounts for optical pumping, Doppler broadening, and transit-time broadening effects and exhibits quantitative agreement (>99%> 99\%>99%) with experimental spectra over a broad range of temperatures (293-353~K), laser probe powers of approximately 10~μμμW-100~μμμW at the 780.24~nm transition and 8~μμμW-80~μμμW at the 420.29~nm transition, and cell lengths (2--100~mm). This method demonstrates a practical and reliable approach for determining the density of alkali vapor cells for quantum sensing, metrology, and quantum communication applications.
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.