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Scientia Sinica Informationis

芯片级铷原子气室的制备

ZhanGuo WANG, Xinkun Li, Fen Wang, Peng Jin, DeChun LIANG

Published May 1, 2015• 1 citation

Abstract

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.

DOI: 10.1360/n112014-00220

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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.

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.

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.

The thin-film deposition and photodecomposition of cesium azide are demonstrated and used to fill arrays of miniaturized atomic resonance cells with cesium and nitrogen buffer gas for chip-scale atomic-based instruments. Arrays of silicon cells are batch fabricated on wafers into which cesium azide is deposited by vacuum thermal evaporation. After vacuum sealing, the cells are irradiated with ultraviolet radiation, causing the azide to photodissociate into pure cesium and nitrogen in situ. This technology integrates the vapor-cell fabrication and filling procedures into one continuous and wafer-level parallel process, and results in cells that are optically transparent and chemically pure.

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.