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European Frequency and Time Forum

Towards wall-coated microfabricated cells: Alkali vapor-cells using indium thin-film low-temperature bonding

Rahel Straessle, Yves Pétremand, D. Briand, Nico F. de Rooij, Matthieu Pellaton, C. Affolderbach, G. Mileti

Published Apr 1, 2012• 3 citations
Publisher

Abstract

We report on the realization and evaluation of micro-fabricated alkali vapor cells using a thin-film indium bonding technique. Optical and double-resonance spectroscopy performed on the cells demonstrates their suitability for atomic clock applications. Long-term measurements show a stable pressure inside the cells and therefore an excellent hermeticity of the bonding over several months. The low bonding process temperature of ≤ 140°C combined with good bond strength makes this method a promising candidate for realizing micro-fabricated alkali cells with anti-relaxation wall-coatings.

DOI
10.1109/eftf.2012.6502389

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We report on the realization and evaluation of micro-fabricated alkali vapor cells using a thin-film indium bonding technique. Optical and double-resonance spectroscopy performed on the cells demonstrates their suitability for atomic clock applications. Long-term measurements show a stable pressure inside the cells and therefore an excellent hermeticity of the bonding over several months. The low bonding process temperature of ≤ 140°C combined with good bond strength makes this method a promising candidate for realizing micro-fabricated alkali cells with anti-relaxation wall-coatings.
We present a microfabricated alkali vapor cell equipped with an anti-relaxation wall coating. The anti-relaxation coating used is octadecyltrichlorosilane and the cell was sealed by thin-film indium-bonding at a low temperature of 140 °C. The cell body is made of silicon and Pyrex and features a double-chamber design. Depolarizing properties due to liquid Rb droplets are avoided by confining the Rb droplets to one chamber only. Optical and microwave spectroscopy performed on this wall-coated cell are used to evaluate the cell's relaxation properties and a potential gas contamination. Double-resonance signals obtained from the cell show an intrinsic linewidth that is significantly lower than the linewidth that would be expected in case the cell had no wall coating but only contained a buffer-gas contamination on the level measured by optical spectroscopy. Combined with further experimental evidence this proves the presence of a working anti-relaxation wall coating in the cell. Such cells are of interest for applications in miniature atomic clocks, magnetometers, and other quantum sensors.
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.
Compared with superconducting quantum interference devices, chip-scale atomic magnetometers using the wafer-level micro alkali vapor cells can be used more conveniently in the applications of geophysical mapping, health care and remote monitoring. Drastic wall collisions and decreased polarization lifetime are the main barriers for the performance of the chip-scale atomic magnetometers. In this study, OTS organic films coated on the inner surface of the micro vapor cells were used to suppress wall collisions and improve polarization lifetime of the alkali atoms in the micro cells. First of all, the fabrication process of wafer-level micro OTS coated vapor cells is introduced, the process is based on anodic bonding technology. Then the prepared wafer-level micro OTS coated vapor cells have been characterized by the water-contact angle test and measured in the micro-machined spin-exchange relaxation free(SERF) atomic magnetometers. Results show that the OTS coating is effective after anodic bonding from the water contact angle test. Results also indicate that the spin exchange relaxation free atomic magnetometer using the micro OTS-coated vapor cell can operate successfully.
We demonstrate the use of an anodic bonding technique for building a vacuum tight vapor cell for the use of Rydberg spectroscopy of alkali atoms with thin film electrodes on the inside of the cell. The cell is fabricated by simultaneous triple stack glass-to-glass anodic bonding at 300 °C. This glue-free, low temperature sealing technique provides the opportunity to include thin film electric feedthroughs. The pressure broadening is only limited by the vapor pressure of rubidium and the lifetime is at least four months with operating temperatures up to 230 °C.
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.