This paper presents a new fabrication method to manufacture alkali reference cells having dimensions larger than standard micromachined cells and smaller than glass-blown ones, for use in compact atomic devices such as vapour-cell atomic clocks or magnetometers. The technology is based on anodic bonding of silicon and relatively thick glass wafers and fills a gap in cell sizes and technologies available up to now: on one side, microfabrication technologies with typical dimensions 2 mm and on the other side, classical glass-blowing technologies for typical dimensions of about 6-10 mm or larger. The fabrication process is described for cells containing atomic Rb and spectroscopic measurements (optical absorption spectrum and double resonance) are reported. The analysis of the bonding strength of our cells was performed and shows that the first anodic bonding steps exhibit higher bonding strengths than the later ones. The spectroscopic results show a good quality of the cells. From the double-resonance signals, we predict a clock stability of ≈3 × 10 −11 at 1 s of integration time, which compares well to the performance of compact commercial Rb atomic clocks.

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 indiumbonding 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 wallcoated 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.Microfabricated alkali vapor cells are widely studied and employed for miniature devices such as atomic magnetometers, 1 atomic clocks, 2,3 or other quantum sensors. 4 The required low relaxation rates of the atomic ground-state polarization are generally achieved by adding buffer gases to the cells, which results in decreased collision rates of the alkali atoms with the cell walls, longer ground-state polarization lifetime, and therefore improved stability in the case of an atomic clock. Alternatively, the lifetime of the polarization can be increased by depositing anti-relaxation coatings on the cell walls. [5][6][7] Macroscopic ($few cm) alkali vapor cells equipped with anti-relaxation wall coatings have shown to be effective for the development of atomic clocks 5,8 and magnetometers.9,10 Such coatings are also of interest for miniature atomic clocks 8,11,12 and several studies aimed at finding an optimal coating that is compatible with microfabrication, 11,13 in particular, in view of the elevated process temperatures.

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A low-temperature sealing technique for micro-fabricated alkali vapor cells for chip-scale atomic clock applications is developed and evaluated. A thin-film indium bonding technique was used for sealing the cells at temperatures of 140 C. These sealing temperatures are much lower than those reported for other approaches, and make the technique highly interesting for future micro-fabricated cells, using anti-relaxation wall coatings. Optical and microwave spectroscopy performed on first indium-bonded cells without wall coatings are used to evaluate the cleanliness of the process as well as a potential leak rate of the cells. Both measurements confirm a stable pressure inside the cell and therefore an excellent hermeticity of the indium bonding. The double-resonance measurements performed over several months show an upper limit for the leak rate of 1.5 Â 10 À13 mbarÁl/s. This is in agreement with additional leak-rate measurements using a membrane deflection method on indium-bonded test structures.

This paper reports on low-temperature and hermetic thin-film indium bonding for wafer-level encapsulation and packaging of delicate and temperature sensitive devices. This indium-bonding technology enables bonding of surface materials commonly used in MEMS technology. The temperature is kept below 140 °C for all process steps and no surface treatment is applied before and during bonding. This bonding technology allows hermetic sealing at 140 °C with a leak rate below 4 × 10−12 mbar l s−1 at room temperature. The tensile strength of the bonds up to 25 MPa goes along with a very high yield.