Lifetime improvement of micro-fabricated alkali vapor cells by atomic layer deposited wall coatings

Alumina (Al2O3), silicon dioxide (SiO2) and titanium dioxide (TiO2) films were examined for their suitability as a protection coating for micro-fabricated cesium vapor cells. The layers were deposited by atomic layer deposition (ALD). This technique enables the deposition of pinhole-free passivation layers with a low roughness, a high conformality and a high homogeneity, even at three-dimensional surfaces. It is shown that Al2O3 and SiO2 films, in contrast to TiO2 films, do not hamper anodic bonding up to thicknesses of 20 nm. The resistance enhancement of the vapor cells by means of the implementation of the ALD films is demonstrated by absorption measurements of the cesium D1 line. SiO2 coatings showed no effects on the resistance of the vapor cells against the reducing behavior of cesium. In contrast, Al2O3 coatings improved the resistance of the vapor cells by a factor of ~ 100.