Microfabricated low cost wafer-level spherical alkali atom vapor cells for chip-scale atomic clock by a Chemical Foaming Process (CFP)

In this paper, we demonstrate an innovative method for fabricating the reference cell for miniaturized atomic clock application, allowing low-cost wafer level batch fabrication and assembly. This method uses silicon micromachining techniques, low temperature anodic bonding technique and Chemical Foaming Process (CFP) for producing spherical alkali atom vapor cells. The Rb (Rubidium) vapor cell has been carefully designed, which consists of two cavities of which the smaller one is used as reaction chamber and the larger one is taken as a working chamber. This design preventing reactant from entering into working chamber improves the purity of Rb in the working chamber. The fabrication is based on etching cavities in silicon, and then anodic bonding of a thin Pyrex7740 glass wafer to the etched silicon wafer. Subsequently, the bonded wafer is heated to the softening point of the glass (Tg=821 °C) and hydrogen gas is released by TiH₂ to blow the glass into spherical shells. Finally a further reaction is performed by heating the reactants at a relatively lower temperature to complete the reaction. The Rb vapor cell is characterized by EDS (Energy Dispersive Spectroscopy). Results are demonstrated that wafer-level Rb vapor cell is fabricated successfully and Rb existed in the cell after chemical reaction.