Low Temperature Hermetic Wafer Bonding and Microfabrication of Wall-Coated Alkali Vapor Cells for Chip-Scale Atomic Clocks

The increasing demand for highly precise, yet compact and low power timing devices pushes the research in the field of chip scale atomic clocks (CSACs). At the heart of a CSAC, there is a vapor cell containing a few micrograms of alkali metal, such as rubidium or cesium. Hermetic sealing of this cell is essential not just to avoid the strong reaction of the alkali metal to oxygen and hydrogen, but also to maintain a stable operation atmosphere that ensures minimized clock shifts. Since alkali atoms loose their spin polarization when colliding with the cell walls, a buffer gas is commonly added in alkali cells to reduce the mean free path. An alternative approach to increase the polarization lifetime is to use antirelaxation wall-coatings, which have been studied extensively for larger glass-blown alkali vapor cells but not yet for microfabricated cells. This is mainly because anodic bonding, which is the standard process for sealing microfabricated alkali vapor cells, degrades the known antirelaxation coatings with its elevated temperatures and the electric field strength. In this work, the problem of implementing the wall-coatings in microfabricated cells is addressed. In a first step, a hermetic and low temperature thin-film indium-bonding technique was developed, which is compatible with the most common antirelaxation coatings and meets the requirements to fabricate alkali vapor cells. The technique is based on the thermocompression of indium and can be applied on wafer level with temperatures ranging from room temperature to temperatures below indium’s melting point at 156 ◦ C. It yields a leak rate below 1.5 × 10−13 mbar l/s and a tensile strength between 10 and 25 MPa, depending on the substrates and the bonding parameters. Wafer-level bonding at such low temperatures with the reported level of hermeticity is unique in the field of MEMS packaging. This novel bonding technique was then successfully applied to microfabricate alkali vapor cells at the lowest temperature alkali cells have been sealed before. The gas contamination in an evacuated cell was lower than 1 mbar and the suitability of indium-bonded cells for atomic clock applications was demonstrated with a buffer-gas filled cell by recording the double-resonance signal. The obtained clock signal linewidth was 2.02 kHz, only slightly higher than the theoretically predicted intrinsic linewidth for this cell design. After validation of the bonding technique for alkali vapor cells, a wall-coating for the cell walls was integrated in the microfabrication process. The bulk and surface properties of selected antirelaxation wall-coatings materials, such as octadecyltrichlorosilane (OTS), paraffin, an alkene mix, and Parylene, were investigated for compatibility with alkali cells and microfabrication. The gathered knowledge led to the fabrication of the first microfabricated rubidium vapor cell with an OTS coating that shows antirelaxation properties.