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arXiv:2405.11088 · Optics Letters

Wafer-scale fabrication of evacuated alkali vapor cells

Yang Li, Donggyu B. Sohn, Matthew T. Hummon, Susan A. Schima, John E. Kitching

Published Aug 14, 2024• 9 citations
arXivHTMLPDF

Abstract

We describe a process for fabricating a wafer-scale array of alkali metal vapor cells with low residual gas pressure. We show that by etching long, thin channels between the cells on the Si wafer surface, the residual gas pressure in the evacuated vapor cell can be reduced to below 0.5 kPa (4 Torr) with a yield above 50%. The low residual gas pressure in these mass-producible alkali vapor cells can enable a new generation of low-cost chip-scale atomic devices such as vapor cell optical clocks, wavelength references, and Rydberg sensors.

DOI
10.1364/ol.527351

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We describe a process for fabricating a wafer-scale array of alkali metal vapor cells with low residual gas pressure. We show that by etching long, thin channels between the cells on the Si wafer surface, the residual gas pressure in the evacuated vapor cell can be reduced to below 0.5 kPa (4 Torr) with a yield above 50%. The low residual gas pressure in these mass-producible alkali vapor cells can enable a new generation of low-cost chip-scale atomic devices such as vapor cell optical clocks, wavelength references, and Rydberg sensors.
This paper describes the progress made in developing wafer-scale fabrication process of alkali vapor cells for chip-scale atomic devices such as Coherent Population Trapping (CPT) clocks, optical frequency standards and Rydberg-atom-based sensors. The wafer level fabrication process uses anodic bonding and introduces alkali atoms by chemical reactions of Alkali Chloride with Barium Azide. This method is compatible to make cells with a wide range of buffer gas species and pressures, including evacuated cells. We improve the yield of the cells to above 50%. We will show the residual gas pressures for evacuated cells are mostly consistent with zero within the resolution of the saturation absorption spectroscopy measurement. The temperature coefficients for the CPT spectroscopy are improved with Ar-N2 mixture buffer gas compared with only N2 buffer gas.
Abstract We demonstrate a new technique for filling mm-scale microfabricated silicon and glass cavities with alkali vapors at the wafer-scale. A single etched silicon wafer contains an array of cavities containing alkali precursor materials offset laterally from the cell array. The wafer is heated to create an array of alkali droplets on an upper glass wafer, which is then translated laterally under vacuum and bonded to create the cells. This technique can be implemented in a commercially available bonding tool, allows the fabrication of cells with arbitrary buffer gas contents and pressures and can potentially produce cells with dimensions below 100 µm.
We demonstrate wafer-scale fabrication of temperature-compensated alkali vapor cells using a mixture of Ar and N2 buffer gases. The temperature coefficient of the fractional frequency shift of the hyperfine clock resonance is reduced from a typical value of ≈30×10−9/ K for cells containing pure N2 to below ≈3×10−9/ K for the buffer gas mixture. This result is an important step toward the mass production of temperature-compensated atomic vapor cell-based devices such as chip-scale atomic clocks.
We report a novel methodology for fabrication of vapor cells for chip-scale atomic clocks (CSAC) using wafer-scale micromachining processes. Alkali metals are enclosed in a chemically inert wax to preform alkali metal-wax micropackets, keeping the alkali metals from reacting with the ambient during the vapor cell fabrication. Apart from the high level of purity of the alkali metals in the resulting vapor cells, this method holds promise for inexpensive and flexible manufacturing of vapor cells, as well as easy handling of alkali metals used for a variety of applications other than for CSAC. The optical absorption on fabricated rubidium-87 (Rb/sup 87/) vapor cells over the D2 transitions of Rb/sup 87/ shows a contrast of more than 65%, important to the frequency stability of CSAC.