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Applied Physics Letters

Wafer-scale fabrication of temperature-compensated alkali vapor cells

Yang Li, Marlou R. Slot, Matthew T. Hummon, Susan A. Schima, John E. Kitching

Published Jun 2, 2025• 6 citations
Publisher

Abstract

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.

DOI
10.1063/5.0264409

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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.
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.
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.
Herein, a microfabricated millimeter-level vapor alkali cell with a high hermeticity is fabricated through a wet etching and single-chip anodic bonding process. The vapor cell, containing Rb and N2, was investigated in a coherent population trapping (CPT) setup for the application of a chip-scale atomic clock (CSAC). The contrast of CPT resonance is up to 1.1% within the only 1 mm length of light interacting with atom. The effects of some critical external parameters on the CPT resonance, such as laser intensity, cell temperature, and buffer gas pressure, are thoroughly studied and optimized. The improved microfabricated vapor cell also exhibited great potential for other chip-scale atomic devices.
This letter reports on the generation of a tunable buffer gas mixture within microfabricated alkali vapor cells. We show that the combination of low-permeation windows with sequential openings of laser-actuated break-seals enables adjustment of a helium-neon (He-Ne) noble gas mixture, fully compatible with alkali metal dispensers. The gas reservoirs and the main cell cavities are initially sealed at the wafer level under distinct helium and neon atmospheres, respectively. Within each cell, after Cs vapor is released from the dispenser, the break-seals are successively actuated to incrementally increase the helium fraction in the buffer gas mixture. This process shifts the atomic clock frequency turnover temperature toward higher values. As an illustration, one of the fabricated cells was operated at 95∘^{\circ}∘C in a coherent population trapping clock, achieving a fractional frequency stability of 9$\times101010^{-11}$ at one-day integration time. These results demonstrate the feasibility of precisely tuning buffer gas compositions in microfabricated vapor cells and support the suitability of He-Ne mixtures for miniature atomic clock applications.
A novel technique for microfabricating alkali atom vapor cells is described in which alkali atoms are evaporated into a micromachined cell cavity through a glass nozzle. A cell of interior volume 1 mm3, containing 87Rb and a buffer gas, was made in this way and integrated into an atomic clock based on coherent population trapping. A fractional frequency instability of 6 x 10(-12) at 1000 s of integration was measured. The long-term drift of the F=1, mF=0-->F=2, mF=0 hyperfine frequency of atoms in these cells is below 5 x 10(-11)/day.
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.