We report 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 $\mathrm{He}$-$\mathrm{Ne}$ 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 $\mathrm{Cs}$ 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 inversion temperature toward higher values. As an illustration, one of the fabricated cells was operated at $95{}^{\circ }\mathrm{C}$ in a coherent population trapping clock, achieving a fractional frequency stability of $9\times {10}^{-11}$ at an integration time of 1 day. These results demonstrate the feasibility of precisely tuning buffer-gas compositions in microfabricated vapor cells and support the suitability of $\mathrm{He}$-$\mathrm{Ne}$ mixtures for miniature atomic clock applications.

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• Atomic, optical & lattice clocks
• Time & frequency standards
• Alkali metals
• Atomic ensemble
• Atomic gases
• Devices
• Optical spectroscopy
• Precision measurements