In recent years, atomic clocks based on coherent population trapping (CPT), which are essential in portable applications, have been miniaturized considerably. Chip-scale atomic clocks are garnering increasing attention owing to their low power consumption and cost, which render them highly suitable for oceanographic exploration. Researchers are focusing on the integration of chip-scale atomic clocks because the discrete nature of optical components in microfabricated atomic vapor cells significantly impedes their miniaturization. CPT atomic clocks function by locking the frequency of a local oscillator to the hyperfine transition levels of atoms in an alkali-metal vapor cell. Most miniature CPT atomic clocks utilize alkali-metal vapor cells fabricated using microelectromechanical system (MEMS) technology and typically feature a glass?silicon?glass trilayer structure, where alkali-metal vapor is stored in a silicon through-hole and light beam propagates along the glass?silicon?glass direction to interact with the alkali-metal vapor. The optical path length is determined based on the silicon-wafer thickness. To increase the interaction path length between light and alkali-metal atoms, a thicker silicon wafer is required. However, owing to the limitations of MEMS manufacturing technology, the thickness of silicon wafers cannot exceed 2 mm, which restricts the number of atoms in the vapor-cell volume, thus adversely affecting the signal-to-noise ratio (SNR). In recent years, the advent of reflective vapor cells has allowed the integration of mirrors into atomic vapor cells, thereby enabling horizontal light propagation within the vapor cell and thus increasing the vapor-cell and optical path lengths for interaction with alkali-metal atoms. Typically, 100-oriented silicon wafers, when anisotropically etched, result in 111-oriented surfaces with an angle of 54.74° between their two surfaces. Researchers have developed an optical path structure that matches a diffraction grating with the anisotropically etched 111 plane of a silicon wafer. However, undesired diffraction on the grating can result in light-intensity loss, and a sufficient distance is required to separate unnecessary diffracted scattering from the grating. By utilizing 100-oriented silicon wafers etched in the 011 direction, a 45° mirror with a 9.74° cutoff angle can be achieved through anisotropic etching, although this configuration is complex. Other studies have proposed the fabrication of 45° reflectors with Bragg mirrors via grinding and polishing, followed by the integration of two 45° reflectors into an atomic vapor cell via local bonding. However, ensuring product consistency through local bonding operations is challenging. Another possibility for forming 45° mirror surfaces using 100-oriented silicon wafers is to apply specific anisotropic etching conditions in an alkaline solution, where surfactants are added to the alkaline etchant and an extremely narrow process window is maintained to achieve a flat 45° mirror surface.

In this study, a simplified anisotropic etching technique for the fabrication of atomic vapor cells was adopted. Specifically, 100-oriented silicon wafers were etched along the 010 direction under anisotropic etching conditions in an alkaline solution. A flat 45° mirror surface was obtained by adding surfactants and operating the process within a narrow window. Additionally, cesium atoms were introduced into the vapor cell via evaporation, and basic microfabrication techniques were used to achieve a low-cost single-chamber atomic vapor cell with aluminum reflector coatings on the sidewalls. This simple and easily integrable approach can replace the conventional bulky optical components, thereby simplifying the challenging fabrication process of current alkali-metal vapor cells. Our method enables the full-chip integration of chip-scale atomic clocks, and the frequency stability of the fabricated atomic vapor cells was evaluated.

The atomic vapor cell was constructed via two rounds of anodic bonding; it features a glass?silicon?glass configuration, with each layer thickness measuring 0.3 mm. Anisotropic etching was performed in alkaline solutions with varying concentrations to obtain 45° mirrors (Fig. 3). The cell was filled with cesium, i.e., an alkali metal, via chemical reactions and evaporation, which significantly reduced the volume of the atomic vapor cell (Fig. 3). The light beam emitted by a vertical cavity surface-emitting laser (VCSEL) is reflected by a 45° mirror and propagates in a planar direction. After interacting with the cesium atoms, the light beam is reflected by another 45° mirror and the signal is received by a photodetector. For this type of atomic vapor cell, the SNR of the CPT signal can be improved by extending the cavity length of the atomic vapor cell. Additionally, performing etching using a stable alkaline solution allows for the large-scale production of atomic vapor cells.

A reflective single-cell atomic vapor chamber was developed via anisotropic etching in an alkaline solution to create 45° mirrors, and cesium vapor was introduced through chemical reactions and evaporation. The performance of these components was evaluated in the context of chip-scale atomic clocks. Short-term stability assessments were conducted on an atomic vapor chamber featuring a 6 mm long single cavity using N?/Ar buffer gas. The VCSEL was operated at a wavelength of 894 nm and a gas pressure of 10000 Pa. An atomic vapor chamber with an optical length of 6 mm was placed in a holder equipped with a C-field coil that generated a magnetic field parallel to the optical path between two 45° reflectors. A permalloy plate was used to shield against the external magnetic field. The incident light was modulated near a CPT resonance frequency of 4.596 GHz, and dark resonance was observed at an operating temperature of 86 ℃, with the CPT resonance peak exhibiting a full width at half maximum (FWHM) of 0.92 kHz. The observed Allan variance is 1.23×10^-10@1 s. This study concludes that the proposed reflective planar vapor cell is promising for applications in chip-scale miniature atomic clocks with system-level packaging.