Micro-Electro-Mechanical System Vapor Cells With Passivated Internal Cavities

Abstract

Micro-Electro-Mechanical, so called 'MEMs,' vapor cells are a key component in atom-based quantum sensors, such as clocks, gyroscopes, electric field sensors and magnetometers. MEMs vapor cell fabrication for Rydberg atom radio frequency sensors is particularly demanding. The Rydberg states used for the sensor can shift in a constant electric field which can be generated by the internal surfaces of the vapor cell cavity. The ratio of the detection wavelength to vapor cell size can span a large range, meaning that the radio frequency field-vapor cell interaction is a critical design consideration. In many radio frequency sensing cases, there is a desire to minimize the interaction between the vapor cell and the target radio frequency field, as well as assure that every vapor cell behaves uniformly. These criterion favor MEMs vapor cells with low background electric fields. Known inert, organic coatings cannot survive the bonding temperatures required for conventional anodic bonding of a MEMs vapor cell. Applying inert, organic coatings to the internal cavities of MEMs vapor cells is a longstanding challenge. In this paper, we present a low temperature bonding scheme that is compatible with coating the internal cavity of a MEMs vapor cell with Octadecyltrichlorosilane (CH$_3\,$(CH$_2$)$_{17}\,$SiCl$_3$, OTS). The coating prevents the Cs used in the vapor cell from sticking to the walls. Spectral linewidths of $\sim300\,$kHz are obtained using Rydberg spectroscopy, with energy shifts corresponding to electric fields $<$10$\,$mV$\,$cm$^{-1}$.

Figures (2)

• Figure 1: (a.) OTS coated vapor cell. (b.) The figure shows a single shot and averaged saturated absorption trace from an OTS coated vapor cell. (c.) The figure shows the spectral lineshapes from the three MEMs vapor cell samples used for the experiments in comparison with the data obtained from the reference vapor cell. The EIA spectra have spectral linewidths in the vapor cells of $2\pi \times 390\,$kHz (sample 1), $2\pi \times 330\,$kHz (sample 2), and $2\pi \times 290\,$kHz (sample 3). The EIA spectral linewidth in the reference vapor cells was $2\pi \times 290\,$kHz. The line shifts were measured to be $2\pi \times -5\,$kHz (sample 1), $2\pi \times 83\,$kHz (sample 2) and $2\pi \times 19\,$kHz (sample 3) relative to the line center of the reference vapor cell. The Rabi frequencies of the lasers were $2\pi \times 1\,$ MHz ($895\,$nm), $2\pi \times 1.5\,$MHz ($636\,$nm) and $2\pi \times 160\,$kHz ($2262\,$nm). The respective laser spectral linewidths were $2\pi \times 600\,$Hz, $2\pi \times 2\,$kHz and $2\pi \times 20\,$kHz. The transit time broadening was estimated to be $2\pi \times 240\,$kHz. The overlap region of the laser beams was $400\,\mu$m in diameter.
• Figure 2: (a) XPS depth profile from a bare glass sample exposed to Cs. The C peak visible on the sample surface, upper $\sim 1\,$nm is from adventitious C on the surface. A distinct non-zero Cs signal is also observed at the surface on the same depth scale. (b) XPS depth profile from a bare $150\,$nm thick SiO$_2$ surface coating on Si. A similar Cs peak to the one found on glass is observed. (c) XPS depth profile from an OTS SAM coated glass surface. Here, the C atomic concentration is much larger and is found to increase at the surface where the Si peak is decreasing. The feature indicates the formation of the SAM. The depth resolution is not high enough to completely separate the signals from the substrate and OTS SAM. The OTS SAM surface is around $2\,$nm thick. A much smaller amount of Cs is found on the surface, see (e). (d) XPS depth profile of an OTS SAM coated SiO$_2$ surface. Again the shape of the C peak and the Si peak shows the OTS SAM formation. A much smaller amount of Cs is detected on the OTS SAM coated surface. Similar to (c), the OTS SAM is around $2\,$nm thick. (e) XPS depth profile showing a comparison of Cs on the glass and OTS SAM coated glass surface, see text for fractions. (f) XPS depth profile showing a comparison of Cs on the SiO$_2$ surface and OTS SAM coated SiO$_2$ surface, see text for fractions.