R. Straessle

Microfabricated rubidium vapour cell with a thick glass core for small-scale atomic clock applications

This paper presents a new fabrication method to manufacture alkali reference cells having dimensions larger than standard micromachined cells and smaller than glass-blown ones, for use in compact atomic devices such as vapour-cell atomic clocks or magnetometers. The technology is based on anodic bonding of silicon and relatively thick glass wafers and fills a gap in cell sizes and technologies available up to now: on one side, microfabrication technologies with typical dimensions <= 2 mm and on the other side, classical glass-blowing technologies for typical dimensions of about 6-10 mm or larger. The fabrication process is described for cells containing atomic Rb and spectroscopic measurements (optical absorption spectrum and double resonance) are reported. The analysis of the bonding strength of our cells was performed and shows that the first anodic bonding steps exhibit higher bonding strengths than the later ones. The spectroscopic results show a good quality of the cells. From the double-resonance signals, we predict a clock stability of approximate to 3 x 10(-11) at 1 s of integration time, which compares well to the performance of compact commercial Rb atomic clocks.

Microfabricated alkali vapor cell with anti-relaxation wall coating

We present a microfabricated alkali vapor cell equipped with an anti-relaxation wall coating. The anti-relaxation coating used is octadecyltrichlorosilane and the cell was sealed by thin-film indium-bonding at a low temperature of 140 °C. The cell body is made of silicon and Pyrex and features a double-chamber design. Depolarizing properties due to liquid Rb droplets are avoided by confining the Rb droplets to one chamber only. Optical and microwave spectroscopy performed on this wall-coated cell are used to evaluate the cells relaxation properties and a potential gas contamination. Double-resonance signals obtained from the cell show an intrinsic linewidth that is significantly lower than the linewidth that would be expected in case the cell had no wall coating but only contained a buffer-gas contamination on the level measured by optical spectroscopy. Combined with further experimental evidence this proves the presence of a working anti-relaxation wall coating in the cell. Such cells are of interest for applications in miniature atomic clocks, magnetometers, and other quantum sensors.

Low-temperature indium-bonded alkali vapor cell for chip-scale atomic clocks

A low-temperature sealing technique for micro-fabricated alkali vapor cells for chip-scale atomic clock applications is developed and evaluated. A thin-film indium bonding technique was used for sealing the cells at temperatures of ≤140 °C. These sealing temperatures are much lower than those reported for other approaches, and make the technique highly interesting for future micro-fabricated cells, using anti-relaxation wall coatings. Optical and microwave spectroscopy performed on first indium-bonded cells without wall coatings are used to evaluate the cleanliness of the process as well as a potential leak rate of the cells. Both measurements confirm a stable pressure inside the cell and therefore an excellent hermeticity of the indium bonding. The double-resonance measurements performed over several months show an upper limit for the leak rate of 1.5 × 10−13 mbar·l/s. This is in agreement with additional leak-rate measurements using a membrane deflection method on indium-bonded test structures.

Low-temperature thin-film indium bonding for reliable wafer-level hermetic MEMS packaging

This paper reports on low-temperature and hermetic thin-film indium bonding for wafer-level encapsulation and packaging of delicate and temperature sensitive devices. This indium-bonding technology enables bonding of surface materials commonly used in MEMS technology. The temperature is kept below 140 degrees C for all process steps and no surface treatment is applied before and during bonding. This bonding technology allows hermetic sealing at 140 degrees C with a leak rate below 4 x 10(-12) mbar l s(-1) at room temperature. The tensile strength of the bonds up to 25 MPa goes along with a very high yield.

Low-temperature indium hermetic sealing of alkali vapor-cells for chip-scale atomic clocks

We present a low-temperature indium hermetic bonding technique on wafer level without using flux, active atmosphere or other pretreatment of the indium. Its simplicity and low temperatures allow encapsulation of sensitive MEMS devices. Bonding stronger than 18 MPa was accomplished with temperatures never exceeding 140°C. Leak rate measurements revealed leak rate below 2.5 × 10-12 atm cc/s. This bonding technique is then applied to fabricate rubidium vapor-cells for chip-scale atomic clocks (CSAC). Saturated absorption spectroscopy performed two and five months after fabrication confirms less than 1 mbar of gas contamination, and the retrieved clock signal demonstrates the suitability of the cell for clock applications.

NEMS based tools for nanoscience and atomic clocks

Nanoscience is a thriving multi-disciplinary activity, which aims at understanding the properties and the interaction of very small objects on the nanometer scale. In this endeavor, tools for the observation, analysis and modification of individual objects like macromolecules, clusters or even single atoms are required. The development of dedicated microfabricated instruments to measure physical and chemical interactions at this scale is therefore required. This talk will give an overview of microfabrication techniques employed to shape such NEMS based tools and introduce the audience to several probing techniques. In a second part, we focus on the principles and fabrication techniques of atomic clocks.

Wall-coated cells for Rb atomic clocks: Study of the ripening process by double-resonance spectroscopy

We present a study of our in-house made tetracontane wall-coated 87Rb vapour cells for rubidium atomic clock applications. Evolution of the double-resonance (DR) signal during the so-called ripening process of these cells is measured and interpreted. Intrinsic properties of the coated cells post ripening are presented. Intrinsic linewidths below 70 Hz, and moderate temperature coefficients (1.5 × 10−10 /K) are promising cell properties in view of highly compact and miniature atomic clocks.

Spectroscopy in a micro-fabricated Rb cell with anti-relaxation wall-coating

We report on the realization and spectroscopic evaluation of a micro-fabricated Rb vapor cell equipped with an anti-relaxation OTS wall coating. By combining different spectroscopic results, we can exclude a hypothetical buffer-gas contamination of the cell as cause of the narrow double-resonance (DR) signals observed. We thus conclude that the narrow DR signal linewidth is due to the anti-relaxation wall-coating present in the cell. This realization presents interests in view of applications in miniaturized atomic devices such as atomic clocks, magnetometers, and others.

Micro-fabricated alkali vapor cells sealed at low temperatures with thin-film metallic bonding

We report on the characterization of micro-fabricated alkali vapor cells, sealed by low-temperature thin-film metallic bonding. The low sealing temperatures ≤ 140°C are of high interest in view of future micro-fabricated cells using anti-relaxation wall coatings. Long-term measurements using saturated-absorption and double-resonance spectroscopy show a stable pressure inside the cells and therefore an excellent hermeticity of the bonding over several months. This is also confirmed by independent leak-rate measurements of indium bonded sample cells by a membrane deflection method.

Towards wall-coated microfabricated cells: Alkali vapor-cells using indium thin-film low-temperature bonding

We report on the realization and evaluation of micro-fabricated alkali vapor cells using a thin-film indium bonding technique. Optical and double-resonance spectroscopy performed on the cells demonstrates their suitability for atomic clock applications. Long-term measurements show a stable pressure inside the cells and therefore an excellent hermeticity of the bonding over several months. The low bonding process temperature of ≤ 140°C combined with good bond strength makes this method a promising candidate for realizing micro-fabricated alkali cells with anti-relaxation wall-coatings.