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International audienceWe describe the fabrication of wafer-scale alkali vapor cells based on silicon micromachining and anodic bonding. The principle of the proposed micromachined alkali cell is based on an extremely compact sealed vacuum cavity of a few cubic millimeters containing caesium vapors, illuminated by a high-frequency modulated laser beam. The alkali cells are formed by sealing an etched silicon wafer between two glass wafers. The technique of cell filling involves the use of an alkali dispenser. The activation of cesium vapors is made by local heating of the dispenser below temperature range causing degradations of cesium vapor purity. Thus, the procedure avoids negative effects of cesium chemistry on the quality of cell surfaces and sealing procedure. To demonstrate the clock operation, cesium absorption as well as coherent population trapping resonance was measured in the cells. (C) 2008 Society of Photo-Optical Instrumentation Engineer

A method for filling alkali vapor cells with cesium from a dispensing paste is proposed and its compliance with miniature atomic clock applications is evaluated. The paste is an organic-inorganic composition of cesium molybdate, zirconium-aluminum powder, and a hybrid organic-inorganic binder. It is compatible with collective deposition processes such as micro-drop dispensing, which can be done under ambient atmosphere at the wafer-level. After deposition and sealing by anodic bonding, cesium is released from the consolidated paste through local heating with a high power laser. Linear absorption signals have been observed over one year in several cells, showing a stable atomic density. For further validation of this technology for clock applications, one cell has been implemented in a coherent population trapping clock setup to monitor its frequency stability. A fractional frequency aging rate around –4.4 × 10−12 per day has been observed, which is compliant with a clock frequency instability below 1 × 10−11 at one day integration time. This filling method can drastically reduce the cost and the complexity of alkali vapor cell fabrication.

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PURPOSEThe objective of this work was to show that the Q-factor of MEMS resonators can be increased and stabilized with the use of getters. [ABSTRACT The need to reach high, reliable and stable values of the Q-factor is one of the key issues of resonant MEMS in order to make highperformance sensors. The Q-factor is strongly influenced by the internal environment of the MEMS packaging, by the total pressure and by the gas composition. The most experienced and technically accepted way to keep the atmosphere stable in a hermetically sealed device is the use of getters that are able to chemically absorb active gasses under vacuum or in inert gas atmosphere for the lifetime of the devices. Several types of hermetically bonded MEMS devices such as gyroscopes, accelerometers, pressure and flow sensors, IR sensors, RF-MEMS and optical mirrors require getter thin film solutions to work properly. A possible getter technical solution for wafer to wafer hermetically bonded MEMS systems is PaGeWafer. PaGeWafer is a silicon, glass or ceramic wafer ("cap wafer") with a patterned getter film, few microns thick. In this paper, a theoretical evaluation of the Q-factor of a MEMS resonant structure in presence of a getter film is investigated and compared to the results of a Residual Gas Analysis of the same MEMS resonant structure and with the conventional measurement of the Q-factor. Using getter thin film technology, total pressures down to 10 -4 mbar with corresponding high and stable Q-factors have been achieved in MEMS resonant structures. We were therefore able to confirm that a getter film can provide high Q-values, stability of the sensor signal, performances stability during the lifetime and removal of dangerous gases like H 2 and H 2 O in hermetically sealed MEMS resonant structures.