Marco Moraja

New approach of fabrication and dispensing of micromachined cesium vapor cell

We 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.

New getter configuration at wafer level for assuring long-term stability of MEMS

The evolution from ceramic packages to wafer to wafer hermetic sealing poses tremendous technical challenges to integrate a proper getter inside the MEMs to assure a long term stability and reliability of the devices. The state of the art solution to integrate a getter inside the MEMs of the last generation consists in patterning the getter material with a specific geometry onto the Si cap wafer. The practical implementation of this solution consists in a 4” or 6” Si wafers with grooves or particular incisures, where the getter material is placed in form of a thick film. The typical thickness of these thick films is in the range of few microns, depending on the gas load to be handled during the lifetime of the device. The structure of the thick getter film is highly porous in order to improve sorption performances, but at the same time there are no loose particles thanks to a proprietary manufacturing method. The getter thick film is composed of a Zr special alloy with a proper composition to optimize the sorption performances. The getter thick film can be placed selectively into grooves without affecting the lateral regions, surrounding the grooves where the hermetic sealing is performed.

Stable and Reliable Q-Factor in Resonant MEMS with Getter Film

The objective of this work was to show that the Q-factor of MEMS resonators can be increased and stabilized with the use of getters

Chemical treatment of getter films on wafers prior to vacuum packaging

The patterned getter film at wafer level has been proven to be the viable technical solution to integrate a getter film in vacuum packaged MEMS. The different MEMS vacuum bonding technologies such as eutectic, direct fusion and anodic bonding guarantee a suitable combination of time and temperature to properly activate the getter film. However, before any MEMS vacuum bonding process it has been discovered that a caustic chemical treatment of the getter film both cleans the film and enhances its performance without measurable degradation of its structural integrity. For example, caustic chemical treatment with SC1 with NH4OH and SC2 with HCl did not affect the morphology and the sorption capacities of the getter film and significantly increased the sorption capacity. The getter film at wafer level can withstand also treatment with a highly aggressive HNO3 process. Therefore, we demonstrated the full compatibility of the getter film towards both temperature and chemical treatment with regards to the activation and capacity of the getter film.

New Vapor Cell Technology for Chip Scale Atomic Clock

Microfabrication techniques are used to realise an original cesium vapor microcell (1mm3). This microcell has a very stable cesium vapor which is generated after the cell is sealed. In addition the microfabrication techniques used allow a production at the industrial scale and in an economic way. The applications aimed are chip scale atomic clocks (CSAC).

Patterned gas absorbing films for assuring long-term reliability and improving performances of mems and omems

Microsensor packaging is one of the most important and challenging technology areas. Many microelectro-mechanical systems (MEMS) and optical MEMS (OMEMS) need to be protected from outside environment stresses: therefore the package must provide an interior environment compatible with the device operation, performances, reliability and lifetime. In addition some MEMS need a specific gas or pressure environment within the package to operate as specified: in particular, OMEMS should be protected against moisture related failures

A new model for vacuum quality and lifetime prediction in hermetic vacuum bonded MEMS

In many MEMS applications the level of vacuum is a key issue as it directly affects the quality of the device, in terms of response reliability. Due to the unavoidable desorption phenomena of gaseous species from the internal surfaces, the vacuum inside a MEMS, after bonding encapsulation, tends to be degraded, unless a proper getter solution is applied. The in situ getter film (PaGeWafer®) is recognised to be the most reliable way to get rid of degassed species, assuring uniform, high quality performances of the device throughout the lifetime. Moreover, post process vacuum quality control and reliability for hermetic bonding is extremely important for overall device reliability and process yield. In this paper we will discuss the main factors that are critical in the attainment of vacuum and will present a novel calculation model that enables the prediction of vacuum level after bonding, making also possible the estimate of the lifetime. Furthermore, a new analytical method based on the residual gas analyses (RGA) will be presented that gives the main characteristics of the materials. Modeling and simulation work support the process optimization and system design.

Development of thin film getters for assuring high reliability and long lifetime to crystal oscillators

The shrinkage of hermetic packages for crystal oscillators poses tremendous challenges to keep constant the pressure during the lifetime of the device because of the considerable effect of surface outgassing and gas permeation. Getters have been used over tens of years in the vacuum tube industry to keep constant vacuum inside sealed devices. To provide a suitable getter solution to miniaturized hermetic packages, a few micron thick getter film has been developed and placed on the lid of the hermetic packages. This technical solution, the getter thin film on the lid, assures a long lifetime and stability to hermetically packaged oscillating structures.

Outgassing and Gettering

Outgassing is an unavoidable issue and although the cleaning process and long baking are carried out, the effect can be limited only by using the getter technology. This technology absorbs the residual gases trapped in the device during the process and maintains the pressure at a very low level for the required lifetime, limiting the gas flux. It has been described that there are gases remaining inside MEMS devices that need a vacuum or other stable pressure to operate properly. The MEMS packaging technology moved from hermetic vacuum discrete packages to vacuum wafer-level packages. Conventional packaging of microelectronic and optoelectronic devices is usually stable and not affected by the external environment except for a few specific cases. Vacuum wafer-level packaging is a very effective technique to produce low-cost, hermetically sealed packages for micromachined sensors and actuators. The main contamination source is the leakage and outgassing phenomena. The leakage is mainly caused by defects in the bonding frame and can be solved by a suitable improvement of the technology or accepting a limited yield in the MEMS production. In the case of outgassing a technically viable solution to assure a long lifetime and high reliability to MEMS device is the integration of a getter film. This has also been explained in detail.

Lifetime prediction in vacuum packaged MEMS provided with integrated getter film

Thin-film getter integration is one of the key technologies enabling the development of a wide class of MEMS devices, such as IR microbolometers and inertial sensors, where stringent vacuum requirements must be satisfied to achieve the desired performances and preserve them for the entire lifetime. Despite its importance, the question about lifetime prediction is still very difficult to answer in a reliable way. Here we present an experimental approach to the evaluation of lifetime, based on an accelerated life test performed varying both the storage conditions and the getter area. A test vehicle based on a resonator device was used. The hermeticity was evaluated by means of specific leak testing, while MEMS behavior during the ageing test was studied monitoring device functional parameters and by residual gas analysis (RGA). Unexpected results were observed leading to the discovery that methane is pumped by the getter below 100°C. These results served as the inputs of a suitable model allowing extrapolating the device lifetime in operating? conditions, and pointed out that RGA is an essential tool to correctly interpret the aging tests.