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.
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.
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.
The need to reach high and stable values of the Q-factor is one of the most important issues of resonant MEMS in order to make high-performance sensors. The Q-factor is strongly influenced by the internal environment of the MEMS packaging, by total pressure and by gas composition. The most experienced and technically accepted way to keep the atmosphere stable in a hermetically sealed device is to use a getter material that is able to chemically absorb active gases under vacuum or in inert gas atmosphere for the lifetime of the devices. MEMS hermetically bonded devices such as gyroscopes, accelerometers, pressure and flow sensors, IR sensors, RF-MEMS and optical mirrors requires getter thin film solutions to work properly. Getter technical solution for wafer to wafer hermetically bonded MEMS systems is PaGeWafer, a silicon, glass or ceramic wafer ("cap wafer") with patterned getter film, few microns thick. In this paper, first the theoretical evaluation of 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 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 getter films can provide high Q-values, stability of sensor signal, performances stability during the lifetime, removal of dangerous gases like H2 and H2O in hermetically sealed MEMS resonant structures.
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.
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.