Many microsensors need to operate in medium vacuum, which is obtained by low temperature vacuum packaging integrating a getter film. By a thermal activation during the sealing process, the getter film aims to compensate the outgassing of the inner surfaces of the micro-cavity and also leaks after sealing. Thin films of getter alloys were coevaporated under ultra-high vacuum on silicon wafers. They were activated by annealing at temperatures ranging from 225°C to 400°C, during one hour under Argon atmosphere with traces of oxidizing species. Three complementary ion beam analysis techniques were performed to obtain depth profiles and to quantify the number of atoms of the different gaseous species absorbed by the getter films: Rutherford Backscattering Spectrometry (RBS), Nuclear Reaction Analysis (NRA) and Elastic Recoil Detection Analysis (ERDA). The results show that both oxygen and hydrogen diffuse inside the getter films. However, hydrogen tends to accumulate near the interface between film and substrate and starts to diffuse inside substrate as well. We demonstrated that the sorption of hydrogen by an yttrium-based getter film is tailored by its composition and depends on its degree of oxidation.
Yttrium, titanium, and yttrium-titanium getter thin films were elaborated on silicon by coevaporation in ultrahigh vacuum. Y-Ti films exhibit nanometric crystallites size (18–35 nm) leading to a very high grain boundary density, which is a favorable microstructure for activation at low temperature. The yttrium content in Y-Ti alloys influences grain size, resistance against room temperature oxidation, and gettering performance for oxygen. Y-Ti films with an yttrium content higher than 30% show strong oxygen sorption during annealing at low temperature (<300 °C). After 1 h of annealing at 250 °C, it was estimated that the yttrium-based getter films can trap between 0.2 and 0.5 μmol of oxygen per cm2, while no oxygen sorption was detected for a single metal titanium film. This makes Y-Ti getter alloys attractive candidates for the packaging of MEMS under vacuum with a low bonding temperature.
Thanks to their reactivity to gases, getter materials are involved in vacuum packaging to compensate leaks and permeation through the package. After deposition, the material is passivated due to the contact with the air, and must be thermally activated to force the diffusion of the passivation layer inside the film. However, new MEMS technologies imply more fragile devices thus processes with a limited temperature. In this frame, we investigate getter materials presenting a low activation temperature, such as Zr-based alloys. Moreover, in most investigations of getter films, activation and sorption are characterized separately by room temperature surface analysis techniques in UHV and by sorption experiments under specific gases, respectively. In this work, we have studied the behavior of getter thin films in vacuum as a function of the temperature and the pressure to be as close as possible to the conditions of integration, i.e. wafer bonding. This consisted in following the sheet resistance measured by using the 4-probes technique and comparing electro-thermal properties (resistivity, Temperature Coefficient of Resistance TCR) to pre- and post- characterization (composition, structure). Thin films of Zr, V, Ti, Zr-Ti, Zr-V and Zr-Co with thicknesses of 300-400 nm have been deposited by co-evaporation in UHV. Their sheet resistance was measured as a function of the temperature under vacuum. The control of the pressure, from 10 -6 to 10 -3 mbar, allowed to activate the films in different conditions. For as-deposited films, results show that the resistivity and TCR are linked and follow the Mooij rule: the TCR decreases with the resistivity and becomes negative for resistivities higher than 150 µW.cm. The values globally depend on the composition, but separated groups are also linked to the disorder of the structure, from the lowest to the highest: pure metals, Zr-Ti, Zr-V then Zr-Co. In case of Zr-Ti, the grain size directly impacts the evolution of the coupled TCR and resistivity. The activation in vacuum involves simultaneous and continuous diffusion inside the films and sorption at the surface. The latter cannot be avoided, even at 10 -6 mbar, and the resistivity increases with the square root of the time. However, differences of behavior as a function of the pressure can be linked to the gettering efficiency and the diffusion length of oxygen in the material, which depends on both the composition and the microstructure. Crystalline Zr-Ti films behave as Zr and Ti and the grain size directly impacts the diffusion, thus the quantity of oxygen sorbed in the film. The structure of Zr-V or Zr-Co is more complex, which can be either crystalline or amorphous depending on the composition. Large variations in the sheet resistance can be obtained and the high disorder of the structure allow them to sorb more gaseous species than Zr-Ti.
It is demonstrated that high quality stress-induced birefringence maps in silicon with a high lateral resolution can be obtained by microphotoelasticity in reflection mode at wavelength of 1.31 μm. This technique was applied to inspection of Si-Si wafer bonding defects with variable magnifications. It is shown that defects with an apparent size as small as 20μm in diameter can be observed with a 2μm lateral resolution.
Non-evaporable getter materials based on transition metal alloys (Zr, V, Co, Fe, etc...) are reactive materials able to sorb species like O, N, H or C. Integrated as thin films inside MEMS packages under vacuum, they can compete with desorption during bonding and leaks during time. Their thermo-electrical characteristics under varying atmospheres reflect the evolution of the microstructure and is dependent on the sorption properties. In this frame, 4-probes electrical resistivity measurements have been performed on different getter binary alloys under partial vacuum as a function of the temperature to investigate their activation and evolution.
