Abstract
Mastering non-evaporable getter (NEG) thin films by elucidating their activation mechanisms and predicting their sorption performances will contribute to facilitating their integration into micro-electro-mechanical systems (MEMS). For this aim, thin film based getters structured in single and multi-metallic layered configurations deposited on silicon substrates such as Ti/Si, Ti/Ru/Si, and Zr/Ti/Ru/Si were investigated. Multilayered NEGs with an inserted Ru seed sub-layer exhibited a lower temperature in priming the activation process and a higher sorption performance compared to the unseeded single Ti/Si NEG. To reveal the gettering processes and mechanisms in the investigated getter structures, thermal activation effect on the getter surface chemical state change was analyzed with in-situ temperature XPS measurements, getter sorption behavior was measured by static pressure method, and getter dynamic sorption performance characteristics was measured by standard conductance (ASTM F798–97) method. The correlation between these measurements allowed elucidating residual gas trapping mechanism and prediction of sorption efficiency based on the getter surface poisoning. The gettering properties were found to be directly dependent on the different changes of the getter surface chemical state generated by the activation process. Thus, it was demonstrated that the improved sorption properties, obtained with Ru sub-layer based multi-layered NEGs, were related to a gettering process mechanism controlled simultaneously by gas adsorption and diffusion effects, contrarily to the single layer Ti/Si NEG structure in which the gettering behavior was controlled sequentially by surface gas adsorption until reaching saturation followed then by bulk diffusion controlled gas sorption process.
Highlights
Non-evaporable getters (NEGs), based on the thin film deposition technique, were until now still the practical technical solution for integrating a scaled-down getter into microelectronic devices, that need special residual gas control within their cavities for keeping their functioning performances optimal after packaging under a controlled atmosphere
Ti and Zr belong to the group V elements in periodic table possessing a high oxygen solubility limit and high oxygen diffusivity physical properties, which are behind the high gas sorption properties manifested by NEGs
The NEGs gettering performances are tightly linked to the NEGs fabricated microstructure, which is required to be essentially formed of a small columnar grain structure that promotes the gas sorption process by diffusion through a NEG grain boundary microstructure
Summary
Non-evaporable getters (NEGs), based on the thin film deposition technique, were until now still the practical technical solution for integrating a scaled-down getter into microelectronic devices, that need special residual gas control within their cavities for keeping their functioning performances optimal after packaging under a controlled atmosphere. To optimize sorption performances and reduce thermal activation temperature of NEGs systems, in the literature, many investigations have been undertaken to improve the NEG materials diffusivity by fabricating them from alloys of several mixed metals [5,9,10,11,12] or by controlling the NEG microstructure and surface roughness using pure multi-metallic-layer thin films forming NEGs in a stacked structure [13,14,15]. The growth of the subsequent getter film layer is partially controlled by a limited mobility of its metal atoms at the Ru sub-layer surface thereby leading to a finer microstructure in the getter film, consisting of a columnar grain structure and a high getter surface roughness Such a microstructure promotes the getter to be activated at a low temperature due to the diffusion process mostly taken along the grain boundaries, as well as provides the getter with a large sorption capacity, sufficient for pumping throughout the MEMS device lifetime. A correlation relating the XPS getter surface chemical state changes to the getter sorption behavior was evidenced, and the getter activation and the corresponding gas sorption mechanisms were elucidated
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