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A broad interest has been showed recently on the study of nanostructuring of thin films and surfaces obtained by low‐energy He plasma treatments and He incorporation via magnetron sputtering. In this paper spatially resolved electron energy‐loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) is used to locate and characterize the He state in nanoporous amorphous silicon coatings deposited by magnetron sputtering. A dedicated MATLAB program was developed to quantify the helium density inside individual pores based on the energy position shift or peak intensity of the He K‐edge. A good agreement was observed between the high density (~35‐60 at/nm3) and pressure (0.3‐1.0 GPa) values obtained in nanoscale analysis and the values derived from macroscopic measurements (the composition obtained by proton backscattering spectroscopy coupled to the macroscopic porosity estimated from ellipsometry). This work provides new insights into these novel porous coatings, providing evidence of highdensity He located inside the pores and validating the methodology applied here to characterize the formation of pores filled with the helium process gas during deposition. A similar stabilization of condensed He bubbles has been previously demonstrated by high‐energy He ion implantation in metals and is newly demonstrated here using a widely employed methodology, magnetron sputtering, for achieving coatings with a high density of homogeneously distributed pores and He storage capacities as high as 21 at%. ; European Union CT‐REGPOT‐2011‐1‐285895 ; Consejo Superior de Investigaciones Científicas PIE 201060E102, PIE 201460E018 ; Ministerio de Economía y Competitividad CSD2008–00023, CTQ2012‐32519 ; Junta de Andalucía TEP217, PE2012‐TEP862

In this work, we present our magnetron sputtering based methodology to produce amorphous silicon coatings with closed porosity, as a strategy to fabricate solid helium targets, in the form of supported or self-supported thin films, for nuclear reactions. We show how by changing the He working pressure it is possible to obtain highly porous homogeneous structures incorporating different He amounts. These porous coatings (a- Si:He) are very reproducible from run to run, and the high He amount incorporated makes them excellent candidates for solid He targets. The possibility of producing self-supported films is illustrated here, and its potential use in inverse kinematics experiments with radioactive beams is shown through the dispersion in forward geometry of a stable 6Li beam. Also the elastic scattering cross-sections for proton from helium were determined using an a-Si:He coating. The results agree well with the ones reported in the literature. These two examples validate our coatings as good candidates to be used as solid He targets in nuclear reactions. The stability of He inside the coatings, fundamental for its use as solid He targets, was investigated, both over time and after irradiation. The coatings proved to be very stable, and the amount of He inside the pores remains unaltered at least 2 years after deposition and after high irradiation fluence (5 × 1017 particles/cm2; with a dose rate of 5 × 1012 particles/(cm2 s)). ; Ministerio de Economía y Competitividad FPA2013-47327-C2-1-R, MAT2015-69035- REDC ; Consejo Superior de Investigaciones Científicas 201460E018 ; European Union CT-REGPOT-2011-285895

The formation of the porous structure in dc magnetron sputtered amorphous silicon thin films at low temperatures is studied when using helium and/or argon as the processing gas. In each case, a-Si thin films were simultaneously grown at two different locations in the reactor which led to the assembly of different porous structures. The set of four fabricated samples has been analyzed at the microstructural level to elucidate the characteristics of the porous structure under the different deposition conditions. With the help of a growth model, we conclude that the chemical nature of the sputter gas not only affects the sputtering mechanism of Si atoms from the target and their subsequent transport in the gaseous/plasma phase towards the film, but also the pore formation mechanism and dynamics. When Ar is used, pores emerge as a direct result of the shadowing processes of Si atoms, in agreement with Thornton's structure zone model. The introduction of He produces, in addition to the shadowing effects, a new process where a degree of mobility results in the coarsening of small pores. Our results also highlight the influence of the composition of sputtering gas and tilt angles (for oblique angle deposition) on the formation of open and/or occluded porosity ; European Union CT-REGPOT-2011-1-285895 ; Consejo Superior de Investigaciones Científicas PIE 201060E102, PIE 201460E018 ; Ministerio de Economía y Competitividad CSD2008–00023, CTQ2012-32519, MAT2013-40852-R ; Junta de Andalucía TEP217 PE2012, TEP862, PE2010-FQM-6900