Plasma-assisted techniques are regularly used to fabricate compact and dense films for microelectronics and protective coatings for a wide range of applications (from turbine blades to food and beverage). The past few years have witnessed the development of new deposition modes at oblique angles that allow the synthesis of nanoporous materials, of interest for numerous technological fields, such as self-cleaning surfaces, fuel cells, microfluidic devices, or solar cells, to name a few. In these cases, the formation of nanopores is the outcome of self-shadowing effects when vapor species arrive at the film surface along a tilted direction.
With these developments in mind, researchers from the Spanish Research Council (CSIC) have developed a fundamental framework to explain the growth of nanoporous thin films when the magnetron sputtering technique is used. They have described the transport of vapor species through a plasma towards a tilted surface by introducing the concept of thermalization degree of sputtered particles, which effectively accounts for the collisional processes in the vapor phase between the traveling atoms and the plasma species. This theoretical framework has been experimentally tested by preparing different nanoporous structures and analyzing them by high energy ion beam tools, obtaining a remarkable agreement in all studied conditions. This research, financed by the FUNCOAT Consolider project, has quantitatively demonstrated the relevance of collisional plasma processes in the development and growth of nanoporous thin film structures.