Si nanospring films for compliant interfaces

Films comprised of dense arrays of Si nanosprings were studied for their potential as compliant interface layers. The films, fabricated via glancing angle deposition (GLAD), were comprised of 10-µm high Si nanosprings with 4 or 10 coil turns and seed spacings of 900 nm or 1500 nm. Unseeded films with the same height and 4 or 10 coil turns were co-fabricated as the control material. The film mechanical behavior was a combination of the mechanical response of the individual springs and their interactions inside the film, resulting in increasing compressive stiffness with applied stress. The shear stiffness of all types of Si spring films was in the narrow range of 17–27 MPa. On the contrary, depending on coil geometry, the low-stress compressive stiffness could be varied in the broad range of 13–150 MPa. Thus, this class of nanostructured films allows for relatively decoupled normal and shear properties with values comparable to those of soft polymers. Despite that unseeded films have the thinnest coil wire, it was shown that seeding can produce equal or lower film stiffness, while also increasing the resistance to permanent compression at high stresses. Similarly, a capping layer increased the coherency of the films and their resistance to permanent compression without affecting significantly the compressive film stiffness. Capped films with 10-turn coils, with 900-nm seed spacing, provided the most resistance to permanent compression for stresses at least as high as 15 MPa, while maintaining compressive stiffness values that were equivalent to unseeded 10-turn springs. Finally, capped 4-turn springs with 900-nm seed spacing were shown to be the most compliant of all film types, including unseeded films, while also exhibiting the best agreement between film-level stiffness and estimates based on individual spring properties. Therefore, seeding of GLAD springs can produce films with increased resistance to permanent deformation while maintaining the low stiffness of unseeded films.