University of Illinois Champaign Urbana United States
This research program investigated the mechanics of thin films comprised of dense arrays of 10-micron high Cu and Si micro and nanosprings fabricated as multifunctional compliant interfaces via Glancing Angle Deposition GLAD. The Cu spring films had 10 coil turns and were deposited with 2000 nm, 2400nm, 2800 nm, or 3200 nm seed spacing on Si wafers as well as on unseeded Si substrates. Larger seed spacing resulted in Cu springs with larger coil diameter and larger wire thickness. Compression tests were conducted with stress amplitudes between 5 MPa and 50 MPa. At 5 MPa, the compressive stiffness of Cufilms was between 1842 and 35315 MPa, with larger values corresponding to larger seed spacing. Notably, the Cu spring films showed no permanent deformation until 20 MPa, when they experienced only 2 permanent strain. The shear stiffness of Cu films was between 21837 and 32285 MPa with unseeded films being the most compliant. Seeding resulted in a weaker interface with the substrate, while unseeded Cu spring films had significant shear strength averaging 160.9 MPa. The normal and shear stiffness values of Cu spring films were several orders of magnitude smaller than bulk Cu and they could be designed to be independent of each other, as opposed to bulk Cu.