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Title:Mechanical response of spring films under compression and shear loading
Author(s):Mott, Ryan Nicholas
Advisor(s):Chasiotis, Ioannis
Department / Program:Aerospace Engineering
Discipline:Aerospace Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Abstract:Thin films of discrete micro and nanostructures fabricated by the method of Glancing Angle Deposition (GLAD) provide a means to build compliant interfaces that maintain important properties of the constituent materials, such as thermal and electrical conductivity while enabling interfacial resilience. In this dissertation research, the normal compressive and transverse stiffnesses of a variety of Si and Cu spring films were determined experimentally in order to assess the effect of geometric parameters on the mechanical response. Si springs of either 4 or 10 coil turns were deposited on unseeded and seeded Si wafers with seed spacing of 900 nm or 1500 nm. The Cu springs had 10 coil turns and were deposited on silicon wafers with 2000 nm, 2400 nm, 2800 nm, or 3200 nm seed spacing or unseeded Si substrates. Larger seed spacing resulted in Cu springs with larger coil diameters and larger wire thickness compared to seeded Si springs. Compression tests were conducted at stress amplitudes between 0.5 MPa and 50 MPa on Si films and between 5 MPa and 50 MPa on Cu films. The test samples were circular areas of ~90 μm diameter, subjected to compression with a flat punch. The force vs. displacement curves were used to compute the film stiffness while scanning electron microscope (SEM) images were collected to measure the residual compression. The stiffness of the Si films at the lowest applied stress of 500 kPa varied between 24 ± 0.2 and 66 ± 3.4 MPa for different spring configurations. At the common test condition of 5 MPa applied stress, the stiffness of Si films was between 44 ± 0.2 and 165 ± 1 MPa, while Cu spring films had stiffnesses between 184 ± 2 and 353 ± 15 MPa. At the other extreme of an applied stress of 50 MPa, the stiffness of Si films ranged between 291 ± 0.5 and 810 ± 6 MPa and of Cu films between 611 ± 5 and 1308 ± 28 MPa. Notably, the Si films experienced more permanent deformation at lower stresses compared to Cu, reaching 6.5% at 5 MPa, while Cu films showed no permanent strain until 20 MPa, at which point they experienced only 2% permanent strain. The maximum permanent strains occurring at 50 MPa were 38% for Si and only 12% for Cu. Shear tests were performed with both types of films using a custom apparatus. The shear stiffness was between 7 ± 0.6 and 27 ± 4 MPa for Si, and between 218 ± 37 and 322 ± 85 MPa for Cu. The higher stiffness of Cu films originated in their significantly larger coil and wire diameter compared to Si. However, the shear strength of seeded Cu springs, between 2 ± 0.4 and 4 ± 1.2 MPa, was approximately the same as that of Si, which had a range of 1 ± 0.05 to 4 ± 0.7 MPa. Cu springs failed at the seeding post which was the most slender point in the structure. Unseeded Cu springs failed within the spring layer with shear strength of 16 ± 0.9 MPa. The largest residual compression was measured for unseeded Cu films reaching a value of 19 ± 2.2 %. In shear, the Si films experienced failure at different locations between the capping layer and just above the seed post, whereas the seeded Cu springs experienced failure directly at the seed post, which prevented the determination of the true shear strength of seeded Cu spring films.
Issue Date:2015-12-11
Rights Information:Copyright 2015 Ryan Mott
Date Available in IDEALS:2016-03-02
Date Deposited:2015-12

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