Date

8-1-2018

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Materials Science and Engineering

First Adviser

Vinci, Richard P.

Abstract

Sub-micron thickness metallic thin films are known to exhibit mechanical size effects, where the thin film mechanical behavior can differ significantly from that of a bulk version of the same material. One such mechanical behavior, and the focus of this work, is the unique thin film viscoelastic deformation response to sub-yield, low-strain stimuli in near-ambient temperature environments. In engineering components using bulk materials, strains within the elastic regime are generally considered instantaneous. It is only at higher operating temperatures that temporal deformation processes, such as creep, are considered in bulk component design. However, in Micro-Electro-Mechanical Systems (MEMS), specifically Radio Frequency MEMS (RF MEMS) devices, the mechanical size effects of thin films result in time-dependent stress relaxation that can degrade component function and lead to reliability issues. For example, low temperature stress relaxation in RF MEMS thin films can cause a reduction in springback forces in moving membranes, leading to stiction failures and low device reliability.The nanocrystalline structure and high purity of typical metallic thin films enable the viscoelastic behavior. In this study, the effect of microstructural features such as grain size and dislocation density are investigated for a series of face-centered cubic (FCC) metal films using the gas pressure bulge test technique at 80 °C. Commonly used MEMS metals including Ag, Al(Mg), Au, Cu, and Pt are grown to a nominally 500 nm thickness using the DC magnetron sputtering technique. The grain size for each material class is altered by heating the film substrate during growth. SEM analysis is used to measure the planar grain size, D, of each film, ranging from 31.9 nm up through 878.4 nm. The dislocation density of each film is measured with TEM and XRD techniques, resulting in a range between 4.1×〖10〗^14 m-2 to 15.1×〖10〗^14 m-2 for all films.The low-strain stress relaxation response of each film is fit using a four-term Prony series. The evaluating metric to compare viscoelastic behavior between films is the plane strain modulus decay after 10,000 s of relaxation. The Al(Mg), Au, and Pt films showed increasing relaxation with increasing grain size while the Ag and Cu films showed no relaxation dependence on grain size. The relaxation dependence is shown to be linearly proportional to Dn where n ranges from ½ to 2.This work suggests that dislocation bowing and unbowing is the driving mechanism for the stress relaxation and respective recovery in metallic thin films. Thermal activation is associated with overcoming the Peierls barrier. For those films that exhibit a dependence of relaxation on grain size, it is shown that the average pinned dislocation segment length, L, increases as the grain size, D, increases, thereby allowing greater bowing and viscoelastic strain. Furthermore, calculated activation volumes for the first 100 s of relaxation change with D for those films that exhibit grain size dependence. This study indicates that the barrier to dislocation bowing was reduced with grain size for Al(Mg), Au, and Pt films but remained constant for Ag and Cu films.A key outcome of this work is that the same relaxation mechanism is active in all FCC metals tested. A second outcome is that grain size dependence of viscoelastic stress relaxation is not an independent mechanism like Hall-Petch strengthening, but rather is a consequence of microstructural changes that accompany grain growth in certain metals.

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