Toward the Acoustic Evaluation of Low Modulus Thin Film Structures

Open Access
Du, Jikai
Graduate Program:
Engineering Science and Mechanics
Doctor of Philosophy
Document Type:
Date of Defense:
March 20, 2008
Committee Members:
  • Bernhard R Tittmann, Committee Chair
  • Joseph Lawrence Rose, Committee Member
  • Clifford Jesse Lissenden Iii, Committee Member
  • Mark William Horn, Committee Member
  • Elizabeth C Dickey, Committee Member
  • Judith Todd Copley, Committee Member
  • acoustic microscopy
  • thin film
  • non-destructive evaluation
Thin films play an important role in the field of materials science and engineering. There is a need to inspect these films. From the literature it seems like acoustic microscopy should work. The currently available systems, however, do not work. As a consequence, a theoretical analysis was carried out to explore what parameters might be necessary to conduct a successful inspection. Results showed that frequency tuning should be useful. An acoustic microscope was modified to do a frequency sweep and an experimental inspection feasibility study was successful. In particular, the functional characteristics of thin films are in part determined by their synthesizing and processing procedure. A thin film component is a layered structure whose proper function depends on the film properties and the film adhesion to the substrate. Therefore, the evaluation of the integrity of thin film structures is very necessary and important. Past efforts reported in the literature have been focused on hard films and coatings on substrate materials. Little or no significant effort, however, has been reported on low modulus or soft films on hard substrates. This dissertation seeks to develop a technique to apply acoustic microscopy to the nondestructive, qualitative and quantitative characterization of thin (micron and submicron) films of low modulus. Well-established theoretical models are applied to the acoustic wave propagation in soft on hard layered structures. Using these for simulations the surface wave velocity is shown to be sensitive to film properties and interface conditions. In the dissertation work reported here, a broadband acoustic microscopy system is assembled in order to optimize the experimental measurement through frequency tuning and amplitude modulation. Using the theoretical models as a guide the experiments on soft film show that its longitudinal wave velocity can be determined. Similarly, the experiments on Au films with differing interface conditions suggest that the surface acoustic wave velocity is sensitive to the interface conditions if the proper frequency range is selected. Comparison between the experimental results agrees well with theoretical predictions. Mechanical destructive tests further support the feasibility of the surface acoustic waves being sensitive to the interface influence. Overall, the study in the dissertation provides a comprehensive feasibility study for the characterization of low modulus film structures through the scanning acoustic microscopy techniques.