Determining Mechanical Properties at the Micron Scale Using Microfabricated Freestanding Structures

Gaskins, John, Mechanical and Aerospace Engineering - School of Engineering and Applied Science, University of Virginia
Barker, Nicolas, Department of Electrical and Computer Engineering, University of Virginia
Begley, Matthew, Department of Mechanical Engineering, University of California, Santa Barbara

Thin films on the order of tens and hundreds of nanometers are being used with increasing frequency in a variety of applications including semiconductor electronics and microelectro-mechanical systems (MEMS). The mechanical properties of these films tend to deviate from those seen in bulk materials which has implications on device design, reliability and functionality. Naturally the size of these films leads to difficulty carrying out traditional mechanical testing, such as tensile or compression testing, to determine their properties. Indentation of free standing structures, specifically beams, has been identified as a means to test films in this size regime. This work focuses on development of indentation testing techniques, analytical solutions and fabrication methods to extract mechanical properties from freestanding thin film beams.

Identification of the point of contact between the indenter probe and compliant beam is necessary in order to accurately determine loads and displacements carried by the beam and to enact time dependent loading schemes (e.g. constant strain-rate testing). Exploiting the under-damped resonant response of the indenter allows for identification of beams with stiffness up to two orders of magnitude less than the indenter springs. The indentation system is well characterized as a single DOF harmonic oscillator. Examining the coupled response of the indenter and beam, detection limits based on operating frequency, beam stiffness, beam damping and environmental noise in the system are identified.

An analysis of the full non-linear load-deflection response of point loaded elastic beams with tensile residual stress is presented. Relevant asymptotic limits corresponding to classical beam response are identified. Further, a simple closed-form expression is identified for non-linear responses that facilitates property extraction when asymptotic expressions are not valid. A critical contribution is the development of an explicit analytical relationship for load-deflection response which avoids the complication of using implicit solutions, which requires non-linear root-finding to determine the mechanical stretch in the beam. An approximate solution is shown to be accurate within 6% of the implicit solution.

Testing of npAu and AuAg beams and cantilevers is used to extract elastic modulus and residual stress. The proposed indentation testing techniques and approximate solution are used to extract mechanical properties from nickel beams created using a novel fabrication procedure utilizing xenon diflouride gas. The mechanical properties of films in their as-fabricated state and annealed at 200 and 300 C are explored. The effects of varying microstructure on modulus, residual strain, yield stress and fracture properties are discussed. Finally, areas of future study are addressed.

PHD (Doctor of Philosophy)
nanoindentation, beam mechanics, nickel thin films, membrane deflection experiment, point-loaded beam solutions
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