Mechanical Characterization of Thin Films by Micromechanical Techniques

Abstract

Evaluating the mechanical properties of thin films on thick substrates is tricky. Some useful techniques for mechanical property measurements on macroscopic film specimens exist, e.g., internal stress measurements by x-ray diffraction or by wafer buckling. Many conventional techniques, however, such as indentation for hardness measurement or scratch testing for evaluation of adhesion or wear resistance, yield results that are difficult or impossible to evaluate in terms of more fundamental film properties. In fact, these techniques are more practical engineering tools than tools for scientific study.One solution is to miniaturize the test probe to dimensions approaching the film thickness. The nanoindentation technique for submicrohardness measurement described elsewhere in this issue by Pharr and Oliver is a recent example of such an approach. Still, for very thin films it is difficult to separate the influence of the substrate properties from the film properties, and the indentation process is inherently too complex to be well suited for determining fundamental materials parameters.The rapidly evolving field of micromechanics offers some new possibilities in thin film characterization. Important film properties such as internal stress, elastic moduli, plastic yield limit, and fracture data can be extracted from experiments with micromachined test structures in the 10–1,000 μm size range.

M-TEST: A test chip for MEMS material property measurement using electrostatically actuated test structures

“…A broad array of material property extraction methods has been proposed in the literature. These methods include a resonant frequency measurement technique on cantilever beam (CB) test structures [2], [3], a direct tensile stress measurement technique [4], a capacitance/voltage (C/V) measurement technique on fixed-fixed beam (FB) bridge structures [5], the direct mechanical bending of CB test structures by a known force and measurement of the resulting deflection [6], a load/deflection technique on suspended thinfilm membranes under tensile stress and known pressure load [1], [4], [7], and an electrostatic pull-in approach using tethered rigid parallel-plate structures [8]. Most of the techniques require special micromachined structures or special test fixtures, which makes them difficult to use in routine wafer-level measurements, for example, in a manufacturing environment.…”

The mechanics and physics of thin film decohesion and its measurement

“…There have been several reviews on aspects of this topic. These include surveys of test methods, [9][10][11][12] the thermomechanical integrity of films and multilayers [13], the mechanics of crack growth along interfaces [ 14], residual stresses and their origin [15]. The present review differs from these by focusing on the quantitative aspects of thin film decohesion and its measurement.…”

“…These tests are simple and effective for routine ranking of bond quality. However, they do not measure Fi, because the strain energy release rate cannot be deconvoluted from the work done by the external load [12]. An ideal test should duplicate the practical situation as closely as possible and be able to modulate the available strain energy.…”

Residual Stress and Fracture of PECVD Thick Oxide Films for Power MEMS Structures and Devices

“…Novel mechanical characterization techniques have been developed to measure both residual stress and the mechanical properties of thin films. Most of them are mainly mechanics-based, such as nanoindentation (Oliver and Pharr 1992;Taylor 1991;Vlassak et al 1997;Baker and Nix 1994;De Boer and Gerberich 1996;Zhang TY et al 1999), uniaxial tensile testing of freestanding films (Mearini et al 1993;Brotzen 1994), micro-cantilever bending (Weihs et al 1988;Najafi and Suzuki 1989;Schweitz 1992;Schull and Spaepen 1996), and bulge testing (Vlassak and Nix 1992;Vinci and Nix 1996;Ziebart et al 1998). Others are mainly based on optical (Sharpe et al 1997;Espinosa 2003), X-ray scattering (Xu et al 2001) and laser-acoustic methods (Hernandez et al 2002) etc.…”