A Method for Analyzing the Critical Adhesion Energy of Thin Film Coatings
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Frederick J. McGarry and Edward 0. Shaffer II Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, Cambridge, MA 02139 ABSTRACT The edge delamination test (EDT) measures the energy required to cause a thin film, under biaxial tensile stress, to debond from a rigid substrate. Circular holes are etched through the films and if the stress is large enough, stable debond rings grow radially around the holes. A finite element analysis is used to find the applied strainenergy release rate as a function of debond crack length, hole radius and side-wall angle. However, the analysis may not be widely available, so, we present tables of reduced debond energy values, Gr, to facilitate the use of the EDT. With observed EDT results, Gr is extrapolated from the tables and then the critical debond energy is calculated by multiplying Gr by the maximum strain energy. Also, we have fit Gr with a semi-empirical equation, which is in good agreement over a broad range of geometries.
INTRODUCTION Good adhesion between dissimilar materials is critical to the performance of many applications. For example, electronic assemblies such as multichip modules develop large residual stresses after thermal processing. These can lead to interfacial failures; thus, to design reliable parts it is necessary to have a true measure of the interfacial integrity. Recent work [1,21 has shown that the edge delamination test (EDT) is an effective method for measuring debond energies of thermally stressed thin coatings. The EDT, illustrated in Figure 1, has circular holes etched into a coating adhered to a rigid substrate. The geometry is characterized by the hole radius, r; the debond length, a; and the side-wall angle, 0. Similar to other tests [3-5], the EDT uses the residual thermal stress to cause debonding. The biaxial tensile stress, co, arises from the differential thermal expansion and, in the simplest case, is: EAoAT (1 - V) where E and v are the Young's modulus and the Poisson's ratio of the film, A0z is the difference in thermal expansion coefficients of the film and the substrate, and AT is the change in temperature for linear elastic materials. (An analysis of Go versus AT for linear viscoelastic materials has been done by Margaritis [6].) The relationship between ao and AT can also be measured: the Tencor Flexus measures the curvature of the wafer
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Mat. Res. Soc. Symp. Proc. Vol. 356 0 1995 Materials Research Society
a
r h
Figure 1: Edge Delamination Test Geometry
as a function of temperature and time that permits the calculation of the stress. In the EDT, the stress can be further increased by cooling the part below room temperature. One parameter characterizing the durability of an interface is the critical strainenergy release rate, Gc, or debond energy. This is the work required to cause an existing flaw to propagate over a unit area of the interface. It is defined so: G
Wa - (W'x - U - WPI)
=
(2)
where Wa is the fracture energy, A is the crack surface area, Wex is the applied external energy, Wp,
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