Acoustic Emission Analysis of Fracture Events in Cu Films with W Overlayers
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University of Minnesota, Dept. of Chem. Engineering and Materials Science, Minneapolis, MN, 55455, [email protected] ABSTRACT Indentation-induced delamination of thin films provides the basis for adhesion
calculations. In the case of ductile Cu films plastic deformation usually prevents a film from debonding from the substrate. Deadhesion is facilitated by the use of a hard W superlayer, which promotes indenter-induced Cu film failure, increasing the delamination area by an order of magnitude. Radial as well as annular cracking acts like a secondary mechanism in the strain energy release, and can be resolved from excursions on the load-displacement curves. For the thicker Cu films no excursions were observed, though radial cracking took place. It is important to identify fracture events as they occur in order to understand the system behavior and accurately apply the analysis. An acoustic emission signal is used to detect the magnitude of fracture events in thin Cu films. For the films of different thickness from 40 nm to 3 microns the corresponding interfacial fracture energy ranged from 0.2 to over 100 JIm 2. Limits of plastic energy dissipation are determined with the lower limit, the true work of adhesion, being associated with a dislocation emission criterion. Crack arrest marks were found upon the blister removal, and are proposed to represent the shape of the crack tip. Total acoustic emission energy was found to be inversely proportional to the strain energy release rate. INTRODUCTION
Acoustic emission (AE) is a very powerful method for examining the behavior of materials deforming under stress. This covers fracture, yielding, fatigue, corrosion, creep, etc. in
bulk materials, including composites [1]. Several researchers have tried to employ AE for the detection of yielding and fracture events in different thin film systems [2-51. In the present study,
we consider indentation-induced delamination in the superlayer structure (Figure 1). •
Indenter W superlayer Cu SiO2 --
_
Si wafer
Figure 1. Schematic of the superlayer indentation.
A superlayer indentation method is used for thin film adhesion determination. In the case of ductile or strongly adhered films, it is often impossible to cause film delamination from the substrate by means of indentation. Ductile thin films cannot store enough strain energy necessary for crack initiation/propagation. Deposition of a hard film, capable of storing sufficient amounts 275 Mat. Res. Soc. Symp. Proc. Vol. 563 © 1999 Materials Research Society
of elastic energy over the film of interest, can result in multilayer debonding, producing much larger delamination radii. 1000 Copper thin film adhesion 0 Cu with Ti underlayer I was shown to increase with the film SUpper bound thickness and with the addition of a 10 nm thin Ti underlayer due to the * Cu, no underlayer 100 higher plastic energy dissipation at the crack tip (Figure 2) [6-8]. For the lower film thicknesses (up to 10 4 j/m2 ------ ----------100 nm) without the Ti underlayer the plastic energy dissipat
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