Investigation of a new fracture mechanics specimen for thin film adhesion measurement
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Investigation of a new fracture mechanics specimen for thin film adhesion measurement Maarten P. de Boer,a) Michael Kriese, and William W. Gerberichb) Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 (Received 11 September 1996; accepted 24 February 1997)
We have investigated mechanical probing of a precracked fine line structure as a new type of thin film fracture mechanics specimen. An idealized mechanics analysis is first presented. Experimentally, two types of precracks are formed. A thin carbon layer to which other layers weakly adhere creates a “processed precrack” by integrated circuit processing techniques. An “indented processed precrack” is formed by precision alignment of a sharp microwedge. The processed precrack is found to reduce the critical tangential load by 50% from a non-precracked line, while the indented processed precrack lowers the load by 200%. From this, a reasonable value of adhesion may be directly calculated. Crack path behavior is observed to depend on strength of the interface. In the case of a weak interface, the crack remains in the interface as it extends. For a strong interface, it kinks into the substrate if the crack is initially short, but remains in the interface if it is initially long. Given the experimental evidence, the mechanics are slightly modified to quantitatively model the experimental data.
I. INTRODUCTION
Indentation testing to measure thin film adhesion has been the subject of numerous studies.1–5 We recently proposed6 the microwedge indentation test (MWIT) of the thin film fine line by a nanoindentation technique as a new method for studying the adhesion Gi and spallation of a thin film on a thick substrate. Experimental investigation7 showed that the method is subject to deviations from ideality which limit the accuracy of adhesion assessment. Cracking of the substrate and densification of the 90% dense film cause the stress inferred from the indentation volume to be overestimated. Even in the case of no substrate cracking, 2% voiding by volume has been determined for sputtered tungsten films by transmission electron microscopy8 for a “dense” zone T film9,10 under compressive stress. We are led to conclude that the “induced residual stress assumption” is inaccurate for many sputtered films. Furthermore, we found the normal to tangential load ratio at spallation to be 17 : 1. These phenomena cause departures from the idealized modeling for bending strains and mode mixity,6 as discussed in Ref. 7. Nonetheless, hard films on brittle substrates in the microelectronics (TiN or tungsten on SiO2 11 ), magnetic recording (carbon on alumina12 ), and optical electronics industries (metal conductors on SiO2 13 ) are used and a)
Present address: Sandia National Laboratory, Department 1114, Albuquerque, New Mexico 87185-1413. b) Author to whom correspondence should be addressed. J. Mater. Res., Vol. 12, No. 10, Oct 1997
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