Macroscopic Modeling of Fine Line Adhesion Tests

Abstract

Microwedge Indentation (MWIT) and Precracked Line Scratch (PLST) tests have been developed for adhesion measurements of microscale fine lines. In the MWIT a symmetric wedge-shaped probe is driven downward into a thin film line debonding it from the substrate. For the PLST, a precrack is introduced at one end of a thin metal line on a substrate. The line is loaded from the precrack end by an asymmetric diamond wedge until the crack propagates. When the crack reaches its critical length at a certain critical load, the film buckles.The mechanics of these tests are based on several assumptions of thin line behavior, which are not experimentally observable due to small line sizes and equipment limitations. Testing similar but larger lines allows for the direct observation of line delamination and buckling, which can be used for calculating in-situ crack lengths as well as instantaneous strain energy release rates.Both MWIT and PLST models [1][2][3][4] were experimentally verified on the macroscopic scale using polycarbonate as a line material bonded to a steel substrate with cyanoacrylate. New features such as initial crack growth as well as instantaneous crack growth at buckling were observed. Modified K IC tests were performed for strain energy release rate comparison. Adhesion values ranging from 200 to 1000 J/m 2 were measured for this system, and had very strong mode mixity dependence. Connectivity to the microscale is through previous experiments by de Boer, et al using fine line W/SiO 2 /Si systems [1][2][3][4].

Fiducial marks as measures of thin film crack arrest toughness

“…Since the analysis is purely elastic, it indirectly proves that there is not much plastic energy dissipation at the crack tip for thin Cu films. Previously we also employed a plasticitybased slow crack growth approach based on the Rice, Drugan and Sham (RDS) analysis of the tearing modulus, T 0 [18] to show a similar result [13]. One can obtain a simple expression for strain energy release rate in terms of the crack-tip opening angle (CTOA in Fig.…”

“…Scanning electron microscopy showed circles that correspond to the original blister diameter, and those were denoted as crack arrest fiducial marks [13,14]. Atomic force microscopy was performed to measure the feature geometry, giving a width over 1 lm, and a height ranging from 5 to 15 nm.…”

“…It was originally believed that the crack arrest mark is formed by crushed W and/or SiO 2 debris during the indentation [13]. More likely, however, radial cracking allowed laboratory air with moisture, hydrocarbons and surface debris to be sucked into the blister [14].…”

“…2. The substrate fiducial mark outlines the crack tip geometry, and its dimensions can be used to extract the thin film crack arrest toughness [13,14]. As discussed in [2,4,15], brittle fracture is observed for thinner Cu films (<100 nm) on Si substrates without a Ti underlayer.…”

“…During indentation experiments into Cu thin films with a W superlayer it was found that the crack arrest marks form and correspond exactly to the blister size [13,14]. Marks are formed of carbon, and outline the crack tip, representing its geometry [13,14].…”