Macroscopic Modeling of Fine Line Adhesion Tests

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A.A. Volinsky, J.C. Nelson, W.W. Gerberich* "University of Minnesota, Dept. of Chem. Engineering and Materials Science, Minneapolis, MN 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 [1-4] 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 Kic tests were performed for strain energy release rate comparison. Adhesion values ranging from 200 to 1000 J/m2 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/SiO2/Si systems [1-4]. INTRODUCTION

For microelectronics reliability it is essential to know thin film adhesion and to be able to measure it quantitatively. Based on the Linear Elastic Fracture Mechanics (LEFM) approach, two new adhesion tests, the Microwedge Indentation (MWIT) and the Precracked Line Scratch (PLST) tests, have been proposed and performed on Pno-m Asymmetric the W/SiO2/Si system by Maarten deBoer et al [1m D Diamond Wedge 41. Both tests are applicable to the thin interconnect lines, and account for thin film geometry. A schematic of the microscopic Precracked Line Scratch test is presented in Figure 1. Carbon Precrack Silicon Substrate Figure 1. Microscopic Precracked Line Scratch test schematic. For the PLST a thin metal line on a substrate is pushed from the end with an asymmetric diamond wedge. The thin line has a processed precrack in the form of a carbon layer, which makes it a real fracture mechanics specimen. Only the cracked portion is assumed to deform elastically. The crack propagates along the film/substrate interface until the line buckles at a critical crack length, a.. (Fixed-fixed end conditions are most applicable in this case):

4

a~ = ah

(1),

where h is the film thickness, Ef is the film's plane strain Young's modulus, and Pc is the critical buckling load (maximum load from the load-displacement curve). 297 Mat. Res. Soc. Symp. Proc. Vol. 563 c 1999 Materials Re