Stress Analysis of 4-Point Bend Test for Thin Film Adhesion
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Stress Analysis of 4-Point Bend Test for Thin Film Adhesion Sassan Roham, Kedar Hardikar and Peter Woytowitz Novellus Systems, Inc. San Jose, CA ABSTRACT Four point bend (4PB) tests are currently used to characterize the adhesive strength of thin films. Of particular interest are low k films whose strength properties are normally less than traditional dielectrics. A finite element analysis (FEA) of a 4PB specimen is conducted in order to better understand the results and limitations of such testing. We discuss the classical equation used to convert 4PB test data into fracture energy and have validated this classical formula using finite element analysis. We also present a theory that explains one possible reason for the occurrence of anomalous test results where the film fails in a cohesive rather than an adhesive mode. 1. INTRODUCTION The reliability of microelectronic devices, which are built by multi-layer film deposition, is a strong function of cohesive strength of the films and adhesive strength of bimaterial interfaces present. As technology drives smaller and faster chips, low dielectric constant materials are replacing silicon oxides. Among other things, the integrity of the interfaces between the new materials and other films during deposition, subsequent processing and device operation is dependent upon whether or not the stresses in the films drive debonding events. A number of techniques have been developed to quantify adhesion of a thin film to its substrate. Several of these methods are reviewed in [1]. Most of the techniques suffer from a common disadvantage, which is that it is difficult to distinguish between delamination energy and other energy dissipation processes. P/2 Width=b
Plane of Symmetry ao
P/2 Pre-crack E, ν E, ν
J-Integral Path
h h
a
Figure 1: 4-Point Bend Test Specimen. The four-point bend test is a technique in which the interface of interest (film stack) is sandwiched between two massive elastic substrates in order to contain the stress relaxation and plastic deformation of films during delamination [2]. The two halves are bonded together using a bond layer (for example, epoxy). Delamination is induced at the interface by application of mechanical load in a standard four-point bend configuration as shown in Figure 1. Fracture energy of the interface is calculated from the critical load required to propagate interface crack in a steady-state manner. Due to controlled loading of the specimen, limited plastic deformation, limited stress relaxation in the film and a fracture mechanics based analysis of the test results,
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this technique is gaining popularity in industry. A recently published paper [3] has correlated the four-point bend data with survival of the film under CMP (Chemical Mechanical Planarization). Availability of such data makes the technique more attractive for practitioners. The success of the four-point bend test among other factors relies on getting a doubly deflected crack configuration from an initially notched specimen as shown in Figure 1. I
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