Quantitative Measurement of Interface Fracture Energy in Multi-Layer Thin Film Structures
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ABSTRACT Interfacial debonding of multi-layer thin film systems can severely affect the reliability of devices. To quantitatively evaluate the interface adhesion strength, a sandwich structure fourpoint bending technique was developed. In the sandwich structure samples, the thin film structure of interest was diffusion bonded between two silicon substrates. A four-point bending method was applied to propagate a crack along the interface of interest. Two thin film systems with nominally the same structure, but processed under different conditions, were measured for their Si0 2 /TiN interface fracture energies. Results showed that the interface fracture energy of one system was about 50% larger than that of the other system. Cross-section TEM observations revealed that the stronger interface was also significantly rougher.
INTRODUCTION Interfacial debonding is increasingly recognized as one of the major reliability problems in electronic devices. The driving force for such debonding is provided by the stresses in the multilayer film structure. These include residual intrinsic stresses that develop during film deposition, and thermal stresses associated with cooling from processing temperatures and temperature fluctuations during normal device operation. Using a fracture mechanics approach [1], the driving force for the extension of an interface crack or debond can be expressed by the strain energy release rate, Gi, which is a function of the loading configuration and elastic properties of the materials on either side of the interface. Interfacial failure depends sensitively on the balance between the driving force, 7i, and the interface fracture energy or "toughness," Fi(y). Crack extension occurs when:
where Wis the phase angle associated with the normal to shear stress ratio ahead of the crack tip. In general, both the value of Fi(/) and the interface crack growth mechanism are dependent on XV [2]. Appropriate design criteria for microelectronic devices should therefore prevent satisfaction of the above fracture criteria both after processing and during device operation. The fracture mechanics-based, damaged tolerant design approach outlined above will necessarily require a detailed knowledge of Fi(y) for constituent interfaces of the device. Direct measurement of Fi(/) relies on the premise that it is possible to determine experimentally the critical load just sufficient for incipient crack growth. By definition, at the critical load, the corresponding criticalstrain energy release rate, Gic, equals the interface fracture energy, i.e.,
Gic
Fi(y)
91 Mat. Res. Soc. Symp. Proc. Vol. 391 ©1995 Materials Research Society
(2)
Therefore, by evaluating the critical driving force, qic, the interface fracture energy is obtained. To perform such an experiment, a crack must be introduced to the interface of interest and the critical load for crack extension determined. The sample geometry and mechanical loading configuration should be designed such that the value of gi can be rigorously calculated and related to the c
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