The Effect of Fatigue on the Adhesion and Subcritical Debonding of Benzocyclobutene/Silicon Dioxide Interfaces
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The Effect of Fatigue on the Adhesion and Subcritical Debonding of Benzocyclobutene/Silicon Dioxide Interfaces Jeffrey M. Snodgrass and Reinhold H. Dauskardt Department of Materials Science and Engineering, Stanford University Stanford, CA 94305-2205, USA ABSTRACT The effect of fatigue loading on microelectronic thin film interfaces has until now been difficult to quantify. Most industrial fatigue testing uses HAST (Highly Accelerated Stress Testing) protocols, which inherently convolutes the effects of mechanical fatigue and the test environment. Our work focuses on isolating the deleterious effects of mechanical fatigue on interfaces, which we have found to be substantial. In this study, the integrity of a low-k polymer interface involving benzocyclobutene (BCB) and silica was examined under a variety of loading conditions. Critical (fast fracture) adhesion values were measured using standard interface fracture-mechanics geometries. Experiments were then conducted to measure the debond growth rate as a function of the applied strain energy release rate under both static and cyclic loading conditions. Our results show that even under room temperature conditions, debond growth rates measured under cyclic fatigue are considerably faster than those observed under static loading. Results are presented detailing the effects of interface chemistry (adhesion promoters), environmental moisture, and test temperature on the resistance of the interfaces to subcritical debonding. Strategies for increasing resistance of dielectric interfaces to fatigue debonding are outlined. INTRODUCTION In order to achieve the dielectric performance needed for next generation devices, silica is increasingly being replaced by polymer or polymer-like materials. Unfortunately these materials face a number of integration challenges related to their low elastic moduli and the large difference between their thermal expansion coefficients those of adjacent materials. These factors can lead to high film stresses and problems with adhesion. Indeed, adhesion and film integrity are often the most important factors in determining whether a new dielectric material can be integrated into a successful process. The techniques outlined in this study describe a methodology that allows for the quantitative measurement of the parameters that influence adhesion and subcritical debonding. Much work has been completed in this area and numerous studies have been published detailing the effectiveness of these techniques in measuring interfacial adhesion [1, 2, 3]. The delaminating beam methods used are analyzed with well-founded fracture mechanics principles. Results are presented in terms of the critical strain energy release rate, GC, measured in J/m2, necessary to cause steady-state debonding. Time-dependent subcritical debonding is obtained by measuring the velocity of debond growth, da/dt, as a function of the applied strain energy release rate, G. Recent work has begun to examine the effects of mechanical fatigue loading on polymer interfaces [4, 5]. It is beli
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