Thermal Stress-Induced Grain Boundary Voiding and Notching in Narrow Al-Based Metallizations
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THERMAL STRESS-INDUCED GRAIN BOUNDARY VOIDING AND NOTCHING IN NARROW Al-BASED METALLIZATIONS Che-Yu Li, R.D. Black, and W.R. LaFontaine Department of Materials Science and Engineering
Cornell University, Ithaca, NY 14853 ABSTRACT Grain boundary voiding has been identified as a diffusional creep mechanism that produces failure of narrow Al-based metallizations during thermal aging. It is considered to be a reliability concern for sub-micron metallizations because the resulting failure rate has been observed to be strongly line width dependent. This paper presents a theoretical model for stress-induced grain boundary voiding. The proposed model is shown to account for the experimentally observed temperature and time dependence of thermal aging-induced line failure data reported in the literature. INTRODUCTION During a constant temperature hold the relaxation of thermal stresses in narrow, thin, Al-based alloy metallizations deposited on a Si substrate can be accomplished via dislocation creep processes or diffusional creep processes, or both. If the nucleation of voids, such as those which nucleate on grain boundaries as a result of grain boundary sliding (GBS), occurs readily, then the existence of void surfaces will provide additional sources of atoms which can participate in diffusional creep. This type of void growth and the resulting failure of the narrow metallization has been extensively reported in the recent literature [1,2,3]. According to Hinode and coworkers, it can be considered to be as important a reliability concern as electromigration [1]. This paper addresses, as a function of thermal history, the role of thermal stress in stress-induced grain boundary cavitation; the kinetics of the line failure process is also considered. The theoretical model proposed by Li, Black, and LaFontaine [4] is shown to accurately predict failure data of narrow Al-based alloy metallizations which have been reported in the literature. PHENOMENOLOGY Nucleation and growth of cavities during creep under an applied load is a common failure mechanism of bulk materials aged at elevated (>0. 4 Tm) temperatures [5]. In this case cavities nucleate on grain boundaries at stress concentration sites, such as steps and second phase particles, as a result of GBS [6]. For the case of voiding in thin, narrow, metallizations nucleation occurs by the same mechanism, except that the stress causing the GBS and subsequent nucleation results not from an applied load, but from the thermal expansion mismatch between the Al-based alloy metallization and the Si substrate. When the metallizations are narrow (-lym) this thermal stress is essentially uniaxial. The traction-free void surface provides a source of atoms and both the grain boundaries and the edges of edge dislocations oriented at a finite angle to the stress axis provide sinks for the atoms. The driving force for cavity growth is the difference in chemical potential of Al atoms, Ap, between the source and the sinks. For dislocations oriented normal to the stress axis a virtual work argu
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