Influence of Interfacial Delamination on Channel Cracking of Brittle Thin Films

Channeling cracks in low-k dielectrics have been observed to be a key reliability issue for advanced interconnects. The constraint effect of surrounding materials including stacked buffer layers has been studied. This paper analyzes the effect of interfacial delamination on the fracture condition of brittle thin films on elastic substrates. It is found that stable delamination along with the growth of a channel crack is possible only for a specific range of elastic mismatch and interface toughness. An effective energy release rate is defined to account for the influence of interfacial delamination on both the driving force and the fracture resistance, which can be significantly higher than the case assuming no delamination. INTRODUCTION Integration of low-k and ultralow-k dielectrics in advanced interconnects has posed significant challenges for reliability issues resulting from compromised mechanical properties. Two common failure modes have been reported, one for cohesive fracture [1] and the other for interfacial delamination [2]. The former pertains to the brittleness of low-k materials subjected to tension, and the latter manifests for poor adhesion between low k and other materials [3]. This paper considers a possible failure mode with concomitant cohesive fracture and interfacial delamination. Fig. 1: (a) A channel crack with no interfacial delamination; (b) a channel crack with stable interfacial delamination of width d on both sides. One common cohesive fracture mode for thin films under tension is channel cracking (Fig. 1). Previous studies have shown that the driving force for the steady state growth of a channel crack (i.e., energy release rate) depends on the constraint effect of surrounding layers [1,4]. For a brittle thin film on an elastic substrate, the driving force increases for increasingly compliant substrates [5,6]. The effect of constraint can be partly lost as the substrate deforms plastically [7] or creeps [8]. More recent studies have focused on the effects of stacked buffer layers [4,9] and patterned film structures [1]. In most of these studies, the interfaces between the film and the substrate or the buffer layers are assumed to remain perfectly bonded as the channel crack grows in the film (Fig. 1a). However, the stress concentration at the root of the channel crack may drive interfacial delamination [10]. While some experimental observations clearly Substrate Film