Interfacial effects during thermal cycling of Cu-filled through-silicon vias (TSV)

Abstract

Large shear stresses may develop at interfaces between dissimilar materials during thermal excursions when there is a significant difference in their coefficients of thermal expansion. The shear stress may cause interfaces to slide via diffusional process, thereby accommodating the relative dimensional changes between the two materials. This phenomenon presents a significant reliability issue in three-dimensional (3-D) interconnect structures involving through-silicon vias (TSVs), which are subjected not only to continuous thermal cycling but also to large electric current densities during service. This paper reports experimental evidence of interfacial sliding between Cu and Si in Cu-filled TSVs during thermal cycling conditions, and in the presence of electric current. Two different thermal cycling conditions were used: (i) small ΔT thermal cycling (-25°C to 135°C) and (ii) large ΔT thermal cycling (25°C to 425°C). Prior to thermal cycling, a few Cu-filled TSV samples were annealed for 30 min at 425°C. Cu intruded inside Si in nonannealed samples during small ΔT thermal cycling, whereas protrusion of Cu relative to Si occurred during all other thermal excursions. Application of electric current biased the net displacement of the Cu in the direction of electron flow, leading to enhanced protrusion (or intrusion) of Cu relative to the thermal cycling only (i.e., without electric current) condition. A simple one-dimensional analytical model and associated numerical simulations are utilized to rationalize the experimental observations. © 2011 TMS.