Electronic Packaging Science and Technology

3Randomly‐Oriented and (111) Uni‐directionally‐Oriented Nanotwin Copper

3.1 Introduction

Why nanotwinned Cu (nt‐Cu) is important? Technically, Cu is the most widely used conductor in microelectronic technology, including both on‐chip and in packaging. Scientifically, nt‐Cu is the fifth mechanism in strengthening metals. This is because in K. Lu’s Science paper in 2004 [1], it is reported that the mechanical yield strength can increase 10 times from about 100 MPa for ordinary Cu to 1000 MPa for nt‐Cu. At the same time, electrical resistivity does not change much. In classical metallurgy, one of the most important challenges was how to strengthen a metal? Typically, there are already four mechanisms. First is by work hardening due to dislocation entanglement. Second is solution hardening. For example, pure gold is too soft, so it cannot hold diamond. Therefore, we need to add some Ag or Cu to make 18 or 14 k gold to make diamond ring. Third is precipitation hardening as in the case of GP zone formation in Al(Cu) alloy, where the precipitation of Al₂Cu has to go through ɵ”, ɵ’, and ɵ phases. Fourth is grain size reduction, as given by the Hall‐Pitch relation. Now, nanotwin formation is the fifth one. [2–16]

Furthermore, nt‐Cu lines possess higher electromigration lifetimes than regular Cu lines. Chen et al. adopted in situ high‐resolution transmission electron microscopy (HRTEM) to observe electromigration of Cu lines, and they observed that the migration rate of Cu atoms was ...