In-Situ Investigation Of Interface Structure Evolution In Copper And Palladium-Coated Copper Wire Bonding

According to Moore’s Law, the feature size of semiconductor devices become smaller and smaller, and the chip integration degree keeps increasing. In particular, accompany with the development of high chip integration and unit size reduction, the metal interconnects, i.e. the wire bonding, are becoming a challenging problem. Copper wire is believed as an important metal for wire bonding instead of gold for its attractive advantages such as low cost, favorable electrical and thermal conductivities, excellent mechanical properties. However, beyond those advantages, bare Cu wires still suffer from oxidization and bad flexibility. Recently, palladium-coated copper (PCC) wire has been introduced to overcome the problems ourred in bare Cu wire. Although the oxidation problem may be prevented by surface coating, the heating treatments during chip manufacturing process, as well as the thermal conditions that the chip may suffer during working, could induce unexpected problems of the bonding interface. Currently, the evolutions of the interfacial microstructures as well as their mechanisms in the bonding interface during thermal condition are still unclear in both copper and PCC bondings. Especially, the behavior of Pd atom diffusion and the formation of a large number of voids in PCC bondings still require detailed explanations.In-situ transmission electron microscope (TEM) has high spatial resolution and strong analysis ability. With fast CCD cameras, TEM can also record the dynamic structure evolution of the target sample in real time. Combined with multi-function holders, TEM can also exert diverse fields and loads on the sample and synchronously monitor their structures and component evolutions. Hence, in situ TEM provides an advanced technique to explore the upper mentioned issues in the bonding interfaces of Cu and PCC. In this thesis, we have prepared TEM samples by cutting Cu and PCC wire bondings using focused ion beam method and studied the evolutions of these two different wire bonding microstructures by in situ annealing inside the TEM. The main findings are summarized as follows:1. In-situ annealing the Cu wire bonding:(1) Composition analysis of interface structure evolution. Before annealing, isolated Cu-Al IMC distributes in the bonding interface. The main component of IMC is Cu9Al4 and the minor component of IMC is CuAl2. After annealing at 50-220℃ for 24 hours, Cu-Al IMC near the Cu layer is Cu9Al4 , while Cu-Al IMC apart from the Cu layer is CuAl2.(2) Calculation of the IMC reaction rate. The evolution of Cu-Al IMC during annealing was recorded in real time to study the evolution mechanism of Cu-Al IMC. The reaction rates of Cu-Al IMC at different temperatures were obtained and the activation energy of Cu-Al IMC was calculated to be 23.8kcal/mol. The more accurate growth equation of Cu-Al IMC was obtained.(3) Thermal reliability analysis. Due to the oxidiation of Al pad surface before bonding, a layer of aluminum oxide has already existed in the Cu-Al wire bonding interface, leading to the generation of voids in the bonding interface during annealing. Void formation reduces bond strength and increases the contact resistance of the bonding connection. At the same time, some results show that there is no voids in Cu-Al IMC layer after annealed 24 hours at 50-220℃.2. In-situ annealing investigation of PCC wire bonding:(1) Composition analysis of interface structure evolution. Before annealing, isolated Cu-Al IMC distributed in the bonding interface, the main component of IMC is Cu9AI4. After annealed at 250’C for 15 hours, Cu-Al IMC near Cu layer is Cu9Al4, while Cu-Al IMC apart from the Cu layer is CuAl2, same as the case of Cu wire bonding. Meanwhile, no signal of Pd atom is detected in the IMC layer, indicating the Pd atoms have no effect on the formation of the bonding interface IMC.(2) Thermal reliability analysis. After annealing at 250 ℃ for 15 hours, a void with diameter of about 3 μm was formed in the copper layer. In this case, we believe that a variety of copper grains can be formed accompany with the formation of free air ball due to the existence of Pd layer in PCC wire. More defects in the grain boundaries can reduce the melting point and accelerate atom diffusion. These induce the formation of more voids during thermal condition of the PCC wire bonding. The reliability of the bonding becomes bad and the contact resistance also increases.In this thesis, interface structure evolution of Cu and PCC wire bonding has been studied. The results provide significant guidance in improving Cu or PCC bonding process and developing the reliability of the wire bonding.