Controlling Copper Electromigration

One of the biggest reliability concerns in copper interconnect lines is electromigration (EM), where electrical current causes atoms to move. This increases resistance and, worst case, creates an open circuit. The phenomenon occurs primarily at the top surface of the copper, where it interfaces with the overlying dielectric, typically silicon carbide (SiC) or a silicon carbon nitride (SiCN) barrier layer (to a lesser degree, it occurs at the interface between copper and surrounding barrier, typically TaN). Since the problem worsens with shrinking dimensions because of higher current densities and thermal effects, there is considerable interest in a solution that will better lock down the copper. Experts say protection against EM needs to improve at least twofold for every device generation.

Currently, several solutions exist. The simplest is a self-aligned barrier (PSAB) approach under development at Novellus (San Jose). PSAB is a selective process that uses SiH4 to create a copper silicide, which adheres better to SiC to reduce EM. In work reported at the 2006 International Reliability Physics Symposium (IRPS) in San Jose, Novellus combined PSAB with an optimized SiC composition to generate >2× improvement in EM, and 10-1000× increase in time-dependent dielectric breakdown (TDDB), another major reliability concern.

Considerable work is also underway to develop a reliable selective electroless copper capping process. The most widely investigated are CoWP and CoWB, followed by CoSnP and palladium (Pd). The selective deposition is achieved through an electroless reaction kinetically activated on the metallic surface only. In a first class of processes, this activation is obtained by selective deposition of palladium by an exchange reaction with copper. At the Advanced Metallization Conference (AMC) in San Diego, a paper by researchers from CEA-LETI (Grenoble, France), STMicroelectronics (Geneva, Switzerland) and Philips AMS (Natick, Mass.) noted that these processes provide superior EM performance, yet can cause excessive corrosion of the copper. Alternative palladium-free solutions are available, which are less corrosive, but at the expense of selectivity because of more reactive deposition baths.¹ Selete (Yokohama, Japan) researchers, however, reported that a combination of chemical wet cleaning and NH₃ plasma treatment after CoWP deposition can improve leakage current to the same level of that without CoWP. “CoWP…will become the most promising solution to improve interconnect reliability in the next generation of technology node,” the Selete team concluded.

A third solution is a relatively new technique being fielded by Epion (Billerica, Mass.), which used gas cluster ion beams (GCIB) to “infuse” various species into the very top surface of the copper, thereby eliminating the problematic abrupt interface between the copper and dielectric film. Presented at AMC, a collaboration between Selete and Epion showed that infusion of silicon and nitrogen into the copper created a SiN film — a mixing layer — that provided better adhesion and improved EM lifetime. EM was improved by roughly an order of magnitude compared with the process of record, which employed a SiCN barrier. The process is not selective, so the dielectric is also infused. This slightly raises the effective k value, but not by much (~5%) since the layer is relatively thin (~5 nm). “We infuse silicon and nitrogen in the appropriate ratios into the copper surface and also the surrounding dielectric material to incorporate silicon nitride film into the subsurface,” said John Hautala, CTO at Epion. “We make a passivated copper surface with a graded layer of silicon nitride in the subsurface. The depth is accurately controlled by beam energy down to as shallow as 5 nm. This infusion process demonstrated the same 10× improvement in electromigration enhancement a customer observed with a single metal-level electroless cobalt capping process.”

In other work presented at AMC, researchers from Renesas (Tokyo) showed how they were able to improve EM and stress-induced voiding with a copper aluminum (Cu-Al) alloy. For the seed layer, a Cu-Al alloy film was deposited by physical vapor deposition (PVD) using a Cu-Al alloy target. This was followed by conventional copper electroplating. Through annealing, aluminum atoms were diffused from seed layer into the plated copper. Diffusion took place along the copper grain boundaries and were localized there, which helped prevent migration of copper atoms and/or vacancies. The researchers noted that this is a relatively simple change, requiring only a switch in target materials. The end result is roughly a half an order of magnitude of improvement in EM.