Self-encapsulated silver metallization and low-k polyimide for ultra-large-scale integration

An integrated approach has been taken to study the interconnect materials system and develop processes related to silver encapsulation, the metal dry etch, diffusion barrier and adhesion promoter, and low-dielectric-constant (low-k) polyimide materials. The objective is to develop a processing scheme for the low resistance metal and the low-k dielectrics, and to use fabricated test structures to demonstrate better performance especially in electromigration and dielectric insulation than the current industry standard for ultra-large-scale integration (ULSI) applications.; Silver is an attractive choice of the low resistivity metal to replace aluminum alloy interconnect. A thin diffusion barrier for Ag metallization can be formed by nitridation of Ti using Ag/Ti bilayers in an ammonia ambient. A linear-parabolic model was proposed to describe the kinetics of the nitridation reaction. The Ti-nitride grows fast initially and a linear kinetics is presumed in a reaction-limiting step. After 15 min annealing, the nitride growth slows down and follows a parabolic growth kinetics.; Silver films in Ag/Ti bilayer structures exhibited a strong {dollar}langle 111rangle{dollar} texture component and a near-bamboo grain structures. In contrast, Ag films with randomly oriented grains were observed on Cr underlayers. The encapsulated silver layers had minimal residual Ti accumulations. X-ray analysis confirmed the absence of intermetallic phase transformation. Therefore, resistivity values of about 2 {dollar}muOmega{dollar}-cm were obtained for the encapsulated Ag bilayer films, which are comparable to that of the bulk values.; A process was first reported to dry etch Ag metallization for microelectronic applications. Silver films oxidize readily in the oxygen plasma to form silver oxides, which subsequently crack and spall away due to incorporation of Ag to the oxide lattice. As a result, the oxide formed can be etched away in the reactive ion etch (RIE) chamber. The silver pattern prepared by the dry etching technique demonstrated desirable stoichiometry and edge profile. Also, Ti can act as an etch stop during the dry etch of Ag metallization.; Electromigration lifetime of Ag metallization was characterized at accelerated current densities and temperatures using NIST test structures prepared by damascene process. Voids were observed to nucleate along the metal line edge and grow in the line width direction that cause electromigration failure. It was found that the median-time-to-failure (MTF) of the silver metallization is 2.5 times better than that of AlCu alloys. In addition, the lifetime of the metallization and electromigration activation energies depending on whether the surface passivation is used for these interconnects.; In a parallel work, a baseline process of a photosensitive polyimide (PSPI) was developed and the material was characterized. The polyimides are low-k materials (k = 2.6), which are thermally stable and compatible to current back-end-of-line (BEOL) environment. Moreover, the polymeric materials can be easily processed with 30-50% cycle time reduction to serve as the interlayer dielectrics as well as passivation stress buffer in multilevel interconnect structures. Electrical characterization suggested that it is critical to reduce the moisture content in the polymer films to ensure electrical isolation during the integration.