Effect of the underlayer on the microstructure and surface evolution in Al-0.5wt.%Cu polycrystalline thin films

Abstract

INTRODUCTIONSputter-deposited thin films have wide application in many microelectronic technologies in which film surface and microstructure have direct implications for device function and reliability. Understanding how factors such as substrate material and roughness, temperature, deposition parameters, residual gas pressure, and others affect the evolution of film grain size, crystallographic texture and roughness is often qualitative. For example, the zone model 1 only categorizes deposited film grain morphology as a function of sputter deposition temperature and pressure. Systematic studies may provide a more detailed understanding of how deposition conditions affect mechanisms such as grain growth and surface diffusion, which control film structure evolution. In an important effect, the substrate material properties may influence the microstructure of deposited films. 2 Examples of extreme substrate control include the heteroepitaxial growth of a single-crystal film above a single-crystal substrate 3 and the growth of a polycrystalline film with a grain-to-grain heteroepitaxial relationship with a polycrystalline substrate. 4 At the other extreme, a noninteracting film and substrate combination is one in which the substrate provides only a suitable surface for deposition and growth. Even in this case, however, energy minimization driving forces may lead to preferred orientation of the depositing film. Additionally, reactions, interdiffusion, and substrate roughness 5 may affect the growing film structure.In a common application, Ti underlayers are used to improve the (111) fiber texture of sputter-deposited Al-based metallizations and the reliability of subsequently patterned interconnects. 6 This effect has been proposed as due to heteroepitaxy between the film and underlayer 7 in which the depositing film texture is "inherited" 8 from the underlayer. Such a mechanism obviously requires some component of grain-to-grain heteroepitaxial matching at least during the initial stages of film nucleation and growth. In this explanation, the Ti (0002) fiber-oriented substrate, with atomic nearest neighbor spacing ϭ2.9505 Å, provides a lattice-matched substrate for Al(111) fiber-oriented grains (Al atomic nearest neighbor spacing ϭ2.863 Å). However, specific evidence for significant heteroepitaxy between a Ti/Al bilayer has not been shown, although heteroepitaxial matching may evolve into Ti/Al multilayer structures. Further, crystallographic texture is only one aspect of film structure to consider when quantifying the effects of The effects of the Ti underlayer on the evolution of grain morphology, crystallographic texture, and surface roughness of Al-0.5wt.%Cu thin films during sputter deposition have been characterized. In comparison to SiO 2 substrates, Ti underlayers reduce the AlCu thickness at which film continuity is reached, reduce the AlCu columnar grain size, and allow exact Al (111) fiber texture development. The AlCu films on both Ti and SiO 2 are primarily randomly oriented at early stages of depositio...