Barrier layer effect of tantalum on the electromigration in sputtered copper films on hydrogen silsesguioxane and SiO2

As IC devices scale down to the submicron level, the resistance-capacitance (RC) time delays are the limitation to circuit speed. A solution is to use low dielectric constant materials and low resistivity materials. In this work, the influence of underlying barrier Ta on the electromigration (EM) of Cu on hydrogen silsesquioxane (HSQ) and SiO₂ substrates was investigated. The presence of a Ta barrier not only improves the adhesion between Cu and dielectrics, but also enhances the crystallinity of Cu film and improves the Cu electromigration resistance. The activation energy obtained suggests a grain boundary migration mechanism and the current exponent calculated indicates the Joule heating effect.     

Relationship between grain structures and texture of damascene Cu lines

The anisotropic spread of the central peak in a (111) pole figure by x-ray diffraction (XRD) was observed for damascene Cu lines of 0.18–2 μm in width and 0.5 μm in depth. The spread originates from the existence of slightly tilted (111) grains because of inclined sidewalls. The tilted (111) orientation is favorable only for polygranular clusters whose sidewall energies can be minimized simultaneously. Consequently, bamboo grains have an exact 〈111〉 orientation, while the polygranular clusters have a tilted 〈111〉 orientation. Using this concept, the volume fraction of the bamboo grains and polygranular clusters in the damascene Cu lines were quantified using the XRD pole plots.     

The incubation time for hole formation due to electromigration in Al and Al/Cu/Al thin films

Transmission electron microscope observations of hole formation in Al and Al/Cu/Al thin film conductors carrying high current densities have revealed the presence of an incubation period for hole growth. The temperature dependence of this incubation period for pure aluminum conductors has been determined and found to follow an Arrhenius relationship with an activation energy of 0.55 ev ± 0.1 ev.     

The effect of grain orientation on the relaxation of thermomechanical stress and hillock growth in AI-1%Si conductor layers on silicon substrates

Hillocks are formed sporadically in Al-l%Si sputter layers on SiO₂/Si substrates during heat treatments in the range from 200 to 500°C. The driving force is the relaxation of thermomechanical stress in the grains induced by the thermal expansion mismatch between the metallization layer and the substrate. The orientations of individual grains and hillocks are measured on-line with a medium voltage transmission electron microscope by the Kikuchi pattern method. Thermomechanical stress in the grains is calculated with a biaxial strain model, considering the glide systems of dislocations for the individual grain orientations. In general, hillocks deviate from the ordinary 〈111〉 fiber texture of aluminum sputter layers. The spatial distribution of grain orientations is illustrated by orientation images using Miller indices or Rodrigues vectors.     

Impact of thermal cycling on the evolution of grain, precipitate and dislocation structure in Al, 0.5% Cu, 1% Si thin films

Thermal cycles have been performed both outside and inside a transmission electron microscope (TEM) in order to analyze the evolution of the microstructure of Al, 0.5% Cu, 1% Si thin films deposited onto oxidized Si substrates. It is shown that grain growth and dislocation activity start almost simultaneously and cooperate throughout the plastic regime of the stress–temperature curve to generate bamboo-type grains with low dislocation density. Si precipitates serve as anchoring points for dislocations and grain boundaries. Thermal cycling and diffusion cause the growth of these precipitates and a diminution of their number. Diffusion also proves to play an important role regarding plastic relaxation at the Al/SiO_x interface and at the grain boundaries where an intense hillock and whisker formation has been observed in scanning electron microscope (SEM). The stress–temperature evolution is discussed in light of these observations.