X-ray determination of thermal strains and stresses in thin aluminum films and lines

Patterned lines of sputtered aluminum are used as device interconnects in the integrated circuit industry. Due to differences in thermal expansion between the Al and its encapsulant, large hydrostatic stresses develop in the Al. These stresses cannot be relaxed by simple plastic flow mechanisms, and thus stress-assisted diffusive voiding of the lines is often observed as a relaxation mechanism.; X-ray diffraction is a proven method for measuring strains and calculating stresses in metallization lines. Two X-ray geometries have been used to successfully determine the strain and stress state of unpassivated and passivated aluminum alloy lines. One measures the interplanar spacing of a particular family of crystallographic planes at several different inclination angles. The principal strain matrix is found from an extrapolation of these data to inclination angles of 0{dollar}spcirc{dollar} and 90{dollar}spcirc.{dollar} The principal components of the strain matrix are also measured directly using grazing incidence X-ray scattering (GIXS). This geometry permits the study of strain relaxation in the metallization.; The strain and stress state in unpassivated lines is measured at room temperature and during thermal cycling using both X-ray geometries. Good agreement is found. The stress gradients and stress relaxation are measured using GIXS. The gradients through the thickness are small, suggesting that plasticity relieves gradients. The strain in passivated Al alloy lines is measured at room temperature and during thermal cycling. The results are compared with strains calculated using finite element methods. The magnitude of the strains predicted by calculations are in general higher than the measured strains. This difference is attributed to the presence of voids and to the deformation of the passivation, neither of which is included in the finite element methods. The relaxation of the principal strain matrix is measured at several temperatures upon cooling from the passivation deposition temperature and is used to calculate the hydrostatic strain relaxation. The calculated deformation in the passivation is on the order of the observed relaxation. The formation and evolution of voids is observed in-situ with a high voltage SEM operated in back scattered mode.