Growth behavior and physical response of Al-Cu intermetallic compounds

This review covers recent investigations and concludes our findings for the growth of Cu/Al intermetallic compounds (IMC). [1, 2] The corresponding copper-aluminum interfaces were either established by a physical vapor deposited (PVD) Cu layer on a PVD aluminum pad or a Cu thermosonic nailhead bond on a PVD aluminum-based pad metallization. The identification, growth kinetics and mechanical strength of the different Al-Cu intermetallic compounds have been investigated. The annealing matrix of these investigations covered the temperature range from 150-300 °C for 25-2000 h. The identification of the Al-Cu phases utilizes X-ray diffraction analysis (XRD), selected area diffraction pattern (SAD) and scanning electron microscopy (SEM) & energy dispersive X-ray spectroscopy (EDX). The main three IMC phases Al₄Cu₉, AlCu and Al₂Cu were identified over the whole temperature range, whereas two additional phases (Al₃Cu₂, Al₆Cu₉₄) contribute to the total IMC growth at temperatures above 200 °C. Individual diffusion constants D₀ and activation energies E_a of 1.0 eV for Al₄Cu₉ and AlCu, 1.2 eV for Al₂Cu and 1.3 eV for the total IMC growth have been obtained. As the two slow growing phases Al₃Cu₂ and Al₆Cu₉₄ were not observed below 200 °C, lower activation energies for the total IMC stack were expected and have been measured to be in the range of 1.05-1.1 eV for thin film and bonded samples for temperatures below 200° C. Therefore it is recommended to use these lower activation energies for lifetime predictions in the typical regime of device application temperatures. The impact of IMC thickness and annealing conditions on bond strength was studied using ball shear test. The test results did not show any hints on interface strength degradation across the full experiment- l matrix even for the groups where Al was already fully consumed, in case of a tungsten barrier or adhesion layer between Al metallization and silicon-based dielectrics was used.