Interdiffusion reactions and degradation in gold-aluminum ball bonds

The present study addresses the interdiffusion reactions and the degradation in Au-Al ball bonds as a function of time and the Cu content of the Al metallization during constant elevated-temperature exposure. Sample preparation methodology was developed to characterize the phase transformations and the degradation in the thermally-exposed Au-Al ball bonds by scanning electron microscopy. The as-bonded microstructure of a Au ball bond consisted of three regions; a columnar grain zone, an equiaxed grain zone and an interdiffusion zone. A line of discontinuous voids (void line) was observed in the interdiffusion zone.; Thermal exposure of Au-Al ball bonds at 175°C and 250°C resulted in the nucleation and the growth of Au₄Al, Au₈Al ₃ and Au₂Al across the ball bond. Once the metallization below the bond is completely consumed, the Al-rich phases reverse transformed eventually resulting in the formation of Au₄Al, in the entire reaction zone. With prolonged thermal exposure, lateral interdiffusion resulted in the nucleation and the growth of AuAl₂, AuAl, Au₂Al and Au₈Al₃ between Al around the ball bond and Au ₄Al.; Theoretical analysis of the partial reactions at each interphase boundary showed that the phase transformations across the ball bond result in volumetric shrinkage, which promote tensile stresses at the void line. It is these shrinkage stresses that aid in the nucleation and the growth of cavities and degrade Au-Al ball bonds during constant elevated-temperature exposure. The line of discontinuous voids acts as a site for the nucleation and the growth of cavities. Aided by thermal exposure, the cavity growth is higher at the crack front due to stress concentration at the crack front, which is initially at the edge of the void line. The crack propagation into the bond occurred by the linking of sufficiently grown cavities at the void line. Crack propagation completely into the ball bond results in the separation of the ball bond from the Si chip. Scanning electron microscopy observations of the thermally-exposed ball bond cross sections revealed lower cavity growth and higher rate of crack propagation in ball bonds on Al-0.75Cu metallization than in bonds on Al metallization.; The interdiffusion coefficients of the phases Au₄Al, Au ₈Al₃ and Au₂Al formed across ball bonds on Al and Al-Cu alloy metallizations were very comparable. From these interdiffusion coefficients and the similar nature of phase transformations in Au ball bonds on Al and Al-Cu alloy metallizations, it is inferred that Cu additions to the Al metallization do not influence the interdiffusion reactions between the Au ball bond and the metallization.