Diffusion Formation of Intermetallic Compounds in Au-Al Couples by Use of Evapolated Al Films

Abstract

Au(bulk)-Al(evapolated film, 10μ) and Au (bulk)-[Al(evapolated film, 10μ)-Al(bulk)] couples were annealed over the temperature range 197 to 399°C.At above 321°C all intermetallic compounds present in the Au-Al equilibrium diagram were detected in a diffusion zone. 80 per cent of the diffusion layer was occupied by the (Au5Al2) phase layer in which Kirkendall markers and voids were observed and the markers moved towards the Au side.In the temperature range 197 to 399°C parabolic rate constants kW for the diffusion layer growth are given by the following equation: kW=2.02×10−2exp(−20400⁄RT). In the same temperature range the rate constants kW for the (Au5Al2) layer growth are given by the following equation: kW2=7.42×10−2exp(−22600⁄RT). The difference of the above equations means that the ratio of the (Au5Al2) layer thickness to the diffusion layer thickness decreases with decrease in annealing temperature. At lower temperatures it would be expected that the diffusion layer is occupied by the Au2Al phase layer.The purple intermetallic compound AuAl2 existed as an extremely thin layer and developed in all cases where cracks were observed in the (Au5Al2) layer. Purple plague i.e. the development of the AuAl2 phase layer at the gold-aluminum bonding interface of semi-conductor devices, may possibly be not a cause but a result of bond failure.