Interfacial reactions in Al-0.5%wtCu/Ti/SiO2/Si structure have been investigated up to the annealing temperature of 600 degreesC for 30 min in Argon ambient. Annealing temperature at above 500 degreesC, Al alloy and Ti start to react and produce Al3Ti, which was already reported. Annealing at higher temperatures (550 degreesC, and 600 degreesC) made Al3Ti transformed into Al5Ti2, which is thermodynamically more stable than Al3Ti. The unreacted 52 nm thick Ti which existed underneath of Al5Ti2 might lead to retardation of the reaction between Al5Ti2 and the underlying SiO2. Hence, the formation of ternary compound (AlxTiySiz) which is believed to be detrimental to the contact metallization layers was protected.