Observation and control of the interfacial reaction of multilayr thin films for lead bonding wire using optical reflectivity technique.

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Formation of intermetallic phases in Al/Au and Al/Cu thinfilm couples annealed at temperatures between 100ﾟC and 250ﾟC has been investigated using X-ray diffraction and optical reflectivity technique. The Au or Cu metals with the thickness of 100nm were first deposited on SiO_2 plate and then Al thin film with the thickness of 100nm was produced on the Au or Cu film in vacuum. Formation of several intermetallic compounds was confirmed by X-ray analysis after annealing. The formation and growth of Al_2Cu, AlCu and Al_2Au could be observed by the variation of optical reflectivity with wavelength. The inter-diffusion of metals in Al/Au thin film occurred faster than that in Al/Cu during annealing. For example, the growth rate constants (k) of Al_2Au and Al_2Cu layrs in thin film couples annealed at 200ﾟC were 3.4x10^<-6> and 2.4x10^<-7> cm/sec^<1/2>, respectively. The growth rate of these intermetallic layrs was faster in thin films than in diffusion couples of bulk metals.The strength of interfacial reaction products between Al and Cu was investigated using indentation test. The strength increased rapidly with increasign time and thickness of reaction zone, and then decreased. An appropriate thickness of the reaction zone at the interface contributed to the improvement of the strength of Al-Cu lead bonding wires.