Recrystallization and intermetallic formation in Au/Al and Au/Zn bimetallic films

Abstract

Bimetallic thin films are of interest for use in microelectromechanical systems (MEMS) for the fabrication of sensors and actuators. The coefficient of thermal expansion (CTE) mismatch between the two metal films of the bimetallic structure can be used to attain the desired level of deflection or actuation in the MEMS device. Gold/aluminum and gold/zinc films have desirable properties for use in bimetallic films because of their differences in CTE. The stress-temperature behavior of these bimetal configurations has been studied. The effect of deposition technique and recrystallization of the metal films on the total stress and strain of the bimetallic films has been investigated. It has been shown that the formation of intermetallic compounds between the metal layers changes the mechanical properties of composite films. The intermetallic compounds are often undesirable because of their uncontrolled composition and poor mechanical properties. The degree of formation of the intermetallic compounds in the gold/aluminum bimetallic films can be reduced by use of a diffusion barrier between the two metals. It is shown that aluminum oxide is an effective diffusion barrier, and it can be easily fabricated. Stress-strain measurements and chemical depth profiling of the bimetallic films with the diffusion barrier have been used to quantify the reduction in the formation of the intermetallic compounds.

Electrostatic Actuators with Intrinsic Stress Gradient

“…The mechanical properties of bimetallic films have been shown to change with time and temperature aging due to intermetallic diffusion and recrystallization. 6,7 Intermetallic diffusion between the two films hinders the long-term reliability and reproducible behavior of these devices. Other material properties that must be considered in the choice of metals in bimetallic beams are ductility, resistance to fatigue or work hardening, electrical conductivity, the temperature of alloy or compound formation, and the ease of processing.…”