Laser micro joining of thin metal films on flexible substrates for mechanical and electrical connections

Abstract

Joining of similar or dissimilar materials with a thickness in the range of micrometers and sub micrometers is of great interest for a number of applications in, e.g., micro technology, photovoltaic and thin-film technology. A laser micro joining process using 25 ns long KrF excimer laser pulses for joining thin films and foils is demonstrated. Metal films of silver, aluminium, copper, molybdenum and titanium with thicknesses down to 500 nm deposited on polyimide substrates were used for bonding to a 12.5 μm thick silver foil. The laser fluencies used for joining of the foil to the metal films are in the range of 3.5 J/cm 2 . The laser-induced joints were investigated by SEM (scanning electron microscope), optical microscopy, and a tensile strength tester. The shear stress calculated from the tensile force measurements and considering the laser-exposed area to be the bonding area is 0.5 N/mm 2 for silver/aluminium bonds. The tensile strength is not only determined by the bond between the metal films but also by the adhesion of the thin film to the substrate. Synergetic effects lead to bond formation comprising of thermal, mechanical, and chemical processes.