Interface Engineering to Enhance the Mechanical Performance of Aluminum Thin Film Deposited on PET Substrate

Abstract

Al films magnetron sputtered on PET substrates with varying interfacial adhesion were produced by controlling the time of the plasma etching prior to the Al deposition. Uniaxial tension and laser-induced thin film spallation tests were performed on the Al film and PET substrate to investigate the effect of the adhesion on the ductility of the Al film. In addition, the effect of substrate thickness was studied using substrates with varying thicknesses. The laser spallation measurement revealed that the adhesion between the Al film and PET substrate increased from less than 5.3 MPa to 37 MPa by applying 10 min-plasma etching. The surface analysis indicated the chemical state of the surface is the dominant factor for the improved interfacial adhesion. The uniaxial tensile test of the PET supported Al film showed that the failure strain of Al film was improved by applying the plasma etching. It is assumed that the shear stress caused by the mismatch in the strain of Al and PET leads to local delamination of the Al film. When the plasma etching was applied, the interfacial adhesion was considered to increase strong enough to prevent the delamination. Accordingly, the PET substrate retarded the strain concentration in Al film by constraining the deformation of Al film, leading to an increase in the failure strain of Al film. The failure strain of Al film with 50 μm-thick PET substrate was substantially lower than the Al film with 12, and 25 μm-thick PET. According to the shear lag theory, the shear stress at the interface increases with an increase in the load acting on the PET substrate. Thus, it is considered that the shear stress was high enough to cause the delamination of Al film even the interfacial adhesion increased with the plasma etching. The Al film deposited on 25μm-thick PET showed the maximum failure strain when the plasma etching time was 5 min. The result of the contact angle measurement indicated the effect of the chemical modification reaches constant when the plasma etching time was more than 5 min. On the other hand, the line surface profile indicated the plasma etching might form a periodic swell on the surface of the PET, and the swell appeared to become sharp as the plasma etching time increased. It is possible that the swell decreased the failure strain of the Al film by initiating the stress concentration during the tensile test. Due to the combination of the two effects, the failure strain of the Al film exhibited the maximum value when the plasma etching time was 5 min.