Effect of surface roughness on the adhesion properties of Cu/Cr films on polyimide substrate treated by inductively coupled oxygen plasma

Abstract

For fabrication of future flexible electronic devices on inexpensive and flexible organic substrates requiring a metallization process, the adhesion of metals to polymer substrate is a very critical issue in realizing required flexibility of metallization lines on the flexible substrates. In this work, effect of the surface morphological roughening and surface chemical modification of the PI surface on the adhesion properties of Cu(100 nm)/Cr(50 nm)/PI systems was investigated by varying the substrate bias power at the fixed top power (40 W) and O 2 flow rate (300 sccm) in an inductively coupled plasma (ICP) treatment system. The results of contact angle measurements and atomic force microscopy (AFM) showed a large increase in surface roughness under the biased condition resulting in the dramatic decrease of contact angle, ≅ 0° (a complete wetting). Analysis of chemical binding states and surface chemical composition by X-ray photoelectron spectroscopy (XPS) showed an increase in the formation of carbon–oxygen functional groups and the concentration of oxygen on the surface. T-peel test for adhesion strength measurement of Cu/Cr/PI systems showed the peel strength as high as ≅ 126 gf/mm, increased by a factor of 2.2 compared to that of the untreated sample. Observed dramatic increase in the adhesion strengths between PI and Cu/Cr films after plasma treatment at the substrate bias power of 125 W is attributed to a large surface roughness of PI induced by reactive etching in addition to the chemical modification of the plasma-treated PI surface.