Adhesion of RF bias-sputtered Cr thin films onto photosensitive polyimide substrates

The adhesion enhancement from inserting a RF bias-sputtered Cr layer between Cu and polyimide (PI) has been studied. The RF bias power applied in this study ranged from 0 to 400 W. Without the RF bias, the peel strength, which measures the adhesion strength, was nearly 0 g/mm. As the RF power was increased, the peel strength rose up to ~130 g/mm at 200 W, which remained constant with further increase of the RF bias power. Cross-sectional transmission electron microscopy (TEM) was used to investigate the interfacial reaction between the Cr film and PI substrate during the bias sputtering. The Cr/PI interface without the application of RF bias showed a clean, sharp interface while the RF biased Cr/PI interface had about 10~30 nm thick atomistically mixed interlayer between the metal film and PI substrate. This interlayer appeared to have resulted from the implantation of high energy adatoms during the RF bias sputtering of Cr film. This mixed layer serves as an interlocking layer, which enhances adhesion between the metal and PI layers