Poly(oxybenzoate-co-oxynaphthoate) (POCO) film surfaces were modified by four plasma bases. Ar, O-2, N-2 and NH3, and the effects of the plasma modification were investigated in order to understand the adhesion with copper metal. The Ar, O-2, N-2 and NH3 plasmas converted the POCO surfaces from hydrophobic to hydrophilic. The effect of the plasma on the hydrophilic modification was in the order: Ar plasma > O-2 plasma > N-2 plasma > NH3 plasma. The plasma modification contributed to the adhesion between the deposited copper metal and the POCO film. The NH3 plasma was most effective in improving the adhesion, and the Ar plasma was ineffective. The plasma-modified POCO film surfaces showed quite different C-1s spectra from that of the original POCO film. There were large differences in the C-1s and N-1s spectra between the NH3 and Ar plasma modifications. The NH3 plasma modification did not show C-1s component #5 due to pi-pi* shake-up satellite, but the Ar plasma modification did show this component. Furthermore, NH3 plasma modification led to a new N-1s spectrum. The plasmas caused etching of the POCO film surfaces, and the etch rate depended on what plasma was used and how much RE power was used. The NH3 plasma-modified POCO film surface showed a larger R-a (25.5 nm) than the other plasma-modified surfaces (R-a = 16.4-19.0 nm), which were comparable to that of the original surface (R-a = 14.8 nm). The NH3 plasma led to a highly-undulated surface, and the other plasmas did not alter the surface roughness. The roughened surfaces showed contribution to enhancement of the adhesion to the deposited copper metal.