Chemical reaction of sputtered Cu film with PI modified by low energy reactive atomic beam

Abstract

Polyimide (PMDA-ODA) surface was irradiated by low energy reactive atomic beam with energy 160-180 eV to enhance the adhesion with metal Cu film. O-2(+) and N-2(+) ions were irradiated at the fluence from 5 x 10(15) to 1 x 10(18) cm(-2). Wetting angle 78 degrees of distilled deionized (DI) water for bare PI was greatly reduced down to 2-4 degrees after critical ion flounce, and the surface energy was increased from 37 to 81.2 erg/cm. From the analysis of O 1s core-level XPS spectra, such improvement seemed to result from the increment of hydrophilic carbonyl oxygen content on modified PI surface. To see more carefully correlation of the peel strength with interfacial reaction between Cu and PI, flexible copper clad laminate with Cu (9 mu m)/Cu (200 nm) on modified PI substrate (25 mu m) was fabricated by successive sputtering and electroplating. Firstly, peel strength was measured by using t-test and it was largely increased from 0.2 to 0.5 kgf/cm for Ar+ only irradiated PI to 0.72-0.8 kgf/cm for O-2(+) or N2O+ irradiated PI. Chemical reaction at the interface was reasoned by analyzing C 1s, O 1s, N 1s, and Cu 2p core-level X-ray photoelectron spectroscopy over the as-cleaved Cu-side and PI side surface through depth profiling. From the C Is spectra of cleaved Cu-side, by the electron transfer from Cu to carbonyl oxygen, carbonyl carbon atom became less positive and as a result shifted to lower binding energy not reaching the binding energy of C-2 and C-3. The binding energy shift of the peak C-4 as small as 1.7 eV indicates that carbonyl oxygen atoms were not completely broken. From the analysis of the 0 Is spectra, it was found that new peak at 530.5 eV (O-3) was occurred and the increased area of the peak O-3 was almost the same with reduced area of the peak carbonyl oxygen peak O-1. Since there was no change in the relative intensity of ether oxygen (O-2) to carbonyl oxygen (O-1), and thus O-3 was believed to result from Cu oxide formation via a local bonding of Cu with carbonyl oxygen atoms. Moreover, from X-ray induced Auger emission spectra Cu LMM which was very sensitive to chemical bonding, Cu oxide or Cu-O-C complex formation instead of Cu-N-O complex was clearly identified by the observation of the peak at 570 eV at higher 2 eV than that of metal Cu. In conclusion, when Cu atoms were sputtered on modified PI by low energy ion beam irradiation, it can be suggested that two Cu atoms locally reacted with carbonyl oxygen in PMDA units and formed Cu+-O--C complex linkage without being broken from carbon atoms and thus the chemically bound Cu was in the form of Cu2O. (c) 2005 Elsevier B.V. All rights reserved.