Assessment of sputtering damage in organic layer surface based on energy distribution of positively charged particles formed during facing-target sputtering of indium–tin oxide

During sputtering formation of transparent conductive oxide (TCO) electrodes, the organic layer surface becomes damaged, limiting transparent organic device performance. Bombardment of the organic underlayer surface with high-energy particles formed during the sputtering process is considered a major source of sputtering damage. In this study, we investigated the energy distribution of positively charged high-energy particles formed during deposition of indium–tin oxide (ITO) by facing-target sputtering (FTS), which is a low-damage sputtering process designed for fabricating TCO on an organic layer. Positively charged particles with mass numbers of 16, 20, 32, 40, and 115 were attributed to O+ or O22+, Ar2+, O2+, Ar+, and In+, respectively. Most of these particles carried kinetic energies below 2 eV, significantly smaller than the energies of most chemical bonds in the organic molecules. Changing the O2 content in the sputtering gas varied the number of positively charged particles without degrading the current–voltage characteristics of the organic devices. Furthermore, X-ray photoelectron spectroscopy measurements revealed that when ITO was deposited on tris(8-hydroxyquinoline) aluminum (Alq)—a typical organic semiconducting molecule—the surface molecules of the Alq underlayer remained intact. These findings suggest that in ITO deposition by FTS, damage is not caused by breakdown of the organic molecules, the widely accepted cause of sputtering damage. However, our ouganic devices showed significantly deterioration of the current-voltage characteristics caused by the sputtering damage. We report that the damage was likely caused by crack formation at the ITO/organic-underlayer interface resulting from internal stress of the ITO layer.