In this study, the effects of O-2 on the etching of photoresist (PR) on Si wafers in atmospheric-pressure Ar and He plasma have been investigated. The PR etch rate in Ar plasma was several times larger than that in He plasma. It is hypothesized that the Ar was more effective because the density of the Ar plasma was higher than that of the He plasma. In addition, the Ar plasma extended more significantly outside the plasma cavity, and approached the PR more closely than the He plasma at equivalent cavity-PR separation distances. The PR etch rate in Ar/O-2 plasma was also higher than that in He/O-2 plasma at equivalent conditions of plasma power, plasma outlet-to-sample distance, and O-2 content. After the treatment, the PR etched surfaces were examined using contact angle goniometry, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). Contact angle measurements demonstrated that gas mixtures (Ar/O-2 or He/O-2) are more effective at raising the hydrophilicity of the treated surfaces than single gases (Ar or He). AFM results showed that the morphology of the Si surface following plasma treatment is comparable when either of the single gases (Ar or He) is applied. However, when gas mixtures were applied, the Ar/O-2 etched surface was rougher than the He/O-2 etched surface. The XPS results showed that Ar/O-2 atmospheric-pressure plasmas at the conditions studied could effectively remove all PR from wafer surfaces. (c) The Electrochemical Study.