DUV stability of carbon films for attenuated phase shift mask applications

Abstract

The UV stability of a-C:H films was investigated by irradiating them with a 248 nm excimer laser or a DUV lamp with a 248 nm filter. Both transmittance and phase angle of the film at 248 nm were measured at different UV dose intervals up to a cumulative dose of 10,000 J/cm2. We found that film stability was dependent on processing conditions. Excellent stability was achieved when the a-C:H films were sputtered from a graphite target in a Ar/hydrocarbon process gas mixture with the substrate held at an rf bias. If no substrate bias was used, films were stable only up to a maximum of 1,000 J/cm2 for a nominal transmittance of about 6%. These instabilities were attributed to adsorption of oxygen on the film and its subsequent diffusion into the films. X-ray photoelectron spectroscopy (XPS) shows that under DUV irradiation the oxygen content increases from 13 to 29 at.% on the carbon surface. Secondary ion mass spectrometry (SIMS) shows also oxygen diffusion into the carbon film. It appears that the oxygen first removes the graphitic content since film transmittance was found to increase. It is believed that rf bias sputtering results in denser films that are more impervious to oxygen penetration and hence more durable to this UV assisted oxidative degradation process. The sputtered films with no substrate bias were found stable when irradiated in a nitrogen atmosphere. ©2003 Copyright SPIE - The International Society for Optical Engineering.