Amorphous carbon films for use as both variable-transmission apertures andattenuated phase shift masks for deep ultraviolet lithography

We describe the development of amorphous carbon (a-C) films grown by magnet ron sputtering for use in optical elements for sub-0.25-mu m deep ultraviol et (DUV) lithography. We have measured the transmittance of a-C films depos ited onto quartz substrates as a function of him thickness, and find that t he films are ideally suited for use in variable-transmission apertures that can be used to improve DUV process latitude: we can achieve essentially an y transmittance (T) desired in the range 0<T<100% by controlling the film t hickness (t) in the range 200>t>0 nm with subnanometer precision. We also f ind that the transmittance remains stable after prolonged exposure to high intensity DUV radiation. We describe a masked deposition technique to produ ce variable-transmission apertures using a-C films of various thicknesses, and also discuss the use of these films in attenuated phase shift masks, gi ven that we can simultaneously achieve similar to 6%-8% transmittance and a phase shift of 180 degrees at either lambda = 248 nm or 193 nm. (C) 1999 A merican Vacuum Society.