Amorphous Carbon Films for Use as Both Variable-Transmission Apertures and Attenuated Phase-Shift Masks for DUV Lithography

We describe the development of amorphous carbon (a-C) films grown by magnetron sputtering for use in optical elements for sub-0.25-micron DUV lithography. We have measured the transmittance of a-C films deposited onto quartz substrates as a function of film thickness, and find that the films are ideally suited for use in variable transmission apertures that can be used to improve DUV process latitude: we can achieve essentially any transmittance (T) desired in the range 0 t > 0 nm with sub-nanometer precision. We also find that the transmittance remains stable after prolonged exposure to high intensity DUV radiation. We describe a masked deposition technique to produce variable transmission apertures using a-C films of various thickness, and also discuss the use of these films in attenuated phase-shift masks, given that we can simultaneously achieve ~6-8% transmittance and a phase shift of 180 degrees at either lambda=248 nm or lambda=193 nm.