Electron cyclotron resonance plasma etching of alpha-Ta for X-ray mask absorber using chlorine and fluoride gas mixture

The etching of alpha -Ta deposited by electron cyclotron resonance (ECR) sp uttering is investigated by an ECR ion stream with a mixture of chlorine an d fluoride gases for use in the X-ray mask process. The addition of fluorid e gas reduced the surface roughness of etched patterns caused by the crysta l structure of ECR-sputtered Ta. However, Ta etching stopped completely whe n the concentration of fluoride gas was sufficiently high. This was because of oxygen generated from the plasma chamber. It was important to keep the amount of fluoride gas very small when etching Ta. Using a mixture of Cl-2 and a small amount of CF4, we obtained patterns with vertical side walls an d minimal roughness at widths less than 100 nm. We also evaluated the criti cal dimension (CD) uniformity and accuracy of actual X-ray masks and confir med that the etching faithfully transferred resist patterns to the Ta. The change in pattern width during etching was less than 1.5 nm on average for three masks.