Electron Cyclotron Resonance Plasma Etching of α-Ta for X-Ray Mask Absorber Using Chlorine and Fluoride Gas Mixture

The etching of α-Ta deposited by electron cyclotron resonance (ECR) sputtering is investigated by an ECR ion stream with a mixture of chlorine and fluoride gases for use in the X-ray mask process. The addition of fluoride gas reduced the surface roughness of etched patterns caused by the crystal structure of ECR-sputtered Ta. However, Ta etching stopped completely when 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 Cl2 and a small amount of CF4, we obtained patterns with vertical side walls and minimal roughness at widths less than 100 nm. We also evaluated the critical dimension (CD) uniformity and accuracy of actual X-ray masks and confirmed 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.