Postfluorination of fluoride films for vacuum-ultraviolet lithography to improve their optical properties

Transmittance of fluoride films prepared by various kinds of deposition methods is not sufficient for a vacuum-ultraviolet (VUV) stepper application. Moreover, moisture penetrates into fluoride films because of porous structures. Then the moisture reacts with the fluoride chemically, and the photoabsorption of the films increases with elapsed time. Postfluorination treatment, in which F-poor areas of fluoride films are sufficiently fluorinated and the film structures are modified to be denser, has been developed. The postfluorination treatment was performed at 0.1 MPa of diluted F2 gas in a special apparatus made of nickel. This treatment reduces photoabsorption of fluoride films and prevents photoabsorption from increasing again with elapsed time. In a long-time F2 laser irradiation test, the transmittance of as-deposited fluoride films remarkably degraded with irradiation. On the other hand, there is no tendency of transmittance degradation in the case of postfluorinated fluoride films. The postfluorination is able to improve the optical performance of fluoride films, promising the highest coating level for VUV lithography.