Sputtering of fibrous-structured low-stress Ta films for x-ray masks

Abstract

In order to achieve precise control for the internal stress of x-ray absorber films, we have controlled the sputtering-gas (Xe) pressure within ±0.002 Pa (at 0.5 Pa) using a newly developed ultrahigh-vacuum sputtering system, but we found that it was difficult to reproducibly obtain sufficiently low-stress Ta films over the long term. We therefore investigated the relationship between the internal stress and substrate temperature at a low deposition pressure (0.45 Pa). It was found that internal stress changes basically toward tensile with increasing substrate temperature because of the bimetal effect, but at substrate temperature from 205 to 220 °C, the internal stress changes toward compressive with increasing temperature. We characterized Ta films deposited at various temperatures using x-ray diffraction and secondary electron microscopy and found that a film deposited at 205 °C was columnar-structure β-Ta and that one deposited at 240 °C was fibrous-structured β-Ta. Films deposited at temperatures higher than 270 °C were rough-surfaced α-Ta. For films deposited at pressures lower than 0.8 Pa and at a substrate temperature of 240 °C, the internal stress was almost constant regardless of gas pressure fluctuation. Thus, at 240 °C and 0.45 Pa, we can deposit low-stress (<5×107 N/m2) Ta films with excellent long-term reproducibility.