Electron cyclotron resonance ion stream etching of Ta film was investigated for preparing x‐ray mask absorber patterns. Ta is etched by the system at a high rate and with high selectivity. Using Cl2as etching gas, the etch rate decreases rapidly with decreasing pattern width below 0.5 μm and large undercutting is observed. The problems are reduced by adding Ar or O2gas to the Cl2. Etching with a mixture of Cl2and O2produces highly accurate Ta absorber patterns for x‐ray masks. The pattern width dependence of the etch rate and the undercutting were simulated with a model that takes account of the angular distribution of active species incident on the sample. The experimental results agree well with those calculated assuming that the incidence angles are distributed between −36° and 36°. The addition of O2or Ar enhances ion assisted etching.