Ion bombardment has been found to enhance the etch rate of amorphous rf‐sputtered Al2O3in hot H3PO4so that this material works as a dry‐etch‐durable ion beam resist. It is shown that the resist properties depend on the substrate temperature and the magnetic field strength near the target surface during the sputter deposition. The optimized Al2O3deposited at 380 °C and 9 mT is insoluble in 80 °C H3PO4when unbombarded, and has a sensitivity of 1×1014cm−2and a contrast (γ) of 1.5 for 50‐keV Ga+ions. Density measurement and x‐ray diffraction revealed that the optimized Al2O3has higher density and a more ordered structure. X‐ray masks were fabricated by defining 200‐nm‐wide grooves in 500‐nm‐thick WNxx‐ray absorber layers using the Al2O3patterned with focused 50‐keV Ga+beams, as a durable mask for electron cyclotron resonance type of plasma etching with SF6; the etch‐rate ratio of WNxto Al2O3was 30. Pattern transfer into poly(methylmethacrylate) from the fabricated mask was successfully carried out with synchrotron radiation.