Focused ion beam lithography using Al2O3 as a resist for fabrication of x-ray masks

Abstract

Ion bombardment has been found to enhance the etch rate of amorphous rf‐sputtered Al2O3 in hot H3PO4 so 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 Al2O3 deposited at 380 °C and 9 mT is insoluble in 80 °C H3PO4 when unbombarded, and has a sensitivity of 1×1014 cm−2 and a contrast (γ) of 1.5 for 50‐keV Ga+ ions. Density measurement and x‐ray diffraction revealed that the optimized Al2O3 has higher density and a more ordered structure. X‐ray masks were fabricated by defining 200‐nm‐wide grooves in 500‐nm‐thick WNx x‐ray absorber layers using the Al2O3 patterned 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 WNx to Al2O3 was 30. Pattern transfer into poly(methylmethacrylate) from the fabricated mask was successfully carried out with synchrotron radiation.