The effects of glancing-angle argon ion bombardment on air-contaminated Si(001) surfaces were studied. Bombarding at substrate temperatureTs=730 °C, impingement angleφ=3–15°relative to the surface plane, ion energyE=1keV and doseD=3×1015ions cm−2gave high-quality reflection high energy electron diffraction (RHEED) patterns and contaminant-free surfaces as observed by ion scattering spectroscopy. Atomic force microscopy images showed roughness value⩽0.5 nm under these conditions, but the roughness increased and RHEED patterns became spotty for higher doses or energies. Secco etching of samples bombarded atTs=730 °C showed etch pits with a density of106–107cm−2that increased with increasingDandE. Room-temperature bombardment withE=1keV,D=3×1015ions cm−2andφ=3°, followed by a 730 °C anneal, yielded a lowest roughness value of 0.2 nm. Secco etching showed no resolvable pits, indicating a dislocation density<4×104cm−2.