Conventional (2 - 4 kV) argon (Ar) ion milling or focused ion beam (FIB) milling tend to leave "mottling" visible on the milled specimens, which is believed to be surface damage caused by ion implantation and amorphization. [1] Therefore, low energy (250 - 500 V) Ar milling at low angles (6 - 8°) is needed to remove such damages. [1]

[1] J. Scott, F. T. Docherty, M. MacKenzie, W. Smith, B. Miller, C. L. Collins and A. J. Craven, Sample preparation for nanoanalytical electron microscopy using the FIB lift-out method and low energy ion milling, Journal of Physics: Conference Series 26 (2006) 223–226.