 Conductive materials and most native oxides of conductors should never need charge referencing.Conductive materials should never be charge referenced unless the topmost layer of the sample has a thick non-conductive film.The charging effect, if needed, can also be compensated by providing suitable low energy charges to the surface by the use of low-voltage (1-20 eV) electron beam from an electron flood gun, UV lights, low-voltage argon ion beam with low-voltage electron beam (1-10 eV), aperture masks, mesh screen with low-voltage electron beams, etc.