We report on the variation of threshold field with oxide thickness for field evaporation of Au nanodots onto ultrathin oxide layers on Si(1 0 0). Au dots of 1-10 nm diameter were fabricated on an in situ oxidized Si(1 0 0) substrate by the application of a voltage pulse between an Au coated STM tip and the sample. Under feedback-controlled tunneling conditions (V-s = 2 V, I-t = 3 nA), the voltage required for field deposition increased roughly linearly from 5.8 to 11 V as average oxide thickness increased from 0 to 8 Angstrom. This corresponds to a change in the threshold field of about 0.3 V/Angstrom, which is roughly consistent with a calculation based on shielding of the image charge by the intervening oxide layer. (C) 2000 Elsevier Science B.V. All rights reserved.