conductor layer 42 is removed from the top surface 36 of insulator layer 28 by one of various approaches removal of the conductor layer 42 leaves the studs 38 40 in an electrically isolated condition separated by an intervening portion of the insulator layer 28 one approach is to mill the conductor layer 42 from the top surface 36 by sputtering using the focused ion beam 34 the focused ion beam 34 is scanned across regions of the top surface 36 coated by the conductor layer 42 the milling process is halted when the conductor layer 42 is removed and the top surface 36 of insulator layer 28 is exposed another approach is to etch the conductor layer 42 using the focused ion beam 34 in a gas assisted etching process a reactive gas is introduced to the top surface 36 and the focused ion beam 34 is scanned across regions of the top surface 36 from which conductor layer 42 is to be etched the focused ion beam 34 in the presence of the reactive gas promotes chemical reactions between the reactive gas and the material constituting the conductive layer the chemical reactions in conjunction with physical sputtering cause the removal of the conductor layer 42 the specific type of reactive gas used in conjunction with the focused ion beam 34 depends on the composition of the conductor in the conductor layer 42 for example the reactive gas may be a halide gas such as chlorine bromine or iodine the gas assisted etching process is halted when the.