Wide variations in the dose enhancement factor observed when milling silicon using Focused Ion Beam (FIB) XeF_2 Gas Assisted Etching (GAE) prompted the development of a simple model of the GAE process. The model accounts for three material removal mechanisms: regular sputtering; gas-assisted sputtering; and spontaneous chemical reactions. An expression linking the dose enhancement factor, ε_d, to the gas and milling parameters has been derived. Experiments show that ε_d behaves as predicted; good quantitative agreement is achieved over wide ranges of milling parameters for ε_d values between 20X and 2500X. Conditions required to minimize variations in _d and maximize material removal rates, M, are derived. It is shown that if the dose per unit area per raster is below a threshold value then ε_d and M depend only on the average current density J (the area of the box divided by the beam current). A consideration of the J regimes used for front-side and back-side FIB work shows why changes in ε_d have not previously been a problem but are inevitable when milling the large trenches characteristics of Flip Chip circuit modification work. While ε_d changes dramatically there is a region of J values for which M is approximately constant.
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