Parametric modeling of focused ion beam induced etching

Focused ion beam induced etching with a (Cl2/GaAs) gas/substrate system was performed and the gas flux dependence analyzed. Using 20 keV Ga⁺ ions, the focused ion beam, with a Gaussian distribution of current density (Iprobe=0.32 nA) and a spot diameter of 0.4 μm, is scanned over a rectangular area to derive an etch. Ion incidence being kept normal to the surface, etched depths are accurately measured as a function of gas flux. Pressures less than 43 mTorr, corresponding to a gas flux of 1.04×10¹⁹ molecules/cm² s are considered, as compared to the peak ion arrival rate of 1.11×10¹⁸ ions/cm² s. A spiked (current) delivery model is presented for the dependence of etch rate on gas flux. The scanned nature of the process is taken into account in this model, so that the ion flux over a differential area arrives with a gaussian spatial distribution in temporal spikes or bursts. The etch mechanism supported by this model, namely, ‘‘chemically enhanced physical sputtering,’’ is discussed in light of the fitted parameters. Finally, a convenient application of the model is elaborated.