Depth control of focused ion-beam milling from a numerical model of the sputter process

A mathematical model of focused ion-beam milling is used to generate dwell times for the vector scanned pixel address scheme of a focused ion-beam deflection system. The model incorporates the absolute sputter yield of the solid as a function of the angle of incidence, and the relationship between the ion-beam current distribution and the pixel size of the deflection pattern. The object of this work is to be able to call for an arbitrary geometric shape to be ion milled and then have the numerical model compute the pixel dwell times for the deflection system such that the final cavity is sputtered. Experimental verification of the procedure was accomplished with parabolic troughs, hemispherical troughs, and cosine troughs. The term "trough" means a plane of symmetry in the ion-milled cavity. These same geometric shapes were also ion milled using a rotational axis uf symmetry, yielding sinusoidal ring patterns, parabolic dishes, and hemispherical dishes. The absolute maximum depth for each of the cavities is consistently between 0.8 and 0.9 of the programed (a priori) depth. The discrepancy between the programed depth and the observed depth is attributed to a slowly decreasing ion-beam current over the duration of the milling and to redeposition. (C) 1999 American Vacuum Society. [S0734-211X(99)20406-4].