Focused ion beam milling: Depth control for three-dimensional microfabrication

Abstract

Ion milling with a focused ion beam (FIB) is a potential method for making micromolds, which will then be the primary elements in the mass production of micro- or mini-objects by embossing or injection molding. The challenge lies in controlling the ion milling to produce cavities with predefined, arbitrary geometric cross-sections. This work involves programming variations as a function of position into the algorithm that generates the dwell times in the pixel address scheme of a FIB. These variations are done according to whether an axis of symmetry or a plane of symmetry determines the final geometry, and the result is 26 new cross-sectional shapes, such as hemispherical pits, parabolic pits, hemispherical domes, etc. The ion milling control programs were used to generate parabolic cross-section trenches, sinusoidal trenches, sinusoidal cross-section rings on an annulus, and hemispherical domes. We observed reasonable agreement between the shapes ion milled in Si(100) and the expected geometry. The dwell times are generated assuming each pixel has a unique dose and the ion yield is constant with angle of incidence. Deviations from ideality are ascribed to the variation in sputter yield with changing angle of incidence, and to the dwell time control algorithm. Redeposition also compounds the deviation from ideality, but it is difficult to estimate the magnitude of this effect.