Medium energy (30 keV) focused gallium ion beam exposure of silicon results in a compressive in-plane stress with a magnitude as large as 0.4 GPa. Experiments involve uniform irradiation of thin polysilicon microcantilevers (200 micron length) over a range of dose from 1 x 1016 to 2 x 1018 ions/cm2. The radii of curvature of microcantilevers are measured using white light interferometry before and after each exposure. The residual stress is determined from these radii and other measured properties using Stoney's equation. The large residual stress is attributed to ion beam damage, microstructural changes and implantation.