Focused ion beam induced deflections of freestanding thin films

Prominent deflections are shown to occur in freestanding silicon nitride thin membranes when exposed to a $50\mathrm{keV}$ gallium focused ion beam for ion doses between ${10}^{14}$ and ${10}^{17}\text{ions}∕{\mathrm{cm}}^2$ . Atomic force microscope topographs were used to quantify elevations on the irradiated side and corresponding depressions of comparable magnitude on the back side, thus indicating that what at first appeared to be protrusions are actually the result of membrane deflections. The shape in high-stress silicon nitride is remarkably flat-topped and differs from that in low-stress silicon nitride. Ion beam induced biaxial compressive stress generation, which is a known deformation mechanism for other amorphous materials at higher ion energies, is hypothesized to be the origin of the deflection. A continuum mechanical model based on this assumption convincingly reproduces the profiles for both low-stress and high-stress membranes and provides a family of unusual shapes that can be created by deflection of freestanding thin films under beam irradiation.