The contrast of ion beam stencil masks

Abstract

Masked ion beam lithography using stencil masks has demonstrated high resolution proximity printing with short exposure times. Stencil masks generally consist of a thin (∼1 μm) membrane with transmission holes etched through the entire membrane thickness. To the first order the mask contrast will be infinite if the membrane is thicker than the range of the exposing ions in the membrane. However, any ion impinging very near a transmission hole may scatter laterally into the transmission hole and escape the mask at an arbitrary angle. These scattered ions reduce the effective contrast of the mask in a manner which is pattern dependent. Grid-support stencil masks, which employ a matrix of small transmission holes instead of completely open transmission areas, are used to eliminate most of the pattern restrictions associated with stencil masks, and to improve mask stability. However, the grid significantly increases the mask area from which scattered ions may escape, and this type of mask requires a higher fluence to accomplish an exposure than simple stencil masks. Both effects will decrease the effective mask contrast. A Monte Carlo program has been developed which models ion scattering in stencil masks. The computer model predicts excellent performance from a grid-support stencil mask which has high-aspect-ratio transmission holes and has a membrane considerably thicker than the ion range.