Lateral growth of focused ion beam deposited platinum for stencil mask repair

Abstract

Focused ion beams can repair conventional lithography masks through milling and deposition. Repairs to stencil masks for ion projection lithography present a greater challenge than conventional mask repair, as the standard model of ion induced deposition requires a surface for deposition to take place. In a stencil mask, a bridge must be formed across the open area, anchored to the mask on either side of a slot. Focused ion beam deposition can induce such horizontal growth of material. Ion-induced growth of platinum over slots up to 3 μm wide in a 2.75 μm silicon membrane is demonstrated for an average beam current density of 7 ${\mathrm{mA/cm}}^{\mathrm{2}}.$ The primary method used to accomplish this is by scanning the focused ion beam in a rectangular pattern that extends over the slot. Material is built up from the edges, and grows inwards to span the slot and form a bridge. The direction of gas flow is an important consideration. When gas flow is perpendicular to the slot, the downstream side of the bridge will grow much faster than the upstream side. Gas flow parallel to the slot results in uniform growth from both sides. Platinum deposition was not found to be gas flow limited within the deposition parameters used. The horizontal growth as a function of time was measured, and found linear. The slope of this line is the horizontal growth rate, 23±0.05 nm/s. The x intercept of this line yields the minimum time for deposition to begin, which is 5.96±0.11 s. Sample bridges were deposited, and trimmed of excess material and redeposited platinum. These are presented as examples that uniformly wide bridges may be obtained at submicron dimensions, both in simple spanning and zig-zag configurations.