High-brightness source of energetic He atoms

Abstract

In atom beam lithography, a beam of kilo-electron-volt helium atoms illuminates a stencil mask and transmitted beamlets transfer the mask pattern to resist on a substrate. It shares the advantages of masked ion beam lithography but is immune to charging artifacts that limit resolution and pattern fidelity. This paper describes a high-brightness source of energetic He atoms, where He+ ions are extracted from a multicusp ion source, focused by two-stage accelerating optics, and neutralized by charge-transfer scattering in a differentially pumped, He-filled cell. Since scattering angles are extremely small, the straight line trajectories of scattered atoms are essentially tangent to the (possibly curved) trajectories of the parent ions. Space-charge repulsion prevents the ion beam crossing over; instead, it converges to a waist of minimum cross section before diverging further downstream. Atom trajectories produced by a cell placed in the region of intense space charge near the waist are strongly affected by the curvature of ion trajectories within the cell. The flaring of the ion beam due to space charge can be used to increase the width of the atom beam, although to the detriment of resolution. In this paper, the authors study a configuration where the cell is placed in the converging ion beam as far as practicable from the ion-beam waist. The atom beam then converges to a crossover, which becomes the virtual source seen by the mask. The source diameter and angular flux density initially increase with increasing cell pressure but saturate at higher pressures; the respective saturation values at 50 keV are 125 μm (2σ) and 8.7 × 1017 particles/s sr. Under these conditions, the beam diameter is ∼2.5 cm, 7 m from the source. A practical system for subnanometer printing is discussed with 0.2 nm (2σ) penumbral blur and 1.25 × 1013 particles/s cm2 flux density over a 1 cm circular field.