Neutral Atom Lithography Using the 389 nm Transition in Metastable Helium

Abstract

Resist based neutral atom lithography with metastable 2<super>3</super>S₁ helium(He. ) has been used to produce small structures in both goldand palladium. A beam of He. from a reverse flow, DC dischargesource is collimated by the bichromatic optical force followed bythree optical molasses velocity compression stages. The He. beamthen crosses a region where a mechanical or optical mask createsthe desired pattern.In the first experiments, a self assembled monolayer (SAM) ofnonanethiol is grown on a gold coated silicon wafer and then exposedto the bright, collimated beam of He. after a Nickel micromesh mask. The mesh protects parts of the SAM from the 20 eVof internal energy deposited by the impact of a He. atom onto thesample surface. The pattern of the mesh is transferred into the~200 &Aring gold layer with a standard selective etch that removes theGold from under the damaged SAM molecules. The samples wereanalyzed with an Atomic Force Microscope and Scanning ElectronMicroscope.The second set of experiments were performed using an opticalmask to pattern a gold coated silicon wafer. In the optical mask,the He. atoms experience the dipole force while traversing a standinglight wave of &lambda<sub>1<\sub> = 1083 nm light tuned 490 MHz above the 2<super>3</super>S<sub>1<\sub>&#8594; 2<super>3</super>P₂ transition or, in separate experiments, &lambda<sub>2<\sub> = 389 nmlight tuned 80 MHz below the 2<super>3</super>S<sub>1<\sub>&#8594; 3<super>3</super>P₂ transition. Using theoptical masks, He. can be focused or channeled into parallel linesseparated by &lambda/2 by varying the intensity of the standing lightwave.The small structures created by the 389 nm optical mask beganapproaching the gold surface granularity. A ~200 &Aring layer of palladium on silicon was chosen as the palladium granularity is muchsmaller and the SAM of nonanethiol will still form on the surface.Experiments using a nickel micro mesh as a mechanical mask havedemonstrated similar features to those in gold.