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Neutral atom lithography with metastable helium

Claire Allred

Published May 1, 2009• 1 citation
OpenAlex

Abstract

Stony Brook University Libraries.\nSBU Graduate School in Physics and Astronomy.\nLawrence Martin (Dean of Graduate School), Harold Metcalf – Dissertation Advisor\nProfessor, Department of Physics and Astronomy, Gorge Sterman – Chairperson of Defense\nProfessor, Department of Physics and Astronomy, Clark McGrew\nProfessor, Department of Physics and Astronomy, Jon Longtin\nProfessor, Department of Mechanical Engineering\nSUNY Stony Brook.

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Resist based neutral atom lithography with metastable 23S1 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 Å 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 &lambda1<\sub> = 1083 nm light tuned 490 MHz above the 23S1<\sub>→ 23P2 transition or, in separate experiments, &lambda2<\sub> = 389 nmlight tuned 80 MHz below the 23S1<\sub>→ 33P2 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 Å 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.
Optical forces using lasers allow precise control over the motion of atoms. The bichromatic optical force (BF) is unique in its large magnitude and velocity range, arising from the absorption and stimulated emission processes. These properties were used to transversely collimate a beam of metastable helium (He∗ ) using the 2^{3}S − 2^{3}P transition. The collimation created a very bright beam of He∗ , allowing experiments of neutral atom lithography. The He∗ beam was used to pattern material surfaces using a resist-based lithography technique, where the pattern was determined by either mechanical or optical masks. The optical masks produced features with a separation of half the wavelength of the light used. Patterning was successfully demonstrated with both IR and UV optical masks. The etched pattern resolution was ∼ 100 nm and limited by the material surface. Further experiments were performed studying the ability of the bichromatic force to cool. The finite velocity range of the BF allows estimation of a characteristic cooling time which is independent of the excited state lifetime, only depending on the atomic mass and optical transition energy. Past experiments, including the helium collimation used for neutral atom lithography, have demonstrated that the BF can collimate and longitudinally slow atomic beams, but required long interaction times that included many spontaneous emission (SE) events. The effect of SE can be mitigated, and is predicted to not be necessary for BF cooling. Since the BF cooling time is not related to the excited state lifetime, a transition can be chosen such that the cooling time is shorter than the SE cycle time, allowing direct laser cooling on atoms and molecules that do not have cycling transitions. Experiments using the helium 2^{3}S− 3^{3}P transition were chosen because the BF cooling time (285 ns) is on the order of the average SE cycle time (260 ns). Numerical simulations of the experimental system were run predicting compression of the atomic velocity distribution. Our experimental results demonstrate the stimulated nature of the force through many atomic recoils from the light in less than one SE cycle time. A large range of the atomic velocity distribution is accelerated and accumulates at the velocity limit of the force. This accumulation results in an increase in the velocity space density, demonstrating cooling. | 112 pages
A high flux metastable Neon atomic beam was designed and characterised. Atom optical enhancement of the beam using a two-dimensional optical collimator, novel two beam Zeeman slower and hexapole magnetic guide was performed to produce a UV-free metastable flux of (4.35 x 109 ± 4 x 107) atoms s−1. Investigations of several resists for neutral atom lithography were undertaken. A quantitative investigation of the wetting properties of ethanethiol (ET) and dodecanethiol (DDT) self-assembled monolayers (SAM's) exposed to various metastable dosages was carried out. A mechanism for the poor lithographic patterning using ET was proposed and the negative contrast patterning observed for this SAM was similar to those observed for bare gold substrates and were attributed to mechanical pump oil (MPO) contamination resists. Negative patterned resists were used to produce 7.5 μm iron dots on a silicon substrate via neutral atom lithography. This scheme was found to be very robust and free from the laser cooling issues of alternative direct depositional schemes. Numerical simulations have shown that two dimensional arrays of magnetic nanodots may be produced this way, paving the way for a magnetic storage option with a density of " 0.1 Gbit mm−2.