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Journal of Applied Physics

Atom lithography with metastable helium

Claire Allred, Jason Reeves, Christopher Corder, Harold J. Metcalf

Published Feb 1, 2010• 20 citations
Publisher

Abstract

A bright metastable helium (He∗) beam is collimated sequentially with the bichromatic force and three optical molasses velocity compression stages. Each He∗ atom in the beam has 20 eV of internal energy that can destroy a molecular resist assembled on a gold coated silicon wafer. Patterns in the resist are imprinted onto the gold layer with a standard selective etch. Patterning of the wafer with the He∗ was demonstrated with two methods. First, a mesh was used to protect parts of the wafer making an array of grid lines. Second, a standing wave of λ=1083 nm light was used to channel and focus the He∗ atoms into lines separated by λ/2. The patterns were measured with an atomic force microscope establishing an edge resolution of 80 nm. Our results are reliable and repeatable.

DOI
10.1063/1.3295903

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•May 1, 1997•4 citations
A bright metastable helium (He∗) beam is collimated sequentially with the bichromatic force and three optical molasses velocity compression stages. Each He∗ atom in the beam has 20 eV of internal energy that can destroy a molecular resist assembled on a gold coated silicon wafer. Patterns in the resist are imprinted onto the gold layer with a standard selective etch. Patterning of the wafer with the He∗ was demonstrated with two methods. First, a mesh was used to protect parts of the wafer making an array of grid lines. Second, a standing wave of λ=1083 nm light was used to channel and focus the He∗ atoms into lines separated by λ/2. The patterns were measured with an atomic force microscope establishing an edge resolution of 80 nm. Our results are reliable and repeatable.
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
Mask based pattern generation with photolithography is a crucial step in microchip production. The next generation Extreme Ultra Violet (EUV) lithography instruments with a wavelength of 13.5 nm is currently under development. In principle this allows patterning down to a few nm resolution in a single exposure. However, there are many technical challenges due to the very high energy of the photons among others. Lithography with metastable atoms has been suggested as a cost effective, less complex alternative to EUV lithography. The big advantage of atom lithography is that the kinetic energy of an atom is much smaller than that of a photon for a given wavelength. However up till now no method has been available for making masks for atom lithography that can produce arbitrary, high resolution patterns. Here we present a solution to this problem. We show that it is possible to make masks based on binary holography that can generate arbitrary patterns down to a few nm resolution using a state of the art metastable helium source. We compare the flux of this source with that of an established EUV source (ASML, NXE:3100) and show that patterns can potentially be produced at comparable speeds. Finally we present an extension of the grid based holography method for a grid of hexagonally shaped subcells. Our method can be used with any wave beam, including other matter wave beams such as helium ions or electrons.
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.
Intense, highly collimated sources of atoms have many potential applications. Bright beams will be important for competitive high flux and high resolution direct-write techniques in lithography, with the added advantage of parallel writing through laser manipulation. Intense sources will also be useful in other atom optic devices e.g. for loading atoms into hollow fiber waveguides. In atomic physics, many collision processes can only be measured with the sensitivity offered by such high flux sources. We report progress on the development of an intense, collimated beam of metastable helium atoms which improves the brightness generated by conventional nozzle discharge sources by several orders of magnitude. The system uses diode lasers to transversely collimate and then to longitudinally slow the atoms, using Zeeman tuning to compensate for the changing Doppler shift. The slowed, collimated beam is then compressed in a 2D magneto-optic trap before a final collimation stage, to achieve the required increase in intensity. Initial experiments using the helium source for some of the applications above are described.
Intense, highly collimated sources of atoms have many potential applications. Bright beams will be important for competitive high flux and high resolution direct-write techniques in lithography, with the added advantage of parallel writing through laser manipulation. Intense sources will also be useful in other atom optic devices e.g. for loading atoms into hollow fiber waveguides. In atomic physics, many collision processes can only be measured with the sensitivity offered by such high flux sources. We report progress on the development of an intense, collimated beam of metastable helium atoms which improves the brightness generated by conventional nozzle discharge sources by several orders of magnitude. The system uses diode lasers to transversely collimate and then to longitudinally slow the atoms, using Zeeman tuning to compensate for the changing Doppler shift. The slowed, collimated beam is then compressed in a 2D magneto-optic trap before a final collimation stage, to achieve the required increase in intensity. Initial experiments using the helium source for some of the applications above are described.