Nano-Lithography with Metastable Helium Claire Allred, Jason Reeves, Christopher Corder, Harold Metcalf IQEC: June 4, 2009 Supported by: ONR and Dept. of Education

Atomic Nanofabrication Direct Deposition. –Done with Na at Bell Labs in 1992 –Since demonstrated with Cr. Resist Assisted. –Patterned atoms interact with a resist changing its wetting properties. –Substrate is processed in a second step producing permanent patterns. –Done with noble gases, alkali & alkaline metals and group III elements. Review article: Meschede et.al. J. Phys. D. 36(2003) R17. Nanometer scale patterning of neutral atoms and subsequent pattern preservation on a surface.

Some Motivation Massive parallel fabrication of nanostructures. Spectroscopically determined accuracy. Detailed study of the limits of optical forces. Applications to meta-material fabrication and photonic crystals.

Brief Outline Experimental System: atomic beam and lithography process. Numerical Simulations: Trajectory calculations. Experimental Results: geometrical mask and light mask. Experimental System Numerical Simulations Experimental Results

Metastable Helium 20eV of internal energy. Doubly disallowed decay gives a lifetime of 8000s. Specific transition information: – nm –  = 98ns Experimental System Numerical Simulations Experimental Results Atom Bichromatic Force Atomic Beam Lithography Process

The Bichromatic Force Two frequencies give an amplitude modulated carrier wave. –Carrier frequency (  1 +  2 )/2 at atomic resonance. –Envelope frequency (  1 -  2 )/2=  bichro. Amplitude modulation satisfies pi pulse condition:  =  bichro /4 Absorption from left gives  p = +  k Stimulated emission from the right gives  p = +  k Experimental System Numerical Simulations Experimental Results Atom Bichromatic Force Atomic Beam Lithography Process  /  bichro

Force Profile Ordinary Optical Molasses { δ/k k/k/ kk Experimental System Numerical Simulations Experimental Results Atom Bichromatic Force Atomic Beam Lithography Process

The Bichromatic Force for Collimation: Experimental System Numerical Simulations Experimental Results Atom Bichromatic Force Atomic Beam Lithography Process Zero velocity is shifted

Collimation Sequence Experimental System Numerical Simulations Experimental Results Atom Bichromatic Force Atomic Beam Lithography Process Source -kv±  +kv±  -kv±  +kv±  -kv±  2.3 mrad 1.1 mrad  =-5   =-  v l =1125±220 m/s

Sample Preparation Si wafers from Montco Silicon Technologies. 5 Å Cr adhesion layer and 250Å Au layer evaporated by Scientific Coatings. Diced in the Lukens Laboratory. Clean wafers: –Acetone. –Ethanol –Pirahna Assemble resist: nonanethiol. Experimental System Numerical Simulations Experimental Results Atom Bichromatic Force Atomic Beam Lithography Process 1-nonanethiol

Neutral Atom Lithography On impact He* deposits energy and becomes g.s. of He. Part of thiol C chain is broken and an extra electron weakens neighboring chain. Wet chemical etch removes Au where thiols are damaged. Samples are examined using an Atomic Force Microscope and a Scanning Electron Microscope. Experimental System Numerical Simulations Experimental Results Atom Bichromatic Force Atomic Beam Lithography Process He* He ~7min Standard Gold Etchant: 1M KOH 0.1M K2S2O3 0.01M K3Fe(CN) M K4Fe(CN)6 3H20

Focusing vs. Channeling Trajectories Experimental System Numerical Simulations Experimental Results Trajectory Calculations Monte Carlo Calculations McClelland, JOSA B 12 (1995)1761 Focusing RegimeChanneling Regime Coordinate System

Geometrical Mask Use micromesh (2000 lines per inch) to pattern resist. Peak dosage of 3 * atoms / mm 2 and 7min etch time gives sharp features. Edge resolution of 80nm. Experimental System Numerical Simulations Experimental Results Geometrical Mask Channeling Light Mask Focusing Light Mask Conclusions

Channeling Regime: P=4P 0 Line spacing of 499±3nm on AFM. Line Spacing of 566±14nm on SEM. Line widths 100nm.  =+490MHz=+300  dosage = 1.5*10 12 atoms/mm 2 Experimental System Numerical Simulations Experimental Results Geometrical Mask Channeling Light Mask Focusing Light Mask Conclusions

Focusing Regime: P=P 0 Transverse velocity distribution in the numerical simulations is probably incorrect. Very faint lines, only visible in SEM or the FFT of AFM scans. Experimental System Numerical Simulations Experimental Results Geometrical Mask Channeling Light Mask Focusing Light Mask Conclusions

Results Performed numerical simulations. Measured the edge resolution to be ~80nm. This is limited by wafer processing techniques. Demonstrated patterning in the focusing and channeling regime. Experimental System Numerical Simulations Experimental Results Channeling Light Mask Focusing Light Mask Velocity Offset Conclusions

Cool wafer pictures Supporting Material

The Radiative Force |e> |g> |e> |g> |e> |g> |e> |g> Atoms and Light Experimental System Numerical Simulations Experimental Results Monochromatic Forces Polychromatic Forces Atoms

Optical Molasses Force [  k  / 2 ] Atoms and Light Experimental System Numerical Simulations Experimental Results Monochromatic Forces Polychromatic Forces Atoms

The Light Shift and Dipole Force Light adds an off diagonal perturbation to the Hamiltonian that describes the atom. –Energies are shifted: –New eigenstates are mixtures of pure states. In a standing wave light field, the intensity of the light changes over half a wavelength. –Spatial modulation of the separation of the energy levels results in a force. –Red detuned light attracts atoms. –Blue detuned light repels atoms. Atoms and Light Experimental System Numerical Simulations Experimental Results Monochromatic Forces Polychromatic Forces Atoms

Laser System Whole system is seeded with an extended cavity DBR diode laser. Laser is kept on resonance with saturation spectroscopy. Double passed AOM produces four frequencies in two beams. –Shift the center velocity. –Phase delay between counter propagating pi pulses. Two AOM's produce light for three optical molasses stages. Atoms and Light Experimental System Numerical Simulations Experimental Results Laser System Vacuum System Atomic Beam Lithography Process  +kv+   +kv-   -kv+   -kv-  To Vacuum System 60 MHz Fiber Amplifiers PZT Diode 82 MHz 89 MHz 90 MHz To Lock  -kv+  m1  -kv-  m1  -kv+  m2  -kv-  m2

SAS Supporting Material

Vacuum System Atoms and Light Experimental System Numerical Simulations Experimental Results Laser System Vacuum System Atomic Beam Lithography Process

OBE's Supporting Material

Initial Population Atoms and Light Experimental System Numerical Simulations Experimental Results Trajectory Calculations Monte Carlo Calculations Position DistributionVelocity Distribution Coordinate System:

Focusing and Channeling Experimental System Numerical Simulations Experimental Results Trajectory Calculations Monte Carlo Calculations

Standard Selective Etch Supporting Material ~7min Standard Gold Etchant: 1M KOH 0.1M K2S2O3 0.01M K3Fe(CN) M K4Fe(CN)6 3H20 Metal Oxidation Or Reduction of Oxidizer