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Nano-Lithographywith 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.
RESISTSUBSTRATEMASKWAFERATOMS
SUBSTRATEMASKATOMS
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 SystemNumerical SimulationsExperimental Results
Metastable Helium
• 20eV of internal energy.• Doubly disallowed
decay gives a lifetime of 8000s.
• Specific transition information:– 1083.33nm = 98ns
Experimental SystemNumerical SimulationsExperimental Results
AtomBichromatic ForceAtomic BeamLithography 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 fromleft gives p = + k
Stimulated emission fromthe right gives p = + k
Experimental SystemNumerical SimulationsExperimental Results
AtomBichromatic ForceAtomic BeamLithography Process
/bichro
€
F =Δp
Δt=
2hk
2π /δbichro
>>hkγ
2
Force Profile
Ordinary Optical Molasses
{
δ/k k/k
Experimental SystemNumerical SimulationsExperimental Results
AtomBichromatic ForceAtomic BeamLithography Process
The Bichromatic Force for Collimation:
Experimental SystemNumerical SimulationsExperimental Results
AtomBichromatic ForceAtomic BeamLithography Process
Zero velocityis shifted
Collimation Sequence
Experimental SystemNumerical SimulationsExperimental Results
AtomBichromatic ForceAtomic BeamLithography Process
Source
-kv± +kv± -kv±+kv±
+kv±+kv±-kv±
-kv±
2.3 mrad
1.1 mrad
=-5=-
=-
vl=1125±220 m/s
Sample Preparation • Si <100> 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 SystemNumerical SimulationsExperimental Results
AtomBichromatic ForceAtomic BeamLithography 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 SystemNumerical SimulationsExperimental Results
AtomBichromatic ForceAtomic BeamLithography Process
He* He*He*
He* He*
He HeHe
HeHe
~7minStandard Gold Etchant:
1M KOH0.1M K2S2O3
0.01M K3Fe(CN)6 0.001M K4Fe(CN)6 3H20
Focusing vs. Channeling Trajectories
Experimental SystemNumerical SimulationsExperimental Results
Trajectory CalculationsMonte Carlo Calculations
€
P0 =πwoxwoz
8I focus
McClelland, JOSA B 12 (1995)1761
Focusing Regime Channeling Regime
€
I focus = amHe*vz
2Isat (γ2 + 4δLM
2 )2
hδLM k 2woz2 γ 2
€
a = 5.37
Coordinate System
Geometrical Mask• Use micromesh (2000
lines per inch) to pattern resist.
• Peak dosage of 3 * 1012 atoms/mm2 and 7min etch time gives sharp features.
• Edge resolution of 80nm.
Experimental SystemNumerical SimulationsExperimental Results
Geometrical MaskChanneling Light MaskFocusing Light MaskConclusions
Channeling Regime: P=4P0
• Line spacing of 499±3nm on AFM.• Line Spacing of 566±14nm on SEM. • Line widths 100nm.
=+490MHz=+300dosage = 1.5*1012 atoms/mm2
Experimental SystemNumerical SimulationsExperimental Results
Geometrical MaskChanneling Light MaskFocusing Light MaskConclusions
Focusing Regime: P=P0
• Transverse velocity distribution in the numerical simulations is probably incorrect.
• Very faint lines, only visible in SEM or the FFT of AFM scans.
Experimental SystemNumerical SimulationsExperimental Results
Geometrical MaskChanneling Light MaskFocusing Light MaskConclusions
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 SystemNumerical SimulationsExperimental Results
Channeling Light MaskFocusing Light MaskVelocity OffsetConclusions
Cool wafer pictures
Supporting Material
The Radiative Force
|e>
|g>
|e>
|g>
|e>
|g>
€
hk
€
hk
|e>
|g>
€
hk
€
hk
€
hk
€
hk
€
hk
€
Fsp = hr k γ p
Atoms and LightExperimental System
Numerical SimulationsExperimental Results
Monochromatic ForcesPolychromatic ForcesAtoms
€
Fsp = hr k γ p =
hr k γ
2
so
1+ so + [2(δ −r k •
r v ) /γ ]2
€
Fsp,max =hkγ
2
Optical Molasses
-4 -2 0 2 4-1.0
-0.5
0.0
0.5
1.0
velocity [/k]
Force
[hk/
2]
€
rF OM ≡
8hk 2δso
r v
γ(1+ so + (2γ /δ)2)2≡ −β
r v
Atoms and LightExperimental System
Numerical SimulationsExperimental Results
Monochromatic ForcesPolychromatic ForcesAtoms
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.
€
Eg =hΩ2
4δ
Atoms and LightExperimental System
Numerical SimulationsExperimental Results
Monochromatic ForcesPolychromatic ForcesAtoms
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 LightExperimental System
Numerical SimulationsExperimental Results
Laser SystemVacuum SystemAtomic BeamLithography Process
+kv++kv-
-kv+-kv-
To Vacuum System
60 MHz
FiberAmplifiers
PZT
Diode
82MHz
89MHz
90MHz
To Lock
-kv+m1
-kv-m1
-kv+m2
-kv-m2
SAS
Supporting Material
Vacuum System
Atoms and LightExperimental System
Numerical SimulationsExperimental Results
Laser SystemVacuum SystemAtomic BeamLithography Process
He inHe outTPH 330TPH 330to Sorption pumpsLithography Region
1st SSD2nd SSDand samplemount
71 cm25 cm33.5 cm68 cmDetectionRegion
CollimationRegion
MCP Phosphor ScreenFeedthrough
BeamDefining
Slits
Source
OBE's
Supporting Material
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
Initial Population
Atoms and LightExperimental System
Numerical SimulationsExperimental Results
Trajectory CalculationsMonte Carlo Calculations
Position Distribution Velocity Distribution
Coordinate System:
Focusing and Channeling
Experimental SystemNumerical SimulationsExperimental Results
Trajectory CalculationsMonte Carlo Calculations
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
Standard Selective Etch
Supporting Material
~7minStandard Gold Etchant:
1M KOH0.1M K2S2O3
0.01M K3Fe(CN)6 0.001M K4Fe(CN)6 3H20
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
Metal Oxidation
Or
Reduction of Oxidizer