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

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


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



/bichro

€

F =Δp

Δt=

2hk

2π /δbichro

>>hkγ

2

Force Profile

Ordinary Optical Molasses

{

δ/k k/k



The Bichromatic Force for Collimation:



Zero velocityis shifted

Collimation Sequence



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.



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.



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


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.


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



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.



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.


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




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




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.



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



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




Position Distribution Velocity Distribution

Coordinate System:

Focusing and Channeling





Standard Selective Etch

Supporting Material

~7minStandard Gold Etchant:

1M KOH0.1M K2S2O3

0.01M K3Fe(CN)6 0.001M K4Fe(CN)6 3H20





Metal Oxidation

Or

Reduction of Oxidizer

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

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