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Focused Ion Beams fromMagneto-Optical Trap Ion Sources

Brenton Knuffman, Adam V. Steele,and Jabez J. McClelland

M. Maazouz, G. Schwind, J. Orloff

7Li MOT

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Focused Ion Beam (FIB) Applications

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B. J. McMorran, et al. Science 331, 192 (2011)Helium Ion Microscope

EIPBN 2010 Micrograph Contest. www.eipbn.org

Imaging NanofabricationHairs on a bee’s wing

FIB milled grating for high orbital angular momentum electron beams

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Focused Ion Beam Applications

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Transmission Electron Microscopy Sample Prep

Gas Assisted Deposition and Etching

Removal of oxide from an integrated circuit by XeF2-assited FIB mill

Large scale material removal to create thin samples (lamellae)

C. Rue, R. Shepherd, R. Hallstein and R. Livengoodhttp://www.fei.com/uploadedFiles/Documents/Content/FIB-Applications-Circuit-Edit_wp.pdf

Wikimedia Commonshttp://en.wikipedia.org/wiki/File:Fib_tem_sample.jpg

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Source Technology

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• Expand variety of available ion species • Tailor choice to application• Enable new applications

• Enable performance at nanoscale• High brightness• Low emittance• Low energy spread

Established: Emerging:

Liquid Metal Ion SourceGas Field Ionization

SourceInductively Coupled

Plasma

Ga+ He+, Ne+ Ar+, Xe+

http://www.fei.com/products/components/ http://www.smt.zeiss.com/ Wikimedia Commons

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MOT

Ion beam

The Magneto-Optical Trap Ion Source (MOTIS)

(1) Laser-cool atoms to ~100 µK in Magneto-Optical Trap (MOT)• Expand ion species selection

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(3) Form ion beam• Enable nanoscale focusing

(2) Photoionize atoms in electric field• Maintain high performance

operation at low energy

chr cUr C

Ua D

D =

100 mVU eE zD = D »

Random transverse motion from thermal energyà Very small at T ~ 100 µK

Ionization volume

Er

zD

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Outline

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MOT

Describe how laser cooling works

Provide overview of MOTIS-based FIB platform

Present results from Cr and Li FIBs

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Spontaneous Light Force & Laser Cooling

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fluorescence

Atomlaser light

k®

1) Absorbed laser photonimparts momentum kick

2) Reradiated via spontaneous emission equally in all directions

p kD =h

0pD =

Spontaneous Light Force

Ground state

Excited state

Laser photon absorption

Spontaneousemission

Doppler shift

1) Laser tuned below resonance

2) Atoms moving toward laser scatter photons due to Doppler shift

• Doppler shift creates velocity-dependent force (cooling)

Laser Cooling

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Laser Cooling and Trapping

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B = 0 at center, increases in all directions, typical gradient ~10 – 50 G/cm

I

B

I

Magneto-Optical Trap (MOT)§ Optical molasses à (cooling)

§ Magnetic field à (trapping)

F vµ -F zµ -

Bose-Einstein Condensation

102

10-1

10-3

100

10-4

10-5

101

10-2

10-6

10-7

Room Temperature

Liquid Helium

MOT atoms, ~100 µK1997 Nobel Prize in Physics:Chu, Cohen-Tannoudji & Phillips

2001 Nobel Prize in Physics:Cornell, Ketterle, & Wieman

1913 Nobel Prize in Physics:Heike Kamerling Onnes

How cold is this?

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MOT

Zeeman Slower

MOT

Atomic Beam

Add Heat

Li OvenIonization

Laser

Ion Beam

MCP +Phosphor

Bulk Metal to Ion Beam

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MOTIS-based FIB Platform

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Two modes of ionization:

MOT

MOT

Transverse mode

Axial mode

• Smaller DU• Less current

• Larger DU• More current

Distributed acceleration reduces DU

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Chromium FIB Platform

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Chromium MOTIS Summary

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10 µm diameter microchannelplate pores à Cr ion microscope at 4 kV

• First successful MOTIS implementation• emittance measured: 6 x 10-7 mm mrad (MeV)1/2

• Small, cold MOT at 100 µK laser-cooled by 425 nm light

• Sub 100 nm resolution at 4 kV with 1 pA of beam current

• Upgrade FIB platform • Correct astigmatism• Test beam performance up to 30 kV

• Test applications• Directly deposit and remove chromium nanowires,

and high aspect ratio vias• Repair lithography masks without staining• Implant single ions

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Lithium FIB Platform

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• Light ion (7 amu)

• Low sputter rate,little collateral damage

• Microscopy applications

• Beam-assisted surface chemistry

• Easy to laser-cool

• l = 671 nm, compact diode laser and fiber optics

• FIB operates at low energy (up to 2 kV)

• Performance enabled by low energy spread

• 30 kV operation coming soon

Lithium (2 kV)

1410 µm diameter microchannel plate pores

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Lithium (2 kV)

Graphite – “Pencil Lead”1 µm

2 µm

5 µm

Comparison between SEM and Li FIB

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SEM: 1 kV, 10 nA Li MOTIS: 2 kV 10 pA

Sample: Si with unknown contamination

5 um 5 um

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Rise Distance Measurement (2 kV)

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Cleaved <100> Silicon Knife Edge

0.7 pA beam current

25/75 26.7 1.0nmd = ±

25/75 26.7 1.0nmd = ±

Measurement Results (@ 2 kV)

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Rise Distance VS Energy

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•Monotonically decreasing trend• Smaller spot size at

higher energy

•Deviation at very low energy• Heating from Coulomb

interactions in source1

1A. V. Steele, B. Knuffman, and J. J. McClelland, J. Appl. Phys. (impending publication).

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Lithium MOTIS Summary

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• MOTIS integrated with commercial FIB system

• Rise distance of 27 nm at 2 kV • Even better performance at 30 kV

• Upgrade FIB platform • Test beam performance up to 30 kV• Improve beam current throughput

• Test applications• Surface modification/damage study• Backscattered ion yield/energy analysis• Ion beam lithography• TEM sample polishing• Imaging/sectioning biological samples

7Li MOT

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Unique Capabilities

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• Wide selection of elements (isotopically pure)

• Extremely low energy spread

• Dual-species system(e.g., lithium & cesium)

• Long term stability

• Single ions on demand

• Relatively insensitive to source vibrations

• Virtual source massively demagnified ~2500x

Enormous number of potential applications to explore

S. B. Hill and J. J. McClelland, Appl. Phys. Lett. 82, 3128 (2003).