What is EBL for? Nanopatterning High Precision Reliable
Versatile LPN
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What is needed? Very narrow, precisely controllable beam of
Electrons Lots of money, a big complex machine, and a lot of
expertise!
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Outline System description Exposure Examples
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Electron Pencil System Schematic
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Electron source Tungsten wire (Thermionic) 2300C Energy Spread
2-3eV Source size 25um Thermal (Schottky) field emitter 1800C
Energy Spread 0.9eV Source size 20nm Cold Field Emitter 20C Energy
Spread 0.22eV Source size 5nm Unstable current 10-20% => SEM
Higher Current density =>EBL I -V Suppressor Tip Tungsten +V
Extractor (Anode) /Zirconium Oxide (High Vacuum ~10e-9mB)
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Electron Lenses Chromatic dispersion Monochromatic beam
Aberrations Use centre of lens Electro-magnetic Electro-static F =
q (E + v B)
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Electron Lenses Magnetic versus Electrostatic Faster deflection
Worse Aberrations EM lenses Simpler to implement
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Beam Blanker Turns the beam/off quickly Control current for
each pixel High speed Electrostatic +V Extractor +V
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Column Source Apertures Blanking Collimation Stigmation control
Deflectors Focus Final Lens (VISTEC VB6)
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Types of Electron Beam Writer SEM (Electron Beam Reader)
Generally magnetic lenses Up to 30kV Converted SEM (RAITH) Addition
of (fast) beam blanker Pattern generator Deflection needs time to
stablise raster scan vector scan
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Types of Electron Beam Writer Purpose Built (LEICA/JEOL) Better
control, calibration Up to 100kV Higher speeds Bigger writefields
Secondary deflection system Can also correct for aberrations in the
primary lens
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Types of Electron Beam Writer Shaped Beam systems Very complex
optics Higher current, lower resolution Photomask making (Not a
research tool) GAUSSIAN
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Patterning Electron sensitive polymer- the paper LPN
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Resist Overview PositiveNegative
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Electron-Solid Interactions Forward scattering Often Small
angles High energy (~95% pass through the resist) Primary
electrons
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Electron-Solid Interactions Back scattering Rare Large angles
Still High energy Primary electrons
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Electron-Solid Interactions Secondary electrons Low energy
(50eV) Low penetrating power Primary electrons Responsible for
exposure
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Electron Sensitive resist Poly methyl methyl acrylate Spin
Coating Long polymer chains C C C C C C O O H H H H H HH H H H H H
H H H H C C C C O O n C H H H H H H H H C C C H H CCC C C C CC C C
C C C C C C C O O H H H H H H H O O O H H H H H H H O O H H H H H H
H O H H H C H Substrate PMMA
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Electron Sensitive resist Bonds broken (induced chain scission)
Dissolved by suitable chemical Methyl Isobutyl ketone
Isopropanol:Water C C C C C C O O H H H H H HH H H H H H H H H H C
C C C O O n C H H H H H H H H C C C H H C CC C C C CC C C C C C C C
C C O O H H H H H H H O O O H H H H H H H O O H H H H H H H O H H H
C H Secondary Electrons Substrate PMMA
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Contrast Curve Threshold electron density (lowerfaster
exposure) Clearing Dose or Base Dose Slope -> Resolution Resist
Thickness Dose (electrons/area) Function of: Voltage Resist
Thickness Substrate e.g. 170uAs/cm2 for 500nm thick PMMA on Silicon
@ 30kV
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Contrast Curve- Experimental Determine for each new situation
Recheck regularly Problem diagnosis 50um Dose
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Negative Resist Microchem SU8 Photo acid generator Baking
Crosslinking Acid Diffusion C C C C C C O O H H H H H H H H H H H H
H H H H C C C C O O n C H H H H H H H H C C C H H C CC C C C CC C C
C C C C C C C O O H H H H H H O O O H H H H H H H O O H H H H H H H
O H H H C H Secondary Electrons Substrate SU8
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Contrast Curve Threshold electron density Chemical
amplification Lower clearing dose Resist Thickness Dose
(electrons/area) e.g. 1uAs/cm2 for 200nm thick SU8 on Silicon @
30kV
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Resolution Most EBL systems -> 1nm spot sizes or less df =
effective beam diameter (nm) Rt = resist thickness (nm) Vb =
acceleration voltage (kV) 1nm dfdf RtRt VbVb J. Vac. Sci. Technol.
B 12, 1305 (1975)
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Resolution EBL advice: Keep resist as thin as possible
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EBL vs Focused Ion Beam etching EBL Modification No removal of
material FIB Energetic Gallium ions Etching of the material
Substrate Heavy ions
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Electron-Solid Interactions Secondary electrons Low energy
(50eV) Low penetrating power Primary electrons Unintended
Exposure!
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Proximity Errors Stray electrons Bias t dose
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Correction Shape Correction Difficult to generalise
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Correction Dose Modulation Calculate the electron distribution
Reduce in certain areas 1 2
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Electron Distribution Forward Scattering- Back Scattering-
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Parameters Depend on voltage/resist/substrate Determine in each
instance Monte Carlo simulations Experiment Beam energy (keV) (um)
(um) 51.330.180.74 100.390.600.74 200.122.00.74 500.0249.50.74
1000.00731.20.74 L. Stevens et al., Microelectronic Engineering 5,
141-150 (1986)
Slide 34
Correction Programs Nanopecs, Proxecco Pattern Fracture
Calculate electron distributions Alter pattern Recalculate Iterate
until convergence is reached
