Electron Beam Lithogr

Electron Beam LithographyWilliam Whelan-Curtin

Nanometre precision1What is EBL for?NanopatterningHigh Precision ReliableVersatile

LPN2What is needed?Very narrow, precisely controllable beam of Electrons

Lots of money, a big complex machine, and a lot of expertise!

3OutlineSystem descriptionExposureExamples4Electron PencilSystem Schematic

5Electron sourceTungsten wire (Thermionic)2300CEnergy Spread 2-3eVSource size 25um

Thermal (Schottky) field emitter1800CEnergy Spread 0.9eVSource size 20nm

Cold Field Emitter20CEnergy Spread 0.22eVSource size 5nm

Unstable current10-20%=> SEMHigher Current density=>EBLI-VSuppressorTipTungsten+VExtractor(Anode)/Zirconium Oxide(High Vacuum ~10e-9mB)

6Electron LensesChromatic dispersionMonochromatic beamAberrationsUse centre of lensElectro-magneticElectro-staticF = q (E + v B)

Electrostatic lenses have worse aberrations but are faster7Electron LensesMagnetic versus Electrostatic

Faster deflectionWorse AberrationsEM lenses Simpler to implement8Beam BlankerTurns the beam/off quicklyControl current for each pixelHigh speedElectrostatic

+VExtractor+V9

ColumnSourceAperturesBlankingCollimationStigmation controlDeflectorsFocusFinal Lens

(VISTEC VB6)10Types of Electron Beam WriterSEM (Electron Beam Reader)Generally magnetic lensesUp to 30kV

Converted SEM (RAITH)Addition of (fast) beam blankerPattern generatorDeflection needs time to stablise

raster scan

vector scan

IMAGE OF ZEISS11Types of Electron Beam WriterPurpose Built (LEICA/JEOL)Better control, calibrationUp to 100kVHigher speedsBigger writefieldsSecondary deflection system

Can also correct for aberrations in the primary lens

12Types of Electron Beam WriterShaped Beam systemsVery complex opticsHigher current, lower resolutionPhotomask making(Not a research tool)

GAUSSIAN

I will return to some of the previous points later13PatterningElectron sensitive polymer- the paper

LPN14Resist OverviewPositiveNegative15Electron-Solid InteractionsForward scatteringOftenSmall anglesHigh energy (~95% pass through the resist)Primary electrons16Electron-Solid InteractionsBack scatteringRareLarge anglesStill High energyPrimary electrons17Electron-Solid InteractionsSecondary electronsLow energy (50eV)Low penetrating powerPrimary electronsResponsible for exposure18Electron Sensitive resistPoly methyl methyl acrylateSpin CoatingLong polymer chainsCCCCCCOOHHHHHHHHHHHHHHHHCCCCOOnCHHHHHHHHCCCHHCCCCCCCCCCCCCCCCCOOHHHHHHHOOOHHHHHHHOOHHHHHHHOHHHCHSubstratePMMA19Electron Sensitive resistBonds broken (induced chain scission)Dissolved by suitable chemicalMethyl Isobutyl ketoneIsopropanol:WaterCCCCCCOOHHHHHHHHHHHHHHHHCCCCOOnCHHHHHHHHCCCHHCCCCCCCCCCCCCCCCCOOHHHHHHHOOOHHHHHHHOOHHHHHHHOHHHCHSecondary ElectronsSubstratePMMA20Contrast CurveThreshold electron density (lowerfaster exposure)Clearing Dose or Base DoseSlope -> ResolutionResist ThicknessDose (electrons/area)Function of:VoltageResist ThicknessSubstrate

e.g. 170uAs/cm2 for 500nm thick PMMA on Silicon @ 30kV21Contrast Curve- ExperimentalDetermine for each new situationRecheck regularlyProblem diagnosis

50umDose

22Negative ResistMicrochem SU8Photo acid generatorBakingCrosslinkingAcid DiffusionCCCCCCOOHHHHHHHHHHHHHHHHCCCCOOnCHHHHHHHHCCCHHCCCCCCCCCCCCCCCCCOOHHHHHHOOOHHHHHHHOOHHHHHHHOHHHCHSecondary ElectronsSubstrateSU823Contrast CurveThreshold electron densityChemical amplificationLower clearing doseResist ThicknessDose (electrons/area)e.g. 1uAs/cm2 for 200nm thick SU8 on Silicon @ 30kVHas its uses24ResolutionMost EBL systems -> 1nm spot sizes or less

df = effective beam diameter (nm)Rt = resist thickness (nm)Vb = acceleration voltage (kV)1nmdfRtVb

J. Vac. Sci. Technol. B 12, 1305 (1975)And indeed if you look at manufacturers website25Resolution

EBL advice: Keep resist as thin as possible26EBL vs Focused Ion Beam etchingEBLModificationNo removal of materialFIBEnergetic Gallium ionsEtching of the material

SubstrateHeavy ions27Electron-Solid InteractionsSecondary electronsLow energy (50eV)Low penetrating powerPrimary electronsUnintended Exposure!28Proximity ErrorsStray electronsBias

tdose29CorrectionShape CorrectionDifficult to generalise30CorrectionDose ModulationCalculate the electron distributionReduce in certain areas

1231Electron Distribution

Forward Scattering-

Back Scattering- 32ParametersDepend on voltage/resist/substrateDetermine in each instanceMonte Carlo simulationsExperiment

Beam energy (keV) (um) (um)51.330.180.74100.390.600.74200.122.00.74500.0249.50.741000.00731.20.74L. Stevens et al., Microelectronic Engineering 5, 141-150 (1986)33Correction ProgramsNanopecs, ProxeccoPatternFractureCalculate electron distributionsAlter patternRecalculateIterate until convergence is reached34GuidelinesPEC Computationally intensive