Electron Beam Lithography William Whelan-Curtin Nanometre precision

What is EBL for? LPN Nanopatterning High Precision Reliable Versatile

What is needed? Very narrow, precisely controllable beam of Electrons Lots of money, a big complex machine, and a lot of expertise!

Outline System description Exposure Examples

Electron “Pencil” System Schematic

Electron source Tungsten wire (Thermionic) /Zirconium Oxide Tip 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 -V Suppressor Higher Current density =>EBL +V Extractor (Anode) Unstable current 10-20% => SEM (High Vacuum ~10e-9mB)

Electron Lenses Chromatic dispersion Aberrations Monochromatic beam Use centre of lens F = q · (E + v  B) Electro-magnetic Electro-static Electrostatic lenses have worse aberrations but are faster

Electron Lenses Magnetic versus Electrostatic Faster deflection Worse Aberrations EM lenses Simpler to implement

Beam Blanker Turns the beam/off quickly Control current for each pixel High speed Electrostatic Extractor +V +V

Column Source Apertures Blanking Collimation Stigmation control Deflectors Focus Final Lens (VISTEC VB6)

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 IMAGE OF ZEISS

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

Types of Electron Beam Writer Shaped Beam systems Very complex optics Higher current, lower resolution Photomask making (Not a research tool) I will return to some of the previous points later GAUSSIAN

Patterning LPN Electron sensitive polymer- the “paper”

Resist Overview “Positive” “Negative”

Electron-Solid Interactions Primary electrons Forward scattering Often Small angles High energy (~95% pass through the resist)

Electron-Solid Interactions Primary electrons Back scattering Rare Large angles Still High energy

Electron-Solid Interactions Primary electrons Secondary electrons Low energy (50eV) Low penetrating power Responsible for exposure

Electron Sensitive resist H H n C H O C C H H H H H H H C C C C C H H C O C O H H O C O C H H H H Poly methyl methyl acrylate Spin Coating Long polymer chains PMMA Substrate

Electron Sensitive resist Secondary Electrons H H H H H H C C C C C C H H H H H H H H H H H H H H H H C H H H H C C C C C C C C C C C C C H H H H H H C O C O C O C O C O C O n H H H H H H O C O C O C O C O C O C H H H H H H H H H H H H Bonds broken (induced chain scission) Dissolved by suitable chemical Methyl Isobutyl ketone Isopropanol:Water PMMA Substrate

Contrast Curve Threshold electron density (lower≡faster exposure) Function of: Voltage Resist Thickness Substrate e.g. 170uAs/cm2 for 500nm thick PMMA on Silicon @ 30kV Resist Thickness Dose (electrons/area) Threshold electron density (lower≡faster exposure) “Clearing Dose” or “Base Dose” Slope -> Resolution

Contrast Curve- Experimental Dose 50um Determine for each new situation Recheck regularly Problem diagnosis

Negative Resist Microchem SU8 Photo acid generator Baking Crosslinking Secondary Electrons H H H H H H C C C C C C H H H H H H H H H H H H H H H C H H H H C C C C C C C C C C C C C H H H H H H C O C O C O C O C O C O n H H H H H H O C O C O C O C O C O C H H H H H H H H H H H H Microchem SU8 Photo acid generator Baking Crosslinking Acid Diffusion SU8 Substrate

Contrast Curve Threshold electron density Chemical amplification Resist Thickness e.g. 1uAs/cm2 for 200nm thick SU8 on Silicon @ 30kV Has its uses Dose (electrons/area) Threshold electron density Chemical amplification Lower clearing dose

Resolution Most EBL systems -> 1nm spot sizes or less Vb Rt df 1nm J. Vac. Sci. Technol. B 12, 1305 (1975) Vb 1nm Rt df And indeed if you look at manufacturer’s website df = effective beam diameter (nm) Rt = resist thickness (nm) Vb = acceleration voltage (kV)

Resolution EBL advice: Keep resist as thin as possible

EBL vs Focused Ion Beam etching Modification No removal of material FIB Energetic Gallium ions Etching of the material Heavy ions Substrate

Electron-Solid Interactions Unintended Exposure! Primary electrons Secondary electrons Low energy (50eV) Low penetrating power

Proximity Errors t dose Stray electrons Bias

Correction Shape Correction Difficult to generalise

Correction Dose Modulation Calculate the electron distribution Reduce in certain areas 1 2

Electron Distribution Forward Scattering- α Back Scattering- β

Parameters Beam energy (keV) α (um) β (um) η 5 1.33 0.18 0.74 10 0.39 0.60 20 0.12 2.0 50 0.024 9.5 100 0.007 31.2 Depend on voltage/resist/substrate Determine in each instance Monte Carlo simulations Experiment L. Stevens et al., Microelectronic Engineering 5, 141-150 (1986)

Correction Programs Nanopecs, Proxecco Pattern Alter pattern Fracture Calculate electron distributions Alter pattern Recalculate Iterate until convergence is reached

Guidelines PEC Computationally intensive β << pattern length scale << β (Homogenous background of scattered electrons) L=2um β =3.5um L=500nm L=50um

Laser Stage Limited Deflection Expose one “writefield” at a time Laser interferometer controlled Stage movements Calibrated Sub 40nm accuracy Pattern stitched together Critical for Photonics

Writefield alignment θ ? 100um

Writefield alignment θ Xum

Examples

Liftoff PMMA Electron beam evaporation Acetone Cannot consider Lithography in Isolation

Liftoff Metal thickness <1/3 of resist Pure PMMA very effective at 30kV (or less)

Liftoff Higher voltages Bilayer Resist Better Resolution Less forward scattering Bilayer Resist Low molecular Weight PMMA

Etch back Liftoff incomplete Deposit metal , spin Expose and develop Dry etch Pmma Metal Substrate

Dry Etching ZEON ZEP 520A Xylene Higher Sensitivity Tougher

Dry Etching ZEON ZEP 520A High resolution, good etch resistance Etch Quality crucial for many applications Low selectivity etch Thick Resist (400nm, @30kV)

Grayscale Lithography SU8 Resist Graded Dose

Grayscale Lithography Dry etch into Silicon Luneberg Lens Di Falco et al. Optics Express 19, pp. 5156 (2011)

The highest Resolution Polymer resists Resolution limited by chain length Hydrogen silsesquioxane Spin on Dielectric Negative Resist Exposure/ Curing

HSQ Highest resolution available Good etch resistance 25nm period Grating 100kV Metal HSQ Substrate Diamond! Lister et al. Microelectronic Engineering 73--74, 319 (2004) <15nm dots

RAITH E-Line Electron beam induced condensation Gas fed into Chamber 3D High Resolution Lithography Substrate

Exposure Guidelines Clearing dose pattern Thinnest resist acceptable Problem Diagnosis Thinnest resist acceptable Etching Always check thickness Low Voltage Larger writefield 20-30nm contamination spots Consistency

Exposure Strategy Step size(pixel size) Moire Effect Much smaller than spot (bottom of resist) Moire Effect Integral multiple of step size 150nm 100nm

Dynamic errors Weakest point of our system Beam out of sync with computer Beam speed <5mm/s PhCs etc does slow the beam down (maybe 20mm/s) Settling time Expose only when beam is in correct place 3ms+!

Position Accuracy Photonic Crystal Low resolution features (200nm) But nanometres still matter High positional resolution University of St Andrews and University of Pavia

Conclusion Photonics High Resolution patterning Versatile Quick turnaround but low throughput Know your process and requirements No “one size fits all” solution Small systems can be as good as the large Know the strengths/weaknesses Design around them Photonics Unique requirements “Lowish” resolution But nanometres really matter References SPIE Handbook of Lithography- http://www.cnf.cornell.edu/cnf_spietoc.html RAITH presentations- www.raith.de

Stitching Errors in device Higher Voltage is not always better Half the voltage, twice the deflection

“Doughnuts” method Dose Inner radius R1 Dose L. Stevens et al., Microelectronic Engineering 5, 141-150 (1986)

I. Haller, M. Hatzakis, R. Srinivasan, "High-resolution positive resists for electron-beam exposure," IBM J. Res. Develop. 12 251 (1968).

How small is a nanometre? Human Hair 100 um 100k nanometres Pin Head 1 mm 1M nanometres Virus ~100 nanometres EBL- Manufacture of ~10nm features, 1nm accuracy