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Electron Beam Lithography

Electron Beam Lithography. William Whelan-Curtin. 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.

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Electron Beam Lithography

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  1. Electron Beam Lithography William Whelan-Curtin

  2. What is EBL for? LPN • Nanopatterning • High Precision • Reliable • Versatile

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

  4. Outline • System description • Exposure • Examples

  5. Electron “Pencil” • System Schematic

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

  7. Electron Lenses • Chromatic dispersion • Monochromatic beam • Aberrations • Use centre of lens F = q · (E+ v  B) Electro-magnetic Electro-static

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

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

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

  11. 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

  12. 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

  13. Types of Electron Beam Writer • Shaped Beam systems • Very complex optics • Higher current, lower resolution • Photomask making • (Not a research tool) GAUSSIAN

  14. Patterning LPN • Electron sensitive polymer- the “paper”

  15. Resist Overview “Positive” “Negative”

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

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

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

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

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

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

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

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

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

  25. Resolution J. Vac. Sci. Technol. B 12, 1305 (1975) • Most EBL systems -> 1nm spot sizes or less Vb 1nm Rt df df = effective beam diameter (nm) Rt = resist thickness (nm) Vb = acceleration voltage (kV)

  26. Resolution EBL advice: Keep resist as thin as possible

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

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

  29. Proximity Errors • Stray electrons • Bias t dose

  30. Correction • Shape Correction • Difficult to generalise

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

  32. Electron Distribution Forward Scattering- α Back Scattering- β

  33. Parameters • Depend on voltage/resist/substrate • Determine in each instance • Monte Carlo simulations • Experiment L. Stevens et al., Microelectronic Engineering 5, 141-150 (1986)

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

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

  36. 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

  37. Writefield alignment θ ? 100um

  38. Writefield alignment θ Xum

  39. Examples

  40. Liftoff • PMMA • Electron beam evaporation • Acetone

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

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

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

  44. Dry Etching • ZEON ZEP 520A • Xylene • Higher Sensitivity • Tougher

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

  46. Grayscale Lithography • SU8 Resist • Graded Dose

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

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

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

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

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