1. Electron Beam Lithography
William Whelan-Curtin
Nanometre precision

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 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)

7. 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

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
IMAGE OF ZEISS

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)
I will return to some of the previous points later
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. Electron Sensitive resistH 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

20. 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

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

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

23. 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

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

25. 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)

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

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

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

29. Proximity Errors t
dose
Stray electrons
Bias

30. Correction
Shape Correction
Difficult to generalise

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

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

33. 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, (1986)

34. Correction Programs Nanopecs, Proxecco Pattern Alter 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)
L=2um
β =3.5um
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
Cannot consider Lithography in Isolation

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

42. Liftoff Higher voltages Bilayer Resist 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

46. Grayscale Lithography
SU8 Resist
Graded Dose

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

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

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

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

51. 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

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

53. 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+!

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

55. 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-
RAITH presentations-

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

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

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

59. 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