1. Proximity Effect in Electron Beam LithographyBy
Hussein Ayedh

2. Electron beam lithography (EBL)
One of the most commonly used methods to pattern structures on a nanometer scale.
EBL systems are a cornerstone of modern micro- and nanofabrication.
Special electron beam sensitive resists have to be used for EBL. The most common one is polymethyl methacrylate (PMMA).

3. Electron beam lithography (EBL)
Two main electron sources:
Thermionic emission source
Based on electron emission from a filament heated to a high T.
Field emission source
Based on a field emission effect from a sharp W-tip.

4. Electron beam lithography (EBL)
Advantages
Extremely high resolution
Direct patterning on a substrate with high degree of
automation (No mask required)
But:
Low throughput (Raster Scan)
Expensive (Vector Scan)

5. The proximity effect
A limiting factor of high resolution and contrast of EBL.
Depends on the pattern density and the substrate material, as well as parameters of the EBL exposure.
Acceleration voltage
Electron dose

6. The proximity effect Source: backscattering and secondary electrons.
 Secondary electrons are produced when an incident electron excites an electron in the sample and loses some of its energy in the process. The excited electron moves towards the surface.

7. The proximity effect
The combined effect of forward scattering and backscattering broadens the electron beam.
The intensity distribution can be approximated by a sum of two Gaussian shapes.
α : Forward scattering dispersion
β : Backscattering dispersion
η : Ratio of backscattered to forward scattered contribution

8. Approach
Study the proximity effect by exposing dot matrices on a resist and varying
Acceleration voltage
Electron dose
Density of dot matrices

9. Approach - patterns

10. Experiment 200 nm of ZEP 520A7 resist on InP substrate.
Patterning in EBL.
Development in O-Xylene.
Evaporation of 20 nm of Au.
Lift-off in remover 1165.
SEM imaging.

11. Results at 10 kV
Doses are in units of 0.01 pC

12. Results at 20 kV
Doses are in units of 0.01 pC

13. Results

14. Discussion Acceleration voltage Electron dose Density
Large voltage = better resolution
Electron dose
Large dose = larger dots and longer time
Density
Higher density = larger dots

15. Conclusion Good resolution in this experiment was achieved by:
High acceleration voltage (20 kV)
Either high dose and low density or
Intermediate dose and intermediate density

16. Discussion

17. Reduction of proximity effect
Proximity effect can be reduced by:
• High electron energy (>100 keV).
• Low electron energy (< 3 keV).
• Thin resists.
• Low Z material of substrate ( secondary electron yield is
generally higher for high atomic number targets)
• can be corrected by software.

18. Reduction of proximity effect
10 keV
Resist
Substrate
100 keV
3 keV

19. Reduction of proximity effect
High energy EBL: (>100 keV)
• Dissipation of energy deep in substrate
• Secondary electrons can not reach the surface
to expose the resist.
• Forward scattering in the resist is very small
Proximity effect is reduced!
But: expensive, big and complicated EBL system

20. Reduction of proximity effect
Low energy EBL: (1-3 keV)
• Dissipation of electron energy in the resist only.
• No generation of secondary electrons
in the substrate!
Limitations:
• Forward scattering is large -> low resolution.
• E-beam size is big due to Column interaction between electrons.
• Electron optics works poor at low energy.
• Resist must be thin for complete exposure (e.g. 70 nm for 2 keV)
difficult to use.
• Hard to focus the e-beam, the beam is very sensitive to external
fields

21. Reduction of proximity effect
Practical proximity effect correction:
Dose scaling: changes in exposure dose in parts of the structure. Dedicated software is used.
Shape correction:
(a) reduction of structure size and
(b) additional structures at underexposed
areas
Dose scaling by software is the main method of proximity effect correction!

22. References
[1] S.M.Sze ’’Semiconductor devices physics and technology ‘’2ND Edition ,willy ,2001.
[2] S.A.Campbell ’’Fabrication Engineering at the Micro and Nanoscale‘’4th Edition ,Oxford ,2013.
[3] G. May and S.M.Sze ’’Fundamentals of Semiconductor Fabrication‘’ ,willy ,2004.
[4] Advanced Processing of Nanostructures Lecture note, FFFN01, Lund University.