1. Electron Beam Lithography (EBL) for High-Volume Manufacturing
Paul Meyer October 10, 2017

2. PSA!
Remember to bring handouts 

3. Electrons Have High Resolution
DUV Litho limited by 𝜆
Luckily, we have quantum mechanics
Electrons are “wavicles!”
Wavelength of 200 keV electron: 3 pm
Relativistic speeds ~ 0.5c
𝑅= 𝑘 1 𝜆 𝑛 𝑠𝑖𝑛 𝜃
𝜆 𝑚𝑎𝑡𝑡𝑒𝑟 = ℎ 𝑝
Electrons have much less diffraction than photons

4. Compare to Optical Lithography
E-beam
Optical
High resolution
(~2 nm)
Can generate unique shapes
Slow
Resolution limited by 𝝀 (~20 nm)
Typically line/space patterns or Manhattan
Fast

5. Two Major Issues with EBL
Scattering
Space Charge e- e-
𝐹= 𝑘 𝑞 1 𝑞 2 𝑟 2

6. Proximity Effect Due to scattering from nearby features
Corrections by Dose Modulation
But this slows things down!

7. Scanning Vs. Projection EBL
Serial Process
Parallel Process
*Pictures taken from (1)

8. Gaussian vs. Shaped Beam

9. Electron Source Tungsten/ LaB6: Thermionic emitters
Heat up until electrons emitted
Thermal Field Emitter:
Heat & use electric potentials to extract e-
Source
Lifetime
Source Size
(𝜇𝑚)
Energy Spread (eV)
Tungsten
~100 hrs 25
2.5
LaB6
~500 hrs 10
Thermal Field Emitter
> ~1,000 hrs
0.02
0.9

10. Lenses Must be Electromagnetic
In comparison to Optical:
Aberrations greater than optical lenses
Can only converge
Magnetic:
Causes rotation along
axis of motion
Electrostatic:
Aberrations worse
*Picture taken from (4)

11. Aberration Sources Spherical: Chromatic: Astigmatism: Diffraction:
Trajectories don’t focus
at same points
Chromatic:
Different wavelength e-
Astigmatism:
Misalignment
Diffraction:
Everyone knows this one 
*Picture taken from (4)

12. Organic Resists Positive Tone: Chain Scission
Poly(methyl methacrylate)
Breaks bonds in polymer chain
Negative Tone: Crosslinking
Poly(glycidyl methacrylate)
Crosslinks polymer together

13. Inorganic Resists All Positive Tone Metal Halides: NaCl, LiF, MgF2
Mechanism of breakdown:
Production of halogen gas
Metal diffuses into resist

14. Attempts at High-Volume E-Beam Lithography

15. Pfeiffer’s Law of E-beam?

16. Electron Beam Exposure System (EBES)
First commercial e-beam tool
~1960’s
Gaussian beam system
Exposed one pixel at a time
Eventually used to make photomasks

17. SCALPEL (‘90s)
SCattering with Angular Limitation Projection Electron-beam Lithography
4x Image Reduction
Scattering Mask: (Projection Litho)
Transmittance: Low atomic #
Scattering: High atomic #

18. PREVAIL (~1995)
e- beam
Projection Reduction Exposure with Variable Axis Immersion Lenses
Improvement on SCALPEL
Electron beam AND wafer scan
Failure: off-axis aberrations
Wafer

19. REBL (2000s) Reflective Electron Beam Lithography
Two main innovations:
Electrons Reflect on a pattern
Wafer rotates like record player
Problem: electrons must be reflected

20. MAPPER (present)
Multiple Aperture Pixel by Pixel Enhancement of Resolution
650,000 beamlets, individually controlled by MEMS structures
40 wafers/hour “not too far away”
Looks to be selling 1st unit soon

21. MAPPER Cont.
Epilepsy Warning:

22. General Applications for EBL
Mask Writing
Mask Inspection Tools
Security Chips (RFID)
Unique patterns easily fabricated

23. Summary
Many attempts made for high-volume manufacturing; none succeeded (so far)
E-beam lithography is useful for:
Complex, small patterns (not line/space)
Low throughput, high resolution
Limitations come from:
Aberrations in lenses
Scattering
Proximity Effects
Space Charge

24. Thank you for Listening!
Questions?

25. References http://eng.thesaurus.rusnano.com/wiki/article1093
SPIE Handbook, Volume 1: Microlithography, Section (accessed Oct 8, 2017).
Electron Beam Lithography. Stephan Kapfinger. January 26, Technische Universität München.
E-Beam Lithography “The Physics of Micro/Nano-Fabrication.” Ivor Brodie, Julius Muray.
Vistec Shaped Beam Technology. Ines Stolberg.
McCord, M. A.; Petric, P.; Ummethala, U.; Carroll, A.; Kojima, S.; Grella, L.; Shriyan, S.; Rettner, C. T. & Bevis, C. F.; “REBL: design progress toward 16 nm half-pitch maskless projection electron beam lithography”,Proc. SPIE, 2012
Harriott, L. R. SCALPEL: Projection Electron Beam Lithography. Proc Part. Accel. Conf. (Cat. No.99CH36366) 1999, 1, 595–599 DOI: /PAC
Disclaimer: All pictures were taken from other sources and are not my own

26. Gaussian vs. Shaped Beam
Spot by Spot Exposure
Expose large areas w/ “shots” of shaped beam
Low Throughput
Higher Throughput
Cheap
Complex optical requirements