Proximity Effect in Electron Beam Lithography By Hussein Ayedh.
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Transcript of Proximity Effect in Electron Beam Lithography By Hussein Ayedh.
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Proximity Effect in Proximity Effect in Electron Beam Electron Beam Lithography Lithography
By
Hussein Ayedh
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Electron beam lithography Electron beam lithography (EBL)(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).
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Electron beam lithography Electron beam lithography (EBL)(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.
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Electron beam lithography Electron beam lithography (EBL)(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)
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The proximity effectThe 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
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The proximity effectThe 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.
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The proximity effectThe 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 dispersionβ : Backscattering dispersionη : Ratio of backscattered to forward scattered contribution
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ApproachApproach
Study the proximity effect by exposing dot matrices on a resist and varying◦Acceleration voltage◦Electron dose◦Density of dot matrices
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ApproachApproach - - patternspatterns
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ExperimentExperiment
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.
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Results at 10 kVResults at 10 kV
Doses are in units of 0.01 pC
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Results at 20 kVResults at 20 kV
Doses are in units of 0.01 pC
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ResultsResults
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DiscussionDiscussion
Acceleration voltage◦ Large voltage = better resolution
Electron dose◦ Large dose = larger dots and longer
time
Density◦ Higher density = larger dots
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ConclusionConclusion
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
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DiscussionDiscussion
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Reduction of proximity Reduction of proximity effecteffect
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.
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Reduction of proximity Reduction of proximity effecteffect
10 keV Resist
Substrate
100 keV
3 keV
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Reduction of proximity Reduction of proximity effecteffect
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
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Reduction of proximity Reduction of proximity effecteffect
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
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Reduction of proximity Reduction of proximity effecteffect
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!
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ReferencesReferences
[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.
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