Post on 29-Jan-2016
description
Proximity Effect in Proximity Effect in Electron Beam Electron Beam Lithography Lithography
By
Hussein Ayedh
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).
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.
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)
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
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.
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
ApproachApproach
Study the proximity effect by exposing dot matrices on a resist and varying◦Acceleration voltage◦Electron dose◦Density of dot matrices
ApproachApproach - - patternspatterns
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.
Results at 10 kVResults at 10 kV
Doses are in units of 0.01 pC
Results at 20 kVResults at 20 kV
Doses are in units of 0.01 pC
ResultsResults
DiscussionDiscussion
Acceleration voltage◦ Large voltage = better resolution
Electron dose◦ Large dose = larger dots and longer
time
Density◦ Higher density = larger dots
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
DiscussionDiscussion
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.
Reduction of proximity Reduction of proximity effecteffect
10 keV Resist
Substrate
100 keV
3 keV
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
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
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!
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.