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.