Part 3:
2 Potential Next Generation Radiative Methods
For Nanostructuring Surfaces
Electron Beam Lithography
Scanning Near Fileld Photolithography
Part 3i:
E-Beam Lithography
Polymers and Molecular Materials
After completing PART 3i of this course you should have an understanding of, and be
able to demonstrate, the following terms, ideas and methods.
(i) The e-beam lithographic process,
(ii) Resist Material requirements,
(iii) Resolution
(iv) Etch durability
(v) Sensitivity
(vi) Resist problems (beam speading, beam penetration, pattern collapse, line
edge roughness),
(vii) Why use low molecular weight materials
(viii) Design considerationss for fullerene and triphenylene derivatives,
(ix) Overcoming the sensitivity problem (low energy methodology and
chemical amplification),
Learning Objectives
• What is a Resist?
• Resist Requirements for EBL
• Polymeric Resist
• Introduction to LMW Resists
• Fullerene and Triphenylene Resists
• Low Energy Resists
• Chemical Amplification
• Conclusions
Overview
Silicon
“Organic”
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ee
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ee
1
2 The unirradiated “organic” is removed with an organic solvent, leaving the cross-linked insoluble network pattern.
The electron beam initiates a chemical reaction in the organic material, either
(i) leading to fragmentation to smaller molecular components, which are soluble in some solvent (positive tone resist), or
(ii) crosslinking to form an insoluble network (negative tone resist).
1
2
Serial Writing is very slow, compared to Photolithography
Spin Coated 10 -100s nm
Electron Beam Lithographic Resist
3
A chemical etchant is employed to remove the exposed silica, and in so doing also etches the irradiated organic material, result in the pattern transfer to the silicon.
3
The Organic Material Requirements For a Negative Tone Resist
· Must interact with the electron beam
· Must cross-link to form a network
· Must have a high sensitivity to the electron beam (energy efficiency)
· The network must be insoluble
· The network must have good mechanical strength
· The network must be resistant to the etchant that is used to remove
the silicon in the pattern transfer step (aspect ratio)
Polymeric ResistsHistorically resists have almost always been polymeric1
PMMA
[1] “Photoresist Materials: A Historical Perspective”, C. Grant Willson et al, SPIE, 3049, p 28
Polymers readily form smooth, amorphous films by spin coating
Poor Resolution Negative Tone
Resist~70 nm
Good EtchDurabilty Resist
6:1
SAL-601
Composite of Novolac Resin, Acid Generator
and Cross Linking Agent
Neither materials have low sensitivity towards the electron beam to make them crosslink efficiently, and neither can make a high resolution (thin) and tall (good etch durabilty) structures, and are not mechanically strong.
Good Resolution Positive Tone
Resist~10 nm
Poor EtchDurabilty Resist
1:1
COOMe
Me
n
PMMA
Resist Resolution and Etch Durability
Photolithography
Resist Sensitivity
0
50
100
1 10 100Exposure Dose (C/cm2)
Norm
alis
ed F
ilm T
hic
kness
(%
) D1 D2
Contrast: = |log10(D2/D1)|-1
Sensitivity=D50%
D50%
PMMA = 140 C/cm2 (20 keV)SAL601 = 8 C/cm2 (20 keV)
Polymer Disadvantages
Beam Spreading
Pattern Collapse
Line Edge Roughness
Beam Spreading and Penetration
electron scattering simulations
20 keV Electrons 3 keV Electrons 1 keV Electrons
PMMA
Si
500 nm
Pattern Collapse
SiO2
Developer
Line Edge Roughness
Image from www.tpd.tno.nl/smartsite910.html
Line edge roughness is affected by factors including lithographic noise, processing, and polymer molecular weight
2
4
6
8
10
2000 2005 2010 2015
Line Edge RoughnessRadius of Gyration
Line
Edg
e R
ough
ness
Year
0 2 104 4 104 6 104 8 104 1 105
Line Edge RoughnessRadius of Gyration
Molecular Weight (g/mol)
4
3
Line Edge Roughness
Negative Tone
Crosslinking Chain Scission
Positive Tone
Avoiding Polymer Problems
Several of the problems with polymers seem to stem from the size of the molecules and low durability. Why not use smaller carbon rich molecules?
Smaller molecules tend to crystallize rapidly after spin coating, giving a rough and unusable polycrystalline film
There are some exceptions to this
Low Molecular Weight Resists
• Amorphous Molecular Materials• Calixarenes
• Catechols
• Fullerenes and its Derivatives
• Molecular Resists/Molecular Glasses
• Oriented materials (Liquid Crystals)• Triphenylene Derivatives
Calixarenes
• Cyclic oligomer around 1 nm in diameter
• Negative tone electron beam resist6
• A chemically amplified epoxidised derivative has been demonstrated7
OR2 R2O
R2O
R2OOR2
OR2
R1 R1
R1
R1
R1
R1
J-I. Fujita et al, Jpn. J. Appl. Phys., 36, 7769 (1997); H. Sailer, et al, Microelec. Eng., 73 - 74, 228 (2004)
Catechols• Cyclic oligmer around 1
nm in diameter with 3 aromatic rings per molecule
• Chemically amplified positive tone electron beam resist8
• Various functional groups allow the solvent to be altered
N. Kihara, et al, J. Photopolym. Sci. Technol., 11, 553 (1998)
RO
RO
RO OR
OR
OR
Fullerenes & Derivatives
• Aromatic cage molecule around 0.7 nm in diameter 60 carbons per molecule
• Negative tone electron beam resist9
• Various functional groups allow the sensitivity and solubility to be altered
T. Tada, et al, Jpn. J. Appl. Phys., 35, L63 (1996)
R
R
Molecular Glasses
• Non-planar (propeller shaped) molecule around 1 to 2 nm in diameter
• Negative or positive tone electron beam resist9
• Chemical amplification has been demonstrated10
M. Yoshiiwa, et al, Appl. Phys. Lett., 69, 2605 (1996); T. Kadota, et al, Chem. Lett., 33, 706 (2004)
OR
RO OR
N
NO O
N
O
O
N
O
O
Triphenylene Derivatives
• Liquid crystalline molecule around 1 to 2 nm in diameter
• Negative or positive tone electron beam resist10
• Various functional groups allow the liquid crystal nature and sensitivity to be altered
A.P.G. Robinson, et al,J. Phys. D, 32, L75 (1999)
RO OR
RO
RO OR
OR
LMW Resist PropertiesResist Sensitivity
(µC/cm2)
Resolution
(nm)
Etch Durability Casting Solvent / Developer
Calixarene > 700 < 10 Moderate Usually chlorinated/Xylene
Calixarene [CA] 10 40 Moderate MCB/MIBK
Catechol [CA] 10 90 Good Methoxymethyl propionate / Aqueous Base
Molecular Glasses
3000 70/150 (+ve/-ve)
- THF/TMAH:IPA or
2-methoxyethyl acetate
MG [CA] 2 25 - THF/TMAH:IPA
Fullerenes & Triphenylenes
We have investigated two families of low molecular weight resists - fullerene derivatives, and triphenylene derivatives.
• Original results for fullerenes and triphenylene derivatives
• Low energy electron beam exposures of fullerene derivatives
• Chemical Amplification of fullerene and triphenylene derivatives
Large flat -surface
Ordering
Introduced strained cyclopropane ring
Crosslinking increasedX
Y
OOO
X
Y
O
n n
ORRO
RO
RO OR
OR
Molecular Design Considerations
High carbon content
Etch Durability?
Large -surface
Enhanced sensitivity?
14 nm
Scanning Electron Micrographs of Resist Patterns (20keV Beam)
Sensitivity ~ 1000 µC/cm2
ORRO
RO
RO OR
OR
100 nm
35 nm
20 nm
Scanning Electron Micrographs
‘A Triphenylene Derivative as a Novel Negative/Positive Tone Resist of 10 nm Resolution A.P.G. Robinson, R.E. Palmer, T. Tada, T. Kanayama, M.T. Allen, J.A. Preece, and K.D.M. Harris, Microelectronic Engineering, 2000, 53, 425-428.
‘Multi-adduct Derivatives of C60 for Electron Beam Nano-Resists’ T. Tada, K. Uekusu, T. Kanayama, T, Nakayama, R. Chapman, W.Y. Cheung, L. Eden, I. Hussain, M. Jennings, J. Perkins, M. Philips, J.A. Preece, E.J. Shelley, Microelectronic Engineering, 2002, 61, 737-743.
2.5 nm
X
Y
OOO
X
Y
O
n n
ORRO
RO
RO OR
OR
PMMA
Resolution
Etch Ratio
20 nm
6
14 nm 20 nm (10 nm)
6 <1 (<1)
Resolution equals or surpassed PMMAEtch ratio much better than SAL 601Sensitivity much better than previous medium molecular weight materials
Sensitivities of around 1000 C/cm2 at 20-30 keVPMMA a factor of ~10 lower
Comparison
The Sensitivity Problem
The resolutions of both fullerene and triphenylene derivatives are comparable with other LMW materials, and the etch durabilities are extremely high.
However, like most LMW resists the best sensitivities (fullerene - 370 µC/cm2; triphenylene - 880 µC/cm2) are still much lower that polymer based materials.
Possible Solutions
Low EnergyElectrons
ChemicalAmplification
Low Energy Exposure
Low energy electrons deposit more of their energy in the resist and less in the substrate. This leads to an increase in sensitivity.
Image after D.F. Kyser et al, J. Vac. Sci. Technol, 12, 1305, (1975)
20 keV MF02-01A 473 µC/cm2 MF03-01 970 µC/cm2
20 keV 1 keVMF02-01A 473 µC/cm2 21 µC/cm2
MF03-01 970 µC/cm2 65 µC/cm2
SAL 601 ~10 C/cm2 (20 keV)
Chemically Amplified Triphenylenes
An alternative two component
crosslinking system, based on
pendant epoxy groups and
using the photoinitiator UVI-
6976 (Triarylsulfonium
hexafluoroantimonate salts)
was developed.
O O
O
O
C5H11
C5H11
C5H11
O
O
O
O
O
Fine Patterning
The pure epoxide has a sensitivity of 600 µC/cm2, which improves to 15 µC/cm2 when the photoinitiator is added (Ratio of derivative to PI - 2:1).
i.e. 45 fold increase in sensitivity. How….?
C5/Epoxy:C5/C0:PI (14:4:9) Film
Line width = 44 nmLine dose = 0.8 nC/cm
PEB 100 °C / 120 sDevelopment in MCBfor 20 s
S S
O O
O
O
C5H11
C5H11
C5H11
O
O
O
O
O
O O
O
O
C5H11
C5H11
C5H11
O
O
O
O
O
O
O O
O
C5H11
C5H11
C5H11
O
O
O
O
O
O O
O
O
C5H11
C5H11
C5H11
O
O
O
O
O
O
O O
O
C5H11
C5H11
C5H11
O
O
O
O
O
‘Photo’-Acid Generator
Further
Cross-Linking
ConclusionsIt is likely that the issue of polymer size will have to be addressed within the next 5 years, based on ITRS line edge roughness requirements.
Several low molecular weight alternatives are approaching viability in terms of sensitivity, but at the cost of resolution, which must instead be maintained.
Fullerene derivatives, with their extremely high etch durability are a good candidate for low energy applications. Sensitivities of 20 µC/cm2 and 30 nm resolution have been demonstrated.
Epoxide functionalised chemically amplified triphenylenes have good sensitivities (15 µC/cm2), and promising resolutions (45 nm).
ThanksDr Alex RobinsonDr H. Mohd ZaidFran Gibbons
Nanoscale Physics Research LaboratoryUniversity of Birmingham
www.nprl.bham.ac.uk
For use of some of their slides
Selected E-Beam Papers