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Lithography for Silicon-based and Flexible Electronics
Christopher K. OberMaterials Science & Engineering
Cornell [email protected]
2
Smaller is BetterMoore’s Law after 40 Years
http://www.chips.ibm.com/gallery/p-n2.htmlhttp://www.intel.com/research/silicon/mooreslaw.htm
• Microprocessors with thousands of transistors operating at a few MHz
• Feature sizes of ~ 0.5 µm
• Now few GHz
• Feature sizes of ~ 100 nm
3
International Technology Roadmap for Semiconductors
4
Sowing the Seeds of Nanotechnology
Richard Feynman, “There is plenty of room at the bottom” (1959)
But…….Gutenberg laid the foundation for microlithography when he invented the printing press (~1450)
5
Expose (193 nm or 157 nm) (seconds)
Resist
WaferCoat & Bake
Mask
Positive
Develop (seconds)
Negative
Typical exposure, bake and development times are in seconds!
Strip
(PEB) Post-Exposure Bake (seconds)
Etch (Plasma)
Lithography: the printing press made small
6
Making the Pattern
• Crosslinking• Chain scission• Polarity change
h ν
h ν
h ν
The March to Smaller Dimensions
193 nmImmersion
?
Photoresist
• Photosensitive material used for transferring pattern to substrate
• Has to– Adhere to substrate– Undergo radiation induced solubility change– Possess etch resistance– Be developable in aqueous base (or other solvent)– Disappear when not wanted
Topics
• High resolution DUV lithography• Without chemical amplification• 193 nm immersion• 157 nm lithography
• E-beam lithography• Thick film lithography• Future directions in lithography
• Imprint lithography• Ink jet printing
10
Resists without Chemical Amplification
• Established technology– Mostly used as electron-beam resists– Was original basis of DUV resists
• High resolution (no acid diffusion problems)• Sub 30 nm feature sizes possible• Problem: Low sensitivity! How to improve?
– Currently low sensitivities are traded for high resolution
11
Electron Beam Lithography
• Characterized by expensive systems and long write times– Typically used for mask making or MEMS devices
e-
CNF NanoCourses
CORNELL NANOSCALE FACILTY • CORNELL NANOSCALE FACILITY • CORNELL NANOSCALE FACILITY • CORNELL NANOSCALE FACILITY
E-Beam Resists and Processing
Positive resists PMMA Toray EBR-9 PBS ZEP Photoresists as e-beam resists
Negative resists COP Shipley SAL NEB-31
Multilayer systems Low/high molecular weight PMMA PMMA/copolymer Trilayer systems
CNF NanoCourses
CORNELL NANOSCALE FACILTY • CORNELL NANOSCALE FACILITY • CORNELL NANOSCALE FACILITY • CORNELL NANOSCALE FACILITY
Poly(methyl methacrylate) (PMMA)
The most popular e-beam resist Extremely high-resolution Easy handling Excellent film characteristics Wide process latitude Usually dissolved in a solvent (e.g. anisole) Exposure causes scission of the polymer chains Solvent developer dissolves exposed (lighter molecular weight)
resist
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PMMA
+
O
E-Beam
• Excellent resolution (<30 nm) + Contrast
• Low sensitivity (800µc/cm2 @ 100kV)
• How to improve sensitivity?
-copolymerize with MAA for 4x increase in sensitivity
E-beam Technology Group, Stanford Nanofabrication Facility
CNF NanoCourses
CORNELL NANOSCALE FACILTY • CORNELL NANOSCALE FACILITY • CORNELL NANOSCALE FACILITY • CORNELL NANOSCALE FACILITY
PMMA Characteristics
Positive acting Several viscosities available, allowing a wide range of resist
thickness Not sensitive to white light Developer mixtures can be adjusted to control contrast and
profile Appropriate processing results in undercut profile for liftoff Poor dry etch resistance No shelf life or film life issues
CNF NanoCourses
CORNELL NANOSCALE FACILTY • CORNELL NANOSCALE FACILITY • CORNELL NANOSCALE FACILITY • CORNELL NANOSCALE FACILITY
P(MMA-MAA) Copolymer Resist
Higher sensitivity than PMMA Can be exposed at a lower dose Faster Less contrast.
Most useful in Bi-level resists with PMMA, to produce undercut profiles useful in liftoff processing
Characteristics Positive acting Several viscosities available, allowing a wide range of resist thickness Not sensitive to white light Developer mixtures can be adjusted to control contrast and profile Poor dry etch resistance No shelf life or film life issues
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Styrene Monomers
insensitive neg. tone resist
insensitive pos. tone resist
Highly sensitive Negative tone
Sensitivity
‘ Introduction to Microlithography’, p 207
E-beam Resists
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Poly (1-Butene Sulphone)
• Very sensitive, but poor dry etch resistance!
+
• Again, favorable decomposition route. Note release of neutral species.
R-SO2-R [RSO2R]+ RSO2+ + R R+ + SO2
20
Key Concepts
• To Improve Sensitivity:
(1) Build in bonds capable of cleavage (2) Ensure stability of intermediates(3) Release of neutral species, i.e. SO2
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UV Lithography
• Only optical lithography can provide the information output needed for high volume production
• Industry loves this and will keep pushing it as long as it can go
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Azo resists
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Azo Absorbance
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Azo Patterning
UV Stepper Tool (248/193 nm)
• Canon FPA-5500iZ step-and-repeat i-line stepper for 300 mm is a mix-and-match companion for the company's 300 mm scanners, the FPA-5000ES3 (KrF) and the FPA-5000AS2 (ArF). The tool can be easily converted to or from 200 mm wafer size and can be used for patterning less-critical IC layers. The unit includes the same third-generation platform as the company's 300 mm scanners.
DNQ Resists
Introduction to Microlithography, 2nd Ed., L. Thompson, C.G. Willons, M. J. Bowden, eds., ACS Books, Washington, 1994.
Interactions of Photoactive Molecule with Matrix
10
100
1000
10,000
R0
Rp
hν
Dissolution Rate(≈/s)
+
+
DNQ / Novolak Photoresists
*Courtesy George Barclay (Shipley)
OH
Limited Light Sources
R = k1λ/NA
Changing Wavelengths
248 nm
248 nm365 nm
193 nm
157 nm
EUV (13 nm)
X-ray
Resists with Chemical Amplification
Resist Components• Polymer• Solvent• Photoacid Generator (PAG)• Additives (e.g. DI,plasticizer)
Positive Chemically Amplified Photoresist Chemistry
0.12µm
0.40µm
PAG
hv H+
*Courtesy George Barclay (Shipley)
Photoacid Generator (PAG) Classes
Non-Ionic PAGsHalogenated Compounds:
Sulfonate Esters/Sulfones:
Ionic PAGsOnium Salts:
*Courtesy George Barclay (Shipley)
Positive Photoresist Technology
Differential in Aqueous Base Solubility - Deprotection Chemistry
Dis
solu
tion
Rat
e
40 A/sec
30,000 A/sec
+ H+
*Courtesy George Barclay (Shipley)
130 °C
33
Photoresists for ArF (193 nm) Lithography
• The current state-of-the-art in the microelectronics industry.
• Capable of producing features as small as 65 nm.
Nikon Precision, Inc.
34
Resist Transparency at 193 nm
• Aromatic groups are highly absorbing at 193 nm wavelength– Phenolic groups used for 248 nm lithography cannot
be used here• Methacrylate groups are transparent
– Low plasma etch resistance• Alicyclic groups are transparent
– Plasma etch resistance similar to aromatics
Kunz RR, Allen RD, Hinsberg WD, Wallraff GM.. Proc. SPIE 1993; 1925: 167-175. Takechi S, Kaimoto Y, Nozaki K, Abe N. J. Photopolym. Sci. Technol. 1992; 5: 439-445
35
First 193 nm Photoresist
• Excellent transparency• Excellent solubility• Poor etch resistance
Poly(t-butyl methacrylate - methacrylic acid)
Kunz RR, Allen RD, Hinsberg WD, Wallraff GM.. Proc. SPIE 1993; 1925: 167-175.
36
Alicyclic Structures Improve Etch Resistance
• Norbornene group adds etch resistance• Maleic anhydride group adds solubility• Carboxylic acid leads to film swelling during
development
Cycloolefin-maleic anhydride (COMA) resist
Allen RD, Wallraff GM, DiPietro RA, Kunz RR. J. Photopolym. Sci. Technol. 1994; 7: 507-516.Allen RD, et al. J. Photopolym. Sci. Technol. 1995; 8: 623-636.
37
Dry Film Photoresists
• polyester support sheet for the photosensitive material
• layer of photoactive monomer mixed with polymeric binder and other materials
• polyolefin cover sheet withich prevents photoresist from sticking or “blocking” when it is wound on a roll
• exposures can take several minutes
Dry Film Initiator Structure
38
N
N
N
N
Cl Cl
N
N
Cl
light
2
Ia
Ia +H3CH2C N
R
R
N
N
Cl
+ H3CHC N
R
R
II IIa
Dry Film Dye Formation
39
Ia + CH NR
R
NR
R
N
R
R
III
C NR
R
NR
R
NR
R
- electronC N
R
R
NR
R
NR
R
Pattern Formation
40
IIa + CH2C
CH2
H2C
H2CO O
O
CH3
OO
O
Polymer Network
IIa + IV +
IV
*HC
H2C
HC
HC *
OO
OH OH
V
nPolymerized matrix
Circuitization
41
42
System Supplier
Cleaning/Wet Process Kraemer KoatingWet Stripper/Developer Hollmuller Siegmund
Large High Vacuum Coater* CHAIn-line Defect Inspection* ECDPrecision Lithography* AzoresPrecision Wet Coat & Bake Frontier IndustrialOLED Evaporation Source* KJL
Small High Vacuum Coater* TBDManual Inspection Table TBD
Defined Systems
*USDC supported
43
Scrub/Rinse
Poly Tank
SSTank
RewindUnwind
Air KnifePoly Tank
• Kraemer Koating, 2001
• 6” to 14” width
• Designed for cleaning and/or wet processing
• Recirculation w/cascading possible
• 0.2 to 10 FPM
• 0.5 PLI to 1.6 PLI
Cleaning/Wet Processing: Capability
44
• Hollmuller Siegmund (MacDermid) 1993
• Up to 15” width
• Designed for Develop & Strip
• Heated tanks (three process and two rinse)
• Stripper: Stainless Steel (DuPont Riston II S-1100X)
• Developer: Polypropylene (DuPont Riston II D-2000)
• Air Knife
• Currently rebuilding web handling
Wet Stripper/Developer: Capability
45
AzoresCorp, 2006
• Based on proven FPD stepper
• 8” width, can handle up to 24” with new chucks
• g-line (436 nm)
• 4 µm L/S
• 230 to 760 mm/min
• 400 ppm distortion compensation
• Requires hole-punch pattern for pre- alignment:
Precision Lithography: Capability
Web handlers in test
46
Other Printing Methods
A
C
E
B
Transducer Ink reservoir
SubstrateNozzle
F
Silicone pad
SubstrateCliche InkD
Inkjet Methods
47
Thermal Inkjet Printing Piezoelectric Inkjet Printing
48
Ink Jet Printing
500 nm
Ink dropletSurface
energypattern
AB
C
49
Drop Spreading
100 µmsource
drain
gate
channel
A
B
C
50
Wetting Control
50 µm
PEDOT/surfactant
PEDOT
PEDOT
Surfactant molecules
A
B
C
51
Ink Jet Circuits
B
C
B
A
52
Printed Designs
53
Soft Lithography
• Umbrella term for ‘unconventional lithography’• Includes molding, embossing and printing.• Recent reviews:
Gates, B.D. et al, Chem. Rev. 2005, 105, 1171
Gates, B.D. et al, Annu. Rev. Mater. Res. 2004, 34, 339
Resnick, D. J. et al, Materials Today, 2005, 8, 34
• Included in ITRS roadmap (2010)
Comparison of Imprint Lithographies
Christie R. K. Marrian and Donald M. Tennant, “Nanofabrication”, J. Vac. Sci. Technol. A 21(5) S207 2003
Step and Flash Process
T. Bailey, B. J. Choi, M. Colburn, M. Meissl, S. Shaya, J. G. Ekerdt, S. V. Sreenivasan, and C. G. Willson, “Step and flash imprint lithography: Template surface treatment and defect analysis”, J. Vac. Sci. Technol. B 3572 18 2000
Sub-100 nm Features
57
Soft Stamp (i.e. PDMS)
Microcontact Printing (µCP)
• Uses a soft stamp to apply ‘ink’ to a substrate
Soft Stamp (i.e. PDMS)
Substrate, typically a metal Transfer ‘Ink’
Wet with ‘Ink’ i.e. thiol.
Press Stamp
Etch • Ink binds by Chemisorption of Physisorbtion
• Forms self assembled monolayer (SAM) at point of contact with substrate
58
Fabrication of Stamps for Soft Lithography
Hard Substrate
Photoresist
Expose + Develop
Etch
Elastomeric pre-polymer
Elastomeric polymer
Cure/HeatPeel off
• Hard substrates include quartz, SiO2, Cr. • Soft stamps made from PDMS, PFPE
Use as Hard Mold or…. Use to make soft stamp
59
Pros and Cons of µCP
• Can generate large patterns of SAM’s (>cm2) across curved surfaces.
(Delamarche, E. et al. Langmuir 2003, 19, 8749)
• Good for fictionalization of surfaces for different applications, i.e. biomaterials
(Brock, A. et al, Langmuir 2003, 19, 1611)
• Resolution depends on binding of ink to substrate. Can’t be considered a universal method.
60
Nanoimprint lithography (NIL)
• Uses rigid mold (i.e. silicon)
Ridged Mold
Polymer Film
Substrate
Ridged Mold
Substrate
Heat > Tg and Imprint
Ridged Mold Ridged Mold
Substrate
Cool < Tg
Release Mold
Etch, etc.
• High Temp., High Pressure• High viscosity medium• Can be difficult to fill all voids in the mold and obtain uniform patterns
61
Applications of NIL
• Extension of process used to make DVD’s, holograms etc.
SEM images of structures patterned by nanoimprint: (a) 10-nm diameter metal dots with a periodicity of 40 nm, and (b) Fresnel zone plates with a 125-nm minimum line width. (c) SEM
image of features patterned by SAMIM. Gates, B.D. et al, Annu. Rev. Mater. Res. 2004, 34, 339.
62
• Density of patterning layer…
Easiest… Easy… Very Difficult!
“Base layer”
Solution? Use a low viscosity patterning layer
(Slide Courtesy of G. Willson)
Problems with NIL
63
Step-and-flash Imprint Lithography (SFIL)
Dispense
template etch barrier
transfer layer
Expose
Separate
release treatment
Imprint
Breakthrough Etch
Transfer Etch
Residual layer
• Etch barrier: UV Curable monomer (low viscosity)
• Avoids density problems with NIL
(Slide Courtesy of G. Willson)
UV Cure
Halogen RI Etch
O2 RI Etch
64
Composition of the Etch Barrier
O2 Etch Resistance
X-Linker (Lowers Viscosity)
UV Free-Radical Initiator
65
• Resolution theoretically limited by template
• Pattern fidelity not so good for small feature sizes-still some interaction between template and etch barrier
(Slide Courtesy of G. Willson)
30 nm 20 nm 20 nm
Resolution of SFIL
66
Step-and-Flash Imprint Lithography (SFIL)
• Low cost, potential for step-and-repeat process
• Formation of multilayer structures possible
SEM images showing cross sections of multi-tiered structures on a template fabricated with alternating layers of ITO and PECVD oxide.
Johnson et al., Microelectron. Eng. 67-68 (2003), 67, 221
67
Soft Lithography: Summary
• Low cost compared to Photolithography
• Potential for Step-and-repeat processes
• SFIL looks most promising technique
• Pattern fidelity issues must be overcome!
Materials Chemistry Solution?