Lithography for Silicon-based and Flexible Electronics

Christopher K. OberMaterials Science & Engineering

Cornell Universitycober@ccmr.cornell.edu

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

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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)

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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

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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

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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

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Electron Beam Lithography

• Characterized by expensive systems and long write times– Typically used for mask making or MEMS devices

e-

CNF NanoCourses

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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

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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

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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

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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

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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

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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.

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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

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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.

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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.

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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

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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

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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

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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

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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

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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

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• 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

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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

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Other Printing Methods

A

C

E

B

Transducer Ink reservoir

SubstrateNozzle

F

Silicone pad

SubstrateCliche InkD

Inkjet Methods

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Thermal Inkjet Printing Piezoelectric Inkjet Printing

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Ink Jet Printing

500 nm

Ink dropletSurface

energypattern

AB

C

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Drop Spreading

100 µmsource

drain

gate

channel

A

B

C

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Wetting Control

50 µm

PEDOT/surfactant

PEDOT

PEDOT

Surfactant molecules

A

B

C

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Ink Jet Circuits

B

C

B

A

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Printed Designs

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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

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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

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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

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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.

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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

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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.

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• 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

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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

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Composition of the Etch Barrier

O2 Etch Resistance

X-Linker (Lowers Viscosity)

UV Free-Radical Initiator

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• 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

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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

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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!

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