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E SC 412

Nanotechnology: Materials, Infrastructure, and Safety

Wook Jun Nam

Lecture 12 Outline

• Overview

• Exposure Systems

• Optical Lithography Parameters

• Photoresist

• Lift-off Process

• EUV Lithography / X-ray Lithography

Copyright 2014 by Wook Jun Nam

Optical Lithography Overview


1. High Resolution

2. Precise Alignment Accuracy

3. Throughput (number of wafers/hr)

Lithography Performance Measure


1. Reliable (e.g., low

defect density)

2. Reproducible

3. High throughput

But, it is

1. Expensive (e.g., >

35% of chip

manufacturing costs)

2. Complicated (e.g.,

light source,

photoresist, optical

elements)

Why optical lithography is the preferred

technique in microelectronic industries ?

cost of exposure tool

http://www.sematech.org/meetings/archives/litho/euvl/7470/Poste

r/FinalS1/1-CO-01%20Goodwin_IFX%20Poster.pdf


http://commons.wikimedia.org/wiki/File:Spectrum_of_lithography_lights.PNG

International Technology Roadmap for

Semiconductors (ITRS) Lithography Roadmap

(2007)


Exposure Systems


Types of Exposure Systems

Contact Proximity Projection


The Karl Suss MJB3 The Karl Suss MA6

Public Domain: Image Generated by CNEU Staff for free use

Contact/Proximity Exposure Systems:

Aligners


Contact Lithography

Optical source

Photomask

Photoresist

/substrate:

• Relatively good resolution:

no gap between a mask

and a resist

• 1x image transfer

• Fast, simple, inexpensive

• Mask

damage/contamination


Proximity Lithography

Optical source

Photomask

Photoresist

/substrate

• Less mask


• 1x image transfer

• Fast, simple, inexpensive

• Relative bad resolution:

the gap between a mask

and a resist causes more

light diffraction


http://www.cnf.cornell.edu/cnf5_tool.taf?_function=detail&eq_id=5&gtitle=PHO

TOLITHOGRAPHY&area=PHOTOLITHOGRAPHY&cacName=Autostep%20i-

line%20Stepper&labUser=&_UserReference=..

Projection Exposure Systems: Stepper


Projection Lithography

Optical source

Reticle

Photoresist

/substrate

• No mask


• Excellent resolution

• 4~10x image reduction:

defect size in a mask is

also decreased

• expensive


Ideal Systems

• Ideal Exposure System: 100% modulation of light over 0

distance.

• Ideal Positive Photoresist: 100% retention if exposed below

Dcrit, 100% removal if exposed above Dcrit.

Perfect exposure system Perfect positive resist


Modulation Transfer Function (MTF)

I max

I min

• MTF describes quantitatively

the relationship between

source and Image.

• MTF is a measure of an

exposure tool’s ability to

modulate the intensity of light

at the wafer surface and

decreases with decreasing

diffraction grating period (due

to more destructive

interference).

S. A. Campbell, Fabrication Engineering at the micro- and nanoscale, Oxford University Press, 2013


Modulation Transfer Function (MTF) -

continued

• Considering a diffraction

grating instead of a single

square aperture, the

Fraunhofer limited (far

field) intensity pattern

(non-normalized intensity

in W/cm2) is shown.



Optical Lithography Parameters


Resolution, R (Rayleigh equation)

For small (good) resolution,

k1 , NA , λ

λ : wavelength of light sourcek1 : a constantNA : numerical aperture

θ

photoresist

Projector

lens

substrate

DOF


Depth of Focus (DOF)

2

2

)(2 NA

kDOF

DOF

DOF

For better DOF,

k2 , NA , λ

Resolution

DOF


Depth of Focus (DOF)

• DOF is the range that light is in focus and can achieve

good resolution of projected image.

• Smaller numerical aperture, larger DOF

• High resolution, small DOF

• Focus at the middle of photoresist layer

2

2

)(2 NA

kDOF


Improvement in Wavelength (λ )

• Need develop light source, PR

and equipment

• Limitation for reducing

wavelength

• UV to DUV, to EUV, and to X-

Ray

θ

photoresist

Projector

lens

substrate

DOF


Spectrum of Light Source (Hg lamp)

http://zeiss-campus.magnet.fsu.edu/articles/lightsources/metalhalide.html


Spectrum of Light Sources (extended)


Light sources should have:

• Short wavelength

• High intensity

• Stable


Improvement in NA

• High NA optics with very low

aberration levels.

• Bigger diameter of lenses

• Immersion lithography (NA>1).

θ

photoresist

Projector

lens

substrate

DOF


Chemical Mechanical Polishing (CMP)

http://www.scsolutions.com/chemical-mechanical-planarization-

cmp-controllers-0


Immersion Lithography

http://www.kitguru.net/components/cpu/dragan/intel-insider-lifts-the-lid-on-advanced-processes-exclusive-interview/

With immersion lenses smaller CDs can be resolved

Resolution enhancement !


http://www.kitguru.net/wp-content/uploads/2011/07/Immersion-Lithography-Intel-KitGuru.jpg

Improvement in k1 : Off Axis Illumination

(OAI)

Nanofabrication: Principles, Capabilities, and Limits, Zheng Cui, Springer (2008)


Mask Image with Diffraction



Improvement in k1 : Phase Shifting Mask



Phase Shift Mask

http://www.tf.uni-kiel.de/matwis/amat/admat_en/kap_5/backbone/r5_3_2.html


http://www.tf.uni-kiel.de/matwis/amat/admat_en/kap_5/backbone/r5_3_2.html

Improvement in k1 : Optical Proximity

Correction



Improvement in k1

• Resolution Enhancement Techniques (RET)s

− Self-aligned Double Pattern

− Double Patterning Technique

Litho.-Etch-Litho.-Etch (L-E-L-E)

Litho.-Freezing-Litho.-Etch (L-F-L-E)

(Please read the suggested reading L5-2 for more

details)

− Dual Tone Development (DTD)

− Double Exposure (DE)

• Phase shift mask


An Example of the RETs

http://spie.org/x35993.xml

Self-Aligned Double Pattern (SADP)


Photoresist


1. High Resolution

2. High Etch Resistance

3. Good Adhesion

4. Conformal Coating (good step coverage)

5. Good Contrast and Sensitivity

6. Easy of Removal : i.e., strip

Requirement of Photoresist


1. Solvent

Controls a resist layer thickness at a certain spin sped.

2. Resin

Material physically maintaining the transferred pattern

after a lithography step.

3. Photoactive molecules

Controls weather UV exposed area is removed or

stayed after a development process step.

Components of Photoresist


Positive & Negative Photoresists

Positive photoresist Negative photoresist


Photoresist Sensitivity

• Sensitivity identifies energy

needs of exposure : i.e.

threshold energy

• Dose = light source power

density (mW/cm2) x

exposure time (sec)

ET

No

rma

lize

d R

esis

t T

hic

kn

ess



Photoresist Contrast

• Contrast is PR’s ability to

distinguish between light

(exposed) and dark

(unexposed) area.

• D100 : the minimum dose for

which the PR will completely

dissolve when developed.

• D0: as the maximum energy

density (mJ/cm2) for which the

PR will not dissolve at all when

developed.Positive PR



Photoresist Contrast (continued)

D0 = D0, D100 = Df

Norm

aliz

ed

Resis

t T

hic

kne

ss

No

rma

lize

d R

esis

t T

hic

kn

ess



• Most common g-line and i-line

photoresists are diazonaphtho-

quinone-sulphonates (DNQ)/

novolac based.

• Novolac resin is soluble in

aqueous base due to the acidic

phenolic OH functionality.

• DNQ dispersed in the phenolic

matrix inhibits the dissolution in

base developer by a factor of

10 or more.

Photoresist Reactions (positive resist)

http://www3.ul.ie/~childsp/CinA/Issue66/TOC25_Discworld.htm


• UV irradiation of photoactive compound liberates N2 from

the C ring, forming a highly reactive carbon site

(carbene).

• The carbene intermediate undergoes the Wolff

rearrangement (C atom moves outside of the ring, O2 is

then covalently bonded) to ketene.

Photoresist Reactions (positive)



• Ketene reacts with ambient water in which double bond

of external C is converted to a single bond with an OH

group –carboxylic acid (RH Cleanroom Stability Crucial).

• Consequently, the exposed areas of the resist film

dissolve much faster than the unexposed in an aqueous

base developer.

Photoresist Reactions (positive)



• Negative resist contain a cross linker which is activated

during exposure and thermally activated crosslinks the

resin during a subsequent baking step.

• Higher exposure doses increase the degree of cross

linking, which improves the thermal and chemical

stability, and makes an impact on the attained resist

profile after development.

Photoresist Reactions (negative)


• Resist molecules are photoacid generators (PAG).

• PAGs produce acid when exposed to light.

• Acid acts like a catalysts in dissolving the molecule

chains.

Chemically Amplified Photoresist

http://www.smtbook.com/popups/resist.htm


Post Exposure Bake (PEB)

SILVACO ATHENA Simulation

PEB :45 min at 115CNo PEB

More exposure More exposure

Standing waves

http://www.utdallas.edu/~walter.hu/teaching/EE6322/Lecture%206%20Lithography%202.pdf


Post Exposure Bake (PEB)

http://www.cnf.cornell.edu/cnf_process_photo_resists.html


Lift-off Process


1. Thermal Stability

2. Chemical Stability

3. Good Adhesion

Requirement of Lift-off Resist

Image resist

Lift-off resist


Lift-Off Process (Process Steps)

Lift-off using a single resist layer Lift-off using double layers


Lift-Off Process (Rabbit Ear)

Lift-off using a single resist layer Lift-off using double layers


Lift-Off Process (Photoresist)

Surface modified single PR layer Double PR layers


Lift-Off Process (Inspection)

Optical Microscope Image FESEM Image


EUV Lithography / X-ray Lithography


Spectrum of Light Sources


Light sources should have:

• Short wavelength

• High intensity

• Stable



International Technology Roadmap for

Semiconductors (ITRS) Lithography Roadmap

(2007)


Light Source Shorter than 193nm

• Nature of light is too absorptive:

– Lenses, pellicles, photomasks (reticles) are the sources

of the light wastes.

– Pellicle is a protective layer coated on a

photomask/reticle.

– Resists designed for 248nm or 193nm is too absorptive

for 157nm or shorter wavelength.

• No more light transmission for pattern transfer: reflection

is used for 13.5nm EUV technology !!


Extreme UV Exposure Tool

No more transmission !



Bragg Reflector

• Called as a distributed Bragg reflectors (DBR) or Bragg

mirror.

• It consists of alternating layers of high- and low-

refractive index films, each having approximately a

quarter-wavelength thickness.

• Resonant reflectivity is achieved when the light whose

wavelength is close to four times the optical thickness of

the layers, the many reflections combine with

constructive interference, and the layers act as a high-

quality reflector.

http://en.wikipedia.org/wiki/Distributed_Bragg_reflectorNanofabrication: Principles, Capabilities, and Limits, Zheng Cui, Springer (2008)


Bragg Reflector (continued)


Bragg Reflector (continued)

• Alternating layers of Mo and Si are reported to offer the

highest reflectivity at the wavelength of about 13nm.

• 50 pairs of Mo(2.76nm)-Si(4.14nm) multilayer mirror

provides higher than 70% (numerical modeling) and ~68%

(experimental) of reflectivity around 13nm wavelength.Nanofabrication: Principles, Capabilities, and Limits, Zheng Cui, Springer (2008)


EUV Masks

• EUV mask is fabricated on a EUV mirror (Bragg reflector).

• Patterned absorber layer (e.g., Al, Cr, Ta, W) is transferred

using ebeam lithography and reactive ion etching (RIE).

• Reduction ratio of the pattern is about 4:1.Nanofabrication: Principles, Capabilities, and Limits, Zheng Cui, Springer (2008)


Requirements for EUV Resist

• High Sensitivity:

– low intensity of source power

– Chemically amplified resist is desirable

• High Resolution

• Low Edge Roughness

– Chemically amplified resist causes high line edge

roughness

• Low Outgassing of Resist

– High energetic light exposure generates gaseous

molecules from a resist which will contaminate EUV

optics (e.g., mirrors).



X-ray Lithography



X-ray Lithography

• X-ray is the radiation wavelength spanning from 1nm at

the soft X-ray ends toward 0.1nm of hard X-ray.

• X-ray can penetrate majority of materials

• Materials (e.g., Au, W, Ta) with high atomic number

should be used for mask

• There is no refraction in X-ray, so the image in a mask is

transferred to a resist with 1:1 ratio.



X-ray Lithography Challenges

• X-ray mask consists of a membrane of low atomic number

material (e.g., Si, SiC) with patterned high atomic number

material as the absorber on top.

• Typical thickness of the Si membrane and absorber layer is

1~2um and 300~500nm, respectively.

• Because of the poor mechanical strength of thin-membrane

mask, the proximity gap is required.

• It is 1:1 ratio pattern transfer

• Local stress caused by heavy metal absorber and internal

stress of supporting membrane can influence on the pattern

resolution Nanofabrication: Principles, Capabilities, and Limits, Zheng Cui, Springer (2008)


X-ray Lithography Challenges (continued)

• X-ray irradiation generates photoelectrons and Auger

electrons in a resist.

• Scattering of these low energy electron in a resist results in

very effective interaction with resist polymer, and causes

degradation of image resolution.

• It is very hard to control the gap between a mask and a

substrate.

• X-ray also has diffraction effects. Highly coherent x-ray

source (e.g., synchrotron radiation source (SRS)) is

needed.



Synchrotron Radiation Source

http://sni-portal.uni-kiel.de/kfs/Infos/Quellen/synchrotronradiationsource.php


What Technology will Continue the Moore’s

Law ?

• Ebeam lithography

• Nano-imprinting lithography

• Extreme Ultra Violet (EUV) lithography : 13.5nm

• Ion Beam lithography

• X-ray lithography

• Something new ??


Lecture 12 Outline

• Overview

• Exposure Systems

• Optical Lithography Parameters

• Photoresist

• Lift-off Process

• EUV Lithography / X-ray Lithography


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