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PHOTOCLASS/ECE580/ARCs/ARCS.ppt Brainerd

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Anti-Reflective Coatings

• Reflection• Anti-reflection Principles• Swing Curves• Reflectivity curve• Top ARCs TARC• Bottom ARCs BARC• Organic BARC• Inorganic DARCs


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Anti-Reflective Coatings: Web sites

• http://www.newport.com/store/product.asp?lone=Optics&ltwo=Technical+Reference&lthree=Optics+Formulas&lfour=&id=3877

• http://webug.physics.uiuc.edu/courses/phys371/RightFrame.htm• http://dustbunny.physics.indiana.edu/~dzierba/P360n/KPAD/Exps/Fres

nel/fresnelnote.pdf• http://www.hep.princeton.edu/~p104/lectures/lecture09.pdf• http://www.mellesgriot.com/pdf/005.4-5.7.pdf• http://www.mellesgriot.com/glossary/wordlist/glossarylist.asp?l=b


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Anti-Reflective Coatingshttp://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/reflectcon.html#c1

• Reflection theory


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Anti-Reflective CoatingsClariant Antireflective Coatings Lecture Notes

• Reflection theoryAbsorption and reflection phenomena can be treated equivalently if the refractive index is written as a complex number:

ñ = n - i kwhere

n = Re (ñ); k = - Im(ñ) =

The real part n of the refractive index is also frequently called the “refractive index”. The imaginary part k is related to the absorbance αof the material by the equation above.The above convention for the sign of k (k > 0) is called the “optical convention”.

α λ4


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• Reflection theory:• Condition: Beam incident normal to surface in a medium with no

refractive index and reflective substrate with nc.

• REFLECTED wave amplitude A = Eo ρ (Eo = electric field )• Coefficient of reflection: ρ = (no - nc)/ (no + nc)• If ρ is negative there is a 180o phase shift.• Then reflectivity R = % = Ir/Io = ρ 2 = [(no - nc)/ (no + nc)]2

• where Ir = reflected intensity, Io = incident intensity• Relative Amplitude = (1- no / nc)/ (1 + no / nc)• Relative reflected intensity = A2 ( note intensity = square of electric

field)


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• External Reflection at a dielectric boundary:• Fresnel’s law of reflection for the s and p polarized components:

• Ordinary and extraordinary rays:• o-ray (oordinary ray): polarized parallel to plane of incidence. Also termed p-

polarization.• e-ray (extraordinary ray): polarized perpendicular to plane of incidence. Also

termed s-polarization.

Non-normal Incidence: o = angle of reflection angle and c = angle of refraction


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Anti-Reflective Coatings• External Reflection at a dielectric boundary:• Fresnel’s law of reflection for the s and p polarized components:

• Non-normal Incidence: o = angle of reflection angle and c = angle of refraction

• r = refection (reflected waves)

• rs = (nocosθo -nccosθc)/(nocosθo + nccosθc) ; rp = (nccos θo -nocosθc)/nccosθo + nocosθc)

• c = transmission ( refracted waves)

• ts = 2nocosθo/(nocosθo + nccosθc) ; cp = 2nocosθo/(nccosθo + nocosθc)

•


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Anti-Reflective Coatings• External Reflection at a dielectric boundary:• Fresnel’s law of reflection for the s and p polarized components:

• Non-normal Incidence: o = angle of reflection angle and c = angle of refraction

• Reflectivity in %:

• for Normal incidence Fresnel’s law becomes: R = ρ 2 = [(no - nc)/ (no + nc)]2

• In air no = 1.00 and at normal incidence Fresnels’s law reduces to

• % Reflectance R = ρ 2 Rp = Rs = ρ 2 = [(n-1)/(n+1)]2 for glass (n=1.50 , then R = 0.04 = 4% at each surface!


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Anti-Reflective CoatingsIMAGE: http://www.hep.princeton.edu/~p104/lectures/lecture09.pdf

• External Reflection at a dielectric boundary:• Sir David Brewster determined experimentally in 1812• The reflected p polarized component vanished at Brewster’s angle.• This angle is when the reflected and refracted rays are normal ( 90o)to

each other. i.e. (θo + θc ) = 90o or here as (θb + θr ) = 90o


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Anti-Reflective CoatingsIMAGE: http://www.mellesgriot.com/pdf/005.4-5.7.pdf

• Reflection at a dielectric boundary:

• KEY IDEA:

• The phase of the light wave reflected from a rare to dense medium is shifted by 180o (π).

• This means a beam in a medium with a lower refractive index than the reflecting medium

• I.e. no < nc

Coefficient of reflection: ρ = (no - nc)/ (no + nc)If ρ is negative there is a 180o phase shift. ( normal incidence)


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Anti-Reflective Coatingsimage: http://www.mellesgriot.com/pdf/005.8-5.11.pdf

• Reflection at a dielectric boundary:• KEY IDEAs:• Optical path length: OPL• Transparent material ( one way path): OPL = nd• Transparent material on reflective substrate ( two way path): OPL = 2nd

• Reflected phase shift in film:

φ = 4ncd π cos θ 2/ λ (1)• nc = coating refractive index• d = coating thickness• θ 2 = angle of incidence

λ = wavelength


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• Thin film interference:• Example: Oil on water


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• Thin film interference


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Anti-Reflective Coatingshttp://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/interf.html#c2


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• Anti - Reflection Coating (ARC):• ARC Primciple:

• If the phase shift between the two reflected wavefronts r1 and r2 is 180o (π) as adjusted by the coating thickness and refractive index, they will interfere destructively and the overall reflected intensity is a minimum.

• If these two reflected wavefronts r1 and r2 also have equal amplitudes ( intensity), then the overall reflected intensity will be zero.

• Therefore the requirement for a “perfect” single coating Anti-reflection film for the cancellation of the reflectedwavefront is:

• 1. Refected wavefronts are 180 out of phase.

• 2. Refected wavefronts have same intensity ( amplitude).

r1r2


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• Anti - Reflection Coating (ARC):• Simple math for calculation of Anti-reflective coating ( TARC or

BARC) thickness and refractive index.

• SIMPLE CASE Assume non-absorbing film ( no k extinction coefficient), un-polarized coherent light, and normal incidence.

• A. ARC Phase condition: require a 180o (π) phase shift to cancel reflections. Find ncd (optical thickness 2 passes) where

• Reflected phase shift: φ = π = 4ncd π cos θ 2/ λ (1)• then ncd = λ / 4cos θ 2• then let θ 2 = 0• then Optical thickness (OPL) = ncd = λ / (4cos θ 2 )= λ /

(4cos 0) = λ / 4• So a film thickness d of λ / 4nc satisfies the 180o

(π) phase shift!


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• Anti - Reflection Coating (ARC):• SIMPLE CASE Assume non-absorbing film ( no k

extinction coefficient ), un-polarized coherent light, and normal incidence.

• no = refractive index of medium ( air or photoresist for BARC))

• nc = refractive index of AR coating

• ns = refractive index of reflective substrate

• B. ARC Equal Amplitude condition: require two reflected wavefronts r1 ( from top surface of ARC in air) and r2 ( from substrate surface in ARC) also have equal amplitudes ( intensity)

• r1 = r2• [(no - nc)/ (no + nc)]2 = [(nc - ns)/ (nc + ns)]2

• [(no - nc)/ (no + nc)] = [(nc - ns)/ (nc + ns)]• [(no nc – no ns + nc

2 – nc ns)] = [(no nc - nc2 + no ns – nc ns)]

• nc = (no ns)0.5


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Anti-Reflective Coatingshttp://physics.bu.edu/~duffy/PY106/Diffraction.html

• ARC: Requires 2 conditions:• Waves are 180o out of phase and same amplitude


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Key photoresist CD variation Effects:

Swing Curve: Interference of r1 and r12

Standing Waves: Interference of i1 and r1

Reflective substrate ns

Medium no

i1

r1

r12


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Key photoresist CD variation Effects ( Swing curve and standing waves) reduced with Anti-reflective coatings:

BARC: Cancel r1 with interference with r2

TARC: Cancel r12 with interference with r22

Substrate ns

Medium no

i1 r1

r12

BARC nc

r22r2

TARC ntop


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Anti-Reflective CoatingsSpecific films n and k

Material Wavelength n kSilicon 365 6.522 2.705

248 1.57 3.565Aluminum 365 0.407 4.43

248 0.19 2.94BARLi 365 1.653 0.3325

248 1.686 0.255AZ7511 (i- line) 365 1.716 0.025

DX981 248 1.76 0.0045


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Anti-Reflective CoatingsSwing curve % CD deviation from Target

Photoresist thickness microns


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Anti-Reflective CoatingsSwing curve Top Antireflective coating: Blue

curve AZ Aquatar: @ 365nm

60

70

80

90

100

110

120

0 .8 0 .85 0 .9 0 .95 1 1 .05 1 .1 1 .15 1 .2 1 .25

Ener

gy [m

J/cm

2 ]

Photoresist Thickness Microns


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Anti-Reflective CoatingsSwing curves

The action of bottom and top antireflective layers on the swing

amplitude S is described in good approximation by Brunner’s formula

R eb t- αdS R= 4

where

Rb: reflectivity at the resist/substrate interface – reduced by bottom antireflective coats BARC

Rt: reflectivity at the resist/air interface - reduced by top antireflective coatings TARC

α: resist absorbance - increased by dye addition (Dill’s B parameter)


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Anti-Reflective CoatingsSwing curves

On anti-reflective coatings, the swing curves can be so shallow that the conventional definition of “swing curve ratio”

can be misleading. In the example below, one would still find a swing ratio of nearly 9% in the limit of the periodic part going to zero. For shallow swing curves, this definition is best replaced with that of the amplitude of the periodic part. The predictions made by Brunner’s formula do not include the linear part of the swing curve, and can therefore only be compared to swing amplitudes.

−S

I I

Imid

= max min


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Anti-Reflective CoatingsReflectivity Curve for a BARC @ 365 nm

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.1 0.2 0.3 0.4 0.5

bottom coat thickness [µm]

Ref

lect

ivity

[rel

ativ

e to

Si]

R into resistR into air


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Anti-Reflective CoatingsReflectivity Curve for a BARC @ 365 nm

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.1 0.2 0.3 0.4 0.5

bottom coat thickness [µm]

Ref

lect

ivity

[rel

ativ

e to

Si]

R into resistR into air

6065707580859095

100105110

0.8 0.85 0.9 0.95 1resist thickness [µm]

Dose

toCle

a r[m

J/cm²

]

Dose

toCl

ear[m

J/cm²

]0 nm (bare silicon)

6065707580859095

100105110


Dose

toCl

ear[

mJ/cm

² ]

90 nm

6065707580859095

100105110


Dose

toC l

ear[

mJ/c

m²]

190 nm


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Top Anti-Reflective CoatingsTARC

* TARC Top Antireflective coating: Spin on top of Photoresist coating

* IBM Invention again!! Only 2 companies manufacture!

* Easy Process Integration: very effective for reduced swing curve

* REQUIREMENTS:

* No intermixing with photoresist:

* AZ Aquatar III-45 (MSDS): Fluoroalkylsulfonic Acid salt <5%; Water > 95%

* JSR NFC-540 ( MSDS): Fluoroacrylic polymer 1-10%;Anionic Surfactant 0.1-1%; Water > 90-99%%

* Non-absorbing: k=0.00 at actinic wavelength

* Water soluble: I.e. comes off with a DI water rinse pre-develop cycle

* Requires a refractive index = nc = (no ns)0.5= 1.26 ( 365nm) or 1.30 (248nm)


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Property Organic A.R.C.s Inorganic A.R.C.sReflectivity and swing ratioreduction - 1st minimum

++ +++

Reflectivity and swing ratioreduction - 2nd and higher minima

+++ 0

Etch rate 0 or + +++Coating conformity 0 ++Thickness tolerance + 0Plasma damage +++ --Refr. index reproducibility +++ 0Throughput ++ +Cost of Ownership + 0Stack issues +++ -Planarization capability ++a) --Rework capability +++ --

Anti-Reflective CoatingsOrganic Vs Inorganic BARC


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Bottom Anti-Reflective Coatingsa. Organic BARC

Organic BARC: Thin films interference effect with strong absorption.

BARC Bottom Antireflective coating: Spin on substrate and cure,photoresist is coated over BARC. Polymer bound dye with acid. Film is spun coated onto wafer abd cured at a high temperature ( 200C). During the cure bake the acts in the film acts to cross-link the polymer chains. This “hardened” coating prevents mixing with thephotoresist.

* IBM Invention again!! Only 3 companies manufacture!

AZ ( Clariant), Shipley, and BrewerSlightly more complex Process Integration: very effective for reduced swing curve and reflective notching. Can use BARC on all substrates

( ) ( )( ) ( )2

212

21

221

221

kknnkknnRBarc=%reflectivity

+++−+−

=


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Bottom Anti-Reflective Coatingsa. Organic BARC

* REQUIREMENTS:

* No intermixing with photoresist:

* AZ KRF 17B (MSDS):

* Shipley AR-3

* Brewer 42P

* Absorbing: k=0.00 at actinic wavelength

PROs: Easy to implement compared to inorganic BARC

Can typically use one BARC effectively on all substrates.

CONs: Need good coating thickness control in spin unit.

Need another chemical line on the wafer track.

Need to modify etch recipe to etch through BARC

Need to strip BARC completely off after etch step

BARC is not 100% conformal so there is a BARC pooling issue: I.e. areas of thicker

BARC causing reflection control and etch variations.

Cannot “tune” BARC for 0% reflectance.

DUV resist can be poisoned by incomplete BARC cure cycle ( I.e. Acid left in film!)


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Bottom Anti-Reflective Coatingsa. Organic BARC Typical components

* POLYMERS: DYES

*

Dye dimers

NH

O

CN

HO

O

NH

ONCO

HO

H3C

H3C

anthracene derivatives (DUV)

X=CH2OH, COOH, NH2, ...X

Coumarin 138 (λmax = 365 nm)

O ON

Curcumin (λmax = 430 nm)

HO

H3CO O

OH

OH

OCH3

Sudan Orange G (λmax = 388 nm)

NN OH

HO

Coumarin 7 (λmax = 437 nm)

O O

N

N

N

H

O

OO

O

N

N

O

O

O

NH

NH

OHOOC

COOH

-2 H2O

polyimide(cured form)

polyamic acid (apply form)

can be made to be developer soluble by partial curing

SO2 O

SO2 CH2 CH

CH2

CH3

PBS


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Anti-Reflective Coatingsb. Inorganic BARC or DARC

Inorganic BARC: Thin films interference effect using dielectric film ( only slight absorption. Film

DARC :Dielectric Bottom Antireflective coating: Deposit films such as silicon oxide or silicon oxynitride Si x ON y

*The thin film deposition tool manufacturers have equipment and recipes to deposit these films. AMAT and NovelusSlightly more complex Process Integration: very effective for reduced swing curve and reflective notching.

PROs: Can tune n and k for optimal reflection control ( 0%) on all substrates.

CONs: Need exact n, k, and thickness control in deposition tool or anti-reflective properties are non-existent.

*There is only one n,k, and t that is optimal for a given substrate. Therefore there are different deposition recipes for each substrate.

* Expensive setup >. Deposition tool

* Film remains on wafer. Can impact electrical performance. So this need to be considered.

* SiON can cause DUV photoresist poisoning, I.e. the SiON acts like a base ( getters H+). Need thin Protective oxide!


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Anti-Reflective Coatings: Complete treatment with absorbing film

Clariant Antireflective Coatings Lecture Notes

t e-ik2x

medium 2

e-ik1x

r e-ik1x

medium 1

( ))kk(inn

iknññ

ñtt2121

22

21

212

22+++

−=

+==

)kk(inn)kk(inn

ññññrr

2121

2121

21

2112 +++

−+−=

+−

==

for normal incidencer12, t12: Fresnel coefficients for reflection and transmission

( ) ( )( ) ( )2

212

21

221

221

2121

2121

2121

2121

1212

kknnkknn

)kk(inn)kk(inn

)kk(inn)kk(inn

rr*rrR = % reflectivity *

+++−+−

=

+−+−−−

⋅+++−+−

=

⋅=⋅=


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Anti-Reflective Coatings: Absorbing Film Reflection at interface of two medium: PR = ñ = 1.60 - 0.02i

Clariant Antireflective Coatings Lecture Notes

( )( )

( ) ( )( ) ( )R

n k

n kR

n n k k

n n k kair

Si Si

Si SiPR

PR Si PR Si

PR Si PR Si

=− +

+ +=

− + −

+ + +

1

1

2 2

2 2

2 2

2 2;

0

1

2

3

4

5

6

7

200 250 300 350 400 450 500

wavelength [nm]

n ASP

k ASP

Si

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

200 300 400 500 600

wavelength [nm]

refle

ctan

ceR into airR into resist

Si


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Anti-Reflective Coatings: Absorbing Film PROLITH SIMULATIONS: n and k effects

Impact of n and k on reflectivity Prolith Simulations

0

0.1

0.2

0.3

0.4

0.5

0.6

0 20 40 60 80 100 120 140 160 180 200

BARC thickness nm

Subs

trat

e R

efle

ctiv

ity %

n= 2.0929 k = 0

n= 1.500 k = 0

n= 2.0929 k = 0.3

n= 2.0929 k = 0.6


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Inorganic BARC: DARCDielectric Anti-Reflective Coating

• SiOxNy Thickness PROLITH SIMULATIONS


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• SiOxNy


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• SiOxNy Thickness for various Polysilicon Thicknesses


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• SiOxNy Thickness for various Top oxideThicknesses


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http://www.semiconductor.net/semiconductor/issues/issues/2000/200010/six001007met.asp

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