Optical Proximity Correction - University of California, Berkeleyee143/fa10/lectures/Lec_05.pdf · 2010-09-08 · Professor N. Cheung, U.C. Berkeley EE143 F2010 Lecture 5 1 Optical

Professor N. Cheung, U.C. Berkeley

Lecture 5EE143 F2010

1

OpticalProximityCorrection

*Auxiliary featuresadded on mask

Mask Wafer



2

Overlay Errors

+

+

+

+

Alignmentmarksfrompreviousmaskinglevel

wafer

alignmentmask

photomaskplate



3

(1) Thermal run-in/run-out errors

sisimm TTrR

T Tsi change of mask and wafer tempcoefficient of thermal expansion of

mask & Si

m

m si

, .,

run-outerror

waferradius



4

run-out

(2) Translational Error

referrer

image

n+

Al

p



5

(3) Rotational Error



6



7

Characterization of Overlay Errors

+ +

+

+

L

T

B

R

wafer

O

O

O

O

+

R

O

O =opticalimage

+ =alignmentmarks onwafer

y

x

C



8

m T R C L Bx 0.0 0.7 0.5 0.3 1.0y 0.7 1.0 0.5 0.0 0.3

* Center of wafer has only translation error

Terror = (0.5, 0.5)

After subtracting Terror,

Example



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T R C L Bx -0.5 0.2 0 -0.2 0.5y 0.2 0.5 0 -0.5 -0.2

T

B

R

wafer

yx

L0.5

0.2



10

radianserrorRotational

cmDisdiameterwaferIf

clockwise][countermerrorRotationalmerroroutRun

510

10"4

5.02.0

R

y



11

Reference markson wafer

Optical imageof maskalignmentmarks

With thermal run-out, the alignment error is 1/2 ofthe image/reference difference [best scenario]



12

Total Overlay Tolerance

2 2total i

i

i = std. deviation of overlay error for ith masking step

total = std. deviation for total overlay error

Layout design-rule specification should be > total



13

Example: Contact to source/drain of MOSFET.

n+

SiO2 SiO2

Al“ideal”

p-Si

SiO2 SiO2

Al

p-Sin+ “short”, ohmic contactAlignment error

between oxide openingand n+ pattern



14

SiO2

n+

SiO2

Solution: Design n+ region larger than contact hole

Al



15

Two Resist Types

• Negative Resist– Polymer (Molecular Weight (MW) ~65000)– Light Sensitive Additive Promotes Crosslinking– Volatile Solvents– Light breaks N-N => Crosslink Chains– Sensitive, hard, Swelling during Develop

• Positive Resist– Polymer (MW~5000)– Photoactive Inhibitor (20%)– Volatile Solvents– Inhibitor Looses N2 => Alkali Soluble Acid– Develops by “etching” - No Swelling.



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hv

mask

P.R.exposedpart isremoved

=E resist sensitivity

Resist contrast

T

log

100%

E1 ETexposurephotonenergy(log scale)

resist thickness remaining

(linearscale)

~ 5 to 10

LOG TO BASE 10

Note: In the 143 Reader, is defined as natural logNote: In the 143 Reader, is defined as natural log

Positive Resist



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Positive P.R. Mechanism

Photons deactivatesensitizer

polymer +photosensitizer

lesscross-linking

dissolvein developersolution



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Positive Resist Exposure Reaction“diazide” “ketene” - a = C = 0 group

moisture

“carboxylic acid”

“soluble ester”

The ketene is short-lived intermediate

The carboxylic acidcan react with thealkaline solution (thedeveloper) to form asoluble ester.

PAC



N Cheung EE243 s2010 Lec 1519

Chemical Amplified Resist (CAR)

Photo-Acidgenerator

For reference only



20

Negative P.R. Mechanism

hv

E1ET

remaining

photonenergy

afterdevelopment

1

1log EET

%

Log to base 10

mask

hv => cross-linking => insoluble indeveloper solution.



21

resistresist

substrate

resist

substrate

resist

Position x

Finitecontrast

Infinitecontrast

Optical image

Why High-Contrast Resist is desirable ?



22



23

Positive vs. Negative Photoresists

• Positive P.R.: higher resolution aqueous-based solvents less sensitive

• Negative P.R.: more sensitive => higher exposure throughput relatively tolerant of developing conditions better chemical resistance => better mask material less expensive lower resolution organic-based solvents



24

Standing Waves

substrate

PositivePhotoresist

hv

substrate

After developmentPositivePhotoresist.

*Photoresist has a finite thickness

Higher Intensity

Lower Intensity

Faster Development rate

Slower Development rate



25

reflectingsurface

OxidePhotoresistAir

E1E2

E3

E4

x=0 x=d x=dr

I23 (x) =1T

0

T(E2(x)+E3(x))2dt

=12 (E2 - E3)2 + 2E2E3sin2[k(d-x)]

I23 (max) =12(E2 + E3)2 ; I 23(min) =

12(E2 - E3)2

I (max)23

I (min)23

d

x

In ten sity m in im a occu r at :2n (d -x) = 0 , , 2 , ... .. ..

In ten sity m a x im a occu r a t :2n (d -x ) = /2 , 3 , 5 , ... .. . .

Resist profile and energydeposition depend onoxide thickness underneath(see handout for derivation)

Resist profile and energydeposition depend onoxide thickness underneath(see handout for derivation)

Standing wave effect



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P.R.

Intensity = minimum whenn

mdx2

x d

m = 0, 1, 2,...

Intensity = maximum whenn

mdx4

m = 1, 3, 5,...

n = refractive index of resist

SiO2/Si substrate



27

Simulated Resist Cross-section as function ofdevelopment time



28

Proximity Scattering



29

Approaches for Reducing Substrate Effects

• Use absorption dyes in photoresist• Use anti-reflection coating (ARC)• Use multi-layer resist process

1: thin planar layer for high-resolution imaging2: thin develop-stop layer, used for pattern transfer to 33: thick layer of hardened resist

(imaging layer)

(etch stop)(planarization layer)



30

Electron-Beam Lithography

V312.

Angstroms

for V in Volts

Example: 30 kV e-beam=> = 0.07 Angstroms

NA = 0.002 – 0.005Resolution < 1 nm

But beam current needsto be 10’s of mA for athroughput of morethan 10 wafers an hour.



31

Low Throughputfor both raster andvector scanning (SerialProcess)

VariableBeam-shapeEBL

StencilMaskEBL



32

The Proximity EffectMonte Carlo simulation of electron trajectories



33

e-beam lithographyresolution factors

• beam quality ( ~1 nm)

• secondary electrons ( lateral range: few nm)

performance records

organic resist PMMA ~ 7 nm

inorganic resist, b.v. AlF3 ~ 1-2 nm



34

•A liquid with index of refraction n>1 is introduced between theimaging optics and the wafer.

Immersion Lithography

liquid = air /n

With water, the index ofrefraction at = 193 nmis 1.44, improving theresolution significantly.



Phase-Shifting Mask

For resolution enhancement . Example shown is an alternating PSM



36

EUV Lithography

=11.2 nm



N Cheung EE243S05 Lec 1637

Mo-SiReflective Mask

Schematic for EUV Litho

reflectivity



N Cheung EE243S05 Lec 1638

Nanoimprinting



N Cheung EE243S05 Lec 1639Empirical : Resolution (in Å) ~ 23 Areal Throughput (in um2/hr) 0.2

Why photolithography ?

Highthroughput



40



41

‘Hands On’ Exploration of Imageshttp://cuervo.eecs.berkeley.edu/

A web browser-based simulator of lithography

Documents

Optical Proximity Correction - University of California, Berkeleyee143/fa10/lectures/Lec_05.pdf · 2010-09-08 · Professor N. Cheung, U.C. Berkeley EE143 F2010 Lecture 5 1 Optical