EE143 F2010 Lecture 15 Reactive Ion Etching (R IE)ee143/fa10/lectures/Lec_… · ·...

Professor N Cheung, U.C. Berkeley

Lecture 15EE143 F2010

1

Reactive Ion Etching (RIE)

~ plasma

wafers

RF13.56MHz

Parallel-PlateReactor

Plasma generates (1) Ions(2) Activated neutrals

Enhance chemical reaction

Sputtering



2



3

Remote Plasma Reactors

Plasma Sources(1) Transformer

CoupledPlasma(TCP)

(2) ElectronCyclotronResonance(ECR)

-bias

pump

e.g. quartz

coilsplasma

wafers

Pressure1mTorr 10mTorrbias~ 1kV



Processes Occurring in Plasma Etching



5

• Synergism of ion bombardment AND chemical reactiongive the high RIE rates.



6

REMOVAL ofsurface filmand DEPOSITIONof plasma reactionproducts canoccursimultaneously



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RIE Etching Sequence

gas flow

1

2 3

5

4X

diffusion ofreactant

diffusion of by productdesorption

chemicalreaction gaseous by productsabsorption

Substrate



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Volatility of Etching Product

* Higher vapor pressure higher volatility

)(..4.. 4

*

pressurevaporlowCuClClCugeSiFFSige

mask

Al-Cu Metal

(high vapor pressure)

Example

Difficult to RIE Al-Cualloy with high Cu content

Do not want CuCl residues



9

kTH v

ePP

0

1/T

P1500oC

AlCl3CuCl

[Al-Cu alloy]

Cl2 as etching gas.200oC

1~2% typical

Example

Difficult to RIE Al-Cualloy with high Cu content

Vapor pressure of by-product has to be high



10



11

Examples

4*

*34

3*

4

4

2

SiFFSieFCFeCF

CFFCF

Use CF4 gasFor etching Si

F* are Fluorine atoms with electrons



12



13

x

xzyx HO

COOOHC

AlClClAl

eClCCleCCl

2

3*

*34

3

2

Photoresist

Aluminum+



14

How to Control Anisotropy ?

1) ionic bombardment to damage expose surface.2) sidewall coating by inhibitor prevents sidewall etching.



15

Sidewall passivation filmsHCl/O2/BCl3 chemistry

Si

Photoresiston top of Si

This exaggerated picture shows a passivation layer so thickthat it can peel off from the sidewall



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How to Control Selectivity ?

E.g. SiO2 etching in CF4+H2 plasma

SiO2

Si

Rates

%H2 in (CF4+H2)

S Rate SiORate Si

2

H2%

P.R.

SiSiO2

Reason:

4

**

SiFcontentFHFHF

S



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1 2

Si

SiO2

Poly-Si

Oxide

Reason:

rateSiOOSirateetchingSiincreasesF

FCOFCFO xx

22

*

)2(*)1(

%O2 in CF4

Rates

Example: Si etching in CF4+O2 mixture



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For reference only



19

effect

Controlvariable

Effect of RIE process variables on etching characteristics



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Temperature Dependence of Selectivity

R A e

R A e

S RR

AA

e

QkT

QkT

Q QkT

1 1

2 2

1

2

1

2

1

2

1 2

77oK if Q1<Q2

1/T

S

R= etching ratesA = proportional constantsQ = activation energies



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P.R.

Al

BCl31

2 Cl2-based RIE

native Al2O3

3 Form oxide again (gently)

Al Al

* It is a three-step sequence :1) Remove native oxide with BCl32) Etch Al with Cl-based plasma3) Protect fresh Al surface with thin oxidation

Example: RIE of Aluminum Lines



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Example: Etching of Deep Trenches

~1m

mask

ballooning

mask

mask erosion

trenching

by-productresidue

Si

“ideal” “problems”



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• Ballooning:– Use chemistry with a good sidewall inhibitor.

• Trenching:-Use high pressure to increase ion-neutral scattering(ion trajectory less directional)

• Bottom Roughness:– Increase vapor pressure of etching byproduct.

Approaches to minimize deep trench etching problems



Hard Mask for Etching

Photoresist

oxide

RIE 1

RIE 2

poly

To minimize CD distortion, sometimes a two-step RIE processis used. Example: Process 1 to transfer pattern from resist;followed by Process 2 to transfer pattern from oxide to poly.

24EE243S2010 Lec22



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A better Solution:Multiple step RIE sequence



26EE243S2010 Lec22



27* Can etch through whole Si wafer thickness



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Local Loading Effect

Wsmall

Wlarge

More etchant consumptionLess etchant consumption



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* smaller trenches etch at a slower rate than larger trenches.

CCl2F2/O2 RIE

RIE Lag



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Etching Profile Simulation(http://cuervo.eecs.berkeley.edu/Volcano/)

Many features such as mask erosion, sidewall angles,undercut, etc. can be predicted from the simple etchingmodels.

Material and Isotropic and Directional Etch rates in nm/s

Resist 0.83 0.0

Oxide 0.0 8.0

Silicon 11.7 5.0

Substrate 0.0 0.0

Q = sin-1 (5.0/(11.7 + 5.)) = 17.4o

Angle appears larger due to unequal scales in x and y.

For reference only



SUMMARY OF ETCH MODULE

• Etch Bias, Degree of Anisotropy, Etch Selectivity• Worst-case considerations for etching• Wet etch – qualitative• KOH/EDP etch of Si (anisotropic)• Reactive Ion Etch equipment- qualitative• Synergism of ion bombardment and chemical etching• Selectivity Control - Gas mixture, Temperature• Anisotropy Control – Inhibitor deposition, Substrate

bombardment• RIE examples: Aluminum, deep trench etching.• Pattern and Aspect ratio Dependence - qualitative

EE143 F2010 Lecture 15 Reactive Ion Etching (R IE)ee143/fa10/lectures/Lec_… · ·...

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