reactive ion etching (rie) - a-tech systematechsystem.co.kr/sub1/pdf/106.pdfreactive ion etching...

Professor Nathan Cheung, U.C. Berkeley EE143 Lecture # 16

Reactive Ion Etching (RIE)

~ plasma

wafers

RF13.56MHz

Parallel-PlateReactor

Plasma generates (1) Ions(2) Activated neutrals

Enhance chemical reaction

Sputtering


Remote Plasma Reactors

Plasma Sources(1) Transformer

CoupledPlasma (TCP)

(2) ElectronCyclotronResonance (ECR)

-bias

pump

e.g. quartz

coilsplasma

wafers

Pressure1mTorr 10mTorrbias~ 1kV≤


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


REMOVAL ofsurface film and DEPOSITIONof plasma reactionproducts canoccur simultaneously


RIE Etching Sequencegas flow

1

2 3

5

4X

diffusion of reactant

diffusion of by productdesorption

chemical reaction gaseous by productsabsorption

Substrate


Volatility of Etching Product

* Higher vapor pressure higher volatility

)(..

4.. 4*

pressurevaporlowCuClClCuge

SiFFSige

→+↑→+

mask

Metal

⇒

(high vapor pressure)


Vapor pressure of by-product has to be high.

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


Examples

↑→+

++⇔+

+→+

4*

*34

3*

4

4

2

SiFFSi

eFCFeCF

CFFCF

Use CF4 gasSi

F* are Fluorine atoms with electrons


x

xzyx HO

COOOHC

AlClClAl

eClCCleCCl

2

3*

*34

3

2

+

↑→+

+++⇔+

Photo Resist

Aluminum


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


Sidewall passivation films Photoresist on top of Si

HCl/O2/BCl3 chemistry


How to Control Selectivity ?

E.g. SiO2 etching in CF4+H2 plasma

SiO2

Si

Rates

%H2 in (CF4+H2)

SRate SiO

Rate Si= 2

H2%

P.R.

Si

SiO2

Reason:↓∴

↓∴→+

4

**

SiF

contentFHFHF

S


Example Si etching in CF4+O2 mixture

12

Si

SiO2

Poly-Si

Oxide

Reason:

↓∴→+

+→+

rateSiOOSi

rateetchingSiincreasesF

FCOFCFO xx

22

*

)2(

*

)1(

%O2 in CF4

Rates


Effect of RIE process variables on etching characteristics


Temperature Dependence of Selectivity

( )

R A e

R A e

SRR

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


Example: RIE of Aluminum Lines

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 :Etching of Deep Trenches

~1µm

mask

ballooning

mask

mask erosion

trenching

by-productresidue

Si

“ideal” “problems”


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

• (3) Trenching– Use high pressure to increase ion.

• neutral scattering (less directional)

• Bottom Roughness– Increase P of byproduct.


Solution: Multiple step RIE sequence


robotto

handlewafers

EtchStep 1

EtchStep 2

EtchStep 3

wafer I/O

We need Cluster Tools

In general, an etching process can be several steps in sequence


Local Loading Effect

Wsmall

Wlarge

More etchant consumptionLess etchant consumption


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

CCl2F2/O2 RIE


XeF2 Etching of Si

* Dry, isotropic, vapor-phase etch XeF2 vapor pressure (~3.8 Torr at 25 °C)

2 XeF2 + Si àà 2 Xe (g)↑↑ + SiF4 (g) ↑↑

Advantages :•Highly selective to silicon with respect to Al, photoresist, and SiO2. •Isotropic, large structures can be undercut.•Fast ( ~10µm per hour)•Gas phase etching, no stiction between freed structure and substrate

Disadvantages: • No known etch stops for Si substrate


Photon-Assisted Etching

reactive ion etching (rie) - a-tech systematechsystem.co.kr/sub1/pdf/106.pdfreactive ion etching...

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