Plasma RIE Etching Fundamentals and Applications

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Birck Nanotechnology CenterPL SM RIE ETCHINGL SM RIE ETCHING

FUND MENT LS ND PPLIC TIONSUND MENT LS ND PPLIC TIONSUND MENT LS ND PPLIC TIONSUND MENT LS ND PPLIC TIONS

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u ne

.

2. Plasma Fundamentals

3. The Physics and Chemistry of Plasmas

. n so ropy ec an sms

5. The Etchin of Si and its Com ounds

6. The Etching of Other Materials

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Electron (e-)

Positive ion (Ar +, Cl+, SiF4+, CF3

+)

Positive ion mass in RIEs >>mass of electron

Radical (F, Cl, O, CF3)

bonding

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cont nue

Mean free ath avera e distance a article travels before collisions

(Dependent on the species)5

)(mT P

1atmosphere= 760 Torr = 1*105 Pascals

Pumping speed (S) [liters/sec]

Gas flow rate (Q) [Torr-liters/sec] or [sccm]

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asma acuum ystem

[Q]Mixed gasvalve

Gas lines

MFC box

Chamber Vacuum pump [S]

ga e

valve

V

Matching

network He backside

cooling

P tS-dP t ⋅V= chamber volume

PSQ:stateSteadyIn

Vdt

⋅=

=

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ec an ca umps

Pumping speed: 20-500 m3/hUltimate pressure: 1-10 mTorr

[BOC Edwards Dry Pump]

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[Kurt J. Lesker]


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ur o umps

= -

Pumping speed: 50-3000 l/sUltimate pressure: 10-5-10-8 Torr

[Wiki]

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[BOC Edwards Turbo Pump]

[TP controller]


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ass ow on ro er

Heater

Thermal-based flow meter

Q [sccm]

T1 T2

T2 – T1= Cp × Q

Cp is specific heat.

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[HORIBASTEC]


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or as ox

Mixed gasline

as

lines

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Panasonic MFC Box


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u oma c ressure on ro er& Gate Valve

Butterfly valvePendulum valve

P×=

10 [VAT]


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amp or ec ros a c uc

Electrostatic Chuck (ESC)Clamp

DielectricSi wafer

Base Plate

+++++ -------

------- +++++

+V -V+ -

He

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enera or a c ng e wor

RF Generator

NetworkPF PL

L

ZIn

PL=PF- PR

PR

ZS= 50Ω

In general: ZL≠ ZS

Purpose of Matching Network: Zin= ZS to

maximize power delivery from source.

Manual or Automatic

Matching NetworkZL

ZS C1L

12

[Gambetti]

ZIn


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2.2. Plasma FundamentalsPlasma Fundamentals



5. The Etching of Si and its Compounds


13 [Plasmas.org]


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a s asma

• Plasma is the fourth state of matter. It is an ionized gas, a gas

into which sufficient energy is provided to free electrons fromatoms or molecules and to allow both species, ions and

, .

14

.


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.

Vc=0 Vc= -100

XRed: n iBlack: ne

x

XVp

V(x)

XVp

-100v

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Vp= a few vol ts


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.

VRF G p RF

G E

ZV (t) V (t)

Z Z

=

+

VRF(t)

ZE ZG

pE G G E

p RF

A A Z Z

V (t) V (t)

⇒

V(x)

X

VRF

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G

p RF

G E

Z

V (t) V (t)Z Z= + Actual RIE

E G G E

p RFV (t) V (t)VRF(t)

AEZGVp(t) ≈ 0

Ion transit time (Tion) is the time it takes the ion

to traverse the sheath.

1/Freq


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asc en s awDescribes how the breakdown voltage depends on electrode

separation and the pressure based on ideal gas law.

a (p.d)× V

l n(p.d) b=

+

600

800

p: Pressure

d: gap distance

a & b: constants

400

[Torr cm]100

10110-1

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n uc ve oup e asma

STS ASE and AOE systems

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y g ens y asmas Lower ion bombardment energies improve selectivity and reduce ion-

bombardment-induced physical damage of the wafer surface.

Lower ion ener ies however result in the lower etch rates and reduced

anisotropy!

,

due to high density plasmas.

.

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3.3. The Physics and Chemistry of PlasmasThe Physics and Chemistry of Plasmas




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ec ron- o ecu e o s ons An energetic electron colliding with a neutral etch gas molecule can

create any of the following processes:

Dissociation AB + e- A+B + e- CF4 + e- CF3+F+e

-

- - - -on za on e e r e r e

Often dissociation and ionization

Occur in one collision:

CF4 + e- CF3

++ F+ 2e-

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a ca s an ons n asmas Positive ions are very important for etching processes.

Radicals are more numerous than ions in gas glowdischarges because:

1. The electron energy required in order to break chemical bonds in themolecules is usually less than the energy required to ionize these

molecules.

2. Radicals have a longer lifetime in the plasma compared to ions becausean ion is almost always neutralized during a collision with a surfacewhile radicals often do not react with a surface and are reflected back

into the plasma.

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a s asma c ngCF4

Chamber

valveCF4

+ e- CF3

++ F+ 2e-

Si + 4F SiF4 (gas)Vacuum pump

Si Wafers

SiF4

CF3+, F

1- Need an etching gas

2- Establish a glow discharge

to form a volatile by-product

4- Pump away the volatile by-product

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y asma c ng

Compatible with automation

Anisotropic etching

rec se pattern trans er espec a y or ano-sca e eatures

Substrate

Isotropic etch Directional etch Vertical etch

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-Solid Etch Gas Etch Product

Silicon CF4,Cl2, SF6 SiF4, SiCl4, SiCl2SiO2, SiNx CF4, C4F8, SiF4, CO, O2, N2,

3, 6

Al BCl3/Cl2 Al2Cl6, AlCl3

Ti, TiN Cl2, CF4 TiCl4, TiF4

Organic Solids O2, O2/CF4 CO, CO2,

GaAs & III-V Cl2/Ar, BCl3 Ga2Cl6, AsCl3

2 2 2 2

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Fe, Ni, Co, Au, Ag, Pt halides not volatile

Cu Cu3Cl3 is volatile above 200C


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z

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[Handbook of Advanced Plasma Processing Techniques by Pearton]


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..



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• Etch rate Mask

e n on

• Mask (Photoresist, Metal, SiO2, …)

o

• Selectivity

Substrate

• Anisotropy degreeL

H

f A 1H≡ −

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eac ve e c ng s an so rop c process

Has very high selectivity!Si + 4F SiF4 (gas)

MaskSiF

4

Substrate

F

Isotropic etch

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process!Has lower selectivit and etch rate!

Si + Ar + Si + Ar +

Mask Ar +

Si

Substrate

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Directional etch Vertical etch


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Has better selectivity and much higher etch rate!

Effect of Ions:

33[J. Appl. Phys. 50, 3189 (1979)]

Breaks bonds, raises temperature locally

on the surface and provides activation energy


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ewa ass va on

where:

(a) Either the carbon is provided by the feed gas through the chamber

3, 4 8.

(b) Or the carbon is provided by the erosion of the photoresist etch

mask.

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ewa ass va on

Oxidation of the sidewall by adding O2 gas.

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osc rocess

500um Silicon Etched by T.

Maleki using STS ASE

(8um/min etch rate)

Switching SF6 and C4F8

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e s ewa m t c ness epen s to t e epos t on or pass vat on t me.


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empera ure e ec s n p asma e c ng

Wafer surface temp. depends on:- Chuck temperature

- on s energy an ens y

Reaction probability of radicals depends on substratetemperature.

or reducing reaction of F and Cl species with sidewalls.

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Notchin effect due to char in

oxide by ions

Can be reduced by using low

in pulsed modee-

+++++++

+

[J. Vac. Sci. Technol. B 19.5., Sep/Oct 2001]

RF biasoff

RF biason

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STS ASE has LF pulsed generator!!


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Grass or micromaskin issue mainl

because of metal mask sputtered on

the wafer

spec a o epen en ec

Typically large open areas etch faster than

smaller features!

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Feature of the same size etch more slowly in dense patterns

Than in wide open areas!

c ng

Species

Density

x

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u.



4. Anisotro Mechanisms

5.5. The Etching of Si and its CompoundsThe Etching of Si and its Compounds


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It’s a 6” ICP Bosch rocess dedicated

Gases:

for silicon etching!

66 44 88

OO22 ArAr

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Hi h etch-rate reci e:

Switchingtime

Pressure RF coil power RF bias power

Gas flow[sccm]

Etch 8.5 sec 40mTorr 2200W 40W 450 SF6

Passivation 3 sec 14mTorr 1500W 20W 200 C4F8

Etch rate ≈ 8µm/min for 500 µm feature

size with ~ 20% exposed area

High selectivity to PR≈ 75-100

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Low etch-rate reci e:

Switchingtime

Pressure RF coil power RF bias power

Gas flow[sccm]

Etch 13 sec 5mTorr 800W 25W 160 SF6

Passivation 7 sec 1mTorr 600W 20W 85 C4F8

Etch rate ≈ 2µm/min for 500 µm feature

size with ~ 20% exposed area

High selectivity to PR≈ 50Smooth side wall

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It’s a 6” ICP tool for etching of silicon,

oxide, nitrite, III-V, polymers, and some

metals!

SFSF66 CCHHFF33 ClCl22 OO22

CFCF44 ArAr BClBCl33 NN22

ases:

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ressure co

power

as

power 6 ow

[sccm]2 ow

[sccm]

Etch

Etch rate ≈ 1µm/min [Panasonic]

Pressure RF coil

power

RF bias

power

Cl2flow

[sccm]

O2 flow

[sccm]

SiEtch

0.8Pa 350W 50W 63 1.2

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c ra e ≈ n m m n [Panasonic]


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power

power 3

[sccm]

Oxide 0.16 Pa 450W 50W 40

Etch rate ≈ 60 nm/min

2

Pressure RF coil

power

RF bias

power

CF4 flow

[sccm]

CHF3 flow

[sccm]

SiN 2 Pa 400W 30W 48 50 SiN3

Etch


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It’s a simple RIE.

It can be used for etching silicon, oxide,nitrite, SiC, and polymers.

SFSF OO ArAr

Gases:

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power

power 2

[sccm] [sccm]

Ti 2 Pa 400W 100W 100 5

Etch rate ≈ 3 µm/min

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Pressure RF coil RF bias BCl3 flow Cl2 flow Ar flow

power power [sccm] [sccm] [sccm]

GaAs 3 Pa 900W 75W 50 150 20

Etch

Etch rate ≈ 5.3µm/minSelectivity to PR≈ 5

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Pressure RF coil RF bias BCl3 flow Ar flow

power power [sccm] [sccm]

GaAs 0.6 Pa 500W 50W 15 60

Etch


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[Panasonic]


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How to Increase Etch Rate?How to Increase Etch Rate?

Increasing Main Coil Power

Increasing Platen or Bias Power Increasing Process pressure

Increasing Etch cycle time (for Bosch process)

How to Reduce Sidewall Roughness/Scallops?How to Reduce Sidewall Roughness/Scallops?

Keep etch and deposition cycle times to minimum, (for Bosch process)

Reduce process pressure

Reduce etch gas flow

, , ,

How to Increase Selectivity?How to Increase Selectivity?

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Reducing platen power


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How to Straighten the Profile?How to Straighten the Profile?

Using low pressure

Decreasing etch cycle time or increasing deposition cycle time (for Bosch process)

Optimizing platen power

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Trends for

Controllingprocess results

Etchrate

Profile Selectivity SidewallRoughness

Etch gas increase ↑↑ ↑ ↑

Pressure increase ↑↑ ↑ ↑

Etch Coil Powerincrease

↑ ↑ ↑

Platen Power

increase

↑↔ ↓ ↔

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c now e gmen

We would like to thank faculty members, staff, andstudents for their support.

Questions?Questions?

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Plasma RIE Etching Fundamentals and Applications

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