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11 Dry Etching

1. Introduction

(1)

Etching bias

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Isotropic etching vs. Anisotropic etching

Complete Isotropic Etching

Vertical Etching Rate = Lateral Etching Rate

B = 2hf

Complete Anisotropic Etching

Lateral Etching Rate = 0 B = 0

Degree of Anisotropy

,

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Etching Selectivity, S

Wet Etching S is controlled by:chemicals, concentration, temperature

RIE S is controlled by:plasma parameters, plasma chemistry,

gas pressure, flow rate & temperature

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(2)

Plasma Etching

reactive gas plasma, low energy ion bombardment

Reactive Ion Etching (RIE)

reactive gas plasma, high energy ion bombardment

Sputtering Etching

inert gas plasma, high energy ion bombardment

Plasma

EtchingRIE

Sputtering

Etching

(Torr) 0.1-10 0.01-0.1 0.01-0.1

(V) 25-100 250-500 500-1000

Yes Yes No

No Yes No

Selectivity

Anisotropy

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(3) /

- anisotropic

- - Clean Process (Vacuum)

-

-

- control

- radiation/plasma damage

-

- (Ni, Cu, Al2O3)

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2. Basic Physics and Chemistry of

Plasma Etching

(1) Plasma Etching Steps

Glow discharge

(, , radical)

gas stream

pumping-out

Example: CF4 + e- -> CF3

+ + F + 2e-

Si + 4F -> SiF4

* RIE

(+) Ion-bombardment

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(2) Glow discharge

Radical (ionization) (dissociation)

() O2 + e- -> O+ + O + e-

Plasma (+)

(+) (+)

Argon Plasma: Ar+ ,

Ar++, Ar2+

Oxygen Plasma: O2+ , O+

, O3+

Freon Plasma: CF3+ ,

CF2+, CF+, C2F5

+,...

F Al : AlF3

Cl Ni : NiCl2

Cl Al2O3 :

Al2O3 + Cl2 -> AlCl3 + O2 ()

Cl SiO2 :

SiO2 + Cl2 -> SiCl4 + O2 ( )

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3. CF4 Si, SiO2

Basic Chemistry: CF4 + e

- -> CF3+ + F + 2e-

Si + 4F -> SiF4 (volatile) or

Si + 2F -> SiF2 (volatile)

(1) CF4 + O2

F

CF3 + F -> CF4 (CF3 F )

CF3 + O -> COF2 + F (O F )

COF2 -> CO + 2F

23% F F 12%

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(2) CF4 + H2

: 40%

F : H2 + 2F -> 2HF F CF3

polymer SiO2 : . SiO2 polymer

(C + O -> CO or C + 2O -> CO2)

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(3) F/C Model (Flourine-Carbon Ratio Model)

F/C : , polymer F/C : , polymer

F/C polymer

Si (anisotropy)

SiO2 Si (selectivity)

F/C polymer

Si (isotropy)

Si SiO2

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4. Anisotropic Etching and Control

of Edge Profile

(1) Ion-assisted Etching of Si

(+) ion profile 50 eV damage radical

50 eV polymer (,

polymer )

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(2) Directionality by blocking layer

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(3) Sidewall Blocking

Blocking

MechanismExample Blocking Materials

Polymer Al - CCl4 (CClx)s

Polymer Si - Cl2 (CClx)s (?)

Oxidation Si - CF4/O2 SiOx

anisotropic etching sidewall blocking

Resist polymer blocking O2 polymer - CF4/O2 Si O2

isotropic etching , O2

sidewall anisotropic

etching

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5. Dry Etching of Various Thin Films

Gas - Solid SystemsSolid Etch Gas Etch Product

Si Cl2, CCl2F2 SiCl2, SiCl4SiO2, Si3N4 CF4, SF6, NH3 SiF4

Al BCl3, CCl4. Cl2 Al2Cl6, AlCl3

Photoresist O2, O2/CF4 CO, CO2, H2O, HF

Refractory metals/

Silicide (Ti, W, Ta, Mo,

TiSi2, WSi2, TaSi2, MoSi2)

CF4, CCl2F2, ... WF6, ....

(1) Si/ Polysilicon Etching

Cl2, CCl4, CF2Cl2, CF3Cl, Br2, CF3Br inert gas (Ar, He) : Cl2/C2F6, Cl2/CCl4, CF3Cl/C2F6 F-based anisotropic

loading effect Cl-based

C2F6, SF6 F-based

Typical Etching Selectivity SiO2: 10 - 50 Si3N4: 5 - 10 Sidewall Blocking Cl resist polymer O2 , oxide sidewall

*

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(2) SiO2 Etching

SiO2 : contact/ via etching

Cl-based F-deficient gas : CF4/H2, CHF3, C2F6 F-scavengers: H2, C2H4, CH4

Si selectivity Si etching blocking material: CFx

Carbon (SF6/H2, F2/H2:

)

10 selectivity

Thermal oxide CVD oxide Oxide

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(3) Si3N4 Etching

Si3N4 Si SiO2 bombardment :

SiO2 < Si3N4 < Si

polymer : SiO2 < Si3N4 < Si : - CF4 + O2 + CBrF3- CH2F2- CHF3- SF6- Cl2 + NF3

F Si3N4 Isotropic etching ER of nitride: ER of Si = 1 : 8

N F NF3 -> F

Interhalogen Molecules(ex; FCl) SiO2 Si3N4 (ex. Cl2 + NF3

, ER of nitride: ER of oxide = 1 : 0)

ER of LPCVD nitride HCN() ->

polymer

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(4) Carbon and Organic Solids

Organic solids(ex: photoresist, polymide) O F

-(CH2) + 2O -> CO + H2O (fast)-(CH2) + 6F -> CF4 + 2HF (slow)

O2 plasma CF4 organic solid

F H HF -> O C

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(5) Silicide Etching

layer - Polycide: gate oxide - polysilicon - silicide

- Interconnection line : ILD - silicide

- Good vertical etch profile

- ER of silicide ER of n+ poly Si- Good selectivity over oxide ( > 10)

- Not too much resist erosion

- Cl-based gas

() TiSi2: CCl4 + O2WSi2: CCl2F2MoSi2: CCl4 + O2TaSi2: Cl2 + CF4

F-based F sufficient : n+ poly silicide undercut

F deficient : oxide selectivity

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(6) Aluminum Etching

Etching Gas Cl-based : BCl3, CCl4, SiCl4, Cl2,

their Mixtures

F-based AlF3 Cl2 Al ( ) Etching Product: ; Al2Cl6, ; AlCl3 Cl-based gas vacuum pump oil

- : HCl, C2Cl3H, Polymers

- : Oil , , Chlorinated

Hydrocarbon

Al2O3 Etching Al Al2O3 ( 30) Cl Al2O3

Al2O3 (+) energetic bombardment

Reducing reaction ()

Al2O3 + CClx (x>4) -> AlCl3 + CO

Al2O3 + BClx (x AlCl3 + B2O3

Al2O3 Al induction time

Al G.B segregation- >

over-etch -> Al

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Al

Al Chloride (AlCl3, Al2Cl6)

/ : SiO2

Photoresist

: Large Gas Flow Rate

Back Ground Gas

Al/ Al Chloride / ,

=>

Al : Load-Locked System Directionality of Al

Cl or Cl2 energetic ion bombardment

Anisotropic sidewall blocking

Resist Degradation

Al Chloride resist : gas flow rate "Hard Bake"

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Post-etch Corrosion

( PR ) chlorine HCl Al

: unloading PR , Al2O3 AlF3(CF4 )

Al Alloy (Al-Si-Cu) Etching

Si Al-Cu , Cu residue . Cu 2%

safe limit

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6. Dry Etching

(1) Plasma Etching Reactors

Barrel System

Planar System

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(2) RIE Reactors

Planar Reactors

Hexode Etcher

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7. Process Issues

(1) Electrode power

rf power dependence of Si etch rate and NF Sputtering Rate = sputtering yield x [+] ion flux

= V x V3/2 = KIV5/2

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(2) Loading Effects

Chamber etching reaction, recombination, pumping out 3

Etching etching reaction -> ->

, reaction rate limited : no loading effect

Loading effect :- large gas flow

- etching

- ion bombardment

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(3) Heating Effect

Heating convection/radiation heat conduction exothermic chemical reaction radical rf inductive heating

rf inductive heating ac magnetic field eddy current" sample 13.5MHz skin depth

(100m) eddy current

ohmic loss/area

t: film , Ie: eddy current, A: magnetic field

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Thermal backing/ no thermal backing

(4) Doping Effects

ER of n+ Si > ER of intrinsic Si > ER of p+ Si ER of Si e- concentration

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Space charge model Si (F, Cl, Br)

(-)

n+ Si : (+) ->

intrinsic Si :

->

p+ Si :

(-) ->

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(5) Plasma Charging Damage (Antenna Effect)