Etching

1
Lecture 11.0

• Etching

2
Etching

• Patterned
• Material Selectivity is Important!!
• Un-patterned

3
Etching
x
y

• Dry Etch
• An-isotropic
• dy/dtdx/dt6
• Gas Phase Reaction with volatile products
• Frequent use of very reactive species in a Plasma
  
• Si Etch
• SiO2 Etch
• Metal Etch

• Wet EtchDissolution
• Isotropic
• dy/dtdx/dt1.2
• Si Etch
• Strong HF
• SiO2 Etch
• Strong NH4OH not NaOH (Na ion is bad)
• Si3N4 Etch
• Phosphoric Acid
• Metal Etch
• Acid Solution (HNO3)
• Photoresist
• Solvent
• H2SO4 Solution

4
Etching

• Wet and Dry Etch have very different chemical
  reactions!
• Wet and Dry Etch have similar rate determining
  steps
• Mass Transfer Limiting
• Surface Reaction Limiting
• Similar mathematics

5
Wet Etch Chemistries

• Layer Etchant
• Photoresist H2SO4, H2O2
• SiO2 HF, NH4F-HCl-NH4F
• Si3N4 ?, HNO3
• Si HF

6
Dissolution of Layer-Wet Etch

• BL-Mass Transfer
• A(l)b B(s)?? ABb(l)
• A
• Acid for metal (B) dissolution
• redox reaction
• Base for SiO2 (B) dissolution
• Solvent for photoresist (B) dissolution

7
Etch Reactions

• Boundary Layer Mass Transfer
• Surface Chemical Reaction
• Like Catalytic reaction
• Product diffusion away from surface

Reactant Concentration Profile
Product Concentration Profile

8
Rate Determining Steps
X
9
Global Dissolution Rate/Time

• Depends on
• Mass Transfer
• Diffusion Coefficient
• Velocity along wafer surface
• Size of wafer
• Solubility
• Density of film being etched

10
Wet Etch Reaction

• Wafers in Carriage
• Placed in Etch Solution
• How Long??
• Boundary Layer MT is Rate Determining
• Flow over a leading edge for MT
• Derivation Mathcad solution

Also a ?C for the Concentration profile
11
Local Dissolution Rate/Time

• Depends on
• Mass Transfer
• Diffusion Coefficient
• Velocity along wafer surface
• Size of wafer
• Solubility
• Density of film being etched
• Position on the wafer
• see photoresist dissolution example

12
Dry Etch

• Physical Evaporation
• Not typically used
• Heating chip diffuses dopants out of position
• Sputtering from a target
• Plasma reactor with volatile reaction product

13
RF Plasma Sputtering for Deposition and for
Etching
RF DC field
14
Removal Rate

• Sputtering Yield, S
• Sa(E1/2-Eth1/2)
• Deposition Rate ?
• Ion current into Target Sputtering Yield
• Fundamental Charge

15
Plasma

• Free Electrons accelerated by a strong electric
  field
• Collide with gas molecules and eject e-
• Collision creates more free electrons
• Free electrons combine with ions to form free
  radicals
• Gas Ions/Free Radicals are very reactive with
  materials at the wafer surface
• Ions non-selective removal
• Free Radicals

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Plasma Conditions

• Reduced Pressure 100 mtorr
• Flow of gases in and out
• DC or AC (rf) electric field
• Parallel plate electrodes
• Other geometries

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Dry Etch Chemistries

• Gas Surface Etched
• O2 Pre-clean
• 95CF4-5 O2 Si
• 50CF4-25HBr-25O2 Poly Si
• 75Cl2-25HBr Metal etch
• CF2 layer on side walls prevents wall etching

18
Plasma

• Temperature of Gas molecules, Tgas ?PVm/Rg
• Temperature of Electrons,
• Te e2E2Mg/(6me2?m2 kB)
• Accelerated by E field between collisions with
  gas molecules
• ?m momentum collision frequencyNg vel ?m(v)
• Te ? E/Ng ? ERgTg/Ptot gtgt Tgas
• kBTe gt Gas Ionization Energy
• kBTe gt Molecular Dissociation Energy

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Plasma Gas Chemistries

• Reactant Gases
• Physical Etch Sputtering from chip target
• Ar
• Chemical Etch
• O2
• CF4
• HBr
• Cl2
• CHF3
• C2F6
• Mixtures
• CF2 deposition (like a teflon polymer layer)
  prevents side wall etch

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Gaseous (Volatile) Products

• SiO(g), SiF4(v), SiCl4(v), SiBr4(v)
• MFx(v), MClx(v), MBrx(v),

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1st Ionization Energies

• O 13.618 eV
• Br 11.814 eV
• Cl 12.967 eV
• F 17.422 eV
• H 13.598 eV
• Ar 15.759 eV

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Plasma Etch Mechanism

• PreClean
• O2 e?O2 2e
• O2 e?2O e
• O e ? O-
• O2 e ? 2O
• O s ? O-s
• O Si(s) ?s-SiO
• SiO-s ? SiO(g)

• Metal (M) Etch
• Cl2 e ? 2Cl e
• Cl2 ? Cl2 e
• Cl s ? Cl-s
• x Cl-M(s) ? MClx(g)
• Simultaneously
• e CF4 ? CF3 F 2e
• e CF3 ? CF2 F
• CF3 CF2? (CF2)nF
• Polymer on wall of etch

Neutrals are main reactive species!!
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Degree of Ionization, a

• a Ni/No Qi N ?D
• N neutral number density
• N NiNo
• ?D Characteristic Diffusion length (mean free
  path)
• Qi ionization collision cross section
• Qi 0.283 x 10-16(cm2) Pi(E)
• Pi(E) ionization probability

24
Plasma Transport Equations

• Flux, J

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Etch Reactions

• Boundary Layer Mass Transfer
• Surface Chemical Reaction
• Like Catalytic reaction
• Product diffusion away from surface

Reactant Concentration Profile
Product Concentration Profile

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Etch Reaction

• A(g)bB(s)?? ABb(g)
• -(1/A) dNB/dt -(1/A)(?/MwB)dVB/dt -(?/MwB)
  dy/dt - JB
• JB b JA b Kg(CAg-CAs) BL-MT of A
• JB b JA b ks Cag Surface Reaction
• 
  may be catalytic
• JB b JABb Kg(CABb-s-CABb-g) BL-MT of Abb
• JB b q/?Hrxn
• q h (Ts Tg) BL-HT
• q k dT/dy Conduction in wafer

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Rate Determining Steps
X
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Plasma Etch Rate of Polymers
Residue Build-up
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Plasma Etch Rate of Polymers
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Clean developed Photoresist off of wafer

• Wet-chemical stripping agents (solvents)
• Incomplete wetting at small scale
• Supercritical CO2.-new technology
• Zero surface tension
• Complete wettability
• Good for small line widths