Lecture 11.0
Etching
Etching

Patterned
– Material Selectivity is Important!!

Un-patterned
x
Etching
y


Dry Etch
An-isotropic
• dy/dt:dx/dt:6


Gas Phase Reaction
with volatile products
Frequent use of very
reactive species in a
Plasma
– Si Etch
– SiO2 Etch
– Metal Etch


Wet Etch=Dissolution
Isotropic
• dy/dt:dx/dt:1.2
– 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
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
Wet Etch Chemistries
Layer
 Photoresist
 SiO2
 Si3N4
 Si

Etchant
H2SO4, H2O2
HF, NH4F-HCl-NH4F
?, HNO3
HF
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
Etch Reactions
Boundary Layer Mass Transfer
 Surface Chemical Reaction

– Like Catalytic reaction

Product diffusion away from surface
Product
Concentration
Profile
Reactant
Concentration
Profile
Rate Determining Steps
X
Global Dissolution Rate/Time

Depends on
– Mass Transfer
• Diffusion Coefficient
• Velocity along wafer surface
• Size of wafer
– Solubility
– Density of film being etched
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
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
Dry Etch

Physical Evaporation
– Not typically used
• Heating chip diffuses dopants out of
position
Sputtering from a target
 Plasma reactor with volatile reaction
product

RF Plasma Sputtering for
Deposition and for Etching
RF + DC field
Removal Rate

Sputtering Yield, S
– S=α(E1/2-Eth1/2)
 
5 .2
Zt
2/3
U (Zt
 Zx )
2 /3 3/4
 Zx 


 Zt  Z x 
2/3
U  surface binding energy
Z i  atomic numbers

of (t) target and (x) gas
Deposition Rate 
– Ion current into Target *Sputtering Yield
–
Fundamental Charge
Plasma





Free Electrons accelerated by a strong
electric field
Collide with gas molecules and eject eCollision 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
Plasma Conditions
Reduced Pressure ~100 mtorr
 Flow of gases in and out
 DC or AC (rf) electric field

– Parallel plate electrodes
– Other geometries
Dry Etch Chemistries






Gas
O2
95%CF4-5% O2
50%CF4-25%HBr-25%O2
75%Cl2-25%HBr
Surface Etched
Pre-clean
Si
Poly Si
Metal etch
CF2 layer on side walls prevents wall
etching
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 frequency=Ng vel m(v)
 Te  E/Ng  ERgTg/Ptot >> Tgas
 kBTe >
Gas Ionization Energy
 kBTe > Molecular Dissociation
Energy
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
Gaseous (Volatile) Products
– SiO(g), SiF4(v), SiCl4(v), SiBr4(v)
– MFx(v), MClx(v), MBrx(v),
st
1
Ionization Energies
O
 Br
 Cl
F
H
 Ar

13.618 eV
11.814 eV
12.967 eV
17.422 eV
13.598 eV
15.759 eV
Plasma Etch Mechanism

PreClean
•
•
•
•

O2+ eO2+ + 2e
O2+ e2O + e
O + e  OO2+ + e  2O
– O + s  O-s
– O + Si(s) s-SiO
– SiO-s  SiO(g)
Neutrals are main reactive species!!
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)n+F
Polymer on wall of etch
Degree of Ionization, α

α = Ni/No= Qi N λD
– N = neutral number density
• N = Ni+No
– λD = Characteristic Diffusion length
(mean free path)
– Qi= ionization collision cross section
• Qi= 0.283 x 10-16(cm2) Pi(E)
– Pi(E)= ionization probability
Plasma Transport Equations

Flux, J
J n  Dn
dn n
for neutrals
dx
J i  Di
dn i
J e  De
dn e
dx
dx
  ini E
  ene E
μ i  ion mobility
μ e  electron
mobility
for ions
for electrons
Etch Reactions
Boundary Layer Mass Transfer
 Surface Chemical Reaction

– Like Catalytic reaction

Product diffusion away from surface
Product
Concentration
Profile
Reactant
Concentration
Profile
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)
JB= b JA= b ks Cag
JB= b JABb = Kg(CABb-s-CABb-g)
BL-MT of A
Surface Reaction
may be catalytic
BL-MT of Abb
JB= b q/Hrxn
• q = h (Ts – Tg)
• q = k dT/dy
BL-HT
Conduction in wafer
Rate Determining Steps
X
Plasma Etch Rate of Polymers
Residue Build-up
Plasma Etch Rate of Polymers
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