Etching
Reading Assignments:
Plummer, Chap 10.1~10.4
Etching
Light
Mask
Etch Rate
Photoresist
Deposited Film
Substrate
Film deposition
Selectivity
Photoresist application
Exposure
Directional
(CD and profile)
Etch mask
Development
Important process
consideration:
Etching
Resist removal
Uniformity
Process that removes material from surface
Damage (Dry etch)
Chemical, physical or combination of the two
Selective or blanket etch
Selective etch transfers IC design image on the photoresist to the surface layer on
wafer
Other applications: Mask making, Printed electronic board, Artwork, etc.
2
Etching Methods
The process can remove the material from the surface
is called ETCHING.
A + B(s) AB N
ETCHING : Wet etching and Dry etching.
Liquid
Chemical solvent
Chemical reaction.
Dry (or called Plasma Etching)
Gas
Physical bombardment. or
Chemical reaction.(or combination)
Jyyang in NDL
Etch Rate
Selectivity
Selectivity is the ratio of etch rates of different
materials.
Very important in patterned etch
Selectivity to underneath layer and to photoresist
ER
S=
ER
Practical Etching Profiles
a)
lateral etch
b)
over etch
More directional etching
Anisotropic Af = 1 rlat/rver
a) isotropic
b) anisotropic
c) completely
anisotropic
Microloading Effect
depth/width ; etch rate
Aspect Ratio Dependent Etch Rate
ARDE effect
wafer
faster etch rate
slower etch rate
Microscopic loading effect
wafer
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Microloading Effect
Smaller hole has a lower etch rate than the
larger holes
Etchants are more difficult to pass through the
smaller hole
Etch byproducts are harder to diffuse out
Lower pressure can minimize the effect.
Longer MFP, easier for etchants reaching the
film and for etch byproducts to get out
Over Etch
Film thickness and etch
rate is not uniform
Over etch: removes the
leftover film
Selectivity of etched
film and substrate
Endpoit Detector
Optical Emission
Spectroscopy, OES
Mass Spectroscopy, MS
Laser-Induce
Fluorescence, LIF
9
Wet Etching
Mechanism
Step 1:
Diffusion (reactant)
Step 2:
Reaction
Step 3:
Desorption (product)
Main factors
Isotropic
Reactant
product
boundary layer
PR
Film
reaction
wafer
reactant concentration
reaction time
reaction temperature
Jyyang in NDL
10
Wet Etch
Chemical solution to dissolve the materials on the wafer
surface
The byproducts are gases, liquids or materials that are
soluble in the etchant solution.
Three basic steps : etch, rinse, and dry
Spin Dryer
Etchant Sink
D.I. Wafer Rinse
11
12
Wet Etch Profiles
7 - 8 m
Photoresist
3m
Film
Substrate
Isotropic
3 m
Etch
Bias
PR
Film
Substrate
Cant be used for feature size smaller than 3 m
Replaced by plasma etch for fine line patterning
13
Si Anisotropic Etching
Si may be etched by direct
dissolution of Si atoms.
The (111)-plane has higher Si bonds density
than the (100)-plane. The etch rate in (111)plane is expected to be lower.
Anisotropic etching or oriented dependent
etching becomes possible.
Wb = W0 2l cot 54.7 D = W0 2l
Etch (100)-oriented Si through a
window creates V-groove (54.7o).
19 wt% KOH AT 80C, (100) : (110) : (111) =
100 : 16 : 1
10 wt% KOH at 80C, (100)-Si etch rate ~
1.1um/min, selectivity to SiO2>600.
14
Plasma Etch or Dry Etch Reactive Ion Etch (RIE)
Combination of chemical and physical etch
Plasma process, ion bombardment (physical) plus free
radicals (chemical)
Misleading name, should be called ion-assisted etch or
ion-induced etch
High and controllable etch rate
Anisotropic and controllable etch profile
Good and controllable selectivity
All patterned etches are RIE processes in 8 fabs
15
Schematic of RIE System
Provide DC bias!
16
Potential Distribution RF Biased
RF Source
Volt
Plasma Potential
DC Bias
RF potential
Time
V1 A2
=
V2 A1
17
Ion-assisted Etch
Free radical only
Ion Bombardment only
(isotropic)
(anisotropic)
18
Ion-assisted Etch
(anisotropic)
(anisotropic)
19
Ion-induced Etching
Etch rate of reactive radicals with etched material ~ 0
Heavy ion bombardment damages chemical bonds
Exposed surface atoms are easier to react with reactive radicals,
i.e., etch reaction is induced by ion bombardment
Ion bombardment is mainly in vertical direction
Etch conducted only on vertical direction anisotropic etch
Ex. Poly-Si gate etching
20
4F + SiO2(s) SiF4(g)N + O2(g)N
Etch chemistry:
Byproduct volatility
Selectivity
Anisotropy
21
Mask
Mask
Film
Film
Inhibitor vs. Etch Profile
Inhibitor
deposition
or formation
Inhibitor: etch byproducts that impede
further etch process. Important for the
control of etching profile.
Etch
Inhibitor
deposition
or formation
Etch
.
..
.
..
Final
profile
a. Inhibitor deposition rate
fast compared to etch rate
b. Inhibitor deposition rate relatively
slow compared to etch rate
22
Inhibitor vs. Etch Profile
Poly-silicon etch
C2H6 increase the sidewall
inhibitor (polymer ) deposition.
SiO2 etch
Higher C/F ratio increase the sidewall
inhibitor (polymer) deposition. O2
reduces the polymer deposition.
23
High-Density Plasma Etching
magnetic coil
In conventional RIE (10-100 mtorr),
For more directed etching, need stronger ion
bombardment. Possible more damage to
substrate.
Lower pressures can be used to attain more
directional etching and less microloading
effect, but reduces the plasma density and thus
the etching rate.
Microwave
supply
(2.45 GHz)
plasma
gas inlet
wafer
gas outlet,
pump
RF
bias supply
(13.56 MHz)
Anisotropy
Selectivity
Energy
Pressure
Sputter Etching
and Ion Beam
Milling
Physical
Processes
High Density
Plasma Etching
Reactive Ion
Etching
Plasma Etching
Wet Chemical
Etching
Chemical
Processes
In HDP etching (1-10 mtorr) ,
Uses separate RF source as wafer bias. This
separates the plasma power (density), from
the wafer bias (ion accelerating field).
High density plasma can be obtained at low
pressure which improves both the directional
etching and reduces the substrate damage
while keeping the compatible etching rate.
24
Endpoint Detect
Each atom has its own emission wavelength
Color of plasma changes when etch
different materials
Optical sensors can be used to detect the
change and indicate the endpoint for plasma etch processes
excited species`
hv
excited species``
25
Residue
Unwanted leftovers
Causes
insufficient over etch
non-volatile etch byproducts
Sufficient ion bombardment to dislodge
Right amount of chemical etch to scoop
Oxygen plasma ashing: Organic residues
Wet chemical clean: inorganic residues
26
Post Etch Clean
Remove PR
O2 plasma ashing (typical method)
UV + O3
H2SO4: H2O2=3:1 or 5:1 (before metal deposition)
Solvents rinse (after metal deposition)
Remove polymer
H2O rinse
Solvents rinse
Remove particles
SC1 + megasonic clean
Scrubber clean
25
0.4
20
0.3
15
0.2
10
0.1
20
40
60
80
Uniformity (%)
Remove radicals
Ashing Rate (um/min)
H2SO4: H2O2=3:1 or 5:1 (before metal deposition)
Solvents rinse (after metal deposition)
H2SO4+H2O2+HF(ppm)+H2O
0.5
100
O2/He Ratio (%)
27
