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1
Etching
Reading Assignments:
Plummer, Chap 10.1~10.4
2
Etching
• Process that removes material from surface • 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.
Substrate
Film deposition Photoresist application
Deposited Film Photoresist
Exposure
Development Etching Resist removal
Mask
Etch mask
Light Important process consideration:
Etch Rate
Selectivity
Directional(CD and profile)
Uniformity
Damage (Dry etch)
3
• The process can remove the material from the surfaceis called “ETCHING”.
• “ETCHING” : Wet etching and Dry etching.
LiquidChemical solventChemical reaction.
Dry (or called Plasma Etching)GasPhysical bombardment. or Chemical reaction.(or combination)
A + B(s) AB
Jyyang in NDL
Etching Methods
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Etch Rate
5
Selectivity• Selectivity is the ratio of etch rates of different
materials.• Very important in patterned etch• Selectivity to underneath layer and to photoresist
1
2
ERSER
=
6
Practical Etching Profiles
a)
b)
a) isotropic b) anisotropic c) completely anisotropic
More directional etching
over etch
lateral etch
Anisotropic Af = 1 – rlat/rver
7
Microloading Effect
wafer
wafer
Aspect Ratio Dependent Etch Rate
ARDE effectARDE effect
depth/width ; etch rate
Microscopic loading effectMicroscopic loading effect
slower etch ratefaster etch rate
Jyyang in NDL
8
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
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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, OESOES• Mass Spectroscopy, MSMS• Laser-Induce
Fluorescence, LIFLIF
10
Mechanism
Main factors • reactant concentration• reaction time• reaction temperature
ReactantReactant
wafer
Film
PR
boundary layer
reaction
product
Wet EtchingIsotropic Step 1:
Diffusion (reactant)
Step 2:ReactionStep 3:Desorption (product)
Jyyang in NDL
11
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 DryerSpin DryerEtchant Sink
D.I. Wafer Rinse
12
13
Wet Etch Profiles
7 - 8 μm
Photoresist
Substrate
3μmFilm
3 μm
PR
Substrate
Film
Etch Bias
• Can’t be used for feature size smaller than 3 μm
• Replaced by plasma etch for fine line patterning
Isotropic
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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.
• Etch (100)-oriented Si through a window creates V-groove (54.7o).– 19 wt% KOH AT 80°C, (100) : (110) : (111) =
100 : 16 : 1– 10 wt% KOH at 80°C, (100)-Si etch rate ~
1.1um/min, selectivity to SiO2>600.
lWlWWb 27.54cot2 00 −=−=
15
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
16
Schematic of RIE System
Provide DC bias!
17
Time
Vol
t
DC Bias RF potential
Plasma Potential
Time
Vol
t
DC Bias RF potential
Plasma Potential
RF Source
Potential Distribution – RF Biased
1 2
2 1
mV AV A
⎛ ⎞= ⎜ ⎟⎝ ⎠
18
Ion-assisted Etch
Free radical only
(isotropic)
Ion Bombardment only
(anisotropic)
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Ion-assisted Etch
(anisotropic) (anisotropic)
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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
21
4F + SiO2(s) SiF4(g) + O2(g)
Etch chemistry:
Byproduct volatility
Selectivity
Anisotropy
22
Inhibitor vs. Etch ProfileFilmMask
FilmMask
Inhibitordeposition
or formation
Etch
Etch
Finalprofile
a. Inhibitor deposition rate fast compared to etch rate
b. Inhibitor deposition rate relatively slow compared to etch rate
. . . . . .
Inhibitordeposition
or formation
Inhibitor: etch byproducts that impede further etch process. Important for the control of etching profile.
23
Poly-silicon etch SiO2 etch
C2H6 increase the sidewall inhibitor (polymer ) deposition.
Higher C/F ratio increase the sidewall inhibitor (polymer) deposition. O2reduces the polymer deposition.
Inhibitor vs. Etch Profile
24
High-Density Plasma Etching
Sputter Etching and Ion Beam Milling
High Density Plasma Etching
Reactive Ion Etching
Plasma Etching
Wet Chemical Etching
Pres
sure
Sele
ctiv
ity
Ani
sotr
opy
Ene
rgy
PhysicalProcesses
ChemicalProcesses
gas inlet
plasma
magnetic coil
RFbias supply
(13.56 MHz)
wafer
gas outlet,pump
Microwavesupply
(2.45 GHz)
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 microloadingeffect, but reduces the plasma density and thus the etching rate.
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.
25
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`
excited species``
hvhv
26
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
27
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– H2SO4: H2O2=3:1 or 5:1 (before metal deposition)– Solvents rinse (after metal deposition)– H2SO4+H2O2+HF(ppm)+H2O
• Remove radicals– H2O rinse– Solvents rinse
• Remove particles– SC1 + megasonic clean– Scrubber clean
0
0.1
0.2
0.3
0.4
0.5
0 20 40 60 80 100
O2/He Ratio (%)
Ash
ing
Rat
e (u
m/m
in)
0
5
10
15
20
25
Uni
form
ity (%
)