etching techniques in nanotechnology...reactive ion etching plasma hits surface with large energy...
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Etching Techniques in Nanotechnology
Shashi Poddar
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Etching is an important process in SEMICONDUCTOR FAB
NMOS
GATE METAL WIDTH defines technology node A typical computer processor involves close to half a billion of
such transistors. Other active elements of any digital or analog
device is also based on large scale integration of transistors in
some form or the other.
e.g: DRAM, CMOS, NAND Flash
So where are we now?.....7 nm node
Where are we headed?
5 nm node using EUVL 2020 mass priduction
G
D
S
World line
BIT line Ground
Charging and discharging of capacitor
CMOS
DRAM
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Bulk micromachining
Silicon wafer
Silicon wafer
Silicon etched
Silicon etched
SiO2
Isotropic etch
Anisotropic etch
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Etching
Chemical reaction resulting in the removal of materialEtching:
Wet etching:
Dry etching:
etchants in liquid form
etchants contained is gas or plasma
ionized gas
Etch rate: material removed per time (μm/min)
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Selectivity and undercutting
etch rate of one material compared to another
Selectivity:
Undercutting
etch rate of one crystalline direction compared to another
[100] [111]
SEM image of a SiO2 cantilever formed by undercutting (S. Mohana
Sundaram and A. Ghosh, Department of Physics, Indian Institute of
Science, Bangalore)
SiO2
(100) Si
54.7°
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Application and properties of different wet etchants
High HF tends to etch SiO2
Acidic etchants tend to etch Si
isotropically
Basic etchants tend to etch Si
anisotropically
Depend on concentration and
temperature
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Rate versus diffusion limited etching
Etchant Products
Etchant Products
Rate limited
reaction
Diffusion
limited reaction
Rate limited
reactions are
preferred → easier
to control and
more repeatable
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Isotropic etching
Estimate of etch depth
depth ≈ (D-d)/2
• Etch rate is the same in all
directions
• Typically acidic
• Room temperature
• Isotropy is due to the fast chemical
reactions
• X μm/min to XX μm/min
→ Reaction or diffusion limited?
D
d
undercutting
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Anisotropic etching
• Etch depths depend on
geometry
• Undercutting also
depends on geometry
• Etch rate is different for different crystal
plane directions
• Typically basic etchants
• Elevated temperatures (70-120°C)
• Different theories propose for anisotropy
• Slower etch rates, ~ 1 μm/min → Reaction or diffusion limited?
d
undercutting
[100]
[111]
54.7°
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Properties of different anisotropic etchants of Si
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Theories for anisotropic etching
The lower reaction rate for the {111} planes is caused by the larger activation energy required to
break bonds behind the etch plane. This is due to the larger bond density of silicon atoms behind
the {111} plane.
Silicon lattice
(100)
(111)
2 dangling bonds1 dangling bond
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Theories for anisotropic etching of Si in aqueous based basic solvents
• Reduction of water believed to be the rate determining step
• OH- believed to be provided by H2O near Si surface
SiOH2++ + 4 e- + 4 H2O → Si(OH)6
-- +2 H2 (reduction step)
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Self-limiting etch and undercutting
D
[111]
D
[111]
• Intersection of {111} planes can cause self-limiting etch.
• Only works with concave corners
Concave corner
Convex corner exposes other
planes
Resulting undercutting can be used to create
suspended structures
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Etch stop
Etch stop: Technique to actively stop the etching process
Insulator etch stopSelf-limiting etch
Timed etch
Etch stop via doping
p-n junction
insulting layer
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Etch stop via doping
Boron etch stop
n type wafer heavily doped with B(called a p+ wafer)
p-n junction
SiOH2++ + 4 e- + 4 H2O → Si(OH)6
-- +2 H2 (reduction step)
p region Si deficient in e-
n region
p region
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Etch stop via doping
Electrochemical etch stop (ECE)
p type wafer doped n-type dopant
p-n junction
“Reverse bias” voltage
applied to p-n junction
keeps current from
flowing
n region
p region
SiOH2++ + 4 e- + 4 H2O → Si(OH)6
-- +2 H2 (reduction step)
diode
-
V
+
e- e-
SiO2
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Dry etching
Chemical reaction resulting in the removal of materialEtching:
Wet etching:
Dry etching:
etchants in liquid form
etchants contained is gas or plasma
Plasma etching:
Chemically reactive gas formed by collision of
• molecules of reactive gas with
• energetic electrons
• Excited/ignited be RF (radio frequency) electric
field ~ 10-15 MHz
Accelerated to target via the electric field
Reactive ion etching (RIE):
mostly chemical etching
In addition to the chemical etching,
accelerated ions also physically etch the
surface
+ + + + + + + +
electrodes
wafer
- - - - - - - - -
excited
ions
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Reactive ion etching
Plasma hits surface with large energy
• In addition to the chemical reaction, there is
physical etching
• Can be very directional—can create tall, skinny
channels
(Intellisense Corporation)
If there is no chemical reaction at all, the
technique is called ion milling.
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Common dry etchant/material combinations
Material Reactive gas
Silicon (Crystalline or
polysilicon)
Chlorine-base: Cl2, CCl2, F2
Fluorine-base: XeF2, CF4, SF6, NF3
SiO2 Fluorine-base: CF4, SF6, NF3
Al Chlorine-base: Cl2, CCl4, SiCl4, BCl3
Si3N4 Fluorine-base: CF4, SF6, NF3
Photoresist O2 (Ashing)
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Deep reactive ion etching (DRIE)
Bosch process
• 1st, reactive ion etching step takes place
• 2nd, fluorocarbon polymer deposited to protect
sidewalls
Kane Miller, Mingxiao Li, Kevin M Walsh and Xiao-An Fu,
The effects of DRIE operational parameters on vertically aligned
micropillar arrays, Journal of Micromechanics and Microengineering, 23 (3)
“Scalloping”