Class 7 - Dry Etching

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Dry Etching, General Principles
Dr. Marc Madou, Fall 2012
Class 7
Content

Dry etching: definition

Pressure units and modes of gas
flow
Plasmas or discharges

How to create a vacuum
 Plasmas: DC and AC
 Paschen curve
Dry etching mechanisms
Dry etching types and equipment
Etching profiles:
 Sputtering
 Chemical
 Ion-enhanced
 Ion-enhanced inhibitor





Etching profiles in physical etching
 Faceting
 Ditching
 Redeposition

Comparing wet with Dry
Etching
Dry etching: definition


Dry etching techniques are those that use
plasmas (hot ionized gases) to drive chemical
reactions or employ energetic ion beams to
remove material. Dry-etching processes yield
finer patterns than wet etching (surface
tension !). These techniques also offers
greater safety as large quantities of corrosive
acids or bases are not required.
Within a dry etching reaction chamber the
wafers lie directly in the plasma glow (also
called a discharge), where reactive ions are
accelerated towards the wafer (often biased).
The ions are a species likely to attack the
substrate material chemically with or without
selectivity.
Pressure units
1 microbar = 0.1 Pa
1 µm Hg =0.1333 Pa
1 N/m2 = 1 Pa
1 mm H2 O = 9.795 Pa
1 mbar = 100 Pa
1 mmHg = 133.3 Pa
1 torr = 133.3 Pa
psi (1)
psi
in.H2 O
in.Hg
kPa
millibar
1.000
3.612710-2
0.4912
0.14504
0.01450
1 in.H2 O = 248.8 Pa
1 kPa = 1000 Pa
1 ft H2 O = 2986 Pa
1 in. Hg = 3386 Pa
1 psi = 6895 Pa
1 bar = 105 Pa
1 atm = 101325 Pa
in.H2 O
(2)
27.680
1.000
13.596
4.0147
0.40147
cm H2 O 1.4223 10-2 0.3937
mm Hg 1.9337 10-2 0.53525
(torr)
in.Hg (3)
kPa
2.036
7.3554 10-2
1.000
0.2953
0.02953
6.8947
0.2491
3.3864
1.000
0.100
2.8958 10-2 0.09806
3.9370 10-2 0.13332
millibar cm H2 O
(4)
68.947 70.308
2.491
2.5400
33.864 34.532
10.000 10.1973
1.000
1.01973
0.9806
1.3332
1.000
1.3595
(1) pounds per square inch
(2) at 39 °F
(3) at 32 °F
(4) at 4 °C
(5) at 0 °C
mm Hg
(5)
51.715
1.8683
25.400
7.5006
0.7500
6
0.7355
1.000
Pressure units
Definitions of Vacuum Regimes:
1) Rough Vacuum: ~0.1- 760 torr (atmospheric pressure is 760 torr)
2) Medium Vacuum:~ 0.1 to 10-4 torr
3) High Vacuum: ~ 10 -8 to 10-4 torr
4) Ultrahigh Vacuum: < 10 -8 torr
 2 modes of gas flow:
 Viscous Flow regime:gas density (pressure) is high enough, many
moleculemolecule collisions occur and dominate the flow process (one
molecule “pushes” another). Collisions with walls play a secondary role
in limiting the gas flow.
 Molecular flow regime: gas density (pressure) is very low, few moleculemolecule collisions occur and molecule- chamber wall collisions dominate the
flow process (molecules are held back by walls). See further below for
mathematical expressions for these two regimes.

How to create a vacuum



Visit on your own time
http://et.nmsu.edu/ETCLASSES/vlsi/files/ARTICLE.HT
M on vacuum pumping (take the quiz at the end).
The first thing that all basic systems have is a roughpumping system. It is used to reduce the pressure from
atmospheric pressure in the chamber to a lower pressure
level that other low-pressure systems can use. Then there
has to be a fine-pumping system that must be able to
attain sufficient pumping speed to handle the outgassing
from the work produced in the chamber of the vessel.
There must also be vacuum gauges that determine the
pressure at certain points of the system.
Pumps:
 Diffusion pumps operate from 10-4 Torr to 5x10-11 Torr.
Diffusion pumps operate by boiling a fluid, often
hydrocarbon oil, and angling the dense vapor stream in a
downward conical direction back into the pump boiler. Gas
molecules from the system that enter the oil curtain are
pushed toward the boiler by momentum transfer from the
large fluid molecules.
Diffusion pump
Plasmas : DC and AC
 Mechanical pump. Pump operation is based on bulk flow of gas; hence the pump works
in the viscous flow regime. Used for obtaining "rough" vacuum (10 -3 Torr), which is the
lower limit of the viscous flow regime
Principle of mechanical operation:
(1) begin expansion cycle
(2) seal off expanded volume
(3) compress gas out exhaust

Simplest plasma chamber is 2 parallel plate electrode set (anode and cathode) in a
low pressure Argon filled chamber (e.g. 0.001 to 1 Torr). The two electrodes are
positioned parallel to each other, with the top electrode and chamber walls
electrically grounded while the lower electrode and substrate holder are connected
through a dc-blocking capacitor and matching network to a 13.56 MHz F generator
(AC plasma case)
Plasmas: DC and AC








Apply 1.5 kV over 15 cm--field is 100-V/cm. Breakdown of
Argon when electrons transfer a kinetic energy of 15.7 eV to the
Argon gas.
These energetic collisions generate a second electron and a
positive ion for each successful strike.
If the two electrons reenergize creating an avalanche of ions and
electrons we get a glow or plasma.
At the start of a sustained gas breakdown a current starts flowing
and the voltage drops to about 150 V.
To sustain a plasma, a mechanism must exist to generate
additional free electrons after the plasma generating ones have
been captured at the anode.
The additional electrons are formed by ions of sufficient energy
striking the cathode (emitting secondary Auger electrons).
This continuous generation provides a steady supply of electrons
and a stable plasma.
Plates too close: no ionizing collisions (not enough energy), too
far too many inelastic collisons in which ions loose energy.
Plasmas : DC and AC


Plasma dark spaces: dark because the higher energy
electrons cause ionization rather than light-generating
excitation.
Plasma is always positive, this follows from kinetics: for
a random velocity distribution the flux of ions and
electrons upon a surface is given by:
j i,e 

n i,e vi, e
4
where n is a density and <v> an average velocity. Ions
are 4000 to 100,000 times more heavy than electrons so
the average velocity of electrons is much larger. Electron
flux to surrounding surfaces is larger resulting in a
positive charge on the plasma.
Assymetry of voltage distribution: electrons move faster
away from the cathode than positive ions are accelerated
towards it larger space charge (also the dark space is
larger at the cathode).
Plasmas : DC and AC
Plasmas : DC and AC


The largest voltage drop is in front of
the cathode where charged particles will
experience their largest acceleration.
The cathode gets etched the anode does
not !! Substrates to be etched are laid
down on the cathode.
Efficiency or ‘strength’ of a particular
plasma is evaluated by the average






electron energy (temperature)
ion energy (temperature)
electron density (e.g. 109 and 1012 cm-3)
ion density (e.g. 108 to 1012 cm-3)
neutral species density (e.g. 1015 to 10 16
cm -3)
ion current (e.g. 1 to 10 mA/cm 2).

The ratio between ionized species
and neutral gas species is 10-6 to
10-4.
v e  kT e (e.g. 1 10 eV)
v i  kT i (e.g. 0.04 eV)
Plasmas : DC and AC



An important quantity to describe a
plasma is the ratio of electrical field
over pressure (Equation I). With
increasing fields the velocity of free
electrons or ions increases (~E) but an
increase in pressure decreases the
electron or ion mean free path
(~1/P).The mean free path (l) is given
by Equation (II) where nv is the number
of molecules per unit volume,
The number of molecules per unit
volume, nv, can be determined from
Avogadro's number and the ideal gas
law, leading to Equation (III)
The bombarding flux of ions on the
cathode is given by Equation (VI):
kTi ,e ~
E
P
(I)
(II)
(Equation III)
j i  qn i i E
(Equation IV)
Plasmas : DC and AC






AC plasma’s for etching insulating surfaces.
Capacitor makes voltage distribution assymetric in this case.
A DC self bias results.
Etching energy: E max  eVdc  V p  eVT
Plasma energy: E max  eV p
Self bias: V  kTe ln T emi
dc
2e T i me
AC Plasma’s
Paschen curve
Dry chemical etching mechanisms






Reactive species generation (1)
Diffuse to the solid (2)
Adsorption at the surface (3)
Reaction at the surface (4)
Reactive cluster desorption (5)
Diffusion away from the surface
(6)
Dry chemical etching mechanisms

Continuous dry-etching spectrum
 low pressure <100 millitorr: physical
sputtering
 unselective
 directional
 radiation damage
 100 millitorr range:RIE
 physical and chemical
 directional
 more selective than sputtering
 higher pressures:plasma etching
 chemical (10-1000 times faster) -see extreme example, gas phase
etching with XeF2 (not really a
plasma)
» isotropic
» more selective
» least damage
Dry chemical etching mechanisms: purely chemical

XeF2 Gas Phase Etching (high
pressure, chemical only)
 no plasma (just pump)
 10 µm/min
 no damage
 isotropic
 very selective (Si over Al,
photoresist, oxide and nitride)
 CMOS compatible
Dry chemical etching mechanisms:
chemical etching: Energy-driven anisotropy
Physical-
Dry chemical etching mechanisms: Physicalchemical etching: Inhibitor-driven anisotropy
Dry etching types and equipment
Dry etching types and equipment

RIE chamber with load lock
Dry etching types and equipment
CAIBE RIBE
IBE
MIE
RIE
Barrel
Etchi ng
PE
~10-4
~10-4
~10-4
10-3-10-2 10-3-10-2 10-310-1
10-1-100
10-1-101
Etch
chem/
Mechani sm phys
chem/
phys
phys
phys
chem/
phys
chem/
phys
chem
chem
Sel ecti vi ty good
good
poor
poor
good
good
excellent
good
Profil e
anis
anis
anis
anis
iso or
anis
iso
iso or
anis
Pressure
(Torr)
anis or
iso
MERIE
Dry etching types and equipment
Acronym/Technique
Explanation
CAIBE
Chemicallyassisted
ionbeametching
Magneticallyenhanced
reactiveionetching
Magneticallyenhancedion
etching
Plasmaetching
Reactiveionbeam
etching
Reactiveionetching
MERIE
MIE
PE
RIBE
RIE
Dry etching types and equipment : RIBE vs.
CAIBE
CAIBE is RIE in a triode
system (e.g. 10, 000Å/min)
 RIBE ion is reactive and etches
(e.g. 100Å/min)

Etching profiles in dry etching




Sputtering: directional, physical.
Chemical: non-directional
(diffusion).
Ion-enhanced energetic:
directional.
Ion-enhanced inhibitor: directional.
Etching profiles in physical etching



Faceting: angle of preferential
etching
Ditching (trenching): sometimes
caused by faceting
Redeposition: rotational stage
might reduce this effect.
Comparing wet vs. dry etching
Parameter
Directionality
Dry Etching
Can be highly directional with
most materials (Aspect ratio of
25 and above)
Good
Production-line
automation
Environmental
Low
impact
Mas king film
Not as critical
adherence
Cos t chemicals
Low
S electivity
Poor
Materials that can be Only certain materials can be
etched
etched (not e.g. Fe, Ni, Co)
Radiation damage
Can be severe
Proces s s cale-up
Difficult
Cleanlines s
Good under the right operational
conditions
CD control
Very good (< 0.1µm)
Equipment cos t
Expensive
S ub micron features Applicable
Typical etch rate
Slow (0.1 µm/min )
Theory
Very complex, not well
understood
Operating
Many
parameters
Control of etch rate Good due to slow etching
Wet Etching
Only directional with single
crystal materials (Aspect
ratio of 100 and above).
Poor
High
Very critical
High
Can be very good
All
None
Easy
Good to very good
Poor
Inexpensive
Not applicable
Fast (1 µm/min, anis. )
Better understood (Chapter
4)
Few
Difficult
Homework
1
2
3.
4.
5.
How is a DC plasma created and how does an RF plasma differ? Why is a
plasma always positive with respect to the reactor vessel walls? In which
etching setup would you prefer to etch an insulator? Is space positively
charged?
Detail the different dry etching profiles available and how you obtain them.
Explain the DC breakdown voltage versus electrode distance curve
(Paschen’s law) and how it is relevant to dry etching. How is
miniaturization of an electrode set equivalent to creating a local vacuum?
Discuss the etch profiles in physical etching. Also draw profiles exhibiting
faceting, ditching, and redeposition.
Design a process to fabricate a polyimide post 100µm high and 10 µm in
diameter on a Si cantilever. The Si cantilever must be able to move up and
down over a couple of microns.
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