Dry Etching Technology
(Reactive Ion Etching
and Plasma Ashing Process)
Ass. Prof. Dr. Uda Hashim
School of Microelectronics Engineering
Outline
Section 1
Purpose and process flow of
Reactive Ion Etching
and Plasma Ashing in wafer
fabrication
Section 2
Introduction to Plasma Concept
Section 3
Methods and Equipment of Plasma
Etching
Section 4
Etching of Silicon and Its Compounds
Section 5
Photoresist removal
Section 1
Purpose and process flow of Reactive Ion Etching and
Plasma Ashing in wafer fabrication
Purpose
Reactive Ion Etch
to transfer pattern from photoresist mask
layer into the desired film by using
reactive gases (dry process)
Photoresist ashing
to remove photoresist mask layer after
etching or ion implantation process using
oxygen plasma
Process Flow
1. Lithography
Resist
AlSiCu
BPSG
FOX
FOX
FOX
P-Well
N-Well
PMOS
NMOS
2. Reactive Ion Etching (metal etch)
Resist
AlSiCu
BPSG
FOX
FOX
FOX
FOX
P-Well
N-Well
PMOS
NMOS
3. Plasma Resist Stripping (ashing)
BPSG
FOX
N-Well
PMOS
FOX
FOX
P-Well
NMOS
Section 2
Introduction to Plasma Concept
Definitions:
Electron (e)
Mass (me) = 9.1 x 10-28 gm, Charge = -1.6 x 10-19 coulomb
Proton (H+)
Mass = 1.67 x 10-24 gm = 1837me, Charge = +1.6 x 10-19
coulomb
Stable molecule
A collection of 2 or more atoms with fully satisfied bonding
Can be chemically active
eg. Cl2, F2, HF, CF4, SiF4
Radical
1 or more atoms with unsatisfied chemical bonding
Uncharged
eg. F, O, OH, CFx (x=1,2,3…)
Positive Ion
An atom, radical or stable molecule which has lost an
electron(s) leaving the particle with a positive charge
eg. Cl+, Cl2+, CF3+, HF+, SiF4+
Negative Ion
An atom, radical or stable molecule which has captured an
electron leaving the particle with a negative charge
eg. Cl-, Cl2-, CF3-, SF5-
General Information
Units of Pressure
1 atm= 760Torr = 1013mbar = 1.013 x 105 Pascals
1 Torr = 133 Pa
1 Pa = 7.52 x 10-3 Torr
1 mbar = 100 Pa = 0.75 Torr
Gas Density
1 Torr = 3.2 x 1016 molecules/cm3
Mean Free Path
The average distance a particle travels between collisions in a
gaseous environment
λ(cm) ≈ 5/P (mTorr) (depending on species)
Vacuum Pumping Equation
P, V, ι
Gas Source
Q
Pumping system
S
Schematic representation of a vacuum system
Vacuum pumping equation
V[dP/dt) = Q – SP
In steady state : Q = SP
The vacuum pumping equation relating the
pressure P in Torr, the gas flow rate Q in Torrlitres/sec, the pumping speed S in litre/sec and
the system volume V in litres.
Dry Etching Methods
Dry Etching Methods with Plasmas and Ion Beams
Glow Discharge
Methods
Plasma
Etching
Ion Beam Methods
Ion
Milling
Chemically
Assisted Ion
Beam Etching
Reactive Ion
Etching
Glow Discharge
Sputter Etching
Reactive Ion
Beam Etching
What is a plasma?
The nature of plasma etching, showing reactions in plasma, reaction at
walls and at the substrate surface, and the desorption of reaction
products.
A plasma is a collection of electrically charged and
neutral particles in which the density of negatively
charged particles (electrons and negative ions) is
equal to the density of positively charged particles
(positive ions).
Plasma-surface boundary
Top electrode
Bulk plasma
Sheath
(dark space)
Bottom electrode
•
•
Whenever a plasma is in contact with a surface, a
boundary layer known as “sheath” forms.
Substantial electric fields develop across sheaths.
Positive ions are accelerated to the surface due to
the polarity of the electric fields, while electrons
and negative ions are held away from surfaces.
The acceleration of the positive ions through
sheaths to surfaces is a very important process in
plasma etching
Electron-molecule
Collisions
An energetic electron striking a
neutral etch gas molecule can
accomplish any of the following
processes:
– Dissociation
– Ionization
– Excitation
Electron-Impact
Dissociation
e + O2
e + CF4
⇒
⇒
O+O+e
CF3 + F + e
• Converts relatively unreactive
etch gas molecules into very
reactive radicals
• Most important plasma-surface
chemistry is accomplished by
radicals
Electron Impact
Ionization
e + O2 ⇒ O2+ + 2 e
e + Cl2 ⇒ Cl2+ + 2 e
e + Ar ⇒ Ar+ + 2 e
Often dissociation and
ionization occur in one collision
(dissociative ionization)
e + CF4 ⇒ CF3+ + F + 2 e
e + O2 ⇒ O+ + O + 2 e
Electron Impact
Excitation
• Electrons colliding with molecules
can introduce vibrational and
rotational excitation of the
molecules.
E + F ⇒ F* + e
F* ⇒ F + hνF
E + Ar ⇒ Ar + e
Ar ⇒ Ar + hνAr
• Light emission from electronically
excited species is the basis of
optical emission spectroscopy
Secondary Reactions
Ions and radical generated
from electrons colliding with
gas molecules can react with
themselves and other species
including the etch gas
molecules
H2 + F ⇒ HF + H
CF3 + O ⇒ COF2 + F
CF2+ + CF4 ⇒ CF3+ + CF3
CF3 + F ⇒ CF4
CF3 + CF3 ⇒ C2F6
Ions in Plasmas
• Bulk plasma is electrically
neutral:
N(X+) = n(e) + n(X+)
• Positive ions are very important
in etching processes. Negative
ions do not play an important
role in etching because usually
they cannot reach surfaces
(surfaces are negative with
respect to plasma).
What kinds of ions are
present?
– Argon plasma – Ar+ is dominant
– Oxygen plasma – O+ and O2+ are
the dominant positive ions
– CF4 (Freon 14) plasma – CF3+ is
the dominant positive ion in low
density plasma, CF+, CF2+, C+
and F+ are sometimes present in
high density plasma
Radicals in plasma
Radicals are more abundant
than ions in molecular gas glow
discharges because radicals
are generated at higher rate
than ions.
– Electron energy required to break
chemical bonds in the molecules
is less then the energy needed to
ionize them.
– Radicals have a longer lifetime in
the plasma compared to ions
What is plasma
etching?
1. Take a molecular gas e.g.
CF4
2. Establish a glow discharge
and create reactive species
3. CF4 + e ⇒ CF3 + F + e
4. Choose chemistry so that the
reactive species react with
the solid to form a VOLATILE
product
5. Si + 4F ⇒ SiF4 ↑
6. Pump away volatile product
Why is plasma etching
important?
• The etching can be anisotropic
• Less consumption of chemicals
– Cost
– Environmental impact
• Clean process (vacuum)
• Compatible with automation
• Precise pattern transfer
In general plasma etching
enables chemical reactions to be
carried out at relatively low
temperatures.
Isotropic and Anisotropic
Etching
mask
substrate
Isotropic etch
Directional etch
Vertical etch
Anisotropic etch
Large feature size
(feature width >> Layer thickness
mask
substrate
Gas Solid System
Solid
Etch Gas
Etch
Product
Si
CF4, Cl2,
NF3, HBr
SiF4,
SiCl4,
SiCl2
SiO2,SiNx
CF4, SF6,
NF3
SiF4
Al
BCl3/Cl2
Al2Cl6,
AlCl3
W, Ta, Nb, Mo SF6, CF4
WF6, TaF6
Ti, TiN
Cl2
TiCl4
Organics, C
O2, O2/CF4
CO, CO2,
HF, H2,
H2O
Parameter Ranges
• Pressure
– 1 mTorr to 10 Torr
– At lower pressure process can have
precise dimensional control
– Isotropic processes tend toward higher
pressure
• Voltage
– Planar diode systems apply hundreds
of volts to the wafer
– High density plasmas and high
frequency plasmas use much lower
voltages
• Frequency
– 13.56 mHz industrial standard
– 2.45 GHz used frequently
– DC voltage not acceptable
Evolution of Plasma
Etching Equipment
• Barrel systems
• Chemical downstream Etching
(DCE)
• Capacitively Coupled rf Diode
System
• Capacitively Coupled Single
Frequency Planar Triode System
• Capacitively Coupled Dual
Frequency Diode
• High density Sources (ICP, ECR)
The current design allow
independent control of the ion
curent density and the ion
bombardment energy
Plasma Etching Apparatus
Barrel Etcher for Plasma Etching
Planar Etcher for Reactive Ion Etching
Etching mechanism
showerhead
plasma
RF
ions, radicals,
electrons, neutrals
By-products
pumped out
resist
film
substrate
Platen/electrode
RIE Process chamber
Steps in an etching reaction
• Adsorption of gas phase reactant onto
solid surface
• Formation of product molecule
• Desorption of product molecule
Section 4
The Etching of Si and Its Compounds and Its Application in
IC Fabrication
Silicon Etching
• Chlorine chemistry
– the most common source of chlorine is
Cl2 gas
– Other possible sources BCl3, SiCl4
and HCl
– Good anisotropy
– Good selectivity to oxide
• Bromine chemistry
– HBr is the most common source of
bromine
– Si etch rates are slower than with
chlorine-based chemistries
– Good anisotropy and selectivity to
oxide
– Most Si etching is carried out with
Cl2/HBr mixtures for critical profile
control
Silicon Etching-Application
•
•
•
•
Polysilicon Gate etch
Polysilicon Resistor etch
Silicon Trench Isolation
First mark etch
Silicon Dioxide Etching
• Fluorine chemistry needed.
– Good anisotropy
– CF4-O2 mixtures have good selectivity
to Si
– CF4-H2 mixtures have good selectivity
to Si
Silicon Dioxide Etch – Application
•
•
•
•
Spacer etch
Contact etch
Via etch
Bond pad etch
Silicon Nitride Etch
• Nitride etch mechanism is not well
understood
• Etch product is SiF4
• CF4-O2 mixtures have good selectivity to
Si
• CF4-H2 mixtures have good selectivity to
Si
Silicon Nitride Etch - Application
– Formation of LOCOS/field oxide
– Active area definition
– Bond pad etch
Aluminum Etching
• Fluorine-based etching cannot be used
because AlF3 is not volatile at
processing compatible temperature
• Aluminum chloride is volatile at room
temperatures. Chlorine based
chemistries are most common in
aluminum etching
– Room temperature etch products is Al2Cl6
– At temp. above 300°C, the etch product is
AlCl3
Cross-section of a metal etch profile
Aluminum etch – Application
• Form wiring for interconnection between
the metal layers
Section 5
Photoresist Removal/Plasma Ashing
Plasma Ashing
• Photoresist removal after etching is
necessary to prevent contamination of
diffusion furnaces or deposition furnaces
• Photoresist ashing can be considered as
a form of etching and can be carried out
by wet chemical or plasma methods
usually known as plasma ashing
• Oxygen (O2) plasma will oxidize resist to
form carbon dioxide and water
• Oxygen plasmas are unreactive with
common microelectronic materials,
therefore can be used on metallized
wafers.
Plasma Asher Equipment
• Barrel type plasma asher is still widely
used
• Single wafer plasma asher offers a
precise control of the ashing process,
especially for removing high dose
implanted resist