Introduction to Plasma Etching

Dr. Steve Sirard

Technical Director
Lam Research Corporation

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Outline – Day 1

► Pattern transfer requirements

► What is plasma and why is it needed?

► General plasma fundamentals

► Basic commercial etch hardware

► General plasma etch process fundamentals

► Specific case: Dielectric (SiO2, Si3N4, etc) etch mechanisms

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Basic Pattern Transfer

► Objective is to produce a Subtractive Additive

patterned thin film on a substrate

► Patterns are commonly formed by

either additive or subtractive
methods
Mask
► To pattern film, a mask is formed Film
with photolithography
Substrate
▪ Resist pattern is a stencil that
protects underlying films/substrate
from dep or etch attack

► Supply etchant (either wet or

gaseous) to remove film in
undesired areas

► We will generally focus on the

subtractive process

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Wafer Fabrication Process Steps

Segments Lam addresses

Incoming
Deposition Lithography Etch Strip Clean Deposition
Wafer

Residue/
Photoresist Particle

Put down Create the Selectively Remove Remove Deposit

the film to pattern remove film to photoresist residues and next
be patterned mask define features mask particles materials

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Often for pattern transfer, final feature dimensions are
required to be different than litho-printed dimensions

Post Litho

Post Pattern
Transfer

Final hole diameter required to be less than litho-printed hole diameter

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For leading edge fabrication, film stacks can get very
complex

Etch Steps
• SOG Open
• SOC Open
• Partial via in
oxide/low-k
• SOC Strip
• Trench etch

Sample Requirements
• Shrink PR CD by 15nm
• Trench depth = ½ via
depth
• Within wafer
uniformity < 2 nm for
trench depth and line
CDs

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For leading edge fabrication, pattern transfer steps can have
vastly different requirements
Singh, SST, 2017

Staircase etch  Control

lateral and vertical etch

Challenges for high-aspect ratio (> 40:1) pattern

transfer

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What do we need to control when transferring patterns?

► Etch rate/Throughput

► Etch rate selectivity (relative etch rate of one film vs another)

► Anisotropy (vertical etch rate vs horizontal etch rate)

► Sidewall angle/Feature Profile (straight, tapered, bowed, re-entrant)

► Faceting (erosion at top of feature)

► Critical dimensions

► Uniformity (within chip, within wafer)

► Repeatability (wafer-to-wafer, chamber-to-chamber)

► Defects (e.g., particles, etc)

► Damage (material modifications that degrade yield or electrical

performance)

► Line edge roughness, line width roughness, local hole uniformity

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What is a plasma??

►A plasma is a quasineutral gas of charged and neutral particles

► “Quasineutral” means that overall the net charge of the plasma is

approximately zero, because fluctuations in charge density in the plasma are
small in magnitude and short in duration

►A plasma is created whenever gases are forced to conduct electric current

▪ Plasmas generate electrons, reactive neutral species, and ions

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What is a plasma?

► Manyof the plasmas used in dry etching

are weakly ionized
▪ Ionization fraction, xi << 1
▪ Quasineutral: ni = ne  densities (~109 – 1012
cm-3); magnitudes lower than the neutral gas
density (ng)

► Plasmagenerated inside etch tool by

feeding electrical power into a gas

► Power transferred to the few free

electrons initially within the gas excites
electrons to higher energies

► Highenergy electrons can then ionize

neutrals and initiate a collision cascade,
thus creating and sustaining the plasma

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What is a plasma?

►A plasma generates reactive species which are not available in a bottle and
“delivers” them to the substrate of interest

► Electrons are the main current-carriers because they are light and mobile

► Energy transfer between light electrons and gas molecules they collide with is
inefficient and electrons can attain a high average energy (thousands of
degrees above the gas temperature)

► Elevated electron temperature permits electron-molecule collisions to excite

high temperature type reactions (forming free radicals) in a low temperature
neutral gas

► Generatingsame reactive species without a plasma would require

temperatures in the 103 – 104 K range!
▪ These temperatures would incinerate organic photoresist and melt many inorganic films

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Characteristics of weakly ionized plasma discharges

1. They are driven electrically

2. Charged particle collisions with neutral gas molecules are important

3. There are boundaries at which surface losses are important

4. Ionization of neutrals sustains the plasma in the steady state

5. The electrons are not in thermal equilibrium with the ions

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The Benefits of Plasma Processing (Etching and Deposition)

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General Plasma Fundamentals

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Anisotropy?  Thank the Boundary Layer Sheath

► Initially
within the system, electrons rapidly move throughout the chamber
and are lost to the walls, as opposed to the slower and heavier ions

► To maintain quasineutrality, a confining potential forms at the wall that acts to

repel electrons back into the bulk, while simultaneously accelerating ions
toward the walls

► Ultimately, this forms a region of net positive charge known as the sheath

► Sheath thickness is typically on the order of a few millimeters (a few debye

lengths)

► Ion acceleration energy is typically 10 – 40eV, but can rise to ~1000eV or so if

further biased

► Sheath is key for achieving anisotropic etching, as at low pressures where

collisions in the sheath are minimized, the ions arrive at near-normal
incidence

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Plasma composition

►A plasma generates reactive species which are not available in a bottle and
delivers them to the substrate

► Typical species in the plasma

▪ Electrons
▪ Neutral/Reactive radicals: F, Cl, O, CFx…..
▪ Ions: Ar+, CF3+, Cl-…..

► Ionmotion is random in the central glow, but when a positive ion drifts to the
sheath boundary, it is accelerated toward the wall/wafer surface

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Ions+Reactants have synergistic effect on etch rate
Key mechanism for anisotropic etching
Silicon Etch

Si(s) + 4F(g)  SiF4(g)

Classic experiment of Coburn and

Winters - Alternately exposing Si
surface to Molecular beam & ion
beam
Ion-Neutral synergy

► Etchrate of combined is order of

magnitude higher than the sum of
individual rates  SYNERGY!

► Shows how enhancement of the

etch requires energy of activation
which is provided by the ion
bombardment

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Collisional processes in the plasma

►Elastic collision:
▪When the internal energies of the two colliding particles do
not change
—The energy exchange is restricted to kinetic energy
▪The sum of the kinetic energies is conserved

►Inelastic collision:
▪When the internal energies of the two colliding particles do
change
—The sum of the kinetic energy is not conserved

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Important Collisional processes in the plasma

 Ionization: e- + Ar  Ar + + 2 e- e-
inelastic
n + e-
collision
e-
 Dissociation: e- + AB  A + B + e-

 Dissociative ionization (molecular gases): e- + AB  A + B + + 2 e-

 Electronic excitation: e- + Ar  Ar* + e- energy loss process that generates light

 Electron attachment:
 Resonance capture (e- + SF6  SF6- ).
 Dissociative attachment (e- + SF6  F + SF5- ).

 Elastic scattering: e- + Ar  Ar + e- Transfers momentum & changes angle

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Collisional processes in the plasma

►Ionization

e- + CF4  CF4+ + 2 e-

An electron can ionize an atom or molecule if it has energy greater than

the ionization potential of the species

e-

inelastic
e- CF4 CF4+
collision

e-

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Collisional processes in the plasma

►Dissociation

e- + CF4  CF3 + F + e-

An electron can dissociate a molecule if it has energy greater than the

weakest bond in the molecule
F

inelastic
e- CF4 CF3
collision

e-
This is the mechanism for generation of free radicals which are the
reactive agents in the plasma
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Excitation processes in the plasma

►Excitation
► Atoms and molecules in their ground states can be excited (by collisions or
radiation ) to higher energy bound states

► Most bound states can emit a photon and return to a lower energy or ground
state

▪ e- + Ar  Ar* + e-  Ar + e- + ħw
Ar*

ħw
ħw is the photon energy
Arf
▪ Here

Ar

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Excitation processes in the plasma

►Excitation
► Thelight emitted by a plasma can provide both a qualitative and
quantitative analysis of the plasma

► Optical
emissions from the plasma are useful for plasma diagnostics
and endpointing etch recipes
Ar = 801 +/- 4nm Endpoint Detection

O = 777 +/- 4nm Endpoint

CN = 390 +/- 5 nm
CO = 520 +/- 5nm  = 520 nm

SiF = 440 +/- 5nm

CF2 = 304 +/- 4nm

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Collisional energy transfer by metastables

► Lifetimeof typical excited state is ~10-9 s and is typically de-excited by

photon emission

► However, certain metastable states are longer lived (up to a few sec) which is
long enough for a collision to occur before it eventually decays

▪ Electronic excitation (excitation transfer):

Xe * + CO  CO* + Xe
▪ Penning ionization:
He* excited (eex = 19.8 eV) + Ar Ar+ (eiz = 15.8 eV) + He + e-

▪ Penning Dissociation:
Ar* excited state + AB  A + B (where ediss < 11.6eV).

Ar* excited state + O2  O + O (where ediss = 5.2eV).

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Energy distribution for collisional processes
Cross-Section can be thought of as a probability of an occurrence. In this case - for
Electron Attachment, Dissociation and Ionization

Dissociation Ionization
Cross Section;
# Electrons

Simultaneously controlled by EEDF

Electron Energy

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Plasma Density and relative energies of species

All neutral particles ng near

RT

electrons ne hotter than ions.

Low energy ions going

through the sheath are
converted to high energy ions.

ions ni near RT

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Important Potentials

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Plasma Potential, Vp

► The high mobility of electrons creates

thin positive ion sheaths near the
walls/electrodes Je
Wall
► Positiveions are left behind, and the
plasma charges up positive Ji
▪ This is the plasma potential, Vp, which is
positive relative to the walls in contact with
the plasma Vp -
-
► With respect to ground (V=0), if the time -
Je -
averaged plasma potential is +100V, then - Wall
ions hitting the ground electrode would -
have an energy of 100eV Ji - V=0
Pos. Ions -

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Floating Potential, Vf

► Electrons will move much faster than

ions to a surface in the plasma, charging
up the surface negative with respect to
the plasma

► Thischarge retards further electron loss

from the plasma

► Ifthe surface is a floating wall

(electrically isolated surface), a steady
state is reached where the reduced flow
of electrons is balanced by the flow of
ions (fluxes balance, so net current is 0)

► Vf is ~ -10 to -20V with respect to the

plasma potential (Vp)

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Self-bias Voltage, Vbias

► Time-averaged value of the powered

electrode voltage is called the self-bias
voltage when measured with respect
to ground

► Vbias
is negative with respect to the
plasma potential, Vp Bulk Plasma, Vp
100V
► The potential drop across the sheath at 85V
Floating, Vf
the powered electrode is the sum of Ground, V0
0V
the plasma potential and the self-bias
▪ Vsh = Vp + IVbiasI -250V
Powered Electrode, Vbias
► The powered electrode will be
bombarded with much higher energy
ions than that of a grounded or floating
wall

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Ion energy distributions

► We’ve been discussing time-averaged potential behavior

► Inactuality, the plasma (e.g., sheath potentials) are oscillating at the applied
RF frequency

► This has implications for the ion energy distribution (IED)

Average
0.015 At 5 mTorr, ie. Plasma Potential Vp
“collisionless”
0.010
Flux
Ion IED

0.005

0.000
0 20 40 60 80 100
DC bias potential Vbias
IonIonEnergy
Energy (eV)
(eV)

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RF excitation frequency has a big effect on the Ion Energy
Distribution Function

► Lower frequency
produces broader
distribution and
higher mean 400W 2MHz
energy (w/ 800W 60MHz)
400W 27MHz
400W 60MHz
400W 100MHz
► Higher
frequencies
produce narrower 400W 2 MHz / 800W 60MHz
distribution and
Ion flux

lower mean 400W 27 MHz

energies
400W 60 MHz

0 100 200 300 400 500

► IEDF plays a key
Ion energy (eV)
role in modulating 400W 100 MHz
etch behavior
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RF excitation frequency has a big effect on the Ion Energy
Distribution Function
► Trends with increasing RF power (single frequency – 27MHz)
▪ Higher mean ion energy
▪ Wider IEDF

0W 2MHz
27MHz power:
200W
400W
800W
Flux 51)

1200W
400 W 27MHz
Ion(mass

1600W
2000W
800 W 27MHz
ion flux

1200 W 27MHz

2000 W 27MHz
0 100 200 300
Ion energy (eV)

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Key points for plasma fundamentals

►A plasma generates reactive species which are not available in a bottle

▪ Plasmas consist of electrons, neutrals/radicals, and ions generated through collisional
processes

► Ions are accelerated through the boundary layer sheath at near normal
incidence (Directional)

► Reactant exposure with simultaneous ion bombardment enhances etch rate of

materials (Synergistic, anisotropy mechanism)

► RF excitation frequency has a big impact on the ion energy distribution

▪ High RF frequency leads to lower mean ion energies, narrower distribution
▪ Low RF frequency leads to higher mean ion energies, broader distribution

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Basic Commercial Etch Hardware

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Conductor and Dielectric Etch Tools

Conductor Etch Dielectric Etch

Inductively Coupled Plasma (ICP) Capacitively Coupled Plasma

Separate ion
energy & flux
3-Frequency

STI, Gate, DPT, TiN Mask Open, Contact, Metal Hard Mask All-in-
Non-volatiles, TSV One, Via, Trench

Common design principles for critical etch performance:

 Symmetrical chambers – including pumping & RF
 Independent tuning knobs – including step-by-step control
 Repeatable performance – die-to-die, wafer-to-wafer, and chamber-to-chamber

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Capacitively Coupled Plasma (CCP) (Voltage coupling)

► Deliver energy to the electrons in the plasma discharge by applying a

RF voltage to electrode

► Typically,when energetic ion bombardment is needed (like in etching

of oxides) capacitively coupled RF power is required

► Multiple RF excitation frequencies can be used individually or

simultaneously to alter plasma characteristics (e.g., ion energies, ion
flux, etc)

3-Frequency

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CCP Etch Chamber Characteristics

► Largefraction of RF power goes to ion

acceleration

► Highion energies but lower plasma

density

► Operating pressure regime 10mT –

2000mT
▪ Most advanced processes operate less than
200mT, rarely above 500mT
3-Frequency

► Low fractional ionization: 10-6 – 10-3

► Low plasma density (108 – 1010/cm3)

► Lowfractional dissociation of species 

Larger fragments remain

► Cannotcontrol plasma density and ion

energy independently

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Inductively Coupled Plasma (ICP) (Current Coupled)

► Inductive coupling is another commonly used method of delivering RF power

to the electrons in a plasma

► High RF current in the external coil generates an RF magnetic field in the

plasma region which, in turn, generates an RF electric field in the plasma
zone
▪ RF electric field can couple energy into the plasma electrons

► ICP tools generate high density plasmas and lower ion bombardment of
surfaces

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ICP Etch Chamber Characteristics

► Generates large RF current as little power

is used for ion accelerations
▪ No high ion energy ion bombardment without
bias power

► With 2 RF generators, both plasma density

and ion energy can be controlled
independently

► Typical operating pressures 1 – 80 mT

► High fractional ionization (10-3 – 10-1)

► High plasma density (1011 – 1013)

► High fractional dissociation, smaller

fragments remain

► Larger gap to give required uniformity

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Comparison of ICP vs CCP Characteristics

Relative Densities and Energies

► Higher pressure operation ► Higher plasma densities

► Higher ion energies ► Decoupled control of plasma

density and ion energy
► Plasmadensity and ion energy are
coupled
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Plasma Etch Process Fundamentals

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Mechanisms for etch directionality & profile control

► Ions are accelerated through

the sheath and the ion flux is
mostly normal to the wafer

► This is the only anisotropic

process in the plasma
discharge, and leads to
anisotropic etching of the
features

► Sidewall etching is usually

chemical in nature and is slow
due to glancing ions or even
ion shading (minimal synergy)

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Etch kinetics: Special etch regimes

► Simple model for etch rate depends on ion flux, ion energy, and neutral
flux/surface coverage
▪ Neglecting role of pure sputtering by inert or reactive ions
▪ Neglecting role of thermally activated neutral etching

where:
1 ei = ion energy JN = neutral flux
EtchRate  eth = threshold energy A ~ ion efficiency
A B

e 
Ji = ion flux B ~ sticking efficiency
1/ 2
i  e th
1/ 2
 Ji JN
Source: Gottscho et al., JVSTB (1992), Steinbruchel (1989)

► Special/Limiting cases
▪ When ion flux is negligible (Ji = 0)  Etch Rate vanishes
▪ When neutral flux is negligible (JN = 0)  Etch Rate vanishes
▪ At constant ion energy flux, the etch rate will initially increase in proportion to the
neutral flux (neutral-limited regime), but then saturate at higher neutral fluxes (ion-
limited regime)

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Etch kinetics: Special etch regimes

Model Prediction Si etching with Cl and Ar+

Etch Yield (Si/Ar+)

Flux Ratio (Cl/Ar+)

Chang et al., JVSTA 15(4), 1997

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Mechanisms for etch directionality & profile control

Often, this type of

undercut is unacceptable

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Mechanisms for etch directionality & profile control

► Condensable species
▪ Tend to form films on surfaces
▪ Very dependent on the surface temperature

► Reactive species
▪ Tend to react chemically with the surface
▪ Often saturate at one monolayer coverage

► Examples at room temperature

▪ Halogen atoms: Cl, F….. – reactive but not condensable
▪ Inert Gas atoms: Ar, Xe, He…. – not reactive or condensable
▪ Polymer Precursors (CxFy radicals): often both condensable and reactive

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Mechanisms for etch directionality & profile control

Ion’s Angular Distribution Neutral’s Angular Distribution

- narrow - wide

Reactive neutrals Ion assisted +

Reactive/Condensable
neutrals

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Mechanism for etch directionality & profile control

Four basic etching processes

1. Pure chemical etching
2. Sputtering
3. Ion enhanced etching
4. Ion enhanced inhibitor etching

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1. Pure Chemical Etch

► Selective, slow process - due to etchant atoms or molecules (like F or O)

reacting at the surface and forming volatile products

► Isotropic

Neutral Equal Rates

Volatile
Mask

Film

Substrate

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2. Sputtering

► Non-selective, slow - physical process due to energetic ion bombardment

ejecting surface atoms

► Anisotropic

Ion

Involatile
Mask

Film

Substrate

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3. Ion Enhanced Etching

► May have lower selectivity than pure chemical etch

► Enhanced vertical etch rate due to synergy between ions and chemical
etching

► Anisotropic

Ion Sidewall etching of resist

Neutral causes loss in Anisotropy
Mask
Volatile
Film

Substrate

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4. Ion Enhanced Inhibitor Etching

► Similar to ion enhanced etching, but may have higher selectivity

► Inhibitor(e.g., polymer film) deposited on the sidewalls where ions are not
effective at removing

► Anisotropic

Ion
Neutral Passivation film
Volatile
Mask

Film
Film removed
Substrate

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What variables influence etch directionality?

►Ion flux
►Ion energy
►Neutral/ion flux ratio
►Deposition or passivation chemistry
►Temperature of surface being etched
►Pressure (sheath collisions may deflect ions at higher
pressures)

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Dielectric Etch Mechanisms

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Overview of SiO2 etch

► Typical process gases: (hydro)fluorocarbons with Ar and O2

▪ Also will see CO, N2, H2

► High bias voltage/wattage for promotion of product formation

► SiO2(s) + CxFy + I+(Ei)  SiF4(g) + CO(g)

► Selectivity (to Si and PR) provided by polymer formation

►F atoms etch silicon dioxide slowly at room temperature; low reaction rate
compared to ion bombardment assisted etch
▪ All observed etching of SiO2 is ion energy driven
▪ Energetic flux breaks bonds and forms reactive sites for F to form volatile products (SiF4)
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Common etchant gases for silicon dioxide

► Perfluorocarbons: CF4, C4F8, C4F6

▪ F/C ratio is a key parameter that can control how polymerizing an etch process is
▪ Important for selectivity and profile control

► Hydrofluorocarbons: CHF3, CH2F2, CH3F, CH4

▪ Addition of hydrogen can scavenge F in the plasma and increase polymerization
— H + F  HF

► Oxygen
▪ Added to increase F and decrease polymer precursors

► Inert gases: He, Ar, Xe

▪ Control the neutral radical/ion flux ratio
▪ Manipulate plasma density and/or electron temperature
▪ Dilute the reactants
▪ Improve heat transfer (He)

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SiO2 etch mechanism

► An SiO2 surface with a CFx radical flux under ion bombardment forms a layer
of SiCxFyOz
▪ Ion beam mixing of CFx radicals plays an important role in the formation

► Thekey etch mechanism is likely the breaking and reforming of bonds of the
SiCxFyOz layer due to energetic ions colliding with and penetrating the surface
▪ This produces easily desorbed etch products, weakly bound to the surface
— SiF4, SiF2, SiOF2, CO, CO2, COF2, O2
▪ A layer of C is prevented from building up due to reaction with O within the film

SiF4 CO2

CFx + CFx + CFx +

SiCxFyOz SiCxFyOz
SiO2 SiO2 SiO2

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SiO2 etch selectivity mechanisms

► For F-rich discharges, there is little selectivity for SiO2 over Si

► Highselectivity is obtained by using unsaturated fluorocarbon gases or by

adding H2 to scavenge fluorine (decrease F/C ratio)

► Theoxygen in SiO2 permits formation of volatile COx, preventing buildup of

carbon on the surface
▪ SiO2 etches while a carbon layer builds up on the Silicon  Etch Selectivity!

SiF4 CO2

CFx + CFx +

SiCxFyOz C-rich film

SiO2 Si

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Effect of Oxygen

► Addition of oxygen can increase photoresist etch rate, thereby

decreasing oxide etch selectivity to resist

► Ifthe etch is more polymerizing (i.e., low F/C ratio), then oxygen
addition will increase the oxide etch rate without as large an increase
in the resist etch rate (thus, increasing selectivity)
▪ Due to additional oxygen liberated by SiO2 as it is etched

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Fluorine/Carbon ratio

► Ifthe plasma is made too fluorine deficient, polymer deposition will dominate
over etching of SiO2  Etch Stop!
▪ The F/C ratio where this occurs is dependent on energetic ion flux
▪ At higher energies, etching will take place
▪ At lower energies, deposition will take place

► For high selectivity, we often have to operate close to this boundary

Bias (V)

Etching

Polymerization

F/C Ratio

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Etching Si3N4

► Mechanistically not as well understood as Si or SiO2 etching

► Often said that Si3N4 etch behavior is in between Si and SiO2

▪ Relative reactivity to F atoms without ion bombardment is in between Si and SiO 2
▪ The effectiveness in removing polymeric blocking material is in between Si and SiO 2

► SiF4 is the dominant Si-containing etch product

► How is nitrogen evolved?

▪ In pure Fluorine plasma (F atoms only), nitrogen leaves as N2
▪ When nitride is etched in a fluorocarbon plasma, optical emission from the CN radical is
observed (FCN has been observed in such situations)

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Nitride etching selectivity

► SiO2/Si3N4 and Si3N4/Si selective etching can be obtained with fluorine-

deficient fluorocarbon plasmas such as CF4/H2, CHF3, C4F8, etc

► The mechanism responsible for SiO2/Si3N4 selective etching is similar to that

discussed previously for SiO2/Si etching

► For SiO2/Si3N4 selective etching, the key factor is that nitrogen is less efficient
than oxygen in removing carbon
▪ Therefore, conditions can be found where SiO2 etches and Si3N4 does not

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