070823 ISPC-18 School (13) · ISPC-18 Summer School Kyoto, Japan Background As integrated circuit...

Plasma Etching

August 24, 2007

Department of Aeronautics and Astronautics, Department of Aeronautics and Astronautics, Graduate School of Engineering, Kyoto University,Graduate School of Engineering, Kyoto University,

YoshidaYoshida--Honmachi, SakyoHonmachi, Sakyo--ku, Kyoto 606ku, Kyoto 606--8501, Japan8501, JapanE-mail: [email protected]

Kouichi OnoKouichi Ono

ISPC-18 Summer SchoolKyoto, Japan

Background�As integrated circuit device dimensions continue to be scaled down to <0.1 �m, strict requirements are being imposed on plasma etching technology.

�Especially, the precise or nanometer-scale control is indispensable for etched profiles and critical dimensions, together with higher selectivity, higher microscopic uniformity, and less damage.

�Moreover, new materials such as metals and low- and high-k*dielectrics are being employed for <0.1 �m devices, and so the etching of such new materials are also required in integrating them into device fabrication. *k : dielectric constant

�This lecture presents the current status of plasma etching tech-nology based on the physical and chemical mechanisms underlying the processing, along with the future prospect towards nano-scale processes.

辻野貴志

タイプライターテキスト

Copyright remains with the author(s).

Outline

1. Introduction1. Introduction�ULSI devices �Requirements for plasma etching technology

2. Fundamentals of Plasma Etching Technology�Etching characteristics �Core technology of Plasma Etching

3. Role of Plasma in Plasma Etching�Gas-phase reactions �Ion acceleration through the sheath

4. Surface Reaction Processes in Plasma Etching�Surface reactions �Ion and neutral transport in microstructures�Feature profile evolution �Microscopic uniformity �Charging

5. Current Issues of Plasma Etching Technology�Current issues �Poly-Si gate etch �High-k gate etch�Metal etch �Deep RIE

6. Summary�Future prospects

1. Introduction� Si-ULSI Devices� Requirements for Plasma Etching

Logic Technology Families

http://www.tsmc.com

Si- ULSI Devices

http://www.intel.com/research/silicon

Si- ULSI Devices (continued)


Trends of Si-ULSI Devices (1)


Planar CMOS Transistor Scaling

Trends of Si-ULSI Devices (2)

MetalMetalMetalPoly-silicon

Poly-silicon

Poly-silicon

Poly-silicon

Poly-silicon

Gate electrode

High-kHigh-kHigh-kSiO2SiO2SiO2SiO2SiO2Gate dielectric

Ultra Low-k(k=1.9)

Ultra Low-k(k=2.1)

Ultra Low-k(k=2.5)

Low-k(k<2.9)

Low-k(k=2.9)

FSG(k<3.6)

FSG(k=3.6)

FSG(k=3.6)

Inter-layer dielectric

?CuCuCuCuCuAlAlInter-connect

300300300300300200/300200200Wafer size (mm)

22 nm32 nm45 nm65 nm90 nm0.13 �m0.18 �m0.25 �mProcess generation

20112009200720052003200119991997Year (1st production)

* FSG: high-density plasma fluorinated SiO2

�As integrated circuit device dimensions continue to be scaled down, increasingly strict requirementsare being imposed on plasma processing technology, including the etching and deposition of newmaterials as well as the more precise control of etching of conventional materials.�Great attention has recently been placed on integrating high and low dielectric constant (k) materials

into the fabrication of gate stacks and multi-level interconnections, respectively, for advancedsub-100 nm microelectronic devices.

High-k Gate Stack

�The technological challenge continues for growing ultra-thin SiO2 film of high quality, to maintainthe gate capacitance without increasing the leakage current and reducing the oxide reliability;however, the ultimate solution would rely on high-k materials such as HfO2 and ZrO2, and theirsilicates and aluminates. �Moreover, for gate stacks with high-k dielectrics, gate electrodes of conventional Poly-Si arerequired ultimately to be replaced by metal gates of TiN, TaN, Ru/RuO2, Pt and Ir.�Plasma processing is indispensable for the fabrication or etching of gate electrodes,and also for the removal or etching of high-k dielectric.

High-k gate stack

(from NEC)

Transistor (Gate stack)

Ultra-thin gate SiO2

(from AMAT)

Multi-level Interconnects andLow-k Inter-layer Dielectric

�Low-k inter-layer dielectrics (ILD) are required for reducing the resistance-capacitance time delay,which is getting more conspicuous as the shrinkage of the spacing between metal lines in high-density multi-level interconnections.�Plasma processing is indispensable for chemical vapor deposition (CVD) of ILD,and also for the fabrication or etching of high-aspect-ratio contact (HARC) and via holes through ILD.

(Low-k,SiOCH)

(Multi-levelinterconnects)


(1) Etch anisotropy and selectivity (�)� Profile control, Critical dimension (CD) control� Selectivity over mask and underlying layers

(2) Plasma damage (�)� Charging damage� Physical damage (ion-bombardment, impurity permeation)� Radiation damage

(3) Microscopic uniformity (on a chip and cell scale) (�)� Etch rate, Profile, Selectivity, Damage, etc.� Dependence on aspect ratio (AR), feature size, and pattern density

(4) Macroscopic uniformity (on a wafer scale) (�)� Etch rate, Profile, Selectivity, Damage, etc.

(5) Etching of new materials and device structures� Low-k, High-k, Metal, Dual gate, etc.

Requirements for Plasma EtchingFor ULSI fabrication

�The ULSI devices are substantially planar (2D), usually 5-10 �m thick, owing to limitations of the traditional fabrication processes for ULSI.

In addition to (1) � (5) for ULSI,

(6) High etch rate (or Etching ofdeep structures)

(7) Etching of 3D microstructures

Requirements for Plasma EtchingFor fabrication of MEMS

�MEMS devices are manufactured using processes based on USLI fabrication technologies, and also using emerging technologies to fabricate 3D, deep (up to 1 mm) microstructures with higher throughputs and lower costs.

*MEMS: Microelectromechanical Systems

Bio-chip, �TASDNA Chip (DNA Microarrays)

SensorIR Sensor

Acceleration Sensor

Actuator

DMD (Digital MirrorDevice)

Optical Switch

RF-MEMS

Microfabrication using Etching

DRAM cell

2 mm0.2 mm

0.2 �m

Micro cantilever Micro turbine

1 �mPhotonic crystal

2. Fundamentals of PlasmaEtching Technology

� Etching Characteristics� Core Technology of Plasma Etching

Plasma reactorFeed gasProcess control

Plasma Etching Technology

� Etching Characteristics :� Etch anisotropy and selectivity� Microscopic uniformity� Plasma damage

� Plasma etching (or processing) technology consists of three coretechnologies: (1) Plasma reactor, (2) Reactive Gas, and(3) Process control.

� Plasma plays two roles in plasma processing:(1) To generate ions and reactive neutrals from feed gases through

electron impact events, which are then transported ontosubstrate surfaces.

(2) To form the sheath above substrate surfaces, which acceleratesthe ions onto substrate surfaces.

Plasma Etching

RF CoilRF PowerSource

FeedGas

DielectricPlate

Plasma

To Pump

Wafer Stage

CoolingWaterSubstrate

RF Bias


FeedGas

DielectricPlate

Plasma

To Pump

Wafer Stage


RF Bias

CCP* mode(100 W)

ICP Cl2 Plasma

ICP mode(300 W)

ICP (Inductively Coupled Plasma)

EtchedProfile(Poly-Si)

*CCP : Capacitively Coupled Plasma

Etch Anisotropy and Selectivity

�Anisotropy :

�CD Loss / Gain :

�Selectivity :

vertical

lateral 1ERERA ��

10 WWW ��

layerunder or mask

film

ERERS �Etching

Process

W0

W1W1

Underetch

AfterLithography

Overetch

Just Etch

After MaskRemoval

Start

MaskFilm

Substrate(Under layer)

(50% Etching)

(100% Etching)

(150% Etching)(50% Overetch)

Isotropic Anisotropic

Profile Irregularities

*The nonuniform surface coverage of neutrals results from the difference in anisotropy between incoming ions and neutral reactants.**The nonvertical ion incidence originates from the thermal motion of ions, scattering of ions through collision with neutrals in the

sheath, and deflection of ion trajectories due to charging of mask dielectrics.***The charging results from the difference in anisotropy between incoming Ions and electrons.

Microtrenching

[Ion Reflection from Feature Sidewalls][Charging of Mask Dielectrics and theConsequent Deflection of IonTrajectories]***

�Ion

(Outwardly) Tapered

[Deposition of Surface Inhibitors/Etch Products on FeatureSidewalls]

Surface Inhibitor

Etch Product

�

[Nonuniformity Surface Coverageof Neutrals in Microstructural Featurescombined with Nonvertical Ion Incidence]*

Inversely Tapered

��

Ion�Neutral Reactant

Undercut

�

Neutral Reactant

[Isotropic Etching by Neutrals]

Mask

Notching

�Ion

[Differential Charging of MicrostructuraFeatures and the Consequent Deflectionof Ion Trajectories] ***

� �� Electron��

Bowing

�Ion

[Nonvetical Incidenceof Ions]**

��

(a) (b) (c)

(d) (e) (f)

SelectivitySelectivity over underlying films is required during overetch.

MaskFilm to be etched

Just Etch

Underlying Film

Overetch

OpenField

Mask

Before Etch

Just Etch

Overetch

Before EtchFilm to be etched

Underlying Film

Mask Mask

Substrate

Selectivity overmask is requiredduring mainand overetch.

Microscopic Uniformity

(c)

(e)

(b)

(f)

(d)

[Shadowing for Incoming (Neutral) Reactants]

RIE Lag

(Neutral) Reactants

OpenField

Large aspect-ratio features etch slower than smaller ones.

OpenField

InverseRIE Lag

Surface Inhibitors

[Shadowing for Incoming Surface Inhibitors][Deposition on Bottom Surfcaes]

Large aspect-ratio features etch faster than smaller ones.

[Shadowing for Incoming (Neutral) Reactants]

RIE Lag

(Neutral) Reactants

OpenField

Large aspect-ratio features etch slower than smaller ones.[Shadowing for Incoming (Neutral) Reactants]

RIE Lag

(Neutral) Reactants

OpenField

Large aspect-ratio features etch slower than smaller ones.

OpenField

InverseRIE Lag

Surface Inhibitors


Large aspect-ratio features etch faster than smaller ones.

OpenField

InverseRIE Lag

Surface Inhibitors


Large aspect-ratio features etch faster than smaller ones.Surface Inhibitors

OpenField

[Shadowing for Incoming Surface Inhibitors][Deposition on Sidewalls]

Large aspect features etch less tapered than smaller ones.

[Transport (Redeposition) of Etch Products]

OpenField

EtchProducts

Large aspect features etch more tapered than smaller ones.

[Differential Charging of Microstructural Featuresand the Consequent Deflection of Ion Trajectories]

Notching occurs at the inner sidewall foot of theoutermost feature of a L&S structure neighboringan open area.

OpenField

Notching+

Ions+ +

OpenField

[Charging of Mask Dielectricsand the consequent Deflection of Ion Trajectories]

Large aspect features exhibit more significant trenchingthan smaller ones.

Micro-trenching

+Ions

+ +

Surface Inhibitors

OpenField




OpenField

EtchProducts


Surface Inhibitors

OpenField



Surface Inhibitors

OpenField


Surface Inhibitors

OpenField




OpenField

EtchProducts

Large aspect features etch more tapered than smaller ones.[Transport (Redeposition) of Etch Products]

OpenField

EtchProducts




OpenField

Notching+

Ions+ +

OpenField



Micro-trenching

+Ions

+ +



OpenField

Notching+

Ions+ +



OpenField

Notching+

Ions+ +

OpenField



Micro-trenching

+Ions

+ +

OpenField



Micro-trenching

+Ions

+ +

(a)

EtchRate

EtchedProfile

Plasma

photon Je Ji

electron ion

CuILD

ion

Si sub.

STISTI

gate dielectricgate electrode

PMD

ILD

ion ion

(i) Charging Damage“plasma current” : Ji , Je

(~ mA/cm2)

(ii) Physical Damageion with high energy

(Ei ~ 100 eV)

(iii) Radiation Damagehigh energy photon

(h�~ 10 eV)

(ii)(iii) (i)

Plasma

photon Je Ji

electron ion

CuILD

ion

Si sub.

STISTI

gate dielectricgate electrode

PMD

ILD

ion ion


(~ mA/cm2)


(~ mA/cm2)


(Ei ~ 100 eV)


(Ei ~ 100 eV)


(h�~ 10 eV)


(h�~ 10 eV)

(ii)(iii) (i)

Plasma Damage

Plasma Etching Reactors

5 �m 3 �m 2 �m 1.3 �m 0.8 �m 0.5 �m 0.35 �m 0.25 �m 0.18 �m 0.13 �m 90 nm 65 nm

16 K 64 K 256 K 1 M 4 M 16 M 64 M 256 M 1 G (4 G) (16G) ---

Year 1977 1980 1982 1985 1988 1991 1994 1996 1998 2001 2004 2007

6” 8” 8”/12”4” 5”WaferDiameter

Barrel type

ICP

HWP

RIE (batch type)

SWP

DCSelf-bias

�Ion-Assisted Reaction(Ion acceleration through the sheath to achievedirectional, energetic ions incident on the substrate�

�Low-Pressure, High-Density �Independent Ion Energy Control

External Magnetic Fields

Two RF/Microwave Sources: for Plasma Source and for RF Biasing

Pulse ModulationPulse BiasingFrequency ControlHigh Flow RateWall Temperature ControlWafer Temperature Control

ECR

Narrow Gap, Triode, Two-frequencyRIE(single type)

MERIE Dipole Ring Magnet

No MagneticFields

Low-Pressure, High-DensityPlasmas

ExternalMagneticFields

FeatureSize

IntegrationLevel

�Low-Pressure, High-Density �Lower Ion Energy

5 �m 3 �m 2 �m 1.3 �m 0.8 �m 0.5 �m 0.35 �m 0.25 �m 0.18 �m 0.13 �m 90 nm 65 nm

16 K 64 K 256 K 1 M 4 M 16 M 64 M 256 M 1 G (4 G) (16G) ---

Year 1977 1980 1982 1985 1988 1991 1994 1996 1998 2001 2004 2007

6” 8” 8”/12”4” 5”WaferDiameter

Barrel type

ICP

HWP

RIE (batch type)

SWP

DCSelf-bias

�Ion-Assisted Reaction(Ion acceleration through the sheath to achievedirectional, energetic ions incident on the substrate�

�Low-Pressure, High-Density �Independent Ion Energy Control

External Magnetic Fields

Two RF/Microwave Sources: for Plasma Source and for RF Biasing

Pulse ModulationPulse BiasingFrequency ControlHigh Flow RateWall Temperature ControlWafer Temperature Control

ECR

Narrow Gap, Triode, Two-frequencyRIE(single type)

MERIE Dipole Ring Magnet

No MagneticFields

Low-Pressure, High-DensityPlasmas

ExternalMagneticFields

FeatureSize

IntegrationLevel

�Low-Pressure, High-Density �Lower Ion Energy

Feed Gas

Cooling Water GroundedElectrode(Anode)

RF Power Source

Plasma Wafer

To Pump

CoolingWater

Powered Electrode(Cathode)

Feed Gas


RF Power Source

Plasma Wafer

To Pump

CoolingWater


Plunger

Microwaves

Waveguide

PlasmaFeed Gas

To PumpWaferStage

Quartz Tube

Wafer

CoolingWater

Plunger

Microwaves

Waveguide

PlasmaFeed Gas

To PumpWaferStage

Quartz Tube

Wafer

CoolingWater

Cathode CouplingRIE (Reactive Ionetching)

Two-Frequency CCP

CDE (Chemical Dry Etching)

AnodeCoupling

Plasma

PoweredElectrode

GroundedElectrode

Wafer

RF Power Source(13.56 MHz)

Plasma

PoweredElectrode

GroundedElectrode

Wafer

RF Power Source(13.56 MHz)

Plasma

PoweredElectrode

Wafer

RF Power Source(40 MHz)

PoweredElectrode


Plasma

PoweredElectrode

Wafer


PoweredElectrode


Plasma Sources (1)

RF Power Source

Powered Electrode

Wafer

Plasma

Grounded Electrode

To Pump

Feed Gas

Powered Electrode

WaferQuartz Boat

Quartz Chamber

Etch Tunnel

RF Power Source

Powered Electrode

Wafer

Plasma

Grounded Electrode

To Pump

Feed Gas

Powered Electrode

WaferQuartz Boat

Quartz Chamber

Etch Tunnel

Barrel type(Multi Wafer Type)

CCP (Capacitively Coupled Plasma) Downstream Plasma

Low and Middle Density


FeedGas

DielectricPlate

Plasma

To Pump

Wafer Stage


RF BiasSource


FeedGas

DielectricPlate

Plasma

To Pump

Wafer Stage


RF BiasSource

Feed Gas


RF Power Source

Plasma Wafer

To Pump

CoolingWater


Magnetic Field Coil

Feed Gas


RF Power Source

Plasma Wafer

To Pump

CoolingWater


Magnetic Field Coil

RF PowerSource RF Coil

Plasma

To PumpRF Bias Source

Dielectric Wall

Substrate

CoolingWaterFeed

Gas

WaferStage

RF PowerSource RF Coil

Plasma

To PumpRF Bias Source

Dielectric Wall

Substrate

CoolingWaterFeed

Gas

WaferStage

Plasma

CoolingWater

FeedGas

To Pump Wafer Stage

RF Bias

RF PowerSource

Wafer PermanentMagnets

MagneticField Coil

Antenna

Dielectric Belljar

Plasma

CoolingWater

FeedGas

To Pump Wafer Stage

RF Bias

RF PowerSource

Wafer PermanentMagnets

MagneticField Coil

Antenna

Dielectric Belljar

MicrowavesWaveguide

DielectricWindow

FeedGas

Feed Gas

RF Bias

Wafer Stage

PlasmaWafer

CoolingWater

To Pump

ECRResonanceRegion

MagneticField Coil

MicrowavesWaveguide

DielectricWindow

FeedGas

Feed Gas

RF Bias

Wafer Stage

PlasmaWafer

CoolingWater

To Pump

ECRResonanceRegion

MagneticField Coil

Microwaves

Waveguide

Feed Gas

Grounded Plate

Wafer Stage

Cooling WaterTo Pump

RF Bias

WaferPlasma

Teflon PlateQuartz Plate

Aluminum Plate

Microwaves

Waveguide

Feed Gas

Grounded Plate

Wafer Stage

Cooling WaterTo Pump

RF Bias

WaferPlasma

Teflon PlateQuartz Plate

Aluminum Plate

ECR (Electron Cyclotron Resonance)

HWP (Helicon Wave-excited Plasma)

SWP (Surface Wave-excited Plasma)

Plasma Sources (2)MERIE (Magnetically Enhanced RIE) ICP (Inductively Coupled Plasma)

Middle and High Density

10-4 10-3 10-2 10-1 100

109

1010

1011

1012

ECP

Helicon

Gas Pressure (Torr)

Pla

sma

Den

sity

(cm

�3)

Barrel

1

RIE

MERIE

ICP

10�4 10�3 10�2 10�1 1

1012

1011

1010

109

10-4 10-3 10-2 10-1 100

109

1010

1011

1012

ECP

Helicon

Gas Pressure (Torr)

Pla

sma

Den

sity

(cm

�3)

Barrel

1

RIE

MERIE

ICP

10�4 10�3 10�2 10�1 1

1012

1011

1010

109

10-4 10-3 10-2 10-1 100

10

100

1000

1

ECR

MERIE

ICP

RIE

Barrel

��

�

��

(eV

)

Helicon

10�4 10�3 10�2 10�1 1

Gas Pressure (Torr)

Ion

Ene

rgy

(eV

)

103

102

10

10-4 10-3 10-2 10-1 100

10

100

1000

1

ECR

MERIE

ICP

RIE

Barrel

��

�

��

(eV

)

Helicon

10�4 10�3 10�2 10�1 1

Gas Pressure (Torr)

Ion

Ene

rgy

(eV

)

103

102

10

� Ion Transport through Collisionless Sheath� Low Flux of Neutral Reactants� High Flux of Ions� Low Ion Incident Energy

Plasma Properties� High Anisotropy� Little Lateral Etching� High Selectivity� Low Damage

Processing

Trends of Plasma Etching Reactors

�Halogen-containing gases are primarilyemployed for plasma etching.

�N2, O2, and CH4 gases are also employedin some cases; e.g., N2/H2 for organiclow-k films, O2/Cl2 for Ru, and CH4/H2for ITO.

�Rare gases such as He and Ar are oftenemployed as diluent gases. (Kr and Xe arealso employed in some cases.)

* Presently not used owing to toxicity problems.

*

*

Reactive Gases for Plasma Etching

From CRC Handbook of Physics and Chemistry

Bond Strengthof Diatomicmolecules

�Feed gases are preferred for etching to give reactive atoms which canbreak the bond of materials and formetch or reaction products.

+ Bond strength in eV* Bond strength in Materials

+ +

Reactive Gases for Plasma Etching(continued)

Boiling temperature of Materials (at a vapor pressure of 1 atm)

Reactive Gases for Plasma Etching

�Feed gases are preferred for etching to give reaction or etch productshaving lower boiling temperatures(or higher vapor pressures).

From CRC Handbook of Physics and Chemistry

(continued)

Process Control in Plasma Processing

Plasma Characteristics(Plasma Parameters and Structures)

Electrons: density, velocity distribution Ions and Neutrals: chemical composition,

concentration, velocity distributionPlasma Structures: plasma and surface

potentials, sheath voltage and thickness

Incident Characteristics on SubstratesIons, Neutrals, and Electrons: flux, chemical

composition, velocity distribution (Incident energy and angular distribution)

1) Gas-Phase Reactions2) Plasma-Wall Interactions

4) Plasma-Surface Interactionsin Microstructural Features

Equipment Functions (Externally Controllable Parameters)Plasma Excitation Source: frequency, power, time modulationRF Bias Source: frequency, power, time modulationFeedstock Gas: species, flow rate, pumping speed, pressure, temperatureReactor and Electrode: geometry / structure, material, temperature, external fieldsWafer / Substrate Stage: geometry / structure, material, temperature

Processing Characteristics (Etching and Deposition Characteristics)

Rate, Profile, Selectivity, Step Coverage, Electrical and Mechanical Properties, Damage, Macroscopic Uniformity, Microscopic Uniformity

Plasma Reactor

3) Plasma-Surface Interactions

Plasma Characteristics(Plasma Parameters and Structures)

Electrons: density, velocity distribution Ions and Neutrals: chemical composition,

concentration, velocity distributionPlasma Structures: plasma and surface

potentials, sheath voltage and thickness

Incident Characteristics on SubstratesIons, Neutrals, and Electrons: flux, chemical

composition, velocity distribution (Incident energy and angular distribution)

1) Gas-Phase Reactions2) Plasma-Wall Interactions


Equipment Functions (Externally Controllable Parameters)Plasma Excitation Source: frequency, power, time modulationRF Bias Source: frequency, power, time modulationFeedstock Gas: species, flow rate, pumping speed, pressure, temperatureReactor and Electrode: geometry / structure, material, temperature, external fieldsWafer / Substrate Stage: geometry / structure, material, temperature

Processing Characteristics (Etching and Deposition Characteristics)

Rate, Profile, Selectivity, Step Coverage, Electrical and Mechanical Properties, Damage, Macroscopic Uniformity, Microscopic Uniformity

Plasma Reactor

3) Plasma-Surface Interactions

3. Role of Plasma in PlasmaEtching Technology

� Gas-phase Reactions(to generate ions and neutrals)

� Ion Acceleration through the Sheath(DC self-bias voltage)

Reaction Processes in Plasma Reactor

Plasma

SheathSubstrate

Electrode (Wafer / Substrate Stage)

FeedGas To

Pump

2) Plasma-Wall InteractionsAdsorption / deposition on wallsCharge neutralization on wallsWall recombination, Wall erosion

1) Gas-Phase ReactionsElectron-impact eventsIon reactionsNeutral reactions

Ions and Neutrals Feed gas speciesProduct species

(from substrates)Impurities

(from Walls)Mask materials

3) Plasma-Surface InteractionsAdsorption, Desorption, Purely chemical reactions,Phsyical sputtering, Ion-assisted reactions,Deposition, Passivation layer formation

Transport to Substrates

�


Shadowing, Surface charging, Surface reemission/ reflection, Surface reactions

Transport to Walls

n

ChamberInner Walls(SiO2, Al2O3, etc.)

�

n

�n

w

s

PlasmaReactor

RF Source

Gas-Phase ReactionsCl/Cl2 Reaction Processes in Chlorine Plasmas

Reaction Process

Eth (eV)

Cross section (cm2)

Rate coefficienta (cm3/s)

Electron-impact reactions: Molecular ionization Dissociative ionization Ion-pair formation Dissociative attachment Dissociative excitation Atomic ionization

Electron detachment

Dissociative recombination Atomic recombination

e + Cl2 Cl2+ + e + e e + Cl2 Cl+ + Cl + e + e e + Cl2 Cl+ + Cl� + e e + Cl2 (Cl2�)* Cl� + Cl* e + Cl2 (Cl2)*

+ e Cl + Cl* + e e + Cl Cl+ + e + e e + Cl+ Cl++

+ e + e e + Cl� Cl + e + e Cl2

+ + e Cl + Cl*

Cl+ + e Cl* Cl+

+ e + e Cl* + e Cl + e Cl� Cl + e + e Cl� + e

11.48 15.48 11.87 0 3.12 13.01 23.80 3.62 0 0 �13.01 0 �3.12

�1 �2 �3 �4 �5 �6 �7 �

� � � � �

k1 k2 k3 k4 k5 k6

k7 k8=2.6�10-8 exp(�5.3/Te)

k9=9.1�10�7/Te0.6

k10=3.2�10�11/Te1/2

k11=7.1�10�7/Te9/2 cm6/s

k12=9.1�10�12/Te1/2

k13=7.1�10�7/Te9/2 cm6/s

Ion reactions: Ion-ion recombination

Charge exchange

Cl2

+ + Cl� Cl2 + Cl Cl+

+ Cl� Cl + Cl Cl+

+ Cl2 Cl + Cl2+ Cl� + Cl2 Cl + Cl� + Cl

��

k14=5.0�10�8 k15=5.0�10�8 k16=5.4�10�10

k17=1.0�10�10

Neutral reactions: Volume recombination Wall recombination Surface reaction

Cl + Cl + M(Cl2) Cl2 + M(Cl2) Cl + Cl + wall Cl2 + wall Cl + wafer (0.25)SiCl4 + wafer

��

k18=2.8�10-32 cm6/s 1 =0.005 for stainless steel 2 =0.05 for silicon

a In k7-k13, the electron temperature Te is in eV. Rate coefficients k13-k17 are values at T=300 K. Coefficients 1 and 2 are reactive sticking probabilities. * Total Ionization Cross Section: Qi = �1 + �2 + �3 Total Cross section for Negative Ion Formation: Qa = �3 + �4 �1 = Qi � (�2 + �3) �3 = Qa for Ee >11.87 eV

�2 = Qi (Ee�4.0)/4 �4 = Qa for Ee <11.87 eV

or 1=0.007 *

* For anodized aluminum 1=0.1 or 0.15

Cl/Cl2 Reaction Processes in Chlorine Plasmas

Collision Cross Section Rate CoefficientElectron-Impact Reactions for Cl/Cl2

� � � �� eeeeeee

eeeeee

��

��

��

��

��

fd

vvfdvk e vv

Gas-Phase Reactions (continued)

rng

eeee

g

nkkkk

nnkknnkkkk

nknnkkkdt

d

��]Cl[]Cl[)]Cl[2]Cl[(]Cl)[]Cl[2]Cl[(

]Cl[)(])Cl[]Cl[]Cl[]Cl[(

]Cl[)2(]Cl[

171615214

13121110298

e6e542

��

��

��

��

��

Reactants and Products0-dimensional Calculation � Cl2 Plasma �

Reactants

Plasma Diagnostics(including diagnostics of the fluxes and energiesof ions and neutrals incident on the surface).

measured

Reactants and ProductsFlux of Ions and NeutralsIncident on Substrate Surfaces

(continued)

�n � (nn/4) (8kTn /�mn)1/2

�i � 0.6ni (kTe / mi)1/2

Neutral Flux :

Ion Flux :

•The neutral-to-ion flux ratio �Cl/�i is of the order of 10at 10 mTorr, decrease wit decreasing pressure.

•The neutral-to-ion flux ratio �Cl2/�i is about one orderof magnitude higher than �Cl/�i.

Wavenumber (cm-1)

50070090011001300A

bsor

banc

e

0.000

0.004

0.008

0.012

0.016

0.020

gas phase

after Ar Plasma Treatment

during Etching

after Etching

Si-OStretch

Si-OBend

SiCl4

SiClx(x=1-3)

surface

Cl2 Plasma 0.5 mTorr

Si etching in Cl2 plasma

Plasma and Surface Diagnostics(by using FTIR absorption spectroscopy)

Reactants and Products (continued)

�Silicon tetrachloride SiCl4 was the only IR-absorbing etch product species detected in the gas phase, while unsaturated SiClx (x=1�3) as well as SiCl4 were observed on the surface.�[SiCl4] � 1�1013 cm�3 � [Cl2] in the gas phase.�A broad absorption feature due to Si�O vibrations or silicon oxides was found to occur both in the

gas phase and on the surface, where oxygen came from the dielectric windows and chamber walls.

Reaction Products (or Etch Products)

e� + SiCl4 � Cl� + SiCl3e� + SiCl4 � Cl� + SiCl2 + Cl e� + SiCl4 � Cl� + SiCl + Cl2e� + SiCl4 � Cl2

� + SiCl2e� + SiCl4 � Cl2

� + SiCl + Cl

At Ee<10 eV, k1�10�7, k2�10�7, k3�10�9, k4�10�9, and k5�10�10 cm3/s

SiCl + O2 � SiO + ClOSiCl2 + O2 � SiO + ClO + ClSiCl3 + O2 � SiO + ClO + Cl2SiCl4 + O2 � SiO + ClO + Cl2 + Cl

At T�300 K, k6�10�12, k7�10�15, k8�10�12, and k9�10�17 cm3/s


[k1][k2][k3][k4][k5]

[k6][k7][k8][k9]

Electron-Impact Reactions and Neutral reactions for Product Species

Electron-Impact Reactions

Neutral Reactions

r (cm)0 2 4 6 8 10

Neu

tral f

luxe

s (10

18 c

m��

s�� )

0.0

0.5

1.0

1.5

2.0

SiCl2SiCl4

Cl

r (cm)

r (cm)r (cm)0 2 4 6 8 10

Ion

fluxe

s (10

16 c

m�2

s�1 )

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Cl2+

Cl+

Ion

Flux

(101

6cm

�2 s�1

)N

eutra

l Flu

x (1

018

cm�2

s�1

)

0.6

0.8

0.8

1.0

1.01.2

1.4

z

r0.6

0.8

0.8

1.0

1.01.2

1.4

z

r

0.20.4

0.4

0.6

0.6

0.60.8

0.8

0.8

1.0

1.0

1.0

1.2

1.2

1.2

z

r0.20.4

0.4

0.6

0.6

0.60.8

0.8

0.8

1.0

1.0

1.0

1.2

1.2

1.2

z

r

z

r

[SiCl2] (1013 cm�3)

[Cl+] (1011 cm�3)r (cm)

(a) (b)

To pump

Wafer Stage

Planar RF Coils

R=15 cm

H=9

cm

Rw=10 cm

z

r

GasInlet

Dielectric Window

RF Bias To pump

Wafer Stage

Planar RF Coils

R=15 cm

H=9

cm

Rw=10 cm

z

r

GasInlet

Dielectric Window

RF Bias

Si etching in Cl2 Plasma

Two-dimensionalFluid Simulation� Cl2 Plasma (10 mTorr) �


a) The situation is similar to Si etching in Cl2/O2 plasmas.b) Silicon oxides SiOx would also occur.c) In case of photoresist mask.

In case of hard mask (or SiO2 mask), Si and/or O would occur.

O2, OReactive species

CxHyc)SiClx, SiOxCly

b)DepositingSpecies

Surface Inhibitors

O2+, O+Cl2

+, Cl+IonCl2, ClNeutral Reactant

Mask Material

Impurity from Wallsand Windows

Reaction Product

Feed Gas

SourceRole in Surface Reactions

Reactive Speciesin Real Etching Environments

During Si etching in Cl2 plasmas a)

�In real etching environments, reactive species responsible for etching surface reactions come not only from feed gases but also reaction products, impurities from walls and/or windows, and mask materials.

Plasma Reactor

FeedGas

0Vs Vp Potential

Position

Plasma

Sheath

Sheath

Substrate

Electrode

PlasmaReactor

PlasmaPotential

Surface Potential

ToPump

�s ��s ER

dsh � 0.1 � 5 mm

D � 1 �m

ni, ne, nn

�n

+n

W � 0.05 � 0.5 �m

s�i � 0.6ni (kTe / mi)1/2

Ei � eVsh = e (Vp�Vs)

Substrate

SubstrateSurface

Microstructure

Plasma

Sheath Mask 0

Blocking Capacitor

RF Source

�s

Ti, Te, Tn

�n � (nn/4) (8kTn /�mn)1/20

DC Self-bias Voltage

r

12.5 cm

z

4 cmPlasma �p

CB

VsI5 cm

Vr

12.5 cm

z

4 cmPlasma �p

CB

VsI5 cm

V

Total current : I = Ii + Ie + Id

Ii : Ion currentIe : Electron current[Id : Displacement current

(a)

0

Pote

ntia

l (V

)

2.012.00.0

1.0

2.0

1.0

�t / 2�

0.0 0.5�0.4

0.2

0.0

�0.2Cur

rent

(A)

Ii

I

Ie

Id

100

1.0

0

�100

�200

�300

V

0.0 0.5

Volta

ge (V

)

�p

r (cm)z (cm)

Pot

entia

l (V

)E

lect

ron

Den

sity

(109

cm�3

)

(b)

r (cm)

z (cm)

z (cm)

r (cm)

(a)

0

Pote

ntia

l (V

)

2.012.00.0

1.0

2.0

1.0

�t / 2�

0.0 0.5�0.4

0.2

0.0

�0.2Cur

rent

(A)

Ii

I

Ie

Id

1.0

�t / 2�

0.0 0.5�0.4

0.2

0.0

�0.2Cur

rent

(A)

Ii

I

Ie

Id

100

1.0

0

�100

�200

�300

V

0.0 0.5

Volta

ge (V

)

�p

0

�100

�200

�300

V

0.0 0.5

Volta

ge (V

)

�p

r (cm)z (cm)

Pot

entia

l (V

)E

lect

ron

Den

sity

(109

cm�3

)

(b)

r (cm)

z (cm)

z (cm)

r (cm)

Vdc

1/2 Vpp � �p + Vdc

Vpp

Two-dimensionalParticle-in-cell/Monte Carlo(PIC/MC) Simulation� Ar Plasma (200 mTorr) �

150 200100Energy (eV)

0.5

0.0

1.0D

istri

butio

n (a

.u.)

10 200Energy (eV)

0.5

0.0

1.0

Dis

tribu

tion

(a.u

.)

0 90Angle (deg.)

0.5

0.0

1.0

Dis

tribu

tion

(a.u

.)

30�30 60�60�9090

1.0

0Angle (deg.)

30�30 60�60�90

0.5

0.0

Dis

tribu

tion

(a.u

.) (a)

(c) (d)

(b)

Ar� e�

Ar� e�

150 200100Energy (eV)

0.5

0.0

1.0D

istri

butio

n (a

.u.)

10 200Energy (eV)

0.5

0.0

1.0

Dis

tribu

tion

(a.u

.)

0 90Angle (deg.)

0.5

0.0

1.0

Dis

tribu

tion

(a.u

.)

30�30 60�60�9090

1.0

0Angle (deg.)

30�30 60�60�90

0.5

0.0

Dis

tribu

tion

(a.u

.) (a)

(c) (d)

(b)

Ar� e�

Ar� e�

Incident Ions and ElectronsEi � �p + Vdc

Energy Distribution of Incident Ions

Cl2 Gas Pressure (Pa)

Ion Energy (eV)

Cur

rent

(re

l. un

its)

Ion Energy (eV)

Dis

tribu

tion

(a.u

.)

* Incident ion energy (or IEDF) depends on rf frequency, voltage, sheath width, ion mass, and pressure.

Incident Angle � (deg.)

-30 -20 -10 0 10 20 30

Dis

tribu

tion

G(�

)cos�

0

5

10

15

20

2050

100

500

1000

R = eVs /kTi�

Low-Pressure, High-Density Plasmas(Typically, kTi � 0.5 eV, Vsh � 50 V)

R = eVsh/kTi � 100

Incident Angle Distribution G(q) of Ion FluxesIn Collisionless Sheath

Angular Distribution of Incident Ions

Sheath

Plasma

Substrate

Maxwellian kTi

G(�)

Vsh

Ion

�

GasMolecules

� Incident angular distribution of ion fluxes (or IADF) depends on sheath voltage, sheath width, ion mass, and pressure.

4. Surface Reaction Processesin Plasma Etching(Plasma-Surface Interactions)� Surface Reactions� Transport of Ions and Neutrals

in Microstructural Features� Feature Profile Evolution� Microscopic Uniformity� Surface Charging

Etching Surface Reactions �Etching occurs through surface reactions with ions and neutrals

incident on surfaces from the plasma, where the positive ions areincident on surfaces after been accelerated through the sheath, while the neutrals are incident isotropically on surfaces.(High-energy electrons are also incident on surfaces after beingdecelerated through the sheath, playing an important role indifferential charging of feature surfaces.)

�Etching characteristics achieved depends primarily on the chemical constituent of ions and neutrals incident on surfaces, flux andenergy and angular distributions of incident ions and neutrals,and surface temperature.

�Moreover, the etching of patterned features (or the feature profile evolution during etching) are determined also by the transport of ions and neutrals in microstructural features, because the etching reactions occur in feature surfaces in microstructures

�Surface reactions responsible for etching have been investigatedby using : Beam experiments (including surface diagnostics)

Plasma experiments (including plasma and surface diagnostics,plasma simulation*) *Particle model and Fluid model

Process simulation (semi-empirical profile simulation, molecular dynamics (MD) simulation)

Study of Etching Surface Reactions

Ion Source

Gas / RadicalSource

(Ar+, Cl+, etc.)

(Cl2, XeF2, etc.)

Quartz CrystalMicrobalance / Wafer Stage

Substrate

To Pump

A+

QMS (QuadrupoleMass Spectrometer)

J.W. Coburn and H.F. Winters, J. Appl. Phys. 50, 3189 (1979).

�The etch rate is enhanced by about an order of magnitude by simultaneous exposure of ions andreactive neutrals.

Time (s)

Si e

tch

rate

(A

/min

)

XeF2 gasonly

Ar+ ion beamonly

Ar+ ion beam + XeF2 gas

Etching Mechanisms

Reaction

Desorption

Adsorption(+ mixing)

EtchingMechanism

Substrate

Purely Chemical Physical / Chemical Sputtering Ion-Assisted Inhibitor DepositionReactiveNeutral

AdsorbedNeutrals

Energetic Ion Energetic Ion (reactive) Energetic Ion

AdsorbedNeutrals

Surface Inhibitor

ReactionProduct

ReactionProduct

ReactionProductProduct

Etching Mechanism vs. Characteristics

�The etch anisotropy relies primarily on the ion-assisted reaction at the bottom of the feature andthe passivation layer formation on feature sidewalls.

�There are two mechanisms for the formation ofpassivation layers on feature surfaces:

(i) Inhibitor deposition.(ii) Inhibitor adsorption or surface oxidation.

�There are two mechanisms for ion-assisted(or ion-enhanced) reaction:

(i) Chemical sputtering (or Physically enhancedchemical sputtering), where energetic ionsenhance the step of Reaction.

(ii) Chemically enhanced physical sputtering(or Physical sputtering), where energetic ionsenhance the step of Desorption

Mechanism for Anisotropic Etching

Si Substrate

Cl+ Cl2+

Energetic Ions

Cl Cl2

��

ChlorosilylLayer(SiClx Layer)

Adsorption

Cl+ Cl2+

Reaction

Etch ProductsSiClx (x=1�4)

Desorption

Cl Cl2

Neutral ReactantsEnergetic Ions

��! �� " � "�

#� � !� � $� %&

�� ' �()*�+,!*

�-� ��

Si

Mask

Cl Cl2

Neutral ReactantsCl+ Cl2+

Energetic Ions

SiClx

Surface InhibitorsSiClxSiOxCly

(Unreactive / Depositing Species)

(Reactive Species)O O2

Reaction Layer(SiClx Layer)

Ion-Assisted Reaction

Passivation LayerFormation

ReactionProducts Passivation Film

(SiClx, SiOx, SiOxCly Layer)


�The etch yield depends on ion energy as Y(Ei)=A(Ei1/2�Eth1/2) at Ei< 1 keV, where A is a constant,in physical sputtering as well as ion-assisted etching reaction.�In ion-assisted etching, the etch yield depends also on neutral-to-ion flux ratio.

Ch. Steinbrucel, Appl. Phys. Lett. 55, 1960 (1989).

Neutral-to-Ion Flux Ratio (Cl / Cl+)

0 50 100 150 200

Etc

h Y

ield

(Si /

Cl+

)

0

1

2

3

4

Ei = 75 eV

Ei = 35 eV

Ei = 55 eV

measured (Chang et al.)calculated (present model)calculated (Langmuir adsorption model)

Neutral-to-Ion Flux Ratio (Cl / Cl+)

0 50 100 150 200

Etc

h Y

ield

(Si /

Cl+

)

0

1

2

3

4

Ei = 75 eV

Ei = 35 eV

Ei = 55 eV

measured (Chang et al.)calculated (present model)calculated (Langmuir adsorption model)

Etch Yield Y(Ei, � ) vs. Incident Ion Energy Ei

J.P. Chang and H.H. Sawin, J. Vac. Sci. Technol.A 15, 610 (1977).

*

*

(� = 0�)

Beam Experiments

(� = 0�)

SiAr+ + Cl2

Cl+

Ar+

F+

Ne+

0 0.5 1.0

E (keV)

ET

CH

YIE

LD

(at

oms/

ion)

3

2

1


T. Mizutani et al., Nucl. Instrum. Methods,B7/8, 825 (1985).

�The etch yield peaks at normal incidence (� = 0�) in ion-assisted etching reaction, while it peaksat around � � 65� in physical sputtering.

Cl2+ ion beam

Ei = 1 kV

T.M. Mayer, R.A. Baker, and L.J. Whitman,J. Vac, Sci. Technol. 18, 349 (1981).

Etch Yield Y(Ei, � ) vs. Ion Incidence Angle �

(continued)

Si

Beam Experiments

Ion-Assisted Reaction (continued)

Cou

nts

(arb

. Uni

ts)

Mass Number (amu)

Ion EnergyEi = 75 eV

Ei = 250 eV

Ei = 1000 eV

SiCl4 Gas

Si+Cl+ SiCl+ SiCl2+ SiCl3+ SiCl4+

Ar+ ion beam+ Cl2 gas on Si

Beam Experiments

Ei = 500 eV

Ei = 250 eV

Ei = 100 eV

CC

MB

MBCC

S=CC+MB

SiC

lFlu

x (a

rb. u

nits

)

Time of Flight (ms)

*MB: Maxwell-Boltzmann *CC: Collision Cascade

�The amount of lower chlorinated SiClx species increases with increasing ion energy.�The velocity distribution of etch product species desorbed from surfaces changes from MB to CC,

as the ion energy is increased.

D.J. Oosatra et al., J. Apl. Phys. 63, 315 (1988)

Reaction Products

Ion-Assisted Reaction (continued)

Surface Reaction Layer (Chlorinated Surface Layer (SiClx Layer)

N. Layadi, V.M. Donnelly, and J.T.C. Lee, J. Appl. Phys. 81, 6738 (1977).

�The chlorinated surface layer consists of SiClx (x=1-3)�The thickness of SiClx layer increases with increasing ion incident energy.

Si etching in Cl2 plasmas

Surface Reaction ProcessesModel of Surface Chemistry in Real Etching Environments(Si etching in Cl2, Cl2/O2 Plasmas)

�

�� Sn0�n

So0�o

Si (s) SiClx (s)�

SiOy (s)�

Ysn�i�

Desorption

Yso�i�

Sputtering�

Coverage: �n

Oxidation

Chlorination�

Coverage: �o�

So0�oOxidation�

Cov.:1-�n-�o-�p�

SiClx (g)�

Decomposition / Reaction �� Deposition : Redeposition :�

SiOy (g)�

Yn�i�

Dechlorination�

Si (g)�

�SiOuClv (s)

Coverage: �p

SiOuClv (g)�Yp�i�

YSi�i�

Deposition�

Sp0�p+ Sq0�q�

Sputtering�

Sputtering�

Sp0�p+ Sq0�qDeposition Deposition

Sp0�p+ Sq0�q

SiSiClx� SiOy�

Clean Surface

OxidizedSurface�

SiOuClv�Deposited Surface�

SiOuClv�Cl� O

Si Substrate

Infinitesimal Reaction Surface�

(Etching)�

�q

�p

�n 1-�n-�o-�p� �o �p�

1-�n-�o-�p�

ChlorinatedSurface�

� � � �� s

nn n n o p sn n i n o o n p p q q n

� � � � � �� t

S xY Y S S S� � � � � � � � �0 0 0 01 ( )

� � � ��s

oo0 o o p so i o p p q q o

��

tS Y S S� � � � � �1 0 0

� �� s

pp0 p q0 q p p i p

��

tS S Y� � � �1

� �� ER Y Y Y� � � � � �1 1 s

i sn n so o s n o p� � � � � �

� �� DR S S Y� � �1

0 0 pp p q q p i p� � ��

v ER DR� �

Interface Evolution Rate :

Etch and Deposition Rates :

Temporal Change of the Coverage of Infinitesimal Reaction Surfaces :

Infinitesimal Reaction Surfaces .Chlorinated SiClx Surface �nOxidized SiOy Surface �oDeposited SiOuClv Surface �pClean Si Surface /��n��o��p

Surface Coverage. 0!�n, �o, �p!1

Surface Reaction Processes(continued)

Langmuir Adsorption Kinetics Model (monolayer adsorption)

0Ion-Enhanced Etch Yield of Si : Ysn = 0.4 (Ei = 50 eV, maximum at � � 01)from a Cl+/Cl2+ ion beam study [M. Balooch et al, JVST A14, 229 (1996)].

0Total Removal Yield of Cl Adsorbed : (xYsn + Yn) = 3 (Ei = 50 eV, maximum at � � 01)determined by comparing an ion-neutral synergy model with aspect-ratio dependence data on etch rate obtained in ECR Ar/Cl2 plasmas [A.D. Bailey III et al, JVST B13, 2133 (1995)].

0Sputter Yields of Si and SiOy : Ys (�=0�) = 0.06 (maximum at � � 601)Yso (�=0�) = 0.02 (maximum at � � 601)

from Cl+/Cl2+ ion beam study [D.J. Oostra et al., Appl. Phys. Lett. 50, 1506 (1987)].[S. Tachi et al., JVST A9, 796 (1991)].

0Sputter Yield of Surface Inhibitors SiuClvOw : Yp (�=0�) = Ysn (maximum at � � 601)assumed.

0Sticking Coefficient of Neutral Reactants on Si : Sn0 = 0.55from beam study of Cl2 on Si [D.J.D.Suullivan et al., J. Phys. Chem. 97, 12051 (1993)].

* � 0.001 for Cl on Si.

0Sticking Coefficient of O on Si and SiClx : So0 = 1from beam study [J.R. Engstrom et al., Phys. Rev. B41, 1038 (1990)].

* � 0.001 for O2 on Si from beam study [J.R. Engstrom et al., Phys. Rev. B41, 1038 (1990)].

0Sticking Coefficient of Etch Products on Si, SiClx, and SiOy : Sp0 , Sq0 = 0.1 ~ 0.5 (assumed)* ! 0.002 for SiCl4 on Si from beam study [L.J. Whitman et al., Surf. Sci. 232, 297 (1990)]. * � 0.1�0.5 for SiClx+ (x=1,2; Ei =30 eV) from beam study [T. Sakai et al., 32, 3089 (1993)].* � 1 for products on themselves.

Surface Reaction ProcessesParameters for Surface Reaction Processes (Si etching in Cl2, Cl2/O2 Plasmas)

(continued)

Transport of Ions and Neutralsin microstructures

�Ions and neutrals coming from the plasma onto substrate surfaces are further transported ontosidewalls and bottom surfaces in microstractural features. Here, neutrals include reactants,etch products and by-products, surface inhibitors, and oxygen.

P

Window (�1!�!�2)

"nP

X

Y

Z

Mask

Substrate

Energetic Ions Neutral Reactants

Surface InhibitorsEtch By-Products

�i0

�d0�p

0

�n0

Ion Sheath

Bulk Plasma

Etched Surface �i (P)

�d(P)�p(P)�n(P)

# =" - �

Etch Products �qs

�1�2

#

�

�o(P)

Oxygen �o0

�q(P)

P

Window (�1!�!�2)

"nP

X

Y

Z

X

Y

Z

Mask

Substrate



�i0

�d0�p

0

�n0

Ion Sheath

Bulk Plasma


�d(P)�p(P)�p(P)�n(P)

# =" - �

Etch Products �qs

�1�2

#

�

�o(P)�o(P)

Oxygen �o0

�q(P)

n

Neutral Shadowing

+

Ion Shadowingn

Surface Reemission of Neutrals(Knudsen Transport)

+

IonReflection

nnn n

nn

nn

Desorption of Adsorbed NeutralsDesorption of Etch Products

Charging of Feature Surfaces(Deflection of Ion Trajectories)

R

+

e

+ + +

++

+

+

- - ------

++

+++

- -------

+ + + +

(a) (b)

(c) (d)

(e) (f)

+

n

Neutral Shadowing

+

Ion Shadowingn

Surface Reemission of Neutrals(Knudsen Transport)

+

IonReflection

nnn n

nn

nn

Desorption of Adsorbed NeutralsDesorption of Etch Products

Charging of Feature Surfaces(Deflection of Ion Trajectories)

R

+

e

+ + +

++

+

+

- - ------

++

+++

- -------

+ + + +

+

e

+ + +

++

+

+

- - ------

++

+++

- -------

+ + + +

(a) (b)

(c) (d)

(e) (f)

+

Incident Angle � (deg.)

-30 -20 -10 0 10 20 30

Dis

tribu

tion

G(�

)cos�

0

5

10

15

20

2050

100

500

1000

R = eVs /kTi�

� � � � � �� i0

i dP Gi� �� " ��

�

1

2 cos

� � � �� n n0 dP � �� ( / )cos1 2

1

2 � " ��

�

� �� $ %� � �� ( / ) ( ) ( ) cos cos1 2 1r Y Q Q S Q Q sn i n n nProfile P Qd� � � & &

(b) Direct incidence (Ion shadowing)

Neutral Flux Incident on Feature Surfaces :

Ion Flux Incident on Feature Surfaces :

(a) Direct incidence (Neutral shadowing)

(e) Incidence through desorptionfrom feature surfaces

(c) Incidence through surface reemission

� �� $ %� �) ( ) ( ) cos cos1r Y Q Q S Q Q sn i n n n P Qd� � � & &P

Window (�1!� !�2)

"nP

X

Y

Z

Mask

Substrate



�i0

�d0�p

0

�n0

Ion Sheath

Bulk Plasma


�d(P)�p(P)�n(P)

# =" - �

Etch Products �qs

�1�2

#

�

�o(P)

Oxygen �o0

�q(P)

P

Window (�1!� !�2)

"nP

X

Y

Z

X

Y

Z

Mask

Substrate



�i0

�d0�p

0

�n0

Ion Sheath

Bulk Plasma


�d(P)�p(P)�p(P)�n(P)

# =" - �

Etch Products �qs

�1�2

#

�

�o(P)�o(P)

Oxygen �o0

�q(P)

Transport of Ions and Neutrals(continued)

(c) (d)

Nor

mal

ized

Flu

x �

i (P) /

�i0

0.0

0.5

1.0

1.5

Sidewall SidewallBottomTop Top

Aspect RatioD / W = 1.0

100

20

R = eVsh / kTi = 1000

0 1 2 3 4 5

Nor

mal

ized

Flu

x �

i (0)

/ �i0

0.0

0.5

1.0

1.5

1000

R = eVsh / kTi = 2050

100

(a) (b)

(a)

Surface Position

Nor

mal

ized

Flu

x �

n(P

) / �n0

0.0

0.5

1.0

1.5

With Surface Reemission

0.1

0.4

1.0

Without Reemission

(c)

Sn= 0.01

Aspect Ratio D / W0 1 2 3 4 5

Nor

mal

ized

Flu

x �

n(0)

/ �n0

0.0

0.5

1.0

1.5

Sn= 0.01

0.4

0.1

1.0Without Reemission

With Surface Reemission

(d)

Sidewall SidewallTop TopBottom

Aspect RatioD / W = 1.0

Ions

Neutrals

Ions

Neutrals

Rectangular Trench

Geometrical Shadowing�Ion shadowing�Neutral shadowing

with surface reemission

n+Ion

Neutral

Side-wall

Side-wall

Top Top

W

Bottom0

D

Geometrical Shadowingof Ions and Neutrals

Transport of Ions and Neutrals(continued)

� RIE lag occurs in (a) and (b), being suppressed in (c) with increasing oxygen flux from the plasma.� Inverse RIE lag occurs in (d) at high oxygen flux.� Sidewall tapering occurs in (b), (c), and (d) where surface inhibitors come from the plasma.

Feature Profile Evolution

X / L0.0 0.5 1.0 1.5 2.0 2.5 3.0

Y / L

-1.0

-0.5

0.0

0.5

X / L0.0 0.5 1.0 1.5 2.0 2.5 3.0

-1.0

-0.5

0.0

0.5

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Y / L

-1.0

-0.5

0.0

0.5

0.0 0.5 1.0 1.5 2.0 2.5 3.0-1.0

-0.5

0.0

0.5

�n0/ �i

0 = 10, R = eVsh / kTi = 100

�p0

/ �i0

= 0.5, �o0

/ �i0

= 0.05 �p0

/ �i0

= 0.5, �o0

/ �i0

= 0.2

�p0

/ �i0

= 0, �o0

/ �i0

= 0 �p0

/ �i0

= 2.0, �o0

/ �i0

= 0

(a) (b)

(c) (d)

Si

Mask Si etching in Cl2/O2 plasmas String Model

Dep

th

Width

ER=1.0 ER=0.91

ER=0.63 ER=0.31

�Etched profiles are simulated through modeling the transport of ions andneutrals in microstructures along with surface reaction kinetics thereat.�

� RIE lag (reactive-ion-etching lag) :�Large aspect-ratio features etch slower than smaller ones.(Narrow space features etch slower than wider ones.)

Inverse RIE lag : ��Large aspect-ratio features etch faster than smaller ones.(Narrow space features etch faster than wider ones.)�The degree of inverse RIE lag increases with increasingO2 concentration in Cl2/O2 plasmas.

Microscopic Uniformity<Example 1>

Poly-Si etching in Cl2/O2 plasmas

Si etching in SF6/Ar plasmas

(continued)

�Large aspect-ratio features etch less tapered than smaller ones.(Narrow space features etch less tapered than wider ones.)

�The degree of sidewall tapering increases with increasing pressure in Cl2 plasmas.


Poly-Si etchingIn Cl2 plasmas

Outwardlytaperedsidewall :

<Example 2>

(continued)

Microscopic Uniformity<Example 3> Poly-Si etching in Cl2/O2 plasmas

�Micropillar:�Miropillars often occurs

in wide space features(or small aspect-ratio features).

�Microscopic uniformity during overetch:�The thinning and breaking of thin gate oxides during overetch step

occurs preferentially in large open spaces in pure Cl2 plasmas, while in dense areas at high level O2 addition in Cl2/O2 plasmas.

(continued)


W (�m)

1 10

�W

(� m

)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.5 52

(b)25% O220%10% 0%

Poly-Si

(P

oly-

Si / S

iO2)

0

10

20

30

40

50

60

O2 / (Cl2 + O2) (%)0 10 20 30

(103 D

/min

)

0

1

2

3

4

5

6

Poly-Si / SiO2

Poly-Si

SiO2 (�2)

(a)

W (�m)

1 100.8

1.0

1.2

1.4

1.6

0.5 52

25% O2

10%

0%

0.5 5

20%

2

SiO2

(d)

W (�m)

1 100.90

0.95

1.00

1.05

1.10

1.15

1.20

25% O2

10%

0%

0.5 5

20%

2

(c)

Poly-Si

Pattern Width

Pattern WidthPattern Width

Cl2/O2 Plasma, 3 mTorrFlow rate : 60 sccmMicrowave power : 400 WRF bias power : 30 W

Nor

mal

ized

Etc

h R

ate

of P

oly-

Si

-�2

!�

34

�5

67

89:

;<

=

Etc

h R

ate

of O

pen

Spa

ce (

102

nm/m

in)

Nor

mal

ized

Etc

h R

ate

of S

iO2

Line

wid

thSh

ift �

W (�m

)Se

lect

ivity

(Po

ly-S

i/SiO

2)

W (�m)

1 10

�W

(� m

)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.5 52

(b)25% O220%10% 0%

Poly-Si

(P

oly-

Si / S

iO2)

0

10

20

30

40

50

60

O2 / (Cl2 + O2) (%)0 10 20 30

(103 D

/min

)

0

1

2

3

4

5

6

Poly-Si / SiO2

Poly-Si

SiO2 (�2)

(a)

W (�m)

1 100.8

1.0

1.2

1.4

1.6

0.5 52

25% O2

10%

0%

0.5 5

20%

2

SiO2

(d)

W (�m)

1 100.90

0.95

1.00

1.05

1.10

1.15

1.20

25% O2

10%

0%

0.5 5

20%

2

(c)

Poly-Si

Pattern Width

Pattern WidthPattern Width

Cl2/O2 Plasma, 3 mTorrFlow rate : 60 sccmMicrowave power : 400 WRF bias power : 30 W

Nor

mal

ized

Etc

h R

ate

of P

oly-

Si

-�2

!�

34

�5

67

89:

;<

=

Etc

h R

ate

of O

pen

Spa

ce (

102

nm/m

in)

Nor

mal

ized

Etc

h R

ate

of S

iO2

Line

wid

thSh

ift �

W (�m

)Se

lect

ivity

(Po

ly-S

i/SiO

2) Linewidth Shift :�W = Wafter - Wbefore

<Example 4>

Wbefore

Poly-Si

Wbefore

Poly-Si

WafterWafter

Poly-Si etching in Cl2O2 plasmas

Etching

*Microscopic uniformity (etch rate, sidewall profile) is achieved at �10% O2 addition in Cl2/O2 plasmas.

(continued)

Passivation Layer Formation

mask

poly-Si

SiO2

mask mask

poly-Si poly-Si

vacuum vacuum

200nm500nm

Passivation layers

(a)

(b)50nm

OpenSpace

200nmspace

~11 nm

micro-trenching

Passivationlayers

~9 nm

Mask

Poly-Si

50nm

OpenSpace

200nmspace

~11 nm

micro-trenching

Passivationlayers

~9 nm

Mask

Poly-Si

mask

poly-Si

SiO2

mask mask

poly-Si poly-Si

vacuum vacuum

200nm500nm

Passivation layers

mask

poly-Si

SiO2

mask mask

poly-Si poly-Si

vacuum vacuum

200nm500nm

Passivation layers

(a)

(b)50nm

OpenSpace

200nmspace

~11 nm

micro-trenching

Passivationlayers

~9 nm

Mask

Poly-Si

50nm

OpenSpace

200nmspace

~11 nm

micro-trenching

Passivationlayers

~9 nm

Mask

Poly-Si

� XPS AnalysisK.V. Guinn, C.C. Cheng, and V.M. Donnelly,J. Vac. Sci. Technol. B13, 214 (1995).

Etched Profile and Profile Simulation �� Atomic-scale Cellular Model �

Y. Osano et al., Jpn. J. Appl. Phys. 45, 8157 (2006).

Thickness

Thic

knes

sSi etching in Cl2/O2 plasmas

�Passivation layers are formed on feature surfaces throughdeposition of surface inhibitors (primarily etch productsand by-products) and/or surface oxidation.�The thickness of passivation layers is significantly large

on feature sidewalls.

�Ion reflection on feature sidewalls is responsible for profile anomalies such as microtrenching and footing on sidewalls near the feature bottom and thereat, which are also affected by deposition of etch products and by-products and surface oxidation.

Ion Reflection

Cl+

Si

�

v0

p

L = 2.7 Å

v'

p

Vacuum Solid

rcutoff = 3.5 Å

r

Solid-vacuum interface

Cl+

Si

�

v0

p

L = 2.7 Å

v'

p

Vacuum Solid

rcutoff = 3.5 Å

r

Solid-vacuum interface

Width (nm)0.0 2.0 4.0 6.0 8.0 10.0

0.0

2.0

4.0

6.0

8.0

10.0

Si (solid)

Vacuum

Cl+ ion.(Ei = 50 eV)

incident angle�i = 751

feature sidewall

�i

xz

y

10.0

Width (nm)0.0 2.0 4.0 6.0 8.0 10.0

0.0

2.0

4.0

6.0

8.0

10.0

Si (solid)

Vacuum

Cl+ ion.(Ei = 50 eV)

incident angle�i = 751

feature sidewall

�i

xz

y

10.0

Dep

th Etched Profile Simulation � � Atomic-scale Cellular Model �Y. Osano et al., Jpn. J. Appl. Phys. 45, 8157 (2006).

Si etching in Cl2/O2 plasmas Without ion reflection on feature sidewalls

Mask

Si

SiO2

Si

Mask

Si

SiO2

Si

Mask

Si

SiO2

Si

With ion reflection on feature sidewalls

Mask

SiSiO2

Si

�Notch is a sharp undercut that occurs on feature sidewalls near the bottom of the feature. �In etching of conducting films on dielectrics (e.g., poly-Si gate etch), notching occurs during overetch

step, at the inner sidewall foot of the outermost feature of a L&S structure neighboring an open area.

�The notch is caused by the deflection of ion trajectories in microstructural features due to the localized charging of feature surfaces, which in turn originates intrinsically from the difference ofthe velocity distribution between ions and electrons incident on substrate surfaces.�Such a phenomena is known as “electron shading effect”, which causes charging damage as well as

profile anomalies.

Surface Charging

Surface ChargingProfile Simulation

�String Model�

X -axis(�m)

-0.9 -0.6 -0.3 0.0 0.3 0.6 0.9

Y -a

xis(

�m

)

3 .0

3.3

3.6

3.9

4.2

4.5

4.8

Open Area

Mask MaskPoly-Si

Dense Line-and-Space

X-axis (�m)

Y-ax

is (�

m)

X -axis(�m)

0 3e-7 6e-7

Y -a

xis(�m

)

4.5

4.6

4.7

4.8

25

8

11

14

14

X -axis(�m)0.0 0.3

Y -a

xis(�m

)

4.50

4.55

4.60

4.65

4.70

4.75

4.80

23456789

101112

131415

131415

(a)

(b)

SiO2

Doped

Pattern Area

X -axis(�m)

-0.9 -0.6 -0.3 0.0 0.3 0.6 0.9

Y -a

xis(

�m

)

3 .0

3.3

3.6

3.9

4.2

4.5

4.8

Open Area

Mask MaskPoly-Si

Dense Line-and-Space

X-axis (�m)

Y-ax

is (�

m)

X -axis(�m)

0 3e-7 6e-7

Y -a

xis(�m

)

4.5

4.6

4.7

4.8

25

8

11

14

14

X -axis(�m)0.0 0.3

Y -a

xis(�m

)

4.50

4.55

4.60

4.65

4.70

4.75

4.80

23456789

101112

131415

131415

X -axis(�m)

0 3e-7 6e-7

Y -a

xis(�m

)

4.5

4.6

4.7

4.8

25

8

11

14

14

X -axis(�m)0.0 0.3

Y -a

xis(�m

)

4.50

4.55

4.60

4.65

4.70

4.75

4.80

23456789

101112

131415

131415

(a)

(b)

SiO2

Doped

Pattern Area

Charging (1): Conductor featuresurfaces (during overetch)

(continued)

X -axis(�m )0.10 0.15 0.20 0.25 0.30 0.35

Y -a

xis(� m

)

1 .2

1.3

1.4

1.5

1.6

1.7

14

14

12

20

18

18

16

16

22

20

20

2222

22

22

X -axis(�m)-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

Y -a

xis(�m

)

0.6

0.8

1.0

1.2

1.4

1.6

Mask Mask

SiO2

X-axis (�m)

Y-ax

is (�

m)

(a)

(b)

X -axis(�m )0.10 0.15 0.20 0.25 0.30 0.35

Y -a

xis(� m

)

1 .2

1.3

1.4

1.5

1.6

1.7

14

14

12

20

18

18

16

16

22

20

20

2222

22

22

X -axis(�m)-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

Y -a

xis(�m

)

0.6

0.8

1.0

1.2

1.4

1.6

Mask Mask

SiO2

X-axis (�m)

Y-ax

is (�

m)

(a)

(b)

Charging (2): Dielectric featuresurfaces

Al etch

a) Al is etched pure-chemically in Cl2, showing no ion-assisted reaction characteristics. Thus, surface inhibitor deposition is indispensable on sidewalls to obtain anisotropic profiles of Al. Selectivity is required over photoresist mask. b) (BOCl)x is volatile.

c) Compounds of AlClx and CxHy are usually primary surface inhibitors during Al etching.

O2, OReactiveCxHy

c)AlClx, AlOxClyc)BxCly, BxOy

b)DepositingSurface Inhibitors

O2+, O+Cl2

+, Cl+, BCl+IonCl2, ClNeutral Reactant

Mask Material


Reaction Product

FeedGas


>S. Park, L.C. Rathbun, and T.N. Rhodin,J. Vac. Sci. Technol.,A 3, 791 (1985).

T. Banjo et al., Jpn. J. Appl. Phys.,36, 4824 (1997). �

Al / Ar++Cl2 Al etching in BCl3/Cl2/O2 plasmas

O2

Reactive species in Al etching with BCl3/Cl2 and BCl3/Cl2/O2 plasmas a)

SiO2 etch

a) SiO2 is etched in fluorocarbon (CF4, C4F8, etc.) plasmas with relatively high rf bias voltage; thus the etched profilesare usually anisotropic. C is indispensable in SiO2 etching through breaking strong Si�O bonds and/or oxygen removal in the form of volatile compounds such as CO. In addition, F is also indispensable for Si removal through the formation of volatile SiF4[e.g., SiO2 + 2CF + 2F � SiCl4 + 2CO].

b) Selectivity is required over Si, where fluorocarbonradicals CxFy play an important role; in some cases,H2 is added to scavenge F and thus to release orincrease surface inhibitors CxFy.

c) Surface reaction layer contains SiCxFyOz. d) Hard mask (e.g., poly-Si) is also often employed.

O2, OReactiveCxHy

d)SiFx, SiOxFyc)CxFy

b), (CFx+)DepositingSurface

Inhibitors

O2+, O+F2

+, F+, CFx+Ion

F2, FNeutral Reactant

Mask Material


Reaction Product

FeedGas


Reactive species in SiO2 etching with fluorocarbon plasmas a)

N.R. Rueger et al., J. Vac. Sci. Technol. A 15, 1881 (1997). �SiO2 etching in CHF3 plasmas

5. Current Issues of PlasmaEtching Technology� Current issues� Poly-Si gate etch� High-k gate etch

(HfO2, Dual metal gate)� Metal etch (Pt, Ru)� Deep RIE of Si

Current Issues� Etching of Conventional Materials :

� Etch anisotropy and selectivity*Profile and CD control on atomic scale*Selectivity over underlying ultra thin films*Surface roughness on atomic scale (LER+, etc.)

� Microscopic uniformity� Plasma damage

� Etching of New Materials :� Substrate : SiGe, Ge� Gate : High-k* dielectrics (HfO2, ZrO2, etc.)

/ Metal electrodes (TaN, Mo, Ru, W, etc.)� Capacitor : High-k* dielectrics (Ta2O5, BST)

/ Metal electrodes (Pt, Ir, Ru)� Inter layer dielectrics (ILD) :

Low-k dielectrics (SiOC, MSQ**, Organic)

These issues are also to be resolvedin etching of new materials

** MSQ: mechylsilsesquioxane* k: dielectric constant

+ LER: line edge roughness

(2) Selectivity�Gate Electrode (poly-Si, metal)

Mask ----- Hard mask (SiO2)Underlying Layer ------- Gate Dielectrics

�Gate Dielectric (SiO2, SiON, High-k dielectric)Mask ----- Gate Electrode ( + Mask)Underlying Layer------- Si

(4) Microscopic Uniformity� Pattern sensitivity* of feature

or profile irregularities� Pattern sensitivity of etch rate or selectivity

RIE lag, inverse RIE lag� Pattern sensitivity of damage

*feature size, aspect ratio, pattern density

(5) Macroscopic Uniformity� Wafer Uniformity

(3) Plasma Damage� Charging Damage

Gate Dielectrics (Dielectric Breakdown)Transistor (deterioration)

� Gate Dielectrics : deterioration of dielectric� Gate Dielectrics : penetrate

(1) Profile (Mainly gate electrode )� Feature. feature size?critical dimension

Minimum feature size L< 0.1 �mCD loss/gain �L< 0.01 �m (=10 nm)

� Profile irregularities. notch, microtrench, (owing to bending of ion trajectory)

*In case of hard etching materialsMore sidewall deposition � CD gain,Needs for removing of residues on sidewalls

Gate Etch

Gate Length . L< 0.1 �m, Spacing < 0.1 �mGate Electrode . Poly-Si (thickness � 300 nm)

metal (W, TiN, TaN, Pt, Ir, Ru, etc.thickness� 50�60 nm)

Gate Dielectric. SiO2, SiONHigh-k dielectric (Al2O3, HfO2, ZrO2)thickness � 1 � 2 nm

Buffer layer . Mask / Gate Electrode Gate Electrode / Gate Dielectrics

Poly-Si Gate Etch

5 s 15 s 60 s 0 s

Overetch Time

10 nm

Poly-Si

SiBrxOy

Mask

Gate Oxide

Removalof PR andBARC

SiO2(Gate Dielectric)

GateElectrode

Hard MaskBeforeEtch Poly-Si

Si

ArF Resist

BARC *HM

PRBARC

Poly-Si

Si

HM

PR

Poly-Si

Si

HM

PR

Etchingof BARC

Etchingof Mask

*BottomAnti-reflectedCoating

BARC

BARC

Poly-Si

Si

Etching of Poly-Si(Overetch)

Poly-Si

Si

HM

Poly-Si

Si

Poly-Si

Si

HM

HM

Etching of Poly-Si (Main etch)

Removalof HM

�A thin passivation layer of featuresidewalls plays a key role in achievingthe nanometer-scale control of theprofile and CD during main etch andoveretch processes.

Poly-Si Gate Etch

In the plasma etching of 10-nm-scale microstructures, the ion-enhanced etching at the bottom of the feature and the passivation layer formation on feature sidewalls are still key mechanisms to be precisely controlled.

(continued)

Main etch& Overetch:

HBr/O2 = 100/7

(b) Two-stepProcess

L0 � 70 nm L0 � 50 nm L0 � 20 nm

�L�15 nm �L�10 nm �L�13 nm

�L�3 nm �L�1 nm �L�3 nm

Si Sub.

Poly-S i

Oxide Mask

GateOxides

Main etch:HBr/O2 =100/3

Overetch:HBr/O2 =100/7

(a) One-stepProcess

High-k Gate Etch�As integrated circuit device dimensions continue to be scaled down, recent efforts have been made to replace gate silicon-oxides with silicon-oxynitridesof slightly higher dielectric constant (k), and nowadays, new high-k (>20) dielectrics or metal oxides such as Al2O3, HfO2, and ZrO2 are being developed to replace SiO2.

�In integrating these high-k dielectric materials intodevice fabrication, selective etching over theunderlying Si is required for their removalprior to forming the source and drain contacts.

�Moreover, the etching of high-k dielectrics at higheretch rates with low ion energies and/or less ionsis indispensable in mass production, for chambercleaning of the chemical vapor deposition (CVD)and atomic layer deposition (ALD) apparatusesto prepare high-k thin films.

�The etching of HfO2 films in BCl3-containing plasmaswithout rf biasing or under low ion-energy conditions,together with a wafer temperature control, would bepromising candidate for the process concerned.

High-k (~2 nm)

Si

Metal (~50 nm)

Mask

< 0.1 @m

Si

Metal

High-kMask

Si

Metal

High-kMask

Si

Metal

High-k

Metaletch

High-ketch

Maskremoval

Etching process for high-k/metal gate stacks

High-k (~2 nm)

Si

Metal (~50 nm)

Mask

< 0.1 @m

Si

Metal

High-kMask

Si

Metal

High-kMask

Si

Metal

High-k

Metaletch

High-ketch

Maskremoval

Etching process for high-k/metal gate stacks

912 *sp331 *sp360 *sp

��

ZrF4ZrCl4ZrBr4

Zr(Z=40)

970 *sp317 *sp323 *sp

HfF4HfCl4HfBr4

��

Hf(Z=72)I

�8657.65

154

� 90.2� 68.85

5.2

SiF4SiCl4SiBr4

Si(Z=14)

1276 �

255

2250 192.6 97.5

AlF3AlCl3AlBr3

Al(Z=13)

BoilingPoint (ºC)

MeltingPoint (ºC)

Halogencompound

Element

*sp: sublimation point

High-k dielectrics are generally difficult materials for etching, owing to strong metal-oxygen bonds and non-volatile etch products or halogen compounds.

Etching of High-k Materials

Physical properties of potential etch product species Bond strengths for diatomic species

8.306.735.16�

Hf-OHf-FHf-ClHf-Br

5.306.885.304.45

Al-OAl-FAl-ClAl-Br

8.036.385.11�

Zr-OZr-FZr-ClZr-Br

11.155.724.112.90

C-OC-FC-ClC-Br

8.295.734.213.813.39

Si-O Si-F Si-ClSi-BrSi-Si

8.387.855.304.11

B-OB-FB-ClB-Br

BondStrength (eV)

BondBondStrength (eV)

Bond

(from CRC Handbook of Physics and Chemistry, 1998)

�Hafnium chlorides are relatively volatile, and thus Cl is expected to be useful for removal of Hf.�Boron-oxygen bonds are relatively strong, and thus B and/or BCl species are expected to be useful

for breaking strong metal-oxygen (Hf-O) bonds.

�In BCl3/Cl2/O2 plasmas, the HfO2 etch rate was enhanced with O2 addition, being �150 nm/min at�5% O2 with a selectivity of �4 over Si and of �1.2 over SiO2. �At high O2 addition > 10%, the heavy deposition occurred to inhibit etching on all sample surfaces.

(i)

(iii)

(iv)

[Cl2]/([BCl3]+[Cl2]) (%)0 20 40 60 80 100

Etc

h R

ate

(nm

/min

)

-50

0

50

100

BCl3/Cl2 5 mTorr

HfO2

SiO2

Si

EtchDepo

[O2]/([BCl3]+[Cl2]+[O2]) (%)0 5 10 15

Etc

h R

ate

(nm

/min

)

-100

0

100

200HfO2

SiO2

Si

BCl3/Cl2/O2 5 mTorr(BCl3/Cl2 = 27.5 / 62.5)

EtchDepo

MW: 600 W (without rf biasing)

�The etching of HfO2 occurred at >20% Cl2 addition. The HfO2 etch rate was maximum at � 60% Cl2,being � 100 nm/min with a selectivity of � 10 over Si and of � 2 over SiO2.

�Note that a high etch selectivity of >50 for HfO2/Si was obtained at 40-50% Cl2 addition, with a HfO2etch rate of �50 nm/min and HfO2/SiO2 selectivity of 1.5�2, where the deposition was still dominantover the etching on Si.

HfO2 etch in BCl3

(continued)Etching of High-k

TiNWMidgap

Pt, Ir, Mo, RuP-MOSFET

TaN, TaCTi, TaN-MOSFET

Nitrides, Carbides

MetalsFET

69.1��

61.3600327180

PtF6PtF4PtCl4PtBr4

Pt(Z=78)

53��

44250763�

IrF6IrF3IrCl3IrBr3

Ir(Z=77)

229.2239.35349

95.1216265

TaF5TaCl5TaBr5

Ta(Z=73)

40�227��

25.4�86.5

> 600 *dec> 500 *dec> 400 *dec

RuO4RuO2RuF5RuF3RuCl3RuBr3

Ru(Z=44)

�136.45230

284�2539

TiF4TiCl4TiBr4

Ti(Z=22)

Boiling point (�C)

Melting point (�C)

Halogencompound

Element

*dec: decomposes

Etching of MetalsPhysical properties of potential etch product species

(from CRC Handbook of Physics and Chemistry, 1998)

Metals for high-k gate stack

NMOS PMOS

Ru

HM

TaNPolyHM

PolyTaN

�Regarding high-k/metal gate stack, one approachis to employ a single metal gate material with amidgap work function for both n- and p-MOSFETs.�However, a dual-work-function metal gate tech-nology would be preferred, because of severaldrawbacks such as high threshold voltage of thesingle metal gate material.

�Etching of Pt relies intrinsically on physical sputtering, because halogen and other compounds of Pt are not volatile; thus, the Pt sidewalls are largely tapered owing to redeposition of Pt sputtered on feature sidewalls during etching.�Etching of Ru is caused by ion-assisted reaction in O2-containing plasmas, because RuO4 is volatile

to be relatively easily desorbed from surfaces; thus, vertical sidewalls are obtained for Ru.

Metal Etch

Etched in Ar/O2 PlasmaSelectivity (Pt/SiO2) � 6

Etched in O2/Cl2 PlasmaSelectivity (Ru/SiO2) �20

A A

BC #�%+CD(E%

F%G(D%#)EH(�)C

-�2 ! IJ*K

LC#)EH(�)C

BC BC

BC BCM

NO #�%+CD(E%

NO-�2 !

NO NOM

NO NO

��$ �,

BC-�2 !

��$ �,

(continued)

Metal Etch (continued)

Dual metal gate

NMOS PMOS

Ru

HM

TaNPolyHM

PolyTaN

NMOS PMOS

Ru

HM

TaNPolyHM

PolyTaN

NMOS PMOS

Ru

HM

TaNPolyHM

PolyTaN

P-metal etch(patterning)

High-k etch(removal)

NMOS PMOS

RuPR

High-k

NMOS PMOS

RuPR

High-k

NMOS PMOS

Ru

HM

TaNPoly-SiHM

P-metal etch

N-metal/ Poly-Sideposition,HM formation

N-metal etch(patterning)

High-k/P-metaldeposition,PR formation

TaNTaN

Deep RIE* (Bosch process)Cyclic process consisting of

isotropic etching with SF6 plasmas sidewall passivation with CxFy plasmas anisotropic etching with SF6 plasmas isotropic etching with SF6 plasmasachieves an etch rate of > 10 �m/min for Si.

To fabricate 3D, deep structures with higher throughputs and lower costs

*RIE: reactive ion etching

Deep RIE� Bosch process (for MEMS) �

Movable Electrode

Beams

Fixed Electrode

P

Movable Electrode Fixed Electrode

Minimum gap: 6 �mHight: 120 �m

Minimum gap: 6 �mHight: 120 �mMovable Electrode

Beams

Fixed Electrode

P

Movable Electrode Fixed Electrode



Deep RIE (continued)

Mask

C4F8/Ar Plasma SF6/Ar Plasma

Ar+

SF5+

SiF4

Si substrate Si substrate Si substrate

F*

CxFy

Passivation layer formation on sidewalls (and on bottom surfaces)

Removal of passivation layerson bottom surfaces

Si etching(at bottom surfaces)

Anisotropic Etch(with low pressure,

high bias)

Isotropic Etch(with high pressure,

low bias)

SF6/Ar Plasma

Ar+

SF5+

F*

*Higher etch rates and the suppress of scallops on sidewalls are issues to be further improved.

6. Summary� Future Prospects

Nanofabrication Technology

0.1 nm(1 A)1 nm10 nm100 nm

1000 nm(1 �m)

Bottom-up approach�manipulation/rearrangement of atoms�self-assembly/re-organization of atoms

to obtain nanostructures(materials, devices, )

Top-down approach�fabrication from bulk materials

and/or thin films of the materialto obtain microstructures(devices)

A combination of top-down and bottom-up approaches will be important in nanofabrication technologies for mass production,because the bottom-up approach isnot available for mass production owing to its slow rate of fabrication.

VLSI ULSI Nanoelectronics

�Top-down approach, Bottom-up approach�

TEOSCVD

CatalystPatterning

SiO2/Si

Metal Deposition

Plasma CVD CMP

Top Metal Layer Deposition

Ti, Mo, Cr, Pt

Ni

At ~ 400 to 8001 C

J. Li, Q. Ye, A. Cassell, H. T. Ng, R. Stevens, J. Han, M. Meyyappan, Appl. Phys. Lett., 82(15), 2491 (2003).

�Bottom-up approach�

Fabrication of Nanoelectrode Arrays(CNT Nanoelectrode Array / CNT Interconnects)

The scale bars are (a) 5, (b) 3, (c) 10, (d) 2, (e) 5, (f) 10, (g) 1, (h) 0.2, and (i) 0.2 �m.

Summary

�Plasma etching is now an indispensable technology for micro- andnano-fabrication of ULSI and MEMS devices.�To meet the requirements for near-future and future devices,

the precise control of etching characteristics is further required, based on the precise control of the etching mechanisms concerned.

*Current issues / requirements for plasma etching technology have been summarized in this lecture.

�In turn, a better understanding of the physical and chemical mechanisms underlying the processing continues to be required in the gas-phase and on the surfaces,

*Ion-assisted reaction and passivation layer formation continue to be key mechanisms to be precisely controlled on surfaces.

�A combination of top-down (plasma etching) and bottom-up approaches would be important for nanofabrication technologyof <10 nm scale.

For further reading� Books *VLSI Technology, edited by S.M/ Sze (McGraw-Hill, New York, 1988).*ULSI Technology, edited by C.Y. Chang and S.M. Sze (McGraw-Hill, New York,1996).*J.D. Plummer, M.D. Deal and P.B. Griffin, Silicon VLSI Technology: Fundamentals, Practice

and Modeling (Prentice Hall, New Jersey, 2000).*K.P. Cheung, Plasma Charging Damage (Springer, Berlin, 2001).*Handbook of Advanced Plasma Processing Techniques, edited by R.J. Shul and J. Pearton(Springer, Berlin, 2000).*M.A. Lieberman and A.J. Lichtenberg, Principles of Plasma Discharges and Materials

Processing , 2nd ed. (Wiley, New York, 2005).*M. Elwenspoek and H.V. Jansen, Silicon Micromachining, (Cambridge Univ. Press, Cambridge,1998). <for MEMS>*M.J. Mandou, Fundamentals of Microfabrication: The Science of Minituarization, 2nd ed.(CRC

Press, New York, 2002). <for MMS>

� Journals :*Journal of Vacuum Science and Technology A, B (AVS/AIP)*Applied Physics Letters (AIP) *Journal of Applied Physics (AIP) *Journal of Physics D : Applied Physics (IOP) *Plasma Sources Science and Technology (IOP)*Japanese Journal of Applied Physics (JSAP)

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