plasma processing and polymers - computational plasma science

PLASMA PROCESSING AND POLYMERS: FRITO BAGS TO MICROELECTRONICS

FABRICATION

Mark J. KushnerUniversity of Illinois

Department of Electrical and Computer EngineeringDepartment of Chemical and Biomolecular Engineering

Urbana, IL [email protected] http://uigelz.ece.uiuc.edu

November 2002

UTA_1102_01

University of IllinoisOptical and Discharge Physics

AGENDA

• Plasmas: Tools for eV physics and chemistry

• Plasmas and Polymers: Extremes in Physics and Applications

• Plasmas for functionalization of polymers

• Polymers for selectivity in plasma etching

• Concluding Remarks

UTA_1102_02

101ELECTRON TEMPERTURE (eV)

ELEC

TRO

N D

ENSI

TY (c

m-3 )

1015

1014

1013

1012

1010

1011

109

ATMOSPHERIC PRESSURE GLOWS LASERS LAMPS REMEDIATION STERILIZATION

LOW PRESSURE ARCS LASERS SWITCHES LAMPS THYRATRONS

LOW PRESSURE GLOWS SWITCHES ECR ETCHERS MAGNETRON DEPOSITION INDUCTIVELY COUPLED (ETCH,DEP) LASERS CAPACITIVELY COUPLED (ETCH,DEP)


PARTIALLY IONIZED PLASMAS• Partially ionized plasmas are gases containing neutral atoms and

molecules, electrons, positive ions and negative ions.

• An air plasma: N2, O2, N2+, O2

+, O-, e where [e] << Neutrals

UTA_1102_03

• These systems are the plasmas of every day technology.

• Electrons transfer power from the "wall plug" to internal modes of atoms / molecules to "make a product”, very much like combustion.

• The electrons are “hot” (several eV or 10-30,000 K) while the gas and ions are cool, creating“non-equilibrium” plasmas

WALL PLUG

POWER CONDITIONING

ELECTRIC FIELDS

ENERGETIC ELECTRONS

COLLISIONS WITHATOMS/MOLECULES

EXCITATION, IONIZATION, DISSOCIAITON (CHEMISTRY)

LAMPS LASERS ETCHING DEPOSITIONE

eA

PHOTONS RADICALS

IONS


COLLISIONAL LOW TEMPERATURE PLASMAS

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• Displays

• Materials Processing

COLLISIONAL LOW TEMPERATURE PLASMAS

• Lighting

• Thrusters

• Spray Coatings

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PLASMAS FOR MODIFICATION OF SURFACES

• Plasmas are ideal for producing reactive species (radicals, ions) for modifying surface properties.

• Two of the most technologically (and commercially) important uses of plasmas involve polymers:

• Both applications utilize unique properties of low temperature plasmas to selectively produce structures.

• Functionalization of surfaces (high pressure)

• Etching for microelectronics fabrication (low pressure)

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SURFACE ENERGY ANDFUNCTIONALITY OF POLYMERS

• Most polymers, having low surface energy, are hydrophobic.

• For good adhesion and wettability, the surface energy of the polymer should exceed of the overlayer by ≈2-10 mN m-1.

0

10

20

30

40

50

60

PTFE

Poly

prop

ylen

e

Poly

ethy

lene

Poly

styr

ene

SUR

FAC

E EN

ERG

Y (m

N m

-1)

POLYMER0

10

20

30

40

50

60

Prin

ting

inks

UV

adhe

sive

s

Coa

tings

Wat

erba

sed

adhe

sive

s

UV

inks

Wat

erba

sed

inks

SUR

FAC

E TE

NSI

ON

(mN

m-1

)

LIQUID

UTA_1102_07


PLASMA SURFACE MODIFICATION OF POLYMERS• To improve wetting and adhesion of

polymers atmospheric plasmas are used to generate gas-phase radicals to functionalize their surfaces.

Untreated PP

Plasma Treated PP

• M. Strobel, 3M

• Polypropylene (PP)

He/O2/N2 Plasma

• Massines et al. J. Phys. D 31, 3411 (1998).

UTA_1102_08


PLASMA PRODUCED WETTABILITY

• Boyd, Macromol., 30, 5429 (1997).• Polypropylene, Air corona

• Increases in wettability with plasma treatment result from formation of surface hydrophilic groups such as C-O-O (peroxy), C=O (carbonyl).

Hydrophobic

Hydrophilic

• Polyethylene, Humid-air• Akishev, Plasmas Polym. 7, 261 (2002).

UTA_1102_09


POLYMER TREATMENT APPARATUS

RAJESH_AVS_02_04A

HIGH-VOLTAGEPOWER SUPPLY

FEED ROLLGROUNDEDELECTRODE

COLLECTORROLL

PLASMA

~

SHOEELECTRODE

POWERED

TYPICAL PROCESS CONDITIONS:

Gas gap : a few mmApplied voltage : 10-20 kV at a few 10s kHzEnergy deposition : 0.1 - 1.0 J cm-2Residence time : a few sWeb speed : 10 - 200 m/min


COMMERCIAL CORONA PLASMA EQUIPMENT

RAJESH_AVS_02_04

Tantec Inc.


CORONA/DIELECTRIC BARRIER PLASMAS

• Laboratory Dielectric Barrier Discharge

• Corona and dielectric barrier discharge plasmas operate in a filamentary mode.

• 1 atm, Dry Air, -15 kV, 30 ns

• Electron Density (2-d Model)

UTA_1102_10


POLYPROPYLENE

• PP is a hard but flexible plastic. 5 million metric tons of PP film are used yearly, much of it functionalized with plasmas

Worldwide market of BOPP by end uses

Snacks

Baked goods

ConfectionaryOthers

Tobacco

Non-foodpackaging

Adhesive Others

UTA_1102_11


FUNCTIONALIZATION OF THE PP SURFACE

• Untreated PP is hydrophobic.

• Increases in surface energy by plasma treatment are attributed to the functionalization of the surface with hydrophilic groups.

• Carbonyl (-C=O) • Alcohols (C-OH)

• Peroxy (-C-O-O) • Acids ((OH)C=O)

• The degree of functionalization depends on process parameters such as gas mix, energy deposition and relative humidity (RH).

• At sufficiently high energy deposition, erosion of the polymer occurs.

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REACTION PATHWAY

RAJESH_AVS_02_05

C C C C C C C C C

C C C C C C C C C C

OH

OHO||

OH, O

C C C C C C C C C C C

LAYER 1

LAYER 2

LAYER 3

H

OH, H2O

OH

OHO2

O2

HUMID-AIR PLASMA

BOUNDARY LAYER

POLYPROPYLENE

H2O

e e

H OH

N2

e e

N NO2

O O

e e

O2O3

O2NO

NO


DESCRIPTION OF THE MODEL: GLOBAL_KIN

RAJESH_AVS_02_08

• Modules in GLOBAL_KIN:• Circuit model• Homogeneous plasma chemistry• Species transport to PP surface• Heterogeneous surface chemistry

N(t+∆t),V,I

CIRCUITMODULE

GAS-PHASEKINETICS

SURFACEKINETICS

VODE -ODE SOLVER

OFFLINEBOLTZMANN

SOLVER

LOOKUP TABLEOF k vs. Te


SPECIES TRANSPORT TO THE POLYMER SURFACE• Species in the bulk plasma diffuse to the PP surface

through a boundary layer (d ~ a few λmfp ≈ µm).

• Radicals react on the PP based on a variable density, multiple layer surface site balance model.

POLYPROPYLENE

BOUNDARYLAYER ~ λmfp

BULK PLASMA

DIFFUSION REGIME

O OH CO2

δ

UTA_1102_14

N2

O2

H2O

N N2(A)

e

e O

H OH

e

O2

NO

O2,OH

HNO2OH

NO2O3, HO2

OHNO3

OHN2

N

O3

HO2

OH

OH

O2

O2(1∆)

HO2

O2HO2H2O2

N2O5

NO3OH


REACTION MECHANISM FOR HUMID-AIR PLASMA

• Initiating radicals are O, N, OH, H

• Gas phase products include O3, N2O, N2O5, HNO2, HNO3.

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POLYPROPYLENE (PP) POLYMER STRUCTURE

• Three types of carbon atoms in a PP chain:

• Primary – bonded to 1 C atom• Secondary – bonded to 2 C atoms• Tertiary – bonded to 3 C atoms

• The reactivity of an H-atom depends on the type of C bonding. Reactivity scales as:

HTERTIARY > HSECONDARY > HPRIMARY

C C C C C CH H H H H H

H H HCH3 CH3CH31

2 31 - Primary C2 - Secondary C3 - Tertiary C

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PP SURFACE REACTION MECHANISM: INITIATION

• The surface reaction mechanism has initiation, propagation and termination reactions.

• INITIATION: O and OH abstract H from PP to produce alkyl radicals; and gas phase OH and H2O.

~CH2 C CH2~

CH3

~CH2 C CH2~

CH3

H O(g)

OH(g), H2O(g)

(ALKYL RADICAL)(POLYPROPYLENE)

OH(g)

UTA_1102_16

~CH2 C CH2~

CH3

(ALKYL RADICAL)

O(g), O3 (g)

(ALKOXY RADICAL)

O2 (g)

O

~CH2 C CH2~

CH3

~CH2 C CH2~

CH3

OO

(PEROXY RADICAL)University of Illinois

Optical and Discharge Physics

PP SURFACE REACTION MECHANISM: PROPAGATION

• PROPAGATION: Abundant O2 reacts with alkyl groups to produce “stable” peroxy radicals. O3 and O react to form unstable alkoxy radicals.

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~CH2 C CH2CH3

OO

C CH2~

CH3

H

~CH2 C CH2CH3

OO

C CH2~

CH3

H

+ O2 (g)

~CH2 C CH2CH3

OO

C CH2~

CH3

HO

O


PP SURFACE REACTIONS: PROPAGATION / AGING

• PROPAGATION / AGING: Peroxy radicals abstract H from the PP chain, resulting in hydroperoxide, processes which take seconds to 10s minutes.

UTA_1102_18


PP SURFACE REACTION MECHANISM: TERMINATION

• TERMINATION: Alkoxy radicals react with the PP backbone to produce alcohols and carbonyls. Further reactions with O eventually erodes the film.

CH2~O

~CH2 CCH3

+

(ALKOXY RADICAL)

O

~CH2 C CH2~

CH3

(CARBONYL)

OH

~CH2 C CH2~

CH3

H(s)

(ALCOHOLS)

O(g)CO2 (g)

UTA_1102_19


BASE CASE: ne, Te

• Ionization is dominantly of N2 and O2,

• e + N2 → N2+ + e + e,

• e + O2 → O2+ + e + e.

• After a few ns current pulse, electrons decay by attachment (primarily to O2).

• Dynamics of charging of the dielectrics produce later pulses with effectively larger voltage.s

• N2/O2/H2O = 79/20/1, 300 K• 15 kV, 9.6 kHz, 0.8 J-cm-2

• Web speed = 250 cm/s (460 pulses)

Time (s)

1

25 - 460

Ele

ctro

n D

ensi

ty (c

m-3

)1014

1013

1012

1011

1010

10910-610-710-810-910-10

1

25 - 460

10-710-810-910-10Time (s)

Ele

ctro

n Te

mpe

ratu

re (e

V)

6

5

4

3

2

1

0

UTA_1102_20


GAS-PHASE RADICALS: O, OH• Electron impact dissociation of O2 and H2O produces O and

OH. O is consumed in the primarily to form O3,

• After 100s of pulses, radicals attain a periodic steady state.

O (1

014

cm-3

)

115

30

45

0

25 - 460

10-610-710-810-9 10-5Time (s)

~~

0.0

0.1

0.2

0.3

2

6

10

1OH

(101

3 cm

-3)

25 - 460

10-610-710-810-9 10-5Time (s)

UTA_1102_21


PP SURFACE GROUPS vs ENERGY DEPOSITION

• Surface concentrations of alcohols, peroxy radicals are near steady state with a few J-cm-2.

•Alcohol densities decrease at higher J-cm-2 energy due to decomposition by O and OH to regenerate alkoxy radicals.

• Air, 300 K, 1 atm, 30% RH

• Ref: L-A. Ohare et al., Surf. Interface Anal. 33, 335 (2002).

UTA_1102_22


GAS-PHASE PRODUCTS: O3, NXOY, HNOX

• O3 is produced by the reaction of O with O2,

O + O2 + M → O3 + M.

• N containing products include NO, NO2, HNO2andN2O5,

N2 + O → NO + N,NO + O + M → NO2 + M,

NO + OH + M → HNO2 + MNO2 + NO3 + M → N2O5 + M

O3

N O52

HNO2

NONO2

0.0

0.5

1.0

1.5

2.0

2.5

0.0

2.5

5.0

7.5

10.0

12.5

0.0 0.5 1.0 1.5Energy Deposition (J cm-2)

O3

(101

7 cm

-3)

NO

, NO

2, H

NO

2, N

2O5

(101

4 cm

-3)

UTA_1102_23

• Increasing RH produces more OH. Reactions with PP generate more alkyl radicals, rapidly converted to peroxy radicals by O2.

PP-H + OH(g) → PP• + H2O(g) PP• + O2(g) → PP-O2•

• Alcohol and carbonyl densities decrease due to increased consumption by OH to form alkoxy radicals and acids.


HUMIDITY: PP FUNCTIONALIZATION BY OH

PP-OH+ OH(g)→PP-O• + H2O(g) , PP=O• + OH(g) → (OH)PP=O

UTA_1102_24


EFFECT OF RH: GAS-PHASE PRODUCTS

•Higher RH results in decreasing O and increasing OH. • Production of O3 decreases while larger densities of HNOx are

generated.

N + OH → NO + H, NO + OH → HNO2, NO2 + OH → HNO3.

UTA_1102_25


EFFECT OF TGas: PP FUNCTIONALIZATION

• Increasing Tgas decreases O3leading to lower alkoxyproduction.

• PP• + O3(g) → PP-O• + O2(g).

• … and lower production of alcohols, carbonyl, and acids.

PP-O• + PP-H → PP-OH + PP•PP-O• → PP=OPP=O → PP=O•

PP=O• + OH → (OH)PP=O•

• Lower consumption of alkyl radicals by O3 enables reactions with O2 to dominate, increasing densities of peroxy.

Peroxy

Alcohol

Carbonyl

Acid

0.0

1.0

3.0

4.0

300 310 320 330 340 350Gas Temperature (K)

2.0

Sur

face

Con

cent

ratio

n (%

)

300 310 320 330 340 350Gas Temperature (K)

UTA_1102_26


RH: PREDICTED CONTACT ANGLE•Relation for wettability / contact angle vs concentration of

functional groups is non-linear and poorly known.• Assume wettability is mainly due to O on PP.

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0 10 20 30 40 50 60 70 80

REL

ATIV

E C

ON

TAC

T AN

GLE

RELATIVE HUMIDITY • Polyethylene, Humid-air• Akishev, Plasmas Polym. 7, 261 (2002).• Model PP, Humid-air

UTA_1102_27


WHAT’S THE UPSIDE: BETTER FRITO BAGS OR ENGINEERED BIOCOMPATIBLE COATINGS?

• The ability to control functional groups on polymers through fundamental understanding of plasma-solid interactions opens the realm of engineered large area specialty surfaces.

•Keratinocyte cells adhere to hydrocarbon polymers containing carboxylic acid groups (PCP, SAM).

• Haddow et al., J. Biomed. Mat. Res. 47, 379 (1999)

UTA_1102_28

• The striking improvement in the functionality of microelectronics devices results from shrinking of individual components and increasing complexity of the circuitry

• Plasmas are absolutely essential to the fabrication of microelectronics.


PLASMAS AND MICROELECTRONICS FABRICATION

Ref: IBM Microelectronics

UTA_1102_29


PLASMAS IN MICROELECTRONICS FABRICATION

• Plasmas play a dual role in microelectronics fabrication.

• First, electron impact on otherwise unreactive gases produces neutral radicals and ions.

• These species then drift or diffuse to surfaces where they add, remove or modify materials.

UTA_1102_30


PLASMAS IN MICROELECTRONICS FABRICATION

• Second, ions deliver directed activation energy to surfaces fabricating fine having extreme and reproducable tolerances.

• 0.25 µm Feature(C. Cui, AMAT)

UTA_1102_31


APPLIED MATERIALS DECOUPLED PLASMA SOURCES (DPS)

PLSC_0901_06


rf BIASED INDUCTIVELYCOUPLED PLASMAS

• Inductively Coupled Plasmas (ICPs) with rf biasing are used here.

• < 10s mTorr, 10s MHz, 100s W – kW, electron densities of 1011-1012 cm-3.

ADVMET_1002_10

PUMP PORT

DOME

GAS INJECTORS

BULK PLASMA

WAFER

30 30 0 RADIUS (cm)

HEI

GH

T (c

m)

0

26

sCOILS

s

rf BIASED SUBSTRATE

SOLENOID

POWER SUPPLY

POWER SUPPLY


SELECTIVITY IN MICROELECTRONICS FABRICATION:PLASMAS AND POLYMERS

• Fabricating complex microelectronic structures made of different materials requires extreme selectivity in, for example, etching Si with respect to SiO2.

• Monolayer selectivity is required in advanced etching processes.

• These goals are met by the unique plasma-polymer interactions enabled in fluorocarbon chemistries.

• Ref: G. Timp

UTA_1102_32


FLUORCARBON PLASMA ETCHING: SELECTIVITY

• Selectivity in fluorocarbon etching relies on polymer deposition.

• Electron impact dissociation of feedstock fluorocarbons produce polymerizing radicals and ions, resulting in polymer deposition.

ADVMET_1002_04

• Compound dielectrics contain oxidants which consume the polymer, producing thinner polymer layers.

• Thicker polymer on non-dielectrics restrict delivery of ion energy (lower etching rates).

SiFn

SiSiO2

COFn, SiFn

CFxCFx

CFn, M+CFn, M+

e + Ar/C4F8 CFn, M+

PolymerPolymer


FLUORCARBON PLASMA ETCHING: SELECTIVITY

• Low bias: Deposition• High bias: etching

ADVMET_1002_05

• G. Oerhlein, et al., JVSTA 17, 26 (1999)

• Etch Rate (SiO2 > Si)

• Polymer Thickness (SiO2 < Si)


FLUORCARBON PLASMA ETCHING: POLYMER

• The polymer composition deposited in fluorocarbon plasmas depends on feedstock, pressure, power, bias power.

• For discussion PTFE (poly)tetrafluoroethylene [C2F4]n is a good approximation for most layers.

• Rueger et al., JVST A 15, 1881 (1997)

UTA_1102_33


SURFACE KINETICS: FLUOROCARBON PLASMA ETCHING Si/SiO2

• CxFy passivation regulates delivery of precursors and activation energy.

• Chemisorption of CFx produces a complex at the oxide-polymer interface.

• 2-step ion activated (through polymer layer) etching of the complex consumes the polymer. Activation scales inversely with polymer thickness.

• Etch precursors and products diffuse through the polymer layer.

• In Si etching, CFxis not consumed, resulting in thicker polymer layers.

CF4

F

Plasma

CFn

I+

SiFn,CxFy

CO2

CFxI+, F

CO2

I+, FSiFn

CFn

SiO2 SiO2 SiFxCO2 SiFxSiO2

CFn

PassivationCxFy

Layer

UTA_1102_34


MODELING OF FLUOROCARBON PLASMA ETCHING

• Our research group has developed an integrated reactor and feature scale modeling hierarchy to model plasma processing systems.

ADVMET_1002_09

• HPEM (Hybrid Plasma Equipment Model)

• Reactor scale• 2- and 3-dimensional• ICP, CCP, MERIE, ECR• Surface chemistry• First principles

• MCFPM (Monte Carlo Feature Profile Model)

• Feature scale• 2- and 3-dimensional• Fluxes from HPEM• First principles

MATCH BOX-COIL CIRCUIT MODEL

ELECTRO- MAGNETICS

FREQUENCY DOMAIN

ELECTRO-MAGNETICS

FDTD

MAGNETO- STATICS MODULE

ELECTRONMONTE CARLO

SIMULATION

ELECTRONBEAM MODULE

ELECTRON ENERGY

EQUATION

BOLTZMANN MODULE

NON-COLLISIONAL

HEATING

ON-THE-FLY FREQUENCY

DOMAIN EXTERNALCIRCUITMODULE

PLASMACHEMISTRY

MONTE CARLOSIMULATION

MESO-SCALEMODULE

SURFACECHEMISTRY

MODULE

CONTINUITY

MOMENTUM

ENERGY

SHEATH MODULE

LONG MEANFREE PATH

(MONTE CARLO)

SIMPLE CIRCUIT MODULE

POISSON ELECTRO- STATICS

AMBIPOLAR ELECTRO- STATICS

SPUTTER MODULE

E(r,θ,z,φ)

B(r,θ,z,φ)

B(r,z)

S(r,z,φ)

Te(r,z,φ)

µ(r,z,φ)

Es(r,z,φ) N(r,z)

σ(r,z)

V(rf),V(dc)

Φ(r,z,φ)

Φ(r,z,φ)

s(r,z)

Es(r,z,φ)

S(r,z,φ)

J(r,z,φ)ID(coils)

MONTE CARLO FEATUREPROFILE MODEL

IAD(r,z) IED(r,z)

VPEM: SENSORS, CONTROLLERS, ACTUATORS


HYBRID PLASMA EQUIPMENT MODEL

SNLA_0102_39


ELECTROMAGNETICS MODULE

AVS01_03

• The wave equation is solved in the frequency domain using sparsematrix techniques (2D,3D):

• Conductivities are tensor quantities (2D,3D):

( ) ( )t

JEtEEE

∂+⋅∂

+∂

∂=

∇⋅∇+

⋅∇∇−

σεµµ

1 12

2

)))((exp()(),( rtirEtrE rrrrrϕω +−′=

( )m

e2

om

2z

2zrzr

zr22

rz

zrrz2r

2

22

mo

mnq

mqiEj

BBBBBBBBBBBBBBBBBBBBB

B

1qm

νσνωασ

ααααααααα

αανσσ

θθ

θθθ

θθ

=+

=⋅=

++−+−+++−+−++

+

=

,/

rv

r


ELECTRON ENERGY TRANSPORT

where S(Te) = Power deposition from electric fieldsL(Te) = Electron power loss due to collisionsΦ = Electron fluxκ(Te) = Electron thermal conductivity tensorSEB = Power source source from beam electrons

• Power deposition has contributions from wave and electrostatic heating.

• Kinetic (2D,3D): A Monte Carlo Simulation is used to derive including electron-electron collisions using electromagnetic fields from the EMM and electrostatic fields from the FKM.

SNLA_0102_41

( ) ( ) ( ) EBeeeeeee STTkTTLTStkTn +

∇⋅−Φ⋅∇−−=∂

∂ κ

25/

23

( )trf ,, rε

• Continuum (2D,3D):


PLASMA CHEMISTRY, TRANSPORT AND ELECTROSTATICS

• Continuity, momentum and energy equations are solved for each species (with jump conditions at boundaries) (2D,3D).

AVS01_ 05

• Implicit solution of Poisson’s equation (2D,3D):

( ) ( )

⋅∇⋅∆+=∆+Φ∇⋅∇ ∑∑

iiqt-- i

iiis Nqtt φρε

r

iiii SNtN

+⋅−∇= )v( r

∂∂

( ) ( ) ( ) ( ) iii

iiiiiii

i

ii BvEmNqvvNTkN

mtvN µ

∂∂

⋅∇−×++⋅∇−∇=rrrrr

r 1

( ) ijjijj

imm

j vvNNm

ji

νrr−−∑

+

( ) 222

2

)()U(UQ Em

qNNPtN

ii

iiiiiiiii

ii

ωννε

∂ε∂

+=⋅∇+⋅∇+⋅∇+

∑∑ ±−+

++j

jBijjij

ijBijjiji

ijs

ii

ii TkRNNTTkRNNmm

mE

mqN 3)(32

2

ν


MONTE CARLO FEATURE PROFILE MODEL (MCFPM)

• The MCFPM predicts time and spatially dependent profiles using energy and angularly resolved neutral and ion fluxes obtained from equipment scale models.

• Arbitrary chemical reaction mechanisms may be implemented, including thermal and ion assisted, sputtering, deposition and surface diffusion.

• Energy and angular dependent processes are implemented using parametric forms.

SCAVS_1001_08

• Mesh centered identify of materials allows “burial”, overlayers and transmission of energy through materials.


TYPICAL ICP CONDITIONS: [e] FOR C4F8, 10 mTORR

• An ICP reactor patterned after Oeherlein, et al. was used for validation.

• Reactor uses 3-turn coil (13.56 MHz) with rf biased substrate (3 MHz)

• Electron densities are 1011-1012 cm-3 for 1.4 kW.

ADVMET_1002_11

• C4F8, 10 mTorr, 1.4 kW, 13.56 MHz


POWER, C4F8 DENSITY

• Large power deposition typically results in near total dissociation of feedstock gases.

ADVMET_1002_12

• C4F8, 10 mTorr, 1.4 kW, 13.56 MHz


MAJOR POSITIVE IONS

ADVMET_1002_13

• C4F8, 10 mTorr, 1.4 kW, 13.56 MHz

• CF3+, CF2

+, and CF+ are dominant ions due to dissociation of C4F8.

CF3+


CF3+ TEMPERATURE

• The ion temperature is peaked near the walls where ions gain energy during acceleration in the presheath.

ADVMET_1002_14

• C4F8, 10 mTorr, 1.4 kW, 13.56 MHz

0

1

2

3

4

5

6

7

8

400 600 800 1000 1200 1400

Exp.Calc.Exp.Calc.

Power (W)

I (m

A)

without magnets

with magnets


IP VERSUS ICP POWER for C4F8

• Extensive validation of the plasma models are performed with available data for densities, temperatures and fluxes.

• Ion saturation current derived from the model are compared to experiments: Ion densities are larger with moderate static magnetic fields.

ADVMET_1002_27

• Experiments: G. Oehrlein, Private Comm.

• C4F8, 10 mTorr, 13.56 MHz, 100 V probe bias


ION/NEUTRAL ENERGY/ANGULAR DISTRIBUTIONS

• The end products of reactor scale modeling are energy and ion angular distributions to the surface.

• In complex gas mixtures the IEADs can significantly vary from species to species.

ADVMET_1002_28

• Ar/C4F8, 40 mTorr, 10b MHz, MERIE


ETCH RATES AND POLYMER THICKNESS

• Etch rates for Si and SiO2 increase with increasing bias due, in part, to a decrease in polymer thickness.

• The polymer is thinner with SiO2 due to its consumption during etching, allowing for more efficient energy transfer through thelayer and more rapid etching.

ADVMET_1002_15

Self-Bias Voltage (-V)

Etch

Rat

e (n

m/m

in)

SiO2

0

100

200

300

400

500

600

700

0 20 40 60 80 100 120 140 160

ExperimentModel

Si

0

2

4

6

8

10

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 20 40 60 80 100 120 140 160Self-Bias Voltage (-V)

Poly

mer

Lay

ers

Tran

sfer

Coe

ffici

ent (λ)

λ

Polymer

SiO2 Si

• C2F6, 6 mTorr, 1400 W ICP, 40 sccm• Exp. Ref: T. Standaert, et al.

J. Vac. Sci. Technol. A 16, 239 (1998).


POLYMERIZATION AIDS SELECTIVITY• Less consumption of polymer on Si relative to SiO2 slows and,

in some cases, terminates etching, providing high selectivity.

ADVMET_1002_16

0.00

0.25

0.50

0.75

1.00

1.25

1.50

0 2 4 6 8 10 12TIME (min)

Etch Depth (µm)

SiInterface

SiO2


TAPERED AND BOWED PROFILES

• In high aspect ratio (HAR) etching of SiO2the sidewall of trenches are passivated by neutrals (CFx, x ≤ 2) due to the broad angular distributions of neutral fluxes.

• Either tapered or bowed profiles can result from a non-optimum combination of processing parameters including:

ADVMET_1002_17

SiO2

PR

BOWED TAPERED

• Degree of passivation• Ion energy distribution• Radical/ion flux composition.


PROFILE TOPOLOGY: NEUTRAL TO ION FLUX RATIO

• The etch profile is sensitive to the ratio of polymer forming fluxes to energy activating fluxes. Small ratios result in bowing, large ratios tapering.

SCAVS_1001_12

8.4 10.2 12

Photoresist

SiO2

Φn/Φion = 6

Wb

/ Wt

Dep

th (µ

m)

Wb / Wt

Depth

Φn / Φion

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.0

0.5

1.0

1.5

2.0

2.5

3.0

7 8 9 10 11 12

Wt

Wb


PROFILE TOPOLOGY: ENGINEERING SOLUTIONS

• Knowledge of the fundamental scaling parameter for controlling sidewall slop enables engineering solutions and real-time-control options.

• Example: Ar/C2F6 ratio controls polymerizing/ion flux ratio, and hence profile topology.

SCAVS_1001_13

Ar/C2F6 = �0/100

Φn/Φion= 12

20/80

8.7

40/60

6.4

Photoresist

SiO2

60/40

4.0

Wb

/ Wt

Φn

/ Φio

n

Ar Fraction

Φn/ΦionWb / Wt

02468

1012

0.00.20.40.60.81.01.2

0 0.1 0.2 0.3 0.4 0.5 0.6


LOW-K DIELECTRICS

• As feature sizes decrease and device count increases, the diameter of interconnect wires shrinks and path length increases.

• L. Peters, Semi. Intl., 9/1/1998

• Large RC-delay limits processor performance.

• To reduce RC-delay, low dielectric constant (low-k) materials are being investigated.

UTA_1102_35


POROUS SILICON DIOXIDE

• Porous SiO2 (xerogels) have low-k properties due to their lower mass density resulting from (vacuum) pores.

• Typical porosities: 30-70%• Typical pore sizes: 2-20 nm

• Porous SiO2 (P-SiO2) is, from a process development viewpoint, an ideal low-k dielectric.

• Extensive knowledge base for fluorocarbon etching ofconventional non-porous (NP-SiO2).

• No new materials (though most P-SiO2 contains some residual organics)

• Few new integration requirements

ADVMET_1002_07


ETCHING OF P-SiO2: GENERAL TRENDS• Etching of Porous SiO2 typically proceeds at a higher rate than NP-SiO2

for the same conditions due to the lower mass density.

ADVMET_1002_08

• When correcting for mass, etch rates are either larger or smaller than NP- SiO2, depending on porosity, pore size, polymerization.

• Standaert et al, JVSTA 18, 2742 (2000).


WHAT CHANGES WITH POROUS SiO2?

• The “opening” of pores during etching of P-SiO2 results in the filling of the voids with polymer, creating thicker layers.

• Ions which would have otherwise hit at grazing or normal angle now intersect with more optimum angle.

ADVMET_1002_18

• An important parameter is L/a (polymer thickness / pore radius).

• Adapted: Standaert, JVSTA 18, 2742 (2000)


ETCH PROFILES IN SOLID AND POROUS SiO2

• Solid • Porosity = 45 %Pore radius = 10 nm

• Porous SiO2 is being investigated for low-permittivity dielectrics for interconnect wiring.

• In polymerizing environments with heavy sidewall passivation, etch profiles differ little between solid and porous silica.

• The “open” sidewall pores quickly fill with polymer.

• Position (µm)• Position (µm)

UTA_1102_36


ETCHING OF POROUS SiO2

• Etch rates of P-SiO2 are generally higher than for non-porous (NP).

• Examples:

• 2 nm pore, 30% porosity• 10 nm pore, 58% porosity

• Higher etch rates are attributed to lower mass density of P-SiO2.

• CHF3 10 mTorr, 1400 W

ADVMET_1002_23

Exp: Oehrlein et al. Vac. Sci.Technol. A 18, 2742 (2000)


PORE-DEPENDENT ETCHING• To isolate the effect of pores on etch

rate, corrected etch rate is defined as

• If etching depended only on mass density, corrected etch rates would equal that of NP- SiO2.

• 2 nm pores L/a ≥1 : C-ER > ER(SiO2).Favorable yields due to non-normal incidence may increase rate.

• 10 nm pores L/a ≤ 1 : C-ER < ER(SiO2).Filling of pores with polymer decrease rates.

ADVMET_1002_24

porosity p p), - (1 ER (ER) Rate Etch regular corrected

=

×=

0 20 40 60 80 100 120 1400

100

200

300

400

Non porous

C-2 nm

C-10 nm

Etch

Rat

e (n

m/m

in)

C - Corrected

Self Bias (V)

EFFECT OF POROSITY ON BLANKET ETCH RATES

• 2 nm pores: Etch rate increases with porosity.

• 10 nm pores: Polymer filling of pores reduces etch rate at largeporosities.

Etch

Rat

e (n

m m

in -1

)

Regular

Corrected

Porosity (%)

2 nm pores0

300

350

400

450

500

0 5 10 15 20 25 30

~~

Porosity (%)

Etch

Rat

e (n

m m

in -1

)

0

100

200

300

400

500

0 10 20 30 40 50 60

Regular

Corrected

10 nm pores

University of IllinoisOptical and Discharge PhysicsADVMET_1002_25


EFFECT OF POROSITY ON HAR TRENCHES

ADVMET_1002_26

• At higher porosities, more opportunity for pore filling producesthicker average polymer layers and lower etch rates.

• Corrected etch rates fall below SiO2 rates when critically thick polymer layers are formed.

0.00

0.20

0.25

0.30

0.35

0.40

0.45

0 10 20 30 40

~~

Porosity (%)

Dep

th (µ

m)

Regular

Corrected

Porosity = 15% 35% 45%

Photoresist

SiO2

Wt

Wb

• 10 nm pores. Wt = 0.1 µm


EFFECT OF PORE RADIUS ON HAR TRENCHES

• With increase in pore radius, L/a decreases, enabling pore filling and a decrease in etch rates.

• Thick polymer layers eventually leads to etch rates falling below NP. There is little variation in the taper.

4 6 8 10 12 14 160.000.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

Pore radius (nm)

Etch

Dep

th (µ

m)

Regular

Wb / Wt

Corrected

NP

50 % porosity

0.000.70

0.75

0.80

0.85

0.90

0.95

1.00

Wb / W

t

Radius = 4 nm 10 nm 16 nm

SiO2

Resist

UTA_1102_37


OXYGEN ETCHING OF PTFE

• After etching, the polymer must be removed from the feature.

• O2 plasmas are typically used for polymer stripping, usually during photoresist mask removal.

• Unlike hydrocarbon polymers which spontanesouly react with O, fluorocarbon polymers require ion activation for etching.

• Removal of polymer from porous materials is difficult due to shadowing of ion fluxes caused by the pore morphology.

• Polymer + Energetic Ion → Activated Polymer Site (P*)• P* + O → Volatile Products

UTA_1102_38


EFFECT OF PORE RADIUS ON CLEANING• Larger pores are more difficult to clean due small view angle of

ion fluxes producing lower fluxes of less energetic ions.

UTA_1102_39

4 nm

Before After Before After

16 nm


CONCLUDING REMARKS

• Plasmas and polymers enjoy a unique relationship in the realm of gas-surface interactions.

• The ability for plasmas to produce reactive species and polymers to “accept” those species at ambient temperatures have enabled a wide range of technological applications.

• Inspite of years of use, lack of fundamental understanding of many of the basic plasma-surface processes has largely limited the technology to empirical development.

• As these processes become better characterized, new technologies will come to the forefront; from biocompatible surfaces to flexible display panels.

UTA_1102_40


ACKNOWLEDGEMENTS

• Dr. Alex V. Vasenkov• Mr. Rajesh Dorai• Mr. Arvind Sankaran• Mr. Pramod Subramonium

• Funding Agencies:

• 3M Corporation• Ford Motor Corporation• Semiconductor Research Corporation• National Science Foundation• SEMATECH

UTA_1102_41

plasma processing and polymers - computational plasma science

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