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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)
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
COLLISIONAL LOW TEMPERATURE PLASMAS
UTA_1102_04
• Displays
• Materials Processing
COLLISIONAL LOW TEMPERATURE PLASMAS
• Lighting
• Thrusters
• Spray Coatings
UTA_1102_05
University of IllinoisOptical and Discharge Physics
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)
UTA_1102_06
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
COMMERCIAL CORONA PLASMA EQUIPMENT
RAJESH_AVS_02_04
Tantec Inc.
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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.
UTA_1102_13
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
REACTION MECHANISM FOR HUMID-AIR PLASMA
• Initiating radicals are O, N, OH, H
• Gas phase products include O3, N2O, N2O5, HNO2, HNO3.
UTA_1102_15
University of IllinoisOptical and Discharge Physics
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
UTA_1102_12
University of IllinoisOptical and Discharge Physics
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.
UTA_1102_17
~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
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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.
University of IllinoisOptical and Discharge Physics
HUMIDITY: PP FUNCTIONALIZATION BY OH
PP-OH+ OH(g)→PP-O• + H2O(g) , PP=O• + OH(g) → (OH)PP=O
UTA_1102_24
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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.
University of IllinoisOptical and Discharge Physics
PLASMAS AND MICROELECTRONICS FABRICATION
Ref: IBM Microelectronics
UTA_1102_29
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
APPLIED MATERIALS DECOUPLED PLASMA SOURCES (DPS)
PLSC_0901_06
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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)
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
HYBRID PLASMA EQUIPMENT MODEL
SNLA_0102_39
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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):
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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.
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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+
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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).
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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.
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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).
University of IllinoisOptical and Discharge Physics
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)
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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)
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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
University of IllinoisOptical and Discharge Physics
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