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47th AVS International SymposiumOctober 2-6, 2000
REACTION MECHANISMS AND SiO2 PROFILEEVOLUTION IN FLUOROCARBON PLASMAS
Da Zhang* and Mark J. Kushner***Department of Materials Science and Engineering
**Department of Electrical and Computer EngineeringUniversity of Illinois at Urbana-Champaign
Chunshi CuiApplied Materials, Inc.
Work supported by SRC, NSF, and Applied Materials.
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
AGENDA
l Introduction
l Hybrid Plasma Equipment Model (HPEM)
l Surface Kinetics Module (SKM)
l Monte Carlo Feature Profile Module (MCFPM)
l C2F6 Plasma Etching of SiO2
l Surface Reaction Mechanism
l Passivation and Etch Rate
l Trench Profiles
l Summary
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UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
INTRODUCTION
l Fluorocarbon plasmas are widely used for SiO2/Si etching due to their high etch
rates and good selectivity.
l CFx radicals passivate the wafer surface by polymeric deposition.
l Passivation-dependent surface reactions define etch rates and feature profiles.
l Our goal:
l Develop a surface reaction mechanism
l Model passivation dependent etching
l Correlate etch rates and feature profiles to process parameters (e.g., bias, chemistry).
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UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
HYBRID PLASMA EQUIPMENT MODEL (HPEM)
l Modular simulation addressing low temperature, low pressure plasma processes.
l EMM calculates electromagnetic fields and magneto-static fields.
l EETM computes electron impact source functions and transport coefficients.
l FKS derives densities, momentum, and temperature or plasma species.
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Electro-MagneticModule(EMM)
ElectronEnergyTransportModule(EETM)
Fluid-chemicalKineticsModule(FKS)
Eφ
B
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
SURFACE MODELING: EQUIPMENT AND FEATURE SCALES
l Equipment scale: Integrating the Surface Kinetics Model (SKM) with the HPEM.l Coupling surface reactions to plasma properties.l Obtaining etch rates along the wafer.
l Feature scale: Monte Carlo Feature Profile Model (MCFPM).
l Obtaining trench profiles.
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HPEM
SKM
n+
MCFPM
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
MECHANISMS FOR SiO2 ETCHING
l Polymerization:l CxFy deposition (e.g., CF, CF2, C2F3, C2F4)
l F atom etching l Ion sputtering l Polymer-SiO2 interaction
l SiO2 etching:
l Neutral passivation --> SiFxCO2 complex
l Ion assisted desorption --> SiFx surface sites
l Fluorination and ion chemical sputtering -- > SiFn volatile products
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CF4
F
Plasma
CFn
I+
SiFn,
CxFy
CO2
CFx
I+, F
CO2
I+, FSiFn
CFn
SiO2 SiO2 SiFxCO2 SiFx
CxFy
PassivationLayer
SiO2
CFn
EnergyTransfer
Diffusion
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
REACTOR GEOMETRY AND GAS REACTIONS
l Simulations of C2F6 etching of SiO2 in an ICP reactor were performed.
l Representative gas phase reactions:
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l C2F6, 6 mTorr, 40 sccm, 1400 W ICP, 100 V bias.
4.7E11
4.9E9
(cm-3)
0 5 100
4
8
12
16
Radius (cm)
l Electron Density
Coils
Nozzle Ring
Wafer
Bias~
e + C2F6 -> CF3+ + CF3 + e + e
e + C2F6 -> CF3 + CF3 + e
e + CF4 -> CF2 + F + F + e
e + CF4 -> CF3+ + F + e + e
e + CF3 -> CF2 + F + e
e + CF3 -> CF2 + F + F-
e + CF2 -> CF2+ + e + e
e + CF2 -> CF + F + e
F + F + M -> F2 + M
CF3 + CF3 + M -> C2F6 + M
CF2 + F2 -> CF3 + F
CF3 + F2 -> CF4 + F
C2F5 + F -> CF3 + CF3
CF3 + F -> CF4CF2 + F -> CF3
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
DENSITY DISTRIBUTIONS: BASE CASE
l The high power, low pressure process produces large and uniform densities of ions and radicals.
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l CF3+ Density
l C2F6, 6 mTorr, 40 sccm, 1400 W ICP, 100 V bias.
3.0E11
3.1E9
(cm-3)
0 5 100
4
8
12
16
Radius (cm)
Coils
Wafer
l CF2 Density
0 5 100
4
8
12
16
Radius (cm)
Coils
Wafer
4.9E13
5.0E11
(cm-3)
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
FLUXES AND ETCH RATE
l Radial distributions of wafer fluxes are uniform as a consequence of the density distributions.
l Polymer thickness and etch rate are uniform along the wafer surface.
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l C2F6, 6 mTorr, 40 sccm, 1400 W ICP, 100 V bias.
Rate
Polymer
Radius (cm)0 1 2 3 4 5 6
0
1
2
3
0
100
200
300
400
500
Radius (cm)0 1 2 3 4 5 6
0
1
2
3
4
5
6CF2 (1017)
CF3+ (1016)
F (1018)
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
SUBSTRATE BIAS
l With increasing bias, the ion energy increases due to increasing sheath voltage.
l The polymer thickness decreases with increasing bias because of increasing ion sputtering consumption. Consequently the etch rate increases.
l Low-energy-ion activation of surface sites for polymerization produces thick passivation at low biases.
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Substrate Bias (V)
Vself
Vdrop
0
100
200
300
0
100
200
300
0 100 200 300Self-Bias Voltage (-V)
0
4
8
12
0
200
400
600
0 40 80 120 160
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
SiO2 AND Si ETCHING
l The polymer thickness is larger on Si than on SiO2 at same process conditions due
to the lack of polymer consumption by the substrate oxygen in Si etching.
l The etch rates of SiO2 are larger than those of Si at the same process conditions.
l Simulations agree well with experiments.
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Self-Bias Voltage (-V)
0
200
400
600
0 40 80 120 160
SiO2
Si
Experiment*
Simulation
Experiment*
Simulation
* G. S. Oehrlein et al., J. Vac. Sci. Technol. A 17, 26 (1999).
Self-Bias Voltage (-V)
0
2
4
6
8
10
0 40 80 120 160
SiO2
Si
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
PASSIVATING NEUTRAL TO ION FLUX RATIO
l The passivation thickness increases with increasing passivating neutral to ion flux ratio (Φn/Φion).
l At low biases, the passivation is thick. The etch rate decreases with increasing Φn/Φion due to decreasing energy and species transfer through the polymer.
l At high biases, the etch rate saturates due to starvation of passivation. Increasing Φn/Φion leads to increasing etch rate.
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0
200
400
600
800
1000
0
2
4
6
8
0 100 200 300 400
Rate
Passivation
Φn/Φion = 12
Φn/Φion = 8.4
Self-Bias Voltage (-V)
MONTE CARLO FEATURE PROFILE MODEL (MCFPM)
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University of Illinois Optical and Discharge Physics
• Profiles of SiO2 in C2F6/Ar plasma etching were investigated with the MCFPM.
• The MCFPM model predicts the time and spatially dependent microscaleprocesses which produce etch profiles.
• The Plasma Chemistry Monte Carlo Simulation(PCMCS) uses HPEM results to producereactive fluxes (energy and angle) to thesurface.
• The MCFPM uses these fluxes to implement auser defined reaction mechanism.
CF2 CF3+ Ar+
PR
Si
SiO2CO2
SiF4
HPEM PCMCS MCFPMΓ(θ, E)
TAPERING OF PROFILES
AVS2000_13
University of Illinois Optical and Discharge Physics
• In high aspect ratio (AR) etching of SiO2 by fluorocarbon plasmas, the sidewall oftrenches are passivated by CxFy neutrals due to the broad angular distributions ofneutral fluxes.
• Tapered trench profiles are produced when the passivation/ion flux ratio is large.
• The critical dimension
shrinks from the top to
the bottom of the trench.
Experiment(Similar Process Conditions)
Simulation
0.25 um
AR = 10:1(C. Cui, AMAT)
SiO2
8
0.25 um
AR = 10:1
SiO2
PASSIVATION/ION FLUX RATIO
AVS2000_14
University of Illinois Optical and Discharge Physics
• Increasing passivating neutral to ion flux ratio (Φn/Φion) leads to more taperedprofiles due to increasing sidewall passivation.
• When the passivating neutral flux is too small, insufficient sidewall protectionby the passivation layer leads to a bowed profile.
100% 125% 150% 175%
Bowed Straight Tapered Tapered
Normalized Φn/Φion
PR
SiO2
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
INFLUENCE OF AR MIXTURE
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0.0
0.4
0.8
1.2
0 0.2 0.4 0.6Ar Fraction
Ar:C2F6 = 0:100 20:80 60:40
PR
SiO 2
l The passivating neutral to ion flux ratio (Φn/Φion) decreases with increasing Ar fraction
in the Ar/C2F6 feedstock gas.
l A tapered trench transitions to a straight and a bowed profile with increasing Ar fraction.
INFLUENCE OF ION ENERGY
AVS2000_16
University of Illinois Optical and Discharge Physics
• With increasing ion energy, the increasing ion sputtering yield of the sidewallpassivation layer produces a less tapered profile.
• The etch rate also increases due to decreasing (but sufficient) passivation.
• Simulations and experiments obtained similar trends.
Experiments(Similar Proccess
Conditions)
Simulations
800 W 1000 W Bias Power(C. Cui, AMAT)
SiO2
100 V 160 V
SiO2
Bias Voltage
INFLUENCE OF ION ENERGY (cont.)
AVS2000_17
University of Illinois Optical and Discharge Physics
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.8
1.6
2.4
3.2
100 150 200 250 300
• When the ion energy is high, the SiO2 etching is limited by the passivation instead ofthe ion sputtering yield.
• The etch rate and the bottom width of the trench are saturated at high ion energies.
Wb: Trench width 0.5 µm above the bottom.
Wt: Trench width at the top.
Depth: Trench depth after equal etch times.
Wb/Wt
Depth
Wb
/ Wt
Dep
th (
µm)
Wt
Dep
th
Wb
Substrate Bias (V)
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
SUMMARY
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l In C2F6 etching of SiO2, a polymer is grown on the surface by CxFy deposition. The
polymer influences etching by limiting diffusion and dissipating ion bombarding energy.
l With increasing substrate bias, the polymer thickness decreases due to increasing ion sputtering, resulting in increasing etch rates and less tapered profiles. The etch rate saturates at high biases due to insufficient neutral passivation.
l The etch selectivity of SiO2 over Si is attributed to the polymer consumption by
oxygen atoms in the SiO2 film.
l Processes with high passivating neutral to ion flux ratios produce tapered profiles. The bottom critical dimension of the trench decreases with increasing ratio of passivating neutral to ion fluxes.
