27th IEEE International Conference on Plasma Science
 

New Orleans, Louisiana
 

June 4 - 7, 2000
 

SURFACE AND GAS PHASE REACTIONS FOR
 

FLUOROCARBON PLASMA ETCHING OF SiO2 

Da Zhang* and Mark J. Kushner**
 

*Department of Materials Science and Engineering
 

**Department of Electrical and Computer Engineering
 

University of Illinois
 

Urbana, IL 61801
 

email: dazhang@uiuc.edu
 

mjk@uiuc.edu
 

Work supported by SRC, NSF, and Applied Materials. 
AGENDA
 

• Introduction 

• Description of the Model 

• Hybrid Plasma Equipment Model (HPEM) 

• Surface Kinetics Module (SKM) 

• Surface Reaction Mechanisms in C2F6 etching of SiO2 

• Surface Passivation 

• Substrate Bias 

• Profile Simulations for C2F6 Etching of SiO2 

• Monte Carlo Feature Profile Model (MCFPM) 

• Ion to Neutral Flux Ratio 

• Ion Energy 

• Summary 

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INTRODUCTION
 

• Fluorocarbon (CmFn) plasmas are widely used for SiO2/Si etching due to

 their high rate and good selectivity. 

• CFx radicals passivate the wafer surface and so influence etching

 behavior. 

• The passivation thickness depends on wafer materials, plasma-wall

 interactions, and processing parameters. 

• Our goals: 

• Develop a surface reaction mechanism 

• Model passivation dependent etching 

• Investigate influences of process parameters (e.g., bias, chemistry). 

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HYBRID PLASMA EQUIPMENT MODEL (HPEM)
 

• Modular simulator addressing low temperature, low pressure plasmas. 

• EMM calculates electromagnetic fields and magneto-static fields. 

• EETM computes electron impact source functions and transport coefficients. 

• FKM derives the densities, momentum and temperature of plasma species. 

• VPEM shell can be added to HPEM for process control. 

Electro- Electron FluidMagnetic 

Module 

(EMM) 

E(r,z) 

B(r,z) 

Energy 

Transport 

Module 

S(r,z), Te(r,z) 

m(r,z) 

chemistry 

Kinetics 

Module 

(EETM) (FKM) 

n(r,z) n(r,z)

V(r,z) V(r,z)

F(r,z) F(r,z)

VPEM: Sensors, Controller, Actuators
 

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SURFACE KINETICS MODEL (SKM)
 

• The Surface Kinetics Model (SKM) is an integrated module of the HPEM. 

• The SKM 

• Uses reactant fluxes from the HPEM 

• Applies a user defined reaction mechanism 

• Updates surface sticking and product reflection coefficient for the HPEM 

• Calculates surface coverages and process rates 

Reaction Mechanism 

Fi 

(1-Si)Fi Process Rate HPEM SKM 

fijFij 

• The SKM uses a multi-layer surface site balance model at every mesh point

 along the plasma-surface boundary. 

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C2F6 PLASMA ETCHING OF SiO2
 

• The reaction mechanism for SiO2 etching is based on: 

• Growth of CxFy passivation layer (balance of deposition and consumption). 

• Formation of complex at the interface between oxide and passivation layer

 resulting from chemisorption of CFx. 

• Ion activated (through passivation layer) etching of complex. Rate of

 activation scales inversely with passivation layer thickness.
 

• Diffusion of etch precursor and etch product. 

CF4 

Plasma 

C Fx y 

Passivation 

Layer 

SiO2 

CFx 

SiO2 SiFx 

F 

CF4 

F 

SiF4 

Film 

Growth 

• Film 

Etching 

F 

Etch 

Diffusion 

I+ 

CFx 

SiO2 SiFx

CO2 SiFx Etch 

CO2 

CFx 

I+ 

CFx 

CO2 

I+, F Energy 

Transfer 

SiF4 

F 

Sputtering 

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SiO2 ETCH REACTION MECHANISM
 

• Simulations of C2F6 etching of SiO2 in an ICP reactor were performed. 

• Representative gas phase reactions: 

e + C • Electron Density 2F6 fi CF3

+

 + CF3 + e + e
 

e + CF3 fi CF3

+

 + e + e
 

e + C2F6 fi CF3 + CF3 + e
 

e + CF3 fi CF2 + F + e
 

e + CF3 fi CF2 + F + Fe + CF4 fi CF2 + F + F + e
 

e + CF4 fi CF3 + F + e
 

F + F + M fi F2 + M
 

CF3 + CF3 + M fi C2F6 + M
 

CF2 + F2 fi CF3 + F
 

CF3 + F2 fi CF4 + F
 rf bias CF Radius (cm) 2 + CF3 fi C2 F5
 

C2F5 + F fi CF3 + CF3
 

• C2F6, 10 mtorr, 200 sccm, 650 W ICP, 100 V bias 

Coils Showerhead (C2F6) 

Wafer 

~ 

Wall 

1.90E11 

3.61E9

 (cm-3) 

5 10 15 

0 

0 

5 

10

Height (cm) 

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4.39E13 

7.31E11 

(cm-3) 

RADICAL DENSITIES
 

• Less uniform CF2 density distribution due to its higher loss rate at the

 reactor wall. 

• CF2 Density • F Density
 

1.67E14 

2.79E12 

(cm-3) 0 5 10 15

10
 

5
 

Height (cm)10
 

5
 

0 0 

0 5 10 15
 

Radius (cm) Radius (cm)
 

• C2F6, 10 mtorr, 200 sccm, 650 W ICP, 100 V bias.
 

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RADIAL DISTRIBUTIONS
 

• Radial flux distribution of CF2 is less uniform than that of F as a consequence of the

 of the density distributions. 

• Polymer distribution follows the CF2 flux distribution for this case. 

• Etch rate increases in the radial direction, inverse to the behavior of polymer distribution. 

6 1.50	 800 6 

CF3+(1016) 

CF2 (1016) 

F (1017) 

0 2 4 6 8 10
 

POLYMER 

ETCH RATE 

0 2 4 6 8 10
 

F, CF

2 Fluxes (cm-2

s-1)
 

CF

3

+ Fluxes (cm-2

s-1) 

Etch Rate (A/min)

5
 1.25
 5
 

Polymer Layers

700
 

600
 

4
 1.00
 4
 

3 0.75 3

2
 0.50
 2 

500 1 0.25 1
 

0 0.00 400 0 

Radius (cm) Radius (cm) 

• C2F6, 10 mtorr, 200 sccm, 650 W ICP, 100 V bias. 

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SUBSTRATE BIAS
 

•	 At low biases, the passivation layer thickness decreases with increasing bias due 

to increasing ion sputtering of the polymer, and the etch rate increases. 

•	 At high biases, the passivation is starved and the etch rate goes to saturation. 

More passivating neutral flux (CFx, x £ 2) is required to increase the etch rate. 

0.0 

0.5 

1.0 

1.5 

2.0 

2.5 

3.0 

100 

200 

300 

400 

500 

600 

700 

: Gn/Gion = 35. 

: Gn/Gion = 25. 

Etch Rate 

Passivation Layers

Passivation

Etch Rate (nm/min) 

0 50 100 150 200 250 300 350 

Substrate Bias (V) 

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MONTE CARLO FEATURE PROFILE MODEL (MCFPM)
 

•	 Profiles of SiO2 in C2F6/Ar plasma etching were investigated with the MCFPM. 

•	 The MCFPM model predicts the time and spatially dependent microscale 

processes which produce etch profiles. 

CF2 CF3

+ Ar+ 

CO2 

SiF4 

• The Plasma Chemistry Monte Carlo Model 

(PCMCS) uses HPEM results to produce PR reactive fluxes (energy and angle) to the 

surface. 

SiO • The MCFPM uses these fluxes to implement 2 

a user defined reaction mechanism. 

Si 

G(q, E) HPEM	 PCMCS MCFPM

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8

TAPERING OF PROFILES
 

•	 In high aspect ratio (AR) etching of SiO2 by fluorocarbon plasmas, the sidewall of 

trenches are passivated by neutrals (CFx, x £ 2) due to the broad angular 

distributions of neutral fluxes. 

•	 Tapered trench profiles are produced when the passivation/ion flux ratio is large. 

Experiment Simulation 

0.25 um 0.25 um 

SiO SiO2 2 

•	 Ar/C2F6 = 20/80. 

•	 1000 W ICP power, 

150 V bias. 

•	 10 mTorr. 

•	 Radial location: 3 cm. 

AR = 10:1 

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AR = 10:1
 

(C. Cui, AMAT) 
PASSIVATION/ION FLUX RATIO
 

•	 Increasing passivating neutral to ion flux ratio (Gn/Gion) leads to more tapered 

profiles due to increasing sidewall passivation. 

•	 When the passivating neutral flux is too small, insufficient sidewall protection 

by the passivation layer leads to a bowed profile. 

Bowed Straight	 Tapered Tapered 

PR 

SiO2 

Normalized Gn/Gion 100% 125%	 150% 175% 

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INFLUENCE OF ION ENERGY
 

• With increasing ion energy, the increasing ion sputtering yield of the sidewall 

passivation layer produces a less tapered profile. 

• The etch rate also increases with increasing ion energy due to decreasing (but 

sufficient) passivation. 

• Simulations and experiments obtained similar trends. 

Experiments Simulations 

SiO2 SiO2

800 W 1000 W 150 V 180 V

 Bias Power Bias Voltage 

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1.0 1.2 1.4 1.6 1.8 2.0

INFLUENCE OF ION ENERGY (cont.)
 

•	 When the ion energy is very high, the SiO2 etching is limited by the passivation 

instead of the ion sputtering yield. 

•	 The etch rate and the bottom width of the trench are saturated at high ion energies. 

Wb: Trench width 0.5 mm

 above the bottom. 

1.0 3.5

Wb/Wt 

Depth 0.8 

0.6 

0.4 

0.2 

0.0 

150 180 210 240 270 300 

Substrate Bias (V) 

Wt: Trench width at the top. 3.0 

2.5
 Depth: Trench depth after

 equal etch times.
 

W

b / Wt 

Depth (

m

2.0
 

m)

1.5
 

1.0 

0.5 

0.0 

Wt 

W

Depth

b 

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SUMMARY
 

• The polymeric passivation at the surface in fluorocarbon plasma etching of SiO2 

strongly influences the process kinetics and feature profiles. 

• With increasing substrate bias, the passivation thickness on the wafer 

decreases due to increasing ion sputtering, resulting in increasing etch rates 

and less tapered profiles. 

• At high ion energies, sufficient passivation is required for the wafer etching. 

• Processes with high passivating neutral to ion flux ratios produce tapered 

profiles. Both the bottom critical dimension and the etch rate decrease with 

increasing ratio of passivating neutral to ion fluxes. 

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