27th IEEE International Conference on Plasma Science New Orleans, Louisiana June 4-7, 2000 SURFACE AND GAS PHASE REACTIONS FOR FLUOROCARBON PLASMA ETCHING OF SiO 2 Da Zhang* and Mark J. Kushner** *Department of Materials Science and Engineering **Department of Electrical and Computer Engineering 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 C 2 F 6 etching of SiO 2 Surface Passivation Substrate Bias Profile Simulations for C 2 F 6 Etching of SiO 2 Monte Carlo Feature Profile Model (MCFPM) Ion to Neutral Flux Ratio Ion Energy Summary DA-ICOPS01
INTRODUCTION Fluorocarbon (C m F n ) plasmas are widely used for SiO 2 /Si etching due to their high rate and good selectivity. CF x 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). DA-ICOPS02
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- Magnetic Module (EMM) E(r,z) B(r,z) Electron Energy Transport Module (EETM) S(r,z), Te(r,z) µ(r,z) Fluidchemistry Kinetics Module (FKM) V(r,z) n(r,z) F(r,z) V(r,z) n(r,z) F(r,z) VPEM: Sensors, Controller, Actuators DA-ICOPS03
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 Φ i HPEM (1-S i )Φ i SKM Process Rate f ij Φ ij The SKM uses a multi-layer surface site balance model at every mesh point along the plasma-surface boundary. DA-ICOPS04
C 2 F 6 PLASMA ETCHING OF SiO 2 The reaction mechanism for SiO 2 etching is based on: Growth of C x F y passivation layer (balance of deposition and consumption). Formation of complex at the interface between oxide and passivation layer resulting from chemisorption of CF x. Ion activated (through passivation layer) etching of complex. Rate of activation scales inversely with passivation layer thickness. Diffusion of etch precursor and etch product. Plasma C x F y Passivation Layer SiO 2 CF x CF x Film Growth Ι + CF 4 F Sputtering F SiF 4 CF 4 CO 2 CO F 2 Ι+ SiF 4 Film Energy CF x Diffusion Etching Transfer Ι +, F CF x F SiO 2 SiF x Etch SiO 2 SiF x CO 2 SiF x Etch DA-ICOPS05
SiO 2 ETCH REACTION MECHANISM Simulations of C 2 F 6 etching of SiO 2 in an ICP reactor were performed. Representative gas phase reactions: e + C 2 F 6 CF 3 + + CF 3 + e + e Electron Density e + CF 3 CF + 3 + e + e e + C 2 F 6 CF 3 + CF 3 + e e + CF 3 CF 2 + F + e e + CF 3 CF 2 + F + F- e + CF 4 CF 2 + F + F + e e + CF 4 CF 3 + F + e F + F + M F 2 + M CF 3 + CF 3 + M C 2 F 6 + M CF 2 + F 2 CF 3 + F CF 3 + F 2 CF 4 + F CF 2 + CF 3 C 2 F 5 Height (cm) 10 5 0 0 rf bias Coils Showerhead (C 2 F 6 ) Wafer Wall 5 10 15 Radius (cm) 1.90E11 3.61E9 (cm -3 ) C 2 F 5 + F CF 3 + CF 3 C 2 F 6, 10 mtorr, 200 sccm, 650 W ICP, 100 V bias DA-ICOPS06
RADICAL DENSITIES Less uniform CF 2 density distribution due to its higher loss rate at the reactor wall. CF 2 Density F Density 4.39E13 1.67E14 Height (cm) 10 5 Height (cm) 10 5 0 0 5 10 15 Radius (cm) 7.31E11 2.79E12 0 (cm -3 ) (cm -3 ) 0 5 10 15 Radius (cm) C 2 F 6, 10 mtorr, 200 sccm, 650 W ICP, 100 V bias. DA-ICOPS07
RADIAL DISTRIBUTIONS Radial flux distribution of CF 2 is less uniform than that of F as a consequence of the of the density distributions. Polymer distribution follows the CF 2 flux distribution for this case. Etch rate increases in the radial direction, inverse to the behavior of polymer distribution. F, CF 2 Fluxes (cm -2 s -1 ) 6 5 4 3 2 1 CF3 + (10 16 ) CF 2 (10 16 ) F (10 17 ) 1.50 1.25 1.00 0.75 0.50 0.25 0 0.00 0 2 4 6 8 10 Radius (cm) CF 3 + Fluxes (cm -2 s -1 ) Etch Rate (A/min) 800 700 600 500 POLYMER ETCH RATE 400 0 0 2 4 6 8 10 Radius (cm) 6 5 4 3 2 1 Polymer Layers C 2 F 6, 10 mtorr, 200 sccm, 650 W ICP, 100 V bias. DA-ICOPS08
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 (CF x, x 2) is required to increase the etch rate. 3.0 700 Passivation Layers 2.5 Etch Rate 600 2.0 500 1.5 : Γ n /Γ ion = 35. 400 1.0 : Γ n /Γ ion = 25. 300 0.5 Passivation 200 0.0 0 50 100 150 200 250 300 100 350 Etch Rate (nm/min) Substrate Bias (V) DA-ICOPS09
MONTE CARLO FEATURE PROFILE MODEL (MCFPM) Profiles of SiO 2 in C 2 F 6 /Ar plasma etching were investigated with the MCFPM. The MCFPM model predicts the time and spatially dependent microscale processes which produce etch profiles. CF 2 CF 3 + Ar + PR The Plasma Chemistry Monte Carlo Model (PCMCS) uses HPEM results to produce reactive fluxes (energy and angle) to the surface. SiF 4 CO 2 SiO 2 The MCFPM uses these fluxes to implement a user defined reaction mechanism. Si Γ(θ, E) HPEM PCMCS MCFPM DA-ICOPS10
TAPERING OF PROFILES In high aspect ratio (AR) etching of SiO 2 by fluorocarbon plasmas, the sidewall of trenches are passivated by neutrals (CF x, 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 0.25 um Simulation 0.25 um SiO 2 SiO 2 Ar/C 2 F 6 = 20/80. 1000 W ICP power, 150 V bias. 8 10 mtorr. Radial location: 3 cm. DA-ICOPS11 AR = 10:1 (C. Cui, AMAT) AR = 10:1
PASSIVATION/ION FLUX RATIO Increasing passivating neutral to ion flux ratio (Γ n /Γ ion ) 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 SiO 2 Normalized Γ n /Γ ion DA-ICOPS12 100% 125% 150% 175%
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 SiO 2 SiO 2 DA-ICOPS13 800 W 1000 W 150 V 180 V Bias Power Bias Voltage (C. Cui, AMAT)
INFLUENCE OF ION ENERGY (cont.) When the ion energy is very high, the SiO 2 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 / Wt 1.0 3.5 0.8 Depth 3.0 2.5 0.6 2.0 W b /W t 0.4 1.5 1.0 0.2 0.5 0.0 0.0 150 1.0 1.2 180 1.4 210 240 1.6 270 1.8 300 2.0 Depth (µm) W b : Trench width 0.5 µm above the bottom. W t : Trench width at the top. Depth: Trench depth after equal etch times. W t W b Depth Substrate Bias (V) DA-ICOPS14
SUMMARY The polymeric passivation at the surface in fluorocarbon plasma etching of SiO 2 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. DA-ICOPS15