46 th AVS International Symposium Oct. 25-29, 1999 Seattle, WA INVESTIGATION of Si and SiO 2 ETCH MECHANISMS USING an INTEGRATED SURFACE KINETICS MODEL Da Zhang* and Mark J. Kushner** *Department of Materials Science and Engineering **Department of Electrical and Computer Engineering University of Illinois Urbana, IL 6181 email: dazhang@uiuc.edu mjk@uiuc.edu Work supported by SRC and LAM Research Corporation.
AGENDA Introduction Description of the Model - Hybrid Plasma Equipment Model (HPEM) - Surface Kinetics Module (SKM) C 2 F 6 etching of Si - Reactor wall temperature C 2 F 6 etching of SiO 2 - Substrate bias Summary AVS99-1
INTRODUCTION Fluorocarbon (C m F n ) plasmas are widely used for Si/SiO 2 etching due to their favorable kinetic properties and high 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 goal: - Develop a surface reaction model - Couple the surface model with the bulk plasma simulator - HPEM - Quantitatively investigate the surface reaction mechanisms - Optimize the processes GEC99-2
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) m(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 GEC99-3
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 etch rates Reaction Mechanism Φ i HPEM (1-S i )Φ i SKM Etch Rate f j Φ j The SKM is a multi-layer surface site balance model at every mesh point along the plasma-surface boundary. GEC99-4
C m F n PLASMA: SURFACE REACTIONS HPEM Surface Reaction Mechanism Incident Species/Fluxes Initial Conditions Surface Composition SKM Reflecting Species/Fluxes Surface Species/Coverage Deposition/Etching Rate AVS99-5
ENERGY DEPENDENCE OF REACTION PROBABILITY All surface reactions involving ion reactants in the SKM allow for ion energy dependence. Ions are accelerated to the surface through the sheath, arriving on the surface with energy of E ion = Q f(r) V sh (r) where Q: Ion charge. f(r): Ratio of ion mean free path to sheath thickness. V sh (r): Sheath voltage drop. Surface reactions have a general energy dependence given by K = K (E ion n - E thn ) / (E ref n - E thn ) where E ion : Incident ion energy. K : Etching yield or reaction probability for ion with energy E ref. E th : Threshold energy for the process. n: Energy dependence (1/2 for this work). GEC99-6
POLYMER LAYER PASSIVATION During Si/SiO 2 etching by fluorocarbon plasmas, a C m F n polymer layer will simultaneously deposit on the surface of the wafer and the reactor wall. The SKM allows the growth of the passivation layer by CF x radical polymerization on the surface. F atoms etching and energetic ion sputtering can consume the layer. The steady state passivation thickness is reached when the film generation and consumption rates are balanced. SKM uses passivation thickness dependent rates for mass and energy transfers through the layer. AVS99-7
C 2 F 6 ETCHING of Si Simulations of C 2 F 6 etching of Si in an ICP reactor were performed. Representative gas phase reactions: e + C 2 F 6 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 + 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 C 2 F 5 + F CF 3 + CF 3 Height (cm) 1 5 rf bias Electron Density Coils Showerhead (C 2 F 6 ) Wafer Wall 5 1 15 Radius (cm) 1.9E11 3.61E9 (cm -3 ) CF 3 + F CF 4 CF 2 + F CF 3 C 2 F 6, 1 mtorr, 2 sccm, 65 W ICP, 1 V bias AVS99-8
REACTION MECHANISM FOR C 2 F 6 ETCHING OF Si The reaction mechanism was based on the work of G. S. Oehrlein et al *. A C x F y polymer layer is formed on the Si surface in coincidence with Si etching. The steady state passivation layer thickness is a balance of CF x deposition, ion sputtering and F etching of the layer. Si etching precursor (F) needs to diffuse through the passivation layer. CF x F I + (Accelerated) Film Growth CF 4 SiF 4 Film Etch F Diffusion F Si Etch CF x Sputtering Plasma C x F y Passivation Layer Si AVS99-9 * T.E.F.M. Standert, M.Scharkens, N.R.Rueger, P.G.M. Sebel, and G.S. Oehrlein, J. Vac. Sci. Technol. A 16(1), 239 (1998)
RADICAL DENSITIES: BASE CASE Radical densities peak on the axis. The less uniform CF 2 density distribution is due to its higher loss rate at the reactor wall. CF 2 Density F Density 4.39E13 1.67E14 Height (cm) 1 5 Height (cm) 1 5 5 1 15 Radius (cm) 7.31E11 2.79E12 (cm -3 ) (cm -3 ) 5 1 15 Radius (cm) C 2 F 6, 1 mtorr, 2 sccm, 65 W ICP, 1 V bias. AVS99-1
RADIAL DISTRIBUTIONS: FLUXES & ETCH RATE Radial flux distribution of CF 2 is less uniform than that for F as a consequence of the low sticking coefficient for F. Polymer thickness follows the CF 2 flux for this case. Etch rate increases in the radial direction, inversely to the polymer thickness due to F atom diffusion. F, CF 2 Fluxes (cm -2 s -1 ) 6 5 4 3 2 1 CF3 + (1 16 ) CF 2 (1 16 ) F (1 17 ) 1.5 1.25 1..75.5.25. 2 4 6 8 1 Radius (cm) CF 3 + Fluxes (cm -2 s -1 ) Etch Rate (A/min) 8 7 6 5 Polymer Etch Rate 4 2 4 6 8 1 Radius (cm) 6 5 4 3 2 1 Polymer Layers AVS99-11
INFLUENCE OF WALL TEMPERATURE Experiments showed a variation of radical densities vs. T wall.* We simulated the consequence of varying T wall by modifying the sticking coefficient of CF 2 to the wall. With increasing T wall, the CF 2 loss rate is smaller due to the lower sticking coefficient, which produces an increase of CF 2 density in the bulk plasma. The resulting gas chemistry favors consumption of F atoms. So F density drops. Normalized Intensity 1.2 1.1 F 1..9 Experiment*.8.7 CF 2.6 5 1 15 2 Wall Temperature ( o C) Normalized Density 1.2 1.1 F 1..9 Simulation.8.7 CF2.6..2.4.6.8 1. 1-Sticking Coef.(CF 2 ) T wall * M.Scharkens, R.C.M. Bosch, and G.S. Oehrlein, J. Vac. Sci. Technol. A 16(1), 239 (1998) AVS99-12
PASSIVATION LAYER AND ETCH RATE As wall temperature increases, the increased CF 2 density produces larger fluxes to the wafer. This leads to a thicker passivation layer. The diffusion flux of the Si etching precursor (F) through the passivation layer decreases with increasing passivation, so the etch rate drops. Passivation Layers 12 1 8 6 4 2 Φ-CF 2 Passivation T wall..2.4.6.8 1. 1-Sticking Coef.(CF 2 ) 5 4 3 2 1 Φ-CF 2 (1 17 cm -2 -S -1 ) Etch Rate (Å/min) 4 T wall 3 2 1..2.4.6.8 1. 1-Sticking Coef.(CF 2 ) GEC99-13
REACTION MECHANISM FOR C 2 F 6 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. AVS99-14 Plasma C x F y Passivation Layer SiO 2 CF x CF x Film Growth F F CF 4 Film Etching SiO 2 Ι + CF x CF 4 Sputtering F CO 2 Energy Transfer SiF 4 Diffusion F SiF x Etch CF x Ι+ CO 2 Ι +, F SiO 2 SiF x CO 2 SiF x SiF 4 Etch
SURFACE COVERAGE The wafer surface sites are occupied by several surface species. The surface coverages at steady state depend on the relative rates of - complex formation - Ion activation - F atom etching - Sputtering.5 Fractional Coverage.4.3.2.1 SiF 3 SiO 2 SiF 2 SiF 2 CO 2 C 2 F 6, 1 mtorr 1 sccm 65 W ICP 1 V bias. 2 4 6 8 Radius (cm) AVS99-15
SUBSTRATE BIAS With increasing substrate bias, the passivation layer thickness decreases and the etch rate increases. Simulations and experiments obtained similar trends. Passivation Layers AVS99-16 5 4 3 2 1 C 2 F 6, 1 mtorr Rate Simulation Passivation 5 1 15 2 25 3 Substrate Bias (V) 7 6 5 4 3 2 1 Etch Rate (nm/min) Passivation Thickness (nm) CHF 3, 1 mtorr * Substrate Bias (V) Rate Experiment Passivation *N.R. Rueger, G.S. Oehrlein et al, J. Vac. Sci. Technol. A 15, 1881 (1997) Etch Rate (nm/min)
SUBSTRATE BIAS With increasing bias, the variation in surface coverage is not big when the processes are not limited by the F atom diffusion. The normalized reactive ion etch efficiency increases with increasing substrate bias due to the increase in sheath voltage and decrease in passivation thickness. Fractional Coverage.5.4.3.2 SiF 3 _s SiO 2 _s.1 4 RIE Efficiency 2 5 1 15 2 25 3 Substrate Bias (V) 14 12 1 8 6 Normalized RIE Efficiency Normalized RIE Efficiency V sheath 2 ( 1+.3 [ P _ s]) AVS99-17
SUMMARY The Surface Kinetics Model (SKM) has been coupled with the HPEM to simulate surface interactions and their influences on the bulk plasma. In Si etching, higher loss of CF 2 on reactor walls leads to lower CF 2 density in the gas phase. This produces thinner passivation layers and higher etch rates. Consumption of C x F y at the polymer-sio 2 interface during the SiO 2 etching leads to thinner passivation compared with Si etching. In SiO 2 etching, with increasing substrate bias, the surface coverages change little when the process is not limited by F diffusion. Increasing bias leads to higher sheath voltage and thinner passivation, so higher RIE efficiency. The etch rate increases accordingly. AVS99-18