Practical Scaling of Multi-Frequency Capacitive Discharges for Etch Applications

Transcription

1 Practical Scaling of Multi-Frequency Capacitive Discharges for Etch Applications Dan Hoffman, Valery Godyak, Jang Gyoo Yang, Steven Shannon Etch Product Business Group Applied Materials, Inc 2005 IEEE International Conference on Plasma Science

2 2 Multifrequency RF Plasma Configurations Very high frequency / low frequency combination independent control of bulk and sheath properties High frequency / low frequency combination ion energy distribution control on the wafer surface Very high frequency / high frequency / low frequency combination independent control of ionization, sheath potential, and ion energy distribution on the wafer Current Presentation overview Overview of RF model used to characterize three frequency systems RF analysis of plasmas generated at various powers and pressures Characterization of three-frequency systems using a matrix that encompasses each of the above configurations

3 3 Scientific methodology (bias power/source power/pressure window) VHF NO PLASMA 1-Port Impedence Sweep and Time Domain Reflectometry Solve for Plasma Impedences Calculate Plasma Parameters Develop Empirical Models Based on RF measurements (V, I) Ar, O 2, C 4 F 6 2 MHz 1356 MHz Pressures = 10mTorr 200mTorr Over 1100 different frequency, power and pressure combinations studied Matchbox with VI Probe Measure VI for Process Window RF Generator Matchbox with VI Probe Control Plasma Parameters BasedonEmpiricalModels and VI Measurements RF Generator

4 4 Plasma Model Homogeneous bulk plasma model Spatial variation ignored Electron temperature assumed constant for all runs (5eV Maxwellian distribution) Bulk ohmic heating only Sharp boundary sheath model oscillating vacuum capacitor approximation for frequencies > 10MHz, parallel resistive element incorporated for lower frequencies 3πX s ω s0 Sheath Impedance: Rs = X s = 2 ω ωε Area Bulk Impedance: Z p = jωc 0 i 1 jωl + Voltage and current across the entire discharge can be determined by RF measurement and chamber characterization Plasma parameters can be determined by either discharge impedance or voltage and current across the discharge + p R p 0 1

5 5 Multi-frequency scaling laws are needed Understanding power deposition mechanisms in a multifrequency plasma Understanding interaction between frequencies that impact plasma parameters Developing empirical models for process control Tracking plasma parameters (electron density, sheath potential, etc) Using plasma parameters for repeatable chucking performance, plasma trend with on-wafer results, etc Each of the current multi-frequency configurations will be studied using a simple plasma model Plasma parameters obtained from the model will be compared to measurements made on an industrial, three frequency (2MHz / 13MHz / VHF) plasma etcher

6 6 Configuration A Very High Frequency / Low Frequency Dual Frequency Combination Electron Density (Arb Units) Source Power (Arb Units) Electron Density (Arb Units) Bias Power (Arb Units) RF Voltage (Arb Units) RF Voltage (Arb Units) Source Power (Arb Units) Bias Power (Arb Units)

7 7 Combination A control independent control of bulk and sheath characteristics true to the first order, not true for all sheath characteristics relevant for processing Distribution (arb units) τ = i 1 6ε 0m e n e i s high = J 2πen high e f high Distribution (arb units) Distribution (arb units) Distribution (arb units) Energy (ev) Energy (ev) Increasing electron density Increasing DC Potential Distribution (arb units) Distribution (arb units) Energy (ev) Energy (ev) Energy (ev) Energy (ev)

8 8 Combination B Dual Low Frequency Control Ion Energy Probabiliy Function Energy (ev) Increasing Power Electron Density (cm^-3) Fraction of Low Frequency Current x DC Potential (V) 0 S0 (AU) Sheath Thickness (cm) VDC (AU) ne (AU) Increasing Power Combination of Frequencies on both sides of ion transit frequency allow for control of IEDF with bulk plasma condition MHz Current / Total Bias Current (%) Frac LF Current

9 9 Density Creation vs Frequency: Bias power is used to control sheath voltage (ion energy) Above 13 MHz, substantial power is devoted to density generation instead of power for the sheath The additional density further loads the cathode and reduces high frequency operating voltages Power in sheath Power that creates density frequency Region where ion density creation dominates Region where ion energy mingles with density creation Region where ion energy creation dominates

10 10 What this means for process n e VHF / LF combination Three Frequency n e Configuration E ion E ion Dual Frequency Sheath Modulation E ion E ion

11 11 Dual Bias 2/13 MHz Power Scan with no Source: Analysis where Both Frequencies in Ion Energy Mode Energy Bulk Density Energy P 2MHz =100% P 2MHz =98% P 2MHz =90% P 2MHz =80% P 2MHz =60% P 2MHz =40% P 2MHz =20% P 2MHz =0% Movement in Parameter Space Is along Ion Energy Plane Only

12 12 Dual Bias Frequency with in Mixed Mode: One Frequency Energy-Dominant, One Frequency Mixed Density/Energy Bulk Density Energy P 2MHz =100% P 2MHz =98% P 2MHz =90% P 2MHz =80% P 2MHz =60% P 2MHz =40% P 2MHz =0% Energy Upper Frequency Limit for Independent Control of IEDF based on % of bulk plasma power coupling

13 13 Combining Source with Dual Frequency Bias Dual Bias Alone Increasing Pressure ( vlow, vhigh, n), ( eminopt, emaxopt, nopt ) ( vlow, vhigh, n), ( eminopt, emaxopt, nopt ) ( vlow, vhigh, n), ( eminopt, emaxopt, nopt ) Dual Bias with VHF Source Increasing Pressure ( vlow, vhigh, n), ( eminopt, emaxopt, nopt ) ( vlow, vhigh, n), ( eminopt, emaxopt, nopt ) ( vlow, vhigh, n), ( eminopt, emaxopt, nopt ) ( vlow, vhigh, n), ( eminopt, emaxopt, nopt ) ( vlow, vhigh, n), ( eminopt, emaxopt, nopt )

14 14 Conclusions 2 frequency configurations are good for first-order independent control of discharge parameters, but can still present second order interactions that cannot be corrected for by frequency selection A three frequency configuration can correct for these second order effects, providing better independent control of discharge parameters and an additional degree of tunability (IEDF control) The trends in electron density, sheath potential, and IEDF shape have been determined for these configurations using RF impedance measurement of the plasma discharge and simple plasma models Empirical models based on these trends have been applied to commercially available tools for enhanced process control and a wider window of operability

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