magnetically enhanced multiple frequency capacitively coupled plasmas: dynamics and strategies yang...

MAGNETICALLY ENHANCED MULTIPLE FREQUENCY CAPACITIVELY COUPLED

PLASMAS: DYNAMICS AND STRATEGIES

Yang Yang and Mark J. KushnerIowa State University

Department of Electrical and Computer EngineeringAmes, IA 50011

http://uigelz.ece.iastate.edu [email protected]

October 2005

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http://uigelz.ece.iastate.edu/

Iowa State University

Optical and Discharge Physics

AGENDA

Introduction to Magnetically Enhanced Reactive Ion Etching (MERIE) reactors and two-frequency plasma sources.

Description of Model

Scaling parameters for single frequency MERIE

Scaling of 2f-MERIE Properties

Concluding Remarks

Acknowledgement: Semiconductor Research Corp., National Science Foundation, Applied Materials Inc.

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MERIE PLASMA SOURCES

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Magnetically Enhanced Reactive Ion Etching plasma sources use transverse static magnetic fields in capacitively coupled discharges for confinement to increase plasma density.

D. Cheng et al, US Patent 4,842,683 M. Buie et al, JVST A 16, 1464 (1998)



SCALING OF MERIE SYSTEMS

• General scalings: More confinement due to B-field has geometric and kinetics effects.

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• More positive bias with B-field• G. Y. Yeom, et al JAP 65, 3825 (1989)

• Larger [e], Te with B-field• S. V. Avtaeva, et al JPD 30, 3000 (1997)



MULTIPLE FREQUENCY CCPs

• Dual frequency CCPs: goals of separately controlling fluxes and ion energy distributions; and providing additional tuning of IEDs.

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• Even with constant LF voltage, IEDs depend on HF properties due to change in sheath thickness and plasma potential

• V. Georgieva and A. Bogaerts, JAP 98, 023308 (2005)

• Ar/CF4/N2=80/10/10, 30 mTorr



MULTIPLE FREQUENCY MERIEs

• Question to answer in this presentation:

• What unique considerations come to light when combining magnetic enhancement, such as in a MERIE, with dual-frequency excitation?

• Ground Rules:

• A computational investigation to illuminate physics. • Ar only in this presentation. Mixtures for another talk.

• Power vs Voltage is important! We are varying power not voltage.

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MODELING OF DUAL FREQUENCY MERIE

2-dimensional Hybrid Model

Electron energy equation for bulk electrons

Monte Carlo Simulation for high energy secondary electrons from biased surfaces

Continuity, Momentum and Energy (temperature) equations for all neutral and ion species.

Poisson equation for electrostatic potential

Circuit model for bias

Monte Carlo Simulation for ion transport to obtain IEADs

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ELECTRON ENERGY TRANSPORT

S(Te) = Power deposition from electric fields

L(Te) = Electron power loss due to collisions

= Electron flux(Te) = Electron thermal conductivity tensorSEB = Power source source from beam electrons

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PLASMA CHEMISTRY, TRANSPORT AND ELECTROSTATICS

Continuity, momentum and energy equations are solved for each species (with jump conditions at boundaries)

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Implicit solution of Poisson’s equation

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Poisson’s equation is solved using a semi-emplicit technique where charge densities are predicted at future times.

Predictor-corrector methods are used where fluxes at future times are approximated using past histories or Jacobian elements are used.

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IMPROVEMENTS FOR LARGE MAGNETIC FIELDS

Iowa State UniversityOptical and Discharge Physics

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MERIE REACTOR

The model reactor is based on a TEL Design having a transverse magnetic field.

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K. Kubota et al, US Patent 6,190,495 (2001)



MERIE REACTOR: MODEL REPRESENTATION

2-D, Cylindrically Symmetric

Magnetic field is purely radial, an approximation validated by 2-D Cartesian comparisons.

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RADIUS (cm)0 10 20

HE

IGH

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cm)

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Conductive Wafer

B-Field



MERIE: Ar+ DENSITY vs MAGNETIC FIELD

Ar, 40 mTorr, 100 W, 10 MHz

Increasing B-field shifts plasma towards center and increases density.

Large B-fields (> 100 G) decrease density.

Plasma is localized closer to wafer.

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The localization of plasma density near the powered electrode with large B-fields is due to the confinement of secondary electrons and more localized heating of bulk electrons.

MERIE: CONFINEMENT OF IONIZATION

Ionization by secondary electrons is uniform across the gap at low B-field; localized at high B-field.

Ar, 40 mTorr, 100 W, 10 MHzGEC05_MJK_14

Secondary Electrons Bulk Electrons

Ionization Sources



MERIE: SHEATH REVERSALAND THICKENING


As the magnetic field increases, the electrons become less mobile than ions across the magnetic field lines.

The result is a reversal of the electric field in the sheath and sheath thickening.

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The dc bias generally becomes more positive with increasing B-field as the mobility of electrons decreases relative to ions.

Constant power, decreasing ion flux, increasing bias voltage More resistive plasma.

VPlasma – Vdc decreases with bias (sheath voltage….)

MERIE dc BIAS,RF VOLTAGE


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Optical and Discharge PhysicsGEC05_MJK_17

Ar+ ENERGY AND ANGLE DISTRIBUTIONS


The more positive dc bias reduces the sheath potential.

The resulting IEAD is lower in energy and broader.



2 FREQUENCY MERIE: GEOMETRY

Ar, 40 mTorr, 300 sccm

B (radial)

Base Case Conditions:

Low Frequency: 5 MHz, 500 W

High Frequency: 40 MHz, 500 W

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2-FREQUENCY CCP (B=0): ELECTRON SOURCES

Mean free paths are long and thermal conductivity is high (and isotropic).

Te is nearly uniform over wafer. Bulk ionization follows electron density.

Secondary electrons penetrate through plasma.

Ar, 40 mTorr, 300 sccm, 0 G, 5 MHz, 40 MHz LF: 500W, 193 V (dc: -22 V) HF: 500 W, 128 V

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2-FREQUENCY MERIE (B=150G): ELECTRON SOURCES

Short transverse mean free paths (anisotropic transport).

Te , bulk ionization peak in sheaths; convect in parallel direction.

Secondary electrons are confined near sheath (trapping on B-field).

dc bias more positive; voltages larger.

Ar, 40 mTorr, 300 sccm, 150 G, 5 MHz, 40 MHz

LF: 500W, 202 V (dc: -1 V) HF: 500 W, 140 V



ION DENSITIES: 2f-CCP vs 2f-MERIE

MERIE achieves goal of increasing ion density due to confinement of beam electrons and slowing transverse diffusion loss.

Spatial distribution changes due to both transport and materials effects.

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B = 0 G (max 9 x 1010 cm-3)

Ar, 40 mTorr, 300 sccm, 5 MHz, 40 MHz LF: 500W, HF: 500 W

B = 150 G (max 1.3 x 1012 cm-3)



2-FREQUENCY CCP (B=0): PLASMA POTENTIAL

Sheaths maintain electropositive nature through LF and HF cycles.

Bulk plasma potential is nearly flat and oscillates with both LF and HF components.

Ar, 40 mTorr, 0 G, 5 MHz, 40 MHz LF: 500W, 193 V (dc: -22 V) HF: 500 W, 128 V

Time dependent Low Frequency High Frequency

2-FREQUENCY MERIE (B=150G): PLASMA POTENTIAL

Iowa State UniversityOptical and Discharge PhysicsGEC05_MJK_23

Sheaths are reversed through portions of both LF and HF cycles.

Bulk electric field is significant to overcome low transverse mobility. Plasma potential oscillates with both LF and HF components.

Ar, 40 mTorr, 150 G, 5 MHz, 40 MHz LF: 500W, 202 V (dc: -1 V) HF: 500 W, 140 V

Time dependent Low Frequency High Frequency



2f-CCP vs 2f-MERIE: ION FLUXES

Larger electric fields to transport electrons results in significantly larger variations in ion flux through cycles.

Ar, 40 mTorr, 5 MHz, 40 MHz LF: 500W, HF: 500 W

B = 0 G B = 150 G



MATERIALS AFFECT UNIFORMITY: PLASMA POTENTIAL

Low mobility of electrons prevent “steady state” charging of dielectrics.

Surface potential of dielectrics is out of phase with plasma potential.

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B = 0 G B = 150 G

RADIUS (cm)0 10 20

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IGH

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Conductive Wafer

B-Field

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SECONDARY EMISSION: IMPORTANT TO SCALING

Scaling of ion flux with HF power is sublinear though better w/B-field.

Increasing HF power reduces LF voltage for constant power. Poor utilization of secondary electrons. Power lost to excitation that does not translate to ionization.


B = 0 G B = 100 G



B=0: Increasing produces nominal increase in ion density and decrease in power as secondary electrons are poorly utilized.

B=100 G: Increasing produces more ionization, larger ion density and increase in power.

Ar, 100 mTorr, 10 MHz

PLASMA PARAMETERS: MERIE B=0, 100 G, V=constant

B = 0 B = 100 G

340 V (p-p) 400 V (p-p)

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IEDS vs B-FIELD

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IEDs broaden and move to lower energy with increase in B-field and more positive dc bias.

Reversal of sheaths slows ions, broaden angle.

Ar, 40 mTorr, 300 sccm, 150

G, 5 MHz, 40 MHz LF: 500W HF: 500 W

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IEDS vs LF POWER

Ability to control IED with LF power is compromised in MERIE.

Redistribution of voltage dropped across sheath and bulk

Change in angular distribution.

Ar, 40 mTorr, 300 sccm, 5 MHz, 40 MHz HF: 500 W

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Maximum ion energy is V(LF)+V(HF)-V(dc). Increasing HF power increases V(HF) and ion current. For

constant LF power, V(LF) decreases. The maximum IED depends on relative increase in V(HF) and

decrease in V(LF). Except that…..

VOLTAGES vs HIGH FREQUENCY POWER

B = 0

Ar, 40 mTorr, 5 MHz, 40 MHz LF: 500W

B = 100 G

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More resistive plasma and field reversal in HF sheath consum voltage otherwise be available for ion acceleration in LF sheath.

The result is a decrease in sheath voltage with a B-field.

Ar, 40 mTorr, 5 MHz, 40 MHz LF: 500W

VOLTAGES vs HIGH FREQUENCY POWER

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LF Sheath Potential B = 100 G



It appears that ability to maintain IED while changing HF power is better without B-field.

That is generally true….but you just got lucky.

IEDs vs HIGH FREQUENCY POWER

B = 0 B = 150 G

Ar, 40 mTorr, 5 MHz, 40 MHz LF: 500WGEC05_MJK_32



IEDS vs LF FREQUENCYB=0

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IED narrows in energy as LF decreases while maintaining nearly the same average energy.

Scaling does not significantly differ from single frequency system.

Ar, 40 mTorr, 300 sccm, LF: 500 W HF 40 MHz: 500 W



PLASMA POTENTIAL vsLF FREQUENCY (B=100 G)

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As the low frequency increases…

The fraction of the cycle during which the LF sheath is reversed increases.

Field reversal occurs in the bulk as well as sheath to attract sufficient electrons across B-field.

More phase dependent.

Ar, 40 mTorr, 300 sccm, LF: 500 W HF 40 MHz: 500 W

LF = 2.5 MHz

LF = 40 MHz



IEDS vs LF FREQUENCYB=100 G

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As the low frequency increases…

The window for allowing ions out of plasma narrows.

The IED narrows and broadens to a greater degree than without B-field.

Ar, 40 mTorr, 300 sccm, 4 MHz, 40 MHz HF: 500 W



CONCLUDING REMARKS

Scaling laws for an industrial MERIE reactor using 2-frequency excitation were investigated.

Reversal of sheaths LF and HF electrodes dominate behavior.

IED shifted to lower energy Broadened in angle Increasing (more positive) bias

Sensitivity to sheath reversal increases with increasing LF.

Ability to maintain constant IED when varying HF power is diminished in MERIE system

Larger voltage drop across bulk plasma and HF sheath leaves less voltage at LF electrode.

Larger plasma resistance with B-field increases RC time constant for charging surfaces thereby impacting uniformity.

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magnetically enhanced multiple frequency capacitively coupled plasmas: dynamics and strategies yang...

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