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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 mjk@iastate.edu
October 2005
GEC05_MJK_01
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.
GEC05_MJK_02
Iowa State University
Optical and Discharge Physics
MERIE PLASMA SOURCES
GEC05_MJK_03
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)
Iowa State University
Optical and Discharge Physics
SCALING OF MERIE SYSTEMS
• General scalings: More confinement due to B-field has geometric and kinetics effects.
GEC05_MJK_04
• 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)
Iowa State University
Optical and Discharge Physics
MULTIPLE FREQUENCY CCPs
• Dual frequency CCPs: goals of separately controlling fluxes and ion energy distributions; and providing additional tuning of IEDs.
GEC05_MJK_05
• 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
Iowa State University
Optical and Discharge Physics
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.
GEC05_MJK_06
Iowa State University
Optical and Discharge Physics
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
GEC05_MJK_07
Iowa State University
Optical and Discharge Physics
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
GEC05_MJK_08
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zrrz2r
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All transport coefficients are tensors:
Iowa State University
Optical and Discharge Physics
PLASMA CHEMISTRY, TRANSPORT AND ELECTROSTATICS
Continuity, momentum and energy equations are solved for each species (with jump conditions at boundaries)
GEC05_MJK_ 09
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
materialsi
jions,e
ii
mg
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GEC05_MJK_10
Iowa State University
Optical and Discharge Physics
MERIE REACTOR
The model reactor is based on a TEL Design having a transverse magnetic field.
GEC05_MJK_11
K. Kubota et al, US Patent 6,190,495 (2001)
Iowa State University
Optical and Discharge Physics
MERIE REACTOR: MODEL REPRESENTATION
2-D, Cylindrically Symmetric
Magnetic field is purely radial, an approximation validated by 2-D Cartesian comparisons.
GEC05_MJK_12
RADIUS (cm)0 10 20
HE
IGH
T (
cm)
4
0
2
Shower Head
PumpFocus RingPowered Substrate
Conductive Wafer
B-Field
Iowa State University
Optical and Discharge Physics
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.
GEC05_MJK_13
Iowa State University
Optical and Discharge Physics
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
Iowa State University
Optical and Discharge Physics
MERIE: SHEATH REVERSALAND THICKENING
Ar, 40 mTorr, 100 W, 10 MHz
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.
GEC05_MJK_15
Iowa State University
Optical and Discharge Physics
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
Ar, 40 mTorr, 100 W, 10 MHz
GEC05_MJK_16
Iowa State University
Optical and Discharge PhysicsGEC05_MJK_17
Ar+ ENERGY AND ANGLE DISTRIBUTIONS
Ar, 40 mTorr, 100 W, 10 MHz
The more positive dc bias reduces the sheath potential.
The resulting IEAD is lower in energy and broader.
Iowa State University
Optical and Discharge Physics
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
GEC05_MJK_18
Iowa State University
Optical and Discharge Physics
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
GEC05_MJK_19
Iowa State University
Optical and Discharge PhysicsGEC05_MJK_20
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
Iowa State University
Optical and Discharge Physics
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.
GEC05_MJK_21
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)
Iowa State University
Optical and Discharge PhysicsGEC05_MJK_22
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
Iowa State University
Optical and Discharge PhysicsGEC05_MJK_24
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
Iowa State University
Optical and Discharge Physics
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.
GEC05_MJK_25
Ar, 40 mTorr, 5 MHz, 40 MHz LF: 500W, HF: 500 W
B = 0 G B = 150 G
RADIUS (cm)0 10 20
HE
IGH
T (
cm)
4
0
2
Shower Head
PumpFocus RingPowered Substrate
Conductive Wafer
B-Field
View
Animation-GIF
Iowa State University
Optical and Discharge PhysicsGEC05_MJK_26
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.
Ar, 40 mTorr, 5 MHz, 40 MHz LF: 500W, HF: 500 W
B = 0 G B = 100 G
Iowa State University
Optical and Discharge Physics
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)
GEC05_MJK_27
Iowa State University
Optical and Discharge Physics
IEDS vs B-FIELD
GEC05_MJK_19
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
GEC05_MJK_28
Iowa State University
Optical and Discharge Physics
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
GEC05_MJK_29
Iowa State University
Optical and Discharge Physics
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
GEC05_MJK_30
Iowa State University
Optical and Discharge Physics
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
GEC05_MJK_31
LF Sheath Potential B = 100 G
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Optical and Discharge Physics
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
Iowa State University
Optical and Discharge Physics
IEDS vs LF FREQUENCYB=0
GEC05_MJK_33
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
Iowa State University
Optical and Discharge Physics
PLASMA POTENTIAL vsLF FREQUENCY (B=100 G)
GEC05_MJK_34
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
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Optical and Discharge Physics
IEDS vs LF FREQUENCYB=100 G
GEC05_MJK_35
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
Iowa State University
Optical and Discharge Physics
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.
GEC05_MJK_36
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