electron transport and power deposition in...
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UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICSRKINDER_AVS_2000_1
Electron Transport and PowerDeposition in Magnetically Enhanced
Inductively Coupled Plasmas
Ronald L. Kinder and Mark J. KushnerUniversity of Illinois, Urbana-Champaign
e-mail : [email protected] [email protected]
http://uigelz.ece.uiuc.edu
Work Supported by :AMAT, SRC, NSF and DARPA/AFOSR
AVS 47th International SymposiumBoston, MA
October 2, 2000
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICSRKINDER_AVS_2000_2
AGENDA
l Motivation
l Plasma Modeling - Hybrid Plasma Equipment Model (HPEM)
l Trikon Mori Helicon Sourcel Validationl Effects of TG Mode
l Analysis of Helicon Component
l Non-Collisional Heatingl Axial Accelerationl Phase Matching
l Conclusions
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
MOTIVATION FOR MAGNETICALLY ENHANCED ICPs
l In order to maintain process uniformity over large areas (> 300 mm) efficient new plasma sources are being developed.
l It is often desirable to produce plasmas in the “volume” of large reactors. This is difficult to accomplish using ICPs due to their finite skin depth.
PLASMA
SUBSTRATE
SOLENOID
B-FIELD ANTENNAE&M WAVE
l Magnetically Enhanced Inductively Coupled Plasma (ME-ICPs) sources are being investigated due to their high ionization efficiency and their ability to deposit power within the volume of the plasma.
l The location of power deposition can substantially vary depending on the mode of operation and reactor conditions.
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UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
PROPERTIES OF MAGNETICALLY ENHANCED ICPs
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l The coupling of electromagnetic fields to the plasma occurs through two channels.
l Helicon Wave l Electrostatic Wave (TG)
l Helicon waves have the property that their parallel phase velocities can be matched to the thermal velocities of 20 - 200 eV electrons.
l Chen and Boswell have suggested Landau damping as a collisionless heating mechanism. If the wave grows fast enough, it can trap thermal electrons and accelerate them to the phase velocity.
l More recently it has been suggested that much of the electron heating comes from the TG component of the wave.
l Here we report on power deposition on MEICPs by these mechanisms.
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
HYBRID PLASMA EQUIPMENT MODEL
l The base two-dimensional HPEM consists of an electromagnetics module (EMM), an electron energy transport module (EETM), and a fluid kinetics simulation (FKS).
l A full tensor conductivity was added to the EMM to calculate 3-d components of the inductively coupled electric field based on 2-d applied magnetostatic fields.
l The plasma current in the wave equation is addressed by a cold plasma tensor conductivity.
l Particle transport:Neutrals: Continuity, Momentum, EnergyIons: Continuity, Momentum, EnergyElectrons: Drift Diffusion, EnergyEEDF: Monte Carlo Simulation
l Potentials: Poisson Equation
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HPEM ELECTROMAGNETICS : TG MODE
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University of Illinois Optical and Discharge Physics
εερ enq
E∆
==⋅∇ ( )ωω
σ
i
q
E
nDamp
e where,
+
⋅⋅∇−
=∆
• If plasma neutrality is not enforced, the divergence term in the waveequation must be included.
! The divergence of the electric field is equal to the wave perturbedelectron density.
• The gradient of the perturbed electron density, represents an effective current sink due to the TG mode.
EiEEE ⋅−=
∇⋅∇−
⋅∇∇ σωεω
µµ 211
TG Wave Helicon Wave
TRIKON MORITM 200 PLASMA TOOL
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
l A commercial Trikon Technologies, Inc., Pinnacle 8000 plasma tool was used to validate the model.
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Nozzle
Wafer
Coils
Bell Jar
Radius (cm)20 20
50
0
Electromagnet
PumpPort
Location ofReference
B Field
0
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
l As static magnetic field increases, the ion saturation current peaks further downstream. Simulations show a similar trend for the ion profile.
l With increasing magnetic field, electric field propagation progressively follows magnetic flux lines and significant power can be deposited downstream.
l Ar, 2.3 mTorr, 1 kW, 50 sccm
ANALYSIS OF TRIKON PLASMA TOOL: VALIDATION
l Ar, 2.3 mTorr, 1 kW (Trikon Technologies, Inc.)
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Theoretical Axial Ion Density
25 G 200 G
0 G
1.0
0
0.5
DownstreamBell Jar
Axial Position (cm)-5 0 5 10 15 20-10
Experimental Axial Ion Saturation Profile
0 G
25 G
Axial Position (cm)-5 0 5 10 15 20-10
50
0
10
20
30
40
200 G
DownstreamBell Jar
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICSl Ar 10 mTorr, 1 kW, 50 sccm, 300 G
l At 300 G, TG mode is strongly damped at the dielectric-plasma surface. The percent of perturbed electrons in this region can reach 1%.
l The TG mode couples power more efficiently per electron produced. At a constant input power, this results in a higher temperature and lower plasma density.
EFFECTS OF TG MODE ON PROPAGATION
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V / cm15 0.15
10-3
107 101
10-7
cm-3 eV3.8 2.0
Eθ (wo / TG) Eθ (w / TG)∆ [e] ∆ [e] / [e] Te (wo / TG) Te (w / TG)
UNIVERSITY OF ILLINOISl Ar 10 mTorr, 1 kW, 50 sccm, 300 G
l Initial studies indicate that the effect of the TG mode is to restructure the power deposition profile near the coils.
l However, the propagation of the helicon component is little affected, particularly at large magnetic fields where the TG modes is damped.
EFFECTS OF TG MODE ON PROPAGATION
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[e] (wo / TG) [e] (w / TG)
(cm-3)5 x 1012 5 x 10101.0 0.01(W / cm-3)
Power (wo / TG) Power (w / TG)
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
l Ar/Cl2 80:20, 10 mTorr,
1 kW, 50 sccm
l For Ar/Cl2, power deposition, at high magnetic fields, cycles back
upstream, resembling an ICP.
l At high enough magnetic fields, the electric field wavelength is larger than the reactor and is unable to sustain a standing wave pattern.
(cm-3)
(W / cm3)1 0.001
ANALYSIS OF PLASMA TOOL : POWER
B = 0 G
[e]Power
B= 250 G
Power
B= 300 G
Power
B= 150 G
Power
B = 40 G
Power [e] [e] [e] [e]
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4 x 1012 4 x 1010
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
l Ar/Cl2 80:20, 10 mTorr, 50 sccm
l Neglecting the TG mode, the ability to deposit power downstream is limited by the wavelength of the helicon-like wave.
l For an m = 0 mode,
ANALYSIS OF HELICON COMPONENT: WAVELENGTH
7.6 x 106
Rcmλz (cm) =
Bo (Gauss)
ne (cm-3)( )
0.6
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Wavelength (cm)
500
1500
0
1000
100 200
Magnetic Field (G)
300
10
30
60
90
120
Helicon
ICP
ICP
0
5
10
15
20
25
30Radius = 4 cm
6 cm
8.5 cm
1 2 3 4 5 6 7Bo / ne (G/cm-3 x 10-11)
0
5
10
15
20
25
30
Radius = 4 cm 6 cm
8.5 cm
Theoretical
Numerical
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
COLLISIONLESS HEATING : AXIAL ACCELERATION
l Ar, 1 kW, 300 G, 2 mTorr
l The electron energy distribution (EED) was obtained from the EMCS. The tail of the EEDF increases with increasing distance from the coil.
l The axial component of the electromagnetic field is responsible for most of the power deposition.
0.5 0.005
0.44.0 V/cm
W/cm-3
IncreasingDistanceFrom Coil
0 10 20 30 40 50
100
10-2
10-4
10-6
Energy (eV)
Power Axial Field
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1
3
21
32
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICSRKINDER_AVS_2000_14
COLLISIONLESS HEATING : PRESSURE
l Ar, 1 kW, 50 sccm, 300 G
l As the pressure is decreased, the collisionless heating mechanisms become more dominant.
l There is significant heating in the downstream region.
1
10-8
50
0
ENERGY (eV)400 302010
ENERGY (eV)40
RADIALLY AVERAGED EEDF
P = 5 mTorr
302010ENERGY (eV)
40
P = 10 mTorr
302010
P = 2 mTorr
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICSRKINDER_AVS_2000_15
PHASE MATCHING
l Ar, 1 kW, 50 sccm, 300 G, 2 mTorr
l The parallel phase velocity of the electric fields is linearly proportional to the input rf frequency.
l As the frequency is decrease non-collisional heating throughout the reactor becomes more prevalent due to better phase matching with thermal electrons.
1
10-8
50
0
ENERGY (eV)400 302010
ENERGY (eV)40
RADIALLY AVERAGED EEDF
f = 13.6 MHz
302010ENERGY (eV)
40
f = 27.1 MHz
302010
f = 1.36 MHz
27.1 MHz
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
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PHASE MATCHING
l Ar, 1 kW, 50 sccm, 300 G, 2 mTorr
50
0Radius (cm) 200
PERCENTAGE OF ELECTRONS IN PHASE
1.36 MHz13.6 MHz
100 %
0.1%
5 %
l Phase matching of the parallel phase velocity with the thermal velocity is required for acceleration.
l At lower frequencies the fraction of electrons in phase with the propagating electric field is larger.
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
CONCLUSIONS
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l MEICPs are being studied for their ability to deposit power within the volume of the plasma.
l Study effects of TG mode on power deposition and ability for the helicon wave component to produce non-local heating.
l Initial studies indicate that the effect of the TG mode is to restructure the power deposition profile near the coils.
l However, the propagation of the helicon component is little affected, particularly at large magnetic fields where the TG modes is damped.
l For conditions where the TG mode is surpressed, the helicon component deposits the majority of the power within the volume of the plasma.
l At low pressures and rf frequencies, non-collisional heating becomes more prevalent since the thermal velocities match input rf phase velocity.