Transcript Document
SiO2 ETCH RATE AND PROFILE CONTROL
USING PULSE POWER IN CAPACITIVELY
COUPLED PLASMAS*
Sang-Heon Songa) and Mark J. Kushnerb)
a)Department
of Nuclear Engineering and Radiological Sciences
University of Michigan, Ann Arbor, MI 48109, USA
[email protected]
b)Department
of Electrical Engineering and Computer Science
University of Michigan, Ann Arbor, MI 48109, USA
[email protected]
http://uigelz.eecs.umich.edu
September 21st, 2011
*
Work supported by DOE Plasma Science Center and Semiconductor Research Corp.
AGENDA
Motivation for controlling f(e)
Description of the model
Typical Ar/CF4/O2 pulsed plasma properties
Etch property with different PRF
Constant Power with DC Bias
Constant Voltage with DC Bias
Without DC Bias
Concluding remarks
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CONTROL OF ELECTRON KINETICS – f(e)
Controlling the generation of reactive species for technological
devices benefits from customizing the electron energy (velocity)
distribution function.
k
CF3 + F + e
e + CF4
dN k r , t
dt
nekij r , t N j
i, j
12
2e
kij r , t
f e , r , t
e d e
0
me
df v , r , t
dt
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v x f r , v
qE r , t
me
f v , r , t
v f v , r , t
t
c
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ETCH RATE vs. FLUX RATIOS
Large fluorine to ion flux ratio enhance etching yield of Si.
Etching Yield (Si/Ar+)
Etching Yield (Si/Ar+)
Large fluorocarbon to ion flux ratio reduce etching yield of Si.
Flux Ratio (F/Ar+)
Flux Ratio (CF2/Ar+)
Ref: D. C. Gray, J. Butterbaugh, and H. H. Sawin, J. Vac. Sci. Technol. A 9, 779 (1991)
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ETCH PROFILE vs. FLUX RATIOS
Large chlorine radical to ion flux ratio makes undercut in etch
profile due to too much chemical reactions.
Etch profile result in ECR Cl2 plasma after 200% over etch with
different flux ratios
Flux Ratio (Cl / Ion) = 0.3
p-Si
Ref: K. Ono, M. Tuda, H. Ootera, and T. Oomori,
Pure and Appl. Chem. Vol 66 No 6, 1327 (1994)
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Flux Ratio (Cl / Ion) = 0.8
p-Si
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HYBRID PLASMA EQUIPMENT MODEL (HPEM)
Electron
Monte Carlo
Simulation
Te, Sb, Seb, k
E, Ni, ne
Fluid Kinetics Module
Fluid equations
(continuity, momentum, energy)
Poisson’s equation
Fluid Kinetics Module:
Heavy particle and electron continuity, momentum,
energy
Poisson’s equation
Electron Monte Carlo Simulation:
Includes secondary electron transport
Captures anomalous electron heating
Includes electron-electron collisions
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MONTE CARLO FEATURE
PROFILE MODEL (MCFPM)
HPEM
PCMCM
Energy and angular
distributions for ions
and neutrals
MCFPM
Etch rates and
profile
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The MCFPM resolves the surface
topology on a 2D Cartesian mesh.
Each cell has a material identity. Gas
phase species are represented by
Monte Carlo pseuodoparticles.
Pseuodoparticles are launched with
energies and angles sampled from the
distributions obtained from the HPEM
Cells identities changed, removed,
added for reactions, etching
deposition.
Poisson’s
equation solved
for charging
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REACTOR GEOMETRY: 2 FREQUENCY CCP
2D, cylindrically symmetric
Ar/CF4/O2 = 75/20/5, 40 mTorr, 200 sccm
Base conditions
Lower electrode: LF = 10 MHz, 500 W, CW
Upper electrode: HF = 40 MHz, 500 W, Pulsed
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PULSE POWER
Use of pulse power provides a means for controlling f(e).
Pulsing enables ionization to exceed electron losses during a portion
of the ON period – ionization only needs to equal electron losses
averaged over the pulse period.
Pmax
Power(t)
Pave
Duty Cycle
Pt dt
1
0
Pmin
= 1/PRF
Time
Pulse power for high frequency.
Duty-cycle = 25%, PRF = 50, 100, 200, 415, 625 kHz
Average Power = 500 W
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Typical Plasma Properties
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PULSED CCP: ne, Te, f(e)
Pulsing with a PRF and moderate duty cycle produces nominal
intra-cycles changes [e] but does modulate f(e).
ANIMATION SLIDE-GIF
40 mTorr, Ar/CF4/O2=75/20/5
LF = 10 MHz, 500 W
HF = 40 MHz, pulsed 500 W
PRF = 100 kHz, Duty-cycle = 25%
[e]
f(e)
MIN
MAX
Te
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CW
ELECTRON DENSITY
At 50% duty, the electron
density is not significantly
modulated by pulsing, so the
plasma is quasi-CW.
Duty = 50%
Duty = 25%
At 25% duty, modulation in
[e] occurs due to electron
losses during the longer
inter-pulse period.
The lower duty cycle is more
likely to reach higher value
of electron density.
40 mTorr, Ar/CF4/O2=75/20/5
LF = 10 MHz, 500 W
HF = 40 MHz, 500 W (CW or pulse)
MIN
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ANIMATION SLIDE-GIF
MAX
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CW
Duty = 50%
ELECTRON SOURCES
BY BULK ELECTRONS
The electrons have two
groups: bulk low energy
electrons and beam-like
secondary electrons.
The bulk electron source is
negative due to electron
attachment and dissociative
recombination.
Duty = 25%
At the start of the pulse-on
cycle, is there a impulsive
positive electron source due
to the overshoot of E/N.
40 mTorr, Ar/CF4/O2=75/20/5
LF 500 W, HF 500 W
MIN
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ANIMATION SLIDE-GIF
MAX
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CW
Duty = 50%
ELECTRON SOURCES
BY BEAM ELECTRONS
The beam electrons result
from secondary emission
from electrodes and
acceleration in sheaths.
The electron source by beam
electron is always positive.
Duty = 25%
The electron source by beam
electrons compensates the
electron losses and sustains
the plasma.
40 mTorr, Ar/CF4/O2=75/20/5
LF = 10 MHz, 500 W
HF = 40 MHz, 500 W (CW or pulse)
MIN
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ANIMATION SLIDE-GIF
MAX
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Etch Properties
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F / POLY FLUX RATIO: CONSTANT POWER
F to polymerizing flux ratio is largest at 200 kHz of PRF.
5.0
4.0
3.0
2.0
1.0
0.0
625
415
200
40 mTorr, Ar/CF4/O2=75/20/5, 200 sccm
LF 10 MHz 500 W, Pulsed HF 40 MHz 500 W
100 kHz
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ETCH PROFILE IN SiO2 & IEAD: CONST. POWER
Etch rate is fastest at 200 kHz PRF with larger ion energy and F to
polymerizing flux ratio.
Cycle Average IEAD
Etch Profile (300 sec)
200
CW
100 kHz
415
CW
415
200
100
CD
Height (mm)
Energy (eV)
70 nm
Bias: -64 V
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Width (mm)
ANIMATION SLIDE-GIF
40 mTorr, Ar/CF4/O2=75/20/5, 200 sccm
LF 10 MHz 500 W, Pulsed HF 40 MHz 500 W
-92 V
-107 V
-134 V
Angle (degree)
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F / POLY FLUX RATIO: CONSTANT VOLTAGE
F to polymerizing flux ratio is controlled not only by PRF, but also
by DC bias.
DC bias is manipulated by the blocking capacitor on the substrate.
With DC Bias
Without DC Bias
5.0
5.0
4.0
4.0
3.0
3.0
2.0
2.0
1.0
1.0
0.0
0.0
CW
625
415
200
100 kHz
40 mTorr, Ar/CF4/O2=75/20/5, 200 sccm
LF 10 MHz 250 V, Pulsed HF 40 MHz 500 W
CW
625
415
200
50 kHz
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ETCH PROFILE IN SiO2 & IEAD: CONST. VOLTAGE
Etch rate is fastest at 415 kHz having larger fluorine flux.
Etch Profile (300 sec)
200
CW
100 kHz
415
CD
Cycle Average IEAD
CW
415
200
100
-103 V
-116 V
-129 V
70 nm
Bias: -88 V
Width (mm)
ANIMATION SLIDE-GIF
40 mTorr, Ar/CF4/O2=75/20/5, 200 sccm
LF 10 MHz 250 V, Pulsed HF 40 MHz 500 W
Angle (degree)
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ETCH PROFILE IN SiO2 & IEAD: NO BIAS
Etch rate is fastest at CW excitation due to continuously delivered
power.
Cycle Average IEAD
Etch Profile (300 sec)
200
CW
100 kHz
415
CW
415
200
100
CD
70 nm
40 mTorr, Ar/CF4/O2=75/20/5, 200 sccm
LF 10 MHz 250 V, Pulsed HF 40 MHz 500 W
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POWER NORMALIZED ETCH RATE
Power normalized etch rate is dependant on the pulse repetition
frequency and DC bias of the substrate.
40 mTorr, Ar/CF4/O2=75/20/5, 200 sccm
LF 10 MHz, Pulsed HF 40 MHz, Duty 25%
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CONCLUDING REMARKS
Extension of tail of f(e) beyond that obtained with CW excitation
produces a different mix of fluxes to substrate.
Ratios of fluxes and IEADs are tunable using pulsed excitation.
Ratios of fluxes are IEADs are tunable using blocking capacitor.
Consequently, etch rate can be controlled by pulsed power with
different blocking capacitors.
With constant power operation, fastest etch rate is achieved at 200
kHz having larger F to polymerizing flux ratio.
With constant voltage operation, fastest etch rate is achieved at
415 kHz having larger fluorine flux.
Without DC bias, the etch rate decrease as pulse repetition
frequency decreases.
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University of Michigan
Institute for Plasma Science & Engr.