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Transcript PPT - DOE Plasma Science Center
SiO2 ETCH PROPERTY 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
Nov. 2011 AVS
*
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 rate with variable blocking capacitor
Etch property with different PRF
Etch rate, profile, and selectivity
Concluding Remarks
SHS_MJK_AVS
University of Michigan
Institute for Plasma Science & Engr.
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
ne k ij r , t N j
i, j
1 2
2e
k ij r , t
f e , r , t
m
0
e
df v , r , t
dt
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v x f r , v
qE r , t
me
e d e
f v , r , t
v f v, r ,t
t
c
University of Michigan
Institute for Plasma Science & Engr.
ETCH RATE vs. FLUX RATIOS
Large fluorine to ion flux ratio enhances etching yield of Si.
Etching Yield (Si/Ar+)
Etching Yield (Si/Ar+)
Large fluorocarbon to ion flux ratio reduces 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)
SHS_MJK_AVS
University of Michigan
Institute for Plasma Science & Engr.
ETCH PROFILE vs. FLUX RATIOS
Large chlorine radical to ion flux ratio produces an undercut in etch
profile.
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
University of Michigan
Institute for Plasma Science & Engr.
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
SHS_MJK_AVS
University of Michigan
Institute for Plasma Science & Engr.
MONTE CARLO FEATURE
PROFILE MODEL (MCFPM)
HPEM
PCMCM
Energy and angular
distributions for ions
and neutrals
MCFPM
Etch rates and
profile
SHS_MJK_AVS
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
University of Michigan
Institute for Plasma Science & Engr.
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
SHS_MJK_AVS
University of Michigan
Institute for Plasma Science & Engr.
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
1
P t dt
0
Pmin
= 1/PRF
Time
Pulse power for high frequency.
Duty-cycle = 25%, PRF = 50, 100, 200, 415, 625 kHz
Average Power = 500 W
SHS_MJK_AVS
University of Michigan
Institute for Plasma Science & Engr.
VARIABLE BLOCKING CAPACITOR
Due to the different area of two electrodes, a “dc” bias is produced
on the blocking capacitor connected to the substrate electrode.
The temporal behavior of “dc” bias is dependent on the magnitude
of the capacitance due to RC delay time.
We investigated variable
blocking capacitor of 10 nF,
1 mF, and 100 F
100 F of blocking capacitor
results in NO “dc” bias on
the substrate.
SHS_MJK_AVS
University of Michigan
Institute for Plasma Science & Engr.
Typical Plasma Properties
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PULSED CCP: Electron Density & Temperature
Electron Density (x 1011 cm-3)
MIN
Electron Temperature (eV)
MAX
Pulsing with a moderate PRF duty cycle produces nominal intracycles changes in [e] but does modulate Te.
40 mTorr, Ar/CF4/O2=75/20/5
PRF = 100 kHz, Duty-cycle = 25%
HF = 40 MHz, pulsed 500 W
LF = 10 MHz, 250 V
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ANIMATION SLIDE-GIF
University of Michigan
Institute for Plasma Science & Engr.
PULSED CCP: ELECTRON SOURCES
by Bulk Electrons
(x 1014 cm-3 s-1) by Secondary Electrons
MIN
MAX
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.
The electron source by beam electrons compensates the electron
losses and sustains the plasma.
ANIMATION SLIDE-GIF
40 mTorr, Ar/CF4/O2=75/20/5
LF 250 V, HF 500 W
University of Michigan
Institute for Plasma Science & Engr.
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PULSED CCP: E-SOURCES and f(e)
Rate coefficient of e-sources is modulated between electron
source (electron impact ionization) and loss (attachment and
recombination) during pulsed cycle.
ANIMATION SLIDE-GIF
40 mTorr, Ar/CF4/O2=75/20/5
PRF = 100 kHz, Duty-cycle = 25%
LF = 10 MHz, 250 V
HF = 40 MHz, pulsed 500 W
University of Michigan
Institute for Plasma Science & Engr.
SHS_MJK_AVS
Etch Properties:
Variable Blocking Capacitor
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PULSED CCP: PLASMA POTENTIAL & dc BIAS
A small blocking capacitor allows the “dc” bias to follow the
change during the pulse period.
Maximum ion energy gain = Plasma Potential – “dc” Bias
1 mF
PRF = 100 kHz, Duty-cycle = 25%
LF = 10 MHz, 250 V
HF = 40 MHz, pulsed 500 W
10 nF
University of Michigan
Institute for Plasma Science & Engr.
ETCH PROFILE IN SiO2 & IEAD: 1 mF
With constant voltage, bias amplitude is constant but blocking
capacitor determines “dc” bias.
Cycle Average IEAD
Height (mm)
Energy (eV)
Etch Profile (600 sec)
SHS_MJK_AVS
Width (mm)
ANIMATION SLIDE-GIF
Pulsed HF 40 MHz 500 W
LF 10 MHz 250 V, Blocking Cap. = 1 mF
Angle (degree)
University of Michigan
Institute for Plasma Science & Engr.
ETCH PROFILE IN SiO2 & IEAD: 10 nF
With smaller blocking capacitor, “dc” bias begins to follow the rf
power and so produces a different IEAD.
Cycle Average IEAD
Height (mm)
Energy (eV)
Etch Profile (600 sec)
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Width (mm)
ANIMATION SLIDE-GIF
Pulsed HF 40 MHz 500 W
LF 10 MHz 250 V, Blocking Cap. = 1 nF
Angle (degree)
University of Michigan
Institute for Plasma Science & Engr.
ETCH PROFILE IN SiO2 & IEAD: NO dc BIAS
In absence of dc bias and for constant voltage, pulse power and is
effect on f(e) in large part determine etch properties.
Cycle Average IEAD
Height (mm)
Energy (eV)
Etch Profile (600 sec)
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Width (mm)
ANIMATION SLIDE-GIF
Pulsed HF 40 MHz 500 W
LF 10 MHz 250 V, Blocking Cap. = 100 F
Angle (degree)
University of Michigan
Institute for Plasma Science & Engr.
POWER NORMALIZED ER: Blocking Capacitor
Power normalized etch rate is dependent not only on the pulse
repetition frequency (PRF), but also the value of the blocking
capacitor on the substrate at lower PRF.
F to Poly Flux ratio
5.0
C
4.0
B
3.0
2.0
A
1.0
0.0
CW
250
100
50 kHz
Pulsed HF 40 MHz 500 W
LF 10 MHz 250 V
NO
Adc
10
BnF
1C
uF
University of Michigan
Institute for Plasma Science & Engr.
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E-SOURCES and FLUX RATIO: PRF
Electron source rate coefficient is modulated with f(e) by pulse
power.
Modulation is enhanced with smaller PRF.
F to Poly Flux ratio
6.0
5.0
4.0
3.0
2.0
1.0
0.0
CW
Pulsed HF 40 MHz 500 W
LF 10 MHz 250 V
Blocking Cap. = 1 mF
250
100
50 kHz
University of Michigan
Institute for Plasma Science & Engr.
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ETCH RATE: POWER NORMALIZED
Power normalized etch rate is large at 250 kHz with ion distribution
extending to higher energies.
Cycle Average IEAD
Energy (eV)
Normalized Etch Rate
CW
250
100
Pulsed HF 40 MHz 500 W
LF 10 MHz 250 V
Without DC Bias on LF electrode
50 kHz
Angle (degree)
University of Michigan
Institute for Plasma Science & Engr.
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ETCH PROFILE: CRITICAL DIMENSION
EPD + Over Etch 50%
A
(1/A)
1
(2/A)
CD is compared at
the middle and
bottom of feature.
CW excitation
produces bowing
and an undercut
profile.
Pulse plasma helps
to prevent the
bowing and undercutting.
100
Smaller PRF has a
50 kHz tapered profile.
Pulsed HF 40 MHz 500 W
LF 10 MHz 250 V
Blocking Cap. = 1 mF
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Institute for Plasma Science & Engr.
CW
250
2
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ETCH SELECTIVITY: Between SiO2 and Si
EPD + Over Etch 50%
Silicon damage
depth is compared
in 2-D etch profile.
Pulsed operation
helps to prevent
the silicon damage.
Lower damage
appears to be
correlated with
smaller F flux ratio
at 250 kHz.
CW
250
100
Pulsed HF 40 MHz 500 W
LF 10 MHz 250 V
Blocking Cap. = 1 mF
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50 kHz
University of Michigan
Institute for Plasma Science & Engr.
CONCLUDING REMARKS
Extension of tail of f(e) beyond that obtained with CW excitation
produces a different mix of fluxes to substrate.
Etch rate can be controlled by pulsed operation with different
pulse repetition frequencies.
Blocking capacitor is another variable to control ion energy
distributions and etch rates. Smaller capacitance allows “dc” bias
to follow the plasma potential in pulse period more rapidly.
Etch rate is enhanced by pulsed power operation in CCP.
Etch profile is improved with pulsed operation preventing
undercut.
Etch selectivity of SiO2 to Si is also improved with PRF of 250 kHz
with a smaller fluorine flux ratio.
SHS_MJK_AVS
University of Michigan
Institute for Plasma Science & Engr.