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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
ssongs@umich.edu
b)Department of Electrical Engineering and Computer ScienceUniversity of Michigan, Ann Arbor, MI 48109, USA
mjkush@umich.edu
http://uigelz.eecs.umich.edu
Nov. 2011 AVS
* Work supported by DOE Plasma Science Center and Semiconductor Research Corp.
AGENDA
Motivation for controlling f()
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
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CONTROL OF ELECTRON KINETICS – f() Controlling the generation of reactive species for technological
devices benefits from customizing the electron energy (velocity) distribution function.
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, , , , ,, , ,
df v r t qE r t f v r tv f r v f v r t
dt m tx ve c
1 2
0
2, , ,ij
ek r t f r t d
m
,
,,k
e ij ji j
dN r tn k r t N
dt
e + CF4 CF3 + F + ek
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ETCH RATE vs. FLUX RATIOS
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Ref: D. C. Gray, J. Butterbaugh, and H. H. Sawin, J. Vac. Sci. Technol. A 9, 779 (1991)
Flux Ratio (F/Ar+) Flux Ratio (CF2/Ar+)
Etc
hin
g Y
ield
(S
i/Ar+
)
Etc
hin
g Y
ield
(S
i/Ar+
)
Large fluorine to ion flux ratio enhances etching yield of Si.
Large fluorocarbon to ion flux ratio reduces etching yield of Si.
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Ref: K. Ono, M. Tuda, H. Ootera, and T. Oomori, Pure and Appl. Chem. Vol 66 No 6, 1327 (1994)
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
p-Si p-Si
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ETCH PROFILE vs. FLUX RATIOS
Flux Ratio (Cl / Ion) = 0.3 Flux Ratio (Cl / Ion) = 0.8
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HYBRID PLASMA EQUIPMENT MODEL (HPEM)
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
E, Ni, ne
Fluid Kinetics ModuleFluid equations
(continuity, momentum, energy)Poisson’s equation
Te, Sb, Seb, kElectron Monte Carlo Simulation
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MONTE CARLO FEATUREPROFILE MODEL (MCFPM) 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.
PCMCM
Energy and angular distributions for ions
and neutrals
MCFPM
Etch rates and profile
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Poisson’s equation solved for charging
HPEM
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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
Time = 1/PRF
Duty Cycle
Power(t)
Pmin
0
1dttPPave
Pmax
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Use of pulse power provides a means for controlling f().
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.
Pulse power for high frequency.
Duty-cycle = 25%, PRF = 50, 100, 200, 415, 625 kHz
Average Power = 500 W
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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.
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We investigated variable blocking capacitor of 10 nF, 1 F, and 100 F
100 F of blocking capacitor results in NO “dc” bias on the substrate.
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PULSED CCP: Electron Density & Temperature
Pulsing with a moderate PRF duty cycle produces nominal intra-cycles changes in [e] but does modulate Te.
Electron Density (x 1011 cm-3) Electron Temperature (eV)
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MIN MAX
40 mTorr, Ar/CF4/O2=75/20/5
PRF = 100 kHz, Duty-cycle = 25%
HF = 40 MHz, pulsed 500 W
LF = 10 MHz, 250 VSHS_MJK_AVS
ANIMATION SLIDE-GIF
PULSED CCP: ELECTRON SOURCES
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.
by Bulk Electrons (x 1014 cm-3 s-1) by Secondary Electrons
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MIN MAX
40 mTorr, Ar/CF4/O2=75/20/5 LF 250 V, HF 500 W
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ANIMATION SLIDE-GIF
PULSED CCP: E-SOURCES and f()
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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
Rate coefficient of e-sources is modulated between electron source (electron impact ionization) and loss (attachment and recombination) during pulsed cycle.
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ANIMATION SLIDE-GIF
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
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PRF = 100 kHz, Duty-cycle = 25% LF = 10 MHz, 250 V HF = 40 MHz, pulsed 500 W
1 F 10 nF
ETCH PROFILE IN SiO2 & IEAD: 1 F
With constant voltage, bias amplitude is constant but blocking capacitor determines “dc” bias.
Cycle Average IEAD Etch Profile (600 sec)
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Angle (degree)Width (m)ANIMATION SLIDE-GIF
Pulsed HF 40 MHz 500 W LF 10 MHz 250 V, Blocking Cap. = 1 F
En
erg
y (
eV
)
He
igh
t (
m)
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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.
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Pulsed HF 40 MHz 500 W LF 10 MHz 250 V, Blocking Cap. = 1 nF
SHS_MJK_AVSAngle (degree)Width (m)
ANIMATION SLIDE-GIF
Cycle Average IEAD Etch Profile (600 sec)
En
erg
y (
eV
)
He
igh
t (
m)
ETCH PROFILE IN SiO2 & IEAD: NO dc BIAS
In absence of dc bias and for constant voltage, pulse power and is effect on f() in large part determine etch properties.
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Pulsed HF 40 MHz 500 W LF 10 MHz 250 V, Blocking Cap. = 100 F
SHS_MJK_AVSAngle (degree)Width (m)
ANIMATION SLIDE-GIF
Cycle Average IEAD Etch Profile (600 sec)
En
erg
y (
eV
)
He
igh
t (
m)
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.
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Pulsed HF 40 MHz 500 W LF 10 MHz 250 V
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CW 250 100 50 kHz0.0
1.0
2.0
3.0
4.0
5.0
NO dc 10 nF 1 uF
A
B
C
A B C
F to Poly Flux ratio
Electron source rate coefficient is modulated with f() by pulse power.
Modulation is enhanced with smaller PRF.
E-SOURCES and FLUX RATIO: PRF
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Pulsed HF 40 MHz 500 W LF 10 MHz 250 V Blocking Cap. = 1 F
F to Poly Flux ratio
0.0
1.0
2.0
3.0
4.0
5.0
6.0
CW 250 100 50 kHz
Power normalized etch rate is large at 250 kHz with ion distribution extending to higher energies.
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Pulsed HF 40 MHz 500 W LF 10 MHz 250 V Without DC Bias on LF electrode
ETCH RATE: POWER NORMALIZED
CW 250 100 50 kHz
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Cycle Average IEAD Normalized Etch Rate
Angle (degree)
En
erg
y (
eV
)
1
Pulsed HF 40 MHz 500 W LF 10 MHz 250 V Blocking Cap. = 1 F
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EPD + Over Etch 50%
ETCH PROFILE: CRITICAL DIMENSION
2
(1/A)
(2/A)
CW 250 100 50 kHz
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 under-cutting.
Smaller PRF has a tapered profile.
A
ETCH SELECTIVITY: Between SiO2 and Si
Pulsed HF 40 MHz 500 W LF 10 MHz 250 V Blocking Cap. = 1 F
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CW 250 100 50 kHz
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.
EPD + Over Etch 50%
CONCLUDING REMARKS
Extension of tail of f() 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.
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