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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
b)Department of Electrical Engineering and Computer ScienceUniversity of Michigan, Ann Arbor, MI 48109, USA
http://uigelz.eecs.umich.edu
September 21st, 2011
* Work supported by DOE Plasma Science Center and Semiconductor Research Corp.
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AGENDA
Motivation for controlling f()
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
University of MichiganInstitute for Plasma Science & Engr.
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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.
University of MichiganInstitute for Plasma Science & Engr.
, , , , ,, , ,
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 enhance etching yield of Si.
Large fluorocarbon to ion flux ratio reduce 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 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
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
University of MichiganInstitute for Plasma Science & Engr.
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
University of MichiganInstitute for Plasma Science & Engr.
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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Typical Plasma Properties
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PULSED CCP: ne, Te, f()
University of MichiganInstitute for Plasma Science & Engr.
Pulsing with a PRF and moderate duty cycle produces nominal intra-cycles changes [e] but does modulate f().
[e]
Te
MIN MAX
f()
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%
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ELECTRON DENSITY CW
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Duty = 50%
Duty = 25%
MIN MAX
ANIMATION SLIDE-GIF
At 50% duty, the electron density is not significantly modulated by pulsing, so the plasma is quasi-CW.
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)
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ELECTRON SOURCES BY BULK ELECTRONS
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CW
Duty = 50%
Duty = 25%
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.
At the start of the pulse-on cycle, is there a impulsive positive electron source due to the overshoot of E/N.
MIN MAX
40 mTorr, Ar/CF4/O2=75/20/5 LF 500 W, HF 500 W
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ELECTRON SOURCES BY BEAM ELECTRONS
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CW
Duty = 50%
Duty = 25%
MIN MAX
40 mTorr, Ar/CF4/O2=75/20/5 LF = 10 MHz, 500 W HF = 40 MHz, 500 W (CW or pulse)
The beam electrons result from secondary emission from electrodes and acceleration in sheaths.
The electron source by beam electron is always positive.
The electron source by beam electrons compensates the electron losses and sustains the plasma.
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Etch Properties
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0.0
1.0
2.0
3.0
4.0
5.0
625 415 200 100 kHz
F / POLY FLUX RATIO: CONSTANT POWER
F to polymerizing flux ratio is largest at 200 kHz of PRF.
40 mTorr, Ar/CF4/O2=75/20/5, 200 sccm LF 10 MHz 500 W, Pulsed HF 40 MHz 500 W
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ETCH PROFILE IN SiO2 & IEAD: CONST. POWER
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40 mTorr, Ar/CF4/O2=75/20/5, 200 sccm LF 10 MHz 500 W, Pulsed HF 40 MHz 500 W
Angle (degree)
En
erg
y (
eV
)
Etch rate is fastest at 200 kHz PRF with larger ion energy and F to polymerizing flux ratio.
Cycle Average IEAD
He
igh
t (
m)
Width (m)
100 kHz CW 415 200 415 200 100 CW
ANIMATION SLIDE-GIF
Etch Profile (300 sec)
CD70 nm
-64 V -92 V -107 V -134 VBias:
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0.0
1.0
2.0
3.0
4.0
5.0
CW 625 415 200 50 kHz
0.0
1.0
2.0
3.0
4.0
5.0
CW 625 415 200 100 kHz
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.
Without DC Bias With DC Bias
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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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ETCH PROFILE IN SiO2 & IEAD: CONST. VOLTAGE
Etch rate is fastest at 415 kHz having larger fluorine flux.
Cycle Average IEAD
100 kHz CW 415 200 415 200 100 CW
Etch Profile (300 sec)
CD70 nm
-88 V -103 V -116 V -129 VBias:
Angle (degree)Width (m)ANIMATION SLIDE-GIF
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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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ETCH PROFILE IN SiO2 & IEAD: NO BIAS Etch rate is fastest at CW excitation due to continuously delivered
power. Cycle Average IEAD
100 kHz CW 415 200 415 200 100 CW
Etch Profile (300 sec)
CD70 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.
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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() 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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