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 Science
University 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(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
SHS_MJK_AVS
 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)
SHS_MJK_AVS
 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.
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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)
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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
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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.