Transcript Document

EFFECT OF BIAS VOLTAGE WAVEFORMS ON
ION ENERGY DISTRIBUTIONS
AND FLUOROCARBON PLASMA ETCH SELECTIVITY*
Ankur Agarwala) and Mark J. Kushnerb)
a)Department
of Chemical and Biomolecular Engineering
Email: [email protected]
b)Department
of Electrical and Computer Engineering
Email: [email protected]
University of Illinois
Urbana, IL 61801, USA
http://uigelz.ece.uiuc.edu
51st AVS Symposium, November 2004
* Work supported by the NSF, SRC and VSEA
AGENDA
 Introduction
 Bias Voltage Waveforms
 Approach and Methodology
 Ion Energy Distribution Functions
 Fluorocarbon Etch Selectivity
 Etching Recipes
 Summary
ANKUR_AVS04_Agenda
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HIGH ETCH SELECTIVITY
 High etch selectivity is a necessary characteristic for semiconductor
manufacturing.
 Prevents erosion of photoresist and/or underlying films.
 Permits over-etching to compensate for process nonuniformities.
 Low Etch Selectivity
 Substrate damage
 Improper etch stop layer
 High Etch Selectivity
 Little Substrate damage
 Proper etch stop layer
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ETCH MECHANISM
 CFx and CxFy form a polymeric passivation layer which regulates
delivery of etch precursors and activation energy.
 Chemisorption of CFx produces a complex at the oxide-polymer
interface.
I*, CF 2
+
CxFy
CFx Ion
Plasma
CxFy
Passi vation
Layer
Ion +
CO 2
CO 2
Ion +,F
SiF 3
SiO 2CxFy
SiOCFy
Ion +,F
F
Plasma
CxFy
Passi vation
Layer
CFx
SiF 3
Polymer
SiF
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 Low energy ion activation of the
complex produces polymer.
 The polymer layer is sputtered by
energetic ions
Polymer
SiO2
Si
Ion +
SiF 2
SiF 3
SiF 3
 The complex formed at the oxidepolymer interface undergoes ion
activated dissociation to form
volatile etch products (SiF3, CO2).
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ACHIEVING HIGH SELECTIVITY
 High etch selectivity is achieved by
controlling the ion energy distribution
at the substrate.
 Sinusoidal bias: Broad ion energy
distribution does not discriminate
thresholds (narrow process window).
Sinusoidal Bias
 Ion activation scales inversely with
polymer thickness, while polymer
thickness scales inversely with
bias.
 Tailored bias: Produce a narrow ion
energy distribution which
discriminates between threshold
energies (broad process window).
Ref: S.-B. Wang and A.E. Wendt, J. Vac. Sci. Technol. A, 19,
2425 (2001)
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VALIDATION OF REACTION MECHANISM
 The reaction mechanism has
been validated with
experiments by Oehrlein et al
using C4F8, C4F8/Ar, C4F8/O2.1
Etch Rate (nm/min)
500
400 SiO2 - M
SiO2 - E
300
 Larger ionization rates result
in larger ion fluxes in Ar/C4F8
mixtures. This increases etch
rates.
200
100
 With high Ar, the polymer
layers thins to
0
0
20
40
60
80
100
submonolayers due to less
Ar Content (%)
deposition and more
sputtering and so lowers etch
Ref: A. Sankaran and M.J. Kushner, J. Vac. Sci. Technol. A, 22,
1242 (2004)
rates.
300
e (nm/min)
C4F8/Ar
1 Li
SiO - M
et al, J. Vac. Sci. Technol. A, 20, 2052 (2002)2
200
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SiO2 - E
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CUSTOM BIAS VOLTAGE WAVEFORMS
 Ion Energy Distribution (IED) traditionally controlled by varying the
amplitude of a sinusoidal voltage waveform.
 Resultant IED – broad; both high and low energy ions
 Specially tailored non-sinusoidal bias voltage waveform
 Narrow IED at the substrate
 Peak of IED can be positioned to achieve desired selectivity
 Synthesized voltage Waveform:
 Periodic
 Short voltage spike
 Ramp down
Ref: S.-B. Wang and A.E. Wendt, J. Vac. Sci. Technol. A, 19,
2425 (2001)
 The “10% Waveform
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INTEGRATED MODELING
 An integrated reactor and feature scale
modeling hierarchy was developed to model
plasma processing systems.
 HPEM (Hybrid Plasma Equipment Model) is
the reactor scale model platform.
 Low pressure (<10’s Torr)
 2-d and 3-d versions
 Address ICP, CCP, RIE
 HPEM is linked to profile simulators – MCFPM
(Monte Carlo Feature Profile Model) to predict
the evolution of submicron features.
 2-d and 3-d
 Fluxes from HPEM
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HYBRID PLASMA EQUIPMENT MODEL
 A modular simulator addressing low
temperature, low pressure plasmas.
 Electro-magnetic Module:
 Electromagnetic Fields
 Magneto-static Fields
 Electron Energy Transport Module:
 Electron Temperature
 Electron Impact Sources
 Transport Coefficients
 Fluid Kinetics Module:
 Densities
 Momenta
 Temperature of species
 Electrostatic Potentials
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MONTE CARLO FEATURE PROFILE MODEL
 Monte Carlo based model to address plasma
surface interactions and evolution of surface
morphology and profiles.
 Inputs:




Initial material mesh
Etch mechanisms (chemical rxn. format)
Energy and Angular dependence
Gas species flux distribution used to
determine the launching and direction of
incoming particles.
 Flux distributions from equipment scale
model (HPEM)
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DYNAMIC SIMULATION – REACTOR SCALE
 Transformer-coupled plasma (TCP)
reactor geometry
 To accelerate ions to the wafer, a rf bias
voltage is applied.
 Base case conditions:
 Ar/C4F8 = 75/25, 100 sccm
 15 mTorr, 500 W
 200 Vp-p, 5 MHz
 “10%” Voltage Waveform
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REACTANT FLUXES
 15 mTorr, 500 W, 200 Vp-p,
Ar/C4F8 = 75/25, 100 sccm
 Dominant Ions: Ar+, CF3+, CF+
 Dominant Neutrals: CF, C2F3, F
 Polymer formation – Low
energy process
 Polymer sputtering and etch
activation – High energy
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ION ENERGY DISTRIBUTION FUNCTIONS
 Custom waveform produces
constant sheath potential
drop resulting in narrow IED.
 Sheath transit time is short
compared to pulse period
 Energy depends on
instantaneous potential drop.
 As duration of positive
portion of waveform IEDs
broaden in energy.
Vdc:
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42
46
56
64
75 -73
 15 mTorr, 500 W, 200 Vp-p,
Ar/C4F8 = 75/25, 100 sccm
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IEAD vs CUSTOM BIAS WAVEFORMS
 As duration of positive
portion of waveform is
increased, IEDs broaden in
energy.
 Waveforms attain form as
sinusoidal waveform
 Increasing waveform beyond
50% narrows the IEDs again
as dc characteristic is
obtained.
Vdc: -73
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-25
-21
-19
-12
13
 15 mTorr, 500 W, 200 Vp-p,
5 MHz, Ar/C4F8 = 75/25, 100
sccm
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IEAD vs CUSTOM BIAS VOLTAGE
 The peak energy of the IEAD
is controlled by amplitude
and frequency.
 IED broadens at higher
biases due to thickening of
sheath and longer transit
times.
 IED still narrower compared
to sinusoidal voltage
waveform.
 15 mTorr, 500 W,
Ar/C4F8 = 75/25, 100 sccm
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ETCH PROFILES – CUSTOM VOLTAGE WAVEFORM
 Low X % have IEADs which produce etch stops.
5%
8%
 X % indicates percent of cycle
with positive voltage
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ANIMATION NEXT SLIDE
10 %
12 %
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ETCH PROFILES – CUSTOM VOLTAGE WAVEFORM
 Low X % have IEADs which produce etch stops.
MASK
SiO2
Si
5%
8%
10 %
 X % indicates percent of cycle
with positive voltage
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ANIMATION SLIDE
12 %
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FLUOROCARBON PLASMA ETCH SELECTIVITY
 Maximum Etch Rate for the
10 % waveform.
 12 % waveform:
 Broader IED
 Lower Etch Rates
 Lower Selectivity
 In a regime where selectivity
is higher, custom waveform
enables higher etch rates
 For same etch rates lower
selectivity with sin waveform.
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ETCH PROFILES – CUSTOM VOLTAGE PEAK-TO-PEAK
 Increasing bias increases etch rate and reduces selectivity.
400 V
500 V
1000 V
 XXX V indicates amplitude of
bias
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ANIMATION NEXT SLIDE
1500 V
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ETCH PROFILES – CUSTOM VOLTAGE PEAK-TO-PEAK
 Increasing bias increases etch rate and reduces selectivity.
MASK
SiO2
Si
400 V
500 V
1000 V
 XXX V indicates amplitude of
bias
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ANIMATION SLIDE
1500 V
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FLUOROCARBON PLASMA ETCH SELECTIVITY
 Increasing bias voltage
increases etch rates.
 Loss of selectivity with
increasing bias voltages.
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ETCHING RECIPES
 Multi-component recipes:
 Main-etch: Non selective; High bias
 Over-etch: Selective; Low bias
 Traditionally, gas mixture is changed to
obtain a selective etch.
 Controlling chemical component
 Clearing of gases is determined by
residence time
 Finite selectivity
 Custom tailored voltage waveform
 Controlling physical component
 Change amplitude – immediate
control
 “Infinite” selectivity
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ETCHING PROFILES – RECIPE
200 V
1500 V
1500/200 V
1500/1000/100/200 V
(Slow, selective)
(Fast, non-selective)
(Fast, selective)
(Fast, selective)
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ANIMATION NEXT SLIDE
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ETCHING PROFILES – RECIPE
1847 s
MASK
713 s
1377 s
1356 s
200 V
1500 V
1500/200 V
1500/1000/100/200 V
(Slow, selective)
(Fast, non-selective)
(Fast, selective)
(Fast, selective)
SiO2
Si
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ANIMATION SLIDE
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SUMMARY
 Higher etch selectivity was obtained by controlling ion energy
distribution.
 Flux, Energy and Angular distribution optimized to attain high etch
selectivity
 Special tailored voltage waveform was synthesized.
 Short voltage spike followed by ramp down
 Results in a narrow IED over wide range of voltages and
frequency.
 New etching recipe
 Based only on bias voltage amplitude without changing gas
chemistry.
 Excellent control over selectivity demonstrated.
ANKUR_AVS_20
University of Illinois
Optical and Discharge Physics