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 EngineeringEmail: [email protected]
b)Department of Electrical and Computer Engineering Email: [email protected]
University of IllinoisUrbana, IL 61801, USA
http://uigelz.ece.uiuc.edu
51st AVS Symposium, November 2004
* Work supported by the NSF, SRC and VSEA
University of IllinoisOptical and Discharge Physics
AGENDA
• Introduction
• Bias Voltage Waveforms
• Approach and Methodology
• Ion Energy Distribution Functions
• Fluorocarbon Etch Selectivity
• Etching Recipes
• Summary
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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.
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• 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.
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CFx Ion+
I*, CF2
SiO2CxFy SiOCFy
CxFyIon+
CO2Ion+
CO2
Polymer
SiF3
Ion+,FSiF3
CFx
Polymer
F
SiF SiF2 SiF3
Ion+,F
SiF3
SiO2
Plasma
Si
CxFy
Plasma
PassivationLayer
CxFyPassivation
Layer
• Low energy ion activation of the complex produces polymer.
• The polymer layer is sputtered by energetic ions
• The complex formed at the oxide-polymer interface undergoes ion activated dissociation to form volatile etch products (SiF3, CO2).
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ACHIEVING HIGH SELECTIVITY
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Ref: S.-B. Wang and A.E. Wendt, J. Vac. Sci. Technol. A, 19, 2425 (2001)
• 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).
• Tailored bias: Produce a narrow ion energy distribution which discriminates between threshold energies (broad process window).
• Ion activation scales inversely with polymer thickness, while polymer thickness scales inversely with bias.
Sinusoidal Bias
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VALIDATION OF REACTION MECHANISM
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• The reaction mechanism has been validated with experiments by Oehrlein et al using C4F8, C4F8/Ar, C4F8/O2.1
• Larger ionization rates result in larger ion fluxes in Ar/C4F8mixtures. This increases etch rates.
• With high Ar, the polymer layers thins tosubmonolayers due to less deposition and more sputtering and so lowers etch rates.
0 20 40 60 80 1000
100
200
300
400
500
C4F8/Ar
SiO2 - E
SiO2 - M
Etch
Rat
e (n
m/m
in)
Ar Content (%)
Ref: A. Sankaran and M.J. Kushner, J. Vac. Sci. Technol. A, 22, 1242 (2004)
1 Li et al, J. Vac. Sci. Technol. A, 20, 2052 (2002)
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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
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• 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
• 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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• An integrated reactor and feature scale modeling hierarchy was developed to model plasma processing systems.
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HYBRID PLASMA EQUIPMENT MODEL
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• 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
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• 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
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• Polymer formation – Low energy process
• Polymer sputtering and etch activation – High energy
• 15 mTorr, 500 W, 200 Vp-p,Ar/C4F8 = 75/25, 100 sccm
• Dominant Ions: Ar+, CF3+, CF+
• Dominant Neutrals: CF, C2F3, F
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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.
• 15 mTorr, 500 W, 200 Vp-p,Ar/C4F8 = 75/25, 100 sccm
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Vdc: 42 46 56 64 75 -73
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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.
• 15 mTorr, 500 W, 200 Vp-p,5 MHz, Ar/C4F8 = 75/25, 100 sccm
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Vdc: -73 -25 -21 -19 -12 13
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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
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5 % 8 % 10 % 12 %
• X % indicates percent of cycle with positive voltage
• Low X % have IEADs which produce etch stops.
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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
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400 V 500 V 1000 V 1500 V• XXX V indicates amplitude of
bias
• Increasing bias increases etch rate and reduces selectivity.
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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
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200 V(Slow, selective)
1500 V(Fast, non-selective)
1500/200 V(Fast, selective)
1500/1000/100/200 V(Fast, selective)
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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.
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