EFFECT OF BIAS VOLTAGE WAVEFORMS ON ION ENERGY ...University of Illinois Optical and Discharge...
Transcript of EFFECT OF BIAS VOLTAGE WAVEFORMS ON ION ENERGY ...University of Illinois Optical and Discharge...
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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