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: aagarwl3@uiuc.edu

b)Department of Electrical and Computer Engineering Email: mjk@uiuc.edu

University of IllinoisUrbana, IL 61801, USA

http://uigelz.ece.uiuc.edu

51st AVS Symposium, November 2004

* Work supported by the NSF, SRC and VSEA

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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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