EFFECT OF BIAS VOLTAGE WAVEFORMS ON ION ENERGY DISTRIBUTIONS AND FLUOROCARBON PLASMA ETCH SELECTIVITY* Ankur Agarwal a) and Mark J. Kushner b) a) Department

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 IllinoisUrbana, IL 61801, USA

http://uigelz.ece.uiuc.edu

51st AVS Symposium, November 2004

* Work supported by the NSF, SRC and VSEA

University of Illinois

Optical and Discharge Physics

AGENDA

Introduction

Bias Voltage Waveforms

Approach and Methodology

Ion Energy Distribution Functions

Fluorocarbon Etch Selectivity

Etching Recipes

Summary

ANKUR_AVS04_Agenda



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.

ANKUR_AVS_01

Low Etch Selectivity Substrate damage Improper etch stop layer

High Etch Selectivity Little Substrate damage Proper etch stop layer



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.

ANKUR_AVS_02

CFx Ion+

I*, CF2

SiO2CxFy SiOCFy

CxFy

Ion+

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).



ACHIEVING HIGH SELECTIVITY

ANKUR_AVS_03

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



VALIDATION OF REACTION MECHANISM

ANKUR_AVS_04

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/C4F8 mixtures. This increases etch rates.

With high Ar, the polymer layers thins to submonolayers due to less deposition and more sputtering and so lowers etch rates.

0 20 40 60 80 1000

100

200

300

C4F8/O2

SiO2 - E

SiO2 - M

Etc

h R

ate

(nm

/min

)

O Content (%)2

0 20 40 60 80 1000

100

200

300

400

500

C4F8/Ar

SiO2 - E

SiO2 - M

Etc

h R

ate

(nm

/min

)

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)



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

ANKUR_AVS_05

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



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

ANKUR_AVS_06

An integrated reactor and feature scale modeling hierarchy was developed to model plasma processing systems.



HYBRID PLASMA EQUIPMENT MODEL

ANKUR_AVS_07

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



MONTE CARLO FEATURE PROFILE MODEL

ANKUR_AVS_08

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)



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

ANKUR_AVS_09



REACTANT FLUXES

ANKUR_AVS_10

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



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

ANKUR_AVS_11

Vdc: 42 46 56 64 75 -73



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

ANKUR_AVS_12

Vdc: -73 -25 -21 -19 -12 13



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

ANKUR_AVS_13



ETCH PROFILES – CUSTOM VOLTAGE WAVEFORM

ANKUR_AVS_14

5 % 8 % 10 % 12 %

ANIMATION NEXT SLIDE

X % indicates percent of cycle with positive voltage

Low X % have IEADs which produce etch stops.



ETCH PROFILES – CUSTOM VOLTAGE WAVEFORM

ANKUR_AVS_14

5 % 8 % 10 % 12 %

ANIMATION SLIDE

MASK

SiO2

Si

X % indicates percent of cycle with positive voltage

Low X % have IEADs which produce etch stops.



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.

ANKUR_AVS_15



ETCH PROFILES – CUSTOM VOLTAGE PEAK-TO-PEAK

ANKUR_AVS_16

400 V 500 V 1000 V 1500 V

XXX V indicates amplitude of bias

Increasing bias increases etch rate and reduces selectivity.

ANIMATION NEXT SLIDE



ETCH PROFILES – CUSTOM VOLTAGE PEAK-TO-PEAK

ANKUR_AVS_16ANIMATION SLIDE

400 V 500 V 1000 V 1500 V

XXX V indicates amplitude of bias

Increasing bias increases etch rate and reduces selectivity.

MASK

SiO2

Si



FLUOROCARBON PLASMA ETCH SELECTIVITY

Increasing bias voltage increases etch rates.

Loss of selectivity with increasing bias voltages.

ANKUR_AVS_17



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

ANKUR_AVS_18



ETCHING PROFILES – RECIPE

ANKUR_AVS_19 ANIMATION NEXT SLIDE

200 V(Slow, selective)

1500 V(Fast, non-selective)

1500/200 V(Fast, selective)

1500/1000/100/200 V(Fast, selective)



ETCHING PROFILES – RECIPE

ANKUR_AVS_19 ANIMATION SLIDE

200 V(Slow, selective)

1500 V(Fast, non-selective)

1500/200 V(Fast, selective)

MASK

SiO2

Si

1500/1000/100/200 V(Fast, selective)

1847 s 713 s 1377 s 1356 s



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

Documents

EFFECT OF BIAS VOLTAGE WAVEFORMS ON ION ENERGY DISTRIBUTIONS AND FLUOROCARBON PLASMA ETCH SELECTIVITY* Ankur Agarwal a) and Mark J. Kushner b) a) Department