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Fundamentals of Plasma Etching Part 1

– Focus on the Plasma and Ion Energy Control

Jim McVittie<mcvittie@stanford.edu>

Stanford Nanofabrication FacilityStanford University

2008 NNIN Etch Workshop

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Outline• Etcher Overview• RF Plasma

• Why we use RF excited plasmas• The Capacitive Coupled Plasma (CCP)• How the rf current across sheath leads the DC bias• Why controlling DC bias is important for etching• Use of Inductive coupled plasmas (ICP) as low bias source• Use of ICP with CCP to control DC bias (Ion Energy)• Beyond simple DC biasing for ion energy control

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Basic Etching Process

• Electrons gain energy from RF or µw fields

• Electrons impact with feed gas to generate ions, reactive neutrals and more electrons

• Ions and reactive neutrals diffuse and drift to wafer surface where they remove and deposit material

RF Power

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Ion Enhanced Etching EffectFrom Coburn and Winters

Physical Etching(Sputtering)Spontaneous (Chemical)

Etching Ion Enhanced Etching

Ions + Adsorbed Reactive Neutral High Etch Rates

25x Etch Rate increase

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Ion Directionality

0

V

PositionWafer

Sheath

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Plasma Etch ReactorsCapacitive Coupled

CCP RIE Type

High DensityInductive Coupled

ICP Type

Downstream

Plasma

Plasma stop

ReactiveNeutral Etch Product

Wafer

Rf Bias

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Why We Use RF• DC plasmas Wafer Damage

• Leads to charging and DC currents through wafer• Microwave Plasmas

• No self (DC) bias (Needed for directional etching)• RF plasmas

• RF current through wafer causes no damage• No charging damage if plasma is uniform

• Exception is electron shading caused charging in high aspect ratio structures

• Easy to get induced self or DC bias

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Capacitive Coupled Plasma (CCP)

Sheaths

MatchingNetwork

RF

To pumpGas In

Vacuum Chamber

Driven electrode

Wafer

13.5 MHz

…

Glow or PlasmaRegion

• To start, initial voltage must exceed Vbreakdown– Depends on gas, pressure and spacing ~ 300 to 600 V

• RF current through gas maintains steady state discharge by heating electrons– Ions and electrons from ionization balance their losses

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Glow or Plasma Region• Generation Region -- Ion, electrons, excited

species and molecular fragments generated here– Relaxation of excited species produces glow (τ ~1 ns)– Reactive fragments important for etching and CVD

• Quasi-neutral gas -- ni+ = ne

- + ni-

– pos ions ni+ , neg ions ni

- , electrons ne-

– ni- can not make it to wafer -- often can be neglected

– Only weak E fields < 10 V/cm

• Neutral density >> ion density no >> ni, ne– ni /no = 10-3 to 10-6 ni ~ 109 – 1012 cm-3

• Electrons carry the RF current in this region• Plasma Potential – V between plasma and gnd

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Ionization, Radical Generation and Electron Temperature

0 Energy, ε

Expontential Boltzmann Tail

εd εiz

IonizationDissociation exp(-ε / Te)

Te/2

dN( ε

)/dε

Te ~ 4 – 5 eV

Electron Energy Distribution

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Sheath Region• Electron depletion region forms at all surfaces to

keep electrons in plasma region (ni+ >> ne

-)– Dark -- Few electrons no excited species no light– Pos Charge High E field (up to a few KV/cm )– Most electrons returned to plasma

• Few percent make it across

– Pos ions accelerated toward surface• Ions gain energy and directionality• Ion current determined by plasma

density ne

– RF current carried by displacement (capacitor) current

+ -Ions electrons

Plasma

Sheath

This is the source for the name capacitive coupling

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CCP Currents

+ -Ions electrons

Plasma

Sheath E

Je

JDisp

Sheath oscJi

Irf

Irf

Plasma Region• Small E field• Quasi neutral

ni+ = ne

-

• e- lighter & fasterve ~ 100x vi

• e- carries currentJrf = Je >> Ji

Sheath Regions• Large E field – to keep mobile e- in plasma region• e- depletion

ni+ >> ne

-

• e- cannot carry current Jrf >> Je ~ 98% of e- are returned to plasma by sheath

• Conduction currents over area balanced over rf cycleJiA= - JeA

• Jrf carried by displacement (capacitor) currentJrf = Jdisp• Charge transfer by sheath width oscillation• Sheath Charge Dc bias

J is current density

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Oscillating RF Sheath• RF current crosses sheath by displacement irf = dq/dt• For irf = io sin ωt, a charge of io /ω cos ωt builds up on each of the

sheath • On plasma side of sheath there is no electrode, displacement current

develops by the sheath moving and generating a dq/dt by depleting and restoring the e’s as the plasma edge oscillates in and out.

Plasma

X

nnsno

ni

ne(t)ne

electrode

0 S(t) Sm

no=ni=ne ni=ne ne~ 0 -----

+ ++ +

+ + +

+Have neglectedpre-sheath region

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RF Sheath Analysis• Assume Jrf = Jo sin ω t• Sheath oscillation is near sinusoidal

s ~ so sin ω t Max Sheath width sm~ 2so

• Analysis gives

– Sheath width, s, increases with Jrf– s decreases with frequency and plasma density

• Charge stored in sheath

• Poisson’s Eq

• DC Sheath voltage– DC sheath voltage increases with RF current

and decreases with RF frequency

snJs ωεοο =

dxnneQ eso ish

m )( −∫=

( ) oei xnxnedxVd ε/)()(/ 22 −=soos neJV 23.1 ωε≈

ne

nin

0 s

++ ++ + +

After Lieberman

ω t

s

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Vdc Depends on Irf and Electrode Geometry

Asymmetric

S2S1

V(t)

• Self bias voltage Vdc is the externally measured voltage• Vdc is sum of two sheath sheath voltages

Vp X0

V

VdcSelf Bias Vrf

Swing

221

s121

21

21

21

15V to10V and Typically

electode) gnd sputtering avoid to(Used

/

sssdc

ss

ss

rfrf

rfrf

VVVV

VV

VV

JJ

AA

AIJ

−≈−=

≈<<

<

<

>

=

223.1 AneIV sorfdc ωε≈

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Summarizing CCP Characteristics • The plasma is generated by RF current flow between electrodes

• Plasma density (ne) tends to increase linearly with RF power

• RF current across a sheath generates a dc voltage

• Ions gain energy from the dc sheath voltage

• In CC plasmas, ne and Eion tend to be coupled and increase together

• Ways to gain independent control of ne and Eion

• Use non-CCP method, such as ICP or ECR, to generate plasma and use CCP for bias (energy) control

• Use high freq ( > 50 MHz) RF to generate ne and low freq (< 10 MHz) for bias plasma generation

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Use of Inductive Coupled Plasmas (ICP) as Low Bias Source

Simple ICP

Current in coil inducescurrent loop in plasma in glass tube

B field lines have been compressedbecause opposing B field from inducedcurrent loop in plasma toroid

Toroid of high density plasma

Lam Style ICP

• In ICP power is transferred to plasma by the oscillating B field.

• There is minimum rf current going across a sheath, so the sheath voltage is usually small

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ICP Configurations

• Inductive coupling can generate high density plasmas with low sheath voltages.

• ICP power controls plasma density, ne .

• Capacitive coupling of a 2nd rf source drives rf current through wafer sheath and is used to control ion energy, Ei .

RF for plasma generat

RF bias

SubstrateChamber

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Ion Directionality

• At 13.6 MHz most ions respond only to the average (DC) sheath field

• Ions gain directionality and energy crossing the sheath• Ion directionality strongly affects

– Etch bow (side wall etching)– Electron shading type charging

Plasma

Wafer

Sheath

Free-fall Collisional

E

Ti

Vsh-

+

Ions enters sheath with transverse energy of Ti

+ +

--

--

+

+

+

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Collisionless Sheath Ion Directionality

• Ion directionality determined by Vs and Ti at sheath edge

• Mean ion arrives at wafer σθ degrees off the normal • Ti is determined by collisions in pre-sheath and energy at ion

creation. Typically, Ti ≈ 0.5 eV

• Example: If Ti = 0.5 eV and Vs = 100V σθ ~ 4.0 °

• For anisotropic etching, typically we need σθ ≤ 4.0 °• Sheath voltage control is essential for etch control

E Vs

Ti

σθ

seViT1tan−=θσ

Direction ofmean ion IAD

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Beyond Simple DC Biasing: RF Effects on Ions Crossing Sheath

Vsdc = 100VTe = 2eVTi = 0.05evne = 5 x 109

After Barnes et al, 1991

Vsdc = 27VTe = 5eVTi = 0.5evne = 1 x 1011

• For an oscillating rf sheath, the ion energy distribution (IED) at wafer surface depends strongly on sheath transit effect• IED tends to be bimodal with ∆εion decreasing with increasing RF frequency• IED strongly affected by ion mass, sheath thickness, and Vsheath waveform

• IED can strongly affect etch profile

• Higher energy ions will have smaller Ion Angular Distribution (IAD)

For Ar @ 1 mT

0.5 MHz1.0 MHz

2.5 MHz5 MHz

10 MHz

50 MHz

25 MHz

0.5 MHz1.0 MHz

2.5 MHz5 MHz

10 MHz25 MHz

50 MHz

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Using Bias Frequency to Control Etch ProfileAfter Schaepkens 1999

-85V, 1.3 MHz -120V, 1.3 MHz -120V, 10.5 MHz-85V, 10.5 MHz

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AMAT Oxide Etcher With Dual Bias Frequency

• 2 MHz and 13 MHz for bias• VHF for plasma generation

• VHF low Vp

• ICP not used for Ox etch

AMAT PEUG 2007 Talk

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Use of Mixed Bias Freq to Improve Ox Etch

13 MHz only

13 MHz/2 MHz

AMAT PEUG 2007 Talk

13 MHz only 13 MHz/2 MHz

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Summary

• DC self bias is a result of rf current flowing across a plasma sheath

• Increases with rf current and decreases with rf frequency

• RF biasing applied to wafer to control Ei in high density plasma systems

• Biasing is needed for controlled anisotropic etching

• Recent etch equipment designs go beyond simple DC biasing to shape

energy distribution of ions bombarding wafer surface to better control

etch characteristics