Fundamentals of Plasma Etching Part 1 – Focus on the Plasma and Ion Energy Control Jim McVittie Stanford Nanofabrication Facility Stanford 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

RF Power • 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 3

Ion Enhanced Etching Effect From Coburn and Winters

Spontaneous (Chemical) Etching

Ion Enhanced Etching

Physical Etching (Sputtering)

25x Etch Rate increase

Ions + Adsorbed Reactive Neutral

High Etch Rates 4

Ion Directionality

V Sheath

0

Wafer Position

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Plasma Etch Reactors Capacitive Coupled CCP RIE Type

High Density Inductive Coupled ICP Type

Downstream

Plasma Plasma stop Reactive Neutral

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) Vacuum Chamber RF

Glow or Plasma Region

13.5 MHz

Sheaths Wafer

… Matching Network

Driven electrode Gas In

To pump

• 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 8

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 9

Ionization, Radical Generation and Electron Temperature dN(ε)/dε

Electron Energy Distribution Dissociation Ionization

0

Te/2

εd εiz

Energy, ε

Expontential Boltzmann Tail exp(-ε / Te)

Te ~ 4 – 5 eV

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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

Plasma Ions +

electrons

-

Sheath

This is the source for the name capacitive coupling 11

CCP Currents E

Ions +

Irf Je -

Ji

Plasma electrons Sheath

Irf Plasma Region • Small E field • Quasi neutral ni+ = ne• e- lighter & faster ve ~ 100x vi • e- carries current Jrf = Je >> Ji

Sheath Regions

JDisp

Sheath osc

J is current density

• 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 cycle JiA= - JeA • Jrf carried by displacement (capacitor) current Jrf = Jdisp • Charge transfer by sheath width oscillation • Sheath Charge Dc bias 12

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. no

n

ns no=ni=ne

ni=ne ni

Plasma Have neglected pre-sheath region 0

ne~ 0

electrode + ne(t) + + + n + + + e+ Sm X S(t) 13

RF Sheath Analysis

After Lieberman

s

• •

Assume Jrf = Jo sin ω t Sheath oscillation is near sinusoidal s ~ so sin ω t Max Sheath width sm~ 2so

•

Analysis gives

• • •

sο = Jο ε ω ns

ωt

– Sheath width, s, increases with Jrf – s decreases with frequency and plasma density Charge stored in sheath Q = e∫ s m (ni −n )dx e sh o Poisson’s Eq

d 2V / dx 2

DC Sheath voltage

= e(n ( x) −n ( x)) / ε i

e

o

Vs ≈ 1.3 J o eε oω 2 ns

n

– DC sheath voltage increases with RF current and decreases with RF frequency

+

ni

+ + + ++ ne 0

s

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Vdc Depends on Irf and Electrode Geometry • •

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

J rf = I rf / A V(t) S2

J rf 1 < J rf 2

S1

Vs1 < Vs 2

V

Typically Vs1 A2 (Used to avoid sputtering gnd electode)

X

Self Bias

Vdc = Vs1 −Vs 2 ≈ −Vs 2 Vrf Swing

Vdc ≈ 1.3 I rf eε oω 2 ns A2 15

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 16

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

Toroid of high density plasma

Current in coil induces current loop in plasma in glass tube

B field lines have been compressed because opposing B field from induced current loop in plasma toroid

• 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 17

ICP Configurations RF for plasma generat

Substrate

Chamber

RF bias

• 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 . 18

Ion Directionality + Ti Ions enters sheath with transverse energy of Ti Plasma

Sheath

+ Vsh -

E +

Wafer

• • •

Free-fall

Collisional

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

-

++ 19

Collisionless Sheath Ion Directionality σ

E

Vs σ θ

θ

= tan − 1

Direction of mean ion

T i eV s

IAD

Ti •

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 Æ σθ ~ 4.0 °

σθ ≤ 4.0 ° • Sheath voltage control is essential for etch control

•

For anisotropic etching, typically we need

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Beyond Simple DC Biasing: RF Effects on Ions Crossing Sheath

After Barnes et al, 1991

For Ar @ 1 mT

Vsdc = 27V Te = 5eV Ti = 0.5ev ne = 1 x 1011

Vsdc = 100V Te = 2eV Ti = 0.05ev ne = 5 x 109

50 MHz 25 MHz 10 MHz 5 MHz 2.5 MHz 1.0 MHz 0.5 MHz

50 MHz

25 MHz 10 MHz 5 MHz 2.5 MHz 1.0 MHz 0.5 MHz

• 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)

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Using Bias Frequency to Control Etch Profile After Schaepkens 1999

-85V, 1.3 MHz

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

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AMAT Oxide Etcher With Dual Bias Frequency AMAT PEUG 2007 Talk

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

low Vp

• ICP not used for Ox etch

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Use of Mixed Bias Freq to Improve Ox Etch AMAT PEUG 2007 Talk

13 MHz only

13 MHz/2 MHz 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

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