Fundamentals of Plasma Etching Part 2 – Focus on plasma/surface interactions Jim McVittie Stanford Nanofabrication Facility Stanford University 2008 NNIN Etch Workshop

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Outline • • • • •

Overview Physical Sputtering Chemical Etching Ion Enhanced Etching Inhibitor Deposition – Profile control – Ion Enhanced deposition

• Bowing, Microtrenching, ARDE • Summary

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Plasma Etching Trends

Anisotropy

Selectivity

Energy

Pressure

Sputter Etching and Ion Beam Milling

Physical Processes

High Density Plasma Etching Reactive Ion Etching Plasma Etching Wet Chemical Etching

Chemical Processes

• Plasma etching uses chemistry to enhance rates and selectivity while keeping anisotropy properties of sputtering

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Etch Mechanisms • Physical Sputtering

• Pure Chemical Etching

• Ion- enhanced Chemical Etching

• Ion- enhanced Inhibitor Etching

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Physical Sputtering •

Physical etching involves momentum transfer from energetic ions to the surface atoms via a collision cascade process. – Atoms which gain energies above their binding energy leave the surface – Sputter Yield Yp = # of sputtered atoms per ion Scattered ions

Collision Cascade

+ Sputtering

Sputter layer 1 or 2 atomic layers

Displacement of lattice Phonons (heat)

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Sputtering: Eth and Yp(E) •

•

Sputter yield Yp depends on – Ion and target masses – Ion energy – Binding energy Ebinding of target material – Crystal structure

Sputtering of Silicon Jin & Sawin

Threshold Energy Eth

>

5 Ebinding

• Model Yp(Ei) = Ypo (Ei1/2- Eth1/2) • Etch Rate = Yeild x Ion Flux

Eth ≈ 25 to 50 eV (Ar)

= Yp(E) Γi Figure from Jin et al, 2002

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Sputter Etch Rates (Ar Ions at Normal Incidence Angle) For 500 eV at 1 mA/cm2 Material Az 1350 SiO2 Si Zr Pt Al GaAs Au GaAs

A/min 250 330 370 570 620 640 650 1080 1600

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Angle Dependent Sputter Yield φ

Reflections

+

≈ 1/cos φ

Γi Sputtering layer

• Energy deposited into sputter layer increases with φ Yp ≈ Yo 1/cos φ for φ < ≈40°

Peak ER

• At high φ the ions begin to reflect or • Physical etching often has peak etch rate ER near 50 ° target material. Causes Y to decrease – Leads to faceting at top corners • Ion flux is measured normal to a surface ~ 50 ° Γi surface = Γi cos φ scatter off surface without sputtering

Etch Rate = Yp Γi cos φ 8

Sputter Etch Example Patterning of Pt using ion milling

Frence left after resist removal ⇒

Downside of sputter etching • Low etch rates • Low selectivity • Re-deposition ⇒ Fences and veils From Werbaneth, Tegal, PEUG Jun 1999 9

Pure Chemical Etching • Reactive neutrals (atoms, radicals or molecules) from plasma spontaneously react with material and form volatile products which are pumped away.

Etchant (free radical) creation e- + Etchant transfer

Byproduct removal

Mask Etchant Etchant/film adsorption reaction Film

– Example: F (gas) + Si (surface) ⇒ SiF4 ↑ O (gas) + resist ⇒ CO ↑ and CO2 ↑ – Cl and Br only chemically etch n+ silicon – Cl and Cl2 spontaneously etch aluminum but product deposition often inhibits this etch without ion bombardment 10

Boiling Points of Typical Etch Products

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Characteristics of Chemical Etching • • • • •

Can be isotropic but usually faster near opening Large undercut Load dependent (More exposed wafer area slower etch rate) A ↑⇒ ER ↓ Suppressed by deposition W More important at low ion energies and high pressure Model: Spontaneous etch rate ERs = ko e(-Ea/kT)Γchem where ko = rate constant, Ea = activation energy, and Γchem = reactive neutral flux T ↑⇒ ER ↑ – Temperature dependent

• Ea (F) ~ 3x Ea (Cl) ⇒ ERs(F) ~ 100x ERs (Cl) Γchem

ko

Ea

#/cm2/s

Acm2s/#min

eV

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Effects of Gas Additives on Chemical Etching

• Gas additives can be used to increase chemical etching by increasing the production of the reactive etch species •

Relative Etch Rate

O2 added to CF4

SiO2

5x increase Si

Example: O2 addition to a CF4 discharge.

Percent O2

– O2 and O react with unsaturated CFx to product more free F. – At high additions of O2 the surface oxidizes and this slows down the etch rate 13

Ion-enhanced Chemical Etching •

Activation - ion activate adsorbed chemical species to form weakly bonded products which slowly desorb – Simulation suggests that ions temporarily break surface bonds which allow reactive species already on surface to quickly react before bonds reform – Needs to form volatile product Chamber

Reactive Species

XeF2 Gas Only

ion Adsorption

Reaction

Desorption

Ar+ Ion Beam + XeF2 Gas

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12x faster

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• Example: Shows etch rate of silicon as XeF2 gas (no plasma) and Ar+ ions are introduced to the silicon surface. Only when both are present does appreciable etching occur.

Figure from Winters, 1980

Ar+ Ion Beam Only

5 4 3 2

From Coburn

1 0

100 200 300

400 500 600 Time (sec)

700

800 900

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Ion Enhanced Si Etch Yields • For F2 chemical etching component has been subtracted. • Cl2 and HBr show very similar yield enhancement. • H enhances yield of HBr over Br2 high etch rates • Smaller size of HBr yields higher surface coverage • H breaks subsurface bonds

From Vitale -- JVST 2001

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Characteristics of Ion-enhanced Etching

Flow Ratio (Cl/ion) • • • •

Ion Angle (deg from normal)

Anisotropic if chemical etching is suppressed Much higher ion yields than pure sputtering and more selective No peak in yield at intermediate angles ion-enhanced case Depends on neutral to ion flux ratio R – Yield saturation at high R

•

Etching Yield (SiO2/Ar+)

Etching Yield (Si/Ar+)

Etching Yield (Si/Cl+)

Cl + 100eV Ar

-- Yield is controlled by [Cl] at low R

Lower temperature sensitivity Figs from J.Chang, 1997

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Etching Yield (Si/Cl+)

Effect of Adsorbing Species: Cl and Cl2 • Cl has more than twice the yield of Cl2 • High fragmentation of Cl2 needed for high etch rates • Recombination of Cl reduces etch rate – Bare metal give high recombination rates Flow Ratio R (Cl/ion)

Figures from Chang, 1997

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Combined Etch Mechanisms

•

For Cl etching of silicon Ys 60° The scattered ions show a distribution about the specular angle Etching has to be ion flux limited Depends on – Ion anglular Distribution (IAD)

– Wall slope -- Less for truly vertical walls – Angle dependent sputter yield curve • Less micro-trenching for HBr than for Cl2

Reflection Efficiency

Bicro-trenching

~60°

Angle of Incidence 29

Ion Scattering Examples Effect of IAD

Effect of Etch Time for Cl2 Etching of Si

Effect of Wall Slope Sloped wall

Vertical wall

From Asbdollahi-Alibeik, 1999

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Aspect Ratio Dependent Etching (ARDE) VLSI Aluminum Etcing Si MEMS Etching SF6/C2F8

Standard

Narrow

Wide

Reverse

78 µ m 128 µm

• •

ARDE, Etch lag or Micro-loading – Etch rate depends on size of opening Characteristics – Worst for Al etching

• Neutral limited etching – Least for poly-Si etching From Ayon, 2000 • Ion limited etching • Larger pump often decreases poly ARDE – Oxide often show reverse ARDE 31

Causes for ARDE • Limited neutral transport – Chemical limited etching -- Al etching – Neutral loss on walls by recombination or by etch product formation

• Deposition on feature bottom – Can show reverse ARDE • Wide ion angle distribution

•

Loss on wall

More dep in wider feature

– Only near normal ions make it + to bottom - Charging effects - - – Lower energy ions +++ deflected to wall

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Summary of Etch Mechanisms Ionic species Reactive neutral species • Free radicals important + Mask erosion

Charging Mask Film

---

+

+

Undercutting Chemical etching • Isotropic, very selective

Micro-trenching • From ion scattering Sidewall-inhibitor Deposition • Sources: etch byproducts, mask erosion, inlet gases • Can inhibits chemical and ion enhanced etching

Physical etching • Anisotropic, non-selective

Ion Enhanced Etching • Anisotropic, selective, high ER • Needs both ions and reactive neutrals Ion Enhanced Inhibitor • Anisotropic, selective, high ER • Thin inhibitor supplies reactive neutrals • Chemical etching after Ion removal of inhibitor 33