Dry Etching 09

8/8/2019 Dry Etching 09

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520/530/580.495

Microfabrication Laboratoryand

520.773Advanced Topics in

Fabrication and Microengineering

Lecture 9

Dry Etching


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Reading for this lecture:(1) May, Chapter 5.2

(2) Williams paper

HW #6: Due Nov. 6


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HFH+

F-

H2OF-

H+H2O

H2OHFHF

Wet Etch

CF4

CF5-CF2

2+

CF4

SF6F

CF3+

Dry Etch

Wet Etching vs Dry Etching

In wet etchants, the etch reactants come form a liquid sourceIn dry etchants, the etch reactants come form a gas or vaporphase source and are typically ionized

-Atoms or ions from the gas are the reactive species thatetch the exposed film

Selectivity : In general, dry etching has less selectivity than wet etching

Anisotropy: In general, dry etching has higher degree of anisotropy than

wet etchingEtch Rate: In general, dry etch has lower etch than wet etching

Etch Control: Dry etching is much easier to start and stop than wetetching


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Etch Mechanism

The slowest dominates !!!

Generation of etching species-Without generating the etching speciesetching will not proceed

Diffusion to surfaces-etching species must get to the surfaceto react with the thin or substrate molecules-the mechanics of getting to the surface canlimit aspect ratio, undercuttig, uniformity

Adsorption-can also effect aspect ratio

Reaction:-strong function of temperature(Arrhenius relationship)

-obviously effect the etch rate

Desorption-can stop etch if the reacted species is notvolatile

Diffusion to bulk gas-can lead to non-uniform etching due to dilution of

un-reacted etching species


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Gas Phase Etch

Reactive Gas

- Gas adsorbs on the surface, diassociates, and then reacts

Xenon Difluoride Etching

XeF2 Adsorb Xe + 2 F

Particular features of XeF2 etching-extremely selective-fast etch rate-isotropic


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RF-Plasma-Based Dry EtchingA plasma is fully or partially ionized gas composed of equal numbers of positive

and negative charges and a different number of unionized molecules.A plasma is produced when an electric field of sufficient magnitude is applied to agas, causing the gas to break down and become ionized.


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Materials Etch Gases Etch Products

Si, SiO2, Si3N4 CF4, SF6, NF3 SiF4

Si Cl2, CCl2F2 SiCl2, SiCl4

Al BCl3, CCL4, SiCl4, Cl2 AlCl3, Al2Cl6

Organics O2, O2 + CF4 CO, CO2, H2O, HF

other: (W, Ta, Mo..) CF4 WF6,..

Dry Etch Chemistries


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Types of Dry Etching Processes

( 1 torr = 1 mmHg)

Types of Etching Methods Geometry Selectivity Excitation Energy Pressure

Gas/vapor Etching Chemical Isotropic Very high noneHigh(760-1torr)

Plasma Etching Chemical Isotropic High 10's to 100's of WattsMedium

(>100 mtorr)

Reactive ion EtchingChemical &Physical

Directional Fair 100's of WattsLow(10-100 mtorr)

Sputtering Etching Physical Directional Low 100's to 1000's of Watts Low(~10 mtorr)


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Degree of Anisotropy

Degree of Anisotropy

vvf

fR

R

tR

tR

h

lA 11 111 ==

For isotropic etching:R

l= R

vandAf= 0

For anisotropic etching:R

l= 0andAf= 1


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Dry Etching Si/SiO2 in F-Based Gases and Plasmas

Prominent etch chemistry in ICs & MEMSCF4 does not etch Si (does not chemisorb) but F2 gas willetch Si with etch products SiF2 and SiF4

Plasma is needed to generate F that must penetrateSiF2-like surface


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Dependence of Etch Rate and Si/SiO2 Selectivityon O2 /CF4 Ratio

Even with plasma the etch rate is slow-insufficient F concentration

Adding O2 to the plasma can increase F concentrationO + CF3 COF2 + F then O+COF2 CO2 + 2F

Etch rate maximizes around 12% O2

Etch rate decreases at higher O2concentrations

-Dilution of F concentration with

overly abundant O2-Trend is similar for SiO2

Etch rate is higher for Si

Si/SiO2 selectivity is good

Isotropic etching


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Adding H2 drastically lowers Si etch rate-lowers F concentration (H+ + F +e- HF)-Nearly 0 at 40% H

2

However, etch rate of SiO2 remains nearlyconstant

Allows etch selectivity to be increasedtremendously

Mechanism for increased selectivity has two

components-deposition of a non-volatile residue-role of O2 in etching of SiO2

Dependence of Etch Rate and SiO2/Si Selectivity

on H2 /CF4 Ratio


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Increase of Degree of AnisotropyFormation of Sidewall Passivating Films

Formation of nonvalatile fluorocarbonsthat deposit on the surfaces (Polymerization)

The deposit can only be removed byphysical collisions with incident ions

Fluorocarbon films deposits on allsurfaces, but the ion velocity is nearlyvertical. As a result, as the etchingproceeds there is little ion bombardment

of the sidewalls and the fluorocarbon filmaccumulates

Adding hydrogen encourages the formationof the fluorocarbon films because hydrogenscavenge fluorine, creating a carbon-rich plasma(same thing happened when C2F6 is usedinstead of CF4 )

Less accumulation is observed on SiO2 than Sisurfaces

Tradeoff betweenSi/SiO2 selectivityandAnisotropy


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Controlling Polymerization through F/C-Ratio

Higher F/C-ratio leads to more etching

Lower F/C-ratio leads to morepolymerization

Can be determined by the gas used

Adding H2 consumes F leads topolymerization

Adding O2

consumes C leads toetching


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Dry Etching Organic Films

O2 plasmas can remove organic films with high selectivity

Adding CF4 can increase etch rate and lowers variation-but selectivity can be reduced if too much is added

Etchrat

eofPloymid

e(um/min)

%va

riationofetc

hrate

etch rate

% etch rate variation

% CF4 added O2


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Damage in Reactive Ion Etching (RIE)

Typical ion flux of 1015 ions/cm2 are delivered at energies of

300 to 700 eV in a RIE.

After a typical etch in a carbon containing RIE, the top 30 isheavily damaged, with an extensive concentration of Si-C bonds.

RIE processes done in a hydrogen-containing ambient also have Si-Hdefects that can be observed as deep

as 300 .

Hydrogen may penetrate severalmicrons into the surface where it candeactivate dopants in the substrate.

Dry Etching 09

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