Etching

Etching

By Deepak RawatGBPEC PauriUttarakhandIndia(246001)

(An important step in IC fabrication )

OutlineWhat is EtchingSteps preceding Etching in IC

FabricationTypes of EtchingSimple Idea of Wet EtchingPlasma EtchingWhat is PlasmaAnisotropy and SelectivityAdvantage of Plasma Etching over Wet

Etching

What is EtchingSimply removing unwanted

materials from the surface to form a required pattern

Steps Preceding Etching in IC Fabrication1. 2.

3. 4.

Types of EtchingTwo types of Etching

◦Wet Etching: ◦Etching is done by liquid chemicals◦Unmasked areas are etched away by the

chemical reactions (Oxidation and Reduction )

◦Plasma Etching (Dry Etching):◦Etching is done exposing the material into

bombardment of ions in Plasma

Simple idea of Wet EtchingEtchants: KOH, HF, BF6, BCl3

SiO26HF H2SiF62H2O

Plasma Etching (Dry Etching)Faster and Easier wayBoth chemical and ionic species

play the roll

Electrode

GroundGas inlet( Ar, CF4, O2)

Gas outlet,pump

Matchingnetwork

RFgenerator

Plasma

RF power input

Electrode

Plasmasheaths

What is PlasmaState of mater which consist free

electrons and cationsPlasma looks similar to the gasesPlasma is also called as ionised

gases

In Plasma high number of electrons does not move around in orbits

Plasma is created when gas is either exposed to high temp. Or high voltage

Due to these free electrons and cations Plasma easily conducts electricity and also produce magnetic field.

Nature also produces plasma by means of fire and lightning.

What happens inside plasma

Ionization: CF3+ e- CF3+ + 2e-

Dissociative ionization: CF4 + e- CF3+ + F + 2e-

Excitation: CF4 + e- CF4* + e-

Recombination: CF3+ + F + e- CF4 F + F F2

Dissociation: CF4 + e- CF3 + F + e-

Physical Etching inside the Plasma

Etching species are ions like CF3+ or Ar+

which remove material by ion-bombardment.

Ion etching is much more directional (anisotropic) due to directional acceleration of ions by high E field

Mask

Film

+ +

Ionic species

++ +

+

AnisotropyEtchant can not distinguishes b/w

vertical or horizontal dimensions (isotropic).

Anisotropy = 1 – dH/dVWet etching is isotropic and dry

etching is anisotropic.

SelectivityEtchant should distinguish b/w

SiO2 and Si wafer.Wet Etching is Selective than Dry

Etching.

Advantages of Plasma Etching over Wet EtchingEliminates handling of dangerous

acids and solvents.Uses small amounts of

chemicals.Anisotropic etch profiles.High resolution and cleanliness.Less undercutting.Better process control.