Etching is a process by which patterns are transferred by selective removal of unmasked portion of a layer.

There are two types of etching:

1. Wet etching –

In this process the removal of unmasked layers is done by selective liquid etchant.

It is a purely chemical process.

The batch of wafers is dipped into highly concentrated pool of acid & the exposed areas of wafer are etched away.

Wet etching gives isotropic profile i.e. it etches equally in all directions.

Isotropic etching profile means removal of material equally in all directions which results in undercutting and uncontrolled etch features.

HF is used as etchant for Si.

• Advantages of wet etching –

  a. Simple equipment.

  b. High etching rate.

  c. High selectivity.

• Disadvantages of wet etching –

  a. Wet etching is difficult to control.

  b. It is prone to high defect levels due to solution particulate contamination.

  c. It produces large volume of chemical waste.

2. Dry etching –

Dry etching involves removal of substrate material by gaseous etchants.

It is a more physical process than chemical process.

It may involve chemical reactions by gases radical species, ion bombardment and/ or combination of both.

Dry etching gives anisotropic etching profile i.e. material is etched in preferred direction only.

It is a crystallographic dependent etching, where etch rate varies according to type of crystal plane exposed to the etchant.

• Advantages of dry etching –

  a. It is easily controllable method.

  b. It also eliminates the chemical waste disposal problem of wet etching.

  c. Dry etching is capable of producing critical geometries that are very small.