Physical dry etching requires high energy kinetic energy (ion, electron, or photon) beams to etch off the substrate atoms.

When the high energy particles knock out the atoms from the substrate surface, the material evaporates after leaving the substrate.

There is no chemical reaction taking place and therefore only the material that is unmasked will be removed.

Sputter etching or ion milling or ion etching:

In sputter etching, air molecules are removed from the chamber and a low pressure inert gas, such as argon, is injected. Inert gas injected at low pressure is used as “milling” tool.

RF plasma is created at low pressure, which creates an environment where the mean free path between the gas atoms is large. The RF energy from the plasma acts on the argon atoms.

It bombards the wafer with Ar+, which knock atoms from the substrate by transferring momentum.

Because the etching is performed by ions, which approach the wafer approximately from one direction, this process is highly anisotropic. On the other hand, it tends to display poor selectivity.

Sputtering is usually carried out at pressure around 50 mtorr.