Plasma etching uses an electrically conductive gas (composed of ionized gas or molecules) to remove unwanted portions of conductive or insulative patterns. The objective of the plasma descum step is to clean the via bottom and dielectric surface after laser ablation or photolithography. The descum step improves the reliability of the via interconnect. Failure to descum can result in poor metal adhesion and increased interconnect resistance. The substrate must be fully cured before performing the plasma descum.

For the plasma descum process to proceed six steps must occur:

1. A feed gas intorduced into the chamber must be broken down into chemically reactive species by the plasma.
2. The chemically reactive species must diffuse to the surface of the substrate and be absorbed.
3. Surface diffusion occurs until the species react with the exposed film.
4. The reaction must be desorbed.
5. Diffused away from the wafer.
6. Transported by the gas stream out of the descum chamber.

The etch rate is determined by the slowest of these steps.
Some typical etch chemistries are shown in the table below:

Si	CF4/O2, CF2Cl2,CF3CL
SiO2	CF4/H2, C2F6
Organics	O2, CF4/O2,SF6/O2
GaAs	BCl3/Ar, Cl2/O2/H2