Radicals are fractions of molecules where the valence electron energy level is not completely occupied, i.e there is at least 1 valence electon on the maximum energy level or at least on electron is missing to complete the valence electron shell.
This causes very high reactivity of any radical because it tends to react as soon as possible with any particle which can provide an electron to its spare position on the valence elctron level or which can absorb its electrons on a not completed valence electron level.
Molecules are cracked to radicals by high energy UV-radiation. This radiation is emitted by a plasma due to recombination of ions with free electrons. By this process radicals are generated within the process gas as well as on a substrate. The process of reactions between process gas radicals as well as the reactions of process gas ions, radicals and stimulated atoms (active species) with radicals of substrate molecules causes the generation of new molecules by the plasma processes.