Surface Processes in Plasma Etching

Physical and chemical surface processes are central to plasma processing. Reactive species are created in the discharge and transported to the surface where they can react and desorp. The reaction and desorption is frequently assisted by the energy of impacting ions. Slide 1 describes these processes for the etching of silicon in fluorine based plasmas.

Besides the reaction at the surface, the discharge equilibrium itself affected by surface processes such as positive ion neutralization and secondary electron emission at surfaces. In processing discharges (ion energy between 10-1000 eV), all positive ions are immediately neutralized at the surface (slide 2).

Heavy particles (ions and neutrals) exhibit the same behavior when they impact surfaces:
- at low thermal energies: physi-and chemisorption and desorption can occur.
- in the tens of electronvolts energy range: molecules can fragment into atoms.
- in the hundreds of electronvolts range: atoms can be sputtered from the surface.
- in the thousands of electronvolts range: implantation is important.

Adsorption and desorption are very important for plasma processing since one or the other of these reactions is the rate limiting step for a surface process (slide 3).

Adsorption A + S -> A:S is the reaction of a molecule with a surface, desorption is the reverse reaction.
Physisorption: Weak attractive van der Waals force between a molecule and a surface. Physisorption is exothermic with Ephys ~ 0.01-0.25 eV. Physisorped molecules are so weakly bound to the surface that they can diffuse rapidly along the surface.
Chemisorption is the formation of a chemical bond with the atom or molecule and the surface. The reaction is strongly exothermic, Echem ~ 0.4-4 eV.

Fragmentation: Ionic and neutral molecules with enough impact energy can fragment into atoms that are reflected or adsorbed when they hit a surface (slide 4). The threshold in energy is of the order of the molecular bond (at energies four or five times above the threshold, 50% of the molecules fragment). High energy molecular ions (energy higher than 50 eV) often fragment when they hit surfaces. Large molecules show frequently delayed fragmentation. The kinetic energy of the impact is transferred into internal energy and redistributed among the bonds as vibrational and rotational energy. If this internal energy is higher than the bonding energy, the ion fragments along this bond. Large molecules with only one type of atoms like C60 are especially resilient to collision induced fragmentation.

Sputtering: At energies above 20-30 V, heavy particles can sputter atoms from a surface. The sputtering yield increases rapidly with energy up to a few hundred volts generating collision cascades in the solid bombarded. Above these energies, the yield is independent of the projectile energy and the target atom density.

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