Ions and Radicals in Plasmas

The energy that is transferred to a plasma is ultimately stored in high energy plasma particles such as fast electrons, photons as well as atoms in high energy states: ions and radicals (slide 1).

Positive and negative ions co-exist in plasmas. Positive ions are very important since they are accelerated through the sheath. Negative ions play an important but secondary role since they don’t reach the wafer under normal conditions. They can influence plasma properties but rarely participate in the surface reactions of the etch process. When electronegative gases are used in plasma etching, the density of negative ions in the plasma can be higher than the electron density. In processing plasmas, the ion to neutral fraction ranges from 10-2 for high density plasmas to the 10-4 to 10-6 range for conventional RIE plasmas.

Radicals are more abundant than ions in molecular gas glow discharges because they are generated at a higher rate than ions (lower threshold energy and ionization is often dissociative) and they survive longer in the discharge than ions.

Slide 2 shows typical relative concentrations of plasma species in low density plasmas (Magnetically Enhanced Reactive Ion Etching (MERIE) and Single Frequency Capacitively Coupled Plasmas (SF-CCP)) and in high density plasmas (Inductively Coupled Plasmas (ICP), Ultra High Frequency Capacitively Coupled Plasmas (UHF-CCP), Double Frequency Capacitively Coupled Plasmas (DF-CCP) and Electron Cyclotron Resonance (ECR) Plasmas).

More plasma etch fundamentals …