Silicon etch rates in CF4/O2 plasma have been studied in a diode reactor which allows both reactive ion etching (RIE) and plasma etching. A fluorine population was also measured from the intensity of optical emission in conjunction with argon actinometry, in order to separate etch rates contributed by the ion enhanced etching and the spontaneous chemical etching. A heavily doped n‐type silicon etches faster than undoped silicon, while a heavily doped p‐type silicon etches slower than undoped silicon. Although this doping effect is present in both ion enhanced etching and chemical etching, it is a more severe influence in the isotropic chemical etching. The thermal activation energy for the chemical etching was measured to be 0.10 eV, independent of the chemical and electrical properties of dopants. The Coulomb attraction between uncompensated donors (As+) and chemisorbed halogens (F-) enhances etch rates in a heavily doped n‐type silicon, whereas the Coulomb repulsion between uncompensated acceptors (B-) and chemisorbed halogens (F-) inhibits etch rates in a heavily doped p‐type silicon.