From measurements of optical emission and silicon etch rate, we are able to separate contributions due to the chemical etching and the ion-bombardment enhanced etching in the CF4+O2 reactive ion etching process. The chemical etching part of undoped polysilicon etch rates is linearly proportional to the ground state fluorine population and the ion bombardment part is proportional to the dc self-bias voltage (V2.3bi). The chemical etching predominates during plasma etching, giving rise to the isotropic etch profile, while both the chemical etching and the ion-bombardment enhanced etching mechanisms coexist during reactive ion etching. A degree of the etch anisotropy in reactive ion etching is determined by competition between the chemical etching and the ion-bombardment enhanced etching, and can be expressed by an equation which only involves two physical quantities, etch rate and fluorine concentration, experimentally measurable in plasma etching and reactive ion etching. The silicon loading effect leads to a substantial decrease in a number density of the ground state fluorine and consequently makes an etch profile more directional by reducing a contribution due to the chemical etching. Also, we have identified important process parameters which influence etch profiles.