Abstract: A method for controllably varying the selectivity of reactive etching process for etching a polysilicon film, involves the addition of an adjustable amount of oxygen (O.sub.2) to a chlorine based chemistry (e.g. Cl.sub.2, HCl) during the etching process. The adjustable amount of oxygen permits the selectivity to be varied from a low to moderate rate at the beginning of the etch and through most of the film thickness to as high a selectivity as is required for overetch, to allow anisotropic removal of polysilicon without penetrating thin oxide. Selectivity is effectively proportional to the oxygen concentration and may be varied from a low to moderate selectivity value (e.g. 20:1) to a high selective value (e.g. 100:1) of polysilicon oxide etch rate in a controlled manner.