The etching of SiO2on Si in a plasma containing 80% helium/20% fluorine has been studied using direct ion sampling. Ionic species in the discharge show the transition from the SiO2surface to the Si surface, and reflect changes in the gas composition in the region adjacent to the specimen very strongly. SiO2etched at a rate of 90–100 A˚/min when mounted on the ground electrode and at 350–450 A˚/min when mounted on the rf electrode. Silicon etches at 2000 A˚/min on the ground electrode and at 6000 A˚/min on the rf electrode. The enhanced etch rates for the rf electrode are attributed to the increased kinetic energy of the ion bombardment. Reactive ion etching could account for the SiO2removal rate, but not for the Si removal rate. A more dilute He‐Fe2mixture was used to etch polycrystalline Si with some degree of anisotropy, and with minimal attack on the photoresist mask.