Silicon dioxide patterns having vertical sidewalls have been obtained without pattern width loss by reactive ion etching in CF4. The polymerization effect, caused by fluorine deficiency in the plasma, is found to influence not only the morphology of the etched resist surface but also the SiO2 etching profiles. The polymerization is greatly reduced by depressing the temperature rise during etching. A temperature‐controlled cathode and a heat sink material are used for this purpose. The polymerization is further reduced by increasing flow rate, by increasing pressure and by decreasing power density. The SiO2 patterns having vertical sidewalls are obtained under the no‐polymerization etching conditions. These SiO2 patterns have been successfully applied to ion‐implantation masks for 16 Mbit bubble devices.