2. Field Threshold Implant and Field Oxidation [Fig. 4.17(b)]. Nitride (Si3N4) is deposited, masked, then etched. The etched nitride regions define the location of the field threshold ion plant [Fig. 4.17(b)] and the locations where oxide Si02 is permitted to grow during field oxidation [Fig. 4.17(c)]. Areas where nitride remains mask (prevent) oxide growth. These regions are where transistors will be built. The thick (approximately 6000 A) field oxide isolates adjacent transistors in order to prevent electrical interactions. After field oxidation, the nitride mask is removed.