u00b0 to 1075 u00b0 c for a time between about 30 to 90 min followed by an additional drive in at a temperature between about 1125 u00b0 to 1175 u00b0 c for a time between about 10 to 120 min the drive in results in the formation of p well region 7 n well region 5 as well as a growth of a silicon oxide layer 6a on the surface of the p well region after removal of oxide layer 6 and oxide layer 6a via use a bufferd hydrofluoric acid solution conventional isolation processing is applied to achieve the field oxide fox region 8 illustarted in fig4 briefly the method used to form the fox insulator is to use a thin thermal oxide and a silicon nitride layer as an oxidation mask the desired fox regions are etched open in the silicon nitride silicon dioxide mask using conventional photolithographic techniques and dry etching processing after removal of the masking photresist and a chemical clean a fox region is grown typically to a thickness of about 4000 to 7000 angstroms after removal of the oxidation mask via wet processes such as hot phosphoric acid for the silicon nitride layer and a buffered hydrofluoric acid for the thin thermal oxide another wet chemical clean is performed followed by the formation of a screen oxide 9 grown as thermally oxidized silicon at a temperature between about 850 u00b0 to 950 u00b0 c to a thickness between about 150 to 250 angstroms this can be seen in fig5 a blanket ion implantation of bf2 10 is then performed for purposes of adjusting the threshold voltage of a n channel device and.