oxide layer 21 to the side of the gates 20 may be selectively etched and the surface of the wafer cleaned as shown alternatively oxide 21 may remain until a later step in the process sequence now a thermal drive in step is performed to drive the n and n regions to appropriate depths these regions form ldds 28 and source drain regions 30 respectively as shown in fig1 e this drive in step is performed in an oxidizing ambient during or after the source drain anneal to grow oxide on the n and n regions that is thicker by about three to four times than the oxide grown on the n well regions i e the future source drain regions of the pmos 12 devices this differential oxide growth to give thick thermally grown oxide layer 32 and thin thermally grown oxide layer 34 results from a differential growth rate caused by the n implant and to some extent the n implant also this phenomemon is well known and is taken advantage of here for its ability to selectively mask for the heavy p implant that follows for example thick oxide layer 32 may be approximately 750 angstroms thick while thin layer 34 may be about 250 angstroms thick these thicknesses are merely presented as representative.