Split-gate eeprom device having floating gate with double polysilicon

Abstract

Methods of forming EEPROM memory cells having uniformly thick tunneling oxide layers include the steps of forming a preliminary field oxide isolation region of first thickness at a face of a semiconductor substrate of first conductivity type (e.g., P-type) and then forming a tunneling oxide layer on the face, adjacent the preliminary field oxide isolation region. The memory cell's drain region dopants are then implanted through the preliminary field oxide isolation region and into the substrate to form a preliminary drain region of second conductivity type. The preliminary field oxide isolation region is then grown to a second thickness greater than the first thickness by oxidizing the portion of the substrate containing the implanted dopants, to form a final field oxide isolation region which may have a thickness of about 2000 .ANG.. To prevent unwanted growth of the tunneling oxide layer, a silicon nitride layer is preferably patterned on the tunneling oxide layer and used as an oxidation mask during the step of growing the preliminary field oxide isolation region to a second thickness. The silicon nitride mask is then removed and then a floating gate electrode and insulated control electrode are patterned on the tunneling oxide layer and channel region to complete the memory cell.