a method is provided to form a split - gate flash memory not susceptible to inadvertent reverse tunneling during programming . this is accomplished by forming a silicon nitride spacer on the negatively tapered walls of the floating gate of the cell which serves as a barrier to reverse tunneling . the negatively tapered walls , in contrast to vertical walls , is disclosed to provide a geometry better suited for forming thicker spacers around the floating gate , which in turn serve to act as a more robust barrier to reverse tunneling . furthermore , it is shown that the method requires fewer steps than practiced in prior art .