Method of making an E.sup.2 PROM cell with improved tunneling properties having two implant stages

DETAILED DESCRIPTION The present invention is directed to a semiconductor memory device and a method for fabricating a semiconductor memory device, in particular an E.
sup.
2 PROM, having an improved tunnel area wherein electrons travel to and from a floating gate of the E.
sup.
2 PROM cell.
An E.
sup.
2 PROM cell designed in accordance with the invention includes a tunneling gate fabricated via two implant stages.
Because these two stages are separate from one another, each of the implant stages is independently optimized to improve the properties of the tunnel area.
These properties enhance operating efficiency of the memory device, and extend the operating life of the memory device.
Further, the windows used to define the implant regions are easily fabricated and provide for easy control of etching.
More particularly, the two separate implant regions permit two separate doses of different materials to be implanted in the tunnel gate.
For example, in a first outer implant region of the tunnel gate, the dose can be increased to reduce resistance.
On the other hand, a second region of the tunnel gate under the thin tunnel oxide layer can be formed with a decreased dose to reduce leakage of the tunnel oxide during normal operation of the memory cell.
In establishing windows to define the implant regions, the same mask used to fabricate an active area of peripheral transistors (e.
g.
, select transistors) can be used to define a first implant region.
Further, a relatively large window can be used to establish a second implant region.
This relatively large window can be defined with a wide tolerance without concern that over-etching will damage the substrate.
In accordance with a preferred embodiment, a method for fabricating a cell for use in a semiconductor memory device such an E.
sup.
2 PROM includes the steps of forming a first oxide layer of varying thickness on a semiconductor substrate, the oxide layer having a first portion formed of a first thickness and a second portion formed of a second thickness greater than said first thickness