Oxide-nitride-oxide stack having multiple oxynitride layers

A method of fabricating a semiconductor device over a semiconductor structure, comprising:

• forming a channel;
• forming a tunnel oxide layer adjacent to the channel, and a blocking oxide layer;
• forming a multi-layer charge storing layer between the tunnel oxide and blocking oxide layers, further comprising;
• forming a first charge storing layer adjacent to the tunnel oxide layer, the first charge storing layer comprising an oxygen-rich silicon-rich oxynitride layer, a stoichiometric composition of the oxygen-rich silicon-rich oxynitride layer resulting in the first charge storing layer having a low charge trap density;
• and
• forming a second charge storing layer directly on the first charge storing layer and adjacent to the blocking oxide layer, the second charge storing layer comprising an oxygen-lean oxynitride layer, a stoichiometric composition of the oxygen-lean oxynitride layer resulting in the second charge storing layer having a high charge trap density, wherein the charge density of the second charge storing layer is at least 1000 times greater than the charge trap density of the first charge storing layer;
• and
• forming a silicon containing gate layer on the blocking oxide layer.