Nitrogen implanted polysilicon gate for MOSFET gate oxide hardening

DETAILED DESCRIPTION The present invention provides a method for hardening of gate oxide without forming low boron concentration regions in polysilicon at the gate oxide-polysilicon interface.
This is accomplished by a process that involves depositing polysilicon onto a gate oxide followed by nitrogen implantation and annealing.
Then the region of the polysilicon layer that contains areas of significant nitrogen concentration is removed.
Because the nitrogen concentration peak in the bulk polysilicon has been removed the dopant concentration from a subsequent implant is sufficiently great near the gate oxide-polysilicon interface that depletion regions are not easily formed in that region during normal device operation.
In one aspect, the present invention provides a method for hardening gate oxide in a partially fabricated electronic device.
First, nitrogen ions are implanted in a first polysilicon layer deposited on gate oxide.
Second, an annealing step is performed that results in areas of significant nitrogen concentration at the gate oxide and at a location in the bulk polysilicon.
Third, sufficient polysilicon is removed from the top of the first polysilicon layer to form a second polysilicon layer in which the region of substantial nitrogen concentration in the bulk polysilicon has been removed.
Optionally, a third polysilicon layer may be deposited on the now thinned second polysilicon layer to replace the polysilicon that was removed.
Preferably, the gate oxide is not more than about 50 .
ANG.
thick.
In one embodiment, an electronically active dopant is implanted in the second polysilicon layer.
In a more specific embodiment, the electronically active dopant is boron.
In another embodiment, the electronically active dopant is phosphorus.
Preferably, the first polysilicon layer is between about 2000 .
ANG.
and about 4000 .
ANG.
thick.
In a more specific embodiment, the second polysilicon layer is between about 750 .
ANG.
and about 2500 .
ANG.
thick.
Preferably, the annealing step is performed at between about 800