Thanks for the document. But how an NMOS will be affected during implantation of Nwell , NMOS portion will be covered by photoresist during fabrication of Nwell.Please check the attachment.
Implantati scattering and subsequent side diffusion during
the well drive are statistical processes and some dopant
tail may get to the P- where the NMOS lives, shifting its
threshold (at least, in the regions near the Nwell).
Some finite safe distance for a given max VT shift has to
be declared.