well proximity effect

Well Proximity Effect
[intro]
implanting N-Well causes Vt increasing near N-Well edges.
The implanting particles above the N-Well edges are clustered at the N-Well edges, i.e. the concentration near N-Well edges are higher than that in the center of the N-Well.

[Environment]
BSIM 4.5 or above
in 90nm/65nm or above CMOS process

[Solution]
Pre-sim:
Cadence: Enable 'Well Proximity Accurate' option for every MOS
HSpice: set SC variable,
    e.g.: M1 VD  VG VS VB NMOS L=0.5U W=2U SC=2u

Layout:
increase the space between N-Well edges and MOSes according to DRC rules or above 3um.
inserting dummy source/drain for sensitive MOSes.

[Ref] http://www.bubuchen.com/2008/09/well-proximity-effect_5006.html