Deep_NWELL
A processing step option available is "Deep NWELL" and is recommended for noise isolation.
Deep NWELL can be a somewhat confusing concept A really great explanation
The NMOS is on the Left and the PMOS is on the right. The lighter yellow indicates the implant ptype layer that defines the active area for the NMOS device. The Red region is the NWELL under the active area of the PMOS.
Imagine if you could cut the substrate out from under the two devices.
At this point if only the source/drain implants were in place and you added an insulator to the bottom you would have fully depleted silicon on insulator FDSOI.
But instead of actually machining anything, cool physics is used to blast n-type dopants deep into the substrate and actually isolate regions of the substrate from the main body.
The cross section of the same NMOS and PMOS with a deep NWELL layer under the NMOS device isolating the substrate from the NMOS.
An 3d cutaway NMOS(Left) and PMOS (Right) Device of the same width and length in deep nwell. Note this is a cut-away view because in order to isolate the substrate region of the NMOS device the entire yellow section must be surrounded by on all sides and underneath. The front and back sides are not show.
Existing side by side, the Deep NWELL NMOS and PMOS are on the Left and the standard versions are on the right, all in the same substrate.