electrical & physical oxide thickness

Hi,
What's the difference between electrical & physical oxide thickness? The variation that literature speak about it on oxide thickness, is on electrical or physical one? Thanks

It depends what you want from it.

If you want FET SPICE model accuracy, then you want the
electrical tox.

If you want inline process control, of the physical thickness,
you want optical (physical) tox.

Electrical tox differs from physical by things such as poly
dopant depletion, "buried channel" doping profiles below,
etc. A depletion region abutting the oxide, at either side,
reduces the measurement because you're measuring a
series capacitance (1/(1/c1+1/c2)) - oxide in series with
depletion region capacitance. So your toxe relation to
toxp will also be operating-point variable, and you can't
depend on the person who wrote the test program to be
the same person who knows what's cared about and what
delivers the best accuracy, about it.