Peter F. Klammer wrote

   I understand what Microchip did, but I still don't like it.  They put the
   code-protect fuse (or *one* of them, anyway) under metalization, so it is
   UV-proof.  The reason being, that the same dice are used for windowed or OTP

I believe that E(E)PROM cells can also be erased by soft Xrays. The problem
with Xrays is that they are 10-50 times more energetic than UV (UV quanta
by definition have energies in the range between 3 and 200 eV, and Xrays
are above that, with most typical Xray generators using 5,000-50,000 eV).
Thus, Xrays can induce radiation damage in the semiconductor, so you have to
be very careful with the dose. On the other hand, a little metalization won't
prevent the erasure by Xrays.

I suppose that a friendly dentist or crystallographer would let someone use
their Xray machine; would someone be willing to experiment a little with
the times necessary?

        p