Roger is correct as we often do 5 micron  up to 15um on Si02. Although
there is a self limiting effect in thermal oxidation, it can be pushed
to higher levels. it is not uncommon to use 2 or 3um for a box layer on
SOI wafers.
Ken

Shile wrote:

>Oxidation of silicon does not stop at 1 micron.  I have wet oxidized Si
>to >3 microns.  This oxide provided sufficient mask to etch through a
>300 micron wafer in KOH.