To my knowledge the etch stop results from the pn-junction between the
n-layer and the p-wafer
when a reverse voltage is applied between the n-layer and an electrode
in the etching solution.
The etching stops at the pn-junction due to the lack of electrons as it
does for the boron etch stop.
The resulting structure thickness is defined by the thickness of the
epilayer.

Cheers, Dirk.


Roger Brennan wrote:

>Recently, I was told n-epi (silicon) could be used as the etch stop.  That
>is, the P substrate is etched away and the n-epi is left
>
>This is contrary to my understanding.  I thought P+ silicon was the only
>silicon etch stop.
>
>Thanks in advance,
>
>Roger Brennan
>
>