I think there are two main reasons. One is that there is
great difference of the lattice constant between the Si and
Si3N4. It will produce a great stress on the silicon. I saw
a guy directly deposited the silicon nitrite on the wafer
and the wafer broke when it came out from furnace. So, a
stress relief layer which is usually SiO2 will be added
between them. The other reason of adding a SiO2 layer is to
produce an etch stop layer for dry etch.

>
> Hi, all
>
> I am a new member on MEMS. I am trying to understand a
> capacitor structure composed of silicon/silicon dioxide/
> silicon nitride sandwich. I am wondering why need a
> silicon dioxide layer in the middle? silicon nitride is
> also a good insulator, why we need two insulators for a
> capacitance? Are there big difference between silicon
> dioxide and silicon nitride, especially on electrical
> property?
> Thanks a lot
>
> Qintao Zhang
>
>
>
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