There are a couple of reason for using a combination of insulators, one is that
it a silicon nitride layer directly on silicon is not particulary good and the
silicon dioxide helps improve this.  A second reason is that these types of
capacitors are commonly used as sensors in an aqueous environment.  In this
case silicon dioxide on its own is not as good a barrier to the movement of
ions as silicon nitride.

Hope this helps,

Helen Berney.

Quoting Qintao Zhang :

>
> 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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