Also, I'm quite sure you can get conductive Stycast. This type of epoxy
is fairly easy to handle, and is ideal for me as it doesn't break at
cryogenic temperatures.

Another thing, you might need to investigate is the actual resistance of
your wafers. If your Si layer, doped or not, has too large resistivity,
almost your entire field will be across it, and not your devices. As far
as I can gather, your setup works as a voltage divider, with the voltage
divided between the heavily doped Si layer, and your devices (where/how
is your counter-electrode mounted?). Of course, this is assuming, that
you are in voltage bias mode.

Best regards
// Morten Aaroe
PhD-student
DTU Physics
Technical University of Denmark

Hongzhi CHEN wrote:
> I have fabricated some devices on the top of the
> silicon dioxide on some heavily doped SiO2/Si wafers.
> And then I want to apply the bias to the heavily doped
> silicon through the backside of substrate, generating
> electric field to change some properties of my devices
> on the top.
>
> I made some PCBs which have big pad on the top. I
> contacted and pressed the back side of the silicon to
> the PCBs. However, it seems the electrical field can
> not be generated in this way, since nothing changed
> after I apply the bias the pads on the PCBs.
>
> I think the contact between the pads and the backside
> of the silicon maybe not good.
>
> Any advice will be appreciated. I heard some people
> put some epoxy at the back of the silicon to make it
> ohmic contact. Does anyone know what kind of epoxy are
> they using? And are there other ways to do it instead
> of using epoxy?