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? Thanks a lot in advanced.

Hongzhi Chen