I trust the copper on the PCB is not protected by a layer of lacquer?

If you are not concerned about scratching the back of the wafer then you
can clamp a few layers of aluminium (aluminum for the Americans out
there) foil between the copper and the silicon. If scratching worries
you, use conductive rubber. The wafer must be clamped- the weight of the
wafer will not be sufficient. Both of these work for creating a
back-side contact onto silicon when making Porous silicon, I imagine
they will work for you.

Jason Milne
Microelectronics Research Group
The University of Western Australia



-----Original Message-----
From: mems-talk-bounces@memsnet.org
[mailto:mems-talk-bounces@memsnet.org] On Behalf Of Hongzhi CHEN
Sent: Sunday, 2 March 2008 9:34 AM
To: mems-talk@memsnet.org
Subject: [mems-talk] apply bias at the back of silicon substrate

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?