If you place a 2nd Pyrex (dummy)wafer between the Pyrex wafer that
you're bonding and the power supply anode, the back of the bonded Pyrex
will appear clean after bonding.  I guess the dummy Pyrex absorbs the
sodium from the surface of the Bonded Pyrex wafer.

Roger Shile

-----Original Message-----

When bonding sodium containing glasses, at the cathode side (top glass
surface) sodium hydroxide can form as a contaminant. This is thought to
be
due to the reaction between the migrated sodium and moisture.

This effect could be avoided by chosing a different electrode geometry
or
material. Can anyone give me ideas or references for avoiding this
specific
problem.