There are actually a few different approaches that are available in a situation
like this - it depends on what the processes are after the bonding step.

In the situation you are listing, it sounds as if you will need a highly uniform
bonding surface, so you will most likely want a spin-on material of some sort as
your intermediate layer. As was mentioned in earlier posts, BCB can make a good
bonding agent for such a process, but in general, there is any variety of
materials that might be able to suit (even various photoresists).  The primary
requisite is that the material be able to survive the thinning process, which is
why BCB could have an advantage. If care is taken with the coating and bonding
process, the BCB should still remain solvent releasable.

Best Regards,
Chad Brubaker