Actually, direct aluminum bonding is even a possibility.  The primary
issue is to deal with the oxide layer on the aluminum, but a cure in
reducing atmosphere will typically take care of this.  If bonding can
also occur in the reducing atmosphere, then so much the better.

The only issue is that the aluminum bonding process requires a
relatively high bonding force Much greater than that for direct Au-Au
bonding, since Al is harder).

Patterned or unpatterned polymer bonding (via Cyclotene or SU-8, for
example) is also a possibility.  The primary deciding factor for bonding
medium is the required charactreristics for the bonded package.  The
characterisitics can include:

Is hermeticity required?
Electrical contact?
Protection of fragile components (common for MEMS devices)
Vacuum (again, common for MEMS resonating components - affects damping
factor).

Best Regards,
Chad Brubaker