High Temperature Metallization for Hybrid Circuits

Hello friends

I am trying to develop Hi Temp Hybrid process which can operate continiously at 250 degree C. I am having problems with metallization of traces connection different die pads and other passive components. Is nickel metal covered with thin gold layer is solution?
or some one can suggest other options.

Substrate on which metallization will be put is 96% alumina.

Thanks

You say you are using 96% Alumina, what is the surface finish? Hopefully not polished or bellow 4u-inches, which in itself will cause poor adhesion.
TiW/Au systems will work up to about 150C. Tiw/Ni/Au will go to your requirement of 250C as long as you use about a minimum of 5000 A or 0.5 µm of Ni which is the diffusion barrier for the Au.
Of course this also assumes your using PVD sputtered thin films.