With the aim to reduce the permittivity of a dielectric substrate, closed pores have been introduced into alumina based ceramics by our innovated superplastisically foaming method. Affect of the ambient humidity on the dielectric constant was compared to those for conventional porous ceramics as well as full densified one. In the dry atmosphere, the dielectric constant (k) decreased with the porosity similar to the conventional porous ceramics. Affect of the ambient humidity on the dielectric property was almost identical to that of fully densified one. The degradation of mechanical strength depended on the pore size introduced.

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Alumina based closed pore dominated substrates have been fabricated by using superplastically foaming method developed by our group. Co-dispersing of magnesium aluminate spinel (Spinel) and zirconia without dopant (0YZ) provided adequate superplastisity to alumina matrix, while individual dispersion of spinel or zirconia resulted in only small deformation. The dielectric constant of superplastically foamed alumina decreased monotonically with porosity. Introduced pores were all closed nature at least up to 30% of porosity, at that porosity the permittivity became half as that of full dense one. Effect of the sweating water or ambient humidity has been proved to be identical to those of full dense one, which is favorable for semiconductor substrate.