Permittivity of modified polyimide layers on LTCC

In this work, the permittivity of a tailored compound material was investigated consisting of a polyimide matrix in which hollow glass microspheres with a mean diameter of 30 μm are implemented as filler material. Choosing this approach the dielectric constant compared to that of the pure polyimide material is further decreased due to the enclosed air targeted to improve the high-frequency performance of patch antennas operated in the GHz range. Furthermore, the thickness of one single layer can be increased substantially from a maximum of about 10 μm for pure polyimide films to values above 80 μm by simply adding this type of filler material to the liquid polyimide precursor so that cavities in LTCC (low temperature co-fired ceramics) substrates can be filled more reliable. Two different variations of this compound material with filler to polymer ratios of 1:7.5 and 1:10 are realized. Basically, the film thickness depends on the spin coating speed and the microsphere content, respectively. The high initial surface roughness can be decreased to an average value of about 3 μm by applying additional layers of pure polyimide on top enabling thin film technology. The dielectric constant of the complete substrate comprising the LTCC and the compound material is measured using a ring resonator in microstrip configuration. From the resonances occurring in the transmission S-parameter |S₂₁| spectrum between 1 and 10 GHz, the relative dielectric constant can be determined. Using 820 μm thick LTCC substrates the permittivity can be reduced from originally ε_r = 7.8-6.6. By applying numerical calculations, a reduced permittivity of the pure polymer film from ε_r = 3.3 to about 2.9 can be determined when adding the glass microspheres.