Modified Organic low-k Dielectric Layers on Fired LTCC Substrates

Abstract

In this study, the reduction of permittivity of LTCC substrates by coating with a polyimide compound containing hollow glass microspheres as filler material is described. By incorporating the filler material, the dielectric constant of the substrate material is locally lowered to improve the high-frequency performance of antennas operated in the GHz range. Furthermore, by adding the filler material to the liquid polyimide precursor the layer thickness is heightened from maximum 10 μm to above 80 μm which is enough to fill cavities in LTCC substrates. Two compound materials with filler to polymer ratios 1:7.5 and 1:10 are mixed. Afterwards they are deposited by spin coating onto LTCC substrates. The film thickness depends on the rotating speed and the filler content. With the higher filler concentration and low rotating speed of 500 rpm 82 μm thick polymer films can be achieved. The high surface roughness can be reduced afterwards by adding additional pure polyimide layers on top to Ra= 3 μm. The dielectric constant of the entire substrate consisting of the LTCC and the resulting compound material is measured using a ring resonator in microstrip configuration. From the resonances occurring in the transmission S-parameter |S21| spectrum between 1 to 10 GHz, the relative dielectric constant can be determined. Using 820 μm thick LTCC substrates a relatively low reduction from εr = 7.8 to 6.6 is achieved. However, due to permittivity can be reduced with higher microsphere amounts, the dielectric constant of pure polyimide of εr= 3.3 can also be reduced. Furthermore due to the sufficiently high film thickness of the modified substrates, the compound layer can be used as single dielectric layer.