Permittivity of LTCC substrates porousified with a wet chemical etching process

AbstractIn this work, the permittivity of porous LTCC (low temperature co-fired ceramics) DP 951 is measured and evaluated. The dielectric constant is locally lowered to improve the performance of e.g. radar sensors operated in the GHz range. Porousification is done at originally dense LTCC substrates in the fired state by a wet chemical etching procedure using hot phosphoric acid. Choosing this approach, areas with tailored permittivity can be generated in one single LTCC layer. The etch time and the bath temperature precisely control the penetration and hence, the porosification depth. Therefore, the decrease in dielectric constant of the LTCC substrate can be correlated to the thickness of the porous layer. The dielectric constant 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 substrates a relatively low reduction from ϵr=7.8 to 6.45 is achieved when a porosification depth of about 35 μm is reached. Based on numerical simulations, the effective dielectric constant for a 100 μm thick glass-ceramic layer which is porousified to a depth of 35 μm is calculated to 5.2, whereas the thickness represents a lower limit for tapes being typically implemented into the fabrication process of monolithic LTCC systems with integrated metallization planes. This value is lower compared to those of commercially available low-k LTCC tape systems