Balancing temperature dependence of on-wafer SOS inductors

The inductance and the quality factor (Q) of on-wafer inductors have a strong temperature dependence. For balancing unwanted temperature-induced variation, two different layout structures for silicon-on-sapphire (SOS) inductors were studied. One test series used metal inductors with a varying number of vias and the other metal inductors with silicon-based coils added. The tested silicon coil materials were polysilicon and n-diffusion and p-diffusion silicon. At the temperature of 423 K, the metal inductor with the highest number of vias gave less decreased Q. A silicon coil increases the parasitic capacitance of the inductor, which decreases the self-resonant frequency. Thus, the Q of the inductor with the polysilicon coil was equal to or better than that of the plain metal inductor up to the frequency of 8 GHz. Furthermore, the polysilicon coil balanced the temperature dependent variations reflected to the inductance and the self-resonant frequency. The inductor with the polysilicon coil had the best and the most stable characteristics in the measured temperature range, from 233 K to 423 K. The present results are important for the design of System-in-Package (SiP) stacked systems in which local power densities may be increased.