On-wafer inductors for SOS-based RF ICs

The effects of additional non-metal coils under SOS inductors on the inductor properties are investigated. N-diffusion silicon, p-diffusion silicon, and polysilicon layers are used as extra conductors. The expected measurement results are found with inductors with p-diffusion and polysilicon layers. A Q factor of 20 is measured for an SOS inductor with an additional p-diffusion coil. An inductor with an n-diffusion layer gives an unexpectedly poor Q factor of 18. The dramatic conductivity loss of n-diffusion silicon cannot be explained by traditional temperature-mobility or frequency-mobility dependence. The conductivity loss is caused by the SOS wafer structure and it is strongest for an n-diffusion silicon layer. The number of vias between the metal coils has an influence on the inductor quality. A minor decrease in inductance value and a clear increase in Q factor are found with an increased number of vias. If the number of vias is increased too much, the via resistance compensates the achieved advantages.