Fabrication and measurement of Si-based inductors integrated in WLP

Nowadays, inductors are prone to be integrated into chip surface for miniaturization considerations in some RF applications, but high Q factor cannot be achieved because of the lossy CMOS Si substrate. In this study, single layer spiral coils of one turn were fabricated on Si wafers with high and low resistivity via redistribution process and a polyimide layer were added on the wafer with low resistivity to reduce the substrate losses. The inductors were tested by vector network analyzer and the Q factors were compared. The highest Q factor was achieved on the wafers with high resistivity. The polyimide insulation layer can reduce the substrate loss effectively and making better Q factor.