Fully 3-D symmetrical TSV monolithic transformer for RFIC

Two integrated passive device (IPD) structures are proposed in this paper. The first structure is a differential symmetrical through-silicon via (TSV) monolithic transformer that exhibited optimal symmetry between the two ends of the primary and secondary coils, enabling the operation of a differential circuit. The second structure is a fully 3-D symmetrical TSV monolithic transformer that differs from 3-D TSV transformers proposed in the literature. In addition to the optimal symmetry between the two ends of the primary and secondary coils, the primary and secondary coils also exhibited optimal symmetry. In this study, test keys with 1∶2 and 1∶3 transformer turns ratios were applied to the structure of a differential symmetrical transformer. The fully 3-D symmetrical TSV monolithic structure was designed as a test key with a true 1∶1 turns ratio. The TSV of all test keys exhibited a 5 µm diameter, 60 µm depth, and 20 µm pitch. These test keys were simulated using HFSS, a fully 3-D simulator, and fabricated on a 300 mm wafer by using the ITRI's 3-D integrated circuit back-end-of-line process. The simulation results indicated that the inductance of the primary and second coils was 514 and 520 pH, respectively. The coils exhibited a Q factor of 12.08 and 11.98, respectively, when operated at 15 GHz, and the relative difference between the two coils was 1.15%. Compared with transformer structures proposed in the literature, the structure proposed in the present study exhibited superior symmetry between the primary and secondary coils.