Modeling of differential vertical transition with through silicon vias (TSVs) in 3D die stack

In this paper, a differential vertical transition with through silicon vias (TSVs) in photonic integrated circuit die is presented. The vertical transition composes of TSVs providing connectivity between the die front side and back side, BEOLs and micro-bumps. No back side redistribution layer is required to keep the fabrication process simple with good electrical performance. The high frequency characteristic of the differential TSV structure is also studied to provide design guidelines for the differential vertical transition. The effects of TSV pitch, liner oxide thickness and TSV mismatch on the high frequency performance are demonstrated. The change in TSV pitch has impact on the differential mode insertion loss and differential mode impedance matching while the liner oxide thickness variation induces negligible effect. Moreover, the TSV mismatch on the differential to common mode conversion is also not critical. The proposed differential transition design acquires simulated differential mode insertion loss of better than 0.75 dB up to 50GHz and simulated differential mode return loss of greater than 14 dB from DC to 50 GHz.