Inductive coupling thruchip interface for 3D integration

ThruChip Interface (TCI) is a low-cost wireless version of through-silicon via (TSV). TCI wirelessly communicates over three-dimensionally (3D) stacked chips through inductive coupling between on-chip coils (Figure 7.1) [1]. The interface coils can be drawn by using existing large-scale integrated (LSI) metal interconnections, and thus TCI is standard complementary metal oxide semiconductor (CMOS) compatible. Unlike TSV, no additional wafer process steps are required, and hence no additional fabrication cost. In addition, the coils are covered under an LSI passivation layer and not exposed for any mechanical contacts to the chip outside. A highly capacitive additional electro-static discharge (ESD) protection circuit can be removed, resulting in small channel loading. As a result, TCI can provide competitive communication performance to TSV even though TCI utilizes a wireless channel (Figure 7.2). TCI is a circuit solution. Fusion combination between the inductive coupling channel characteristics and legacy wireline 176circuit techniques further enhances TCI communication performance. Utilizing capacitive coupling is one of the alternative solutions to the wireless interface between stacked chips [2]. However, the capacitive coupling has a limitation in using stacked chip communication. It can be applied only to two chips stacked face-to-face. In a field plot of the capacitive coupling shown in Figure 7.3(a), the vertical electric field is shielded by the silicon substrate in the stacked chips. On the other 177hand, the magnetic field of the inductive coupling can penetrate through the silicon substrate (Figure 7.3(b)). Although there is a small eddy current loss in the substrate, the inductive coupling can communicate through the chips. That is the reason why this technology is called ThruChip Interface. In this chapter, the TCI technology is comprehensively overviewed together with recent research activities. In Section 7.2, TCI basics and fundamentals are overviewed. Detailed channel characteristics and design guidelines are explained. In Section 7.3, TCI circuit techniques inspired by legacy wireline arts are introduced for communication performance enhancement. In Section 7.4, TCI applications are described. This section covers from practical to emerging applications. Finally, in Section 7.5, a summary will be given. Figure 7.1 ThruChip Interface (TCI). Figure 7.2 TSV vs. TCI. Figure 7.3 Electric and magnetic field plot of (a) capacitive and (b) inductive coupling.