According to Noriyuki Miura, et al. in “A scalable 3D heterogeneous multi-core processor with inductive-coupling thruchip interface”
It communicates through inductive coupling between coils in the stacked chips. TCI is a low-cost wireless version of a through-silicon via (TSV). It can provide competitive performance to TSV while maintaining its cost at close to the same level as wire bonding. Because the interface coils are drawn using existing IC interconnections, we can use a standard CMOS process (without any RF options) for fabrication. Unlike TSV, no additional wafer process steps are required, resulting in low cost. In addition, TCI is covered under the passivation layer of the chip. No electrostatic discharge (ESD) protection devices are needed, resulting in small channel loading. We can achieve an over-Gbit/s data transfer rate with < 10 mW power dissipation. Parallel data bits can be multiplexed into a single coil and burst-transferred.