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This paper presents a novel die-to-die clock synchronization method that is independent of the inter-die wire delay. Through a 2-Phase All-Digital Delay Locked Loop (2P-DLL) and a Dual Locking Mechanism (DLM), this method can be used to maintain a global clock signal between two dies in a 3-D IC, and thereby enabling the synchronous 3-D IC design methodology. Unlike previous designs, ours does not need to replicate the delay of the inter-die clock wire. This property can make our scheme more adaptive to various 3-D technologies and more robust to PVT variation. Such a method has several other benefits. For example, it can accommodate the ever-increasing process variation easily through its silicon tracking ability. Simulation results indicate that it can support clock signals running up to 2.8 GHz. Silicon measurements of a test chip in a 90 nm CMOS technology show that the phase error can be locked constantly to less than 9.6 ps at a clock frequency of 600 MHz, with a peak-to-peak jitter of 9.778 ps and a power consumption of only 1.8 mW.