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ssss1 (a) Synchrotron radiation tomographic images of a similar device as shown in ssss1. Due to the weak absorption of X‐ray, the two Si chips and the polymer substrate become invisible. But the solder balls and the Cu wires are shown clearly. (b) Synchrotron radiation tomographic image of a 2.5D IC device, having a length about 4 mm, and a thickness and a height of about 0.5 mm. The vertical TSV pillars can be seen.

Why do we emphasize electromigration and Joule heating? This is because electronic devices are current–voltage (I‐V) devices, so the applied electric current goes in and out of the devices in an open system. It causes Joule heating and electromigration, which are of key reliability concern. ssss1 is a schematic diagram of the cross‐section of a typical 3D IC device. The structure, in essence, is the same as that shown in ssss1, except that on the right‐hand side, there is a stack of memory chips on a logic chip as the CPU. If we replace the stack by an optical or compound semiconductor or MEMS chip, it becomes heterogeneous integration.