With traditional air or watercooling, the bottom layers of a chip will have more trouble dissipating their heat, which will result in performance throttling. The top layer will also have extra strain because the heat of the entire package moves through it to the dissipation layer.
To address these future needs, TSMC ran some experiments with on-chip liquid cooling solutions. The Taiwanese foundry cooked up Thermal Test Vehicles (TTVs) and experimented with three water cooling designs:
The company further tested three types of water cooling designs: one with only direct water cooling (DWC), where water has its own circulating channels etched directly into the chip's silicon as part of the manufacturing process; another design with water channels being etched into their own silicon layer on top of the chip proper, with a Thermal Interface Material (TIM) layer of OX (Silicon Oxide Fusion) that carried heat from the chip to the watercooling layer; and lastly a design which swapped the OX layer for a simpler, cheaper liquid metal solution.Tom's Hardware has more details over here. The direct water cooling method offered the best cooling performance -- with a dissipation of up to 2600W and a temperature delate of 63°C. A pillar-based water flow design offered by far the best performance characteristics.
Just don't expect to see this in consumer applications anytime soon.