Investigation of tier-swapping to improve the thermal profile of memory-on-logic 3DICs

In 3DICs heat dissipating devices are stacked directly on top of each other leading to a higher heat density than in a comparable 2D chip. 3D integration also moves the majority of active devices further away from the heatsink. This results in a degraded thermal path which makes it more challenging to remove heat from the active devices. Gradient HeatWave-3DIC was used to perform an appropriate 3D thermal analysis on a 1024-point, memory-on-logic 3DIC FFT processor for synthetic aperture radar (SAR). The chip was simulated with a spatial resolution of 80 nm, and was modeled to include the effect of each line of interconnect, as well as each via and fill structure exactly as drawn in the layout. Large isolated temperature spikes were found near groups of clock buffers at the edge of the SRAMs on the middle tier. The effect of reordering the stackup of memory and logic tiers has been investigated. It was found that placing the memory tier closer to the heatsink improved not only the thermal profile of the memories but also of the logic tiers. Temperature spikes in the memories no longer significantly impacted the logic tiers, yielding a design where the thermal profile of the tiers were significantly less dependent on each other.