What layout choices worsen self-heating in FinFETs?

Tight fin spacing is one of the biggest layout choices that worsen self-heating in FinFETs. When fins are placed too close, there’s less space for heat to escape through the STI or substrate, so temperature builds up faster.

For example, in one of our 7nm analog blocks, we saw higher temperature in simulation when we used a dense fin layout for current mirrors. After increasing fin spacing slightly, the temperature dropped, and the performance became more stable. So, sometimes, spreading out fins helps a lot in reducing self-heating.

Using high metal density or stacked routing over active fins can also worsen self-heating. The dense metal layers act like a thermal blanket and trap heat inside the device region. For instance, in a FinFET layout where we routed power and signal lines right above the fins, we observed a noticeable temperature rise. When we shifted some routing to higher metal layers, the heat dissipation improved. So, I always say — keep hot devices clear from heavy metal routing above them whenever possible.

In my view, large multi-finger transistors without proper thermal breaks make self-heating worse. When we place many fins together under a single gate or connect them with continuous diffusion, the heat generated in the centre fins cannot dissipate easily.For example, I once worked on a driver circuit where all fins were merged to reduce resistance. During stress testing, the central fins ran hotter than the edge ones, leading to mismatch. So, now I prefer adding dummy fins or isolation gaps to let heat spread more evenly.