Why set different layers for trunk, leaf, and top clock nets? Why not shield all clock layers?

o   Why Different Layers:

o   Top Nets: These are the initial long branches of the clock tree. That distributes clock all over the block. They are placed on higher metal layers because these layers have thicker wires with lower resistance, which is critical for minimizing the overall tree latency.

o   Trunk and Leaf Nets: As the tree branches out closer to the flops, it moves to intermediate and lower metal layers. These layers are better for shorter, high-density routing.

o   Why Not Shield All Layers:

o   Routing Resource Congestion: Shielding consumes routing resources. Adding a VSS/VDD shield next to every clock wire would make it impossible to route the rest of the design's signals.

o   Not all the clock net’s need shielding, Shielding is most effective on the long nets that are most susceptible to coupling. Leaf-level nets are shorter and may not show critical level of coupling.