PD interview Questions - Routing

1. What is the basic difference between 9 track and 12 track in standard cells? Who does a track means?

Firstly track can be related to lanes, eg like we say 6 lane road, implies 6 vehicles can run in parallel. So 10 tracks would imply we can route 10 wires in parallel with minimum pitch.

Now, 9/12 track implies 9 routing tracks are available for routing 9 wires in parallel with minimum pitch. For example if for 28nm we are using 9 track standard cell library with minimum metal pitch of 100n. In this case we will have standard cell of height 0.9um (100n*9) where 9 horizontal tracks are available for routing.

2. Why do we use alternate layer horizontal/vertical/horizontal (HVH or VHV) routing?

Some advantages of VHV/HVH:
To make best utilization of placement and routing resources.
To nullify crosstalk from layer beneath and above the routing layer.
More uniform distribution of power.