Usage of horizontal and vertical metal layers

[venugopala_0202]

Hello

Are horizontal and vertical metal routing layers strictly used only in horizontal and vertical directions? If not, what is the point of defining them as horizontal and vertical? I have this question because I have come across instances where, in the power ring a vertical metal layer was used for the top and bottom portions of the ring.

 

[rca]

The metal direction is mainly defined to help the std cell routing by the PnR tool.
You could route in the opposite direction, but the PnR router will be disturbe.
if your design does not respect the direction over the std cell, and the std cell density is "high", you could have some issue during the routing phase.

you could unrespect the direction over the macros, because normaly the density of nets which will cross over the macro is low.

During the floorplan phase, some time, I did a prototype placement+routing to "check" quickly the phasability.

 

[mask_layout]

Aside from the reason already pointed by rca, Restricting routing directions helps reduce routing congestion and speed up the routing and drc fixing in place and route.
Use of metal route in opposite direction in power rings for top most chip may have been done to strengthen power connections by using two consecutive layers and power shield. On macro/block level, power ring implementation may also serve as block isolation/shield. But it could also have been implemented wrongly effectively blocking metal access coming from top and bottom.