columns to route 2000 pins. However, keeping a uniform 40um wastes tracks at the bottom, since the two bottom macros contribute only 400 pins there. To use the space efficiently, do not keep 40um at the bottom - keep only a gap equal to the VDD-VSS pitch. Pins accumulate as you move upward, and you need the full 40um at the top where all 2000 pins must be routed. This means the macros should be arranged in a V-shape for efficient area and routing-track utilisation.

KEY Channel grows from VDD-VSS pitch at the bottom to about 40um at the top - arrange the macros in a V-shape.

Getting a Macro's Lower-Left Coordinates

• set llx [lindex [lindex [dbGet [dbGet -p top.instance.name $macro].box] 0] 0]
• set lly [lindex [lindex [dbGet [dbGet -p top.instance.name $macro].box] 0] 1]

KEY Use dbGet on the instance box and lindex into element 0 to extract the macro's lower-left x and y.

Die, Scribe, Seal Ring, Mask and Corner Cells

Special corner cells are placed at the corners of a shutdown block to turn the power signals around those corners, maintaining power-rail continuity along the block boundary.

KEY Corner cells route the power rails around the corners of a shutdown block to keep the rail continuous.

The Metal-Density Rule

A layer must maintain both a minimum and a maximum density within a specified area, because etch and CMP processes are sensitive to how much material is present. If polysilicon density is too high or too low, gates may be over- or under-etched, causing channel-length variation; non-uniform metal density can cause copper dishing during CMP. A typical rule might require 30% minimum and 70% maximum