Systematic and Random variation
There are two types of process variation in VLSI, Systematic and Random variation
Random variation is proportionate to the logic depth of each analyzed path. The random component of variation arises across different lots, wafers, on-dies, and die-to-die. Examples of random variation include variations in gate-oxide thickness, implant doses, and metal or dielectric thickness.
Systematic variation is proportional to the cell location of the analyzed path. The systematic component of variation can be predicted based on the location of the wafer or the nature of the surrounding patterns. These variations are connected to proximity effects, density effects, and the relative distance of devices. Examples of systematic variation include variations in gate length or width and interconnect width.