Defect density (D₀)
Beginner
How many tiny flaws, like specks of dust, land on each patch of a wafer, on average.
Novice
The average number of fatal manufacturing flaws per square centimeter of wafer. Multiply it by a chip’s area to get the expected number of flaws per chip.
Expert
Falls as a manufacturing process matures, so a new process starts with a higher than an established one. Factories track it layer by layer; in the Poisson model the per-layer yields multiply, so each layer’s contribution can be separated.