Lithography systems print patterns onto wafers. As many as 100 of these patterns are needed to make a microchip – and they all have to align with each other precisely for the chip to work. A lithography system’s ability to line up one pattern with the previous one is measured by its overlay performance. ASML’s current most advanced lithography systems deliver overlay performance better than 2 nm.

To achieve that kind of overlay, a lithography system needs to know the position of a wafer to within fractions of a nanometer before it exposes the pattern. When it enters the system, the wafer is initially positioned with a precision of around 80 micrometers – about the diameter of an average human hair. But that isn’t nearly accurate enough. To make the jump to sub-nanometer accuracy, ASML’s systems use an optical alignment sensor that calculates the wafer’s position based on how light reflects from special features on the wafer called 'alignment marks'.