To meet its quality control roadmap goals by 2010, researchers at the National Institute of Standards and Technology (NIST) developed the Improved Interferometer (IR³), which accurately measures differences in thickness across the new 300-mm wafer with repeatability to 5 N-m.

Optics and materials for printing nanoscale circuit lines require silicon wafers to be perfectly flat and of uniform thickness.

Using intersecting waves of light to create interference patterns as a ruler, the IR³ can illuminate the top and bottom on a 300-mm wafer and produce a detailed spatial map of differences in thickness in one pass. Traditionally, the wafer was spun and measured at multiple locations.

Precision measurements are made of the wafer's index of refraction — the amount of light that is bent as it passes through silicon — to interpret interference patterns. Increased precision of the refractive index is necessary before absolute measurements of thickness can be made.