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.