Optical system and method for measuring small dimensions

A method of manufacturing a semiconductor device, the method comprising:

• forming a film of a first material;
• forming features in the film, the features having a feature size that deviates from a nominal size, the features being formed of a second material;
• illuminating the film and the features;
• analyzing light reflected from the film and the features and determining therefrom an index of refraction of the film and the features;
• comparing the index of refraction of the film and the features to an index of refraction derived from a known film having formed therein nominal features of the nominal size;
• and
• determining a deviation from a nominal feature size based upon the comparing.

A system for measuring features of a workpiece having a film, the system comprising:

• an illumination tool for illuminating the features and the film using radiation having a wavelength and a spot size;
• a detection tool for detecting a reflected light with respect to the features and the film, wherein the detection tool determines an effective index of refraction of the features and the film based upon the reflected light;
• and
• a processor for determining a feature size based upon the effective index of refraction of the features and the film and an index of refraction of a reference film having thereon precisely measured features.