A process of modeling a diffracting structure with normally incident radiation
and the radiation diffracted from the structure includes constructing an optical
model of the diffracting structure and calculating spectral information for the
optical model based on a plurality of diffracted orders using either the positive
or negative of each of said plurality of diffracted orders and the zero order.
The process may be used to measure a diffracting structure, in which spectral information
from a diffraction structure is extracted and compared to the calculated extracted
information. The optical model is adjusted and the spectral information recalculated
until an adequate fit is found, at which time it is known that the optical model
accurately describes the actual diffraction grating. The process may be used with
any normally incident metrology device, such as a reflectometer, ellipsometer and scatterometer.
