  We theorize the surface step characterization by reflected incoherent-light
differential interference microscopy with consideration of the optical
diffraction effect. With the integration of localization analysis, we develop a
quantitative differential interference optical system, by which we demonstrate
that the axial resolution of measuring surface height variation is sensitive to
the shear distance between the two spatially differentiated beams. We fabricate
three nanometer-size steps by photolithography, and successfully characterize
their 1D height variations with 0.13 nm Hz^(-1/2) axial precision. Our result
suggests that the optical differential interference microscopy can be used for
real-time characterization of surface structure with a subnanometer accuracy
and a large field of view, which is greatly beneficial to the surface
characterization of micro/nano-electromechanical systems.
