Reflected Darkfield Observation
This observation method allows the user to observe the scattering beam and/or diffraction beam from a specimen by illuminating it obliquely with ring light from outside an objective lens so as not to directly enter into the observation optical system. With the use of this microscopy method, the user can identify the existence of even a minute scratch and flaw of 8nm level smaller than the resolving power limit of an optical microscope. Since the resolving power under brightfield observation using an N.A. 1.4 objective lens is 0.24μm (at wavelength 550nm), the use can detect a defect at a resolving power of about 30 times that under normal illumination observation with the aid of darkfield observation. The reflected darkfield observation method as in the figure below illuminates a specimen at an N.A. larger than that of the objective through the ring condenser lens at the circumference of the objective via the ring reflection mirror (or direct from the ring reflection mirror) by using a ring-shaped illumination beam formed by the ring diaphragm installed in the illumination optical system. Therefore, this microscopy requires a reflected brightfield/darkfield illumination unit and brightfield/darkfield objective lens with a ring-light optical path at the circumference.
With this method a scratch or a minute step on a mirror specimen such as wafer and glass, which is difficult to observe under bright field observation, can be observed remarkably.
Optical System Configuration of Darkfield Microscopy: Reflected Type
Wafer
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