Abstract
It is well known that sub-wavelength structures can modify the polarization of incident light. Due to the coupling of electric fields at the sub-wavelength boundary, the responses to TE and TM polarized light are different. This effect has been intensively investigated in optical data storage [1, 2], metrology [3-6] and artificial dielectric synthesis [7, 8]. In optical metrology applications, this effect can be used as a sensitive measure of the sub-wavelength structures. In this summary, we report a measurement technique capable of measuring unresolved isolated features using a high resolution imaging ellipsometer. The imaging ellipsometer technique has been reported in details in a separate publication [9]. Illustrated in Fig. 1, a high numerical aperture (NA=0.8) objective is used in the setup. The high NA illumination gives rise to both a good ellipsometric signal and high spatial resolution. The transmission axes of the polarizer and the analyzer are aligned with the x-axis and y-axis, respectively. The sample is placed at the focal plane of the objective. A fiber bundle with a narrow band filter (10 nm bandwidth with central wavelength at 633 nm) is used as the illumination source to form an image on the CCD. The condenser collimates the light from each point from the source. Since individual points from the source in the field-of-view of the condenser subtend very small angles with respect to the optical axis, we can analyze each image point in the same way as the on-axis point. For a ray from the on-axis point entering the pupil at position (θ, ϕ), where θ is the corresponding incident angle on the sample of this ray after the lens and ϕ is the azimuth angular position from the x-axis, it can be shown that the power contributed from this ray on the detector is:
© 2002 Optical Society of America
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