Because of the numerous technological applications of gratings, especially in nanolithography, there is great interest in grating metrology, especially in the metrology of gratings with very small features. We are concerned here with the optical metrology technique of scatterometry, which estimates the dimensions and the shape of grating ridges from reflectometric and ellipsometric measurements done on gratings. One aid for the scatterometrist is provided by an interesting property of the gratings, a certain anomalous behaviour of the reflectance curves. Grating anomalies are generally associated with increased sensitivity to changes of the grating parameters. In the present paper the anomalous behaviour is studied, and an approximate model for explaining and predicting the anomalous behaviour, based on the Fabry-Perot resonance combined with the Effective Medium Theory (EMT) is proposed.
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