It is natural, before using an optical imaging instrument, to understand its resolving power. Traditionally, resolution is a measure of the ability of an imaging system to separate images of two neighboring object points. Although, within the confines of geometrical optics, the image of a point is sharp, the actual image is smeared because of diffraction. When the diffraction patterns of two point objects overlap and their principal maxima draw closer, it becomes more difficult to distinguish the objects.
Hence there is no precise resolution limit because it depends on the recording medium. An approximate measure of resolution can be specified using the heuristic criterion of Rayleigh (J. W. Strutt) [20â22]. Images of two point objects of equal brightness are considered just resolved when the principal maximum of one coincides with the first minimum of the other.
In optical lithography and integrated circuit fabrication, we are concerned not only with delineation of patterns that are placed closely with one another, but also with reliable replication of small features. To refine Rayleigh's criterion in order to distinguish between these two requirements, let us define minimum half-pitch as the measure of object denseness; and denote the smallest size of an individual feature by minimum dimension. Minimum dimension relates to the speed of a transistor, while minimum half-pitch determines transistor integration density.
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