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Doubly Darkfield Microscopy

IP.com Disclosure Number: IPCOM000040501D
Original Publication Date: 1987-Nov-01
Included in the Prior Art Database: 2005-Feb-02
Document File: 1 page(s) / 11K

Publishing Venue

IBM

Related People

Batchelder, JS: AUTHOR

Abstract

The sensitivity of optical detection of surface features by grazing angle illumination can be advanced by using grazing angle detection. Standard darkfield microscopy is performed by illuminating the surface to be inspected with light at grazing incidence. Because light that reflects off the surface interferes destructively with the incoming light at the surface under these conditions, nodes of destructive and constructive interference are set up near the surface such that features that project up from the plane of the surface are more strongly illuminated. As long as the height of the features is small compared to the node spacing then under standard darkfield illumination, small features are illuminated in proportion to the square of their height.

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Doubly Darkfield Microscopy

The sensitivity of optical detection of surface features by grazing angle illumination can be advanced by using grazing angle detection. Standard darkfield microscopy is performed by illuminating the surface to be inspected with light at grazing incidence. Because light that reflects off the surface interferes destructively with the incoming light at the surface under these conditions, nodes of destructive and constructive interference are set up near the surface such that features that project up from the plane of the surface are more strongly illuminated. As long as the height of the features is small compared to the node spacing then under standard darkfield illumination, small features are illuminated in proportion to the square of their height. However, if the above feature illumination is detected at a grazing angle above the surface, then the detected signal will be proportional to the fourth power of the feature height. This yields a greatly increased ability to discriminate features of slightly different heights.

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