Dark field surface inspection illumination technique

Abstract

A method and apparatus for observing imperfections on the surface of and imbedded in an optical sample using a dark field technique. Linearally polarized laser light is entered by a prism-fluid index matching method which causes internal reflection at the critical angle. The internally reflected beam then coherently combines with the incident beam in the vicinity of the sample surface. This results in a standing wave pattern which can be adjusted by changing the laser wavelength, the angle of incidence or polarization to selectively illuminate variously regions at and below the surface. One polarization will have maximum intensity at the surface level while the alternate polarization will have a null at the surface level. Defects within the optical sample scatter light such that it does not reach the surface at an angle equal to or greater than the critical angle. This light is emitted from the sample surface and appears as a pattern of bright spots on a dark background. A viewing piece is used to systematically scan the surface of the sample. Using the above standing wave pattern, it is possible to illuminate the surface with a maximum intensity, to learn the position of optical sample imperfections and to determine the sizes of those imperfections. Placing a drop of oil on the surface of the sample removes surface irregularities as a source of scattered light. The oil drop also permits greater control of standing wave patterns within the sample.