Process and apparatus for filament or slit size monitoring

Abstract

Method and apparatus for determining the width of an elongated element, such as a filament or slit which comprise producing a moving interference fringe zone by converging two beams of coherent light of the same intensity but slightly different frequency; positioning the element within the fringe zone in such manner that the longitudinal axis of the element is substantially normal to the plane of the convergent beams, namely, parallel to the plane of the fringes; maintaining the element substantially spatially stationary relative to the zone, whereby the moving fringe pattern continuously sweeps across the element; and determining the ratio of the AC to DC signal components of the radiation scattered or transmitted by the element. The method and system include respectively the step and means therefor of adjusting or scanning the fringe period value around the element to determine the size capable of producing a minimum AC/DC ratio. In the case of a system substantially free from noise, including non-compensating optical noise factors introduced by the element being measured, a zero ratio indicates that the filament diameter or slit width equals the known fringe period, and a non-zero ratio indicates diameter or slit width deviation from the fringe period, the degree of which can be determined from the signal ratio with the use of known means by adjusting the fringe period until the AC component of the signal becomes zero. In the case of a system having a substantial amount of noise, the AC/DC minimum ratio obtained at a given fringe period may have a finite value and indicates an element width equal to the given fringe period times a constant, the constant being the ratio of the width of the given element and the given fringe period. The element, though spatially stationary in the fringe zone, can be continuously moving longitudinally. The process and system are particularly useful for monitoring the width of manufactured elements and, by automatic feedback, correcting deviations. In some applications where change in the magnitude of the DC signal component during the fringe period scan is sufficiently nominal with respect to permissible width-sensing error, the AC/DC ratio measurement can be dispensed with and determination of the AC signal component minimum alone can be employed.