Automatic detector for microscopic dust on large-area, optically

Abstract

A system for automatically detecting dust or other minute particles on a large-area, optically unpolished surface such as a face of a glass reticle plate used in the production of microelectronic circuits. The system irradiates the surface with a narrow, high intensity beam of monochromatic radiation at a grazing angle, typically 1/2 degree with respect to the surface. An oscillating mirror scans the beam across the moving surface in a direction generally perpendicular to the direction of plate advance. A beam splitter provides separate inspecting beams for each surface of the plate. Optical systems characterized by a high numerical aperture are positioned on opposite sides of the plate to collect radiation which is scattered from the particles. The optical systems are oriented to accept scattered radiation, and typically have their optical axes at an angle in the range of 60.degree. to 160.degree. measured from the direction of advance. They each utilize a multiplet of cylindrical lenses characterized by excellent resolution and a large numerical aperture, preferably in the range of 0.15 to 0.20. A fiber optic concentrator transmits the scattered radiation from the image plane of the lenses to a detector which generates an electrical signal proportional to the size of the particle. In the preferred form, a comparator circuit with an adjustable threshold level generates a digital signal only when the size of the particle exceeds a predetermined value. Also, the oscillation of the scanning mirror and the advance of the plate provide a timing reference for the particle detection signal to locate the particle on the plate.