Critical dimension measurement method and apparatus capable of measurement below the resolution of an optical microscope

The present invention relates to a critical dimension measurement method and apparatus utilizing an optical microscope and a two-dimensional image sensor such as a CCD camera. In particular, the present invention relates to a very narrow line size measurement method and apparatus suitable for, although not restricted to, measuring a critical dimension, such as a magnetic head track width and a line width of a photomask for semiconductor fabrication, in a contactless manner. A critical dimension measurement method and apparatus capable of measurement even for a object below a resolution of an optical microscope. An image of an object is picked up by using an optical microscope and an image sensor. From an obtained video signal, signal positions of two points coinciding with a predetermined luminance level are extracted. A difference in position between the two points is multiplied by a ratio of maximum luminance between the two points to maximum luminance serving as a reference. On the basis of a resultant product, a size of the object is measured.