A new precise optical autofocus system

A new optical configuration, called the inverse confocal microscope, is used in the automatic focusing system of a microspectroreflectometer that is used for film thickness measurement. At best focus, a conventional confocal microscope maximizes the transmission of light through an aperture. In contrast, the inverse confocal arrangement minimizes transmission. The focussing precision so far demonstrated is comparable to the 0.08 micron quantization error of the control system used.