Slit confocal autofocus system

A technique for focusing and maintaining the focus of an inspection or review system upon a specific layer of a multi-layered specimen is described. In one embodiment, a confocal autofocus system can be used to focus an optical inspection or review system upon the top layer of a semiconductor wafer thin-film stack. The confocal autofocus system utilizes a tilted mask having a linear array of apertures or a continuous slit that is aligned so that a respective linear array of focal points or a focal slit is parallel with a scanning axis of the inspection system. Appropriate processing of the profile depth information yields knowledge of the depth of various layers in the specimen and allows for selection of the layer or location of interest upon which to focus the inspection or review system.