In situ measurement of an image during lithographic exposure

This letter describes a method which can directly observe the aerial image of a lithographic exposure tool. Submicrometer resist structures, doped with a laser dye, are swept through the lithographic image as fluorescence is monitored. Experimental aerial image profiles are reported for a 5X reduction lens at varying focus parameters. The location of the fluorescent structures with respect to the image can be accurately determined to yield real-time overlay information.