Technique for separately viewing multiple levels

Abstract

It is often desired to measure the accuracy of image placement in an integrated circuit, as well as the overlay between two levels printed on the same wafer. Such measurements yield information not only about alignment, but also about wafer, mask, and imaging distortions. Image placement measurements require state-of-the-art equipment. Overlay measurements are difficult to make if the patterns overlap each other, since one pattern may obscure parts of the other. In addition, the presence of an edge in one pattern may produce an error in the location of a nearby edge in the other pattern. This article describes a method of patterning that permits optical separation of the two levels so that they can be viewed independently. The method is based on the use of a resist containing a dye, such that the resist has a high contrast at one wavelength, but is completely transparent at another. In addition, the use of a precision grating for one of the levels provides a simple way to obtain highly accurate absolute image placement measurements.