Portable coordinate measuring tool

Abstract

The ability to accurately measure the placement of features within a level is becoming increasingly critical in integrated circuit fabrication. In this article the level to be measured is patterned on a transparent wafer in poly(methylmethacrylate) (PMMA) resist to which a blue absorbing dye has been added. The dyed resist is on top of a thin partially transparent aluminum layer and is highly visible when viewed in reflected blue light in an oil immersion microscope. It is placed over a diffraction grating, which is not visible, both because the aluminum layer reflects most of the light and because the grating is far out of focus. In transmitted red laser light the grating can be seen, since various techniques may be used to increase the depth of focus of gratings in collimated monochromatic light. However, the resist pattern is not seen, because the resist is transparent to red light and the immersion oil matches its index of refraction. Measuring the separation between features reduces to measuring their location with respect to individual grating lines, and then counting the number of lines between them. The image placement accuracy of levels measured with two-dimensional gratings will be limited by the accuracy of the gratings—at present less than 2nm.