High-resolution lithography with projection printing

Very large-scale integration (VLSI) will want to increase circuit density and to reduce power dissipation at no sacrifice in speed. It will, therefore, ask for ever smaller circuit geometries, which, because of the increased sensitivity to defects, can economically be only projection printed. Projection optics are diffraction limited. Only blurred images of the mask expose the photoresist on the silicon wafer: the whole photoprocess becomes increasingly sensitive to variations of its many parameters. The parameters´ interdependencies have been modeled identifying four distinct process steps. Measurement techniques are described, which allow to quantize each step independently. With standard IC technology one may measure the illumination profiles of the mask image directly in the projection printer. Characteristic curves describe the photoresist response to exposure. A simple time measurement allows to real-time control the photoresist development. Finally, the concept of an operating point on a characteristic is introduced. It helps to define an optimized set of process parameters.