Automatic registration in an electron-beam exposure system

In the manufacture of integrated circuits, each succeeding pattern must overlay the preceding ones precisely. After one pattern is exposed, the wafer must be removed from the tool, subjected to a series of processes, and then returned for the next pattern. Mechanical handling tolerances cause errors, which are measured by sensing the location of processed patterns; the next pattern is then exposed with appropriate corrections. This pattern registration has become increasingly complicated, because the large-scale integration of more circuits into the same silicon area reduces pattern detail sizes and overlay tolerances.