Measurement of minimum line widths using Fallon ladders

A test structure is proposed to assess resolution both optically and electrically. The structure consists of a ladder of conductors with incrementally varied widths. The maximum resolution can be assessed by either measuring the resistance or counting the rungs that have been resolved. The results indicate a plateau from zero towards a negative value of focus offset. There is a sharp resolution reduction as the stepper focus is increased above the nominal focus setting. The electrical measurements correlate well with scanning electron microscopy (SEM) results, and prove to be considerably faster, while removing subjectivity from the measurements. It is shown that the change in resistance of the Fallon ladder as resolution decreases is sensitive enough to enable this technique to be applicable to current processes