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Monitoring of lithography modules using defect density inspection systems

The aim of this paper is to present two process monitor methods concerning the early detection of process problems resulting from different components of a lithography module (so-called cluster) under conditions of deep ultraviolet lithography. The lithography clusters are monitored with bare silicon wafers which run through a normal process sequence of the cluster. Then, measurements of critical dimension and defect density are performed. The first measurement indicates whether the focus or the exposure dose run out of their target values, the latter detects defects on, within or underneath the resist which may indicate a process problem in one of the cluster parts. Since defect density measurement inspects all dies on the wafer, it was able to detect process problems that result in massive variations of pattern size when the measured values of critical dimensions were still within their specifications. This helped us to derive instructions for operators and stepper and track maintenance personnel that specify a maximum delay time between exposure and development.