Controlling lithographic imaging performance at sub-100 nm CD with optical measurements

We present a new technique for accurate and fast evaluation of lithographic imaging performance at critical dimensions (CDs) of 100 nm and below. Its advantages over traditional methods that use either SEM or electrical CD metrologies are based on two key factors. First, it exploits a specially designed mark corresponding to a particular CD. Second, instead of mark dimensions the mark image irradiance is measured with a CCD TV camera. In combination, these provide an easy-to-implement and inexpensive technique for controlling exposure tool imaging performance. In actual application, best focus determination shows a repeatability (3σ) of less than 5 nm.