Optimization of a SCALPEL ® exposure tool using a diffractive image quality technique

Abstract

Determining and maintaining the best exposure parameters, such as dose, focus, or illumination uniformity, is a primary concern in operating a modern day stepper or scanner. A commonly adopted means of obtaining lithographic image quality information relies on accurately measuring CD values across a wafer, die, or image field. Continually shrinking Linewidth have pushed this type of characterization into the realm of the CD-SEM; though greatly improved and automated in recent years, the accuracy required to reliably identify trends makes this a tedious and time consuming practice. We present here an alternative, flexible, and potentially rapid method for optimizing lithography conditions using a diffractive image quality (DIQ) technique, emphasizing its utility in a SCALPEL-based exposure system. A DIQ technique uses the fact that the maximum intensity of higher-order diffraction dispersions is a function of the (image) quality of the grating being illuminated so interrogation of gratings that have been exposed in resist can provide knowledge of how the aerial image quality can be varied for a given set of exposure conditions. We have evaluated grating diffraction efficiency using a probe beam (HeNe laser) introduced at a glancing angle. This arrangement has the advantage of looking at higher order intensity distributions while avoiding the zero order, as well as being fast since one is only collecting signal (eg. No feature recognition searches). Further, it enables the imaging optimization of a new system, such as SCALPEL, without tying up quantitative tools such as a CD-SEM. We have found that this metrology technique provides a quite sensitive and rapid identification of the best exposure conditions. Cross-correlation with a CD-SEM metrology tool has also been performed, in order to verify that the optimum imaging conditions identified through this DIQ technique correspond to the best CD conditions.