Characterizing and understanding stray tilt: the next major contributor to CD SEM tool matching

Measurement using electron beam tilt has recently been highlighted as holding the promise of future sidewall angle and thickness determinations in the CD SEM in a manufacturing environment. But even before robust tilted beam measurements can be made, a thorough understanding of stray tilt, its characterization and control, is needed to provide the foundation for tilt calibrations and measurements. Stray tilt is the amount of unintended angular deviation of the electron beam from the normal to the specimen's (wafer) surface. Stray tilt is common to all SEMs used in manufacturing due to the following contributors: mechanical tolerances, acting both within the SEM column and between the SEM column and the chamber and the sample holder; and also residual and parasitic magnetic and electrostatic fields - these fields are generated both within and outside of the SEM integrated stand-alone unit. Past characterization attempts addressed this issue through an asymmetry specification. Recent data has suggested that stray tilt errors can have significant negative effects on today's critical dimension measurements, especially on a fleet of CD SEM tools with different amounts of stray tilt. This paper explores the measurement, monitoring and minimizing of stray tilt and the consequences on tool matching.