Focusing in instruments, such as SEMs and CRTs

摘要

A significantly improved focusing technique is set forth for use with electron beams, particularly in scanning electron microscopes and/or CRTs. This technique utilizes an in-situ differential signal measurement of an object surface to form a signal which is particularly sensitive to edges in the sample at a superimposed frequency. Perfect focus is obtained when the signal strength at the superimposed frequency is a maximum thereby resulting in a minimum spot size.