Effects of lateral tip control in CD-AFM width metrology

Abstract

Critical dimension atomic force microscopes (CD-AFMs) use flared tips and two-dimensional sensing and position control of the tip-sample interaction to enable scanning of features with near-vertical or reentrant sidewalls. Sidewall sensing usually involves lateral dither of the tip, which was the case in the first two generations of CD-AFM. Current, third-generation instruments also have a fast dither tube actuation (FDTA) mode where a control algorithm and fast response piezo actuator are used to position the tip in a manner that resembles touch-triggering of coordinate measuring machines (CMMs). All methods of tip position control, however, induce an effective tip width that may deviate from the actual geometrical tip width. When lateral dithering is involved, this effect is readily understood as the addition of a dither envelope to the geometrical tip width.The effective tip width is a key correction parameter for accurate feature width measurements and is typically estimated using a tip calibration procedure. However, the possibility exists of small errors in the estimated tip width due to variations and dependencies of the effective width on tip, tool, material, and environmental parameters. We are investigating this possibility through a systematic study of the dependence of the apparent width on measurement mode, dither amplitude, tip type, and sample composition. While we believe that there are potential effects that should be considered carefully, we also conclude, particularly for silicon features, that most potential biases can be removed by performing the calibration and measurement exercises under the same conditions.