VERTICAL METROLOGY USING SCANNING-PROBE MICROSCOPES - IMAGING DISTORTIONS AND MEASUREMENT REPEATABILITY

We report a study of the repeatability as limited by instrumental imag ing distortions in scanning-probe microscope (SPM) measurements of the heights of nominal 44 and 88 nm steps in calibration artifacts. By im aging the same series of locations on different days, we are able to d istinguish sample variations from variations originating in the imagin g process. In particular, the value and repeatability of the measured step heights are found to depend upon the algorithm used to infer the step height from the SPM image. The three general approaches tested ar e: a manual single-point method, which represents the most commonly us ed practice in the SPM community; a histogram-based method, which is a vailable in commercially available SPM image-analysis software; and th e polynomial step-function fit (PSFF), which explicitly removes image bow and sample tilt. Factors related to variations in the sample measu red such as image size and location on the sample cause up to 10% vari ations in the step-height measurements. Methods of statistical analysi s of variance are used to separate sample variations from instrumental and algorithmic variations. Once image bow and sample tilt have been corrected properly using PSFF, SPM instrument-related variations are u nder 1%. Without PSFF, the step height exhibits a systematic dependenc e upon sample tilt and image bow which distorts the measured step heig ht. In our study, these distortions occur at a level of up to 2% of th e step height. However, these step-height distortions can be far more extreme in some cases. We show an example of a step-height measurement in which the image bow is so extreme that the step can barely be iden tified without first correcting the image bow. We also show that, for small images of a rough sample, tip wear can contribute a significant systematic error in the step height measurement, of up to 1% per image in this case, which can lead to large cumulative errors if the tip is not changed often enough. Thus, understanding SPM image distortions a nd their effect on step-height measurement repeatability is crucial to SPM metrology. (C) 1998 American Institute of Physics.