Vertical metrology using scanning-probe microscopes: Imaging distortions and measurement repeatability

Abstract

We report a study of the repeatability as limited by instrumental imaging distortions in scanning-probe microscope (SPM) measurements of the heights of nominal 44 and 88 nm steps in calibration artifacts. By imaging the same series of locations on different days, we are able to distinguish sample variations from variations originating in the imaging 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 are: a manual single-point method, which represents the most commonly used practice in the SPM community; a histogram-based method, which is available in commercially available SPM image-analysis software; and the polynomial step-function fit (PSFF), which explicitly removes image bow and sample tilt. Factors related to variations in the sample measured such as image size and location on the sample cause up to 10% variations in the step-height measurements. Methods of statistical analysis 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 under 1%. Without PSFF, the step height exhibits a systematic dependence upon sample tilt and image bow which distorts the measured step height. In our study, these distortions occur at a level of up to 2% of the 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 identified 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 and their effect on step-height measurement repeatability is crucial to SPM metrology.

Surface modification of AFM silicon probes for adhesion and wear reduction

“…Edwards et al [9] reported that tip wear could contribute a significant systematic error in step height measurement. When a measured sample surface includes sharp features with radius smaller than the tip radius, the scanned image consists of a combination of the tip shape and the surface, referred to as tip artifacts [10,11].…”

Characteristics of fracture during the approach process and wear mechanism of a silicon AFM tip

“…For example, the lateral resolution of an AFM can be worsened due to tip wear and increase in contact diameter between the tip and the surface [6]. Also, artifacts are generated in the topography image due to degradation of the tip by the convolution effect [6,7]. In addition, the realization of SPM-based applications has been often limited due to the reliability problem induced by the tip damage.…”

An energy-based model to predict wear in nanocrystalline diamond atomic force microscopy tips

“…1 This tip degradation introduces artifacts in imaging, which leads to errors in step height measurements. 1,2 The wear of tip material during scanning also affects nanopatterning techniques, such as dip pen nanolithography, 3,4 nanofountain probe patterning, [5][6][7][8][9] and scanning probe contact printing, 10,11 as the size of patterned features depends on the radius of the tip. Knowing the extent of tip wear is also important for hardness measurements using AFM nanoindentation or atomic force acoustic microscopy, 12,13 as the interpretation of results depends on the tip radius.…”