Inevitable tip bending of modern probes with fine end has non-negligible effects in AFM applications

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Poster Session Two
Dr Xinyue Chen (2), Dr Patrick Hole (1), Prof Andrew Humphris (1), Prof Jamie Hobbs (2)
1. Infinitesima Ltd
2. University of Sheffield
Abstract text

We experimentally determined the tip bending effect for different types of popular modern probes used in atomic force microscopy (AFM) applications. Such effect can be significant in a broader range of applications than that has been recognised so far, including both imaging and mechanical characterisation, and should be taken into account properly for more accurate AFM quantifications.

The majority of AFM applications rely on analysing the cantilever motions while assuming the tip is a solid attachment. In the field of critical dimension AFM, probes with high aspect ratio tip (> 10:1) were often used to examine the metrology of semiconductor devices at nanometre scales. The bending of such high aspect ratio tip, in addition to cantilever deflections, had significant effects in the quantified results. This had been gradually understood and improved in last decade [1, 2]. However, such effect has not yet discussed for other modern probes, often with fine end for achieving high resolution but without such high tip aspect ratio. Our work expanded the understanding of tip bending to several popular probes that are widely used in AFM applications.  

Different modern AFM probes, including Biotool High-resolution (Bio-HR, Nanotools, tip aspect ratio > 10:1), USC-f0.3-k0.3 (USC, NanoWorld, tip aspect ratio > 5:1) and AC40 (Bruker), were used to line scan over a reference semiconductor structure, with near-rectangular trenches of approximately 60 nm in width and 100 nm in depth. Scans were done under Quantitative Imaging (QI) mode on JPK Nanowizard III system. This method enabled proper tracking of both the vertical and the lateral deflections of cantilevers, which is essential for controlling the trigger forces and analysing the cantilever motions during scanning. The scanning direction was perpendicular to the flexural axis of cantilevers, so that any interactions beyond the standard repulsion would be reflected in the lateral deflection but not vertical deflection that may destabilise the trigger forces. The tip stiffness vs. distance from the tip end of all these AFM probes were also measured by QI scan along the tip pointing direction, using a tipless MLCT-F cantilever.

The resultant morphological profiles over the reference trench structure indicated very significant tip displacements due to both slippage across the rough edge and attraction from the trench sidewall. For high aspect ratio tip, the displacement was dominated by tip bending (could be up to more than 10 nm) but not cantilever deflection. Such tip bending is also not trivial for all the tested probes with lower tip aspect ratio. The experimentally measured tip stiffness of all tested AFM probes also showed a dramatic drop getting closer to the fine tip end, which could be as low as the nominal spring constant of the cantilever (e.g. ~ 0.1 N/m). These findings suggested the tip bending effect is more significant than that has been commonly recognised by the AFM community and should definitely be considered for better quantifications. Proper corrections of the tip bending can potentially improve the AFM application performance at the analysing stage (e.g. enhancing resultant “resolution” of AFM images).


[1] M. Watanabe, et. al. (2012) Atomic force microscope method for sidewall measurement through carbon nanotube probe deformation correction, J. Micro/Nanolith. MEMS MOEMS 11(1) 011009. 

[2] V. A. Ukraintsev, et. al. (2013) Distributed force probe bending model of critical dimension atomic force microscopy bias, J. Micro/Nanolith. MEMS MOEMS 12(2), 023009.