title: Modelling, quantifying and attenuating multi-material thermal bend in atomic force microscopy cantilevers
Creators: name: Mordue, Christopher William
Publisher: name: University of Glasgow
Year: 2022

description: Atomic Force Microscopy (AFM) is a technique that generates images of surfaces with a resolution down to the atomic scale through the use of a micro-cantilever. Like many machines that operate at such a scale, it is prone to thermal drift which can result in imaging artefacts. This body of work comprehensively explores the micro-cantilever’s contribution to this phenomenon, particularly in the situation where the cantilever is constructed from two materials (typically an insulator and metal). This work made use of a specific type of AFM called Scanning Thermal Microscopy (SThM), that provided simultaneous temperature and deflection quantification. Modelling demonstrated that an AFM tip can deflect up-to 166 nm/K when out-of-contact resulting in variable and erroneous measurement of deflection and topography in AFM. The latter is a highly unique insight and is a consequence of AFM’s commonly employed optical lever system that measures cantilever rotation (rather than deflection) at the laser spot focused on the cantilever. The AFM converts this signal into tip deflection, using a tip-force defined sensitivity factor, meaning the tip deflection measurement is indirect. This thermal induced deflection was similarly modelled in standard contactmode AFM cantilevers, emphasising its widespread occurrence. With AFM cantilever tips in-contact with surfaces and so their degrees of freedom limited, all cantilevers were theoretically predicted to deflect like a bridge when undergoing a temperature change. This manifested as a humped deflection profile along each cantilever’s length. As a result, this provided the conclusion that an AFM can interpret thermal induced deflection either positively or negatively depending on the longitudinal position ofthe optical lever’s laser. Both out-of-contact and in-contact experimental measurements on SThM cantilevers showed that AFM systems employing optical lever set-ups do have variable and inherently incorrect responses to thermal induced cantilever deflection. This was also seen for commercial contact-mode AFM cantilevers. Measured deflection profiles of all cantilevers when in-contact agreed with models, demonstrating inconsistent AFM measured deflection direction depending on laser location on the cantilever. This provides clear evidence of a new phenomenon not previously documented. Contact AFM scans were employed to confirm this effect’s direct impact on topographic scans with images of the same area varying positively or negatively over a 600 nm range for 2 K of temperature change. For a technique that measures sub-nanometre features, this is a significant artefact. However, with the humped cantilever deflection profile seen in-contact, it offers a point-of-inflexion where very little thermal bending induced tip-deflection would be measured by the optical lever. Measurements demonstrated an improvement of up-to 97.7 % when the laser was focused at this position on the cantilever. This presents a useful technique to mitigate thermal bending artefacts in-contact AFM scan modes, although cantilever bend itself remains present. To address this latter aspect, a simple solution was explored, where the native metal is counteracted by another metal on the backside of the cantilever. Using SThM cantilevers to study this, up-to 99 % reduction in thermal bend induced deflection of the cantilever is possible. New SThM cantilevers with evaporated aluminium on their backside to counteract the native gold were fabricated and showed complete attenuation of thermal bending for laser locations along the whole cantilever length when out-of-contact and in-contact. These were further improved by fabrication of modified SThM cantilevers with gold patterning that better complement the planar aluminium deposition to further improve the effect not only longitudinally, but also laterally in their deflection. This translated into in-contact AFM scans showing variation of only 10s nm for 2 K of temperature change in contrast to 100s nm without any design alteration. These findings were mirrored in commercial, contact-mode AFM cantilevers with a similar trend of pronounced insensitivity to thermal bending when out and in-contact with surfaces, translating into greatly reduced scan artefacts.