Methods for Enhancing the Accuracy, Precision and Spatial Resolution of the Atomic Force Microscope

Author: Ashley D. Slattery

Slattery, Ashley D., 2015 Methods for Enhancing the Accuracy, Precision and Spatial Resolution of the Atomic Force Microscope, Flinders University, School of Chemical and Physical Sciences

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Atomic force microscopy (AFM) is an extremely powerful characterisation technique, its versatility has resulted in applications across a broad range of fields from direct visualisation of walking motor proteins to the characterisation of nanoscale semiconductor devices. Since the invention of the AFM in 1986, there has been significant development in the technique and instrumentation; there are now a number of manufacturers which offer a variety of AFMs to suit a range of applications. One of the instruments' greatest strengths is the ability to perform a variety of measurements, however it is the measurement of sample topography and nanoscale forces which are at the heart of the AFM. This thesis reports the development of these two fundamental aspects; the initial focus is on the challenge of accurate force measurement, whereby a number of different approaches are used to improve the process of AFM cantilever spring constant calibration. The accuracy by which the cantilever spring constant can be determined is directly responsible for uncertainty in AFM force measurement, and as such is an important area of development. Focused ion beam (FIB) milling is a common theme throughout this work, and was used to greatly improve the accuracy with which the cantilevers' spring constant can be determined. The techniques reported here provide excellent accuracy, avoid tip damage and are applicable to a wide variety of the many types of cantilevers available. The measurement of cantilever deflection sensitivity is another critical aspect of force measurement, and FIB milling was again used to improve this measurement. By inverting the measurement geometry and milling spatial markers on the cantilever, the deflection sensitivity was measured without any damage to the cantilevers' delicate imaging tip, an otherwise unavoidable aspect of this measurement. The chapter on force calibration concludes with an investigation of the recently commercialised fast-scanning cantilevers; these cantilevers offer video-rate imaging by virtue of their ultra-small size and resonant frequencies in the MHz regime. The vastly different properties of these "next-generation" cantilevers' is expected to have an effect on the measurement of their spring constant, and here the applicability of standard calibration techniques is reported.For the first time, a number of fast-scanning cantilevers were calibrated using a variety of methods and compared to determine their effectiveness. The later chapters switch focus to the improvement of spatial resolution, by reducing the size of the AFM tip. Carbon nanotubes (CNTs) are attached to AFM probes by a variety of different methods, resulting in an imaging tip which is of high aspect ratio, small diameter and incredibly high wear resistance. CNT attachment using a micromanipulator in a scanning electron microscope (SEM) was found to be an efficient approach which yielded high quality CNT tips, and the final chapter reports two different applications of these specialised probes. Using the method reported herein, CNTs were attached to fast-scanning cantilevers for the first time, allowing a CNT probe to be scanned with a tip velocity of 109 microns per second. The CNT tips also demonstrated superior wear resistance in comparison to standard silicon tips, which are expected to wear at accelerated rates for fast-scanning probes. Application of the recently-developed PeakForce tapping (PFT) imaging mode was demonstrated with CNT probes; this mode shows great promise for providing simple, stable imaging with CNT probes which are notoriously difficult to apply. The PFT mode is used to demonstrate high resolution imaging on samples with very small features, and artifacts associated with the technique were investigated. In addition to stable operation, the PFT mode is shown to eliminate the "ringing" artifact which affects CNT probes in tapping mode near large vertical steps. This will allow characterisation of high aspect ratio structures using thin CNT probes, an exercise which has previously been challenging with other imaging modes.

Keywords: AFM, cantilever, spring constant, calibration, CNT, FIB, deflection sensitivity,
Subject: Physics thesis

Thesis type: Doctor of Philosophy
Completed: 2015
School: School of Chemical and Physical Sciences
Supervisor: Jamie Quinton