Correction of Artifacts and Optimization of Atomic Force Microscopy Imaging: A Case of Thin Aluminum Films for Prosthetic Applications

Introduction

Atomic force microscopy (AFM) is one of the scanning probe microscope (SPM) techniques that provides lateral information and heterogeneity of surface topography at the nanoscale level, and its resolution can reach atomic measurements for perfectly flat surfaces (Anandhan 2003). The major advantage of AFM microscopy over the other microscopy techniques (such as optical, SEM, TEM, etc.) is that it does not require the tedious surface preparation, and therefore, it finds applications in a wide spectrum of nanomaterials including metals, polymers, composites and biological materials (Bandyopadhyay 2008). The versatility of AFM technology is further enhanced by its capability to image in different media, including vacuum, fluid and gas environment and as such its application cuts across all disciplines from life to physical sciences and engineering. It is possible to capture the atomic arrangement or molecules of various surfaces; for instance, at images of 5 nm scale, the crystallographic structure of materials in both life and physical sciences can be studied (Eaton and West 2010). The other advantage of AFM is that it can image nearly all types of materials’ surfaces; be it very hard (metals, ceramics or polymers) or soft (animal cells, very thin lone standing films or genetic molecules).

The above advantages are attributable to the unique design and operating mechanism of the atomic force microscope. Unlike, SEM, TEM or optical microscopes, which uses light to capture images, AFM scans (‘touches’) the surface of the sample under imaging (Eaton and West 2010; Jacobs, Junge, and Pastewka 2017). In operation, the AFM tip (usually having a radius range of 2-100 nm) scans the surface through contact, non-contact or tapping mode. The detection and imaging of the surface topography by the AFM probe are based on the concept of inter-atomic force distance as illustrated in Figure 1. As the tip nears the specimen surface, it senses the distance-dependent (also known as inter-atomic) forces such as a capillary, van der Waals, electrostatic and magnetic forces. The measurement of these forces in AFM is based on Hooke’s principle (force as a product of deflection and spring constant of the AFM cantilever beam). As the tip scans over the surface, the resultant tip-sample forces make the cantilever to deflect according to the topography of the surface under analysis. The AFM assembly is fitted with position photosensors, which use a laser to detect any deflections and raster motions of the cantilever. Also, AFM facilities consist of piezo-vibrator that is meant to oscillate the tip at a range of frequencies or near the natural frequency for tapping mode operation.