ATOMIC FORCE MICROSCOPY (AFM)

AFM is a high-resolution imaging technique that can resolve features as small as an atomic lattice in the real space. It is also commonly referred to as

Fig. 5.5 Nano-quasicrystalline particles in Zr-Cu-Al-O alloy. (Source: BS Murty, IIT Madras).

50 nm

Fig. 5.6 High-resolution TEM image of amorphous nano-pockets in ultrafine-grained Al. (Source: BS Murty, IIT Madras).

2 nm Amorphous

scanning probe microscope (SPM). It allows researchers to both observe as well as manipulate molecular- and atomic-level features. The AFM is being used to solve processing and materials problems in a wide range of technologies affecting the electronics, telecommunications, biological, chemical, automotive, aerospace and energy industries. Almost every material ranging from polymers to ceramics to composites are being investigated using AFM. AFM can be handy in studying the effect of processing/synthesis parameters on the microstructure of the specimen as well as to study the effect of the external environment like chemical or mechanical forces on material behaviour. AFM involves measurement of surface atomic forces in the range of a few nano-Newtons to image the surface topography. Figure 5.7 shows the schematic diagram of an atomic force microscope.

Gerd Binnig and Christoph Gerber invented the AFM in 1985 for which they were awarded the Nobel Prize. The AFM works on the principle of a cantilever, where a small hook is attached to one end of the cantilever and the force between the tip and sample is measured by tracking the deflection of the cantilever as the hook is pressed against the sample surface. This was done by monitoring the tunnelling current to a second tip positioned above the cantilever. Lateral features as small as 30 nm could be seen. The breakthrough in the production of tips/probes has made AFM a reality rather than a curiosity. The first tip prepared was a silicon micro- cantilever, and using it the atomic structure of boron nitride was observed. Today, most of the tips are microfabricated from Si or Si₃N₄. With the imaging of silicon (111) surface, AFM became very popular.

In AFM, a very small force of the order of <10–9 N is used between the tip and the sample surface. In AFM, the force is not directly measured, but the deflection of the micro-cantilever or the probe/tip is monitored. The first device introduced by Binnig was

Fig. 5.7 Schematic showing the principle of atomic force microscope (AFM). (Source: http://commons.wikimedia. org/wiki/File:AFMsetup.jpg)

a tunnelling tip placed above the metallized surface of the cantilever. This is a sensitive system where a change in spacing of 0.1 nm between the tip and the cantilever changes the tunnelling current by an order of magnitude. It is straightforward to measure deflections smaller than 0.001 nm.

The AFMs that came later are based on optical principles. Small changes in the tilt of the tip can cause changes in optical scattering, which influences the interference pattern and hence the surface can be studied for force variations. In this technique, light is reflected from the surface of the cantilever onto a position-sensitive detector. Thus, even a small deflection of the cantilever will cause a tilt in the reflected light, changing the position of the beam falling on the detector. In another optics-based AFM, the tip acts as one of the mirrors of a diode laser. Any change in the position of the cantilever influences the laser output, which is exploited by the detector. Based on the interaction of the tip with the sample surface, the AFM can be classified as repulsive or contact mode and attractive or non-contact mode. Tapping mode is more widely used.

In the AFM, an atomically sharp tip is scanned over a surface with feedback mechanisms that enable the piezoelectric scanners to maintain the tip at either

1. a constant force (to obtain height information), or

2. at constant height (to obtain force information) above the sample surface.

Tips are typically made from Si₃N₄ or Si, and extended down from the end of a cantilever. In the nanoscope AFM, the tip is attached below a reflective cantilever. A diode laser is focussed on this reflective cantilever. The tip moves on the surface of the sample up and down, tracing the contour of the surface and the laser beam is deflected off the cantilever into a photodiode. The photodetector measures the difference in light intensity between the upper and lower photodetectors, and then converts to voltage. Feedback from the photodiode helps the tip to work in a constant force or a constant height mode. In the constant force mode, the deviation in height is monitored by a piezotransducer. In the constant height mode, the AFM records the deflection force on the sample.

In some AFMs, samples as large as 200 mm wafers can be studied. The primary purpose of these instruments is to quantitatively measure surface roughness with a nominal 5 nm lateral and 0.01 nm vertical resolution on all types of samples. In some AFMs, the sample is translated under the cantilever, while in others the cantilever is moved over the sample. Both these methods can be used to measure the local height of the sample. Plotting of the local sample height as a function of horizontal tip position can give three-dimensional topographical maps of the surface. The concept of resolution in AFM is different from radiation-based microscopy techniques because AFM imaging is a three-dimensional imaging technique. Resolution of the images in optical-based techniques is limited by diffraction, while that in scanning probe techniques is controlled by probe and sample geometry. Usually the width of a DNA molecule is loosely used as a measure of resolution, because it has a known diameter of 2.0 nm in the B form. Some of the best values of resolution for AFM imaging are 3.0 nm based on the DNA in propanol. Unfortunately, this definition of resolution can be misleading because the sample height clearly affects this value.

5.5.1 Comparison of AFM and other imaging techniques

AFM versus STM It is interesting to compare AFM and its precursor, scanning tunnelling microscopy. In some cases, the resolution of STM is better than AFM because of the exponential dependence of the tunnelling current on distance. Only conducting samples can be studied by STM, while AFM can be applied to both conducting and non-conducting samples. AFM is more versatile than STM. In AFM, the voltage and tip-to-substrate spacing can be controlled independently, while in STM these two parameters are connected.

AFM versus SEM Compared with the scanning electron microscope, AFM provides extraordinary topographic contrast, direct height measurements and unobscured views of surface features (no coating is necessary) (Fig. 5.8).Both these techniquesmeasure surface topography. However, both types of microscopes can also measure other surface physical properties. SEM is preferred for measuring chemical composition and AFM for measuring mechanical properties of surfaces.

AFM versus TEMCompared with the transmission electron microscope, three-dimensional AFM images are obtained without expensive sample preparation and yield far more complete information than the two-dimensional profiles available from cross-sectioned samples.

AFM versus optical microscope Compared with the optical interferometric microscope (optical profiles), AFM provides unambiguous measurement of step heights, independent of reflectivity differences between materials.

5.5.2 The common AFM modes

Many AFM modes have appeared for special purposes while the technique of AFM is becoming mature. Three commonly used techniques are discussed here: contact mode, non-contact mode and tapping mode.