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Chapter_4_ - CHAPTER 4 Imaging Modes The Atomic Force...

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Imaging Modes CHAPTER 4 68 The Atomic Force Microscope is a member of the family of scanning probe microscopes that includes the scanning tunneling microscope and the near field optical microscope, see Figure 4-1. Scanning Probe Microscope Family Air, Liquid, Vacuum FIGURE 4-1 There are several types of Scanning Probe Microscopes that are used for measuring surface topography and physical properties. The primary types of SPM’s are the AFM, STM and NSOM. AFM’s account for about 80% of the total number of scanning probe microscopes. Atomic Force Microscope (AFM) Contact Mode Lateral Force (LFM) Scanning Thermal (SThM) Lithography Shark Vibrating Mode Close Contact (CC) Intermittent Contact (IC) Magnetic Force (MFM) Electric Force (EFM) Kelvin Probe (SKPM) Electrochemistry Scanning Tunneling Microscope (STM) Topography Spectroscopy Lithography EChem. BEEM Near Field Optical Microscope (NSOM) Aperture Aperture-less Reflection Transmission Each of these microscopes measures surface topography by raster scanning a small probe across a surface and monitoring the probes motion. A scanning tunneling microscope (STM) 1 operates by monitoring the current flow between a probe and surface. In the atomic force microscope
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69 Chapter 4 • Imaging Modes Soon after the invention of the AFM it was realized that these instruments were capable of measuring far more than surface topography. In fact, it is possible to measure almost any physically observable phenomena at the nanometer scale. The only requirement is that a nanoscopic sensor must be developed for the end of a probe. For example, magnetic fields, electric fields, temperature, and hardness may be measured with the AFM probe. Additionally, it is possible to use the AFM probe to modify surfaces. By definition, an AFM mode is a non-topographical measurement made with an AFM. For the most part, atomic force microscopes are operated in ambient air. At the surface of samples maintained in ambient air, there is always a contamination layer comprised of water and hydrocarbons. Thus, in an AFM, the probe tip is typically immersed in the contamination layer (see Figure 4-2). Because the contamination layer can vary from one environment to the next, the layer can cause uncertainty in AFM measurements. This is especially true for mode measurements made with AFM. FIGURE 4-2 In ambient air, the AFM probe must pass through a surface contamination layer to touch the surface. AFM probes contribute a lot of uncertainty of topography and mode measurements. The uncertainty is due to variations in probe geometry. As provided by a vendor 4 , the typical AFM probe has a diameter of < 15 nm. That is to say it could be 15 or 5 or 10 nm in diameter. The uncertainty goes up when the probe is coated with a thin film of metal or other type of material. Not only are there variations in the probe coating thickness, there can be variations in the integrity of the probe. For example, the coating on an AFM probe may have grains. Figure 4-3 shows an
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