Deflection atomic force microscope

Meyer G and Amer N M 1990 Simultaneous measurement of lateral and normal forces with an optical-beam-deflection atomic force microscope Appl. Phys. Lett. 57 2089... 

Mayer, G. and Amer, N. M., Simultaneous Measurement of Lateral and Normal Forces with an Optical-Beam-Deflection Atomic Force Microscope, AppZ. Phys. Lett., Vol. 57, 1990, pp. 2089-2091. 

Atomic Force Microscopy. A homemade beam deflection atomic force microscope equipped with a single tube scanner, an optical deflection scheme, Si3N4 tip-cantilever assemblies (Digital Instruments, CA), and RHK AFM 100 and RHK STM 100 electronics was used to measure the lattice constants and frictional properties of... 

Fig. 5. Block diagram of contact atomic force microscope system in which cantilever deflection monitored optically with position-sensitive photodiode...

Binnig et al. [48] invented the atomic force microscope in 1985. Their original model of the AFM consisted of a diamond shard attached to a strip of gold foil. The diamond tip contacted the surface directly, with the inter-atomic van der Waals forces providing the interaction mechanism. Detection of the cantilever s vertical movement was done with a second tip—an STM placed above the cantilever. Today, most AFMs use a laser beam deflection system, introduced by Meyer and Amer [49], where a laser is reflected from the back of the reflective AFM lever and onto a position-sensitive detector. 

FIG. 8 Schematic of an atomic force microscope with optical beam deflection detection showing a typical angle of 10° between lever and sample. 

Figure 3.2 The essential elements of an atomic force microscope. The sample is moved beneath a tip mounted on a cantilever a laser beam reflected off the back of the tip and on to a photodiode amplifies deflections of the cantilever.

Plate 4.1 Atomic force microscope image of a synthetic goethite crystal scanned in deflection mode (see Weidler et al., 1996, with permission, courtesy P. Weidler). 

Fig. 13.2. Addition of piezoelectric transducers to an atomic force microscope for acoustically excited probe microscopy. The forces acting between the tip and the sample are measured by the vertical and lateral deflections of the cantilever...

Fig. 4. Principle of a atomic force microscope. A sharp tip is brought close to the sample. The forces acting between tip and sample lead to a deflection of the spring.

Atomic force microscopes have been built in many different versions, with at least six different ways of measuring the deflection of the cantilever [36, 37, 40-42], The commercially available AFM systems use the double photo detector system shown in Figure 7.17 and described by Meyer and Amer [44], Here, a lens focuses a laser beam on the end of the cantilever, which reflects the beam onto two photo detectors which measure intensities T and f2. When the cantilever bends towards the surface, detector 2 receives more light and the difference (h — h) becomes larger. If the tip is scanned over the sample by means of the x- and y-components of the piezo crystal, the difference signal (T — h)/(h + h)... 

The restriction to mica was overcome by a relatively recent technique the atomic force microscope (AFM). sometimes also called the scanning force microscope [69. AFMs are usually used to image solid surfaces. Therefore a sharp tip at the free end of a cantilever spring is scanned over a surface. Tip and cantilever are microfabricated. While scanning, surface features move the tip up and down and thus deflect the cantilever. By measuring the deflection of the cantilever, a topographic image of the surface can be obtained. 

Since the introduction of the STM a number of variations have been devised, such as ATM (atomic force microscope). The basic concept is that piezoelectric actuators move a miniature cantilever arm (with a nm-sized tip) across the sample while a non-contact optical system measures the deflection of the cantilever caused by atomic scale features. The deflection is proportional to the normal force exerted by the sample on the probe tip and images are generated by raster scanning the sample [201]. One application of this technique was to measure the thickness and size distribution of sub-micron clay particles with diameters in the 0.1 to 1 pm size range and thickness from 0,01 to 0.12 pm [202]. 

A new alternative to solve this problem is atomic force microscopy (AFM) which is an emerging surface characterization tool in a wide variety of materials science fields. The method is relatively easy and offers a subnanometer or atomic resolution with little sample preparation required. The basic principle involved is to utilize a cantilever with a spring constant weaker than the equivalent spring between atoms. This way the sharp tip of the cantilever, which is microfabricated from silicon, silicon oxide or silicon nitride using photolithography, mechanically scans over a sample surface to image its topography. Typical lateral dimensions of the cantilever are on the order of 100 pm and the thickness on the order of 1 pm. Cantilever deflections on the order of 0.01 nm can be measured in modem atomic force microscopes. 

The atomic force microscope can be configured in several ways, the most obvious (contact mode) merely involving scanning the tip over the sample at regular intervals, rasper fashion, and recording the deflection. Because of the proportionately very large capillary forces that arise from a contamination layer, imaging of polysaccharides in direct contact mode is carried out under a solvent... 

Fig. 6.7. Diagrammatic representation of the atomic force microscope showing the important components. A position-sensitive diode is used to monitor changes in the cantilever deflection as the sample moves under the tip.

Semiconductor fabrication processes permit construction of small, sensitive, stress sensors. In fact the levers used in atomic force microscopes are almost ideal for this purpose. The combination of the mechanical properties of silicon nitride and the geometry of the cantilever mean that the lever has a high resonant firequency and a low spring constant [32]. The low spring constant is beneficial for sensor applications because it means that a small applied force can be transduced to a measurable deflection, which lies at the heart of any sensor [33]. When combined with the highly sensitive optical lever AFM detection system, both of these factors mean that this arrangement is a fast and highly sensitive stress sensor. 

Force spectroscopy, though originally conceived as a tool for calibrating the atomic force microscope, has become an invaluable tool for studying adhesive interactions on the nanometer scale [29 - 31]. In force spectroscopy the deflection of an atomic force microscopy (AFM) tip is measured as a sample is moved into and then out of contact with the tip. The characteristic hysteresis observed as the sample is retracted is due to adhesion between the tip and sample. The point at which the adhesion is broken and the AFM tip pulls off the sample surface is characterized by a sharp discontinuity in the... 

Figure 4. A schematic of an atomic force microscope (AFM), showing the primary components including the piezoelectric translator, the sample, the cantilever-tip assembly, and the photodetector. In its simplest operating mode (contact mode), the feedback loop of the AFM maintains a constant force between the tip and sample. The force is monitored by measuring the deflection of the cantilever-tip assembly using a laser beam scattered into a quadrant photodetector.

The atomic-force microscope (AFM) can explore contact and hardness on the atomic scale. Analogous to the STM, the AFM uses a feedback loop to control the distance between sample and a probe tip at the end of an cantilever arm. As opposed to tunneling current however, the AFM monitors an optical signal as feedback to measure lever deflection. Thus both attractive and repulsive interactions of tip and sample can be monitored. As the microscope tip approaches the surface, attractive... 

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