Piezoelectric tube scanners

How can the tunneling barrier width and the lateral position of the tip be controlled so accurately Tire tip is mounted on an actuator consisting of piezoelectric ceramics for reliable and exact positioning in all three dimensions (particular for the scanning). Only the nanometer positioning ability of piezo ceramics (see Tutorial 2 on Piezoelectric Tube Scanners and Translational Stages) made SPM techniques initially possible. 

Tutorial 2. Piezoelectric Tube Scanners and Translational Stages... 

The movement of the tip is usually carried out by drivers based on piezoelectric elements, similar to those used in STM, as described in Chapter 2. Typically, inchworm drivers (Burleigh Instruments, Fishers, NY) are used, since they can move larger distances than simple piezoelectric tube scanners. However, where higher resolution is needed, piezoelectric pushers can be added, so that the inchworms provide coarse drives and the pushers nm-resolution drives. Generally the direction normal to the substrate is taken as the z direction, while x and y are those in the plane of the substrate. 

J Akila, SS Wadhwa. Correction for nonlinear behavior of piezoelectric tube scanners used in scanning tunneling and atomic force microscopy. Rev Sci Instrum 66 2517-2519, 1995. 

Figure 2. Schematic of a STM. The atomically sharp tip is mounted in a piezoelectric tube-scanner and is controlled by a feedback loop, which ultimately produces an image on the computer screen.

The atomically sharp STM tip is mounted on a piezoelectric tube scanner. When a voltage is applied across piezoelectric materials, they expand or contract. The piezoelectric tube scanner used in the STM is often built in three parts so that the application of voltages to each part individually gives the possibility of very accurate movement in the x, y, and z directions (76). 

The geometry of the tube scanner also provides a purely electrical method to self-test and self-calibrate, especially for measuring the piezoelectric constants in a cryogenic environment. The piezoelectric constant varies with temperature in a complicated manner, and also with the particular batch of materials by the manufacturer and time (the aging effect). 

Fig. 9.13. Double piezoelectric response of a tube scanner with symmetric connections. (A) The two y quadrants are connected to an ac voltage source. The two x quadrants are connected to the ground through an ac ammeter. (B) The stress in the x quadrants of the piezoelectric ceramics is equal in magnitude and opposite in sign to the y quadrants. (Reproduced from Chen, 1992a, with permission.)...

Actual measurements of the double piezoelectric response also indicated that the double piezoelectric responses from individual quadrants vary significantly. As shown by the example in Fig. 9.16, the currents from the two x quadrants differ by about 40%. The currents from two y quadrants differ by about 16%. Therefore, the double piezoelectric response provides a sensitive method for testing the tube scanner. 

Fig. 12.7. The concentric-tube STM. The inner piezoelectric tube acts as the scanner. The outer piezoelectric tube acts as an inertial walker. (Reproduced from Lyding et al., 1988, with permission.)...

Three-dimensional positioning of the scanner is obtained by placing three piezoelectric tubes in an orthogonal arrangement as shown in the illustration of the STM structure (Figure 5.2). The positioning accuracy of the scanner is determined by the characteristics of piezoelectric... 

A tube scanner moves the tip along the section of a circle or a parabola. This nonlinearity can lead to distortions towards the edges of an image, especially when dimensions of the scan areas come close to the maximum elongation specified for that piezoelectric ceramic at the typical operation voltage (approximately... 

Controllers for piezoelectric tube or tripod scanners require high-stability, low-noise voltage amplifiers. Since piezoelectric materials have a large... 

The two mounts described above are too large for probes with nanometer-sized tips. In addition to vibration problems, the mounts would not fit onto piezoelectric tube or tripod scanners. Small SECM probes are very similar to electrochemical STM probes. These probes are often mounted by plugging the probe into a small electrical pin-socket glued to the tube scanner. The socket thus serves as a mechanical and electrical contact to the probe. 

Figure 6.28 STM tubes and biomorph Illustrations of a piezoelectric tube (a), biomorph (b) and a complete single tube scanner (c). The tube and biomorph are explained in the text. In the case of the scanner, external electrodes (Yac, Yjc, Xac, Xjc) are applied parallel to the tube axis. As voltage is applied to an external electrode, the tube bends away from that electrode. Otherwise, voltage applied to an inside Z electrode causes uniform tube elongation.

Other types of piezoelectric STM scanners were developed such as the tube scanner shown in Figure 4.21. 

Fig. 6.14 Piezoelectric scanners used in STM and AFM. (A) Tube scanner. This is a monolithic tube of piezoelectric ceramic, with the tip attached at one end - attached to the tube, or mounted centrally as shown. A voltage applied across the tube wall causes it to lengthen. The outer electrode is divided into four segments, so opposite voltages applied to opposite segments make the tube bend and the tip scan. (B) Tripod scanner. Three stacked piezoelectric ceramics are placed orthogonally to give independent motion in x, y and z. The mechanical design is not so simple as the tube, but it allows the z-range to be chosen independently of the xy scan range.

Figure 9. Configurations of piezoelectric elements (a rectangular beam, thin-wall tube, and sandwich bimorph structure) for positional control, (a) and (b) are polarized uniformly, while (c) has opposing polarizations on the two sides of the interface. The electrostatic field is perpendicular to the long dimension (a) and (c), or radial (b). The two common implementations of. v-y-z positioning are shown a tube scanner (middle) and a tripod (lower).

From Fig. 9.16, we obtain c/33 1.05 kN, a value consistent with the value listed in the catalog (1.27 kN). The value might be somewhat lower than the true value because the bonding of the tube ends is not perfectly rigid. If one end of the tube is free, or both ends are free, the deformation pattern varies significantly at the end(s). The net end effect is to reduce the value of the double piezoelectric response. Even if the end-bonding condition is unknown, an accurate measurement of the temperature or time variation of the piezoelectric constant can still be achieved. In other words, if the piezoelectric scanner is calibrated by a direct mechanical measurement or by the scale of images at one temperature, then its variation can be precisely determined by the electrical measurements based on double piezoelectric responses. 

Another application of the idea of an inertial stepper is the STM with four identical tube piezoelectrical elements developed by Besocke (1987). Each of the tube piezoelectric elements has its outer metallization quartered, similar to that of Binnig and Smith (1986). The center piezo tube is used as the scanner, whereas the three outer tubes are used for sample manipulation. 

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