Tube scanner

In this section, we describe the tube scanner, which has high piezo constants as well as high resonance frequencies. Moreover, it is much simpler than both the tripod scanners and bimorph-based scanners. The tube scanner soon became the predominant STM scanner after its invention by Binnig and Smith (1986). 

One of the problems with this design is that the motions driven by the x, y, and z voltages are nonlinear and not precisely orthogonal. This can be taken care of by proper programming in the control system, as we will discuss it in Chapter 11. Because the simplicity of the mechanical and electrical design, this arrangement is still widely used. Substantial improvement can be 

The constant a is to be determined by the condition of zero torque. The torque can be evaluated easily by integrating over the angle 0 see Fig. 9.10. The condition of zero torque is 

The neutral plane, where the stress is zero, lies beyond the periphery of the tube  

using elementary geometry, the deflection is found to be 

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Usually, in AFM the position of the tip is fixed and the sample is raster-scanned. After manual course approach with fine-thread screws, motion of the sample is performed with a piezo translator made of piezo ceramics like e. g. lead zirconate tita-nate (PZT), which can be either a piezo tripod or a single tube scanner. Single tube scanners are more difficult to calibrate, but they can be built more rigid and are thus less sensitive towards vibrational perturbations. 

The tip is first brought near to the sample via a coarse Z positioner, such as a controlled approach piezo-electric motor. This provides a 10 A step size and so is capable of very precise and controlled movement. As soon as a tunnelling current is detected, the tip is stopped and the fine control system is operated. Thus the lip is mounted on the end of a hollow, cylindrical piezo crystal, a tube scanner (a piezo-electric crystal can be made to deform under the influence of a voltage applied across it). The tube scanner has four strip electrodes on the outside, providing movement in the x and y directions (lateral movement), and one electrode covering the whole of the inside. A... 

Figure 2.25 Schematic representation of the STM head and electrochemical assembly. (I) Inchworm motor, (2) Inch worm, (3) Faraday cage around tube scanner, (4) Teflon electrochemical cell, (5) working electrode (i.e. sample), (6) stainless steel plates, (7) halved rubber O rings, (8) elasticated ropes attatched to baseplate. The counter and reference electrodes and the various electrical connections arc not shown for clarity. From Christensen (1992).

Sonnenfeld, R. SPIE 88 Conference Proceedings, in press). Such tips have also been employed in a newer STM, shown in Figure 1, that employs a single tube scanner, and allows for fluid delivery and removal via a fluid transfer line. This approach, discussed in greater detail below, was successful in that Au could be plated from, and imaged with, these tips (58). 

Fig. 9.9. The tube scanner. (Reproduced from Binnig and Smith, 1986, with permission.)...

Fig. 9.10. Deflection of a tube scanner. (A) Opposite and equal voltages are applied to the y electrodes of a tube scanner. The x, z electrodes are grounded. A positive stress (pressure) is generated in the upper quadrant, and a negative stress (tension) is generated in the lower quadrant. (B) At equilibrium, a distribution of stress and strain is established such that the total torque at each cross section is zero. This condition determines the deflection of the tube scanner in the y direction. (Reproduced from Chen, 1992, with permission.)...

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. 

Once the tube scanner was invented (Binnig and Smith, 1986), it soon became the primary choice of piezo scanners in STM. Its small size and high natural resonance frequency make the mechanical design and vibration isolation much easier. Currently, most of the commercial STMs as well as home-made STMs use tube scanners. 

Carr, R. G. (1988). Finite element analysis of PZT tube scanner for scanning tunneling microscopy. /. Microscopy 152, Part 2, 379-385. 

For stand alone-type AFMs any hot stage may be used, while for tube scanner design, miniature hot stages can be built. 

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... 

For SPM applications, a piezo stage must be designed to move in x-, y- and z-directions. Several types of scanners have been technically realized (Fig. 10.4). Most common is a tube scanner in configuration B. Overall, piezo materials show a reliable performance for thousands of operation hours " , however, one has to be aware of possible hysteretic behavior" and piezo creep . 

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... 

Figure 16-32. Molecularly resolved AFM topograph (unfiltered image) of a thin (10 nm) film of Ooct-OPV5, annealed at 12O C for 5 min (Park Scientific Instruments AFM, 10 pm tube scanner and triangular microfab-licated 813X4 cantilevers with integrated tips...

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. 

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