Tripod scanner

In the early years of STM instrumentation, tripod piezoelectric scanners were the predominant choice, as shown in Fig. 9.6. The displacements along the X, y, and z directions are actuated by three independent PZT transducers. Each of them is made of a rectangular piece of PZT, metallized on two sides. Those three PZT transducers are often called x piezo, y piezo, and z piezo, respectively. By applying a voltage on the two metallized surfaces of a piezo, for example, the x piezo, the displacement is 

The accurate calculation of the resonance frequencies of a tripod scanner is a complicated problem. The flexing modes are effectively coupled with the stretching modes. An evaluation of the lowest resonance frequency of the flexing mode provides an order-of-magnitude estimation of the lowest resonance frequency of the tripod scanner. For a piezo made of PZT-5A, 20 mm long and 2 mm thick, the radius of gyration is 2 mm/y/ 2 = 0.577 mm. The speed of sound is about 2.8 km/sec. Using Eq. (9.44), the resonance frequency is found to be 3.3 kHz, which is close to the values often observed experimentally. 

Beeause the piezo constant is proportional to Uh, whereas the resonance frequency is proportional to hlU, it is clear that by reducing the length and thickness in proportion, the piezo constant remains the same, and the resonance frequency will increase. This is in general true. The natural limit of such a reduction is the depoling field of the material. 

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Fig. 9.6. Tripod scanner. Three PZT bars to control the x, y, and z displacements, respectively. The tip is mounted at the vertex of the tripod. (Reproduced from Binnig and Rohrer, 1987, with permission.)...

However, it is very difficult to construct a three-dimensional scanner based on bimorphs. An example of such a design is reported by Muralt et al. (1986). It is much more complicated than the tripod 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). 

Even with the foot, the vibration problem in such STM designs has not been resolved sufficiently. In fact, the tripod scanner has a relatively low natural vibration frequency, and the approaching mechanism is relatively bulky. In actual application, a four to-six element metal-plate stack with viton separators is used for vibration isolation. In order to achieve atomic resolution, a spring stage, either suspension spring or compression spring, is neces-... 

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. 

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.

Actuating the opposed pairs of electrodes on a tube scanner bends the tube so that a tip attached to the end moves in a circular arc. Thus motion in x or y involves motion in z. Ideally, in tripod scanner, the x, y and z motions are independent, but inaccuracy of construction or the use of a pivot point to amplify the motion in X and y couples the motions. Correction for this and other scanner errors is an important feature of AFM design, and image artifacts may result from uncorrected non-ideal behavior (see Section 3.3.7). 

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 heart of STM is a piezoelectric scanner, sometimes called piezodrive or simply piezo. In this chapter, we provide a brief summary of the basic physics of piezoelectricity and piezoelectric ceramics relevant to the applications in STM. Three major types of piezodrives, the tripod, the bimorph, and the mbe, are analyzed in detail. 

The positioning of the sample at better than atomic precision is carried out with piezo crystals, which are ceramic electromechanical transducers that distort when a voltage is applied. As the distortion is proportional to the applied voltage, one can displace a sample by means of piezos at any desired precision, provided that the electronics are sufficiently accurate and stable. The rapid development of the scanning probe microscope would not have been possible without the availability of modern, stable electronics. In order to obtain optimum atomic resolution, one usually selects a tubular piezo element with a small scan range (ca. 1 pm) for larger scan ranges, the scanner is a tripod with a separate piezo element for each dimension. 

The set up is now in principle ready to start an experiment. The coarse approach is carried out to position the tip close to (but not in contact with) the sample surface. Using an eyepiece, the cantilever can be viewed from the side (or alternatively one can use a top-view CCD camera for this purpose). With the eyepiece, we locate the reflection of the laser light on the cantilever and its reflection (mirror image) on the sample (red spots). Using the stepper motor, we lower the optical head until the spots are close however, we still want to clearly be able to detect a gap between the spots. In older scanner types, the optical head is lowered by using the stepper motor for one pod of a tripod, while the other two are lowered manually using the corresponding screws. In this case, it is essential that the head is lowered such that it stays leveled at all times. 

Fig. 10.4. Two scanner realizations tripod and tube, (A) contact scheme with the z-signal added to x- and y-signal, (B) contact scheme with z-electrode in the middle.

Figure 3.24. Two types of piezo scanners used in commercial AFMs the tripod (left) and tube (right).

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).

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