Piezo Scanners

Almost all STMs use piezo translators to scan the tip, or seldom, to scan the sample. Even the first STM[84,85] and some of the predecessor instruments[86,87] used piezo translators for scanning. Microscopes using magnetostrictive materials[88] or electromagnetic drives[89] have been proposed. We will concentrate on piezo electric materials.

An electric field applied across a piezo electric material causes a change in the crystal structure, with expansion in some directions and contraction in others. Also, a net change in volume occurs. Detailed descriptions of the piezo electric effect can be found in solid state physics textbooks[90]. The transverse piezo electric effect is by far the most important for scanning probe microscopes. The expansion perpendicular to the applied electric field $\vec{E}$ for a long slab of material with the field applied across the small sides is

$$\Delta l = l \vert\vec{E}\vert d_{31} = l \frac{V}{t} d_{31}$$ (931)

where $d_{31}$ is the piezoelectric constant, $V$ the applied voltage and $t$ the thickness of the piezo slab or the distance between the electrodes where the voltage is applied. This allows to choose the sensitivity of a piezo actuator within the limits of its mechanical stability.

Abbildung 4.273: Types of piezo scanners: a) the tripod; b) the thermally compensated scanner and c) the piezo tube.

The first STMs all used piezo tripods for scanning (see for instance Binnig and Rohrer[45]). The piezo tripod (figure 4.273a)) is an intuitive way to generate the three dimensional movement of a tip attached to its center. However, to get a suitable stability and scanning range, the tripod needs to be fairly large (about 5 cm). Its size and its asymmetric shape make it very susceptible to thermal drift. The design of van Kempen and van de Walle[82] (figure 4.273b)) tries to circumvent this problem by using a symmetrical design. Its thermal drift performance is much better than the simple tripod. However a complicated assembly of many piezo pieces is required. The tube scanner (figure 4.273c)) is now widely used in scanning tunneling and scanning probe microscopy for its simplicity and its small size[91]. The outer electrode is segmented in four equal sectors of 90 degrees. Opposite sectors are driven by signals of the same magnitude, but opposite sign. This gives, through bending, a two dimensional movement on, approximately, a sphere. The inner electrode is normally driven by the $z$ signal. It is possible, however, to use only the outer electrodes for scanning and for the $z$-movement. The main drawback of applying the $z$-signal to the outer electrodes is, that the applied voltage is the sum of both the $x$- or $y$-movement and the z-movement. Hence a larger scan size effectively reduces the available range for the $z$-control.

Piezo scanners, tubes and tripods, are made of piezo ceramic material. Piezo materials with a high conversion ratio, i.e., a large $d_{31}$ or small distances between the electrodes, allowing large scan ranges with low driving voltages, do have substantial hysteresis resulting in a deviation from linearity by more than 10 %. The sensitivity of the piezo ceramic material (mechanical displacement divided by driving voltage) increases with reduced scanning range, whereas the hysteresis is reduced. A careful selection of the material for the piezo scanners, the design of the scanners, and of the operating conditions is necessary to get optimum performance.