Piezoelectric scanners for SPMs are usually fabricated from lead zirconium titanate, or PZT, with various dopants added to create specific materials properties. Scanners are made by pressing together a powder, then sintering the material. The result is a polycrystalline solid. Each of the crystals in a piezoelectric material has its own electric dipole moment. These dipole moments are the basis of the scanner's ability to move in response to an applied voltage.
After sintering, the dipole moments within the scanner are randomly aligned. If the dipole moments are not aligned, the scanner has almost no ability to move. A process called poling is used to align the dipole moments. During poling the scanners are heated to about 200°C to free the dipoles, and a DC voltage is applied to the scanner. Within a matter of hours most of the dipoles become aligned. At that point, the scanner is cooled to freeze the dipoles into their aligned state. The newly poled scanner can then respond to voltages by extending and contracting.
Occasional use of the scanner will help maintain the scanner's polarization. The voltage applied to enact the scanning motion realigns the stray dipoles that relax into random orientation. If the scanner is not repoled by regular use, a significant fraction of the dipoles will begin to randomize (depolarize or depole) again over a period of weeks. Depoling is accelerated markedly if the scanner is subjected to temperatures above 150°C. This means that if you want to add a heated stage to your SPM, you must isolate it thermally from the scanner. (The Curie temperature for PZT materials is about 150°C.)
Many SPMs use variations of the simple tube design
depicted in Figure 2-2. In this design, the scanner is a hollow
tube.
Electrodes are attached to the outside of the tube, segmenting it electrically into vertical quarters, for +x, +y, -x, and -y travel. An electrode is also attached to the center of the tube to provide motion in the z direction. When alternating voltages are applied to the +x and -x electrodes, for example, the induced strain of the tube causes it to bend back and forth in the x direction. Voltages applied to the z electrode cause the scanner to extend or contract vertically.
In most cases, the voltage applied to the z electrode of the scanner at each measurement point constitutes the AFM (constant-force) or the STM (constant-current) data set. In some cases, an external sensor is used to measure the height of the scanner directly (see Hardware Correction). For the purposes of the discussion in the following sections, it is assumed that the data set consists of voltages applied to the z electrode.
The maximum scan size that can be achieved with a particular piezoelectric scanner depends upon the length of the scanner tube, the diameter of the tube, its wall thickness, and the strain coefficients of the particular piezoelectric ceramic from which it is fabricated. Typically, SPMs use scanners that can scan laterally from tens of angstroms to over 100 microns. In the vertical direction, SPM scanners can distinguish height variations from the sub-angstrom range to about 10 microns.
Piezoelectric scanners are critical elements in SPMs, valued for their sub-angstrom resolution, their compactness, and their high-speed response. However, along with these essential properties come some challenges.
