Use of Ferroelectric Single-crystal Bimorphs for Precise Positioning in Scanning Probe Microscope

Abstract

Scanning probe microscopy and its modes are among the most widely used techniques for surface characterization and modification. Modern scanning probe microscopes (SPMs) are commercially available equipment which combines dozens of methods for different purposes. These devices provide an opportunity to study objects with an atomic resolution and to influence their physical properties in situ. Though a modern multifunctional SPM is a very sophisticated scientific tool including many state-of-theart devices, one part of it remains almost unchanged for years. Indeed, the probe positioning system that generally includes piezoelectric actuators based on PZT ceramic and controlled by capacitive sensors has been utilized since the first SPM was invented. There are different configurations of the positioning system, but in the most widespread case the table with a specimen is moved by stack piezoelectric transducers in the XY-coordinates, while the probe is moved by a PZT-tube in the Z direction. Despite large displacements provided by ceramic actuators, such disadvantages of PZT as electromechanical creep, non-linearity of the deformation vs. applied voltage and narrow range of operating temperatures limit the possibility to create high-precision actuators based on this material [1]. This is the reason why an extra feedback loop based on capacitive or interferometric sensors of distance needs to be used. The presence of the feedback loop complicates the device, decreases the scanning speed and finally leads to an increased price of an SPM. The reason of the abovementioned disadvantages of PZT is mostly the ceramic nature of the material, thus they cannot be fully eliminated simply changing the composition or processing. On the other hand, piezoelectric single crystals do not possess these drawbacks, demonstrate a high thermal and electrical stability and almost do not degrade. Despite this fact, the main disadvantage of single-crystal piezoelectricssmall piezoelectric coefficientsis the reason why PZT is still used in most cases. The problem of the weak conversion of a mechanical deformation into an electrical signal by single-crystal piezoelectric materials can be solved by utilizing complex constructions, such as unimorphs, bimorphs, or multilayer composites, but the presence of adhesive layers or grain boundaries in these composite transducers decreases the sensitivity, as well as the accuracy and thermal stability of the sensors [2]. However, there is a way to manufacture a series bimorph for the piezoelectric sensing element and avoid bonding of separate plates by the formation of two domains with oppositely directed spontaneous polarization vectors in a ferroelectric single-crystal plate. If the crystallographic cut is correctly selected, such a "bidomain" crystal demonstrates a bimorph-like behavior but does not comprise any interface except for an interdomain wall. The application of a voltage across the bidomain plate causes the expansion of one domain and contraction of its counterpart, which leads to the bend...