Piezo creep

The controlled manipulation of single atoms and molecules demands a higher stability and lower thermal drift of the STM than that required for surface imaging. Most experiments up to now have been performed at low temperatures since instrumental effects like piezo creep, hysteresis and thermal drift are then negligible. Due to the above-mentioned requirements a much lower precision was achieved in the experiments at room temperature than at low temperature. Nevertheless vertical manipulation can be done with atomic precision at room temperature as well. 

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 . 

When a new scan size is actuated with a piezo scanner (e.g. 60 pm), the piezo will further stretch (to 61.8 pm) over time. This effect causes an apparent bending of rows at the beginning of an SPM image. Hence, to avoid piezo creep in an image, the measurement should be restarted after the first 20% of the imaged area, when a new scan range has been selected. 

During later operation it suffices to measure the electrical quantities in order to compute the mechanical actuator quantities on the basis of the model, thereby compensating the hysteresis and creep of the piezo actuator. Since the computing process is quite complex and has to be executed in realtime, this control method is presently only implemented for low-frequency operations. However, the applicable frequency range of this inverse control method will grow in conjunction with the technological progress in the field of microelectronics [364]. 

The response of a piezo to a large step change in voltage is not a step change in position. Most of the motion occurs very quickly, but then there is a slow continuing motion, called creep. This means that after a rapid change, the tip position will not be stable until the creep is... 

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