IMPROVED SCANNING TUNNELING MICROSCOPE FEEDBACK PERFORMANCE BY MEANS OF SEPARATE ACTUATORS

Scanning tunneling microscopes (STMs) are usually operated in current- feedback mode when scanning over wide areas or rough surfaces. The spe ed of image acquisition is determined by the scanning rate, which (in this mode) is limited by the bandwidth of the feedback control loop. T he control loop bandwidth is usually restricted to frequencies lower t han the lowest mechanical resonant frequency (f(r)) of the STM scanner and tip. This resonant frequency is typically in the range 5-15 kHz a nd is generally ascribed to mechanical resonance of the piezoelectric scanner tube or scanner tripod. In this article we describe some measu red sources of mechanical resonance in STMs, and present a design whic h eliminates the need to control the scanner at high frequencies. We h ave established that the lowest mechanical resonance mode may be ascri bed to the base mounting structure (in addition to the scanning actuat or), depending on the specific design. The complex origins of this mod e prevent the use of conventional control techniques at frequencies ab ove f(r). We have developed an STM design in which a separate feedback actuator is used to modulate the z position of the sample, independen tly to the scanning (x and y) actuator. The effective resonance freque ncy of this actuator is extremely high (in excess of 100 kHz) and the control bandwidth is correspondingly improved.