PERFORMANCE LIMITS FOR THE SCANNING TUNNELING MICROSCOPE

The electromechanical performance limits of a tube scanner type, scanning tunneling microscope are analyzed with respect to the three figures of merit: noise level, scan speed, and scan range. A simple tradeoff between the tube resonant frequency and scan range is defined. For a critically damped scanner tube, a good balance between scan speed and scan range is achieved with a preamplifier bandwidth equal to the resonant frequency of the scanner tube, in which case the closed loop control system bandwidth (90°phase margin) is 2/π times the resonant frequency of the tube, for a proportional-integral controller. This control loop bandwidth is shown to be compatible with a noise performance that is limited by shot noise in the tunnel junction.