Abstract
In this article, we propose a technique for highly stabilized atom-tracking control of a scanning tunneling microscope (STM) tip by referring to an atomic point (or atomic array) on a regular crystalline surface. Graphite crystal, whose lattice spacing is approximately 0.25nm, was utilized as the reference. To enhance stability of the atom-tracking control against external disturbances, a new atom-tracking controller, which consists of integrator, tracer and limiter units, was developed. The integrator unit is designed to eliminate the steady-state error due to thermal drift. A phase-lag low-pass filter is utilized as the tracer unit to compensate the disturbance due to vibration/acoustic noise. To improve the phase margin of the controller, the limiter unit consists of a phase-lead high-pass filter and a saturator whose output is less than one-half of the lattice spacing. The performance of the stabilized technique, which is to combine the new tracking controller with enhanced STM stiffness, was evaluated using internal/external artificial disturbance generators. The experimental results show that the proposed method has a high capability for maintaining atom-tracking control without any jumping of the STM tip to neighboring atoms, even in a noisy environment. The method was also applied to atom-stepping control of the STM tip by referring to some crystalline axis. The atom-stepping control atom by atom along the crystalline axis over a range of 200 atoms, at a rate of 10 atoms/sec, was performed without missing the atomic array.
