Simultaneous Estimation of Differential Surface Parameters with Ultra-fast Feedback Loop in Scanning Tunneling Microscopy

We report an enhanced feedback control method to enable the simultaneous estimation of differential surface pa-rameters di/dV and d(ln(Ri))/d8 during atomic-scale imaging of a surface with a Scanning Tunneling Microscope (STM). The proposed system enables exceptional signal-to-noise ratio (SNR) measurements of surface parameters by adding notch filters in the feedback loop. We then analyze the performance of a conventional PI controller for the ultra-fast feedback loop and demonstrate that changes in the work function (), a quantum mechanical property of the tip-sample junction, can destabilize the closed-loop system, leading to a possible tip crash. These tip crashes can compromise the STM's efficiency and reliability, thus necessitating measures to address this loss of robustness. The effectiveness of this new STM feedback control system is validated by applying it to a set of STM images obtained from a Si(100)-2×1: H passivated surface. We provide guidelines for PI tuning to optimize the system's performance.