Control System Design for High Performance Scanning Tunneling Microscopy

Scanning Tunneling Microscope (STM) is scientific instrument that is used to generate atomic-resolution images from material surface. Since its invention in early 1980s, STM has played a crucial role in advancements and many breakthroughs in nanotechnology. The early works on STM concentrated on imaging. However, soon it was realized that the STM tip could be used as an effective tool for patterning the surface with a resolution down to a single atom through lithography. This capability of STM has turned it to an important instrument in atomically-precise manufacturing. Tip-sample crash is a prevalent failure in STM which severely limits its performance. Adverse effects of such failure are even worse in lithography applications which need preserved non-changing tip shape.

In this research, we focus on the STM control system to address the tip-sample crash problem. Based on frequency-domain closed-loop system identification tests, we show that the DC gain of the open-loop plant depends on the Local Barrier Height (LBH) which is a quantum mechanical property of the tip and sample. Since LBH is highly variable due to local changes in surface and tip properties, the control loop gain is subject to large changes. Such variations adversely affect the closed-loop stability and increase the chance of tip-sample crash if the controller gains are kept fixed.

We propose a method for estimating LBH on-the-fly and use that estimation to adaptively tune the gains of a proportional-integral (PI) controller. Results of the proposed LBH measurement method are not dependent on the feedback parameters, despite a method prevalently used in STM research. We report experimental results confirming variability of LBH, enhanced closed-loop stability in the presence of the tuning method, and extended tip life-cycle.
Furthermore, we study the effect of proposed control method on the STM performance in Hydrogen Depassivation Lithography (HDL), and show that it results in more stable current and improves the STM performance. Moreover, we investigate the HDL procedure and suggest effective ways for conducting the HDL from a control system perspective to minimize damages to the tip during lithography.