Constant di/dz Scanning Tunneling Microscopy: Atomic Precision Imaging and Hydrogen Depassivation Lithography on a Si(100) - 2 x 1 : H Surface

TL;DR

This work presents a constant di/dz feedback method for scanning tunneling microscopy, achieved by modulating the controller output with a high-frequency signal and closing the loop on $\ln(Rdi/dz)$. By using a lock-in amplifier to extract the amplitude of the di/dz response and maintaining a constant $\ln(Rdi/dz)$, the approach enhances sensitivity to surface variations and improves image contrast and dimer-resolution on Si(100)-2×1:H surfaces. Experimental validation across multiple STM systems demonstrates superior imaging and hydrogen depassivation lithography (HDL) performance, including AP-mode-like atomic precision patterns with the constant di/dz loop. The method holds promise for high-throughput, multi-tip HDL and scalable nanoscale fabrication, offering improved stability and reduced tip-sample-disturbance-related artifacts. The combination of modulation, LIA demodulation, and system-identification-guided controller design provides a robust framework for advancing STM imaging and lithography applications.

Abstract

We introduce a novel control mode for Scanning Tunneling Microscopy (STM) that leverages di/dz feedback. By superimposing a high-frequency sinusoidal modulation on the control signal, we extract the amplitude of the resulting tunneling current to obtain a di/dz measurement as the tip is scanned over the surface. A feedback control loop is then closed to maintain a constant di/dz, enhancing the sensitivity of the tip to subtle surface variations throughout a scan. This approach offers distinct advantages over conventional constant-current imaging. We demonstrate the effectiveness of this technique through high-resolution imaging and lithographic experiments on several Si(100)-2x1:H surfaces. Our findings, validated across multiple STM systems and imaging conditions, pave the way for a new paradigm in STM control, imaging, and lithography.

Figures (18)

• Figure 1: Schematics of the scanning tunneling microscope working in conventional constant current mode.
• Figure 2: Control block diagram of a constant current imaging feedback loop. The controller command, $u$, is amplified by the high voltage amplifier ${G_{h}(s)}$, which drives the piezo-actuator ${G_{p}(s)}$. The tunneling current, $i$, changes momentarily when the tip encounters an unknown surface feature, ${{h}}$. This change is regulated by the controller ${K(s)}$ by adjusting the tip-sample gap ${\delta}$ (where ${\delta}$ is obtained as the tip displacement due to controller output and true sample topography, $z_t - h$) as shown in the image within the circle. The preamplifier ${G_{A}(s)}$ converts the small tunneling current, $i$ to a measurable voltage. Then, the logarithmic amplifier is applied to the absolute value of the signal obtained from the preamplifier as we are working with negative bias voltage.
• Figure 3: Block diagram of a Lock-in amplifier (LIA) implementation. For better readability, the explicit time-dependency of signals is dropped in this block diagram.
• Figure 4: Schematics of the scanning tunneling microscope working in the constant $di/dz$ mode.
• Figure 5: The control block diagram of the z-axis of STM in the constant ${di/dz}$ mode. In this mode, a modulation signal is applied to the controller output, $u$. The controller command is amplified by the high voltage amplifier ${G_{h}(s)}$, which drives the piezo-actuator ${G_{p}(s)}$. The ${di/dz}$ changes momentarily when the tip encounters an unknown surface feature, ${{h}}$. This change is regulated by controller ${K(s)}$ by adjusting the tip-sample gap ${\delta}$ (where ${\delta}$ is obtained as the tip displacement due to controller output and true sample topography, $z_t - h$). The preamplifier ${G_{A}(s)}$ converts very small tunneling current to a measurable voltage. The amplified current is passed through LIA to obtain ${di/dz}$. The positive valued signal obtained from the LIA is then applied to a logarithmic.