# DC-free Method to Evaluate Nanoscale Equivalent Oxide Thickness: Dark-Mode Scanning Capacitance Microscopy

## TL;DR

A new DC-free method using dark-mode scanning capacitance microscopy is developed to evaluate and image nanoscale equivalent oxide thicknesses with high accuracy.

## Contribution

The study introduces a DC-free technique using DM-SCM for evaluating EOTs with a small-area contact electrode.

## Key findings

- The DM-SCM method achieved EOT values with less than 0.5% difference compared to the conventional C–V method.
- DM-SCM enables EOT imaging and visualization of EOT variations caused by DC-stress-induced charge trapping.
- The method is effective for both thermal and high-k oxide films.

## Abstract

This study developed a DC-free technique that used dark-mode scanning capacitance microscopy (DM-SCM) with a small-area contact electrode to evaluate and image equivalent oxide thicknesses (EOTs). In contrast to the conventional capacitance–voltage (C–V) method, which requires a large-area contact electrode and DC voltage sweeping to provide reliable C–V curves from which the EOT can be determined, the proposed method enabled the evaluation of the EOT to a few nanometers for thermal and high-k oxides. The signal intensity equation defining the voltage modulation efficiency in scanning capacitance microscopy (SCM) indicates that thermal oxide films on silicon can serve as calibration references for the establishment of a linear relationship between the SCM signal ratio and the EOT ratio; the EOT is then determined from this relationship. Experimental results for thermal oxide films demonstrated that the EOT obtained using the DM-SCM approach closely matched the value obtained using the typical C–V method for frequencies ranging from 90 kHz to 1 MHz. The percentage differences in EOT values between the C–V and SCM measurements were smaller than 0.5%. For high-k oxide films, DM-SCM with a DC-free operation may mitigate the effect of DC voltages on evaluations of EOTs. In addition, image operations were performed to obtain EOT images showing the EOT variation induced by DC-stress-induced charge trapping. Compared with the typical C–V method, the proposed DM-SCM approach not only provides a DC-free approach for EOT evaluation, but also offers a valuable opportunity to visualize the EOT distribution before and after the application of DC stress.