Imaging ultra thin layers with helium ion microscopy: Utilizing the channeling contrast mechanism

Abstract

Summary Background: Helium ion microscopy is a new high-performance alternative to classical scanning electron microscopy. It provides superior resolution and high surface sensitivity by using secondary electrons. Results: We report on a new contrast mechanism that extends the high surface sensitivity that is usually achieved in secondary electron images, to backscattered helium images. We demonstrate how thin organic and inorganic layers as well as self-assembled monolayers can be visualized on heavier element substrates by changes in the backscatter yield. Thin layers of light elements on heavy substrates should have a negligible direct influence on backscatter yields. However, using simple geometric calculations of the opaque crystal fraction, the contrast that is observed in the images can be interpreted in terms of changes in the channeling probability. Conclusion: The suppression of ion channeling into crystalline matter by adsorbed thin films provides a new contrast mechanism for HIM. This dechanneling contrast is particularly well suited for the visualization of ultrathin layers of light elements on heavier substrates. Our results also highlight the importance of proper vacuum conditions for channeling-based experimental methods.

To see or not to see: Imaging surfactant coated nano-particles using HIM and SEM

“…Helium Ion Microscopy is characterized by a high surface sensitivity in particular for thin carbon layers [14]. The small escape depth of secondary electrons (SE) generated by swift ions [21] results in the superior surface sensitivity and resolution [13] of the HIM SE image.…”

“…Alternatively, a new charged particle scanning beam microscopy method has entered the market a few years ago. Helium Ion Microscopy (HIM) [12] has an ultimate resolution as small as 0.29 nm [11,13] and a very high surface sensitivity [14]. It uses helium ions to generate a multitude of signals including secondary electrons (SE), backscattered helium (BSHe) and photons.…”

“…However, applying appropriate cleaning procedures to the chamber as well as the sample prior to imaging this problem can be eliminated. Provided that deposition of carbon from the chamber vacuum can be excluded a very high sensitivity for intentionally deposited ultra-thin carbon layers is possible in HIM [14].…”

Helium ion microscopy of graphene: beam damage, image quality and edge contrast

“…The HIM also has enhanced topographical contrast and enhanced material contrast over SEM imaging [9]. A backscattered ion imaging mode can provide complementary information such as channelling contrast [10] and material contrast from sub-surface particles [11].…”

From Graphene and Topological Insulators to Weyl Semimetals

“…Silicon carbide (SiC) is coated on a wafer, with high temperatures being required 43 . Recent studies of the Raman topography of epitaxial graphene, show that the electron mobility is highly dependent upon thickness and monolayer strain uniformity 62,63,64,65 . The highest mobility of epitaxial graphene reaches a value of 18,000 cm 2 V −1 s −1 at room temperature and significantly larger at low temperatures.…”