[Paper Review] Transmission Helium Ion Microscopy of Graphene

K. L. Kavanagh, Aleksei Bunevich+1 more|arXiv (Cornell University)|Apr 3, 2020

Advanced Electron Microscopy Techniques and Applications4 citations

TL;DR

This study demonstrates transmission helium ion microscopy (THIM) of graphene using a modified focused ion beam (FIB) system with a direct-detection camera, achieving transmission scattering patterns that closely match SRIM simulations. The key contribution is the first experimental validation of coherent He+ ion transmission through few-layer graphene, showing potential for high-sensitivity, low-damage imaging of 2D materials with superior mass contrast compared to TEM.

ABSTRACT

We compare transmission He ion microscopy (HIM) to transmission electron microscopy (TEM) of graphene support films. We present spot transmission patterns that compare with scattering and range of ions in materials (SRIM) predictions, and show examples of scanning He$^+$ transmission images, based on integrated camera intensity. We also consider the potential for coherent HIM scattering.

Motivation & Objective

• To investigate the feasibility of transmission helium ion microscopy (THIM) for imaging graphene at the nanoscale using a modified FIB system.
• To compare THIM results with transmission electron microscopy (TEM) in terms of image contrast, diffraction patterns, and scattering behavior.
• To evaluate the spatial and temporal coherence of He+ beams in transmission geometry using experimental scattering patterns and SRIM simulations.
• To assess the potential of THIM for detecting thickness variations and mass contrast in low-Z 2D materials like graphene.
• To identify technical limitations such as camera damage from ion implantation and explore mitigation strategies like annealing.

Proposed method

• A commercial He+ ion microscope (Zeiss, Nanofab) was modified with a 14×14 mm² digital camera (55 µm pixels) placed 20 cm below the sample stage to detect transmitted He+ ions.
• Transmission scattering patterns were recorded using single-spot exposures at 35 keV He+ beam energy, with signal integrated over time to generate 2D intensity maps.
• Scanning transmission HIM (STHIM) was implemented by rastering the focused He+ beam and integrating camera response at each position, analogous to annular dark-field STEM.
• Scattering probability as a function of angle was extracted from experimental data and compared with SRIM simulations for amorphous carbon layers of equivalent mass (2.5 nm).
• TEM and HIM secondary electron imaging (SEI) were performed in parallel for direct comparison, using 200 keV electrons and 30 keV He+ ions with low-dose protocols to minimize damage.
• Dead pixels in the camera were attributed to He ion implantation, with post-operation annealing investigated as a mitigation strategy.

Experimental results

Research questions

• RQ1 Can transmission helium ion microscopy (THIM) produce measurable scattering patterns from few-layer graphene, and do they match SRIM predictions?
• RQ2 How does the coherence of the He+ beam in transmission geometry compare to that of electrons in TEM, and can diffraction effects be resolved?
• RQ3 What is the sensitivity of THIM to thickness variations in graphene, and how does it compare to annular dark-field STEM in terms of mass contrast?
• RQ4 To what extent does ion beam damage affect the camera's performance, and can annealing mitigate long-term degradation?
• RQ5 Can STHIM imaging be achieved with sufficient spatial resolution to resolve sub-pixel scattering features in 2D materials?

Key findings

• The experimental transmission scattering pattern from 6-8 layer graphene is rotationally symmetric and matches SRIM simulations for 2.5 nm amorphous carbon, with maximum scattering angles between 5 and 10 mrad.
• Scattering probability profiles from the graphene samples align well with SRIM predictions, confirming the validity of the model for low-Z, thin materials.
• The camera detected transmitted He+ ions over a 1.4×1.4 cm² area with a maximum scattering angle of ±70 mrad, indicating effective beam transmission through the sample.
• Dead pixels were observed in the camera, likely due to He ion implantation at doses exceeding 10¹⁷ cm⁻³, suggesting a need for mitigation strategies like annealing.
• STHIM imaging was successfully demonstrated by integrating camera response during beam rastering, showing potential for thickness- and mass-sensitive imaging with low beam current (<0.1 pA).
• The expected first-order Bragg angle for {1100} graphene planes (0.2 mrad) is 100× smaller than for TEM, requiring either longer camera distances or higher pixel density to resolve diffraction patterns.