A Time-of-Flight Secondary Ion Mass Spectrometer Add-on for the Helium Ion Microscope

In a Helium Ion Microscopes (HIM) a Gas Field Ion Source (GFIS) is used to create a Helium or Neon ion beam with a diameter smaller than 0.5 nm and 1.8 nm, respectively. The method is well known for its high resolution imaging and nano-fabrication capabilities which it is able to provide not only for conducting but also insulating samples without the need for a conductive coating.

However, the existing HIM tools suffered from the lack of a well integrated analytic method that can enrich the highly detailed morphological images with materials contrast. Recently, we designed, implemented and reported on the first time-of-flight secondary ion mass spectrometry (TOF-SIMS) add-on that can be retrofitted to existing microscopes [1,2,3].

After a brief introduction to the HIM, I will focus on the new time-of-flight setup. It is based on fast blanking electronics that chop the primary beam into pulses with a minimal length of 20 ns. In combination with a MCP based stop detector this enables TOF backscattering spectrometry with 54 nm lateral resolution [1,2] - the world record for spatially resolved backscattering spectrometry. In order to extend the TOF setup for SIMS an ion optic has been designed and optimized for high transmission by ion transport simulations and an evolutionary algorithm. The high transmission is crucial to collect enough signal from nanoparticles prior to their complete removal by ion sputtering.

The setup can obtain SIMS data from a region of interest or can be used in imaging mode to obtain elemental line profiles and maps of the surface. For m/q ≤ 80 u a m/∆m > 200 has been achieved. This is sufficient for many life science applications that rely on the isotope identification of light elements (e.g. C, N). The lateral resolution has been evaluated to 8 nm using the knife edge method and a 75%/25%. The results will be compared to the theoretical achievable lateral resolution and the limiting experimental and physical constraints of this approach will be reviewed.

References

[1] Klingner, N.; Heller, R.; Hlawacek, G.; von Borany, J.; Notte, J. A.; Huang, J. and Facsko, S. (2016). Nanometer scale elemental analysis in the helium ion microscope using time of flight spectrometry, Ultramicroscopy 162 : 91-97.
[2] Heller, R.; Klingner, N.; Hlawacek, G. (2016). Backscattering Spectrometry in the Helium Ion Microscope: Imaging Elemental Compositions on the nm Scale. In: Hlawacek, G. & Gölzhäuser, A. (Ed.), Helium Ion Microsc., Springer International.
[3] Klingner, N.; Heller, R.; Hlawacek, G.; Facsko, S. and von Borany, J.; (2018) Time-of-flight secondary ion mass spectrometry in the helium ion microscope, submitted.