Time of Flight Backscattering and Secondary Ion Mass Spectrometry in a Helium Ion Microscope

Helium ion microscopes (HIM) have developed into a frequently used imaging device in several laboratories around the world. Beside a sub nano-meter resolution and a high depth of focus the latest generation of HIM devices (Zeiss Orion NanoFab) offers the ability to make use of Neon ions enabling additional possibilities for surface modifications on the nm scale [1].
While the image generation in a HIM is based on evaluating the amount of secondary electrons (SE) the information carried by the energy of the backscattered He/Ne projectiles (BS) is not taken into consideration at the moment. Thus the HIM offers excellent topographic imaging capabilities but chemical information (in terms of elemental composition) of the surface is barely accessible. Nevertheless back-scattered particles carry that information and may be used to provide additional contrast mechanism(s). First attempts to measure BS energy spectra were carried out by Sijbrandij et al. [2] and gave evidence for the general feasibility but also revealed that a quantitative chemical analysis of thin layers would require the development of more sophisticated detection concepts than those used in their experiments (silicon surface barrier detector).

Since the primary He/Ne energy is rather low (10-35 keV) back-scattering spectrometry is suffering various difficulties like high contribution of multiple scattering, non-Rutherford backscattering cross sections and an energy dependent charge fraction. Further the angular spread in the collision cascade as well as the high sputtering yields in this energy regime define physical limits on the maximum achievable lateral resolution and the detection sensitivity. In this contribution we will address these challenges and present our experimental approach and the corresponding results of performing BS spectrometry in a HIM.

We show that pulsing the primary ion beam and measuring the Time of Flight (ToF) of the BS He/Ne ions presents a promising technique of performing BS spectrometry in a HIM in terms of sensitivity, energy resolution and lateral resolution.
Our approach enables us also to perform Secondary Ion Mass Spectrometry (SIMS) by just biasing the sample to a positive potential (of several 100V). Advantages and limitations of this technique will be discussed and compared to those of BS spectrometry.
REFERENCES
[1] [1] G. Hlawacek, V . V eligura, R. van Gastel, and B. Poelsema, J. Vac. Sci. Technol. B 32(2), 2014, 020801.
[2] [2] S. Sijbrandij, B. Thompson, J. Notte, B. W. Ward and N. P. Economou, J. Vac. Sci. Technol. B, 26(6), 2008, 2103-2106