Time of Flight Spectrometry in the HIM

Time of Flight Spectrometry in the HIM
N. Klingner1,2*, R. Heller1, G. Hlawacek1, J. von Borany1 and S. Facsko1
1 Ion Beam Center (IBC), Institute for Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Bautzner Landstraße 400, 01328 Dresden, Germany
2 Technical University Dresden, School of Science, Helmholtzstraße 10, 01069 Dresden, Germany
*n.klingner@hzdr.de
Helium Ion Microscopy (HIM) is well known for its high-resolution imaging and nano fabrication capabilities. However, in terms of analytic capabilities it lacks behind comparable techniques such scanning electron microscopy (SEM) or transmission electron microscopy (TEM). Although several primary and secondary particles are available, to date none of them has been exploited in a practical way to obtain analytic information.
Here, we present the first successful attempt to use time of flight backscattering spectrometry (TOF-BS) and secondary ion mass spectrometry (TOF-SIMS) in the HIM for materials characterization [1]. The successful use of sputtered particles for analytic purposes has also been demonstrated by adding a sophisticated SIMS spectrometer to the HIM [2].
For the TOF measurements the start pulse is generated by chopping the primary beam of the ion microscope using the build-in blanker and a custom made electronics that allows pulse length of 10 ns to 250 ns. The stop signal is given by the arrival of the backscattered particles at the counting micro channel plate. The setup provides high lateral resolution and a good time resolution. Moreover it is minimal invasive to the microscope and therefore the high-resolution capabilities of the device are not derogated when the TOF setup is not in use.
TOF-BS spectra of thin HfO2 films on Si are presented in fig. 1. The time resolution is limited by the physical length of the microscope blanker to approximately 17 ns or 5.4%. This value can be decreased to 2.7% by using a longer flight path. Thanks to a home built scan electronic to control the beam, TOF data can be recorded also in imaging mode. This allows an efficient post acquisition analysis by applying energy filters to extract the elemental distribution. A lateral resolution of 54 nm has been achieved so far. Although this is significantly worse than the native resolution of the tool, this value is close to the physical limit and can be overcome by using correlative approaches in connection with the high resolution SE data available in the HIM.
Modifying the sample holder slightly one can also perform TOF-SIMS. The sputtered particles are accelerated towards the stop detector of the TOF setup by means of a high voltage applied to the sample and a grounded grid. TOF-SIMS spectra obtained from different samples are presented in fig. 2. TOF-BS and TOF-SIMS performed in-situ complement each other and therefore deliver a maximum of compositional information on the sample.

References
[1] N. Klingner, R. Heller, G. Hlawacek, J. von Borany, J. A. Notte, J. Huang, S. Facsko: “Nanometer scale elemental analysis in the helium ion microscope using time of flight spectrometry”, Ultramicroscopy 162 (2016), 91-97. DOI:10.1016/j.ultramic.2015.12.005
[2] T. Wirtz, P. Philipp, J.-N. Audinot, D. Dowsett, S. Eswara. “High-resolution high- sensitivity elemental imaging by secondary ion mass spectrometry: from traditional 2D and 3D imaging to correlative microscopy”, Nanotechnology 26 (2015), 434001. DOI:10.1088/0957-4484/26/43/434001