Scanning ionoluminescence microscopy with a helium ion microscope

Abstract

The ORIONR PLUS scanning helium ion microscope (HIM) images at sub nanometer resolution. Images of the secondary electron emission have superior resolution and depth of field compared to a scanning electron microscope (SEM). Ionoluminescent imaging is not an area that has been extensively explored by typical ion beam systems as they have large spot sizes in the region of microns, leading to poor spatial resolution. This thesis confirms that the ORIONR PLUS can form images from the ionoluminescent signal, resolutions of 20nm can be obtained for images of bright nanoparticles. Ionoluminescence spectra can also be obtained from some samples. The position of emission peaks in samples under the ORIONR PLUS does not deviate significantly from cathodoluminescence (CL) peaks under SEM. However, the relative heights of the emission peaks in a sample can vary between ionoluminescence (IL) and CL. In addition, It is found that there exists a proportional relationship between acceleration voltage and ionoluminescent signal in the ORIONR PLUS, this relationship is also exhibited in CL. However, when normalised for current and acceleration voltage there appears to be no samples that show greater luminescence under ionoluminescence than cathodoluminescence, with ionoluminescent intensities up to an order of magnitude lower.

Ionoluminescence under the ORIONR PLUS is found to be a poor candidate for the analysis of direct band gap semiconductors, this is attributed to the smaller interaction volumes and achievable beam current of the ORIONR PLUS. It is also found that some direct band gap materials are very susceptible to beam damage under the ion beam at beam doses typically used for secondary electron (SE) imaging. It is possible to obtain simultaneous IL and SE images of organic fluorospores in a biological sample. However, the luminescence of the fluorospores was only just sufficient to form images with a 200nm resolution. Rare earth based nanoparticles show brighter luminescence and greater resistance to beam damage than organic fluorospores. If such particles could be utilised for immunofluorescence it would make combined secondary electron and immunofluorescence imaging under the ORIONR PLUS a viable technique.

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