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Co-registered In-Situ Secondary Electron and Mass Spectral Imaging on the Helium Ion Microscope demonstrated using Lithium Titanate and Magnesium Oxide Nanoparticles David Dowsett, and Tom Wirtz Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b01481 • Publication Date (Web): 03 Aug 2017 Downloaded from http://pubs.acs.org on August 9, 2017
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Analytical Chemistry
Co-registered In-Situ Secondary Electron and Mass Spectral Imaging on the Helium Ion Microscope demonstrated using Lithium Titanate and Magnesium Oxide Nanoparticles D. Dowsett and T. Wirtz 1
Advanced Instrumentation for Ion Nano-Analytics (AINA), MRT Department, Luxembourg Institute of Science and Technology (LIST), Belvaux, LUXEMBOURG *
[email protected] Abstract The development of a high resolution elemental imaging platform combining co-registered secondary ion mass spectrometry and high resolution secondary electron imaging is reported. The basic instrument setup and operation are discussed and in-situ image correlation is demonstrated on a lithium titanate and magnesium oxide nanoparticle mixture. The instrument uses both helium and neon ion beams generated by a gas field ion source to irradiate the sample. Both secondary electrons and secondary ions may be detected. Secondary ion mass spectrometry (SIMS) is performed using an in-house developed double focusing magnetic sector spectrometer with parallel detection. Spatial resolutions of 10 nm have been obtained in SIMS mode. Both the secondary electron and SIMS image data are very surface sensitive and have approximately the same information depth. While the spatial resolutions are approximately a factor of ten different, switching between the different images modes may be done insitu and extremely rapidly allowing for simple imaging of the same region of interest and excellent coregistration of data sets. The ability to correlate mass spectral images on the 10 nm scale with secondary electron images on the nanometer scale in-situ has the potential to provide a step change in our understanding of nanoscale phenomena in fields from materials science to life science. Keywords: Helium Ion Microscopy, Mass Spectrometry Imaging, SIMS, correlative microscopy, high resolution imaging, image co-registration, lithium titanate, magnesium oxide Techniques for nano-metrology and nano-analysis are crucial for the ongoing development of nanotechnology products and processes in disciplines from materials to life sciences1. The Helium Ion Microscope (HIM) has become an ideal tool for imaging2,3 and nano-patterning4. For SE based imaging, resolutions of 0.5 nm and 2 nm are typical for helium ions and neon ions respectively. While structures with sub 20 nm feature sizes may be rapidly patterned using neon, even smaller structures may be patterned using helium. The HIM is based on a gas field ion source5-7 (GFIS). The source consists of a cryogenically cooled, atomically sharp tip. When a bias is applied to the tip, the local electric field can exceed the threshold for field ionisation. In the presence of atoms of helium or neon gas, ionisation occurs at the apex of the tip, producing one of the brightest ion beams known (B>4109 A/cm2 sr). The ion beam is then accelerated to several tens of kilovolts and focussed onto the sample by an electrostatic column. The interaction of the ion beam with the sample gives rise to several possible
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Analytical Chemistry
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imaging modes. Secondary electrons8 (SE), secondary ions9 (SI), backscattered ions10 (BSI) and ionoluminescence11 have all been investigated. The basic SE imaging mode has several advantages compared with low voltage SEM. The shorter wavelength of ions compared to electrons eliminates the probe size limitations of diffraction. The lower convergence angle of the ion beam also gives rise to higher depth of field. The higher stopping power for ions compared with electrons gives rise to higher SE yields improving signal to noise ratios for low currents. The lower contribution of secondary electrons arising from BSI makes the technique more surface sensitive. Because the ion beam injects positive charge into the sample, charge compensation may be easily obtained over a wide range of primary beam energies using an electron flood gun. The ability to imaging charging samples without having to apply conductive coating has been widely used in for imaging of biological specimens3,12. Boden et al. were able to show not only superior resolution and surface detail compared with environmental SEM when imaging the photonic structures of Lepidoptera wings but also extract height information from stereo pairs thanks to the HIM’s superior depth of field13. Schumann et al. have used the HIM to observe nanodomains on the surface of mammalian cells. Such domains are no longer visible when sputter coated with 10 nm gold layers to make them conductive for SEM analysis14. The HIM has also been used for imaging of dopant profiles in semiconductor materials15 and visualisation of the conductivity of graphene layers16. Channelling contrast (both in SE and BSI modes) has been used to determine crystallographic orientation in gold films17,18. The high resolution milling capabilities of helium have been used to fabricate a wide range of nanoscale structures/devices including nanopores for biomolecule identification19, graphene nano-devices4,20,21, nanostructured silicon nitride membranes22 and nanophotonic structures with smaller feature sizes and improved optical properties23-26. The addition of neon27-30 has extended the milling/machining capabilities of the tool by providing increased milling rates and lower implantation and subsurface damage. Despite these advantages, the HIM currently lacks a of state-of-the-art analytical capability, limiting its fields of applications. At beam energies of 35 keV neither helium nor neon ions lead to the emission of characteristic X-rays from a sample, so that X-Ray Spectroscopy is not possible. While some compositional information can be obtained from backscattered helium2,10, identifying elemental information is more difficult due to the multiple scattering events that occur at impact energies below 100 keV. This requires comparison of measured spectra with simulations for interpretation. Nevertheless, Klingner et al. have obtained elemental maps on a test sample coated with bulk patches of Si, Ni and Au using a time of flight based approach to measure the energy of backscattered helium. While lateral resolutions in the 50 nm range31 were obtained from the patches, other ion beam techniques such as Secondary Ion Mass Spectrometry (SIMS) offer similar lateral resolution but with much better sensitivity (detection limits in the ppm range are possible32) and elemental identification is much easier. As both the helium and neon beams sputter material locally from the sample, the sputtered material can be used as the basis of an analytical signal. As some fraction of the sputtered material will be ionised, it can be analysed using secondary ion mass spectrometry. SIMS is a powerful ion beam based technique for analyzing surfaces, capable of high sensitivity and high mass resolution32-34. SIMS is based on the generation and identification of characteristic secondary ions by irradiation with a primary ion
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Analytical Chemistry
beam (in this case helium or neon). The fundamental lateral information limit in SIMS is determined by the area at the surface from which secondary ions are emitted. This depends both on the primary beam parameters (species, energy) and the sample composition. For primary ion beams with energies in the range of a few keV up to a few tens of keV and masses from 4 up to 133 amu this area is between 2 and 10 nm9,32,35. Currently the imaging resolution on commercial SIMS instruments is limited by the probe size of the primary beam rather than such fundamental considerations. While Kollmer et al. have shown results approaching this limit on a standard TOF-SIMS instrument, special care had to be taken to set up their primary column and optimisation of source parameters was necessary to achieve a small probe size. Using a 70 keV Bi3++ beam they achieved a lateral resolution (80%-20%) of 16 nm imaging the read/write head of a hard disk36. More typically, resolutions in the 50 nm range are possible on the Cameca NanoSIMS 50. As the probe size in the HIM is substantially smaller (both for helium and neon) than SI emission area, the lateral resolution is in principal limited only by fundamental considerations9,37 and not by the probe size. The prospect of adding SIMS to the HIM yields not just a powerful analytical capability, but opens the way for in-situ correlative imaging combining ultra-high resolution SE images with elemental maps from SIMS38. We have previously shown that combining SIMS with other high resolution microscopies in-situ can be used to gain insights not possible with standalone techniques and correct for certain artefacts39-41. While secondary electron imaging on the HIM yields topographical information on the nanometers and even sub nanometer scale, SE imaging alone is often not enough to obtain a deep understanding of the sample. SIMS imaging provides elemental/mass filtered imaging on the tens of nm scale, however as the signal level of SIMS images is typically 10-100 times lower than that of corresponding SE images, the lower signal to noise ratio and poorer lateral resolution can make interpretation of SIMS images more difficult. SIMS images often produce hot spots of a few pixels in size that are statistically significant due to the low background of SIMS (
