Conditions for Low Voltage Microanalysis and X-ray Mapping

richard wuhrer

Conditions for Low Voltage Microanalysis and X-ray Mapping

Microscopy and Microanalysis, 2016

Conditions for Low Voltage Microanalysis and X-ray Mapping Richard Wuhrer 1 and Ken Moran 2 1 Western Sydney University, Advanced Materials Characterisation Facility (AMCF), Australia. 2 Moran Scientific Pty Ltd, 4850 Oallen Ford Road, Bungonia, NSW, Australia. Traditional microanalysis using energy dispersive X-ray spectroscopy (EDS) involves using higher accelerating voltages in the range of 15kV to 25kV in order to excite the well-known K-Lines in the atomic spectra. However, the use of higher accelerating voltages usually results in larger interaction volumes [1], which consequently degrades the spatial resolution of the X-ray image and increases the analytical volume. With the development and advancement of large area silicon drift detectors (SDD’s), the sensitivity for X-rays in the low energy part of the spectrum has substantially improved. This, and high count rate throughput, is now allowing the possibility of operating the SEM at much lower accelerating voltages and subsequently reducing the interaction volume of the electron beam with the material as well as achieving higher spatial resolution information. Furthermore, SDD’s can now be purchased with varying windows [2] and also windowless systems [3-6] allowing greater sensitivity of the lower energy X-rays. Figure 1 shows the EDS spectrum from an Amptek detector with two different window materials (C1 and C2). These windows are made from 90nm and 40nm silicon nitride (Si3N4) with a very thin aluminium coating to extend the low energy response [2]. The C2 window has excellent transmission for low Z elements with ten times more carbon X-rays. By lowering the accelerating voltage, this forces the selection of X-ray lines with low excitation energy such as L and M family lines between 0-4kV, rather than using K and L lines between 0-20kV [1, 3,6]. The measurement of low energy L line X-rays is complicated by low fluorescence yield, an increase in X-ray absorption, numerous X-ray interferences from other elements within the sample, and less accurately determined mass absorption coefficients [1, 3-6]. The overlaps of K-line X-rays from light elements with L and M line X-rays from heavier elements limit the low voltage analysis and mapping capabilities of conventional microanalysis systems [7]. The use of transition element L-lines generally results in poor accuracy as well as poor sensitivity [8- 11]. Furthermore, operating at low voltages has limitations due to the large quantification errors existing on the lower energy X-ray lines [11]. As can be seen in Figure 1c, the EDS spectra shows the hard facing material of NbC particles in white cast iron, has numerous L lines overlapping each other. The subsequent generation of X-ray maps collected at 8kV using K lines (Figure 2b-e) and L lines (Figure 2f, h-j) shows major errors for Mn L line x-ray. Furthermore, the quantification results using the L lines are totally incorrect. This can be minimized by more precise calibration of the detector parameters (possibly due to non-linearities in the detector electronics) to enable better correction of the overlaps in the low energy range. Extensive work is still required on fundamental parameters [8], algorithms and mass absorption coefficients before any accurate quantitative analysis can be performed [9-11], especially with the low voltage regime of multi-element materials. These multi-element materials that have many overlaps, are only good for qualitative analysis. 406 doi:10.1017/S1431927616002889 Microsc. Microanal. 22 (Suppl 3), 2016 © Microscopy Society of America 2016 . https://doi.org/10.1017/S1431927616002889 Downloaded from https://www.cambridge.org/core. IP address: 168.151.112.8, on 29 Sep 2017 at 03:02:56, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms

High quality quantitative analysis at low voltage is best carried out using a standards based method [8]. Standardless analysis has many more parameters to be determined for accurate analysis compared to standards based analysis [8], however there are still many sources of errors. The SDDs have to be calibrated more carefully and using different calibration standards than that used for conventional standards at higher accelerating voltages. References: [1] D. C. Bell and N. Erdman, “Low Voltage Electron Microscopy”, Wiley, 2013. [2] http://www.amptek.com/products/c-series-low-energy-x-ray-windows/ [3] M. Meisnara, S. Lozano-Pereza, M. Moodya and J. Holland, Micron 66, 16–22, 2014. [4] R. Wuhrer, L. Guja, D. Merritt and K. Moran, Microsc. Microanal. 20 (Suppl 3), 634-635, 2014. [5] S. Burgess, X. Li,, J. Holland, Microsc. Anal. 27, 8, 2013. [6] L. Moran, K. Moran and R. Wuhrer, Microsc. Microanal. 20 (Suppl 3), p678-679, 2014. [7] J. J. Friel, C. E. Lyman, Microsc. Microanal. 12, 2-15, 2006. [8] R. Gauvin, Microscopy and Microanalysis, 18, 915-940, 2012. [9] P. K. Carpenter, AMAS XIII-The 13 th Biennial AMAS Symposium, Hobart, 132-133, Feb, 2015. [10] J. Donovan, P. Pinard, and S. Richter, AMAS XIII Symposium, Hobart, 122-123, Feb, 2015 [11] P. Pinard and S. Richter, AMAS XIII-The 13 th Biennial Australian Microbeam Analysis Symposium, Hobart, 124-125, Feb, 2015. [12] R. Wuhrer and K. Moran, EMAS Proceedings 2015, Slovenia, Nov 2015. Figure 1. EDS spectra from an Amptek Fast123 detector, collected at Eo=8kV. a) 90nm (C1) window of a contaminated Cu-Al grid, b) 40nm (C2) window of a contaminated Cu-Al grid (same area) and c) Spectra (from C2 window detector) of a white cast iron (WCI) hard-facing material that has NbC particles present Reproduced from [12]. Figure 2. Corrected x-ray maps taken at 8kV of hard-facing material with NbC particles in white cast iron. a) BSE image, b) Carbon K-Line, c) Chromium K-line, d) Mn K-line, e) Fe K-line, f) Nb L-line, g) Carbon K-Line, h) Chromium L-line, i) Mn L-line and j) Fe L-line. Notice the Mn X-ray map for the K- line and L-line X-ray maps are totally different, due to the linearity and overlap problems occurring at the L-lines for Cr, Mn, and Fe. Reproduced from [12]. C1 Window (a) C2 Window (b) C2 Window (c) Microsc. Microanal. 22 (Suppl 3), 2016 407 . https://doi.org/10.1017/S1431927616002889 Downloaded from https://www.cambridge.org/core. IP address: 168.151.112.8, on 29 Sep 2017 at 03:02:56, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms

406 doi:10.1017/S1431927616002889 Microsc. Microanal. 22 (Suppl 3), 2016 © Microscopy Society of America 2016 Conditions for Low Voltage Microanalysis and X-ray Mapping Richard Wuhrer 1 and Ken Moran 2 1 2 Western Sydney University, Advanced Materials Characterisation Facility (AMCF), Australia. Moran Scientific Pty Ltd, 4850 Oallen Ford Road, Bungonia, NSW, Australia. Traditional microanalysis using energy dispersive X-ray spectroscopy (EDS) involves using higher accelerating voltages in the range of 15kV to 25kV in order to excite the well-known K-Lines in the atomic spectra. However, the use of higher accelerating voltages usually results in larger interaction volumes [1], which consequently degrades the spatial resolution of the X-ray image and increases the analytical volume. With the development and advancement of large area silicon drift detectors (SDD’s), the sensitivity for X-rays in the low energy part of the spectrum has substantially improved. This, and high count rate throughput, is now allowing the possibility of operating the SEM at much lower accelerating voltages and subsequently reducing the interaction volume of the electron beam with the material as well as achieving higher spatial resolution information. Furthermore, SDD’s can now be purchased with varying windows [2] and also windowless systems [3-6] allowing greater sensitivity of the lower energy X-rays. Figure 1 shows the EDS spectrum from an Amptek detector with two different window materials (C1 and C2). These windows are made from 90nm and 40nm silicon nitride (Si3N4) with a very thin aluminium coating to extend the low energy response [2]. The C2 window has excellent transmission for low Z elements with ten times more carbon X-rays. By lowering the accelerating voltage, this forces the selection of X-ray lines with low excitation energy such as L and M family lines between 0-4kV, rather than using K and L lines between 0-20kV [1, 3,6]. The measurement of low energy L line X-rays is complicated by low fluorescence yield, an increase in X-ray absorption, numerous X-ray interferences from other elements within the sample, and less accurately determined mass absorption coefficients [1, 3-6]. The overlaps of K-line X-rays from light elements with L and M line X-rays from heavier elements limit the low voltage analysis and mapping capabilities of conventional microanalysis systems [7]. The use of transition element L-lines generally results in poor accuracy as well as poor sensitivity [811]. Furthermore, operating at low voltages has limitations due to the large quantification errors existing on the lower energy X-ray lines [11]. As can be seen in Figure 1c, the EDS spectra shows the hard facing material of NbC particles in white cast iron, has numerous L lines overlapping each other. The subsequent generation of X-ray maps collected at 8kV using K lines (Figure 2b-e) and L lines (Figure 2f, h-j) shows major errors for Mn L line x-ray. Furthermore, the quantification results using the L lines are totally incorrect. This can be minimized by more precise calibration of the detector parameters (possibly due to non-linearities in the detector electronics) to enable better correction of the overlaps in the low energy range. Extensive work is still required on fundamental parameters [8], algorithms and mass absorption coefficients before any accurate quantitative analysis can be performed [9-11], especially with the low voltage regime of multi-element materials. These multi-element materials that have many overlaps, are only good for qualitative analysis. Downloaded from https://www.cambridge.org/core. IP address: 168.151.112.8, on 29 Sep 2017 at 03:02:56, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms . https://doi.org/10.1017/S1431927616002889 Microsc. Microanal. 22 (Suppl 3), 2016 407 High quality quantitative analysis at low voltage is best carried out using a standards based method [8]. Standardless analysis has many more parameters to be determined for accurate analysis compared to standards based analysis [8], however there are still many sources of errors. The SDDs have to be calibrated more carefully and using different calibration standards than that used for conventional standards at higher accelerating voltages. References: D. C. Bell and N. Erdman, “Low Voltage Electron Microscopy”, Wiley, 2013. http://www.amptek.com/products/c-series-low-energy-x-ray-windows/ M. Meisnara, S. Lozano-Pereza, M. Moodya and J. Holland, Micron 66, 16–22, 2014. R. Wuhrer, L. Guja, D. Merritt and K. Moran, Microsc. Microanal. 20 (Suppl 3), 634-635, 2014. S. Burgess, X. Li,, J. Holland, Microsc. Anal. 27, 8, 2013. L. Moran, K. Moran and R. Wuhrer, Microsc. Microanal. 20 (Suppl 3), p678-679, 2014. J. J. Friel, C. E. Lyman, Microsc. Microanal. 12, 2-15, 2006. R. Gauvin, Microscopy and Microanalysis, 18, 915-940, 2012. P. K. Carpenter, AMAS XIII-The 13th Biennial AMAS Symposium, Hobart, 132-133, Feb, 2015. J. Donovan, P. Pinard, and S. Richter, AMAS XIII Symposium, Hobart, 122-123, Feb, 2015 P. Pinard and S. Richter, AMAS XIII-The 13th Biennial Australian Microbeam Analysis Symposium, Hobart, 124-125, Feb, 2015. [12] R. Wuhrer and K. Moran, EMAS Proceedings 2015, Slovenia, Nov 2015. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] C1 Window (a) C2 Window (b) C2 Window (c) Figure 1. EDS spectra from an Amptek Fast123 detector, collected at Eo=8kV. a) 90nm (C1) window of a contaminated Cu-Al grid, b) 40nm (C2) window of a contaminated Cu-Al grid (same area) and c) Spectra (from C2 window detector) of a white cast iron (WCI) hard-facing material that has NbC particles present Reproduced from [12]. Figure 2. Corrected x-ray maps taken at 8kV of hard-facing material with NbC particles in white cast iron. a) BSE image, b) Carbon K-Line, c) Chromium K-line, d) Mn K-line, e) Fe K-line, f) Nb L-line, g) Carbon K-Line, h) Chromium L-line, i) Mn L-line and j) Fe L-line. Notice the Mn X-ray map for the Kline and L-line X-ray maps are totally different, due to the linearity and overlap problems occurring at the L-lines for Cr, Mn, and Fe. Reproduced from [12]. Downloaded from https://www.cambridge.org/core. IP address: 168.151.112.8, on 29 Sep 2017 at 03:02:56, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms . https://doi.org/10.1017/S1431927616002889

Although Earth has lost at least 35% of its pre-agricultural forest cover over the past three centuries 1 , forests are still widely distributed, covering a total of 40 million km 2 (~25%) of Earth's terrestrial surface 2. Of the remaining forests, as much as 82% is now degraded to some extent as a result of direct human actions such as industrial logging, urbanization, agriculture and infrastructure 3,4. This figure is probably an underestimate of the true level of anthropogenic impact as it does not incorporate other, more cryptic forms of degradation, such as over-hunting 5. As the human footprint continues to expand 4 , remaining forest free of significant anthropogenic degradation is in rapid decline (Fig. 1). Over the past decade, there has been increasing international concern around the loss of forest and the impact this has on climate change, the loss of biodiversity and the provision of ecosystem services 1. The 2015 Paris Agreement, together with earlier agreements under the United Nations Framework Convention on Climate Change (UNFCCC), acknowledges the importance of forests for limiting a future temperature increase to well below 2 °C above pre-industrial levels 6. The United Nations' Sustainable Development Goals (adopted in 2016) have the ambitious aim of fully halting deforestation by 2020 7. However, while these targets are clearly warranted , they fall short of specifically prioritizing the crucial qualities of a forest that contribute most to achieving each convention's specific goals 1. For example, indicators tracking progress towards the 2015 New York Declaration on Forests — among the most significant global forest conservation targets to date — focus on forest extent and make almost no acknowledgement of forest condition 8. In this Perspective, we argue that to achieve the goals of global international environmental accords it is insufficient to treat all forests as equal regardless of their condition. Instead, forest that is free of significant anthropogenic degradation (which we term 'intact forest') should be identified and accorded special consideration in policymaking, planning and implementation. Anthropogenic degradation here includes all human actions that are known to cause physical changes in a forest that lead to declines in ecological function 9,10. Well-studied examples include forest fragmentation, stand-level damage due to logging, over-harvesting of particular species (such as over-hunting) and changes in fire or flooding regimes. We first summarize published evidence that intact forests support an exceptional confluence of globally significant environmental values relative to forests that have experienced those damaging human actions. We show that intact forests are indispensable not only for addressing rapid anthropogenic climate change, but also for confronting the planet's biodiversity crisis, providing critical ecosystem services and supporting the maintenance of human health. We then show that the relative value of intact forests is likely to become magnified as already-degraded forests experience further intensified pressures (including anthropogenic climate change). While it is beyond the scope of this paper to set thresholds for acceptable forest fragment size and configuration, logging intensity or any other measure of damage, we provide evidence that human activity that exceeds the natural range of variation in a forested system reduces key ecological functions, and the greater the level of alteration, the greater the reduction in function. Here we outline the significant, As the terrestrial human footprint continues to expand, the amount of native forest that is free from significant damaging human activities is in precipitous decline. There is emerging evidence that the remaining intact forest supports an exceptional confluence of globally significant environmental values relative to degraded forests, including imperilled biodiversity, carbon sequestration and storage, water provision, indigenous culture and the maintenance of human health. Here we argue that maintaining and, where possible, restoring the integrity of dwindling intact forests is an urgent priority for current global efforts to halt the ongoing biodiversity crisis, slow rapid climate change and achieve sustainability goals. Retaining the integrity of intact forest ecosystems should be a central component of proactive global and national environmental strategies, alongside current efforts aimed at halting deforestation and promoting reforestation.

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Conditions for Low Voltage Microanalysis and X-ray Mapping