TY - JOUR
AU - Chinone,, Kazuo
AB - Abstract A scanning electron microscope transition edge sensor has been developed to analyze the minor or trace constituents contained in a bulk sample and small particles on the sample under a low accelerating voltage (typically <3 keV). The low accelerating voltage enables to improve the spatial analysis resolution because the primary electron diffusion length is limited around the sample surface. The characteristic points of our transition edge sensor are 1) high-energy resolution at 7.2 eV@Al-Kα, 2) continuous operation by using a cryogen-free dilution refrigerator and 3) improvement of transmission efficiency at B-Kα by using thin X-ray film windows between the sample and detector (about 30 times better than our previous system). Our system could achieve a stabilization of the peak shift at Nd-Mα (978 eV) within 1 eV during an operation time of 27 000 s. The detection limits with B-Kα for detection times 600 and 27 000 s were 0.27 and 0.038 wt%, respectively. We investigated the peak separation ability by measuring the peak intensity ratio between the major constitute (silicon) and the minor constitute (tungsten) because the Si-Kα line differs from the W-Mα line by only 35 eV and a small W-Mα peak superimposed on the tail of the large Si-Kα peak. The peak intensity ratio (I(W-Mα)/I(Si-Kα)) was adjusted by the W particle area ratio compared with the Si substrate area. The transition edge sensor could clearly separate the Si-Kα and W-Mα lines even under a peak intensity ratio of 0.01. X-ray analysis, superconducting, electron microscope, energy-dispersive spectrometry Introduction Microanalysis using characteristic X-rays is a powerful tool when combined with a scanning electron microscope (SEM) and scanning transmission electron microscope (STEM) to analyze elements contained in a sample at multiscale from atomic resolution to mm order. Energy-dispersive spectrometry (EDS) has the merit of a wide detection range (0.05–20 keV), and wavelength dispersive spectrometry (WDS) has the merit of a high-energy resolution [1]. Although the silicon drift detector (SDD) has a lower energy resolution compared with that of WDS, it can be compensated by the peak separation function with a high-count rate to increase the statics accuracy [2]. However, EDS with a high-energy resolution has been expected to improve the element detection accuracy and detection limit especially in the case where multi X-ray peaks are conjugated. A transition edge sensor (TES) is a calorimetric photon detector that uses superconducting phase transition [3] to detect visible rays [4] to Gamma rays [5] by adjusting the geometrical design and superconducting transition temperature. The National Aeronautics and Space Administration (NASA) has reported their best energy resolution at about 0.87 eV @Al-Kα line and 1.56 eV @Mn-Kα line [6]. A prototype of the scanning electron microscope transition edge sensor (SEM-TES) was reported by the National Institute of Standard and Technology (NIST) in 1997 and showed its performance as a new X-ray EDS with high-energy resolution (7.2 eV@Mn-Kα line) [7]. We originally developed and commercialized the SEM-TES on the basis of a single pixel detector and dilution refrigerator using liquid helium. The dilution refrigerator could cool the X-ray detector near to 100 mK. The high-energy resolution X-ray detector enables to set the primary electron beam voltage under 5 kV and improve the analysis spatial resolution because the TES can clearly separate the K lines of light elements, L lines of transition metals and M lines of heavy metals. Our system showed the existence of small Sr-Lα line (1806 eV) by the minor constitute between the W-Mα line (1775 eV) and W-Mβ line (1836 eV) under an accelerating voltage of 3 keV [8]. A transmission electron microscope (TEM)-TES and a STEM-TES have been developed to observe and analyze the small precipitates and minor or trace constituents around or on the grain boundary [9,10]. The STEM-TES used a cryogen-free dilution refrigerator to achieve a continuous operation [11] and multipixel detectors to increase the detection area [12]. In the success of the TEM-TES and STEM-TES development, we started developing a new SEM-TES on the basis of the four-pixel detectors and a cryogen-free dilution refrigerator to detect minor or trace elements in a bulk sample. In this paper, we report the performance of our SEM-TES and the detection limit of trace constituents of boron and peak separation ability using a small peak intensity with W-Mα and large peak intensity with Si-Kα where the difference of characteristic energies between the two peaks is only 35 eV. Instrumentation Our TES system uses the cryogen-free refrigerator developed for the STEM-TES. The system is composed of a TES body, GM refrigerator, operation unit and compressors. The X-ray detectors are installed in the snout of the TES body as that does not generate any mechanical and sonic vibration. However, the GM refrigerator has a mechanical and sonic vibration source. The TES body is connected to the GM refrigerator by a flexible tube to dampen the vibration noise, and the sound-absorbing material surrounding the GM refrigerator attenuates the sonic vibration. The GM refrigerator is a precooler that cools the TES body from room temperature to under 4 K and is connected to the side of the TES body with a 1.5 m flexible tube. The gas handling system contains a compressor, vacuum pump and 3He-4He gas tank. The 3He-4He gas cycles between the TES body and the gas handling system. The 3He gas is then evacuated from the TES body by the vacuum pump, and the compressor pushes the 3He gas into the TES body. The cycle of the liquefaction and vaporization of the 3He gas is achieved under 100 mK in the TES body. Our system can cool the TES body to under 100 mK by controlling the temperature from a touch panel on the gas handling system. The TES body is mounted on a SEM (SU5000: Hitachi High-Tech Corp.) using the WDS port located behind the SU5000 specimen chamber. The SU5000 is a versatile electron microscope suitable for the observation and analysis of magnetic/nonmagnetic samples because this SEM adopts an out-lens objective lens, a Schottky emission electron source, a draw-out stage and a concentric backscatter detector (BSD). The low vacuum mode helps to observe and analyze nonconductive samples. Figure 1(a) is a photograph taken from the front of the SEM-TES. Figure 1(b) is an enlarged photograph around the attachment connected between the TES body and SEM. The setup of this system is similar to that of the STEM-TES; however, the take-off angle is set at 30 degree for our system that enables to analyzing the sample without tilting. Fig. 1 Open in new tabDownload slide (a) Overview of the SEM-TES. (b) Photograph of the attachment between the SEM and TES body. Fig. 1 Open in new tabDownload slide (a) Overview of the SEM-TES. (b) Photograph of the attachment between the SEM and TES body. The X-ray lens and X-ray windows are located between the sample and the four-pixel X-ray detector set in the TES body. The X-ray lens collects the X-rays emitted from the sample and focuses on the detectors [13]. The detectors are located about 200 mm from the X-ray lens because X-rays can be transmitted in a vacuum without degrading their intensity over a wide energy range excluding the attenuation by the X-ray windows. The designed solid angle of our X-ray lens is about 10 msr. at the Si-Kα line and 8 msr. at the Cu-Kα line. The transmission efficiency with similar X-ray lenses is described in Ref. [13]. The X-ray lens position adjuster is used to align the X-ray generation point with the input focal position of the lens, and the TES body position adjuster is used to align the output focal position with the TES device position. The X-ray windows comprise one window set at room temperature and 1 and 40 K cooled windows. The room temperature window is a grid-supported polymer window (AP3.3: MoxTek Inc.) [14]. The TES detector is a type of thermometer with high sensitivity (typically dlnR/dlnT = 100 where R is the resistance, and T is the temperature). The Joule heating by the current flowing through the TES detector and the heat through the thermal link from the TES device to the heat bath are balanced to keep the operation point between the superconducting state and the normal state [3]. The Joule heating power is estimated to be about several tens of pW. This means that thermal fluctuations on the order of pW cause fluctuations in the TES operation point, yielding a degradation of energy resolution. The black-body radiation from the room-temperature X-ray window to the TES detector (0.1 K) entering through the collimator set over the TES is calculated to be a few μW. The cooled X-ray windows must reduce the black-body radiation from the room temperature X-ray window to the TES device less than 1 pW. Empirically, we could reduce the cooled X-ray windows’ thickness to about 0.67 times less than that of our previous system without degrading the energy resolution. The room temperature X-ray window is used to separate the SEM chamber and TES body because the TES body has to be kept in a vacuum state in the atmospheric release of the SEM chamber. Also, the room temperature window enables to detect the X-rays from a sample under the low vacuum mode prepared for the SEM function. However, the room temperature window is not needed when the vacuum level between the two chambers is equal. With our system, the room temperature window can be placed and removed by using the gate valve as shown in Fig. 1(b). Reducing the thickness of all X-ray windows can improve X-ray transmittance by about 37 times at 183 eV, which is better than our previous system. We evaluated the transmittance with and without the room temperature X-ray window. The transmittance should improve by about 3.4 times at 183 eV by calculating the transmittance of the AP3.3. The peak intensity of the B-Kα line (183 eV) without the room temperature X-ray window increased by about 2.7 times that with the window. Instrument performance The transition temperature of the four-pixel detectors was set from 171 to 172 mK, and the heat bath temperature was set at 120 mK. The expected energy resolution calculated by the signal-to-noise ratio (S/N) is expressed as the following equation: $$\begin{equation} \Delta \mathrm{E}=2.355{\left({\int}_0^{\infty }{SN}^2(f) df\right)}^{-\frac{1}{2}} \end{equation}$$(1) Table 1 shows the expected and obtained energy resolution at Al-Kα for the four-pixel detectors. Figure 2 is a summed X-ray spectrum of the four detectors, of which the energy resolution is 7.6 eV for Al-Kα. We can see that Al-Kα, Al-Kβ and the satellite peaks of Al-Kα3,4 which arise from doubly ionized atoms are observed in Si-K lines [1,15]. Fig. 2 Open in new tabDownload slide Energy spectrum by TES (solid line) and by SDD (solid line plus closed circle) around the Al-K lines. Fig. 2 Open in new tabDownload slide Energy spectrum by TES (solid line) and by SDD (solid line plus closed circle) around the Al-K lines. Table 1 Energy resolution calculated by S/N ratio and fitting results to the obtained spectrum for each pixel . Energy resolution (eV) calculation by signal-to-noise ratio . Energy resolution (eV) fitting by the obtained spectrum . CH1 7.3 7.3 CH2 8.8 9.2 CH3 7.3 7.5 CH4 6.8 6.2 . Energy resolution (eV) calculation by signal-to-noise ratio . Energy resolution (eV) fitting by the obtained spectrum . CH1 7.3 7.3 CH2 8.8 9.2 CH3 7.3 7.5 CH4 6.8 6.2 Open in new tab Table 1 Energy resolution calculated by S/N ratio and fitting results to the obtained spectrum for each pixel . Energy resolution (eV) calculation by signal-to-noise ratio . Energy resolution (eV) fitting by the obtained spectrum . CH1 7.3 7.3 CH2 8.8 9.2 CH3 7.3 7.5 CH4 6.8 6.2 . Energy resolution (eV) calculation by signal-to-noise ratio . Energy resolution (eV) fitting by the obtained spectrum . CH1 7.3 7.3 CH2 8.8 9.2 CH3 7.3 7.5 CH4 6.8 6.2 Open in new tab Our system could maintain the minimum temperature for about 6 months with the temperature stabilizing within 40 μK peak to peak. The minimum temperature could have been maintained for longer, but the six-month period was determined not by any limitations in cooling ability but by an intentional system stop (ex. legal inspection with water or electrical circuits of the building). We evaluated the detection limit of the B-Kα without the room temperature X-ray window using a conventional NdFeB magnet. The accelerating voltage and probe current were 3 kV and 8–10 nA, respectively. We obtained the X-ray spectra for 45 measurement points by changing the position of each measurement at set intervals. The beam irradiation time for each pixel was 600 s. Figures 3(a) and (b) show the X-ray spectra for the B-K line and Nd-M line summarized for the total number of measurement points (27 000 s) and the X-ray spectrum for the first measurement point (600 s). The high-energy resolution can separate the X-ray peaks between Nd-Mα (978 eV) and Nd-Mβ (997 eV). Our system can indicate sufficient stabilization (<1 eV) to identify the element correctly because the peak position of the summarized Nd-Mα spectra deviates to within 1 eV compared with that of the first measurement spectrum. Figure 4 shows the net counts of B-Kα plots for the 45 measurement points. The background is subtracted from the spectrum within the region of interesting (ROI) set from 171 to 191 eV. The average net count is about 200 with the overall net count scattered from 150 to 250 counts. The background count within the ROI is B = 320 counts, yielding the 3σof the background counts (3√B) as 54 counts, which is almost equal to the scattering width of the boron net counts plot as shown in Fig. 4. The detection limit for B-Kα in this experimental condition (600 s) was estimated as 0.27 wt% when the average count (200 counts) is assumed to be 1 wt % as the average boron density. The summarized spectrum denotes the boron detection limit as 0.038 wt%. Fig. 3 Open in new tabDownload slide (a) X-ray spectrum of B-Kα for measurement times of 600 s (open circle) and 27 000 s (solid line). (b) X-ray spectrum of W-Mα and Mβ for 600 s (open circles) and 27 000 s (solid line). Fig. 3 Open in new tabDownload slide (a) X-ray spectrum of B-Kα for measurement times of 600 s (open circle) and 27 000 s (solid line). (b) X-ray spectrum of W-Mα and Mβ for 600 s (open circles) and 27 000 s (solid line). Fig. 4 Open in new tabDownload slide Net counts of B-Kα measured from the spectra of 45 different points in the NdFeB sample. The dashed line shows the three-sigma lines (3-sigma (54 counts)) calculated by the background counts within ROI (171 to 191 eV) to the average net counts of the B-Kα (200 counts). Fig. 4 Open in new tabDownload slide Net counts of B-Kα measured from the spectra of 45 different points in the NdFeB sample. The dashed line shows the three-sigma lines (3-sigma (54 counts)) calculated by the background counts within ROI (171 to 191 eV) to the average net counts of the B-Kα (200 counts). We then checked the peak separation ability by measuring the small peak intensity with the W-Mα and W-Mβ lines and comparing it to the large peak intensity with the Si-Kα and Si-Kβ lines. The peak separation among the W-M lines and Si-K lines is a difficult case for a conventional Si-based X-ray detector. The characteristic X-ray energy of the W-Mα line (1775 eV) is close to that of the Si-Kα line (1740 eV), and the difference between the two lines (35 eV) is narrow compared with the energy resolution of the Si-based X-ray detector from 70 to 80 eV@Si-Kα. Figure 5 shows the simulation curves with X-ray spectra for 1) Si only and 2) Si + 10% W. The energy resolutions for this simulation are set at 7.6 and 76 eV, respectively. The 10% W spectrum composed of W-Mα and Mβ slightly overlap on the tail of the Si spectrum from 1740 to 1900 eV for 76 eV. On the other hand, the Si-Kα, W-Mα and W-Mβ lines are clearly separated for Si + 10% W for 7.6 eV. Fig. 5 Open in new tabDownload slide Calculation curves for Si only (solid line) and Si + 10% W (open circle) by TES and for the Si only (dotted line) and the Si + 10% W (dash-dot line) by SSD. Fig. 5 Open in new tabDownload slide Calculation curves for Si only (solid line) and Si + 10% W (open circle) by TES and for the Si only (dotted line) and the Si + 10% W (dash-dot line) by SSD. To check the peak separation ability, we prepared a small W particle on the Si substrate. Figures 6(a) and 6(b) show the substrate taken from an angle about a few tens degree from surface parallel direction and from the top, respectively. This particle was made by the electron beam deposition with W gas using FIB-SEM (NX5000: Hitachi High-Tech Science Corp.), where the accelerating voltage and the deposition time was 1 kV and 38 s, respectively. The particle is triangular pyramid-shaped, and the base diameter and height are 95 nm and 25 nm, respectively. First, the electron beam irradiated the center of the particle, and we enlarged the scan area including the particle in the following measurements as shown in Fig. 6(b). The X-ray spectra of not only the TES but also the SDD (X-MAX80 by Oxford Instruments) were obtained to compare both spectra. The scan time of TES and SDD was 5160 and 4300 s, respectively, for each measurement with drift correction. Figures 7(a) and (b) show the X-ray spectra with SDD and TES, respectively, for a spot analysis at the position indicated by the plus mark in Fig. 6(b) and the four-area analysis shown for Areas 1 to 4. The enlarged X-ray spectrum with TES from 1700 to 1900 eV is inserted in Fig. 7(b). Compared with the intensity of Si-Kα line, that of the W-Mα line with TES was about 0.03 for the spot analysis. The intensity of the W-Mα line decreased monotonically as the scan area increased. On the other hand, the W-Mα line was buried into the tail of the Si-Kα line in a spot and area analysis for the SDD. This result is similar to the simulation data shown in Fig. 5. Fig. 6 Open in new tabDownload slide (a) Side view of the W particle. (b) Top view of the particle overlapping the spot and four scan areas. Fig. 6 Open in new tabDownload slide (a) Side view of the W particle. (b) Top view of the particle overlapping the spot and four scan areas. Fig. 7 Open in new tabDownload slide (a) X-ray spectra for spot and four scan areas from 1200 to 1900 eV by SDD. (b) X-ray spectra for spot and four scan areas from 1200 to 1900 eV by TES and the enlarged spectra from 1700 to 1850 eV are inserted. Fig. 7 Open in new tabDownload slide (a) X-ray spectra for spot and four scan areas from 1200 to 1900 eV by SDD. (b) X-ray spectra for spot and four scan areas from 1200 to 1900 eV by TES and the enlarged spectra from 1700 to 1850 eV are inserted. To calculate the peak intensity of Si-Kα and W-Mα for the spectra with the spot and the four-areas analysis, we prepared a spectrum (hereafter the standard spectrum) with the pure Si and pure W sample to determine the peak intensity ratio between the set of Si-Kα, Si-Kα34 and Si-Kβ for Si and that of W-Mα and W-Mβ for W using TES and SDD. We fitted the summation of the standard spectrum of pure Si and W to the spot and the four-area analysis. In this fitting, the peak intensity of the standard spectrum was treated as a free parameter. Table 2 shows the intensity ratio between the net area of Si-Kα and W-Mα for TES and SDD. The intensity of Si for TES and SDD was intentionally normalized at 100. The intensity of the W-Mα with TES monotonically decreased, while that of the W-Mα with SDD seemed to saturate around Si-Kα:W-Mα = 100:1. Our results suggest that TES would have an advantage to detect the W spectrum when the peak intensity ratio between Si-Kα:W-Mα is under 100:1. Table 2 Peak intensity ratio between Si-Kα and W-Mα to the measured spectra for the spot and four scan areas by TES and SDD. The peak intensity ratio was obtained by curve fitting using the summation of the pure Si and W spectra as the peak intensities of Si and W are free parameters Mode . TES (Si-Kα:W-Mα) . SDD (Si-Kα:W-Mα) . Spot 100:2.56 100:4.36 Area1 100:1.31 100:2.88 Area2 100:0.73 100:2.20 Area3 100:0.59 100:1.53 Area4 100:0.32 100:1.37 Mode . TES (Si-Kα:W-Mα) . SDD (Si-Kα:W-Mα) . Spot 100:2.56 100:4.36 Area1 100:1.31 100:2.88 Area2 100:0.73 100:2.20 Area3 100:0.59 100:1.53 Area4 100:0.32 100:1.37 Open in new tab Table 2 Peak intensity ratio between Si-Kα and W-Mα to the measured spectra for the spot and four scan areas by TES and SDD. The peak intensity ratio was obtained by curve fitting using the summation of the pure Si and W spectra as the peak intensities of Si and W are free parameters Mode . TES (Si-Kα:W-Mα) . SDD (Si-Kα:W-Mα) . Spot 100:2.56 100:4.36 Area1 100:1.31 100:2.88 Area2 100:0.73 100:2.20 Area3 100:0.59 100:1.53 Area4 100:0.32 100:1.37 Mode . TES (Si-Kα:W-Mα) . SDD (Si-Kα:W-Mα) . Spot 100:2.56 100:4.36 Area1 100:1.31 100:2.88 Area2 100:0.73 100:2.20 Area3 100:0.59 100:1.53 Area4 100:0.32 100:1.37 Open in new tab Discussion The detection limit using net counts for TES estimated from the data of Area 4 was estimated to be about 0.15% (Si-Kα:W-Mα = 100:0.15). Figure 8 shows the experimental curve (solid line) and the fitting curve (plus mark), and the inserted curve shows an enlarged curve around W-Mα. The W-Mα spectrum exists on the tail of the X-ray spectra composed of the Si-Kα line (1740 eV) and the Si-Kα3,4 line (around 1750 eV). The detection limit of W-Mα would not be determined by the height of the bremsstrahlung but by the summation between the tail height of the Si-Kα lines and the bremsstrahlung. As previously explained, the best energy resolution has been reported at 0.87 eV at Al-Kα [6]. We estimated the improvement of the detection limit by improving the energy resolution from 7.6 to 0.87 eV. The resolution improvement not only decreases the background by ignoring the tail of Si-Kα but also decreases the ROI from 20 to 5 eV. The decreasing of the ROI improves the peak to background (P/B) ratio because the net counts with the background can decrease without the peak net counts, yielding an improvement to the detection limit from 0.15 to 0.0078%. Fig. 8 Open in new tabDownload slide Measurement spectrum for Area 4 (solid line) and the calculated spectrum (plus mark). The inserted graphs show the enlarged graph around W-Mα. Fig. 8 Open in new tabDownload slide Measurement spectrum for Area 4 (solid line) and the calculated spectrum (plus mark). The inserted graphs show the enlarged graph around W-Mα. We investigated the minimum detection time of small W-Mα peak for large Si-Kα and Si-Kα3,4 peaks with Area 4. Figure 9 shows the X-ray spectra for 500 s. The dashed line and solid line are the experimental data and fitting data, respectively. We assumed the presence of W-Mα and W-Mβ. The net peak count (54 counts) was larger than the three sigma (45.5 counts) of the background. In this case, the detection time (500 s) was the most effective detection time. Fig. 9 Open in new tabDownload slide X-ray spectra for Area 4 when the measurement time was 500 s. The dashed line and solid line are the experimental data and the fitting data, respectively. Fig. 9 Open in new tabDownload slide X-ray spectra for Area 4 when the measurement time was 500 s. The dashed line and solid line are the experimental data and the fitting data, respectively. Conclusion We developed a TES with high-energy resolution (7.2 eV@Al-Kα) to be mounted on a SEM with a high transmission efficiency below 1 keV and continuous cooling operation system. We determined the detection limit with boron in NdFeB. The boron detection limit was 0.27 and 0.038 wt% when the detection time was 600 and 27 000 s, respectively. 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TI - High sensitivity X-ray analysis for a low accelerating voltage scanning electron microscope using a transition edge sensor
JF - Microscopy
DO - 10.1093/jmicro/dfaa026
DA - 2020-10-30
UR - https://www.deepdyve.com/lp/oxford-university-press/high-sensitivity-x-ray-analysis-for-a-low-accelerating-voltage-XG95WarWRV
SP - 298
EP - 303
VL - 69
IS - 5
DP - DeepDyve
ER -