For many years now, analytical scanning/transmission electron microscope systems (S/TEM systems) have combined information from high-resolution imaging with elemental composition maps obtained from either energy dispersive X-ray spectroscopy (EDX) or by electron energy loss spectroscopy (EELS), but with only limited resolution in chemical mapping well above the atomic length scale. These performance boundaries in chemical mapping applications have now been overcome by the revolutionary ChemiSTEM technology combined with the Titan G2 series of Cs-corrected S/TEMs.
This new technology greatly enhances EDX detection sensitivity due to a number of innovations in the system architecture, including: the X-FEG (a high-brightness Schottky FEG source), the Super-X EDX detector system (4 windowless silicon drift detectors with shutters integrated deeply into the objective lens), and high-speed electronics capable of 100,000 spectra/second readout.
This new system architecture provides tremendous performance benefits, including:
- Improved light element detection
- Better sample tilt response
- Faster mapping
- Atomic chemical mapping capabilities on crystalline structures
In the past, atomic analytical performance was attainable only by EELS mapping, which was limited to elements suitable for EELS applications and which required ultrathin specimens to avoid scattering artifacts.
Finally, as the number of elements in the periodic table accessible for chemical mapping by EDS is much higher than for EELS, EDS mapping provides chemical information at the atomic level, on complex multiphase materials, which was not accessible via EELS in the past.
Advantages of the ChemiSTEM Technology System Architecture
For some time, Silicon Drift Detectors (SDDs) have been replacing Si(Li) detectors for EDX in SEMs but only recently have they entered the S/TEM world. One of the biggest advantages of SDDs is their ability to handle very high count rates without saturation. Although count rates of over 100,000/second can easily be generated with bulk samples in the SEM, this was traditionally less likely, even impossible, with thin specimens in the S/TEM. However, with the advent of probe-corrected S/TEMs and high brightness Schottky field emission sources (X-FEGs), there has been a considerable increase in available beam currents, even in small atomic-sized electron probes, and therefore higher achievable EDX count rates. Moreover, SDDs can be designed in a very compact way allowing the integration of multiple detectors inside the S/TEM column as opposed to just attaching them to ports close to the objective lens.
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Figure 1: ChemiSTEM™ design, showing the X-FEG and the Super-X™ geometry. Not to scale.
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Figure 2: Comparison of relative EDX count rates of the Super-X system (on Tecnai Osiris with 0.9 sr collection angle) and a single Si(Li) detector system with 0.3 sr nominal solid angle. Both S/TEMs were operated at 200 kV with the same (constant) beam current. NiOx films were used as samples for both tilt series. Positive tilt angles represent specimen tilts towards the
single detector for the Si(Li) system. Diagrams above the graph show the effects of detector shadowing for the 4 Super-X detectors, and diagrams below show shadowing effects for the single detector system.
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Figure 3: X-ray transmission efficiency versus energy for a windowless SDD detector (red curve) and an SDD detector with thin polymer window (blue curve).
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ChemiSTEM Technology, now available on the Titan G2 series S/TEM, incorporates four SDD detectors symmetrically placed around the optical axis close to the sample area (figure 1) in combination with an ultra-stable X-FEG high-brightness electron source. The resulting total sensor area of 120mm2 and its integration deep inside the electron-optical column result in a solid angle of 0.7 steradian (sr) allows for the acquisition of chemical information on the atomic scale. Additionally, the detected X-ray count rate is greatly improved since the EDX signal can be obtained under all tilt conditions. By contrast, a single detector solution, whether Si(Li) or SDD, always suffers from its asymmetrical geometrical placement. For negative tilt angles above approximately -10°, the count rate drops to zero in most conventional EDX systems due to the sample and/or specimen holder completely shadowing the single detector. Figure 2 shows (lower blue curve) an example of this single detector tilt dependency on a conventional EDX system with a 0.3 sr solid angle. The ability to acquire high EDX count rates irrespective of sample tilt adds a valuable new degree of flexibility.
The four SSDs are cooled for optimum performance by a direct connection to the cold trap of the Titan G2 with no additional Dewar required. The windowless design of the SDDs improves the sensitivity for light elements, compared to detectors with thin polymer windows, while mechanical shutters protect the SDDs against high-energy electrons. Specially designed front-end electronics and an ultrafast multi-channel pulse processor are also employed, and the entire EDX system is fully embedded in the system control of the Titan G2 S/TEM. Pixel dwell times down to 10 µs can be used for fast mappings, which are acquired and processed using Bruker's ESPRIT Software for EDS. Additionally, the use of a probe Cs-corrector allows atomic chemical maps to be acquired at lower acceleration voltages, such as 80 kV, in order to reduce knock-on damage effects caused by the high electron doses required for atomic chemical imaging.
Sensitive Detection of Light Elements
The detection of light elements is not regarded as a traditional strength of EDX as a technique; however the combination of SDD technology with a windowless design considerably enhances the sensitivity for light elements like oxygen and nitrogen. Figure 3 shows X-ray transmission efficiency versus energy for a windowless SDD detector and an SDD detector with thin polymer window. Loss due to both the Si grid bars and the polymer window contributes to the lower efficiency across all energies of the detector with window. The inset schematic shows loss due to holder shadowing and the detector window. The loss due to the window includes total absorption by the Si support grid bars (at all energies) and selective absorption by the polymer window (at energies below 1 keV). The benefits of this design are illustrated in the example of atomic oxygen mapping in a SrTiO3 crystal investigated in [110] and [100] projection at 200 kV.
Fast Mapping of Large Areas
By utilizing ChemiSTEM Technology, along with fast electronics, elemental maps can be recorded with speed enhancements factors of up to ~50 when benchmarked against traditional 0.3 sr EDX Si(Li) systems. New system software allows for spectrum image maps of up to 1,000x1,000 pixels to be collected while also providing a complete EDX spectrum for each individual point on the map. This allows post acquisition searching for further elements in the stored data cube. Since the EDX spectra are easy to analyze, due to the low background signal and the symmetric peak shapes, the elemental maps can be processed live during the acquisition with mapping results obtained immediately afterward
Atomic Chemical Mapping:
The benchmark on SrTiO3 versus EELS spectroscopy
By combining the sub-atomic resolution imaging capabilities of the Titan G2 with the revolutionary EDX detection sensitivity of ChemiSTEM technology, routine atomic-level spectroscopy is now a reality. The images shown below are chemical maps of a thin strontium titanate (SrTiO3) crystal in which the individual atomic positions can be distinguished by their unambiguous chemical signal (Sr is red, Ti is green). To compare the results, raw data from EDX and EELS is presented with no post-processing. The individual atomic columns are not only visible; they are also clearly distinguished from their neighbors by a very high contrast in EDX. The sampling of these chemical maps was 0.075 Angstroms/pixel, representing the highest pixel sampling obtained so far by any atomic spectroscopy S/TEM technique while maintaining excellent signal-to-noise quality. Additionally, these images were obtained in just minutes thanks to the unprecedented sensitivity of the ChemiSTEM technology detection system on the Titan G2 platform. Lastly, the number of elements in the periodic table accessible for chemical mapping in EDX is much higher than EELS. EDX mapping allows chemical information to be obtained on the atomic level from a variety of complex multiphase materials, many of which were not previously accessible via EELS measurements.
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On the left side data extracted from EELS are shown, and on the right side the corresponding chemical imaging with the EDX signal is shown. The top images are showing the composite maps, while below the elemental maps of the single elements are presented. The EDX maps for each element were extracted using background corrected integrated intensities for the corresponding edges. No post data acquisition processing was applied other than colorization of the elemental signals. The sampling in EDX is 0.075 Angstroms/pixel and in the EELS data is 0.32 Angstrom/pixel. The EDX map has the highest sampling ever obtained in an atomic level chemical map. Both the EDX and EELS results were obtained on a Titan G2 at 200 kV acceleration voltage using 10ms total dwell time per pixel.
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Atomic Chemical Mapping:
The benchmark data with ChemiSTEM Technology on SrTiO3 versus
conventional EDX technology
SrTiO3 is a well understood perovskite structure and this
structure is a good benchmark to illustrate the recent progress
in atomic chemical mapping. The map obtained on a Titan G2
with ChemiSTEM Technology shows the breakthrough progress
in atomic spectroscopy and is superior not only in the chemical
signal and contrast, but also in the mapping speed and map size
compared to the conventional technology. The raw data is of
such high signal-to-noise quality that noise filtering is not even
necessary to identify the positions of strontium and titanium in
the lattice.
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Comparison of atomic chemical mapping performance obtained with ChemiSTEM Technology on a Titan G2 with probe Cs-corrector compared
to a Titan with conventional Si(Li) EDX detector. The chemical map of SrTiO3 in the [100] direction is acquired with 128x128 pixels with 250 pA probe current
in 259 s exposure time. The map on a Titan G1 was acquired with 40x40 pixels with a probe current of 10-20pA with 800 s exposure time (see reference:
PRB 81 (2010) A.J. D’Alfonso, B. Freitag, D. Klenov, and L.J. Allen). The atomic structure of SrTiO3 is shown in the middle diagram.
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High lateral resolutions in chemical mapping:
GaAs dumbbells in [110] direction at 80 & 200 kV using the K and L edges
The GaAs dumbbell of 0.14 nm can be clearly resolved by chemical mapping using energy dispersive x-ray analysis. Both elements - the Ga and the As - can be resolved by using not only the low energy L- peak but also the high energy K-peak, as shown in figure 5. In energy dispersive x-ray analysis all x-rays with different energies are acquired simultaneously up to very high energy losses (40 keV). This allows comparison of the performance in atomic chemical mapping using different characteristic edges of the same element with perfect correlation of the results. In the second example above, the contrast of Ga/As-K and Ga/As-L peaks are compared and differences in contrast can be observed. The contrast at the higher energy loss K-edges is better than for the L-edges. The contrast is calculated by dividing the difference of maximum and minimum by the sum of the two. This result indicates that higher energy loss x-rays have higher localization than lower energy loss x-rays. Theoretical calculations can reproduce this result, but further examinations are required[1]. This suggests that the ability of atomic EDX to access higher energy loss peaks can be an advantage in obtaining the highest possible spatial resolution in chemical mapping.
The use of the probe corrector allows atomic sized probes to be maintained at lower voltages, such as 80 kV. Therefore the GaAs dumbbell of 0.14 nm can be resolved in chemical mapping even
at 80 kV (upper right image of figure below).
[1] Chemical mapping on the atomic level using energy dispersive x-ray spectroscopy, D.O. Klenov, H.S. von Harrach, B. Freitag, A.J. D'Alfonso, and L.J. Allen, M&M2011, Nashville, USA,MC Kiel, 2011, Germany.
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Atomic chemical mapping GaAs in [110] projection using ChemiSTEM with probe Cs-corrector at 80&200 kV acceleration voltage. The upper images (left and middle) of Ga and As are acquired at 200 kV using 200 pA probe current per pixel. In the upper right corner the GaAs map acquired at 80kV with 50 pA Probe current and 50 μs dwell time is shown. The atomic structure is shown on the lower right side of the figure. An intensity profile of the Ga/As-K peaks and the Ga/As-L peaks reveals differences in contrast between the K&L signals.
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Seeing mixed occupancies in atomic columns:Atomic EDX on In0.53Ga0.47As in [110] projection at 200 kV
Not only can the dumbbell structure of InGaAs be resolved
using atomic chemical mapping with ChemiSTEM Technology,
but it also permits distinguishing between a mixed (In,Ga)
atomic column and pure atomic column (As), as shown below. This application is even more demanding in the
required sensitivity than atomic chemical imaging of pure
atomic columns - and the data below demonstrates the excellent
sensitivity of the ChemiSTEM Technology detector system of
4 SDD detectors. Since the signal in EDX is proportional to the
chemical concentration illuminated with the beam, the signal
is intrinsically more weak in case of mixed atomic columns due
to the small volume excited and the smaller concentrations
compared to pure atomic columns.
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Atomic
chemical mapping
of In0.53 Ga0.47 As
in [110] projection
obtained with
ChemiSTEM
Technology on a
Titan G2 with probe
Cs-corrector at
200 kV acceleration
voltage. The raw
data (left) and
filtered data is
shown as pairs. The
atomic structure is
shown in the middle
of the figure. The
map size is 64x64
pixels, recorded for
174 s with 50 pA
beam current.
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Atomic chemical analysis at interfaces:
Interface of SrTiO3 / PbTiO3 in [100] projection
In the figures above, perfect crystals were examined, and their
sub-lattices were imaged using EDX spectroscopy. This is an
important application, but the study of chemical variations
on interfaces, surfaces and defects is an even more exciting
application in atomic chemical analysis. Therefore the interface
of SrTiO3 / PbTiO3 has been studied with atomic EDX. The
positions of the Sr, Ti and Pb atomic columns can clearly be
visualized in the x-ray maps revealing the chemical change at
the interface. No change in contrast over the field of view shows
that the titanium concentration is stable across the interface.
The chemical maps of lead and strontium reveal a change at
the interface through the thickness of the sample. Simulations
(shown and referenced below) of atomic resolution EDX maps
of a SrTiO3/PbTiO3 interface have been made where the 200 keV
probe was assumed to be aberration free with a convergence
semi-angle of 21.5 mrad. The specimen was assumed to be
50 nm thick, and under these conditions agreement with the
experimental data is found.
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Atomic chemical mapping of the interface of SrTiO3 / PbTiO3 obtained with ChemiSTEM Technology on a Titan G2 with probe Cs-corrector at
200 kV acceleration voltage. The underlaying image shows the HAADF STEM image of the interface. The composite image of the Pb and Sr signals is shown
in the bottom center and the maps of the individual elements are shown in raw and filtered on the left and right sides. The mapping size is 200×200 pixels
with a total mapping time of ~300 s. In the insert we show theoretical calculations in a color composite map constructed from the Pb L2,3 signal (cyan),
the Sr K signal (magenta) and the Ti K signal (yellow) (L.Allen et al., Melbourne, Australia). Sample courtesy M. Kurasawa, Y. Chen and P.C. McIntyre, Stanford
University.
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Seeing mixed occupancies:
In the [110] projection of Y2Ti2O7 the concentration of Y,Ti is
changing in the atomic columns depending on the position
in the unit cell. Three different columns are present in this
projection. A pure Y, pure Ti and a mixed (50:50) Y,Ti column
(See structural model below). The different occupancies of the
structure can be clearly seen in the atomic EDX maps. Since the
concentration is known in this case a sensitivity in mixed column
detection of 50% for atomic resolution can be established.
This is the first time in EDX analysis that a 50% occupied column
can be resolved in an atomic chemical map. Hence atomic
chemical mapping using Chemistem Technology is not only
suitable to obtain atomic maps of pure columns, but also is
suitable to visualize mixed columns in complex structures.
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Atomic chemical mapping of Y2Ti2O7 in [110] projection obtained with ChemiSTEM Technology on the Titan G2 with probe Cs-corrector at 200 kV
acceleration voltage. The maps have a size of 64x64 pixels with a mapping time of 306 s. A probe current of 76 pA was used. EDX maps of filtered data using
averaging over 3 pixels are shown. A model of the structure in [110] projection indicates that in this projection three different occupancies are present. Pure Y,
pure Ti and 50:50 mixed Y/Ti columns.
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Detection of light elements:
Since ChemiSTEM Technology employs windowless detectors,
the sensitivity for light elements is higher than on detector
systems with polymer windows that absorb x-rays at lower
energies. The benefit of this design feature is illustrated by the
detection of single oxygen columns in atomic chemical mapping.
The oxygen edge has a very low energy loss of 532 eV.
Nevertheless, in the example of SrTiO3 the high performance
is shown by the visualization of the oxygen sub lattice in [100]
projection where the oxygen distribution is imaged clearly.
The composite image of Sr,Ti,O reveals the position of the pure
oxygen columns and mixed Ti/O columns.
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Atomic chemical mapping SrTiO3 in [100] projection obtained with ChemiSTEM Technology on a Titan G2 with probe Cs-corrector at 200 kV
acceleration voltage. The maps have a size of 64x64 pixels with a mapping time of 306 s. The atomic structure is plotted on the right side. Two composite
images are shown. The left one shows only the Sr and Ti signal and consists of raw data. The upper right one shows the composite of Sr,Ti and O, and
shows filtered data.
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