Parallel-detection Electron Energy Loss Spectrometer Research Articles

High spatial resolution PEELS characterization of FeAl nanograins prepared by mechanical alloying

We investigate the nanograin “chemical” structure in a nanostructured material of possible industrial application (Fe–Al system) prepared by conventional mechanical alloying via ball milling in argon atmosphere. We restrict ourselves to the structural and nanochemical behaviour of ball-milled nanocrystalline Fe–Al powders with atomic composition Fe 3Al, corresponding to a well-known intermetallic compound of the Fe–Al system. Scanning transmission electron microscopy (STEM) equipped with a parallel detection electron energy loss spectrometer (PEELS) has provided an insight on the “chemical” structure of both nanograins and their surface at a spatial resolution of better than 1 nm. The energy loss near edge structure of the Al L loss reveals that the Al coordination is similar to a B2 compound and the oxidation of the powder during processing may play a significant role in the stabilization of the intermetallic phases. Conventional transmission electron microscopy (TEM) was used for the structural characterization of the material after the ball milling; powder X-ray diffraction (XRD) aided the investigation.

RDF Analysis of Radiation-Amorphized SiC using a field Emission Scanning Electron Microscope

AbstractFast electrons are a particularly useful chemical and structural probe for the small sample volumes associated with ion- or fast electron-irradiation-induced amorphization, because of their much stronger interaction with matter than for X-rays or neutrons, and also because they can be readily focused to small probes. Three derivative signals are particularly rich in information: the angular distribution of scattered electrons (which is utilized in both diffraction and imaging studies); the energy loss spectrum of scattered electrons (electron energy loss spectroscopy, or EELS); and the emission spectrum of characteristic X-rays resulting from ionization energy losses (energy dispersive X-ray spectroscopy, or EDXS). We have applied the first two to the study of three amorphized compounds (AIPO4, SiO2, SiC) using MIT's Vacuum Generators HB603 field-emission (FEG) scanning transmission electron microscope (STEM), operating at 250 kV and equipped with a Gatan digital parallel-detection electron energy-loss spectrometer (digiPEELS).

RDF Analysis of Ion-Amorphized SiO2 and SiC from Electron Diffraction using Post-Specimen Scanning in the Field-Emission Scanning Transmission Electron Microscope

ABSTRACTRadial density functions (RDFs) provide important information about short- and ntermediaterange structure of topologically-disordered materials such as glasses and irradiation-amorphized materials. We have determined RDFs for irradiation-amorphized SiO2, AIPO4 and SiC by energy-filtered electron diffraction methods in a field-emission scanning transmission electron microscope (FEG-STEM) equipped with a digital parallel-detection electron energy-loss spectrometer. Post-specimen rocking was used to minimize the effects of spherical aberration in the objective lens, which interfere with the acquisition of data collected by pre-specimen rocking. Useful energy-filtered data has been collected beyond an angular range defined by q = 2 sin(Θ/2)/λ = 25 nm−1

A 200kV Comprehensive Analytical Electron Microscope

With the use of a high-brightness electron source, the performance of the analytical electron microscope (AEM) has significantly improved. It features a variety of sophisticated analytical functions, such as X- ray spot analysis, area analysis, electron beam energy analysis, electron diffraction, and phase analysis using a biprism for holography. Here, we report the results of our analysis using a 200kV comprehensive AEM which has an integrated ZrO/W emitter as its optic source. The AEM (JEM- 2010F) is equipped with an X-ray energy dispersive spectrometer (EDS), scanning image observation device (ASID), parallel detection electron energy loss spectrometer (PEELS), and TV camera.(1) X-ray mapping using EDS and ASID With FEG, the probe current in a 1 nm probe is increased by two-order orders of magnitude, and the background noise generated near the sample and in the illumination system is minimized. As a result, EDS analysis features high sensitivity at a minimum detection of 1% weight fraction from an ultra-micro area of lnm diameter.

High-resolution TEM and AEM study in (Au, Tin) thin film on (100) silicon

The cross-sectional microstructure of a (Au, TiN) thin film deposited on a (100) Si substrate without further heat treatment was studied by CTEM, HRTEM, and AEM. HTREM was performed in a Topcon 002B microscope with interpretable resolution limit of 0.18 nm, and high spatial AEM was done in a Philips 400ST field emission gun microscope at 100 kV using a Gatan 666 parallel-detection electron energy loss spectrometer. Cross-section specimens of the interface were prepared by traditional polishing and ion milling. Temperatures used in the whole process of TEM specimen preparation were below 100°C and a liquid-nitrogen-cooled cold stage was used in ion milling to prevent interaction between Au and Si.Ti was co-deposited with Au by sputtering in a nitrogen atmosphere to increase the hardness of the thin Au contact film. Fig. 1a shows the microstructure of the (Au, Ti, N)/Si contact. The features of this microstructure are very similar to the those of pure gold thin films on Si substrates annealed below the Au-Si eutectic temperature (363°C). The thin, white layer was considered to be the original Au/Si interface by Chang et al.

EXELFS as a tool for quantifying phase distributions in materials

A method has been developed to quantify phase distributions in multiphase materials by using the information contained in the EXtended Energy Loss Fine Structure (EXELFS) of a probe atom in an Electron Energy Loss Spectroscopy (EELS) spectrum. The EELS and EXELFS determine, in addition to the atomic composition, the near-neighbor atomic environment about each atom type separately. The method is applicable to electron-transparent foils observed in a Transmission Electron Microscope (TEM) equipped with a parallel-detection electron energy loss spectrometer. The relative amounts of phases that have a common element can be determined even in the presence of additional phases which do not share that common element. The method is particularly applicable to layered materials and can be used, for example, to determine the local relative thickness of a metal and its oxide layer. The Mg MgO system was used to demonstrate the method, and it was found that the accuracy in relative thickness determination is about 5%. The near-edge fine structure information was consistent with the results of the EXELFS analysis.

Characterization of iron-implanted silicon carbide

As part of extensive fundamental studies of ion-implanted ceramics, analytical electron microscopy (AEM) has been used to characterize [0001] single crystal 6H α-silicon carbide implanted at room temperature with 160 keV 57Fe ions to fluences of 1,3, and 6 x 1016 ions/cm2. AEM was performed at 100 kV with a Philips EM400T equipped with a field emission gun, a 6585 STEM unit, an EDAX 9100/70 energy dispersive X-ray spectrometer (EDS), and a Gatan 666 parallel-detection electron energy loss spectrometer (PEELS).A micrograph of a cross-sectioned specimen implanted with 6 × 1016 ions/cm2 is shown in Fig. 1. The implanted region extending to 195 nm from the surface is amorphous, as confirmed by convergent beam electron diffraction (CBED) patterns (insets), and contains iron at levels easily detectable by EDS (Fig. 2). The 15 nm lattice fringes of the edge-on 6H basal planes in the substrate provide an unambiguous depth calibration scale. The transition layer at the end of the ion range exhibits the expected strain contrast. The depth profile of the implanted iron was measured by EDS line scans perpendicular to the implant surface; the mean of three of these scans is shown by the filled symbols in Fig. 3. The integrated iron profile agrees with the expected 6.0 × 1016 ions/cm2 only if the density of the amorphous SiC is equal to that of its crystalline forms, rather than the 20% lower density reported earlier.

Nanoanalysis of impurity precipitation on grain boundaries in polysilicon

Polycrystalline silicon (polysilicon) has been the subject of extensive studies in recent years due to its wide applications in microelectronics and solar cells, see for example references. In most cases of interest impurities segregated to the grain boundaries strongly modify the electronic properties of this material. Understanding the behavior of impurities in polysilicon requires a detailed knowledge of their distribution, phase and thermodynamical stability. In the present study, the structure and chemistry of grain boundaries in polysilicon were characterized by HREM and very high spatial resolution FEG/AEM.The specimen materials are Solarex cast polysilicon used for solar cells, which are manufactured under a proprietary method called the ubiquitous crystallization process. The samples for TEM observations were cut from the ingot perpendicular to its growth direction and then thinned to electron transparency by mechanical polishing and ion milling. HREM was performed at 200kV in an ISI-002B. Microanalysis was done using a Philips 400ST/FEG microscope coupled to a Gatan parallel-detection electron energy loss spectrometer, at 100kV. A liquid nitrogen cooled specimen holder was used for nanoprobe experiments to minimize specimen contamination and local specimen heating.

Parallel EELS as a complementary tool in an Analytical TEM study of metal alloys

The advent of efficient parallel-detection electron energy loss spectrometers (EELS) in analytical transmission electron microscopes (ATEM) promotes rapid elemental characterization of various materials containing light, as well as heavy, elements at high spatial resolution. In the current study, the microstructures of Udimet 520 and HK-25 were investigated to integrate advances in parallel EELS methods with other ATEM techniques.Thin foils of Udimet 520, a nickel based heat-resistant superalloy, were obtained using common ion milling techniques. Precipitates of the HK-25, a heat resistant steel alloy, were obtained by etching (with Marble's reagent) a polished surface, extracting with a replicating Parlodian film, dissolving the film with amyl acetate and dispersing the resulting particles on holey carbon grids. A JEOL 2000FX ATEM fitted with a Gatan Model 666 EELS system was operated at 200 kV for studies of these alloys.

Improved detection limits with parallel-detection EELS

Parallel-detection electron energy-loss spectrometers (PEELS) have improved the detection limits of electron energy-loss spectroscopy (EELS) so much that established comparisons between EELS and energy-dispersive X-ray spectroscopy (EDXS) no longer apply. We have therefore decided to reinvestigate the comparative advantages of EELS as a microanalytical technique.Two types of detection limits are of interest: minimum detectable mass (MDM), and minimum detectable mass fraction (MMF). MDM has been improved by parallel-detection EELS so much that single atoms of elements with favorable cross-sections have become detectable. Since detecting a single atom requires a probe with at least 0.1 nA of current and a diameter of no more than 1 nm, this level of performance needs an electron microscope equipped with an efficient field-emission gun (FEG). On the other hand, because the ratio of the signal from a uniformly dispersed element to the signal from the matrix does not depend on the size of the probed area, MMF depends on the total available probe current rather than the probe current density. This means that MMF should be better for microscopes equipped with thermionic guns than for ones equipped with FEGs.

Materials science applications of a 120kV FEG TEM/STEM: Triskaidekaphilia

The introduction by several manufacturers of 200kV transmission electron microscopes (TEM) equipped with field emission guns affords the opportunity to assess their potential impact on materials science by examining applications of similar 100-120kV instruments that have been in use for more than a decade. This summary is based on results from a Philips EM400T/FEG configured as an analytical electron microscope (AEM) with a 6585 scanning transmission (STEM) unit, ED AX 9100/70 or 9900 energy dispersive X-ray spectroscopy (EDS) systems, and Gatan 607 serial- or 666 parallel-detection electron energy-loss spectrometers (EELS). Examples in four areas that illustrate applications that are impossible or so difficult as to be impracticable with conventional thermionic electron guns are described below.

Advances in Parallel-Detection EELS

Parallel-detection electron energy-loss spectrometers (PEELS) now routinely provide spectra in which the main source of noise is the statistical fluctuation in the number of arriving electrons (i.e., they have DQE > 0.5), achieve an energy resolution which is more than 90% limited by the electron microscope gun, and are fairly easy to operate. They have pushed the minimum detectable mass (MDM) obtainable by PEELS close to the single atom level, and have improved the minimum detectable mass fraction (MDF) so that it is now comparable or better than MDF detectable by EDXS even for elements as heavy as Fe. The attainable energy resolution is now 0.3-0.5 eV on a routine basis when the spectrometer is mounted on a cold field-emission gun (S)TEM operating at 100 kV (Fig. 1). This impressive progress has opened up an important question: where next?Our answer is three-fold: towards greater integration of EELS with other techniques of electron microscopy, towards new applications of the technique, and towards more quantitative and yet more user-friendly analysis of the results.

Computer-aided parallel EELS techniques: acquisition, processing, & imaging

The recent commercial introduction of the parallel detection electron energy-loss spectrometer has undoubtedly made electron energy-loss spectroscopy (EELS) more viable as a technique for routine microanalysis. Additionally, the increased recording efficiency of parallel EELS (PEELS) warrants the use of more involved acquisition and processing techniques than was necessary, or even possible with serial EELS. This increased complexity places greater demands on the computer systems controlling data acquisition. Multichannel analyzers systems with small resources and limited programming facilities are not capable of exploiting the full capabilities of the PEELS spectrometer.Preliminary efforts of the author with the Gatan PEELS spectrometer were concentrated on development of a flexible acquisition system at National Institutes of Health. Hardware control is performed through machine-language drivers called from high-level languages (HLL) such as FORTRAN and C. The software drivers and hardware were designed to minimize processor involvement in the data collection process, resulting in the capability to collect data while processing continues within the parent HLL. This design simplifies the HLL program structure and minimizes data collection dead time.

Fine structure of inner shell electron energy loss edges of manganese oxides

Rask et al. showed that electron energy loss spectroscopy (EELS) may be used to identify oxidation states of polyvalent cations. For the case of manganese, differences in oxidation state are reflected in the shape and position of the edges in the EELS spectrum due to inelastic scattering of incident electrons by the inner shells of manganese and oxygen. We have used a scanning transmission electron microscope (STEM) with a cold field emission gun (FEG) and a parallel-detection EELS system to re-investigate the variations in energy loss fine structure of EELS spectra of various manganese oxides with a significantly improved energy resolution.Spectra were recorded at 100 kV using a VG HB501 STEM with post-specimen lenses, and a Galan Model 666 parallel-detection electron energy loss spectrometer. The operation and performance of this system are discussed elsewhere in these proceedings. For the inner shell edges, the energy dispersion produced by the spectrometer was set to give 0.05 eV per channel. so that a complete spectrum of 1024 channels sampled a range of energy loss of 50 eV. The energy resolution, of the order of 0.4 eV, was therefore not limited by the spatial resolution of the photodiode array in the spectrometer. The oxygen K and manganese L2,3 edges were recorded separately.

Performance of the Gatan PEELS™ on the VG HB501 STEM

Parallel-detection electron energy-loss spectrometers offer several hundred times the detection efficiency of serial-detection spectrometers, as well as improved energy resolution. These advantages should be especially important when using a scanning transmission electron microscope (STEM) with a cold field emission gun (FEG), in which the available beam current is typically 10 to 100 times less than in a conventional TEM, while the beam energy spread is typically only 0.3 eV. We have therefore investigated the performance of the Gatan parallel-detection spectrometer (Gatan model 666 PEELS™) when mounted on the VG HB501 FEG STEM.

Progress in parallel-detection electron energy loss spectroscopy

Parallel-detection electron energy loss spectrometers improve the detection efficiency by several hundred times compared to the traditional serial-detection spectrometers, but they have their own set of difficulties, such as the limited dynamic range of solid state detectors, the possibility of stray reflections of the intense zero loss beam giving rise to spurious background, and channel-to-channel gain variation. Fortunately, none of these difficulties is turning out to be insoluble. Here we report on improvements of the Gatan 666 parallel detection electron spectrometer in the areas of increasing the dynamic range of the detector, and in eliminating stray reflections.The increase in the dynamic range of the detector was needed especially for low energy losses (high spectral intensities), which usually saturated the detector even at the minimum acquisition time of 12 msecs. Accordingly, we have developed an electron attenuator which uses a magnetic dipole to sweep the spectrum across the detector perpendicular to the dispersion direction (Fig. 1).

Minimum-dose electron energy-loss spectroscopy

Parallel-detection electron energy-loss spectrometers have greatly reduced the irradiation doses needed to acquire electron energy-loss spectroscopy (EELS) data. This raises the possibility that changes in chemical bonding due to radiation damage, which usually precede changes incomposition, may be studied by examining alterations in low-loss and core-edge fine structure with increasing dose.The quality of data attainable with low-doses in parallel-detection EELS is illustrated by Fig. 1, which shows the K-edge region for a thin amorphous carbon film. This spectrum was collected in 2sec with a 1pm-diameter probe at 100kV in (microscope) diffraction mode. A 20cm camera length ensured that virtually all scattered electrons passed through the 3mm entrance aperture of the spectrometer. Spectrum dispersion is 3eV/channel and the energy resolution is ∼1eV/channel as the π* peak is clearly resolved. The total incident current, as determined in image mode (no objective aperture) from the viewing-screen meter, was only 40pA. The total dose for this K-edge spectrum was consequently only 6.4e-/Å2.