Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Energy-Dispersive X-ray Microanalysis in the TEM Anthony J. Garratt-Reed Neil Rowlands •One result of the interaction of an electron beam with matter is the emission of x-rays •One result of the interaction of an electron beam with matter is the emission of x-rays •The energy and wavelength of the Xrays is different for, and characteristic of, each element •One result of the interaction of an electron beam with matter is the emission of x-rays •The energy and wavelength of the Xrays is different for, and characteristic of, each element •Analysis of the X-rays can, therefore, be used as a tool to give information about the composition of the sample In today's talk: In today's talk: i. X-ray emission from materials In today's talk: i. X-ray emission from materials ii. X-ray detectors (brief!) In today's talk: i. X-ray emission from materials ii. X-ray detectors (brief!) iii. Quantitative chemical analysis In today's talk: i. X-ray emission from materials ii. X-ray detectors (brief!) iii. Quantitative chemical analysis iv. Spatial Resolution X-ray emission from materials X-ray emission from materials • 2 independent processes X-ray emission from materials • 2 independent processes • Characteristic X-rays (discrete energies) X-ray emission from materials • 2 independent processes • Characteristic X-rays (discrete energies) • Bremsstrahlung (continuum) Characteristic X-rays • 2-step process involving the atomic electrons Characteristic X-rays • 2-step process involving the atomic electrons • Firstly, the atom is excited by ionization of one of the core-level electrons Characteristic X-rays • 2-step process involving the atomic electrons • Firstly, the atom is excited by ionization of one of the core-level electrons • This is followed by an outer-shell electron losing energy by emission of a photon (the X-ray), and dropping to the core state Bremsstrahlung • “Braking radiation” Bremsstrahlung • “Braking radiation” • All charged particles radiate energy when accelerated Bremsstrahlung X-ray detectors X-ray detectors • Lithium-drifted Silicon (Si(Li)) X-ray detectors • Lithium-drifted Silicon (Si(Li)) Used since around 1970 on SEMs X-ray detectors • Lithium-drifted Silicon (Si(Li)) Used since around 1970 on SEMs • Silicon Drift detector X-ray detectors • Lithium-drifted Silicon (Si(Li)) Used since around 1970 on SEMs • Silicon Drift detector Over the last 5 years X-ray detectors • Lithium-drifted Silicon (Si(Li)) Used since around 1970 on SEMs • Silicon Drift detector Over the last 5 years • Crystal detectors – Electron Microprobe X-ray detectors • Lithium-drifted Silicon (Si(Li)) Used since around 1970 on SEMs • Silicon Drift detector Over the last 5 years • Crystal detectors – Electron Microprobe Different characteristics Si(Li) crystal Si(Li) crystal •Crystal of pure silicon, with lithium diffused in to compensate for any residual carriers Si(Li) crystal •Crystal of pure silicon, with lithium diffused in to compensate for any residual carriers •About 3mm thick and 3-6 mm diameter Si(Li) crystal •Crystal of pure silicon, with lithium diffused in to compensate for any residual carriers •About 3mm thick and 3-6 mm diameter •Electrodes plated on front and back Si(Li) crystal •Crystal of pure silicon, with lithium diffused in to compensate for any residual carriers •About 3mm thick and 3-6 mm diameter •Electrodes plated on front and back •Front electrode is thin to allow X-rays to enter Si(Li) crystal •Crystal of pure silicon, with lithium diffused in to compensate for any residual carriers •About 3mm thick and 3-6 mm diameter •Electrodes plated on front and back •Front electrode is thin to allow X-rays to enter •Biased by a voltage of 3-500V Si(Li) crystal •Crystal of pure silicon, with lithium diffused in to compensate for any residual carriers •About 3mm thick and 3-6 mm diameter •Electrodes plated on front and back •Front electrode is thin to allow X-rays to enter •Biased by a voltage of 3-500V •Cooled to Liq. N2 Si(Li) crystal •Energy of an x-ray generates electronhole pairs Si(Li) crystal •Energy of an x-ray generates electronhole pairs •These are swept from the crystal by the bias voltage, and are detected in the external circuitry as a pulse of charge Si(Li) crystal •Energy of an x-ray generates electronhole pairs •These are swept from the crystal by the bias voltage, and are detected in the external circuitry as a pulse of charge •Since the average energy required to create an electron-hole pair is constant and predictable (about 3.8eV), the external charge is proportional to the x-ray energy Quantitative Analysis Quantitative Analysis • Different techniques for: Quantitative Analysis • Different techniques for: • SEM Quantitative Analysis • Different techniques for: • SEM • Organic thin sections Quantitative Analysis • Different techniques for: • SEM • Organic thin sections • Materials thin sections Quantitative Analysis • Different techniques for: • SEM • Organic thin sections • Materials thin sections – Today's talk! Characteristic X-rays • 2-step process involving the atomic electrons • Firstly, the atom is excited by ionization of one of the core-level electrons • This is followed by an outer-shell electron losing energy by emission of a photon (the X-ray), and dropping to the core state Characteristic X-rays • 2-step process involving the atomic electrons • Firstly, the atom is excited by ionization of one of the core-level electrons • This is followed by an outer-shell electron losing energy by emission of a photon (the X-ray), and dropping to the core state Fluorescence Ionization cross-section Ionization cross-section •The Ionization cross-section is defined as the probability of ionizing a single atom in a region of uniform current density of electrons. Ionization cross-section •The Ionization cross-section is defined as the probability of ionizing a single atom in a region of uniform current density of electrons. •Usually denoted by “QA” where the “A” denotes the particular element of interest Ionization cross-section •The Ionization cross-section is defined as the probability of ionizing a single atom in a region of uniform current density of electrons. •Usually denoted by “QA” where the “A” denotes the particular element of interest • It has units of area Ionization cross-section • Units are generally Barns, where 1 Barn=10-24 square centimeters Ionization cross-section • Units are generally Barns, where 1 Barn=10-24 square centimeters • Typical values of the cross-section are 100-1000 Barns. Ionization cross-section • Units are generally Barns, where 1 Barn=10-24 square centimeters • Typical values of the cross-section are 100-1000 Barns. • For practical purposes, the cross-section can be regarded as a function of the electron energy alone, and is independent of the chemical surroundings. Ionization cross-section • For practical purposes, the cross-section can be regarded as a function of the electron energy alone, and is independent of the chemical surroundings. • Various equations have been proposed to predict the value of the ionization crosssection for all the elements at different beam voltages Characteristic X-rays • 2-step process involving the atomic electrons • Firstly, the atom is excited by ionization of one of the core-level electrons • This is followed by an outer-shell electron losing energy by emission of a photon (the X-ray), and dropping to the core state Fluorescence Fluorescence Yield Fluorescence Yield • Generally given the symbol “wA” where, again, the subscript “A” denotes the particular element. Fluorescence Yield • Generally given the symbol “wA” where, again, the subscript “A” denotes the particular element. •For practical purposes again, the fluorescence yield can be considered to be a constant for a particular transition. (No significant dependence on chemical bonding, for example) Fluorescence Yield • For practical purposes again, the fluorescence yield can be considered to be a constant for a particular transition. • The fluorescence yield has been measured for a wide range of lines; an equation has been developed to fit these measurements to predict the fluorescence yield in those cases where measurements have not been made. Putting this together -- Putting this together -• We can write, for a sample of thickness t and density r: No i p IA r t C A QA w A s A t AA e where IA is the number of x-rays generated, ip is the probe current in Amps, e is the electron charge, CA is the concentration (weight fraction) of element A in the sample, AA is the atomic weight of element A, s is a partition function to account for the fraction of x-rays in the detected line, and t is the analysis time in seconds. Writing the same equation for element B and dividing: I A AB Q A w A A s A C A I B AA Q B w B B s B C B Writing the same equation for element B and dividing: I A AB QA w A s A C A I B AA QB w B sB CB or I A CA .k AB I B CB Since the detector sensitivity varies for different elements, I ' A CA A .k AB ' I B CB B where the I’s are now the measured x-ray intensities for the various elements Since the detector sensitivity varies for different elements, I ' A CA A .k AB ' I B CB B where the I’s are now the measured x-ray intensities for the various elements The Cliff-Lorimer equation Limitations of Cliff-Lorimer • Valid for “thin” samples only Limitations of Cliff-Lorimer • Valid for “thin” samples only Limitations of Cliff-Lorimer • Valid for “thin” samples only The more common reality! Limitations of Cliff-Lorimer • Valid for “thin” samples only • Variations of detector parameters (espec. ice) Limitations of Cliff-Lorimer • Valid for “thin” samples only • Variations of detector parameters (espec. ice) • Only works when all elements can be detected Limitations of Cliff-Lorimer • Valid for “thin” samples only • Variations of detector parameters (espec. ice) • Only works when all elements can be detected • Spectral Processing Limitations of Cliff-Lorimer Limitations of Cliff-Lorimer • Valid for “thin” samples only • Variations of detector parameters (espec. ice) • Only works when all elements can be detected • Spectral Processing • Spurious effects - Spurious effects: • Fluorescence Spurious effects: • Fluorescence • Escape peaks Spurious effects: • Fluorescence • Escape peaks • Coherent Bremsstrahlung Spurious effects: • Fluorescence • Escape peaks • Coherent Bremsstrahlung • Detector imperfections Spurious effects: • Fluorescence • Escape peaks • Coherent Bremsstrahlung • Detector imperfections • Etc., etc. Limitations of Cliff-Lorimer Limitations of Cliff-Lorimer • Valid for “thin” samples only • Variations of detector parameters (espec. ice) • Only works when all elements can be detected • Spectral Processing • Spurious effects • Statistics! Statistics • Counting of x-rays is a random phenomenon Why do we need counts? 2 sec, low count rate Why do we need counts? 10 secs, low count rate Why do we need counts? 100 secs, low count rate Why do we need counts? 100 secs, high count rate Statistics • Counting of x-rays is a random phenomenon • In counting N events, there is an uncertainty s (the standard deviation) which is equal to the square root of N Statistics • Counting of x-rays is a random phenomenon • In counting N events, there is an inherent uncertainty s (the standard deviation) which is equal to the square root of N • N has a 95% probability of being within +2s of the “Correct” answer Statistics • N has a 95% probability of being within +2s of the “Correct” answer • Hence if 1% precision is required 95% of the time, 40,000 counts must be acquired Statistics • N has a 95% probability of being within +2s of the “Correct” answer • Hence if 1% precision is required 95% of the time, 40,000 counts must be acquired •Likewise for 0.1% precision, 4,000,000 counts are required Statistics • Likewise for 0.1% precision, 4,000,000 counts are required • Approximately half the counts are in the major peak of an element, so 8,000,000 counts must be acquired in the spectrum Statistics • Likewise for 0.1% precision, 4,000,000 counts are required • Approximately half the counts are in the major peak of an element, so 8,000,000 counts must be acquired in the spectrum • Maximum count rate for Si(Li) detector is about 30,000cps, so this will take about 250 seconds (SDD will count at 250,000 cps) Spatial Resolution Spatial Resolution Spatial Resolution • There is no single definition of “Spatial Resolution” Spatial Resolution • There is no single definition of “Spatial Resolution” • Analyzing a small particle on a thin support film has very different requirements from analyzing a diffusion gradient in a foil Spatial Resolution • There is no single definition of “Spatial Resolution” • Analyzing a small particle on a thin support film has very different requirements from analyzing a diffusion gradient in a foil • Consider the diffusion example: Spatial Resolution Putting this together -• We can write, for a sample of thickness t and density r: No i p IA r t C A QA w A s A t AA e where IA is the number of x-rays generated, ip is the probe current in Amps, e is the electron charge, CA is the concentration (weight fraction) of element A in the sample, AA is the atomic weight of element A, s is a partition function to account for the fraction of x-rays in the detected line, and t is the analysis time in seconds. But … ip 8 3 d B 2 4C s 2 3 (B is brightness of electron source, Cs is spherical aberration coefficient of objective lens) Source Brightness: Source Brightness: •Inherent function of emitter Source Brightness: •Inherent function of emitter •Thermionic W: 5 Vo A/cm2/Sr Source Brightness: •Inherent function of emitter •Thermionic W: 5 Vo A/cm2/Sr •Thermionic LaB6: 200 Vo A/cm2/Sr Source Brightness: •Inherent function of emitter •Thermionic W: 5 Vo A/cm2/Sr •Thermionic LaB6: 200 Vo A/cm2/Sr •Field Emitter: 5000 Vo A/cm2/Sr AND • Beam Broadening: Z b 6.25 10 Eo 5 r A 1/ 2 t 3/ 2 Spatial Resolution AND • Beam Broadening: Z b 6.25 10 Eo 5 r A 1/ 2 t 3/ 2 Inserting values: Z=26 (Iron), r=8gm/cc, A=56, t=4E-6 cm (40 nm), Eo=200KV We find that b= 2.4x10-7 cm (2.4 nm) Optimizing, • We can estimate a spatial resolution of about 2 nm with 1% analytical precision Optimizing, • We can estimate a spatial resolution of about 2 nm with 1% analytical precision • Or, much better resolution if the required precision is not so high Optimizing, • We can estimate a spatial resolution of about 2 nm with 1% analytical precision • Or, much better resolution if the required precision is not so high • Requires VERY good sample! (e.g. thickness of ~10nm)