Electron probe microanalysis Electron - Specimen Interaction Revised 9/10/2003 UW- Madison Geology 777

What’s the point? Electrons from a source interact with electrons in specimen yielding a variety of photons and electrons via elastic and inelastic scattering processes. These are the “signals” that we monitor and measure to characterize our specimens. UW- Madison Geology 777

Overview Elastic and inelastic processes Characteristic and continuum X-rays K,L,M etc: families of X-rays Energy versus wavelength Moseley’s relation Absorption or critical excitation energy Interaction volume and ranges Monte Carlo models Odds of X-ray production UW- Madison Geology 777

Elastic and inelastic scattering of HV electron by sample Elastic (a): incident electron’s direction altered by Coulombic field of nucleus (Rutherford scattering), screened by orbital electrons. Direction may be changed by 0-180° (ave 2-5°) but velocity remains virtually constant. <1 eV of beam energy transferred. Inelastic (b): incident electron transfers some energy (up to all, E 0 ) to tightly bound inner-shell electrons and loosely bound outer-shell electrons. Direction barely changes (<0.1°) (Goldstein et al, 1992, p.72) E 0 = accelerating voltage (of electrons emitted from gun); usually keV UW- Madison Geology 777

Elastic and inelastic scattering of HV electron by sample (Goldstein et al, 1992, p.72) This represents 1000 electron trajectories (idealized), in a cross-section--both elastic and inelastic scattering. UW- Madison Geology 777

Scattering lexicon Cross section: a measure of the probability that an event of a certain kind will occur, e.g. K-shell cross section. Defined as Q = N/n i n t, where N=events of certain type/vol (sites/cm 3 ), n i =number incident particles/unit area (particles/cm 2 ), and n t =number target sites/vol (sites/cm 3 ). Q has units of cm 2 and is thought of as an effective ‘size’ which the atom presents as a target to incident particle. The Q for elastic scattering is ~ cm 2 and for K-shell ionization is ~ cm 2. Mean free path: average distance an electron travels within a specimen between events of a specific type. MFP=A/(N A  Q) where A is atomic wt (g/mol), N A is Avogadro’s number,  is density (g/cm 3 ). UW- Madison Geology 777

Elastic and inelastic scattering Elastic : Backscattering of electrons (~high energy) Inelastic : Plasmon excitation (in metals, loosely bound outer- shell electrons are excited) Phonon excitation (lattice oscillations: heating) Secondary electron excitation Inner-shell ionization (Auger electrons, X-rays) Bremsstrahlung (continuum) X-ray generation Cathodoluminescence radiation (non-metal valence shell phenomenon) UW- Madison Geology 777

Backscattered Electrons High energy beam electrons may suffer multiple elastic scattering events in the solid, with cumulative effect of escaping from the material. The fraction of beam electrons that scatter back (  ) was found experimentally to vary directly as a function of composition (atomic number Z). This provides a valuable imaging tool: a rapid means to discriminate phases that have different mean Z values. Intensity (grey level) varies from black (voids/epoxy), to plagioclase, olivine, basaltic glass, with Ti- magnetitethe brightest phase. UW- Madison Geology 777

Secondary Electrons Inelastic scattering of HV beam electron can promote loosely bound electrons from valence to conduction band in semiconductor or insulator with enough energy to move thru the solid (in metals, promotion from conduction-band directly). Backscattered electrons can also produce secondary electrons. a) Complete energy distribution of electrons emitted from target. Region I and II are BSE, Region III secondary. b) Secondary electron energy distribution, measured (points) and modeled (lines) By definition, these secondary electrons are <50 eV, with most <10 eV. (Goldstein et al, 1992, p. 107) UW- Madison Geology 777

SE images Secondary electrons are generated throughout the interaction volume, but only secondary electrons produced near the surface are able to escape (~5 nm in metals, ~50 nm in insulators). For this reason, secondary electron imaging (SEI) yields high resolution images of surface features.These have grey-scales, though pseudo-coloring is sometimes done. Pollen, cat flea, and Si nanowires on alumina sphere. 20  m UW- Madison Geology 777

SE and BSE coefficients (Goldstein et al, 1992, p. 109) Coefficients for backscattered-electron (  ) and secondary electron (  ) as function of Z. Tilt of specimen from 90° beam incidence (  ) is 0. E 0 =30 keV. Data from 1966; more recent views suggest the flat SE curve may be due to carbon contamination on specimen hindering SE escape. UW- Madison Geology 777

Inner-shell ionization: Production of X-ray or Auger e- (Goldstein et al, 1992, p 120) HV electron knocks inner shell (K here) electron out of its orbit (time=1). This is an unstable configuration, and an electron from a higher energy orbital (L here) ‘falls in’ to fill the void (time=2). There is an excess of energy present and this is released internally as a photon. The photon has 2 ways to exit the atom (time=3), either by ejecting another outer shell electron as an Auger electron (L here, thus a KLL transition), or as X-ray (KL transition). K shell L shell (=photoelectron) Blue Lines indicate subsequent times: 1 to 2, then 3 where there are 2 alternate outcomes Time UW- Madison Geology 777

X-ray Lines - K, L, M (Goldstein et al, 1992, p 121) K  X-ray is produced due to removal of K shell electron, with L shell electron taking its place. K  occurs in the case where K shell electron is replaced by electron from the M shell. L  X-ray is produced due to removal of L shell electron, replaced by M shell electron. M  X-ray is produced due to removal of M shell electron, replaced by N shell electron. UW- Madison Geology 777

All possible K, L, M X-ray Lines (Originally Woldseth, 1973, reprinted in Goldstein et al, 1992, p 125) UW- Madison Geology 777

X-ray Lines with initial + final levels (Reed, 1993) UW- Madison Geology 777

Nomenclature of X-rays (Reed, 1993) There is some movement now to change the way X- rays are described, from the traditional Siegbahn notation (e.g. K  1 ) to the the IUPAC (K-L 3 ). (International Union of Pure and Applied Chemistry). This table is from their 1991 recommendation. UW- Madison Geology 777

Absorption Edge Energy Example: Pt (Z=78) X-ray line energies and associated critical excitation (absorption edge) energies, in keV Edge or Critical ionization energy: minimum energy required to remove an electron from a particular shell. Also known as critical excitation energy, X-ray absorption energy, or absorption edge energy. It is higher than the associated characteristic (line) X- ray energy; the characteristic energy is value measured by our X-ray detector. UW- Madison Geology 777

Overvoltage Overvoltage is the ratio of accelerating (gun) voltage to critical excitation energy for particular line*. U = E 0 /E c Maximum efficiency (cross-section) is at 2-3x critical excitation energy. Example of Overvoltage for Pt: for efficient excitation of this line, would be (minimally) thisß accelerating voltage La keV Ma -- 4 keV Example: Pt (Z=78) X-ray line energies and associated critical excitation (absorption edge) energies, in keV * recall: E 0 =gun accelerating voltage; E c =critical excitation energy UW- Madison Geology 777

Fluorescence yield (Goldstein et al, 1992) Fluorescence yield (  ) is fraction of ionizations that yield characteristic X-ray versus Auger yield (  ) within a particular family of X-rays.  +  =1 UW- Madison Geology 777

Continuum X-rays HV beam electrons can decelerate in the Coulombic field of the atom (+ field of nucleus screened by surrounding e-). The loss in energy as the electron brakes is emitted as a photon, the bremsstrahlung (“braking radiation”). The energy emitted in this random process varies up from 0 eV to the maximum, E 0. On an EDS plot of X-ray intensity vs energy, the continuum intensity decreases as energy increases. The high energy value where the continuum goes to zero is known as the Duane-Hunt limit. Duane-Hunt Limit UW- Madison Geology 777

Continuum and Atomic Number At a given energy (or ), the intensity of the continuum increases directly with Z (atomic number) of the material. This is of critical importance for minor or trace element analysis, and also lends itself to a timesaving technique (Mean Atomic Number,“MAN”). MAN plot (Z-bar = average Z = MAN) Continuum intensity around the Si Ka peak, varying with Z: Mo (42), Ti (22), B (5). X axis is sin theta position units. UW- Madison Geology 777

X-ray units: A, keV, sin , mm = hc/E 0 where h=Plancks constant, c=speed of light = /E 0 where is is in Å and E 0 in keV also, the 2 main EMPs plot up X-ray positions thusly: Cameca: n = 2d sin  so for n=1 and a given 2d, an X- ray line can be given as a sin value (or 10 5 times sin  ) JEOL: distance (L, in mm) between the sample (beam spot) and the diffracting crystal, i.e. L= R/d, where R is Rowland circle radius (X-ray focusing locus of points) and d is interlayer spacing of crystal. UW- Madison Geology 777

Moseley’s Relation (Goldstein et al, 1992, p. 123) Moseley (1913, 1914) found that there is a regular relationship between the atomic number of a material and its characteristic X-ray wavelength. =B/(Z-C) 2, where B and C are constants for each family of X-rays. UW- Madison Geology 777

Cathodoluminesce When insulators and semiconductors are hit by HV electrons, long photons (UV, visible, IR light) may be emitted. The light may be bright enough to be seen in the reflected light image (examples are benitoite, scheelite, zircon, corundum, diamond, wollastonite, YAG, GaAlAs). Incident electrons may promote valence shell electrons across the band gap to the empty conduction band, creating electron-hole pairs. With no bias to sweep the electron away, it will recombine with the hole. The excess energy (= gap energy) will be emitted as a long photon. Impurity atoms as well as dislocations increase the possibilities for additional gap energies, yielding different wavelengths of emitted light.These may be valuable for production of diagnostic images.

CL Images Impurity atoms as well as dislocations increase the possibilities for additional gap energies, yielding different wavelengths of emitted light.These may be valuable for production of diagnostic images. CL is a cheap way to view overgrowths (inherited cores) and healed fractures in quartz and zircons. CL image of zircon from Yellowstone tephra (Lava Creek Tuff). Note faint oscillatory zoning surrounding sector-zoned core, and healed fractures. These are not visible in the BSE image. ~50 um grain. (courtesy Ilya Bindeman) UW- Madison Geology 777

Electron interaction volumes (Goldstein et al, 1992, p 80) Effect of beam interaction (damage) in plastic (polymethylmethacrylate), from Everhart et al., All specimens received same beam dosage, but were etched for progressively longer times, showing in (a) strongest electron energies, to (g) the region of least energetic electrons. Note teardrop shape in (g). Same scale for all.

Ranges and interaction volumes It is useful to have an understanding of the distance traveled by the beam electrons, or the depth of X-ray generation, i.e. specific ranges. For example: If you had a 1 um thick layer of compound AB atop substrate BC, is EPMA of AB possible? UW- Madison Geology 777

Electron and X-ray Ranges Several researchers have developed physical/mathematical expressions to approximate electron and X-ray ranges. Two common ones are given below. Electron range. Kanaya and Okayama (1972) developed an expression for the depth of electron penetration: R KO =( A E )/(  Z 0.89 ) X-ray range. Anderson and Hasler (1966) give the depth of X- ray production as: R AH =(0.064)(E E c 1.68 )/  where E c is the absorption edge (critical excitation) energy. There are nomograms for these ranges, given on the next slides.

Ranges UW- Madison Geology 777

Ranges From Will Bigelow, now emeritus U MI (Ann Arbor) UW- Madison Geology 777

Monte Carlo simulations With the development of PCs, Monte Carlo simulations of electron-beam interactions have been very easy to perform. You can input your specific sample composition and run various “what if” scenarios, e.g. what is the maximum penetration of the electron beam through a thin film, or what is the smallest size crystal in a glass matrix that can be analyzed. You will be performing some of these MC simulations in a take home exercise. Each MC run has distinct conditions: specific E 0, specific composition (Atomic wt and average Z), density, and potentially different tilt angle. UW- Madison Geology 777

Specimen Heating Castaing (1951) derived the maximum temperature rise in a solid impacted by electrons of E 0 energy and i current (in  A) and beam diameter d (  m):  T = 4.8 E 0 i /kd where k is thermal conductivity (W/cmK). For E 0 =20 keV and 20 nA, d=1 um, in a metal (k=1),  T is 2 K. In a typical mineral (k=0.1),  T is 20 K. And in organic material, (k=0.002),  T is 1000 K! (e.g. epoxy) Difficult materials: carbonates, hydrated materials, halides, phosphates, glasses, feldspars. (Reed 1993, p 158) UW- Madison Geology 777

“Harper’s Index” of EPMA 1 nA of beam electrons = coulomb/sec 1 electron’s charge = 1.6x coulomb ergo, 1 nA = electrons/sec Probability that an electron will cause an ionization: 1 in 1000 to 1 in 10,000 ergo, 1 nA of electrons in one second will yield 10 6 ionizations/sec Probability that ionization will yield characteristic X-ray (not Auger electron): 1 in 10 to 4 in 10. ergo, our 1 nA of electrons in 1 second will yield 10 5 xrays. Probability of detection: for EDS, solid angle < 0.01 (1 in 100). WDS, <.001 ergo 10 3 X-rays/sec detected by EDS, and 10 2 by WDS. These are for pure elements. For EDS, 10 wt%, 10 2 X-rays; 1 wt% 10 X-rays; 0.1 wt % 1 X-ray/sec. ergo, counting statistics are very important, and we need to get as high count rates as possible within good operating practices. UW- Madison Geology 777 Acknowledgement: I first encountered this treatment at the Lehigh Microscopy Summer School

Sources of X-ray data J.A. Bearden, 1964 (NBS; AEC) White et al (“Penn State” 1965) tables main lines in tables in Goldstein et al, and Reed texts Probe for Windows database (includes higher order lines for WDS), also online at perry.geo.berkeley.edu/geology/labs/epma/xray.htm NIST database: click on “X-ray Database” at bottom of page: Lawrence Berkeley National Lab online at xdb.lbl.gov for data and how to order handy free reference book UW- Madison Geology 777